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YOKOSUKA Cruise Report YK08-08 Leg 1 Mariana Forearc/Trough 25 June - 6 July 2008 Yokosuka - Guam Japan Agency for Marine-Earth Science and Technology (JAMSTEC)

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Page 1: YOKOSUKA Cruise Report...YOKOSUKA Cruise Report YK08-08 Leg 1 Mariana Forearc/Trough 25 June - 6 July 2008 Yokosuka - Guam Japan Agency for Marine-Earth Science and Technology This

YOKOSUKA Cruise Report YK08-08 Leg 1

Mariana Forearc/Trough

25 June - 6 July 2008 Yokosuka - Guam

Japan Agency for Marine-Earth Science and Technology

(JAMSTEC)

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This cruise report is a preliminary documentation as of the end of the cruise. It may not be corrected even if changes on content (i.e. taxonomic classifications) are found after publication. It may also be changed without notice. Data on the cruise report may be

raw or not processed. Please ask the Chief Scientist for the latest information before using. Users of data or results of this cruise are requested to submit their results to Data

Integration and Analysis Group (DIAG), JAMSTEC.

このクルーズレポートは、航海終了時点での情報に基づく資料である。作成後

に記載内容(分類学上の分類など)に修正、変更等が生じた場合でも、必ずし

も訂正されるとは限らず、予告なく訂正される場合もある。またクルーズレポ

ートに掲載されているデータは生データや暫定値である場合もある。もし記載

内容を利用・引用する場合は、首席研究者に事前に確認すること。データ等を

利用した場合には、成果を機構事務局(情報管理部署)に提出すること。

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YK08-08 Leg 1-1 Cruise Report Geological expedition of the serpentinite seamounts

in the Mariana forearc

June 25 – July 1, 2008 (Yokosuka to Guam)

Hirokazu MaekawaGraduate School of Science, Osaka Prefecture University,

Hisayoshi YokoseGraduate School of Science and Technology, Kumamoto University,

Patricia FryerHawaii Institute of Geophysics & Planetology, University of Hawaii

Hajime SatoGraduate School of Science and Technology, Kumamoto University,

Shogo YoshidaFaculty of Science, Osaka Prefecture University,

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Contents

Acknowledgements --------------------------------------------------------------------- 2ConfidentialityCruise Structure

I. Cruise summary ----------------------------------------------------------------------- 3II. General background and objectives ----------------------------------------------- 4 III. Dive Report -------------------------------------------------------------------------- 7

#1085 (H. Yokose) ------------------------------------------------------------------ 8 #1086 (P. Fryer) --------------------------------------------------------------------- 22 #1087 (H. Maekawa) --------------------------------------------------------------- 31

IV. Shore base study -------------------------------------------------------------------- 42

Appendix --------------------------------------------------------------------------------- 44 Rock description

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Acknowledgements

We are grateful to Captain Mr. S. Ryono, Chief Officer N. Kimura, and Chief Engineer K. Kajinishi for their safe navigation and their skillful handling of “R/V Yokosuka”. Great thanks are due to Commander Mr. T. Sakurai and “Shinkai 6500” operation team for taking us to deep sea floor and accurate operation of “Shinkai 6500”. We also thanks Mr. M. Okada, Nippon Marine Enterprise, Ltd., for their attentive supports. Finally, we would like to appreciate all the JAMSTEC personnels who have supported us for this cruise.

Confidentiality

Data and samples obtained during this cruise, most of which are described in this report, should be treated as carefully as possible, in order to protect the priority of the cruise participants. Confidential and publication policies are as follows: (1) The participants aboard the YK08-08 Leg 1-1 cruise agreed the future study plan discussed in wrap-up meeting and the plan will be listed in the cruise report. Those appeared in the lists have the priority for each study topics. (2) No one other than the cruise participants or those listed in the future study plan can submit papers or give oral (or poster) presentations using any data and/or samples of this cruise within 12 month after the completion of the cruise (July 7, 2009).

Cruise Structure

The YK08-08 Leg 1 cruise aboard the R/V Yokosuka (JAMSTEC) were planned to carry out the two projects in the Mariana forearc and back-arc areas. The first project (Leg 1-1, June 25-July 1, 2008) was planned to investigate the serpentinite seamounts in the Mariana forearc to clarify the tectonic development of forearc area, formation mechanism of serpentinite seamounts, and the nature of subduction zone metamorphism (PI: H. Maekawa). The second (Leg 1-2, July 2-6, 2006) was planned by Jamstec (PI: T. Fujiwara) to clarify the morphological and geophysical characteristics of Mariana trough. We will describe the outlines and results of Leg 1-1 cruise in this report.

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I. Cruise summary

Geological expedition was conducted by means of submersible vessel "Shinkai 6500" and its mother vessel R/V Yokosuka in the forearc seamounts west to the Mariana Trench from June 25 to July 1, 2008. The ship left Yokosuka on June 25, 2008 for the survey area. After three days, we started our survey, and successfully conducted three dives at three forearc seamounts, that is, "Pan -lid Seamount", north to Conical Seamount (Dive #1085), small serpentinite mud volcano (Dive #1086), and Twin Peaks Seamount (Dive #1087). Based on these dives, we understood that the main constituents of these seamounts are serpentinite mud matrix and serpentinized peridotite, and gravity flows of serpentinite materials often made ridges on the surface of seamounts. Dive #1085 was planned to move up the fault valley of the western slope of the Pan-lid Seamount. The seamount is situated at further north region from any serpentinite seamounts we have ever studied. Dive observations and sample descriptions reveal that this seamount is an old serpentinite seamount and are consistent with the morphological consideration. According to the thickness of manganese-iron oxide crusts on serpentinite breccia, this seamount has been inactive at least from 4Ma. Medium-grained sandstone that were formed by reworking of serpentinite breccias probably prevail in the low acoustic reflection area in the sidescan image and are restricted to gentle submarine flanks of the seamount. The interval of reworking process may be every tens of thousands year. The target of Dive #1086 is a relatively small serpentine mud volcano that represents the first stages of growth of serpentinite seamount. The first stages of formation may produce fresher peridotites because the conduit of the seamount has not grown large enough to have heavily serpentinized protolith. As we planned, we sccefully recover fresh peridotite, cores of serpentinite muds and collect any chimney materials discovered during the dive. The materials collected will be studied for composition, petrology/mineralogy and the muds will be analyzed for pore fluids and microbial populations. It may help us to understand the initial stage of formation of serpentinite seamount. Dive #1087 was planned to visit again the large ault scarp of Twin Peaks Seamount where we investigated during KR06-15 and found large blocks of blueschist facies rock, and aimed to reveal inner structure of serpentinite seamount in detail. According to our preliminary observation, the seamount is composed of mafic to ultramafic blocks of varying sizes up to several tens meter and fine serpentine mud materials (=matrix). It probably forms so-called "block-in-matrix fabrics", typical of on-land serpentinite melange. We successfully sampled many serpentinized peridotites, metagabbros, and green rocks. Based on many data obtained during YK08-08 Leg 1 cruise, we strolgly believe that we are sure to make great progress on morphological, petrological, and geochemial aspects of Mariana forearc seamounts.

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II. General background and objectives Dredging and drilling in the circum-Pacific regions have revealed that the serpentinized peridotites are often exposed in the nonaccretionary convergent plate margins (e.g. Fisher and Engel, 1969; Hawkins et al., 1972; Bloomer and Fisher, 1987; Ogawa et al., 1985a; Ogawa et al., 1985b; Honza and Kagami, 1977; IGCP Working Group, 1977; Fryer et al., 1985; Ishii, 1985; Fryer, 1996, Fryer and Fryer, 1987). In the Izu-Bonin (=Ogasawara) and Mariana forearcs, enormous amounts of serpentinized peridotite have been raised from the mantle wedge to the seafloor to form a zone of seamounts (Fryer et al., 1985). Fryer et al. (1990) described the fluids seeping from the chimneys at the summit of one of the seamounts (Conical Seamount), and indicated that the fluids were derived from the dehydration process of descending slab. Ultramafic rocks obtained from the serpentinite seamounts are mainly harzburgite with subordinate dunite, which are more depleted than the abyssal peridotites from the mid-oceanic ridge (Ishii et al., 1992). They were often highly tectonized. Common occurrences of kink band in olivine and pyroxene crystals provide evidence of penetrative deformation. All of them are serpentinized to some degree. Serpentine minerals are antigorite, chrysotile and lizardite (Saboda et al., 1992; D'Antonio and Kristensen, 2004). In addition to these minerals, serpentinized peridotites often contain brucite, and rarely contains minor amounts of acicular diopside and tremolite. Antigorite commonly associate with brucite. Two types of serpentine mineral association are recognized in serpentinized peridotites; one is antigorite bearing and one is antigorite free. In the rocks with the former association, chrysotile and/or lizardite also occur in vein or matrix as later-stage secondary serpentine minerals. Antigorite is stable to higher temperatures than chrysotile (Iishi and Saito, 1973; Evans et al., 1976), and stable association of antigorite and brucite gave stability range of 300˚C~450˚C (Evans et al., 1976). Lizardite favours low temperature condition of less than 300˚C, and is considered to have the same P-T stability field as chrysotile (Peacock, 1987). The high-temperature antigorite-bearing assemblages are only found in the serpentinites from Conical and South Chamorro Seamounts, which are situated in the forearc areas 80-90 km west to the trench. The distances indicate that these two seamounts are located at the most far-off places from the trench

Serpentinite Seamount

YK08-08 #1085Conical SMT

Pacman SMT

Quaker SMTTwin Peaks SMT

Big Blue SMT

Turquoise SMT

Celestial SMT

Peacock SMT

Blue Moon SMT

North Chamorro SMT

South ChamorroSMT

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in all the forearc serpentinite seamounts, suggesting the possibility that the serpentinites of these seamounts were originated from the high-temperature deeper part of subduction zone than those of other seamounts situated near the trench. One of our aims of this cruise is to examine mineral assemblages of serpentinites from the many seamounts as much as possible, to establish the distributions of above two serpentine mineral assemblages in the forearc area, and to clarify the origin and process of serpentinization in the subduction zone. We believe that the results obtained this study could be important to proceed the scismological study around the subduction zone. During Ocean Drilling Program Leg 125, small clasts of high-pressure/low-temperature blueschist facies metabasites were recovered from Conical seamount. It suggests the blueschist facies metamorphism beneath the forearc (Maekawa et al., 1992 and 1993). Recent discoveries of blueschist-facies rocks and sediments including fragments of blueschist-facies minerals from other seamounts suggests that the blueschist-facies rocks must be common in the Izu-Bonin and Mariana forearc seamounts (Maekawa, 1995; Maekawa et al., 2004). To understand the behavior of slab-derived fluids during serpentinization and metamorphism is important because the fluids must be one of the main controlling factors of physical properties of subduction zone materials, which could affect the modes of material transport and earthquakes within the subduction zone. Metamorphic rock clasts recovered from Hole 778A and 779B at Conical seamount provide essential information on interaction between forearc materials and water. Geochemical study of the 778A metabasites indicates that the rocks have chemical affinity with mid-ocean ridge basalts, some of which have zigzag REE patterns due to intense interaction with seawater. These MORB-type rocks probably have originated from trapped oceanic crust in the Mariana forearc when the subduction of the Pacific plate started. The trapped oceanic crust has been tectonically eroded by the subducting slab, contaminated with seawater, and dragged into depth, where blueschist facies metamorphism has prevailed. The Hole 778A metabasites commonly contain quartz veins, which have been produced prior to or during blueschist facies metamorphism because high-pressure minerals, lawsonite, pumpellyite, and aragonite were often crystallized in the vein. When the trapped oceanic crust has been eroded by the subducting slab, the fragmented oceanic crust had encountered the pelagic sediments at the top of the subducting slab. The Si-rich fluids having permeated the Hole 778A rocks were probably derived from pelagic sediments on top of the subducting slab. A metasomatic rock clast rich in phengite and chlorite was obtained from Hole 779B (125-799B-01R-06, 19-22) at Conical Seamount (Maekawa et al., 2001), and abundant tremolite-chlorite schists were reported from South Chamorro Seamount during ODP 195 (Shipboard Scientific Party, 2002). Rocks with similar mineral associations and similar geochemical characteristics are often found in metasomatic reaction zone developed at the boundary between serpentinite and pelitic schist in the high-pressure Sanbagawa metamorphic belt, Japan. The clast may have been

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formed at the boundary between hanging-wall peridotite and subducting oceanic crust where the hydrothermal metasomatic reactions have pervasively occurred (Maekawa et al., 2001 and 2004). We wish to establish the general ideas of metamorphism and metasomatism along and above the subduction boundary by examining these rocks obtained during this cruise. Further more, we will make effort to clarify the modes of occurrence ( = morphological features) of the forearc seamounts, petrological and geochemical characteristics of serpentinized peridotite, metamorphic rocks, and metasomatic rocks occupying the seamounts, and consider the phisico-chemical conditions below the forearc and the behavior of water squeezed from the subducting slab in the formation of serpentinite seamounts. Along the Mariana forearc many serpentinite seamounts were discovered for the first time during the Kana Keoki cruise in 1981 (Fryer, 1984). Before ODP drilling pre-site survey was car-ried out for deep-tow, submersible Alvin dive as well as multi-channel seismic profile (Fryer, Pearce, Stokking, 1989). 5 drill holes were spudded in Conical Seamount (Hole 778-780) and Izu Torishima seamount (783 and 784) during ODP Leg 125 (1989), and in the Chamorro Seamount (Hole 1200, A-E) during ODP Leg 195 (2001).The results gave us the stratigraphic and petrologic insights, but the recoveries of the cores were so poor that origin and evolution of the serpentinite seamount remain unsolved. The morphology of serpentine seamount was summarized by Fryer et al. (1998) during several cruises. They recognized several seamounts from north to south, Conical, Pacman, Big Blue, Celestial, Peocock, Blue Moon, North Chamorro and south Chamorro, respectively. They also named four more other SS (=serpentine or serpentinite seamount) other than the above, called Ms Pacman, Quaker, Blue Eye and Deep Blue, that correlate to the three big SS south of Pacman from north to south and SS between Celestial and South Chamorro. Fryer described chimney and cold seep at the Quaker and Pacman (Cerulean Spring).Fryer et al. (1999) described serpentine seamount and discussed the origin of the SS. They demonstrated that the SS was formed by the hydration and serpentinization of wedge upper mantle then buoyant uplift by their density difference to make serpentine seamount. High-pressure and low-temperature metamorphic rocks were transported by this process. Fryer et al. (2000) described each SS using the sss data and proposed fault drived uplift and transportation of serpentine mud and the lateral variation of high pressure blueschist facies metamorphic rocks by the depth of the subducting Pacific slab. Stern and Smoot (1998) summarize the tectonic resume of the Mariana forearc including forearc basin and serpentine seamounts by the side scan sonar data of this region. They recognize the major tectonic difference, oblique and normal subduction of the Pacific Plate. Fujioka et al. (2003) proposed a new biosphere “Serpentine Biosphere” along the Mariana forearc and suggest way how to estimate the lower limit of the subcrustal biosphere by the elaborated study of microbiological method. Fujioka et al. (2003) conducted the bathymetric survey on the forearc serpentine seamount and made a brief summary of the SS (Onboard report of YK03-11 cruise). They described morphological features of each SS with dive results. Other than these studies Jonson et al. (1991) reported the existence of allochthonons such as the Cretaceous chert, foraminifers and volcanic rocks that were accreted at the trench lower slope by the subduction of the Pacific Plate. Therefore as for the origin and development of the Mariana Forearc Serpentine Seamount still remain unsolved even though almost 50 years had passed with many cruises since 1948 when Hess (1948) first pointed out the origin of the Mariana Arc system.

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3-1. Dive list & dive site mapDive# Observer

Main objectives SiteDatum Start Lat.(N) Lon.(E) Depth

(m)date Affiliation Nav. End(LCT)

6K#1085 Hisayoshi YOKOSE Sampling rocks and pebbles, Push cores, and observation at the summit of the seamount

Small serpentinite

mud volcano

WGS-84 12:04 19º31.70' N 146º38.88' E 3160

29th June 08 Kumamoto Univ. SSBL 14:40 19º32.41' N 146º38.95' E 2946

6K#1086 Patricia FRYER Sampling rocks and pebbles, Push cores, and observation at the summit of the seamount

Small serpentinite

mud volcano

WGS-84 11:20 18º45.68’ N 147º7.67’ E 3160

30th June 08 Univ. of Hawaii SSBL 15:54 18º44.48’ N 147º7.78’ E 2946

6K#1087 Hirokazu MAEKAWA Observation of the steep slope showing the cross-cut section of serpentinite seamount

Twin Peaks Seamount

WGS-84 10:50 18º35.90' N 146º58.57' E 2948

1st July 08 Osaka Prefecture Univ. SSBL 14:43 18º35.49' N 146º58.26' E 2632

-7000

-600

0

-6000

-6000

-6000

-600

0

-5000

-5000

-5000

-500 0

-500

0

-5000

-5000

-4000

-4000

-4000

-4000

-4000

-4000

-400 0

-4000

-4000

-4000

-4000

-4000

-4000

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0

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0

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0

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-3000

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0

-300 0

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-3000

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-2000

-20000 50

km

146°E 147°E 148°E18°N

19°N

20°N

21°N

contour interval 200 m

YK08-08 Leg1 Dive Site

#1085

#1086

#1087

-10000 -5000 0

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Dive Report: SHINKAI 6500 DIVE # 1085

Date: 29 June, 2008Site Name: Pan -lid Seamount (temporally name), Mariana forearcLanding: 11:17; 19_53.2401’ N, 146_29.7547’ E, -2461 m Leaving: 16:11; 19_54.4120’ N, 146_31.4252’ E, -1959 mObserver: Hisayoshi Yokose (Graduate school of science and Technology, Kumamoto University)Pilot: K. Matsumoto, Co-Pilot: F. Saito

Objectives:Hydrous wedge mantle obtains buoyancy resulted in the dehydration of subducting plate and it can move upward through fracture zones that are created by tectonic movement. As a result, serpentinite mud volcanoes, which have a similar growth history of a mud volcano, are formed. If we adopt this model to the Mariana arc-trench system, the distribution pattern of the serpentinite seamounts should be controlled by the geometry of the down going slab and are expected to align equidistantly like a volcanic front. However, recent investigations reveal that serpentine seamounts are not aligned equidistantly but distributed locally around the horst-graben structure.Investigations on serpentinite seamounts in the Mariana forearc region have been concentrated in the central and southern part of this arc. Conical seamount is the northern end of the well studied area. Therefore, geological investigation of the father north Mariana forearc region is scarce. Spacial distribution pattern of serpentinite seamount is expected to provide us the detailed chemical interactions in the serpentinized wedge mantle.Previous deep-sea investigations of serpentinite seamounts were concentrated summit region where remarkable acoustic reflection is observed in the sidescan sonar image. Therefore, geological information about serpentine seamount may be biased by serpentinite mud flows around the summit region. To understand the whole spectrum of the geological information about serpentinite seamounts, we need to investigate the other part of the seamount. Because geomorphological characteristics are similar to the other serpentinite seamount, but the summit region cannot delineate the serpentinite mud flows using high acoustic reflection image, the dive site was chosen. The objectives of this dive are (1) to clarify the geological characteristics of the acoustically low reflection area which has the topographically similar to the other serpentinite seamount; (2) to discuss the internal structure of serpentinite seamount collecting geological information from outcrops that are expected to appear on the fault scarps; (3) to collect samples of serpentinized peridotite for petrological study of the wedge mantle; (4) to find chimneys, which are symbolic trace of hydrothermal activity, at around the summit region. These data will provide useful clue to construct complete geological images and growth process of the serpentinite seamounts.

Dive summary: Dive #1085 was designed to move up the fault valley of the western slope of the seamount. The valley is inferred to have been developed along one of the regional scale lineaments (NE-SW direction). We started from the southwest submarine flank of the seamount at 2500 m depth, and move to the shallowest eastern summit, at 1900m depth, through the central mountain pass. The length of the dive course is about 3.5 km. One push-core sample and 22 rocks were recovered during the dive. Based on the collected rock samples and observations from the submersible, outcrops can be divided into three geological units: (1) surface medium-grained sand which commonly indicates ripple structure, (2) massive or layered medium-grained sandstone, (3) serpentinite breccia including various sizes of peridotite blocks. The geological route map of the dive #1085 is shown in appendix 3-A.

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The seafloor was covered by very thin medium-grained sand. The ripple structure was seen almost everywhere during the dive. The ripple structures imply a constant weak current above the seafloor. The thickness of the surface sediment was examined using push-core sampler and was less than 10 cm (Stop 9) even in the flat summit region. Many foraminiferal fossils were observed in medium-grained sand recovered by the push-core sampler. The crests of ripple structure are appeared as a white bund in the video images. It could be a concentrated part of the microfossils. The rippled medium-grain sand covers widely underlying massive sandstone layer. Outcrops of the medium-grained sandstone layer were observed on fault scarps (Appendix 1a: (1), (3), (6)). Most of the outcrops indicated massive sandstone, but some of them have platy joints with 20 cm intervals. More thin layers (~5 cm thick) also can be found in a hand specimen (R15). The dipping of the platy-jointed medium-grained sandstone is almost parallel to the slope angle. The occurrence implies that the underlying strata are draped by the sandstone layer.Talus breccias have tabular or box shape when occurred at the base of the medium-grained sandstone outcrops. Vertically fractured medium-grained sandstones layers were often observed and are caused by translational extension on the relatively steep slopes. Fracture surface of the talus breccias and floats, which are derived from medium-grained sandstone, are very smooth. The characteristics of fracture surface are very important tool for identifying the rock variations.Most samples of the medium-grained sandstone indicate massive in their appearance, but one sample has three layers (R15). Each layer shows sedimentary flow unit with normal grading and includes some angular fragments of serpentinite. The boundaries between layers are not flat and sometimes show wavy surface (appendix 2. (d)). As the boundaries consist of manganese-iron oxide crusts (less than 1mm in thickness), there are no constant sediment supply from the water column and the sedimentary processes are episodic. The manganese-iron oxide crust layers suggest that there are no sedimentation periods after formation of each layer, or the currents on the sea floor, which produce the ripple structure, are preventing the sedimentation from the water column. Because constituents of the sandstone are very similar to the matrix of serpentinite breccias, the sandstone could be reworked sediment of underlying the serpentinite breccias. Serpentinite breccia with peridotite blocks were often observed at the base of fault scarps and the summit region. Serpentinized dunit and harzbergite were recovered from the serpentinite breccias. Because outcrops of serpentinite breccias are thickly coated by manganese-iron oxide crust, occurrence of the peridotite blocks could not be observed in large area. In many case, occurrences of the serpentine breccia are very similar to talus breccias of medium-grained sandstone. The serpentinite breccias that are rough surface with thickly coated manganese-iron oxide crust are distinguishable from talus breccia of medium-grained sandstone. (Appendix 1b-c, cf. Appendix 1a-(1), (3) and (5)). Matrix of the serpentinite breccia indicates greenish cream white when manganese-iron oxide crust was removed. Matrix of the serpentinite breccias (see appendix 2) consists of angular fragments that are from coarse- to fine-grained sand in size. The grains are angular polymictic fragments and are mainly consist of serpentinite, peridotite and talc. It is difficult to recover the matrix by manipulators due to its unconsolidated nature. Samples, R11 and R12, have abundant euhedral acicular aragonite crystals (appendix 2). In sample R18, abundant broken acicular aragonite crystals are included. These euhedral acicular aragonite crystals suggest in situ hydrothermal activities in this fault valley. The thickness of manganese-iron oxide crust developed on the serpentinite breccia are more thicker than those of the sandstone and is exceeded 1mm. The maximum thickness of the manganese-iron oxide crust is reached to 10 mm. Because growth rate of manganese-iron oxide crust is considered to be 2.5mm /1Ma (Moore and Clague, 2004), the serpentinite breccias on the summit region have been exposed in the sea from at least 4Ma.On the other hand, the thicknesses of the manganese-iron oxide crust of massive or weakly laminated medium-grained sandstones are very thin (less than 1mm). The difference in the crust thickness suggests that the medium-grained

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sandstones are relatively younger than the serpentinite breccias. There are two remarkable large lineaments in this region, directed NE-SW and NW-SE. They are crossing at approximately right angle. Fault scarps appeared in the dive course also mainly consist of two different directions, NNE-SSW and WNW-ESE and are little bit different from the regional scale lineaments. These fault scarps arrange an echelon fashion (appendix 3-A). The difference in the direction between the regional scale lineaments and outcrop scale fault scarps may interpret as a difference in the observational scale. Dive observations and sample descriptions reveal that this seamount is an old serpentinite seamount and are consistent with the morphological consideration. According to the thickness of manganese-iron oxide crusts on serpentinite breccia, this seamount has been inactive at least from 4Ma. Medium-grained sandstone that were formed by reworking of serpentinite breccias probably prevail in the low acoustic reflection area in the sidescan image and are restricted to gentle submarine flanks of the seamount. The interval of reworking process may be every tens of thousands year. On the basis of the above observations, interpretive geological map is shown in appendix 3.

Biological observations Representative animals are indicated in appendix 4.

Keywords: Serpentinite breccias, peridotite blocks, sandy matrix, euhedral aragonite, reworked sediment

Payloads:(1) Sample boxes x2, (2) Push core x6, (3) Sub-bottom profiler, (4) Magnetometer

Location of Events:Time Position Depth 11:17 19_53.2401’ N, 146_29.7547’ E; 2461m Stop 01: Sampling two platy-jointed floats (R01and R02) of float near the landing site

11:42 19_53.3105’ N, 146_29.7891’ E; 2464m Stop 02: Sampling three rectangular blocks (R03, R04, and R05) from talus breccia at the base of outcropping layered sandstone 12:05 19_53.3544’ N, 146_29.8184’ E; 2434m Stop 03: Sampling three floats (R06, R07, and R08) at the base of outcropping layered sandstone. Two medium-grained sandstones and one serpentinite breccia with a peridotite block. The breccia is thickly coated with manganese-iron oxide crust (3mm) but the other two sandstones have thin film of the crust

12:23 19_53.4282’ N, 146_29.8527’ E; 2363m Stop 04: Sampling massive sandstone (R09) from smoothly surfaced outcrop with thin manganese-iron oxide crust 13:03 19_53.6103’ N, 146_30.0166’ E; 2259m Stop 05: Sampling a platy-jointed boulder (R10) from talus breccias at the base of outcropping massive sandstone, a serpentinite breccia (R11) from outcrop, and a float of dunite (R12)

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13:22 19_53.6564’ N, 146_30.0900’ E; 2207m Stop 06: Sampling a platy-jointed boulder (R13) and sub angular boulder (R14) from talus breccias at the base of outcropping massive sandstone 14:05 19_54.0325’ N, 146_30.3834’ E; 2052m

Stop 07: Sampling a subangular float (R15) and two angular peridotite blocks (R16 and R17) from outcropping serpentinite breccia

14:28 19_54.1680’ N, 146_30.5398’ E; 1991m Stop 08: Sampling two boulders of serpentinite breccia with peridotite blocks (R18 and R19) from talus breccias at the base of outcropping fault scarp

15:00 19_54.2905’ N, 146_30.7691’ E; 2007m Stop 09: Sampling a push-core of surface sediment at summit area 16:07 19_54.4131’ N, 146_31.3845’ E; 1961m Stop 10: Sampling two peridotite blocks (R20 and 22) and one matrix of the breccia (R21) from outcropping massive serpentinite breccia

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Appendix 1a. Representative outcrops and samplesa. Rippled sandy sediment

b. Layered medium-grained sandstone

(1) No. 2 camera, 11:32, near the landing site (2) No. 2 camera, 15:05, near the sumit region

serpentinite breccia

(1) -(2) Stop 3:

Sample:

R3, R4 and R5,

medium-grained

sandstone with thin

manganese-iron oxide

crust

(3) near the stop 4.

Sample( R07 and R08)

(4) stop 7.

sample: R13 and R14

medium-grained

sandstone with thin

manganese-iron oxide

crust

(5) talus breccia

formed at the base of

layered medium-

grained sandstone

(6) Platy-jointed

sandstone bed

,

Dive Report: SHINKAI 6500 DIVE # 1085 Appendix

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Appendix 1b. Representative outcrops and samplesc. Outcrops of serpentinite breccia with peridotite blocks

d. Fault scarp

Serpentinite breccia

Layered medium-grained sandstone

Depression(3m ~ 6m)

Serpentinite breccia

(samples: R20, R21, and R22) Serpentinite breccia

(samples: R18, R19)

Sample (R16 and R17)

Depression(~ 2m)

(1) Stop #07 (14:05)

(2) Stop #08 (14:28) (3) Stop #11 (16:07)

(1) No. 2 camera 14:20 (2) No. 2 camera 15:25

Dive Report: SHINKAI 6500 DIVE # 1085 Appendix

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Appendix 2. Matrix of serpentinite breccia (a) Dive 1085_R06 (12:05) matrix: coarse ~ fine grained sand polymictic angular fragments

(b) dive 1085_R18 (14:28) matrix: coarse ~ fine grained sand polymictic angular fragments with abundant fragmented accicular aragonite crystals

bioturbation

Angular fragments of serpentinite

Thin maganese-oxide crust

Accicular crystals (euhedral aragonite)

accicular crystal

(euhedral aragonite)

Talc

(c) dive 1085_R21 (16:07) matrix: coarse ~ fine grained sand polymictic angular fragments with abundant accicular aragonite crystals

(d) dive 1085_R11(13:03)

Reworked serpentinite breccia (Medium-grained sandstone with angular fragments)

Serpentinite

Serpentinite

Talc

Serpentinite

Talc

broken aragonite

Dive Report: SHINKAI 6500 DIVE # 1085 Appendix

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Dive 1085Rippled sandy sedimentDraped sandstonelayer (reworked sed.)Serpentinite breccia withperidotite blocks

Dip &Strike

Fault

-2000 m

-2100 m

-2200 m

-2300 m

-2400 m

500 m

146_31.5’ E146_31.0’ E146_30.5’ E146_30.0’ E

19_54.0’ N

19_53.5’ N

Appendix 3. Route map & geological interpretation

B. Sidescan image C. Preliminary geologic interpretation of the dive site

Muddy sediment

Basement rocks

Serpentinitemud volcano

19_55’ N

20_00’ N

20_05’ N

20_10’ N

19_50’ N

19_45’ N

19_40’ N

19_35’ N

A. Route map of the dive 1085

Stop 11

Stop 10Stop 9

Stop 8

Stop 4Stop 5

Stop 3

Stop 7

Stop 6

Stop 2

Stop 1

The draped sandstone layer probably correspond to the low acoustic reflection region in the sidescan image.

Dive Report: SHINKAI 6500 DIVE # 1085 Appendix

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Appendix 4. Biology(a) Octopoda Opisthoteuthidae

Grimpoteuthis sp.

~ 70 cm long,

No.2 camera, 12:36 ~ 12:40

(b) Elasmobranchii Batoidea

Myliobatiformes ?

~ 40 cm long,

No.2 camera, 15:05 ~ 15:06

(c) Anguilliformes Anguilloidei

or Congroidei

~ 40 cm long,

No.2 camera, 15:33

Dive Report: SHINKAI 6500 DIVE # 1085 Appendix

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H. Yokose 2008/06/29Time(JST)

Dep.(m)

Alt.(m)

Head(Deg)

BoxNo. Description Remarks

10:06 driving for the sea floor10:29 1000 depth: 1000m, driving for the sea floor10:52 2000 443 107 depth: 2000m, driving for the sea floor11:14 2458 3 23 visible on the bottom11:18 2461 3 155 arrive to the bottom,serpentinite blocks on the sea floor&ripple11:21 2461 2 153 S1 get the serpentinite block #111:26 2461 3 75 S1 get the serpentinite block (about 20 cm diameter) #211:31 2469 2 282 ripple11:35 2465 2 21 outcrop

11:40 2464 3 19 S2 get surpentinite blocks #3,#4,#5 X:-1030Y:-1340

11:47 2459 3 30 set the course 35 degree

11:53 2436 3 30 check the position X:-970Y:-1300

12:06 2434 3 42 B2 get serpentinite blocks #6,#7,#8 X:-900Y:-1240

12:08 2430 1 31 set the course 30 degree

12:16 2372 2 31 check the position X:-670Y:-1190

12:19 2363 2 45 ripple

12:21 2363 3 64 S3 get the serpentinite block #9 X:-630Y:-1170

12:24 2358 1 44 set the course 45 degree12:31 2327 2 45 ripple and some blocks

12:34 2307 2 45 check the position X:--520Y:-1050

12:37 2293 2 65 Octopus12:43 2279 5 46 set the course 45 degree12:49 2261 2 46 ripple12:50 2259 3 63 serpentinite blocks

13:04 2259 3 65 P1 get the serpentinite rocks #10,11,12 X:-240Y:-940

13:06 2250 2 53 set the course 55 degree13:08 2245 2 55 ripple and a little blocks13:10 2233 3 55 outcrop13:11 2230 2 56 ripple and serpentinite blocks13:16 2207 1 92 outcrop? white fish

13:20 2207 3 123 P2 get the serpentinite block #13 X: -180Y:-780

13:22 2207 1 65 P2 get the serpentinite block #1413:25 2205 2 69 leave the sampling site. Large blocks (>1 m)13:27 2190 2 52 ripple and blocks. Set the course 55 degree

13:30 2168 3 53 check the position X:-90Y:-750

13:36 2129 2 52 check the position X:30Y:-630

13:40 2107 3 52 ripple

13:46 2074 3 53 check the position X:260Y:-380

13:47 2063 4 32 set the course 35 degree13:53 2052 5 1 set the course to high reaction of soner13:57 2052 1 1 outcrop

13:59 2052 2 38 P3 get a serpentinite block #15 X:490Y:-270

14:01 2052 3 43 P3 get a serpentinite block #1614:05 2052 2 36 P3 get a serpentinite block #1714:07 2051 2 36 set the course 40 degree

14:15 2013 3 40 check the position X:640Y:-220

14:16 2009 3 50 set the course 50 degree

14:20 1994 2 52 check the position X:720Y:-100

14:24 1990 3 37 blocks

14:26 1991 3 305 P4 get the gabbro #18 X:750Y:-10

14:28 1991 207 292 P4 get the serpentinite block #1914:32 1988 2 92 set the course 80 degree, ripple14:35 1987 2 78 serpentinite blocks

Dive Log of 6k DiveLatitude/

Longitude

X:-1100Y:-1420

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H. Yokose 2008/06/29Time(JST)

Dep.(m)

Alt.(m)

Head(Deg)

BoxNo. Description Remarks

Dive Log of 6k DiveLatitude/

Longitude

14:45 1999 1 50 check the position X:790Y:230

14:53 2007 1 50 check the position X:940Y:350

15:00 2007 3 351 push core (blue) X:970Y:410

15:04 2006 1 89 set the course 90 degree15:10 1999 1 90 serpentinite blocks15:14 1995 1 100 sponges

15:17 1999 3 90 check the position X:970Y:610

15:24 1997 1 92 ripple

15:32 1994 1 102 give up to get stone,set the course 90 degree X:980Y:820

15:40 1975 2 90 check the position X:1050Y:1030

15:45 1963 2 90 check the position X:1090Y:1220

15:53 1959 3 57 check the position X:1140Y:1450

16:08 1957 2 91 P5 get the serpentinite rocks #20,#21,#22 X:1190Y:1470

16:10 1959 3 91 leave the bottom

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-4500

-4000

-4000

-400

0

-4000

-400

0

-400

0

-4000

-350 0

-3500

-3500

-350

0

-3500

-350

0

-3000

-300

0

-300

0

-3000

-3000

-300

0

-300 0

-3000

-300 0

-3000

-3000

-2500

-2500

-2500

-2500

-2500

-2000

-2000 -1500

-8000 -6000 -4000 -2000 0 m

0 10 20

km

2008 Jul 02 13:15:34 contour:50m

146 00'E 146 20'E 146 40'E

19 40'N

20 00'N

20 20'N

20 40'N

depth

21 00'N°

°

°

°

°

°°°

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-300

0

-290

0

-2800 -2700

-260

0

-2500 -240

0

-2300

-2300

-2200

-2200

-2100

-2100

-2000

0 1 2

km

2008 Jul 04 08:01:19 contour:20m Stars are sampling sites.

146 28'E 146 30'E 146 32'E19 52'N

19 54'N

19 56'N°

°

°°

°°

Dive1085

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Dive Report: SHINKAI 6500 Dive #1086

Date: 30 June, 2008Site Name: Small serpentinite mud volcano, Mariana forearcLanding: 11:20; 18°45.6831’N, 147°7.6659’E, 3160mLeaving: 15:54; 18°44.4823’N, 147°7.7802’ E, 2946mObserver: Patricia Fryer (School of Earth Science and Technology, Hawaii Institute of Geophysics and Planetology, University of Hawaii) Pilot: T. Komuku, Co-Pilot: K. Chiba

Objectives: The objectives of this dive are 1) to return to a site where in 2003 a gravity core of serpentine mudflow material revealed a strong geochemical indication, from analysis of the pore fluids, that they had derived from the subducting slab and recover serpentinite mud and serpentinized peridotite; 2) to determine the nature of early eruptions at serpentinite mud volcanoes; and 3) to collect samples for microbiological analysis. This site is on a relatively small serpentine mud volcano and likely represents the first stages of growth of these seamounts. The first stages of formation of these seamounts may produce fresher peridotites because the conduit of the seamount has not grown large enough to have heavily serpentinized the surrounding protolith. The sampling plan is to recover peridotite, cores of serpentinite muds and collect any chimney materials discovered during the dive. The materials collected will be studied for composition, petrology/mineralogy and the muds will be analyzed for pore fluids and microbial populations.

Dive Summary: At 11:24 the sea floor came into view. We landed slightly east of the proposed landing point at about mid-way up the northern flank of the seamount. The seafloor was sedimented and the sediment had numerous animal trails and burrows. There were also long (~2 m maximum) narrow, but shallow, scours in the sediment, possibly where fish had rubbed themselves on the sea floor. A very small number of pebble-sized to cobble-sized manganese nodules lay scattered on the sea floor at the landing site. We started on a southeasterly course up hill to examine the flank of the seamount. After examining the sea floor surface at an altitude of 1 m for ~10 minutes we increased altitude to ~3 m to optimize for the sub-bottom profiler. The data on the sub-bottom profiler display indicated a “basement” reflection at about 1 m depth. At 11:41 we located some outcropping boulders at a depth of 3146 m and maneuvered to get into position to sample them (Stop #1). The boulders were coated with manganese oxide and appear to be composed of a red-brown sediment. The outcrop gives the appearance of having been broken apart by horizontal extension as it has mainly vertical fractures. Some of the boulder surfaces do not have manganese coating. We continued on a southerly course up slope after sampling and encountered the same heavily bioturbated sediment surface until 12:05 (3108 m depth) where we observed thin (~8-10 cm thick) sheets of manganese-oxide coated sediment protruding out of the sediment on a slope that was slightly more steeply inclined. The slope is sediment covered. At 12:13 another outcropping of boulders appeared through the sediment (3077 m depth) and we stopped to sample them (Stop #2). Again at 12:22 we observed a small patch of boulders protruding from the sediment. At 12:53 a small octopus is visible in the external still camera picture and can be seen swimming on CAM2 between 12:53:20 and 12:53:34 (2964 m depth). At 12:56 we saw the first indication of serpentinite mud at the surface (2952 m depth). Small boulders appeared set in a matrix of light-green serpentinite mud with a thin veneer of sediment. The sediment cover on the sub-bottom profiler disappears at about this point. We maneuvered to sample the serpentinite. This appears to be the newest eruptive site on this mud volcano. We changed course to a more NE direction and proceeded up the slope until 1:08 at a depth of 2952 where we sampled a large

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boulder of serpentinite mud matrix (Stop #3). We changed course back to a southerly direction after sampling and proceeded up slope, where we began to observe undulations in the surface. The wavelength of the undulations is less than 1 m and they are 20-30 cm high at about 13:02 then become larger up-slope until at 13:20 they have wavelengths of about 3 - 4 m and the height is about 1 – 1.5 m. Serpentine mudflow matrix is present everywhere on the slope and dusted with a thin veneer of sediment (the sub-bottom profiler shows essentially no sediment cover in the area). As we neared the summit of the mound the wavelength of the undulations and the height of the ridges again diminished. We stopped at 1:34 to sample the mudflow (Stop #4). The depth at the summit is 2933 m, thus the mound is ~20 m high and the map shows it to be ~350 m in diameter. We changed course to go eastward because sonar indicated this direction was up-slope, but we almost immediately began to descend a slope to the east. At 13:50, at a depth of 2949 m, a sharp ledge striking ~NNE and dropping-off to the southeast marks an abrupt change in slope. We changed course again to follow this steeper slope and at 13:59 we encountered some large boulders on the side of the slope (at 2950 m) that have a different texture. There are protuberances reminiscent of chimney structures on one of them. We stopped to reorient the submersible to sample them (see external still photos at 13:59 and 14:00). We observed a light colored structure projecting from one of the boulders and we tried to sample it (Stop #5). It was very fragile and broke apart in the manipulator and fell between the boulders. At 14:06:30 on CAM2 the video shows a close-up of the chimney material that was too friable to handle with the manipulator. We picked up a piece of the part of the large boulder that was projecting out of the boulder that the lighter colored “chimney” was sitting on. This material was a light gray color, speckled with white dots. After sampling the boulders we changed course, at 14:22, toward the SW and headed back up slope. At the summit at 14:44 we stopped to take 3 push cores (Stop #6) for, mineralogical, microbial and pore fluid studies. After sampling, we rose to 3 m to optimize the sub-bottom profiler data and moved SW down the slope of the mudflow eruption site and headed for the southern mound. By partway down the slope (14:58), the amount of sediment cover increased and remained continuous throughout the traverse to the next summit area. The bottom of the slope is at 2969 m. There were few if any outcrops of boulders to see or sample and the sub-bottom profiler showed ~2 m of sediment for most of the traverse to the southern mound. In some places particularly toward the southern end of the traverse there are multiple sub-bottom reflectors. We reached the base of the ridge that marks the southern boundary of the southern mound at 15:43 and sampled (Stop #7) some boulders protruding from the sediment. We left the bottom after collecting these rocks (mudstone) and returned to the surface.

Keywords:Mariana, forearc, serpentinite, peridotite, serpentine mudflow, chimney

Payloads:(1) Sample boxes x2, (2) Push core x6, (3) Sub-bottom profiler, (4) Magnetomete

Location of Events:Time Position Depth Event11:51 18˚45.6831’N, 147˚7.7248’E, 3141m STOP 01: sampling boulders outcropping from

sediment12:16 18°45.6055’N, 147°7.7996’E, 3077m STOP 02: Sampling boulders outcropping

through sediment13:08 18°45.2593’N, 147°7.9309’E, 2950m STOP 03: Sampling three rocks in serpentine

mud outcrop13:47 18°45.1473’N, 147°7.9665’E, 2933m STOP 04: Sampling two rocks at the summit

of the serpentine mud mound

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14:21 18°45.1917’N, 147°8.0219’E, 2946m STOP 05: Sampling one rock (chimney fragment)

14:51 18°45.1267’N, 147°7.9346’E, 2932m STOP 06: Sampling three push-cores of serpentine mud at summit area of mud mound (blue, yellow, green)

15:54 18°44.4823’N, 147°7.7802’E, 2946m STOP 07: Sampling 2 rocks outcropping at the base of the scarp to the south of the mud mounds

Video Digest :11:48:15 – 11:48:25 12 CAM1First outcrop of sediment coated with manganese oxide

12:58:50 - 12:59:10 20 CAM1First outcrop of serpentinite mud with several cobbles of peridotite (dark) under a thin cover of sediment (also a shrimp swims by)

13:26:30 – 13:27:10 40 CAM1Serpentine mud with little sediment cover

13:36:30 – 13:36:50 20 CAM1Serpentine mudflow matrix outcrops at summit area of the seamount, with very light dusting of sediment. You can see blue-green serpentine in some areas (not reacted with seawater).

14:02:25 – 14:02:55 30 CAM1Panning across the eastern slope of the summit knoll where we discovered chimneys

14:09:25 – 14:09:55 30 CAM1While sampling the chimney structure, a fish swims around the base of it___2’ 32”_______________________________11:33:00 – 11:33:15 15 CAM2animal trails in the sediment

11:43:15 – 11:43:30 15 CAM2manganese coated sediment outcropping from loose sediment on a gentle slope. The outcrop exposes some brown sediment beneath to manganese coating.

12:13:45 – 12:13:51 6 CAM2outcrop of manganese coated rock with white starfish and crinoid

12:14:21 – 12:13:33 12 CAM2sampling manganese-coated sediment fragment near the starfish outcrop.

13:02:27 – 13:02:37 10 CAM2observing the first serpentinite mud outcrop and preparing to sample

13:03:22 – 13:04:17 45 CAM2sampling the first peridotite rock from the mudflow

13:28:32 – 13:28:52 10 CAM2

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undulating surface near the summit of the young serpentinite mudflow depth is 2935 m

13:59:40 – 14:00:00 20 CAM2 Boulders of chimney material, note the white material projecting from the middle of 3 large boulders

14:08:07 – 14:08:22 15 CAM2Close-up of light chimney material (with guardian shrimp), but we were unable to obtain a sample of this piece.

2’ 28”

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Dr. P. FryerTime(JST)

Dep.(m)

Alt.(m)

Head(Deg)

BoxNo. Description Remarks

driving for the sea floor

10:49 2000 155 depth: 2000m X:950Y:20

10:53 2163 349 set heading 350 degree

10:58 2358 347 check the position X:1120Y:-20

10:59 2361 330 set heading 330 degree, start running11:01 2500 625 327 depth: 2500m

11:02 2543 582 327 check the position X1220Y:-90

11:03 2583 559 327 stop running,driving for the sea floor11:13 3000 160 342 depth: 3000 m

11:29 3160 1 208 arrive to the bottom, check the position X:1260Y:-60

11:32 3155 3 150 set heading 150 degree11:35 3156 1 150 track

11:40 3144 3 152 check the position X:1250Y:10

11:42 3139 4 161 rocks. Starting observation11:50 3142 1 180 start sampling (st.1)

11:51 3142 1 133 P3 (st.1) get the rock (mud stone?) #1 X:1250Y:40

11:51 3142 1 133 sandy floor, no ripple11:55 3134 2 162 set heading 160 degree11:55 3133 2 162 sea cucumber11:56 3131 2 160 rocks12:03 3109 4 160 sandy sea floor, no ripple12:06 3100 2 160 rocks

12:06 3100 2 160 check the position X:1150Y:140

12:08 3094 3 181 set heading 180 degree12:10 3090 3 181 sandy sea floor, no ripple12:14 3077 155 200 rocks. Starting observation12:14 3077 155 200 starfish

12:14 3077 155 201 P2 (st.2) get the rocks (mud stone?) #2 X:1100Y:180

12:19 3070 3 180 sandy floor. fish?12:22 3058 4 182 rocks12:25 3047 3 180 tracks

12:26 3042 4 180 check the position X:990Y:200

12:35 3007 3 180 check the position X:800Y:220

12:36 3007 3 168 set heading 170 degree

12:45 2977 4 170 check the position X:620Y:250

12:27 2974 2 133 set heading 130 degree12:53 2958 5 131 fish

12:56 2950 4 149 set heading 160 degree. Check the position X:460Y:400

12:56 2948 4 145 rocks. Starting observation (st.3)13:05 2950 175 139 P1 (st.3) get the rock #313:05 2950 175 139 P1 (st.3) get the rock #4 X:48013:08 2950 3 197 P4 (st.3) get the rock #5 Y:41013:14 2949 3 150 set heading 150 degree

13:21 2942 1 147 check the position X:410Y:460

13:22 2941 1 148 serpentine mud13:23 2941 1 151 starting observation

13:29 2934 1 185 check the position X:310Y:470

13:34 2934 150 154 start sampling (st.4)13:35 2934 156 172 P5 (st.4) get the rock #6 X:27013:37 2934 156 172 P5 (st.4) get the rock #7 Y:47013:37 2934 156 172 continuing observation

13:48 2946 3 97 check the position X:270Y:540

14:06 2949 4 238 dead chimney14:08 2950 2 306 shrimp

#1086Latitude/

Longitude

2008/06/30

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Dr. P. FryerTime(JST)

Dep.(m)

Alt.(m)

Head(Deg)

BoxNo. Description Remarks

#1086Latitude/

Longitude

2008/06/30

14:21 2950 1 335 S3 (st.5) get the dead chimney #8 X:350Y:570

14:25 2945 3 303 start sampling (st.6)14:29 2946 1 307 S1 (st.6) get the rock #914:30 2946 1 306 S1 (st.6) get the rock #1014:31 2944 2 290 continuing observation14:42 2932 3 231 start coring (st.7)14:44 2932 3 231 blue (st.7) push the core. blue14:47 2932 102 231 yellow (st.7) push the core. yellow X:23014:50 2932 206 232 green (st.7) push the core. green Y:41014:53 2930 2 200 set heading 200 degree

15:00 2965 1 201 check the position X:100Y:360

15:10 2966 2 201 check the position X:-80Y:290

15:20 2960 2 201 check the position X:-300Y:190

15:22 2955 3 201 tracks

15:29 2961 2 201 check the position X:-540Y: 110

15:31 2963 2 180 set heading 180 degree15:33 2967 2 180 tracks

15:37 2969 1 184 check the position X:-790Y:120

15:45 2948 3 137 S4 get the rock (st.8) #11 X:-960Y:140

15:53 2946 84 170 S4 get the rock (st.8) #1215:54 2945 3 170 leave the bottom

-8000-6500

-6500

-6000

-6000

-5500

-550

0-5

000

-5000

-450

0

-4500

-450

0

-4000

-400

0

-4000-4000

-4000

-4000

-3500

-3500

-3500

-3500

-3500

-3500

-3500

-300

0

-300 0

-300

0

-300

0

-3000

-3000

-3000

-3000

-2500

-2500

-2500

-2000

-1500

-5000 0

0 10 20

km

2008 Jul 02 16:09:38 contour:50m

146 40'E 147 00'E 147 20'E 147 40'E18 00'N

18 20'N

18 40'N

19 00'N°

°

°

°

°°

° °

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Dive Report: SHINKAI 6500 Dive #1087

Date: 1 July, 2008Site Name: A N-S trending fault scarp of Twin Peaks Seamount, Mariana forearcLanding: 11:29; 18°35.6322’N, 146°59.0306’E, 3015mLeaving: 15:54; 18°35.9486’N, 146°57.5404’ E, 2670mObserver: Hirokazu Maekawa (Graduate School of Science, Osaka Prefecture University) Pilot: Y. Sasaki, Co-Pilot: H. Ueki

Objectives: This dive site is on the N-S trending major fault scarp of Twin Peaks Seamount. It gives us a good and rare opportunity to see inner structure of serpentinite seamount in Mariana forearc. During KR06-15 cruise in 2006, we investigated near this site by the ROV “Kaiko 7000II”, and successfully sampled a 20 cm rock fragment of amphibolite which was later metamorphosed under low-temperature blueschist-facies conditions. Analyses of video image suggested that the fragment have come from just above exposure which has about 5 m width and height. We had long believed that small fragments of high-pressure metamorphic rocks can only be brought up from the depth of subduction zone to seafloor by serpentinite diapir since we found blueschist-facies rock clast from Conical Seamount during ODP Leg 125 for the first time (Maekawa et al., 1993). The finding from Twin Peaks Seamount is important, so we must change our previous idea and consider a thoroughly new mechanism for the large blueschist block in serpentinite seamount. Thus, the objectives of this dive are 1) to visit again a site and examine the mode of occurrence of of amphibolite and gabbroic rocks which have possibly undergone later blueschist-facies metamorphism; 2) to investigate the inner structure of serpentinite seamount in detail ; and 3) to collect rock samples for constituents of the seamount for petrological and geochemical studies.

Dive Summary: We landed 150 m east of the proposed landing point at the bottom of N-S trendig fault scarp of the seamount at a depth of 3015 m. The landing point was occupied by muddy sediments. Muddy sediments are commonly composed of altered and clayey serpentine mud materials. We could see ripple marks and many animal trails on the seafloor. Pebble- to cobble-sized rock fragments or manganese nodules commonly scatter on the seafloor. We started on westerly course up the scarp to observe the modes of occurrence of rocks and sediments. Aggregates of several centimeter to several tens centimeter blocks are sporadically exposed on muddy seafloor. At 11:46 we located some exposures of rocks at a depth of 2985 m. The sizes of exposures are a few meters to more than ten meters, but rocks seem to be highly fragmented. We sampled two basalts and one harzburgite, which have manganese crust less than 1 mm thick (Stop 1). We continued on a westerly course up muddy slope. At 12:04, we encountered abundant angular to sub-rounded blocks of several centimeter to a few meter in size, and sampled two metagabbros and one harzburgite (Stop 2). After we passed through muddy seafloor for about 80 m, we found pebble to cobble-sized rocks are scattered in muddy sediments (Stop 3). We took three samples, one metabasalt and two harzburgites. After we observed rocks at Stop 3, we kept on westerly course up sloop. We frequently encounterd large exposures with rugged surfaces of fragmented rock. We collected one metabasalt and two harzburgite at Stop 4, three harzburgites at Stop 5, two harzburgites and one dunite at Stop 6, and three harzburgites at Stop 7. Mn coating of samples is commonly thin less than 1 mm, but rarely attains up to 4 mm. After sampling at Stop 7, the slope is gentle, and at 13:39 slope become again steep and white serpentine mud matrix between blocks became obvious. Consolidated serpentine mud matrix with black fragments are also recognized. Three harzburgites were sampled at Stop 8. At 14:00, we arrived at a rocky place, where bedding like foliation were recognized in large greenrock body (Stop 9). We took one dunite

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and one greenrock there. On the way to Stop 10, slope of muddy and white serpentine sediments was widespread. Blocks of varying sizes are often exposed. We tried to take some cobbles in mud matrix, and two metagabbros and one harzburgite were successfully obtained. At Stop 10, we changed course to go northeastward along rige line. Gentle and smoos surface composed of serpentine mud materials and serpentinite fragments were observed. Boulders are sometimes were found. Animal trails and seacucumber were often observed. We encountered roky place again at 14:40, but we skipped here because no enough time is left. After the rock exposures, seafloor with frequent animal trails and ripples continued, and we saw many red shrimps. After one our travel from Stop 10, we encounterd large outcrop. We tried to take sample, but most of rocks are not enough hard to take by manupilator. We sampled one mudstone, suggesting that the outcrop is occupied by mudstone. At. 15:38, we reached the last exposure extending to several tens meters. We could see some bedding. We sampled one serpnetine mudstone and one brownish mudstone. We left the bottom after collecting these rocks. During the dive we collected 33 rocks, 293 and 59 still photos of the external and inner camera, respectively, and No. 1 video and 2 video records.

Biological observations Some kinds of fish, red shrimps, seacucumber glassy sponges were oftern found. Animal tracks are also commonly observed on the muddy seafloor.

Keywords:Mariana, forearc, serpentinite, peridotite, metagabbro, green rock

Payloads:1) Sample boxes x22) Push core x6

Location of Events:Time Position Depth Event11:52 18˚35.6481’N, 146˚58.8684’E, 2985 m STOP 01: Sampling three rocks from rocky

outcrop covered partly by mud sediments.12:12 18°35.6481’N, 146°58.7425’E, 2930 m STOP 02: Sampling three rocks from talus

breccia. Subrounded pebbles and cobbles are common in this outcrop.

12:31 18°35.6401’N, 146°58.6796’E, 2909 m STOP 03: Sampling three rocks from talus breccia. Subrounded to rectangular pebbles and cobbles are common in this outcrop.

12:51 18°35.6286’N, 146°58.6607’E, 2892 m STOP 04: Sampling three rocks from rocky outcrop. Subrounded to subangular pebbles and cobbles are common in serpentine mud materials.

13:05 18°35.6091’N, 146°58.6015’E, 2849 m STOP 05: Sampling three rocks from talus breccia. Pebble and cobble-sized rocks are scattered in serpentine mud materials.

13:23 18°35.6033’N, 146°58.5614’E, 2813 m STOP 06: Sampling three rocks from talus breccia. Pebble and cobble-sized rocks are scattered in serpentine mud materials.

13:33 18°35.6033’N, 146°58.5341’E, 2798 m STOP 07: Sampling three rocks from

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continuous rocky outcrops occupied by serpentnized harzburgite. Rectangular blocks of varying sizes are predominant.

13:53 18°35.5910’N, 146°58.4969’E, 2770 m STOP 08: Sampling three rocks of harzburgite. The modes of occurrence is similar to those at Stop 07. White colored serpentinite mud materials are partly found between harzburgite blocks.

14:08 18°35.5876’N, 146°58.4812’E, 2736 m STOP 09: We took a sample from weakly schistosed green rock layer of 30 cm thick. We also took a dunite sample from talus breccia at seafloor.

14:20 18°35.5666’N, 146°58.4289’E, 2702 m STOP 10: Sampling three rocks. Angular blocks of varying sizes are predominant in this outcrop. Two metagabbro and one harzburgite were recovered from this outcrop.

15:27 18°35.9049’N, 146°57.6685’E, 2691 m STOP 11: Rectangular or irregular blocks of varying sizes in muddy matrix.Sampling one mudstone.

15:53 18°35.9486’N, 146°57.5404’E, 2670 m STOP 12: Sampling three rocks from talus deposits just below outcrop. One pumice, one mudstone, and one serpentine mudstone were recovered.

Video Digest :

11:39:35 – 11:39:45 15 CAM1An isolated block in muddy seafloor.

11:47:15 – 11:47:30 15 CAM1Outcrop view of Stop 1. Rocks covered partly by sediments are well exposed.

12:16:45 – 12:17:00 15 CAM1A view of muddy seafloor.

13:40:22 – 13:40:37 15 CAM1Harzburgite at Stop 7. White serpentine mud materials between blocks characterize the outcrop.

13:57:15 – 13:57:45 30 CAM1Modes of occurrence of brecciated sertpentinite or sedimentary serpentinite. We can see nice block-in-matrix fablics here.

11:45:40 – 11:45:55 15 CAM2Basalt and harzburgite boulders in muddy sediment (Stop 1).

13:43:38 – 13:43:53 15 CAM2An outcrop of harzburgite.

14:03:15 – 14:03:30 15 CAM2Weakly schistosed green rock.

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14:04:03 – 14:04:10 7 CAM2Several red shrimps pass through the vessel.

15:16:41 – 15:16:46 5 CAM2Animal tracks on muddy seafloor.

15:39:15 – 15:39:30 15 CAM2Sampling manganese-coated mudstone outcrop.

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H. Maekawa 2008/07/01

Time (JST) Dep.(m)

Alt.(m)

Head(Deg)

BoxNo. Description Remarks

09:55 driving for the sea floor10:27 1000 depth: 1000 m

10:40 1500 depth: 1500 m X:-750Y:2010

10:50 2000 290 depth: 2000 m X:-600Y:1840

10:55 2142 check the position X:-550Y:1800

11:05 2550 450 12 depth: 2550 m11:26 3000 11 260 depth: 3000 m11:29 3015 2 263 mud and small rocks (< 10 cm)

11:30 3015 2 236 arrive to bottom X:-490Y:1810

11:34 3013 2 258 set the heading 260 degree11:39 2999 2 262 mud, pebbles (< 5 cm) and few blocks (<2 m)

11:42 2992 3 260 check the position X:-500Y:1650

11:49 2985 2 156 s1 (st.1)get the rocks #1#2#3 X:-460Y:1530

11:54 2985 2 260 set the heading 260 degree11:56 2973 2 261 mud11:58 2962 2 260 mud, pebbles (< 5 cm) and few blocks (<2 m)

12:03 2935 3 261 check the position X:-470Y:1340

12:07 2930 3 158 mud, blocks (10~20 cm)12:07 2930 3 158 s3 (st.2) get the rock (ca. 10 cm) #412:08 2930 3 151 s3 (st.2) get the rock (ca. 25 cm) #512:13 2929 1 260 s3 (st.2) get the rock #612:17 2922 3 240 mud,set the heading 240 degree12:22 2911 1 158 s1 (st.3) get the rock (ca. 10cm) #712:29 2909 1 194 s4 (st.3) get the rock (ca. 20cm) #812:32 2909 4 168 s4 (st.3) get the rock (ca. 25cm) #912:35 2908 2 240 set the heading 240 degree12:42 2897 2 193 p1 (st.4) get the rock (ca. 10cm) #1012:45 2897 1 199 p1 (st.4) get the rock (ca. 12~13cm) #11#1212:53 2889 3 240 set the heading 240 degree

12:59 2853 3 240 check the position X:-570Y:1070

13:04 2850 2 252 P2 (st.5) get the rock #1313:04 2850 2 252 P2 (st.5) get the rock #1413:06 2850 2 272 P2 (st.5) get the rock #1513:07 2847 2 243 set the heading 240 degree13:07 2847 2 243 large blocks (< 2 m), pebble and mud

13:13 2814 2 240 check the position X:-550Y:990

13:15 2814 3 286 start sampling (st.6)13:19 2813 1 236 P4 (st.6) get the rock (ca. 20 cm) #1613:24 2812 1 228 P4 (st.6) get the rocks #17, #1813:26 2806 2 244 set the heading 240 degree13:30 2798 3 190 start sampling (st.7)13:32 2796 201 P5 (st.7) get the rock (ca. 10 cm) #1913:32 2798 4 200 P5 (st.7) get the rock (ca. 30 cm) #2013:33 2798 4 200 P5 (st.7) get the rock #2113:36 2796 2 249 set the heading 240 degree13:38 2785 3 241 angular to sub-rounded blocks and pebbles13:45 2771 1 219 (st.8) get the rock (ca. 10 cm) #2213:48 2770 1 218 (st.8) get the rock (ca. 20 cm) #2313:54 2769 2 237 (st.8) get the rock (ca. 10 cm) #2413:56 2759 4 240 set the heading 240 degree. large block (~ 3 m)14:00 2750 2 229 start sampling (st.9)14:02 2750 2 238 S2 (st.9) get the rock #2514:08 2744 3 239 S2 (st.9) get the rock #2614:12 2721 2 240 set the heading 240 degree14:15 2702 2 243 start sampling (st.10)14:16 2702 1 258 S3 (st.10) get the rock #2714:20 2701 2 222 S3 (st.10) get the rock #28, #2914:24 2693 1 311 set the heading 310 degree14:25 2691 2 312 mud and small angular rocks (< 10 cm)14:25 2689 2 321 set the heading 320 degree

Dive Log of 6k Dive #1087Latitude/

Longitude

X:-550Y:990

X:-540Y:1060

X:-500Y:1160

X:-480Y:1190

X:-460Y:1300

X:-550Y:940

X:-570Y:870

X:-580Y:850

X:-620Y:750

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H. Maekawa 2008/07/01

Time (JST) Dep.(m)

Alt.(m)

Head(Deg)

BoxNo. Description Remarks

Dive Log of 6k Dive #1087Latitude/

Longitude

14:30 2673 3 320 check the position X:-500Y:580

14:36 2685 2 278 set the heading 320 degree. Mud, sub-angular to sub-rounded pebbles and cobbles

X:450Y:520

14:42 2680 2 281 check the position X:-460Y:370

14:46 2673 3 280 mud

14:48 2668 3 284 Check the position X:-460Y:180

14:50 2668 2 301 set the heading 300 degree.

14:54 2677 2 301 Check the position X:-410Y:20

14:55 2683 1 311 set the heading 310 degree. Muddy floor

14:58 2686 2 311 Check the position. Ripple mark (NNW-SSE strikingridges)

X:-270Y:-110

15:03 2693 1 310 Check the position X:-170Y:-230

15:08 2690 1 310 Check the position X:-40Y:-380

15:14 2690 3 262 set the heading 260 degree.

15:16 2687 1 262 Check the position X:20Y:-540

15:20 2682 1 160 ripple (NNW-SSE striking ridges)15:21 2680 1 86 outcrop15:23 2681 2 start sampling (st.11)

15:26 2681 3 73 B2 (st.11) get the rock (sub-angular, 20 cm) #30 X:10Y:-590

15:30 2675 5 301 set the heading 300 degree.15:31 2678 3 301 ripple (NNW-SSE striking ridges)15:40 2671 3 246 start sampling (st.12)15:42 2671 3 241 P1 (st.12) get the rock (20 cm)15:47 2670 1 228 P1 (st.12) get the rock15:54 2664 5 320 leave the bottom

X:90Y:-810

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GM T 2006 Dec 11 07:49:25 NME_marine_Tech..Scale=1:200000.Cont_int=20m../grd/datalist150.grd.N

KR06-15.7K.Dive.371.TwinPeaks3.SMT

146° 50' E 146° 55' E 147° 00' E 147° 05' E 147° 10' E

18° 25' N

18° 30' N

18° 35' N

18° 40' N

18° 45' N

- 4 2 0 0

-4 0 0 0

-380

0

-3 6 0

0 -3400

-340

0

-3200

-320 0

-3000

-3 0 0 0

-28 0 0

GM T 2006 Dec 11 07:49:27 Lindquist_et_al(2004)30s.grd+YK0307+KR0615.seabeam2112data

-4000 -3000

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Stop 1. Metabasalts and harzburgite were obtained from this outcrop.

Stop 2. Metagabbros were obtained from this talus deposits in sediments.

Stop 2. Rectangular blocks of metagabbros and harzburgite were common in this place.

Stop 6. An outcrop of dunite and harzburgite.

Stop 8. An outcrop of harzburgite. Three harzburgite samples were recovered from this outcrop.

Stop 8.

Stop 9. Weakly schistosed green rock occupies the outcrop.

Stop 9.

Modes of occurrence of constituent rocks of Twin Peaks Seamount

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Stop 10. Rectangular blocks of metagabbro are common in this outcrop.

Stop 10.

We found red shrimps frequently in this dive. Animal trails between Stop 10 and Stop 11.

Rippled seafloor prevailed between Stop 11 to Stop 12. Mud layers developed near Stop 12.

Stop 12. Mudstone coveres the ridge to northeastern peak of Twin Peaks Seamount. Stop 12.

Biology

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IV. Shore base study

1. Metamorphic evolution beneath the Mariana forearc, western Pacific.Hirokazu Maekawa et al.

Since high-pressure/low-temperature metamorphic rocks were found from Conical Seamount during ODP Leg 125, our understandings on the idea of subduction zone metamorphism appear to have made large progress. However, there still remains an ambigrous and controvertial problem. Newly found blueschist clasts from South Chamorro Seamount (ODP Leg 195) are strikingly different in metamorphic conditions from the previously found blueschsit-facies clasts from Conical Seamount. In this paper, we will describe metamorphic rocks obtained from the seven seamounts during this cruise, clearlify the nature of Mariana subduction metamorphism and establish the Mariana blueschist-facie metamorhpic belt beneath the forearc.

2. Serpentinization of wedge mantle peridotites beneath the Mariana forearc.Hirokazu Maekawa et al.

We will examine the serpentine mineral associations of serpentinized peridotites from seven seamounts investigated in this cruise and those from South Chamorro Seamount recovered from ODP Leg 195, clarify the distributions of rocks with antigorite-bearing or antigorite-free assemblages, and inspect our hypothesis that antigorite only occurs in seamounts far from the trench (more than 80 km).

3. Cooling history of wedge mantle peridotites in the Mariana forearc.Keiko Murata (Kobe Women’s Univ.) and Hirokazu Maekawa et al.

Using geospeedmeter (Ozawa, 1983), we will estimate cooling history of peridotites for each serpentinite seamount investigated in this cruise. We already obtained the cooling rates of 10-2~10-4 ºC/year for peridotites both from Conical and South Chamorro Seamounts. In this paper, will compare the retes with peridotites from other seamounts investigated in this cruise, and discuss the significance of the cooling within the subduction zone in general.

4. Geology of the Mariana forearc regionYokose et al.

To understand the general geology of the Mariana forearc, we will synthesis with newly acquired bathymetric data, both KR06-15 and YK03-07 cruises, and seven dive observations from Kaiko 7000 II, lithological characteristics. This paper will be focused on the geological description of this area.

5. Growth model for the serpentine seamounts on the Mariana forearc regionYokose et al.

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Ten serpentine seamounts on the eastern flanks of the Mariana fore arc region have been discovered. However, their topographical features have been discussed insufficiently due to low resolution of the bathymetric data. High resolution bathymetric surveys carried out during YK08-08, KR06-15 and YK03-07 cruises and revealed topographical details of the seamounts. They show three basic topographical types: plateau, conical, and bell shape. The types could be depended on the physical properties, such as viscosity, effusion rate, and yield strength, of the serpentinite flows. Therefore the differences are expected to reflect its development history and effusion style. The shape analysis will offer a realistic model for serpentinization and transportation processes in the wedge mantle.

6. Initial stages of formation of a serpentine mud volcano on the Mariana forearc.P. Fryer et al.

7. Raman spectroscopy of serpentinized peridotite from a young serpentine mud volcano on the Mariana forearc.P. Fryer et al.

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Date 2008/6/29

Sample No. Weight(kg) Rock Name Location Mn coating

(mm) Distribution

#1085-R-01 22 18 11 5.84 harzburgite st. 1 thin film Archive, P. Fryer, H. H. Yokose, H. Maekawa

#1085-R-02 26 22.5 12 8.35 harzburgite st. 1 thin film Archive, P. Fryer, H. Maekawa

#1085-R-03 10.5 8 4.5 0.32 sandstone st. 2 thin film Archive, H. Yokose

#1085-R-04a 10 9.5 4.5 0.3 sandstone st. 2 thin film Archive, H. Yokose

#1085-R-04b 10 8.5 3 0.23 sandstone st. 2 thin film Archive, H. Yokose

#1085-R-05 23 14 10 2.38 siltstone st. 3 thin film Archive

#1085-R-06 14 10.5 7.5 0.5 siltstone with harzburgitefragments st. 3 thin film Archive, H. Yokose, H. Maekawa

#1085-R-07 14 8.5 7 0.4 siltstone st. 3 thin film Archive, H. Maekawa

#1085-R-08 19 12 12 2.08 siltstone st. 3 thin film Archive, H. Yokose

#1085-R-09 35 24 15 10.02 sandstone st. 4 1.2 Archive, H. Yokose

#1085-R-10 38 20 8 5.02 sandstone st. 5 1.2 Archive, H. Yokose

#1085-R-11 15 10 5 0.78 siltstone st. 5 thin film Archive, H. Yokose

#1085-R-12 10.5 8 7.5 0.5 dunite st. 5 0.1 Archive, P. Fryer, H. Maekawa

#1085-R-13 32.5 23 15.5 7.96 sandstone st. 6 0.5 Archive, H. Yokose

#1085-R-14 19.5 12 9.5 1.34 sandstone st. 6 0.4 Archive, H. Yokose

#1085-R-15 25 17 11 3.96 siltstone st. 7 0.1 Archive, H. Yokose

#1085-R-16 13 12 8.5 1.48 harzburgite st. 7 2 Archive, P. Fryer, H. Maekawa

#1085-R-17 20 13 11.5 4.45 harzburgite st. 7 2.5 Archive, P. Fryer, H. Maekawa

#1085-R-18 18 15 6.5 1.01 siltstone st. 8 2.5 Archive

#1085-R-19 8.5 8.5 4.5 0.34 harzburgite st. 8 2 Archive, H. Yokose

#1085-R-20 22 12 10.5 6.2 harzburgite st. 9 2 Archive, H. Maekawa

#1085-R-21 14 10 5.5 0.62 siltstone st. 9 3 Archive, H. Yokose

#1085-R-22 18 12.5 12 2.1 harzburgite st. 9 10 Archive, P. Fryer, H. Maekawa

Total=66.18 kg

Rock Samples collected from YK08-08 Leg 1 Dive #1085Size (cm)

Appendix

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Date 2008/6/30

Sample No. Weight(kg)

Rock Name Location Mn coating(mm)

Remarks Distribution

#1086-R-01 10.5 9.5 5 0.4 mudstone st. 1 thin film Archive, H. Yokose

#1086-R-02 8 6 3 0.2 mudstone st. 2 thin film Archive, H. Yokose

#1086-R-03 17 12 7 1 sepentine mud st. 3 Archive, P. Fryer, H. Maekawa

#1086-R-04 15 11.5 6.5 1.6 peridotite st. 3 thin film P. Fryer, H. Maekawa

#1086-R-05 19 16 16 5.6 sepentine mud st. 3 thin film H. Maekawa

#1086-R-06 st. 4 lost before on board

#1086-R-07 st. 4 lost before on board

#1086-R-08 17.5 17 13 1.6 chimney (dead) st. 5 none H. Maekawa

#1086-R-09 17 14 10 3.4 harzburgite st. 6 none Archive, P. Fryer, H. Yokose, H. Maekawa

#1086-R-10 14 13 9 2 peridotite st. 6 5 P. Fryer, H. Maekawa

#1086-R-11 26 21 16 11.8 peridotite st. 8 1 Archive, H. Maekawa

#1086-R-12 27 17 9 2.6 peridotite st. 8 2 Archive, H. Maekawa

#1086-P-01 mud st. 7 Push core P. Fryer

#1086-P-02 mud st. 7 Push core P. Fryer

#1086-P-03 mud st. 7 Push core P. Fryer

Total =30.2 kg

Size (cm)

Rock Samples collected from YK08-08 Dive #1086

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Date 2008/7/1

Sample No. Weight(kg) Rock Name Location Mn coaing

t(mm) Remarks Distribution

#1087-R-01 22 18 12 6.18 basalt St.1 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-02 12 8 6 1.1 harzburgite St. 1 0.7 H. Yokose, P. Fryer, H. Maekawa

#1087-R-03 9 8 5 0.26 basalt St. 1 0.5 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-04 30 22 16 10.8 harazburgite St. 2 3 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-05 19 16 12 4.5 metagabbro St. 2 0.1 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-06 29.5 11 8 3.4 metagabbro St. 2 none H. Yokose, P. Fryer, H. Maekawa

#1087-R-07 15 9 6 0.9 metabasalt St. 3 0.2 H. Yokose, P. Fryer, H. Maekawa

#1087-R-08 27 21 17 11.4 harzburgite St. 3 3 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-09 23.5 11 9 2.5 harzburgite St. 3 4 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-10 13 10 8 0.98 harzburgite St. 4 0.9 H. Yokose, P. Fryer, H. Maekawa

#1087-R-11 14 10 4.5 0.6 basalt St. 4 thin film Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-12 10 8 5 0.4 harzburgite St. 4 thin film H. Yokose, P. Fryer, H. Maekawa

#1087-R-13 16 13 7 1.9 harzburgite St. 5 1 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-14 14 11 7 1 harzburgite St. 5 0.1 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-15 12 10 7 1 harzburgite St. 5 2 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-16 30 16 13 3.6 harzburgite St. 6 1 Archive, P. Fryer, H. Maekawa

#1087-R-17 10 8 6 0.58 dunite St. 6 thin film H. Yokose, P. Fryer, H. Maekawa

#1087-R-18 8 7 4 0.4 harzburgite St. 6 0.3 H. Maekawa

#1087-R-19 22 15.5 11 4.8 harzburgite St. 7 thin film P. Fryer, H. Maekawa

#1087-R-20 13 10 6 0.9 harzburgite St. 7 0.7 H. Yokose, P. Fryer, H. Maekawa

#1087-R-21 12 6 5 0.3 harzburgite St. 7 1 H. Yokose, P. Fryer, H. Maekawa

#1087-R-22 19 16 13 4.2 harzburgite St. 8 thin film H. Yokose, P. Fryer, H. Maekawa

#1087-R-23 12 10 9.5 1.8 harzburgite St. 8 2 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-24 12 10 5 0.8 harzburgite St. 8 0.7 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-25 20 15 10 7.54 dunite St. 9 0.1 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-26 15 12 8 1.36 greenrock St. 9 thin film weak schistose Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-27 43 21 11 13.6 metagabbro St. 10 thin film Archive, H. Yokose, H. Maekawa

#1087-R-28 21.5 18.5 3.5 2 metagabbro St. 10 thin film Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-29 14.5 12 11 1.9 harzburgite St. 10 0.7 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-30 28 22 13 6.9 mudstone St. 11 6 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-31 18 11 7 1.15 pumice St. 12 thin film Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-32 19 13 6 0.9 sepentine mudstone St. 12 2 Archive, H. Yokose, P. Fryer, H. Maekawa

#1087-R-33 13 10 7 0.6 mudstone St. 12 0.7 H. Yokose, H. Maekawa

Total =100.25 kg

Size (cm)

Rock Samples collected from YK08-08 Leg 1 Dive #1087

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YK08-08 Cruise Report Leg 1: Mariana Trough

25 June - 6 July 2008 Yokosuka - Guam

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Acknowledgements

Onboard scientists express great thanks to the R/V Yokosuka crew and to the Shinkai 6500 team of their excellent operations. We sincerely appreciate Mr. Satoshi Okada, Nippon Marine Enterprises Ltd., who supports our daily jobs during the cruise. We

thank Mr. Kazuki Iijima of JAMSTEC for shore-base support. We thank Prof. Kyoko Okino and Mr. Taichi Sato, ORI Univ. Tokyo, for providing a deep-sea magnetometer and instructions. Also we thank Drs. Hidenori Kumagai and Masataka Kinoshita,

JAMSTEC, for providing a deep-sea sub-bottom profiler and instructions. We thank Drs. Kiyoyuki Kishimoto, Masato Joshima, Kiyokazu Nishimura, AIST, for their advise regarding installation and operation of the sub-bottom profiler. Part of this work is a

contribution of the research program at the IFREE, JAMSTEC, and the Grant-in-Aid for Scientific Research from the MEXT, Japan (No. 20540417).

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Contents

Acknowledgements 0. Participants 0-1. Onboard Scientists ------------------------------------------------------------ 1 0-2. Crew & 6K Staffs ------------------------------------------------------------- 2 1. Introduction --------------------------------------------------------------------------------- 3 2. Cruise Summary 2-1. Cruise Log --------------------------------------------------------------------- 9 2-2. Cruise Summary ------------------------------------------------------------- 14

3. Geophysical Survey 3-1. Swath Bathymetry ----------------------------------------------------------- 15 3-2. Gravity ------------------------------------------------------------------------ 18

3-3. Magnetics --------------------------------------------------------------------- 19 3-4. A Part of YK08-08 Cruise Report ----------------------------------------- 20 3-5. Deep-Sea Magnetometer ---------------------------------------------------- 26

3-6. Deep-Sea Sub-Bottom Profiler --------------------------------------------- 29 4. Preliminary Reports 4-0. Dive Tracks ------------------------------------------------------------------- 48 4-1. Dive Results 4-1-1. 6K#1088 -------------------------------------------------------- 50

4-1-2. 6K#1089 -------------------------------------------------------- 58 4-1-3. 6K#1090 -------------------------------------------------------- 66 4-2. Sample Descriptions

4-2-1. Rocks ------------------------------------------------------------ 73 4-2-2. Sediments ------------------------------------------------------- 76

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Appendices A-1. Rock Photos ------------------------------------------------------------------ 78 A-2. Sediment Photos ------------------------------------------------------------- 90

A-3. References -------------------------------------------------------------------- 93

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0. Participants

0-1. Onboard Scientists Toshiya Fujiwara (Chief Scientist) IFREE, JAMSTEC

Susumu Umino Kanazawa University

Miho Asada ORI, University of Tokyo

Yuki Koike Kochi University

Satoshi Okada Nippon Marine Enterprises

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0-2. Crew & 6K Staffs Shinya Ryono (Captain of R/V Yokosuka)

Naoto Kimura (Chief Officer) Kiyonori Kajinishi (Chief Engineer) Hiroyasu Saitake (Chief Radio Officer)

Toshiaki Sakurai (Submersible Operation Manager) Kazuhiro Chiba (Assistant Operation Manager)

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1. Introduction

As a prologue of our study, in 2003, we conducted a deep-towed sidescan sonar (~100 kHz) survey in the central Mariana Trough back-arc spreading centers (R/V Kairei KR03-12 cruise). We surveyed a segment at 17°N which is characterized by a

dome-shaped topography shoaling at the segment center (Figure 1-1). The morphology is supposed to be a consequence of enhanced magma supply. As the result of our survey, we discovered that smooth surfaces of sidescan imagery

with high backscattering intensities are dominant in the median valley of the segment center (Figure 1-2). On the other hand, small ridges around the axis of the median valley and areas toward the segment end are associated with sidescan imagery of bumpy

surfaces (Deschamps et al., 2004; Deschamps et al., 2005; Asada et al., 2007). The bumpy sidescan images suggest pillow mounds and ridges, and the smooth surfaces suggest sheet-like lava flows or lobate flows. The different flow morphologies primarily

reflect the rate at which the lava erupted. The presence of sheet flow morphology suggests a high rate of eruption, pillow flow morphology suggests a low rate of eruption, and lobate flow morphology indicates an intermediate eruption rate. The smooth

surfaces occupy half of the survey area. Such a broad area of the smooth surfaces is unusual characteristic among slow spreading centers (Mariana Trough full spreading rate ~30 km/myr).

Mantle Bouguer gravity anomaly in the 17°N segment shows bulls-eye low with large amplitude (Kitada et al., 2006). Consistently, seismic velocity structure shows that the thickness of crust in the spreading axis at 17°N is ~2 km thicker than that in the off-axis

(Takahashi et al., 2007; 2008) (Figure 1-3). These observational results indicate that enhanced magma supply yielded lava flows of the high effusion rate and produced the thick crust. According to the crustal structure, the abnormal crustal thickness extends

~30 km distance. It may indicate that duration of the enhanced magma supply is ~1 myr (30 km ÷ 30 km/myr). Moreover, morphology of lava flows has spatial variation within the median valley. The spatial variation may indicate temporal variation of the magma

supply. Why such large amount of magma has been provided to the 17°N segment? Contribution of water and/or volatile to decrease the melting temperature is a possible

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explanation. Or mantle temperature temporally increased. The previous petrological study did not suggest influence of water at 17°N (Gribble et al., 1996). However there was only one dredge point in this segment. The sample may not represent the character

of the segment. Therefore, to investigate chemistry of magma in this segment, we need to collect more rock samples at sites carefully selected by utilizing visual observation. If chemistry of the rock samples is ordinary, tectonics of the back-arc spreading may be

considered. Bow-shaped Mariana back-arc spreading may introduce reorganization of spreading geometry and local high spreading rate, then decompression melting enhances. Detail tectonic evolution will have key to understand. However the details of magnetic

stripe patterns are still not clear (Iwamoto et al., 2002; Deschamps et al., 2005). Because the trough is situated in low latitudes and survey line interval is sparse. Therefore, acquisition of additional magnetic data is significant.

Are the structural change of crustal thickness and/or the lava morphology consequence of temporal variation of the magma supply? To investigate this issue, we need to examine spatial variation of the rock chemistry and age difference within the median

valley and the off-axis. To deduce age difference, radiometric age of lava, sedimentary layers covering lava flows, and magnetization intensities of lava flows will be examined.

We proposed seven submersible dives for the study (combination of dives across the spreading axis, along-axis, and off-axis). Three dives finally were approved. Initially, the cruise was assigned in 2007. However, the cruise was canceled because of the

submersible's hatch problem. At last, the YK08-08 Leg-1 cruise was on schedule in June-July 2008. The cruise took place in June 25 and July 6 in 2008 (Yokosuka-Guam) using the JAMSTEC R/V Yokosuka and the submersible Shinkai 6500. The Shinkai

6500 dives focused on two specific sites. The first area encompassed the Mariana fore-arc region, where serpentine seamounts evolved. Three submersible dives in this area were proposed by Prof. Hirokazu Maekawa, Osaka Prefecture University, and will

be discussed separately. The second area that will be described in this report encompassed the Mariana back-arc spreading center at 17°N. Three dives were devoted to the median valley in the central portion of the 17°N segment.

Objectives of the submersible survey are 1) geological observation of lava flow morphology and its spatial variation from the center to the edge of the median valley, 2) collection of rock and sediment samples for chemical and age analysis, and 3)

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geophysical observation using a deep-sea magnetometer and a sub-bottom profiler to investigate magnetization of the lava flow and thickness of sedimentary layers that cover the lavas.

During the cruise, a surface-ship geophysical survey including swath bathymetry was conducted in nighttime and occasional days. Survey lines of YK08-08 were designed to be parallel to the survey lines previously obtained to increase spatial density of

magnetic and gravity data.

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Figure 1-1. (a) Bathymetry of the central Mariana Trough showing the location of the study area. The blue line indicates the position of seismic profile shown in Figure 1-3. (b) Bathymetry of the Mariana Trough 17°N segment. The strike of spreading axis

trends in the direction of NNW-SSE (~N20°W). The black box shows the KR03-12 sidescan sonar survey area in 2003.

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Figure 1-2. Sidescan backscattering image of the median valley in the spreading center

at 17°N (modified from Asada et al., 2007). Light color shows high backscattering intensity. The blue star points a rock sampling site of Gribble et al. (1996).

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Figure 1-3. P-wave velocity structure across the Mariana Trough. The spreading axis at 17°N is located at ~100 km in the horizontal axis (Velocity data from Takahashi et al.,

2007; 2008).

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2. Cruise Summary

2-1. Cruise Log

Figure 2-1-1. Ship track of YK08-08 Leg 1 cruise.

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Date Time Description Remark

25,Jun,08 9:00 embarkation science group 6/25 12:00

14:00 departure from JAMSTEC JAMSTEC

15:00~16:00 on board education for safety YOKOSUKA life

16:00~16:30 on board meeting introduction for scientists

26,Jun,08 10:00~11:00 on board meeting for survey plan 6/26 12:00

19:00~20:00 scientific meeting 29-52.7N, 140-42.2E

cloudy

SW-5(Fresh breeze)

27,Jun,08 13:00~14:00 on board training for safety YOKOSUKA life 6/27 12:00

14:30~16:00 on board meeting introduction for 6K dive 24-26.0N, 141-31.0E

19:00~20:00 scientific meeting fine but cloudy

0:00 put ship's clocks ahead 1hour SSW-4(Moderate breeze)

28,Jun,08 5:57 released XBT 6/28 12:00

9:57 set on Proton magnetometer 20-30.6N, 146-00.0E

10:15~10:37 figure eight turn Magnetic calibration fine but cloudy

11:06 commenced MBES mapping survey ENE-3(Gentle breeze)

19:00~20:00 scientific meeting

29,Jun,08 5:36 finished MBES survey 6/29 12:00

8:02 recovered Proton magnetometer 19-53.2N, 146-29.7E

9:50 launched 6K fine but cloudy

10:05 started 6K#1085 dive ESE-3(Gentle breeze)

11:17 arrived at bottom D=2461m

16:11 leave the bottom D=1959m

16:56 surfaced 6K

17:20 recovered 6K

17:56 set on Proton magnetometer

19:00~20:00 scientific meeting

30,Jun,08 5:51 finished MBES survey 6/30 12:00

Shipboard Log & Ship Track(YK08-08 Leg.1 08/06/25 - 08/07/06) Position/Weather/Wind/Seacondition (Noon)

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Date Time Description Remark

Shipboard Log & Ship Track(YK08-08 Leg.1 08/06/25 - 08/07/06) Position/Weather/Wind/Seacondition (Noon)

6:03 recovered Proton magnetometer 18-45.7N, 147-07.7E

6:05 released XBT fine but cloudy

6:38~7:22 carried out MBES mapping survey SE-3(Gentle breeze)

9:55 launched 6K

10:04 started 6K#1086 dive

11:29 arrived at bottom D=3160m

15:54 leave the bottom D=2946m

17:03 surfaced 6K

17:09 recovered 6K

18:06 set on Proton magnetometer

19:00~20:00 scientific meeting

19:19 commenced MBES mapping survey

01,Jul,08 5:03 finished MBES survey 7/1 12:00

6:55 recovered Proton magnetometer 18-35.6N, 146-59.0E

9:56 launched 6K fine but cloudy

10:04 started 6K#1087 dive ESE-2(Light breeze)

11:29 arrived at bottom D=3015m

15:54 leave the bottom D=2670m

16:55 surfaced 6K

17:22 recovered 6K

19:00~19:30 scientific meeting

02,Jul,08 5:30 arrived at research area 7/2 12:00

5:46 released XBT 17-00.0N, 144-51.0E

6:43~7:36 carried out MBES mapping survey fine but cloudy

9:54 launched 6K SE-2(Light breeze)

10:01 started 6K#1088 dive

11:32 arrived at bottom D=3408m

15:46 leave the bottom D=3425m

16:59 surfaced 6K

17:24 recovered 6K

17:55 set on Proton magnetometer

18:32 commenced MBES mapping survey

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Date Time Description Remark

Shipboard Log & Ship Track(YK08-08 Leg.1 08/06/25 - 08/07/06) Position/Weather/Wind/Seacondition (Noon)

19:00~20:00 scientific meeting

03,Jul,08 6:47 finished MBES mapping survey 7/3 12:00

7:06 recovered Proton magnetometer 17-01.0N, 144-52.5E

9:58 launched 6K fine but cloudy

10:05 started 6K#1089 dive ENE-3(Gentle breeze)

11:34 arrived at bottom D=3508m

15:46 leave the bottom D=3351m

17:06 surfaced 6K

17:30 recovered 6K

18:02 set on Proton magnetometer

18:38 commenced MBES mapping survey

19:00~19:30 scientific meeting

04,Jul,08 7:41 finished MBES mapping survey 7/4 12:00

8:04 recovered Proton magnetometer 17-03.9N, 144-49.7E

9:53 launched 6K fine but cloudy

10:00 started 6K#1090 dive SE-2(Light breeze)

11:39 arrived at bottom D=3795m

15:39 leave the bottom D=3616m

16:59 surfaced 6K

17:24 recovered 6K

17:59 set on Proton magnetometer

19:00~19:30 scientific meeting

19:20 commenced MBES mapping survey

05,Jul,08 16:41 finished MBES mapping survey 7/5 12:00

16:44~17:07 figure eight turn 16-22.2N, 143-50.7E

17:15 recovered Proton magnetometer fine but cloudy

17:20 left the research area for YOKOSUKA E-1(Light air)

19:00~19:30 scientific meeting

06,Jul,08 9:00 arrived at Guam 7/6 12:00

13:00 left the ship and concluded YK0808 Leg.1 Guam Apra Port

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Date Time Description Remark

Shipboard Log & Ship Track(YK08-08 Leg.1 08/06/25 - 08/07/06) Position/Weather/Wind/Seacondition (Noon)

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2-2. Cruise Summary During the R/V Yokosuka YK08-08 Leg-1 cruise, the submersible Shinkai 6500 survey of the central Mariana Trough at 17°N was carried out to study a formation process of

oceanic lithosphere in the active back-arc spreading center. Three dives were devoted to the median valley in the central portion of the 17°N segment. Visual observation of geology, lava flow morphology, and fault structure provided ground references of the

deep-towed sidescan sonar survey conducted in 2003 to discuss volcanotectonic features. Smooth surfaces of sidescan imagery with high backscattering intensities are found out to be jumbled-wrinkled, folded sheet, or lobate lava flows. The visual

observation proved that the 17°N segment is covered with sheet lava flows of high effusion rates. Quantifiable degrees of sedimentation superposed on the lavas suggest relative age differences of formation. Lava flows in the center portion of the median

valley are youngest among the three dive observations. Sedimentation more or less gradually increases toward the western margin of the median valley. The observed eastern margin of the median valley is covered with sedimentary layers of ~0.5-2 m

thickness, thus the area is oldest in our dive's observation. North-south trending tectonic structures (i.e. faults, fissures) could be younger than NNW-SSE trending structures because they show relatively little accumulation of sediment, and they cut the other

structures in some places. Basaltic rock samples are collected at total of 22 stations where several types of lava flow morphology are shown. Spatial variations in the rock faces are found (phyric/aphyric, manganese coating, vesiculation). Hemipelagic

sediment samples including nanofossils are also collected at 6 stations. Geophysical instruments were installed on the submersible. Sub-bottom profiler observation to reveal sedimentary thickness was successfully performed. Combined

with the visual observation, the sedimentary thickness is quantifiable to discuss age relationship of the lava flows. Deep-sea magnetic data were collected successfully through all dives. Because the magnetization intensities of lava flows relate to age, the

magnetic data may provide geophysical evidences for discussion of age relationship of the lava flows. In addition, underway geophysical survey was conducted in nighttime and occasional days. Survey lines were designed to be parallel to the previous lines to

increase spatial density of magnetic and gravity data. Bathymetric survey for dive sites data with a speed of 8 knots can be utilized for discussion combined with the swath bathymetry, the sidescan images, and the submersible observation.

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3. Geophysical Survey

3-1. Swath Bathymetry Underway geophysical survey including SeaBeam swath bathymetry was conducted in nighttime and occasional days. The section 3-4 is of reference to sections 3-1 to 3-3.

Survey lines of YK08-08 Leg 1 were designed to be parallel to the survey lines previously obtained to increase spatial density of magnetic and gravity data. As shown in Figure 3-1-1, the ship tracks are laid out in the direction of WSW-ENE which is the

general direction of seafloor spreading of the Mariana Trough. The track spacing is about 11 km (6 nautical miles). The survey was conducted at a ship speed of ~15 knots. Thanks to good weather condition, high quality of data was acquired. Full coverage in

the trough area was already accomplished by previous cruises as for swath bathymetry. Newly in this cruise, coverage in boundary areas between the trough and the West Mariana Ridge were slightly extended (Figures 3-1-1 and 3-1-2). Bathymetric surveys

for dive sites inspections provide high resolution data of which measurement points are densely distributed because the ship ran at a slow speed of 8 knots (Figures 3-1-3 and 3-1-4). The high resolution swath bathymetry can be correlated with the sidescan

images and the submersible observation for discussion of the volcano-tectonics in the vicinity of dive sites.

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Figure 3-1-1. Bathymetry of the central Mariana Trough around 17°N.

Figure 3-1-2. Backscatter intensity of the central Mariana Trough around 17°N. Dark color indicates high intensity.

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Figure 3-1-3. Bathymetry around #1088 and #1089. The map was made by using the site survey data "200807012043e.mb41".

Figure 3-1-4. Bathymetry around #1090. The map was made by using the site survey data "200807030838e.mb41".

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3-2. Gravity Gravity data were collected throughout the YK08-08 cruise. Gravity anomaly is variable for discussion of spatial and temporal variations of crustal thickness and/or crustal

density. In the Mariana Trough, gravity and magnetic data were already obtained by previous cruises along survey lines at an interval of 6 miles (~11 km) in the direction of WSW-ENE which is sub-parallel to seafloor spreading direction. Thus the data were

densely distributed to discuss the flowline variation. However correlation between survey lines was unclear because of the sparsely distribution, therefore the along axis variation was obscure. By conducting our survey in this cruise, the track interval is

improved to be 3 nautical miles (~5.5 km).

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3-3. Magnetics Magnetic data were collected to investigate magnetic anomaly. The tectonic evolution of the central Mariana Trough was described by Iwamoto et al. (2002) using magnetic

anomaly. However the details are still not clear. Because the trough is situated in low latitudes and survey line interval is sparse. Therefore, acquisition of additional magnetic data is significant. Magnetic field was measured using a proton precession

magnetometer and a shipboard three-component magnetometer. "Figure-8" sailings for calibration of three-component magnetic data were conducted at 2 locations in this Leg (Table 3-3-1). In the beginning of the Leg, total force magnetic data was noisy due to a

ground problem. The data were successfully collected in the trough area. Table 3-3-1. Log of "Figure-8".

00:15~00:37 28/June/2008 (UT): 146° 02'E, 20° 48'N, IGRF X: 33913, Y: -318, Z: 16494, F: 37712 nT 06:44~07:07 5/July/2008 (UT):

145° 02'E, 16° 46'N, IGRF X: 34962, Y: 312, Z: 11695, F: 36867 nT Figure 3-3-1. Wiggle map of total force magnetic anomaly. The magnetic anomaly was

calculated by subtracting the International Geomagnetic Reference Field (IGRF) 10th generation.

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3-4. A Part of YK08-08 Cruise Report Aims of our (Fujiwara-trough group) cruise 1. know relative age-relationship between smooth-surface and hummocky lava flows 2. know relative age-relationship between NS-trending and along-axis trending

geological features (AVRs and smaller linear features) 3. speculate absolute age of lava flows 4. and speculate a reason why robust volcanism forming smooth-surface lava flow was

(were) occurred …subduction component effect on it or not??

Schematic diagram of seafloor formation

Tools for our studies 1. Seabeam2100 (bathymetry, backscattering intensity, and sub-bottom data) 2. geomagnetic data obtained by Proton magnetometer 3. geomagnetic data obtained by submersible 3 component magnetometer 4. gravity data obtained by ship mounted gravity meter 5. data of sediment thickness obtained by submersible sub-bottom profiler Details for observation tools 1. GPS

Two GPS antennas are settled on funnel deck. Ship’s position indicates where the radar pole is, which is corrected from position of GPS antennas, 20m behind the radar pole.

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Two GPS antennas System of GPS: Trimble SV6 GPS receiver system General features: Carrier frequency = 1575.42 MHz, using C/A code (standard

positioning service), 6-channel continuous tracking receiver Position accuracy: standard 15 m without selective availability Velocity accuracy: 0.1 m/s without selective availability Time accuracy: 1µs with 1 pulse-per-second output synchronized to UTC Datum: WGS-84 Acquisition rate: signal interruption = 2 sec. 2. Seabeam2100 (bathymetry, backscattering intensity, and sub-bottom data)

The Seabeam2100 multi-beam sonar seafloor mapping system was run over night before every dive between 29 June and 4 July, and during site survey on 28 June and 5 July. -----

Seabeam2100.004 is a multibeam surveying system which collects data and produces wide-swath bathymetry map along ship’s track and/or sidescan sonar image. The system transmits many different frequencies and suits for both shallow and deep water survey.

The Seabeam2100.004 system produces single sonar signal orthogonal to ship’s track from signal projector mounted at the bottom of ship. Reflected acoustic signals are received by hydrophones at the bottom. The hydrophone changes receiving acoustic signals to electric signals, through “Octal Acoustic Receiver Board” which is echo electric processing equipment controlled by “Acoustic Receiver Control Board”, and send them to CPU. Sonar data and positioning data which obtained by navigation system are combined during this process. The Seabeam2100.004 system measures vertical topography across ship’s track for every ping cycle. Ship’s motion is also measured along time and data distortion is collected against ship’s pitch and roll even if the system receiving acoustic signals. Less than 7.5º of pitch and 20º of roll will be corrected.

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Production and receive of echo will be stop when ship’s roll becomes more than 20º.

Sampling Specifications Beam number, beam range, and ping interval are switched-over automatically as a

depth of survey area. During this cruise, beam number is 120~121, beam range 64°~65° for fore-arc area and 60°~63° for back-arc area, and ping interval 9~11 seconds. Frequency: 12 kHz fixed (bathymetry) and 4 kHz for (sub-bottom profiler) Beam width: 2 º, note that output resolution of beam’s

width becomes 1° after on board calculation by the Seabeam system

Ship’s speed during mapping: ~8 knots or 15~16 knots

Software dealing the Seabeam2100 data is “SeaView real-time processing software”. A parameter of velocity of acoustic signals in water, which is important for reliable mapping, is obtained by XBT observations. Sub-bottom profiler did not work during our cruise.

3. Proton magnetometer (Kawasaki geologiiiii Electooo??? 川崎地質)

Geomagnetic observation using proton magnetometer was performed with Seabeam bathymetric survey. The proton magnetometer censor was towed just after Shinkai6500 on board, and was towed until … (next morning, or we arrive to the next diving target, or during geophysical survey… or…??).

Data format: jjj:hh:mm:ss[space]dddd.dCRLF

4. Three component flux magnetometer (SFG-1212, Terra Technica?? テラテクニカ)

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The censor of the three component magnetometer is settled on the funnel deck.

Measuring range: ±99999 nT Resolution: 1 nT Sampling rate: 8 Hz Data filtering: automatic removal for more than 10 nT (-3dB) singnals Output rate: 1/8 sec. Data format: -30065-12345_34567_0.54321_-9.345_-9.456_359.999_12.321_10.234 HX HY HZ Bearing Pitch Roll Bearing Pitch Roll (magnetic record) (Ship [Volt]) (Gyro [degree])

(63 characters/each output)

5. On board gravity meter (S-63)

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6. Shinkai6500 submersible Length: 9.5 m Width: 2.7 m Height: 3.2 m Weight in air: 26 ton Radius of pressure core: 2 m Radius of entry gate: 50 cm Vehicle’s speed in water: 0~2 knots Positioning system: SSBL

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Position accuracy: ±1.5% of distance between mother ship and submersible Calculate position accuracy: ~21+ m for 2000 m depth and ~31+ m for 3000m depth

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3-5. Deep-Sea Magnetometer Measurement using a deep-sea magnetometer (owned by ORI) was conducted to know magnetization intensity of lava flows in the median valley of the Mariana Trough.

Because the magnetization intensities relate to age of lava, deep-sea magnetic data may provide geophysical evidences for discussion of old and new lava flows. The flux-gate sensor was settled on the front panel of the submersible (Photo 3-5-1).

The recording unit was housed in the cockpit of submersible. Z-component of the sensor was directed downward, Y-component was directed forward, and thus X-component was directed portside. The sensor cable was connected to payload

junction box (a702) (Photos 3-5-2 and 3-5-3). The control unit of magnetometer was installed in the cockpit of submersible (Photo 3-5-4). Output format of the submersible's gyrocompass was set to GYROCOMPAS1-V1.txt (bit rate 9600, primary, level RS232C,

sampling period 100, parity N, stop bit 1, input bit rate 4.8K, protocol None, timeout -1) to incorporate the magnetic measurement. Deep-sea magnetic data were collected successfully from descending to leaving bottom in all dives of this cruise (Table 3-5-1).

Photo 3-5-1. Flux-gate sensor

settled on the Shinkai 6500.

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Photo 3-5-2. Connection to the payload junction box during the YK08-08 Leg 1.

Photo 3-5-3. Connection to the payload junction box during the YK08-08 Leg 1.

Photo 3-5-4. Control unit in the cockpit of submersible.

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Table 3-5-1. Log of Deep-Sea Magnetic Measurement. #1085 (Observer: Yokose): Start Recording 23:34:30 (UT), File Name 080629_1085/SD000000.DAT, Memory Card 2, IGRF X: 34037, Y: -77, Z: 15371, T:

37347 nT #1086 (Fryer): 23:35:00 (UT), 080629_1086/SD233426.DAT, 080630_1086/SD000000.DAT, Card 1, X: 34168, Y: 226, Z: 13993, T: 36924

#1087 (Maekawa): 23:37:24 (UT), 080630_1087/SD233650.DAT, 080701_1087/SD000000.DAT, Card 2, X: 34230, Y: 237, Z: 13803, T: 36909 #1088 (Fujiwara): 23:34:36 (UT), 080701_1088/SD233436.DAT,

080702_1088/SD000000.DAT, Card 1, X: 34953, Y: 239, Z: 11996, T: 36955 #1089 (Umino): 23:38:14 (UT), 080702_1089/SD233756.DAT, 080703_1089/SD000000.DAT, Card 2, X: 34941, Y: 241, Z: 12015, T: 36950

#1090 (Asada): 23:33:29 (UT), 080703_1090/SD233259.DAT, 080704_1090/SD000000.DAT, Card 1, X: 34941, Y: 226, Z: 12075, T: 36969

*) Observation data are divided into 2 files for one dive at 0:00 (UT) due to the recording software setting. Starting time of #1085 were reset by an accident, the time 00:00:00 in the record should be 23:34:30.

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3-6. Deep-Sea Sub-Bottom Profiler Measurement using a deep-sea sub-bottom profiler (owned by IFREE) was conducted to reveal sediment thickness that overlay lava flows (Figure 3-6-1). Quantifiable degrees

of sedimentation superposed on the lavas suggest relative age differences of formation. Lava flows in the center portion of the median valley are youngest among the three dive observations. Sedimentation more or less gradually increases toward the western margin

of the median valley. The observed eastern margin of the median valley is covered with sedimentary layers of ~0.5-2 m thickness, thus the area is oldest in our dive's observation (Figures 3-6-2 and 3-6-3).

Figure 3-6-1. Schematic figure of SBP observation.

Table 3-6-1. Log of Measurement

#1085 (Yokose): Box C Battery C, DC Gain 20 db, Range 10 m, File Name 20080629084854.odc #1086 (Fryer): Box D Battery A, DC Gain 15 db, Range 10 m, 20080630100558.odc

#1087 (Maekawa): Box C Battery C, DC Gain 15 db, Range 10 m, 20080701084343.odc #1088 (Fujiwara): Box D Battery A, DC Gain 15 db, Range 10 m, 2008070208644.odc

#1089 (Umino): Box C Battery C, DC Gain 15 db, Range 10 m, 20080703102123.odc #1090 (Asada): Box D Battery A, DC Gain 15 db, Range 10 m, 20080704084435.odc

*) Time in the records is UT.

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Figure 3-6-2. Example of SBP profile (#1089 03:59 (UT)). Sediment covers rough lava flow with a thickness of ~1 m.

Figure 3-6-3. Example of SBP profile (#1090 05:04 (UT)). Sediment covers are not distinguished due to rough seafloor.

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「しんかい 6500」に搭載されたサブボトムプロファイラを

用いたマリアナトラフ背弧拡大軸付近の表層堆積物観察

小池悠己¹*,富士原敏也²,海野進³,浅田美穂⁴,岡田聡⁵ ¹高知大学大学院理学研究科,²海洋研究開発機構地球内部変動研究センター, ³金沢大学

自然システム学系,⁴東京大学海洋研究所,⁵日本海洋事業

Abstract マリアナトラフ背弧拡大軸 17°N付近では,枕状溶岩とシート状溶岩の 2種が分布し

ており,それぞれ拡大軸谷端,中央に分布している.これらは,溶岩噴出量の変化を示唆

しているとともに,形成年代の変化も示している可能性がある.YK08-08 Leg.1 では,多種にわたる海洋調査が行われたが,本稿では「しんかい 6500」に搭載したサブボトムプロファイラ(SBP)の観察結果を報告する.行われた 3潜航#1088,1089,1090での SBPデータと潜水船外写真を照らし合わせ,表層堆積物を観察した.その結果,#1088 では,堆積層をほとんど確認できず,新しい溶岩流であることが推定され,#1089 は,最も堆積層を確認できた測線であり,堆積層中に基盤面と平行な反射面も確認できた.#1090 では,各地点の目視観察で堆積物の被覆を確認できたが,SBP では堆積層厚を確認できなかった.これら 3潜航測線沿いの溶岩流は#1089,#1090,#1088付近の順に形成された可能性がある. Keyword:マリアナトラフ背弧拡大軸,表層堆積物,サブボトムプロファイラ,しんかい6500 欄外見出し:マリアナトラフ背弧拡大軸付近の表層堆積物観察

高知大学大学院理学研究科自然環境科学専攻

(JAMSTEC: IFREE1研究生) 小池 悠己

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1. はじめに 背弧海盆における海底拡大に関する研究は,背弧下ウェッジマントル内でのマントル対流,背

弧と島弧で生成されるマグマが相互作用する複合系の海洋リソスフェア形成を解明するのに重

要である.ウェッジマントルでは,海洋プレート沈み込みによる強制流動とマントル深部からの

逆流,拡大軸での 2 つの上盤プレートの分離による受動的流が複合したマントル対流が起こっ

ている.背弧海盆は,弧状に拡大していることから,年代によってマントル対流自体が変化して

いることが考えられる.

中部マリアナトラフの背弧海盆拡大軸谷内で行われた深海曳航サイドスキャンソナーを用い

た海洋調査(Deschamps et al., 2005; Asada et al., 2007)では,17°Nのセグメントでは滑ら

かな海底表面を推定させる後方散乱パターンと凸凹した表面を推定させるパターンの 2 パター

ンが確認された.これら滑らかなパターン,凸凹なパターンはそれぞれシート状溶岩流,枕状溶

岩流と推定され,マリアナトラフは両側 3cm/yrという低速拡大であるのにもかかわらず,噴出

率が高いことを示唆するシート状溶岩が分布しているとされている.

地震波速度構造の結果(Takahashi et al., 2007)からは,およそ 1myrだけ溶岩噴出率が高

くなっていると考えることもできる.またサイドスキャンソナーでの観測に見られるシート状溶

岩流から枕状溶岩流への変化も,溶岩噴出率の時間変化を示唆している可能性がある.これを議

論するためには,溶岩流の年代差の情報が不可欠である.

「よこすか」YK08-08 Leg1(2008年 6月~7月)では西太平洋マリアナトラフ 17°Nの背

弧拡大軸付近(Fig.1)で有人潜水調査船「しんかい 6500」を用いて,岩石採集と目視観察,物

理探査を行った(Fujiwara et al., 2008).本稿では,「しんかい 6500」に取り付けたサブボトム

プロファイラ(SBP)について報告する.海底形成後,年代が古いほど堆積層厚は厚くなること

が考えられるので,SBP での調査目標は堆積層厚を測り,その変化から相対的な年代を推定す

ることである.これらは局所的な溶岩流の新旧を判別するため,重要な情報であると考えられる.

2. 手法 2.1サブボトムプロファイラ(StrataBox) サブボトムプロファイラは,潜水船内の StrataBox本体と表示・収録を行うノート PC,潜水

船外の深海用トランスデューサで構成されおり,浅海用の地質探査機を深海用に改造し,トラン

スデューサを深海用のものに交換している.本体電源は電池ボックスを使用し 12V,トランス

デューサの使用周波数は 10kHz で,最大探査深度 40m,地質分解能は約 6cm である.浅海部

のリミットは 2.5mで底質に依存する(Table.1).

2.2 YK08-08航海におけるサブボトムプロファイラ設置と設定,計測 本航海では,「よこすか」YK05-16 Leg1(Kumagai, 2005)と同様,「しんかい 6500」のマニ

ュピレータ下の荷台にトランスデューサを取り付けた(Fig.2).YK08-08 航海では,潜航直前

に電池ボックスとデータ保存・モニターするノート PCを潜水船内に設置し,テスト(起動・収

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録)後,正常に起動するか確認するため,潜水船が吊り上げられると同時に起動と収録を行った.

対象海域は堆積層が薄く,溶岩の基盤が露出していることが予想されたので,基盤からくる強い

反射で SBPデータ上の地下浅部の微細な構造をつぶしてしまわないよう,Range:10mと短く,

DT Gain:15dbと小さめに設定し観測を行った.潜水船揚収後,船内に入りノート PCと電池

ボックスを回収した.電池ボックス内の使用済み電池は取り出して充電し,ノート PCからはデ

ータを取り出した.そして,生データを StrataBox付属ソフトで再生(Fig.3),100~200秒毎

にキャプチャし,イベントリスト,log ファイル等を用いて,船外写真と SBP データの場所と

時刻を合わせ,3潜航の堆積層厚分布の推定を行った.

「しんかい 6500」に設置した SBPは,船体~海底の距離が 2m以上離れていないと送信バー

ストによりデータを得ることはできないため,高度 2.5m以上が好ましく,目視観察は高度 1~

2m が適している.目視観察に適した高度と SBP の良質データを取得できる高度は異なってい

る.そのため,Fig.6~8 でみられるように,SBP データで海底面が明瞭である場合,高度が高

く写真では観察しづらいことが多い.しかし,SBP海底面と目視観察を確認できる地点もあり,

これらは堆積層厚や地下構造から相対的な海底の新旧を予想することができる重要な情報であ

る.

3. 結果・考察 本航海では,合計6潜航が行われ,マリアナトラフ拡大軸17°N付近では 3潜航(#1088,#1089,

#1090)が行われた(Fig.4).#1088では,背弧拡大軸から西へ向かう測線を取り(Fig.5),潜

水船外カメラでの写真では,堆積物に覆われていない岩石が目視できた地点が多く(Fig.6

12:53), SBP データも強い反射を示していた.しかし,SBP データから海底面が検知できな

い地点も多く(Fig.6 11:51),これは起伏の多い溶岩の基盤による多方面からの反射によるもの

であると考えられる.堆積層厚分布は確認することができなかった(Fig.9, Table.2).表層堆積物

がほとんど確認されなかったことから,新しい溶岩流である可能性が高い.岩石サンプルでは年

代測定ができないほど新しい可能性もあるが,堆積物サンプル中の化石から相対的な年代を推定

されることが期待される.

#1089は,潜航開始地点付近では堆積物を見ることはできなかったが,南下するにしたがっ

て,拡大軸東側斜面では堆積層が確認された.測線上(Fig.5)の潜航開始地点の南側から火山性海

丘の東側では,表層堆積物が 1m以上(試料サンプリングポイント付近では,明瞭な SBPデー

タが取れなかった (Fig.7 12:57))あり,海丘頂上付近では,堆積層は薄くなっている(Fig.7

14:57).行われた3潜航中,最も堆積層が明瞭に確認できた潜航であり,また堆積層中に平行な

もう一つの反射面を見ることもできた(Fig.7 12:32).堆積層厚分布が唯一推定することができ

た潜航である(Fig.9, Table.2). #1090では,各地点の目視観察で堆積層を確認できることが

多かったが,SBP データでは確認できなかった.また,起伏の多い溶岩の基盤による多方面反

射による SBPデータの荒れもみられた(Fig.8 15:02).Fig.8 12:33では,船外写真でなだらかな

地形と堆積物が確認でき,SBP データでもわずかに堆積層が見えているように思えるが,基盤

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面-海底面境界が明瞭ではなく,主観性が生じると考え,SBPでは観測できないと判断した.明

瞭に確認できない原因は,#1089の SBP結果では 0.5m 以上の堆積層を確認できたことを考慮

すると,0.5mより薄い堆積層であるためと予想される.そのため,堆積層厚分布を推定するこ

とはできなかった(Fig.9, Table.2).SBPの分解能が足りず図示できなかったのか,解析ソフト

によるものなのか,現時点では不明であるが,別の解析ソフトを使用することでより明確に確認

できるかもしれない.

各潜航を行った場所の海底は,最も堆積層が厚かった#1089,目視観察では確認できたが,SBP

では確認できなかった#1090,溶岩の基盤が目視観察で確認できるほど堆積物が少なかった

#1088 の順に形成されたことが推定される.これら3潜航のみから相対的な新旧が判別できた

ことで,より多くの測線を取ることができれば,より多様な年代変化を知ることができるかもし

れない.

4. まとめ YK08-08 Leg.1では,有人潜水調査船「しんかい 6500」を用いて,岩石・堆積物採取と目視

観察,海底地磁気測定,堆積層厚測定を行った.マリアナトラフ背弧拡大軸 17°N 付近での各

潜航#1088,1089,1090(Fig.4)のサブボトムプロファイラのデータと潜水船外写真を照らし合わ

せ,その地点での海底の様子(堆積物の有無,溶岩の種類)を観察した.観察の結果,ほぼ堆積

層が確認できなかった#1088, 0.5~2m の堆積層が確認できた#1089,写真では確認できたが

SBPデータには反映されなかった#1090.これら 3潜航を行った海底は#1089,#1090,#1088

の順に形成された可能性がある.SBP での観測に成功しデータを取得することはできた.しか

し,SBPデータが荒れ,海底面を検知できないことも多かった.対象海域の形成年代が新しく,

堆積層が薄いことと枕状溶岩が多く分布する複雑な地形なために起こる多方向反射,StrataBox

付属ソフトの分解能の限界が原因であると考えられる.地形や解析ソフトがこれからの表層堆積

物観察の課題となるが,改善できればより詳しい堆積層厚分布が解明される可能性が高い.それ

でも,本航海の 3 潜航のみから海底の新旧を判断できたことは,マリアナトラフ背弧拡大軸の

溶岩噴出率の局所的な変化を解明する助力となると考えられる.より多くの測線を取れば,より

多様な相対的な年代変化がわかるかもしれない.

参考文献 Asada, M., A. Deschamps, T. Fujiwara, and Y. Nakamura, Submarine lava flow

emplacement and faulting in the axial vally of two morphologically distinct

spreading segments of the Mariana back-arc basin from Wadatsumi side-scan soner

image, Geochem. Geophys. Geosyst., 8(4), Q04001, 10.1029/2006GC001418,2007

Deschamps, A., T. Fujiwara, M. Asada, L. Montési, and P.Gente, Faulting and volcanism in

the axial valley of the slow spreading center of the Mariana back arc basin from

Wadatsumi side-scan soner images, Geochem. Geophys. Geosyst., 6(5), Q05006,

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35

10.1029/2004GC000881,2005.

Fujiwara, T., S. Umino, M. Asada, Y. Koike, T. Kanamatsu, and S. Okada, A submersible

study of the Mariana Trough back-arc spreading center at 17°N, JAMSTEC Rep.

Res. Dev., submitted, 2008.

Kumagai, H. (edit), Preliminary cruise report YK05-16 Leg 1 Yokosuka/Shinkai 6500

“URANIWA” expedition, 2005.

Takahashi, N., S. Kodaira, S. L. Klemperer, Y. Tatsumi, Y. Kaneda, and K. Suyehiro,

Crustal structure and evolution of the Mariana intra-oceanic island arc, Geology, 35,

203-206, 2007.

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図説 Fig.1 今航海で取得した 144°E,17°N付近の海底地形

四角部分が潜航ポイント周辺を示す(Fig.4参照).0.05分間隔でグリッドデータ

を作成

Fig.2 しんかい 6500による SBP観測概念図 トランスデューサは潜水船外のマニュピレータ下に荷台をつけ設置し,トランス

デューサケーブルを「しんかい」ペイロード分岐箱に接続した.船内では,

StrataBox 本体と「しんかい」ターミナルボード,電池ボックス,ノート PC を

接続した.

Fig.3 SBPデータ表示例

縦軸は 1メモリ 0.5m間隔で 10m表示しており,横軸は約 1分間のデータを示す.

時間は UTで表示されており,+10時間でグアム時間となる.青破線で示すよう

に,2m以浅は記録が得られないことがわかる.この地点では約 1mの堆積層が確

認できる.

Fig.4 YK08-08 Leg1で行った 3潜航の測線と周辺海底地形 50m間隔でグリッドデータを作成.各潜航の測線,星は離着底ポイントを示す.

Fig.5 各潜航の地点と測線 赤字はサブボトムプロファイラの各データと海底写真をキャプチャした時間を示

す(グアム時間).黒丸は岩石,黄色三角はプッシュコアの採取地点を示し,赤星

は離着底地点を示す.

Fig.6~8 3潜航の各時刻の SBP生データと船外カメラ写真 黒字はデータの時刻を示しており,上図は潜水船外側カメラ写真で,下図は SBP

データ(Fig.3参照),それぞれ同時刻のデータに対応している.

Fig.9 各潜航の堆積層厚分布 赤:堆積層を確認できない 黄:SBPでは確認できないが,写真では確認できる

緑:約 0.5m 青:1m以上 赤星:離着底地点

Table.1 「しんかい 6500」に搭載した SBP(Strata Box)のスペック概要 Kumagai 2005を引用

Table.2 各潜航の SBPと写真による表層堆積物の有無 SBPと船外写真で観察できた表層堆積物の有無を示す.SBPデータから厚さを確

認できた場合は数値を示す.

○:確認できる △:わずかに堆積物が確認できる ☓:確認できない

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Fig.1

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Fig.2

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Table.1 Depth Ranges 5,10,20,40,80,150m

Resolution 6cm

Penetration 40m

Depth Accuracy 0.5%

Upper limit 2.5m

Sending rate Max 10Hz

Date File Output ODEC format

Frequency Output 10kHz

Transmit Output

Power 300W (Pulsed)

Input Power 12V DC,Nominal power 8W

(Kumagai 2005)

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Fig.3

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Fig.4

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Fig.5

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Fig.6 #1088

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Fig.7 #1089

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Fig.8 #1090

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Table.2 1)#1088 2)

#1089 Time(hh:mm) SBP(m) Picture

11:51 ☓ ☓

12:08 ☓ △

12:21 ☓ △

12:53 ☓ ☓

13:38 ☓ ☓

14:07 ☓ ☓

14:47 ☓ ☓

15:06 ☓ △

15:28 ☓ ☓

3) #1090

Time(hh:mm) SBP(m) Picture

12:00 ☓ △

12:30 ☓ △

12:33 ☓ ○

12:47 ☓ △

13:15 ☓ △

13:34 ☓ △

14:17 ☓ △

14:23 ☓ △

14:48 ☓ △

15:02 ☓ △

15:13 ☓ ○

15:26 ☓ △

Fig.9

Time(hh:mm) SBP(m) Picture

12:16 ☓ △

12:32 2 ○

12:57 ☓ △

13:12 0.5 ○

13:56 1 ○

14:07 1 ○

14:27 1 ○

14:34 0.5 ○

14:57 △ ○

15:16 ☓ ☓

15:35 0.5 ○

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4. Preliminary Reports 4-0. Dive Tracks Dive #1088 traversed the small ridge situated in the center of the median valley and took a route on the western flank of the median valley of ~3420 m depth. The small

ridge corresponds to bumpy sidescan sonar images and the western flank is smooth surfaces with high backscatter intensity (Figures 4-0-1 and 4-0-2). Dive #1089 climbed a broad ridge formed by a bunch of large domes 100-300 m in the eastern side of the

valley. The dive track followed along the ridges from the northern plain of the bumpy area to the fan-shaped terraces to the south, and then turned to the west and went up and beyond the broad hummocky ridge (Figures 4-0-1 and 4-0-3). Dive #1090 traversed the

western side of the valley to the three miles north of #1088 and #1089 toward a flat-topped seamount. The dive took a route across linear structures trending in the NNW-SSE (parallel to the strike of spreading axis) and the N-S and in smooth surfaces

recognized in the sidescan sonar image.

Figure 4-0-1. Locations of the Shinkai 6500 dives in the Mariana Trough 17°N.

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Figure 4-0-2. Shinkai 6500 #1088 and #1089 dive tracks superposed on the sidescan sonar image and swath bathymetry with 10 m contours. Stars and hexagons mark the

landing and leaving points of #1088 and #1089, respectively.

Figure 4-0-3. Shinkai 6500 #1090 dive track superposed on the sidescan sonar image

and swath bathymetry with 10 m contours. Triangles mark the landing and leaving points.

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4-1. Dive Results 4-1-1. Dive Report: Shinkai 6500 Dive #1088

Date: 2 July 2008 Site Name: Mariana Trough 17°N Landing: 11:32; 17°00.1711'N, 144°51.3784'E, 3408 m

Leaving: 15:46; 16°59.9297'N, 144°49.9328'E, 3425 m Observer: Toshiya Fujiwara (IFREE, JAMSTEC) Pilot: Keita Matsumoto, Co-Pilot: Fumitaka Saito

Objectives: Dive objectives are 1) geological observation of lava flow morphology and its spatial

variation from the center to the edge of the median valley, 2) collection of rock and sediment samples for chemical and age analysis, and 3) geophysical observation using a deep-sea magnetometer and a sub-bottom profiler to investigate magnetization of the

lava flow and thickness of sedimentary layers that cover the lavas. Dive Summary: The landing point is an eastern foot of the small ridge of ~150m relative height elongating in NNW-SSE (~N20°W) located in the center of the median valley of the 17°N segment. The dive traverses the ridge and takes the route on the western flank of

the median valley (Figures 4-0-1 and 4-0-2). Morphology at the landing point is jumbled-wrinkled lavas (Photo 4-1-1-1). The lava morphology suggests sheet and turbulent flow at high eruption rates. The lava flows could be very young, because they

show no or little accumulation of sediment. It seems to be the freshest lava flows observed among the three dives (#1088-#1090). The small ridge mainly consists of pillow and pahoehoe lavas covered with thin

sediment. Pillows are ~2 m in length and ~1m in diameter (Photo 4-1-1-2). The top of the small ridge is covered with thin sediment. In the western half of the dive route beyond the small ridge, there are various types of lava morphology. It shows mixture of

pahoehoe, folded sheet, lobate sheet, and jumbled flows produced by sheet flows with high effusion rate (Photos 4-1-1-3 and 4-1-1-4). Topography is a series of ridges and troughs. The relative height is ~5-10 m. The topography may be originated to be

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tectonic structures such as faults, or volcanic structures such as fissures or lava channels. Strikes of the ridges and troughs are in the NNW-SSE or N-S direction. It is difficult to distinguish this small angle difference from the submersible visual observation because

their topographic edges are not sharp. According to tectonic interpretation of the sidescan sonar images (Asada et al., 2007), the submersible passes across an area of N-S lineated structures, and then across an area of NNW-SSE lineated structures. Sediment

coating in the area of N-S structures tends to be thinner than that in the area of NNW-SSE structures. It may suggest that the N-S structure is newer inferred from the sediment accumulation.

Sediment cover more or less gradually increases toward the margin of median valley although the spatial variation in detail is not monotonically. Onboard records of the sub-bottom profiler indicate very thin sediment cover through the dive route although it

is difficult to obtain clear reflection image from the sediment/lava boundary due to rough seafloor relief. Basaltic rock samples were collected at 9 stations from landing to leaving points. These

samples have grass rims and almost no manganese coating indicating very fresh lavas (Photo 4-1-1-5). Basaltic lavas taken at R1 (the landing point) are highly vesiculated. Sediment samples in sediment pockets between lavas were collected at 3 stations using

push corers. These samples are brown colored clay. Microfissils are included in the sediment samples. Deep-sea magnetic data were collected successfully from descending to leaving bottom.

Keywords: lava flow, pillow, pahoehoe, lobate sheet, jumbled flow

Payloads: Sample box, Basket divider, Push corer X3, Scoop, Marker, Sub-bottom profiler, Magnetometer

Location of Events: See an attachment of #1088 dive log.

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Photo 4-1-1-1

Photo 4-1-1-2

Photo 4-1-1-3

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Photo 4-1-1-4

Photo 4-1-1-5

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#1088Observer: Toshiya Fujiwara 2008.7.2Chief-pilot: K. Matsumoto Co-pilot: F. Saito page:1/

Dive logger: S. UminoDep.(m)

Alt.(m)

Head(deg)

Xm(N)

Ym(E)

10 0111 24 335411 27 3408

1050 138011 40 24011 43 3406 25111 44 3402 25211 46 3384 25411 48 3377 248 1050 1350

Run across a valley onto the western ridgePahoehoe-lobate sheets; no sediments

11 51 3370 23012 02 3372 142 1000 128012 02 24612 06 3385 12 23812 10 3389 240 900 120012 12 3385 25112 14 3368 25012 15 3364 250 860 112012 18 3368 249

12 213359 251 830 1030

12 24 335312 26 3352 850 100012 30 3347 25012 32 3350 5 24812 33 335012 40 3351 1 810 92012 42 3348 2 25012 45 3336 25112 48 3327 2 24512 49 3325 3 25112 51 3321 249 810 89012 55 3300 24512 58 3277 24912 59 3273 4 232 800 79013 00 3276 11 20813 07 3342 4 25113 11 3389 5 250 720 58013 12 3409 3 24913 16 3421 4 25113 17 3420 4 234

SHINKAI 6500 Dive #1088

ObservationTime(LCL)hh|mm|ss

Shinaki on surfaceStart DiveBegin descendingOn bottom; jumbled-wrinkled flows, rubble; pitSampled crust; R1A: big, R1B: small chunk, R1C:? into S1Start movingPit on lobate sheet; Jumbled flow on top of the lobate sheetPahoehoe-flattened knobby pillows directing 140Elongate-flattend lobes directing 120; sedimentedElongate-bulbous-knobby pillows; thinly sedimented

Bulbous pillow with trapdoor buds; no sedimentsR2A, B, C: stained bulbous pillows into S4Start movingFly overPillows-pahoehoeLobae sheet with a collapse pit shwing stacked pahoehoe-pillow lobesSedimented elongates; faultedPahoehoe-pillows; bulbous pillow on top; no sedimentsPahoehoe nto lobate sheets

Some knobby lobes among pahoehoe-lobate sheets; thinly sedimented

Stop for coring; deep sedimentsBlue S1Satart moving; deeply sedimented elong pillowsRubbleStop for sample rubble; talus rubble at the foot of a fault?R3A: stained small chunk, R3B: chunk into B3Start movingPillows with rubble intersticesFractured pillows; unsedimentedSedimented pillow-pillow rubblePillow rubble; irregular shaped blocksPillows with rubble intersticesBroken pillows with rubble intersticesBulbous pillow on lobate-pahoehoe sheetPassed the summit; descending onto the western flankKnobby-elongate pillows directing 10Fly overSedimented? Flat floorRubbly floor with a pit on the leftLava pillar; flying over a pit

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#1088

13 19 3420 3 198

700 470

13 27 3420 18013 31 3414 25913 33 3415 1 26013 33 3415 26013 34 3415 2 26013 36 3422 4 259 870 38013 37 3425 1 26113 39 3425 2 26013 41 3424 2 26013 42 3425 2 259 670 240

13 44 3426 3 25813 45 3426 3 249

13 473426 275

13 54 850 200

13 57 3426 3 277 Pahoehoe-lobate sheet; hollow lobes present13 59 3427 2 26014 01 3423 2 26114 05 3418 3 257 650 5014 08 3411 2 26114 10 3408 26114 12 3410 12 261 650 -10014 12 3422 2 26214 15 3421 2 261 650 -15014 18 3415 3 26114 25 3415 236 640 -240

14 31 3414 1 26114 37 3424 3 260 630 -37014 42 3421 1 25914 45 3419 2 26114 47 3415 2 260 650 -52014 51 3416 3 26114 52 3415 3 26415 01 3415 258 630 -600

15 0215 03 3413 2 26115 05 3413 3 26115 09 3414 4 261 620 -76015 12 3413 2 26115 16 3419 11 26115 18 3419 245 Lobate-pahoehoe; broken crust; stop for sample

Wrikled flow directin 160?2nd pit on the wrinkled flowStart movingSedimented sheet with some rubble?Sedimented lobate-pahoehoeFly over a pit on lobate sheet >4 m deepRubble on the pit floorSedimented pahoehoe lobes

Lobate sheet with sporadic hollow lobes; sedimentedSedimented lobate sheetRubble?Jumbled sheet; rubbly surface

R7B: dittoR7C: dittoStart movingHeading to W

Wrinkled flow directing 10Lobate-pahoehoe lobesJumbled crust on lobate sheet; stop for sample; few sedimentR7A: jumbled crust into P2

Start moving on sedimented jumbled sheetJumbled flow? Blocky-rubbly surface partially buried by sedimentWrinlked flow among jumbledSlabby-jumbled sheet

R6A: stained crust of wrinkled sheet flow into P1R6B: dittoetc.Yellow S2:brown sediment on jumbled crust? or what?

Fly over a pitWrinkled lobate sheets; jumbledInto a pit; jumbled crustJumbled crust; stop for sampling

R5A: crust of a stained pahoehoe lobe into S3R5B: dittoR5C: dittoR5D: ditto

Start moving; broken clasts on pahoehoe lobesInto a pitRubbly?Knobby?Stacks of collapsed hollow pahoehoe lobes; rubbleRubble; into a pit

R4A: stained ropy crust into B2

Stop for sampling on pahoehoe lobes; few sediments

No sediments on pahoehoe lobesOnto a pit roof; lobate-pahoehoeLava pillar; lobate roofInto a pitSedimented lobate-hollow pahoehoe lobesFew sedimented pahoehoe-lobate sheet

R4B: dittoR4C: ditto

Stop for sampling; smooth-wrinkled flow on the bottom of a pitThin sediments; stained ropy crust sampled

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#1088

15 20 3419 19215 28 3419 2 176 620 -1010

15 29 3418 2 21315 30 3414 5 26115 33 3413 6 263 600 -110015 36 3425 25015 45 3425 610 -1190

15 46R9C: dittoLeave bottom

Lobate-pahoehoe; stop for sample; sedimentedGreen S3: brown sediment on pahoehoe lobesR9A: crust of hollow pahoehoe lobe into P5R9B: ditto

R8B: dittoStart movingLobate sheet-pahoehoe lobesFly over a valley

Sedimented rubble of rugged crust like "aa"R8A: rubble into P4

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4-1-2. Dive Report: Shinkai 6500 Dive #1089 Date: 3 July 2008

Site Name: Mariana Trough 17°N Landing: 11:34; 17°01.0559'N, 144°52.525157'E, 3505 m Leaving: 15:46; 16°59.9297'N, 144°49.9328'E, 3425 m

Observer: Susumu UMINO (Kanazawa Univ.) Pilot: Tetsuya KOMUKU, Co-Pilot: Kazuhiro CHIBA

Objectives: Dive objectives are 1) to observe volcanic landforms and products to establish correlations between the Wadatsumi sidescan sonar images and volcanotectonic features,

2) to collect rock and sediment samples from varieties of volcanic landforms for chemical analyses and age determinations, and 3) to investigate magnetization of the lava flow and thickness of sedimentary carapace on the lavas using a deep-sea

magnetometer and a sub-bottom profiler attached to submersible Shinkai 6500. Dive Summary: The Wadatsumi sidescan sonar imagery in the eastern half of the axial valley at 17°N shows bumpy aggregates of dome-like structures along the foot of the eastern valley wall, which continues to superposed fan-shaped terraces like traditional Japanese

“Tanada” to the south (Figure 4-0-2; "#1088-1089wadatsumi.jpg"). The terraces appear to be overlain by a bunch of large domes 100-300 m across that form a broad ridge 1 km east-west and 700 m north-south and 30-40 m in height. The dive track followed

along the above ridges from the northern plain of the bumpy area to the fan-shaped terraces to the south, and then turned to the west and went up and beyond the broad hummocky ridge. The dive summary is listed in Table 4-1-2-1; "S1089_Table.xls".

The northern plain where Shinkai landed is underlain by lobate sheets covered with thin sediments. Spherical to bulbous pillows are exposed on the sedimentary cover. The submersible landed on a tumulus several meters in height and 10 m in diameter with

wrinkled surface and wide and deep inflation cracks. Sample 6K#1089R-01 was collected from the folded wrinkles of the surface crust of the tumulus. Just 100 m to the south, swollen and fractured pillows and irregular pillow lobes were present on the

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sedimentary plain. Sample 6K#1089R-02 was collected from a pillow. The lobate sheet with pillowy knobs continue further 400 m to the south, before entering into a field of pillow mounds. Folded crust on a tumulus was sampled (6K#1089R-03) near the end of

the lobate-bulbous pillow field. These samples (6K#1089RR-01, R-02, R-03) are all aphyric basalt as is observed in other two dives at the axial valley floor (#1088 and #1090).

The submersible moved southward into the bumpy area, a field of pillow mounds several to a few tens meters high and swam along the curved steep slopes of elongate pillows directing downslope.

Passed through the pillow mounds, Shinkai went into the terrain of terraces underlain by thick sedimentary plains with sporadically exposed big pillows. Sample 6K#1089R-04 was taken from a big pillow on the sedimentary plain and a push core 6K#1089S-01

was sampled. Sample 6K#1089R-04 has abundant plagioclase and rare olivine phenocrysts and tiny hypersthene microphenocrysts. The flat plain is covered with 40-60 cm thick sediments, with a sporadic holes and knobs of bulbous-round pillows.

Interiors of hollow lobes can be seen from the holes on the sedimentary plain, suggesting the presence of lobate sheets beneath the sediments. The submersible turned southeast and traversed the edges of terraces fringed with elongate pillows. Shinkai then

turned its head to southwest and passed through a sedimentary terrace to the foot of the hummocky ridge with an apron of elongate pillows. Sample 6K#1089R-05 was taken from an elongate pillow on the steep slope of the ridge. Sample R5 is abundantly

plagioclase-phyric, but differ from Sample 6K#1089R-04 and R-06 in that it lacks olivine and hypersthene. The submersible went up a very steep slope (60-70°) of elongate and knobby pillows

directing downslope with a thin sedimentary carapace. The summit of the hummocky ridge is cut by faults running NNE and NNW, which expose truncated pillow lobes. Swam over a few pillow mounds, the submersible went down slopes of elongate pillows.

Sample 6K#1089R-06 was collected from an elongate pillow, which like the Sample 6K#1089R-04, has abundant plagioclase phenocrysts and rare olivine phenocrysts with tiny hypersthene microphenocrysts.

The western foot of the lobate sheet flows with domal uplifts of tumuli several meters in diameter. We tried sample a tumulus, but abandoned as the time to return to the surface came.

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It should be noted the presence of plagioclase-rich samples from the pillows on the terraces and the hummocky ridge, that carry tiny but not rare microphenocrysts of hypersthene. However, hypersthene was identified only based on the optical

characteristics using a hand lens, it needs to be tested by a polarized microscope and electron microanalysis.

Keywords: lava flow, bulbous pillows, elongate pillows, tumulus, lobate sheet flow Payloads: Sample box, Basket divider, Push corer X3, Scoop, Marker, Sub-bottom profiler, Magnetometer

Location of Events: See an attachment of #1089 dive log.

Samples: 6 lava samples, 1 push core of mud

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DiveLog_YK0808_Leg1Observer: Susumu Umino 2008.7.3Chief-pilot: T. Komuku Co-pilot: K. Chiba page: 1/

Dive logger: M. AsadaDep.(m)

Alt.(m)

Head(Deg)

Pos.Xm

Pos.Ym

9 5510 511 35 3508 1030 1270

350811 45 351011 47 3509 103 990 130011 50 3504 2 190

11 51 3503 111 56

11 59 3503 2

12 8 3500 2910 1270

12 12 3496

12 17 3492 6 19112 18 3496 2 19112 20 3496 3 19112 21 3494 2 190 790 125012 24 3492 212 25 3491 2 18012 29 3485 2 620 122012 31 3482 1

12 32 3478 4

12 45 3481 1 570 1240

12 47 3474 412 49 3468 412 50 3462 3

12 54 3452 312 55 3442 3 18212 57 3441 2 182 500 1200

13 0 3449 272

13 4 3440thin sediment covers elongated pillows with few cracksStop for observation

steep slope covered by wany pillows at the left side of cameraslightly sedimented pillowsfly over? No seafloor on TV斜面に正対し航走する

still heavy sediments surrounding斜面に正対し航走する

bulbous and elongated, wany and small pillows; sedimentedchange course to 180*

Sample crust (brown colored) of lava and other rocks, total two into P1[R3]start moving to 190*cliff of pillow? Or a wall of pillow mound?change course to 210*

heavily sedimented lavaschange course to 190*Big and sedimented flow with fresh-looking fractureStop for observation & sampling

heavily sedimented areaheavily sedimented and cracked flatten(?) lavas and fragmentschange course to 180*Sediment…sediment….sometimes cracked flows come out

heavier sedimented pillows than R2 pointcracked flatten-top lavas, and fragments, under sedimentflyfind cross section of sheet flow!?

Cracked big pillows…Get sample form crust of cracked pillow; sediments surroundingsample two rocks into S3 [R2]start moving to 190*

Stop for observationstart moving to 190*cracked bulbous (partly elongated) pillows on sedimentStop for sampling

Sedimented-cracked-sheet flowssample several rocks into S1 [R1]start moving to 190*cracked bulbous pillows and fragmented lavas; sedimented

Shinkai on surfaceBegin descendingOn bottom; massive lava flowsRock sampling, crust

SHINKAI 6500 Dive #1089

ObservationTime (LCL)hh|mm|ss

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DiveLog_YK0808_Leg1

13 8 3432 5 34913 11 3422 3 8

13 13 3423 2 1413 20 3424 3 490 1170

13 29 3448 313 30 3442 2 18013 33 3446 3 18013 3413 35 18513 37 3441 2 21813 38 3441 1 218 360 116013 41 3441 113 44 3438 2 17813 4613 47 3441 12 18013 49 3456 2 17713 50 3443 4 181 260 113013 55 3424 2 184 190 116013 59 3415 2 180 90 112014 3 3429 5 14714 5 3437 2 150 -20 109014 614 6 3427 2 14314 10 3403 3 118 -80 116014 13 3387 2 12114 15 3384 5 120 -120 122014 19 3409 2 11914 20 3404 4 -150 130014 25 3402 4 17814 26 3383 10 181 -220 130014 29 3374 3 181 -290 129014 30 3365 214 36 3365 2 24814 38 3371 5 251 -410 1120

14 41 3387 2 24314 42 3385 3 22214 42 3384 2 214

14 52 3361 ? -410 105014 53 335814 54 3357 27014 59 3375 3 271 -410 910

15 4 3350 3 27115 7 3333

15 9 3316 5 271

partly lobate-like flows are recognized, start move長い舌に丸いのが乗っかっているおかしな形態の溶岩発見

cumulated pillow mound covered by thin sedimentfracture or steep slope on the front~right side of cameraまだ舌ピアス溶岩が見えてる

finish observation, start moving to 180*fly over? No seafloor on TVsheet-like flow, elongated pillows, thinly sedimentedvarious directions of flow lines are recognizedsedimneted flat areapillows are recognized at end (edge?) of the flat areapillows on sedimentstop for samplingsample one rock into P2 [R4], keep stop for observationsample Green piston core [S1]start moving to 180*Heavily sedimented area --> no seafloor imagefly overelongated and bulbous pillows on sediment, thinly covered by sedimentHeavily sedimented areasediment…sediment….sediment…sediment…change course to 150*sediment…sediment…elongated lava and fragmented(?) angular lava on thick sedimentchange course to 120*cumulated bulbous pillows covered by thin sedimentstill heavily sedimented area, few lavas are recognizedsediment…sediment… -->change course to 180*sediment…top of pillows sometimes appear on heavy sedimentno seafloor imagesediment…change course to 250*sediment…sediment…elongated pillows appearrrrrrs!! on thick sediment, after long silence!!Shinkai may on local ridgebulbous pillows with few cracks on sedimentfractured pillowStop for observationcliff of broken pillows on front of Shinkaimany bulbous and elongated pillows on thick sedimentsample one rock into S4 [R5]start moving to 270*fragmented lava on thick sedimentcumulated elongated pillows, partly fractured, ridgeShinkai at near the top of small seamount.. Still heavily sedimentedcumulated pillows make mound. Most of pillows seems be broken.Foot of the mound is heavily sedimented.

altered flatten-top flow and pillows make moundcliff of broken lavaflow line of lava is sometimes visible on slope of the pillow mound

small bulbous lavas clings on surface of lava tubeupslope on left side of camera

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DiveLog_YK0808_Leg1

15 103303 4? 270 -400 780

15 14 3373 5 27115 17 3354 3

15 24 3350 4 ? 690

15 28 3330 26415 29 3329 5 26015 31 3345 3

15 36 3342 3 286

15 44 3349 3 16015 46 3351 3 138 -360 54017 5

fly over? No seafloor image on TV

Shinkai going down slope?lavas on thick sedimentbroken flatten-top flow (sheet flow?) on sedimentcumulated lava tubes make mound, sediment surroundingstart moving 270*Sample two rocks into P1 [R6]narrow and long lava tube, pillows, both broken, no sediment!Stop for observation

overview

A relative height of the target seamount is ~120 m. Most of lava flows observed during this dive is kinds of pillows and lava tubes.Sediments are much thicker than we expected. No chimneys are found.

Shinkai on peak of the target seamount. Still sedimented but many lavasare observed

Shinkai on deckShinkai on surfacegiving up sampling and leave bottommassive sheet flow on right side of camerafresh-looking fracture on massive lavaStop observation and sampling

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Table 4-1-2-1. Dive summary of YK08-08, S1089.

TIme Waypoint X(Northing),Y(Easting) Latitude Longitude Depth [m] Notes Sediment

1146 WP-01 On bottom 17deg1.0375'N 144deg52.5311'E 3508 Tumulus with lava wrinkles, lobate flow field very thin R1 Wrinkled flow surface of a tumulus AphyricWP-02 17deg 0.9916'N 144deg52.5150'E 3501 Pillow field; inflated & fractured spherical-irregular pillows < several cm R2 Pillows AphyricWP-03 910, 1270 Sedimented plainWP-04 Flat lava field with circular pits; trapdoor pillows and tumuli thick >several cmWP-05 Lobate sheet with folded crust 30 cmWP-06 600, 1200 Pillows on slopeWP-07 570, 1240 17deg0.8083'N 144deg52.5003'E 3481 Tumulus 6-7 m across several cm R3 Folded crust on tumulus AphyricWP-08 490, 1170 Entering into field of pillow mounds on the right; elongates directing 80thick

1343 WP-09 360, 1160 17deg0.6977'N 144deg52.4560'E 3441 Pillows on sedimentary floor < 30 cm R4 Pillow Pl-phyric17deg0.6977'N 144deg52.4560'E 3441 S1 mud

WP-10 260, 1130 Pillows buried by mud thick1355 WP-11 190, 1160 Pillow mounds 60 cm

WP-12 90, 1120 Sedimented downslope 50 cmWP-13 Go up elongate pillow slope 30-60 cmWP-14 Bulbous pillows on hollow lobate sheet; elongates direct 240WP-15 -80, 1110 Elongates direct 150WP-16 -120, 1120 40 cmWP-17 -150, 1300 Elongate pillows burried by sediment

1427 WP-18 -220, 1300 Sporadic pillows exposed on sediment 40-50 cmWP-19 -290, 1290 60 cmWP-20 Downslope 40 cmWP-21 Pillow moundWP-22 -410, 1050 17deg0.2777'N 144deg52.3906'E 3361 Elongates on sediment; direct 30 thin R5 Elongate pillow Opx-Ol-Pl-phyricWP-23 Top of Mons Faulted elongate pillows thin

1511 WP-24 -400, 780 Pillow mound1524 WP-25 -390, 690 17deg0.2907'N 144deg52.1900'E 3354 Elongate pillows directing 100 thin R6 Elongate pillow Opx-Ol-Pl-phyric1546 WP-26 Take off 17deg0.3066'N 144deg52.1021'E 3351 Lobate sheets with tumuli thin, 1 cm

Sample

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4-1-3. Dive Report: Shinkai 6500 Dive #1090 Date: 4 July 2008

Site Name: Mariana Trough 17°N Landing: 11:41 (LTC); 17°03.8'N, 144°49.6'E, 3820 m Leaving: 15:41 (LTC); 17°03.2'N, 144°48.7'E, 3611 m

Observer: Miho Asada (HADEEP, ORI, Univ. of Tokyo) Pilot: Yoshitaka Sasaki, Co-Pilot: Hirofumi Ueki

Objectives: 1. Recognize age relationship between lavas 2. Detect reliable ground-reference of Wadatsumi Imagery

3. Find out new volcanic mounds and to get very fresh lavas (and to find first hydrothermal site in Seg-17 area, with any luck)

Dive Summary: At first of my summary, I deeply thank to pilot Mr. Sasaki and co-pilot Mr. Ueki. In general, several types of (in terms of size and shape) pillows and lava tubes are

thickly sedimented over the observed area. The most important result of this dive is that we could recognize clear age-relationship between older NW-SE trending (ridge axis parallel) structure and newer ~N-S trending (oblique to axis) structure. The assumption

that local stress field over the Seg-17 area is changing along time was confirmed. For dive#1090, SHINKAI 6500 found the bottom of seafloor along small ridge

(time, 11:40). The ridge is recognized clearly on both Seabeam bathymetry map and

Wadatsumi sidescan sonar imagery, so that we expect the ridge may consisted by fresh lava flow. However, the ridge is covered by thick sediments. Large pillows and lava tubes, these diameters are ~= 1.5 m, are found under the thick sediments. There are few

cracks on the pillows and lava tubes, so that sampling rocks was difficult. We obtained rocks from 3 points and 1 push core (sediment sample) from the higher level along the ridge (11:40, 12:11, 13:11). One push core was fully buried by thick sediment when we

obtained the core sample, thus calculated thickness of the sediments become more than 30 cm. Nearly North-South trending fissures are developed with very fresh crack surface. The SHINKAI 6500 landed on floor of large depression along the ridge (12:38).

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Sonar system of the SHINKAI 6500 indicates surrounding steep scarps around us. Floor of the depression is also buried by thick sediments.

We change the course from south to west and going off the ridge and fly over a

western lava flow plain (~13:30). Very shallow valley nearby the ridge can also recognized by Seabeam bathymetry map and sidescan sonar image, and was indicated as an older lava flow terrain because of lower backscattering intensity of Wadatsumi

image. Thick sediment also covers the shallow valley. A difference of sediment thickness between the ridge and the shallow valley was not recognized by eye observation, but I hope that deep sea sub-bottom profiling system attached to the

SHINKAI 6500 may clarify the thickness difference. Seabeam bathymetry map indicate that the area is relatively flat, but was not. Low (~several meters high?) but continuous ridge and valleys make tiny up and down, and prevents our moving. Direction of these

ridge and troughs are ~NW-SE, based on SHINKAI 6500s sonar instruction. Some very fresh fissures trending ~N-S cuts these ~NW-SE trending structures, with ~5 m width and more than ~10 m depth, and are frequently observed. Cut section of pillows and

lava tubes are obvious on the fissure walls. It indicate that ~NW-SE trending and heavily sedimented old structures cut by newer ~N-S trending structures.

The older lava plain, however, distributed more than 500 m width which broader area

than we expected before (up to 300m) based on sidescan sonar imagery. The lava plain was ceased suddenly and thinly sedimented surface-fragmented sheet flow was appeared at the end of a smooth ridge (14:30). We could not recognize any

bathymetrically boundary, and fragmented lava flow which seems similar to lava flow found during Dive#1088 survey. The fragmented lava flow gave way to sedimented and cobbly pillow and lava tube soon (14:46), but appeared again as a floor of large hole

(14:58). The SHINKAI 6500 touch a bottom of the hole and observed numerous fragmented fresh lava flows. The second fragmented sheet flow, forming large depression, may be one of arm of single lava flow.

Western part of the two fragmented lava flow areas was small ridge and small mounds of pillows and lava tubes again (~15:00~15:10). Mound (~15 m) settled on a foot of western axial valley wall, which seems newer structure because it does not cut

by any tectonic deformations on bathymetry map, was expected as a relatively new lava flow mound, but it is also false. The mound also covered by thick sediment and large pillows and lava tubes comes out from the sediments. At the last of our dive, the

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SHINKAI 6500 reach to the western axial valley wall. The lower level of the western axial valley wall was consisted by smaller pillows and lava tubes than the mound, and seems cover a western edge of the mound (15:23).

The SHINKAI 6500 observed go up along steep scarp of an axial valley wall as long as possible until we leave the bottom. Just before the leaving bottom, we sampled fragment of pillow and push core. Crack was few on the pillow again, so that sampling

was difficult. However, excellent skill of pilot gives us enough samples and time to observe at the point.

Curiously, I could not focus inner camera because of very high turbidity of seawater,

which became worse when we going westward over the plain ridge. The causation is not known yet.

Keywords: N-S trending fresh fissures, axial valley-parallel ridges and troughs, heavily sedimented pillows and lava tubes, Mn coating

Payloads: Sample box, Sample scoop, Basket divider, Push corer X3, Sub-bottom profiler, and Magnetometer

Location of Events: See attached file named "Dive#1090Event_List"

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#1090Observer: Miho Asada 2008.7.4Chief-pilot: Y. Sasaki Co-pilot: H. Ueki page:1/

Dive logger: T. FujiwaraDep.(m)

Alt.(m)

Head(deg)

Xm(N)

Ym(E)

10 0511 31 375511 36 379411 40 560 101011 47

3796 18011 52 14011 59 3803 16012 11 560 1010

170 470 1030 Start moving12 23 270 1020 Lobate lava thin sediment12 34 3743 100 108012 40 3755 0 106012 45 185 -40 104012 51 -130 1060

3749 -210 105013 10 374913 13 27013 21 374213 23 374213 30 3734 27013 31 3733 -260 100013 40 3733 -230 84013 46 270 -260 77013 54 3744 3 265 -270 71013 58 373114 13 3724 -250 68014 15 3724 2 27014 26 3724 27014 30 3728 2 266 -230 35014 36 3730 -220 33014 45 3723 249 -240 22014 51 3716 230 -280 18014 5414 58 3720 225 -320 8015 05 3710 231 -430 3015 09 369315 15 3706 -600 -17015 17 3703 25015 22 3674 3 270 -620 -35015 2315 27 3633 3 268

SHINKAI 6500 Dive #1090

ObservationTime(LCL)hh|mm|ss

Shinaki on surfaceStart DiveBegin descendingOn bottom; bulbous lava sedimented

Sampled pillow lava #R1 1 piece into S1Start movingPile of knobby pillowsGive up sampling pillow thinly sedimentedSampling #R2 into S2

Sedimented seafloorStacks of pillow fragments

Sample bulbous lava #R3 into S3Sample #S1 into greenStart movingCube worm?Sample pillow lava thinly sedimented #R4 into S4Pillow lavas covered with sediment

Moving sedimented slope

Rubble, piles of pillowsPillow piles faulted?Sample crust of lava tube or pillow #R5 into P1Start moving; crack or scarp in N-S?Pillows covered with sedimentPassing graben of fractured pillows; fault, fissure?Sample rubbled lava #R6 into P2Pillow rubble with sediment

Sheet-like, sedimented lobate-pahoehoe?Bulbous pillow on lobate-pahoehoe sheet? difficult to see due tosediment

Fish

Climbing slopes of lobate flow?

Sedimented jumbled flow?

Rubbly floor with thinly sedimentSedimented lobate flow?

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#1090

15 30 3618 -580 -49015 33 361615 39 3616

15 40 3611 -560 510 Leave bottom

Pillows with thin sedimentSample pillow basalt #R7 2 pieces into P5Sample sediment #S2 into blue

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LEGEND

Pillows and lava tubesFragmented sheet flowSediment

Shinkai6500 track

Depression

large pillows and lava tubesR ~= 1.5 m

many small pillow moundss

Fissure Pillow ridge

Sampling pointRock and core

Sampling pointone rock

Landing point, &Sampling point

Sampling point

axis-parallel fissurehere, but buried bysediment Very sharp and deep fissure trending N-S

Fissure width ~5 m

Large pillows and lava tubeson sediment. R~= 2 m

Very small and narrow seamount here,heavily sedimented

Thinner sedimented pillows and lava tubes than small seamount on foot

Sampling point

Large pillows and lava tubes, R ~= 1.5 m?

Pillows and lava tubes cover a foot of small seamount.Pillow size R ~< 1 m?

Heavily sedimented pillows and lava tubes

Sampling point

Sampling point, rock and core,Leave the bottom

Surface-fragmented sheet flow,similar to the lava flow observed during #1088

Large depression,broken lavas on floor

Large depression,broken lavas on floor

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4-2. Sample Descriptions 4-2-1. Rocks Basaltic rock samples were collected at total of 22 stations in the three dives. Rock

samples obtained from #1088 have glass rims and almost no manganese coating indicating very fresh lavas. Basaltic lavas taken at sheet lava in #1088 are highly vesicular (e.g. 6K#1088R-01). Compared to the samples of #1088, rock samples

collected in #1089 and #1090 look slightly old because of manganese coating and/or on-site sediment accumulation. All samples collected in #1088, #1090, and at the northern plain of #1089 are aphyric

basalts. On the other hand, plagioclase-rich samples were collected from the pillows on the terraces and the hummocky ridge in #1089. Samples collected from pillow lavas (6K#1089R-04 and R-06) have abundant plagioclase and rare olivine phenocrysts and

tiny hypersthene microphenocrysts. However, hypersthene was identified only based on the optical characteristics using a hand lens, it needs to be tested by a polarized microscope and electron microanalysis. The sample (6K#1089R-05) is abundantly

plagioclase-phyric. The sample lacks olivine and hypersthene.

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Table 4-2-1. Rock samples collected by dives #1088, #1089 and #1090.

Dive# Sample I.D.* Size [cm] Mn coating[mm]**

Palagonite[mm] Vesicularity Rock name Phenocrysts Note

1088 6K#1088R-01A 30x27x14 none 0.1 3%, 0.5 mm (max 2.0 mm) Grey aphyric basalt none Folded crust of sheet flow. Core is highly vesicular. Slightlyaltered surface glass.

1088 6K#1088R-01B 14x12x8 none -1088 6K#1088R-01C 14x9x6 none -

1088 6K#1088R-02A 34x21x18 Fr <1.0 0.6 <<0.1%, 0.1 mm (max 5.0 mm) Grey aphyric basalt Pl <0.1%, <4.0 mm

1088 6K#1088R-02B 8x6.5x6.5 Fr 1.0 0.21088 6K#1088R-02C 8x7x5 Fr 0.1 0.21088 6K#1088R-02D 5x4.5x3 Fr 0.5 0.11088 6K#1088R-03A 21x12x8 Fr 0.1 1.01088 6K#1088R-03B 17x12x12 Fr 0.1 - <0.1%, 0.2 mm (max 5.0 mm) Grey aphyric basalt Pl <0.5%, <4.0 mm Stain along fractures1088 6K#1088R-04A 18x14x9 Fr <<0.1 0.2 - Aphyric basalt glass none Black lustered glass with a slightly hydrated surface1088 6K#1088R-04B 13x9x7 - -1088 6K#1088R-04C 6x4x2 - -1088 6K#1088R-05A 21x15x12 none -1088 6K#1088R-05B 17x12x8 none 0.051088 6K#1088R-05C 14x11x9 none -

1088 6K#1088R-05D 8x8x7.5 none - 8%, 0.8 mm (max 3.0 mm) Grey vesicular aphyric basalt noneCrust of lobate sheet with surface glass and lava curtains andstalactites on the back. Elongate and pipe vesicles 17x6 mm.Stain along fractures.

1088 6K#1088R-05E <2 0.3 0.11088 6K#1088R-06A 33x23x22 Fr 3.0 0.31088 6K#1088R-06B 19x12x12 Fr 0.8 0.51088 6K#1088R-06C 11x9x8 Fr 0.1 0.5 1%, 0.2 mm (max 4.0 mm) Grey aphyric basalt none Folded crust of wrinkled sheet flow with glassy surface.1088 6K#1088R-06D <8 Fr 3.0 0.11088 6K#1088R-07A 26x14x12 Fr 5.0 1.0 3%, 0.8 mm (max 5.0 mm) Dark grey aphyric basalt none Folded crust of wrinkled sheet flow. Vesicular core1088 6K#1088R-07B 18x14x7 Fr 1.5 0.81088 6K#1088R-07C 18x15x6 Fr 2.0 2.01088 6K#1088R-07D <9 Fr 0.3 0.3

1088 6K#1088R-08A 12x9x8 Fr 0.7 0.6 <1%, 0.2 mm (max 2 mm) Grey aphyric basalt none Folded crust of wrinkled sheet flow that are coalescedtogether.

1088 6K#1088R-08B 12x9x7 0.3 0.11088 6K#1088R-08C <5 0.1 0.51088 6K#1088R-09A 17x14x10 Fr 0.3 0.2 10%, 1.0 mm (max 4.0 mm) Grey aphyric basalt none Radially jointed pillow fragment. Stain along fractures.1088 6K#1088R-09B 18x15x14 Fr .0.7 0.61088 6K#1088R-09C 17x15x8 Fr 0.5 0.11089 6K#1089R-01 <18x15x10.5 Fr 0.3 0.3 3%, 0.5 mm (max 6x3 mm) Dark grey aphyric basalt none Folded crust with glassy surface of a tumulus.1089 6K#1089R-02 24x24x14 Fr 1.0 0.8 0.5%, 0.8 mm (max 4.0 mm) Grey aphyric basalt none Radially jointed pillow fragment. Stain along fractures.

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1089 6K#1089R-03 21.5x14x2.5 Fr 1.0 0.5 5%, 0.5 mm (max 5x3 mm) Dark grey aphyric basalt noneSlabby crust of sheet lava with glassy surface and inner roofwith lava curtains and stalactites. Linear streaks on the glasssurface.

1089 6K#1089R-04 38x34x42 Cr 2.0 1.0 none Dark grey hyp-ol-bg. pl-phyric basalt

Pl 30%, 5.0 mm (max 10x5 mm);ol rare, 1.0 mm; hyp tiny grains Big pillow lobe with abundant plagioclase.

1089 6K#1089R-05A 12.5x6.5x6 Fr 2.0 0.2 none Dark grey pl-phyric basalt Pl 15%, 3.0 mm (max 6.0 mm)

1089 6K#1089R-05B 3.3x2.8x2.8 none 0.5 10-15%, 0.2 mm (max 1.5x0.6mm) Dark grey aphyric basalt none Contaminated from other sample?

1089 6K#1089R-06 36x31x24 Fr 0.7 0.6 none Dark grey hyp-ol-bg. pl-phyric basalt

Pl 35%, 5.0 mm (max 7.0 mm);ol rare, 3.0 mm; hyp rare tinygrains

Big pillow lobe with abundant plagioclase.

1090 6K#1090R-01 12x13x8 Fr 0.5 0.5 Very poor, tiny vesicles Grey aphyric basalt none Magician's cap shaped protrusion. Slightly altered with stainalong cracks; chunks in a bag

1090 6K#1090R-02 18x14x13 Fr 0.5 0.5 0.1%, max 0.5 mm Grey aphyric basalt none Radial jointed pillow clast. Stain along cracks1090 6K#1090R-03 19.5x18x12 Fr 0.5 0.6 0.3%, 0.2 mm (max 1.0 mm) Dark grey apyric basalt rare ol Small finger-like protrusion1090 6K#1090R-04 14x11x8 Fr 2.0 2.0 <0.1%, tiny vesicles Grey aphyric basalt rare ol Hemi-cylindrical lobe. Stain along cracks

1090 6K#1090R-05 20x18x17 Fr 1.0 0.4 Very poorly vesiculated, < 0.5mm Grey aphyric basalt none Cylindrical pillow lobe. Vesicles filled by secondary mineral

(limonite?). Stains along cracks

1090 6K#1090R-06 31x17x16 Fr 0.1 - 15%, max 0.1x3.0 mm pipevesicles Dark grey aphyric basalt none Jumbled glassy crust of a sheet flow. Stains along cracks.

Chunks denoted as R-06B

1090 6K#1090R-07 25x17x16 Fr 1.0 0.7 none Dark grey aphyric aug-hyp-ol-bg. pl basalt

pl 0.5%, 1x5 mm; ol 0.1%, <3mm; hyp 0.1%, <1 mm; aug rare,<1 mm

Clyndrical pillow lobe. Stain along cracks

* Alphabets postfixing sample numbers from dive #1088 are either pieces taken from the same rock bodies or samples broken into pieces after recovery..** Fr: franvoidal, Cr: crustChunks are fragments collected from sample boxes; could be contaminated with other samples

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4-2-2. Sediments Sediment samples that coated lavas flows were collected at 6 stations in all using push corers (Table 4-2-2-1). These samples are hemipelagic olive brown colored clay or silty

sand including volcanic glass, oxides of iron or manganese, radiolarian, and foraminifer. These samples each other do not show any material difference by optical inspection. Therefore a degree of sediment accumulation on in-situ lava flows could indicate age

differences. Nanofossils (coccolith) are included in the sediment samples. These fossils may give information of age underlaid lava flows. Although specific age within a narrow range is not determined, species of the fossils of planctonic foraminifer are

identified as shown in Table 4-2-2-1. Population of foraminifer or nanofossils is small and only fragments of foraminifer are found in the samples obtained in #1090, thereby their species are cannot be identified. Fossils of foraminifer are also partly dissolved

even in the samples of #1088 and #1089. It may be caused by water depth where the samples are taken. The depth may be as deep as carbonate compensation depth (CCD).

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Table 4-2-2-1. Sediment sample description.

Name Latitude Longitude Depth Description 6K#1088S-01 17° 00.0591' N 144° 51.1631' E 3253 m Olive brown clay including volcanic glass & foraminifer

Globigerinoides ruber (Early Miocene-recent) Globigerinoides tenellus (Late Pliocene-recent) 6K#1088S-02 16° 59.9477' N 144° 50.4666' E 3415 m Olive brown clay including volcanic glass & foraminifer

Sphaeroidinella dehiscens (Early Pliocene-recent) 6K#1088S-03 16° 59.9297' N 144° 49.9328' E 3425 m Olive brown clay including volcanic glass & foraminifer Beella digitata (Pleistcene-recent)

6K#1089S-01 17° 00.6977' N 144° 52.4560' E 3441 m Olive brown silty sand including volcanic glass & foraminifer Globorotalia truncatulinoides (Late Pliocene-recent) 6K#1090S-01 17° 03.3862' N 144° 49.5912' E 3743 m Olive brown clay including volcanic glass & foraminifer

6K#1090S-02 17° 03.1950' N 144° 48.7122' E 3616 m Olive brown clay including volcanic glass & foraminifer

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Appendices A-1. Rock Photos

6K#1088R-01

6K#1088R-02

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6K#1088R-03

6K#1088R-04

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6K#1088-R05

6K#1088R-06

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6K#1088R-07

6K#1088R-08

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6K#1088R-09

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6K#1089R-01

6K#1089R-02

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6K#1089R-03

6K#1089R-04

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6K#1089-R05

6K#1089R-06

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6K#1090R-01

6K#1090R-02

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6K#1090R-03

6K#1090R-04

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6K#1090-R05

6K#1090R-06

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6K#1090R-07 *) There are more photos in digital files.

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A-2. Sediment Photos

6K#1088S-01

6K#1088S-02

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6K#1088S-03

&K#1089S-01

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6K#1090S-01

6K#1090S-02

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A-3. References Asada, M., A. Deschamps, T. Fujiwara, and Y. Nakamura, Submarine lava flow

emplacement and faulting in the axial valley of two morphologically distinct

spreading segments of the Mariana back-arc basin from Wadatsumi side-scan sonar images, Geochem. Geophys. Geosyst., 8(4), Q04001, 10.1029/2006GC001418, 2007.

Deschamps, A., T. Fujiwara, M. Asada, P. Gente, Y. Nakamura, A. Heuret, K. Naito, H. Horikawa, and S. Suganuma, Deep-tow sonar survey of the Mariana spreading axis: initial results of the KR03-12 cruise, InterRidge News, 13, 6-9, 2004.

Deschamps, A., T. Fujiwara, M. Asada, L. Montési, and P. Gente, Faulting and volcanism in the axial valley of the slow spreading center of the Mariana back arc basin from Wadatsumi side-scan sonar images, Geochem. Geophys.

Geosyst., 6(5), Q05006, 10.1029/2004GC000881, 2005. Gribble, R. F., R. J. Stern, S. H. Bloomer, D. Stuben, T. O'Hearn, and S. Newman,

MORB mantle and subduction components interact to generate basalts in the

southern Mariana Trough back-arc basin, Geochim. Cosmochim. Acta, 60, 2153-2166, 1996.

Iwamoto, H., M. Yamamoto, N. Seama, K. Kitada, T. Matsuno, Y. Nogi, T. Goto, T.

Fujiwara, K. Suyehiro, and T. Yamazaki, Tectonic Evolution of the Central Mariana Trough, American Geophysical Union, Eos Trans. AGU, 83(47), Fall Meet. Suppl., abstract T72A-1235, 2002.

Kitada, K., N. Seama, T. Yamazaki, Y. Nogi, and K. Suyehiro, Distinct regional differences in crustal thickness along the axis of the Mariana Trough, inferred from gravity anomalies, Geochem. Geophys. Geosyst., 7, Q04011,

10.1029/2005GC001119, 2006. Koike, Y., T. Fujiwara, S. Umino, M. Asada, and S. Okada, Observation of a

sub-bottom profiler installed on the Shinkai 6500 in a back-arc basin spreading

center of the Mariana Trough, JAMSTEC Rep. Res. Dev., submitted, 2008. Kisimoto, K., S. Umino, N. Geshi, T. W. Hilde, H. Kumagai, J. Sinton, S. M. White,

Largest off-axis lava flow field from the southern East Pacific Rise at 14°

-Preliminary, rsults of NIRAI-KANAI cruise leg, YK04-07 Yokosuka/Shinkai Shinkai 6500 dives, AGU, 85(47), Fall Meet. Suppl., abstract V53A-0612, 2004.

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Kumagai, H. (edit), Preliminary cruise report YK05-16 Leg 1 Yokosuka/Shinkai 6500 "URANIWA" expedition, 2005.

Sato, T., Magnetic structure of an oceanic core complex at the southernmost part of the

Central Indian Ridge, the analysis of shipboard and deep sea three component magnetometers, Master thesis, Univ. Tokyo, 2007.

Takahashi, N., S. Kodaira, S. L. Klemperer, Y. Tatsumi, Y. Kaneda, and K. Suyehiro,

Crustal structure and evolution of the Mariana intra-oceanic island arc, Geology, 35, 203-206, 2007.

Takahashi, N., S. Kodaira, Y. Tatsumi, Y. Kaneda, and K. Suyehiro, Structure and

growth of the Izu-Bonin-Mariana arc crust: 1. Seismic constraint on crust and mantle structure of the Mariana arc-back-arc system, J. Geophys. Res., 113, B01104, doi:10.1029/2007JB005120, 2008.