36. diagenetic evolution of neogene volcanic …€¦ · element compositions of bulk sample, ......

Silver, E. A., Rangin, C., von Breymann, M. T., et al., 1991Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 124

36. DIAGENETIC EVOLUTION OF NEOGENE VOLCANIC ASHES (CELEBES AND SULU SEAS)1

Alain Desprairies,2 Marc Riviere,2 and Manuel Pubellier3

ABSTRACT

Mineralogical and geochemical analyses were performed on 40 ash layers of Pleistocene to late Miocene age,recovered during Leg 124 in the Celebes and Sulu Seas (Sites 767, 768, and 769). They provide information onalteration processes related to burial diagenesis. The zonal distribution of secondary volcanic products emphasizesa major diagenetic change, characterized by the complete replacement of volcanic glass by an authigenicsmectite-phillipsite assemblage, in tephra layers dated at 3.5-4 Ma. This diagenetic "event" occurs simultaneouslyin the two basins, and, on the basis of isotopic data, under low-temperature conditions. It is independent of distinctsedimentation rates and related to a relative quiescence of on-land volcanic activity. This period suggests a moreuniform paleooceanographic situation having tectonic significance, and probably reflects a kinetic and environmen-tal control of diagenetic reactions.

INTRODUCTION

During ODP Leg 124, tephra ranging in age from middleEocene to Quaternary were recovered from five holes in theCelebes and Sulu Seas (Rangin, Silver, von Breymann, et al.,1990). Middle Eocene to early Miocene ash beds are scatteredin brownish pelagic claystones from the Celebes Sea (Site767). Late, early Miocene pyroclastic flows occur only in theSulu Sea (Sites 768 and 769). In both basins, middle Miocenetime is a period of quiescence of volcanic activity, and nodiscrete ash layers are recognizable in hemipelagic claystones.The last period of Cenozoic explosive volcanism extends fromlate Miocene (about 8 Ma) to the present with an apparent lackof volcaniclastic material centered on 3 Ma (Pubellier et al.,this volume; Pouclet et al., this volume).

We only considered in this paper air-fall ash deposits ofPleistocene to late Miocene age. The frequency of ash-layerdecreases markedly during the Pliocene and may record eithera gap in the on-land plinian volcanic activity, dilution bysedimentary processes, and/or progressive vanishing of pri-mary volcanic components by diagenetic effects. Andesitic torhyolitic glasses altered to clay were frequently reported onshipboard smear-slide descriptions (Rangin, Silver, von Brey-mann, et al., 1990). Our purpose is to specify both the natureof secondary volcanic products and their location within theNeogene ash distribution as a result of diagenetic processesthat emphasize the geodynamic evolution of Celebes and Suluseas.

METHODOLOGY AND ANALYTICALPROCEDURES

Forty discrete ash layers were chosen (Table 1) as repre-sentative of lithologic Units I and II from Sites 768 and 769(Sulu Sea) and 767 (Celebes Sea). Mineralogical and chemicalanalyses were performed on bulk samples, silt (35-63 µra) andclay (< 2µm) fractions separated by sieving or centrifugingand cleaned by ultrasonic methods. Two X-ray diffractionmethods (randomly oriented and suspension sedimented sam-

1 Silver, E. A., Rangin, C , von Breymann, M. T., et al., 1991. Proc. ODP,Sci. Results, 124: College Station, TX (Ocean Drilling Program).

2 Université de Paris Sud - Laboratoire de Géochimie des Roches Sédimen-taires - Bát. 504 - 91405 Orsay, France.

3 Université Pierre et Marie Curie - Laboratoire de Géotectonique T26 -O - 75252 Paris, France.

pies) were used for mineralogical determinations. 4 Major-element compositions of bulk sample, coarse and fine frac-tions, or single separate particles were determined either byelectron microprobe (EM) or by scanning electron microprobe(SEM) fitted with an energy dispersive spectrometer.5 Eachtechnique has advantages and drawbacks. For the EM work,the major problem is the loss of alkalis under beam, especiallyNa. We analyzed scanning areas of 25-100 µm2 consideringthe size of the particles. The EM used was a CAMEBAX-typeautomated microprobe with TRACOR system, working under15 kV and 5 nA. Under these conditions and with a countingtime of 5 s, the detection limit of major elements is ca. 0.3%with analytical precisions better than 5%. The lack of impor-tant alkali loss was checked by a CIPW norm calculation onglass analyses (lack or scarcity of normative corundum). Forthe SEM/EDS (energy dispersive spectrometry) studies, largescanning areas (100 µm2) were analyzed under 15 kV withreal-time acquisition of 120 s. Under these conditions, nosignificant loss of alkalis was observed. The relative errors areless than 3% for elements with concentrations between 5%and 10%, and 20% for elemental concentrations < 5%. Thedetection limit is 0.1%. The weight deficit to 100% is oftenunrelated to water content. Therefore the results for clayminerals were recalculated to 100% (oxide values in percentby weight, in a water-free basis). To allow for comparisonwith EM, data were normalized to 90% for zeolites, 95% and97% for silicic and basic glasses, respectively, and to 98% forK-feldspars.

Oxygen isotope analyses have been made on authigenicphillipsite and smectite extracted from bulk altered-ash sam-ples and on a variety of mixed fresh-glass/altered-glass, andauthigenic zeolite/clay assemblages. Oxygen was liberatedfrom samples by reacting with BrF5 and then converted to CO2for mass spectrometric analysis following the procedure de-scribed by Clayton and Mayeda (1963). Phillipsite was left 5days in a drybox before analysis to eliminate the effect of 18Ocomposition of zeolitic water (Savin and Epstein, 1970b).Analytical precision was generally ± 0.5 %o (average devia-tion).

Finally, at each Site, the ash layers were dated usingbiostratigraphic markers and extrapolation between markers

4 (XRD INEL with curved position sensitive detector CPS 120).5 (SEM PHILIPS 505 with LINCK System).

489

A. DESPRAIRIES, M. RIVIERE, M. PUBELLIER

Table 1. Location and age of studied ash-layers samples. Tephra are dated using biostratigraphic markers and according to the sedimentation rates(Rangin, Silver, von Breymann, et al., 1990).

Hole

767B767B767B767B767B767B767B767B767B767B767B767B767B767B767B

Core

1H4H6H9H

10H13X18X23X26X29X35X36X40X42X47X

Sect.

66236

CC415252161

Interval(cm)

124-126041-043090-092061-063079-081005-007029-031054-056044-046117-119008-010114-116007-009094-098054-056

Depth(mbsf)

8.7435.9149.4075.1189.29

119.15162.79203.84233.04273.67325.48331.74367.87395.53435.94

Age(Ma)

0.0880.3620.5150.9601.3091.7522.5103.2613.7954.5395.6865.8807.0007.8598.913

Hole

768A768B768B768B768B768B768B768B768B768B768B768B768B768B

Core

1H4H7H

10H12H14H14H18H18H21H28X29X33X38X

Sect.

55455236623314

Interval(cm)

069-071014-016097-099113-115072-075132-134098-100052-053053-054115-120064-066037-039053-055033-035

Depth(mbsf)

6.6729.1456.9787.13

105.72120.82121.98164.02164.03185.98242.24251.67287.33339.93

Age(Ma)

0.0700.2870.5841.2221.6152.0012.0614.0224.0224.8695.9546.1376.8287.848

Hole

769B769B769A769B769B769B769B769B769B769B769B

Core

2H4H6H7H9H

12H14H15H15H16H24H

Sect.

22124342255

Interval(cm)

068-070130-132055-058030-033126-128101-103122-124020-022056-058050-052002-004

Depth(mbsf)

7.5827.2046.%54.7077.66

104.41125.12130.62130.%144.90217.32

Age(Ma)

0.0760.2730.4880.6861.2712.3503.3573.5483.6424.2689.134

according to the sedimentation rates (Rangin, Silver, vonBreymann, et al., 1990).

PETROGRAPHY OF ASH LAYERSSeveral types of assemblages consisting of primary and

secondary volcanic components were distinguished throughmineralogical and chemical analyses of ash layers. The occur-rence of alteration products can be related to a zonal distri-bution within lithologic Units I and II from Sites 767, 768, and769 (Fig. 1).

Celebes Sea (Site 767)Tephra layers are present in two lithostratigraphic units

(Rangin, Silver, von Breymann, et al., 1990).Unit I (0-56.8 mbsf; Core 124-767B-1H through Section

124-767B-7H-1, 34 cm) is Pleistocene to Holocene in age. Itconsists of volcanogenic clayey silts with sparse ash interbeds.

Unit II (56.8-406.5 mbsf; Section 124-767B-7H-1, 34 cmthrough Core 124- 767B-43X) ranging from late Miocene toPleistocene in age, is divided into three subunits.

Subunit IIA (56.8-109.6 mbsf; Section 124-767B-7H-1, 34cm through Core 124-767B-11X) is Pleistocene in age. Volca-nogenic clayed silt with rare interbeds of volcanic ash andcarbonate turbidites is the dominant lithology.

Subunit HB (109.6-300.1 mbsf; Cores 124-767B-13Xthrough -767B-32X) is early Pliocene to Pleistocene in age.Volcanogenic clayey silt is also dominant lithology. Thissubunit differs from subunit IIA in that carbonate turbiditesare more abundant and discrete ash layers are almost com-pletely absent (Rangin, Silver, von Breymann, et al., 1990).

Subunit HC (300.1-406.5 mbsf; Cores 124-767B-33Xthrough -767B-43X) is late Miocene to early Pliocene in age.Volcanogenic silty claystone contains less abundant carbon-ate turbidites and more tuff beds compared to Subunit HB.

The spatial distribution of diagenesis, compared to litho-stratigraphic units, is as follows (Table 2):

Zone I "Fresh Glass"

Unaltered zone (0-71.5 mbsf; Cores 124-767B-1H through-8H). Tephra layers consist mainly of fresh glasses associatedwith unaltered plagioclases and ferro-magnesian silicates(magnesio-hornblende and biotite). Glasses are of rhyoliticcomposition and, on the basis of the K2O vs. SiO2 plot, they lie

in the field of calc-alkaline (Samples 124-767B-1H-6, 124-126cm; -4H-6, 41-43 cm) or high-K calc-alkaline (Sample 124-767B-6H-2, 90-92 cm) series (Peccerillo and Taylor, 1976)(Table 3; Fig. 2). Plagioclases are mainly andesine (37%—51%An) and labradorite (59%-70% An).

Zone II "Glass Hydrolysis"

Leaching zone (71.5-158 mbsf; Cores 124-767B-9Hthrough 17X). The characteristic of this zone is the coexist-ence of fresh and "altered" glasses in each individual ashlayer. The composition of the fresh glasses ranges fromandesites (56%-63% SiO2) to rhyolites (70%-76% SiO2) andbelongs to the field of calc-alkaline (Sample 124-767B-9H-3,61-63 cm) or high-K calc-alkaline series (Samples 124-767B-10H-O6, 79-81 cm; -13X-CC, 5-7 cm). K-feldspar (49%-66%Or), plagioclases and ferro-magnesian minerals remain unal-tered. Apart from typical dissolution features, altered glassesare chemically distinguished by their water content, a markedenrichment in alkalis, especially K, and an increase of theAl/Si ratio (Table 3; Fig. 2). Leaching affects both basic(andesitic) as well as acidic (rhyodacitic) glasses. The degreeof alteration increases with depth, leading to the first authi-genic mineral formation, i.e., smectite (iron beidellite type)and, secondarily, opal CT.

Zone HI "Phillipsite formation"

Zeolite zone (158-329 mbsf; Cores 124-767B-18X through-35X). This zone is characterized by the crystallization ofphillipsite. Feldspars (K-feldspars and plagioclases) and ferro-magnesian minerals remain still unaltered. Association, ornot, of zeolite with more or less "altered" glasses allows threesubzones or levels to be distinguished.

An upper level (Zone Ilia) (158-233 mbsf; Cores 124-767B-18X through -25X), where fresh and "altered" andesiticglasses of high-K calc-alkaline series are mixed with grainsshowing phillipsite-smectite assemblages (Samples 124-767B-18X-4, 29-31 cm; -23X-1, 54-56 cm).

A medium narrow level (Zone Illb) (233-329 msbf; Core124-767B-26X), well defined by the complete replacement ofglass by zeolite-smectite paragenesis.

A lower level (Zone Ule) (242-329 msbf; Cores 124-767B-27X through -35X) Phillipsite is less abundant but there ismore than in the upper level. Altered glasses reappear. Their

490

NEOGENE VOLCANIC ASHES

767

(1)

I0.643

IIA

1.556

HB

Illb -.

5.022

HC

8.200

r

(2)

I

0.897— —

II

2.422

Ilia

3.785

3~959~

IIIc

5.796

IV

7.905

768

(1)

I

2.115

II

9.789

—

_

(2)

T

1

1.272

Ilia

3.624

Illb4~71~IIIc5.023

IV

6259_

769

(1)

I

2.230

—

—

-

(2)

T1

1.269

Ilia

3.466

Illb3.920_IIIc

4~24~

IV

7.039

Figure 1. Relationship between lithostratigraphic Units (1) and diage-netic zones (2) for Sites 767, 768, and 769. Limits are given in Ma.

original chemical composition is difficult to define but seems inagreement with the alteration, as above, of both acidic andbasic glasses.

Zone TV "Smectite formation"

Clayed zone (329 to 397 msbf; Cores 124-767B-36X through-42X). Ash layers in this zone are markedly bioturbated andreworked. Inherited minerals, quartz, and phyllosilicates areoften mixed with volcanogenic material, leading to a high"Continentality Index" 6 (Rangin, Silver, von Breymann, etal., 1990). Phillipsite and altered glass are always present butin low amounts. The main characteristic of these ash layers isthe massive growth of authigenic smectite (Samples 124-767B-

6 Importance of continental sources, expressed by the logarithm of the sumof the abundances of smectite, illite, and chlorite, divided by the abundance ofsmectite.

36X-2, 114-116 cm; -40X-1, 7-9 cm; -42X-06, 22-24 cm). Thelower limit of this mixed zone is difficult to define. However,we presume that the sudden disappearance of hornblende andbiotite below 410 mbsf could be correlated to the beginning ofon-land volcanic explosive activity. This assumption is cor-roborated by the lack of any primary and secondary volcano-genic components at about 435 msbf (Sample 124-767B-47X-01, 54-56 cm).

Sulu Sea (Site 768) and Cagayan Ridge (Site 769)The volcanic ashes of aeolian origin occur in two litho-

stratigraphic units (Fig. 1).

Site 768Unit I (0-123 msbf; Cores 124-768B-1H to Section -768B-

14H-3) is latest Pliocene to Holocene in age. It consists ofinterbedded marls and calcareous turbidites with sparse thinbeds of volcanic ash.

Unit II (123-652 mbsf; Section 124-768B-14H-3 throughCore 124-768C-31R) is middle Miocene to late Pliocene inage. It is composed mainly of alternating clay stone, clayedsiltstone, and siltstone with minor carbonate turbidites.Volcanic ashes occur in the upper part of this unit. A clearbreak appears at a depth of 260 msbf. The lack, of anyvolcanogenic component below this level, could signify, asnoted in the Celebes Sea, the lower limit of the explosivevolcanic activity.

Site 769Unit I (0-102.15 mbsf; Cores 124-769A-1H through -769A-

7H; Core 124-769B-1H through Section 124-769A-12H-2) islate Pliocene to Holocene in age. The dominant lithologyconsists of nannofossil marl; volcanic ashes altered to claybecome dominant at the base (Rangin, Silver, von Breymann,et al., 1990).

Unit II was divided into three subunits. Unit Ha (102.15—262.35 msbf; Section 124-769B-12H-2 through Section-29X-2) is middle Miocene to late Pliocene in age. It iscomposed mainly of massive hemipelagic clay with abun-dant interlayered laminae representing deeply altered ashlayers. No volcanogenic component of pyroclastic originwas observed below 185 mbsf, leading to the same interpre-tation as the one given for the Celebes Sea and for Site 768discussed above.

The spatial distribution of previously defined diageneticzones in the Celebes Sea is, as a whole, recognizable in theSulu Sea and on the Cagayan Ridge (Table 2).

Zone I "Fresh Glass"(Site 768: 0-89.5 mbsf, Cores 124-768B-1H through -10H.

Site 769: 0-81.5 mbsf, Cores 124-769B-1H through -9H).Tephra consists mainly of fresh glasses associated with freshalkali-feldspars, plagioclases and ferro-magnesian silicates.The chemical composition of glasses ranges from basalticandesites (52% 56% SiO2) and andesites (56‰63% SiO2) todacites (63‰-70% SiO2) and rhyolites (709£ 76% SiO2). On thebasis of the K2O content, they belong, as in the Celebes Sea,either to the calc-alkaline series (Samples 124-768A-7H-4,97-99 cm and -769B-7H-2, 30-33 cm) or to high-K calc-alkaline series (Sample 124-768A-1H-5, 67-69 cm) (Fig. 2).Some samples are located at the limit of these two series(124-768B-4H-5, 14-16 cm and -768B-10H-5, 113-115 cm),Plagioclases are mainly andesines (27%-51% An) and alkali-feldspars are K-rich feldspars (66%-81% Or). Ferro-magne-sian minerals consist of amphiboles (magnesio-hornblende),orthopyroxene (70%-77% En) and biotite.

491


Table 2. Location and age of diagenetic zones.

ZONE

I

II

III

IV

Fresh glass

Glass hydrolysis

Phillipsite formation

Ilia

lub

Ule

Smectite formation

CoreDepth (mbsf)Age (Ma)







Site 767

1H-8H0-71.50-0.897

9H-17X71.5-158

0.897-2.422

18X-35X158-329

2.422-5.796

18X-25X158-233

2.422-3.795

26X233-242

3.795-3.959

27X-35X242-329

3.959-5.796

36X-42X329-397

5.796-7.905

Site 768

1H-10H0-89.50-1.272

11H-21H89.5-193

1.272-5.023

11H-17H89.5-156

1.272-3.624

18H156-165

3.624-4.071

19H-21H165-193

4.071-5.023

22H-29X193-258

5.023-6.259

Site 769

1H-9H0-81.50-1.369

10H-16H81.5-148

1.369-4.424

11H-14H81.5-129

1.369-3.466

15H129-138

3.466-3.920

16H138-148

3.920-4.424

16H-21H148-185

4.424-7.039

Zone II "Glass Hydrolysis"

This zone, showing gradational evolution of alteration pro-cesses at Site 767, is apparently missing at Sites 768 and 769.

Zone III "Phillipsite Formation"

(Site 768: 89.5-193 msbf, Cores 124-768B-11H through-21H. Site 769: 81.5- 148 mbsf, Cores 124-769B-10H through-16H). The contact with the above zone is very sharp-Feldspars and ferro-magnesian minerals remain unaltered.Phillipsite is always associated with smectite and also withfresh and altered glass. The relative abundance of phillipsitepermits us to distinguish three levels.

Zone Ilia - Upper Level

At the top, phillipsite coexists with fresh and altered glass(Site 768: 89.5 to 108 mbsf, Cores 124-768B-11H through 12H.Site 769: 81.5 to 120 mbsf, Cores 124-769B-10H through -13H)and subsequently only with altered glass (Site 768, 108-156mbsf; Cores 124-768B-13H through -17H. Site 769: 120-129mbsf; Core 124-769B-14H). Fresh glasses are of rhyolitic com-position (Sample 124-768B-12H, 72-75 cm; high-K calc-alkalineseries) and altered glasses are related to either acidic (rhyo-dacitic) or basic (andesitic) composition (Samples 124-768B-12H, 72-75 cm; -14H-02, 132-134 cm; -14H-03, 98-100 cm).

Zone IHb - Medium Level

Limited to Cores 124-768B-18H and -769B-15H (156 to 165mbsf and 129 to 138 mbsf, respectively), this zone is never-theless well characterized by a massive phillipsite-smectiteassemblage with complete disappearance of vitric glasses(Samples 768B-12H-6, 52-53 cm and 53-54 cm; -769B-15H-2,20-22 cm and 56-58 cm).

Zone Ule - Lower Level

(Site 768: 165-193 mbsf, Cores 124-768B-19H through-21H. Site 769B: 138- 148 mbsf, Core 124-769B-16H). Alteredglasses coexist again, as in level Ilia with authigenic minerals(Samples 124-768B-21H-2, 115-120 cm and -769B-16H-5,50-52 cm). However, there are no fresh glass subunits at thislevel.

Zone IV "Smectite Formation"

(Site 768: 193-258 mbsf; Cores 124-768B-22H through29X). This zone is marked by the abundance of smectiteassociated with a few amount of altered glass and phillipsite.In some cases, volcanogenic components, of primary andsecondary origin, are more or less diluted by inherited mate-rial, as emphasized by a high "Continentality Index" (Rangin,Silver, von Breymann, et al., 1990). As mentioned before, weplace the lack of pyroclastic material to the lower limit of thiszone.

CHEMICAL COMPOSITION OF ALTERATIONPRODUCTS

"Altered Glass"Bulk analysis on an individual glass shard showing distinc-

tive dissolution features was, compared to analysis of freshglass, inferred as representative of the initial composition. Theresults indicate that the alteration is characterized by: (1) anincrease of the H2O content; (2) a decrease of the Si/Al ratio;(3) an increase of the content of Na and especially K (Fig. 2,Table 3). Punctual analyses with a SEM on the same "al-tered" glass shard reveal large heterogeneity and a scale ofcompositions from residual (?) glass and residual laths of

492


× FG 767

× AG 767

- Ph 767

- FG 768

• AG 768D Ph 768

• FG769

O AG 769

* Ph769

Δ KF 767,768, 769

Figure 2. K2O vs. SiO2 variation diagram. Each symbol represents, for a given sample, the mean valueof individual glass shards or mineral analyses (See Table 3). FG: fresh glass; AG: altered glass; Ph:phillipsite; KF: K-feldspar, from Sites 767, 768, and 769. Representative fields of "altered" glasses,K-feldspars and phillipsite, more-or-less associated with "altered" glasses, were plotted. Solid linesindicate boundaries of low-K calc-alkaline, calc-alkaline, high K calc-alkaline series after Peccerilloand Taylor (1976). Dashed line is the limit of fresh glasses analyzed.

K-feldspars or plagioclases to intergrown zeolite and clayminerals. In another environmental context, this mixture,confirmed by XRD studies, would be noted as "palagonite"(Honnorez, 1981). All stages exist between fresh vitric glass,acidic or basic, "altered" glass, and pure phillipsite and/orsmectite.

Zeolites

On SEM observations, glomerules composed of agglomer-ates of well-developed prismatic crystals of phillipsite are verycommon. Structural formulas deduced from punctual analyseson individual crystals, on the basis of 32 oxygens, range from[Si1165 AL,.̂ ] Fe2+

o.oi(Cao.o3 Na2.i6 K1 7 8) (Sample 124-767B-26X-5, 44-46 cm) to [SiH 3 4 Al480] Fe2 +

0.0 2 Mg^Cao 12 Na, 65

K2 2 8) (Sample 124-768B-18H-6, 52-53 cm) (Table 3). Si/Alratios ranging from 2.79 to 2.36 and a very low Ca contentcompared to alkalis Na and K, are characteristics of pelagicphillipsite (Stonecipher, 1978). Mafic glasses and acidicglasses are often cited in deep-sea sediments as preferentialparental precursors of phillipsite and clinoptilolite, respec-tively (Kastner and Stonecipher, 1978; Iijima, 1978). In ourstudy, both basic andesitic and rhyolitic glasses are linked tophillipsite formation. Nevertheless, their chemical parentalcomposition could be reflected in the Si/Al and Na/K varia-tions (Fig. 3).

Alkali-Feldspars

Two types of K-feldspars can be distinguished. Some (49%to 66% Or) are concentrated in the fraction > 63 µm and,evidently, are of primary magmatic origin. Others, of morerestricted composition (80% Or), and only found in associa-tion with phillipsite, are probably authigenic (Table 3). Nev-ertheless, they are relatively rare and sparse compared to

zeolite, and any zonal distribution similar to the latter wasobvious within the three studied sites.

Clay Minerals

Inherited minerals, illite, kaolinite, chlorite, and also smec-tite, can be mixed by bioturbation or reworking with authi-genic smectite (Nicot et al., this volume). Detrital smectite isusually of aluminous composition (b parameter deduced fromthe (06.33) band on an XRD diagram, at about 9 Å). Authi-genic smectite shows larger scale of 060 peaks, ranging from1.496 to 1.518 Å and reflecting various chemical compositions(Desprairies, 1983). Heterogeneity was confirmed by punctualanalyses on SEM and leads us to consider each sample as amixture of species ranging from aluminous beidellite to iron-rich beidellite and even nontronite (Table 4). This heteroge-neity could reflect successive stages of formation and disso-lution-recrystallization connected to the increasing degree ofalteration. Parental composition of the glass plays a prominentpart in the chemical composition of the final product. Finally,a positive correlation appears between the K2O and ironcontent of the smectite (Table 4). Using the position of thecombination peaks (001)10/(002)17 and (002),o/(003)17 on ethyl-ene-glycol solvated samples (Reynolds and Hower, 1970), wefound values of randomly interstratified illite/smectite with80% to 100% of expandable layers.

DISCUSSION

Correlation of Diagenetic Zones with theLithostratigraphy

Ages of the diagenetic zones defined above, based ondistribution of ash layers, can be inferred from biostrati-graphic zonations and sedimentation rates (Rangin, Silver,

493


Table 3. Major-element analyses of fresh glasses (FG), altered glasses (AG), phillipsite (Ph), and K-feldspars (KF). For a given sample (Table 1), datarepresent mean of individual EM analyses (Sites 767 and 768) or individual SEM/EDS analyses (Site 769) recalculated to 95%-97% (glasses), 90%(zeolites) 98% (K-feldspars). (See Methodology and Analytical Procedure section).

Core, section

Interval (on)

Mean

S i O 2

T i O 2

A 1 2 O 3

F e OMnOM g OC a O

N a 2 OK 2 O

Σ

Core, section

Interval (cm)

Mean

SiO2

TiO2

AljOjF e OMnOM g OC a O

N a 2 OK 2 O

Σ

Core, section

Interval (cm)

Mean

S i O 2

T i O 2

AI2O3F e OM n OM g OC a O

N12OK 2 O

Σ

1H-6

124-126

FG/10

72.910.18

12.571.010.040.211.213.732.95

94.80

23X-1

54-56

FG/4

63.161.63

15.375.390.101.513.874.322.47

97.80

26X-5

44-46

Ph/8

59.580.04

19.360.080.040.000.155.707.14

92.09

4H-6

41-43

FG/12

72.560.08

12.890.960.030.221.623.602.47

94.42

29X-2

117-119

Ph/2

53.860.14

18.340.460.070.240.645.275.43

84.42

6H 2

90-92

FG/6

71.930.06

12.280.880.040.030.763.125.00

94.09

9H-3

61-63

AG/6

67.340.25

15.631.650.130.351.463.406.22

96.42

29X-2

117-119

PWI

60.330.00

18.370.000.000.000.074.757.22

90.74

9H-3

61-63

FG/5

72.470.08

13.130.850.030.241.393.132.08

93.41

10H-6

79-81

AG/3

62.160.56

18.462.830.110.643.095.155.23

98.23

35X-5

8-10

Ph/3

57.940.00

19.150.090.050.000.355.806.44

89.81

SITE 767

9H-3

61-63

¥Cß

60.350.73

17.864.480.190.765.004.672.94

96.98

13X-CC

5-7

AGβ

55.430.56

17.145.130.071.164.743.925.55

93.71

10H-6

79-81

FG/4

73.850.17

12.070.920.000.090.823.473.61

94.99

18X-4

29-31

AG/4

61.790.28

16.993.290.150.792.594.634.89

95.38

10H-6

79-81

KF/1

65.300.02

18.540.210.000.000.363.46

10.71

98.60

13X-CC

5-7

FG/5

70.300.02

12.521.070.070.141.023.653.62

92.42

23X-1

54-56

AG/2

61.060.09

22.320.830.000.235.175.214.26

99.15

29X-2

117-119

KF/3

64.250.02

18.450.150.040.020.704.379.42

97.44

13X-CC

5-7

FG/3

59.710.61

15.535.260.151.174.114.712.32

93.57

26X-5

44-46

AG/1

58.320.17

22.040.400.000.072.205.784.83

93.81

35X-5

8-10

KF/1

64.700.19

19.430.460.000.051.843.539.10

99.30

18X-4

29-31

FGß

63.860.35

17.393.660.110.543.495.823.62

98.85

29X-2

117-119

AG/3

62.990.02

20.030.600.000.002.415.116.71

97.86

40X-1

7-9

KF/4

64.560.03

19.350.210.020.010.643.869.96

98.63

18X-4

29-31

FG/8

59.910.42

19.783.900.070.575.804.962.32

97.75

40X1

7-9

AGO

66.660.01

13.740.100.000.152.311.335.31

89.61

40X-1

7-9

KF/2

64.070.00

17.840.030.080.070.060.94

14.20

97.28

1H-5

69-71

FG/7

73.170.13

11.470.870.090.100.943.583.76

94.11

12H-5

72-75

FG/5

73.030.27

12.491.070.030.191.103.813.16

95.15

14H-3

98-100

Ph/2

61.630.09

19.000.710.020.041.654.008.47

95.59

4H-5

14-16

FG/10

73.260.07

12.630.650.030.121.393.943.27

95.35

12H-5

72-75

FG/2

68.780.38

12.621.360.000.260.933.663.83

91.80

18H-6

52-54

Ph/11

57.130.04

20.540.140.000.050.554.288.99

91.73

4H-5

14-16

FG/2

67.240.62

15.922.430.050.422.995.422.65

97.73

4H-5

14-16

FG/14

73.820.10

13.080.890.030.321.563.383.08

96.27

12H-5

72-75

AG/3

62.330.00

16.170.110.000.491.821.316.02

88.25

12H-5

72-75

KFΠ

62.980.00

18.250.670.000.220.681.60

12.55

96.95

SITE 768

7H-4

97-99

FG/2

73.840.14

12.850.710.050.191.353.792.02

94.92

12H-5

72-75

AG/3

61.000.04

20.840.880.010.622.682.488.81

97.36

18H-6

52-54

KF/2

63.370.00

18.890.110.000.000.432.92

11.58

97.28

7H-4

97-99

FG/2

64.300.70

18.014.070.051.184.495.911.82

100.50

14H-2

132-134

AG/8

61.860.13

18.731.420.050.752.624.234.91

94.69

7H-4

97-99

FGP

56.651.09

17.208.490.183.337.613.561.37

99.48

14H-3

98-100

AGß

58.450.14

18.660.630.090.001.162.72

10.55

92.37

10H-5

113-115

FG/2

76.100.10

12.740.580.000.081.003.923.37

97.86

21H-2

115-120

AG/4

59.280.00

19.930.790.000.012.634.746.46

93.83

10H-5

113-115

FG/2

67.120.54

14.642.140.060.682.334.552.70

94.74

10H-5

113-115

FG/11

63.890.63

15.763.170.031.233.354.812.40

95.26

von Breymann, et al., 1990). At each site, the zone boundariesare obviously independent of lithostratigraphic units (Fig. 1).The main diagenetic "event", i.e., the complete replacementof glass by a smectite-phillipsite assemblage, occurs between3.5 and 4 Ma both in the Celebes and Sulu Seas: between 3.8and 3.96 Ma at Site 767, 3.62 and 4.07 Ma at Site 768, 3.47 and3.92 Ma at Site 769. This "event" is located within thelithostratigraphic Unit II (Subunit HB at Site 767) in whichlithofacies are very uniform and not linked to noticeablechanges in sedimentation rate (50 m/m.y. at Site 767; 20m/m.y. at Sites 768 and 769).

However, the sharp boundary at Sites 768 and 769 of ZoneI ("fresh glass") and Zone Ilia ("phillipsite formation")coincides with the rapid decrease in carbonate content con-sistent with the observed calcareous oozes (Rangin, Silver,von Breymann, et al., 1990). At site 767, the distribution ofcarbonate content is more sporadic and mostly linked tograded carbonate turbidites. The appearance at this site of thediagenetic "Transition Zone" II ("glass hydrolysis") couldreflect the origin and distribution of carbonate content withinthe three sites.

Correlation of Diagenetic Zones with Volcanic ActivityThe variation of secondary volcanogenic products can be

compared with that of primary volcaniclastic material ob-served on shipboard smear slides, specifically the presenceof vitric glasses vs. crystals and lithic fragments and the

frequency of ash layers (number or thickness of layers vs.time).

Zone I records a maximum in the on-land volcanic activityin the Pleistocene (0-0.90 Ma at Site 767; 0-1.27 and 0-1.37 atSites 768 and 769 respectively).

Zones II and Ilia show the progress of alteration processes(increase in the percentage of crystals relative to fresh glass).However, enhanced alteration is concurrent with a decreasein the signal of explosive volcanism. This period extends up toabout 3.5 Ma on the upper part of Pliocene time (3.80 at Site767; 3.62 and 3.47 at Sites 768 and 769, respectively).

Zone Illb, related to a maximum in zeolitization processbetween 3.5 and 4 Ma, reveals a gap in the volcanic activity.

Zone Ule is the recrudescence of explosive volcanismstarting at different times: about 4.4 Ma at Site 769, 5 Ma atSite 768, 5.8 Ma at Site 767. The characteristics are the sameas in Zone Ilia, i.e., an opposite correlation between expres-sions of alteration and volcanism.

Zone IV is the period of mixed epiclastic-pyroclastic vol-canism beginning at about 6.8 Ma. The paleoenvironmentalsignal of alteration seems completely different from thatregistered in the above zones.

Finally, the diagenetic zonation coincides with the volcaniccyclicity. Nevertheless, we emphasize that, sometimes, adeep alteration process can prevent any precise estimation ofprimary volcaniclastic material and thus its associated vol-canic activity.

494


Table 3 (continued).

7H-2

30-32

FG/2

71.860.31

11.971.140.000.272.153.743.57

95.00

15H-2

56-58

Ph/11

59.260.00

18.430.140.000.001.543.686.95

90.00

7H-2

30-32

FG/4

59.531.00

15.375.510.002.835.664.952.16

97.00

16H-5

50-52

PWI

57.620.27

17.740.790.000.620.575.137.26

90.00

9H-4

126-128

FG/3

69.760.54

13.671.590.000.231.183.634.40

95.00

16H-5

50-52

Ph/1

58.960.30

15.391.140.001.240.564.248.17

90.00

9H-4

126-128

FG/11

67.270.77

15.592.680.000.852.903.793 . 1 6

97.00

SITE 769

9H-4

126-128

FG/4

63.031.04

16.673.260.001.573.505.032.90

97.00

15H-2

20-22

KF/1

65.060.00

17.490.460.000.000.833.17

10.98

98.00

14H-4

122-124

FG/2

68.050.51

15.152.380.001.032.184.603.10

97.00

15H-2

20-22

KF/1

62.480.26

18.120.530.000.000.000.67

15.94

98.00

14H-4

122-124

FG/1

61.310.00

20.600.460.000.004.228.122.29

97.00

14H-4

122-124

AG/1

62.360.00

17.610.470.000.001.674.48

10.42

97.00

16H-5

50-52

AGß

62.620.33

18.620.830.000.591.987.234.80

97.00

Correlation of Diagenetic Zones with Interstitial WaterConcentration Gradients

Interstitial water analyses from Sites 767, 768, and 769show marked changes in concentration depth profiles of thedissolved constituents and their carbon, oxygen, and stron-tium isotopes (Rangin, Silver, von Breymann, et al., 1990; vonBreymann, et al., this volume). Through the diagenetic zones,concentration gradients are as follows.

1. Dissolved silica values, below initial diffusion profilenear the surface, remain virtually constant in Zone I andincrease markedly in Zone II. A drastic change takes place inZone Ilia: a strong depletion, which occurs faster at Sites 768and 769 than at Site 767 and leads to minimum concentrationsin Zone Ule. Values remain low and unchanged in Zones Illband IV (Fig. 4).

2. Dissolved strontium shows concentration profiles com-parable to that of dissolved silica (Fig. 5). Alkalinity anddissolved potassium values drop gradually within Zones II andIlia of Site 767. An identical downward curvature, lessdemonstrative, is observed at Site 769. A similar trend existsat Site 768 for Zones I and Ilia, but alkalinity increases belowthis zone. Change in pH gradient is inversely correlated to thealkalinity (Fig. 5), remaining nearly constant from Zones I toIlia and then increasing downward.

3. "The concentration depth profiles of calcium and mag-nesium are nonlinearly correlated throughout the sedimentcolumn" (von Breymann, et al., this volume). Opposite trends

with calcium enrichment and magnesium depletion, occurwithin Zone III. At Site 767, the intersection of curvescoincides exactly with the beginning Zone Illb (Fig. 6).

4. The oxygen and strontium isotopic composition of thepore fluids change gradually with depth (von Breymann, et al.,this volume). The δ 1 8θ values decrease from 0 %o SMOW toapproximately -2 %c in Zone Illb and those of 87Sr/86Sr from0.709 to 0.707.

The diagenetic reactions corresponding to the observedconcentration gradients can be identified:

1. Zone I (Site 767). The increase in the dissolved silica andstrontium concentrations and the decrease in dissolved potas-sium and alkalinity can be linked to the first stage of diageneticreaction. Superimposed on the effect of glasses alteration, theaddition of strontium to the pore fluids can be attributed to therecrystallization of biogenic calcite to inorganic calcite andaragonite (von Breymann, et al., this volume).

2. Zones Ilia and Illb (Sites 767, 768, and 769). Theobserved δ 1 8θ depletion in the pore fluids, and isotopic valuesof dissolved strontium, support an explanation of exchangebetween volcanic ash material and interstitial water (vonBreymann, et al., this volume).

Gradual depletion of alkalinity, dissolved silica, potassium,strontium, and magnesium reflects progressive alteration andreplacement of glass by a dominant phillipsite-smectite assem-blage sometimes associated with minor K-feldspar andopal-CT or quartz. Calcium is easily released from glass;phillipsite and smectite are sinks for K and Mg (Kastner, 1976;Kastner and Gieskes, 1976; Gieskes and Lawrence, 1981).Uptake of K can be proved by bulk and fraction > 35 µmchemical analyses of fresh and altered ash samples (Fig. 7).Al/Fe ratio remains unchanged through the diagenetic zones,reflecting only the primordial chemical variations through theash layers distribution and emphasizing that reactions wereisochemical and conservative for these two elements. Incontrast, Al/K ratio decreases abruptly, reaching minimumvalue in Zone Illb, and obviously is linked to the uptake of Kby authigenic minerals, mainly phillipsite but also smectitehaving interstratification with illite layers. Finally, smectite ismagnesium-poor and the consumption of magnesium is mainlyrelated to dolomitization reactions (von Breymann, et al., thisvolume).

3. In Zones Ule and IV, alkalinity, silica, and magnesiumconcentrations show low values and steady state. Glasses arecompletely altered to smectites, without zeolites. This alter-ation process is very common in open marine environments(Honnorez, 1981) and contrasts with the "zeolite fades"usually developed in more restricted environmental condi-tions (Riviere, 1989).

Correlation of Diagenetic Zones with a PaleogeothermalGradient

To evaluate paleotemperatures of authigenic minerals for-mation, we performed isotopic analyses on smectite, phillip-site, and some associated components (glass, detrital smec-tite) (Table 5). Isotope ratio on three samples of authigenicsmectite is about 26%o. Downhole distributions of δ 1 8θ of thepore water at Sites 767 and 768 show values of approximately-2%o in diagenetic Zone Illb (von Breymann, et al., thisvolume).

Using a smectite-water isotopic fractionation factor of1.03083 at 1°C (Yeh and Savin, 1976) the calculated pale-otemperature is about 12°C. Isotope ratio of phillipsite isabout 24.5%o. Using a fractionation factor of 1.034 (Savinand Epstein, 1970a, b), the estimated paleotemperature isabout the same. Therefore, isotopic data rule out any

495


57 58 59

SiO2

60 61

Figure 3. Example of oxide variations in phillipsite within a single ash layer (Sample 124-769B-15H-2,56-58 cm).

possibility of formation of either mineral under high-temper-ature gradients.

SUMMARY AND CONCLUSIONS

1. On the basis of mineralogical and chemical analysesperformed on primary and secondary volcanic products, wehave shown a zonal distribution of diagenesis through theNeogene tephra from the Celebes and Sulu Seas.. 2. The largest of diagenetic alteration is observed in ashlayers dated 3.5 to 4 Ma. No specific sedimentary environ-ment is particular to this period except for the fact that it isrelated to a gap in on-land volcanism. Nevertheless, thealteration processes can partly obliterate signals of volcanicactivity, i.e., frequency of ash layers.

3. This diagenetic "event" is synchronous in both basins.If the alteration is contemporaneous with sedimentation, this"event" corresponds to a change in the geodynamics of thebasins at 4 Ma, when subduction of the Awe Sea northward toMindanao Island ceased. It is relayed by incipient subductionalong the Cotabato trench and renewal of activity along theSulu trench (Pubellier et al., this volume).

4. Isotopic data imply low-temperature formation for au-thigenic smectite and phillipsite. Sites of diagenetic reactionsare well correlated with interstitial water concentration gradi-ents, suggesting time, as kinetic variable, and lithology as themain factors controlling the secondary mineralogy in a semi-closed model system.

ACKNOWLEDGMENTSThe authors express their sincere thanks to J. C. Alt and J.

M. Gieskes for the critical review of manuscript. We grate-fully acknowledge help and guidance from M. T. von Brey-mann and ODP Staff members. We would like thank Ph.Pradel for providing oxygen isotopic analyses. This study wassupported by ASP-ODP France.

REFERENCES

Clayton, R. N., and Mayeda, T. K., 1963. The use of brominepentafluoride in the extraction of oxygen from oxides and silicatesfor isotopic analyses. Geochim. Cosmochim. Acta, 27:43-52.

Desprairies, A., 1983. Relation entre le paramètre b des smectites etleur contenu en fer et magnesium. Application à 1'étude dessediments. Clays Clay Miner., 18:165-175.

Gieskes, J. M., and Lawrence, J. R., 1981. Alteration of volcanicmatter in deep-sea sediments: evidence from the chemical compo-sition of interstitial waters from deep-sea drilling cores. Geochim.Cosmochim. Acta, 45:1687-1703.

Honnorez, J., 1981. The aging of the oceanic crust at low temperature.In Emiliani, C. (Ed.), The Sea (Vol. 7): New York (Wiley),525-587.

Iijima, A., 1978. Geological occurences of zeolite in marine environ-ments. In Sand, L. B., and Mumpton, F. A. (Ed.), NaturalZeolites: Occurrence Properties Use: Oxford (Pergamon Press),175-198.

Kastner, M., 1976. Diagenesis of basal sediments and basalts of Site322 and 323, Leg 35, Bellingshausen Abyssal Plain. In Hollister C.D., Craddock, C , et al., Init. Repts. DSDP, 35: Washington (U.S.Govt. Printing Office), 513-527.

Kastner, M., and Gieskes, J. M., 1976. Interstitial water profiles andsites of diagenetic reactions, Leg 35, DSDP, Bellingshausen Abys-sal Plain. Earth Planet. Sci. Lett., 33:11-20.

Kastner, M., and Stonecipher, S. A., 1978. Zeolites in pelagicsediments of the Atlantic, Pacific, and Indian oceans. In Sand,L. B., and Mumpton, F. A. (Eds.), Natural Zeolites: Oc-currence, Properties, Use: New York (Pergamon Press), 199—218.

Peccerillo, A., and Taylor, S. R., 1976. Geochemistry of Eocenecalc-alkaline volcanic rocks from the Kastamonu area, northernTurkey. Contrib. Mineral. Petrol, 58:63-81.

Rangin, C , 1989. The Sulu Sea, a back arc basin setting within aNeogene collision zone. Tectonophysics, 161:119-141.

Rangin, C , Silver, E. A., von Breymann, M. T., et al., 1990. Proc.ODP, Init. Repts., 124: College Station, TX (Ocean DrillingProgram).

Reynolds, R. C , and Hower, J., 1970. The nature of interlayering inmixed-layer illite-montmorillonites. Clays Clay Miner., 18:25—36.

Riviere, M., 1989. En avant et en arrière du bloc d'Alboran, lesparagenèses diagénétiques à zeolites des depots volcanogéniquesmarins témoignent d'un episode de confinement burdigalien. C. R.Acad. Sci. Ser. 2, 309:1407-1412.

Savin, S. M., and Epstein, S., 1970a. The oxygen and hydrogenisotope geochemistry of clay minerals. Geochim. Cosmochim.Acta, 34:25-42.

496


Savin, S. M., and Epstein, S., 1970b. The oxygen and hydrogen Yeh, H. W., Savin, S. M., 1976. The extent of oxygen isotopeisotope geochemistry of ocean sediments and shales. Geochim. exchange between clay minerals and sea water. Geochim. Cosmo-Cosmochim. Acta, 34:43-63. chim. Acta, 40:743-748.

Stonecipher, S. A., 1978. Chemistry of deep-sea phillipsite clinopti-lolite, and host sediments. In Sand, L. B., and Mumpton, F. A. Da*e of initial receipt: 25 June 1990(Eds.), Natural Zeolite: Occurrence, Properties, Use: New York Date of acceptance: 9 January 1991(Pergamon Press), 221-234. Ms 124B-178

497

Table 4. Chemical analyses, and structural formulas on the basis of 11 oxygens, and b-parameter for smectite from altered ash layers, Site 767. Clay fraction ( 2 µm) or mean values ofindividual analyses (No) are normalized to 100.

Core, section

Fraction, No

S i O 2

A 1 2 O 3

F e O

MgOC a ON a 2 OK 2 O

Σ

T i O 2

IV SiAl

VI A 1 2 +

F eMg

Σ

N aKC a

bÅ

26X-5

< 2 µm

60.9919.447.065.521.483.251.54

99.28

0.72

3.810.19

1.240.370.512.12

0.390.120.10

35X-5

1

55.3724.5013.993.010.100.352.13

99.45

0.36

1

35X-5

2

58.1816.0816.236.270.770.001.97

99.50

0.50

9.048-

35X-5

3

57.5417.0014.836.270.680.173.35

99.84

0.16

9.090

35X-5

MEAN

57.0319.1915.025.180.520.172.48

99.60

0.34

3.680.32

1.140.810.502.45

0.020.200.04

40X-1

1

58.7416.4312.317.482.041.001.74

99.74

0.25

40X-1

2

62.4020.656.685.842.600.630.75

99.55

0.45

40X-1

3

62.9123.613.296.512.870.550.20

99.94

0.06

8.982 -

40X-1

4

63.5521.727.004.262.550.000.70

99.78

0.21

9.012 -

40X-1

5

61.9421.647.624.801.890.261.00

99.15

0.85

9.024

4 0 X 1

MEAN

61.9120.81

7.385.782.390.490.88

99.63

0.36

3.800.20

1.310.380.532.22

0.060.070.16

42X-6

1

66.0021.905.624.720.610.000.54

99.39

0.60

42X-6

2

62.7323.436.214.450.450.121.97

99.36

0.63

42X-6

3

63.4321.985.773.861.831.101.57

99.54

0.45

42X-6

4

64.4921.816.044.400.420.531.89

99.58

0.41

8.970 -

42X-6

5

62.6326.135.144.040.580.230.54

99.29

0.71

9.018

42X-6

6

66.1620.986.264.180.510.430.36

98.88

1.12

42X-6

7

65.3722.265.973.970.360.511.09

99.53

0.46

42X-6

8

65.0720.406.514.250.530.402.12

99.28

0.72

42X-6

MEAN

64.4822.365.944.230.660.421.26

99.36

0.64

3.900.10

1.500.300.382.18

0.050.100.04

42X-6

< 2 µm

61.7820.75

6.514.411.583.121.27

99.42

0.57

3.820.18

1.330.340.412.08

0.370.100.10


1000

800

io

600

400

200

α-α—π o — DZONE I x

ZONE Ix II x Ilia xbK Ule IV

1 4 5Age (Ma)

x

8

769

10

— SiO 2(µM)767 •α• SiO2 (µM) 768 — SiO2 (µM) 769 --SAMPLES 767 o• SAMPLES 768

— SAMPLES 769 X ZONES 767 X ZONES 768 × ZONES 769

Figure 4. Plot of dissolved silica vs. age (Interstitial water analyses: Rangin, Silver, von Breymann, et al., 1990) showing relationship betweenconcentration gradients and diagenetic zones.

499


"0- Alk 767 — SAMPL. 767 •-pH 767 × ZONES 767

X

1000

—Sr •-87Sr/86Sr × ZONES 767

I0

0.710

0.709

0.708

0.707

0.706

10

1000

800 .

600

§ 400 )

200

SiO-, —Sr ••-87sr/86sr × ZONES 768

0.710

0.709

0.708

0.707

0.706

4 6Age (Ma)

10

Figure 5. Strontium isotopic composition, alkalinity, pH, dissolved silica, and strontium contents of the porefluids vs. age (interstitial water analyses: Rangin, Silver, von Breymann, et al., 1990; von Breymann et al., thisvolume), and location of diagenetic zones.

500


0.710

•- 87Sr / 86Sr × ZONES 767

— — ^«*..— — Q - — — -π

δ

0.706

4 6Age (Ma)

10

50

10

< Ca + M g •O K... 87 S r / 86 S r •× ZONES 768

IV

J O-H3-

0.710

0.709

0.708 ^

0.707

0.706

0 4 6Age (Ma)

8 10

Figure 6. Strontium isotopic composition, dissolved calcium, magnesium, and potassium contents of the pore fluids vs.age (interstitial water analyses: Rangin, Silver, von Breymann, et al., 1990; von Breymann et al., this volume), andlocation of diagenetic zones.

501


10-

-

-

o oπ

π ^

> OD

Δ

A Δ

µ-

D

o o

o

4

0Δ

•A

o

αo

A

A

o

- 1 —

o

— ^ —

D

o

µ-

D

o

µ

D

o

A

A

µ-

767

768

769

4 5 6Age (Ma)

10

9,

OCS

10

1 -

Φ Δ O OΔ O

3 n π uz r ""~ """ • aO D

a

1 1 µ->

Illb

ZO

NE

D

s

: — :

_ α 76/

v /Oo

Δ 769

•v•

D

O u

Δ ° AD Δ

: 1 1 \ \ 1

Age (Ma)10

Figure 7. AI2O3/K2O and Al2O3/FeO ratios vs. time respectively, given by the 35- through 63-µ.m fraction and bulkchemical analyses of ash layers samples.

502

Table 5. Oxygen isotope data for glasses, clay minerals, and zeolite.


Hole Core, section, Natureinterval (cm)

δ 1 8 O%O

SMOW

767B 4H-6, 41-43 Fresh glass 9.0767B 29X-2, 117-119 Altered glass 12.4767B 26X-5, 44-46 Phillipsite 24.5768B 4H-5, 14-16 Detrital clay 17.6767B 35X-5, 8-10 Altered glass and authigenic smectite 22.8767B 26X-5, 44-46 Authigenic smectite 26.1 - 26.3768B 18H-6, 52-53 Authigenic smectite 26.5767B 42X-6, 94-98 Authigenic smectite 24.9

503

36. diagenetic evolution of neogene volcanic …€¦ · element compositions of bulk sample, ......

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