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Variable Impact of the Subducted Slab on Aleutian Island Arc Magma Sources: Evidence from Sr, Nd, Pb, and Hf Isotopes and Trace Element Abundances

B.R. Jicha1, B. S. Singer1*, J.G. Brophy2, J.H. Fournelle1, C.M. Johnson1, B.L. Beard1, T.J. Lapen1, N.J. Mahlen1

1 Department of Geology and Geophysics, University of Wisconsin-Madison, 1215 West Dayton Street, Madison WI 53706, USA

2 Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA

* corresponding author Telephone: 001-608-265-8650

Fax: 001-608-262-0693 Email: bsinger@geology.wisc.edu mail to: bjicha@geology.wisc.edu

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ABSTRACT

Major and trace element compositions and Sr, Nd, Pb, and Hf isotope ratios of 33

Aleutian Island arc lavas from Kanaga, Roundhead, Seguam, and Shishaldin volcanoes provide

constraints on the composition and origin of the material transferred from the subducted slab to

the mantle wedge. 40Ar/39Ar dating of 17 flows indicates that these lavas erupted mainly during

the last ~ 400 kyrs. Along arc geochemical and isotopic variations are consistent with variable

degrees of fluid input to the mantle. Addition of bulk sediment, partially melted sediment, or a

combination of sediment and fluid components may also produce the major, trace element and

isotopic compositions of some Aleutian lavas. Mass-balance modeling suggests that the fluid is

derived from subducted sediment (10-25%) and underlying oceanic crust (75-90%). Hf-Nd

isotope data suggest that relative to Nd, little Hf is transferred to the mantle wedge via fluid.

Lavas from Seguam Island in the central Aleutian arc have distinctly elevated B/La, U/Th,

87Sr/86Sr, and 207Pb/204Pb ratios, which probably reflect a large volume of fluid released from

serpentinized oceanic crust plus the overlying layer of subducted sediment. We propose that the

Amlia Fracture Zone, which was subducted beneath Seguam Island in the past 1 myr, contains

excess sediment and larger quantities of H2O-rich serpentine near the surface of the Pacific plate,

and hence more fluid was available for transfer into the wedge in this section of the arc. The

degree of partial melting of the mantle, modeled from incompatible trace element contents of the

lavas, correlates with the estimated mass of fluid fluxing of the mantle wedge. Seguam lavas,

which show the largest quantity of fluid addition, have compositions which can be matched by a

22% partial melt of a fluid-modified mantle, whereas Shishaldin and Roundhead lava

compositions are consistent with an order of magnitude less partial melting of the mantle wedge.

KEY WORDS: Aleutian Island arc; 40Ar/39Ar dating; fluids; Hf isotopes; magma sources

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INTRODUCTION

Trace element, isotopic, and experimental studies of arc lavas suggest that the transfer of

elements from the subducted plate into the mantle causes flux melting within the overlying

mantle wedge (e.g., Kushiro, 1987; Luhr, 1992; Elliot et al., 1997). Specific chemical and

isotopic tracers (e.g., B, Be, and Li isotopes) have become increasingly used to identify and

quantify the contributions from sediments and subducted oceanic crust (e.g., Morris et al., 1990;

Leeman et al., 1994; Chan et al., 2002). However, debate continues regarding how subducted

components, which control many geochemical features of arc magmas, are transferred to the

mantle wedge. One way to address this question is to better understand the origin of across-arc or

along-arc variations in volcanic geochemistry. Where along-arc changes in magma chemistry can

be correlated to specific features of the subducting plate, it becomes possible to constrain the role

of particular plate kinematics, structures, lithologies, and mechanisms that affect the transfer of

subducted material into the mantle (Leeman et al., 1994; Singer et al., 1996; Rüpke et al., 2002).

Chemical and isotopic differences between Aleutian Island arc lavas have been broadly

interpreted to reflect along-arc variability in either the overriding plate (e.g., Kay et al., 1982;

Singer & Myers, 1990) or, less commonly, in the subducting Pacific plate (Singer et al., 1996).

Here we expand upon the initial study of Singer et al. (1996) to further delineate the role of the

subducted Pacific plate on the genesis of Aleutian Island arc magmas.

In many island arcs, including the Aleutians, it has been proposed that transport of

elements from the subducting plate into the mantle wedge occurs via: (1) fluid alone (Morris et

al., 1990); (2) fluid plus bulk sediment (Miller et al., 1994); (3) fluid plus sediment melt (Elliot

et al., 1997; Class et al., 2000); or (4) melt of an eclogite-facies MORB (Kay et al., 1978;

Brophy and Marsh, 1986; Yogodzinski et al., 1995; Kelemen et al., 2003). High pressure trace

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element partitioning experiments such as those of Tatsumi et al. (1986), Brenan et al. (1998),

Keppler (1996), Kogiso et al. (1997), and Ulmer & Trommsdorf (1995) have emphasized the

importance of a fluid component derived from dehydration of minerals comprising altered and

unaltered oceanic crust and sediments (e.g., amphibole, phlogopite, phengite, lawsonite,

serpentine). Notably, relative to other hydrous minerals, serpentine can carry an order of

magnitude more H2O to depths of 150-200 km (Ulmer & Trommsdorf, 1995). The

experimentally determined partitioning behavior of trace elements between minerals and aqueous

fluids is highly variable, but certain conclusions can be drawn. Specifically, the high-field-

strength elements (HFSE) Zr, Hf, Nb, Ta are relatively immobile compared to large-ion-

lithophile elements (LILE) Cs, Rb, K, Ba, Sr, and Pb. Because of the dramatically different

behavior of these two groups of elements, comparative analyses of the abundances and isotopic

compositions of representative HFSE and LILE could provide insights into the processes

involved in island arc magma genesis.

Here we present new major element, trace element, Sr, Nd, Pb, and Hf isotope

compositions of 33 lavas from Shishaldin, Seguam, Roundhead, and Kanaga volcanoes. These

lavas span the major element range observed within the eastern and central Aleutian arc (Fig.

1a). Compositional and isotopic contrasts suggest that each volcano evolved by markedly

different processes. For example, the composition of Seguam lavas is most likely controlled by

repeated episodes of closed-system differentiation of basalt to rhyolite (Singer et al., 1992a, b),

whereas magma from Kanaga volcano may have been subject to wall-rock assimilation and

contamination by the lower crust (Brophy, 1990; Singer et al., 1992c). Pleistocene-Recent lavas

from the three centers are geochronologically constrained on the basis of 17 new 40Ar/39Ar ages

determined using furnace incremental-heating methods. These age determinations facilitate an

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assessment of changes in magma sources over the last ca. 400 kyr for the first time at these

volcanoes. In particular, B/La ratios in conjunction with Sr, Nd, and Pb isotope ratios suggest

that the Aleutian mantle wedge is variably modified by fluid released from a combination of

hydrated basalt or serpentinite comprising the oceanic crust plus the overlying layer of subducted

sediment. Our model thus contrasts with recent Aleutian petrogenetic models (e.g., Miller et al.,

1994; Class et al., 2000; George et al., 2003; Kelemen et al., 2003) in that it does not require

significant addition of bulk sediment, sediment melt, or eclogite to the mantle wedge. Further,

we propose that the quantity of fluid transferred to the mantle wedge varies by an order of

magnitude from volcano to volcano. The most likely explanation is that the availability of

serpentinite, and hence H2O in the subducted oceanic crust, is greatest where the Amlia Fracture

Zone of the Pacific plate has subducted beneath Seguam Island during the Late Pleistocene.

TECTONIC SETTING

The Aleutian Island arc sits atop a narrow ridge that extends 2000 km westward from the

Alaska Peninsula to its intersection with the Kamchatka Peninsula (Fig. 1a). The Aleutian ridge

is presumed to have formed in the latest Cretaceous to earliest Tertiary in response to a

southward shift in the convergence zone of the Kula plate, which trapped oceanic crust in the

Bering Sea and isolated the Beringian continental margin behind the current subduction zone

(Scholl et al., 1975). Since ridge formation, a back-arc spreading zone has never developed, thus

distinguishing the Aleutians from other island arc systems (e.g., Tonga-Kermadec, Scotia, New

Britain, Marianas).

The central (Okmok to Atka) and western (west of Adak) Aleutian arc is structurally

segmented into several blocks that have undergone clockwise rotation accompanied by arc-

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parallel extension (Geist et al., 1987, 1988). Seismic reflection and refraction data indicate that

the sub arc crust is 25-30 km thick (Holbrook et al., 1999; Fleidner & Klemperer, 1999), whereas

earlier gravity and seismic refraction data suggested that the thickness of the Aleutian arc is 20-

25 km (Grow, 1973). P-wave velocity data suggest an overall mafic composition for the arc,

consisting mainly of metabasalt, diorite, and diabase in the upper crust, greenschist-facies

MORB, gabbro, and anorthosite in the middle crust, and garnet granulite, amphibolite,

hornblendite, or the mafic residua of calc-alkaline and tholeiitic fractionation in the lower crust.

A small volume of granitoid plutons crop out on Adak, Amchitka, Kagalaska, and Unalaska

Islands (Kay et al., 1990), thereby supporting the three-dimensional velocity models of Fleidner

& Klemperer (1999) which estimated that 40% of the upper crust is intermediate to silicic in

composition. The Aleutian arc lacks seismic evidence for a silicic middle crust (Holbrook et al.,

1999).

South of the Aleutian trench, the subducting Pacific plate contains three north-south

trending fracture zones (Fig. 1a). The most prominent of these is the 25 km-wide Amlia Fracture

Zone (AFZ) which offsets west-trending magnetic anomalies by at least 220 km (Scholl et al.,

1982). East-facing escarpments of the AFZ pond west-flowing terrigenous sediment in the

Aleutian trench and may prevent much of the terrigenous and pelagic sediment from being

scraped off below the accretionary wedge during subduction. Terrigenous sediment thickness is

typically 2.0-2.5 km in the Aleutian trench, but sediment flux beneath the arc is variable in the

central-eastern Aleutians (Fig. 1b). Because of the focusing effect of the AFZ, the wedge of

terrigenous sediment is 3.7-4.0 km where the AFZ intersects the trench (Scholl et al., 1982).

Maximum sediment flux, estimated by Kelemen et al. (2003), beneath the arc also occurs at this

location (Fig 1b). Where fracture zones have been sampled directly through dredging, drilling, or

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diving, peridotite is commonly exposed to ocean water causing serpentinization (Bonatti &

Crane, 1973; Schroeder et al., 2002). The AFZ is therefore likely to expose serpentine near the

surface, which upon breakdown at depths of > 100 km in subduction zones, may release up to 13

wt% H2O (Ulmer & Trommsdorf, 1995).

At the oceanic-continental arc transition (~163oW), subduction of the Pacific plate is

nearly orthogonal to the arc, but becomes increasingly oblique in the central and western

Aleutians (Isacks et al., 1968; Grow & Atwater, 1970; Fig. 1a). As the rate of orthogonal

convergence decreases, the subaerial volumes of Quaternary volcanoes also generally decrease

(Marsh, 1982; Fournelle et al., 1994). Fournelle et al. (1994) also suggested that erupted lavas

from smaller volcanoes have more evolved compositions than those from large Aleutian edifices.

Other variations in the major and trace element and isotopic compositions of Aleutian lavas have

been attributed to: (1) the tectonic positions of volcanic centers within an arc segment (Kay et

al., 1982; Singer & Myers, 1990); (2) lithospheric contamination (Myers et al., 1985; Brophy,

1990; Kelemen et al., 2003); (3) subduction rate/obliquity (Keleman et al., 2003; Yogodzinski et

al., 1995; George et al., 2003); and (4) variations in the downgoing plate (i.e. fracture zones)

(Kay, 1980; Singer et al., 1992a, b; Miller et al., 1994; Singer et al., 1996).

GEOLOGY OF VOLCANIC CENTERS

Shishaldin

Shishaldin, the largest (300 km3) and tallest (2587 m) volcano in the Aleutian Islands, is

composed of a wide range of basalt types with minor andesite. One of the most active volcanoes

in the Aleutian arc, Shishaldin has erupted 28 times since 1775. The most recent eruptive activity

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occurred in April and May of 1999. Shishaldin was mapped at a reconnaissance level by Finch

(1934), and Fournelle (1988) conducted a petrologic study of the volcano. Based on erosion and

glaciation of the three volcanoes on eastern Unimak Island (Roundtop, Isanotski, and

Shishaldin), Fournelle (1988) suggested that Shishaldin is the youngest center in an east to west

progression of volcanism. The modern Shishaldin edifice, along with the 24 monogenetic cones

on its flanks, is believed to have formed after the last glaciers retreated from the Aleutians about

10-12 ka (Black, 1983).

Seguam

Seguam is a Pleistocene-Recent shield volcano (~80 km3) with multiple eruptive centers

comprising a bimodal suite of tholeiitic, low-K basalt/basaltic andesite and dacite and rhyolite

with up to 71 wt.% SiO2 (Singer et al., 1992a, b, c). Pyre Peak, a basaltic cinder cone, is the

highest of the centers, rising to 1054 meters.

Deeply glaciated Late Pleistocene lavas and tephras are capped by Holocene lava flows

and ash deposits consisting of ~1.0 km3 rhyolitic domes in the east and more voluminous basalt

flows and scoria beds in the west. Historical activity in 1977 and 1992-1993 included basaltic

ash and lava eruptions from a 2.5 km long fissure ~2 km south of Pyre Peak. Based on the K-Ar

dating, major and trace element data, Sr, Nd, Pb, and O isotope compositions, and because

Pleistocene basalts, basaltic andesites, and crystal poor rhyolites are strikingly similar in

composition to the Holocene and historical eruptive products, Singer et al. (1992a, c) suggested

that tholeiitic basalt underwent repeated episodes of closed-system differentiation to produce

rhyolite beneath Seguam over the past 1 myr.

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Kanaga

In contrast to the shield-type structure of Seguam, Kanaga is a small (~25 km3) volcanic

center with a 1307 m high, calc-alkaline, andesite-dominated central vent complex that has

undergone repeated episodes of stratocone growth and destruction. The most prominent

historical eruption of Kanaga volcano occurred in 1906 when lava poured down both the east and

west sides of the cone. Recent activity from 1993 through 1995 produced blocky lava flows and

debris avalanches that covered the northwestern flank.

The geologic history of Kanaga is known mainly from field mapping and relative

stratigraphic relations determined by Coats (1952, 1956). Brophy and co-workers (1990, 1999)

described the petrography and geochemistry of lavas from Kanaga and Roundhead volcanoes on

northeastern Kanaga Island. Kanaga volcano is flanked to the south and east by Kanaton Ridge,

an 800m high arcuate ridge comprised of nearly horizontal basaltic andesitic and andesitic flows.

A sample from the top of Kanaton Ridge gave a whole rock K-Ar age of 184 ± 180 ka (Bingham

& Stone, 1972)(here and throughout, ages are reported with ±2σ errors). The low outward dips

of these flows imply a common source from a broad volcano herein called Mount Kanaton.

Roundhead volcano, a <1 km3 parasitic cone along the eastern shore, comprises interlayered

high-alumina basalt flows and pyroclastics. Holocene activity on Kanaga Island has created the

modern edifice, Kanaga volcano, which formed inside the Mt. Kanaton caldera. Singer et al.

(1992c) concluded that oxygen isotope disequilibrium and heterogeneity of Kanaga lavas reflects

fractionation, assimilation of crust, and magma mixing during petrogenesis, consistent with the

petrologic interpretations of Brophy (1990), who suggested that quenched inclusions in Kanaga

andesites were the result of magma mingling and/or mixing. Mafic and ultramafic xenoliths

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found in Tertiary lavas on southern Kanaga Island suggest that during some periods in the past,

lavas may have been contaminated by lower crustal and mantle material (DeLong et al., 1975;

Conrad et al., 1983).

SAMPLE DESCRIPTION AND PETROGRAPHY

Shishaldin, Seguam, and Kanaga volcanoes were chosen for this study because an

extensive sample suite exists for each volcano. Thirty-three samples were chosen to represent the

compositional, geographical, and temporal spectrum preserved at these volcanoes. The suite

includes 13 basalts (47-52 wt.% SiO2), 8 basaltic andesites (52-56 wt.% SiO2), 9 andesites (56-

62 wt.% SiO2), and 3 rhyolites (> 69 wt.% SiO2), which covers most of the observed major

element compositional range in the central-eastern Aleutian arc lavas.

Shishaldin high-Mg basalts (> 8.5 wt.% MgO) contain up to 20% diopsidic

clinopyroxene and two populations of olivine: Fo92-93 and Fo72-74. High-alumina basalts (HAB)

have 35-50 modal percent phenocryts of plagioclase (30-45%), olivine (< 5%), and rare

clinopyroxene. Plagioclase cores range from An77-82 in HAB to An60 in aphyric Fe-Ti basalts

(Fournelle, 1988). Seguam lavas have plagioclase (up to 42%), olivine (0.4-9.3%),

clinopyroxene (0.3-5.8%), and rare orthopyroxene and titanomagnetite microphenocrysts (Singer

1992a). The unusually phyric Roundhead HAB contain 68-70% phenocrysts of plagioclase (43-

45%), clinopyroxene (15-19%), titanomagnetite (3-4%), and olivine (2-3%). Roundhead HAB

also contains 1.5 cm diameter megacrysts of concentrically zoned augite that formed as the result

of HAB decompression, volatile (H2O-rich) exsolution, augite supersaturation, and rapid augite

crystallization (Brophy et al., 1999). Kanaga andesites and mafic andesites have plagioclase (23-

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34%), clinopyroxene (7-9%), titanomagnetite (1-3%), and minor amounts of olivine and

orthopyroxene (Brophy, 1990). Small euhedral crystals of amphibole are present in samples KG

31 and KG 34. Biotite is not present in any of the samples.

ANALYTICAL METHODS

40Ar/39Ar dating

40Ar/39Ar furnace incremental heating experiments were undertaken on 200-375 mg

aliquots of carefully separated holocrystalline groundmass from 17 samples using the methods of

Singer et al. (2002). The experiments consisted of 3-13 steps from 800-1325oC. System blanks

were measured prior to each experiment at temperatures between 800-1200oC. These signals

were 6x10-19 moles for 36Ar and 1.5x10-16 for 40Ar, which are 1 to 2 orders of magnitude smaller

than the samples. Corrections for undesirable neutron-induced reactions on 40K and 40Ca are:

[40Ar/39Ar]K= 0.00086; [36Ar/37Ar]Ca = 0.000264; [39Ar/37Ar]Ca = 0.000673.

All ages were calculated using the decay constants of Steiger and Jäger (1977) relative to

either the 28.34 Ma Taylor Creek (TCs) or 1.194 Ma Alder Creek (ACs) rhyolite sanidine that

were used to monitor fluence. Because isochron regressions (York, 1969) agreed with plateau

ages and did not reveal evidence that excess argon is present in any of the lavas, we consider the

plateau ages to give the best estimate of time elapsed since eruption (Fig. 2).

Major and trace elements

Whole rock major and trace element and Sr, Pb, Nd, and Hf isotope compositions listed

in Table 1 were determined from fresh ~ 200 g slabs that were cut from each sample, crushed in

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a steel jaw crusher and powdered in an alumina shatterbox. Major and trace element

concentrations were measured by ICP and ICP-MS techniques, respectively, at Actlabs in

Ontario, Canada. Boron concentrations were determined via prompt gamma neutron activation

analysis (PGNAA). For boron, one gram powdered samples were encapsulated in polyethylene

vials and placed in a thermalized beam of neutrons produced from the nuclear reactor at

McMaster University. Samples were measured for the Doppler-broadened prompt gamma ray at

478 keV using a high purity GE detector following Hoffman et al. (1984). Precision is 10 to15%

for concentrations > 5 ppm, and 20-25% for concentrations near 2.5 ppm. Precision of the

ICP/ICP-MS data is ± 0.6-4.7% for the major elements and ± 4.2-5.9% for most trace elements.

Sr, Nd, Pb, and Hf isotopes

Sr, Nd, and Pb isotope ratios were measured on a Micromass Sector 54 TIMS at the

University of Wisconsin-Madison Radiogenic Isotope Laboratory following Johnson &

Thompson (1991). For Sr and Nd, 75 mg of powdered sample were dissolved in 4 ml of doubly

distilled 29M HF and 1 ml of once distilled 14M HNO3 for two days on a hotplate. Samples were

not spiked. The elements were separated by cation exchange techniques using HCl for Sr and

HCl and α-HIBA for Nd. A separate100 mg aliquot of powder was dissolved in HF-HNO3 and

separated for Pb using a HBr-HCl anion exchange procedure (Johnson & Thompson, 1991).

Strontium isotope measurements were measured using a dynamic multi-collector analysis

routine, with exponential normalization to 86Sr/88Sr = 0.1194. Twelve measurements of NBS

987 yielded an 87Sr/86Sr ratio of 0.710263 ± 0.000002 (2σ) and 3 measurements of BCR-1

averaged 0.705031 ± 0.000010 (2σ). Neodymium isotope ratios were measured as NdO+ using

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dynamic multi-collection, and were exponentially corrected for instrumental mass fractionation

using 146Nd/144Nd = 0.7219. Six measurements of an internal laboratory standard, AMES II,

yielded a 143Nd/144Nd ratio of 0.511977 ± 0.000003 (2σ), and 2 measurements of BCR-1

averaged 0.512643 ± 0.000005 (2σ). Fifteen measurements of NBS 981 and NBS 982 standards

yielded a mass fractionation correction of 0.14 ± 0.013 (2σ) percent per atomic mass unit

(a.m.u.) for Pb isotope ratios. Procedural blanks were typically 175 pg for Nd, 450 pg for Sr, and

600 pg for common Pb, all of which are negligible.

All Hf isotope analyses were obtained on a Micromass IsoProbe at the University of

Wisconsin-Madison. For Hf isotope analyses, 100 mg of powdered sample was dissolved in 4 ml

of 29M twice-distilled HF and 1 ml of 14M HNO3 for two days. Samples were redissolved three

times with 5 ml of 14M HNO3 and then three more times with 5 ml of 8M HCl. Hafnium was

separated from the sample matrix using Ln-spec resin following procedures modified from

Münker et al. (2001). Complete chemical separation and Hf isotope analysis procedures, are

given in Lapen et al. (in press). Samples yielding high Ti (Ti/Hf > 1) after one pass through ion-

exchange columns were passed again to remove excess Ti. The concentration and elemental

purity of samples after chemical separation is monitored by analysis of a very dilute aliquot of

sample solution prior to isotopic analysis of the main solution. Concentrations of Yb, Lu, Zr, Ti,

W, and Ta are determined by comparing the ion intensity of the sample with standards of

variable, but well-known concentration. 176Lu and 176Yb interference corrections on 176Hf are

less than 0.05% of the 176Hf peak for each. Corrections on 180Hf volts are generally < 30 ppm for

180W and < 0.5 ppm for 180Ta. Isotope analyses are performed in static mode. No collector biases

are applied beyond those determined by a constant-current gain calibration. Instrumental mass

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bias may be corrected using internal normalization to a constant 179Hf/177Hf = 0.7325. A second

correction for “residual mass bias” is determined from the average of standards measured

throughout each analytical session. The UW-AMES Hf standard, which is analyzed during each

session, is isotopically indistinguishable from JMC-475 Hf: 176Hf/177Hf = 0.282160; 178Hf/177Hf

= 1.467168; 179Hf/177Hf = 0.7325; 180Hf/177Hf = 1.88666. Seventeen measurements of the JMC-

475 Hf standard yielded values of 176Hf/177Hf = 0.282162 ± 0.000009 (2σ), 178Hf/177Hf =

1.46716 ± 0.000003 (2σ), 179Hf/177Hf(measured) = 0.7463 ± 0.000006 (2σ), 180Hf/177Hf =

1.88669 ± 0.000013 (2σ). εHf values for each sample were calculated based on 176Hf/177Hf(CHUR)

= 0.282772 (Blichert-Toft et al., 1997). Procedural blanks were less than 150 pg for Hf, which

are negligible. Complete duplicate Hf isotope analyses of 5 of the 31 samples were performed.

The average spread in the Hf duplicate measurements was 0.5 εHf units.

RESULTS

40Ar/39Ar dating

Multiple 40Ar/39Ar incremental heating experiments were conducted to identify the time

interval over which magma source evolution has occurred at each volcano. Forty one

experiments yielded nearly concordant spectra comprising 63-100% of 39Ar released that defined

plateau ages between 33.3 ± 0.7 ka and 713.3 ± 9.6 ka (Table 2, Figure 2, complete 40Ar/39Ar

data in electronic supplement, http://www.petrology.oupjournals.org). The 17 new 40Ar/39Ar age

determinations add to the limited geochronologic data set for the Aleutian Island arc, which only

consists of 42 published K-Ar (e.g., Singer et al., 1992a; Romick et al., 1990; Bingham & Stone,

1972) and three 40Ar/39Ar (e.g., Layer, 1997) ages, most without supporting analytical data.

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Eight 40Ar/39Ar age determinations on basaltic andesite to rhyolite from Seguam Island

yielded ages between 93.1 ± 9.5 to 33.3 ± 0.7 ka (Fig. 3, Table 2). A 60 m sequence of

interbedded till, pyroclastics, and Fe-Ti enriched and high-alumina basalt (HAB) flows on the

northwestern flank of Shishaldin gave 40Ar/39Ar plateau ages between 713.3 ± 9.6 and 28.0 ± 3.9

ka. Four incremental heating experiments on samples from the uppermost section of Kanaton

Ridge on Kanaga Island gave weighted mean plateau ages of 199.1 ± 2.5 ka and 198.1 ± 2.1 ka,

whereas lavas that underlie Kanaton Ridge are ca. 150 to 180 ka older and define plateau ages of

383.9 ± 4.0 ka and 352.0 ± 3.9 ka (Fig. 3, Table 2). Finally, five incremental heating

experiments from two Roundhead high alumina basalt (HAB) flows yielded 40Ar/39Ar mean

plateau ages of 133.9 ± 3.1 ka and 112.5 ± 5.1 ka, indicating that Roundhead preserves the

youngest Pleistocene lavas on Kanaga Island.

Major elements

The 33 lavas range from basalt (47.6 wt.% SiO2) to rhyolite (69.8 wt.% SiO2) with Al2O3

(19.38-14.47 wt.%), FeO* (13.12-3.71 wt.%), MgO (7.27-0.69 wt.%), CaO (11.30-2.63 wt.%)

and TiO2 (2.36-0.56 wt.%) showing a systematic decrease with increasing SiO2 (Table 1, Fig. 4).

Conversely, Na2O (2.34-5.01 wt.%) and K2O (0.34-2.36 wt.%) increase with SiO2. The major

element range in these lavas is a representation of the vast majority of compositions erupted in

the central and eastern Aleutian Island arc (e.g., Kelemen et al., 2003). Mg# (molar Mg/Mg+Fe)

ranges from 0.60 in primitive basalts to 0.22 in rhyolites. A distinctive feature of the Shishaldin

lavas is the high TiO2 (1.10-2.63 wt.%) and P2O5 (0.19-0.63 wt.%) contents relative to Kanaga

and Seguam (Fig. 4). As noted by Singer et al. (1992a), at a given SiO2 content, Seguam lavas

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have distinctly lower K2O, Na2O, and P2O5 abundances than most lavas in the Aleutian arc (Fig.

4).

Trace Elements

Abundances of Cs, U, Ba, Zr, Hf, and Nb increase with SiO2, whereas Sr decreases. At a

given SiO2 content, Seguam lavas have the lowest abundances of these elements (Fig. 5).

Shishaldin basalts have much higher abundances of Zr (87-201 ppm), Hf (2.3-5.4 ppm), Y (17.6-

43.6 ppm), Nb (3.3-8.5 ppm) and Ta (0.20-0.67 ppm) than other Aleutian mafic lavas. These

elevated abundances are similar to those observed from lavas in the continental sector of the

Aleutian arc (Nye & Turner, 1990; Brophy, 1987; George et al., 2003).

Boron is a fluid-mobile, strongly incompatible element that is concentrated in marine

sediments and hydrothermally altered oceanic crust (Leeman et al., 1994; Leeman, 1996) It has

been used in conjunction with other geochemical and isotopic tracers including 10Be to evaluate

sediment recycling at convergent margins and the transfer of subducted material via fluid to the

source of arc magmas (Morris et al., 1990; Edwards et al., 1993; Leeman et al., 1994). Boron

concentrations (3.2-72 ppm) in Seguam, Kanaga, and Shishaldin lavas fall within the range

previously reported for Aleutian lavas (Morris et al., 1990; Ryan & Langmuir, 1993; Class et al.,

2000, George et al., 2003). In contrast to other incompatible elements, B abundances in Seguam

basalts are up to 15% higher than basalts from the other two centers, and Seguam rhyolites have

some of the highest B concentrations in the Aleutians (Table 1, Fig. 5).

Shishaldin basalts have high total REE contents (64.3-145.2 ppm), light-rare-earth

enriched (LREE) patterns (La/Yb = 4.42-7.64) and both positive and negative Eu anomalies (Fig.

17

6a). The Fe-Ti enriched basalts have among the highest total REE contents for Aleutian Island

arc lavas (141.9-145-2 ppm) (Fig. 6a). The most striking features of the Seguam basalts are their

low total abundances (23.7-30.5 ppm), nearly flat REE patterns (La/Yb = 1.81-2.57), and strong

positive Eu anomalies (Fig. 6b). The Seguam dacites and rhyolites have moderate total REE

contents (98.0-107.6 ppm), slight LREE-enriched patterns (La/Yb = 3.23-3.61), and negative Eu

anomalies. The most REE-enriched Seguam rhyolite has a lower total REE content than many of

the basalts from Shishaldin. Kanaga lavas have moderate total REE abundances (55.8-90.0 ppm),

LREE-enriched patterns (La/Yb = 3.97-6.25), and no Eu anomalies (Fig. 6c). Roundhead and

Seguam basalts exhibit Nb and Ta anomalies that are characteristic of island arc basalts, but the

high-field-strength-element depletion in Shishaldin basalts is not as extreme. Mafic lavas from

each of the volcanoes show strong positive Sr anomalies (Fig. 6b), which can be explained by

either plagioclase accumulation or Sr-rich fluid flux into the magma source.

Sr, Nd, and Pb isotope compositions

143Nd/144Nd ratios of Shishaldin lavas (0.51310-0.51295, εNd = 7.4-8.9) are among the

most radiogenic in the Aleutians (Fig. 7). In contrast, Seguam lavas have, on average, the least

radiogenic 143Nd/144Nd ratios in the Aleutians, and those of Kanaga lie in between Shishaldin and

Seguam (Table 1, Fig. 7). 87Sr/86Sr isotope ratios vary from 0.70296 to 0.70370, where Seguam

lavas have the highest ratios and Shishaldin the lowest. The Pb isotope ratios (206Pb/204Pb =

18.70-18.95) mainly lie between compositions for Pacific MORB and N. Pacific sediments, and

plot above the Northern Hemisphere Reference Line (NHRL) in terms of 207Pb/204Pb ratios.

However, unlike Sr and Nd, the Pb isotope ratios do not represent the entire range of Aleutian

18

lavas. Among the lavas measured here, the most radiogenic Pb isotope ratios are from Seguam

and the lowest at Roundhead and Shishaldin. An along-arc plot of isotopic and selected trace

element ratios shows that Seguam lavas have the highest 87Sr/86Sr, 207Pb/204Pb, B/La and U/Th

ratios and lowest 143Nd/144Nd and La/Yb ratios in the Aleutian arc (Fig. 8).

Hf isotope compositions

Prior to the current study, only three samples of Aleutian lavas from Little Sitkin volcano

were analyzed for Hf isotope compositions (White & Patchett, 1984). 176Hf/177Hf ratios from

Shishaldin, Kanaga, and Seguam lavas are limited to between 0.28312 and 0.28318 (εHf = +12.3

to +14.4) (Table 1, Fig. 7). Each Aleutian volcano plots distinctly in Hf-Nd space. Seguam and

Kanaga lavas exhibit a limited range for both Hf and Nd isotope compositions, whereas

Shishaldin lavas show a correlation between Hf and Nd (Fig. 7). 176Hf/177Hf ratios from the three

volcanoes are less radiogenic than those of Little Sitkin (White & Patchett, 1984). Aleutian and

Marianas arc lavas display a similar range in 143Nd/144Nd ratios, but the Marianas lavas have

distinctly higher 176Hf/177Hf ratios.

DISCUSSION

K-Ar vs. 40Ar/39Ar ages of Seguam lavas

Whereas 11 whole-rock K-Ar age determinations suggest a ~1 myr subaerial eruptive

history (Singer et al., 1992a), eight new 40Ar/39Ar age determinations constrain the duration of

Pleistocene volcanism at Seguam from 93.1 ± 9.5 to 33.3 ± 0.7 ka. We suspect that the ~1 Ma K-

Ar ages obtained from a Seguam basalt and basaltic andesite are most likely the result of very

19

low K2O contents (0.33-0.39 wt.%) of the lavas or the incorporation of xenocrysts into the large

(~25 g) whole rock samples melted for the Ar analyses. Three incremental heating experiments

on purified groundmass separated from the basaltic andesite flow (sample SB87-63), which gave

a K-Ar age of 1.07 ± 0.32 Ma, yielded a mean 40Ar/39Ar plateau age of 52.9 ± 13.7 ka. This lava

is from one of the most deeply eroded and presumably oldest sections exposed on the island.

Thus, we find it unlikely that a substantial volume of subaerially exposed lavas and tephras are

significantly older than the oldest 40Ar/39Ar age of 93.1 ± 9.5 ka (Fig. 3, Table 2).

Mantle heterogeneity beneath Kanaga Island

On the basis of low whole rock deuterium contents and calculated pre- and post-eruption

H2O contents, Brophy et al. (1999) proposed that Roundhead basalt, which contains large sector-

zoned augites, represents a mantle-derived low-alumina basalt that fractionated at the base of the

crust, and crystallized rapidly at shallow depth (<3 km) due to decompression and volatile

exsolution. The new isotopic and age data (Fig. 9) reveal that 134-112 ka Roundhead basalt has

distinctly low 87Sr/86Sr and 206Pb/204Pb ratios compared to both older and younger basaltic

andesites and andesites at the adjacent Kanaga volcano.

Because textural, oxygen isotope, and chemical evidence suggest that open-system

mixing or assimilation in crustal reservoirs was widespread beneath Kanaga volcano (Brophy,

1990; Singer et al., 1992c), two possible explanations for the unusually low Sr and Pb isotope

compositions are: (1) Roundhead basalt represents ascending melt that reacted with and

incorporated 87Sr- and 206Pb-poor mantle peridotite components (e.g., Myers et al., 1985;

Kelemen et al., 2003), or (2) Roundhead basalt is a relatively uncontaminated magma, and the

20

more radiogenic Kanaga basaltic andesites represent Roundhead magma that has assimilated

relatively radiogenic (87Sr/86Sr > 0.7036) crustal rocks. To test the first hypothesis we searched

for mantle xenocrysts by measuring the 87Sr/86Sr ratio of clinopyroxene and groundmass

separates from two Roundhead basalts and one Kanaga andesite. 87Sr/86Sr ratios of the

clinopyroxene and groundmass separates are virtually identical to the whole rock values (Table

3), indicating that contamination of these magmas with mantle-derived material, if it occurred,

took place before the clinopyroxene phenocrysts grew. Assimilation-fractional crystallization

calculations indicate that in order to generate basaltic andesite like that erupted f

rom Kanaga volcano, the minimum proportion of crust (~ 300 ppm Sr; 87Sr/86Sr = 0.7036) that

must be assimilated by the Roundhead basalt is unrealistically large (~70% of initial magma

mass), making the second proposed scenario unlikely.

If contamination of Roundhead basalt was minimal prior to clinopyroxene crystallization,

our data imply that the mantle beneath Kanaga volcano is isotopically distinct from that under

Roundhead (Fig 3). On the basis of similar isotopic data, mantle heterogeneity at this ~10 km

scale is thought also to exist beneath Umnak and Adak Islands (Fig. 1; Miller et al., 1992; Kay et

al., 1985). This could reflect intrinsic variability of the mantle wedge, or recent focusing of large

slab-derived additions to the wedge beneath the major stratovolcanoes like Kanaga relative to

smaller volcanoes like Roundhead that tap adjacent mantle domains (e.g., Hickey-Vargas et al.,

2002).

Crustal contamination

Chemical and isotopic variations observed in arc lavas are commonly explained by

21

shallow level contamination of magmas within the crust, which has been well documented for

volcanic arcs on older continental crust (Hildreth & Moorbath, 1988). With the exception of

Kanaga volcano, Sr, Nd, Pb, and Hf isotopes in this study show no correlation with SiO2, which

suggests that contamination processes have had a minimal effect on magmas at Seguam,

Shishaldin, and Roundhead. However, it is also likely that Aleutian sub-arc crust has a Sr, Nd,

Pb, and Hf isotope composition that is similar to those of the arc lavas. Nevertheless, to

minimize complications that involve differentiated lavas, we have omitted lavas with > 54 wt.%

SiO2 (Mg# < 0.4) in our discussion on mantle source characteristics.

Role of sediment in the magma source

The pioneering work of Kay et al. (1978) noted that a mixture of several percent

sediment with a mantle source may explain the Sr and Pb isotope compositions of Aleutian

volcanic rocks, assuming that the unmodified mantle component had the same isotopic

compositions as MORB. Subsequent studies using Pb isotopes and 10Be have confirmed that

sediments play an important role in modifying the source regions for Aleutian magmas (e.g.,

Myers and Marsh, 1987; Morris et al., 1990; Miller et al., 1994; Singer et al., 1992b). Debate on

the specific role played by sediments has centered on the physical mechanisms by which

sediment or sediment-derived components are transported from the slab to the Aleutian mantle

wedge and/or wedge-derived magmas. Class et al. (2000) proposed that a sediment melt and two

distinct fluid components are responsible for the geochemical characteristics of Aleutian arc

magmas. We propose that the abundances of Pb, Sr, Nd and their isotopic compositions, together

with B, LREE, and LILE contents in Seguam Island lavas are best explained by the addition to

22

the mantle wedge of a fluid component that shares characteristics of both sediment and oceanic

crust. Alternatively, Roundhead and Shishaldin lavas can be explained by either the addition of

fluid and/or a sediment melt component. We explore the evidence for these hypotheses below.

Fluid addition to the mantle wedge

Aleutian lavas have high Ba/La and low Th/Yb ratios that are similar to arcs that have

been characterized as ‘fluid-dominated,’ such as the Kermadec, Marianas, and New Britain arcs.

In contrast, the Sunda and Lesser Antilles arcs, which have low Ba/La and high Th/Yb ratios,

have been interpreted to reflect a significant component of sediment-dominated melts

(Woodhead et al., 2001). However, Kelemen et al. (2003) note that because Ba/La is positively

correlated with Th/La ratios in Aleutian arc lavas, it cannot be used to distinguish between fluid-

rich and sediment melt components, and it suggests that Ba is transported from the subducted

crust to the mantle wedge via a silicate melt. We find that Ba/La ratios in Aleutian lavas are not

correlated with B/La ratios, an indicator of aqueous fluids, thereby supporting the idea that Ba is

not mobilized by aqueous fluids (Fig. 10).

Arc lavas with extreme 238U excesses and U/Th ratios higher than those in MORB, have

been used to support the hypothesis of hydrous fluid input to the magma source. U/Th ratios of

Seguam lavas (0.53-0.57) are indeed higher than Pacific MORB (0.10-0.42) whereas Shishaldin

and Roundhead lavas have U/Th ratios which overlap Pacific MORB. In addition, the

(238U/232Th) ratio measured by George et al., (2003) on a Seguam basalt is the highest yet

measured in the Aleutians.

Enrichment of boron and high δ11B isotope compositions of arc lavas also provide strong

23

evidence for fluid addition to the subarc mantle. Basalts from Seguam Island have elevated

boron contents, δ11B values between 1.9 and 3.5‰, and B/Nb ratios (up to 33), which are

significantly greater than Pacific MORB or DSDP 183 sediments. These data suggest that the

subducting sediments provide a source of boron at Seguam (Chan et al., 2002; L.H. Chan,

personal communication). In contrast, Kanaga and Shishaldin lavas show low B/Nb ratios and

lower δ11B values (-0.69 to 2.7‰). We focus on the incompatible-element ratio B/La, in

conjunction with isotopic data, to explore the potential role that slab-derived fluids may have

played in modifying the mantle wedge beneath the Aleutian arc. Seguam lavas contain the most

radiogenic Sr and Pb isotope ratios and the highest B/La ratios in our sample suite (Fig. 11).

These trends suggest that Seguam lavas are derived from a mantle wedge that has been modified

by fluid, whereas Shishaldin and Roundhead lavas have B/La ratios and Sr and Pb isotope

compositions which reflect slight fluid modification or sediment melt addition to the mantle

wedge. The positive correlation between 87Sr/86Sr and 207Pb/204Pb and B/La ratios for Seguam

lavas strongly suggests that Sr and Pb were transported by fluids. However, the negative

correlation between Hf and Nd isotope compositions and Th/Yb ratios of Shishaldin and

Roundhead lavas hints at the involvement of sediment melts or bulk sediment mixing with the

magma source.

143Nd/144Nd-Th/Nd variations

The isotope and trace-element compositions of Aleutian lavas have been used to argue

that sediment may be added to the mantle wedge as a siliceous partial melt (Class et al., 2000;

Plank & Langmuir, 1993). Class et al. (2000) proposed that the melt has a sediment-like isotopic

24

composition for Nd and Pb and high Th/Nd and Th/Yb ratios compared to both global and

regional sediments. We note, however, that 13 of the 18 lavas analyzed by Class et al. (2000)

have evolved compositions ranging from 54 to 71 wt.% SiO2 such that Th/Nd ratios are

correlated with SiO2 contents (Fig. 12a). Thus, inferences based on trends defined by these

evolved lavas may be problematic because of shallow-level differentiation effects. Most of the

mafic lavas (< 54% SiO2) from Class et al. (2000) and our study lie in an array between the

mantle wedge and a potential sediment melt component in terms of 143Nd/144Nd-Th/Nd variations

(Fig. 12b). Seguam lavas lie along a mixing trend between the mantle wedge and DSDP 183

sediment or sediment-derived fluid.

Three component model

Using Ce/Pb ratios and Pb isotope compositions, Miller et al. (1994) proposed that

Recheshnoi and Okmok lavas on Umnak Island could not be explained solely by sediment

addition to the mantle wedge, but required enrichment of the magma source by a fluid

component derived from subducted basalt which contained relatively unradiogenic Pb and low

Ce/Pb ratios. Kelemen et al. (2003) point out that low Ce/Pb ratios in Umnak lavas could reflect

transport of Pb from subducted basalt to the mantle wedge via either an aqueous fluid, or a

partial melt of subducted basalt. Moreover, Kelemen et al. (2003) suggest that because sediments

have low Ce/Pb ratios, this ratio cannot be used to distinguish sediment-derived fluid from a

sediment-derived melt in Umnak lavas. B/La ratios in conjunction with Pb and Sr isotope ratios

can distinguish between a fluid derived from the oceanic crust, and a sediment-derived fluid.

Accordingly, we propose a three component model for Aleutian magma genesis involving the

25

migration of two fluids from the subducted slab to the mantle wedge (Fig. 13). First, fluid

released from altered basaltic crust and serpentinized peridotite rises through and mixes with

fluid extracted from the overlying section of subducted sediment, thereby further leaching

mobile components from the sediments. These fluid fluxes into the overlying mantle wedge

would be the primary means for partial melting. Mass-balance modeling calculations suggest that

Roundhead and Shishaldin lavas require < 0.2% fluid modification of the magma source (Fig.

13). In contrast, Seguam lavas reflect 1-5% fluid addition to their mantle source regions prior to

melting, obtained predominantly (~ 75-90%) from oceanic crust, plus a subordinate component

from sediment (~ 10-25%).

The proportions of fluid addition to the mantle wedge in this model are comparable to

those recently proposed for the Izu-Bonin and Aleutian Island arcs. For example, incompatible-

element abundances and Sr, Nd, and Pb isotope compositions of Izu-Bonin arc lavas suggest that

approximately 2% fluid fluxed the mantle wedge. The fluid was derived from Izu sediments and

altered oceanic crust (AOC) with a sediment fluid:AOC fluid proportion of 12:88 (Hochstaedter

et al. 2000). Based on similar modeling, Class et al. (2000) suggested that the mantle wedge

beneath Umnak Island in the Aleutian Island arc experienced 0.1 to 3.2% fluid addition prior to

melting.

Although the fluid component reflects a significant contribution from the thick layer of

terrigenous sediment being subducted in the Amlia Fracture Zone below Seguam Island, its

overall composition remains dominated by the altered oceanic crust component. Fracture zones

contain highly faulted oceanic crust, which commonly exposes large areas of peridotite at the

seafloor (Bonatti & Crane, 1973), where reaction with seawater causes serpentinization

26

(Schroeder et al., 2002). Serpentine may contain up to 100 ppm B (Thompson & Melson, 1970),

and therefore serpentine breakdown at high pressure (Ulmer & Trommsdorf, 1995) will result in

large quantities of B-rich fluid transferred to the mantle wedge. Boron rich fluids derived from

serpentinized components have been proposed for the Central American arc, where along-arc

changes in B/La and Ba/La ratios of arc lavas reflect a change in the source of slab-derived fluids

(Rüpke et al., 2002). High B/La and Ba/La ratios occur in lavas above deeply faulted,

serpentinized, lithosphere beneath Nicaragua, whereas lavas with low B/La and Ba/La ratios are

erupted from volcanoes in Costa Rica above less deeply faulted subducting lithosphere. We

propose that in the Aleutian arc, the anomalously large B/La ratios observed in Seguam lavas

reflect subduction of the highly faulted Amlia Fracture Zone (Fig. 1a).

High field strength element mobility in subduction zone fluids

If our model is correct and fluid addition is variable along the arc, it offers a means to test

the mobility of Hf along the arc. Pearce et al. (1999) concluded that the absence of Hf-Nd

isotope covariation in Izu-Bonin-Mariana arc lavas indicates that Hf behaves as a conservative

element. Yet, Woodhead et al. (2001) argue that Hf may not show conservative-element

behavior in island arcs systems because the Hf isotopic compositions of Marianas, New Britain,

and Kermadec arc lavas are less radiogenic than their associated back-arc spreading centers. In

light of these conflicting models, we have examined Hf isotope compositions of Aleutian lavas

to address the mobility of Hf in the Aleutian subduction zone.

Aleutian lavas have Nd isotope compositions less radiogenic than those of Pacific

MORB (Fig. 7), but Hf isotope ratios identical to MORB (Fig. 14). This may be explained by

27

mixing of a sediment-derived fluid or melt with a MORB-source mantle. Hydrothermal

experiments indicate that subduction zone fluids are enriched in B, Cs, Li, Pb, and LREEs

(including Nd) and depleted in HFSEs (Brenan et al., 1994; You et al., 1996). Therefore,

because subducted sediment has a Nd/Hf ratio of ~ 6, fluid derived from this sediment should

have a Nd/Hf ratio greater than 6. Element partitioning experiments (i.e., Green et al., 1994)

indicate that Nd and Hf behave similarly during mantle melting. Significant sediment melt

contribution to the mantle wedge may result in lavas that have a Nd/Hf of approximately six. The

Hf-Nd isotope compositions of Seguam and Roundhead lavas are most likely derived from the

addition of a sediment-derived fluid to the MORB-like mantle source. Shishaldin lavas, however,

define a linear trend between the two endmembers which can be explained by simple mixing

between the mantle source and either bulk sediment or a sediment melt (Fig. 14). Seguam lavas

are interpreted to contain the largest sedimentary influence, which is likely due to the focusing of

sediments into the Amlia fracture zone. Because the isotopic composition of the mantle

endmember used in the mixing calculations is arbitrarily defined within the broad field of Pacific

MORB, Seguam and Roundhead lavas could be explained by the addition of sediment, sediment

melt, or a mixture of melts and fluids to the mantle wedge, however the boron concentration and

Sr and Pb isotope data presented earlier argue for a fluid transfer process. If Hf was mobilized

relative to Nd in sediment-derived fluids, these lavas would plot along a concave up mixing line

between the sediment and mantle endmembers. Because this trend is not observed for Aleutian

lavas (Fig. 14), we infer that Hf is “conserved” in the slab beneath the Aleutians during fluid

addition to the mantle wedge. Thus, we concur with Pearce et al. (1999) and You et al. (1996),

but contradict the conclusions of Woodhead et al. (2001) from the Mariana, Kermadec, and New

28

Britain arcs.

Relationship between fluid addition and partial melting of the mantle wedge

Experiments and thermodynamic modeling indicate that addition of water to spinel

lherzolite lowers its solidus temperature and leads to greater melting at a given temperature

(Kushiro, 1969; Eiler et al., 2000). Observed H2O contents, oxygen fugacities, and trace-element

ratios in Mariana, Mexican Volcanic Belt, and Cascade arc lavas broadly suggest that higher

degrees of partial melting may be linked to a greater degree of mantle hydration (Stolper &

Newman, 1994; Luhr, 1992; Grove et al., 2002). Carr et al. (1990) and Leeman et al. (1994) also

suggested that lavas which have high B/La ratios and low La/Yb ratios reflect a source that has

been enriched in subduction-related fluids and undergone large degrees of partial melting. Fluid

addition to the mantle appears to have a profound effect on the degree of partial melting of the

Aleutian mantle as well. Modal batch-melting models of a lherzolite indicate that the

compositions of Roundhead and Shishaldin lavas require a 1.5-2.0% percent partial melt of a

slightly modified MORB-source mantle (Fig. 15), whereas Seguam lavas require a larger (1-5%)

fluid addition to the mantle wedge and 22% partial melting of the fluid-enriched source. The

remarkably low HFSE abundances and low La/Yb ratios, and high B/La and B/Be ratios in

Seguam basalts (Fig. 8, Table 1, Singer et al., 1996) are consistent with a source that has been

modified by fluid addition and undergone extensive partial melting. The high degree of partial

melting beneath Seguam reflects subduction of unusually water rich materials atop the Pacific

Plate in the Amlia Fracture zone.

29

CONCLUSIONS

Based on geologic mapping, 40Ar/39Ar dating, Sr, Nd, Pb, and Hf isotope compositions,

and select trace element abundances of Pleistocene lavas from Shishaldin, Seguam, and Kanaga

volcanoes, we conclude the following:

1. Groundmass separates from low-K tholeiitic to high-K calc-alkaline lavas comprise

excellent material for 40Ar/39Ar dating of Late Pleistocene volcanic eruptions in the

Aleutian arc. The 40Ar/39Ar incremental heating method allowed us to identify a change

in the magma source tapped beneath Kanaga for a short period of time ~ 130 ka.

Moreover, we have shown that subaerial volcanism preserved at Seguam occurred over

the last 100 kyrs, an order of magnitude shorter duration than implied by previous K-Ar

dating. 2. The major, trace element, and Sr, Nd, Pb, and Hf isotope compositions of basaltic

magmas from the Seguam Island are best explained by partial melting of a mantle wedge

that has been variably modified by fluid, but it is less clear which subduction components

(e.g. bulk sediment, sediment melts, or fluids) have modified the mantle beneath

Roundhead and Shishaldin. The fluid likely comprises at least two sources, a sediment-

derived component, which contributes 10-25% of the total fluid component to the wedge,

and a much larger slab component that is derived through breakdown of serpentinized

peridotite. The volume of fluid added to the mantle wedge may reflect the high

availability of serpentinite in structures like the Amlia Fracture Zone on the downgoing

plate.

3. Roundhead and Shishaldin lavas appear to require 0.2% fluid addition and 1.5 to 2.0%

30

partial melting of a slightly fluid modified MORB source, whereas Seguam lavas reflect

1-5% fluid addition and possibly 22% partial melting of a fluid enriched source. Hf-Nd

isotope systematics of Aleutian lavas suggest that Hf most likely behaves more

conservatively than Nd during fluid addition to the mantle wedge.

4. As in other arcs such as the Marianas, Mexican Volcanic Belt, and the Cascades, the

Aleutians illustrate a strong connection between the degree of inferred source hydration,

percentage partial melting, and the major and trace element and isotopic compositions of

erupted lavas.

ACKNOWLEDGMENTS

We thank Garret Hart for helpful discussions, Anthony Koppers for his ArArCalc

software, and the staff of Oregon State University Radiation Center for support during numerous

irradiations. Jon Woodhead, Jeff Vervoort, and Pat Castillo are thanked for their critical and

helpful reviews. This research was supported by NSF grants EAR-99-80512 (Johnson), EAR-99-

03252 (Johnson), EAR-99-09309 (Singer), EAR-01-14055 (Singer), and The Louis G. Weeks

Foundation grants.

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Table 1: Whole rock major element (wt. %), trace element (ppm), and isotope compositions of Aleutian lavas. Sample SH 5 SH 15 SH 131 SH 7 SH 134 SH 130 SH 129 SH 128 KG 21 KG 34 KG 23 KG 31 Center Shishaldin Shishaldin Shishaldin Shishaldin Shishaldin Shishaldin Shishaldin Shishaldin Kanaga Kanaga Kanaga Kanaga Age (ka) < 2 28.0 30.2 713.3 1906 A.D. 198.1 383.9 SiO2 52.57 48.80 52.32 50.69 59.58 50.04 50.53 51.12 54.62 59.43 57.93 56.92 TiO2 1.56 1.19 1.94 2.36 1.09 1.49 1.49 1.26 0.81 0.66 0.81 0.79 Al2O3 17.71 15.44 16.46 16.22 16.65 17.44 17.51 19.38 17.13 17.24 16.72 17.74 FeO* 9.71 10.50 11.97 13.12 7.48 11.34 11.33 9.05 8.00 6.27 7.26 7.72 MnO 0.19 0.17 0.23 0.24 0.18 0.20 0.20 0.15 0.16 0.16 0.16 0.18 MgO 2.99 8.71 3.87 4.20 1.97 5.52 5.43 3.89 5.20 2.71 3.32 3.31 CaO 7.70 10.80 7.69 7.92 5.24 9.66 9.66 9.66 9.19 6.57 7.23 7.37 Na2O 3.81 2.47 3.78 3.59 4.70 3.09 3.14 3.42 3.06 3.68 3.60 3.67 K2O 1.00 0.80 1.12 1.16 1.58 0.67 0.69 1.22 1.37 2.27 1.90 1.87 P2O5 0.40 0.19 0.50 0.63 0.40 0.30 0.32 0.22 0.17 0.25 0.19 0.21 LOI 1.07 -0.05 -0.34 -0.32 0.72 -0.37 -0.55 0.02 0.01 0.45 0.56 0.19 Total 98.71 99.03 99.53 99.81 99.60 99.39 99.76 99.37 99.71 100.40 99.67 99.97 B 17 8 18 15.2 18.9 7.1 10.2 3.2 27 17 33.1 23.2 V 179 309 228 200 61 242 232 223 231 170 192 183 Cr - 328 - - - 31 27 - 110 - - - Co 20 36 26 24 9 28 27 22 26 18 16 17 Ni 74 96 90 - - - - - 93 55 - - Cu 44 84 40 32 - 40 48 30 82 58 29 177 Zn 78 66 76 141 97 113 98 88 55 76 116 107 Ga 19 18 20 18 16 17 16 17 18 20 17 17 Ge 1.3 1.0 1.3 1.4 1.4 1.5 1.4 1.2 1.1 1.4 1.3 1.7 Rb 24 13 25 27 34 13 13 23 34 62 42 37 Sr 477 493 421 461 390 455 442 601 367 472 383 427 Y 33.1 18.5 43.6 42.6 40.2 26.7 25.9 17.6 20.8 20.5 23.1 21.8 Zr 167 87 201 185 218 122 120 110 119 160 133 122 Nb 6.9 3.3 8.5 7.7 8.6 5.1 4.9 4.4 2.7 3.3 2.6 2.4 Cs 1.2 0.6 0.9 1.3 1.5 0.7 0.5 1.0 2.1 1.5 2.6 1.0 Ba 394 258 363 463 544 252 249 535 478 919 651 614 La 15.1 8.80 19.0 20.2 21.0 12.5 12.3 13.2 9.11 15.0 11.9 11.3 Ce 35.3 21.5 44.6 46.9 47.4 28.4 28.2 27.5 20.2 32.7 25.3 24.4 Pr 4.99 2.88 6.42 6.38 6.13 3.75 3.72 3.50 2.82 4.16 3.21 3.09 Nd 25.7 14.2 32.5 31.9 29.6 18.6 18.6 16.2 14.0 19.1 15.5 14.8 Sm 6.02 3.31 7.76 8.05 7.30 4.71 4.73 3.92 3.20 3.97 3.72 3.71 Eu 1.97 1.23 2.31 2.39 2.13 1.54 1.52 1.31 1.04 1.19 1.07 1.11 Gd 5.87 3.29 7.68 8.42 7.33 4.98 4.98 3.58 3.27 3.55 3.81 3.63 Tb 1.03 0.59 1.37 1.39 1.22 0.84 0.83 0.58 0.59 0.62 0.63 0.61 Dy 6.25 3.51 8.25 7.85 7.19 4.85 4.86 3.24 3.68 3.62 3.87 3.72 Ho 1.21 0.70 1.63 1.60 1.50 1.00 1.00 0.66 0.75 0.75 0.84 0.81 Er 3.44 1.94 4.65 4.64 4.48 2.95 2.98 1.91 2.23 2.29 2.56 2.42 Tm 0.510 0.285 0.680 0.660 0.655 0.435 0.419 0.264 0.338 0.340 0.391 0.383 Yb 3.27 1.84 4.31 4.22 4.23 2.75 2.69 1.73 2.21 2.30 2.66 2.48 Lu 0.500 0.273 0.641 0.623 0.623 0.397 0.400 0.260 0.348 0.363 0.408 0.385 Hf 4.3 2.3 5.4 4.6 5.4 3.1 3.1 2.7 3.1 4.2 3.3 3.3 Ta 0.50 0.20 0.67 0.61 0.68 0.44 0.38 0.33 0.15 0.26 0.20 0.18 Tl 0.31 0.01 0.09 0.07 0.53 0.06 0.06 0.11 0.10 0.37 0.19 0.22 Pb 9 - 7 - 10 - - - 8 11 7 10 Th 2.14 3.12 2.64 3.09 3.72 1.89 1.88 2.97 3.21 6.11 4.69 4.50 U 0.90 0.65 1.14 1.27 1.47 0.77 0.76 1.17 1.34 2.69 2.05 1.97 87Sr/86Sr 0.702961 0.702955 0.702992 - 0.703110 0.703004 0.702988 0.703081 0.703301 0.703280 - 0.703268 + 2σ + 10 + 10 + 10 - + 11 + 11 + 10 + 10 + 11 + 13 - + 11

143Nd/144Nd 0.513068 0.513046 0.513097 - 0.513042 0.513069 0.513070 0.513018 0.513041 0.513005 - 0.513033 + 2σ + 4 + 6 + 7 - + 8 + 7 + 7 + 9 + 7 + 8 - + 8

176Hf/177Hf 0.283146 0.283121 0.283163 0.283155 0.283131 0.283151 0.283161 0.283119 0.283179 0.283168 0.283170 0.283169 + 2σ + 5 + 5 + 6 + 4 + 4 + 4 + 10 + 13 + 5 + 12 + 7 + 7

206Pb/204Pb 18.804 18.789 18.755 18.778 18.779 18.766 18.769 18.866 18.799 18.788 18.818 18.778 207Pb/204Pb 15.563 15.544 15.508 15.525 15.537 15.541 15.552 15.573 15.555 15.558 15.583 15.574 208Pb/204Pb 38.241 38.304 38.117 38.178 38.243 38.256 38.304 38.416 38.346 38.355 38.432 38.383 (-) Element concentration below detection limit or no analysis performed

Table 1: continued Sample KG 13 KG 40 KG 33 KG 8 KG 3 KG 1 KG 4 KG 5 SB87-40 SB87-10 SB87-22 SB88-16 Center Kanaga Kanaga Kanaga Roundhead Roundhead Roundhead Roundhead Roundhead Seguam Seguam Seguam Seguam Age (ka) 352.0 199.1 112.5 133.9 < 10 < 10 1977 A.D. 61.4 SiO2 58.53 56.11 58.66 47.60 49.21 49.80 49.61 48.71 69.81 69.55 51.17 55.55 TiO2 0.70 0.76 0.68 1.08 0.91 0.98 1.00 1.15 0.56 0.61 0.67 0.89 Al2O3 17.04 18.02 17.21 18.58 16.14 17.79 17.41 18.72 14.47 14.51 18.20 17.74 FeO* 7.22 7.94 7.23 11.66 10.45 10.78 11.31 11.37 3.71 4.39 8.84 8.00 MnO 0.14 0.16 0.17 0.18 0.18 0.18 0.18 0.19 0.10 0.12 0.15 0.14 MgO 2.68 3.23 2.90 5.77 7.01 5.29 6.11 5.05 0.79 0.88 7.27 4.14 CaO 6.22 7.52 6.54 11.30 11.27 10.27 10.62 10.86 2.70 2.89 10.29 8.68 Na2O 3.69 3.64 3.64 2.86 2.62 2.98 2.74 2.97 4.93 5.01 2.44 3.42 K2O 2.36 2.25 2.16 0.98 1.01 1.33 1.23 1.04 2.36 2.03 0.55 0.94 P2O5 0.19 0.29 0.23 0.21 0.16 0.26 0.22 0.20 0.14 0.16 0.08 0.21 LOI 0.66 0.15 0.47 -0.60 -0.26 -0.15 -0.38 -0.43 0.43 0.18 -0.30 -0.10 Total 99.44 100.07 99.88 99.62 98.69 99.50 100.04 99.82 99.98 100.33 99.35 99.61 B 23.7 19.9 25.9 5.4 6 6.9 6.8 4.2 72 62 17 15 V 188 179 148 332 344 306 324 346 25 28 243 242 Cr - - - - 108 - 28 - - - 34 60 Co 16 17 15 31 29 28 32 29 5 3 39 18 Ni - - - - 33 - - - 49 48 100 92 Cu 103 54 56 77 157 85 68 18 24 18 104 41 Zn 112 111 101 114 44 106 108 110 55 72 76 57 Ga 17 18 17 18 18 18 18 18 19 19 18 20 Ge 1.5 1.4 1.6 1.5 0.9 1.4 1.6 1.3 1.3 1.3 1.1 1.0 Rb 39 57 53 15 13 20 14 15 58 54 12 23 Sr 405 468 441 574 581 614 620 548 184 192 315 331 Y 22.5 22.7 19.1 17.8 16.7 17.8 16.5 18.1 38.1 37.7 13.8 22.2 Zr 127 154 138 64 66 77 68 66 213 187 48 88 Nb 2.5 3.4 2.8 1.6 1.4 1.6 1.3 1.5 4.0 3.7 1.2 1.7 Cs 1.8 1.5 1.7 0.2 0.2 0.3 0.3 0.2 4.4 3.9 0.9 0.6 Ba 633 772 834 357 274 442 346 368 748 720 176 353 La 11.6 15.1 13.0 7.48 7.27 10.2 8.73 7.43 15.6 14.4 3.62 7.51 Ce 24.9 31.5 26.7 17.4 17.6 23.1 19.4 17.2 34.0 33.6 8.22 17.8 Pr 3.15 4.05 3.33 2.46 2.55 3.08 2.65 2.44 4.54 4.41 1.19 2.41 Nd 15.1 19.0 15.1 12.7 12.9 15.4 13.7 12.9 22.1 20.9 6.28 11.9 Sm 3.72 4.24 3.66 3.39 3.11 3.83 3.38 3.20 5.36 5.13 1.72 3.08 Eu 1.09 1.18 1.10 1.12 1.12 1.15 1.09 1.11 1.39 1.46 0.732 1.12 Gd 3.72 3.95 3.38 3.53 3.02 3.53 3.27 3.38 5.73 5.82 1.98 3.55 Tb 0.62 0.64 0.57 0.57 0.52 0.57 0.52 0.56 1.04 1.04 0.39 0.64 Dy 3.79 3.83 3.33 3.28 3.14 3.23 3.11 3.34 6.77 6.64 2.53 4.09 Ho 0.82 0.81 0.72 0.67 0.63 0.67 0.64 0.71 1.40 1.41 0.52 0.84 Er 2.49 2.47 2.25 2.03 1.78 2.00 1.88 2.04 4.01 4.11 1.48 2.39 Tm 0.387 0.385 0.345 0.282 0.250 0.279 0.265 0.304 0.679 0.618 0.225 0.358 Yb 2.57 2.41 2.32 1.78 1.64 1.86 1.73 1.87 4.32 4.22 1.41 2.38 Lu 0.397 0.385 0.363 0.275 0.252 0.270 0.247 0.280 0.678 0.667 0.234 0.373 Hf 3.3 4.0 3.8 2.0 2.0 2.3 2.0 1.9 6.0 5.4 1.3 2.6 Ta 0.19 0.23 0.20 0.11 0.06 0.10 0.08 0.10 0.33 0.29 0.07 0.13 Tl 0.20 0.31 0.23 - - 0.11 0.05 - 0.32 0.27 0.05 0.12 Pb 8 13 9 - - 9 - - 20 18 - 6 Th 4.59 7.73 6.43 1.88 2.38 2.89 2.39 1.85 4.27 4.15 0.84 2.30 U 2.01 3.30 2.75 0.78 0.75 1.23 0.98 0.76 2.40 2.18 0.45 0.98 87Sr/86Sr 0.703252 0.703285 0.703257 0.703045 0.703087 0.703053 - 0.703061 0.703571 0.703625 0.703700 0.703644 + 2σ + 11 + 9 + 13 + 11 + 11 + 10 - + 12 + 10 + 6 + 9 + 10 143Nd/144Nd 0.513018 0.513006 0.513019 0.513032 0.513026 0.513006 - 0.513021 0.512984 0.512991 0.512971 0.512989 + 2σ + 7 + 7 + 6 + 5 + 7 + 5 - + 5 + 7 + 7 + 8 + 7 176Hf/177Hf 0.283169 - 0.283177 0.283158 0.283155 - 0.283141 0.283169 0.283149 0.283145 0.283145 0.283144 + 2σ + 5 - + 8 + 7 + 6 - + 8 + 4 + 6 + 4 + 5 + 4 206Pb/204Pb 18.788 18.785 18.813 18.726 18.697 18.703 - 18.716 18.932 18.908 18.930 18.897 207Pb/204Pb 15.555 15.555 15.579 15.538 15.550 15.528 - 15.528 15.596 15.567 15.589 15.573 208Pb/204Pb 38.333 38.331 38.412 38.249 38.254 38.206 - 38.222 38.532 38.440 38.516 38.450

Table 1: continued Sample SB88-23 SB87-9 SB87-49 SB87-56 SJ87-79 SB88-21 SJ88-11 SJ88-6 Center Seguam Seguam Seguam Seguam Seguam Seguam Seguam Seguam Age (ka) 49.2 48.9 93.1 33.3 SiO2 53.83 53.52 56.77 69.49 50.65 55.35 56.32 51.75 TiO2 0.71 0.81 1.13 0.60 0.76 0.90 1.15 0.68 Al2O3 17.53 17.30 15.94 14.07 19.78 17.80 17.17 18.62 FeO* 8.08 8.61 9.73 4.375 8.88 9.04 9.75 8.35 MnO 0.15 0.15 0.17 0.11 0.15 0.16 0.17 0.14 MgO 5.35 5.39 3.35 0.69 6.04 4.43 3.63 7.07 CaO 9.24 9.55 7.32 2.63 11.18 8.79 7.44 10.80 Na2O 3.01 2.99 3.73 4.96 2.34 3.25 3.59 2.36 K2O 0.67 0.66 0.92 2.04 0.34 0.70 1.10 0.40 P2O5 0.09 0.10 0.15 0.155 0.07 0.11 0.16 0.08 LOI 0.19 -0.12 0.13 0.19 -0.14 -0.40 -0.08 -0.15 Total 98.83 98.95 99.33 99.305 100.03 100.12 100.39 100.11 B 29 18 34 48 16.5 26.3 35.3 17.4 V 209 254 337 19 241 242 272 193 Cr 52 55 - - - - - 88 Co 27 47 21 35 26 21 20 27 Ni 76 89 56 53 - - - - Cu 70 75 88 18 58 63 92 61 Zn 65 51 85 44 84 60 106 97 Ga 19 18 20 18 16 17 18 15 Ge 1.2 1.1 1.4 1.2 1.2 1.0 1.4 1.4 Rb 17 14 21 53 6 13 23 7 Sr 297 273 277 175 304 318 313 289 Y 18.8 16.8 25.0 36.7 11.5 16.8 22.5 13.6 Zr 73 59 86 186 31 58 88 38 Nb 1.3 0.9 2.4 4.2 0.5 1.5 1.8 0.8 Cs 1.4 0.4 1.7 1.1 0.5 0.8 1.6 0.6 Ba 251 215 325 694 130 248 356 184 La 5.62 4.24 7.08 13.4 2.45 5.32 7.38 3.67 Ce 12.8 9.86 16.2 29.5 6.11 11.5 16.2 7.64 Pr 1.80 1.40 2.26 4.13 0.82 1.51 2.12 1.06 Nd 9.24 7.60 11.6 20.3 4.45 7.56 11.0 5.41 Sm 2.45 2.08 3.12 5.20 1.42 2.24 3.15 1.63 Eu 0.951 0.845 1.15 1.54 0.652 0.883 1.08 0.702 Gd 2.76 2.37 3.64 5.56 1.76 2.60 3.35 2.01 Tb 0.52 0.46 0.69 1.02 0.32 0.47 0.63 0.38 Dy 3.40 3.07 4.40 6.62 2.15 2.89 3.80 2.29 Ho 0.70 0.63 0.93 1.35 0.46 0.63 0.82 0.50 Er 2.11 1.85 2.68 3.97 1.38 1.89 2.51 1.49 Tm 0.308 0.286 0.384 0.642 0.206 0.286 0.370 0.216 Yb 2.04 1.76 2.64 4.14 1.35 1.79 2.41 1.45 Lu 0.307 0.263 0.392 0.639 0.201 0.279 0.368 0.211 Hf 2.1 1.7 2.5 5.4 0.9 1.6 2.4 1.1 Ta 0.10 0.15 0.11 0.39 0.04 0.11 0.14 0.06 Tl 0.15 - 0.13 0.15 - - 0.15 0.05 Pb 8 - 11 - - - 7 5 Th 1.20 1.02 1.66 4.21 0.52 1.16 1.97 0.71 U 0.68 0.58 0.89 2.36 0.27 0.64 1.09 0.39 87Sr/86Sr 0.703659 0.703670 0.703660 0.703700 0.703625 0.703648 0.703713 0.703703 + 2σ + 11 + 10 + 10 + 10 + 10 + 11 + 8 + 10 143Nd/144Nd 0.512971 0.512950 0.512958 0.512948 0.512979 0.512992 0.512994 0.512989 + 2σ + 8 + 16 + 21 + 13 + 4 + 9 + 9 + 10 176Hf/177Hf 0.283123 0.283154 0.283153 0.283142 0.283179 0.283127 0.283142 0.283153 + 2σ + 4 + 5 + 6 + 6 + 10 + 13 + 5 + 5 206Pb/204Pb 18.941 18.916 18.927 18.950 18.924 18.938 18.917 18.938 207Pb/204Pb 15.584 15.580 15.575 15.600 15.622 15.591 15.578 15.584 208Pb/204Pb 38.504 38.478 38.470 38.553 38.553 38.527 38.485 38.518

Table 2: Summary of 40Ar/39Ar incremental heating experiments of Aleutian Island arc lavas Age Spectrum Isochron Analysis Weight K/Ca total fusion Increments Age (ka) SUMS 40Ar/36Ari Age (ka) Sample (mg) total age(ka) +2σ used (oC) 39Ar % + 2σ MSWD N (N-2) + 2σ + 2σ SEGUAM SB87-56 200 1.026 31.8 + 2.2 925-1140 93.6 33.0 + 1.7 a 0.12 7 of 8 0.07 297.9 + 4.2 29.6 + 6.2 200 1.071 31.4 + 3.0 935-1170 93.2 34.4 + 2.6 a 0.80 8 of 9 0.93 296.5 + 8.1 33.0 + 11.0 225 1.037 31.3 + 2.5 925-1275 100.0 32.9 + 1.8 a 1.12 5 of 5 1.07 293.9 + 2.8 35.1 + 4.4 Weighted mean plateau and combined isochron: 33.3 + 0.7 0.62 20 of 22 0.62 294.5 + 1.2 34.6 + 1.9 SB87-9 200 0.091 57.3 + 18.0 800-1260 100.0 48.2 + 12.2 a 0.55 6 of 6 0.52 296.7 + 3.0 39.5 + 22.2 200 0.105 52.7 + 14.2 850-1185 100.0 49.3 + 10.3 a 0.52 6 of 6 0.54 296.9 + 4.2 40.8 + 25.3 Weighted mean plateau and combined isochron: 48.9 + 7.9 0.49 12 of 12 0.43 296.7 + 2.4 40.7 + 16.2 SB88-23 200 0.079 54.9 + 21.1 825-1050 87.2 46.2 + 20.1 b 0.36 5 of 7 0.43 297.7 + 10.7 33.4 + 43.2 200 0.108 50.9 + 23.3 875-1150 100.0 54.3 + 22.4 b 0.74 3 of 3 0.38 297.9 + 4.6 34.1 + 41.4 200 0.077 52.5 + 13.2 850-1135 96.5 51.5 + 10.3 b 0.95 5 of 6 1.22 296.1 + 3.8 47.8 + 22.1 225 0.065 41.7 + 17.8 900-1200 100.0 42.0 + 17.0 a 0.33 4 of 4 0.25 294.3 + 3.5 50.4 + 28.5 Weighted mean plateau and combined isochron: 49.2 + 7.6 0.56 17 of 20 0.59 295.9 + 2.1 47.1 + 13. 3 SB87-63 200 0.060 56.5 + 32.8 850-1050 87.7 71.0 + 38.3 a 1.41 5 of 7 1.21 293.2 + 3.6 133.5 + 85.1 225 0.059 40.8 + 24.5 900-1100 84.6 47.6 + 21.6 a 0.28 4 of 5 0.16 294.3 + 3.3 75.7 + 77.5 375 0.057 44.5 + 29.7 875-1025 78.9 51.6 + 21.1 b 0.59 4 of 6 0.84 294.8 + 4.5 68.4 + 75.4 Weighted mean plateau and combined isochron: 52.9 + 13.7 0.82 13 of 18 0.76 294.2 + 2.0 82.7 + 37.7 SB88-18 225 0.218 66.3 + 14.0 900-1250 93.9 56.8 + 6.8 b 0.63 4 of 5 0.59 294.6 + 2.1 62.2 + 14.5 225 0.256 61.4 + 11.9 975-1240 100.0 59.1 + 9.3 b 1.23 4 of 4 1.82 295.7 + 2.0 57.9 + 16.9 Weighted mean plateau and combined isochron: 57.7 + 5.3 0.82 8 of 9 0.95 295.4 + 1.2 58.6 + 9.5 SB88-16 200 0.201 58.4 + 12.1 930-1085 72.1 63.1 + 10.8 b 0.65 5 of 8 0.77 289.2 + 23.2 96.1 + 85.0 200 0.205 57.2 + 13.7 930-1150 100.0 60.1 + 12.3 b 0.56 4 of 4 0.38 292.8 + 5.5 76.8 + 36.0 200 0.198 66.1 + 12.6 920-1130 89.6 68.4 + 13.3 b 2.77 5 of 7 1.78 301.0 + 6.2 36.9 + 27.9 225 0.237 55.5 + 7.3 970-1240 100.0 55.8 + 8.9 a 1.63 4 of 4 2.40 295.3 + 1.9 57.3 + 18.5 Weighted mean plateau and combined isochron: 61.4 + 5.5 1.53 18 of 23 1.60 295.2 + 1.4 63.4 + 10.4 SJ87-47 200 0.379 97.7 + 5.8 875-1325 100.0 98.0 + 5.4 b 0.25 5 of 5 0.28 296.0 + 2.6 95.6 + 13.2 200 0.371 90.5 + 4.6 875-1325 100.0 90.9 + 3.3 b 0.40 5 of 5 0.51 295.1 + 2.5 92.3 + 9.8 Weighted mean plateau and combined isochron: 92.8 + 2.8 0.85 10 of 10 0.89 296.1 + 1.8 90.4 + 7.3 SB87-49 200 0.149 80.3 + 25.6 900-1100 64.5 105.3 + 24.8 a 1.29 6 of 8 1.42 297.4 + 4.9 65.0 + 55.8 200 0.138 68.8 + 18.2 880-1125 62.7 106.6 + 27.5 a 0.97 6 of 10 0.88 296.6 + 2.0 77.7 + 47.2 225 0.131 75.3 + 20.2 940-1100 88.4 87.6 + 11.2 a 0.34 4 of 6 0.16 294.1 + 3.3 118.4 + 72.7 Weighted mean plateau and combined isochron: 93.1 + 9.5 1.03 16 of 24 1.04 296.3 + 1.6 75.6 + 27.0 KANAGA KG 3 200 0.202 131.9 + 6.7 900-1185 94.6 133.0 + 5.2 a 0.77 9 of 13 0.82 294.7 + 2.5 136.4 + 12.0 200 0.205 135.5 + 8.4 800-1250 100.0 137.4 + 5.9 a 0.12 6 of 6 0.03 294.7 + 2.1 140.4 + 10.7 240 0.224 124.5 + 5.4 1085-1300 81.7 132.2 + 5.6 a 1.35 4 of 6 0.40 293.7 + 2.0 139.4 + 9.2 Weighted mean plateau and combined isochron: 133.9 + 3.1 0.71 19 of 25 0.56 294.4 + 1.2 138.6 + 6.1 KG 8 200 0.185 120.2 + 9.2 955-1150 73.9 120.4 + 8.6 a 0.96 6 of 12 1.15 296.6 + 5.3 115.2 + 15.8 200 0.183 120.0 + 6.0 1000-1200 87.0 109.8 + 5.1 a 0.96 4 of 5 0.45 298.5 + 4.4 95.2 + 21.4 Weighted mean plateau and combined isochron: 112.5 + 5.1 1.34 10 of 17 1.32 297.4 + 3.7 103.5 + 18.3 KG 33 200 0.600 195.1 + 6.9 825-1275 98.0 199.1 + 2.8 a 0.43 6 of 7 0.39 295.9 + 1.2 198.1 + 3.8 200 0.683 198.0 + 4.8 950-1300 100.0 199.1 + 4.2 a 1.17 5 of 5 1.53 295.8 + 2.7 198.4 + 7.6 Weighted mean plateau and combined isochron: 199.1 + 2.5 0.68 11 of 12 0.68 295.9 + 1.0 198.2 + 2.3 KG 34 200 0.701 195.5 + 2.9 850-1090 96.1 198.5 + 2.7 a 0.08 8 of 9 0.06 294.4 + 4.5 199.5 + 4.7 200 0.622 197.8 + 2.9 850-1320 100.0 197.7 + 2.7 a 0.45 5 of 5 0.38 296.5 + 2.6 196.7 + 3.7 Weighted mean plateau and combined isochron: 198.1 + 2.1 0.21 13 of 14 0.22 296.0 + 2.2 197.6 + 3.0 KG 40 200 0.055 347.3 + 5.9 850-1200 95.7 352.0 + 3.9 a 0.29 9 of 12 0.33 295.7 + 3.2 351.6 + 6.4 KG 31 200 0.491 383.9 + 5.4 850-1270 99.7 383.9 + 4.0 a 0.54 6 of 7 0.24 294.5 + 1.4 386.7 + 5.8 SHISHALDIN SH 131 200 0.178 29.4 + 10.0 840-1220 100.0 28.9 + 6.7 a 0.68 8 of 8 0.74 296.3 + 2.7 23.5 + 14.2 200 0.178 33.7 + 10.5 875-1325 100.0 34.2 + 8.6 a 0.05 6 of 6 0.06 295.4 + 2.2 34.9 + 17.4 260 0.177 25.0 + 6.5 850-1300 100.0 24.6 + 7.1 a 1.57 5 of 5 0.15 297.1 + 1.3 10.1 + 12.2 Weighted mean plateau and combined isochron: 28.0 + 3.9 0.82 19 of 19 0.74 296.2 + 1.0 22.3 + 7.1 SH 129 200 0.119 32.9 + 10.1 800-1310 100.0 30.5 + 4.8 a 0.15 6 of 6 0.03 296.1 + 1.6 26.0 + 12.5 200 0.114 40.7 + 14.3 825-1225 91.5 27.6 + 12.4 a 0.77 4 of 5 0.77 293.3 + 3.8 48.0 + 35.2 225 0.123 31.2 + 9.8 900-1300 100.0 30.3 + 6.9 a 0.33 5 of 5 0.33 296.0 + 1.9 26.6 + 14.1

Weighted mean plateau and combined isochron: 30.2 + 3.7 0.32 15 of 16 0.33 295.7 + 1.1 28.3 + 8.6 SH 128 200 0.206 704.5 + 14.2 865-1200 95.5 713.3 + 9.6 a 0.04 8 of 11 0.04 295.8 + 2.8 711.2 + 18.2 a Ages calculated relative to 28.34 Ma Taylor Creek Rhyolite sanidine (Renne et al., 1998); uncertainties reported at 2σ precision b Ages calculated relative to 1.194 Ma Alder Creek Rhyolite sanidine (Renne et al., 1998); uncertainties reported at 2σ precision

Table 3: Sr concentrations and isotope compositions of clinopyroxene and groundmass separates from Kanaga lavas Sample material Sr (ppm) 87Sr/86Sr +2σ KG 3 wr 581 0.70309 + 1 cpx 54 0.70310 + 1 gmass 839 0.70309 + 1 KG 8 wr 582 0.70305 + 1 cpx 43 0.70309 + 1 gmass 640 0.70308 + 1 KG 34 wr 472 0.70328 + 1 cpx 25 0.70328 + 1 gmass 373 0.70328 + 1 Errors on isotope ratios are in the last decimal place

Table 4: Model parameters for source-fluid mixing calculations Component B La Sr Nd Pb 87Sr/86Sr 143Nd/144Nd 207Pb/204Pb Sediment (DSDP 183)a 100 17.1 250 18.4 12.1 0.7063 0.51264 15.625 MORB-source mantleb 0.5 2.5 21 0.815 0.3 0.7028 0.51310 15.500 Oceanic crustc 24 2.5 90 7.3 0.3 0.7028 0.51310 15.500 Crustal-derived fluidd 960 93.3 2733 151 16.9 0.7028 0.51310 15.500 Sediment-derived fluidd 4666 638 6833 380 682 0.7063 0.51264 15.625 Mobility (sediment)e 0.7 0.56 0.41 0.31 0.85 Mobility (MORB)e 0.6 0.56 0.41 0.31 0.85 a Boron concentration in sediment from Sano et al. (2001); Leeman (1996). Other sediment data from Plank & Langmuir (1998). b Mantle values from Kogiso et al. (1997). c Oceanic crust values from Moran et al. (1992); Tatsumi & Kogiso (1997). d Concentration of an element in a fluid is determined by using the equations given in Tatsumi & Kogiso (1997). e Trace element mobility from Sano et al. (2001); Kogiso et al. (1997).

Table 5: Mixing end-members and partition coefficients used in melting models. chondrited MORBe fluide element olivinea opxb cpxc spinelb Bulk D (ppm) source (ppm) Ba 0.0003 0.00001 0.00068 0 0.0003 2.34 0.288 3550 Th 0.00005 0.00001 0.01 0 0.0020 0.0294 0.006 14 U 0.00002 0.00001 0.001 0 0.0002 0.0081 0.0025 8 Nb 0.00005 0.0029 0.2 0.01 0.0413 0.246 0.200 3 Ta 0.00005 0.0029 0.053 0.01 0.0119 0.0142 0.0075 0.1 K 0.00003 0.00001 0.0072 0 0.0015 558 250 70559 La 0.00003 0.001 0.089 0.0006 0.0181 0.2347 0.150 99 Ce 0.00009 0.003 0.16 0.0006 0.0328 0.6032 0.490 150 Sr 0.0015 0.003 0.062 0 0.0139 7.8 5.770 5000 Nd 0.0004 0.009 0.36 0.0006 0.0745 0.4524 0.654 100 Sm 0.0008 0.02 0.67 0.0006 0.1394 0.1471 0.293 20 Zr 0.0008 0.04 0.27 0.07 0.0679 3.94 7.200 300 Hf 0.001 0.04 0.55 0.07 0.1240 0.104 0.200 3 Eu 0.0015 0.03 0.38 0.0006 0.0843 0.056 0.119 3 Gd 0.002 0.04 0.99 0.0006 0.2090 0.1966 0.467 7 Yb 0.02 0.1 1.43 0.0053 0.3213 0.1625 0.402 3 mode 0.5 0.25 0.2 0.05 a Partition coefficents for olivine from Green (1994) b Partition coefficents for orthopyroxene and spinel from Kelemen et al. (2003) c Partition coefficents for clinopyroxene from Blundy et al. (1998) d Chondrite values from Anders & Grevesse (1989) e MORB source and fluid compositions from Stolper & Newman (1994), Borg et al. (1997)

Figure Captions Figure 1. a) Tectonic setting of the Aleutian arc. Figure adapted from Geist et al. (1988). b) Updip sediment flux vs. Longitude plot, modified from Kelemen et al. (2003), showing the locations of the 3 volcanoes in this study. Figure 2. Several 40Ar/39Ar plateau and isochron diagrams for the Aleutian lavas showing the various ages and spectra obtained from multiple incremental heating experiments. Figure 3. Simplified geologic maps of a) Seguam and b) Kanaga Islands showing sample locations and new 40Ar/39Ar ages. Figure 4. Harker variation diagrams for Aleutian lavas. Shaded area encompasses virtually all analyses of Aleutian arc lavas as summarized in Kelemen et al. (2003) and George et al. (2003). Figure 5. Trace element variation diagrams for Aleutian lavas. Shaded area is the same as in Figure 4. Boron data from Singer et al. (1996), George et al. (2003), Class et al. (2000), and Ryan and Langmuir, 1993. Figure 6. Chondrite normalized multi-element plots of Aleutian samples. a) Shishaldin b) Seguam c) Kanaga. Rhyolites from Seguam display negative Sr and Ti anomalies whereas the more mafic lavas do not. Chondrite values from Anders & Grevesse, (1989). Figure 7. a) 207Pb/204Pb vs. 206Pb/204Pb, b) 143Nd/144Nd vs. 87Sr/86Sr, c) 176Hf/177Hf vs. 143Nd/144Nd plots for Aleutian lavas. Light gray area represents previously analyzed central and eastern Aleutian lavas. Aleutian Sr-Nd isotope data from McCulloch & Perfit, (1981); White & Patchett, (1984); Fournelle, (1988); von Drach et al., (1986); Nye & Reid, (1986); Morris & Hart, (1983); Singer et al. (1992a, b); Singer et al. (1996); Class et al. (2000); Myers et al. (2002); George et al. (2003). Aleutian Pb-Pb isotope data from Kay et al. (1978); Morris and Hart (1983); Nye and Reid (1986); Myers and Marsh (1987); Romick et al. (1990); Singer et al. (1992a, b); Miller et al. (1994); Class et al. (2000); Myers et al. (2002). Sr, Nd, and Pb isotope data for Pacific MORB is from White et al. (1987) and Hegner and Tatsumoto (1989). DSDP 183 sediment data from Plank and Langmuir (1998). d) εHf vs. εNd plot of Pacific MORB, various volcanic arcs, and ocean island basalts (OIB) worldwide. Data sources for arcs and OIB include Patchett & Tatsumoto, (1980); White & Patchett, (1984); Salters and Hart, (1991); Chauvel et al. (1992); Salters (1996); Nowell et al. (1998); Salters & White (1998); Woodhead et al. (2001) and references therein. Hf-Nd isotope data for Pacific MORB from Nowell et al. (1998); Pearce et al. (1999); and Chauvel & Blichert-Toft (2001). Two sigma errors for all isotope measurements are less than the symbol size. All Hf isotope data is normalized to JMC-475 (176Hf/177Hf = 0.28216). Figure 8. B/La, La/Yb, U/Th, 87Sr/86Sr, 143Nd/144Nd, and 207Pb/204Pb vs. longitude. Seguam lavas have higher B/La, U/Th, 87Sr/86Sr, 207Pb/204Pb ratios and lower 143Nd/144Nd and La/Yb ratios than most Aleutian lavas. Shaded area represents section of the arc affected in the last 1 myr by the subduction of the Amlia Fracture Zone. Aleutian isotope and trace element data sources same as Figure 7. White circles represent an average of published values and those from this study for the

3 volcanoes. Figure 9. a) 87Sr/86Sr, b) 206Pb/204Pb, c) 143Nd/144Nd, and d) 176Hf/177Hf versus 40Ar/39Ar age (ka) plot for Aleutian lavas. The ~113-130 ka Roundhead lavas have Sr and Pb isotope compositions which are significantly less radiogenic than the historical to 400 ka Kanaga andesites. Figure 10. B/La versus Ba/La plot for Aleutian arc lavas showing no correlation between the two ratios. Data from Singer et al. (1996), George et al. (2003), Class et al. (2000) and this study. Figure 11. 87Sr/86Sr, ) 7/4, 143Nd/144Nd and 176Hf/177Hf versus B/La and Th/Yb ratios. 87Sr/86Sr, and ) 7/4, show correlation with B/La which most likely indicates fluid involvement in magma genesis. Roundhead and Shishaldin lavas show a correlation with Th/Yb, a common indicator of bulk sediment addition or sediment melt addition to the mantle wedge. Boxes with diagonal lines represent mantle compositions. Shaded area represents Aleutian data from Singer et al. (1996) and Class et al. (2000). Figure 12. a) Th/Nd vs. SiO2 plot for Aleutian lavas showing that Th/Nd ratio is strongly affected by intracrustal differentiation of the magma. Arrows represent general differentiation trends exhibited at each volcano. b) 143Nd/144Nd vs. Th/Nd diagram adapted from Class et al. (2000) which includes only lavas with < 54% SiO2. A fluid derived from DSDP 183 sediment has Th/Nd ratios similar or slightly less than DSDP sediment. Dashed lines represent mixing fields between Pacific MORB and sediment endmembers. Seguam and most of the Shishaldin lavas lie on mixing trend between the mantle wedge and DSDP 183 sediment/fluid. Okmok, Recheshnoi, and Roundhead lavas follow the mixing trend between Pacific MORB and a sediment melt. Most of the lavas could also be explained by a mixture of MORB and both sediment components. Figure 13. a) ) 7/4, b) 87Sr/86Sr, and c) 143Nd/144Nd versus B/La plots illustrating the three components involved in Aleutian magma genesis. For parameters used in mass balance model calculations see Table 4. Sano et al. (2001) and Tatsumi & Kogiso(1997) suggested that trace-element concentrations in a slab-derived fluid (Cf) can be calculated using: Cf = C0M/F, where C0 is the original abundance of an element in the subducted sediment or oceanic crust, M is the mobility of each element in a fluid, and F is the weight fraction of hydrous fluid extracted. For calculations using the above equation, we assume F = 1.5%, which is based on high pressure experiments by Poli & Schmidt (1995). Vertical mixing lines represent percentage of fluid addition, whereas horizontal mixing lines represent various mixtures of sediment-derived and crustal-derived fluids. Figure 14. ,Hf vs. ,Nd plot of all mafic lavas from this study. Aleutian lavas lie between Pacific MORB and the average composition of DSDP 183 sediments, but all have Nd isotope compositions which are less radiogenic than Pacific MORB. Tick marks represent percentage of fluid or melt addition. Hf and Nd isotope compositions of DSDP 183 sediments from Vervoort & Plank (2002). Hf and Nd isotope compositions of Pacific pelagic sediments and clastic turbidites from Vervoort et al. (1999). Hf and Nd abundances in DSDP 183 sediments from Plank & Langmuir (1998). Hf and Nd isotope composition of Pacific MORB from Nowell et al. (1998);

Pearce et al. (1999); Chauvel & Blichert-Toft (2001) and references therein. Hf and Nd abundances in Pacific MORB from Patchett & Tatsumoto (1980) and Pearce et al. (1999). Figure 15. Partial melting and fluid addition models. Models reproduce the compositions of Aleutian lavas through the addition of fluid to source followed by modal batch melting of the fluid-enriched source. MORB source and fluid compositions from Stolper & Newman (1994); Borg et al. (1997). Normalization to chondrite values of Anders & Grevesse (1989). For Shishaldin and Roundhead, open triangles represent mixing of 0.2% fluid with 99.8% MORB. Open triangles at Seguam correspond to mixing of 1.0% fluid and 99.0% MORB. Percent fluid modification to the MORB-source at each volcano is based on the results of the three component model in Figure 12. Open diamonds on each diagram represent compositions generated by modal batch melting of a spinel lherzolite with Ol:Opx:Cpx:Spl in the proportions 50:25:20:5. a) Shishaldin and b) Roundhead lavas require 0.2% fluid addition and 1.5 to 2.0% partial melting respectively. c) Seguam lavas require at least 1.0% fluid addition and 22% partial melting. Mixing and melting parameters are listed in Table 5.

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Kanaga

Shishaldin

Seguam

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crustalcontamination

sediment meltaddition

bulk sediment/sediment-derivedfluid addition

F

SiO2

Th

/Nd

0.15

0.25

0.35

0.45

0.05

45 50 55 60 65 70

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Rou

ndhe

ad� �

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SiO2

Th

/Nd

0.15

0.25

0.35

0.45

0.05

45 50 55 60 65 70

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Recheshnoi

Rou

ndhe

ad� �

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0.5127

0.5129

0.5131

0.5133 mantle wedge

0.5125

143

144

Nd

/N

d

sedimentmeltDSDP 183 sediment or

sediment derived fluid

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anaga

Seguam/Okmok

Jicha, Singer, Brophy, Fournelle, Johnson, Beard, Lapen, MahlenFigure 12

(a)

(b)

0.5126

0.5127

0.5129

0.5130

0.5131

0.5132

0

14

31

44

Nd

/N

d

2 4 6 8 10 12

sediment-derived fluid

slab-derivedfluid

mantle wedge

B/La

0.5128

�

10

20

40

0.5 1 2 5

0.5

5

60

801

25

30

0.7020

0.7030

0.7040

0.7050

0.7060

0.7070

87

86

Sr/

Sr

sediment-derived fluid

slab-derived fluidmantle wedge

5

10

1

5

0.5

-2

0

4

6

8sediment-derived fluid

slab-derived

fluidmantle wedge

51

0.5

����

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20

30

40

60

80

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0.5 1 5

2

7/4

5

20

10

5

1

0.5

30

�

Jicha, Singer, Brophy, Fournelle, Johnson, Beard, Lapen, MahlenFigure 13

(a)

(b)

(c)

�

AverageDSDP 183

eNd

-3 3 6 90-6 12

6

9

12

15

18

eHf

3

sediment-derived

fluid with Nd/Hf >> 6

sediment melt or bulk

sediment

addition (N

d/Hf = 6) Pacific MORB

Shishaldin

Roundhead

Seguam

MORB

Pacificturbidites

Pacificpelagicsediments

sediment 3 ppm 18 ppmMORB 2.3 ppm 9.0 ppm

Hf Nd

��

�

sediment-derived

fluid withNd/Hf << 6

1%5%

10%20%

30%

30%

40%

50%

20%20%

30%

40%

10%

5%5%5%1%

�

Jicha, Singer, Brophy, Fournelle, Johnson, Beard, Lapen, MahlenFigure 14

0.1

1.0

10

100

1000

10000

MORB-source

fluid

MORB-source + 0.2% fluid

Average Shishaldin basalt

1.5% partial melt of MORB + fluid

Mo

del/ch

on

dri

te

Shishaldin

Mo

del/ch

on

dri

teM

od

el/ch

on

dri

te

Jicha, Singer, Brophy, Fournelle, Johnson, Beard, Lapen, MahlenFigure 15

(a)

0.1

1.0

10

100

1000

10000

MORB-source

fluid

MORB-source + 1.0% fluid

Average Seguam basalt

22% partial melt of MORB + fluid

Seguam

(c)

0.1

1.0

10

100

1000

10000

Ba Th U Nb Ta K La Ce Sr Nd Sm Zr Hf Eu Gd Yb

MORB-source

fluid

MORB-source + 0.2% fluid

Average Roundhead basalt

2.0% partial melt of MORB + fluid

Roundhead

(b)