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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: [email protected] mail to: [email protected]
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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
4
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
5
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
7
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
8
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.
9
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
10
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
12
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
13
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
14
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.
15
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
16
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.
NorthAmerican Plate
Kanaga
Shishaldin
Seguam
7-8 m/year
Okmok
Little Sitkin
AdakAtka
Recheshnoi
Komandorsky Islands
Pacific Plate
Alaska
Ala
skaPen
insu
la
Ratfracturezone
Amliafracturezone
Adakfracturezone
25
26
27
29
3031
2526
27
2526
28
2425
28
28
6600 m6600 m
AleutianTrench
Quaternary volcanic center
Eocene-Miocene arc rocks
Fracture zone
Magnetic anomalies25
4000 km
DSDP 183
50 No
55 No
170
Wo
175
Wo
180
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175
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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
��
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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)