Journal of Petrology

Journal of Petrology Advance Access originally published online on August 5, 2004
Journal of Petrology 2004 45(9):1845-1875; doi:10.1093/petrology/egh036

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Journal of Petrology 45(9) © Oxford University Press 2004; all rights reserved

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. SINGER^1,*, J. G. BROPHY², J. H. FOURNELLE¹, C. M. JOHNSON¹, B. L. BEARD¹, T. J. LAPEN¹ and 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

RECEIVED JANUARY 3, 2003; ACCEPTED APRIL 7, 2004

	ABSTRACT

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Major and trace element compositions and Sr, Nd, Pb, and Hf isotope ratios of 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 indicates that the lavas erupted mainly during the last 400 kyr. Along-arc geochemical and isotopic variations are consistent with variable degrees of fluid input to the mantle wedge. Addition of bulk sediment, partially melted sediment, or a combination of sediment and fluid components may also explain the major and 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 the 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 that can be matched by a 22% partial melt of a fluid-modified mantle source, 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

	INTRODUCTION

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Trace element, isotopic and experimental studies of arc lavas suggest that the transfer of elements from the subducted plate 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) increasingly have been 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 understand better 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, 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 in 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 mid-ocean ridge basalt (MORB) (Kay et al., 1978; Brophy & Marsh, 1986; Yogodzinski et al., 1995; Kelemen et al., 2003). High-pressure trace element partitioning experiments such as those of Tatsumi et al. (1986), Ulmer & Trommsdorf (1995), Keppler (1996), Kogiso et al. (1997) and Brenan et al. (1998) have emphasized the importance of a fluid component derived from dehydration of minerals that make up 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, and Ta are relatively immobile compared with 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, and Sr, Nd, Pb, and Hf isotope compositions of 33 lavas from Shishaldin, Seguam, Roundhead, and Kanaga volcanoes (Figs 1a and 2). These lavas span the major element range observed within the eastern and central Aleutian arc. Compositional and isotopic contrasts suggest that each volcano evolved by markedly different processes. For example, the composition of Seguam lavas is probably controlled by repeated episodes of closed-system differentiation of basalt to rhyolite (Singer et al., 1992a, 1992b), 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 assessment of changes in magma sources over the last c. 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 that forms 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 melt 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.

View larger version (39K): [in this window] [in a new window]   Fig. 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 three volcanoes in this study.

 

View larger version (87K): [in this window] [in a new window]   Fig. 2. Simplified geological maps of (a) Kanaga and (b) Seguam islands showing sample locations and new 40Ar/39Ar ages.

 

	TECTONIC SETTING

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
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 arc 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). A back-arc spreading zone has never developed, thus distinguishing the Aleutians from many 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-parallel extension (Geist et al., 1987, 1988). Seismic reflection and refraction data indicate that the sub-arc crust is 25–30 km thick (Fleidner & Klemperer, 1999; Holbrook et al., 1999); earlier gravity and seismic refraction data suggested that the thickness of the Aleutian arc crust 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 basalt 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; however, 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 thick where the AFZ intersects the trench (Scholl et al., 1982). Maximum sediment flux beneath the arc, estimated by Kelemen et al. (2003), also occurs at this location (Fig. 1b). Where fracture zones have been sampled directly through dredging, drilling, or 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 the subduction zone may release up to 13 wt % H₂O (Ulmer & Trommsdorf, 1995).

At the oceanic–continental arc transition (163°W), 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 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 or obliquity (Yogodzinski et al., 1995; George et al., 2003; Kelemen et al., 2003); (4) variations in the downgoing plate (i.e. fracture zones) (Kay, 1980; Singer et al., 1992a, 1992b, 1996; Miller et al., 1994).

	GEOLOGY OF THE VOLCANIC CENTERS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Shishaldin
Shishaldin, the largest (300 km³) and highest (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 occurred in April and May 1999. Shishaldin was mapped at a reconnaissance level by Finch (1934), and Fournelle (1988) conducted a petrological study of the volcano. On the basis of 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 at 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 lavas with up to 71 wt % SiO₂ (Singer et al., 1992a, 1992b, 1992c). Pyre Peak, a basaltic cinder cone, is the highest of the centers, rising to 1054 m (Fig. 2b).

Deeply glaciated Late Pleistocene lavas and tephra 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. On the basis of 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, 1992c) suggested that tholeiitic basalt underwent repeated episodes of closed-system differentiation to produce rhyolite beneath Seguam over the past 1 Myr.

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 (Fig. 2a). 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 to 1995 produced blocky lava flows and debris avalanches that covered the northwestern flank.

The geological history of Kanaga is known mainly from field mapping and relative stratigraphic relations determined by Coats (1952, 1956). Brophy (1990) and Brophy et al. (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 800 m high arcuate ridge composed 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 ±2 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 earlier Mt. Kanaton caldera. Singer et al. (1992c) concluded that the oxygen isotope disequilibrium and heterogeneity of Kanaga lavas reflects fractionation, assimilation of crust, and magma mixing during petrogenesis, consistent with the petrological 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 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

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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), eight basaltic andesites (52–56 wt % SiO₂), nine andesites (56–62 wt % SiO₂), and three 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: Fo_92–93 and Fo_72–74. High-alumina basalts (HAB) have 35–50 modal % phenocryts of plagioclase (30–45%), olivine (<5%), and rare clinopyroxene. Plagioclase cores range from An_77–82 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 contain 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 (23–34%), 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

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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 to 1325°C. System blanks were measured prior to each experiment at temperatures between 800 and 1200°C. These signals were 6 x 10^–19 moles for ³⁶Ar and 1·5 x 10^–16 for ⁴⁰Ar, which are 1–2 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 & 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. 3).

View larger version (48K): [in this window] [in a new window]   Fig. 3. 40Ar/39Ar plateau and isochron diagrams for the Aleutian lavas showing the various ages and spectra obtained from multiple incremental heating experiments.

 
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 a steel jaw crusher and powdered in an alumina shatterbox. Major and trace element concentrations were measured by inductively coupled plasma (ICP) and ICP-mass spectrometry (ICP-MS) techniques, respectively, at Actlabs in Ontario, Canada. Boron concentrations were determined via prompt gamma neutron activation analysis (PGNAA). For boron, 1 g 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 10–15% for concentrations >5 ppm, and 20–25% for concentrations near 2·5 ppm. Precision of the ICP and ICP-MS data is ±0·6–4·7% for the major elements and ±4·2–5·9% for most trace elements.

View this table: [in this window] [in a new window]   Table 1: Whole-rock major element (wt %), trace element (ppm), and isotope compositions of Aleutian lavas

 
Sr, Nd, Pb, and Hf isotopes
Sr, Nd, and Pb isotope ratios were measured by thermal ionization mass spectrometry on a Micromass Sector 54 instrument 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 29 M HF and 1 ml of once distilled 14 M HNO₃ for 2 days on a hot plate. Samples were not spiked. The elements were separated by cation exchange techniques using HCl for Sr and HCl and -HIBA for Nd. A separate 100 mg aliquot of powder was dissolved in HF–HNO₃ and separated for Pb using a HBr–HCl anion exchange procedure (Johnson & Thompson, 1991).

Strontium isotopes were measured using a dynamic multi-collector analysis routine, with exponential normalization to ⁸⁶Sr/⁸⁸Sr = 0·1194. Twelve measurements of NBS 987 yielded a ⁸⁷Sr/⁸⁶Sr ratio of 0·710263 ± 0·000002 (2) and three measurements of BCR-1 averaged 0·705031 ± 0·000010 (2). Neodymium isotope ratios were measured as NdO⁺ using 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 (2), and two measurements of BCR-1 averaged 0·512643 ± 0·000005 (2). Fifteen measurements of NBS 981 and NBS 982 standards yielded a mass fractionation correction of 0·14 ± 0·013 (2) 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 29 M twice-distilled HF and 1 ml of 14 M HNO₃ and then three more times with 5 ml of 8 M HCl. Hafnium was separated from the sample matrix using Ln-spec resin following procedures modified from those of Münker et al. (2001). Complete chemical separation and Hf isotope analysis procedures have been given by Lapen et al. (2004). 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 was 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 were 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 are generally <30 ppm for ¹⁸⁰W and <0·5 ppm for ¹⁸⁰Ta. Isotope analyses were performed in static mode. No collector biases were applied beyond those determined by a constant-current gain calibration. Instrumental mass bias may be corrected for using internal normalization to a constant ¹⁷⁹Hf/¹⁷⁷Hf = 0·7325. A second correction for ‘residual mass bias’ was determined from the average of standards measured throughout each analytical session. The UW-AMES Hf standard, which was analyzed during each session, was 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 (2), ¹⁷⁸Hf/¹⁷⁷Hf = 1·46716 ± 0·000003 (2), ¹⁷⁹Hf/¹⁷⁷Hf(measured) = 0·7463 ± 0·000006 (2), ¹⁸⁰Hf/¹⁷⁷Hf = 1·88669 ± 0·000013 (2). _Hf 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 five of the 31 samples were performed. The average spread in the Hf duplicate measurements was 0·5 _Hf units.

	RESULTS

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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, Fig. 3; the complete ⁴⁰Ar/³⁹Ar data are included in Electronic Appendix 1, available at http://www.petrology.oupjournals.org). The 17 new ⁴⁰Ar/³⁹Ar age determinations add to the limited geochronological dataset for the Aleutian island arc, which consists of only 42 published K–Ar (e.g. Bingham & Stone, 1972; Romick et al., 1990; Singer et al., 1992a) and three ⁴⁰Ar/³⁹Ar (e.g. Layer, 1997) ages, most without supporting analytical data.

View this table: [in this window] [in a new window]   Table 2: Summary of 40Ar/39Ar incremental heating experiments of Aleutian island arc lavas

 
Eight ⁴⁰Ar/³⁹Ar age determinations on basaltic andesite to rhyolite from Seguam Island yielded ages between 93·1 ± 9·5 and 33·3 ± 0·7 ka (Figs 2 and 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 c. 150–180 ka older and define plateau ages of 383·9 ± 4·0 ka and 352·0 ± 3·9 ka (Figs 2 and 3, Table 2). Finally, five incremental heating experiments from two Roundhead HAB flows yielded ⁴⁰Ar/³⁹Ar mean plateau ages of 133·9 ± 3·1 ka and 130·5 ± 7·1 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-number [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 have distinctly lower K₂O, Na₂O, and P₂O₅ abundances than most lavas in the Aleutian arc (Fig. 4).

View larger version (40K): [in this window] [in a new window]   Fig. 4. Harker variation diagrams for Aleutian lavas. Shaded area encompasses virtually all analyses of Aleutian arc lavas as summarized by George et al. (2003) and Kelemen et al. (2003).

 
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 for lavas in the continental sector of the Aleutian arc (Brophy, 1987; Nye & Turner, 1990; George et al., 2003).

View larger version (42K): [in this window] [in a new window]   Fig. 5. Trace element variation diagrams for Aleutian lavas. Shaded area is the same as in Fig. 4. Boron data from Ryan & Langmuir (1993), Singer et al. (1996), Class et al. (2000) and 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 in 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. 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), slightly 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 HFSE 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.

View larger version (30K): [in this window] [in a new window]   Fig. 6. Chondrite-normalized multi-element plots of Aleutian samples. (a) Shishaldin; (b) Seguam; (c) Kanaga–Roundhead. Rhyolites from Seguam display negative Sr and Ti anomalies whereas the more mafic lavas do not. Chondrite values from Anders & Grevesse (1989).

 
Sr, Nd, and Pb isotope compositions
¹⁴³Nd/¹⁴⁴Nd ratios of Shishaldin lavas (0·51310–0·51295, _Nd = 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 between the values for 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 North 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 lavas. Among the lavas measured here, the most radiogenic Pb isotope ratios are from Seguam and the lowest are from 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).

View larger version (44K): [in this window] [in a new window]   Fig. 7. (a) 207Pb/204Pb vs 206Pb/204Pb, (b) 143Nd/144Nd vs 87Sr/86Sr and (c) 176Hf/177Hf vs 143Nd/144Nd for Aleutian lavas. Light gray area in (a) and (b) represents previously analyzed central and eastern Aleutian lavas. Aleutian Sr–Nd isotope data from McCulloch & Perfit (1981), Morris & Hart (1983), White & Patchett (1984), Nye & Reid (1986), von Drach et al. (1986), Fournelle (1988), Singer et al. (1992a, 1992b, 1996), Class et al. (2000), Myers et al. (2002) and George et al. (2003). Aleutian Pb–Pb isotope data from Kay et al. (1978), Morris & Hart (1983), Nye & Reid (1986), Myers & Marsh (1987), Romick et al. (1990), Singer et al. (1992a, 1992b), Miller et al. (1994), Class et al. (2000) and Myers et al. (2002). Sr, Nd, and Pb isotope data for Pacific MORB are from White et al. (1987) and Hegner & Tatsumoto (1989). DSDP 183 sediment data from Plank & Langmuir (1998). (d) 176Hf/177Hf vs 143Nd/144Nd 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 & 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 are normalized to JMC-475 (176Hf/177Hf = 0·28216).

 

View larger version (37K): [in this window] [in a new window]   Fig. 8. B/La, La/Yb, U/Th, 87Sr/86Sr, 143Nd/144Nd, and 207Pb/204Pb vs longitude (W). Seguam lavas have higher B/La, U/Th, 87Sr/86Sr and 207Pb/204Pb ratios, and lower 143Nd/144Nd and La/Yb ratios than most Aleutian lavas. Shaded area represents the section of the arc affected in the last 1 Myr by the subduction of the Amlia Fracture Zone (AFZ). Aleutian isotope and trace element data sources are the same as for Fig. 7. , average of published values and those from this study for the three volcanoes.

 
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 (_Hf = +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

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
K–Ar vs ⁴⁰Ar/³⁹Ar ages of Seguam lavas
Whereas 11 whole-rock K–Ar age determinations suggest an 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 probably the result of very 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) as a result of decompression and volatile exsolution. The new isotopic and age data (Fig. 9) reveal that 133 ka Roundhead basalt has distinctly low ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb ratios compared with both older and younger basaltic andesites and andesites at the adjacent Kanaga volcano.

View larger version (22K): [in this window] [in a new window]   Fig. 9. (a) 87Sr/86Sr, (b) 143Nd/144Nd, (c) 206Pb/204Pb, and (d) 176Hf/177Hf vs 40Ar/39Ar age (ka) for Aleutian lavas. The 113–130 ka Roundhead lavas have Sr and Pb isotope compositions that are significantly less radiogenic than the historical to 400 ka Kanaga andesites.

 
Because textural, oxygen isotope, and chemical evidence suggests 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 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, to generate basaltic andesite like that erupted from 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.

View this table: [in this window] [in a new window]   Table 3: Sr concentrations and isotope compositions of clinopyroxene and groundmass separates from Kanaga lavas

 
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. 9). 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; Kay & Kay, 1985; Miller et al., 1992). This could reflect intrinsic variability of the mantle wedge, or recent focusing of large slab-derived additions to the wedge beneath the major stratovolcanoes such as Kanaga relative to smaller volcanoes such as 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 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 the 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-number <0·4) in our discussion on mantle source characteristics.

Role of sediment in the magma source
In their pioneering study 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 & Marsh, 1987; Morris et al., 1990; Singer et al., 1992b; Miller et al., 1994). 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 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 the addition of either 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 those of 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) noted that because Ba/La is positively correlated with Th/La ratios in Aleutian arc lavas, this ratio cannot be used to distinguish between fluid-rich and sediment melt components, and suggested 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).

View larger version (17K): [in this window] [in a new window]   Fig. 10. B/La vs Ba/La plot for Aleutian arc lavas showing no correlation between the two ratios. Data from Singer et al. (1996), Class et al. (2000), George et al. (2003) and this study.

 
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 those of Pacific MORB (0·10–0·42) whereas Shishaldin and Roundhead lavas have U/Th ratios that overlap Pacific MORB values. 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 ¹¹B isotope compositions of arc lavas also provide strong evidence for fluid addition to the sub-arc mantle. Basalts from Seguam Island have elevated boron contents, ¹¹B values between 1·9 and 3·5, and B/Nb ratios up to 33, which are significantly greater than values for Pacific MORB or DSDP (Deep Sea Drilling Project) 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, 2002). In contrast, Kanaga and Shishaldin lavas have low B/Nb ratios and lower ¹¹B 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 exhibit 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 that 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.

View larger version (31K): [in this window] [in a new window]   Fig. 11. 87Sr/86Sr, 7/4Pb, 143Nd/144Nd and 176Hf/177Hf vs B/La and Th/Yb. Arrows indicate that 87Sr/86Sr, 7/4Pb, and 143Nd/144Nd show correlation with B/La, which probably indicates fluid involvement in magma genesis. Circled 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. Dark gray boxes represent mantle compositions. Pale gray shaded area represents Aleutian data from Singer et al. (1996) and Class et al. (2000).

 
¹⁴³Nd/¹⁴⁴Nd–Th/Nd variations
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 (Plank & Langmuir, 1993; Class et al., 2000). Class et al. (2000) proposed that the melt has a sediment-like isotopic composition for Nd and Pb and high Th/Nd and Th/Yb ratios compared with 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.

View larger version (29K): [in this window] [in a new window]   Fig. 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 to or slightly less than those of DSDP sediment. Dashed lines represent mixing fields between Pacific MORB and sediment end-members. Seguam and most of the Shishaldin lavas lie on a 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.

 
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 that contained relatively unradiogenic Pb and low Ce/Pb ratios. Kelemen et al. (2003) pointed 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) suggested 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 migration of two fluids from the subducted slab to the mantle wedge (Fig. 13, Table 4). 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%).

View larger version (24K): [in this window] [in a new window]   Fig. 13. (a) 7/4Pb, (b) 87Sr/86Sr and (c) 143Nd/144Nd vs B/La, illustrating the three components involved in Aleutian magma genesis. Parameters used in mass balance model calculations are given in Table 4. Tatsumi & Kogiso (1997) and Sano et al. (2001) suggested that trace element concentrations in a slab-derived fluid (Cf) can be calculated using the equation 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 = 0·5%, 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 slab-derived fluids.

 

View this table: [in this window] [in a new window]   Table 4: Model parameters for source–fluid mixing calculations

 
The proportions of fluid addition to the mantle wedge in this model are comparable with 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 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). On the basis of similar modeling, Class et al. (2000) suggested that the mantle wedge beneath Umnak Island in the Aleutian island arc experienced 0·1–3·2% 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 that commonly exposes large areas of peridotite at the seafloor (Bonatti & Crane, 1973), where reaction with seawater causes serpentinization (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.

HFSE 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. However, Woodhead et al. (2001) argued that Hf may not show conservative-element behavior in island arc systems because the Hf isotopic compositions of Marianas, New Britain, and Kermadec arc lavas are less radiogenic than those of 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 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 LREE (including Nd) and depleted in HFSE (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 >6. Element partitioning experiments (i.e. Green, 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 6. The Hf–Nd isotope compositions of Seguam and Roundhead lavas are probably 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 end-members that 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 probably due to the focusing of sediments into the Amlia Fracture Zone. Because the isotopic composition of the mantle end-member 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 above 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 end-members. 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 You et al. (1996) and Pearce et al. (1999), but contradict the conclusions of Woodhead et al. (2001) from the Mariana, Kermadec, and New Britain arcs.

View larger version (28K): [in this window] [in a new window]   Fig. 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 that are less radiogenic than Pacific MORB. Each arrow represents a mixing curve between Pacific MORB and a sediment-derived end-member. (See text for detailed explanation of each sediment-derived end-member.) Tick marks represent the 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).

 
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 (Luhr, 1992; Stolper & Newman, 1994; Grove et al., 2002). Carr et al. (1990) and Leeman et al. (1994) also suggested that lavas that 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% partial melt of a slightly modified MORB-source mantle (Fig. 15, Table 5), 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.

View larger version (33K): [in this window] [in a new window]   Fig. 15. Chondrite-normalized trace element variation diagrams that model partial melting and fluid addition. Models reproduce the compositions of Aleutian lavas through the addition of fluid to the mantle source followed by modal batch melting of the fluid-enriched source. MORB-source and fluid compositions from Stolper & Newman (1994) and Borg et al. (1997). Normalization to chondrite values of Anders & Grevesse (1989). For Shishaldin and Roundhead: , mixing of 0·2% fluid with 99·8% MORB-source mantle. For Seguam: , mixing of 1·0% fluid and 99·0% MORB-source mantle. Percent fluid modification to the MORB source at each volcano is based on the results of the three-component model in Fig. 13. , melt 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 1·5–2·0% 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.

 

View this table: [in this window] [in a new window]   Table 5: Mixing end-members and partition coefficients used in melting models

 

	CONCLUSIONS

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
On the basis of geological 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 are 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 at 130 ka. Moreover, we have shown that subaerial volcanism preserved at Seguam occurred over the last 100 kyr, an order of magnitude shorter duration than implied by previous K–Ar dating.

(2) The major and trace element, and Sr, Nd, Pb, and Hf isotope compositions of basaltic magmas from 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 probably comprises at least two sources, a sediment-derived component that 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 such as the Amlia Fracture Zone on the downgoing plate.

(3) Roundhead and Shishaldin lavas appear to require 0·2% fluid addition and 1·5–2·0% 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 probably 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.

	SUPPLEMENTARY DATA

TOP ABSTRACT INTRODUCTION TECTONIC SETTING GEOLOGY OF THE VOLCANIC... SAMPLE DESCRIPTION AND... ANALYTICAL METHODS RESULTS DISCUSSION CONCLUSIONS SUPPLEMENTARY DATA REFERENCES

 
Supplementary data for this paper are available on Journal of Petrology online.

	ACKNOWLEDGEMENTS

 
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.

	FOOTNOTES

 

^* Corresponding author. Telephone: 001-608-265-8650. Fax: 001-608-262-0693. E-mail: bsinger{at}geology.wisc.edu

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