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

JOURNAL OF PETROLOGY VOLUME 45 NUMBER 9 PAGES 1845–1875 2004 DOI: 10.1093/petrology/egh036 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 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 ADVANCE ACCESS PUBLICATION AUGUST 5, 2004 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. *Corresponding author. Telephone: 001-608-265-8650. Fax: 001-608262-0693. E-mail: Journal of Petrology 45(9) # Oxford University Press 2004; all rights reserved KEY WORDS: Aleutian island arc; 40 Ar/39Ar dating; fluids; Hf isotopes; magma sources INTRODUCTION 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 JOURNAL OF PETROLOGY VOLUME 45 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; R€ upke 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 NUMBER 9 SEPTEMBER 2004 et al., 1992c) Pleistocene–Recent lavas from the three centers are geochronologically constrained on the basis of 17 new 40Ar/39Ar ages determined using furn (...truncated)