VARIABLE IMPACT OF THE SUBDUCTED SLAB ON ALEUTIAN ISLAND ARC MAGMA SOURCES: EVIDENCE FROM SR, ND, PB, AND HF ISOTOPES AND TRACE ELEMENT ABUNDANCES

Abstract

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.