HR: 1340h
AN: V43D-1626    [Abstracts]
TI: Mantle Wedge Processes from Primitive Lavas in the Western Aleutians
AU: * Yogodzinski, G M
EM: gyogodzin@geol.sc.edu
AF: University of South Carolina, 701 Sumter St., Columbia, SC 29208, United States
AU: Kelemen, P B
EM: peterk@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades, NY 10964, United States
AU: Wyatt, W C
EM: cwyatt@geol.sc.edu
AF: University of South Carolina, 701 Sumter St., Columbia, SC 29208, United States
AB: The 2005 Western Aleutian Volcano Expedition mapped more than 19,000 sq km of Aleutian seafloor and dredged >4000 kg of fresh lava from volcanic cones and other young, constructional features along more than 1600 km of arc length. The most interesting feature is the field of active seafloor volcanism discovered in the Ingenstrem Depression - a rectangular basin, approximately 60 km in length by 10-15 km wide along the Aleutian Ridge west of Buldir island, the western-most emergent Aleutian volcano. We found more than 90 volcanic cones and associated lava flows within and on the margins of this basin, which is bounded by WNW-oriented strike-slip faults. The largest cones are 2-4 km in diameter and 300-500 m high. Some appear to be offset by strike-slip faulting, but most are un-deformed, constructional features. A striking feature of Ingenstrem dredge samples is that they include an unusually large proportion of primitive lavas with whole-rock Mg#>0.60, including basalt, andesite and dacite. Basalts and basaltic andesites have moderately enriched trace element patterns (e.g., La/Yb 4-8, Sr/Y<30) and relatively radiogenic Sr (87/86Sr=0.7031-0.7034), typical of lavas from the Aleutians and other island arcs. In contrast, Ingenstrem primitive andesites and dacites have high Sr (700-2300 ppm) and strongly fractionated trace element patterns (Sr/Y>50), with low Y (<12 ppm) and HREE. Among these, andesites and dacites with high Sr/Y also have systematically higher SiO2, lower Y, and higher Mg#, Cr and Ni. Strontium isotope ratios in Ingenstrem lavas are inversely correlated with Sr/Y and SiO2, so the most felsic samples (66-67% SiO2) have the most fractionated trace element patterns (Sr/Y>120) and the least radiogenic Sr (87Sr/86Sr<0.7029). Interestingly, samples that span the full range of geochemical variability are found in close spatial association within the Ingenstrem Depression. In one case, primitive lavas within an individual dredge span almost the entire range of SiO2 and Sr/Y. The small length scale of compositional variation in primitive lavas implies significantly different melt temperatures (basalts ~1250C vs dacites ~1000C) and source compositions (87Sr/86Sr=0.7027-0.7034), over short distances at sub-Moho depths. This, in turn, requires advective transport of cold material through a hot mantle wedge `cold plumes': e.g., Ringwood, JGSL 74; Marsh AJS 76; Kelemen et al. ToG 03; Gerya & Yuen EPSL 03) or advective transport of hot material through a cold shallow mantle conductive layer (`hot fingers': e.g., Tatsumi et al. JGR 03; Tamura et al EPSL 02). In the first case, primitive dacites with high Sr/Y could be formed by reaction between low T melts of subducting eclogite and hotter residual peridotite in the wedge. In the second case, primitive dacites could be formed by reaction between fractionating basalt in sub-crustal magma chambers and surrounding, colder residual mantle in the lithosphere.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1037 Magma genesis and partial melting (3619)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting