HR: 09:45h
AN: V51G-08    [Abstracts]
TI: High Field Strength and Rare Earth Element Systematics of the Kurile arc.
AU: * Tollstrup, D L
EM: darrent@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth and Planetary Sciences, Santa Cruz, CA 95064, United States
AU: Gill, J B
EM: jgill@pmc.ucsc.edu
AF: University of California Santa Cruz, Earth and Planetary Sciences, Santa Cruz, CA 95064, United States
AU: Dreyer, B
EM: dreyer@levee.wustl.edu
AF: Washington University, Earth and Planetary Sciences, Saint Louis, MO 63130, United States
AB: The depletion of high field strength elements (HFSE) relative to other trace elements with similar mantle/melt partition coefficients (e.g., rare earth elements; REE) is characteristic of subduction-related volcanic rocks. Although this behavior is common to most western pacific arcs (e.g., Mariana, Izu, Japan, Kamchatka arcs) the exact cause of HFSE depletions is still debated. This debate centers on the relative importance of mass transfer processes from the subducting slab to the overlying mantle wedge: how important are fluids versus melts in slab-mantle mixing in arcs? New Hf and Nd isotope and HFSE concentration data for lavas erupted from the length of the Kurile arc, both at the volcanic front and within the backarc, are reported. Relative to the Mariana, Izu, and Kamchatka arcs, the Kurile arc is characterized by more radiogenic Nd and Hf isotope ratios at the volcanic front. Concentrations and ratios of HFSE and REE are similar to those reported for the Izu and much of the Kamchatka arcs. When corrected for the effect of slab contributions, the Kurile arc is isotopically "Indian". Across-arc variations are also similar, suggesting that the nature of slab contributions changes with depth. Lavas erupted in the backarc have lower Nd and slightly lower Hf isotope ratios, and lower Ba/Th and Hf/Sm, than those at the volcanic front. Slab-mantle mixing can account for these isotope and trace element observations if HFSE are effectively fractionated from REE. Attributing HFSE/REE depletions in arcs to the relative fluid-immobility of HFSE relative to REE cannot explain both the isotope and concentration data, because neither the HFSE nor the REE are fluid mobile enough to be added to the mantle wedge in any substantial quantity. Hf and Nd isotope ratios and HFSE/REE concentration data require a melt transport agent, in which case HFSE are fractionated from REE by the presence of trace quantities of residual accessory phases (rutile and zircon) in the subducting slab. Slab melt is, therefore, ubiquitous along and across the Kurile arc.
DE: 1009 Geochemical modeling (3610, 8410)
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting