HR: 10:05h
AN: V51B-08    [Abstracts]
TI: Izu Backarc Extension-related Basalts: The Importance of Accessory Phases in Island arc Magma Genesis
AU: * Tollstrup, D L
EM: darrent@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
AU: Gill, J B
EM: jgill@pmc.ucsc.edu
AF: University of California Santa Cruz, 1156 High St., Santa Cruz, CA 95064, United States
AB: Studies of Izu's Oligocene-Miocene backarc and Quaternary volcanic front suggest that slab-derived components have been consistently partitioned between the rear arc that receives slab melts versus the volcanic front that receives only slab fluids. Th/La ratios increase whereas Ba/La ratios and Sr, Pb, Nd and Hf isotope ratios all decrease across the arc. The volcanic front mantle source is also more depleted than the mantle source of the rear arc volcanoes. More recent and widespread basaltic volcanism (3-0 Ma) is associated with “backarc” extension and rifting between the volcanic front and rear arc. The spatial and temporal variability of these basalts is used to constrain the sources and processes of magmagenesis. Extension-related backarc basalts have major element systematics that are MORB-like and similar to those of Manus and Lau�"-two backarc basins with the fastest spreading rates and highest inferred mantle potential temperatures�"-even though spreading is not yet occurring at Izu. REE patterns are also MORB-like. However, ratios of fluid-mobile elements to fluid- immobile elements (e.g., Ba/Nb) and Pb isotope ratios suggest variable but minor amounts of slab-derived fluid components. Nd and Hf isotope systematics and Hf*/Hf and Ce*/Ce ratios are consistent with some slab-derived sediment melt contribution but, if so, then Hf*/Hf, Zr/Hf, La/Nb, and Th/La ratios require trace accessory phases (e.g., ilmenite, rutile, and zircon) to be present in either the subducting slab or mantle wedge, or both. Distinguishing between pre-existing and subduction-related mantle heterogeneities is still problematic, given the small contributions of sediment inferred from slab-mantle mixing calculations, but trace accessory phases such as ilmenite would be required in the mantle wedge even if sediment is not the source of elevated REE/HFSE ratios.
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, and Petrology [V]
MN: 2007 Joint Assembly