HR: 1400h
AN: V23B-05    [Abstracts]
TI: Re-Os Isotope Systematics in Subduction Zones: Izu, Indonesia and Kamchatka
AU: * Widom, E
EM: widome@muohio.edu
AF: Miami University, Dept. of Geology, 114 Shideler Hall, Oxford, OH 45056, United States
AU: Meng, Q
EM: mengq@muohio.edu
AF: Miami University, Dept. of Geology, 114 Shideler Hall, Oxford, OH 45056, United States
AB: The Re-Os isotope system has the potential to serve as a sensitive tracer of crustal recycling processes during subduction due to the extremely radiogenic signatures of most sediment and altered oceanic crust compared to depleted mantle. Slab-derived fluid-fluxing of the mantle wedge should impart a radiogenic Os isotope signature to the arc mantle and associated arc magmas if Os is sufficiently mobile in slab-derived fluids. Results from several studies of arc mantle xenoliths show that some arc mantle is radiogenic in Os compared to non-arc mantle despite depleted Re/Os ratios, suggesting that radiogenic slab Os may be mobile in some subduction environments. Most arc volcanic rocks also are radiogenic in Os, but determining the cause of the radiogenic Os is complicated by very low Os abundances in most arc magmas, which make them highly susceptible to minor amounts of crustal assimilation. Potential subduction-related Os isotope signatures are thus easily overprinted. Recent studies of arc lavas from numerous subduction systems demonstrate low Os abundances and radiogenic Os in arc magmas globally, interpreted to reflect either crustal assimilation or radiogenic slab-derived Os in the mantle wedge. In order to further address the origin of radiogenic Os in arc lavas, our studies in Izu, Indonesia and Kamchatka focus on the most MgO-rich samples available. In general, Os isotope ratios are negatively correlated with MgO and Os concentrations. In each arc Os abundances and isotope ratios exhibit an extreme range (Izu backarc: 1- 130 ppt Os, ¹⁸⁷Os/¹⁸⁸Os = 0.131-0.734; Indonesia: 1-360 ppt Os, ¹⁸⁷Os/¹⁸⁸Os = 0.135 - 0.788; Kamchatka: 0.5-278 ppt Os, ¹⁸⁷Os/¹⁸⁸Os = 0.138-1.24). The least radiogenic Os isotope signature in each arc is similar (0.131-0.138), and despite relatively high Os concentrations in these magmas, all are more radiogenic than depleted or primitive upper mantle, but within the range found in arc peridotites. In all three arcs, two sample groups can be defined with a gap in ¹⁸⁷Os/¹⁸⁸Os between groups. We suggest that the Os isotope signatures of the more radiogenic sample groups from Izu, Indonesia and Kamchatka are caused by crustal assimilation, but propose that the less radiogenic sample group from each arc may record radiogenic slab-derived Os in the respective mantle sources.
DE: 1031 Subduction zone processes (3060, 3613, 8170, 8413)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
SC: Volcanology, Geochemistry, and Petrology [V]
MN: 2007 Joint Assembly