SUBDUCTION MODIFIED PELAGIC SEDIMENTS AS THE ENRICHED COMPONENT IN BACK-ARC BASALTS FROM THE JAPAN SEA - OCEAN DRILLING PROGRAM SITE-797 AND SITE-794

Ocean Drilling Program Legs 127 and 128 in the Yamato Basin of the Jap an Sea, a Miocene-age back-are basin in the western Pacific Ocean, rec overed incompatible-element-depleted and enriched tholeiitic dolerites and basalts from the basin floor, which provide evidence of a signifi cant sedimentary component in their mantle source. Isotopically, the v olcanic rocks cover a wide range of compositions (e.g., Sr-87/Sr-86 = 0.70369 - 0.70503, Pb-206/Pb-204 = 17.65 - 18.36) and define a mixing trend between a depleted mantle (DM) component and an enriched compone nt with the composition of EM II. At Site 797, the combined isotope an d trace element systematics support a model of two component mixing be tween depleted, MORB-like mantle and Pacific pelagic sediments. A best estimate of the composition of the sedimentary component has been det ermined by analyzing samples of differing lithology from DSDP Sites 57 9 and 581 in the western Pacific, east of the Japan are. The sediments have large depletions in the high field strength elements and are rel atively enriched in the large-ion-lithophile elements, including Pb. T hese characteristics are mirrored, with reduced amplitudes, in Japan S ea enriched tholeiites and northeast Japan are lavas, which strengthen s the link between source enrichment and subducted sediments. However, Site 579/581 sediments have higher LILE/REE and lower HFSE/REE than t he enriched component inferred from mixing trends at Site 797. Sub-arc devolatilization of the sediments is a process that will lower LILE/R EE and raise HFSE/REE in the residual sediment, and thus this residual sediment may serve as the enriched component in the back-are basalt s ource. Samples from other potential sources of an enriched, EM II-like component beneath Japan, such as the subcontinental lithosphere or cr ust, have isotopic compositions which overlap those of the Japan Sea t holeiites and are not ''enriched'' enough to be the EM II end-member.