Tracing Subducted Sediment Inputs To The Volcanic Rocks From The Ryukyu Arc-Okinawa Trough System

Okinawa Trough is an active back-arc basin behind the Ryukyu Arc in the western Pacific.We report the geochemical and Sr,Nd isotopic compositions of volcanic rocks,which are recovered as lavas and pumices,dredged from northern Okinawa Trough.The volcanic rocks are composed of basaltic andesite,andesite,dacite and trachydacite.Both intermediate and felsic rocks are characterized by enrichments in large ion lithophile elements?LILEs?and depletion in high field strength elements?HFSEs?.Three groups of volcanic rocks have been identified.Andesite D5-1 and trachydacite D10 have identical Sr and Nd isotopic ratios,and more enriched REE patterns and incompatible element values,compared with those of basalts in middle Okinawa Trough?MOT?.Andesite D5-1 and trachydacite D10 are derived from basaltic magma by crystal fractionation.Andesite D5-2,D5-3 and D5-4have lower ⁸⁷Sr/⁸⁶Sr and higher ¹⁴³Nd/¹⁴⁴Nd than those of MOT basalts,transitional REE patterns and trace element values between Adakite rocks and MOT basalts.Features of these samples can be interpreted by mixing of the interaction magma of mantle wedge and subducted slab-melt,with evolved basaltic magma.Dacite D22,occurring as pumice,has identical trace elements,including REE,and Sr,Nd isotopic compositions with rhyolites from the southern Ryukyu volcanic front,which indicates that this pumice was not emplaced in situ and drifted from southern Ryukyu volcanic front by Kuroshio Current.Sediments are actively subducted in virtually every arc worldwide.However,quantifying their contributions to arc lavas and thereby establishing budgets of how sediments participate in slab-mantle interaction is challenging.In this contribution we use thallium?Tl?abundances and isotopic compositions of lavas from the Ryukyu arc?including south Kyushu?and its back-arc basin,Okinawa Trough,to investigate the influence of sediments from arc to back-arc.We also present extensive geochemical data for sediments and altered oceanic crust?AOC?outboard of the northern?DSDP Sites 296,442B,443 and 444?and central?DSDP Sites 294 and 295?part of the Ryukyu arc.The Tl isotopic compositions of sediments change systematically from lighter outboard of northern Ryukyu arc to heavier outboard of central Ryukyu arc.The feature reflects the dominance of terrigenous material and pelagic sedimentation outboard of the northern and central Ryukyu arc,respectively.Along-arc Tl isotopic variation of the lavas mirror those found for the sediments,which implies that sediment inputs are the main control on Tl budgets for lavas in the Ryukyu arc.Identical Tl,Sr,Nd and Pb isotope variations are found when comparing arc and back arc lavas,which indicates that sediments fluxes also account for the Tl isotopic variations in the Okinawa Trough lavas.Two-end-member mixing models of Tl with Pb,Sr and Nd isotopes require sediment inputs of<1%,0.1-1%and 0.3-2%by weight to the depleted mantle source to account for all these isotopic compositions of lavas from northern,central and southern portion of the Ryukyu arc and Okinawa Trough.Bulk mixing between mantle and sediment end members predict very similar sediment fluxes when using Tl,Sr,Nd and Pb isotopes,which indicates that fractionation of these elements must have happened after mixing between mantle and sediments.This conclusion is corroborated by model calculations of mixing between sediment melts with fractionated Sr/Nd ratios and mantle wedge,which show that no arc lava plot on such mixing lines.Thus bulk sediment mixing,rather than sediment melt,is required for the generation of the lavas from the Ryukyu arc and Okinawa Trough.The requirement of bulk sediment mixing occurring before trace element fractionation in the subarc mantle is consistent with models where mélange layers form at the top of the slab and are the principle source material for arc lavas.In addition,the fact that sediment components observed in the Ryukyu arc and Okinawa Trough lavas are similar,suggests that transport of mélange material to the source regions of the arc and back arc is equally efficient.This feature is most readily explained if mélange material is transported from the slab as diapirs.Two lavas from northern Okinawa Trough and central Ryukyu arc reveal slightly lighter Tl isotopic compositions than the mantle wedge.These samples are also characterized by adakite-like geochemical characterizations such as high Sr/Y and La/Yb ratios and less radiogenic Sr and Pb isotope compositions than lavas from same locations.Thus,these lavas might represent melts derived from high-temperature AOC or subsequent interaction of low-temperature AOC-melts with the overlying mantle wedge.