THE CHEMICAL-COMPOSITION OF SUBDUCTING SEDIMENT AND ITS CONSEQUENCES FOR THE CRUST AND MANTLE

Subducted sediments play an important role in are magmatism and crust- mantle recycling. Models of continental growth, continental compositio n, convergent margin magmatism and mantle heterogeneity all require a better understanding of the mass and chemical fluxes associated with s ubducting sediments. We have evaluated subducting sediments on a globa l basis in order to better define their chemical systematics and to de termine both regional and global average compositions. We then use the se compositions to assess the importance of sediments to are volcanism and crust-mantle recycling, and to re-evaluate the chemical compositi on of the continental crust. The large variations in the chemical comp osition of marine sediments are for the most part linked to the main l ithological constituents. The alkali elements (K, Rb and Cs) and high field strength elements (Ti, Nb, Hf, Zr) an closely linked to the detr ital phase in marine sediments; Th is largely detrital but may be enri ched in the hydrogenous Fe-Mn component of sediments; REE patterns are largely continental, but abundances are closely linked to fish debris phosphate; U is mostly detrital, but also dependent on the supply and burial rate of organic matter; Ba is linked to both biogenic barite a nd hydrothermal components; Sr is linked to carbonate phases. Thus, th e important geochemical tracers follow the lithology of the sediments. Sediment lithologies are controlled in turn by a small number of fact ors: proximity of detrital sources (volcanic and continental); biologi cal productivity and preservation of carbonate and opal; and sedimenta tion rate. Because of the link with lithology and the wealth of lithol ogical data routinely collected for ODP and DSDP drill cores, bulk geo chemical averages can be calculated to better than 30% for most elemen ts from fewer than ten chemical analyses for a typical drill core (100 -1000 m). Combining the geochemical systematics with convergence rate and other parameters permits calculation of regional compositional flu xes for subducting sediment. These regional fluxes can be compared to the compositions of are volcanics to asses the importance of sediment subduction to are volcanism. For the 70% of the trenches worldwide whe re estimates can be made, the regional fluxes also provide the basis f or a global subducting sediment (GLOSS) composition and flux. GLOSS is dominated by terrigenous material (76 wt% terrigenous, 7 wt% calcium carbonate, 10 wt% opal, 7 wt% mineral-bound H2O+), and therefore simil ar to upper continental crust (UCC) in composition. Exceptions include enrichment in Ba, Mn and the middle and heavy REE, and depletions in detrital elements diluted by biogenic material (alkalis, Th, Zr, Hf). Sr and Pb are identical in GLOSS and UCC as a result of a balance betw een dilution and enrichment by marine phases. GLOSS and the systematic s of marine sediments provide an independent approach to the compositi on of the upper continental crust for detrital elements. Significant d iscrepancies of up to a factor of two exist between the marine sedimen t data and current upper crustal estimates for Cs, Nb, Ta and Ti. Sugg ested revisions to UCC include Cs (7.3 ppm), Nb (13.7 ppm), Ta (0.96 p pm) and TiO2 (0.76 wt%). These revisions affect recent bulk continenta l crust estimates for La/Nb and U/Nb, and lead to an even greater cont rast between the continents and mantle for these important trace eleme nt ratios. GLOSS and the regional sediment data also provide new insig hts into the mantle sources of oceanic basalts. The classical geochemi cal distinction between 'pelagic' and 'terrigenous' sediment sources i s not valid and needs to be replaced by a more comprehensive understan ding of the compositional variations in complete sedimentary columns. In addition, isotopic arguments based on surface sediments alone can l ead to erroneous conclusions. Specifically, the Nd/Hf ratio of GLOSS r elaxes considerably the severe constraints on the amount of sediment r ecycling into the mantle based on earlier estimates from surface sedim ent compositions. (C) 1998 Elsevier Science B.V. All rights reserved.