Sediment subduction: geochemical implications for the sub-arc mantle
Sediment subduction regulates the volume of the continental crust and has important implications for the geochemistry for the sub-arc mantle and ultimately, arc magmas. Arc magmas are estimated to contain only 3-6 wt.% sediment component, however isotopic studies and geochemical correlations between arc-input and arc-output emphasize the importance of this small ‘slab component’ to the unique signature of arc magmas: in particular, their enrichment in H2O, K, LILE, LREE, Th and U, compared to N-MORB and primitive mantle. Sediments are the main sink of LILE in the subducted slab, with recycling occurring through the release of fluids generated in the fore-arc (<70 km depth) or fluids/melts in the sub-arc mantle (70-170 km depth). The specific nature of the phase (aqueous fluid vs silicate melt vs supercritical fluid) and the composition of such fluids, however, remains unclear. Even less is known to what extend fluids/melts infiltrate and react with surrounding mantle rock and more importantly, how their signature is transported to the locus of wet-melting in the mantle wedge. A series of piston-cylinder experiments were conducted at P-T conditions relevant to the sub-arc mantle (800-1100 °C, 2.0 GPa) to investigate sediment recycling processes through the generation of C-O-H fluid and silicate partial-melts, secondly, how these fluids/melts react with harzburgite. This study characterises modal and cryptic metasomatism by melts and C-O-H fluid, emphasizing the importance of the former for the recycling of LILE, Th, U and the latter for the recycling of H2O, Cl and chalcophile elements (Ag, As, Cu, Sb, Sn, Zn, Pb).