Enrichment of HFSE in chlorite-harzburgite produced by high-pressure dehydration of antigorite-serpentinite: Implications for subduction magmatism

Abstract

[ 1] Depletion of high- field- strength trace elements ( HFSE) relative to normal mid- ocean basalts ( N- MORB) is the most distinctive geochemical fingerprint of subduction magmatism. Proposed hypotheses advocate that this " subduction'' signature is acquired during melting and/ or fluid transfer either in the mantle wedge or in the crust of the subducting oceanic plate. Here we provide field- based and geochemical evidence showing that high- pressure dehydration of antigorite- serpentinite produces chlorite-harzburgite relatively enriched in HFSE due to the stabilization of F- OH- Ti- clinohumite intergrowths with prograde olivine. Available experimental data indicate that in hydrated, intermediate to warm subduction zones, clinohumite- olivine intergrowths can be stable in prograde chlorite- harzburgite olivine at subarc depths. In these settings, deserpentinization may act as a source of fluids leaching large- ion lithophile elements ( LILE), Pb, and Sr from the overlying crust and sediments on their way up to the mantle wedge. Stabilization of chlorite- harzburgites with clinohumite- olivine intergrowths in the mantle wedge, on the other hand, acts as a sink of HFSE by selectively fractionating them from other incompatible trace elements in fluids emanating from the slab. Resulting arc fluids in equilibrium with wedge chlorite-harzburgite are strongly depleted in HFSE and transfer this depletion to the overlying hot mantle wedge, where subduction magmas are generated. Show more