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

[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 chloriteharzburgite 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 chloriteharzburgite are strongly depleted in HFSE and transfer this depletion to the overlying hot mantle wedge, where subduction magmas are generated.