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

Garrido, C. J.; Sanchez-Vizcaino, V. L.; Gomez-Pugnaire, M. T.; Trommsdorff, V.; Alard, O.; Bodinier, J. L. and Godard, M. (2005). Enrichment of HFSE in chlorite-harzburgite produced by high-pressure dehydration of antigorite-serpentinite: Implications for subduction magmatism. Geochemistry Geophysics Geosystems, 6(Q01J15)

DOI: https://doi.org/10.1029/2004GC000791

Abstract

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.

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