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On the stability of acetate in subduction zone fluids

Recent theoretical studies have suggested that, under certain intermediate redox conditions, organic anions, in particular acetate, may be the dominant carbon species in deep subduction zone fluids. This could have major consequences for the properties of these fluids, for the formation of diamonds, and for the deep carbon cycle in gen-eral. We have tested these predictions by carrying out both ex situ piston cylinder experiments and in situ Raman spectroscopic experiments in the externally heated diamond cell, with a temperature up to 600 °C and pressure up to 5 GPa, the predicted stability field of acetate. We observed that upon heating and pressurisation to these conditions, an aqueous solution of sodium acetate undergoes several inter-esting reactions, including the formation of higher hydrocarbons. However, at 5 GPa and 600 °C, almost all organic species appear to have decomposed to a graphite-like material. Our experiments, therefore, do not support the stability of acetate and other organics as main carbon species in deep subduction fluids. However, the stability of minor concentrations of such species is still possible and requires further study. During deep subduction, most of the reduced carbon in sediments may be retained and recycled into the mantle to great depth.