Fluid-mediated carbon release from serpentinite-hosted carbonates during dehydration of antigorite-serpentinite in subduction zones

Abstract

Subduction of serpentinite-hosted carbonate rocks formed during the oceanic stage in the oceanic lithosphere (ophicalcite) or by metasomatism at the slab mantle-wedge interface (hybrid carbonate–talc rocks) constitute important fluxes of carbon in subduction zones. If hosted in the hydrated mantle lithosphere of the slab, these carbonate-rocks may be preserved from the infiltration of fluids produced by prograde metamorphism of the overlying meta-sediments and the hydrated oceanic crust to subarc depths, where they are fluxed by fluids derived from dehydration of their host serpentinite. Because the dissolution of carbonate in aqueous fluids is enhanced at high P and T, fluid-mediated carbon release at subarc depth is critical to understand the global carbon balance and magnitude of carbon fluxes from the subducting plate into the deep mantle. This contribution presents thermodynamic modelling —using the implementation of the DEW aqueous database in Perple_X— of prograde devolatilization reactions (solid = fluid + solid) and infiltration-driven devolatilization reactions (external fluid + solid = fluid + solid) of serpentinite-hosted carbonate rocks. In line with previous studies of prograde devolatilization of carbon-bearing oceanic crust, our models show that, in warm and cold subductions zones, the carbon released into fluids produced by prograde devolatilization reactions of meta-ophicalcite and carbonate-talc rocks is limited even if electrolytic complexing in the fluid phase is taken into account. Therefore, serpentinite-hosted meta-carbonate rocks are likely to be preserved to subarc depths, where they undergo infiltration-driven devolatilization by fluids derived from Atg-serpentinite dehydration. At the P–T conditions of Atg-breakdown in cold to warm subduction zones, our models of infiltration-driven devolatilization of meta-ophicalcite and carbonate-talc rocks indicate that carbon release is accompanied by significant Ca loss, particularly at high pressures (P>3.5 GPa), due to the high concentration of CaHCO+3 in the fluid. The carbon solubility in fluids equilibrated with meta-ophicalcite and carbonate–talc rocks is markedly higher than for pure aragonite. Mass balance considerations combined with the parameterization of the P–T conditions of Atg-serpentinite dehydration as a function of the thermal parameter of subduction zones allow us to investigate the carbon loss caused by infiltration of serpentinite dehydration fluids. Unlike the dissolution of pure CaCO3, carbonate dissolution in serpentinite-hosted meta-carbonate rocks is highest at the slab surface of warm subduction zones and lowest at Moho depths of cold subduction zones, where Atg-serpentinite dehydration occurs at greater depths. Carbon in subducted meta-ophicalcite —with thicknesses of up to 70 m typically found in oceanic ophicalcite— will be preserved beyond the subarc depths of cold and hot subduction zones, and recycled into the deep mantle as carbonate-garnet-clinopyroxene-olivine rocks. At the subarc depths of most subduction zone regimes, infiltration-driven devolatilization of subducted carbonate-talc rocks is a highly efficient decarbonation mechanism transforming these rocks into carbon-free orthopyroxenite.