Thermodynamic constraints on the fate of carbon mobilized from subducted sediments in the overlying mantle wedge

Abstract

In subduction zones, carbon from the surface can be transported into Earth's interior. Fluids released during subduction can mobilize carbon bound in solid minerals in the subducting plate, and transport it into the overlying mantle. While there is increasing attention on the composition and redox state of carbon-bearing, subduction-mobilized fluids, the fate of fluids as they migrate through and react with overlying mantle rocks is less well characterized. Using thermodynamic modeling, we quantify the evolution of carbon-bearing aqueous fluids mobilized from subducting sedimentary rocks, as they follow several possible pathways through the overlying mantle. The extent of carbon mineralization in the overlying mantle depends on the source (siliciclastic vs carbonate rock) and composition of the fluids as well as the fluid path (isothermal, cooling, heating). A heating path towards the hot core of the mantle wedge does not favor sequestration of carbon into solid phases. Along cooling trajectories, most of the dissolved carbon (61 to 98 %) forms in solid minerals when carbon-rich, siliciclastic-equilibrated fluids react with mantle rocks. In contrast to fluids from siliciclastic sediments, less carbon is sequestered in the mantle when relatively carbon-poor, limestone-equilibrated fluids react with mantle rocks. The formation of quartz-rich (birbirite), quartz-magnesite (listvenite), talc-magnesite (soapstone), and serpentine-rich (serpentinite) parageneses is predicted at decreasing water-rock ratios. Soapstones are less favored to form along fractional crystallization paths and/or lower temperatures. Listvenite assemblages form from migrating fluids with dissolved carbon concentration > 20,000 ppm. Graphite is favored to form along closed reaction, cooling paths. Moreover, several cooling paths form reduced fluids rich in hydrogen (H2), methane (CH4), and organic species. Overall, this work provides quantitative models that can comprehensively inform studies on the movement of carbon in subduction zones and into the deep Earth.