Carbon transfer by slab-fluids: in-situ experimental constraints

Carbon represents an essential element for the origin and evolution of life and profoundly contributes to the well-being and sustainability of our planet. Over recent years, understanding carbon cycling on a global scale has become a central objective within the Earth science community, and the study of subduction zone fluids has become a crucial topic, as this geological setting represents the primary carbon input into the mantle. At subsolidus conditions, carbon transfer is mediated by mineral dissolution, triggered by aqueous fluids released from the subducting slab. While carbonate solubility has been extensively investigated, the contribution of reduced carbon forms, such as graphite and amorphous carbon, has been only recently taken into consideration, and their role in the deep carbon cycle is still unconstrained. Several issues remain open, especially whether carbon-rich fluids, generated from reduced carbon dissolution, can be transferred across oxidized conditions in the subduction mélange and eventually reach the mantle wedge.Here we present in-situ results on the solubility of glass-like carbon, considered a proxy for disordered subducted organic material, in aqueous fluids and in equilibrium with quartz at pressures up to 2 GPa and 800 °C. Experiments were conducted in Hydrothermal Diamond Anvil Cells1, employing Rhenium gaskets to ensure oxidized conditions2 and mimic fluids released by dehydrating slabs in fore to back-arc settings. The solubility in aqueous fluids of glass-like carbon and quartz was determined by in-situ observations of the complete dissolution of samples, while the speciation of the fluids was monitored by Raman spectroscopy. Our results constrain the mutual solubility of carbon and silica in natural slab fluids and provide new constraints for the transfer of carbon operated by aqueous fluids. [1] Bassett W.A., Shen A.H., Bucknum M. & Chou I.M. Rev. Sci. Instrum. 64, 2340–2345 (1993)[2] Foustoukos D.I. & Mysen B.O. Am. Mineral. 100, 35–46 (2015)