Oxygen fugacity-mediated carbonate reactions with siliceous fluids in shallow subduction zones

Abstract

Sediments are one of the main carbon sinks in subduction zones, with CaCO₃ and SiO₂ being the main components in sediments. Their chemical stability plays a significant role in the form of carbon in the Earth’s mantle. Here we report the reactions of CaCO₃ with SiO₂ in hydrated sediments at 0.8–2.0 GPa, 400–500 ℃ and redox-buffered conditions relevant to shallow subduction zones. Our results show that the reaction CaCO₃ + SiO₂ = CaSiO₃ + C + O₂(fluid) occurs under CoCoO and IW buffered conditions to generate wollastonite (CaSiO₃) and carbonaceous material (CM). Moreover, wollastonite is formed by the dissolution-crystallization process, which may be significantly affected by oxygen fugacity, leading to distinct crystallization habits (Yui, 1966, Schott et al., 2012). Anhydrous experiments indicate that the reaction proceeds only in the presence of H₂O within the pressure and temperature (P-T) range of this study. The reaction occurs more rapidly with aragonite-structured than calcite-structured CaCO₃. Further, the experiment buffered with natural olivine at 1.0 GPa and 400 ℃ proves that the above reaction can occur during serpentinization processes in shallow subduction zones. More importantly, nanoscale CM may be generated under relatively reducing conditions, exhibiting Raman characteristics of kerogen. These results provide new insights into how deep carbon is distributed in the Earth’s interior.