An experimental investigation of the redox reactions between carbonates and pyrrhotite at high P-T conditions: insights into diamond formation in deep subduction zones

Abstract

Subducted carbonates can be reduced into graphite/diamond through redox reactions involving metallic alloys (e.g., Fe⁰) or silicate minerals (e.g., Fe²⁺-bearing minerals) in the deep mantle. Pyrrhotite (FeS), an essential carrier of reduced sulfur (S²⁻), is commonly found as inclusions in natural diamond, mantle-derived xenoliths, and subducted slabs alongside various carbonates. Although carbonate/diamond and pyrrhotite show strong associations, the interplay of carbon and sulfur and the sulfide-induced formation mechanism of diamonds in deep subduction zones remain poorly constrained. In this study, we conducted a series of experiments to investigate possible redox reactions between aragonite/magnesite (CaCO₃/MgCO₃) and pyrrhotite (FeS) with or without SiO₂ and Al₂O₃ added to the bulk compositions under pressure and temperature conditions of 4–10 GPa and 800–1600 °C. Based on high-pressure experimental results combined with thermodynamic modelling, we found that the transformation of pyrrhotite to pyrite (FeS₂) under high-pressure and low-temperature conditions can simultaneously reduce carbonates to graphitic carbon (C). High contents of SiO₂ and Al₂O₃ in the bulk compositions could promote the formation of pyrite + graphite + Fe²⁺-rich silicates (e.g., hedenbergite and almandine). Therefore, we propose that the subduction of oceanic slabs, including sialic sediments and carbonated eclogites (e.g., those enriched in carbonate, quartz, feldspar, and mica minerals), along a relatively cold geotherm into the deep mantle can drive efficient redox reactions between carbonates and sulfides, potentially contributing to the formation of diamonds in the deep mantle.