An effect of reduced S-rich fluids on diamond formation under mantle-slab interaction

Experimental study, dedicated to understanding the effect of S-rich reduced fluids on the diamond-forming processes under subduction settings, was performed using a multi-anvil high-pressure split-sphere apparatus in Fe₃C-(Mg,Ca)CO₃-S and Fe⁰-(Mg,Ca)CO₃-S systems at the pressure of 6.3 GPa, temperatures in the range of 900–1600 °C and run time of 18–60 h. At the temperatures of 900 and 1000 °C in the carbide-carbonate-sulfur system, extraction of carbon from cohenite through the interaction with S-rich reduced fluid, as well as C⁰-producing redox reactions of carbonate with carbide were realized. As a result, graphite formation in assemblage with magnesiowüstite, cohenite and pyrrhotite (±aragonite) was established. At higher temperatures (≥1100 °C) formation of assemblage of Fe³⁺-magnesiowüstite and graphite was accompanied by generation of fO₂-contrasting melts - metal-sulfide with dissolved carbon (Fe-S-C) and sulfide-oxide (Fe-S-O). In the temperature range of 1400–1600 °C spontaneous diamond nucleation was found to occur via redox interactions of carbide or iron with carbonate. It was established, that interactions of Fe-S-C and Fe-S-O melts as well as of Fe-S-C melt and magnesiowüstite, were С⁰-forming processes, accompanied by disproportionation of Fe. These resulted in the crystallization of Fe³⁺-magnesiowüstite+graphite assemblage and growth of diamond. We show that a participation of sulfur in subduction-related elemental carbon-forming processes results in sharp decrease of partial melting temperatures (~300 °C), reducting the reactivity of the Fe-S-C melt relatively to FeC melt with respect to graphite and diamond crystallization and decrease of diamond growth rate.