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 Fe-0-(Mg,Ca)CO3-S systems at the pressure of 6.3 GPa, temperatures in the range of 900-1600 degrees C and run time of 18-60 h. At the temperatures of 900 and 1000 degrees C in the carbide-carbonate-sulfur system, extraction of carbon from cohenite through the interaction with S-rich reduced fluid, as well as C-0-producing redox reactions of carbonate with carbide were realized. As a result, graphite formation in assemblage with magnesiowustite, cohenite and pyrrhotite (+/- aragonite) was established. At higher temperatures (>= 1100 degrees C) formation of assemblage of Fe3+-magnesiowustite and graphite was accompanied by generation of fO(2)-contrasting melts - metal-sulfide with dissolved carbon (Fe-S-C) and sulfide-oxide (Fe-S-O). In the temperature range of 1400-1600 degrees 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 magnesiowustite, were C-0-forming processes, accompanied by disproportionation of Fe. These resulted in the crystallization of Fe3+-magnesiowiistite+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 (similar to 300 degrees C), reducting the reactivity of the Fe-S-C melt relatively to Fe-C melt with respect to graphite and diamond crystallization and decrease of diamond growth rate. (C) 2019 Elsevier B.V. All rights reserved.