Experimental determination of carbon isotope fractionation between graphite and carbonated silicate melt under upper mantle conditions

•Carbon isotope fractionation observed at high temperature mantle melting.•Heavy carbon isotope preferentially fractionates in to carbonate silicate melt.•Progressive melting results in large carbon isotope heterogeneities in the mantle.•Carbon isotope systematics depends on redox states of mantle melting.

Carbon isotope fractionation between graphite and carbonated silicate melt was determined at 5 GPa and in the temperature range between 1400 and 1900 °C. High pressure experiments were carried out in the carbon-saturated model harzbergite system (Enstatite-Magnesite-Olivine-Graphite), where carbonated silicate melt and graphite were the two stable carbon-bearing phases in the run products. Carbonated silicate melting resulted in an isotopic fractionation between graphite and carbon in the silicate melt, where the carbon in the melt is 13C enriched than co-existing graphite. 13C enrichment in carbonate melt when compared to graphite was further confirmed in experiments where redox melting between olivine and graphite produced carbonate melt as well as carbonate reduction experiments to form graphite. Although a quantitative estimate of fractionation between carbonate melt and graphite could not be obtained, our results indicate that mantle melting in the presence of graphite can result in progressive 13C carbon isotope enrichment in carbonate melt and depletion in graphite, which can be an alternate explanation for the carbon isotopic heterogeneity observed in the mantle derived carbon.