CARBON ISOTOPE FRACTIONATION BETWEEN CALCITE, GRAPHITE AND CO2: AN EXPERIMENTAL STUDY

Abstract

The partitioning of stable carbon isotopes between calcite, graphite and CO2 was experimentally determined at temperatures from 500 to 1200 °C and 1 to 15 kbar pressure. Attainment of carbon isotope equilibrium in CO2-calcite runs was proven by achieving the same fractionation from isotopically opposite directions. The resultant CO2-calcite fractionation curve for carbon differs from Bottinga's calculation by 1.2‰ and confirms recent experiments of Chacko et al. and Mattey et al. In CO2-graphite experiments equilibrium fractions were extrapolated by applying the partial-exchange technique of Northrop and Clayton and by optimizing the contribution of surface reaction in graphite. CO2-graphite fractionations at temperatures up to 800 °C are in fair agreement with Bottinga's calculation, but yield a surprisingly high fractionation of ≈5‰ at upper mantle temperatures. The combination of CO2-calcite (carbon) and CO2-graphite fractionation results in a new experimentally determined calcite-grapite fractionation curve, expressed by the equation: 103 ln αcc - gr=7.99×106/T2−9.58×103/+5.76 (873 - 1473 Kelvin) Applying the experimentally determined fractionation curve on graphite-bearing metacarbonates yields metamorphic temperatures distinctly higher than those obtained by Valley and O'Neil.