Carbon isotope fractionation between calcite,graphite and CO2: an experimental study |
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| Authors: | Norbert Scheele Jochen Hoefs |
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| Institution: | 1. Geochemisches Institut, Universit?t G?ttingen, Goldschmidtstrasse 1, W-3400, G?ttingen, Federal Republic of Germany
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| 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:
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