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13C 12C exchange between calcite and graphite: A possible thermometer in Grenville marbles

J.W. Valley·J. R. O’Neil
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Abstract

The fractionation of 13 C between calcite and graphite, Δ(Cc-Gr). is consistently small (2.6–4.8 permil) in 34 assemblages from upper amphibolite- and granulite-facies marbles of the Grenville Province. In 25 samples from the Adirondack Mountains, New York, it decreases regularly with increasing metamorphic temperature. The fractionations are independent of absolute δ 13 C values of calcite (−2.9 to +5.0). For T = 600–800° C , the Adirondack data are described by Δ ( Cc - Gr ) = −0.00748 T (° C ) + 8.68. This good correlation between Δ and T suggests that carbon isotope equilibrium was attained in these high-grade marbles and that the theoretical calculations of this fractionation by Bottinga are approximately 2 permil too large in this temperature range. Because of the relatively high temperature sensitivity suggested by these results and by Bottinga's calculations, and the pressure independence of isotope fractionation, Δ(Cc-Gr) may provide a very good thermometer for high-grade marbles. Comparison of this field calibration for Δ(Cc-Gr) vs temperature with results from other terranes supports the utility of Δ(Cc-Gr) for geothermometry and suggests that graphite is much more sluggish to exchange than calcite, that exchange between calcite and graphite occurs at temperatures as low as 300°C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500–600°C. Because 13 C exchange is an unavoidable metamorphic process at temperatures above 300°C, high values of δ 13 C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite.

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J.W. Valley, J. R. O’Neil. 1981. 13C 12C exchange between calcite and graphite: A possible thermometer in Grenville marbles. https://doi.org/10.1016/0016-7037(81)90249-0

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