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

Abstract

The fractionation of 13C between calcite and graphite, [Delta](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 [delta]13C values of calcite (-2.9 to +5.0). For T = 600-800[deg]C, the Adirondack data are described by [Delta](Cc-Gr) = -0.00748T ([deg]C) + 8.68. This good correlation between [Delta] 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, [Delta](Cc-Gr) may provide a very good thermometer for high-grade marbles.Comparison of this field calibration for [Delta](Cc-Gr) vs temperature with results from other terranes supports the utility of [Delta](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[deg]C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500-600[deg]C.Because 13C exchange is an unavoidable metamorphic process at temperatures above 300[deg]C, high values of [delta]13C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite.