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

The fractionation of 13C between calcite and graphite, Δ(Cc-Gr), is consistently small (2.6 to 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 delta13C values of calcite (- 2.9 to + 5.0). For T = 600 to 8000C, the Adirondack data are described by Δ(Cc-Gr) = - 0.00748T (0C) + 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 3000C, and that equilibrium may normally be attained only when peak metamorphic temperatures are greater than 500 to 6000C. Because 13C exchange is an unavoidable metamorphic process at temperatures above 3000C, high values of delta13C(Gr) in moderate- to high-grade carbonate-bearing rocks do not provide a sufficient criterion to infer an abiogenic origin for the graphite. (author)