Experimental determination of carbon isotope fractionation between CaCO3 and graphite

Carbon isotopic exchange between graphite and three polymorphs of CaCO3 was investigated at temperatures of 600–1400 °C and at pressures from 1.4 to 2.3 GPa. Fractionation factors at all temperatures were determined by the partial exchange treatment of Northrop and Clayton (1966).Graphite starting material for the majority of the experiments was milled in water for 20–25 h, producing aggregates of nanosheets. The sheets range in width from 50 to 1000 nm and in thickness from 20 to 30 nm, and they retain hexagonal symmetry.Isotopic exchange appears to be the sum of surface exchange and interior exchange. At 1100–1400 °C, interior exchange exceeded surface exchange, probably by a combination of grain growth, as determined by increase in crystallite size, recrystallization, as observed in FESEM images, and diffusion. In some runs at 1200 and 1400 °C with an isotopic contrast between the initial graphite and calcite of close to 50‰, equilibrium fractionation was actually overstepped due to a kinetic effect. A weighted regression of fractionation factors from the high-temperature runs yields the line of equilibrium interior exchange:1000lnαcarbonate–graphite=3.28(0.07)×106/T2.Our calibration lies between the empirical geothermometers of Kitchen and Valley (1995) and Valley and O’Neil (1981) and, accordingly, with a substantial body of data from granulite-facies metamorphic rocks.At 600–700 °C surface exchange greatly exceeded interior exchange, with a much lower activation energy. Interior exchange was slight to nonexistent because there was no crystal growth, no recrystallization, and, probably, little diffusion. Fractionation factors are ∼1‰ higher than the interior exchange factors. Surface exchange probably occurred in the outer one or two unit cells of nanosheets. In previous experimental studies, similar surface-dominated fractionations apparently were measured, even at high temperatures.At 750–1000 °C, exchange rates and fractionation factors followed the low-temperature surface trends. But higher progress toward equilibrium indicates that exchange may also have occurred along rapidly-migrating subgrain boundaries. At those temperatures, crystallite size decreased, but the physical appearance of nanosheets did not change, indicating internal rearrangement of the grains.