Experimental carbonatite/graphite carbon isotope fractionation and carbonate/graphite geothermometry

Carbon isotope exchange between carbon-bearing high temperature phases records the carbon (re-) processing in the Earth's interior, where the vast majority of global carbon is stored. Redox reactions between carbonate phases and elemental carbon govern the mobility of carbon, which then can be traced by its isotopes. We determined the carbon isotope fractionation factor between graphite and a Na2CO3-CaCO3 melt at 900-1500 degrees C and 1 GPa; The failure to isotopically equilibrate preexisting graphite led us to synthesize graphite anew from organic material during the melting of the carbonate mixture. Graphite growth proceeds by (1) decomposition of organic material into globular amorphous carbon, (2) restructuring into nano-crystalline graphite, and (3) recrystallization into hexagonal graphite flakes. Each transition is accompanied by carbon isotope exchange with the carbonate melt. High-temperature (1200-1500 degrees C) equilibrium isotope fractionation with type (3) graphite can be described by Delta C-13(carbonate-graphite )= 3.17(+/- 0.07) . 10(6)/T-2 (temperature T in K). As the experiments do not yield equilibrated bulk graphite at lower temperatures, we combined the >= 1200 degrees C experimental data with those derived from upper amphibolite and lower granulite facies carbonate-graphite pairs (Kitchen and Valley, 1995; Valley and O'Neil, 1981). This yields the general fractionation function Delta C-13(carbonate-graphite )= 3.37(+/- 0.04) . 10(6)/T-2 usable as a geothermometer for solid or liquid carbonate at >= 600 degrees C. Similar to previous observations, lower-temperature experiments (<= 1100 degrees C) deviate from equilibrium. By comparing our results to diffusion and growth rates in graphite, we show that at <= 1100 degrees C carbon diffusion is slower than graphite growth, hence equilibrium surface isotope effects govern isotope fractionation between graphite and carbonate melt and determine the isotopic composition of newly formed graphite. The competition between diffusive isotope exchange and growth rates requires a more careful interpretation of isotope zoning in graphite and diamond. Based on graphite crystallization rates and bulk isotope equilibration, a minimum diffusivity of D-graphite 2 x 10(-17) m(2) s(-1 )for T > 1150 degrees C is required. This value is significantly higher than calculated from experimental carbon self-diffusion constants (similar to 1.6 x 10(-29) m(2) s(-1)) but in good agreement with the value calculated for mono-vacancy migration (similar to 2.8 x 10(-16) m(2) s(-1)). (C) 2019 The Authors. Published by Elsevier Ltd.