Kinetic carbon isotope fractionation links graphite and diamond precipitation to reduced fluid sources.

At high temperatures, isotope partitioning is often assumed to proceed under equilibrium and trends in the carbon isotope composition within graphite and diamond are used to deduce the redox state of their fluid source. However, kinetic isotope fractionation modifies fluid- or melt-precipitated mineral compositions when growth rates exceed rates of diffusive mixing. As carbon self-diffusion in graphite and diamond is exceptionally slow, this fractionation should be preserved. We have hence performed time series experiments that precipitate graphitic carbon through progressive oxidization of an initially CH 4 -dominated fluid. Stearic acid was thermally decomposed at 800 °C and 2 kbar, yielding a reduced COH-fluid together with elemental carbon. Progressive hydrogen loss from the capsule caused CH 4 to dissociate with time and elemental carbon to continuously precipitate. The newly formed C0, aggregating in globules, is constantly depleted by − 6.2 ± 0.3 ‰ in 13C relative to the methane, which defines a temperature dependent kinetic graphite-methane 13C/12C fractionation factor. Equilibrium fractionation would instead yield graphite heavier than the methane. In dynamic environments, kinetic isotope fractionation may control the carbon isotope composition of graphite or diamond, and, extended to nitrogen, could explain the positive correlation of δ 13 C and δ 15 N sometimes observed in coherent diamond growth zones. 13C enrichment trends in diamonds are then consistent with reduced deep fluids oxidizing upon their rise into the subcontinental lithosphere, methane constituting the main source of carbon. • Tracking the 13C/12C distribution during oxidation of a methane-dominated COH-fluid. • CH 4 dissociation causes kinetic isotope effects recorded in co-precipitated C0. • At 800 °C, co-precipitated C0 is depleted in 13C by 6.2 ± 0.3 ‰ relative to CH 4. • Kinetic control of C isotope composition of fluid-precipitated graphite and diamond. [ABSTRACT FROM AUTHOR]