Carbon isotopic composition of solid carbon and C–H–O fluid produced through the decomposition of stearic acid under HP-HT condition.

We estimated δ13C of the C–H–O fluid produced by the decomposition of stearic acid at high-pressure and high-temperature conditions (10 and 17 GPa, 800–1600°C) from δ13C of the solid carbon products and the initial stearic acid. The δ13C of the stearic acid and the recovered graphite/diamond were −28.0‰ and −27.08 to −29.35‰, respectively. Then, the δ13C of the coexisting C–H–O fluid was estimated to be −26.3 to −29.2‰ based on the stoichiometry of the decomposition reaction C18H36O2 → 10C + 8CH4 + 2H2O. In the studied P-T-t conditions, carbon isotope fractionation through the stearic acid decomposition is as small as < 3‰. This means that the δ13C of the C–H–O fluid is almost identical to that of the initial stearic acid and can be a useful tracer of carbon source for diamond forming reaction such as in fluid/carbonate systems. [ABSTRACT FROM AUTHOR]