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

ABSTRACT

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.

KEYWORDS:

• C–H–O fluid
• diamond
• carbon isotope
• isotope fractionation
• high-pressure and high-temperature experiments

Acknowledgements

The authors thank S. Kakizawa, T. Inoue, and K. Litasov for their technical advices in performing high-pressure experiments using the multi-anvil apparatus. The authors also thank K. Amano for her advice on the data visualization of Figure 4. This work was supported by the Joint Usage/Research Center PRIUS, Ehime University, Japan.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This study was supported by JSPS KAKENHI grants: grant numbers JP20J21968 (given to HK), JP20H02009 (to HO), JP19H01985 (to AS), JP15H05831 and JP20KK0081 (to MS-K).