Natural diamond formation by self-redox of ferromagnesian carbonate

• Published in: Proceedings of the National Academy of Sciences - PNAS Vol. 115; no. 11; pp. 2676 - 2680
• Main Authors: Chen, Ming, Shu, Jinfu, Xie, Xiande, Tan, Dayong, Mao, Ho-kwang
• Format: Journal Article
• Language: English
• Published: United States National Academy of Sciences 13.03.2018
• Subjects: Carbon; Diamonds; Earth mantle; Graphite; Impact loads; Iron; Lower mantle; Oxidation; Physical Sciences; Pressure; Reduction; self-redox; shock-metamorphism; diamond; ferromagnesium carbonate; lower mantle
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1720619115

Formation of natural diamonds requires the reduction of carbon to its bare elemental form, and pressures (P) greater than 5 GPa to cross the graphite–diamond transition boundary. In a study of shocked ferromagnesian carbonate at the Xiuyan impact crater, we found that the impact pressure–temperature (P-T) of 25–45 GPa and 800–900 °C were sufficient to decompose ankerite Ca(Fe2+,Mg)(CO₃)₂ to form diamond in the absence of another reductant. The carbonate self-reduced to diamond by concurrent oxidation of Fe2+ to Fe3+ to form a high-P polymorph of magnesioferrite, MgFe3+₂O₄. Discovery of the subsolidus carbonate self-reduction mechanism indicates that diamonds could be ubiquitously present as a dominant host for carbon in the Earth’s lower mantle.