Natural diamond formation by self-redox of ferromagnesian carbonate.
basic_science · Level V
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- Record sourced from PubMed, PMID 29483248.
- Also identified by DOI 10.1073/pnas.1720619115 and PMC identifier 5856551.
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Abstract
Formation of natural diamonds requires the reduction of carbon to its bare elemental form, and pressures (<i>P</i>) 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 (<i>P-T</i>) of 25-45 GPa and 800-900 °C were sufficient to decompose ankerite Ca(Fe<sup>2+</sup>,Mg)(CO<sub>3</sub>)<sub>2</sub> to form diamond in the absence of another reductant. The carbonate self-reduced to diamond by concurrent oxidation of Fe<sup>2+</sup> to Fe<sup>3+</sup> to form a high-<i>P</i> polymorph of magnesioferrite, MgFe<sup>3+</sup><sub>2</sub>O<sub>4</sub> 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.