Abstract
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(Fe(2+),Mg)(CO(3))(2) to form diamond in the absence of another reductant. The carbonate self-reduced to diamond by concurrent oxidation of Fe(2+) to Fe(3+) to form a high-P polymorph of magnesioferrite, MgFe(3+)(2)O(4) 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.