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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 degrees 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, MgFe23+O4. 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.