Natural diamond formation by self-redox of ferromagnesian carbonate

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(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.

Keywords: diamond; ferromagnesium carbonate; lower mantle; self-redox; shock-metamorphism.