Diamond was synthesized at high temperature and pressure in a Walker-style multianvil apparatus from various supercritical C-O-H–rich fluid compositional systems. Depending on the type and amount of impurities added to the system, diamonds of varying morphology and bearing solid oxide inclusions were found within the run product. These characteristics bear a striking resemblance to metamorphic microdiamonds recovered from the Kokchetav massif, Kazakhstan; these natural diamonds share morphological traits with our synthesized diamonds, and both the natural and synthesized diamonds contain similar solid oxide inclusions. Our data favor formation of Kokchetav diamonds via crystallization from a supercritical C-O-H fluid, rather than from carbonate or carbonatite melt, as others have suggested.

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