A time-series experimental study by Dionysis Foustoukos  illustrates that the formation of a poorly crystallized or "disordered" phase of graphite may serve as an intermediate state in a range of redox conditions outside the thermodynamic stability field of the crystalline graphite structure. The current findings suggest a key role for amorphous carbon as a substrate towards the deposition of fully crystalline graphite, especially at moderate temperatures. Subsequently, the intermediate state represented by metastable graphite could potentially facilitate plate boundary subduction of carbonaceous material, while imposing constraints on the formation mechanisms and the 13C/12C isotopic systematics of deep-seated carbonaceous fluids and minerals such as diamonds.
In natural environments metastable graphite, similar to that found in these experiments, might serve as a precursor for diamond formation in ultrahigh-pressure metamorphic zones associated with low temperature (<600oC) oceanic subduction complexes. In these settings, poorly organized carbonaceous material has been documented to exist as a coating or intergrown with microdiamonds in enriched garnet fluid inclusions. Accordingly, the circulation of C-O-H-bearing aqueous fluids within the subducting slab along with the possible precipitation of metastable graphite suggests an additional carbon sink - potentially serving as a source of carbon for eclogitic diamonds.
Figure: Raman spectra of graphitic carbon precipitated under oxidizing conditions (HM-redox buffer) shown in the inset microphotograph as the dark grain adjacent to the Raman microbeam (green dot). Based on the relative distribution of the integrated peak areas and the broad FWHH values of the D and G bands, this is inferred as a highly disordered structure of fluid-deposited graphite. (Color online.)