A critique of using epitaxial criterion to discriminate between protogenetic and syngenetic mineral inclusions in diamond.

ABSTRACT

Distinguishing syngenetic from protogenetic inclusions in natural diamonds is one of the most debated issues in diamond research. Were the minerals that now reside in inclusions in diamonds born before the diamond that hosts them (protogenesis)? Or did they grow simultaneously and by the same reaction (syngenesis)? Once previously published data on periclase [(Mg,Fe)O] and magnesiochromite (MgCr2O4) inclusions in diamond have been re-analysed, we show that the main arguments reported so far to support syngenesis between diamond and its mineral inclusions, definitely failed. Hence (a) the epitaxial relationships between diamond and its mineral inclusion should no longer be used to support syngenesis, because only detecting an epitaxy does not tell us which was the nucleation substrate (there are evidences that in case of epitaxy, the inclusion acts as a nucleation substrate); (b) the morphology of the inclusion should no longer be used as well, as inclusions could be protogenetic regardless their shapes. Finally, we advance the hypothesis that the majority of inclusions in diamonds are protogenetic, e.g., they are constituent of rocks in which diamonds were formed and not products of reactions during diamond growth.