Is the Imposition of Diamond Morphology on Mineral Inclusions a Syngenetic or Post-Genetic Process with Respect to Diamond Formation?

Abstract

Recent studies showed that inclusions of the same phase within a single diamond are relicts of an original monocrystal that underwent a dissolution event during diamond growth. Interestingly, these inclusions developed both diamond-imposed (i.e., cubic-octahedral shape) and lobed morphologies with rounded shapes and/or embayments. Whether the diamond-imposed morphology is developed during or after entrapment of the inclusion is unknown. We addressed the problem in two ways: (i) by determining the thermodynamic conditions under which mineral inclusions can modify their size (e.g., a single inclusion separates to give two inclusions with different size) and morphology, when trapped in diamond and (ii) by critically reviewing and discussing the recent observations on mineral inclusions in diamond. Accordingly, we developed a thermodynamic model which considers all the involved energetic contributions (i.e., surface and strain energies) to completely describe the Gibbs energy of a closed system designed to forecast the size evolution of two adjacent (or a single) inclusions, at constant T and P. Based on this model and analyzing the existing scientific literature on diamond, we propose an experimental/observational protocol to evaluate whether post-entrapment modification of inclusions has occurred.

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

“…This interpretation can no longer be considered valid as many inclusions in diamonds showing imposed cubo-octahedral morphology are protogenetic 2 , therefore the GC must be rejected. Indeed, a mineral inclusion with cubo-octahedral morphology implies a modification of the inclusion/host interface during or after its entrapment in the diamond 16 , thus contradicting the initial hypothesis of the GC, which states that the shape of a protogenetic inclusion is not altered during the diamond growth.…”

“…This model assumes a protogenetic origin of the Fe-rich Fper inclusions, in analogy with the inclusions of silicates and sulphides ( Supplementary Information ), but with the difference that in this case the nucleation of the diamond is triggered by the phase itself which will form the iso-oriented inclusions. Moreover, this model also assumes that post-entrapment modification of the inclusions shape cannot occur: Bruno et al 16 ruled out that an epitaxial relationship can develop after trapping of inclusions.…”