Non-Destructive 3D Petrologic Analysis of Inclusions in a Carbonado Diamond Using Micro-Scale Computed Tomography: Relation to Impact Events?

Carbonado is a polycrystalline diamond variety been found in placer deposits in Central Africa and Brazil. Various hypotheses exist for the origin of this unusual diamond type, including derivation from the Earth's mantle, formation in subduction-related metamorphic processes, meteorite impact, extraterrestrial sources, or irradiation of carbonaceous materials at or near the surface of the Earth. Carbonados host an esoteric collection of inclusions that are extremely difficult to analyze due to the toughness and polycrystalline nature of the diamond matrix. Some samples contain inclusions of florencite, metals (e.g., Ta, Sn), Fe-Cr-Ni spinel, rutile, and anhydrite. Recent advances in micro-scale computed tomography (microCT) provide unprecedented opportunities to nondestructively image inclusions within opaque solid materials. A series of inclusions in a carbonado from Central Africa were imaged using the new XRadia microCT scanner at the University of Texas High-Resolution X-ray CT Facility. Resolution at the 1 µm level allows detailed examination of the three-dimensional morphology and habit of inclusions of 10 µm or less in size. Using dual-energy acquisition, the contributions to X-ray attenuation made by density and composition can be to some extent differentiated. Using this approach the density and weighted mean atomic number can be determined for many inclusions, which combined with crystal morphology permits confident mineral identification in many cases. Because CT imagery is three-dimensional, we are also able to analyze the spatial distribution of inclusions within the sample, both overall and with respect to composition. The inclusions occur throughout the sample, but also feature a fair degree of clustering. Along some faces an inclusion-poor rim is evident and in others it is absent, indicating breakage of the sample at some time in the past. Such an internal structure, and the population of inclusions, is unlike those of known impact diamonds.