Diamond-Silicate-Sulphide-Oxide Textural Relationships in Diamondiferous Eclogites from the Kalahari Craton Revealed by 3D X-Ray Computed Tomography

X-ray computed tomography (CT) scanning provides a non-destructive way to image diamonds within their host mantle rocks. The CT scans produce thousands of 2D slices, referred to here as pseudo-thin sections, allowing us to observe the textural relationships between diamonds and the surrounding minerals, such as silicates, oxides, and sulphides. In this study, we present CT scans of 18 diamond-bearing eclogites from three locations across the Kalahari craton. The scans reveal that the diamonds are mostly well-formed, step-faced crystals. These step-faced diamonds are associated with metasomatic textures, either forming in distinct veins that cut through the silicate minerals of the xenoliths or along the grain boundaries between garnet and clinopyroxene. The data suggest that diamonds typically form at the expense of garnet, indicating simultaneous garnet dissolution and diamond-friendly metasomatism. In some cases, diamonds appear in specific planes, though there is no visible evidence of the metasomatic minerals responsible for their formation. These instances may represent original diamond growth through metasomatic alteration of the eclogite, followed by recrystallization that erased signs of the earlier diamond-friendly event. We also find evidence of in situ diamond destruction due to metasomatic modification of the eclogites, linked to silicate- and sulphide-rich metasomatism through melt or fluid percolation along garnet–clinopyroxene grain boundaries. While sulphide-rich metasomatism is common in these eclogites, it does not show a clear spatial or temporal link to diamond-forming metasomatic events. Overall, the textural evidence suggests that these diamondiferous eclogites have experienced multiple stages of both diamond-forming and diamond-destructive metasomatism involving silicate and sulphide-rich fluids over their billions of years in the sub-continental lithospheric mantle.