Composition and origin of crystalline inclusions in natural diamonds

Abstract Natural diamonds from a variety of African localities, Venezuela and Thailand contain abundant monomineralic inclusions with a grain size that is generally in the range 10–200 μm. Many of these were incorporated in diamond at the time of crystallization of the host and have been insulated from further reaction with their surroundings. Inclusions of olivine, garnet, chromite, enstatite and Ca-rich pyroxene have been studied by electron probe and X-ray techniques. The chemical compositions of many inclusions resemble those of similar minerals that occur as xenocrysts and in peridotite xenoliths in kimberlite, but there are a number of consistent differences. A minority of the garnet and pyroxene inclusions show some resemblances to minerals from eclogites and grospydites. There are few experimental data that can be used to interpret the differences between the inclusions and their counterparts in peridotite xenoliths. However, it can be hypothesized that the diamonds formed in igneous events and that the inclusions they contain crystallized in equilibrium with liquid. The minerals in the peridotite xenoliths, in contrast, may represent assemblages formed after cooling and equilibration at a lower temperature, perhaps in a volatile-rich liquid or perhaps in the solid state. We suggest that the inclusions have not participated in lower temperature equilibration because they were armored by diamond.