A comparison of the microdiamonds from kimberlite and lamproite of Yakutia and Australia

Microdiamonds from kimberlite pipes of Yakutia and kimberlite and lamproite pipes of Australia exhibit a range of physical and chemical characteristics which suggest that at each location many different microdiamond populations are present. Octahedral crystals dominate the majority of microdiamond suites examined, and the proportions of cubes and cubooctahedra, together with resorbed and aggregated forms vary from host to host. Type I (nitrogen-bearing) microdiamonds generally predominate, though type II (nitrogen-depleted) stones, particularly amongst Australian hosts, constitute significant proportions. The degree of nitrogen aggregation can be used to constrain the temperature and residence interval of microdiamonds within the source region. Type I microdiamonds generally contain aggregated nitrogen (type Ia) and are enriched in A-defects relative to B-defects. FT-IR studies suggest type IaAB microdiamonds have formed in multiple or long, continuous growth events, and have resided in the mantle for substantial periods prior to eruption. Type Ib characteristics are restricted to cubic crystals and imply at least one generation of cubes and cubic overgrowths form close to time of eruption at the Earth's surface.