Abstract Kimberlites are the deepest derived magmas on Earth, and together with the mantle xenoliths and xenocrysts they carry, provide an unprecedented look into the composition and evolution of the mantle from the Archaean to Cenozoic. Although superficially similar to kimberlites, olivine lamproites of the Kaapvaal Craton, previously known as Group II kimberlites, are highly micaceous and represent a distinct magmatic episode to kimberlites. Both kimberlites and olivine lamproites are derived by small degrees of partial melting in the upper convective asthenospheric mantle, which then traverse the non-convective, sub-continental lithospheric mantle (SCLM) en route to Earth’s surface. The major element compositions of the parent magmas are modified during complex melt-SCLM interactions, and this leads to significant diversification of the magmas. During their ascent, they also entrain diamonds, and consequently, kimberlites and olivine lamproites form primary volcanic ore deposits on emplacement near the surface. These volcanic pipes, ~3 km in depth and several hundred metres in diameter, formed from predominantly juvenile volatile-driven eruptions. In most cases across the Kaapvaal Craton, kimberlite pipes have been significantly eroded post-emplacement and the diamonds within them have been transported toward the west coast, forming secondary alluvial diamond deposits along the way. Diamond exploration and evaluation relies on kimberlite indicator minerals, e.g., garnet, ilmenite, and clinopyroxene, which are mantle xenocrysts from the SCLM transported to the surface in kimberlite and olivine lamproite magmas. The composition of these minerals is controlled by the pressure-temperature (P-T) conditions at which they last equilibrated. The majority of diamonds form at relatively low temperatures (900 to 1200°C) but high pressures (4 to 7 GPa). These conditions are met in the SCLM and this region is termed the ‘diamond window’. A smaller proportion of diamonds have a sub-lithospheric origin likely forming in the mantle transition zone (MTZ). Thermobarometry is the approach used to constrain the P-T conditions of indicator minerals based on their chemistry. This provides a means to map the composition and thermal state of the SCLM to depths of ~200 km, and to assess the extent of a possible diamond window sampled by individual kimberlites. Ultimately, kimberlites and olivine lamproites, along with their mantle cargo, are invaluable in our understanding of the deep Earth and are economically valuable as diamond deposits.
G.H. Howarth (2026) studied this question.