Primary petrology, mineralogy and age of the Letšeng-la-Terae kimberlite (Lesotho, Southern Africa) and parental magmas of Group-I kimberlites

Abstract

The Letseng-la-Terae kimberlite (Lesotho), famous for its large high-value diamonds, has five distinct phases that are mined in a Main and a Satellite pipe. These diatreme phases are heavily altered but parts of a directly adjacent kimberlite blow are exceptionally fresh. The blow groundmass consists of preserved primary olivine with Fo86−88, chromite, magnesio-ulvospinel and magnetite, perovskite, monticellite, occasional Sr-rich carbonate, phlogopite, apatite, calcite and serpentine. The bulk composition of the groundmass, extracted by micro-drilling, yields 24–26 wt% SiO2, 20–21 wt% MgO, 16–19 wt% CaO and 1.9–2.1 wt% K2O, the latter being retained in phlogopite. Without a proper mineral host, groundmass Na2O is only 0.09–0.16 wt%. However, Na-rich K-richterite observed in orthopyroxene coronae allows to reconstruct a parent melt Na2O content of 3.5–5 wt%, an amount similar to that of highly undersaturated primitive ocean island basanites. The groundmass contains 10–12 wt% CO2, H2O is estimated to 4–5 wt%, but volatiles and alkalis were considerably reduced by degassing. Mg# of 77.9 and 530 ppm Ni are in equilibrium with olivine phenocrysts, characterize the parent melt and are not due to olivine fractionation. 87Sr/86Sr(i) = 0.703602–0.703656, 143Nd/144Nd(i) = 0.512660 and 176Hf/177Hf(i) = 0.282677–0.282679 indicate that the Letseng kimberlite originates from the convective upper mantle. U–Pb dating of groundmass perovskite reveals an emplacement age of 85.5 ± 0.3 (2σ) Ma, which is significantly younger than previously proposed for the Letseng kimberlite.