Lead, uranium, strontium, potassium and rubidium in inclusion-bearing diamonds and mantle-derived xenoliths from Southern Africa

Abstract

Measurements are reported of K, Rb and Sr levels and Sr isotopic compositions in silicate inclusions in diamonds, and of U, Th and Pb levels and Pb isotopic compositions in sulphide inclusions in diamonds. Samples are from the Premier Mine (Transvaal), the Finsch Mine (northern Cape Province) and the Kimberley mines (northern Cape Province). The isotopic composition of Pb in sulphide inclusions indicates that the diamonds containing these inclusions are xenocrysts in the host kimberlite. Model Pb ages for the sulphide inclusions in the diamonds from Finsch and Kimberley are in excess of 2 b.y., although the host kimberlites are Cretaceous. The sulphide inclusions from Premier, however, have a model age similar to the emplacement age of the Premier kimberlites (approximately 1.2 b.y.). In addition, K, Rb, Sr, U and Pb concentrations and Sr and Pb isotopic compositions in clinopyroxenes from 14 mantle-derived xenoliths were measured. Samples were eclogites from the Roberts Victor Mine (Orange Free State), peridotites from the Matsoku Pipe (Lesotho) and diopside megacrysts from the four mines in Kimberley. The samples from the Roberts Victor Mine define a large spread in Sr and Pb isotopic composition. The slope of a regression line through the Pb data in the 207/204 vs. 206/204 plot defines an age of roughly 2.5 b.y. The Matsoku samples have 87Sr/ 86Sr ratios around 0.704. One group of samples (“fertile peridotites”) shows uniform, anomalously radiogenic Pb isotopic compositions, and bears evidence of a young event involving isotopic homogenisation. Lead isotopic heterogeneity, indicating an old age, has persisted in another group (“common peridotites”). The megacrysts from the four Kimberley mines have 87Sr/ 86Sr ratios around 0.704 and uniform, anomalously radiogenic Pb isotopic compositions, indicating a young age. Their Pb isotopic uniformity contrasts with the spread shown by kimberlites in Kimberley, and shows that the megacrysts did not crystallise in equilibrium with their host kimberlites. The similarity between the Pb isotopic data obtained on clinopyroxenes from xenoliths and the data obtained on sulphide inclusions in diamonds from Finsch and Kimberley suggests that these diamonds might be cogenetic with old rock provinces in the subcontinental mantle.