Os, Sr, Nd, and Pb isotope systematics of southern African peridotite xenoliths: Implications for the chemical evolution of subcontinental mantle

Abstract

Twelve peridotite xenoliths from the Jagersfontein, Letseng-la-terae, Thaba Patsoa, Mothae and Premier kimberlites of southern Africa were analysed for Os, Sr, Nd and Pb isotopic composition. 187Os186Os for 9 of the 12 samples, including both low-and high-temperature peridotites, are below any published value for the present mantle. Initial 187Os186Os in the peridotites show some correlation with Mg/Fe suggesting that the Re-Os fractionation was caused by melt removal that occurred at least 2 Ga ago. Sr and Nd isotopic compositions (87Sr86Sr = 0.7059 to 0.7078, ϵNd = −7.4 to −21) for acid washed clinopyroxenes from samples equilibrated at low temperature (<1200°C) are outside the range observed for oceanic basalts. Clinopyroxenes from high-temperature samples have Sr and Nd isotopic composition (87Sr86Sr = 0.7028 to 0.7032; ϵNd = 3.5 to 4.3) plotting within the oceanic mantle array, but have distinct Pb isotopic compositions (e.g. 207Pb204Pb = 15.07 and 16.18). Initial Os, Sr and Nd isotopic compositions for the Letseng-la-terae (Group I) kimberlite are within the range observed for rocks from the oceanic mantle, whereas these isotopic compositions in a Group II kimberlite from the Bellsbank group suggest a source for Group II kimberlites that is isotopically similar to the low-temperature peridotites. The compositional and Os isotopic characteristics of the low-temperature samples indicate that they may be residues from high-degrees of partial melt extraction. The distinct Os and Pb isotopic characteristics of the samples analysed here suggest that both low- and high-temperature peridotites reside in an ancient stable lithospheric “keel” to the craton that has been isolated from chemical exchange with the sub-lithospheric mantle for time periods in excess of 2 Ga.