In situ U[sbnd]Pb age determination and Sr[sbnd]Nd isotopic analysis of perovskite from the Premier (Cullinan) kimberlite, South Africa

Abstract

South Africa is characterized by multiple episodes of kimberlite intrusion. However, the age and SrNd isotopic compositions of the primary magma of most of the earlier emplacement events remain unresolved. Using in situ secondary ion mass spectrometry and laser ablation multi-collector inductively coupled plasma mass spectrometry, the UPb ages and SrNd isotopic compositions were obtained for perovskite from the Premier (Cullinan) kimberlite: one of the oldest diamondiferous kimberlites. The UPb perovskite ages are 1150±16, 1151±9, and 1156±12Ma for the brown, black, and green kimberlites, respectively, demonstrating that the different phases of the kimberlite were emplaced contemporaneously at ~1150Ma. In situ SrNd isotopic analyses of perovskite yield initial Sr isotopic ratios and εNd(t)1150 values of 0.7031±4 and 1.9±0.9 for the brown; 0.7025±1 and 1.8±0.4 for the black; and 0.7030±2 and 0.8±0.7 for the green kimberlites. These are essentially identical within analytical uncertainties and suggest that the kimberlites were derived from a moderately-depleted mantle source. In contrast, whole-rock data yield variable SrNd isotopic compositions, confirming that whole-rock isotopic data cannot be used to constrain the character of the source of kimberlitic magma due to crustal contamination, later alteration and/or weathering. Our data also indicate that kimberlite emplacement might be considered as a prelude to mantle plume activity beneath a craton.