African kimberlites revisited: In situ Sr-isotope analysis of groundmass perovskite

New in situ Sr-isotope data are presented for perovskite grains from 54 samples, representing 42 separate localities of Group I, Group II and transitional kimberlites from southern Africa. Although broadly consistent with published whole-rock data from the same intrusions, a clear offset towards less radiogenic compositions is evident in the perovskite results. This suggests that even bulk-rock data acquired after the most careful selection and treatment of chips can be compromised by alteration and contamination processes. Consistent with the bulk-rock data, on- and off-craton samples from the Group I kimberlites of our study cannot be distinguished on the basis of initial 87Sr/86Sr ratios. Our dataset also contains 4 samples of transitional kimberlite from 3 off-craton intrusions. The perovskite results from Springbank lie within the range for Group I samples, whereas Droogfontein and Silvery Home are more typical of the Group II kimberlites. An apparent gap in 87Sr/86Sr between ~ 0.705 and ~ 0.706 suggests that at least in terms of Sr-isotope values, so-called transitional kimberlites need not have intermediate compositions between Group I and Group II magmas.