New constraints on the petrogenesis of the Nuanetsi picrite basalts from Pb and Hf isotope data

Previous studies established strong linear correlations between 143Nd/144Nd and a variety of incompatible element abundances and inter-element ratios for a suite of picrite basalts from Nuanetsi in the Karoo CFB province. While a heterogeneous source was clearly indicated, the extent to which mantle heterogeneity exceeds that sampled by magmatism was unconstrained because the linear correlations can be extrapolated to extreme end-members well beyond the picrite basalt compositions. The Nuanetsi samples showed a small variation in Pb isotope composition and any coherent Pb-Nd isotope relationship was obscured by the likely analytical uncertainty. New Pb isotope data obtained by MC-ICP-MS reveal coherent Pb-Nd isotope arrays that are suggestive of hyperbolic mixing curves. New Hf isotope data reveal very similar Pb-Hf relationships. Least-squares hyperbolic fits are used to constrain the asymptotes of these putative mixing hyperbolae, thereby defining end-member isotopic compositions. The low εNd, low εHf end-member equates with the enriched lithospheric mantle component previously postulated, and it is now possible to define the Pb isotope characteristics of this end-member more precisely. The higher εNd and εHf end-member is constrained to have sub-chondritic 143Nd/144Nd and 176Hf/177Hf. These characteristics are not readily reconciled with an asthenospheric source because there are few oceanic basalts with sub-chondritic εNd and it seems unlikely that the Karoo would fortuitously have sampled an extreme asthenospheric composition. Either, the Nuanetsi picrite basalts are derived largely from lithospheric mantle, or they sample a more complex mixture of source materials than previously inferred.