Mantle heterogeneity beneath southern Africa: evidence from the volcanic record

Abstract

The scale, timing and development of mantle heterogeneity beneath southern Africa is assessed by reference to data obtained from mantle derived igneous rocks and, to a lesser extent, from peridotites contained in kimberlite. Sr isotope data for ultrabasic and basic igneous rocks ranging in age from 3.5 Ga komatiites to Tertiary oblivine melilitites indicate that heterogeneity existed at 3 Ga and was well established by 2 Ga, and also suggest progressive development of variability in Sr isotope ratios in the mantle source regions involved. Detailed evaluation of Sr isotope and incompatible element and inter-element ratios, together with rare earth element patterns, of the widespread Jurassic Karroo volcanics shows that the overall compositional variability of these volcanics is best explained by (horizontal) mantle heterogeneity. Both depletion and enrichment pre-Karroo processes appear to have affected the parental mantle source regions. Evidence for such enrichment is provided by kimberlite peridotite nodules that have been subjected to mantle metasomatic processes, leading to the development of phlogopite and the amphibole potassic richterite, with consequent enrichment of incompatible elements.