Evolution of the South African mantle—a case study of garnet peridotites from the Finsch diamond mine (Kaapvaal craton); Part 2: Multiple depletion and re-enrichment processes

Abstract

Recalculated whole rock major-, trace-element and isotope compositions from mineral analyses of Finsch mantle peridotites indicate a complex history of the South African sub-continental lithospheric mantle (SA-SCLM). According to the Lu–Hf whole rock isotopic data last depletion coupled with the final cratonization of the Kaapvaal craton underneath Finsch occurred at ~2.6Ga. The data of this study indicate several depletion and enrichment processes during that early history of Finsch SCLM. Calculated degrees of partial melting using major and rare earth element (REE) compositions show that all depletion episodes together have removed more than 40% of melt from the SA-SCLM.As indicated by elevated Hf concentrations and subchondritic Lu/Hf, many Finsch peridotites were metasomatically overprinted between the last and earlier depletion stages. Silica enrichment, expressed in high modal abundance of orthopyroxene in the majority of the samples likely took place at that early stage. The last coupled melting and re-enrichment event (at ~2.6Ga) resulted in complete re-equilibration of trace element and Hf isotope signatures at the whole rock scale and thus, erased most signs of previous events. After the final depletion and before the final kimberlite eruption, the Finsch mantle suffered from further metasomatic overprints. Metasomatism is expressed in the enrichment of light REE (LREE) and some large ion lithophile elements (e.g. Sr, Rb, Th). Additionally, the Nd isotope signatures of all samples as well as the Hf isotope compositions of some samples were modified.Late enrichment is documented by a modally metasomatized sample that contains rutile and sulfide. This sample is characterized by strong enrichment of high-field-strength elements and LREE and was probably overprinted with a sulfide bearing silico-carbonatitic melt with high contents of Ti, Fe and Ni. This melt likely originated from an enriched mantle that was isolated for a long time during which it developed a very unradiogenic Hf isotope signature, characteristic for this type of metasomatism.