Proterozoic highly siderophile element enrichment of the Kaapvaal cratonic Archean mantle

The c. 86 Ma Jagersfontein kimberlite in the southern portion of the Kalahari craton in South Africa erupted a suite of refractory peridotite xenoliths with regionally anomalous rhenium+osmium (ReOs) isotope systematics and highly siderophile element (HSE) abundances. A portion of this suite comprises peridotites with 2.7 to 3.0 Ga Re- depletion¹⁸⁷Os/¹⁸⁸Os model ages, refractory major elements and HSE abundances that represent some of the most depleted and least metasomatised known components of Archean cratonic nuclei. Other peridotites in the Jagersfontein suite show enrichment in Os, Ir, Pd, Pt and, especially, Ru, as well as having more radiogenic¹⁸⁷Os/¹⁸⁸Os. Ruthenium abundances reach up to ∼22 times “Primitive Upper Mantle” levels and correspond to samples containing microscopic laurite plus pentlandite. The more radiogenic¹⁸⁷Os/¹⁸⁸Os in HSE-enriched samples, along with the modern mantle like Ru isotope composition (ε¹⁰⁰0.00 ± 0.12, ε¹⁰²–0.03 ± 0.16), indicate that the prominent HSE enrichment is not related to an excess of unequilibrated Late Veneer material in the mantle source. Instead, the HSE characteristics and Os isotope ratios of these peridotites are inferred to have been caused by metasomatic addition of small volumes of HSE-rich sulfide/alloy to the refractory Archean peridotites. Although there is some variation in the metasomatic characteristics, the calculated¹⁸⁷Os/¹⁸⁸Os of the contaminant causing the strongly Ru-enriched peridotites is ∼0.116, which is equivalent to the convecting mantle composition in the early Mesoproterozoic. Unlike the erosion and Proterozoic replacement of older mantle interpreted to explain post-Archean¹⁸⁷Os/¹⁸⁸Os mantle beneath the central Kaapvaal nucleus, the Archean mantle lithosphere beneath Jagersfontein therefore appears to have been modified by the passage of Proterozoic melts. The 2400 km²1.9 Ga layered mafic Trompsburg Complex, less than 10 km southeast of Jagersfontein, indicates that the mantle lithosphere in the area has experienced substantive transfer of large volumes of post-Archean melt, and we suggest that the passage of those melts may have caused the mantle HSE enrichment.