Mantle metasomatism: Isotope and trace-element trends in xenoliths from Kimberley, South Africa

Abstract

Modal metasomatism in peridotite xenoliths from Kimberley, South Africa, is observed in the development of phlogopite, K-richterite, diopside and LIMA. The average bulk-rock minor- and trace-element contents of different peridotite types range up to 100× primitive mantle, and the bulk-rock compositions reflect the modal proportions of metasomatic minerals in individual samples. Present-day whole-rock 87 Sr 86 Sr =0.7045–0.7118 and 206 Pb 204 Pb =17.48–21.24 , and they have been little affected by interaction with the host kimberlite which has 87 Sr 86 Sr =0.7042 and 206 Pb 204 Pb =18.6–19.2 . The preferred interpretation is that these isotope ratios reflect Rb/Sr and U/Pb fractionation during the modal metasomatism ∼150 Ma ago, i.e. after Karoo magmatism at ∼190 Ma and prior to emplacement of the kimberlite at 90 Ma. Present-day ϵ Nd= −9.3 to +2.4, reflecting both the presence of low-Sm Nd Proterozoic mantle prior to metasomatism at 150 Ma, and the inferred shift to relatively high ϵ Nd during that metasomatism. Rb/SrU/Pb fractionation during metasomatism has resulted in a distinctive negative correlation between present-day 87 Sr 86 Sr and 206 Pb 204 Pb . This contrasts with positive SrPb isotope trends in MORB glasses and selected OIB, but it is similar to that observed in certain CFB provinces. The trend to relatively high Rb/Sr at low U/Pb appears to be linked to the development of a potassic phase, typically phlogopite, and as such it arguably offers the first evidence for a style of minor- and trace-element enrichment which is restricted to shallow levels in the Earth's mantle.