Progressive metasomatism of the mantle by kimberlite melts: Sr–Nd–Hf–Pb isotope compositions of MARID and PIC minerals

Abstract

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks occur as mantle-derived xenoliths in kimberlites and other alkaline volcanic rocks. Both rock types are alkaline and ultramafic in composition. The H2O and alkali metal enrichments in MARID and PIC rocks, reflected in abundant phlogopite, have been suggested to be caused by extreme mantle metasomatism. Radiogenic (Sr–Nd–Hf–Pb) isotope and trace element compositions for mineral separates from MARID (clinopyroxene and amphibole) and PIC (clinopyroxene only) samples derived from Cretaceous kimberlites (Kimberley) and orangeites (Newlands) from South Africa are used here to examine the source(s) of mantle metasomatism. PIC clinopyroxene (n=4) is relatively homogeneous, with narrow ranges in initial isotopic composition (calculated to the emplacement age of the host Bultfontein kimberlite; 87Sr/86Sri: 0.7037–0.7041; εNdi: +3.0 to +3.6; εHfi: +2.2 to +2.5; 206Pb/204Pbi: 19.72–19.94) similar to kimberlite values. This is consistent with PIC rocks representing peridotites modified by intense metasomatic interaction with kimberlite melts.The MARID clinopyroxene (n=9) and amphibole separates (n=11) studied here display broader ranges in isotope composition (e.g., 87Sr/86Sri: 0.705–0.711; εNdi: −11.0 to −1.0; εHfi: −17.9 to −8.5; 206Pb/204Pbi: 17.33–18.72) than observed in previous studies of MARID rocks. The Nd–Hf isotope compositions of kimberlite-derived MARID samples fall below the mantle array (ΔεHfi between −13.0 and −2.4), a feature reported widely for kimberlites and other alkaline magmas. We propose that such displacements in MARID minerals result from metasomatic alteration of an initial “enriched mantle” MARID composition (i.e., 87Sr/86Sri = 0.711; εNdi = −11.0; εHfi = −17.9; and 206Pb/204Pbi = 17.3) by the entraining kimberlite magma (87Sr/86Sr∼i0.704; εNd∼i+3.3; εHf∼i+2.3; 206Pb/204Pb∼i19.7). A model simulating the flow of kimberlite magma through a mantle column, thereby gradually equilibrating the isotopic and chemical compositions of the MARID wall-rock with those of the kimberlite magma, broadly reproduces the Sr–Nd–Hf–Pb isotope compositions of the MARID minerals analysed here. This model also suggests that assimilation of MARID components could be responsible for negative ΔεHfi values in kimberlites. The isotopic composition of the inferred initial MARID end-member, with high 87Sr/86Sr and low εNd, εHf, and 206Pb/204Pb, resembles those found in orangeites, supporting previous inferences of a genetic link between MARID-veined mantle and orangeites. The metasomatic agent that produced such compositions in MARID rocks must be more extreme than the EM-II mantle component and may relate to recycled material that experienced long-term storage in the lithospheric mantle.