The geochemistry and petrogenesis of MARID and PIC xenoliths

MARID (Mica-Amphibole-Rutile-Ilmenite-Diopside) and PIC (Phlogopite-Ilmenite-Clinopyroxene) rocks are rare mantle samples, most often occurring as xenoliths in kimberlites. Both MARID and PIC xenoliths contain high modal abundances of phlogopite, which does not occur in the “depleted mantle” – i.e. the residue that represents the protolith of Archean mantle roots, complementary to the formation of continental crust. Consequently, these rocks are frequently interpreted as the products of extreme mantle metasomatism. Previous studies of these rocks have focussed on acquiring isolated datasets on small sample suites. This approach has made it difficult to explore possible relationships within the spectrum of metasomatised, phlogopite-rich xenoliths, even from a single locality. It therefore remains unclear how MARID and PIC rocks are formed (whether by metasomatism of peridotites, or by melt crystallisation in mantle veins). The nature of MARID parental melts/fluids also remains unidentified. Finally, MARID geochemical compositions (e.g., high K2O content) and “enriched mantle” radiogenic isotope signatures are often cited as evidence for their presence in the source of mantle-derived potassic magmas, such as lamproites. This study seeks a greater understanding of the causes and effects of mantle metasomatism by studying the petrographic, geochemical, and isotopic (radiogenic Sr-Nd-Hf-Pb and stable N-O-S-Tl) features of 26 MARID and PIC samples from southern African kimberlites (Kimberley) and orangeites (Newlands). Previous inferences of a genetic link between PIC rocks and kimberlite melts are substantiated by clinopyroxene trace element compositions in equilibrium with kimberlite melts, and the similar radiogenic isotopic compositions of PIC clinopyroxene (87Sr/86Sri = 0.7037–0.7041) and southern African Cretaceous kimberlites (87Sr/86Sri = 0.7032–0.7048). Based on petrographic observations of kimberlite groundmass minerals (e.g., calcite, perovskite) as inclusions