School of Earth Sciences - Theses
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ItemThe geochemistry and petrogenesis of MARID and PIC xenoliths( 2019)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 in secondary clinopyroxene rims, PIC rocks must be the result of kimberlite metasomatism, and not the melt source for kimberlite magmas. Furthermore, PIC rocks appear to be closely related to some phlogopite-bearing peridotites and wehrlites from the lithospheric mantle beneath Kimberley, due to similarities in their mineral geochemical compositions. It can therefore be inferred that these lithologies are the result of variable degrees of interaction between kimberlite melts and the lithospheric mantle. This study also presents mineralogical, geochemical, and isotopic evidence that kimberlites infiltrate and affect the mineral and bulk-rock compositions of the MARID rocks that they host, complicating interpretations of MARID genesis. By filtering out such contamination effects, new interpretations regarding these rocks can be reached. Although many studies support a magmatic genesis, the ranges in primary MARID mineral major element compositions presented in this thesis overlap with those in extremely metasomatised peridotites, opening the possibility of MARID genesis by metasomatic overprinting of peridotite protoliths. Some lherzolites studied herein have similar geochemical and isotopic signatures to MARIDs, suggestive of a common parental metasomatic fluid. Prior to kimberlite infiltration, MARID rocks appear to have had “enriched mantle” radiogenic isotopic signatures, which may indicate that the source of MARID parental melts contained recycled material – comprising both oceanic and continental crust, based on stable isotope considerations. Finally, bulk-rock reconstructions of MARID compositions were conducted to eliminate secondary features, from which the trace element composition of MARID-derived melts was modelled. These melts closely match the compositions of mantle-derived magmas, such as orangeites, lamproites, and some ultramafic lamprophyres, suggesting that MARID-veined mantle could indeed be the source of alkaline ultramafic magmas in intracontinental settings.