Micro-geochemistry of the mantle and its volcanic rocks

Abstract

This thesis comprises three studies which utilise detailed micro-sampling techniques to understand the effects of melt infiltration in the continental lithosphere and the relationship of deeply derived kimberlite melts to lithospheric processes. To be able to make these links it is necessary to better constrain the geochemical composition of the primary kimberlite melt.

• The first study investigates inter-grain trace element and Sr isotope variation in clinopyroxene (cpx), amphibole and melt from two spinel lherzolite xenoliths from the Middle Atlas Mountains, Morocco. These results support a multi-stage metasomatic process in which cpx and amphibole are recent additions to the lithospheric mantle, most recently equilibrated with a carbonatite melt. The limited Sr isotope variation in the cpx from this study is typical of global off-craton cpx implying that the majority of cpx in off-craton settings may have a recent, metasomatic origin.

• The second micro-sampling study examines inter-grain trace element and Sr isotope variation in garnet and cpx in garnet lherzolite xenoliths within cratonic peridotites from Bultfontein, South Africa, and Pyramidfels, Greenland. The Sr isotope and trace element variation, combined with a lack of trace element equilibrium between cpx and garnet, imply that the majority of cpx is a recent addition, associated with the host kimberlite-type magma, whereas the garnet is added by an older event, probably by a melt related to either Group II kimberlite or lamproite. A global database of Sr and Nd isotope data for cpx and garnet indicates that these conclusions can be applied more widely. The metasomatic addition of cpx and garnet is limited to vein-wallrock reactions and therefore the majority of lithospheric mantle is concluded to be depleted harzburgite or dunite.


• The third micro-sampling study focuses on constraining the primary geochemical characteristics of kimberlite magma by analysing the constituent phases (perovskite, olivine, apatite, phlogopite, calcite) for trace element and Sr isotope ratios from a sample of hypabyssal kimberlite from Jos, Somerset Island, Canada. An analogue study was carried out on an olivine melilitite from Saltpetre Kop, South Africa with a similar mineral suite (with the addition of melilite). These results show that phenocrystal olivine and apatite provide the best record of the source Sr isotope composition whereas perovskite, typically analysed by laser ablation, is more variable and influenced by crustal interaction, albeit to a limited extent. The results indicate that previously recorded differences between the Sr isotope compositions of whole rock kimberlites and the low-Cr megacryst suite, used to rule out a genetic relationship between the two, can be ascribed to later stage crustal interaction with the kimberlite. As a result, the link between kimberlites and the formation of the low-Cr megacryst suite is strengthened.