Petrogenesis of kimberlites from South Africa and lamproites from Western Australia and North America

Fraser, Kirsten Jane (1988). Petrogenesis of kimberlites from South Africa and lamproites from Western Australia and North America. PhD thesis The Open University.



Group 2 kimberlites from South Africa, and lamproites from Western Australia and North America are relatively unfractionated mantle-derived igneous rocks, situated on or close to ancient cratonic areas. They are characterised by high trace element contents, while the range in N d and Sr isotopes encompasses much of that reported for various upper and lower crustal rocks. It is argued that these features are not due to crustal contamination during magma ascent, rather they are source and extraction phenomena.

The mantle source regions of these rocks were ancient (~ 1.0 to 2.5 Ga) and variably trace element enriched. Preservation of such regions within the mantle is most probable in the relatively 'cold' and 'rigid' subcontinental mantle lithosphere, which is believed to have been isolated from the convecting asthenosphere for a long time. The source regions of the kimberlites and lamproites were situated at various depths within the subcontinental mantle lithosphere, from within the amphibole stability field « 100 km) to within the diamond stability field (> 150 km). Low degrees of partial melting « 1 %), together with volatile composition and depth of melting, have significantly influenced the composition of the resultant kimberlite and lamproite magmas. Those magmas that originated from within the diamond stability field contain abundant entrained and dis aggregated mantle peridotite. This feature is related to melt migration and rapid ascent to the surface, from these mantle depths.

The Sr, Nd and Pb isotope data record evidence of variable, but related trace element enrichment styles. The origin of these trace elements is either from recycled continental crust (e.g. pelagic sediment), or from intra-mantle processes (e.g. the migration and crystallisation of small volume silicate melts with variable volatile compositions). The available data are insufficient to determine between the models and further work in this area is required.

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