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