An isotopic and geochemical study of Kimberlites and associated alkaline rocks from Namibia

Abstract

Abstract taken from Dissertation Abstracts International, vol 50, no 10, April 1990, p. 4412-B [agrees with abstract in original thesis] :

One kimberlite pipe (33/K2) is enriched in Pb, Rb, Ba, K, Sr and P and has enriched isotopic characteristics, interpreted as the result of assimilation, during ascent, of older phlogopite-K-richterite-peridotite (PKP) type metasomatised lithosphere. Isotopically and geochemically, this kimberlite is intermediate between previously recorded group I and II kimberlites

Garnet and pyroxene megacrysts (hosted by the kimberlites) have distinctly different Sr, Nd and Pb isotopic compositions to the kimberlite indicating their xenocrystal origins. Geochemical and isotopic variations in the megacrysts indicate their origin from an OIB-type, fractionating parental magma which was synchronously assimilating enriched lithosphere in an AFC process

Hotspot traces extending from the Gibeon area to the present day Vema and Discovery seamounts are also coincident with non-kimberlite alkali magmatism. These alkaline intrusions have geochemical and isotopic signatures equivalent to the kimberlites and Bouvet. It is concluded that a gradation exists between kimberlite and alkali basalt, with depth of melting being the dominant control affecting magma type