Helium-ratios of rocks and minerals from the diamond pipes of South Africa

Abstract

It is well known that the diamond pipes of South Africa are occupied by kimberlite in varying states of brecciation and decomposition, together with inclusions of widely different sizes, shapes, and petrographic types. Indeed, so many of the “phenocrysts” of kimberlite are xenocrysts transported from below that it is difficult to determine the composition of the essential kimberlite magma which was responsible for the perforation of the crust and the formation of the pipes. The larger inclusions— commonly referred to as “nodules”—can be conveniently divided into three groups, according to whether they represent: 1—strata now removed by denudation which were penetrated by the uppermost parts of the pipes; 2—formations now exposed in the walls of the pipes opened up by mining, including granites, gneisses, and amphibolites belonging to the Pre-Cambrian basal complex; and 3—metamorphic and igneous rocks brought up from below.