Schonland Research Centre for Nuclear Sciences

Ion-Beam Microprobe

Diamond Genesis and Mantle Geophysics as revealed by the trace element composition in selected samples containing peridotitic sulphide inclusions

Council for GeoSciences : RJ Hart
SRCNS : SH Connell

Most researchers of Archaean geology now believe that diamonds that originate beneath the continents are syngenetic minerals of rocks that formed during the early evolution of the lithosphere, and that fragments of these diamond-bearing rocks were then later transported to surface during kimberlite or lamproite eruptions. The most important clues as to the nature of the rocks that host the diamonds are minute minerals that are often found encapsulated in some of the diamonds. These minerals provide information about the physical and chemical environment in which diamonds crystallised. Most of the diamond inclusions belong to one of two suites of rock: a peridotitic suite (mainly garnet-harzburgite and garnet- dunite) or an eclogitic suite (mainly clinopyroxene and garnet), and the peridotite suite is by far the most abundant, and makes up about 80% of the inclusion types. The inclusions also broadly correspond to the two most abundant ultramafic xenoliths types found in kimberlite. So in general our current understanding is that the deep seated rocks beneath the Archaean cratons, such as the Kaapvaal craton in South Africa, consist dominantly of peridotite and lesser amounts of eclogite, and there are essentially two schools of thought on the way these rocks that make up the cratons originated. Previous studies reported on compared the chemistry of diamond inclusions to ocean floor peridotites and basalt to try and find a link between Archaean cratonic rocks and rocks formed at mid ocean ridges. In the current phase, the Platinum group elements are focussed on, as indicators that could illuminate the debate further. The project to study the PGE distributions and relative concentrations is well underway.

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                                                                                                                            Last updated: 5 October 1998