Formation of garnet kelyphite rims on kimberlite-bourne peridotite xenoliths

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Garnet peridotite xenoliths are fragments of the deep cratonic lithopshere, accidentally entrained in and transported to the surface by kimberlite magmas. Kimberlite magmas are very deeply sourced volcanic rocks, which sometimes host economic grades of diamonds.

The garnet grains in the xenoliths invariably exhibit evidence of breakdown to kelyphite rims. These rims normally consist of spinel, amphibole, phlogopite and other minerals, but they have never been fully investigated. They have typically been assumed to represent garnet decomposition at magmatic temperatures during transport in the host kimberlite from very high pressures to the surface. Alternatively or additionally, they may have formed during reaction of garnets with infiltrating host kimberlite magma, or with deep metasomatic fluids or melts in the lithopshere before eventual entrainment in the kimberlite. If the latter, they may provide important information about early stages of kimberlite magmatism and metasomatism in the cratonic lithopshere.

The aim of this project is to test these hypotheses by more carefully examining some examples of garnet kelyphitic breakdown rims from xenoliths from the Kaapvaal Craton, southern Africa. Your mission (should you choose to accept it) will be to fully characterise their mineral assemblages and the major, minor and trace element chemistries of the constituent minerals. Techniques to be used will include optical microscopy, Field Emission Scanning Electron Microscopy, Electronprobe Microanalysis and Laser Ablation – ICPMS.

 

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