Confocal Raman spectroscopic study of melt inclusions from peridotite xenoliths in economic and barren kimberlites from Kaapvaal Craton

Abstract

Melt inclusions trapped in minerals inside xenoliths from kimberlites can help to examine the composition of kimberlite melt and/or metasomatic processes in the subcratonic lithospheric mantle as well as shed more light on the role of these melts in diamond destruction. In this study, confocal Raman spectroscopy of secondary melt inclusions in olivine from xenoliths in five different Kaapvaal Craton kimberlites was used for testing any compositional differences between melt inclusions from economically productive (Bultfontein and Frank Smit) and uneconomic diamond barren (Matsoku, Thaba Putsoa, and Pipe 200) kimberlite pipes. The xenoliths represent a range of pressures (37–45 kbar) and temperatures (1000–1300°C). The 26 daughter minerals identified within melt inclusions include Ca–Mg (±Na, K, P, Cl)-bearing carbonates, alkali (±Ca, Ba, Cl, F, H₂O, CO₂)-bearing sulfates, phosphates, oxides, silicates, and a rare nitrate. The mineral assemblages in melt inclusions are similar in both economic and barren kimberlite pipes from the interior and the edge of the craton, indicating the similar composition of the entrapped melts in all studied samples. However, the petrographic study revealed different metasomatic processes recorded by xenoliths from barren and economic kimberlites. Metasomatism by a melt enriched in K, Ca, and H₂O could be instrumental in diamond destruction and the low diamond grade of the three barren kimberlites from Lesotho. Our study revealed no effect of kimberlite melt composition on diamond preservation in the studied kimberlites: instead, diamond grade is most likely affected by diamond destruction in the mantle source prior to kimberlite emplacement with kimberlite ascent.