Geochemical processes in peridotite xenoliths from the Premier diamond mine, South Africa: Evidence for the depletion and refertilisation of subcratonic lithosphere

This paper serves to document evidence for lithospheric depletion and enrichment processes through the analysis of mineral and bulk rock compositional trends in garnet-bearing peridotite xenoliths from the Premier kimberlite. The xenolith suite at Premier shows petrographic and compositional trends comparable to kimberlite-derived peridotite xenoliths world-wide, although hot, deformed xenoliths are unusually abundant. Observed differences between xenolith groups e.g. between coarse and deformed xenoliths follow trends which, by now, have been well documented for xenolith suites world-wide. However, there are also important similarities in mineral and bulk chemistry between lherzolite and harzburgite xenoliths from Premier. For instance, xenoliths containing garnets with 4 to ~ 10 wt.% Cr₂O₃ are recognised amongst both varieties, and all of these have bulk Al₂O₃ < 2 wt.%, with many < 1 wt.% Al₂O₃. Furthermore, examples with Y in garnet < 10 ppm and/or Zr < 30 ppm are recognised amongst all coarse xenolith varieties analysed. This is also a feature of the deformed harzburgites containing subcalcic garnet, and it is evident that xenolith deformation is not necessarily always accompanied by melt-related metasomatism. Evidence for high levels of depletion as measured by low bulk Al₂O₃ is not only seen in subcalcic and calcic harzburgite xenolith varieties, but is also a feature of a number of coarse and deformed lherzolites. This suggests that all xenolith varieties probably derive from the same (or similar), highly depleted protolith(s) but with varying levels of metasomatic overprint associated with a refertilisation process.