Abstract
Megacrysts (clinopyroxene, orthopyroxene, garnet, olivine and ilmenite) from different kimberlites of the northern Lesotho—eastern Orange Free State Cluster are characterised by Closely overlapping Compositional fields. Minerals in sheared peridotites from these pipes define Continuous chemical fields that straddle the compositional gap between Corresponding phases in coarse, relatively low temperature peridotites, and the Mg-rich (high temperature) extreme of the megacryst suite. The highest temperatures recorded for the sheared peridotites are from rocks with minerals closest in composition to the high temperature megactysts. These relationships are consistent with models that interpret sheared peridotites as representing metasomatised mantle samples that have responded chemically and thermally to the magma, here inferred to be the host kimberlite, which was parental to the megacryst suite. The latter crystallised in pegmatitic veins from small volumes of liquid injected into a fracture network in the thermal aureole surrounding the main kimberlite magma reservoir in the mantle prior to eruption. Crustal contamination of the kimberlite magma during ascent may in part explain Sr-isotopic signatures that differ from those of the entrained megacrysts. However, such differences can also be ascribed to melting of a Sr-rich carbonate in the thermal aureole surrounding the kimberlite magma in the mantle prior to eruption. This carbonate melt would mix with the small volumes of kimberlitic liquid injected into the fracture network surrounding the main magma reservoir. An important implication of this model is that sheared xenoliths cannot be used to infer vertical mantle zonation, and thus chemical and isotopic differences between the lithosphere and asthenosphere. The model accounts for the link between ilmenite chemistry and kimberlite diamond grade. It also provides an explanation for the range in zircon ages which are reported in some kimberlites
