Degassing in kimberlite: oxygen isotope ratios in perovskites from explosive and hypabyssal kimberlites

Interest in the radiogenic and trace element characteristics of uncontaminated kimberlitic magma has been rekindled by recent isotope studies on groundmass perovskite, which is a major sink of most of the trace elements including rare earth elements (REE) in kimberlite magma. Perovskite is also quite resistant to sub-aerial weathering. In-situ trace element, oxygen isotope ratios and conventional TIMS Sr isotopic analyses of perovskites are presented for two kimberlites: one from Orapa, a crater facies group I kimberlite from Botswana, and the other from Wesselton, a hypabyssal facies group I kimberlite from South Africa. The perovskites from Orapa exhibit greater ranges in trace element abundances and 87Sr/86Sr ratios than those from Wesselton (La = ~ 7000–13,600 times primitive mantle and 87Sr/86Sr = 0.70310–0.70541 in Orapa perovskites, and La = ~ 4000–7000 times primitive mantle and 87Sr/86Sr = 0.70441–0.70558 in Wesselton). The δ18O values from Orapa perovskites have two distinct peaks around + 3.6‰ and − 0.6‰, which are unlikely to reflect crustal assimilation processes. Wesselton perovskites in contrast are clustered around δ18O values of + 4‰. Perovskite in equilibrium with the mantle has lower δ18O than other common upper mantle minerals (olivine, orthopyroxene). The group of Orapa perovskites with δ18O clustering around + 3.6‰, and the Wesselton perovskites are interpreted to have δ18O values (~ 4.2‰) that reflect the δ18O of uncontaminated kimberlite magma of an upper mantle origin (~ 5.2‰). The negative δ18O values from the second group of Orapa perovskites are attributed to crystallisation of perovskite after magma degassing that depleted 18O in the residual magma. Abundance of magmatic carbonate in the Wesselton sill, together with the absence of any perovskite with very low δ18O, indicates a lack of significant degassing for the Wesselton kimberlite.