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 Sr-87/Sr-86 ratios than those from Wesselton (La = similar to 7000-13,600 times primitive mantle and Sr-87/Sr-88 = 0.70310-0.70541 in Orapa perovskites, and La = similar to 4000-7000 times primitive mantle and Sr-87/Sr-88 = 0.70441-0.70558 in Wesselton). The delta O-18 values from Orapa perovskites have two distinct peaks around +3.6 parts per thousand and -0.6 parts per thousand, which are unlikely to reflect crustal assimilation processes. Wesselton perovskites in contrast are clustered around delta O-18 values of +4 parts per thousand. Perovskite in equilibrium with the mantle has lower delta O-18 than other common upper mantle minerals (olivine, orthopyroxene). The group of Orapa perovskites with delta O-18 clustering around +3.6 parts per thousand, and the Wesselton perovskites are interpreted to have delta O-18 values (similar to 4.2 parts per thousand) that reflect the delta O-18 of uncontaminated kimberlite magma of an upper mantle origin (similar to 5.2 parts per thousand). The negative delta O-18 values from the second group of Orapa perovskites are attributed to crystallisation of perovskite after magma degassing that depleted O-18 in the residual magma. Abundance of magmatic carbonate in the Wesselton sill, together with the absence of any perovskite with very low delta O-18, indicates a lack of significant degassing for the Wesselton kimberlite. (C) 2011 Elsevier B.V. All rights reserved.