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

Chiranjeeb Sarkar, Craig D. Storey, Chris J. Hawkesworth, R. Stephen J. Sparks

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    Abstract

    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.

    Original languageEnglish
    Pages (from-to)291-299
    Number of pages9
    JournalEarth and Planetary Science Letters
    Volume312
    Issue number3-4
    DOIs
    Publication statusPublished - 15 Dec 2011

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