LOW-CA GARNET HARZBURGITES FROM KIMBERLEY, SOUTH-AFRICA - ABUNDANCE AND BEARING ON THE STRUCTURE AND EVOLUTION OF THE LITHOSPHERE

Based on composition and abundance of garnets from heavy mineral conce ntrates and from ultramafic nodules, it is estimated that low-calcium garnet harzburgites, thought to be the host rocks' of most diamonds in the upper mantle, form 2% of the mantle (5% of the garnet-bearing por tion) sampled by Group I kimberlites at Kimberley, South Africa. As bo th sampling approaches (xenocryst abundance and xenolith abundance) yi eld virtually identical estimates, there is no need to invoke the pres ence of magnesite or interstitial liquid to cause disaggregation of lo w-Ca garnet harzburgites during eruption. Six of the eleven low-Ca gar net harzbur,oite xenoliths identified contain zoned garnets and althou gh there is considerable variation between samples, typically Ca incre ases and Cr decreases from core to rim. All other minerals are homogen eous (except in one rock) and magnesian (e.g., Mg/(Mg + Fe) in olivine = 0.929 - 0.949). Four of the eleven samples contain magnesio-chromit e (59.7 - 62.6 wt % Cr2O3), and two have primary phlogopite. These roc ks have apparently equilibrated at pressure and temperature conditions equivalent to those on a conductive subcontinental geothermal gradien t, mostly in the diamond stability field, and within the range of equi libration of the majority of garnet lherzolites from Kimberley. Zoning is interpreted as due to re-equilibration of the Ca-poor domains towa rd the local bulk composition of the diopside-saturated Iherzolite upp er mantle. Introduction of Ca may have been fluxed by the presence of a fluid (also introducing phlogopite), but uniformly low TiO2 values a rgue against interaction with a silicate melt. Such Ca metasomatism, i f post diamond formation, could ultimately result in transformation of low-Ca garnet harzburgites into garnet lherzolites that could contain diamonds with low-Ca garnet harzburgite paragenesis inclusions.