Olivine: a monitor of magma evolutionary paths in kimberlites and olivine melilitites |
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| Authors: | A E Moore |
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| Institution: | (1) Department of Geochemistry, University of Cape Town, Private Bag, 7700 Rondebosch, South Africa;(2) Present address: AMPAL, Box 10072, Gaborone, Botswana |
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| Abstract: | Petrographic and chemical criteria indicate that the overwhelming majority of olivines in kimberlites are probably cognate phenocrysts. The implied low volume of xenocryst olivines requires that primitive kimberlite magmas are highly ultrabasic liquids. Two chemically distinctive olivine populations are present in all of the kimberlites studied. The dominant olivine population, which includes large rounded olivines and smaller euhedral crystals, is Mg-rich relative to late-stage rim compositions. It is characterized by a range in 100 Mg/(Mg + Fe) and uniform Ni concentration, reflecting Rayleigh-type crystallization during magma evolution. The most Mg-rich of these olivines are considered to be similiar to those in the mantle source rocks. The second compositional population, generally very subordinate, though markedly more abundant in the megacrystrich Monastery kimberlite, is Fe-rich relative to rim compositions. This group of olivines crystallized from evolved liquids in equilibrium with iron-rich megacrysts, both entrained by the kimberlite magma during ascent. Differences between the chemical fields of Fe-rich olivines in Group I and Group II kimberlites point to relatively deeper derivation of the latter suite. Olivine chemistry can be used to characterize kimberlite magma sub-types, and may prove to be a useful tool for evaluating the diamond potential of kimberlites. |
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