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Some aspects of the mineralogy and petrology of ultramafic xenoliths in kimberlite

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Abstract

Brief petrographic descriptions are given of 32 types of ultramafic xenoliths that occur in the kimberlites of Lesotho, Rhodesia, South Africa, and Tanzania. Six new mineral assemblages, which are believed to be primary, are recorded for such xenoliths. These are: olivine-orthopyroxene-clinopyroxene-amphibole; garnet-spinel; garnet-clinopyroxene-quartz; garnet-clinopyroxene-high sanidine; garnet-clinopyroxene-plagioclase-quartz; and garnet-clinopyroxene-scapolite-amphibole.

Volumetric data for 224 xenoliths have shown that the eclogites are distinct from the peridotites and pyroxenites and do not appear to grade into them. The latter rock types have mineral proportions suggestive of two, and possibly three, disconnected series; a pronounced division exists at 25 % olivine.

The optical properties of olivine and orthopyroxene in 122 peridotite/pyroxenite xenoliths have been measured. Both sets of data indicate that these minerals have remarkably constant, and high magnesium, compositions, the former ranging from Fo88–94% and the latter from Of1.5–11% with one unique sample of hypersthene (Of34.5%). One orthopyroxene from an eclogite was found to have a composition of Of15%.

Similar optical data were obtained for clinopyroxene from 71 peridotites/pyroxenites and 50 eclogites, but compositional information could not be derived.

The physical properties (n and a 0) of garnet from 136 peridotites and pyroxenites have indicated a compositional range of very similar almandine: (uvarovite + andradite) ratio, but varying appreciably in both pyrope and (uvarovite + andradite). Similar measurements of garnet from 57 eclogitic rocks have enabled us to locate 4 new compositional areas in addition to the four previously proposed. Garnets from plagioclase-eclogites, quartz-plagioclase-eclogites, high sanidine-eclogite plus scapolite-amphibole-eclogite, and garnet-spinel plus garnet-corundum rocks form the new groups.

Optical data for kyanite, corundum, plagioclase (An18–42%), scapolite (Me61–63%) and spinel are reported together with unit cell edge lengths for the last-named phase. The spinel in the garnet-spinel rocks is essentially pleonaste but it contains unusually large amounts of Ni and Zn.

These results have been interpreted to indicate the possibility of two distinct fractionation trends within the Upper Mantle. Calcium-aluminium enrichment that operates in the lower regions results in the series Eclogite→Kyanite-Eclogite→Corundum-Eclogite as crystal cumulates from a melt formed by partial fusion of garnet-peridotite. A similar melt, possibly derived from a similar depth but transported to high levels in the Upper Mantle, is the parent for a silica enrichment trend Eclogite→Plagioclase-Eclogite→Plagioclase-Quartz-Eclogite.

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Author notes

  1. J. C. Siebert

    Present address: Dept. of Geology, University of the Witwatersrand, Johannesburg, South Africa

  2. P. C. Rickwood

    Present address: School of Applied Geology, University of New South Wales, Kensington, N.S.W., Australia

Authors and Affiliations

  1. Division of Geological Sciences, University of Cape Town, South Africa

    M. Mathias, J. C. Siebert & P. C. Rickwood

  2. Department of Geology, University of Cape Town, Rondebosch, South Africa

    M. Mathias

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Mathias, M., Siebert, J.C. & Rickwood, P.C. Some aspects of the mineralogy and petrology of ultramafic xenoliths in kimberlite. Contr. Mineral. and Petrol. 26, 75–123 (1970). https://doi.org/10.1007/BF00371259

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