Pyrope-spinel (alkremite) xenoliths from kimberlite |
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| Authors: | P H Nixon N A Chapman J J Gurney |
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| Institution: | (1) Department of Earth Sciences, University of Leeds, LS2 9JT Leeds, UK;(2) Department of Geochemistry, University of Cape Town, Cape Town, South Africa;(3) Present address: Dept. of Geology, University, P.O. Box 4820, Papua New Guinea;(4) Present address: Environmental Pollution Unit, Institute of Geological Sciences, Building 151, A.E.R.E., Harwell, Didcot, Oxon., UK |
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| Abstract: | Xenoliths consisting of two thirds pyrope (Py73Alm14Gr13-Py15Alm18Gr31) and one third hercynite-bearing spinel with minor chromium, from Bellsbank and Jagersfontein kimberlites, South Africa, are compared with similar rocks,  alkremites , from the Udachnaya pipe, U.S.S.R. From published experimental data and textural relationships these formed as early dense cumulates in aluminous mantle melts under restricted pressure conditions equivalent to about 75 km depth. At greater pressures very pyrope-rich garnets (Py80) are capable of being formed. The garnet spinel xenoliths are considered to have become separated from the magma prior to crystallisation of clinopyroxene, whereas complete uninterrupted fractionation and reaction would produce the more common griquaites (eclogites). |
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