A Unique Enstatite Megacryst with Coexisting Cr-Poor and Cr-Rich Garnet, Weltevreden Floors, South Africa

  1. F.R. Boyd and
  2. Henry O.A. Meyer
  1. Henry O.A. Meyer1,
  2. Hsiao-ming Tsai1 and
  3. John J. Gurney2

Published Online: 19 MAR 2013

DOI: 10.1029/SP016p0279

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

The Mantle Sample: Inclusion in Kimberlites and Other Volcanics

How to Cite

Meyer, H. O.A., Tsai, H.-m. and Gurney, J. J. (1979) A Unique Enstatite Megacryst with Coexisting Cr-Poor and Cr-Rich Garnet, Weltevreden Floors, South Africa, in The Mantle Sample: Inclusion in Kimberlites and Other Volcanics (eds F.R. Boyd and H. O.A. Meyer), American Geophysical Union, Washington, D.C.. doi: 10.1029/SP016p0279

Author Information

  1. 1

    Department of Geosciences, Purdue University, West Lafayette, Indiana 47907

  2. 2

    Department of Geochemistry, University of Cape Town, Rondebosch, C. P. South Africa

Publication History

  1. Published Online: 19 MAR 2013
  2. Published Print: 1 JAN 1979

ISBN Information

Print ISBN: 9780875902135

Online ISBN: 9781118664858

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Keywords:

  • Chromite;
  • Diopsides;
  • Garnets;
  • Ilmenite;
  • Mineralogy;
  • Olivine;
  • Single crystal enstatite;
  • Sulfides

Summary

A large (17 cm) single crystal of enstatite (En88) contains Mg-ilmenite (MgO ∼ 12 wt%) and inclusions of polyphase garnets. The major garnet is orange in color and is predominantly pyrope-almandine (Py71Al21Gr8) whereas inside this garnet is a second one that is pink in color, and Cr-rich (Cr-Py33Py28Al19Gr20). This Cr-rich garnet itself contians inclusions of Cr-diopside, chromite and ilmenite. The enclosing orange almandine garnet at the boundary with the pink Cr-rich garnet contain rounded Mg-ilmenites together with a string of elongated olivines (Fo86) that are in optical continuity. Also present in the same zone is diopside. Within the enstatite host and in close association with the polyphase garnet assemblage are calcite, Ti-phlogopite and serpentine. Although it is conceivable the Cr-rich garnet and related phases represent the remnant of an earlier pre-existing rock that underwent considerable partial melting it must also be considered that the second garnet may be metamorphic in origin. However, irrespective of the origin, based on orthopyroxene geothermometry and A12O3 content possible P and T of equilibration are in the region of 60kb and 1200°C.