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Contributions to Mineralogy and Petrology

, Volume 150, Issue 1, pp 119–130 | Cite as

Origin of phlogopite-orthopyroxene inclusions in chromites from the Merensky Reef of the Bushveld Complex, South Africa

  • Chusi LiEmail author
  • Edward M. Ripley
  • Arindam Sarkar
  • Dongbok Shin
  • Wolfgang D. Maier
Original Paper

Abstract

About 30% of the chromite grains of variable sizes in a chromitite seam at the base of the Merensky Reef of the Bushveld Complex on the farm Vlakfontein contain abundant composite mineral inclusions. The inclusions are polygonal to circular with radial cracks that protrude into the enclosing chromite. They vary from a few microns to several millimeters in diameter and are concentrated in the cores and mantles of chromite crystals. Electron backscattered patterns indicate that the host chromites are single crystals and not amalgamations of multiple grains. Na-phlogopite and orthopyroxene are most abundant in the inclusions. Edenitic hornblende, K-phlogopite, oligoclase and quartz are less abundant. Cl-rich apatite, rutile, zircon and chalcopyrite are present at trace levels. Na-phlogopite is unique to the inclusions; it has not been found elsewhere in the Bushveld Complex. Other minerals in the inclusions are also present in the matrix of the chromitite seam, but their compositions are different. The Mg/(Mg+Fe2+) ratios of orthopyroxene in the inclusions are slightly higher than those of orthopyroxene in the matrix. K-phlogopite in the inclusions contains more Na than in the matrix. The average compositions of the inclusions are characterized by high MgO (26 wt%), Na2O (2.4 wt%) and H2O (2.6 wt%), and low CaO (1.1 wt%) and FeO (4.4 wt%). The δ18O value of the trapped melt, estimated by analysis of inclusion-rich and inclusion-poor chromites, is ∼7‰. This value is consistent with the previous estimates for the Bushveld magma and with the δ18O values of silicate minerals throughout the reef. The textural features and peculiar chemical compositions are consistent with entrapment of orthopyroxene with variable amounts of volatile-rich melts during chromite crystallization. The volatile-rich melts are thought to have resulted from variable degrees of mixing between the magma on the floor of the chamber and Na-K-rich fluids expelled from the underlying crystal pile. The addition of fluid to the magma is thought to have caused dissolution of orthpyroxene, leaving the system saturated only in chromite. Both oxygen and hydrogen isotopic values are consistent with the involvement of a magmatic fluid in the process of fluid addition and orthopyroxene dissolution. Most of the Cr and Al in the inclusions was contributed through wall dissolution of the host chromite. Dissolution of minor rutile trapped along with orthopyroxene provided most of the Ti in the inclusions. The Na- and K-rich hydrous silicate minerals in the inclusions were formed during cooling by reaction between pyroxene and the trapped volatile-rich melts.

Keywords

Chromite Footwall Hangingwall Bushveld Complex Base Metal Sulfide 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgements

We thank Alan Boudreau and Edmond Mathez for their thoughtful reviews of the manuscript. Partial financial support for this study was provided through NSF grant EAR0335131 to EMR and CL, and through a Korean PDF fellowship to DS.

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Copyright information

© Springer-Verlag 2005

Authors and Affiliations

  • Chusi Li
    • 1
    Email author
  • Edward M. Ripley
    • 1
  • Arindam Sarkar
    • 1
  • Dongbok Shin
    • 2
  • Wolfgang D. Maier
    • 3
  1. 1.Department of Geological SciencesIndiana UniversityBloomingtonUSA
  2. 2.Korea Institute of Geoscience and Mineral ResourcesDaejeonSouth Korea
  3. 3.Sciences de la TerreUniversité du Québec à ChicoutimiChicoutimiCanada

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