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Phenocrystic fluorite in peralkaline rhyolites, Olkaria, Kenya Rift Valley

Published online by Cambridge University Press:  05 July 2018

A. S. Marshall ,
R. W. Hinton  and
R. Macdonald
A. S. Marshall
Affiliation:
Environmental Science Division, Lancaster University, Lancaster LA1 4YQ, UK
R. W. Hinton
Affiliation:
Department of Geology and Geophysics, Edinburgh University, Edinburgh EH9 3JW, UK
R. Macdonald
Affiliation:
Environmental Science Division, Lancaster University, Lancaster LA1 4YQ, UK
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Abstract

The first occurrence of phenocrystic fluorite in a peralkaline rhyolite is reported from Quaternary lavas (agpaitic index 1.11–1.37) erupted from the Greater Olkaria Volcanic Complex, Kenya. Fluorite compositions are almost stoichiometric, with 0.65–1.49 ΣREE + Y oxides. Relative to coexisting glass (melt), the fluorites are enriched in REE, Y and Sr, are strongly depleted in all other trace elements and concentrate MREE relative to LREE and HREE. Y partitions into fluorite more strongly than the HREE. Fractionation of fluorite in its modal abundance would not significantly affect the REE abundances in the residual liquids, nor result in stronger Sr depletion. The P, T and composition conditions which stabilize fluorite in rhyolitic magmas are still obscure.

Keywords

fluorite phenocrystsperalkaline rhyolitesKenya Rift Valley
Type
Research Article
Information
Mineralogical Magazine , Volume 62 , Issue 4 , August 1998 , pp. 477 - 486
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1998

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