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Chemical changes accompanying spherulitic crystallization in rhyolitic lavas, Central Volcanic Region, New Zealand

Published online by Cambridge University Press:  05 July 2018

A. Ewart
A. Ewart*
Affiliation:
Department of Geology and Mineralogy, University of Queensland, St. Lucia, Queensland, 4067
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Summary

Spherulitic devitrification is a post-eruptive process affecting many rhyolitic lavas of the Taupo region. The spherulites consist of cryptocrystalline intergrowths of α-cristobalite and alkali feldspar (calcic anorthoclase), with minute granules of magnetite, hematite, and secondary (?)goethite. The effects of chemical fractionation occurring during progressive spherulite growth has been studied from a suite of samples from the Aratiatia rhyolite. The most significant effect is the progressive enrichment of both bulk spherulite compositions and the coexisting residual glass in potash with increasing spherulite development. This effect is due both to the very low potash in the earliest formed spherulites and to the consistently higher Na/K ratios of the spherulites relative to the total rock compositions. These differences progressively decrease with increasing spherulite crystallization. The bulk rock compositions, however, evidently remain essentially constant. The degree of potash enrichment in the residual glasses during advanced stages of devitrification is greater than expected by reference to the ternary feldspar and quartz-feldspar systems. This post-eruptive alkali fractionation during spherulite formation is superimposed on the pre-eruptive phenocryst-liquid fractionation.

Type
Research Article
Information
Mineralogical Magazine , Volume 38 , Issue 296 , December 1971 , pp. 424 - 434
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1971

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