Thumbnail
Access Restriction
Open

Author Ewart, A.
Source CiteSeerX
Content type Text
File Format PDF
Subject Domain (in DDC) Computer science, information & general works ♦ Data processing & computer science
Subject Keyword Chemical Change ♦ Rhyolitic Lava ♦ Spherulitic Crystallization ♦ Central Volcanic Region ♦ Residual Glass ♦ Post-eruptive Process ♦ Chemical Fractionation ♦ Minute Granule ♦ Low Potash ♦ Cryptocrystalline Tergrowths ♦ Rhyolitic Volcanic Product ♦ Significant Effect ♦ Total Rock Composition ♦ Na Ratio ♦ Bulk Spherulite Composition ♦ Progressive Enrichment ♦ Advanced Stage ♦ Calcic Anorthoclase ♦ Spherulite Crystallization ♦ Potash Enrich-ment ♦ Aratiatia Rhyolite ♦ Progressive Spherulite Growth ♦ Spherulite Formation ♦ Taupo Region ♦ Quartz-feldspar System ♦ Pre-eruptive Phenocryst-liquid Fractionation ♦ Spherulitic Devitrification ♦ Spherulite Development ♦ Post-eruptive Alkali Fractionation ♦ Reference Tothe Ternary Feldspar ♦ Alkali Feldspar ♦ Bulk Rock Composition ♦ Many Rhyolitic Lava
Abstract SUMMARY. Spherulitic devitrification is a post-eruptive process affecting many rhyolitic lavas of the Taupo region. The spherulites consist of cryptocrystalline tergrowths ofc~-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 as 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 enrich-ment in the residual glasses during advanced stages of devitrification is greater than expected by reference tothe ternary feldspar and quartz-feldspar systems. This post-eruptive alkali fractionation during spherulite formation issuperimposed on the pre-eruptive phenocryst-liquid fractionation. I T is now well established that post-eruptive processes may significantly modify the chemistry of rhyolitic volcanic products. For example, alkali leaching (especially
Educational Role Student ♦ Teacher
Age Range above 22 year
Educational Use Research
Education Level UG and PG ♦ Career/Technical Study
Publisher Date 1971-01-01