Summary
A detailed TEM study of dusty plagioclase revealed for the first time that the dust materials are mainly composed of glass, alkali feldspar, quartz and other fine-grained minerals. Detailed textures and the chemistry of glass inclusions are also described. b-domains with smoothly curving boundaries are found in the dusty zone and the clear rim. The plagioclase in the dusty zone inverts from C\(\bar 1\) into I\(\bar 1\) by (1) annealing in a stable C\(\bar 1\) field and then quenching on eruption and, (2) compositional change from less calcic to more calcic during partial dissolution or (3) annealing in a metastable C\(\bar 1\) field. The calcic clear plagioclase rim crystallizes metastably as C\(\bar 1\) and then inverts into I\(\bar 1\) on eruption. The glass inclusions are more enriched in SiO2 (> 80 wt.%) than residual glasses in the groundmass. It is estimated that alkali feldspar, quartz and other fine-grained minerals were crystallized from the melts entrapped in the dusty zone during eruption. It is concluded that the dusty plagioclase was formed by the dissolution of less calcic plagioclase in basic magma during magma mixing.
Zusammenfassung
Eine detaillierte TEM-Studie von durchstäubtem Plagioklas zeigte erstmals, daß das Staubmaterial hauptsächlich aus Glas, Alkali-Feldspat, Quarz und anderen feinkörnigen Mineralphasen besteht. Weiters werden Details der Texturen, sowie der Chernismus von Glaseinschlüssen beschrieben. In der durchstäubten Zone und im klaren Rand des Plagioklases wurden b-Domäne mit sanft gebogenen Begrenzungen gefunden. Der Plagioklas in der durchstäubten Zone wandelt sich durch folgende Prozesse von C\(\bar 1\) in I\(\bar 1\) um: 1) durch Tempern in einem Stabilitätsfeld von C\(\bar 1\) und darauffolgender Abschreckung während der Eruption, und 2) durch Änderung der Zusammensetzung von weniger Careich zu mehr Ca-reich während partieller Auflösung, oder 3) durch Tempern in einem metastabilen C\(\bar 1\) Feld. Der Ca-reiche, klare Plagioklasrand kristallisiert als metastabile C\(\bar 1\) Phase und wandelt sich dann, während der Eruption, in die 11 Phase um. Die Glaseinschlüsse sind deutlich an SiO2 (> 80 wt.%) angereichert, verglichen mit Residualgläsern in der Grundmasse. Es wird vermutet, daß Alkali-Feldspat, Quarz und andere feinkörnigen Mineralphasen aus jener Schmelzphase kristallisiert sind, die während der Eruption in die durchstäubte Zone eingeschlossen worden ist. Es wird der Schluß gezogen, daß der durchstäubte Plagioklas durch Auflösung von Caärmeren Plagioklas in einem basischen Magma während Magmen-Mischung gebildet worden ist.
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Konishi, H., Akai, J. A transmission electron microscopic study of dusty plagioclase in calc-alkaline andesite from the Oze-Hiuchigatake volcano, central Japan. Mineralogy and Petrology 53, 173–187 (1995). https://doi.org/10.1007/BF01171955
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DOI: https://doi.org/10.1007/BF01171955