A TRANSMISSION ELECTRON-MICROSCOPIC STUDY OF DUSTY PLAGIOCLASE IN CALC-ALKALINE ANDESITE FROM THE OZE-HIUCHIGATAKE VOLCANO, CENTRAL JAPAN

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 che mistry 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 (1) over bar into I (1) over bar by (1) annealing in a stable C (1) over bar field and then qu enching on eruption and, (2) compositional change from less calcic to more calcic during partial dissolution or (3) annealing in a metastabl e C (1) over bar field. The calcic clear plagioclase rim crystallizes metastably as C (1) over bar and then inverts into I (1) over bar on e ruption. The glass inclusions are more enriched in SiO2 (>80 wt.%) tha n residual glasses in the groundmass. It is estimated that alkali feld spar, quartz and other fine-grained minerals were crystallized from th e melts entrapped in the dusty zone during eruption. It is concluded t hat the dusty plagioclase was formed by the dissolution of less calcic plagioclase in basic magma during magma mixing.