| 概要 |
The olivine andesite, a member of the Setouchi volcanic rocks from the Arita area, Saga Prefecture comprises megacrysts of olivine (Fo82-80), phenocrysts of olivine (Fo85-84) and microphenocrysts of b...ronzite (En78-75). The megacrysts of olivine and its spine! inclusions were studied on thier chemical compositions and texture to see whether they are in equilibrium with the host magma or not. It is considered that the megacrysts are not a cumulus phase of the host magma, but a xenocryst which once existed was captured in the process of an ascent of the olivine andesite magma. The olivine andesite is characterized, irrespectively of its high Si02 content (up to 61 wt%), by the presence of olivine phenocryst. Crystallization order of constituent minerals is spinel+olivine orthopyroxene→clinopyroxene→plagioclase→agnetite+ilmenite. At the groundmass stage, orthopyroxene (bronzite) crystallize abundantly in place of olivine. This can be explained by the expansion of stability field of olivine to that of bronzite due to high water content of magma before eruption. The above explanation is discussed on the normative O1-Di-Qz diagram. From the available experimental data and plagioclase geothermometer, eruption temperature is estimated about 1150°C.続きを見る
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Mendeley出力
