Compositions of spinels from cumulative anorthites of the Mutnovskii,Ksudach, Golovnina,and Malyi Semyachik Volcanoes,in Kamchatka |
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| Authors: | V V Anan’ev |
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| Institution: | 1.Institute of Volcanology and Seismology, Far East Branch,Russian Academy of Science,Petropavlovsk-Kamchatskii,Russia |
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| Abstract: | Microprobe studies of unzoned plagioclases (An92–96) from crystal tuff of Mutnovskii Volcano and allivalite nodules of rocks from the Ksudach, Malyi Semyachik, and Golovnina
Volcanoes revealed small inclusions of a dark-colored mineral that was later identified as spinel. Microprobe analyses showed
that the grains are unzoned and spinel inclusions of different chemical compositions may occur in one plagioclase crystal.
The spinel compositions form a clear extended single trend corresponding to the solvus zone of a solid solution that has not
been described in the literature. The existence of this spinel trend in the solvus zone might have been due to early capturing
of spinel grains by growing plagioclase crystals and their rapid cooling soon after eruption, resulting in hardening of the
metastable solution. These spinels are supposed to form synchronously with plagioclase crystallization. The diversity of spinel
compositions is explained by thermo diffusive equalizing of originally zonal spinel crystals after they were captured by plagioclase
crystals or by their growth in crystallization haloes of anorthite. |
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