| Research Abstract |
For the purpose of specifying the phenocrysts which were crystallized within the magma chamber before its eruption, we have been managing to observe and analyze a number of specimens from many volcanic localities, for example, Usu Miyake-jima, Sakura-jima, Funagata, and Takadate volcanoes and so on. The criteria are as follows, (1) the rock contains pigeonite phenocrysts, (2) the phenocrysts are surrounded by the overgrown rims, (3) the compositions of these overgrown rims and grains in groundmass are almost similar, and (4) the growth temperature of the core parts of these phenocrysts can be estimated with such as a pyroxene geothermometry. Unfortunately, however, no specimens but one can be found satisfying all of these criteria - the only one exceptional specimen is a tholeiitic basalt from the Kurohana area in Funagata volcano.
This sample has already been studied by us (Miyake & Shimobayashi, 2000), and we inferred that the phenocrysts of pyroxene and plagioclase crystallized at above 1200℃ in the magma chamber and that they were kept in the chamber less than half a year, which was determined on a basis of the coarsening kinetics of pigeonite-augite intergrowth. Another study (Yoshimura, private communication) using the same sample that we used, on the other hand the residence time in the chamber is estimated as 50-560 years based on a theory of crystal size distribution (CSD). In the present study, we minutely examined these two significantly different durations and considered that the latter duration (50-560 years) could be shortened to the value, which is not so largely inconsistent with ours. Consequently, we suggest that the CSD theory is very useful to estimate the residence time in the chamber even if the crystal growth rate in the chamber can be properly determined.
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