Glass Inclusions: an Indicator of Geothermal Heat Source

Abstract

Many geothermal fields are located in the terrains where volcanic rocks are dominant. These rocks are considered to be products prior to the present geothermal activity on a series of magmatism. In the Kirishima geothermal field, southwestern Japan, many glass inclusions are found in phenocrysts of plagioclase, pyroxene and olivine of the volcanic rocks. At present, most of them consist of a bubble and a clean or devitrified glass with or without daughter and/or trapped minerals. Each inclusion should have consisted of a silicate melt and trapped minerals when it was isolated, and a bubble and daughter crystals were formed during the cooling and solidification process of the melt. The homogenization temperature at which a bubble disappears, therefore, gives an information concerning the temperature of trapped magma. The glass inclusions homogenize at temperatures ranging from 800?C to 1400?C. The values may suggest that the present temperature of geothermal heat source in the Kirishima geothermal field is 800?C at the highest.