Abstract
Miyake-jima has erupted about every 20 years (1962, 1983, 2000) since 1940. We have investigated pre-eruption processes for magmas erupted in 1983, and it was shown that the 1983 magma was a mixing product between basaltic and andesitic magmas (Kuritani et al., 2000). In this study, we examine the compositions of the two end-member magmas and the pressure conditions of the magma reservoirs, using constraints of multicomponent thermodynamics. By such information, recent magmatic processes beneath the volcano are also investigated. The whole-rock composition trends of the 1983 products are principally smooth and linear (SiO2 52.6-54.7 wt.%). Phenocryst contents of the products are 3-5 vol.%, and the mineral assemblage is olivine, plagioclase, augite, titanomagnetite, and minor orthopyroxene. Phenocrysts can be divided into two groups: those consisting of olivine, high-An plagioclase, high-Mg# augite, and low-Usp# titanomagnetite (referred to as A group), and those composed of low-An plagioclase, low-Mg# augite, orthopyroxene, and high-Usp# titanomagnetite (referred to as B group). Kuritani et al. (2000) discussed that the 1983 magma was produced by mixing between a homogeneous basaltic magma (referred to as BEM magma) containing the A group phenocryst and an heterogeneous andesite magma (referred to as AEM magma) with the B group phenocryst, just prior to eruption. Unlike the case of the mixing of two homogeneous magmas, estimation of the compositions of the end-member magmas is difficult, due to the heterogeneity of the AEM magma. The BEM compositions are, therefore, estimated using the constraint that the A-group phenocrysts were present in equilibrium in the BEM magma before the magma mixing. Thermodynamic models for both plagioclase-melt and olivine-melt pairs are applied to the calculated melt phase in the possible compositional area for the BEM magma, and the calculated equilibrium Mg# of olivine and An content of plagioclase are compared with the observed compositions, by which the BEM composition was constrained. The BEM magma is estimated to have a composition of SiO2 ~51 wt.% and a phenocryst content of ~15 %. The pressure condition of the magma chamber for the AEM magma is estimated using compositions of glass and plagioclase of the crystal aggregate of the B group. Application of the thermodynamic models of plagioclase-melt pair suggests that the magma chamber was present at the depth corresponding to about 1 kbar pressure. The estimated composition and phenocryst content of the BEM magma are similar to those of eruptive products of August 2000 eruption. This suggests that the products of the August 2000 eruption represent the one end-member magma in the recent activity of the volcano. The composition trend of the 1962 products is parallel to the composition trend of the AEM magma, and the 1962 products have similar phenocrysts to the A and B groups of the 1983 products. These observations suggest that the 1962 magma might also have been produced by mixing between the AEM and BEM magmas with different proportion from the case of the 1983 magmas.
