Continuous Existence and Evolution of a Magma Chamber Beneath Usu Volcano, Japan: Evidence From Phenocryst Compositions and Textures

A long-lived magma chamber is often postulated beneath a volcano. The continuous existence of the same magma chamber is very difficult to prove, however. There could be a short-lived magma chamber that repeated solidification and remelting. Here, we show evidence of continuous existence of a silicic magma chamber beneath Usu volcano, which is one of the most active volcanoes in Japan. We investigated systematic petrological and petrographic analysis (e.g., Tomiya &Takahashi, 2005: J.Petrol.) of a series of eruptive products (1663, 1769, 1822, 1853, 1943, 1977 and 2000 A.D. eruptions; rhyolite ~ dacite), and revealed the followings: all these products contain similar types of plagioclase and orthopyroxene phenocrysts that consist of a homogeneous core with uniform composition and a zoned mantle; the texture of these phenocrysts changes systematically, showing progressive crystal growth, dissolution and diffusion; the compositions of these phenocrysts vary regularly with time, as well as the bulk-rock compositions. These observations demonstrate the continuous existence of the magma chamber since 1663. Magma Mixing and Evolution of the Magma Chamber. The change of magmatic conditions with time is clearly shown by magnetite composition because magnetite has large diffusion coefficients and should represent magmatic conditions immediately before the eruption. Most pumices from Usu volcano contain two types of magnetite phenocryst, and this means that two magmas mixed shortly before each eruption. There are three combinations of the end-members: (i) rhyolite + basaltic andesite (1663); (ii) dacite ± rhyolite (1769, 1822, 1853); (iii) dacite ± dacite (1977, 2000). Corresponding to these combinations, textures of the rocks are also classified into three categories: (i) nearly aphyric with homogeneous phenocrysts; (ii) porphyritic with inhomogeneous phenocrysts; (iii) nearly aphyric with inhomogeneous phenocrysts. This textural change is quantitatively shown by our CSD analysis for plagioclase phenocrysts. The P-T Conditions of the Magma Chamber. The temperature of the magma apparently increases with time, from 780 °C at 1663 to about 950 °C at 2000, according to Fe-Ti oxide geothermometry. This is consistent with the change in bulk rock compositions (e.g., Oba et al., 1983: J.Fac.Sci., Hokkaido Univ.) from SiO2 = 74 wt.% at 1663 to 68 wt.% at 2000. The depth of the Usu magma chamber has been estimated by our experimental petrological study (e.g., Tomiya &Takahashi, 1996: AGU Fall Meeting). It is about 250 MPa (10 km) at 1663, and after 1663 a shallow chamber newly formed at about 100 MPa (4-5 km). Since then, each eruption occurred after mixing of magmas from these two magma chambers. The existence of the deeper magma chamber was detected by geodetic observation as an inflation/deflation source at the 2000 eruption (Murakami et al., 2001: J.Geog.Surv.Inst.).