1988 Volume 33 Issue SPCL Pages S234-S254
During the 1986 Izu-Oshima eruption, various kinds of lavas and ejecta were formed. From the central cone (A vent), plagioclase-phyric basalt (SiO2, 52.5-53.2 wt%) was erupted as scoria, bombs and lava flows. From the fissure on the caldera floor (B vent), aphyric basaltic andesite and aphyric andesite (SiO2 54-58 wt%) were erupted forming scoria, bombs and lava flows. Along the fissure on the outer slope of the main stratovolcano (C vent), aphyric basaltic andesite (SiO2 54-55 wt%) formed many cones and erupted lava flows. Detailed analysis of both major and minor elements indicates that A vent and B and C vents belong to different magma plumbing system. The high alumina nature of the basalt from A vent is explained by flotation and accumulation of plagioclase phenocryst at the top of the magma reservoir. Based on the compositions of coexisting pyroxenes, the temperature of the magmas on eruption are estimated to be 1100-1150℃ for basalt from A vent, 1100℃ for basaltic andesite from B and C vents, and 1070-1100℃ for andesite from B vent. Crystallization fractionation at relatively shallow magma reservoir (less than 2 k bar or shallower than 8 km) can account for compositional variation from basaltic to andesite magmas. During the fractionation, H2O increased from 0.7% or less in basaltic magma to about 1% in andesite magma. This low pressure crystallization fractionation can also account for the compositional variation within each major eruption of the recent 1500 years. Comparison of major and trace element concentrations in the effusive rocks of 1986 eruption and the recent 1500 years eruptions indicates that they were derived from a single parental magma. The parental magma has been continuously fractionated at least these 1500 years within a deeper and probably much larger magma reservoir than the shallow magma reservoir mentioned above. Because of this fractionated nature of the supplied magma, the andesite magma, which was very rare in Oshima volcano, was easily formed by the successive fractionation in the small scale magma reservoir. Analysis of dacitic pumice fragments from B vent indicates that they are not the fragments of the dacite layer of the Senzu group (Tatsunokuchi dacite), but are probably fragments of the dacite pumice of the Younger Oshima group, which is not yet found within Oshima Island.
