2015 年 110 巻 2 号 p. 65-70
The trace element and Sr–Nd isotopic compositions of Quaternary magmas from the Pre–Komitake volcano were investigated. The Sr and Nd isotope ratios ranged from 0.703320–0.703476, and 0.512885–0.513087, respectively, which are very similar to those of the lavas from Fuji and Komitake volcanoes that erupted subsequently. Enrichment of large ion lithophile elements, Pb and Sr, can be seen in the primitive mantle–normalized multi–element diagram of the Pre–Komitake, Komitake, and Fuji lavas. These collectively show island arc lava signatures; however, the middle to heavy rare earth elements are more depleted in the Pre–Komitake lavas, compared to those from Fuji. Positive Eu anomalies are observed, although the extents of these anomalies decrease with increasing SiO2 in the Pre–Komitake lavas, whereas this is not observed in Fuji lavas. The Sr/Y ratios of Pre–Komitake lavas increase from basalt to basaltic andesite, but decreases through andesite to dacite. This occurs in combination with a rapid increase in La/Yb ratios, followed by a more gradual increase. A gradual decrease in Dy/Yb ratios is also seen over the entire compositional range. These data suggest deep (>12 kbar) fractionation of garnet and amphibole followed by shallow (i.e., ~ 5 kbar) fractionation of amphibole and plagioclase. Such variations are not observed in the Komitake and Fuji lavas, for which deep fractionation of clinopyroxene and shallow fractionation of plagioclase have been suggested. All three lavas, including those from the Pre–Komitake volcano, show similar isotopic, major, and trace element compositions in the unfractionated basalts. The differing geochemical trends found in the Pre–Komitake lavas are likely to be due to different mineral fractionations occurring in the hydrous Pre–Komitake basalts compared to the dry Fuji and Komitake basalts.
