Masatsugu Yamamoto

Picritic primary magma and its source mantle for Oshima-Ōshima and back-arc side volcanoes, Northeast Japan arc

Oshima-Ōshima volcano is an endmember of a geochemical variation which is characterized by a low FeO content toward the back-arc side across the NE Japan arc. Analyses of the basalts show primitive characteristics. Variation trends of the chemical compositions indicate initial olivine control then olivine+clinopyroxene control from a picritic to a differentiated basalt. The more magnesian basalts have the more magnesian olivine phenocrysts. The most magnesian (MgO 15%) of all rock samples, contains olivine phenocrysts with a composition of Fo 93.7 as a liquidus phase and is considered a product of a mantle-derived magma. The possible range in FeO and MgO content of source mantle for the Oshima-Ōshima magma can be demonstrated. Ichinomegata lherzolite inclusions, also from the back-arc side of NE Japan, is unlikely to be a candidate for the source mantle for high FeO. The upper mantle beneath the back-arc side is considered to be compositionally zoned; a Fe-rich mantle (Ichinomegata lherzolite) at shallower place and a Fe-poor mantle (the source mantle for back-arc side volcanoes).

Contemporaneous ascent of within-plate type and island-arc type magmas in the Beppu-Shimabara graben system, Kyushu island, Japan

Abstract Within-plate (WP) type and island-arc (IA) type magmas have erupted in north Kyushu and south Kyushu, respectively, during Quaternary time. The former is related to the eastern end of the East Asian extensional province and the latter to the subduction zone of the Philippine Sea plate. The Beppu–Shimabara graben system in the central part of Kyushu island is located in the area of overlap between them. Quaternary volcanic rock samples from the graben system can be chemically classified as WP- and IA-types by their multi-element patterns and positions on a Zr–Nb–Y discrimination diagram. Rocks of both type and of similar ages occur throughout the graben system. Furthermore, the rocks of WP-type give systematically higher 87Sr/86Sr ratios (0.7048–0.7049) than those of the associated IA-type (around 0.7040). The relations between 87Sr/86Sr and Nb/Y ratios of the former are clearly distinguished from those of the latter. These results demonstrate that this graben system is characterized by the contemporaneous ascents of WP- and IA-type magmas to the same Quaternary volcanic areas. Such rivalrous ascents of WP- and IA-type magmas seem to be a common feature of magmatism associated with graben in extensional provinces related to subduction zones of island arcs and their boundary areas.

Characteristic Features of Ra‐Pb‐Sr‐Rare Metal‐bearing Tamagawa Acidic Thermal Water Derived from a Deep‐seated Magmatic System in Akita Prefecture, Japan

Ohbuki hot spring in the Tamagawa hot spring area, which is associated with volcanic activity, is a famous hot spring characterized by strong acidity and presence of radium. Many studies on the thermal water of Ohbuki hot spring have been carried out. Iwasaki et al. (1963) reported that the anions in thermal water of Ohbuki hot spring are transported by volcanic gas derived from active magma, while Yoshiike (1996) proposed that the cations in the thermal water are derived from volcanic rocks in the shallow underground part around Tamagawa. We examined the chemical characteristics of thermal water on the basis of concentrations of major and trace elements and Sr, Nd and Pb isotopic ratios of the thermal water and estimated the source of elements in the thermal water.