Cooling history of a tholeiitic basalt from the Kurohana area in Funagata volcano is inferred from the microstructures of clinopyroxene and plagioclase. The sample contains pigeonite, orthopyroxene, olivine and plagioclase as phenocrysts. The phenocrysts are surrounded by the overgrown rims. From the compositional difference between phenocrysts and the overgrown rims, the crystallization sequence has been divided into two stages. At the first stage, the phenocrysts of pyroxene and plagioclase crystallized at above 1473K in the magma chamber. They were kept in the chamber less than half a year, which was determined on a basis of the coarsening kinetics of pigeonite-augite intergrowth. Then, at the second stage, the rims of phenocrysts overgrew and groundmass minerals crystallized during the rapid cooling after the eruption. The cooling duration was estimated as about 4-5 years from the size of antiphase domains in plagioclase.
There are a number of Cenozoic ultramafic xenolith localities on the Southwest Japan arc, which enable us to deduce a petrological model of the upper mantle of the island arc or active continental margin on a regional scale. On the basis of mineral chemistry and petrology the mantle peridotites (harzburgite and lherzolite) from the Southwest Japan arc can be divided into two types, unmetasomatized and metasomatized types. The former type of peridotites, which are less predominant than the latter, have characteristics of mantle restite (olivine of Fo88−91) and are not accompanied with Fe-rich black pyroxene-bearing xenoliths and related pyroxene megacrysts. They are found exclusively in isolated monogenetic volcanoes not forming volcano clusters. In contrast, the latter type of peridotites, which are always accompanied with large amounts of Fe-rich black pyroxenite xenoliths and related black pyroxene megacrysts, have less magnesian minerals (e.g., olivine of Fo78−90) depending on the degree of metasomatism. They have textural and modal characteristics as mantle restites but also have chemical characteristics as metasomatites. They are mainly found in monogenetic volcanoes constituting volcano clusters. The Fe-rich pyroxenites and related rocks are young precipitates of possibly Tertiary ages from alkali basaltic melts, genetically related with the xenolith-carrying basalts. A large scale asthenospheric upwelling in the Miocene time may have been responsible both for the intra-plate alkali basalt magmatism, namely the formation of xenolith-bearing basalts, metasomatism and precipitation of Fe-rich pyroxenites and megacrysts, and for the Japan Sea opening. The formation of Fe-rich pyroxenites and related rocks strongly modified the pre-existing mantle-crust both in chemistry and structure, leaving a chemically and geophysically intermediate Moho transition zone. This may be one of the characteristics of the upper mantle of active continental margin or island arc in the Western Pacific.
Crystals with a hexagonal lath-like morphology were found in glass products for cathode ray tubes. These crystals are preserved at the interfaces between glass and ZrO2 ceramics in a glass melting tank. An electron microprobe analysis gives SiO2: 45.6, ZrO2: 31.5, K2O: 19.0, Na2O: 1.92, BaO: 1.46, SrO: 0.33, in wt%. These values and X-ray analyses indicated the crystals to be precipitated with the same structure as mineral wadeite, K2ZrSi3O9, whose K atom sites are partially substituted by Ba, Sr or Na atoms. The structure is hexagonal with the space group P63/m, and the unit cell dimensions of a=6.918(2) and c=10.151(3)Å. The structure was refined using X-ray diffraction intensities from a single crystal. The bond distances and angles for Si3O9 rings are well comparable with those of benitoite, BaTiSi3O9, which is another example mineral including Si3O9 rings, with slight but significant difference in non-bridging Si-O bond distances. The mean square displacements of atoms in Si3O9 rings show that the tetrahedron is rigid for Si-O bond but strongly distorted for the O-O edges of non-bridging O atoms during thermal vibrations, coordinating to Zr and K. The thermal vibrational behaviors for the non-bridging oxygen are significantly different from those in benitoite.
Large amounts of chips of hydrothermal minerals were ejected with steam and hot water at the initial opening of a shallow production well (Well-4; total depth of 1448 m) after well completion at the Kakkonda geothermal field, northeast Japan. The ejectas yield important information to specify an indicative hydrothermal mineral of the productive fractures as an essential factor on development of the geothermal energy resource.The ejectas consist mainly of euhedral quartz, epidote (pistacite mole fraction of 0.22 to 0.25), prehnite and wairakite. The quartz and calc-silicate minerals precipitated as drusy minerals in a productive fracture where is connected to a feed point of the well. On the microscopic observation of the ejectas, cavities in the fracture are seen to have been partly self-sealed with precipitation of these minerals.Thermodynamic consideration shows that the fossil reservoir fluids are in equilibrium with epidote or prehnite-wairakite as well as quartz and calcite, whereas the present reservoir fluids are in equilibrium with prehnite as well as quartz and calcite. The decreasing of temperature and fCO2 in the reservoir fluids has shifted the calc-silicate mineral stabilities from epidote or prehnite-wairakite to prehnite. The process may result from the degassing of CO2-rich reservoir fluid. Prehnite appears to be a good indicator of the productive fractures in the shallow reservoir of the Kakkonda geothermal field.
Uralborite, CaB2O2(OH)4, was found in a vein consisting of borate minerals that cut into crystalline limestone at Fuka, Okayama Prefecture, Japan. It occurs as aggregates of fibrous crystals up to 0.2 mm long and as euhedral crystals up to 7 mm long and 3 mm wide, in association with sibirskite, borcarite, fluorite and calcite. Electron microprobe analyses and ICP gave the empirical formula Ca1.006B2.019O2.069(OH)3.931 on the basis of O=6. X-ray powder diffraction were indexed on the monoclinic cell, a=6.923(1), b=12.326(1), c=9.831(1)Å, β=97.09(1)°, determined by a single crystal method. The mineral was optically biaxial positive with refractive indices α=1.605(2), β=1.611(2), γ=1.618(2). The Vickers microhardness was 372 kg mm−2 and the Moh's scale of hardness was 4.5. The density was 2.58(2) g cm−1. It is likely that uralborite at Fuka was formed as a secondary mineral by a late-hydrothermal alteration of sibirskite.
A small amount of greenstone is found in the Jurassic accretionary complex of the Keisoku Massif. It occurs as lavas, dykes, volcanic breccias and tuffs in the melange zone of the chert-clastics sequences. The greenstones metamorphosed into the greenschist facies contain igneous augite, titaniferous augite and titaniferous hornblende. Fourteen samples of the greenstones were analyzed in this study. On the basis of discriminating diagrams, most greenstones have a chemical affinity with T- or P-type MORB, AB, OIT, OIA, WPB, WPA or WPT. Three greenstones have high ratios of Nb/Y versus Nb/Zr at high Nb/Zr ratios. The Permian Izuru Formation occurs in the Ashio Mountains, and mainly consists of greenstones. The chemical characteristics of Izuru greenstones are of the types OIT and OIA. Geological evidence also indicates that the Izuru Formation originated from a Paleozoic seamount. Greenstones of the Keisoku Massif are interbedded with olistostrome in the melange zone of the Jurassic accretionary complex, and have a chemical affinity with OIT and/or OIA. It is suggested that most greenstones of the Keisoku Massif were accreted as slices derived from the Izuru seamount.