Serpentine is one of the candidates to explain low-velocity and high Vp/Vs anomaly in subduction zones. However, the extremely high Vp/Vs found beneath Kanto and southwest Japan suggests the presence of aqueous fluids in addition to the serpentinized mantle. In this study, we investigated the effect of pore fluid pressure on elastic wave velocity of antigorite serpentinites at confining pressure (Pc) of 10-200 MPa, pore pressure (Pp) of 10-100 MPa and room temperature. Experimental results under dry conditions show that compressional and shear-wave velocities increase with increasing confining pressure, and that Vp/Vs increases slightly with pressure. Under wet conditions, the effect of pore fluid pressure depends on the sample porosity. In the sample with relatively high porosity (0.8%), elastic wave velocities are sensitive to pore pressure and are mostly unchanged at a constant differential pressure, Pc-Pp, within the studied range of confining pressure. In contrast, the sample with low porosity shows a little influence of pore fluid pressure, in which the elastic velocities are insensitive to differential pressure, but are sensitive to confining pressure. In the former sample, Vp/Vs increases with increasing pore pressure at a given confining pressure; however, the change in Vp/Vs is relatively small, suggesting that the observed extremely high Vp/Vs requires the infiltration of large amounts of water probably due to a hydro-fracturing at the plate interface.
Many Bi-bearing ore deposits are distributed in and around the Wasada granodiorite, located in the Tsuruoka district, Yamagata Prefecture, Japan. We examined Bi-minerals from the Kuromori, Hongo, Obari, Yakuwa, Honko, Asashi and Obiro mines. Most of the Bi-minerals are sulfides and sulfosalts, and the occurrence of native Bi is very rare. Cu-Bi-S minerals such as wittichenite occur and are closely associated with bornite and tetrahedrite-tennantite in the Cu-Bi mineralization of the Kuromori, Hongo Obari, Yakuwa and Obiro mines. The Aikinite-bismuthinite series occurs within chalcopyrite in the Obari, Honko and Obiro mines. Pb-Bi-S minerals such as cosalite occur and are intergrown with galena in the Cu-Bi-Pb-Zn mineralization of the Obari, Asashi and Obiro mines. Therefore, the formations of Bi-mineral phases were affected by coexisting major Cu and/or Pb minerals. A mineral assemblage of coexisting iron sulfides indicates a lack of native Bi in this region, attributed to higher fS2 in the studied deposits than other Bi-deposits in the Inner Zone of SW Japan.
Mafic and feldspar minerals in the two contrasting granitoid plutons of the Ohgi Granodiorite and O'hara Tonalite were examined chemically and texturally, using electron microprobe analyzers. The results show distinct differences between the two plutons. The chemical variation trends of amphiboles and biotites record the magmatic crystallization and succeeding hydrothermal reconstruction. The feldspars show some textural differences between the two plutons. Magmatic resorption of the primary plagioclase, associated with the later alkali feldspar crystallization, is likely to have occurred more intensely in the O'hara Pluton than in the Ohgi Pluton. Microperthitic textures common in both the plutons produced by subsolidus hydrothermal reactions are mainly of bead type, and they are different from patch microperthites characteristically to other granite plutons around Lake Biwa. The mineralogy of granitoid plutons provides important information on cooling conditions and processes different from those of more evolved granite plutons.