The Rausu geothermal field is situated in the middle part of the Shiretoko Peninsula, east Hokkaido, Japan. It has been classified into “A-type” (characterized by high temperature/depth ratio) in terms of its Geothermal Activity Index. The geology of the area consists of Tertiary volcaniclastic rocks, a hornblende dacite intrusion and overlying Tertiary to Quaternary lavas. The dacite is 63 wt.% SiO₂ and belongs to the calc-alkaline suite. The solidus and subsolidus temperatures of the dacite magma, calculated from mineral equilibrium, are 830°C and 741°C, respectively. The K-Ar age obtained from hornblende phenocryst is 3.0±0.8 Ma. The geological, geochronological and petrological data suggest that the heat source of the Rausu geothermal field is a long-lived, high-temperature granodioritic pluton existing beneath the intrusion. It is suggested that the thermal activity is caused by the upwelling of high-temperature fluids related to the pluton, through fracture networks within or around the intrusion.

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The Oligo-Miocene Anamizu Formation in the Ushitsu-Matsunami area consists of a lower volcano-sedimentary member and an upper volcanic member. The upper member is essentially composed of three lava series; tholeiite (basalt and basaltic andesite), calc-alkaline (basaltic andesite) and high-magnesian andesite (bronzite andesite), in ascending order. The tholeiitic basalt is further divided into K-poor and K-rich (K-feldespar bearing) types. The tholeiitic basaltic andesite has higher FeO^*/MgO (∼3) than the others (1-1.5). The calc-alkaline basaltic andesite has higher Cr and Ni than tholeiitic basaltic andesite. Bronzite andesite contains higher MgO, Cr and Ni than common calc-alkaline andesite of the same SiO₂ content (∼60 wt%), but lower than those of boninite and sanukite. All the rock series are depleted in HFSE (Nb and Ti) in comparison with N-MORB and OIB, suggesting typical subduction-related arc magmas. High Zr/Y ratios of the tholeiitic basalt resemble those of active continental margin magmas rather than island-arc magmas. The HFSE, Ni and Cr compositions indicate a progressive depletion or increasing degree of partial melting of the mantle wedge source in the order tholeiite basalt (basaltic andesite)→calc-alkaline basaltic andesite→bronzite andesite. Spinel in the three series shows different trends: Cr-poor (Cr# 0.49 to 0.54) in the tholeiitic basalt, fairly Cr-rich (Cr# 0.61) in the calc-alkaline basaltic andesite, and Cr-rich (Cr# 0.73) in the high-magnesian andesite. These trends indicate different mantle restites; lherzolite for the tholeiitic basalt and harzburgite for the calc-alkaline basaltic andesite and high-magnesian andesite, and increasing degree of depletion in the order as above. On the contrary, LILE and LREE exhibit a gradual enrichment of the source in the same order. Corresponding decrease of TiO₂/K₂O suggests that the enrichment has been due to addition of fluids derived from the descending slab. These data indicate that the K-poor tholeiitic basalt magma has been formed by partial melting of the lherzolitic upper mantle wedge under almost anhydrous conditions; whereas the K-rich tholeiitic basalt magma may have been produced by partial melting of the metasomatized lherzolitic mantle source under slightly hydrous conditions. The calc-alkaline and bronzite andesite magmas have been produced by partial melting of the hydrous, metasomatized, harzburgitic mantle wedge. The stratigraphy of the lava succession in the studied area (tholeiite basalt-basaltic andesite→calc-alkaline basaltic andesite→bronzite andesite) indicates that depletion and hydration (metasomatism) of the mantle source have progressed simultaneously.

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Five samples of fine-grained, authigenic pyrite seams in silicified woods from Tertiary marine to brackish sediments in Japan were examined on surface morphology by scanning electron microscopy (SEM) in combination with energy dispersive X-ray spectrometry (EDX) and on sulfur isotopic composition (δ³⁴S). The surface morphology as revealed by SEM varied from long fibrous (pseudomorph after original plant fiber), spongy, blocky/cubic to framboidal (spherical aggregates of minute crystals). Except two samples from Chikuho coal mine area in northern Kyushu (Ebitsu: spongy fabric, δ³⁴S=+3.5‰, +12.2‰), all pyrite samples were characterised by fairly low δ³⁴S values ranging from −40.0 to −20.1‰, suggesting to be the products from steady state sulfate reduction, under an open system, of seawater-sulfate by sulfate-reducing bacteria. Pyrite in coal deposits from the Ebitsu area is also bacteriogenic, but probably formed under a closed system where original plant debris were deposit. Any clear trend between surface morphology and δ³⁴S of the pyrite was not found.

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