The geochemical study carried out on seven natural gas emissions from Sardinia island (western-central Italy) has allowed to distinguish two different groups: i) CO2-high and He-low gases in the Logudoro area (northern Sardinia) associated with low temperature Na-HCO3 type waters and ii) N2 and He-rich gases bordering the western limit of Paleozoic basement crystalline rocks associated with long-term Na-Cl and Na(Ca)-Cl thermal water circulation therein. The emission of CO2 is prevalently related to outcrop areas of recent Quaternary extensional basaltic volcanism. The different origin of the two types of gas is even more evident when considering the helium (as R/Ra) and argon isotopic ratios for the Logudoro and the remaining areas, being 3.0-3.5 Ra and >320 and <0.4 Ra and 295, respectively. Both CO2 and such isotopic ratios suggest: i) a deep source for the Logudoro samples where a contribution of 40-50% mantle gas can be assessed; ii) a prevalent atmospheric origin for the N2-rich gases emerging in association with meteoric-derived thermal waters circulating in aquifers reaching 2, 000 m in depth and 80°-110°C maximum temperature inside the Paleozoic formations. Although both the N2/Ar and 40Ar/36Ar ratios in the N2-rich gases point, roughly, to an atmospheric origin, their relative Ne concentration, being this element almost exclusively of atmospheric origin, suggests that contributions of N2 from other sources than air can be envisaged.
The distribution of elements and chemical state of aluminum along the growth direction of a siliceous deposit formed from geothermal water at the Hachobaru geothermal power station were examined by chemical analysis and 27Al MAS NMR. The content of 4-coordinated Al, Al(4), was essentially constant along the growth direction. The content of Na was also constant because Na+ ion is neutralized by a negative charge induced by the isomorphous substitution of Al(4) in silica. Elements except Al(4) and Na were concentrated outward in the siliceous deposit. Since the chemical property of geothermal water was essentially constant during the growth of the siliceous deposit, the other elements including Al(6) would be expelled outward due to a change in silica structure by condensation of silica particles during the growth of siliceous deposit.
The mineralogy of fracture system and the chemistry of groundwater have been studied in deep boreholes drilled in granitic rock to understand the chemical evolution of groundwater and redox conditions within the deep underground which preserved the Tono uranium deposit. Geological studies revealed that fracture system within the granitic rock can be classified into intact zones, moderately fractured zones and intensely fractured zones, using the degree of fracturing. Fracture fillings within the fractured zones consist mainly of clay minerals such as montmorillonite. Hydrogen and oxygen isotopic data show that young, shallow groundwater has been traveled into a depth of at least approximately 186 m. The precipitation of iron hydroxide provides an evidence to support the idea that the oxidizing surface water has traveled down to a depth of approximately 130 m from the surface. Groundwater is rich in Na+, Ca2+ and HCO3- near the surface and rich in Na+ and HCO3- in depth. Mineral saturation indices suggest that the dissolution of plagioclase and calcite is more significant in the shallower part than in the deeper part of the groundwater system. Na+ concentration of the groundwaters increases with depth and Ca2+, Mg2+ concentrations decrease at a depth of 200 m and over. Below this depth, the increase in Na+ concentrations is nearly equivalent to the decrease in combined Ca2+ and Mg2+ concentrations, suggesting that a Na-(Ca2+, Mg2+) ion exchange reaction occurs between the montmorillonite and groundwater. The chemical evolution of groundwater in the granitic rock is controlled mainly by the dissolution of plagioclase and calcite, and by the ion exchange reaction between montmorillonite in the fractured zones and groundwater. In-situ Eh-value of groundwater in the fractured zones is approximately 0 mV at a depth of approximately 180 m. This value is on the boundary between the Fe(OH)3, and Fe2+ stability field on the Eh-pH diagram for the Fe-S-H-O system. The redox conditions of deep groundwaters in the fracture system are probably controlled by Fe2+ and Fe3+ in the granitic rocks. On the other hand, the amount of pyrite in the granite is limited. Pyrite crystals that maintain their original form are observed in the intact zones, whereas the pyrite with dissolution texture is found in the fractured zones. This can be explained that the minerals in the fractured zones have reacted with oxidized groundwater over a long period of time. There is a possibility that the redox condition of groundwater is controlled by aqueous reduced sulfur species and pyrite, and that the Eh value of groundwaters may be around -300 mV.
Experimental data have been analyzed thermodynamically over temperature and pressure ranges from 900 to 1550°C and 0.8 to 7.5 GPa, using a two-site regular solution model for the equilibria between olivine and clinopyroxene in the CaO-FeO-MgO-SiO2 system: (1)CaMgSi2O6+1/2FeFeSiO4=CaFeSi2O6+1/2 MgMgSiO4CpxOlCpxOl(2)CaMgSi2O6+MgMgSiO4=MgMgSi2O6+CaMgSiO4CpxOlCpxOl(3)CaFeSi2O6+FeFeSiO4=FeFeSi2O6+CaFeSiO4CpxOlCpxOl(4)1/2MgMgSi2O6+1/2FeFeSiO4=1/2FeFeSi2O6+1/2MgMgSiO4CpxOlCpxOl Calibrations were achieved using thermodynamic data on the Fe-Mg partitioning between olivine and clinopyroxene (ΔGlo, WFeMgOl and WFeMgCpx; Kawasaki and Ito, 1994), where the Fe-Mg interactions in M1 and M2 sites were assumed to be a ual to each other for both olivine and clinoroxene (WFeMgOl, M1=WFeMgOl, M2=WFeMgOl and WFeMgCpx, M1=WFeMgCpx, M2=WFeMgCpx). The free energy change of reaction at the standard state for the Mg-Ca partitioning between olivine and clinopyroxene, ΔG2o, was estimated from Adams and Bishop's (1982) data. Volume changes of Reactions (3) and (4) (ΔV3o and ΔV4o) were calculated from unit cell volume data of olivine (Akimoto and Fujisawa, 1968; Mukhopadhyay and Lindsley, 1983; Adams and Bishop, 1985) and clinopyroxene (Newton et al., 1979; Hugh-Jones et al., 1994). Mixing parameters of Ca-Fe and Mg-Ca olivines were evaluated from solvi of kirschsteinite-fayalite (WCaFeOl, M2; Mukhopadhyay and Lindsley, 1983) and forsterite-monticellite (WFo and WMo; Adams and Bishop, 1985). Asymmetric parameters WEn and WDi were estimated from calorimetric data on CaMgSi2O6-Mg2Si2O6 clinopyroxene (Newton et al., 1979). Kawasaki's (1998) model is available for the Fe-Mg site occupancy of olivine. The effect of Ca2+ on the Fe-Mg occupancy in clinopyroxene (McCallister et al., 1976) was empirically formulated. The present results show that the Ca-Fe-Mg distribution between olivine and clinopyroxene is highly sensitive to compositional variation and is temperature-dependent. The Ca partition coefficient, DCa (=XCaOl, M2/XCaCpx, M2) reaches a maximum at intermediate values of Fe/(Fe+Mg). The position of the maximum DCa shifts towards the Mg-rich side with increasing temperatures. The CaO content of clinopyroxene significantly affects KD [=(XMgOlXFeCpx)/(XFeOlXMgCpx)]. The KD increases drastically for a slight decrease in CaO content of clinopyroxene in the Mg-rich system at low temperatures, whereas a small increase in KD is found in the Fe-rich system. At high temperatures, KD changes only slightly when XCa decreases in the system.
Fission track (FT) zircon ages for eight samples, and K-Ar ages using biotite and hornblende for two samples were determined on four granitic rock bodies and a dyke in the Hida Mountain Range, Central Japan. FT zircon ages were determined <3 Ma for six samples, and >40 Ma for two samples. K-Ar ages were always older than the FT ages, reflecting the difference in closure temperature. For a sample of the Okukurobe Granite, the evidence of secondary thermal overprinting was found according to the statistical test for single grain FT ages and FT length distribution aspect. The apparent FT age of 44 ± 5 Ma (2σ level) should not correspond to any geological events. On the basis of the radiometric age data, the emplacement order of the rock bodies could be reconstructed as follows: The Kitamatadani Tonalite intruded at ∼90 Ma, the Tsurugidake Granite at ∼60-70 Ma, the Okukurobe Granite prior to ∼60 Ma, the Kurobegawa Granite (KG) prior to ∼7 Ma, and a dyke intruded into KG at ∼1 Ma. Because a simple cooling process is not applicable to the rock bodies in this region, it is necessary to analyze systematically the cooling histories of relatively small parts of each rock body prior to the thermo-tectonic history analysis.
Sulfur isotope compositions (δ34S) were analysed on elemental sulfur and cellular protein in sulfur-oxidizing bacterial mats and on hydrogen sulfide and sulfate in the associated geothermal waters which were collected from nine locations in central and northeastern Japan. The δ34S values of elemental sulfur and cellular protein in the mats were comparable to those of hydrogen sulfide, but far from the associated sulfate in the waters, indicating the positive use of dissolved hydrogen sulfide. There could be observed slightly negative (rod-shaped bacteria) and positive (sickle-formed bacteria) sulfur isotope fractionations (up to 2‰) during the bacterial use of hydrogen sulfide.