Equilibrium conditions of methane hydrate in saline waters such as seawater and pore waters were predicted by statistical thermodynamic method based on the clathrate model proposed by van der Waals and Platteeuw (1959). In this study, equilibrium conditions of methane hydrate in saline waters were calculated by thermodynamic parameters proposed by Avlonitis (1994), with the evaluation of water activity by Englezos and Bishnoi (1988). It is found that equilibrium conditions of methane hydrate in saline waters predicted by this study are in good agreement with those determined by laboratory experiments. The present results indicate that the predicted equilibrium temperatures of methane hydrates in seawater and pore waters are about 1.2℃ lower than that in pure water.

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The sulfur isotope ratios of sulfate and sulfide mineral pairs in hydrothermal ore deposits have been often used for the estimation of the formation temperature of the mineral pairs. The oxygen isotopic ratios of sulfate and phosphate have also been useful means to assess the formation temperatures of various sulfate- and phosphate-deposits in hydrothermal to marine low temperature environments. In these applications, however, one must have the adequate knowledge on the following three parameters; 1) the temperature dependence of the equilibrium fractionation factors of sulfur or oxygen isotopes between coexisting phases in question, 2) the exchange rate of isotopes between the coexisting phases under given conditions such as temperature and pH, and 3) the residence time of the pairs in the given systems. I and my colleagues have been engaged in experimental determinations of exchange rates and equilibrium fractionation factors of sulfur isotopes in sulfate/sulfide system and of oxygen isotopes in sulfate/water and phosphate/water systems. In this article, I will review and summarize the results of these works with special emphases on their application to hydrothermal sulfate/sulfide system and to sulfate and phosphate in marine environments. Recent works on phosphate/water oxygen isotope exchange will also be mentioned.

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This paper reviews geochemical study of crustal evolution in the early stage of the Earth on the basis of La-Ce and Sm-Nd isotopic and REE geochemistry on the Archean gneisses and iron-formations from West Greenland and Archean cherts from Western Australia. The results are summarized as follows. (1) The REE patterns of original materials for the Amitsoq gneisses, West Greenland, had been fractionated in the very early stage of the Archean. (2) The sources for the Archean iron-formations from West Greenland had a chiefly chondritic or rather mantle-like depleted nature without a notable long-term negative Ce anomaly. (3) Large contribution of hydrothermal solution derived from depleted or chondritic mantle is suggested to the Archean Marble Bar Cherts, Western Australia, in contrast to Phanerozoic biogenic cherts. These results imply REE fractionation (i.e., chemical fractionation) of the solid Earth in the very early stage of the Archean, less development of light-REE enriched continental crust in the Archean, and the lower oxygen contents in the Archean atmosphere-ocean system. Finally, it is noted that Precambrian episode and protolithic features in East Asia can be understood from Precambrian orthogneiss clasts in Mesozoic Kamiaso conglomerate, since the clasts have not undergone post-depositional thermo-tectonic influences.

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Analyses of 250 rocks from the world showed that mantle-derived rocks such as tectonized peridotite in ophiolite sequences (tectonite) and mantle xenolithes in alkali basalts contained high molecular weight hydrocarbons from C₁₄ to C₃₃. The occurrence of the hydrocarbons indicated that they were not derived either from laboratory contamination or from field contamination; those compounds found in the mantle-derived rocks were called here "mantle hydrocarbons." Possible origins of the mantle hydrocarbons were as follows. (1) They were delivered to the early earth by meteorites and comets. (2) They were recycled by subduction. (3) They were inorganically synthesized by Fischer-Tropsch type reaction in the mantle. The mantle hydrocarbons in the case of (1) and (3) are abiotic and (2) is mainly biotic. In order to examine the first possible origin for the mantle hydrocarbons, shock experiments for CO and H₂ mixture, benzene, and hexane as reactants were carried out. Shock waves generated by projectile impacts were transmitted into starting materials. The shock reaction for CO and H₂ mixtures produced hydrocarbon gases up to C₄H₁₀. The conversion of CO to hydrocarbons increased with increasing the projectile velocity. The composition of produced hydrocarbons depended on that of the starting material (CO/H₂). The shock reaction for benzene produced hydrogen, light alkanes from C₁ to C₃, light alkenes from C₁ to C₃, acetylene, aromatic hydrocarbons with high molecular weight from 102 (phenylacetylene) to 306 (quaterphenyl). Examination of the yield relationships among structural isomers in products suggested that concerted cycloaddition reactions controlled by Woodward-Hoffmann rules explained the formation of some products better than do radical addition reactions. The shock reaction for hexane produced hydrogen, light alkanes from C₁ to C₄, light alkenes from C₂ to C₃, heavy alkanes from C₈ to C₁₂ and soot-like materials. In this shock reaction, the dehydrogenation was one of the important reactions and the recombination of hexyl radicals might play a role in the formation of n-C₁₂. The shock reactions showed that organic materials in meteorites and comets would survive and would develop into organics with higher molecular weights than initial materials, when the meteorites and the comets impacted on the earth. However, it is still unclear whether organic materials can survive in high temperature-pressure environment such as the mantle or not. Author plans to analyze organic materials in high-grade metamorphic rocks. This study will provide us with important information on the stability of organic materials.

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