Yasuhiro Kiyosu

Chemical reduction and sulfur-isotope effects of sulfate by organic matter under hydrothermal conditions

Abstract Under hydrothermal conditions sulfuric acid, sodium bisulfate and sodium sulfate solutions were reduced by dextrose to hydrogen sulfide in order to clarify the origin of sulfide species in hot-springs, geothermal water and ore-forming fluids. At temperatures above 250°C, reduction of sulfuric acid and at above 300°C of sodium bisulfate and sodium sulfate was observed. The reduction rate depends fairly well on the temperatures, pH and sulfate species. The reduction of sulfate seems to be a first-order reaction. Sulfur-isotope compositions of sulfate and hydrogen sulfide were measured in order to disclose isotope effects in the reduction of sulfate. The reduction of sulfuric acid and sodium bisulfate solution results in enrichment of 3S in the hydrogen sulfide and of the heavy isotope into residual sulfate. The fractionation factor in the reduction is independent of the temperature and is seen to be 1.007 to 1.009, in agreement with previously published values.

Origin of sulfur species in acid sulfate-chloride thermal waters, northeastern Japan

Abstract Acid sulfate-chloride thermal water samples collected together with fumarolic gases from various volcanic areas in northeastern Japan were studied chemically and isotogdically. δ 34 S (COT) values of sulfate and hydrogen sulfide from these volcanic hot springs range from +4.0 to +31 and from −15.0 to −2.0% respectively, with δ 34 S ys value of +2.5 to +31. The δ 34 S of the sulfate in the more saline waters tends to become smaller with increasing ratio of SO 4 to Cl, although the chemical and isotopic composition of acid thermal water within some areas may be altered by secondary processes during the discharge of the thermal waters. This trend can be explained by the reaction of the volcanic gases, having S/Cl of 4 ~ 7 and total sulfur of ~0% in δ 34 S, with ground water at 200°C, and/or the removal of sulfide phase depleted in 34 S from the acid thermal water formed by the disproportionation of volcanic sulfur. The sulfur species in acid sulfate-chloride thermal water are shown to be volcanic exhalations.

Hydrogen isotopic compositions of hydrogen and methane from some volcanic areas in northeastern Japan

Abstract D/H ratios of volcanic condensates, hydrogen and methane collected from Hachimantai, Zao¯, Azuma, Nasu and Kusatsu shirane volcanic areas in northeastern Japan were determined together with chemical components. On the basis of the isotopic ratios for volcanic condensates, it is concluded that most of the water vapor is essentially local surface water which has been heated and slightly enriched in18O by exchange with silicates. δD values of hydrogen and methane range from −220 to −515‰ (SMOW) and from −180 to −487‰ (SMOW), respectively. The D/H ratio in volcanic hydrogen indicates that hydrogen may have isotopically equilibrated with water vapor at temperatures between 200 and 400°C. The δD value of methane also suggests that volcanic methane has been in isotopic disequilibrium with the water vapor and hydrogen and that most of the methanes may be thermogenic methanes produced by pyrolysis of carbonaceous materials.

Carbon isotope effect during abiogenic oxidation of methane

Abstract The oxidation of methane during flow over CuO and Fe2O3 has been examined in the temperature range of 400–650°C. The reaction rate and carbon isotope fractionation are dependent upon the choice of oxide and temperature. The activation energy is lower for hematite (8.0 kcal mole−1) than for cupric oxide (16.6 kcal mole−1). The measured ratios of the isotopic rate constants α =k12/k13 were found to have temperature dependences given by: 103(α − 1) =2.93 × 106/T2 + 8.11 (cupric oxide) 103(α − 1) =7.44 × 106/T2 + 6.56 (hematite) Abiogenic oxidation of methane is probably a significant mechanism for fractionating carbon isotopes in nature.

Carbon and hydrogen isotope fractionation during oxidation of methane by metal oxides at temperatures from 400° to 530°C

Abstract The oxidation of methane by metal oxides (Fe2O3, MnO2 and CuO) in closed systems is controlled by the oxides and temperature. Abiogenic oxidation of methane can readily occur, even at temperatures below 200°C in the Earths upper crust. The existence of residual CH4 relatively enriched in 13C during oxidation can be explained by a kinetic isotope effect. The deuterium concentration of residual methane in the MnO2 and CuO oxidation systems is controlled by a kinetic isotope effect, whereas for the Fe2O3 system, an isotope exchange between CH4 and H2O was observed. For the MnO2 system, the temperature dependence of hydrogen isotope fractionation is similar to that for CuO, and was obtained as: 10 3 (α H − 1) = 134 × 10 6 T 2 −127 The relative change in the isotope concentrations in CH4 during abiogenic oxidation is ΔH ΔC ≈ 21 at 200°C. This change is greater than that of other methane oxidation processes, indicating that carbon and hydrogen isotope compositions of methane may be useful in identifying abiogenic oxidation from other oxidation processes.

Isotopic geochemistry of acid thermal waters and volcanic gases from Zaō volcano in Japan

Abstract The chemical composition and D/H, 18 O 16 O and 34 S 32 S ratios have been determined for the acid hot waters and volcanic gases discharging from Zaō volcano in Japan. The thermal springs in Zaō volcano issue acid sulfate-chloride type waters (Zaō) and acid sulfate type waters (Kamoshika). Gases emitted at Kamoshika fumaroles are rich in CO2, SO2 and N2, exclusive of H2O. Chloride concentrations and oxygen isotope data indicate that the Zaō thermal waters issue a fluid mixture from an acid thermal reservoir and meteoric waters from shallow aquifers. The waters in the Zaō volcanic system have slight isotopic shifts from the respective local meteoric values. The isotopic evidence indicates that most of the water in the system is meteoric in origin. Sulfates in Zaō acid sulfate-chloride waters with δ34S values of around +15‰, are enriched in 34S compared to Zaō H2S, while the acid sulfate waters at Kamoshika contain supergene light sulfate ( δ 34 S = ∼ + 4‰ ) derived from volcanic sulfur dioxide from the volcanic exhalations. The sulfur species in Zaō acid waters are lighter in δ34S than those of other volcanic areas, reflecting the difference in total pressure.

Isotopic composition of acid sulfate-chloride waters and volcanic steam from some volcanoes in northeastern Japan

Abstract Acid sulfate-chloride water and volcanic steam samples collected from andesitic volcanoes in northeastern Japan have been examined for chemical and isotopic compositions. The acid thermal waters and fumarolic condensates are significantly enriched in D as well as 18 O compared to the respective local surface waters, although their compositions are variable. The chemical and isotopic evidence suggests that these waters may be mixtures of magmatic and meteoric waters. This fact is also supported by the N 2 /Ar and He/Ar ratios of fumarolic gases and δ 34 S values of sulfur species in the acid thermal waters.

Variation in fumarolic H2 gas and volcanic activity at Nasudake in Japan

Abstract The chemical and isotopic compositions of fumarolic gases collected from Nasudake in northeastern Japan were determined in order to investigate the correlations between the behaviour of H 2 and volcanic activity. On the basis of the hydrogen isotope ratio for volcanic H 2 , the temperature of the thermal fluid reservoir producing H 2 was estimated at ~ 160 †C. Large variations in the δD and δ 18 O values of fumarolic condensates around the summit were observed. Hydrogen content in the fumarolic gases has been gradually increasing since 1980, resulting from rock-water interaction under redox conditions governed by a mineral buffer such as pyrite and magnetite in the underlying shallow hydrothermal systems. Hydrogen concentration and isotopic composition of volcanic steam are strongly influenced by the proportion of local meteoric water circulating underground. The measured content and D H ratios of volcanic H 2 suggest that the contribution of magmatic gases to the fumarolic discharges has decreased with time.