| 1. |
- Gunnarsson-Robin, Jóhann, et al.
(författare)
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Sulfur isotopes in Icelandic thermal fluids
- 2017
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 346, s. 161-179
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Tidskriftsartikel (refereegranskat)abstract
- Multiple sulfur isotope compositions of thermal fluids from Iceland were measured in order to evaluate the sources and reactions of sulfur and sulfur isotope fractionation in geothermal systems at Icelandic divergent plate boundaries, characterized by MORB-like basalts. The geothermal systems studied had a wide range of reservoir temperatures of 56-296 degrees C and Cl concentrations of 18-21,000 ppm. Dissolved sulfide (Sigma S-II) and SO4 concentrations in liquid water measured <0.01-165 ppm and 1.3-300 ppm, respectively, and H2S(g) concentrations in the vapor 4.9-2000 ppm. The delta S-34 and Delta S-33 values for different phases and oxidation states were highly variable: delta S-34 Sigma s(-II) = -11.6 to 10.5 parts per thousand (n = 99), Delta S-33 Sigma s(-II) = -0.12 to 0.00 parts per thousand (n = 45), delta(34)Sso(4) = -1.0 to 24.9 parts per thousand (n = 125), Delta(33)Sso(4) = -0.04 to 0.02 parts per thousand (n = 50), delta(34)SH(2)s(g) = -2.6 to 5.9 parts per thousand (n = 112) and Delta(33)SH(2)s(g) = -0.03 to 0.00 parts per thousand (n = 56). The multiple sulfur isotope values of the thermal fluids are interpreted to reflect various sources of sulfur in the fluids, as well as isotope fractionation occurring within the geothermal systems associated with fluid-rock interaction, boiling and oxidation and reduction reactions. The results of isotope geochemical modeling demonstrate that the sources of S-II in the thermal fluid are leaching of basalt (MORB) and seawater SO4 reduction for saline systems with insignificant magma gas input, and that the observed ranges of delta S-34 and Delta S-33 for Sigma S-II and H2S(g) reflect isotope fractionation between minerals and aqueous and gaseous species upon fluid -rock interaction and boiling. The sources of SO4 are taken to be multiple, including oxidation of S-II originating from basalt, leaching of S-VI from the basalts and the seawater itself in the case of saline systems. In low-temperature fluids, the delta S-34 and Delta S-33 values reflect the various sources of sulfur. For high-temperature fluids, fluid -rock interaction, Sigma S-II oxidation and SO4 reduction and sulfide and sulfate mineral formation result in a large range of delta S-34 and Delta S-33 values for Sigma S-II and SO4 in the fluids, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements like sulfur. Such effects needed to be quantified in order to reveal the various sources of an element.
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| 2. |
- Luo, Genming, et al.
(författare)
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Multiple sulfur-isotopic evidence for a shallowly stratified ocean following the Triassic-Jurassic boundary mass extinction
- 2018
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Ingår i: Geochimica et Cosmochimica Acta. - : Elsevier BV. - 0016-7037. ; 231, s. 73-87
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Tidskriftsartikel (refereegranskat)abstract
- The cause of the Triassic-Jurassic (Tr-J) boundary biotic crisis, one of the ‘Big Five’ mass extinctions of the Phanerozoic, remains controversial. In this study, we analyzed multiple sulfur-isotope compositions (δ33S, δ34S and δ36S) of pyrite and Spy/TOC ratios in two Tr-J successions (Mariental, Mingolsheim) from the European Epicontinental Seaway (EES) in order to better document ocean-redox variations during the Tr-J transition. Our results show that upper Rhaetian strata are characterized by 34S-enriched pyrite, low Spy/TOC ratios, and values of Δ33Spy (i.e., the deviation from the mass-dependent array) lower than that estimated for contemporaneous seawater sulfate, suggesting an oxic-suboxic depositional environment punctuated by brief anoxic events. The overlying Hettangian strata exhibit relatively 34S-depleted pyrite, high Δ33Spy, and Spy/TOC values, and the presence of green sulfur bacterial biomarkers indicate a shift toward to euxinic conditions. The local development of intense marine anoxia thus postdated the Tr-J mass extinction, which does not provide support for the hypothesis that euxinia was the main killing agent at the Tr-J transition. Sulfur and organic carbon isotopic records that reveal a water-depth gradient (i.e., more 34S-, 13C-depleted with depth) in combination with Spy/TOC data suggest that the earliest Jurassic EES was strongly stratified, with a chemocline located at shallow depths just below storm wave base. Shallow oceanic stratification may have been a factor for widespread deposition of black shales, a large positive shift in carbonate δ13C values, and a delay in the recovery of marine ecosystems following the Tr-J boundary crisis.
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| 3. |
- Stefánsson, Andri, et al.
(författare)
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Isotope systematics of Icelandic thermal fluids
- 2017
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Ingår i: Journal of Volcanology and Geothermal Research. - : Elsevier BV. - 0377-0273 .- 1872-6097. ; 337, s. 146-164
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Forskningsöversikt (refereegranskat)abstract
- Thermal fluids in Iceland range in temperature from <10 degrees C to >440 degrees C and are dominated by water (>97 mol%) with a chloride concentration from <10 ppm to >20,000 ppm. The isotope systematics of the fluids reveal many important features of the source(s) and transport properties of volatiles at this divergent plate boundary. Studies spanning over four decades have revealed a large range of values for delta D (-131 to +3.3%o), tritium (-0.4 to +13.8 TU), delta(18) O(-20.8 to + 2.3%o), He-3/He-4 (3.1 to 30.4 R-A), delta B-11 (-6.7 to+25.0%o), delta C-13 Sigma co(2) (-27.4 to+ 4.6%o), C-1 Sigma co(2), (+0.6 to + 118 pMC), delta C-l3(CH4) (-523 to-17.8%o), delta N-15 (-10.5 to+3.0%o), 8(34)C Sigma s(-ll) (-10.9 to (+)3.4%o), delta S-34(SO4) (-2.0to + 21.2%) and delta Cl-37 (-1.0 to + 2.1%o) in both liquid and vapor phases. Based on this isotopic dataset, the thermal waters originate from meteoric inputs and/or seawater. For other volatiles, degassing of mantle-derived melts contributes to He, CO2 and possibly also to Cl in the fluids. Water-basalt interaction also contributes to CO2 and is the major source of H2S, SO4, Cl and B in the fluids. Redox reactions additionally influence the composition of the fluids, for example, oxidation of H2S to SO4 and reduction of CO2 to CH4. Air water interaction mainly controls N-2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as delta C-13 Sigma co(2)and(34)S Sigma S-u indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as delta C-13 Sigma co(2), and delta S-34 Sigma S-u, are heavily affected by processes occurring within the geothermal systems, including fluid-rock interaction, depressurization boiling, and isotopic fractionation between secondary minerals and the aqueous and vapor species. Variations due to these geothermal processes may exceed differences observed among various crust and mantle sources, highlighting the importance and effects of chemical reactions on the isotope systematics of reactive elements.
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