Multiple sulfur isotope systematics of Icelandic geothermal fluids and the source and reactions of sulfur in volcanic geothermal systems at divergent plate boundaries

TitleMultiple sulfur isotope systematics of Icelandic geothermal fluids and the source and reactions of sulfur in volcanic geothermal systems at divergent plate boundaries
Publication TypeJournal Article
Year of Publication2015
AuthorsStefánsson A, Keller NS, Robin JGunnarsson, Ono S
JournalGeochimica et Cosmochimica Acta
Volume165
Pagination307 - 323
ISSN0016-7037
Abstract

Abstract Multiple sulfur isotope systematics of geothermal fluids at Krafla, Northeast Iceland, were studied in order to determine the source and reactions of sulfur in this system, as an example of a geothermal system hosted on a divergent plate boundary. Fluid temperatures ranged from 192 to 437 Â°C, and the fluids have low Cl concentration between ∌10 and ∌150 ppm, with liquid water and vapor being present in the reservoir. Dissolved sulfide (S-II) and sulfate (SVI) predominated in the water phase with trace concentrations of thiosulfate ( S 2 O 3 2 - ) whereas sulfide (S-II) was the only species observed in the vapor phase. The reconstructed sulfur isotope ratios of the reservoir fluids based on samples collected at surface from two-phase and vapor only well discharges indicated that ÎŽ34S and Δ33S of sulfide in the reservoir fluid ranged from −1.5 to +1.1‰ and −0.001 to −0.017‰, respectively, whereas ÎŽ34S and Δ33S of sulfate were significantly different and ranged from +3.4 to +13.4‰ and 0.000 to −0.036‰, respectively. Depressurization boiling upon fluid ascent coupled with progressive fluid–rock interaction and sulfide mineral (pyrite) formation results in the liquid phase becoming progressively isotopically lighter with respect to both ÎŽ34S and Δ33S. In contrast, \{H2S\} in the vapor phase and pyrite become isotopically heavier. The observed Δ33S and ÎŽ34S systematics for geothermal fluids at Krafla suggest that the source of sulfide in the reservoir fluids is the basaltic magma, either through degassing or upon dissolution of unaltered basalts. At high temperatures, insignificant \{SO4\} was observed in the fluids but below ∌230 Â°C significant concentrations of \{SO4\} were observed, the source inferred to be \{H2S\} oxidation. The two key factors controlling the multiple sulfur isotope systematics of geothermal fluids are: (1) the isotopic composition of the source material and (2) the isotope fractionation associated with aqueous and vapor speciation and how these change as a function of processes occurring in the system, including boiling, oxidation and fluid–rock interaction.

URLhttp://www.sciencedirect.com/science/article/pii/S0016703715003828
DOI10.1016/j.gca.2015.05.045