Isotope systematics of Icelandic thermal fluids

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Stefansson, Andri, Hilton, David R., Sveinbjornsdottir, Arny E., Torssander, Peter, Heinemeier, Jan, Barnes, Jaime D., Ono, Shuhei, Halldorsson, Saemundur Ari, Fiebig, Jens, Arnorsson, Stefan
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2017
• Subjects: Iceland; Isotopes; Thermal fluids; Volatiles; SUBMARINE HYDROTHERMAL VENTS; DIVERGENT PLATE BOUNDARIES; GEOTHERMAL SYSTEMS; CARBON-ISOTOPE; MANTLE PLUME; SUBGLACIAL BASALTS; LAKAGIGAR ERUPTION; HELIUM-ISOTOPES; REDOX REACTIONS; TRACE-ELEMENTS
• Online Access: https://pure.au.dk/portal/en/publications/8e9b5808-2559-40d4-b81a-f7ae145464d3; https://doi.org/10.1016/j.jvolgeores.2017.02.006

Thermal fluids in Iceland range in temperature from 440 degrees C and are dominated by water (>97 mol%) with a chloride concentration from 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 ...