Isotope systematics of Icelandic thermal fluids

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Stefánsson, Andri, Hilton, David R., Sveinbjörnsdóttir, Árný E., Torssander, Peter, Heinemeier, Jan, Barnes, Jaime D., Ono, Shuhei, Halldórsson, Sæmundur Ari, Fiebig, Jens, Arnórsson, Stefán
• Other Authors: Massachusetts Institute of Technology. Department of Earth, Atmospheric, and Planetary Sciences
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier BV 2017
• Subjects: Iceland
• Online Access: https://hdl.handle.net/1721.1/123818

Thermal fluids in Iceland range in temperature from < 10 °C to > 440 °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 δD (− 131 to + 3.3‰), tritium (− 0.4 to + 13.8 TU), δ¹⁸O (− 20.8 to + 2.3‰),³He/⁴He (3.1 to 30.4 R[subscript A]), δ¹¹B (− 6.7 to + 25.0‰), δ¹³C[subscript ∑ CO₂](− 27.4 to + 4.6‰), ¹⁴C[subscript ∑ CO₂](+ 0.6 to + 118 pMC), δ¹³C[subscript CH₄](− 52.3 to − 17.8‰), δ¹⁵N (− 10.5 to + 3.0‰), δ³⁴S[subscript ∑ S− II] (− 10.9 to + 3.4‰), δ³⁴S[subscript SO₄](− 2.0 to + 21.2‰) and δ³⁷Cl (− 1.0 to + 2.1‰) 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 N2, Ar and Ne concentrations. The large range of many non-reactive volatile isotope ratios, such as δ³⁷Cl and ³He/⁴He, indicate heterogeneity of the mantle and mantle-derived melts beneath Iceland. In contrast, the large range of many reactive isotopes, such as δ¹³C[subscript ∑ CO₂] and δ³⁴S[subscript ∑ S− II], 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 ...