Deep magma degassing and volatile fluxes through volcanic hydrothermal systems: Insights from the Askja and Kverkfjöll volcanoes, Iceland

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Ranta, Eemu, Halldórsson, Sæmundur Ari, Barry, Peter H., Ono, Shuhei, Robin, Jóhann Gunnarsson, Kleine, Barbara Irene, Ricci, Andrea, Fiebig, Jens, Sveinbjörnsdóttir, Árný E, Stefánsson, Andri
• Other Authors: Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Marine Chemistry and Geochemistry Department, Woods Hole Oceanographic Institution, Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione Palermo, Palermo, Italia, Institute of Geosciences, Goethe University, Altenh¨oferallee
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2023
• Subjects: Askja; Kverkfjöll; Iceland
• Online Access: http://hdl.handle.net/2122/17076; https://doi.org/10.1016/j.jvolgeores.2023.107776

Mantle volatiles are transported to Earth’s crust and surface by basaltic volcanism. During subaerial eruptions, vast amounts of carbon, sulfur and halogens can be released to the atmosphere during a short time-interval, with impacts ranging in scale from the local environment to the global climate. By contrast, passive volatile release at the surface originating from magmatic intrusions is characterized by much lower flux, yet may outsize eruptive volatile quantities over long timescales. Volcanic hydrothermal systems (VHSs) act as conduits for such volatile release from degassing intrusions and can be used to gauge the contribution of intrusive magmatism to global volatile cycles. Here, we present new compositional and isotopic (δD and δ18O-H2O, 3He/4He, δ13C-CO2, Δ33S- δ34S-H2S and SO4) data for thermal waters and fumarole gases from the Askja and Kverkfj¨oll volcanoes in central Iceland. We use the data together with magma degassing modelling and mass balance calculations to constrain the sources of volatiles in VHSs and to assess the role of intrusive magmatism to the volcanic volatile emission budgets in Iceland. The CO2/ΣS (10???? 30), 3He/4He (8.3–10.5 RA; 3He/4He relative to air), δ13C-CO2 (???? 4.1 to ???? 0.2 ‰) and Δ33S- δ34S-H2S (???? 0.031 to 0.003 ‰ and ???? 1.5 to +3.6‰) values in high-gas flux fumaroles (CO2 > 10 mmol/mol) are consistent with an intrusive magmatic origin for CO2 and S at Askja and Kverkfj¨oll. We demonstrate that deep (0.5–5 kbar, equivalent to ~2–18 km crustal depth) decompression degassing of basaltic intrusions in Iceland results in CO2 and S fluxes of 330–5060 and 6–210 kt/yr, respectively, which is sufficient to account for the estimated CO2 flux of Icelandic VHSs (3365–6730 kt/yr), but not the VHS S flux (220–440 kt/yr). Secondary, crystallization-driven degassing from maturing intrusions and leaching of crustal rocks are suggested as additional sources of S. Only a minor proportion of the mantle flux of Cl is channeled via VHSs whereas the H2O flux remains poorly constrained, because ...