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

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 magmatic signals in Icelandic VHSs are masked by a dominant shallow
groundwater component of meteoric water origin. These results suggest that the bulk of the mantle CO2 and S
flux to the atmosphere in Iceland is supplied by intrusive, not eruptive magmatism, and is largely vented via
hydrothermal fields.