Helium isotopes in geothermal fluids reveal off-rift plume degassing and localized seismicity-induced processes in North Iceland

Iceland is a location of geological interest due to the combination of upwelling mantle plume and divergent plate boundary, which resulted in the formation of its extensive surface area (>100,000 km2) that rises above sea-level. This unique setting facilitates assessing the role of the underlying mantle plume and tectonic activity on crust-forming processes. Helium isotopes provide a useful tool in this regard, as they can identify physical processes and resolve deep and shallow fluid sources in the crust. In Iceland, the highest 3He/4He for geothermal fluids are found in Vestfirðir with values up to 29 Ra (where Ra is the 3He/4He of air), more than 110 km away from current active rift zones. Such locations are key to understand the extent of mantle degassing processes associated with the high buoyant Icelandic mantle plume. Other off-rift regions, such as most of North Iceland, have not been extensively investigated, despite the widespread presence of geothermal activity. Although North Iceland has been volcanically inactive for the past 0.8 Ma, severe earthquake hazards associated with mature and partially on-land transform zones have occurred, rendering the monitoring of the full tectonic-hydrogeochemical system of societal importance. Our study in North Iceland aimed to (i) assess temporal variations in helium isotopic signatures in low-T geothermal water and their relationship with regional earthquakes, (ii) diminish the helium isotope data gap in geothermal fluids of this region, and (iii) elucidate both local and regional processes controlling the He isotope systematics in this region as a case study for other off-rift contexts on Earth. In order to achieve these goals, we report helium isotope time series data collected from June 2020 to October 2022 from a borehole in Hafralækur, Aðaldalur valley (95 samples collected on a near-weekly basis), along with an isotope survey (δ2H-3He/4He-δ13CTDIC-δ18O-δ34SSO4) of North Iceland geothermal fluids (T < 130 °C, n = 36 samples). The results indicate a large regional variability in helium isotope ratios (4 to 27 Ra) that is comparable to the entire range evident in geothermal fluids across Iceland (∼1 to 29 Ra) where the maximum 3He/4He signature is among the highest measured in geothermal fluids from oceanic and continental hotspots globally. Several processes, both on regional and local scales, are needed to account for this large range: (i) influence of a deeply-derived mantle flux evidenced by a high 3He/4He mantle component, degassing via fault systems, (ii) release of local radiogenic helium components, potentially associated with seismic events along the Dalvík Lineament, and (iii) local groundwater mixing, for example evident at the Hafralækur site and documented by periodic M > 5 seismic events. The estimated magmatic helium flux for the entire study region is comparable to that of the mid-ocean ridge, where mantle-derived melts intruded in the crust are actively degassing, confirming the large-scale degassing of the Iceland plume. As basalts and their source materials can be affected by radiogenic additions and temporal variations, we postulate that geothermal fluids may better reflect the maximum present-day 3He/4He plume signature.