Medieval eruptions in the Reykjanes Peninsula, Iceland: magma storage depths and SO2 emission potentials for future eruption hazard assessment

The Reykjanes Peninsula, in southwest Iceland, has recently undergone magmatic reactivation with the 2021, 2022 and 2023 AD Fagradalsfjall eruptions. Considering the eruptive history of the peninsula in the past 4000 years, characterized by ~400-year-long rifting episodes at time intervals of 800-1000 years, the current magmatic reactivation could mark the onset of a new rifting episode in the most populated area of Iceland. In this contribution, we present results about magma plumbing configurations¹ and SO₂ emission potentials² during past eruptions across the peninsula, which are vital aspects for interpreting pre-eruptive signals and assessing the impact of sulfur release on human health, respectively. We target 16 basaltic lava units erupted in the volcanic systems of Reykjanes, Svartsengi, Krýsuvík and Brennisteinsfjöll during the last medieval rifting episode: the 800-1240 AD Fires. We analysed major and minor element contents of glasses, mineral phases and melt inclusions, and we reconstructed magma storage depths and SO₂ emission potentials across Reykjanes Peninsula. Independent clinopyroxene-melt and melt-based barometry show consistent results and suggest that magmas from the western part of the peninsula were extracted from magma reservoirs located at about 5-10 km depth. In contrast, and similar to observations from recent eruptions at Fagradalsfjall, the easternmost system, Brennisteinsfjöll, was fed from deep crustal reservoirs, at about 14-21 km depth. Starting from published lava volumes, we calculate SO₂ emission potential across the peninsula to be in the range 0.004-7.4 Mt. These estimates correspond to daily SO₂ emissions in the range 600-53000 tons, higher than the mean SO₂ field measurements of 5240 ± 2700 tons/day during the 2021 AD Fagradalsfjall eruption. Also, we develop an empirical approach to calculate end-member SO₂ emission potentials of any past or ongoing RP eruption of known volume or effusion rate. We conclude that the potential sulfur emissions across the RP can be significantly higher than observed during the 2021 AD Fagradalsfjall eruption, mainly because of the more evolved nature and higher sulfur contents of magmas erupted during the 800-1240 AD Fires. In the light of the ongoing magmatic unrest at Svartsengi, our work emphasizes the significance of petrological and geochemical studies of past eruptions for interpreting future pre-eruptive signals and developing future eruption mitigation strategies.

¹ Caracciolo, A. et al. Magma plumbing systems and timescale of magmatic processes during historical magmatism on Reykjanes Peninsula. Earth and Planetary Science Letters  621, 118378 (2023). https://doi.org/10.1016/j.epsl.2023.118378

² Caracciolo, A. et al. Reykjanes Peninsula's historical eruptions: SO₂ emissions and future hazard implications (GPL, under review). Preprint https://doi.org/10.31223/X5TX05