The 2021-23 Fagradalsfjall fires: geochemical and petrological insights

After 7000 years of quiescence, three eruptions occurred in 2021, 2022 and 2023 AD in the Fagradalsfjall volcanic system, in Reykjanes peninsula in southwest Iceland. Looking at 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. The 2021 eruption, which is a olivine tholeiite lava (mean WR MgO=9.5 wt%), was directly sourced from Moho depths, providing unique insights into the deep parts of the Fagradalsfjall volcanic system1. Furthermore, the 2021 eruption featured remarkable geochemical changes in the first 50 days, characterized by great variability in geochemical tracers used as a proxy for mantle enrichment such as K2O/TiO2  and La/Yb, in the range 0.14-0.26 and 2.1-4.51. After this period and to the end of the eruption, incompatible element ratios record only minor fluctuations2. The 2022 and 2023 AD lavas have lower WR MgO, in the range 8.4-8.7 wt%, with the 2023 eruption being slightly less differentiated. Incompatible element ratios do not change throughout the 2022 AD and 2023 AD eruptions. However, the 2022 and 2023 lavas are slightly more enriched than any 2021 lava, with K2O/TiO2 and La/Yb in the range 0.24-0.28 and 4.3-4.9 respectively. Hence, the 2022 and 2023 lavas are very similar to the most enriched 2021 lavas. The crystal cargo in the Fagradalsfjall lavas is made of plagioclase, olivine and clinopyroxene macrocrysts, with olivine and clinopyroxene becoming increasingly rare in the 2022 and 2023 products. Plagioclase macrocryst cores are in the range An84-91 throughout the 2021, 2022 and 2023 eruptions, without significant variability between eruptions. Conversely, the olivine and clinopyroxene cargo varies over the course of the Fagradalsfjall fires. In the 2021 products, they are mostly in the range of Fo86-89 and Mg#84-91, respectively—more primitive than in the 2022 and 2023 eruptions and out of equilibrium with the melts that carried them to the surface. In the 2022 and 2023 products, most of olivine and clinopyroxene crystals are in the range Fo84-87 and Mg#83-86, respectively. These are found to be in chemical equilibrium with carrier melts, suggesting that they likely represent true phenocrysts of the 2022-23 crystal cargo, as opposed to plagioclase crystals. This compositional variability of the crystal cargo suggests different mush erosion and/or incorporation processes throughout the Fagradalsfjall fires, with preferential incorporation of mush plagioclase crystals and less efficient mush erosion processes over time.1Halldórsson, S. A. et al. Rapid shifting of a deep magmatic source at Fagradalsfjall volcano, Iceland. Nature 609, (2022).2Marshall, E. W. et al. Rapid geochemical evolution of the mantle-sourced Fagradalsfjall eruption, Iceland. Abstract, AGU 2021.