A dynamic mid-crustal magma domain revealed by the 2023-24 Sundhnúksgígar eruptions, Iceland

Following periods of unrest in 2020 and late 2023, a sequence of eruptions started in December 2023 in the Svartsengi system, on Iceland’s Reykjanes peninsula. By the end of 2024, seven eruptions had occurred along the Sundhnúksgígar crater row. We undertook extensive sampling campaigns throughout and following each eruption in order to identify and characterise the properties of the pre-eruptive magma accumulation region and the chemical variability it hosts [1]. While the 2021 eruption in the neighbouring Fagradalsfjall complex provided a near real-time view of magmatic processes occurring near the Moho [2,3]; the eruptions in Svartsengi provide us with a view into the workings of mid-crustal magma domains.

Eruptions in Iceland fed from magma reservoirs in the mid-crust generally erupt lavas with next to zero mantle-derived geochemical variability (e.g., in radiogenic isotopes or incompatible trace element ratios) [4,5]. Extraordinarily, within the first hours of the eruptions at Svartsengi, high amplitude mantle-derived variability was erupted from the same fissure, indicating the involvement of multiple magma reservoirs. The amplitude and mean composition characterising the variability have changed from eruption to eruption, showing that the magma domain feeding these eruptions is dynamic. Magma storage and mobilisation in the mid-crust may be more complex than geochemical and petrological observations often suggest, which must be considered when interpreting real-time monitoring data.

[1] Matthews et al., Science (2024); [2] Halldórsson et al., Nature (2022); [3] Marshall et al. AGU Advances (2024); [4] Halldórsson et al., CMP (2018); [5] Caracciolo et al., EPSL (2023)