Thorium, Sr and Nd isotope study of basalt erupted 2021-2024 on the Reykjanes Peninsula, Iceland

Seven short-lived basaltic fissure eruptions took place from December 2023 to December 2024 at the Sundhnúkur crater-row on the Svartsengi intra-transform spreading centre. The duration and magma volume of each eruption increased with time until the last event. The whole-rock major-element composition of the basalt produced is limited (with 7-8.5% MgO) whereas incompatible element and Sr-Nd isotope ratios vary (e.g. ⁸⁷Sr/⁸⁶Sr: 0.70312 - 0.70323 and ¹⁴³Nd/¹⁴⁴Nd: 0.51302-0.51295) irregularly with time (Matthews et al. 2024) until June 2024. Similar variability was observed for the isotope ratios during the six-months long 2021 eruption at Fagradalsfjall (⁸⁷Sr/⁸⁶Sr: 0.70310 - 0.70323 and ¹⁴³Nd/¹⁴⁴Nd: 0.51302-0.51295); Halldórsson et al., 2022; Marshall et al. 2024). An important difference is that the Sr isotope ratios are significantly higher for a given Nd isotope ratio in the 2024 Sundhnúkur basalt. Mass-balance criteria exclude significant crustal interaction and consequently, the two eruptions are fed from different deep-seated magma aggregation zones. The main difference is that since late 2023 the basalt has ascended into a mid-crustal domain causing a regular inflation until eruption.  The eruptions at Sundhnúkur crater-row all start as an intense but short-lived phase on several km long erupting fissures reflecting pressure release in a magma chamber and consequent deflation.

After June 2024, the Sr- and Nd isotope ratios as well as the incompatible element ratios reached the background values of the historical basalts on the Reykjanes Peninsula. Therefore, the evacuation of the “enriched” (high ⁸⁷Sr/⁸⁶Sr and K₂O/TiO₂) basalt compositions, erupted from late April 2021, through 2022 and July 2023 in the Fagradalsfjall region and from December 2023 to June 2024 at Sundhnúkur, from the trans crustal magma domain may be over.

Correlations between Th, Sr and Nd with Th/U revealing magma mingling in an aggregation zone and/or during transport and corresponding timescales of ascent through the oceanic crust will be discussed.

References:

Halldórsson, S.A. et al 2022. Rapid source shifting of a deep magmatic system revealed by the Fagradalsfjall eruption, Iceland. Nature 609, 529-534.

Marshall et al. 2024. The near-Moho magmatic system of the 2021 Fagradalsfjall eruption, Iceland: insights into melt aggregation processes at divergent plate boundaries. AGU Advances  5, AV001310.

Matthews et al. 2024. A dynamic mid-crustal magma domain revealed by the 2023-24 Sundhnúksgígar eruptions, Iceland. Science DOI: 10.1126/science.adp8778.