Eruption at Sundhnúksgígar, May 2024 – preliminary petrographic and geochemical data

Eruption at Sundhnúksgígar, May 2024 – preliminary petrographic and geochemical data

Samples of tephra and quenched lava were collected north of Fiskidalsfjall and east of Sýlingarfell on the 1^st day and 4^th day of the eruption at Sundhnúksgígar that started on May 29^th. The volcanic glass in the samples was analysed with the electron microprobe of the Institute of Earth Sciences, University of Iceland. The lava and tephra are composed of vesicular glass, plagioclase, olivine and clinopyroxene crystals (Fig. 1). The tephra glass is microlite-free, whereas quenched lava samples contain variable amounts of microlites. Overall, the petrographic features of the new lava resemble those of previous lavas erupted at Sundhnúksgígar since December 2023 (see previous reports).

Fig. 1. Backscattered electron image of a quenched lava sample from day 4 of the May 2024 eruption at Sundhnúksgígar. The quenched lava contains crystals of plagioclase, olivine and clinopyroxene.

Geochemical characteristics

The basaltic glass has ~6.2-7.0 wt% MgO, similar to the glass that erupted at Sundhnúksgīgar since December 2023. The K₂O/TiO₂ ratio of the glass is 0.13-0.14 across all analysed samples. The K₂O/TiO₂ ratio of the new lava differs significantly from the compositions that dominated the previous eruptions at Sundhnúksgígar, for which it was 0.21-0.23 (Fig. 2). The difference in K₂O/TiO₂ with respect to previous eruptions in the area since December 2023 suggests that a distinct batch of magma is currently erupting at Sundhnúksgígar.

Fig. 2. Comparison of the glass compositions from the May 2024 eruption with preliminary glass data from previous eruptions in the area since 2023, and with glass data from the first 50 days of the 2021 Fagradalsfjall eruption (Halldórsson et al. 2022).

Ground inflation started in Svartsengi at the beginning of April (IMO reports), while the March 2024 eruption was ongoing. Our preliminary geochemical data likely suggests that a different magma has been accumulating under Svartsengi since the beginning of April. This magma was most likely stored in a separate, isolated mid-crustal reservoir distinct from the one feeding the eruption in March. The magma now erupting at Sundhnúksgígar has a K₂O/TiO₂ ratio resembling that of the magma which erupted in Fagradalsfjall at the beginning of the 2021 eruption. One possible explanation for this is that the two systems are connected at depth, close to the crust-mantle boundary. For the first time since the start of the 2021 Fagradalsfjall eruption, low K₂O/TiO₂ melts are dominating the eruptions.

References:

Halldórsson et al. (2022): https://www.nature.com/articles/s41586-022-04981-x