Eruption at Sundhnúksgígar February 2024 - Preliminary petrographic and geochemical data

Eruption at Sundhnúksgígar February 2024

Samples were taken of tephra and lava erupting from the southernmost vents near Sundhnúkur and from close to Grindavíkurvegur. Preliminary petrographic and geochemical data were obtained on February 9th and are reported here. Images and glass compositional data were obtained using an electron microprobe.

Petrological characteristics

The lava and tephra contain abundant macrocrysts and microphenocrysts of olivine and plagioclase (Fig. 1). Smaller clinopyroxene and Cr-spinel crystals are present but are more rare. The lava has abundant microlites of predominantly plagioclase, indicating degassing and cooling during lava flow (Fig. 1B).

Figure 1: Back scatter electron images showing the petrographic characteristics of a lava sample and tephra sample collected from the Febuary 2024 eruption at Sundhnúksgígar. Panel A shows the more microlite rich texture of the lava flows, and Panel B shows the glassier texture of the tephra.
 

Geochemical characteristics

The lava composition is very similar to the previous two eruptions at Sundhnúksgígar in December and January, and is typical for Icelandic fissure eruptions. The K2O/TiO2 ratio, which traces magmatic sources, is ~0.22, identical within uncertainty to the lavas erupted in December and January. The MgO content varies between 5.7 wt% for one of the lavas, and 6.6 wt% for the tephra, consistent with the lavas having undergone crystallisation during flow away from the fissure.

Summary

· The lava erupted in February has a very similar composition to the previous two eruptions at Sundhnúksgígar, indicating they are fed from the same magma storage region.

· The lavas from all three eruptions at Sundhnúksgígar are more evolved (lower MgO) than the lavas erupted at Fagradalsfjall. This is consistent with storage in the mid-crust, indicating the inflation source at Svartsengi corresponds to the magma storage region.

· Inter-sample variation in the MgO content of the glass corresponds to the degree of crystallisation during solidification at, or near to, the surface.