First results of the characterisation of rock samples collected in the eruption at Sundhnúksgígar crater row

Eruption near Sundhnúksgígar crater row, December 18^th 2023

Petrological characteristics

The rock is composed of vesicular glass, plagioclase, olivine, and chromian spinel (Fig.1). Olivine and plagioclase are present as microlites (<100 microns in length), microphenocrysts (100-1000 microns in length) and macrocrysts (>1 mm in length), whereas chromian spinel occurs as microphenocrysts and inclusions in olivine microphenocrysts and macrocrysts. (Fig.1). Microphenocrysts and macrocrysts commonly contain silicate melt inclusions (Fig. 1b).

Figure 1: Petrographic characteristics of the Sundhnúkur lava collected in the early morning hours on the 19^th of December 2023. A) Stereomicroscopic image of glassy lava fragments with plagioclase (plag) crystals. B) Backscattered electron image of the lava, containing olivine (ol), plagioclase (plag), chromian spinel (Cr-Sp) and basaltic glass. Note the inclusions of basaltic glass in plagioclase microphenocrysts.

Geochemical characteristics

The lava is of tholeiitic composition, typical for fissure eruptions in Iceland. The tholeiitic basalt glass was analysed with an electron microprobe. It contains ~6.0 wt% MgO and ~2.29 wt% TiO₂, with a K₂O/TiO₂ of 0.23. Whole rock powders analysed with ICP-OES, have less evolved compositions as expected, whereas K₂O/TiO₂ is almost identical to those of the glass.

The new lava by Sundhnúksgígar is:

-More evolved (lower MgO) than recent Fagradalsfjall eruptions, indicating that modifications (cooling) of parental melts following crustal storage is more important than in recent Fagradalsfjall eruptions.

-The level of K₂O/TiO₂ enrichment is similar to that of the melt that came to dominate the 2021 eruption and erupted in 2022 and 2023. This suggests a similar magma source for all recent events on Reykjanes Peninsula

-The K₂O/TiO₂ is higher than seen in all historical eruptions (Illahraun, Eldvarpahraun and Arnarseturshraun) and some pre-historical eruptions, including the ~2400 years old Sundhnúkshraun lava. This suggests that the current magma source supplying the recent Reykjanes Peninsula eruptions is different than the magma source that supplied at least the medieval cycle of eruptions.