Eruption North of Grindavík - Preliminary petrographic and geochemical data

Eruption North of Grindavík, January 14th 2024

Samples were taken from both the northern and southern fissures in the afternoon on January 14th 2024. Preliminary petrographic and geochemical data were obtained on January 15th and are reported here.

Petrological characteristics

The rocks are composed of vesicular glass, plagioclase, olivine, clinopyroxene, and Cr-spinel (Fig. 1). Olivine and plagioclase are present as microlites (<100 microns in length), microphenocrysts (100-1000 microns in length), and macrocrysts (Fig. 1). Clinopyroxene and Cr-spinel are present as microphenocrysts (100-1000 microns in length). The rock from the southern fissure is more crystal rich than the rock from the north fissure.

Figure 1: Petrographic characteristics of the samples collected from the Grindavík lavas. Panels A and B show electron backscatter images of polished sections of the lava containing plagioclase crystals (pl) and olivine crystals (ol) surrounded by basaltic glass. Panel A shows the rock sampled from the northern fissure; panel B shows the rock sampled from the southern fissure.
 

Geochemical characteristics

The lava is of tholeiitic composition, typical for fissure eruptions in Iceland. The basalt glass was analysed with an electron microprobe. The glass from the northern fissure has ~5.9 wt% MgO and a K2O/TiO2 ratio of 0.23, very similar to the glass erupted at Sundhnúkar in December 2023 (Fig. 2). The glass from the southern fissure has a lower MgO content, ~5 wt%, but a similar K2O/TiO2 ratio of 0.23 (Fig. 2).

Figure 2: Comparison of the glass compositions from the January 2024 eruption at Grindavík with the December 2023 eruption at Sundhnúkar (data reported by IES), the early part of the first eruption at Fagradalsfjall (Halldórsson et al., 2022) and lavas from the Reykjanes Fires, 800-1240 AD (Caracciolo et al., 2023). The darker shaded Reykjanes fires symbols are from historical eruptions in Svartsengi.

Summary

• The lava erupted at Grindavík is related to the magmas erupted at Fagradalsfjall (2021-2023) and Sundhnúkar (December 2023), as shown by the similar K2O/TiO2 ratios, but is derived from a more evolved homogenised magma storage zone. This reflects increased cooling during prolonged storage in the crust, which causes the MgO content to decrease.
• The glass in the lava from the southern fissure has a lower MgO content than the lava from the northern fissure, consistent with the southern lava having crystallised more than the northern lava. This might reflect greater cooling and/or degassing immediately before or during eruption. The consistency of the K2O/TiO2 ratios, however, suggests it is ultimately derived from the same magma storage zone.
• The lavas are compositionally distinct from historical Reykjanes lavas, demonstrating the Sundhnúkar and Grindavik eruptions are a continuation of the magmatic unrest seen on Reykjanes since 2021.