Melt compositions during the 2023-2024 Sundhnúkur events

Melt compositions during the 2023-2024 Sundhnúkur events

The composition of silicate glass in quenched lava and tephra has been analyzed by the electron microprobe (EPMA) of the Institute of Earth Sciences, University of Iceland throughout the activity along the Sundhnúkur crater row. The MgO content of the glass varies between 5.6 and 6.7 wt%, whereas TiO₂ changes from 1.8 to 2.5 wt% (Figure 1). The MgO contents are slightly higher in glasses analyzed in quickly cooled tephra, while TiO₂ contents are somewhat lower. This is related to microlite crystallization in the lava after eruption and before quenching in a bucket of water (Figure 2).

Figure 1: Compositional variation of silicate glass in lava and tephra erupted between December 2023 and April 2024 along the Sundhnúkur crater row. Error bars are 1 standard deviations. In case of TiO₂, they are smaller than the symbol size.

Importantly, the MgO contents, the increase of which can be used as a proxy for magma originating deeper in the crust, do not change significantly as the eruption progresses (Figure 1) despite the input of primitive magma from the deep reservoir, suggested by ground deformation data. This suggests that the magma that feeds the mid-crustal reservoir below Svartsengi either re-equilibrates very quickly at mid-crustal conditions or causes eruption of magma already present.

Figure 2: Backscatter electron images of (a) a quenched lava and (b) a tephra sample, showing the high microlite content in the lava. Microlites are rare in (b) the highly vesicular tephra, as the tephra was instantly quenched after reaching the surface.