Tephra and lava were regularly sampled during the August 2022 eruption at Meradalir. Figure 1 shows representative backscatter electron images of (A) tephra, (B) artificially and (C) naturally quenched lava crust, and (D) slowly cooled pahoehoe lava samples collected between 3rd and 5th of August 2022. The samples contain macrocrysts, microphenocrysts and microlites of plagioclase, olivine and clinopyroxene, in order of decreasing abundance. Cr-rich spinel is found as inclusions in olivine and by its own. The petrographic features of the 2022 samples are very similar to those erupted in 2021, and there is no significant difference in the composition of the mineral phases.
The MgO content of the glass is generally between 7 and 8 wt%, whereas it is ~8.5 wt% in the whole rock samples (Fig. 2). The negative correlation between TiO2 and MgO is consistent with fractional crystallization of the observed mineral phases. Glasses analyzed later are generally more primitive than those from the start of the eruption. The K2O/TiO2 ratio is constant throughout the eruption and is similar to that measured at the end of the 2021 eruptive period or about 0.25-0.26.
The tephra is relatively degassed, with SO2 on the order of 100-400 ppm. Melt inclusions with similar MgO content have higher amount of SO2, generally in the range 2000 to 2300 ppm, indicative of sulfur-undegassed melt compositions. The SO2 contents were very similar throughout the eruption, and also similar to the contents observed in 2021. The difference between the highest concentrations measured in melt inclusions and lowest concentration measured in the groundmass glasses enables us to estimate the maximum amount of SO2 that could have been released during the eruption as 6 x 10**7 kg.
Olivine-plagioclase-augite-melt barometry indicates that the crystal cargo initially came from the shallow dyke. As the eruption progressed, the crystals came from increasing depth (close to the Moho on day 8). Clinopyroxene-melt barometry shows that clinopyroxene microphenocrysts crystallized throughout the whole vertical extent of the dyke, whereas macrocrysts were transported to the surface from the deep crust. The calculated magma temperature was between 1190 and 1200 °C throughout the eruption. This is very similar to the temperature observed during the 2021 eruptive period.
Fig.1: Backscatter electron images of (A) tephra and (B-D) lava samples collected between 3rd and 5th of August 2022. These images are representative for samples throughout the eruption.
Fig. 2: Variation in glass and whole-rock compositions over the 2022 eruption.