Shallow conduit processes and sulfur release in the phreatomagmatic stages of the 1211 CE Younger Stampar eruption, Iceland

The 2021 Fagradalsfjall basaltic eruption in Iceland was effusive, but a different eruptive scenario could have unfolded if its location had been shifted a few kilometres to the south to an offshore setting. Namely a shallow marine event similar to the phreatomagmatic stages of the 1211 CE Younger Stampar eruption. The 1211 CE eruption was the initial event of the 1211-1240 Reykjanes Fires and its first stage was a Surtseyan eruption just offshore of the point of Reykjanes. It constructed the ~0.006 km³ Vatnsfellsgígur tuff cone that featured a short-lived dry phase towards the end. A second phreatomagmatic stage took place ca. 500 m off the current Reykjanes coastline to produce the larger Karlsgígur tuff cone (~0.044 km³), with a combined cone/tephra volume of ~0.15 km³. Later, the activity migrated onshore onto a 4km-long fissure with an effusive eruption that generated the Yngri-Stampar crater row and associated lava flow fields. The Vatnsfellsgígur and Karlsgígur tuff cones consist of alternating pyroclastic surge-tephra fall units, intercalated with units formed by simultaneous deposition from surge and fall. The 3.5m-thick Vatnsfellsgígur section is composed of 8 units, whereas the 5.5m-thick Karlsgígur section consists of 9 units. Chemical analysis reveals that the cones are tholeiitic basalt (MgO 6.0-7.5 wt%) with sporadic olivine phenocrysts (Fo₇₈ to Fo₈₄) and dispersed plagioclase macrocrysts with core composition of An₈₇ to An₉₁. Two compositionally distinct groups of plagioclase-hosted melt inclusions are identified: one with composition comparable to the host magma and another more primitive in composition with lower FeO, TiO₂ and K₂O and higher MgO (ranging from 9-10 wt% and 9-11.5 wt% for Vatnsfellsgígur and Karlsgígur, respectively). This suggests that whilst upper crustal storage zones may have facilitated melt evolution, the erupting magma originated from a deeper, crystal-mush-dominated storage zone. Original and residual sulfur contents of ~2221.7 ± 150 ppm and ~966.2 ± 120 ppm respectively, indicate that ~0.658 ± 0.034 Tg of SO₂ were released into the atmosphere during these two stages of phreatomagmatic activity. Moreover, vesicularity measurements on lapilli reveal unimodal, left-skewed vesicularity distributions with modes of 90% and 95% and a range of ~40% for Vatnsfellsgígur and Karlsgígur, respectively. These results indicate that magma had gone through vesicle nucleation to free growth and coalescence and probably initial dry (magmatic) fragmentation prior to contact with external water. The evidence strongly suggests that expansion of exsolved magmatic gases was the driver of explosivity and that the role of external water in these phreatomagmatic stages of the 1211 CE eruption was confined to secondary quench granulation. The analysed juvenile clasts also displayed sharp-bound domains of contrasting vesicularity with boundaries that cross-cut the clast margins. This confirms early mingling of melt batches with different histories of ascent and/or stalling in the shallow conduit. Given such heterogeneity, regions of contrasting vesicularity were analysed separately to construct two vesicle size and number distribution (VSD/VND) datasets. Results from the ongoing micro-textural and additional analysis of volatile degassing shall also be presented here.