Stratigraphic reconstruction of the Víti breccia at Krafla volcano (Iceland): insights into pre-eruptive conditions priming explosive eruptions in geothermal areas

Abstract

Krafla central volcano in Iceland has experienced numerous basaltic fissure eruptions through its history, the most recent examples being the Mývatn (1724‒1729) and Krafla Fires (1975–1984). The Mývatn Fires opened with a steam-driven eruption that produced the Víti crater. A magmatic intrusion has been inferred as the trigger perturbing the geothermal field hosting Víti, but the cause(s) of the explosive response remain uncertain. Here, we present a detailed stratigraphic reconstruction of the breccia erupted from Víti crater, characterize the lithologies involved in the explosions, reconstruct the pre-eruptive setting, fingerprint the eruption trigger and source depth, and reveal the eruption mechanisms. Our results suggest that the Víti eruption can be classified as a magmatic-hydrothermal type and that it was a complex event with three eruption phases. The injection of rhyolite below a pre-existing convecting hydrothermal system likely triggered the Víti eruption. Heating and pressurization of shallow geothermal fluid initiated disruption of a scoria cone “cap” via an initial series of small explosions involving a pre-existing altered weak zone, with ejection of fragments from at least 60-m depth. This event was superseded by larger, broader, and dominantly shallow explosions (~ 200 m depth) driven by decompression of hydrothermal fluids within highly porous, poorly compacted tuffaceous hyaloclastite. This second phase was triggered when pressurized fluids broke through the scoria cone complex “cap”. At the same time, deep-rooted explosions (~ 1-km depth) began to feed the eruption with large inputs of fragmented rhyolitic juvenile and host rock from a deeper zone. Shallow explosions enlarging the crater dominated the final phase. Our results indicate that at Krafla, as in similar geological contexts, shallow and thin hyaloclastite sequences hosting hot geothermal fluids and capped by low-permeability lithologies (e.g. altered scoria cone complex and/or massive, thick lava flow sequence) are susceptible to explosive failure in the case of shallow magmatic intrusion(s).

Soil CO2 emission and stable isotopes (13C/12C, 18O/16O) of CO2 and calcites reveal the fluid origin and thermal energy in the supercritical geothermal system of Krafla, Iceland

“…Recent drilling of the KJ-39 and IDDP-1 geothermal wells encountered rhyolitic melts at shallow depth (~2 km; Elders et al, 2011;Mortensen et al, 2010), overlying the deeper mafic reservoir of the magmatic plumbing system. The similar petrological data of these rhyolites with those erupted in 1724, during the formation of the Viti maar (Montanaro et al, 2021;Fig. 1), suggest they extend throughout the center of the caldera (Rooyakkers et al, 2021).…”

Sulfur origin and flux variations in fumarolic fluids of Vulcano Island, Italy

“…The role of elemental sulfur and sulfate minerals (alunite, jarosite, anhydrite, and gypsum) as sealers in volcanichydrothermal systems has been a key focus during the quest for precursory signals for phreatic eruptions (Takano et al, 1994;Christenson et al, 2010, Christenson et al, 2017Montanaro et al, 2017, Montanaro et al, 2021, Montanaro et al, 2023Rouwet et al, 2017;Rouwet, 2021;Pappaterra et al, 2022). Variations in gas chemistry often turned out to be indicative of the source and mechanism of phreatic eruptions at several volcanoes.…”