Insights into the caldera cycle of the Torfajökull central volcano, South Iceland
Creators: name: Moser, Zoe
Publisher: name: ETH Zurich
Year: 2023
Subjects: subject: Torfajökull
subject: Caldera
subject: Geochemistry
subject: FOS: Earth and related environmental sciences
subject: U-Th dating
subject: info:eu-repo/classification/ddc/550
subject: Earth sciences

description: Caldera-forming volcanoes can be associated with the highest volcanic hazards on our planet, but also with areas of potential georesources. Understanding the evolution and activity patterns of these magmatic systems is of great interest, both from a risk and economic perspective. From what is currently known, their large volume magmatic reservoirs tend to evolve along a cyclic progression, involving an incubation period, maturation and fermentation, which can result in a caldera-forming eruption, followed by either the death of the system or its recovery. Testing how this cycle applies to different volcanic systems, understanding what causes the death or recovery of the system and finding ways to verify the current stage of a caldera-forming volcano is an important challenge in volcanology. Here, we approach these aspects on Torfajökull, a silicic caldera volcano in South Iceland. We present a geochronological and petrological dataset encompassing pre-caldera to post-caldera eruptions to better constrain the magmatic storage environment related to the different stages in the life-cycle of Torfajökull. We use U-Th disequilibrium dating on zircon crystals to determine the ages of the units and the crystallization history of the magma, while the chemical composition of the volcanic glass and minerals yield information required to reconstruct and estimate the magma chamber evolution and conditions preceding the eruptions.
The oldest samples studied here are from the so-called ring fracture rhyolites (Laufafell: 56.4 ± 4.3 ka, Rauðufossfjöll: 63.3 ± 7.5 ka). They contain homogeneous anorthoclase feldspars, magnesium-poor clinopyroxenes, and fayalitic olivines. Based on zircon crystallization ages, which hint at the efficient evacuation of the magma reservoir during the Thórsmörk eruption, we confirm that this ignimbrite is indeed associated with the caldera-forming event of Torfajökull, which we date to 38.0 ± 4.4 ka. Its feldspar chemistry is similar to the ring fracture rhyolites but with distinctly lower Mg# for the clinopyroxenes, indicating more evolved conditions. In the post-glacial period over the past 10 ky, multiple small rhyolitic units were emplaced. Geochemically, they record slightly less evolved magmatic conditions than the pre-caldera units, with the three youngest lavas containing significant recharge crystals, through the influence of the neighboring Veiðivötn fissure swarm intersecting Torfajökull from the north.
We estimate a trend of cooling from the pre-caldera units (800-880°C) at ~60 ka, down to 740°C for a post-caldera unit emplaced at 15.4 ± 3.3 ka, recorded by the Ti-in-zircon thermometer. However, the youngest post-glacial magmas seem to have had more variable temperatures, indicating more heterogenous conditions. Local variations of higher storage temperatures towards the north of the caldera are likely due to the stronger influence of the basaltic fissure system. Based on dissolved water content estimates, the pre-caldera units were water poor and water-undersaturated (3.25-3.75 wt.% H2O), but the system reached the 'explosive window' of dissolved water content (4.25-4.75 wt.% H2O) before the Thórsmörk erupted, enabling efficient fragmentation.
The ring fracture rhyolites, with their evolved and homogeneous mineralogy, can be linked to the late stage of the maturation phase, at which a large and unified magma reservoir was present. Further cooling and differentiation led finally to the emplacement of the Thórsmörk ignimbrite during the caldera-forming eruption. The volcanism of the past 10 ky is characterized by small but frequent eruptions with heterogeneous and slightly less evolved mineral signatures. The magmatic plumbing system seems to consist of multiple potentially unconnected melt pockets, some of them recording water-rich conditions of >5 wt.% dissolved H2O. We believe the system is currently still in a recovery stage, with isolated melt pockets at different evolutionary stages, some of them in the maturation stage, others probably already reaching fermentation and possibly water-saturation.