Insights into the caldera cycle of the Torfajökull central volcano, South Iceland

• Main Author: Moser, Zoe
• Other Authors: Bachmann, Olivier, Popa, Răzvan-Gabriel
• Language: English
• Published: ETH Zurich 2023
• Subjects: Torfajökull; Caldera; Geochemistry; U-Th dating; info:eu-repo/classification/ddc/550; Earth sciences; Laufafell; Iceland
• Online Access: https://hdl.handle.net/20.500.11850/661902; https://doi.org/10.3929/ethz-b-000661902

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 ...