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Reservoir reconstruction at Torfajökull volcano after the Thórsmörk super-eruption, identified through zircon age distribution and mineral chemistry

Zoe Moser 1,Razvan Popa1, Marcel Guillong1, Sæmundur Ari Halldórsson2, Kristján Jónasson3, Olivier Bachmann1

  • Affiliations:  1Institute of Geochemistry and Petrology, ETH Zürich, CH-8092 Zürich, Switzerland; 2Institute of Earth Sciences, University of Iceland, IS-102 Reykjavík, Iceland; 3Natural Science Institute of Iceland, IS-210 Garðabær, Iceland 

  • Presentation type: Talk

  • Presentation time: Thursday 10:45 - 11:00, Room R380

  • Programme No: 3.11.2

  • Theme 3 > Session 11

Abstract

Torfajökull is a rhyolite-dominated volcanic system at the edge of the active Eastern rift zone in Iceland's Southern Highlands. This highly geothermally active system has long been speculated to have experienced at least one caldera-forming eruption, based on its central topographic depression, dipping sediment features, and surrounding large tuyas, known as "Ring-fracture rhyolites". Additionally, an ash layer found in a Greenlandic ice core and marine sediments (North Atlantic Ash Zone II) is thought to originate from Torfajökull due to its distinctive glass chemistry, which also matches an ignimbrite deposit called "Thórsmörk Ignimbrite", located about 30 km south of Torfajökull. To test the connection between the Thórsmörk ignimbrite and a catastrophic event at Torfajökull, we examine zircon crystallization timescales and differentiation trends in mineral chemistry, building on previous evidence from field observation and glass chemistry correlation. U-Th crystallization ages (N > 1000) from a large suite of samples (N = 34) collected across Torfajökull, including the Thórsmörk ignimbrite, reveal a notable decline in zircon crystallization frequency around the time of the ignimbrite's emplacement. Zircon crystallization ages in younger units are limited to the post-Thórsmörk period, suggesting a complete reorganization of the subvolcanic magma reservoir. These observations strongly indicate a high-intensity eruption, which significantly emptied the upper crustal reservoir and was followed by a prolonged reconstruction stage. Mineral chemistry, particularly of Fe-Ti oxide and clinopyroxene, supports this interpretation, showing increased differentiation leading up to the Thórsmörk eruption, followed by less evolved and more compositionally heterogeneous units with increasing influence of mafic recharge.