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Hydraulically linked reservoirs simultaneously fed the 1975--1984 Krafla Fires eruptions

Shane M. Rooyakkers1,2, Kate J. Carroll1, Alexandra F. Gutai3, Ben Winpenny3, Enikő Bali1, Guðmundur H. Guðfinnsson1, John Maclennan3, Freysteinn Sigmundsson1, Kristján Jónasson4, Euan J.F. Mutch5,6, David A. Neave7, Jóhann Gunnarsson Robin1, Karl Grönvold1, Sæmundur A. Halldórsson1

  • Affiliations: 1Nordic Volcanological Center, Institute of Earth Sciences, University of Iceland, Reykjavík, Iceland 2GNS Science, Lower Hutt, New Zealand 3Department of Earth Sciences, University of Cambridge, Cambridge, UK 4Icelandic Institute of Natural History, Garðabær, Iceland 5Earth Observatory of Singapore, Nanyang Technological University, Singapore 6Asian School of the Environment, Nanyang Technological University, Singapore 7Department of Earth and Environmental Sciences, The University of Manchester, Manchester, UK

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 187

  • Programme No: 1.8.18

  • Theme 1 > Session 8

Abstract

The 1975--1984 Krafla Fires in northeast Iceland was the first plate-boundary rifting episode to be tracked using seismic and geodetic monitoring. We present a detailed petrologic and geochemical study of all Krafla Fires eruptions. New whole-rock, matrix glass and mineral analyses show a clear compositional bimodality in the erupted magmas that persisted across the episode, with evolved quartz tholeiite (MgO = 5.8 ± 0.2 wt.%) erupted inside the caldera and more primitive olivine tholeiite (MgO = 7.3 ± 0.8 wt.%) erupted north of the caldera. Barometric calculations indicate tapping of these magmas from distinct reservoirs: a primitive lower-crustal reservoir at ~14--19 km depth, and a more evolved reservoir at ~7--9 km depth beneath the caldera. These reservoirs were tapped simultaneously in several of the eruptions, and in three events the two magma types mixed near the northern caldera margin. Clinopyroxene rims on gabbroic nodules from primitive September 1984 lavas record lower crustal pressures, while diffusion models suggest that these rims grew up to within a few months before eruption. Transcrustal ascent of the primitive magma thus occurred over timescales much shorter than eruptive reposes. These observations are inconsistent with the view that the eruptions were entirely fed by lateral magma outflow from the shallow reservoir. They instead require some decoupling of the flow paths of the two magmas: the primitive magma either bypassed the subcaldera reservoir laterally or ascended vertically beneath the northern vents. The two reservoirs nonetheless shared a hydraulic connection and jointly responded to rifting.