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Protracted assembly of the magmas feeding the 1783-84 Laki fissure eruption, Iceland

Euan J. F. Mutch1,2; John Maclennan3 ; Margaret Hartley4

  • Affiliations: 1Earth Observatory of Singapore, Nanyang Technological University, Singapore; 2The Asian School of the Environment, Nanyang Technological University, Singapore; 3Department of Earth Sciences, University of Cambridge; Downing Street, Cambridge, U.K., CB2 3EQ; 4Department of Earth and Environmental Sciences, University of Manchester, Williamson Building, Oxford Road, Manchester, U.K., M13 9PL

  • Presentation type: Talk [Invited]

  • Presentation time: Monday 08:30 - 08:45, Room R280

  • Programme No: 1.2.1

  • Theme 1 > Session 2

Abstract

The A.D. 1783-84 Laki eruption in Iceland was one of the largest basaltic fissure eruptions in historical times and serves as one of the best small-scale modern analogues of a flood basalt eruption. Understanding the timescales over which large volumes of eruptible magma are assembled remains a key challenge in Earth sciences and volcano monitoring. We combine textural observations, microanalysis (EPMA and SIMS) and diffusion chronometry of high-anorthite (An85-90) plagioclase crystals from Laki eruptive fissure VII to gain new insights into timescales and magma storage conditions underneath Grímsvötn. We identified three plagioclase subpopulations based on crystal zoning and Mg trace element geochemistry. Type I plagioclases have oscillatory zoned mantles and core Mg contents in equilibrium with an evolved interstitial mush liquid at a temperature of 1120 °C. 3D diffusion modelling indicates a minimum residence time for Type I crystals of 500 years. Type II and Type III plagioclase crystals have simple zoning and exhibit major Mg disequilibrium in their cores representing melt replenishment events approximately 100 years to 150 days before eruption. This corresponds well with a cyclical increase in eruptive activity at Grímsvötn central volcano. Combined, our observations and modelling results suggest that the magma reservoir that fed the Laki eruption grew incrementally over hundreds to thousands of years with regular replenishment by new melts supplied under the Grímsvötn volcanic system. These replenishment events may have been closely associated with stress release events in southern Iceland. This creates an interesting framework in which monitoring strategies can be devised.