HR: 1340h
AN: V53A-1533    [Abstracts]
TI: How Degassing Determined the Crystallization Style of the Lava Flows Produced During the Laki Eruption (South Iceland)
AU: * Guilbaud, M
EM: m.guilbaud@open.ac.uk
AF: Volcano Dynamics Group, The Open University, Walton Hall, Milton Keynes, MK11 1BE United Kingdom
AU: Blake, S
EM: s.blake@open.ac.uk
AF: Volcano Dynamics Group, The Open University, Walton Hall, Milton Keynes, MK11 1BE United Kingdom
AU: Thordarson, T
EM: moinui@soest.hawaii.edu
AF: Department of Geology and Geophysics, University of Hawaii at Manoa, Post Building room 614A, 1680 East-West Road, Honolulu, HI 96822 United States
AU: Self, S
EM: stephen.self@open.ac.uk
AF: Volcano Dynamics Group, The Open University, Walton Hall, Milton Keynes, MK11 1BE United Kingdom
AB: Basaltic eruptions mainly produce lava flows. Degassing of basalt as it rises in the conduit can control the late-stage crystallization of the magma. Microlites formed by degassing-induced undercooling may then influence the flow regime of the lava, control the formation of surface morphologies, and thus determine the final extent of the flows. We present evidence that this mechanism operated during the AD1783-84 Laki eruption. During the eruption, 15 km³ of dominantly rubbly pƒhoehoe lava was emitted from along a fissure system. We studied crystal compositions, morphologies, glass-crystal gradients, glass chemistry and water content, and quantified crystal numbers and sizes. The observations were then compared to models of equilibrium crystallization and finally combined in a temperature-time model of the magma crystallization path. In this paper, we concentrate on showing snapshots of the four stages that controlled the thermal regime, degassing, and crystallization style of the magma. Phenocrysts formed at water under-saturated, low-undercooling conditions in the magma chamber (stage 1). Compositional zoning in plagioclase evidences the control of degassing on the nucleation and growth of plagioclase-clinopyroxene microphenocrysts in the conduit (stage 2). When the magma reached the surface, nucleation rates were significantly raised that there was a burst of crystallization of olivines and plagioclases skeletal microlites, and relative suppression of clinopyroxene crystallization (stage 3). The lava then partially re-equilibrated to equilibrium conditions during further crystallization in the flow (stage 4). The whole crystallization path of the magma during the eruption may have been nearly isothermal, mainly driven by the undercooling induced by degassing during magma ascent. The high density of microlites in lava from the early phases of outflow from the fissure created conditions favouring periodic disruption of the thickening lava crust, and hence development of rubbly pahoehoe surface morphology.
DE: 3610 Geochemical modeling (1009, 8410)
DE: 3625 Petrography, microstructures, and textures
DE: 3640 Igneous petrology
DE: 3652 Pressure-temperature-time paths
DE: 3690 Field relationships (1090, 8486)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005