Role of Syn-eruptive Cooling and Degassing on Textures of Lavas from the ad 1783-1784 Laki Eruption, South Iceland

Abstract

The Laki eruption involved 10 fissure-opening episodes that produced 15-1 km3 of homogeneous quartz-tholeiite magma. This study focuses on the texture and chemistry of samples from the first five episodes, the most productive period of the eruption. The samples comprise pumiceous tephra clasts from early fallout deposits and lava surface samples from fire-fountaining and cone-building activity. The fluid lava core was periodically exposed at the surface upon lobe breakout, and its characteristics are preserved in glassy selvages from the lava surface. In all samples, plagioclase is the dominant mineral phase, followed by clinopyroxene and then olivine. Samples contain <7 vol. % of euhedral phenocrysts (>100μm) with primitive cores [An* = 100 × Ca/(Ca + Na) >70; Fo >75; En* = 100 × Mg/(Mg + Fe) >78] and more evolved rims, and >10 vol. % of skeletal, densely distributed groundmass crystals (<100μm), which are similar in composition to phenocryst rims (tephra: An*58-67, Fo72-78, En*72-81; lava: An*49-70, Fo63-78, En57-78). Tephra and lava have distinct vesicularity (tephra: >40 vol. % lava: <40 vol. %), groundmass crystal content (tephra: <10 vol. % lava: 20-30 vol. %), and matrix glass composition (tephra: 5.4-5.6 wt % MgO; lava: 4.3-5.0 wt % MgO). Whole-rock and matrix glass compositions define a trend consistent with liquid evolution during in situ crystallization of groundmass phases. Plagioclase-glass and olivine-glass thermometers place the formation of phenocryst cores at ∼10km depth in a melt with ∼1 wt % H2O, at near-liquidus temperatures (∼1150°C). Phenocryst rims and groundmass crystals formed close to the surface, at 10-40°C melt undercooling and in an ∼10-20°C cooler drier magma (0-0.1 wt % H2O), causing an ∼10 mol % drop in An content in plagioclase. The shape, internal zoning and number density of groundmass crystals indicate that they formed under supersaturated conditions. Based on this information, we propose that degassing during ascent had a major role in rapidly undercooling the melt, prompting intensive shallow groundmass crystallization that affected the magma and lava rheology. Petrological and textural differences between tephra and lava reflect variations in the rates of magma ascent and the timing of surface quenching during each eruptive episode. That in turn affected the time available for crystallization and subsequent re-equilibration of the melt to surface (degassed) conditions. During the explosive phases, the rates of magma ascent were high enough to inhibit crystallization, yielding crystal-poor tephra. In contrast, pervasive groundmass crystallization occurred in the lava, increasing its yield strength and causing a thick rubbly layer to form during flow emplacement. Lava selvages collected across the flow-field have strikingly homogeneous glass compositions, demonstrating the high thermal efficiency of fluid lava transport. Cooling is estimated as 0.3°C/km, showing that rubbly surfaced flows can be as thermally efficient as tube-fed pāhoehoe lavas. © The Author 2007. Published by Oxford University Press. All rights reserved.