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

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 100 μm) 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 (40 vol. %; lava: