Spatter-Rich and Lava-Like Ignimbrites of the Halarauður Eruption, Krafla (Iceland): An Unusually Violent Eruption From a Basalt-Dominated Caldera

Basalt-dominated calderas, unlike their silicic counterparts, commonly lack voluminous ignimbrite deposits indicative of catastrophic collapse events. Because of this scarcity, ignimbrite eruptions from these systems are not well-studied, and their causes and dynamics are poorly understood. Here we present a detailed field study of the c. 110 ka Halarauður eruption ( 5 km³ DRE), the largest known eruption of the basalt-dominated Krafla caldera. Our new eruptive stratigraphy and whole-rock chemical data offer new insights into the style and dynamics of this eruption, and the formation of rare spatter-rich and basaltic welded ignimbrites.

Three major stratigraphic units, H1-H3, are defined. The basal unit (H1) occurs as laterally discontinuous, m-scale lenses of poorly sorted, spatter-bearing pumiceous lapilli tuff (proximally as lithic breccia) containing white (rhyolitic) and streaky (hybrid intermediate) pumices, interpreted as ponded ignimbrite. Co-eruption of spatter and pumice may indicate contrasting styles of activity from discrete vents, with spatter possibly generated by squeezing of degassed magma through a developing caldera fault system. Abrupt intensification of the eruption is marked by a transition into the volumetrically dominant, regionally dispersed units H2 and H3. H2 is an intermediate spatter-rich agglomerate, usually welded, lithic-rich (up to 20 vol%) and strongly oxidised. Lithic concentration zones, dense welding >7 km from vent, rapid local thickness changes, and very poor sorting of non-welded facies imply emplacement as a spatter-rich pyroclastic density current(s). The high lithic content of H2 likely reflects partial collapse of the caldera, with an upward increase in deep-derived (plutonic) lithics indicating deepening of the vent system. H2 grades up into H3, a very strongly welded (lava-like) basaltic unit with similar dispersal, interpreted as ignimbrite from the climactic phase.

Major element compositions of juvenile products define a mixing line between basalt and high-silica rhyolite. Compositions are more mafic on average up-section, implying progressive depletion of the rhyolitic endmember. Future work will focus on determining the pre-eruptive storage conditions of the endmember magmas, and the timescale of their interaction.