EVOLUTION OF WATER CONCENTRATION DURING THE MARCH 28-29, 1875 ERUPTION OF ASKJA VOLCANO, ICELAND

The Askja central volcano and its associated fissure swarm are located in the northeastern rift zone in central Iceland. The bulk of the eruption of Askja on March 28-29 1875 consisted of a plinian eruption that lasted 6-7 hours, produced approximately 0.2 km³ of ash and rhyolitic pumice, and created a surge deposit and partially welded ash/pumice fall deposit that crop out on the northeastern shore of the modern caldera lake (Sparks et al. 1981). This series of deposits was described by Sigurdsson & Sparks (1981) and divided into layers A through E corresponding to distinctive phases of the eruption. The present study is an evaluation of the volatile budget of the magma during the eruption and focuses on water concentration of glass fragments and shards, glass adjacent to crystals and melt inclusions (MIs) hosted in those crystals. Sparks et al. (1981) estimated that the gas exit velocity at the vent was 380 m/s during the plinian phase, and estimated the water content at 2.8 wt%. Measurements of water concentration in discrete glass shards and fragments of basaltic, dacitic and rhyolitic composition from layers C through E range from 0.15 to 0.5 wt%, with distinctive variations within layers, a steep drop in water concentration in layer D, and recovery in layer E. In contrast, plagioclase and pyroxene crystals from layers C through E contain glassy rhyolitic MIs with water concentrations ranging from 0.1 to 1.8 weight percent, some significantly higher than the matrix glass. Rhyolitic glass adjacent to host crystals contains the highest measured water concentration, from .4 to 2.18 wt%. In one sample, crystals hosting rhyolitic MIs with water concentrations approximately 3 times higher than surrounding rhyolitic glass indicate that the magma underwent significant degassing on its way to the surface. Rhyolitic glass adjacent to crystals hosting MIs with higher water concentration than those MIs and the initial phreatoplinian eruptive style suggest extensive interaction with meteoric water prior to and during the eruption. Intimate mixtures of basaltic glasses of differing compositions within the same sample and basaltic glass fragments surrounded by rhyolitic glass suggest that magma batches mingled and possibly mixed prior to and during the course of the eruption.