AN FTIR STUDY OF WATER CONCENTRATIONS IN GLASS FROM THE MARCH 28-29, 1875 ERUPTION OF ASKJA VOLCANO, ICELAND

The Askja central volcano and its associated fissure swarm are located on the northeastern rift zone in central Iceland. The eruption of Askja on March 28-29 1875 had a plinian phase 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 outcrop on the northeastern shore of the caldera lake (Sparks et al. 1981). This series of deposits was described by Sigurdsson & Sparks (1980) and divided into layers A-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 and carbon dioxide concentrations of glass, glass shards, crystals and melt inclusions. Sparks et al. (1981) estimated that the gas exit velocity at the vent was 380 m/s during the plinian phase, and based on relationships between water content, gas pressure, temperature and gas velocity, estimated the water content at 2.8 weight percent. Measurements of water concentration in glass from layers C through E range from 0.15 to 0.5 weight percent water, with distinctive variations within layers, a steep drop in water concentration in layer D, and recovery in layer E. These results indicate that either the magma largely degassed on its way to the surface or that earlier estimates of magma water concentrations were high.