Petrological relations in some Icelandic basaltic hyaloclastites

Analyses of a number of subglacially formed basaltic hyaloclastites are used to evaluate various properties of the magmas on eruption. Glass inclusions in olivine and plagioclase crystals have similar composition as the surrounding glass and crystals are therefore true phenocrysts. These data combined with simplified experimental systems can be used to show that crystallization of the lavas studied took place under varying water pressure conditions. The maximum total pressure did not exceed 2 kb. Mineral liquid calculations show that variation between samples from any single area can be explained by fractionation less than 20 % olivine and plagioclase