H2O IN BASALT AND BASALTIC ANDESITE GLASS INCLUSIONS FROM 4 SUBDUCTION-RELATED VOLCANOS

Total dissolved H2O and major element abundances were measured in basa lt and basaltic andesite glass inclusions in olivine phenocrysts from Quaternary eruptions of four subduction-related volcanoes to test the hypothesis that low-MgO high-alumina basalts contain high H2O at depth [1] and to reveal any petrogenetically significant correlations betwe en arc basalt compositions and H2O contents. Total dissolved H2O (comb ined molecular H2O and OH groups) measured by ion microprobe in mafic glass inclusions from the 1974 eruption of Fuego, Guatemala, reaches 6 .2 wt.%. Dissolved H2O contents decrease in more evolved Fuego glasses . Correlations of H2O with MgO, Na2O, K2O, S and Cl indicate that aque ous fluid exsolution during magma ascent forced crystallization and di fferentiation of residual liquids. Low-K2O magnesian high-alumina basa lt glass inclusions from the 3 ka eruption of Black Crater (Medicine L ake volcano, California) have low H2O contents, near 0.2 wt.%, which a re consistent with the MORB-like character of these and other primitiv e lavas of the Medicine Lake region. Basalt and basaltic andesite glas s inclusions from Copco Cone and Goosenest volcano on the Cascade volc anic front north of Mt. Shasta have H2O contents of up to 3.3 wt.%. Th e range of H2O contents in Cascade mafic magmas is too large to have r esulted solely from enrichment by crystallization and indicates the pa rticipation of an H2O-rich component in magma generation or crustal-le vel modification. Whereas fluid-absent melting of amphibole-bearing pe ridotite can account for the H2O in most mafic arc liquids, the very h igh H2O/alkali ratios of the 1974 Fuego eruptives suggest that an aque ous fluid was involved in the generation of Fuego basalts.