Water in Some Hypersthenic Magmas

The concentrations of $$H_{2}O$$ in inclusions of glass in phenocrysts are estimated indirectly by summation difference using electron probe microanalysis. The method probably is valid if inclusions are glassy (not devitrified) and larger than about 30 μm diameter and if more than about $$2 wt\% of H_{2}O$$ is present. Inclusions of melt may lose $$H_{2}O$$ by diffusion and/or oxidation after initial entrapment. The concentrations of $$H_{2}O$$ quenched in basaltic and andesitic glasses in olivine phenocrysts are mostly between 2 and 4 wt % for Pavlof Volcano, Alaska; Asama Volcano, Japan; Fuego Volcano, Guatemala; and Mt. Shasta, California. The revised estimate of the rate of igneous outgassing of $$H_{2}O$$ in subduction zones is twice that for fresh submarine ridge basalts and suggests that most of the $$H_{2}O $$added to oceanic crust during alteration and weathering either is recycled by some means other than magmatism or is stored deep in the earth. The $$Cl/K_{2}O$$ ratios of associated silicic glasses commonly are less than for the low-silica glasses. Silicic glasses commonly have equal or lesser concentrations of $$H_{2}O$$ compared to associated less silicic glasses. Probably many silicic hypersthenic magmas develop in vapor saturated bodies of magma within a few kilometers of the surface. Some silicic hypersthenic magmas appear to have about $$7 wt\% of H_{2}O$$ and probably developed at greater depths either by differentiation from hydrous basaltic parents or by contamination with $$H_{2}O$$-rich dacitic melts derived from partial fusion of hydrous peridotites within the crust.