Degassing of H2O from rhyolite magma during eruption and shallow intrusion, and the isotopic composition of magmatic water in hydrothermal syste

Study results were reviewed on degassing of H2O from rhyolite magma during eruption and shallow intrusion, and on the isotopic composition of magmatic water in hydrothermal systems. Recently, hydrogen isotope studies on obsidian from tephra and related rhyolite flows revealed the common co-variation between [delta]D and wt% of H2O . The [delta]D and water content of magma decreased during depressurization, eruption and shallow intrusion, and the second boiling of magma at shallow levels had a more limited effect on [delta]D because of a small amount of the original water remaining. These studies suggested that variation of [delta]D in magmatic rocks is an indicator of related magmatic processes. The obsidian data allowed to estimate accurately the [delta]D of magma prior to degassing, and it was similar enough among several different volcanic systems. Since the composition of magmatic water accumulated in hydrothermal systems fell within a restricted range, it was difficult to represent the composition by conventional magmatic water box on a [delta]D-[delta][sup 18]O diagram. 9 refs., 2 figs.