HYDROGEN ISOTOPE FRACTIONATION BETWEEN COEXISTING VAPOR AND SILICATE GLASSES AND MELTS AT LOW PRESSURE

Abstract

A series of experiments was performed to determine the hydrogen isotope fractionation factors between water vapor and “water” dissolved as hydroxyl groups in rhyolitic and feldspathic glasses and melts. Experiments were carried out in quartz tubes with a large excess of water at 530–850°C and P_(H2O) = 1.4–2.8 bars. Infrared measurements of water concentration profiles and isotopic reversal experiments indicate that equilibrium was attained during the 210–5400 hr runs. The fractionation factors ([D/H_(vapor)]/[D/H_(glass)]) between the vapor and dissolved water in the glasses decrease from about 1.051 to 1.040 for rhyolite glass and 1.049 to 1.035 for albite-orthoclase glass as temperature increases from 530 to 750°C. These fractionations are greater than those observed for most hydrous mineral-water systems at these temperatures, perhaps reflecting the strong hydrogen bonding of hydroxyl groups in the studied glasses. The measured fractionation factor for rhyolitic melt accounts for the observed decreases in δD with decreasing water contents of obsidians erupted from young rhyolitic volcanoes and supports a two-stage degassing history of these magmas. The variation of δD values with water contents of explosively erupted obsidians can be explained by a closed-system vapor-melt partitioning, whereas the low water contents and δD values of the quiesciently erupted domes and flows are interpreted to be a result of open-system degassing.