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Published October 1989 | public
Journal Article

Hydrogen isotope fractionation between coexisting vapor and silicate glasses and melts at low pressure


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.

Additional Information

© 1989 Pergamon Press. Received 9 January 1989. Accepted 1 August 1989. A Earlier work on vapor-glass fractionation conducted in our laboratories was done by Dr. S. Newman, and we thank her for the important role she has played in our study. We thank P. Ihinger, J. O'Neil, and G. Rossman for their suggestions and comments and I. Friedman, B. Taylor, and D. Wenner for their reviews of the manuscript. Dr. G. Fine of Coming Glass prepared the synthetic albite-orthoclase glass used in our experiments. G. Miller wrote the computer programs used for the degassing models. This work was supported by NSF grant EAR 85-04096 and DOE grant DE-FG-03-85ER13445. Division contribution #4734. Editorial handling: J. R. O'Neil.

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October 18, 2023