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Published February 1, 2003 | public
Journal Article

Experimental determination of oxygen isotope fractionations between CO_2 vapor and soda-melilite melt


We report results of experiments constraining oxygen isotope fractionations between CO_2 vapor and Na-rich melilitic melt at 1 bar and 1250 and 1400°C. The fractionation factor constrained by bracketed experiments, 1000.lnα_(CO2-Na melilitic melt), is 2.65±0.25 ‰ (±2σ; n=92) at 1250°C and 2.16±0.16 ‰ (2σ; n=16) at 1400°C. These values are independent of Na content over the range investigated (7.5 to 13.0 wt. % Na_2O). We combine these data with the known reduced partition function ratio of CO2 to obtain an equation describing the reduced partition function ratio of Na-rich melilite melt as a function of temperature. We also fit previously measured CO_2-melt or -glass fractionations to obtain temperature-dependent reduced partition function ratios for all experimentally studied melts and glasses (including silica, rhyolite, albite, anorthite, Na-rich melilite, and basalt). The systematics of these data suggest that reduced partition function ratios of silicate melts can be approximated either by using the Garlick index (a measure of the polymerization of the melt) or by describing melts as mixtures of normative minerals or equivalent melt compositions. These systematics suggest oxygen isotope fractionation between basalt and olivine at 1300°C of approximately 0.4 to 0.5‰, consistent with most (but not all) basalt glass-olivine fractionations measured in terrestrial and lunar basalts.

Additional Information

© 2003 Elsevier Science Ltd. Received 1 March 2002. Accepted 30 July 2002. Available online 27 January 2003. We thank John Beckett for invaluable assistance on experimental procedures, Nami Kitchen for help in the stable isotope laboratory, and Reid Cooper and his colleagues at Corning for their help preparing NaMel/1 glass. We thank Juske Horita and anonymous reviewers for help in improving our manuscript. This work was supported by BSF Grant BSF 97–00454, DOE grant DE-FG03–85ER13445, and NSF grants EAR-0095897 and OCE-0095897. Associate editor: J. Horita.

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