Wang, Zhengrong and Schauble, Edwin A. and Eiler, John M. (2004) Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases. Geochimica et Cosmochimica Acta, 68 (23). pp. 4779-4797. ISSN 0016-7037. http://resolver.caltech.edu/CaltechAUTHORS:20121030-111501633
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Isotopologues of molecular gases containing more than one rare isotope (multiply substituted isotopologues) can be analyzed with high precision (1σ <0.1‰), despite their low natural abundances (∼ ppm to ppt in air), and can constrain geochemical budgets of natural systems. We derive a method for calculating abundances of all such species in a thermodynamically equilibrated population of isotopologues, and present results of these calculations for O_2, CO, N_2, NO, CO_2, and N_2O between 1000 and 193 to 77 K. In most cases, multiply substituted isotopologues are predicted to be enriched relative to stochastic (random) distributions by ca. 1 to 2‰ at earth-surface temperatures. This deviation, defined as Δ_i for isotopologue i, generally increases linearly with 1/T at temperatures ≤ 500 K. An exception is N_2O, which shows complex temperature dependences and 10’s of per-mill enrichments or depletions of abundances for some isotopologues. These calculations provide a basis for discriminating between fractionations controlled by equilibrium thermodynamics and other sorts of isotopic fractionations in the budgets of atmospheric gases. Moreover, because abundances of multiply substituted isotopologues in thermodynamically equilibrated populations of molecules vary systematically with temperature, they can be used as geothermometers. Such thermometers are unusual in that they involve homogeneous rather than heterogeneous equilibria (e.g., isotopic distribution in gaseous CO_2 alone, rather than difference in isotopic composition between CO_2 and coexisting water). Also, multiple independent thermometers exist for all molecules having more than one multiply substituted isotopologue (e.g., thermometers based on abundances of ^(18)O^(13)C^(16)O and ^(18)O^(12)C^(18)O are independent); thus, temperatures estimated by this method can be tested for internal consistency.
|Additional Information:||© 2004 Elsevier Ltd. Received October 7, 2003; accepted in revised form May 21, 2004. Associate editor: D. Cole. We thank Desheng Wang, Yiqin Gao, Wei-Chen Chen, Mitchio Okumura, and Paul Asimow for discussions and inspirations, Paul Wennberg and Zhonghua Yang for the explanation of HITRAN, L. S. Rothman for offering HITRAN2k dataset, and David Cole and two other anonymous reviewers for providing constructive comments and suggestions. This work is supported by a Packard Foundation grant to J.M.E., and support to J.M.E. from the Davidow endowment to Caltech.|
|Official Citation:||Zhengrong Wang, Edwin A. Schauble, John M. Eiler, Equilibrium thermodynamics of multiply substituted isotopologues of molecular gases, Geochimica et Cosmochimica Acta, Volume 68, Issue 23, 1 December 2004, Pages 4779-4797, ISSN 0016-7037, 10.1016/j.gca.2004.05.039. (http://www.sciencedirect.com/science/article/pii/S001670370400451X)|
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|Deposited By:||Ruth Sustaita|
|Deposited On:||30 Oct 2012 20:16|
|Last Modified:||30 Oct 2012 20:16|
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