Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements Weifu Guo a, * , Jed L. Mosenfelder a , William A. Goddard III b , John M. Eiler a a Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA b Materials and Process Simulation Center, California Institute of Technology, Pasadena, CA 91125, USA Received 28 July 2008; accepted in revised form 27 May 2009; available online 23 June 2009 Abstract Phosphoric acid digestion has been used for oxygen- and carbon-isotope analysis of carbonate minerals since 1950, and was recently established as a method for carbonate ‘clumped isotope’ analysis. The CO 2 recovered from this reaction has an oxygen isotope composition substantially different from reactant carbonate, by an amount that varies with temperature of reaction and carbonate chemistry. Here, we present a theoretical model of the kinetic isotope effects associated with phosphoric acid diges- tion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H 2 CO 3 reaction intermediary. We test that model against previous experimental constraints on the magnitudes and temperature dependences of these oxygen isotope fractionations, and against new experimental determinations of the fractionation of 13 C– 18 O-containing isotopologues (‘clumped’ isotopic species). Our model predicts that the isotope fractionations associated with phosphoric acid digestion of carbonates at 25 °C are 10.72&, 0.220&, 0.137&, 0.593& for, respectively, 18 O/ 16 O ratios (1000 lna * ) and three indices that measure proportions of multiply-substituted isotopologues ðD  47 ; D  48 ; D  49 Þ. We also predict that oxygen isotope fractionations follow the mass dependence exponent, k of 0.5281 (where a 17 O ¼ a k 18 O ). These predictions compare favorably to independent experimental constraints for phosphoric acid digestion of calcite, including our new data for fractionations of 13 C– 18 O bonds (the measured change in D 47 = 0.23&) during phosphoric acid digestion of calcite at 25 °C. We have also attempted to evaluate the effect of carbonate cation compositions on phosphoric acid digestion fractionations using cluster models in which disproportionating H 2 CO 3 interacts with adjacent cations. These models underestimate the magnitude of isotope fractionations and so must be regarded as unsucsessful, but do reproduce the general trend of variations and temperature dependences of oxygen isotope acid digestion fractionations among different carbonate minerals. We suggest these results present a useful starting point for future, more sophisticated models of the reacting carbonate/acid interface. Examinations of these theoretical predictions and available experimental data suggest cation radius is the most important fac- tor governing the variations of isotope fractionation among different carbonate minerals. We predict a negative correlation between acid digestion fractionation of oxygen isotopes and of 13 C– 18 O doubly-substituted isotopologues, and use this relationship to estimate the acid digestion fractionation of D  47 for different carbonate minerals. Combined with previous theoretical evaluations of 13 C– 18 O clumping effects in carbonate minerals, this enables us to predict the temperature calibra- tion relationship for different carbonate clumped isotope thermometers (witherite, calcite, aragonite, dolomite and magnesite), and to compare these predictions with available experimental determinations. The success of our models in capturing several of the features of isotope fractionation during acid digestion supports our hypothesis that phosphoric acid digestion of carbonate minerals involves disproportionation of transition state structures containing H 2 CO 3 . Ó 2009 Elsevier Ltd. All rights reserved. 0016-7037/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2009.05.071 * Corresponding author. Present address: Geophysical Laboratory, Carnegie Institution of Washington, Washington DC 20015, USA. Tel.: +1 202 478 8993; fax: +1 202 478 8901. E-mail address: wfguo@ciw.edu (W. Guo). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 73 (2009) 7203–7225