Isotopic fractionations associated with phosphoric acid digestion of carbonate minerals: Insights from first-principles theoretical modeling and clumped isotope measurements

Geochimica et Cosmochimica Acta (Impact Factor: 4.25). 12/2009; DOI: 10.1016/j.gca.2009.05.071
Source: OAI

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 digestion of carbonates, based on structural arguments that the key step in the reaction is disproportionation of H_2CO_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 lnα^*) and three indices that measure proportions of multiply-substituted isotopologues (Δ^*_(47), Δ^*_(48), Δ^*_(49). We also predict that oxygen isotope fractionations follow the mass dependence exponent, λ of 0.5281 (where α_(17)_O = α^λ_(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 Δ_(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_2CO_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 factor 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 Δ^*_(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 calibration 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_2CO_3.


Available from: William A. Goddard, Jul 02, 2014
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    ABSTRACT: The analysis of mass 47 isotopologues of CO2 (mainly 13C18O16O) is established as a constraint on sources and sinks of environmental CO2, complementary to δ13C and δ18O constraints, and forms the basis of the carbonate clumped isotope thermometer. This measurement is commonly reported using the Δ47 value — a measure of the enrichment of doubly substituted CO2 relative to a stochastic isotopic distribution. Values of Δ47 for thermodynamically equilibrated CO2 approach 0 (a random distribution) at high temperatures (≥ several hundred degrees C), and increase with decreasing temperature, to ≈ 0.9% at 25 °C. While the thermodynamic properties of doubly substituted isotopologues of CO2 (and, similarly, carbonate species) are relatively well understood, there are few published constraints on their kinetics of isotopic exchange. This issue is relevant to understanding both natural processes (e.g., photosynthesis, respiration, air-sea or air-groundwater exchange, CO2 degassing from aqueous solutions, and possibly gas-sorbate exchange on cold planetary surfaces like Mars), and laboratory handling of CO2 samples for Δ47 analysis (e.g., re-equilibration in the presence of liquid water, water ice or water adsorbed on glass or metal surfaces). We present the results of an experimental study of the kinetics of isotopic exchange, including changes in Δ47 value, of CO2 exposed to liquid water between 5 and 37 °C. Aliquots of CO2 gas were first heated to reach a nearly random distribution of its isotopologues and then exposed at low pressure for controlled periods of time to large excesses of liquid water in sealed glass containers. Containers were held at 5, 25 and 37 °C and durations of exchange ranging from 5 min to 7 days. To avoid the formation of a boundary layer that might slow exchange, the tubes were vigorously shaken during the period of exchange. At the end of each experiment, reaction vessels were flash frozen in liquid nitrogen. CO2 gas was then recovered from the head space of the reaction vessel, purified and analyzed for its Δ47, δ13C and δ18O by gas source isotope ratio mass spectrometry. Equilibrium was reached for both δ18O and Δ47 after durations of a few hours to tens of hours. δ18O values at equilibrium were consistent with known fractionation factors for the CO2-H2O system. The evolution of δ18O and Δ47 with experiment duration were consistent with first-order reactions, with rate constants equal to each other (within error), averaging 0.19 h− 1 at 5 °C, 0.38 h− 1 at 25 °C and 0.65 h− 1 at 37 °C. We calculate an activation energy for this isotopic exchange reaction of 26.2 kJ/mol. By comparison, Mills and Urey (1940) measured the rate of 18O exchange between CO2(aq) and water to have a rate of 11 h− 1 at 25 °C and an activation energy of 71.7 kJ/mol. Our finding of a slower rate and lower activation energy is consistent with the rate limiting step of our experiment being the CO2(g)-CO2(aq) exchange, even when samples are shaken during the partial equilibration. Our results broadly resemble those from the study of (Affek, 2013), though this prior study found a lower rate constant for Δ47. We propose that the difference is due to analytical uncertainties and explore the theoretical consequences of unequal reaction rates between 12C18O16O and 13C18O16O with a forward model.
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    Geological Society of America Bulletin 01/2015; DOI:10.1130/B31169.1 · 4.40 Impact Factor
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