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

Geochimica et Cosmochimica Acta (Impact Factor: 4.33). 12/2009; 73(24). DOI: 10.1016/j.gca.2009.05.071
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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.

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Available from: William A. Goddard, Jul 02, 2014
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    • "and the data are reported in the " carbon dioxide equilibrium scale " (CDES) of Dennis et al. (2011). An acid correction factor of + 0.069‰ was added to all measurements of Δ 47 CDES following Guo et al. (2009) and Wacker et al. (2013). Masses 48 and 49 were monitored to check for possible sample contamination following Affek and Eiler (2006) and Huntington et al. (2009, 2011). "
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    ABSTRACT: Ancient marine radiaxial calcite cements are commonly exploited as archives of marine porewater properties based on the argument that they lack metabolic effects often assigned to biogenic carbonates. Here we critically test the significance of conventional versus (with respect to these fabrics) less-conventional proxy data from Pennsylvanian, Triassic, and Cretaceous case examples. Conventional proxies include: cathodoluminescence, carbon and oxygen isotope ratios, main and trace elemental concentrations. Less conventionally applied proxies are: clumped isotope " Δ 47 " , redox-sensitive, and rare earth elements sampled across a succession of Triassic radiaxial fibrous calcites. Radiaxial calcites are subdivided in three groups based on their luminescence characteristics: non-luminescent, patchy luminescent, and bright luminescent. Luminescence patterns are in fair agreement with isotope ratios, in particular with those of oxygen. The data fall into, or are close to, the range of reconstructed marine seawater values and often plot to the positive end member of the isotopic range. These results disagree with the commonly held view that isotope data from luminescent cements reflect a priori non-marine values. Further evidence for this comes from REE concentration patterns and cerium-anomalies suggesting normal marine porewater values for all except the very last generation of radiaxial calcites. This implies that luminescent radiaxial calcites must not necessarily represent significant diagenetic resetting. Kinetic effects during precipitation and different activator elements must be considered. Marine and earliest burial porewater temperatures of ~12–26 °C are suggested by conventional calcite δ 18 O thermometry. Conversely, the application of the clumped isotope thermometer to the same radiaxial calcites suggests temperatures of 180–200 °C, reflecting solid-state resetting of fully cemented limestones under a low water:rock ratio. Redox-sensitive elements, particularly Zn, Cd, U, and Cu are affected by kinetic processes overriding fluid Eh. Manganese concentrations and Ce-anomaly data point to gradually decreasing marine porewater oxygen levels from outer to inner cement fringes. Judging from REE patterns and Ce-anomalies, the cement layers in the central portions of the pore filling cement succession witnessed the end of marine precipitation and the onset of shallow marine diagenesis. Consequently, radiaxial calcite precipitation is suggested to continue in the early shallow (marine) burial domain. This study underscores the potential of radiaxial calcite successions as archives of marine porewater to shallow burial diagenetic pathways. The combination of conventional and less conventional proxies is a clear strength of this study and documents that abiogenic carbonate archives are often underexplored.
    Full-text · Article · Dec 2015 · Chemical Geology
    • "to predictions based on transition state theory (0.015&) (Guo et al., 2009). Furthermore, the observed AFF of 0.280& for calcite reconciles observations of high-temperature calcite mineral equilibrium (Passey and Henkes, 2012) with theoretical predictions (Schauble et al., 2006; Hill et al., 2014; this study). "
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    ABSTRACT: “Clumped-isotope” thermometry is an emerging tool to probe the temperature history of surface and subsurface environments based on measurements of the proportion of ^(13)C and ^(18)O isotopes bound to each other within carbonate minerals in ^(13)C^(18)O^(16)O_2^(2−) groups (heavy isotope “clumps”). Although most clumped isotope geothermometry implicitly presumes carbonate crystals have attained lattice equilibrium (i.e., thermodynamic equilibrium for a mineral, which is independent of solution chemistry), several factors other than temperature, including dissolved inorganic carbon (DIC) speciation may influence mineral isotopic signatures. Therefore we used a combination of approaches to understand the potential influence of different variables on the clumped isotope (and oxygen isotope) composition of minerals. We conducted witherite precipitation experiments at a single temperature and at varied pH to empirically determine ^(13)C–^(18)O bond ordering (Δ_(47)) and δ^(18)O of CO_3^(2−) and HCO_3^− molecules at a 25 °C equilibrium. Ab initio cluster models based on density functional theory were used to predict equilibrium ^(13)C–^(18)O bond abundances and δ^(18)O of different DIC species and minerals as a function of temperature. Experiments and theory indicate Δ_(47) and δ^(18)O compositions of CO_3^(2−) and HCO_3^− ions are significantly different from each other. Experiments constrain the Δ_(47)–δ^(18)O slope for a pH effect (0.011 ± 0.001; 12 ⩾ pH ⩾ 7). Rapidly-growing temperate corals exhibit disequilibrium mineral isotopic signatures with a Δ_(47)–δ^(18)O slope of 0.011 ± 0.003, consistent with a pH effect. Our theoretical calculations for carbonate minerals indicate equilibrium lattice calcite values for Δ_(47) and δ^(18)O are intermediate between HCO_3^− and CO_3^(2−). We analyzed synthetic calcites grown at temperatures ranging from 0.5 to 50 °C with and without the enzyme carbonic anhydrase present. This enzyme catalyzes oxygen isotopic exchange between DIC species and is present in many natural systems. The two types of experiments yielded statistically indistinguishable results, and these measurements yield a calibration that overlaps with our theoretical predictions for calcite at equilibrium. The slow-growing Devils Hole calcite exhibits Δ_(47) and δ^(18)O values consistent with lattice equilibrium. Factors influencing DIC speciation (pH, salinity) and the timescale for DIC equilibration, as well as reactions at the mineral–solution interface, have the potential to influence clumped-isotope signatures and the δ^(18)O of carbonate minerals. In fast-growing carbonate minerals, solution chemistry may be an important factor, particularly over extremes of pH and salinity. If a crystal grows too rapidly to reach an internal equilibrium (i.e., achieve the value for the temperature-dependent mineral lattice equilibrium), it may record the clumped-isotope signature of a DIC species (e.g., the temperature-dependent equilibrium of HCO_3^−) or a mixture of DIC species, and hence record a disequilibrium mineral composition. For extremely slow-growing crystals, and for rapidly-grown samples grown at a pH where HCO_3^− dominates the DIC pool at equilibrium, effects of solution chemistry are likely to be relatively small or negligible. In summary, growth environment, solution chemistry, surface equilibria, and precipitation rate may all play a role in dictating whether a crystal achieves equilibrium or disequilibrium clumped-isotope signatures.
    No preview · Article · Sep 2015 · Geochimica et Cosmochimica Acta
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    • "to predictions based on transition state theory (0.015&) (Guo et al., 2009). Furthermore, the observed AFF of 0.280& for calcite reconciles observations of high-temperature calcite mineral equilibrium (Passey and Henkes, 2012) with theoretical predictions (Schauble et al., 2006; Hill et al., 2014; this study). "

    Full-text · Dataset · Aug 2015
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