Article

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.

1 Bookmark
 · 
121 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: The 13 C-18 O bonds in carbonates are potential single-phase geo-thermometers. However, their theoretical distributions (noted as ∆ 47 s) in CO 2 degassed from calcite and aragonite with phospho-ric acid are unclear. Thus, the isotope reactions of 13 C-18 O bonds on the growing surfaces of calcite (0001) and aragonite (001) planes were investigated using ab initio techniques. It was found that these reactions determined 13 C-18 O clumped isotope signatures in bulk calcite and aragonite min-erals with novel ∆ 47 polynomials: -8 6 5 2 2 47 calcite 4 3 2 8.14673 10 9.31441 10 1.42742 10 1.64905 10 = + + 5.65986 10 − × × × × ∆ − − − × T T T T and -8 7 5 2 2 47 aragonite 4 3 2 5.81530 10 1.18267 10 1.50830 10 1.71999 10 = + + 5.86981 10 − × × × × ∆ − − − × T T T T for temperatures T ranging from 260 to 1500 K. These theoretical results were in good agree-ment with the experimental data. In addition, the influence of phosphoric acid on these polynomi-als was at the level of 0.01‰.
    Open Journal of Geology 09/2014; 4(9):436-480.
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Despite their utility for bio- and chemo-stratigraphy, many carbonate platform sequences have been difficult to analyze using paleomagnetic techniques due to their extraordinarily weak natural remanent magnetizations (NRMs). However, the physical processes of magnetization imply that stable NRMs can be preserved that are many orders of magnitude below our present measurement abilities. Recent advances in reducing the noise level of superconducting magnetometer systems, particularly the introduction of DC-SQUID sensors and development of a low-noise sample handling system using thin-walled quartz-glass vacuum tubes, have solved many of these instrumentation problems, increasing the effective sensitivity by a factor of nearly 50 over the previous techniques of SQUID moment magnetometry.Here we report the successful isolation of a two-polarity characteristic remanent magnetization from Middle-Late Permian limestone formed in the atoll of a mid-oceanic paleo-seamount, now preserved in the Jurassic accretionary complex in Japan, which had proved difficult to analyze in past studies. Paleothermometric indicators including Conodont Alteration Indices, carbonate petrology, and clumped-isotope paleothermometry are consistent with peak burial temperatures close to 130 °C, consistent with rock-magnetic indicators suggesting fine-grained magnetite and hematite holds the NRM. The magnetic polarity pattern is in broad agreement with previous global magnetostratigraphic summaries from the interval of the Early-Middle Permian Kiaman Reversed Superchron and the Permian-Triassic mixed interval, and ties the Tethyan-Panthalassan fusuline zones to it. Elevated levels of hematite associated with the positive δ13Ccarb of the Kamura event argue for a brief spike in environmental oxygen. The results also place the paleo-seamount at a paleolatitude of ~ 12° S, in the middle of the Panthalassan Ocean, and imply a N/NW transport towards the Asian margin of Pangea during Triassic and Jurassic time, in accordance with the predicted trajectory from its tectono-sedimentary background. These developments should expand the applicability of magnetostratigraphic techniques to many additional portions of the Geological time scale.
    Palaeogeography Palaeoclimatology Palaeoecology 11/2014; · 2.75 Impact Factor
  • Current Genetic Medicine Reports. 09/2014; 2(3):151-161.

Full-text (2 Sources)

Download
14 Downloads
Available from
Jul 2, 2014