Oxygen isotope analysis of carbonates in the calcite-dolomite-magnesite solid-solution by high-temperature pyrolysis : initial results

Crowley, S.F.; Spero, H.J.; Winter, D.A.; Sloane, Hilary; Croudace, I.W.. 2008 Oxygen isotope analysis of carbonates in the calcite-dolomite-magnesite solid-solution by high-temperature pyrolysis : initial results. Rapid Communications in Mass Spectrometry, 22. 1703-1713.

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Accurate and efficient measurement of the oxygen isotope composition of carbonates (C18O) based on the mass spectrometric analysis of CO2 produced by reacting carbonate samples with H3PO4 is compromised by: (1) uncertainties associated with fractionation factors () used to correct measured oxygen isotope values of CO2 to C18O; and (2) the slow reaction rates of many carbonates of geological and environmental interest with H3PO4. In contrast, determination of C18O from analysis of CO produced by high-temperature (>1400°C) pyrolytic reduction, using an elemental analyser coupled to continuous-flow isotope-ratio mass spectrometry (TC/EA CF-IRMS), offers a potentially efficient alternative that measures the isotopic composition of total carbonate oxygen and should, therefore, theoretically be free of fractionation effects. The utility of the TC/EA CF-IRMS technique was tested by analysis of carbonates in the calcite-dolomite-magnesite solid-solution and comparing the results with C18O measured by conventional thermal decomposition/fluorination (TDF) on the same materials. Initial results show that CO yields are dependent on both the chemical composition of the carbonate and the specific pyrolysis conditions. Low gas yields (<100% of predicted yield) are associated with positive (>+0.2) deviations in compared with . At a pyrolysis temperature of 1420°C the difference between C18O measured by TC/EA CF-IRMS and TDF was found to be negatively correlated with gas yield (r = -0.785) and this suggests that C18O values (with an estimated combined standard uncertainty of ±0.38) could be derived by applying a yield-dependent correction. Increasing the pyrolysis temperature to 1500°C also resulted in a statistically significant correlation with gas yield (r = -0.601), indicating that C18O values (with an estimated uncertainty of ±0.43) could again be corrected using a yield-dependent procedure. Despite significant uncertainty associated with TC/EA CF-IRMS analysis, the magnitude of the uncertainty is similar to that associated with the application of poorly defined values of used to derive C18O from measured by the H3PO4 method for most common carbonate phases. Consequently, TC/EA CF-IRMS could provide a rapid alternative for the analysis of these phases without any effective deterioration in relative accuracy, while analytical precision could be improved by increasing the number of replicate analyses for both calibration standards and samples. Although automated gas preparation techniques based on the H3PO4 method (ISOCARB, Kiel device, Gas-Bench systems) have the potential to measure efficiently for specific, slowly reacting phases (e.g. dolomite), problems associated with poorly defined remain. The application of the Principle of Identical Treatment is not a solution to the analysis of these phases because it assumes that a single fractionation factor may be defined for each phase within a solid-solution regardless of its precise chemical composition. This assumption has yet to be tested adequately.

Item Type: Publication - Article
Digital Object Identifier (DOI):
Programmes: BGS Programmes 2008 > NERC Isotope Geoscience Laboratory
ISSN: 0951-4198
Additional Keywords: Oxygen isotopes, Geochemistry (analysis)
NORA Subject Terms: Earth Sciences
Date made live: 01 Sep 2008 13:00 +0 (UTC)

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