Oxygen isotope analysis of carbonates in the calcite‐dolomite‐magnesite solid‐solution by high‐temperature pyrolysis: initial results
Authors: Crowley Stephen F. , Spero Howard J. , Winter David A. , Sloane Hilary J. , Croudace Ian W. 
Published: April 2008

Accurate and efficient measurement of the oxygen isotope composition of carbonates (δC18O) based on the mass spectrometric analysis of CO2 produced by reacting carbonate samples with H3PO4 is compromised by: (1) uncertainties associated with fractionation factors (${\bf \alpha }_{{\bf CO}_{\bf 2} {\bf C}}$) used to correct measured oxygen isotope values of CO2 ${\bf (}{\bf \delta }_{{\bf CO}_{\bf 2} } ^{{\bf 18}} {\bf O)}$ to δC18O; and (2) the slow reaction rates of many carbonates of geological and environmental interest with H3PO4. In contrast, determination of δC18O 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 δC18O 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 ${\bf \delta }_{{\bf C}_{{\bf TC/EA}} } ^{{\bf 18}} {\bf O}$ compared with ${\bf \delta }_{{\bf C}_{{\bf TDF}} } ^{{\bf 18}} {\bf O}$. At a pyrolysis temperature of 1420°C the difference between δC18O measured by TC/EA CF‐IRMS and TDF ${\bf (\Delta }_{{\bf C}_{{\bf TC/EA,TDF}} } ^{{\bf 18}} {\bf O)}$ was found to be negatively correlated with gas yield (r = −0.785) and this suggests that δC18O 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 δC18O 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 ${\bf \alpha }_{{\bf CO}_{\bf 2} {\bf C}}$ used to derive δC18O from ${\bf \delta }_{{\bf CO}_{\bf 2} } ^{{\bf 18}} {\bf O}$ 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 ${\bf \delta }_{{\bf CO}_{\bf 2} } ^{{\bf 18}} {\bf O}$ efficiently for specific, slowly reacting phases (e.g. dolomite), problems associated with poorly defined ${\bf \alpha }_{{\bf CO}_{\bf 2} {\bf C}}$ 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. Copyright © 2008 John Wiley & Sons, Ltd.