Effect of temperature on the oxygen isotope composition of carbon dioxide (δCO218O)(δCO218O) prepared from carbonate minerals by reaction with polyphosphoric acid: An example of the rhombohedral CaCO3–MgCO3 group minerals

Measurement of the ratio of 18O to 16O in CO2(δCO218O) produced from rhombohedral carbonate minerals in the compositional range CaCO3–MgCO3 by reaction with polyphosphoric acid (PPA), at temperatures of between 25 and 110 °C, shows that values of δCO218O are linearly correlated (ro > 0.99) with the reciprocal of absolute reaction temperature (K/T). This observation is consistent with earlier studies documenting the effect of temperature on the kinetic fractionation of oxygen isotopes between parent carbonate and product CO2 and H2O during acid decomposition. However, analysis of the resultant data reveals: (1) a progressive increase in dδ18O/dT-1dδ18O/dT-1 with increasing Mg content, and (2) a significant variation in dδ18O/dT-1dδ18O/dT-1 between individual samples of carbonate of identical lattice symmetry and similar chemical composition. The overall increase in gradient with increasing Mg content is assumed to reflect cation radius dependent factors that control the bonding environment at the interface between the metal cation exposed at the surface of the reacting carbonate solid and a H2CO3 transitional species during disproportionation of H2CO3 to CO2 and H2O (“cluster model” of Guo et al., 2009). Phase-specific variations in dδ18O/dT-1dδ18O/dT-1 might result from differences in lattice structure variables (e.g., degree of lattice distortion, extent of positional disorder, and non-ideal mixing of substituent cations where carbonates depart from end-member compositions). Lattice structure variables may be dependent on geochemical conditions pertaining at the time of carbonate precipitation (e.g., biosynthetic versus inorganic precipitates) and suggests that dδ18O/dT-1dδ18O/dT-1 has the potential to vary, within limits, in response to both the chemical composition and structure of each carbonate sample.Because the oxygen isotope composition of carbonate minerals (δMCO318O) measured on the VPDB scale is defined by the oxygen isotope composition of CO2 prepared from NBS19 (calcite) by reaction with PPA at 25 °C (Coplen et al., 1983), the variation in dδ18O/dT-1dδ18O/dT-1 potentially introduces a non-systematic uncertainty to the calibration of the oxygen isotope composition of CO2 produced at higher temperatures. Calibration of other carbonate phases, using a reference frame defined by δCO218O produced from calcite at 25 °C, is subject to additional uncertainty associated with poorly defined PPA fractionation factors (α(T)CO2,MCO3) required to compute values of δMCO318O from δCO218O. These additional sources of uncertainty suggest that levels of analytical precision routinely reported from the measurement of δCO218O at higher temperatures, and used as estimates of uncertainty for δMCO318O, do not adequately reflect real uncertainties in δMCO318O relative to VPDB.