A model for 18O/ 16O variations in CO 2 evolved from goethite during the solid-state α-FeOOH to α-Fe 2O 3 phase transition

Abstract

“Plateau” δ 18O values of CO 2 that evolved from the Fe(CO 3)OH component during isothermal vacuum dehydrations (200–230 °C) of 18 natural goethites range from 8.2 to 28.1‰. In contrast, the measured δ 18O values of the goethite structural oxygen range from −11.3 to 1.7‰. The results of this study indicate that the apparent oxygen isotope fractionation factor ( 18α app) between plateau CO 2 and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the 18α app values are predicted by a relatively simple mass balance model with the following assumptions: (1) the rate of isothermal vacuum dehydration of goethite (for the interval from 0 to ∼60 to 80% loss of structural hydroxyl hydrogen) can be reasonably well represented by first-order kinetics and (2) isotopic exchange between evolving H 2O vapor and solid occurs only in successive, local transition states. The generally good correspondence between the model predictions and the experimental data seems to validate these assumptions. Thus, the 18O/ 16O ratios of the evolved CO 2 can act as probes into the transient processes operating at the molecular level during the solid-state goethite-to-hematite phase transition. For example, the activation energy for the rate constant associated with the transition state, oxygen isotopic exchange between solid and H 2O vapor, is tentatively estimated as 28 ± 11 KJ/mol. Such knowledge may be of consequence in understanding the significance of 18O/ 16O ratios in hematites from some natural environments (e.g., Mars?). Kinetic data and δ 18O values of CO 2 are routinely obtained in the course of measurements of the abundance and δ 13C values of the Fe(CO 3)OH in goethite. The observed correlation between 18α app and dehydration rates suggests that plateau δ 18O values of evolved CO 2 may provide complementary estimates of the δ 18O values of total goethite structural oxygen (O, OH, CO 2) with an overall precision of about ±1‰. However, because of isotopic exchange during the dehydration process, δ 18O values of the evolved CO 2 do not reflect the original δ 18O values of the CO 2 that was occluded as Fe(CO 3)OH in goethite.