18O/16O in CO2 evolved from goethite during some unusually rapid solid state α-FeOOH to α-Fe2O3 phase transitions: Test of an exchange model for possible use in oxygen isotope analyses of goethite

Abstract The initial ∼60% of an isothermal vacuum dehydration of goethite can commonly be approximated by first order kinetics. Also, natural goethites contain small amounts of an Fe(CO 3 )OH component in apparent solid solution. The 18 O/ 16 O of CO 2 evolved from the Fe(CO 3 )OH during isothermal vacuum dehydrations is related to the 18 O/ 16 O of the goethite by an apparent fractionation factor ( α app ) that is, in turn, correlated with a first order rate constant, | m |. A kinetic exchange model predicts that α app should decrease as | m | increases for a range of | m | that corresponds to relatively slow rates of dehydration. This pattern has been observed in published results. In contrast, for rapid rates of dehydration, α app is predicted to increase with increasing | m |. Isothermal vacuum dehydrations of two natural goethites had unusually large values of | m | and provided serendipitous tests of this rapid-rate prediction. For these experiments, the measured values of α app were consistent with patterns of variation predicted by the model. This allowed an estimate of the activation energy ( E 2 ) of a model parameter, K 2 , which is the rate constant for oxygen isotope exchange between CO 2 and H 2 O during the solid-state goethite to hematite phase transition. The estimated value of E 2 is only ∼9 kJ/mol. Heterogeneous catalysis tends to decrease the activation energies of gas reactions. Consequently, the inferred value of E 2 suggests that goethite and/or hematite catalyze oxygen isotope exchange between CO 2 and H 2 O during the solid-state phase change. Yield, δ 13 C, and δ 18 O values are routinely measured for increments of CO 2 evolved from the Fe(CO 3 )OH component during isothermal vacuum dehydration of goethite. Model-predicted values of α app can be combined with plateau δ 18 O values of the evolved CO 2 to estimate the δ 18 O of the goethite with a less than optimal, but potentially useful, precision of about ±0.8‰. Therefore, a single analytical procedure (incremental dehydration) applied to one aliquot of a sample could provide not only δ 13 C and mole fractions ( X ) of the Fe(CO 3 )OH component, but also hydrogen yield, δD, and the approximate δ 18 O value of the goethite. Recovery of multiple types of geochemical data from a single aliquot would be particularly useful if the amount of sample available for analysis were limited. Also, the method could be used to estimate the δ 18 O value of goethite in mixtures of minerals not amenable to selective dissolution – e.g., goethite admixed with hematite.