Авторы: Yapp C.

2003 г.

Geochim.Cosmochim.Acta

    “Plateau”
18 O values of CO2 that evolved from the Fe(CO3)OH component during isothermal vacuum dehydrations (200–230 °C) of 18 natural goethites range from 8.2 to 28.1‰. In contrast, the measured
18 O 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 ( 18 app) between plateau CO2 and initial goethite is systematically related to the rate of isothermal vacuum dehydration. The nonlinear correlation and the magnitudes of the 18 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 H2O 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 18 O/ 16 O ratios of the evolved CO2 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 H2O vapor, is tentatively estimated as 28  11 KJ/mol. Such knowledge may be of consequence in understanding the significance of 18 O/ 16 O ratios in hematites from some natural environments (e.g., Mars?). Kinetic data and
18 O values of CO2 are routinely obtained in the course of measurements of the abundance and
13 C values of the Fe(CO3)OH in goethite. The observed correlation between 18 app and dehydration rates suggests that plateau
18 O values of evolved CO2 may provide complementary estimates of the
18 O values of total goethite structural oxygen (O, OH, CO2) with an overall precision of about 1‰. However, because of isotopic exchange during the dehydration process,
18 O values of the evolved CO2 do not reflect the original
18 O values of the CO2 that was occluded as Fe(CO3)OH in goethite.

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