Oxygen isotope effects associated with the solid-state α-FeOOH to α-Fe2O3 phase transformation

Abstract

Samples of synthetic and natural goethites were subjected to isothermal dehydration under both closed- and open-systein conditions at various teinperatures ranging from 160 to 300°C. The oxygen isotope ratios of the dehydration product (hematite) were systematically different for open- and closedsystein dehydration. In every instance, open-systein dehydration resulted in 18 O enrichments of the product hematite relative to the starting goethite. For all but a few cases, closed-system dehydration produced an 18 O depletion in the residual mineral relative to the starting goethite. These oxygen isotope effects indicate that significant mineral-vapor isotope exchange occurred during the solid-state, closed-system transformation of goethite to ∗∗ hematite. Calculated values of the apparent closed-system oxygen isotope fractionation factor between hematite and the corresponding liquid water suggest that equilibrium isotope exchange may have been approached in those samples for which the extent of closed-system dehydration was at least 95%. The rapidity of the mineral-vapor isotope exchange associated with the solid-state goethite to hematite transformation is indicated by the fact that the dehydration times (for those samples apparently approaching equilibrium) ranged from only 24 to 68 hours. These results suggest that the oxygen isotope ratios of many hematites in low temperature geological systems reflect the environmental conditions associated with their formation from some hydrous precursor. Information about the 18 O 16 O ratio in the hydrous precursor itself is not likely to be directly preserved in the hematite