OXYGEN ISOTOPES IN IRON (III) OXIDES .1. MINERAL-WATER FRACTIONATION FACTORS

Experimental data obtained in this laboratory indicate that goethite and hematite may be isotopically indistinguishable at equilibrium. The hematite (goethite)-water oxygen isotope fractionation based on synthesis experiments at five temperatures over the range from 25° to 120°C is represented by the expression: 1000 ln 18αmin-H=1.63·106 T-2-12.3. This equation predicts a "crossover" at 91°C. The hematite (goethite)-water curve of the current work is approximately parallel to the magnetite-water curve of Blattner et al. at temperatures of ≲ 200°C. Furthermore, the hematite (goethite)-water curve suggests that hematite should be enriched in 18O by ∼ 2.5-3‰ relative to coprecipitated magnetite defined by the curve of Blattner et al. This predicted temperature insensitivity of the hematite-magnetite oxygen isotope fractionation and the relative enrichment of 18O in hematite are generally consistent with the characteristics of hematite-magnetite oxygen isotope fractionation noted previously by Clayton and Epstein for natural samples. © 1990.