OXYGEN ISOTOPE FRACTIONATION IN HEMATITE AND MAGNETITE - A THEORETICAL CALCULATION AND APPLICATION TO GEOTHERMOMETRY OF METAMORPHIC IRON-FORMATIONS

Thermodynamic oxygen isotope factors for hematite and magnetite are calculated using the modified increment method and incorporating the theoretical and experimental reduced partition function ratios determined for quartz and water in the temperature range 0 to 1200-degrees-C. The obtained oxygen isotope fractionation factors between hematite, magnetite and water are expressed as 10(3)ln alpha-Fe2O3-H2O = 2.69 x 10(6)/T2 - 12.82 x 10(3)/T + 3.78 10(3)ln alpha-Fe3O4-H2O = 3.02 x 10(6)/T2 - 12.00 x 10(3)/T + 3.31 and those between quartz and hematite or magnetite are expressed as 10(3)ln alpha-SiO2-Fe2O3 = 1.55 x 10(6)/T2 + 9.05 x 10(3)/T - 4.82 10(3)ln alpha-SiO2-Fe3O4 = 1.22 x 10(6)/T2 + 8.22 x 10(3)/T - 4.35 The present results are well in agreement with existing theoretical, experimental and empirical calibrations involving magnetite and hematite. A significant oxygen isotope fractionation is obtained between hematite and magnetite, with hematite being depleted in O-18 relative to magnetite. This matches with the observation from Precambrian iron-formation in the Hamersley Range, Australia. The rule of "effect of oxidation state" on the oxygen isotope fractionation seems not to work for the two iron oxides. Previous experimental calibration of magnetite-water system by reduction of hematite to magnetite is concordant with the present result for hematite-water sytem. This is attributed to the quantitative redox of iron in the experiments, that can in turn explain the nearly identical isotope composition of oxygen in coexisting hematite and magnetite pairs from the banded iron-formation in the Iron Quadrangle, Brazil.