Oxygen isotope fractionation in hematite and magnetite; a theoretical calculation and application to geothermometry of metamorphic iron-formations

The application of the modified increment method provides original thermodynamic isotope factors (beta -factors) of O in hematite and magnetite which show that a consistently different isotopic behaviour can be expected for these two minerals. The partition of O isotopes in hematite and magnetite is determined by crystal structure and cation distribution features. Hematite is depleted in (super 18) O relative to magnetite; this matches with observations from the Precambrian iron formation in the Hamersley Range, Australia. Previous experimental calibration of the magnetite- water system by reduction of hematite to magnetite is agreement with the present result for the hematite-water system. This is attributed to the quantitative redox of iron in the experiments that in turn can explain the nearlyidentical isotope composition of O in coexisting hematite and magnetite pairs from the banded iron formation of the Iron Quadrangle, Brazil.