Equilibrium high-temperature Fe isotope fractionation between fayalite and magnetite: An experimental calibration

The iron isotopic fractionation factor between magnetite and fayalite was measured at high temperature in a piston-cylinder apparatus. For the first time, the three-isotope method [Matsuhisa, J., Goldsmith, J. R., and Clayton, R. N., 1978. Mechanisms of hydrothermal crystallisation of quartz at 250 degrees C and 15 kbar. Geochim. Cosmochim. Acta 42, 173-182.] has been used to determine equilibrium fractionation between two minerals directly. The experimentally-determined temperature-dependent iron isotope fractionation between fayalite and magnetite is described by 10(3)ln alpha(57)(Mag-Fa) congruent to Delta(57) FeMag-Fa = 0.30 (+/- 0.024) x 10(6) / T-2. The equation predicts measurable fractionation at magmatic temperatures. Our work bears on the magnitude of Fe isotope fractionation attending differentiation in magmatic systems and provides a new isotope thermometer for coexisting fayalite and magnetite. (c) 2008 Elsevier B.V. All rights reserved.