OXYGEN ISOTOPE FRACTIONATION IN MAGNETITES: STRUCTURAL EFFECT AND OXYGEN INHERITANCE

Abstract

The modified increment method is applied to the calculation of oxygen isotope fractionation in different structures of magnetite. The results show that, at isotopic equilibrium, the magnetite of a spinel-type structure has the very similar behavior of oxygen isotope partitioning to hematite, but it is considerably depleted in 18O relative to the magnetite of an inverse spinel-type structure. This provides a resolution to the systematic discrepancy in the quartz-magnetite oxygen isotope fractionation between previous theoretical calculations of the author and the experimental determinations of H. Chiba and coworkers. An oxygen isotope inheritance is suggested in which the magnetites used in the experiments could have inherited the oxygen isotope feature of a precursor mineral hematite. The oxygen isotope inheritance may be common in the formation of secondary minerals in laboratory and nature. Most magnetite would be of the spinel-type structure in the infancy of its crystallization. The oxygen isotope study on magnetite may provide insight into the mechanism of mineralogical reactions during magnetite formation under laboratorial or natural conditions.