TEMPERATURE AND REDOX PATH OF BIOTITE-BEARING INTRUSIVES - A METHOD OF ESTIMATION APPLIED TO S-TYPE AND I-TYPE GRANITES FROM AUSTRALIA

Estimations of the oxidation state and development of f(O2) during magmatic evolution are usually not possible because f(O2) is a function of temperature (and pressure). If two independent equations for f(O2) = f(O2)(T) can be obtained, f(O2) and the corresponding temperature can be estimated. For biotite-bearing rocks an estimation of f(O2)(bio) can be combined with an estimation of f(O2)(rock). This latter estimation requires an extrapolation of high-temperature data because low-temperature data are not available. The combination of the two equations provides a quadratic equation in T with the common (negative) solution: T(int) = 1/4c2{- k1 - 2c1 - square-root [(k1 + 2c1)2 + 8c2k2]} which permits back calculation of f(O2). The usefulness of the method is demonstrated for typical S-type, ilmenite, and I-type, magnetite granites from Australia. Two distinct oxidation states are obtained. It is suggested that the availability of H2O during granite emplacement largely determines f(O2) conditions.