I have calculated the oxygen fugacity (fO2) as delta LogfO2 (FMQ) from the olivine-spinel pairs by the method of Balhaus et al. (1991). The results of this calculation is related to the FMQ buffer (e.g. - 2.53 to + 1.34 FMQ). How to convert these log-bar deviation from the FMQ line to the normal LogfO2 values (e.g., - 15.6 or so) to be plotted on the T - LogfO2 diagram (like what has been attached here).

Your help will be highly appreciated.

Ahmed