2Fe3Fe2Si3O12Gt=8FeSi0.5O2Ol+2FeSiO3Opx+O2(2) Comparison of calculated fO2s with those measured by the Pt-Fe sensors demonstrated that either barometer gives the correct answer within the expected uncertainty. Data from the first (Luth et al. 1990) has an uncertainty of about 1.6logfO2 units, however, while that from equilibrium (2) (Woodland and ONeill 1993) has an error of +/ 0.6log units, comparable to that of the spinel peridotite oxybarometer. We therefore conclude that equilibrium (2) may be used to calculate the fO2 recorded by garnet peridotites with an uncertainty of about +/ 0.6log units, providing the potential to probe the oxidation environment of the deep continental lithosphere. Preliminary application based on data from Luth et al. (1990) indicates that garnet peridotite xenoliths from Southern Africa record oxygen fugacities about 3.0 log units below the FMQ (fayalite-magnetite-quartz) buffer. These are substantially more reducing conditions than those recorded by continental spinel lherzolites which typically give oxygen fugacities close to FMQ (Wood et al. 1990).