Analyses of sodic amphiboles from a blueschist metamorphic terrain of constant grade in the Tavsanli area, Northwest Turkey, show that practically the whole range of sodic amphibole compositions is stable in the glaucophane-lawsonite facies. By analysing sodic amphiboles coexisting with magnetite and hematite, it has proved possible to plot for a constant pressure and temperature an oxygen fugacity isopleth. Through the analyses of sodic amphiboles and reference to the extended oxygen fugacity contours drawn on the Miyashiro compositional diagram, relative rock oxygen fugacities can be obtained. The oxygen fugacity isopleth corresponding to the magnetite/hematite buffer from two blueschist terrains with differing P-T regimes, namely Tavsanli in Turkey and Shuksan in Cascades, lies along the same loci on the amphibole diagram indicating that the intrinsic $fO_{2}$ slope of sodic amphiboles in the $fO_{2}$-T plane is approximately the same as the $fO_{2}$ slope of the magnetite/hematite buffer. Changes in t...