The oxygen fugacity of the mantle exerts a fundamental influence on mantle melting, volatile speciation, and the development of the atmosphere. However, its evolution through time is poorly understood. Changes in mantle oxidation state should be reflected in the$Fe^{3+}/Fe^{2+}$of mantle minerals, and hence in stable iron isotope fractionation. Here it is shown that there are substantial (1.7 per mil) systematic variations in the iron isotope compositions ($\delta^{57/54}Fe$) of mantle spinels. Spinel$\delta^{57/54}Fe$values correlate with relative oxygen fugacity,$Fe^{3+}/\Sigma Fe$, and chromium number, and provide a proxy of changes in mantle oxidation state, melting, and volatile recycling.