The oxidation state of Fe in garnets in a garnet peridotite xenolith from the Wesselton kimberlite (South Africa) was quantitatively mapped using X-ray absorption near edge structure (XANES) spectroscopy. Maps of Fe3+/ΣFe were produced by recording the fluorescence intensity at discrete energies rather than recording the full spectrum at each point. The intensity at each point in the map was quantitatively converted to Fe3+/ΣFe with reference to a linear calibration derived from garnet standards for which Fe3+/ΣFe had been determined previously by Mössbauer spectroscopy. The resolution of these maps approaches that of elemental maps obtained using an electron microprobe. The maps reveal zoning in Fe3+/ΣFe between the core (0.075) and rim (0.125) that correlates with zoning of other elements. The rims record an oxidizing metasomatic event in the lithospheric mantle. The oxygen fugacity (fO2) of this metasomatism is considerably higher than expected from studies of homogeneous garnets that exhibit metasomatic signatures; such garnets may represent a re-equilibrated average of the original (core) and metasomatic (rim) fO2 values. Metasomatism of the lithospheric mantle may thus have a greater impact on diamond stability than previously thought.