Iron oxidation state in (Mg Fe)O : calibration of the Flank method on synthetic samples and applications on natural inclusions from lower mantle diamonds O ferropericlase is the most common mineral found in diamonds originating in the lower mantle (more than 50% of occurrences). It is well known that the Fe3+ concentration in (Mg Fe)O is sensitive to oxygen fugacity even at high pressures. Therefore the determination of Fe3+ Fetot in such inclusions provides a direct method for investigating lower mantle redox conditions during diamond formation. The goal of the present research is to calibrate the flank methodʺ by electron microprobe using synthetic (Mg Fe)O and then apply the method to determine in situ Fe3+ Fetot in ferropericlase inclusions from lower mantle diamonds. Up to now a calibration of the flank method is available only for garnets. Initially the flank method was calibrated for garnets to test the reproducibility of the method on the Jeol XA 8200 electron microprobe in use at Bayerisches Geoinstitut. Results showed that for garnets a new calibration curve needs to be established at each working session. Then the flank method was calibrated for the Jeol XA 8200 electron microprobe in use at Bayerisches Geoinstitut for a homogeneous set of (Mg Fe)O ferropericlase crystals over a wide range of composition (xFe = 2 to 60 at.%) and Fe3+ Fetot (1 to 15%). Samples were obtained by performing high pressure high temperature experiments in a multi anvil apparatus. In order to avoid compositional effects on flank method measurements the high sample homogeneity was essential. Moreover the determination of the Fe3+ Fetot ratio needed to be extremely accurate...