Micro-analytical determination of Fe3 /[summation operator]Fe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure ([mu]-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe3 /[summation operator]Fe ratios of 0.16 ± 0.01 (n = 103), which trace back to primary MORB melts equilibrated at the conditions of the quartz-fayalite-magnetite (QFM) buffer. Our results necessitate an upward revision of the Fe3 /[summation operator]Fe ratios of MORBs, mantle oxygen fugacity, and the ferric iron content of the mantle relative to previous wet chemical determinations. We show that only 0.01 (absolute, or < 10%) of the difference between Fe3 /[summation operator]Fe ratios determined by micro-colorimety and XANES can be attributed to the Mössbauer-based XANES calibration. The difference must instead derive from a bias between micro-colorimetry performed on experimental vs. natural basalts. Co-variations of Fe3 /[summation operator]Fe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe3 behaving incompatibly in shallow MORB magma chambers. MORB Fe3 /[summation operator]Fe ratios do not, however, vary with indices of the extent of mantle melting (e.g., Na2O(8)) or water concentration. We offer two hypotheses to explain these observations: The bulk partition coefficient of Fe3 may be higher during peridotite melting than previously thought, and may vary with temperature, or redox exchange between sulfide and sulfate species could buffer mantle melting at ~ QFM. Both explanations, in combination with the measured MORB Fe3 /[summation operator]Fe ratios, point to a fertile MORB source with greater than 0.3 wt.% Fe2O3.
