A re-assessment of the oxidation state of iron in MORB glasses

The oxidation state of Fe, Fe/3+ΣFe (where ΣFe=Fe²⁺+Fe³⁺), in glass samples of mid-ocean ridge basalt (MORB), from a wide range of localities, was determined by XANES spectroscopy to be 0.10(2) (n=42). This value is lower than that reported previously by XANES, 0.16(1) (n=103), but consistent with the most recent value determined by redox titrations, 0.11(2) (n=104), all for similar sets of samples. We attribute the anomalously high XANES value of 0.16 to a calibration error resulting from the interpretation of Mössbauer spectra and the resulting Fe/3+ΣFe values of the standards. Our alternative interpretation removes the problem of resolving Fe/3+ΣFe values <∼0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe³⁺ that are independent of Fe³⁺/ΣFe (as is the case for Fe²⁺), and gives Fe/3+ΣFe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO₂). Fe/3+Fe²⁺ is related to fO₂ for our synthetic MORB composition by the temperature independent expression ΔQFM=4log⁡(Fe³⁺/Fe²⁺)+4.23(5), where ΔQFM is the fO₂ in log units relative to the quartz–fayalite–magnetite buffer. The average fO₂ of natural MORB was estimated to be QFM+0.1.