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

The oxidation state of Fe, Fe3+/Sigma Fe (where Sigma Fe = 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 MOssbauer spectra and the resulting Fe3+/Sigma Fe values of the standards. Our alternative interpretation removes the problem of resolving Fe3+/Sigma Fe values < similar to 0.1 in basaltic glasses, produces isomer shift and quadrupole splitting values for Fe3+ that are independent of Fe3+/Sigma Fe (as is the case for Fe2+), and gives Fe3+/Sigma Fe values that are consistent with the thermodynamically expected dependence on oxygen fugacity (fO(2)). Fe3+/Fe2+ is related to fO(2) for our synthetic MORB composition by the temperature independent expression Delta AQFM = 4 log(Fe3+/Fe2+)+ 4.23(5), where Delta AQFM is the f O-2 in log units relative to the quartz-fayalite-magnetite buffer. The average fO(2) of natural MORB was estimated to be QFM+0.1. (C) 2017 Published by Elsevier B.V.