The oxidation state of iron in Mid-Ocean Ridge Basaltic (MORB) glasses: Implications for their petrogenesis and oxygen fugacities

摘要

The oxidation state of a basaltic liquid may be obtained from the average valence state of Fe in its quenched glass. Suitable glasses are widely available from the basalts erupted at Mid-Ocean Ridges (MORB). Measurements of Fe3+/Sigma Fe, where Sigma Fe = Fe2+ + Fe3+, by XANES spectroscopy of a globally representative sample of these MORB glasses are in good agreement with the most recent literature values determined by wet chemistry, but have improved precision, with an expected standard deviation of +/- 0.01 in Fe3+/Sigma Fe for each independent measurement. These precise data allow the geochemical controls on MORB redox systematics at both global and local scales to be evaluated. At the global scale, the relationship between log[Fe2O3] and [MgO] shows that Fe3+ behaves like a lightly incompatible element (LICE) during the crustal evolution of MORB, with its incompatibility between those of the redoxinsensitive elements Li and In. The variability of Fe3+ about this global trend is also consistent with that of redox-insensitive elements of similar incompatibility, implying no external buffering of oxygen fugacity (fO(2)). The variability of Fe3+ is anti-correlated with the variabilities of Sr and Na, which are compatible in plagioclase, but is positively correlated with the variabilities of elements that partition preferentially into clinopyroxene and/or olivine rather than plagioclase, showing that its variability is controlled at least partly by varying ratios of plagioclase to clinopyroxene and olivine during the crustal evolution of MORB. Deviations of Fe3+ concentrations from the global trend are, like those of other LICE, anticorrelated with the deviations from their global trends of the very incompatible elements (VICE). Extrapolation of the global trend to an assumed parental melt at 10.4 wt% [MgO] gives [Fe2O3](o), the concentration of Fe2O3 in the global average MORB parental melt, of 0.6 wt%, which is consistent with 15-20% partial melting of a spinel lh