The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle
Document Type
Article
Date of Original Version
5-15-2011
Abstract
Micro-analytical determination of Fe3+/ΣFe ratios in mid-ocean ridge basalt (MORB) glasses using micro X-ray absorption near edge structure (μ-XANES) spectroscopy reveals a substantially more oxidized upper mantle than determined by previous studies. Here, we show that global MORBs yield average Fe3+/ΣFe 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+/ΣFe 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+/ΣFe 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+/ΣFe ratios in global MORB with indices of low-pressure fractional crystallization are consistent with Fe3+ behaving incompatibly in shallow MORB magma chambers. MORB Fe3+/ΣFe 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+/ΣFe ratios, point to a fertile MORB source with greater than 0.3wt.% Fe2O3. © 2011.
Publication Title, e.g., Journal
Earth and Planetary Science Letters
Volume
305
Issue
3-4
Citation/Publisher Attribution
Cottrell, Elizabeth, and Katherine A. Kelley. "The oxidation state of Fe in MORB glasses and the oxygen fugacity of the upper mantle." Earth and Planetary Science Letters 305, 3-4 (2011). doi: 10.1016/j.epsl.2011.03.014.