The Fe/ΣFe ratios of MORB glasses and their implications for mantle melting

The Fe³⁺/ΣFe ratio of 104 MORB glasses from the Pacific, the Atlantic, the Indian, and the Red Sea spreading centers have been determined using wet chemical Fe²⁺ analyses and electron microprobe FeO_total measurements. The data provide a new estimate for the MORB oxygen fugacity (fO₂) of 0.41 ± 0.43 (1sigma, N = 100) log units below the fayalite-magnetite-quartz buffer (FMQ), equivalent to a Fe³⁺/ΣFe = 0.12 ± 0.02 (1sigma, N = 104). This new fO₂ estimate is 0.8 log units more oxidized than the average fO₂ proposed by Christie et al. (1986) (FMQ-1.20 ± 0.44; Fe³⁺/ΣFe = 0.07 ± 0.01; N = 87). This slight difference may be related in part to the 3.5% underestimation of the Fe²⁺ concentration determined by Christie et al. (1986) compared with this study. MORB oxygen fugacity does not display any significant difference between the three main oceanic domains, or between enriched and depleted MORB. Yet, the iron red-ox state ratio shows a broad increase during fractional crystallization. Detailed study of magmatic suites highlights the lack of systematic Fe³⁺/ΣFe ratio fractionation during differentiation. Despite the large variations of inferred partial melting degrees (from 5 to 20%), the present data set does not provide any evidence of Fe³⁺/ΣFe relationships with partial melting proxies such as Na_8.0.Based on the Fe³⁺ systematics during partial melting, it is suggested that the oxidation state of MORB reflects a “buffered mantle melting process” resulting in the apparent compatible behavior of Fe³⁺ during partial melting, and in the relatively constant Fe³⁺/ΣFe ratio irrespective of the extent of melting. This result implies that partial melting processes may be open relative to oxygen. We propose a model where the Fe³⁺/ΣFe ratio in the melt is buffered during partial melting. The MORB Fe₂O₃ systematics can be accounted for by using a fO₂ of FMQ-1 that is equivalent to the average fO₂ reported for abyssal peridotites.