Oxygen Content, Oxygen Fugacity, the Oxidation State of Iron, and Mid-Ocean Ridge Basalts

Oxygen is the third most abundant element in the Solar System and the most abundant in the silicate Earth. As the second most electronegative element its concentration in silicate rocks is fixed by the concentrations and oxidation states of the cations to which it bonds as O 2- . All of the major cation forming elements in terrestrial silicates have only one oxidation state, except for Fe. Thus, charge balancing with Fe is the only significant reason for variability in the amount of O in silicate rocks. It is difficult to measure differences in the oxygen content of rocks directly and instead it is usually proxied by Fe 3+ /Fe 2+ . The Fe 3+ /Fe 2+ of a rock is often used to define an oxygen fugacity ( f O 2 ), however, the relationship between Fe 3+/Fe 2+ and f O 2 is not monotonic, as illustrated by the quartz-fayalite-magnetite (QFM) oxygen buffer where at constant f O 2 Fe 3+ /Fe 2+ can vary from ~ 0 to 2. Since 1991 the empirical expression of Kress and Carmichael has been used to relate the oxidation state of Fe to f O 2 , temperature, composition, and pressure. A feature of this expression is the non-ideal dependence of Fe 3+ /Fe 2+ on f O 2 , suggesting Fe-Fe interactions. However, recent studies have shown that Fe 3+ /Fe 2+ behaves ideally. This has necessitated a refit of the available data. The new expression, for compositions with < 60 wt% SiO 2 , is log 10 (Fe 3+ /Fe 2+ ) = 0.25 QFM - 1.35 + 0.023[CaO] + 0.034[Na 2 O] + 0.044[K 2 O] - 0.018[P 2 O 5 ], where f O 2 is in log units relative to QFM (and thus temperature does not need to be specified) and the oxide components are in weight percent. This equation reproduces the data from the Carmichael laboratory better than their original expression and identifies offsets in the literature data that distorted their fit. The average Fe 3+ /ΣFe (where ΣFe = Fe 2+ + Fe 3+ ) value of <termDefinition xml:id="c8-tdef-0001">mid-ocean ridge basalt</termDefinition> ( <term definitionRef="#c8-tdef-0001" type="abbreviation" xml:id="c8-term-0001">MORB</term> ) glasses was found to be 0.10 by XANES spectroscopy, which is consistent with the result of the most recent redox titration study, both for global datasets of samples. The XANES result was based on a new interpretation of the Mössbauer spectra used to determine Fe 3+ /ΣFe of standards, which removes several long-standing anomalies: (i) the non-ideal dependence of Fe 3+ /Fe 2+ on f O 2 , (ii) the inability to resolve Fe 3+ /ΣFe values < ~0.10, and (iii) variation of the isomer shift of Fe 3+ , but not Fe 2+ , with f O 2 . Using the new expression the average f O 2 of MORB is QFM+0.1. The large dataset of Fe 3+ /ΣFe values for MORB can be used to show that Fe 3+ has an incompatibility between that of In and Mn. The bulk partition coefficient of Fe 3+ is 0.32 and the concentration of Fe 2 O 3 in a primitive MORB melt (with 10.4 wt% MgO) is 0.61 wt%. The Fe 3+ /ΣFe values of MORB are as expected for closed system evolution, with both Fe 3+ and Fe 2+ behaving like other moderately incompatible elements during partial melting and fractional crystallisation.