Oxygen fugacity (fO₂) has a primary control on element speciation in minerals, melts, and fluids. Accordingly, fO₂ can modulate: the capacity of the mantle to store water, the depth of mantle phase transitions, and transport of volatiles from the mantle to Earth’s atmosphere. Currently, estimates of the upper mantle fO₂ span 3 orders of magnitude. It is unknown, however, whether this range reflects real spatial variation in fO₂ or, instead, different degrees of chemical disequilibrium in the mineral assemblage used to estimate fO₂. In order to apply the spinel-olivine-orthopyroxene oxybarometer, chemical equilibrium is assumed. With the capability to make high precision isotope measurements we can now test whether this assumption of equilibrium is valid by measuring isotopes of the elements involved.

As a Chateaubriand Fellow hosted by the University de Lille in France, I synthesized spinel crystals with Fe³⁺/Fe²⁺ ratios ranging from 0 to 1. I then measured the force constant (a measure of bond strength) for Fe in spinel using NRIXS (nuclear resonant inelastic X-ray scattering) at beamline-3ID at APS. I found that the force constant increases with increasing Fe³⁺/Fe²⁺ in spinel. Combined with the force constant for Fe measured for olivine, I was able to calculate the spinel-olivine iron isotope fractionation at equilibrium as a function of Fe³⁺/Fe²⁺. Comparing this calibration with literature spinel-olivine data, it is apparent that the majority of samples do not fall on the equilibrium trend. The lack of iron isotope equilibrium warrants caution regarding the assumption of chemical equilibrium for mineral assemblages used for oxybarometry.

References:

1. Roskosz, M., Sio, C. K., Dauphas, N., Bi, W., Tissot, F. L., Hu, M. Y., Zhao, J. & Alp, E. E. (2015). Spinel–olivine–pyroxene equilibrium iron isotopic fractionation and applications to natural peridotites. Geochimica et Cosmochimica Acta, 169, 184-199.
2. O’Neill H. StC., Wall V.J. (1987) The olivine-orthopyroxene-spinel oxygen geobarometer, the nickel precipitation curve, and the oxygen fugacity of the Earth’s upper mantle. Journal of Petrology, 28(6), 1169-1191.
3. Frost, D. J., & McCammon, C. A. (2008). The redox state of Earth’s mantle.  Rev. Earth Planet. Sci., 36, 389-420.