Controls of P-T-X-fO2 on Iron Isotopic Fractionation in Igneous Rocks

Studies of the stable isotopic composition of Earth’s mantle for major rock-forming elements other than oxygen are rapidly expanding to new elements but much uncertainty remains as to what those measurements have to tell about the evolution of our planet. Iron is particularly interesting because it possess three oxidation states (metallic Fe0, ferrous Fe2+, and ferric Fe3+) that were partially separated during core formation and crust extraction. We have used the synchrotron technique of NRIXS to measure the mean force constant of iron bonds in geologically relevant silicate glasses, olivine and spinels. The mean force constant directly controls equilibrium isotopic fractionation at high temperature. The Fe force constant measured in these glasses increases linearly with Fe3+/Fetot ratio measured by conventional Mössbauer spectroscopy. The NRIXS results reveal a 0.2 to 0.4 permil equilibrium fractionation on 56Fe/54Fe ratio between Fe2+ and Fe3+ end-members in basalt, andesite, dacite glasses and spinels at magmatic temperatures. For reference, iron isotopic analyses routinely achieve precisions better than ±0.03 permil. Water saturation and pressure up to 9 GPa do not affect the force constants significantly. Structural changes and potential anharmonicity effects will be evaluated in the future by measuring samples at higher temperature. Our results demonstrate for the first time that stable isotope variations of Fe, and of possibly other heterovalent elements such as Ti, V Eu, Cr, Ce, or U, record redox and structural changes in igneous rocks.