The Electronic State of Iron in Peridotitic Glasses up to 172 GPa and Its Implications for Deep Magma Oceans

Iron, as the most abundant redox-variable transition metal, plays a crucial role in setting the oxygen fugacity in magma oceans, a parameter that controls the speciation of multivalent elements in the early Earths mantle and potentially the composition of its primordial atmosphere. Despite its importance, how the electronic and structural state (redox, co-ordination and spin state) of iron changes as a function of pressure in complex silicate melt compositions representative of the Earths mantle remains poorly constrained. To quantify the behaviour of iron in a terrestrial magma ocean, we performed synchrotron Mossbauer spectroscopy in-situ at high pressures in a diamond anvil cell (DAC) on peridotitic silicate glasses based on the composition of KLB-1 and synthesized under different redox conditions, up to pressures of 172 GPa. The Mossbauer spectra show that at ambient conditions (1 bar, room temperature), Fe2+ high spin occupies two different sites (D1 and D2) in subequal proportions. The relative fraction of the D2 feature is much higher than that observed for Fe2+-bearing Mid-Ocean Ridge Basalt glasses at the same pressure (Berry et al. 2018). At pressures starting from ~60 GPa a third component appears, which we attribute to a transition in Fe2+ from high- to low-spin that becomes predominant at extreme pressure at the expense of the high spin state. The likely presence of a spin transition in Fe2+ may induce a change in its molar volume at high pressures relevant for core formation. Furthermore, the spin state may also affect the volume change of reaction between Fe3+ and Fe2+, and hence affecting fO2 in deep magma oceans. This work has been supported by Swiss National Science Foundation (SNFS) References Berry, A. J., Stewart, G. A., O'Neill, H. S. C., Mallmann, G., & Mosselmans, J. F. W. (2018). A re-assessment of the oxidation state of iron in MORB glasses. Earth and Planetary Science Letters, 483, 114-123.