Probing magnetic transitions in (Mg,Fe)GeO3-perovskite with Mössbauer Spectroscopy

The effect of iron on the properties of major lower mantle minerals must be understood for proper interpretation of seismic and geodynamic data. The role of Fe in bridgmanite in the deep earth is complicated as Fe can occupy two different crystallographic sites (8-fold site or octahedral site) and adopt different valence states (2+,3+) and electronic configurations (high or low spin). Previous experimental and theoretical work on this material has reported a pressure-induced low- to high-QS (quadrupole splitting) transition at 30 GPa, explained by a small lateral displacement of the Fe2+ ion (e.g. Jackson et al., 2005, Hsu et al., 2010). Further insight into the nature of this transition can be obtained through the study of germanates which are well-known to be effective analogues for silicates. The perovskite (Pv) to post-perovskite (pPv) transition is reduced by 50 GPa in MgGeO3 compared with MgSiO3. Despite this, a recent theoretical study predicts that in the Ge analogue the low- to high-QS transition should be 20 GPa higher in the germanate due to its larger unit cell (Shukla et al., 2015). 57Fe-enriched (Mg0.8Fe0.2)GeO3 perovskite was synthesized at 40 GPa with laser heating at Sector 13-ID-D, as confirmed with X-ray diffraction. Conventional and synchrotron Mössbauer spectroscopy was conducted at Sector 3 and Sector 16 of the Advanced Photon source, Argonne National Laboratory over the stability field of germanate perovskite: 39-61 GPa. This study took advantage of the new capability of synchrotron Mössbauer spectroscopy conducted during the APS operations in hybrid mode, which expanded the experimental time window from 150 to 800 ns. Preliminary analysis indicates that iron is predominately Fe2+ with some Fe3+ contribution at low pressure. With increasing pressure, we find the appearance of a third high-QS site, consistent with similar observations in the silicate. Our results provide new insights into high-pressure behavior of Fe in perovskite-structured materials, as well as providing a test of theoretical predictions. We also demonstrate the advantages of collection synchrotron Mössbauer data in hybrid mode.