The lower mantle is dominated by a magnesium- and iron-bearing mineral with the perovskite structure. Iron has the ability to adopt different electronic configurations, and transitions in its spin state in the lower mantle can significantly influence mantle properties and dynamics. However, previous studies aimed at understanding these transitions have provided conflicting results1,2,3,4. Here we report the results of high-pressure (up to 110 GPa) and high-temperature (up to 1,000 K) experiments aimed at understanding spin transitions of iron in perovskite at lower-mantle conditions. Our Mössbauer and nuclear forward scattering data for two lower-mantle perovskite compositions demonstrate that the transition of ferrous iron from the high-spin to the intermediate-spin state occurs at approximately 30 GPa, and that high temperatures favour the stability of the intermediate-spin state. We therefore infer that ferrous iron adopts the intermediate-spin state throughout the bulk of the lower mantle. Our X-ray data show significant anisotropic compression of lower-mantle perovskite containing intermediate-spin ferrous iron, which correlates strongly with the spin transition. We predict spin-state heterogeneities in the uppermost part of the lower mantle associated with sinking slabs and regions of upwelling. These may affect local properties, including thermal and electrical conductivity, deformation (viscosity) and chemical behaviour, and thereby affect mantle dynamics.
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We acknowledge the European Synchrotron Radiation Facility for provision of synchrotron radiation facilities (ID18 and ID27) and we would like to thank A. Chumakov and R. Rüffer for assistance in using beamline ID18 and V. Dmitriev for additional technical assistance. Use of the Advanced Photon Source (beamline 13-ID-D) was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. We are grateful to S. Übelhack and S. Linhardt for technical assistance at Bayerisches Geoinstitut. The project was partly supported by funds from the German Science Foundation (DFG) Priority Programme SPP1236 under project Mc 3/16-1.
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McCammon, C., Kantor, I., Narygina, O. et al. Stable intermediate-spin ferrous iron in lower-mantle perovskite. Nature Geosci 1, 684–687 (2008) doi:10.1038/ngeo309
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