Spin transition of ferric iron in the calcium‐ferrite type aluminous phase

Abstract

AbstractWe investigated Fe‐free and Fe‐bearing CF phases using nuclear forward scattering and X‐ray diffraction coupled with diamond anvil cells up to 80 GPa at room temperature. Octahedral Fe3+ ions in the Fe‐bearing CF phase undergo a high‐spin to low‐spin transition at 25–35 GPa, accompanied by a volume reduction of ~2.0% and a softening of bulk sound velocity up to 17.6%. Based on the results of this study and our previous studies, both the NAL and CF phases, which account for 10–30 vol % of subducted MORB in the lower mantle, are predicted to undergo a spin transition of octahedral Fe3+ at lower mantle pressures. Spin transitions in these two aluminous phases result in an increase of density of 0.24% and a pronounced softening of bulk sound velocity up to 2.3% for subducted MORB at 25–60 GPa and 300 K. The anomalous elasticity region expands and moves to 30–75 GPa at 1200 K and the maximum of the VΦ reduction decreases to ~1.8%. This anomalous elastic behavior of Fe‐bearing aluminous phases across spin transition zones may be relevant in understanding the observed seismic signatures in the lower mantle.