Spin transition of ferric iron in the NAL phase: Implications for the seismic heterogeneities of subducted slabs in the lower mantle

- A spin transition of the octahedral Fe3+ in the NAL phase occurs at 30∼47 GPa.
- The spin transition is associated with a volume reduction of the NAL phase.
- Significant softening in KT and VΦ are observed within the spin transition region.
- The NAL phase may help to explain the seismic heterogeneities in the lower mantle.

Al-rich phases (NAL: new hexagonal aluminous phase and CF: calcium-ferrite phase) are believed to constitute 10∼30 wt% of subducted mid-ocean ridge basalt (MORB) in the Earth's lower mantle. In order to understand the effects of iron on compressibility and elastic properties of the NAL phase, we have studied two single-crystal samples (Fe-free Na1.14Mg1.83Al4.74Si1.23O12 and Fe-bearing Na0.71Mg2.05Al4.62Si1.16Fe2+0.09Fe3+0.17O12) using synchrotron nuclear forward scattering (NFS) and X-ray diffraction (XRD) combined with diamond anvil cells up to 86 GPa at room temperature. A pressure-induced high-spin (HS) to low-spin (LS) transition of the octahedral Fe3+ in the Fe-bearing NAL is observed at approximately 30 GPa by NFS. Compared to the Fe-free NAL, the Fe-bearing NAL undergoes a volume reduction of 1.0% (∼1.2 Å3) at 33∼47 GPa as supported by XRD, which is associated with the spin transition of the octahedral Fe3+. The fits of Birch-Murnaghan equation of state (B-M EoS) to P-V data yield unit-cell volume at zero pressure V0=183.1(1) Å3 and isothermal bulk modulus KT0=233(6) GPa with a pressure derivative KT0′=3.7(2) for the Fe-free NAL; V0-HS=184.76(6) Å3 and KT0-HS=238(1) GPa with KT0-HS′=4 (fixed) for the Fe-bearing NAL. The bulk sound velocities (VΦ) of the Fe-free and Fe-bearing NAL phase are approximately 6% larger than those of Al, Fe-bearing bridgmanite and calcium silicate perovskite in the lower mantle, except for the spin transition region where a notable softening of VΦ with a maximum reduction of 9.4% occurs in the Fe-bearing NAL at 41 GPa. Considering the high volume proportion of the NAL phase in subducted MORB, the distinct elastic properties of the Fe-bearing NAL phase across the spin transition reported here may provide an alternative plausible explanation for the observed seismic heterogeneities of subducted slabs in the lower mantle at depths below 1200 km.

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