The chemical composition of Earth’s lower mantle can be constrained by combining seismological observations with mineral physics elasticity measurements1,2,3. However, the lack of laboratory data for Earth’s most abundant mineral, (Mg,Fe,Al)(Al,Fe,Si)O3 bridgmanite (also known as silicate perovskite), has hampered any conclusive result. Here we report single-crystal elasticity data on (Al,Fe)-bearing bridgmanite (Mg0.9Fe0.1Si0.9Al0.1)O3 measured using high-pressure Brillouin spectroscopy and X-ray diffraction. Our measurements show that the elastic behaviour of (Al,Fe)-bearing bridgmanite is markedly different from the behaviour of the MgSiO3 endmember2,4. We use our data to model seismic wave velocities in the top portion of the lower mantle, assuming a pyrolitic5 mantle composition and accounting for depth-dependent changes in iron partitioning between bridgmanite and ferropericlase6,7. We find excellent agreement between our mineral physics predictions and the seismic Preliminary Reference Earth Model8 down to at least 1,200 kilometres depth, indicating chemical homogeneity of the upper and shallow lower mantle. A high Fe3+/Fe2+ ratio of about two in shallow-lower-mantle bridgmanite is required to match seismic data, implying the presence of metallic iron in an isochemical mantle. Our calculated velocities are in increasingly poor agreement with those of the lower mantle at depths greater than 1,200 kilometres, indicating either a change in bridgmanite cation ordering or a decrease in the ferric iron content of the lower mantle.
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This research was supported through the projects ‘GeoMaX’ funded under the Emmy-Noether Program of the German Science Foundation (MA4534/3-1) and the ERC advanced grant number 227893 ‘DEEP’ funded through the EU 7th Framework Programme. The FEI Scios FIB machine at BGI Bayreuth is supported by grant INST 91/315-1 FUGG. H.M. acknowledges support from the Bavarian Academy of Sciences. We thank J. Buchen for assistance in creating Fig. 1c, H. Schulze for sample polishing and K. Marquardt for help with the FIB device.
The authors declare no competing financial interests.
Reviewer Information Nature thanks I. Jackson and the other anonymous reviewer(s) for their contribution to the peer review of this work.
An erratum to this article is available online at https://doi.org/10.1038/s41586-018-0115-1.
Extended data figures and tables
The dashed line corresponds to PREM.
Extended Data Figure 3 Modelled sound wave velocities for (Mg0.9Fe0.1Si0.9Al0.1)O3 (red curves) along with the experimental data from this study.
For (Mg0.9Fe0.1)O (red curves) along with experimental data29. The data below 35 GPa have been employed to constrain the physical properties of the FeO and MgO components in the model as the effects of the iron spin transition are not captured by the model. At the temperature conditions of Earth’s mantle, the effects of the iron spin crossover will be shifted to depths beyond those modelled in this study21 and are, therefore, irrelevant to the present contribution.
Extended Data Figure 7 Standard deviation as a function of the signal-to-noise ratio in Brillouin spectra.
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Kurnosov, A., Marquardt, H., Frost, D. et al. Evidence for a Fe3+-rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data. Nature 543, 543–546 (2017) doi:10.1038/nature21390
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