Evidence for a Fe3+-rich pyrolitic lower mantle from (Al,Fe)-bearing bridgmanite elasticity data

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Abstract

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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Figure 1: Experimental setup and results.
Figure 2: Pressure dependence of the average acoustic velocities of (Al,Fe)-bearing bridgmanite (red circles).
Figure 3: Mineral-physics-based seismic models.
Figure 4: Deviation of modelled seismic velocities from PREM with depth.

Change history

  • 16 May 2018

    In this Letter, some of the elastic constants reported in Extended Data Table 1 and the values of the elastic constants at room pressure cited in the manuscript on page 544 were incorrect, as a result of an error in the script used to generate the numbers. These errors have been corrected online.

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Acknowledgements

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.

Author information

A.K., H.M., D.F. and T.B.B. designed the research. L.Z. synthesized the bridgmanite sample. A.K. performed the experiments and analysed the Brillouin data. T.B.B. performed the XRD analysis. D.F. and H.M. discussed the content of the paper and performed the modelling. H.M. wrote the paper draft. All authors commented on the manuscript.

Correspondence to H. Marquardt.

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Competing interests

The authors declare no competing financial interests.

Additional information

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

Extended Data Figure 1 Effect of FeAlO3 incorporation on the room pressure elastic moduli of MgSiO3 bridgmanite.

The red circles represent data based on this study on (Al,Fe)-bearing bridgmanite and earlier Brillouin scattering work on single-crystal MgSiO3 bridgmanite13. The blue and green symbols refer to computational studies14,18.

Extended Data Figure 2 Densities (red curve) as a function of depth calculated from our model.

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.

Extended Data Figure 4 Modelled sound wave velocities.

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 5 Fe-partitioning coefficient KD(app) from our model in comparison to previous experimental data measured on a pyrolitic mantle composition.

The thermodynamic model not only matches available elasticity data for bridgmanite and ferropericlase (Extended Data Figs 3, 4) but also reproduces changes in Fe–Mg partitioning between these phases with depth in both Al-free and Al-bearing systems.

Extended Data Figure 6 Secondary electron image of polished single crystals of bridgmanite.

Crystals have been cut to half-circles by a focused ion beam12,34.

Extended Data Figure 7 Standard deviation as a function of the signal-to-noise ratio in Brillouin spectra.

Extended Data Table 1 Summary of high-pressure elastic constants derived for (Mg0.9Fe0.1Si0.9Al0.1)O3
Extended Data Table 2 Summary of ambient pressure elastic constants reported for MgSiO3 and (Fe,Al)-bearing MgSiO3 from both experimental and theoretical work
Extended Data Table 3 Summary of parameters used for the model calculations

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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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