Seismic velocities of CaSiO3 perovskite can explain LLSVPs in Earth’s lower mantle

Abstract

Seismology records the presence of various heterogeneities throughout the lower mantle1,2, but the origins of these signals—whether thermal or chemical—remain uncertain, and therefore much of the information that they hold about the nature of the deep Earth is obscured. Accurate interpretation of observed seismic velocities requires knowledge of the seismic properties of all of Earth’s possible mineral components. Calcium silicate (CaSiO3) perovskite is believed to be the third most abundant mineral throughout the lower mantle. Here we simultaneously measure the crystal structure and the shear-wave and compressional-wave velocities of samples of CaSiO3 perovskite, and provide direct constraints on the adiabatic bulk and shear moduli of this material. We observe that incorporation of titanium into CaSiO3 perovskite stabilizes the tetragonal structure at higher temperatures, and that the material’s shear modulus is substantially lower than is predicted by computations3,4,5 or thermodynamic datasets6. When combined with literature data and extrapolated, our results suggest that subducted oceanic crust will be visible as low-seismic-velocity anomalies throughout the lower mantle. In particular, we show that large low-shear-velocity provinces (LLSVPs) are consistent with moderate enrichment of recycled oceanic crust, and mid-mantle discontinuities can be explained by a tetragonal–cubic phase transition in Ti-bearing CaSiO3 perovskite.

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Fig. 1: Compressional- and shear-wave velocities of cubic CaSiO3 perovskite from this and previous studies.
Fig. 2: X-ray diffraction patterns demonstrating the cubic–tetragonal phase transition in CaSiO3 perovskite.
Fig. 3: Acoustic velocities of Ca-Pv samples at high-PT conditions.
Fig. 4: Modelled velocity profiles of lower-mantle phase assemblages incorporating Ca-Pv based on this study.

Data availability

Raw data were collected at the European Synchrotron Radiation Facility in Grenoble and are available from https://doi.org/10.5285/6db95d87-365f-4018-abec-00e96e8fcf8d. Derived data from this study, which includes source data for Figs. 2 and 3 and Extended Data Figs. 1 and 5, are provided in the Supplementary Tables.

Change history

  • 23 August 2019

    Owing to a technical error, this Letter was not published online on 14 August 2019, as originally stated, and was instead first published online on 15 August 2019. The Letter has been corrected online.

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Acknowledgements

We acknowledge the support of NERC grants NE/PO17657/1 and NE/M00046X/1, and ESRF beamtime proposals ES-464 and ES-636. We thank G. Manthilake and D. Freitas for their assistance and for lending us ultrasonic equipment from Laboratoire Magmas et Volcans for use during the initial experiments of this study. Use of the Pixirad-8 detector was supported by the French Government via the ‘Investissements d’Avenir’ programme, under the reference ANR-10-AIRT-05.

Author information

A.R.T. designed, performed and analysed the experiments, gathered data from the literature and wrote the manuscript. W.A.C. designed and developed the experimental procedure at ID06 of the ESRF. I.G.W. assisted with interpretation and refinement of diffraction data. J.P.B., D.P.D, W.A.C and N.C.S. helped perform experiments over two sessions at the ESRF. J.M.R.M. performed the computational simulations. S.A.H. assisted with data analysis. All authors contributed to the scientific discussion and preparation of the manuscript.

Correspondence to A. R. Thomson.

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The authors declare no competing interests.

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Peer review information Nature thanks Ian Jackson and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 Lattice and diffraction peak parameters for CaSiO3 and Ca[Si0.6Ti0.4]O3 perovskite.

ad, Refined lattice parameters and pseudo-cubic unit cell volumes from Ca[Si0.6Ti0.4]O3 (a, c) and CaSiO3 (b, d) plotted as a function of experimental temperature with 2σ uncertainties. e, Full-width at half-maximum (FWHM) of diffraction peaks (see key) of the CaSiO3 perovskite sample, normalized to the FWHM at high temperature, measured at 100 K intervals in a separate experiment to that in Fig. 2.

Extended Data Fig. 2 X-ray diffraction patterns from CaSiO3 perovskite.

Shown are stacked diffraction patterns of CaSiO3 perovskite; each panel shows data at 300 K, 373 K and 473 K (see key in a). a, Full patterns; b, c, patterns limited in the 2θ range to allow indication of weak superlattice peaks. The positions of the diffraction peaks from the Ca-Pv sample, MgO, NaCl and Au are indicated by markers—other small peaks are from boron epoxy and/or furnace components. Cubic Ca-Pv peaks are labelled with indices, hkl, in bold. The diffraction patterns reveal the appearance of small superlattice reflections at T = 373 K and 300 K at 2θ values of about 6.1°, 8.05°, 12.1° and 13.2° (we note there is believed to be an additional superlattice reflection obscured at 2θ = 10.5°) labelled with hkl indexed on the tetragonal (I4/mcm) unit cell and marked with gold stars.

Extended Data Fig. 3 Refined X-ray diffraction patterns from Ca[Si0.6Ti0.4]O3 perovskite.

ac, Rietveld refinements of Ca[Si0.6Ti0.4]O3 samples: a, in P21/c with LaB6 calibrant, at 300 K and ambient pressure; b, in the tetragonal I4/m structure (with other cell components) at 890 K and high pressure (about 12 GPa); and c, in \(Fm\bar{3}m\) at 1,336 K and high pressure (12 GPa). In each panel, the black dots are the collected data, the blue curve the model pattern and the green curve the residual. The coloured tick-marks indicate the positions of diffraction peaks of each phase.

Extended Data Fig. 4 X-ray diffraction patterns from Ca[Si0.6Ti0.4]O3 perovskite.

a, Complete diffraction pattern of the Ca[Si0.6Ti0.4]O3 sample as a function of temperature at about 12 GPa, with diffraction intensity indicted by colour scaling. bh, Magnified panels from a focusing on the temperature evolution of the 311, 222, 400, 422, 440, 620 and 444 diffraction peaks (bh, respectively; indexed using a cubic lattice with a ≈ 7.3 Å), demonstrating the change in thermal expansivity between cubic and tetragonal/monoclinic structures, and allowing visual identification of the observed phase transitions.

Extended Data Fig. 5 Phase diagram of calcium perovskite throughout the mantle from ab initio simulations and experiments.

Shown is the cubic–tetragonal transition extrapolated throughout the mantle based on ab initio (solid circles) and experimental (triangles) constraints from this study. Vertical error bars (1σ) and the grey envelope (80% confidence interval) represent the uncertainty in computational results from this study. A 1,500 K mantle adiabat and cold slab temperature profile are plotted as red curves, with dashed red arrows indicating the warming occurring during slab stagnation at 700–1,000 km depth. Results from previous experimental18,23 and computational3 studies are plotted as open symbols and grey curves, respectively.

Extended Data Fig. 6 Equations of state for CaSiO3 perovskite.

a, PV EoS for tetragonal CaSiO3 at 300 K, fitted to data from this study only (purple line) and combined with data from previous studies (thick black curve). Only data with large symbols, those that used pressure transmitting media, have been included in fitting the EoS. All small symbols are from experiments that did not employ a pressure-transmitting medium so have been excluded as volumes are expected to be affected by residual sample stress. Additionally, data from Wang et al.47 were excluded as they used energy dispersive diffraction in the large volume press, which can be subject to larger uncertainties in volume. Error bars represent pressure and volume uncertainties as reported in previous studies. Computational EoS for tetragonal Ca-Pv are plotted as dashed curves for comparison3,17,19,45,47,59,60,61,62,63,64. b, PVT EoS for cubic CaSiO3 perovskite at 298 K and along a 1,600 K adiabat fitted to data from this and previous studies. Small, partially transparent symbols are literature data that were not included in the fitting, either due to falling below the calculated slope of the cubic–tetragonal transition (Methods) or due to concerns about data accuracy. The inset histogram shows the, approximately normal, distribution for the residuals for the fitted data compared with the best-fit model, demonstrating the lack of outliers45,46,47,48.

Extended Data Fig. 7 Bulk sound velocity and bulk modulus of CaSiO3 perovskite.

a, Bulk sound velocity of Ca-Pv predicted from the EoS in this study along a 1,600 K mantle adiabat and at 300 K, compared with results from previous computational studies on a 1,600 K adiabat4,5,6, a fit to previously published PVT diffraction data, and PREM15. b, The adiabatic bulk modulus of CaSiO3 perovskite calculated at 300 K and along a 1,600 K mantle adiabat using the finite strain model from this study, compared with thermodynamic results in Stixrude et al.6 and previous high-temperature computational studies4,5.

Extended Data Fig. 8 Exemplar radiographic image, ultrasonic data and schematic of the experimental cell design.

a, Example of synchrotron radiographic image in plan view used to measure sample length in a Ca[Si0.6Ti0.4]O3 sample. b, Ultrasonic signals from the ‘runa’ experiment on a CaSiO3 sample. c, Cross-section of the experimental assembly (to scale) used in ultrasonic experiments throughout this study.

Extended Data Fig. 9 Comparison of bulk sound velocity from diffraction and ultrasonic measurements.

Shown is a comparison of bulk sound velocity calculated from PVT EoSs, fitted to literature diffraction data (solid curves) with 2σ uncertainties shown by shaded regions, and only to that reported by Gréaux et al.20 (dashed curves), with bulk sound velocities calculated from ultrasonic measurements via vbulk = (vP2 − 4/3vS2)0.5 for data from Gréaux et al.20 (squares) and this study (circles). All curves and symbols are coloured for temperature (colour scale at right).

Supplementary information

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Supplementary Tables 1–7

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