Sound velocity of CaSiO3 perovskite suggests the presence of basaltic crust in the Earth’s lower mantle

Abstract

Laboratory measurements of sound velocities of high-pressure minerals provide crucial information on the composition and constitution of the deep mantle via comparisons with observed seismic velocities. Calcium silicate (CaSiO3) perovskite (CaPv) is a high-pressure phase that occurs at depths greater than about 560 kilometres in the mantle1 and in the subducting oceanic crust2. However, measurements of the sound velocity of CaPv under the pressure and temperature conditions that are present at such depths have not previously been performed, because this phase is unquenchable (that is, it cannot be physically recovered to room conditions) at atmospheric pressure and adequate samples for such measurements are unavailable. Here we report in situ X-ray diffraction and ultrasonic-interferometry sound-velocity measurements at pressures of up to 23 gigapascals and temperatures of up to 1,700 kelvin (similar to the conditions at the bottom of the mantle transition region) using sintered polycrystalline samples of cubic CaPv converted from bulk glass and a multianvil apparatus. We find that cubic CaPv has a shear modulus of 126 ± 1 gigapascals (uncertainty of one standard deviation), which is about 26 per cent lower than theoretical predictions3,4 (about 171 gigapascals). This value leads to substantially lower sound velocities of basaltic compositions than those predicted for the pressure and temperature conditions at depths between 660 and 770 kilometres. This suggests accumulation of basaltic crust in the uppermost lower mantle, which is consistent with the observation of low-seismic-velocity signatures below 660 kilometres5,6 and the discovery of CaPv in natural diamond of super-deep origin7. These results could contribute to our understanding of the existence and behaviour of subducted crust materials in the deep mantle.

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Fig. 1: Radiography images and X-ray measurements of the sample during synthesis of CaSiO3 perovskite at high pressure and temperature.
Fig. 2: Travel time of P and S waves through the CaPv sample at high pressure and temperature.
Fig. 3: Shear and longitudinal sound velocities of tetragonal and cubic CaSiO3 perovskite.
Fig. 4: Comparison of sound velocities of pyrolite, MORB and harzburgite compositions with representative seismological models in the lower MTR.

Data availability

Source data for Fig. 3 are provided in Supplementary Information.

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Acknowledgements

We thank T. Kunimoto for technical assistance in the experiments at beamline BL04B1 in SPring-8. We acknowledge Y. Kono for providing data analysis software; H. Dekura, Y. Nishihara and Y. Kudo for discussions; and G. Helffrich for comments on the manuscript. This work was supported by the JSPS Kakenhi programmes (to T.I.; number 25220712 and 15H05829).

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Nature thanks J. Buchen, L. Stixrude and the other anonymous reviewer(s) for their contribution to the peer review of this work.

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Contributions

S.G. and T.I. designed the research project. A.Y. and S.G. prepared the glass starting material. Y.H., Y.T. and S.G. prepared the ultrasonic experiments in SPring-8. S.G., T.A. and Z.L. carried out the ultrasonic experiments at SPring-8 and S.G. and Y.T. analysed the data. T.I. and S.G. wrote the manuscript. All authors discussed the results and commented on the manuscript.

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Correspondence to Steeve Gréaux.

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Extended data figures and tables

Extended Data Fig. 1 Pressure and temperature conditions of sound-velocity and density measurements conducted in the stability field of CaPv.

The pressure is calibrated against the NaCl scale34. The red line indicates the decomposition boundary of CaPv to Ca2SiO4 larnite and CaSi2O5 titanite39. Dashed lines represent the pressure–temperature paths followed during the travel-time measurements in the two cooling cycles at 15 GPa (blue) and 17 GPa (red) shown in Fig. 2b.

Extended Data Fig. 2 Schematic diagram of the high-pressure cell used in the present ultrasonic experiments.

Buffer rod, Al2O3; sample, CaSiO3 glass (Fig. 1a); pressure marker, NaCl and BN; soft medium, MgO; pressure medium, (Mg,Co)O; insulator, LaCrO3. denotes the thermocouple hot junction.

Extended Data Fig. 3 Cross-section of the experimental cell recovered after the ultrasonic experiment.

The elemental energy-dispersive spectroscopy mappings of Ca, Si, Al, Mg, Na, Cl and Au, superimposed on the backscattered-electron image of the cross-section, showed no chemical zoning in the CaSiO3 sample or at the interface with the sample and the other cell components.

Extended Data Fig. 4 Experimental longitudinal and shear moduli.

a–f, Results are shown for tetragonal CaPv at 300 K (a, b) and cubic CaPv at 900 K (c, d) and 1,500 K (e, f) as a function of unit-cell volume of CaPv. The solid lines represent fits of the experimental data with the finite-strain EOS (equations (1)–(12)) and the shaded areas represent the 95% confidence intervals, calculated from the trade-off between KS0 and K′ (Extended Data Fig. 4a–e) and G0 and G′ (Extended Data Fig. 4b–f). Most of our experimental data agree within the 95% confidence intervals, demonstrating the small correlation coefficients between the fitted variables.

Extended Data Table 1 Thermoelastic properties of tetragonal and cubic CaSiO3 perovskites
Extended Data Table 2 Correlation matrix of elastic parameters of tetragonal CaPv
Extended Data Table 3 Correlation matrix of thermoelastic parameters of cubic CaPv
Extended Data Table 4 Thermoelastic parameters of major mantle minerals

Supplementary information

Supplementary Table 1

Pressure and temperature conditions of the P- and S-wave velocity and 567 density measurements.

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Gréaux, S., Irifune, T., Higo, Y. et al. Sound velocity of CaSiO3 perovskite suggests the presence of basaltic crust in the Earth’s lower mantle. Nature 565, 218–221 (2019). https://doi.org/10.1038/s41586-018-0816-5

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