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Calcium dissolution in bridgmanite in the Earth’s deep mantle

Abstract

Accurate knowledge of the mineralogy is essential for understanding the lower mantle, which represents more than half of Earth’s volume. CaSiO3 perovskite is believed to be the third-most-abundant mineral throughout the lower mantle, following bridgmanite and ferropericlase1,2,3. Here we experimentally show that the calcium solubility in bridgmanite increases steeply at about 2,300 kelvin and above 40 gigapascals to a level sufficient for a complete dissolution of all CaSiO3 component in pyrolite into bridgmanite, resulting in the disappearance of CaSiO3 perovskite at depths greater than about 1,800 kilometres along the geotherm4,5. Hence we propose a change from a two-perovskite domain (TPD; bridgmanite plus CaSiO3 perovskite) at the shallower lower mantle to a single-perovskite domain (SPD; calcium-rich bridgmanite) at the deeper lower mantle. Iron seems to have a key role in increasing the calcium solubility in bridgmanite. The temperature-driven nature can cause large lateral variations in the depth of the TPD-to-SPD change in response to temperature variations (by more than 500 kilometres). Furthermore, the SPD should have been thicker in the past when the mantle was warmer. Our finding requires revision of the deep-mantle mineralogy models and will have an impact on our understanding of the composition, structure, dynamics and evolution of the region.

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Fig. 1: Stability of a single-perovskite mineralogy at high pressure–temperature conditions.
Fig. 2: In situ XRD and ex situ electron microscopy analysis of Ca-pyrolite and the komatiitic compositions.
Fig. 3: A steep increase in the solubility of CaSiO3 in bridgmanite and mineralogical models for the lower mantle.

Data availability

Datasets for this research are available online at https://doi.org/10.5281/zenodo.6979058.

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Acknowledgements

K. Mossman and M. R. Gutierrez assisted with the FIB and STEM measurements at Arizona State University; and Y.-J. Chang assisted with the FIB and STEM measurements at University of Arizona. This work was supported by National Science Foundation (EAR-1725094). This research used resources of the Advanced Photon Source (APS), a US Department of Energy (DOE) Office of Science User Facility operated for the DOE Office of Science by Argonne National Laboratory under contract number DE-AC02-06CH11357. We acknowledge the support of GeoSoilEnviroCARS (Sector 13), which is supported by the National Science Foundation (NSF) - Earth Sciences (EAR-1634415), and the Department of Energy, Geosciences (DE-FG02-94ER14466). Use of the COMPRES-GSECARS gas loading system was also supported by COMPRES under NSF Cooperative Agreement EAR -1606856. High-Pressure Collaborative Access Team (Sector 16) is supported by DOE-NNSA Grant DE-NA0001974 and DOE-BES Grant DE-FG02-99ER45775.

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B.K. conceptualized the project, designed and performed the experiments, analysed data and wrote the manuscript. S.-H.S. conceptualized and supervised the project, acquired funding, designed experiments and wrote the manuscript. E.G., V.P., Y.M. and D.Z. provided resources for the in situ XRD measurements and supervised the experiments. E.E.A. and W.B. provided resources for the SMS measurements and supervised the experiments. All authors reviewed the manuscript.

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Correspondence to Byeongkwan Ko or Sang-Heon Shim.

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Extended data figures and tables

Extended Data Fig. 1 Previous reports of existence (small symbols) or absence (large symbols) of CaSiO3 perovskite from experiments on pyrolitic and peridotitic compositions.

The data are collected from refs. 2,3,6,7,8,28,48,49,50,51,52,53,54,55,56,57,58,59. Mantle geotherms are shown as a grey area with the lower bound from ref. 4. and the upper bound from ref. 5. Diamonds: bridgmanite + ferropericlase + CaSiO3 perovskite. Circles: bridgmanite + ferropericlase or bridgmanite only. Squares: bridgmanite (+ post-perovskite) + ferropericlase + CaSiO3 perovskite. Pentagons: bridgmanite (+ post-perovskite) + ferropericlase. The open symbols are from this study.

Extended Data Fig. 2 In situ X-ray diffraction patterns measured at high P−T conditions for the (a) komatiitic composition and (b) Ca-pyrolite.

a,b, Miller indices of bridgmanite, CaSiO3 perovskite, and Rh2O3(II)-type Al2O3 are provided as the black, blue, and magenta labels. The diffraction peak positions of CaPv are shown as the blue bars.

Extended Data Fig. 3 Effects of Ca dissolution on crystal structure of bridgmanite.

a,b, The molar volumes of bridgmanite (Brg) at high pressures and 300 K. The red and blue symbols represent Ca-rich Brg and Brg, respectively. The solid curves are fits of the solid symbol data points to the Vinet equation with fixed K0′ = 4. The data shown as open symbols were not used for fitting. The calculated molar volume of the Brg + CaSiO3 perovskite (CaPv) assemblage is plotted as the dashed black curve for comparison with uncertainties (the grey shade). a, The blue dashed curve is from Mg,Fe,Al-bearing Brg in KLB-1 peridotite2. b, The inset is a magnified view for the purple triangles. c,d, The octahedral tilting of Brg at high pressures and 300 K. The octahedral tilting angle for MgSiO3 endmember Brg is shown as the black dashed line. d, The red circles (K62 and K100) and diamonds (K59B and K73B) denote Ca-rich Brg. The blue circles (K33) and diamonds (K59B and K73B) denote Brg. b,d, The purple triangles represent the data points of Ca-rich Brg observed together with CaPv in XRD patterns (K100-2; Methods). The error bars are estimated 1σ uncertainties.

Extended Data Fig. 4 X-ray diffraction patterns and chemical analysis of komatiitic composition synthesized at 62 GPa and 2,350 K (K62) and 48 GPa and 2,000 K (K48).

a,b, Miller indices of Ca-rich bridgmanite (Brg), CaSiO3 perovskite (CaPv), stishovite (St), and corundum (Crn) are provided as the black, blue, red, and magenta labels, respectively. The expected diffraction peak positions of CaPv are shown as the blue bars. c,d, A high-angle angular dark-field image (left) and chemical maps (right) of the recovered samples. A small grain of CaPv is rarely observed in c, as the majority of Ca exists in Ca-Brg because of an increase in Ca solubility in Brg with temperature. However, at lower temperature CaPv is frequently observed because of low Ca solubility in Brg at the conditions.

Extended Data Fig. 5 X-ray diffraction patterns of the Ca30Fe13 composition at high pressures and 300 K.

Pressures were measure at 300 K after heating. The Miller indices of bridgmanite and CaSiO3 perovskite are provided as the black and blue labels, respectively. The expected diffraction peak positions of CaSiO3 perovskite (tetragonal, space group: I4/mcm) is shown as the blue bars.

Extended Data Fig. 6 Synchrotron Mössbauer spectra of bridgmanite (Brg) and Ca-rich Brg at 59 GPa and at 58 GPa after laser heating.

The synthesis temperatures were 2,150 K and 2,400 K for Brg and Ca-rich Brg, respectively. The circles are measured spectral data points and the curves are spectral fitting results.

Extended Data Fig. 7 Differences in the seismic properties between the single-perovskite phase (Ca-rich bridgmanite) case and the two perovskite phases (bridgmanite + CaSiO3 perovskite) case.

The differences of Ca-rich bridgmanite from bridgmanite + CaSiO3 perovskite are shown for bulk sound speed (Φ), bulk modulus (KT), and density (ρ) for the komatiitic composition at 300 K. The grey shades show the estimated 1σ uncertainties.

Extended Data Fig. 8 The depth of the transition zone from a two-perovskite domain (TPD; bridgmanite + CaSiO3 perovskite) at shallower depths to a single-perovskite domain (SPD; Ca-rich bridgmanite) at greater depths over time in the lower mantle.

a, The depth of the TPD-to-SPD transition zone (TSTZ) was calculated for the mantle geotherms from ref. 4 (green) and ref. 5 (orange) for Urey ratios (Ur) of 0.23, 0.38 (ref. 25) and 0.8 (ref. 26). The thickness of the TSTZ was not considered for the calculation. b, A 3-D plot of the TSTZ depths for Ur = 0.38. The green and orange planes intersect the grey plane which represents the TSTZ at 2,300 K.

Extended Data Fig. 9 High-angle annular-dark-field images of the samples of Ca-pyrolite (a) and komatiitic composition (b).

a,b, The yellow boxes indicate the areas where chemical compositions were measured, which are presented in Fig. 2b, d. a, Chemical compositions were analysed at the bottom left of the sample where the thickness was smaller. The presented area of the sample shows texture consistent with heating centre, that is, well crystallized bridgmanite and ferropericlase. b, Chemical compositions were analysed at the top area of the sample. The Ca-rich bridgmanite matrix is nearly indistinguishable from the unheated glass since the Ca-rich bridgmanite matrix takes roughly 95 vol% of the sample. However, the well crystallized grains of other phases (Al2O3 and SiO2) show that the area is well heated.

Extended Data Table 1 Chemical compositions of the starting materials (mol%)
Extended Data Table 2 Fitting results for synchrotron Mössbauer spectroscopy for Ca30Fe13 composition at high pressure and 300 K
Extended Data Table 3 Summary of the experimental runs

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Ko, B., Greenberg, E., Prakapenka, V. et al. Calcium dissolution in bridgmanite in the Earth’s deep mantle. Nature 611, 88–92 (2022). https://doi.org/10.1038/s41586-022-05237-4

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