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Abstract Thermoelastic properties of mantle candidate minerals are essential to our understanding of geophysical phenomena, geochemistry, and geodynamic evolutions of the silicate Earth. However, the lower-mantle mineralogy remains much debated due to the lack of single-crystal elastic moduli (Cij) and aggregate sound velocities of (Al,Fe)-bearing bridgmanite, the most abundant mineral of the planet, at the lower mantle pressure-temperature (P-T) conditions. Here we report single-crystal Cij of (Al,Fe)-bearing bridgmanite, Mg0.88Fe0.1Al0.14Si0.90O3 (Fe10-Al14-Bgm) with Fe3+/ΣFe = ~0.65, up to ~82 GPa using X-ray diffraction (XRD), Brillouin light scattering (BLS), and impulsive stimulated light scattering (ISLS) measurements in diamond-anvil cells (DACs). Two crystal platelets with orientations of (–0.50, 0.05, –0.86) and (0.65, –0.59, 0.48), that are sensitive to deriving all nine Cij, are used for compressional and shear wave velocity (νP and νS) measurements as a function of azimuthal angles over 200° at each experimental pressure. Our results show that all Cij of singe-crystal Fe10-Al14-Bgm increase monotonically with pressure with small uncertainties of 1–2% (±1σ), except C55 and C23, which have uncertainties of 3–4%. Using the third-order Eulerian finite-strain equations to model the elasticity data yields the aggregate adiabatic bulk and shear moduli and respective pressure derivatives at the reference pressure of 25 GPa: KS = 326 ± 4 GPa, µ = 211 ± 2 GPa, KS′ = 3.32 ± 0.04, and µ′ = 1.66 ± 0.02 GPa. The high-pressure aggregate νS and νP of Fe10-Al14-Bgm are 2.6–3.5% and 3.1–4.7% lower than those of MgSiO3 bridgmanite end-member, respectively. These data are used with literature reports on bridgmanite with different Fe and Al contents to quantitatively evaluate pressure and compositional effects on their elastic properties. Comparing with one-dimensional seismic profiles, our modeled velocity profiles of major lower-mantle mineral assemblages at relevant P-T suggest that the lower mantle could likely consist of about 89 vol% (Al,Fe)-bearing bridgmanite. After considering uncertainties, our best-fit model is still indistinguishable from pyrolitic or chondritic models.
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This content will become publicly available on May 21, 2026
Elasticity of β-Mg2SiO4 containing 1.2 wt% H2O to 10 GPa and 600 K by Ultrasonic Interferometry with synchrotron X-radiation
Abstract We have measured the sound velocities and elasticity of synthetic polycrystalline β-Mg2SiO4 containing 1.2 wt% H2O to 10 GPa and 600 K using ultrasonic interferometry with synchrotron X-radiation. We determined sample length at high pressure and temperature using the sample’s X-radiographic image and applied travel times bond corrections appropriate to the experimental cell assembly configuration. Fitting the entire moduli data to third-order finite strain equations yields the adiabatic bulk [KS0 = 153.3(4) GPa] and shear [G0 = 101.8(2) GPa] moduli, their pressure derivatives (∂KS/∂P)T = 5.15(6) and (∂G/∂P)T = 1.68(3) and temperature derivatives (∂KS/∂T)P = −0.0179(9) GPa/K and (∂G/∂T)P = −0.0151(7) GPa/K. Comparing the bulk sound velocity contrast between the new hydrous wadsleyite data and olivine (0.38 wt% H2O) with seismic bulk sound velocity contrasts of 3.5% and 4.0% yields 53% and 60% olivine content, respectively, assuming an iso-chemical mantle model of the Earth. The results suggest that a hydrous mantle transition zone with a pyrolite model composition could explain the 410 km seismic velocity jump.
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- Award ID(s):
- 2100985
- PAR ID:
- 10617909
- Editor(s):
- Xu, Hongwu
- Publisher / Repository:
- American Mineralogist
- Date Published:
- Journal Name:
- American Mineralogist
- ISSN:
- 0003-004X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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