New ways to probe deep-Earth minerals at ID06-LVP

Understanding the chemistry, mineralogy, and temperature of planetary interiors is key to deciphering internal dynamics and the long-term evolution, including atmospheric and surface processes. However, direct access to Earth’s deep interior remains elusive. Seismic observations provide indirect insights, but accurate interpretations require reliable data on how acoustic properties of mantle minerals behave at relevant pressure and temperatures – a longstanding experimental challenge.

Ringwoodite, a spinel-structured mineral ([Mg,Fe]₂SiO₄), is the most abundant phase in the Earth’s mantle between depths of 520 and 660 km. Its seismic velocities vary with pressure, temperature, iron content, and hydration [1]. However, previous experimental studies have been limited by sample characterization or the need to extrapolate results to mantle conditions, introducing significant uncertainties into global seismic models. 

Recent technical advancements at beamlines ID06-LVP and ID14 now allow direct and simultaneous high-frequency ultrasonic velocity measurements, synchrotron X-ray diffraction and imaging at pressures above 20 GPa and temperatures up to 1700 K. These capabilities enable accurate determination of acoustic velocities in well-characterized mineral samples under realistic Earth’s mantle conditions.

In this work, fully dense monomineralic ringwoodite samples containing negligible water (<0.035 wt.% H₂O) were synthetized at UCL Earth Sciences, UK. Iron speciation was characterized by Mössbauer spectroscopy at ID14. In-situ experiments at ID06-LVP were conducted in the Large Volume Press (LVP) using multi-anvil assemblies equipped with custom-made X-ray transparent furnaces [2], enabling high-quality data at pressures up to 21 GPa and temperatures up to 1650 K. Diffraction data were recorded continuously (every three seconds) throughout the experiments, while sample imaging and ultrasonic spectra were acquired at ~100 K intervals (Figure 1). 

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Fig. 1: Schematic cross-section of the ceramic assembly used for ultrasonic high-pressure experiments at ID06-LVP. MHz-frequency ultrasonic signals are generated and collected in pulse-echo mode along a direction perpendicular to a monochromatic synchrotron X-ray beam used for diffraction and imaging. The ringwoodite sample (shown in green) is surrounded by an acoustic buffer rod (blue) and a pressure marker (red) within a high-pressure environment.
 

Compared to other high-pressure diffraction methods (e.g., diamond anvil cells) the ID06-LVP setup offers exceptional data quality and resolution [3]. Seismic velocities were determined by combining travel-time measurements with direct imaging of sample lengths. The results featured small uncertainties (< 0.1 GPa in pressure, < 10 K in temperature, and < 0.03 km.s^-1 in velocity), with minimal extrapolation (200-300 K) required to model mantle conditions. 

The data reveal that previous studies underestimated both the absolute velocities of dry ringwoodite (by up to ~1%) and their temperature dependence at high pressures. These refinements are significant, as seismic properties at depths of 410 – 660 km have been widely used to infer mantle temperatures and hydration. Furthermore, when combined with existing data on other mantle minerals, the results suggest that the canonical model of the upper mantle is inconsistent with global 1D seismic profiles. 

In conclusion, the developments at ID06-LVP now enable accurate acoustic velocity measurements at > 20 GPa and 1700 K, with concurrent high-resolution diffraction. When coupled with iron speciation studies at ID14, this facility offers unmatched capabilities for investigating the interior structures of the Earth, Moon, and Mars.

Principal publication
Sound velocity measurements of γ-(Mg0.91Fe0.09)2SiO4 show that the ringwoodite to bridgmanite and ferropericlase phase transformation does not produce the seismically observed 660 km discontinuity, R. Huang et al., Earth Planetary Sci. Lett. 663, 119416 (2025); https://doi.org/10.1016/j.epsl.2025.119416

References
[1] W. Wang et al., Earth Planetary Sci. Lett. 554, 116626 (2021).
[2] F. Xu et al., High Pressure Res. 40(2), 257-266 (2020).
[3] W. Crichton et al., J. Synchrotron Radiat. 30(6), 1149-1155 (2023).