Xu et al. (2018)

Melting of silicate minerals and rocks is common to the mantles and crusts of the Earth and other terrestrial planets in the solar system and has played an important role in the differentiation of these planetary bodies. The equation of state (EOS) of silicate melts (i.e., density as a function of pressure, temperature and melt composition) is critical to our understanding of the dynamics and evolution of magmatic systems in Earth, including the stability of melt layers under deep mantle conditions, the separation and migration of melts in source regions, and the solidification process of the early magma ocean. However, for alkali-rich silicate melts, due to their high viscosity, high reactivity and low-X-ray absorption, it is difficult to determine their densities at high P-T conditions using traditional method such as sink-float experiment. We have successfully employed the pink-beam X-ray microtomography combined with a high-pressure tomography device at GSECARS beamline 13-BM-D to determine the 3D volume of sodium-rich jadeite melt at high P-T conditions up to ~5 GPa and 1950 K. The results show that jadeite melt is highly compressible, and has significant implications for the stability of alkali-rich, polymerized silicate melts in the Earth's upper mantle.

Besides direct density measurements at high P-T, sound velocity measurements on silicate melts at high pressures can provide better constraints on the EOS, as they can provide direct information on the bulk modulus and its pressure derivative of the melts. In addition, in order to explain seismic observations of the low-velocity anomalies in the Earth's interior, or use seismology as a tool to detect mantle melting and melt distribution, knowledge of melt sound velocity is required. We have successfully developed the high-pressure ultrasonic technique for melt measurements at 13-ID-D beamline using the 1000-ton large volume press equipped with a T25 Kawai type module. This technique has been applied to measure relaxed sound velocities of silicate melts in the diopside-hedenbergite join up to ~6 GPa and 2400 K, which are important to understand the seismic signatures of silicate melts in the Earth's interior.

Below are some of our major findings:

  • Polymerized sodium aluminosilicate melts, if generated by low-degree partial melting of mantle peridotite at ~250-400 km depth, are likely denser than surrounding mantle

  • The amount of velocity reduction in the mantle increases with melt fraction and decreases with pressure

  • Silicate glasses may not be good analogs for studying the acoustic and elastic properties of corresponding melts

  • Fe reduces the sound velocity while increasing the density of silicate melts

  • A small amount of unextractable melts distributed in film geometry may explain the low-velocity zone in mantle asthenosphere

  • ~0.7% of Fe-rich melts distributed in equilibrium geometry may explain the low-velocity layer atop the mantle transition zone

Recently, we are trying to determine the sound velocity of titanium-rich basaltic melts in deep lunar interior, which would help us better understand the lunar mantle structure. Stay tuned...

Relevant Publications

Xu, M., Jing, Z., Chantel, J., Jiang, P., Yu, T., & Wang, Y. (2018). Ultrasonic Velocity of Diopside Liquid at High Pressure and Temperature: Constraints on Velocity Reduction in the Upper Mantle Due to Partial Melts. Journal of Geophysical Research: Solid Earth, 123(10), 8676-8690.

Xu, M., Jing, Z., Van Orman, J. A., Yu, T., & Wang, Y. (2020). Density of NaAlSi2O6 Melt at High Pressure and Temperature Measured by In-Situ X-ray Microtomography. Minerals, 10(2), 161.

Jing, Z., Yu, T., Xu, M., Chantel, J., & Wang, Y. (2020). High-Pressure Sound Velocity Measurements of Liquids Using In Situ Ultrasonic Techniques in a Multi-anvil Apparatus. Minerals, 10(2), 126.

Xu, M., Jing, Z., Yu, T., Alp, E. E., Lavina, B., Van Orman, J. A., & Wang, Y. (2021). Sound velocity and compressibility of melts along the hedenbergite (CaFeSi2O6)-diopside (CaMgSi2O6) join at high pressure: Implications for stability and seismic signature of Fe-rich melts in the mantle. Earth and Planetary Science Letters. (Accepted)