To understand Earth’s internal composition and structure, researchers track seismic and sound waves as they travel through the planet. Low-velocity zones, where seismic waves travel more slowly than they do in the surrounding mantle, may be explained by the presence of molten iron-bearing minerals. However, the behavior of iron in silicate melts is complex and the elastic properties of iron-rich melts are not well constrained. This research, carried out in large part at the U.S. Department of Energy’s Advanced Photon Source (APS), can shed light on the nature of the two low-velocity zones in Earth’s mantle and may someday help reveal the seismic detectability of deep magmatism on the Moon and Mars. Results of this study were published in Earth and Planetary Science Letters.
Earth’s mantle is composed primarily of solid silicate minerals. Iron is one of the heaviest major elements contained in silicates and so iron-rich silicate minerals tend to have higher densities. During partial melting of mantle rocks, iron, as an incompatible element, preferentially partitions into the melt phase, making these melts high in iron oxide (FeO) content. Due to their high density, these melts may sink deep into the mantle, where they may become neutrally buoyant.
For this work, the researchers took high-pressure and high-temperature ultrasonic measurements of silicate melts along the hedenbergite (Hd,CaFeSi2O6)–diopside (Di, CaMgSi2O6) join, an analog for mantle iron-rich melts, at pressure and temperature conditions up to 6 gigapascals and 2329K, corresponding to the conditions in the Earth’s upper mantle. At the APS, an Office of Science user facility at Argonne National Laboratory, they combined the high-pressure ultrasonic technique with synchrotron radiation in a multi-anvil apparatus. This apparatus is the 10-MN multi-anvil press equipped with a T-25 Kawaii-type module at the GSECARS beamline 13-ID-D of the APS.
Based on the measured sound velocity data derived from the study at GSECARS, the researchers proposed an empirical equation of state to describe the sound velocity and melt density along the Hd-Di join as a function of pressure, temperature, and melt composition. At high pressures, iron strongly affects the elastic properties of silicate melts. As iron content in the melt increases, melt density also increases, but sound velocity decreases. Using this experimentally constrained model, the researchers were able to quantify the seismic velocity and gravitational stability of iron-bearing silicate melts at deep mantle conditions. By comparing the results with seismic observations, whether the low-velocity zones in the Earth’s mantle are caused by the presence of silicate melts or not can be evaluated.
The low-velocity zone (LVZ), where seismic wave velocity is relatively slow, characterizes the boundary of the lithosphere and asthenosphere in the upper mantle and plays an important role in plate tectonics. The velocity reduction is commonly attributed to partial melting, with a small fraction of melt distributed through the LVZ mantle rocks. Observed velocity reduction in the LVZ requires small fractions of melts, which are likely present as thin films along grain boundaries. In such a geometry, even when the melt is relatively less dense than solid rocks, it would maintain gravitational stability at LVZ depths. In addition, a low-velocity layer (LVL) atop the mantle transition zone observed seismically may be explained by the presence of iron-rich melts (FeO>~10wt%) distributed in textural equilibrium with the ambient mantle.
Low-velocity zones are important for understanding plate tectonics, the origin and nature of the Earth’s crust, and the structure and dynamics of the Earth’s interior. Understanding the nature of these low-velocity zones requires better knowledge of the elastic properties of mantle minerals and melts at high temperatures and pressures. It is possible that similar iron-rich melt layers are present in the mantles of the Moon and Mars, an understanding of which also requires knowledge of the elastic properties of iron-bearing silicate melts. ― Dana Desonie
See: Man Xu1, 2, Zhicheng Jing3, Tony Yu2, E. Ercan Alp4, Barbara Lavina4, James A. Van Orman1, Yanbin Wang2*, “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 Planet. Sci. Lett. 577 117250, (2022). DOI: 10.1016/j.epsl.2021.1172
Author affiliations: 1Case Western Reserve University, 2The University of Chicago, 3Southern University of Science and Technology, 4Argonne National Laboratory
Correspondence: * [email protected]
This study was supported by the National Natural Science Foundation of China (41974098) and the National Science Foundation (EAR-1619964 and 1620548). GSECARS is supported by the National Science Foundation-Earth Sciences (EAR-1634415) and Department of Energy (DOE)-GeoSciences (DE-FG02-94ER14466). Use of the GSECARS SEM system was supported by the Natural Science Foundation MRI Proposal (EAR-1531583). This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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