Ultrasonic Velocity and Density Measurements on Silicate Melts at Upper Mantle Pressures

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Melts exist in the Earth's interior and play important roles in various geological processes, such as volcanic eruption and lava flow, the formation and migration of magmas in the mantle, and the convection of liquid outer core. For instance, precise knowledge of sound speed of melts helps us diagnose the presence of melts in the Earth's deeper depths, and the information about the density contrasts between melts and crystals determines the sink/float of crystals in magmas in the evolution of the Earth. Characterization of the behavior and properties of melts at the Earth's upper mantle depths, however, remains a challenging task. The proposed project will (i)develop new techniques for precise sound speed measurements using ultrasonic interferometric method on liquids/melts at pressures of the Earth's upper mantle, and (ii)conduct measurements on melts of olivine, the most abundant mineral of the Earth's upper mantle. Currently, density measurements using static sink-float and shock wave methods have been the major tools in constraining melt densities at mantle pressures to test the density crossovers, such as between the basic silicate liquids and the co-existing olivine. To date, sound velocity measurements on melts have only been conducted at ambient pressure using varying-sample-depth technique; measurements at high pressures, however, have been hindered by the lack of means of precisely measuring melt thickness or varying the sample depths. The PIs have developed the techniques to overcome these difficulties by using an X-radiographic imaging technique. They propose to conduct measurements on the iron end-members as well as intermediate compositions of Mg2SiO4-Fe2SiO4. These measurements and the derived thermal equation of states will complement and provide benchmarking data for static (sink-float, X-ray absorption), shock wave, and computer simulations, allowing for a refined characterization of the locations of crystal/liquid density crossovers in the upper mantle, transition zone, and lower mantle.