The thermal evolution and chemical differentiation of the Earth involves the segregation of silicate and metallic melts, and coexisting minerals due to differences in their density. This single factor impacts the initial condition as the Earth formed its hydrosphere and biosphere (to become a habitable planet), the efficiency of magmatic differentiation, evolution of volcanic systems and character of eruptions, formation of some economic ore deposits, transportation of volatiles through the deep mantle, and heat transfer across the core mantle boundary which, in turn, impacts generation of the magnetic field. The buoyancy relations between minerals and melts are strong functions of pressure caused by the greater compressibility of melts compared to minerals. Quantifying the response of increasing pressure on melt properties is challenging given the refractory nature, absence of long range structural order, and complex structural change that accommodates densification of melts. Critical measurements must be made under well-controlled laboratory conditions. The results of this research will greatly improve our understanding of dynamic processes within our planet and links to motions of the tectonic plates and global geochemical cycle.
This research employs novel high-pressure experimental methods utilizing in situ X-ray techniques (microtomography, absorption and diffraction) and acoustic velocity measurements to determine the volumetric properties and structural changes in melts at deep Earth conditions. The results will help place tighter constraints on properties of the magma ocean, core formation, segregation and convection dynamics, and, ultimately, the thermal and chemical evolution of the Earth. These data will also contribute to our understanding of present-day dynamics involving the storage and transport of magma through the crust and mantle, spreading of tectonic plates from mid-ocean ridges, mantle plume upwelling, and possible reactions at the core-mantle boundary. Additionally, our efforts in connection with this project will help to further expand the opportunities for microtomography research at high pressures for the study of partial melting and deformation, among other phenomena at high pressures requiring detailed microstructure characterization in 3D. It is anticipated that the results of this work will have long-term benefits in the search and characterization of new exotic materials synthesized at high temperature and pressure for industrial applications.
This project (Federal Award ID: 1214376) is a continuation of a previous three-year project (0711057), both are collaborations with the UD Davis group led by Prof. C.E. Lesher. This grant has enabled us to make several important technical advances in the continuing development of the high pressure x-ray tomography microscope (HPXTM). Within the one-year funding period, we have designed a new cupped Drickamer anvil system, which is capable of higher temperatures at high pressures. Several successful tests have demonstrated that we can now reach 10+ GPa and 1500 K routinely over long periods of time. Another new development is the Paris-Edinburgh (PE) cell. The new PE cell design now allows us to reach 2300 K over a long period of time. Both developments are critical for future experiments using the HPXTM - we are confident now that we can conduct volumetric property measurements using HPXTM. Withinthe combined project periods, we have published more than a dozen papers in peer-reviewed scientific journals and one book chapter (7 during the funding period 2013-2014). The published results cover a wide range of topics relevant to this project: structural responses of silicate melts and glasses to pressure and temperature, density and viscosity of melts, and relationship between structure and physical properties of melts. These publications include one paper in Nature-Geoscience, one paper in Nature Communications, and two in Earth Planet. Sci. Lett. As an example, in the Nature Communications paper (Wang et al., 2014: Nat Comms, 5, 3241. doi: 10.1038/ncomms4241), we compiled literature viscosity and elasticity data of silicate liquids and reveal interesting trends as a function of structure characteristics. Under upper mantle conditions, structure of silicate liquids can be divided into two end-member groups, depending on the average number of "non-bridging" oxygen atoms per tetrahedral structure unit (SiO4 or AlO4), or NBO/T. Through high pressure x-ray diffraction and molecular dynamics simulations, we are able to provide an atomistic model to explain the trends. Throughout these efforts, we have trained two post-docs, who have moved on to permanent positions at universities, and one female Ph.D. Candidate (UC Davis), who is a lead author of a paper on compression behavior of basalt (in preparation).
