The heat flow across the Earth's core-mantle boundary controls the thermal evolution of the whole Earth, which is ultimately expressed on the surface by plate tectonics. However the thermal properties of the core/mantle boundary are not well known, leading to significant uncertainties in our understanding of the temperature and heat profile of the Earth - both currently and in the past. The goal of this proposal is to determine the thermal properties -especially thermal conductivity- of the Earth's mantle, through a combination of experiment and modeling. This information will be used to help constrain the temperature profile throughout the Earth's interior, the style of mantle convection, and the timing for the growth of the inner core.
We will combine methods from theoretical and experimental mineral physics and heat flow modeling in crystalline bulk and in composite materials to measure the thermal conductivity of the deep Earth. Starting with a data set describing the lattice thermal conductivity of MgO and MgSiO3 at deep mantle conditions, we will measure how the presence and behavior of iron changes those values at high pressures and temperatures. The experimental data will be interpreted with help from computational models of heat flow in the laser-heated diamond cell, used to make these measurements. Finally, we examine the implications for the mantle using a heat flow model for composite materials and including both conduction and radiation. The overall outcome will be an estimate of the thermal conductivity of the lower mantle at core/mantle boundary conditions, and how the thermal conductivity varies with pressure, temperature, and composition.
This project brings together two scientists from very different disciplines: a mechanical engineer with expertise in heat transfer and a geophyscist with expertise in measurements of physical properties under extreme conditions. This interdisciplinary project will support the training of a graduate student for three years. In addition, this project will support summer research for undergraduates at UCLA. Measurements of thermal properties of materials under extreme conditions are important not only for Earth & planetary science, but also for engineering, materials science, and physics. Both researchers are actively involved in science education and outreach at all levels, and have active research groups including undergraduate and graduate students, and postdoctoral scholars.
The major goal of this research program is to understand the thermal evolution of the whole Earth, especially its mantle, which helps govern the heat flow driving convection and ultimately generates plate tectonics, earthquakes, and volcanism activity on the Earth's surface. To do this, we are measuring the thermal conductivity and thermoelastic properties--especially thermal expansion-- of materials that comprise the lowermost mantle, especially silicate perovskite and the oxide phase periclase. This is an experimental study that relies upon a detailed physical model of the heat flow in our high pressure apparatus, the diamond anvil cell, as well as precise measurements of the temperature and pressures of samples inside the laser heated diamond cell. The models and measurements together provide information about the pressure dependence, temperature dependence, and compositional dependence of the thermal conductivity of deep Earth materials. Intellectual Merits: We have completed a theoretical study that shows that the pure magnesium silicate perovskite has a lower thermal conductivity than previously believed. This implies lower-than-expected heat flow across the Earth's core/mantle boundary. We have performed a series of laboratory experiments both in our UCLA mineral physics lab and also at beamline 12.2.2 of the Advanced Light Source to measure how temperature increase as a function of laser heating power, which when combined with our new heat flow model, provides information on sample thermal conductivity. We have shown that pressure lowers the thermal conductivity for silicate perovskite. We have measured the thermal conductivity of two major minerals of the lower mantle: MgSiO3 perovskite and MgFeO oxide as a function of iron content and pressure. We have measured the thermoelastic properties at high temperatures and pressures of a series of metals, oxides, and silicates at mantle conditions. These results help determine the stability of transition metal cations in the oxide phase vs. as elements in the metal phase deep in the Earth. Broader impacts: The funding has been used primarily to support the research and training of a UCLA graduate student, Ms. Emma Rainey. Some funds were also used to support a postdoctoral researcher at UC Berkeley, and undergraduate research at UCLA. We have developed, published, and distributed a new numerical model describing heat flow in the laser heated diamond anvil cell. Innovations in temperature measurement that were developed in our laboratory were implemented at the X-ray user facility at beamline 12.2.2 at the Advanced Light Source, Berkeley. The PI has participated in a broad array of outreach programs, including a public talk about deep Earth geophysics at LA's Mindscape program, a popular weblog and maintains a professional twitter account (@mineralphys) which updates highlights from a life of a scientist.
