An Experimental and Computational Study of the Radiative Thermal Conductivity of Upper Mantle Minerals and Rocks

California Institute Of Technology, Pasadena CA

Investigators

Abstract

The Earth is a dynamic planet — with earthquakes, volcanoes, mountain-building, and supply of life-essential compounds to the surface — because of forces arising from the transport of heat through its interior. Such transport occurs by three mechanisms. The first mechanism is advection: the upward motion of hot material and downward motion of cold material that make the planet active. The others represent “waste heat” that moves through Earth materials without necessarily contributing to planetary activity. One is conduction, the familiar experience of heat flowing from hot objects to cold objects in direct contact. This mechanism is well-studied. The third mechanism is transmission of heat by visible and infrared light, or radiative heat transfer. This latter mechanism has been largely neglected in geophysics. Here the team builds on their initial discovery that mineral grains can become dramatically more transparent at elevated temperatures. This allows for more radiative heat transfer. The researchers, thus, re-evaluate the role of radiative heat transfer in the Earth’s mantle. They systematically extend their experimental and computational test to the minerals that dominate the interior of the Earth. This project involves two important components. The first is measurements of the optical absorption properties of relevant minerals at elevated temperatures. The second is to use the measured optical absorption data to improve the mathematical models that describe radiative conductivity in the Earth. The project’s overarching goal is to assess the consequences of a more transparent mantle for global tectonic plate motions. The project support one graduate and several undergraduate students. Its outcomes, which include new techniques, methods, codes, and data products, are openly shared with the scientific community. The project results, technical and theoretical, have broad implications beyond Earth sciences in materials science and engineering. Thermal conduction represents inefficiency of the Earth’s convective heat engine. Indeed, if the sum of lattice and radiative thermal conductivity become large enough, convective vigor decreases. Consequently, a greater heat flow or lower viscosity becomes necessary to maintain dynamic motions. In the upper mantle, radiative thermal conductivity has largely been ignored. This is because the opacity of Fe-bearing mantle minerals is thought to be high enough to make radiative transport negligible. Yet, the team’s preliminary work has shown that the optical spectra of minerals collected at room temperature can be quite different from those collected at elevated temperatures. At room temperature, the most important optical absorption mechanism in many mantle minerals at visible wavelengths is intervalence charge transfer (IVCT). This mechanism involves pairs of Fe2+ and Fe3+ ions or Fe2+ and Ti4+ ions. Since it depends on electrons moving between orbitals of different ions, intuition suggests that elevated temperature should reduce the barrier to such hopping and increase the probability of absorption. But initial experimental results on model minerals have shown that the IVCT absorption fades with increasing temperature and is gone at the temperatures of Earth’s asthenosphere; this makes the minerals increasingly transparent to visible light. Here the researchers investigate the consequences of this discovery for geodynamics through two parallel and integrated efforts: (1) experimental work to characterize the optical spectra of key mantle minerals at elevated temperature; and (2) application of a flexible numerical code to model radiative transport in a multiphase medium - given measured spectral properties of the constituent phases - and to incorporate the resulting bulk radiative conductivity into geodynamic models of mantle convection. Their initial studies involved analogues for mantle minerals. The team now investigates actual mantle minerals. The goal is to gain a systematic understanding of mineral high-temperature optical spectra. The researchers study important (relatively transparent) mantle minerals such as olivine and garnet. In addition, they work with collaborators who grow thin films of nearly opaque minerals (spinels, orthopyroxene) that can be used for spectroscopy. These novel measurements call for upgrades to the laboratory centered around improved broadband detectors, a heating stage for the microscope, and environmental control to minimize mineral oxidation at high temperature. Although the researchers suspect that the temperature dependence of IVCT is the main effect in need of study, they also conduct some measurements at elevated pressure in a diamond-anvil cell to evaluate the effect of pressure on the phenomenon. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.