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

地球是一颗充满活力的行星--地震、火山、造山和生命供应--这些都是地表必不可少的化合物--因为热量在地球内部的传输所产生的力量。这种传输通过三种机制发生。第一种机制是平流：使地球活跃的热物质向上运动和冷物质向下运动。其他代表的是“废热”，它们在地球物质中流动，但不一定对行星活动有贡献。一种是传导，即热物体直接接触时热从热物体流向冷物体的常见体验。这一机制已经得到了充分的研究。第三种机制是可见光和红外光的热传递，或辐射热传递。地球物理学在很大程度上忽略了后一种机制。在这里，研究小组建立在他们最初的发现基础上，即矿物颗粒在高温下可以显著变得更加透明。这允许更多的辐射热传递。因此，研究人员重新评估了地幔辐射热传输的作用。他们系统地将他们的实验和计算测试扩展到主导地球内部的矿物。该项目涉及两个重要组成部分。首先是测量相关矿物在高温下的光吸收性能。二是利用测得的光吸收数据，改进描述地球辐射传导性的数学模型。该项目的首要目标是评估更加透明的地幔对全球构造板块运动的影响。该项目资助了一名研究生和几名本科生。它的成果，包括新的技术、方法、代码和数据产品，与科学界公开分享。该项目的技术和理论成果在材料科学和工程领域具有超越地球科学的广泛影响。热传导代表了地球对流热机的低效。事实上，如果晶格导热系数和辐射导热系数之和变得足够大，对流活力就会降低。因此，需要更大的热流或更低的粘度来保持动态运动。在上地幔，辐射导热系数在很大程度上被忽略了。这是因为含铁地幔矿物的不透明度被认为足够高，使得辐射传输可以忽略不计。然而，该团队的初步工作表明，在室温下收集的矿物的光谱与在高温下收集的矿物的光谱可能有很大不同。在室温下，许多地幔矿物在可见光波段的最重要的光吸收机制是价间电荷转移(IVCT)。这种机制涉及成对的Fe2和Fe3离子或Fe2和Ti4离子。由于它依赖于电子在不同离子的轨道之间移动，直觉表明，温度的升高应该会降低这种跳跃的障碍，并增加吸收的可能性。但对模型矿物的初步实验结果表明，IVCT吸收随着温度的升高而减弱，并在地球软流层的温度下消失；这使得矿物对可见光越来越透明。在这里，研究人员通过两个并行和综合的努力来研究这一发现对地球动力学的影响：(1)实验工作，以确定主要地幔矿物在高温下的光谱；(2)应用灵活的数值代码来模拟多相介质中的辐射传输--给定组成相的光谱特性--并将由此产生的整体辐射传导性纳入地幔对流的地球动力学模型。他们最初的研究涉及地幔矿物的类似物。该团队现在正在调查实际的地幔矿物。其目的是对矿物高温光谱有一个系统的了解。研究人员研究重要的(相对透明的)地幔矿物，如橄榄石和石榴石。此外，他们还与合作者合作，生产可用于光谱分析的几乎不透明的矿物(尖晶石、斜方辉石)薄膜。这些新颖的测量要求以改进的宽带探测器、显微镜的加热阶段和环境控制为中心对实验室进行升级，以最大限度地减少高温下的矿物氧化。尽管研究人员怀疑IVCT的温度依赖性是需要研究的主要影响，但他们也在钻石顶压室中进行了一些加压测量，以评估压力对现象的影响。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

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.