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

地球是一个充满活力的星球--地震、火山、造山运动和向地表供应生命所必需的化合物--这是因为热量通过其内部传输所产生的力。这种运输通过三种机制发生。第一个机制是平流：热物质的向上运动和冷物质的向下运动使行星活跃。其他代表“废热”，通过地球材料移动，不一定有助于行星活动。一种是传导，即热从热的物体流向直接接触的冷的物体。这一机制已得到充分研究。第三种机制是通过可见光和红外光传递热量，或辐射传热。 后一种机制在天体物理学中基本上被忽视了。在这里，该团队建立在他们最初的发现基础上，即矿物颗粒在高温下可以变得更加透明。 这允许更多的辐射热传递。 因此，研究人员重新评估了地球地幔中辐射热传递的作用。他们系统地将实验和计算测试扩展到地球内部占主导地位的矿物。该项目包括两个重要组成部分。第一个是测量相关矿物在高温下的光吸收特性。第二是利用测量的光吸收数据来改进描述地球辐射传导率的数学模型。 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。该项目支持一名研究生和几名本科生。 其成果，包括新技术、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率。事实上，如果晶格热导率和辐射热导率之和变得足够大，则对流活力降低。因此，需要更大的热流或更低的粘度来维持动态运动。在上地幔中，辐射热导率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可能与在高温下收集的矿物的光谱完全不同。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）。该机制涉及Fe 2+和Fe 3+离子或Fe 2+和Ti 4+离子对。由于它依赖于电子在不同离子的轨道之间移动，直觉表明，升高的温度应该会降低这种跳跃的势垒，并增加吸收的概率。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，并在地球软流圈的温度下消失;这使得矿物对可见光越来越透明。在这里，研究人员通过两个平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，以表征高温下关键地幔矿物的光学光谱;以及（2）应用灵活的数值代码来模拟多相介质中的辐射传输-给定组成相的测量光谱特性-并将由此产生的整体辐射传导率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。 该团队现在正在研究实际的地幔矿物。目的是获得矿物高温光谱的系统性认识。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，他们生长了几乎不透明的矿物（尖晶石，斜方辉石）薄膜，可用于光谱学。这些新的测量要求对实验室进行升级，重点是改进宽带探测器、显微镜加热台和环境控制，以最大限度地减少高温下的矿物氧化。虽然研究人员怀疑IVCT的温度依赖性是需要研究的主要效应，但他们也在金刚石压砧室中进行了一些高压测量，以评估压力对该现象的影响。该奖项反映了NSF的法定使命，并通过使用基金会的智力价值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.