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

上地幔矿物和岩石辐射热导率的实验和计算研究

• 批准号: 2148727
• 负责人: George Rossman
• 金额: $ 50.99万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2022
• 资助国家: 美国
• 起止时间: 2022-02-01 至 2025-01-31
• 项目状态: 未结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2148727&HistoricalAwards=false
• 关键词: Experimental; Computational; Study; Radiative; Thermal

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.

地球是一个充满活力的星球——有地震、火山、造山活动，并向地表提供生命必需的化合物——这是由于通过其内部的热量传输产生的力。这种运输通过三种机制发生。第一种机制是平流：热物质的向上运动和冷物质的向下运动使行星变得活跃。其他代表的是“废热”，它们穿过地球物质，但不一定有助于行星活动。一种是传导，即热量从直接接触的热物体流向冷物体的常见经历。该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输热量，或辐射传热。 后一种机制在地球物理学中很大程度上被忽视了。在这里，该团队基于他们最初的发现，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幔中辐射传热的作用。他们系统地将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。第一个是测量相关矿物在高温下的光学吸收特性。第二个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。 该项目的总体目标是评估更透明的地幔对全球板块运动的影响。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，与科学界公开共享。 该项目的技术和理论成果在地球科学之外的材料科学和工程领域具有广泛的影响。热传导代表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，对流活力就会降低。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，足以使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有很大不同。在室温下，许多地幔矿物在可见波长下最重要的光学吸收机制是间隔电荷转移（IVCT）。该机制涉及 Fe2+ 和 Fe3+ 离子对或 Fe2+ 和 Ti4+ 离子对。由于它取决于电子在不同离子轨道之间移动，直觉表明升高的温度应该会减少这种跳跃的势垒并增加吸收的可能性。但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，研究人员通过两项并行且综合的努力研究了这一发现对地球动力学的影响：(1) 表征关键地幔矿物在高温下的光谱的实验工作； （2）应用灵活的数字代码来模拟多相介质中的辐射传输（给定测量的组成相的光谱特性）并将所得的整体辐射电导率纳入地幔对流的地球动力学模型中。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。目标是系统地了解矿物高温光谱。研究人员研究了重要的（相对透明的）地幔矿物，例如橄榄石和石榴石。 此外，他们还与合作者合作，生长可用于光谱学的几乎不透明矿物（尖晶石、斜方辉石）薄膜。这些新颖的测量需要对实验室进行升级，重点是改进宽带探测器、显微镜加热台以及环境控制，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效应，但他们还在金刚石砧室中在高压下进行了一些测量，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。