CSEDI: Integrated seismic, geodynamic, and mineral physics studies of scatterers and other multi-scale structures in Earth’s lower mantle

地幔中物质的行为限制了驱动板块构造的流动。由深地幔源驱动的大量火山喷发被认为是造成全球环境变化的原因。剧烈的成分和热变化发生在核幔边界（CMB），大约3000公里以下的地球表面。这些变化对地球的冷却产生了主要影响。它们还影响着地核动力学、地球磁场和地幔热对流。然而，了解地球深处的动力学并不是微不足道的。事实上，需要多学科的努力和最先进的技术来解决地球系统的复杂性。在这里，研究人员调查了在核幔边界观察到的神秘特征。为了揭开它们的起源，该团队结合了地震学，地球动力学和实验矿物物理学的专业知识。研究人员在地幔中普遍存在的极端压力下进行实验。他们在国家同步加速器设施上使用强大的X射线和红外光测量预计存在于海洋地壳物质下沉到CMB中的物质的性质。利用计算设施的最新进展，他们模拟了候选海洋地壳物质与深地幔物质的相互作用，这些物质是由贯穿地球历史的构造力作用在一起的，从而在深地幔中产生复杂的结构。模型的结果将与地球内部的地震观测结果进行比较，以检验我们对地球深部动力学的理解。该项目为加州理工学院的研究生培训提供支助，并促进与澳大利亚的国际合作。 地震学家已经揭示，CMB的地幔一侧是非常不均匀的，具有公里级的精细结构，可能含有不同的化学储层。热和化学不均匀性、固-固相变、弹性各向异性、可变粘度和熔化可能都需要解释观察到的复杂性。凭借地震学、地球动力学和实验矿物物理学方面的专业知识，该团队将原子尺度与构造尺度联系起来，并通过动力学将其与时间维度联系起来。这是通过测量地球深部相的热弹性性质来实现的，与地震观测到的结构相比，地震观测到的结构是由地球板块构造历史重建所产生的地幔动力学预测的。研究人员将对太平洋大型低地震速度区（LLSVP）及其邻近环境进行系统研究。他们使用完整的地震图与增强层析成像模型和热化学对流模型生成的合成图进行比较。这些模型整合了受观测约束的板块构造重建，并考虑了材料的物理特性，包括弹性张量和流变特性。实验评估了俯冲板片中候选深部含水相的地震特征的来源。 它们包括：（1）利用非弹性x射线散射技术测量剪切波速度;（2）利用下地幔条件下的x射线衍射和同步加速器红外光谱学测量热状态方程和稳定性约束。这项研究解决了一些基本问题，例如：俯冲板片的存在是否会使LLSVP变形为具有明显各向异性的地震可分辨的3-D形状，并影响D”地形？如果含水相可以被输送到地幔的最低层，它们是否可以被地震探测到，它们是否在热化学堆的稳定性中起作用？这项工作将通过对三名研究生进行尖端技术培训来完成，合作项目将加强与澳大利亚国立大学的伙伴关系。通过地震研究员计划开展的外联活动将包括来自地球科学背景不足的高中生。该奖项反映了NSF的法定使命，并通过使用基金会的知识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 behavior of materials in the Earth’s mantle constrains the flows that drive plate tectonics. Voluminous volcanic eruptions driven by deep mantle sources are thought to have caused global environmental changes. Dramatic compositional and thermal changes occur at the core-mantle boundary (CMB), about 3000 km below the Earth’s surface. These changes exert a primary influence on the cooling of the planet. They also influence the core dynamics, the Earth’s magnetic field, and mantle thermal convection. Yet, understanding the dynamics of the deep Earth is not trivial. Indeed, multidisciplinary efforts and state-of-the art techniques are required to tackle the complexity of the Earth system. Here, the researchers investigate enigmatic features observed at the core-mantle boundary. To unveil their origin, the team combines expertise in seismology, geodynamics, and experimental mineral physics. The researchers carry out experiments at the extreme pressures prevailing in the mantle. They measure the properties of materials anticipated to exist in of oceanic crustal material sinking to the CMB using powerful x-rays and infrared light at national synchrotron facilities. Taking advantage of recent advances in computational facilities, they simulate the interaction of candidate oceanic crustal materials with deep-mantle materials brought together by tectonic forces acting throughout Earth’s history to produce complex structures in the deep mantle. The results of the models will be compared with seismic observations of the Earth’s interior, testing our understanding of the dynamics of the deep Earth. The project provides support for the training of graduate students at the California Institute of Technology and fosters international collaboration with Australia. Seismologists have revealed that the mantle side of the CMB is extraordinarily heterogeneous, with km-scale fine structure that could harbor distinct chemical reservoirs. Thermal and chemical heterogeneity, solid-solid phase transitions, elastic anisotropy, variable viscosity, and melting are probably all required to explain the observed complexity. With expertise in seismology, geodynamics and experimental mineral physics, the team connects the atomic scale to the tectonic scale as linked to the temporal dimension through dynamics. This is accomplished through the measurement of the thermoelastic properties of deep Earth phases as compared to seismically observed structures predicted by mantle dynamics arising from reconstructions of Earth’s plate tectonic history. The researchers will conduct a systematic study of the Pacific large low seismic velocity province (LLSVP) and proximal surroundings. They use whole seismograms compared against synthetics generated from enhanced tomographic models and thermo-chemical convection models. The models integrate plate tectonic reconstructions constrained by observations and account for materials’ physical properties, including elastic tensors and rheological properties. The experiments assess the sources of the seismic signatures of candidate deep hydrous phases in subducted slab. They include: (1) shear-wave speed measurements using inelastic x-ray scattering techniques; and (2) thermal equation of state and stability constraints using x-ray diffraction and synchrotron infrared spectroscopy at lower mantle conditions. The study addresses fundamental questions, such as: can the presence of subducted slabs deform LLSVPs into seismically resolvable 3-D shapes with distinctive anisotropy and affect D" topography? If hydrous phases can be transported into the lowermost mantle, are they seismically detectable and do they play a role in the stability of a thermo-chemical pile? This work will be accomplished through the training of three graduate students in cutting-edge techniques, and the collaborative project will enhance partnerships with Australian National University. Outreach activities through the Earthquake Fellows Program will include high school students from backgrounds underrepresented in the geosciences.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.