CSEDI: Integrated seismic, geodynamic, and mineral physics studies of multi-scale structures in the lowermost mantle

地球深处物质的行为限制了驱动板块构造的流动。由深部地幔来源驱动的大规模喷发被认为是导致全球环境变化的原因。在地核-地幔边界(CMB)--地球表面下约3000公里处--发生了戏剧性的成分和热变化。这些变化对地球的降温产生了主要影响。它们还影响地核动力学(因此地球磁场)和撞击地幔热对流。然而，了解地球深处的动态并不是一件微不足道的事情。事实上，要解决地球系统的复杂性，需要多学科的努力和最先进的技术。在这里，研究人员调查了在核-地幔边界观察到的神秘特征。为了揭示它们的起源，该团队结合了地震学、地球动力学和实验矿物物理方面的专业知识。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中使用强大的X射线和红外光来测量地球深处材料的性质。利用计算设备的最新进展，他们模拟了地壳材料与由多尺度结构组成的下地幔材料之间的相互作用。这些物质是通过地球复杂的历史中的构造力聚集在一起的。模型的输出与地震观测结果进行了比较，从而逐渐揭示了地球深部的动力学。该项目为加州理工学院的研究生提供支持。它还促进了与澳大利亚和英国的国际合作。地震学家揭示，CMB的地幔一侧异常不均匀，具有公里级的精细结构，可能蕴藏着不同的化学储集层。热和化学的不均一性、固-固相变、弹性各向异性、可变粘度和熔化可能都是解释观察到的复杂性所必需的。凭借在地震学、地球动力学和实验矿物物理学方面的专业知识，该小组将原子尺度(地球深部相的热弹性性质)与构造尺度(通过地震观测到的构造及其动力学)联系起来，并将所有过程与时间维度(重建构造板块历史)联系起来。研究人员对太平洋大地震低速省(LLSVP)和邻近地区，如超低速带(ULVZ)进行了系统研究。他们使用完整的地震记录，并与增强层析成像模型和热化学对流模型生成的合成数据进行了比较。这些模型集成了受观测约束的板块构造重建，并考虑了材料的物理属性，包括弹性张量。这些实验评估了俯冲板块中候选深水相地震特征的来源。它们包括：(1)用非弹性X射线散射技术测量横波速度；(2)用X射线衍射和同步辐射红外光谱测量下地幔条件下的热状态方程和稳定性约束。这项研究解决了基本问题，例如：俯冲板块的存在是否会将LLSVP变形为地震可分辨的3D形状(具有明显的各向异性)，并影响LLSVP边缘附近的D“地形和化学上不同的结构？所有ULVZ是否都是同等创建的？如果含水相可以被输送到地幔最下部，它们是否在地震中可检测到，它们是否有助于热化学堆的稳定性？这项裁决反映了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，认为值得支持。

The behavior of materials in the deep Earth constrains the flows that drive plate tectonics. Voluminous eruptions driven by deep mantle sources are thought to have caused global environmental changes. At the core-mantle boundary (CMB) – about 3000 km below the Earth’s surface - dramatic compositional and thermal changes occur. These changes exert a primary influence on the cooling of the planet. They also influence the core dynamics (hence Earth’s magnetic field) and impact 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 combine 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 deep Earth materials using powerful x rays and infrared light at national synchrotron facilities. Taking advantage of recent advances in computational facilities, they simulate the interaction of crustal materials with lower-mantle materials made of multi-scale structures. These materials are brought together by tectonic forces through Earth’s complex history. Outputs of the models are compared with seismic observations, hence gradually unveiling the dynamics of the deep Earth. The project provides support for graduate students at the California Institute of Technology. It also fosters international collaboration with Australia and the UK. 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 (thermoelastic properties of deep Earth phases) to the tectonic scale (seismically observed structures and their dynamics) and link all processes to the temporal dimension (reconstruction of tectonic plate history). The researchers conduct a systematic study of the Pacific large low seismic velocity province (LLSVP) and proximal surroundings such as ultralow velocity zones (ULVZs). 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. 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 3D shapes (with distinctive anisotropy) and affects D" topography and chemically–distinct structures near the edges of LLSVPs? Are all ULVZs created equally? If hydrous phases can be transported into the lowermost mantle, are they seismically detectable and can they contribute to the stability of a thermo-chemical pile?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.