Collaborative Research: A Multidisciplinary Approach to Investigate the Origin of Anisotropy at the Base of the Mantle

合作研究：研究地幔底部各向异性起源的多学科方法

• 批准号: 1464036
• 负责人: Allen McNamara
• 金额: $ 9.57万
• 依托单位: Arizona State University
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2015
• 资助国家: 美国
• 起止时间: 2015-07-01 至 2016-10-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1464036&HistoricalAwards=false
• 关键词: Collaborative; Research; Multidisciplinary; Approach; Investigate

The lowermost part of the earth's mantle, referred to as the D" region, is a dynamic region that is both a thermal and chemical boundary layer between the solid, silicate mantle and the fluid, mostly iron outer core. A better understanding of the deformation processes that occur in this region would provide important constraints on the current dynamics of the entire mantle, the processes of heat transfer from the core to the mantle, the thermal evolution of our planet, and the existence and extent of geochemical heterogeneity. To study deformation processes in the deepest mantle, the investigators combine expertise from several disciplines: seismology, mineral physics and geodynamical modeling of mantle convection, linked together around a common object of study: seismic anisotropy, i.e. the difference in propagation speeds of seismic waves depending on the orientation of the path travelled, a proxy for macroscopic deformation. The latter's characteristics reflect mineral properties as well as flow strength and geometry. In this project, the team will apply the multi-disciplinary tools developed in a previous joint study funded by the CSEDI program of NSF to further characterize the possible causes of seismic anisotropy in the earth's deep mantle. They will begin with 3D fluid dynamical modeling of mantle convection, in which they will model the deformation of descending tectonic plates as they come in contact with Earth's core-mantle boundary. It is unclear how strong these plates are, and their strength will control how they deform. Strong plates will buckle and bend, whereas weak plates will deform is a more ductile fashion. The team will examine numerous scenarios, each assuming a different strength for descending plates. They will also examine scenarios in which descending plates interact with hypothesized compositional heterogeneity in the deep mantle. Deformation data from the dynamical calculations will be used as input for mineral physics calculations to predict the alignment of minerals which will control the nature of seismic anisotropy. By comparing predicted seismic anisotropy from various models to that observed by seismic studies, they will constrain deformation characteristics of descending plates and compositional characteristics of the D" zone. This will provide important information on how sinking plates drive larger-scale mantle convection.The presence of anisotropy in the D" region of the earth's mantle is now well established, although its cause remains unclear. Much progress was recently achieved in mineral physics, to characterize elastic and deformation properties of lowermost mantle minerals including the post-perovskite (pPv) phase, as well as in geodynamics, tracking strain evolution in mantle convection modeling. There are now precise ways to compute synthetic seismograms in a 3D anisotropic earth down to body wave frequencies. This study will advance our understanding of the structure and dynamics of an important boundary layer region in the earth. In previous collaborative work funded by CSEDI, the investigators developed a multi-disciplinary approach combining elements from geodynamic modeling, mineral physics and material science experiments and computations, to perform forward modeling of crystal preferred orientation (CPO) anisotropy in a 3D spherical earth, in the deep mantle part of a subducted slab, under different starting assumptions, and compared them with seismic observations. The ultimate goal is to gain better understanding of the origin of seismic anisotropy in D", and determine which microscopic and macroscopic processes may or may not be at play. So far, they investigated the case of a 3D geodynamical model under rather simple rheological assumptions. Now, they will explore varying rheologies producing slabs of variable strength, including the effect of the pPv phase-change, and how slabs will deform in the presence of hypothetical thermochemical piles. For each of these different calculations, they will provide the deformation information to serve as input for the mineralogical texture development within a polycrystalline mineral aggregate. While the team will still focus on three phases, perovskite, pPv and ferropericlase, they will also explore the effect of a variety of Fe and Al substitution mechanisms, both theoretically and experimentally. The predicted seismic anisotropy from these models will be confronted with seismological observations of radial and azimuthal anisotropy both acquired during this project and from the literature.

地幔的最下层，被称为D区，是一个动态区域，它既是固体硅酸盐地幔和流体（主要是铁外核）之间的热和化学边界层。更好地了解该区域发生的变形过程将对整个地幔的当前动力学、地核到地幔的热传递过程、地球的热演化以及地球化学非均质性的存在和程度提供重要的约束。为了研究最深地幔的变形过程，研究人员结合了几个学科的专业知识：地震学、矿物物理学和地幔对流的地球动力学建模，围绕一个共同的研究对象联系在一起：地震各向异性，即地震波传播速度的差异取决于传播路径的方向，这是宏观变形的代表。后者的特征反映了矿物性质以及流动强度和几何形状。在这个项目中，该团队将应用之前由美国国家科学基金会CSEDI项目资助的联合研究中开发的多学科工具，进一步表征地球深部地幔地震各向异性的可能原因。他们将从地幔对流的三维流体动力学建模开始，在这个模型中，他们将模拟构造板块在与地球核心-地幔边界接触时下降的变形。目前还不清楚这些板块有多强，它们的强度将控制它们如何变形。坚固的板会弯曲弯曲，而脆弱的板则会以一种更具延展性的方式变形。科考队将研究多种情况，每种情况都假设板块下降的强度不同。他们还将研究下降板块与深部地幔中假设的成分不均匀性相互作用的情景。动力学计算的变形数据将用作矿物物理计算的输入，以预测矿物的走向，从而控制地震各向异性的性质。通过将各种模型预测的地震各向异性与地震研究的观测结果进行比较，可以约束沉降板块的变形特征和D”带的成分特征。这将提供关于下沉板块如何驱动更大规模的地幔对流的重要信息。地幔D区各向异性的存在现在已经确定，尽管其原因尚不清楚。近年来，在矿物物理学和地球动力学方面取得了很大进展，包括表征最下层地幔矿物的弹性和变形特性，包括后钙钛矿（pPv）阶段，以及在地幔对流建模中跟踪应变演化。现在有精确的方法来计算三维各向异性地球的合成地震图，精确到体波频率。这项研究将促进我们对地球上一个重要边界层区域的结构和动力学的认识。在之前由CSEDI资助的合作研究中，研究人员开发了一种多学科方法，将地球动力学建模、矿物物理学和材料科学实验与计算相结合，在不同的起始假设下，对俯冲板块深部地幔部分的三维球形地球中的晶体优先取向（CPO）各向异性进行了正演模拟，并将其与地震观测结果进行了比较。最终目标是更好地了解D ‘ ’地震各向异性的起源，并确定哪些微观和宏观过程可能起作用，哪些可能不起作用。到目前为止，他们在相当简单的流变假设下研究了三维地球动力学模型。现在，他们将探索产生可变强度板的不同流变学，包括pPv相变的影响，以及在假设的热化学桩存在下板将如何变形。对于这些不同的计算，它们将提供变形信息，作为多晶矿物集合体矿物结构发育的输入。虽然该团队仍将专注于钙钛矿、pPv和铁方长石这三种相，但他们也将从理论上和实验上探索各种铁和铝取代机制的影响。这些模型预测的地震各向异性将与本项目和文献中获得的径向和方位角各向异性地震学观测结果相矛盾。