Investigating the long-term spatial stability of LLSVPs

地球表面是高度动态的，以地震、火山和由地表构造板块运动引起的活动断层为特征。众所周知，板块构造的驱动力是地幔内的对流，地幔是地核和地壳之间的一层。在过去的几十年里，在加深我们对构造板块如何形成、在地表移动以及如何下沉到地球内部的理解方面，已经取得了很大进展；然而，关于地幔如何对流和驱动板块构造，我们仍然有一些基本的、一阶的问题。地震学提供了最好的线索来回答这些问题，通过使用地震引起的地震波来生成内部的3D图像。我们从地震学中发现的一个令人费解的观察结果是，在非洲和太平洋下面的最低地幔中存在两个大型结构。这些结构直接位于地核，并延伸到地幔数百公里处。地震波在这些结构中的传播速度比平时慢，而且它们的边界似乎很尖锐，这表明它们可能是由一些与周围地幔岩石不同的东西组成的。有趣的是，这些构造形成了大量异常活火山(称为热点)，如夏威夷。通过重建过去的板块运动，我们发现许多灭绝的古代火山也在这些相同的地点上方形成。因此，如果非洲和太平洋下面的大型结构是这些热点火山的原因，那么它们肯定已经在现在的位置很长一段时间了，可能是过去5亿年。这与预测地幔对流的计算模型相矛盾，该模型预测，这些结构将很容易因构造板块运动的变化而移动。PI假设这些深部地幔结构可能具有与地幔其余部分不同的矿物学粒度，这可能导致它们具有更高的粘度。该团队将进行数值流体动力学，以检查与这些结构相关的较高粘度是否会导致地幔对流在地质时间内稳定下来，以解释这些地理区域上火山的长期形成。更重要的是，研究人员将研究这些非洲和太平洋构造的动力学如何指导和控制地幔对流的长期模式。这项研究的结果将为理解地幔对流如何引起和控制板块构造提供关键的洞察力。地震层析成像揭示了最下面的地幔(非洲和太平洋之下)存在大片剪切波速度低于平均速度的区域，通常被称为大的低剪切速度省(LLSVPs)。有人假设它们是由大规模的成分不均质性(如热化学堆积)造成的。近年来，由于LLSVP在理解大尺度地幔对流、热输运以及地球的热和化学演化方面的关键作用，发现LLSVP的性质和动力学意义涉及到许多活跃的研究。有趣而有些自相矛盾的是，最近的古地磁研究暗示，LLSVP可能已经保持了数亿年的相同位置。这是令人费解的，因为动力学计算表明，它们应该很容易通过改变俯冲模式而被席卷。这是一个需要解决的至关重要的问题，这里探讨了以下问题：一个合理的热化学地幔对流的动力学/流变学概念模型能否解释LLSVP如何保持空间固定(或非常缓慢的移动)？大多数地球动力学计算采用依赖温度的流变学，导致LLSVP较弱(由于其温度较高)，因此更容易被下沉的岩板横向推。然而，扩散蠕变流变学对矿物粒度高度敏感(以幂规律的方式)，因此粒度的微小变化可以导致粘度的大变化。如果LLSVP在地质时间尺度上保持了与背景地幔不同的成分，那么就没有理由期望它们的平均粒度与周围的地幔相同。如果LLSVP的平均粒度仅略大于背景地幔的平均粒度，则LLSVP的组成粘度将增加。初步工作表明，当与粘度的温度相关时，这会导致围绕LLSVP的强大外壳或包层，产生不寻常的热化学结构，其动态行为与传统的被动热化学堆截然不同。这种类型的热化学对流还没有被探索过，拟议的工作将检验这样一个假设，即颗粒大小引起的粘性成分依赖可以导致LLSVP结构，这些结构不太容易被俯冲板块推来推去，因此在地质时间尺度上变得更稳定。将进行广泛的地球动力学研究(通过数值模拟)，以探索与传统模型相比，流变学的颗粒尺寸相关性如何影响系统的流体动力学性质。此外，将在模型的子集中使用粒度演化，以检查LLSVP在相关地质时间尺度上具有比背景地幔更大的粒度是否可行。总之，将开展和执行数值实验，以确定粘度的颗粒尺寸相关性如何影响热化学堆的空间稳定性，以及这是否是产生长寿命、稳定位置的LLSVP的动态可行机制。

The Earth's surface is highly dynamic, characterized by earthquakes, volcanoes, and active faulting caused by the movement of tectonic plates at the surface. It is well-understood that the driver of plate tectonics is convection within Earth's mantle, the layer between the core and the crust. Over the past several decades, there has been much advancement toward increasing our understanding of how tectonic plates form, move about the surface, and sink back into the Earth's interior; however, we still have fundamental, first-order questions regarding how the mantle convects and drives plate tectonics. Seismology provides the best clues to answer these questions, by using seismic waves caused by earthquakes to generate a 3D image of the interior. One puzzling observation that we find from seismology is the presence of 2 large structures in the lowermost mantle, beneath Africa and the Pacific. These structures rest directly on the Earth's core, and extend several hundred kilometers into the mantle. Seismic waves travel slower than usual through these structures, and they seem to have sharp boundaries, indicating that they may be composed of something somewhat different than the surrounding mantle rock. Interestingly, these structures underlie a large number of anomalous active volcanoes (called hotspots), such as Hawaii. By reconstructing plate motions back through time, we find that many extinct, ancient volcanoes were also formed above these same locations. Therefore, if the large structures beneath Africa and the Pacific are the cause of these hotspot volcanoes, they must have been in their present location for a long time, perhaps for the past 500 million years. This is contradictory to computational models of mantle convection that predict that such structures would be easily moved by changing tectonic plate motions. The PI hypothesizes that these deep mantle structures may have a different mineralogical grain size than the rest of the mantle, which could cause them to have a higher viscosity. The team will perform numerical fluid dynamics to examine whether the higher viscosity associated with these structures could lead to a stabilizing of mantle convection currents over geologic time to explain the long-lived creation of volcanoes over these geographic areas. More importantly, the investigators will examine how the dynamics of these African and Pacific structures can guide and control the long term patterns of mantle convection. Results from this study will provide critical insight into understanding how mantle convection causes and controls plate tectonics.Seismic tomography reveals the presence of large regions in the lowermost mantle (beneath Africa and the Pacific) that exhibit lower-than-average shear wave velocity, commonly referred to as the Large Low Shear Velocity Provinces (LLSVPs). It has been hypothesized that they are caused by large-scale compositional heterogeneity (e.g., thermochemical piles). Discovering the properties and dynamical implications of the LLSVPs has involved much active research in recent years due to their critical role toward understanding large-scale mantle convection, heat transport, and thermal and chemical evolution of the Earth. Interestingly and somewhat paradoxically, recent paleomagnetic research has hinted that LLSVPs may have remained in the same positions for hundreds of millions of years. This is perplexing because dynamical calculations indicate that they should be easily swept around by changing subduction patterns. This is a critically important issue to address, and here, the following question is explored: Can a reasonable dynamical/rheological conceptual model of thermochemical mantle convection explain how LLSVPs could remain spatially fixed (or very slow moving)? Most geodynamical calculations employ a temperature-dependent rheology, resulting in LLSVPs being weaker (due to their high temperature) and therefore, easier to be laterally pushed around by downwelling slabs. However, diffusion creep rheology is highly sensitive to mineral grain-size (in a power-law manner), so small changes in grain-size can lead to large changes in viscosity. If LLSVPs have remained compositionally distinct from the background mantle over geologic timescales, there is no reason to expect that they would have the same average grain-size as the surrounding mantle. If the average grain-size of LLSVPs is only slightly larger than that of the background mantle, LLSVPs would have an increased compositional viscosity. Preliminary work has shown that when combined with temperature-dependence of viscosity, this leads to a strong rind or envelope surrounding the LLSVPs, producing unusual thermochemical structures that dynamically behave quite differently than conventional, passive thermochemical piles. This type of thermochemical convection has not been explored, and the proposed work will test the hypothesis that grain-size induced compositional dependence of viscosity can lead to LLSVP structures that are less easily pushed around by subducting slabs and therefore, become more spatially-stable over geologic timescales. An extensive geodynamical study (through numerical modeling) will be performed to explore how grain-size dependence of rheology influences the fluid dynamical properties of the system, compared to traditional models. Additionally, grain-size evolution will be employed in a subset of models, to examine whether it is feasible for LLSVPs to have larger grain size than background mantle over the relevant geological timescales. In summary, numerical experiments will be developed and performed to determine how grain-size dependence of viscosity influences the spatial stability of thermochemical piles and whether this is a dynamically feasible mechanism to produce long-lived, stable positions of LLSVPs.