CSEDI: Geochemical Evolution of the Earth's Mantle Constrained by Observations and Dynamical Modeling

该项目旨在推进有关地球内部如何随着地质时间的变化而发生变化的科学知识，这是由于构造板块下沉到内部造成的非常缓慢的运动，以及来自地球深处的热浮力物质的上升，所有这些都是为了混合地球的地幔。这些对流运动有可能将构造板块从地表一路移动到核-地幔边界(CMB)，并从CMB返回物质，在那里它可以促进近地表的融化，从而在大洋中脊和海洋热点(如夏威夷和其他地区)产生火山。研究人员将使用这些对流运动的高性能计算机模拟来研究地球内部的流动，以及在地球深部高压下发生的矿物学变化如何改变这种流动；计算模型将得到加强，包括数百万个被动示踪粒子，这些粒子记录了地幔在表面融化时发生的地球化学变化，当板块下沉并混合到地球内部时，以及当大陆侵蚀的沉积物通过这些过程带入地球内部时。示踪粒子记录的地球化学变化将与火山的地球化学数据进行比较，以测试地球深处的流动是否一直持续到核心，或者内部是否作为独立的层流动，两者之间的混合最少。从这个项目中获得的知识将提供有关地球表面的构造运动如何影响内陆混合的信息，以及这种混合如何反过来改变从内陆到大陆的各种元素的流动，从而提供关于大陆如何在地球地质历史上增长和侵蚀的信息。在这个项目中，研究人员将专注于在一系列高分辨率数值对流模型中记录地幔的地球化学演化，这些模型检验了地幔分层、分层和对流隔离的程度，当使用现实的可压缩状态方程来描述地幔的物理属性时。可压缩地幔的使用将使我们能够以严格的方式模拟矿物学相变。当结合关于地幔粘度、导电性和热膨胀的P-T应变率依赖的实验室数据时，该模型的这些特征将对由于与已知相边界相关的深度相关的密度和粘度而可能发生的地幔分层的程度产生最真实的描述。模型系列将进一步限于那些与主要地球物理和地球化学观测(对流能量、地幔温度、热流、板块速度、内部加热率、大气和大陆地壳的成分和质量等)的已知范围相匹配的模型。模型空间的地球化学将从嵌入地幔流动中的数百万活跃的数字示踪剂计算出来，每个示踪剂都将记录当它们的演化被发生在模型表面的熔融、脱气和大陆提取事件打断时它们经历的微量元素分馏和相关的同位素演化。该团队将对对流地幔、大陆地壳和大气中亲石和稀有气体同位素体系的演化同位素地球化学进行显式建模，并将模型输出与地球化学数据进行定量比较，以测试我们模型的可行性。

This project seeks to advance scientific knowledge about how the Earth's interior has changed over geologic time, due to the very slow motions caused by the sinking of tectonic plates into the interior and the rise of hot buoyant material from the deep Earth, all of which work to mix the Earth's mantle. These convective motions have the potential to move tectonic plates from the surface all the way to the core-mantle boundary (CMB), and to return material from the CMB where it can contribute to near-surface melting that produces volcanoes at mid-ocean ridges and at oceanic hotspots (such as Hawaii and others). The investigators will use high-performance computer simulations of these convective motions to study the flow of the Earth's interior and how this flow is modified by changes in mineralogy that occur at the high pressures of the Earth's deep interior; the computational models will be enhanced by the inclusion of millions of passive tracer particles that record changes in geochemistry that occur when the mantle melts at the surface, when plates sink and mix into the Earth's interior, and when sediment eroded from the continents is brought into the Earth's interior by these processes. The geochemical changes recorded by the tracer particles will be compared with geochemical data from volcanoes to test whether the flow in the deep Earth continues all the way to the core, or whether the interior flows as separate layers with minimal mixing between them. The knowledge gained from this project will provide information on how the tectonic motions at the surface of the Earth affect mixing in the interior, and how this mixing in turn has altered the flow of various elements from the interior to the continents thus providing information on how the continents have grown and eroded over Earth's geologic history.In this project the investigators will focus on documenting the geochemical evolution of the mantle in a series of high-resolution numerical convection models that examine the degree of mantle layering, stratification, and convective isolation that occur when realistic compressible equations of state are used to describe the physical properties of the mantle. The use of a compressible mantle will allow us to model mineralogical phase transitions in a rigorous way. When combined with laboratory data on the P-T-strain rate dependence of mantle viscosity, conductivity and thermal expansion, these features of the model will produce the most physically realistic description of the extent of mantle layering that can occur due to depth-dependent density and viscosity associated with known phase boundaries. The model series will be further constrained to only those models that match the known ranges in major geophysical and geochemical observations (convective vigor, mantle temperature, heat flow, plate velocities, rate of internal heating, composition and mass of the atmosphere and continental crust, etc.). The geochemistry of the model space will be calculated from millions of active numerical tracers embedded in the mantle flow, that will each record the trace element fractionation and associated isotopic evolution they experience when their evolution is punctuated by melting, degassing, and continental extraction events that occur at the surface of the model. The team will model explicitly the evolving isotope geochemistry of both lithophile and noble gas isotope systems in the convecting mantle, continental crust and atmosphere, and quantitatively compare the model output with geochemical data to test the viability of our models.