CSEDI: Layering within cratonic lithosphere: Integrated constraints from xenoliths, seismic structure and geodynamical modeling

克拉通是大陆古老而稳定的核心。这些地区在过去的25亿年里没有经历过明显的变形。各种地球化学和地球物理数据表明，它们的下面是厚厚的地幔岩石圈，相对于周围的地幔来说异常寒冷。克拉通地幔的内部结构在物理和化学性质上也具有分层性。然而，关于这种内部结构的起源，还有很多有待研究。研究人员计划利用地震波在克拉通地幔中传播的速度来提供地幔岩石的温度、化学成分、颗粒大小和岩石结构的界限。多学科研究小组计划直接测量已经喷发到地表的克拉通地幔样本的地球化学和岩石结构，以提供有关其化学演化和变形历史的补充信息。提出的工作目标是：1)更好地约束克拉通地幔岩石圈内部地球化学和地震速度结构的分层；2)探索不同类型分层之间的关系；3)揭示克拉通形成的过程和使它们在数十亿年内保持稳定的机制。了解大陆稳定的地核将有助于我们了解地球及其大陆的演化过程。该项目将有助于两名或多名研究生和几名本科生的教育和职业发展，该项目的跨学科性质将有助于拓宽他们的研究专长。我们的教师团队还将在布朗大学为高年级本科生和研究生开设一个学期的研讨会，主题是克拉通。该团队提出了一个地震学和基于包体的地球化学和微观结构分析以及地球动力学建模的综合计划，重点关注在过去20亿年中经历不同程度破坏的三个地幔岩石圈：奴拉通，怀俄明克拉通和科罗拉多高原。每个区域都提供了优秀的捕虏体套件，可以对深克拉通地幔和宽带站进行采样，这将有助于解决地震速度结构的问题。他们计划用各种新技术研究克拉通地幔中不同类型的层序（岩石圈中部地震不连续层序、方位各向异性层序、衰竭层序、再成矿层序、粒度层序、橄榄石组构层序）之间的关系。散射波、表面波、环境噪声和SKS分裂数据联合反演可以更好地约束地震结构。在一系列地幔深度的捕虏体中，我们将使用完善的分析技术来确定其组成矿物中的大量和微量元素组成以及主要，微量和水分含量，以确定：最后一次构造岩浆事件的压力-温度条件，地幔的水化程度，以及影响岩石圈地幔的交代流体/熔体的来源（与俯冲有关或与俯冲无关）。捕虏体微观结构分析将产生晶粒尺寸、含水量和晶格优先取向的限制。热气压计、模态分析、挥发性含量和颗粒大小将通过弹性模型（包括成分的影响）和非弹性效应的组合来预测地震速度；这些预测将与观测到的地震分层进行比较。基于这些比较，将定义一系列最适合地球化学、微观结构和地震学约束的模型。为了探索这些模型对克拉通地幔稳定性的影响，我们将使用捕虏体约束，利用橄榄石的实验流动规律计算有效粘度，以及密度。每个研究区域可能的密度和粘度结构范围将被纳入岩石圈稳定性的地球动力学数值模拟，包括它们对边缘俯冲过程的脆弱性。这项工作将为几个问题提供新的见解。1)克拉通地幔岩石圈的内部（物理和化学）分层是什么？不同类型的分层是如何相互关联的？2)它们的内部结构是如何使稳定的克拉通在十亿年的时间尺度上基本保持完整的？克拉通边缘的俯冲作用如何影响克拉通地幔岩石圈的稳定性？3)不同类型和规模的克拉通层理如何“检验”克拉通的形成模式？

Cratons are the old, stable cores of continents. They are regions that have not experienced significant deformation for the last 2.5 billion years. A variety of geochemical and geophysical data indicate that they are underlain by thick mantle lithosphere that is unusually cold relative to the surrounding mantle. The internal structure of the cratonic mantle also includes layering in both physical and chemical properties. However, much remains to be learned about the origin of this internal structure. The researchers plan to use the velocities at which seismic waves propagate through the cratonic mantle to provide bounds on the temperature of the mantle rocks, their chemical composition, and their grain size and rock fabric. The multidisciplinary research team plans to directly measure the geochemistry and rock fabric of samples of the cratonic mantle that have been erupted to the surface to provide complementary information on their chemical evolution and deformation history. The goals of the proposed work are to: 1) better constrain layering in geochemical and seismic velocity structure internal to cratonic mantle lithosphere, 2) explore the relationships among different types of layering, and 3) shed new light on the processes that formed the cratons and the mechanisms that permit them to remain stable over billions of years. Understanding the stable cores of continents will help us understand the evolution of the Earth and its continents through time. This project will contribute to the education and career development of two or more graduate students and several undergraduates, and the interdisciplinary nature of the project will serve to broaden their research expertise. Our faculty team will also teach a semester-long seminar at Brown on the topic of cratons for upper-level undergraduates and graduate students.The team proposes an integrated program of seismological and xenolith-based geochemical and microstructural analyses and geodynamical modeling focused on three mantle lithospheres that have experienced varying degrees of disruption in the last 2 billion years: the Slave craton, the Wyoming craton and the Colorado Plateau. Each region provides excellent xenolith suites that sample the deep cratonic mantle and broadband stations that will allow progress on resolving seismic velocity structure. They plan to investigate the relationships between different types of layering in the cratonic mantle (mid-lithospheric seismic discontinuities, layering in azimuthal anisotropy, depletion, refertilization, grain size, olivine fabrics) with a variety of new techniques. Joint inversions of scattered wave, surface wave, ambient noise and SKS splitting data will provide better constraints on seismic structure. In xenoliths from a range of mantle depths we will use well-established analytical techniques to determine bulk and trace element compositions and major, trace and water contents in their constituent minerals to establish: the pressure-temperature conditions of the last tectonomagmatic event, the degree of hydration of the mantle, and the source of the metasomatic fluids/melts that affected the lithospheric mantle (subduction-related versus subduction-unrelated). Xenolith microstructural analyses will yield constraints on grain size, water content, and lattice preferred orientation. Thermobarometry, modal analyses, volatile content and grain size will be used to predict seismic velocities via a combination of elastic models (which include the effects of composition) and anelastic effects; these predictions will be compared to the observed seismological layering. Based on these comparisons, a range of models will be defined that reflect the best fits to geochemical, microstructural and seismological constraints. To explore the implications of these models for the stability of the cratonic mantle, we will use the xenolith constraints to calculate effective viscosity using experimental flow laws for olivine, as well as density. The range of possible density and viscosity structures for each study region will be incorporated in geodynamical numerical modeling of lithospheric stability, including their vulnerability to subduction processes at their margins. This work will provide new insight on several questions. 1) What is the internal layering (physical and chemical) of the cratonic mantle lithosphere? How do different types of layering correlate with each other? 2) How has their internal structure permitted stable cratons to remain largely intact over billion-year time-scales? How does subduction at the edges of a craton affect the stability of the cratonic mantle lithosphere? 3) How do the different types and scales of cratonic layering 'test' models of cratonic formation?