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

克拉通是古老而稳定的大陆核心。这些地区在过去的25亿年里没有经历过显著的变形。各种地球化学和地球物理数据表明，它们被厚厚的地幔岩石圈覆盖，相对于周围的地幔来说，岩石圈异常寒冷。克拉通地幔的内部结构还包括物理和化学性质上的分层。然而，关于这种内部结构的起源仍有许多需要了解的地方。研究人员计划利用地震波在克拉通地幔中传播的速度，为地幔岩石的温度、化学成分、颗粒大小和岩石组构提供界限。这个多学科研究小组计划直接测量克拉通地幔喷发到地表的样品的地球化学和岩石组构，以提供关于其化学演化和变形历史的补充信息。拟议工作的目标是：1)更好地约束克拉通地幔岩石圈内部的地球化学和地震速度结构中的分层；2)探索不同类型分层之间的关系；3)揭示形成克拉通的过程和使它们能够保持数十亿年稳定的机制。了解大陆稳定的核心将有助于我们了解地球及其大陆随时间的演化。该项目将有助于两名或两名以上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本科生和研究生讲授为期一学期的克拉通专题研讨会。该团队提出了一个综合计划，内容包括地震学和基于捕虏体的地球化学和微结构分析以及地球动力学模拟，重点是过去20亿年来经历了不同程度破坏的三个地幔岩石圈：奴隶克拉通、怀俄明州克拉通和科罗拉多高原。每个区域都提供了采集深部克拉通地幔样品的极好的捕虏体套装和宽带站，这将使解决地震速度结构的工作取得进展。他们计划用各种新技术研究克拉通地幔中不同类型的分层之间的关系(岩石圈中部地震间断、方位各向异性分层、亏损、参考、粒度、橄榄石组构)。散射波、面波、环境噪声和SKS分裂数据的联合反演将对地震结构提供更好的约束。在一系列地幔深度的包体中，我们将使用成熟的分析技术来确定主体和微量元素组成及其组成矿物中的主要、痕量和水分含量，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(与俯冲有关与俯冲无关)。捕虏体显微结构分析将给出颗粒大小、水含量和晶格择优取向的限制。热压计、模式分析、挥发分含量和颗粒大小将通过弹性模型(包括成分的影响)和滞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。在这些比较的基础上，将定义一系列模型，以反映最符合地球化学、微结构和地震学限制的模型。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来计算有效粘度，使用橄榄石的实验流动定律以及密度。每个研究区域可能的密度和粘度结构的范围将被纳入岩石圈稳定性的地球动力学数值模拟，包括其边缘对俯冲过程的脆弱性。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分层(物理和化学)是什么？不同类型的分层如何相互关联？2)它们的内部结构如何使稳定的克拉通在数十亿年的时间尺度上保持基本完好？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分层如何“检验”克拉通形成的模型？

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?