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

南极洲是古老而稳定的大陆核心。 它们是在过去25亿年里没有经历过显著变形的区域。 各种地球化学和地球物理数据表明，它们下面是厚地幔岩石圈，相对于周围地幔异常冷。 地幔的内部结构还包括物理和化学性质的分层。 然而，关于这一内部结构的起源仍有许多问题有待了解。 研究人员计划利用地震波在地幔中传播的速度来提供地幔岩石的温度、化学成分、粒度和岩石结构的界限。 多学科研究小组计划直接测量已喷发到地表的地幔样品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。 拟议工作的目标是：1）更好地约束地球化学和地震速度结构的分层内部的地幔岩石圈，2）探索不同类型的分层之间的关系，和3）揭示新的光的过程，形成的地幔和机制，使他们能够保持稳定的数十亿年。 了解大陆的稳定核心将有助于我们了解地球及其大陆随时间的演变。 该项目将有助于两个或两个以上的研究生和几个本科生的教育和职业发展，该项目的跨学科性质将有助于扩大他们的研究专长。我们的教师团队还将在布朗大学为高水平的本科生和研究生讲授一个为期一个学期的研讨会，主题是地幔。该团队提出了一个地震学和捕虏体为基础的地球化学和微结构分析和地球动力学建模的综合计划，重点关注在过去20亿年中经历了不同程度的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。 每个区域都提供了优秀的捕虏体套件，可以对深部地幔进行采样，并提供了宽带台站，可以在解决地震速度结构方面取得进展。他们计划利用各种新技术研究地幔中不同类型的分层（中岩石圈地震不连续性，方位各向异性分层，耗尽，再施肥，粒度，橄榄石组构）之间的关系。 散射波、面波、背景噪声和SKS分裂数据的联合反演将对地震构造提供更好的约束。 在一系列地幔深度的捕虏体中，我们将使用成熟的分析技术来确定其组成矿物中的大量和微量元素成分以及主要、微量和水含量，以建立：最后一次构造岩浆事件的压力-温度条件，地幔水化程度，以及影响岩石圈地幔的交代流体/熔体的来源（俯冲相关与俯冲无关）。捕虏体显微结构分析将产生对粒度、含水量和晶格优选取向的约束。将结合弹性模型（包括成分的影响）和滞弹性效应，利用热压测定法、模态分析、挥发物含量和粒度来预测地震速度;这些预测将与观测到的地震分层进行比较。 根据这些比较，将定义一系列模型，反映最适合地球化学，微观结构和地震学的限制。为了探索这些模型的稳定性的影响，我们将使用捕虏体的约束来计算有效粘度使用实验流动定律橄榄石，以及密度。每个研究区域可能的密度和粘性结构范围将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。 这项工作将对几个问题提供新的见解。1)地幔岩石圈的内部分层（物理和化学）是什么？不同类型的分层是如何相互关联的？2)它们的内部结构是如何让稳定的中微子在数十亿年的时间尺度上保持基本完整的呢？克拉通边缘的俯冲作用如何影响地幔岩石圈的稳定性？3)不同类型和尺度的重力分层如何“检验”重力形成的模型？

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?