High-resolution seismic constraints to reveal mid-mantle processes

地幔的动力学，夹在地壳和地核之间的2900公里层，塑造了我们今天所知道的地球表面。例如，地幔中的上涌物质，称为地幔柱，导致局部的，增加了表面的火山活动，形成了今天的大部分海洋岛屿。大陆的起源和早期形成也被归因于地幔柱，沿着更近的大陆规模的火山流出，被称为“大火成岩省”。深部地幔的过程也被假设控制着板块构造的模式，以及地球历史上的超大陆循环。我们目前还没有一个完整的地幔动力学历史的图片来解释所有这些观测。例如，我们不知道地幔对流是一层还是两层，也不知道地幔柱是否直接将核幔边界的过程连接到地表，以及地幔是否随着时间的推移而充分混合。我们也不知道更深地幔的异质性的本质，也不知道这如何影响整体动态。我们对地球深部结构和动力学的大部分知识来自全球地震层析成像，它使用地震波来制作地幔中地震速度变化的图像。地震层析成像的图像显示出具有快速地震波速度的特征，解释为冷的下行板，以及具有慢速地震波速度的特征，解释为热的上涌地幔柱。这些图像的分辨率随着数据和计算能力的迅速增加而不断提高。最近最引人注目的发现之一是在中地幔1000公里深处的一些板块和地幔柱的积水。这里有一个未解的问题：在这个深度发生了什么影响对流运动？地震层析成像的缺点是它扩大了成像特征，低估了它们的真实振幅。这部分是因为所使用的波的周期相对较长，从而降低了在深度成像的异质性的分辨率。在这里，我们提出了有针对性的研究，使用更高的频率波比可以纳入地震层析成像的小规模的不均匀性约1000公里的中地幔。具体来说，我们将使用被这些不均匀性反射或转换的波，因此对这些特征的边界具有很强的敏感性。不同相位的独特灵敏度使我们能够绘制异常不均匀性的大小，形状，速度对比度，密度对比度和锐度。在一项初步研究中，使用转换的地震波在欧洲，我们映射了广泛的补丁的异质性一致在1000公里的深度。我们将扩展这项技术，在全球范围内绘制这些特征，并了解它们与观测到的板块和地幔柱的关系，以及它们在1000公里左右聚集的程度。接下来，我们需要用不同的反射波和转换波的组合来瞄准非均匀性。关于这些地幔不均匀性的性质和作用的问题基本上是跨学科的。我们将联合收割机这些高分辨率的地震学约束的实验和计算的特定组合物的热弹性行为在高压和高温下。通过这种方式，我们将测试一些关键的假设地球深部结构：不均匀性起源于表面，他们介绍了俯冲板？或者它们是否代表原始物质，要么是从更深的地幔中产生的，要么是在地球历史上停滞在这个深度的？是否存在不同类型的异质性？通过有针对性的地球深部成像来了解观测到的不均匀性的组成，我们可以确定它在控制整个地幔动力学中的作用。

The dynamics of Earth's mantle, the 2900 km layer sandwiched between the crust and core, have shaped the Earth's surface, as we know it today. For example, upwelling material in the mantle, known as mantle plumes, causes localized, increased volcanism at the surface, forming most of today's ocean islands. The initiation and early formation of continents has also been attributed to mantle plumes, along with more recent continent-sized volcanic outflows known as 'large igneous provinces'. Processes in the deep mantle have also been hypothesized to control the pattern of plate tectonics, and the supercontinent cycle over the history of the Earth. We currently do not have a full picture of the dynamical history of the mantle that explains all these observations. For example, we do not know whether the mantle convects as one layer or two layers, and thus if mantle plumes directly connect processes at the core-mantle boundary to the surface, and if the mantle is well mixed over time. We also do not know the nature of heterogeneity in the deeper mantle, nor how this influences the overall dynamics. Much of our knowledge of the deep Earth's structure and dynamics comes from global seismic tomography, which uses earthquake waves to make an image of seismic velocity variations in the mantle. The images of seismic tomography show features with fast seismic wave speeds, interpreted as cold downgoing slabs, and features of slow seismic wave speeds, interpreted as hot upwelling mantle plumes. Resolution of these images has been ever improving with the burgeoning increase in data and computational power. One of the most remarkable recent discoveries has been the ponding of some slabs and mantle plumes around a depth of 1000 km in the mid-mantle. An unanswered question lies here: What is happening at this depth that affects the convective motion?The downside of seismic tomography is that it broadens imaged features and underestimates their true amplitudes. This is partly because the periods of the waves used are relatively long, thus reducing the resolution of the heterogeneity imaged at depth. Here we propose targeted studies using higher-frequency waves than can be incorporated in seismic tomography to image the small-scale heterogeneities around 1000 km in the mid-mantle. Specifically, we will use waves that are reflected or converted by these heterogeneities and therefore have strong sensitivity to the boundaries of these features. The unique sensitivities of the different phases allow us to map the size, shape, velocity contrast, density contrast and sharpness of the anomalous heterogeneities. In a preliminary study using converted seismic waves beneath Europe, we mapped broad patches of heterogeneity consistently at 1000 km depth. We will expand this technique to map these features on a global scale and understand how they relate to the observed slabs and mantle plumes and to what degree they are clustered around 1000 km. Next, we need to target the heterogeneities with a combination of different reflected and converted waves. The questions about the nature and role of these mantle heterogeneities are fundamentally interdisciplinary. We will combine these high-resolution seismological constraints with experiments and calculations of the thermo-elastic behaviour of specific compositions under high pressures and temperatures. In this manner we will test a number of key hypotheses on deep Earth structure: Do the heterogeneities originate from the surface and are they introduced by subducting slabs? Or do they represent primordial material, either brought up from the deeper mantle or stagnating at this depth throughout the history of the Earth? Are there different types of heterogeneities present?By understanding the composition of the observed heterogeneities through targeted deep Earth imaging, we can determine its role in controlling the overall mantle dynamics.