High-resolution seismic constraints to reveal mid-mantle processes

地幔是夹在地壳和地核之间长达2900公里的一层，它的动力学塑造了我们今天所知道的地球表面。例如，地幔中上涌的物质，被称为地幔柱，导致地表局部的、增加的火山活动，形成了今天大多数的海洋岛屿。大陆的起源和早期形成也归因于地幔柱，以及最近被称为“大火成岩省”的大陆大小的火山流出物。深地幔中的过程也被假设控制着地球历史上的板块构造模式和超大陆旋回。我们目前还没有一个完整的地幔动力学历史的图景来解释所有这些观测结果。例如，我们不知道地幔对流是一层还是两层，因此不知道地幔柱是否直接将核心-地幔边界的过程与地表联系起来，也不知道地幔是否随时间混合得很好。我们也不知道深部地幔非均质性的本质，也不知道这如何影响整体动力学。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断层扫描，它利用地震波来绘制地幔中地震速度变化的图像。地震层析成像显示地震波速度快的特征，解释为冷的下降板块，地震波速度慢的特征，解释为热的上涌地幔柱。随着数据和计算能力的迅速增长，这些图像的分辨率一直在不断提高。最近最引人注目的发现之一是在地幔中部约1000公里深处有一些板块和地幔柱。这里有一个悬而未决的问题：在这个深度发生了什么，影响了对流运动？地震层析成像的缺点是它使成像特征变宽，并低估了它们的真实振幅。部分原因是使用的波周期相对较长，从而降低了深度成像的非均质性的分辨率。在这里，我们提出了针对性的研究，使用比地震层析成像更高频的波来成像中地幔约1000公里处的小尺度非均质性。具体来说，我们将使用由这些非均质性反射或转换的波，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出异常非均质性的大小、形状、速度对比、密度对比和清晰度。在一项利用欧洲地下转换地震波的初步研究中，我们在1000公里深度连续绘制了广泛的非均质斑块。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公里附近聚集的程度。接下来，我们需要结合不同的反射波和转换波来瞄准非均匀性。关于这些地幔异质性的性质和作用的问题基本上是跨学科的。我们将把这些高分辨率的地震学约束与高压和高温下特定成分的热弹性行为的实验和计算结合起来。通过这种方式，我们将测试一些关于地球深部结构的关键假设：非均质性是来自地表还是由俯冲板块引入的？或者它们代表了原始物质，或者是从更深的地幔中带出来的，或者在地球历史上一直停滞在这个深度？是否存在不同类型的异质性？通过目标地球深部成像了解观测到的非均质成分，我们可以确定其在控制整体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.