High-resolution seismic constraints to reveal mid-mantle processes

地幔的动力学，夹在地壳和核心之间的2900公里层，塑造了我们今天所知的地球表面。例如，地幔中的上升物质，即所谓的地幔热柱，导致地表局部的火山活动增加，形成了今天的大部分海洋岛屿。大陆的形成和早期形成也归因于地幔热柱，以及最近大陆规模的火山流出，被称为“大型火成岩省”。深部地幔中的过程也被假设为控制着板块构造的格局，以及地球历史上的超大陆旋回。我们目前还没有解释所有这些观测到的地幔动力学历史的全貌。例如，我们不知道地幔是以一层还是两层的形式对流，因此，地幔热柱是否直接将核-地幔边界的过程连接到地表，以及随着时间的推移，地幔是否充分混合。我们也不知道深部地幔中非均质性的性质，也不知道这如何影响整体动力学。我们对地球深层结构和动力学的大部分知识来自全球地震层析成像，它使用地震波来拍摄地幔中地震速度变化的图像。地震层析成像图像显示了地震波速较快的特征，解释为冷下沉板块，以及地震波速较慢的特征，解释为热上涌地幔热柱。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中地幔约1000公里深的地方发现了一些板块和地幔羽流。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扩大了成像特征，低估了它们的真实幅度。这在一定程度上是因为所用波的周期相对较长，从而降低了在深度成像的非均质性的分辨率。在这里，我们建议使用比地震层析成像可以并入的更高频率的波进行有针对性的研究，以成像中地幔约1000公里处的小尺度非均质性。具体地说，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。不同相位的独特敏感性使我们能够绘制出异常非均质的大小、形状、速度对比度、密度对比度和锐度。在使用欧洲地下转换的地震波进行的初步研究中，我们绘制了1000公里深度的大片不均匀区域。我们将扩展这项技术，在全球范围内绘制这些特征的地图，并了解它们与观测到的板块和地幔热柱之间的关系，以及它们聚集在1000公里左右的程度。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质性。关于这些地幔不均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地震学约束与特定成分在高压和高温下的热弹性行为的实验和计算结合起来。通过这种方式，我们将检验一些关于地球深部结构的关键假设：非均质性是否起源于地表，它们是通过俯冲板块引入的吗？或者它们代表着原始物质，要么是从更深的地幔中带出来的，要么是在整个地球历史上一直停滞不前的？是否存在不同类型的非均质性？通过目标地球深部成像了解观测到的非均质性的组成，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.