Detecting melt in the deep mantle with seismic anisotropy and attenuation

融化是地球系统的一个至关重要的组成部分。地壳和上地幔的化学成分是由大洋中脊、热点和岛弧的融化和结晶控制的。板块构造、火山和热流以及许多其他地球过程都受到浅层融化的影响。然而，在地幔的另一端——地核-地幔边界（CMB），熔融可能同样重要。客观地说，这个边界在地球系统中是最重要的，因为它代表了大多数物理性质（例如，温度、密度、化学、粘度）的巨大反差。在最底地幔中存在熔融相将显著影响整个地幔的热力学和化学。熔体的存在会改变地幔底部的粘度，并可能影响地幔柱的形成。它可以为上地幔“丢失”的原始化学成分提供一个持久的隐藏储层。其中包括不相容的放射性元素，这些元素可能会提高地幔底部的温度，改变从核心流出的热流。这将对地球发电机可用的能量产生影响：地核中液态铁的快速对流产生磁场。它还可能隔绝像水和二氧化碳这样的挥发相，改变我们对地表附近丰富的挥发相在深时间里的演化的看法。实验已经为这种熔化阶段提供了貌似合理的解释。其中包括在洋中脊（MORB）形成的玄武岩的融化，这些玄武岩在俯冲板块中下降到核心地幔边界。另一种可能性是，这是整个地球融化时（数十亿年前）遗留下来的。虽然这些已经被实验证明是可能的，但我们希望能够直接观察它们的存在——这是地球科学中长期存在的挑战。地震学提供了对地球最深处的唯一直接探测。最下面的地幔显示出一系列有趣的地震特征，包括地震各向异性（地震波速度随方向的变化）的强烈特征。这通常归因于下地幔矿物的变形，但也可能是由包含熔融相的首选排列引起的。为了区分这两种机制，我们提出了一种新的技术，其中包括另一个参数的测量：地震衰减。我们有一个大的地震波形数据集，这些数据集描绘了世界上最下层的地幔，我们将把我们的新方法应用于这些数据集。这将使我们能够在D的广阔地带测试熔体的存在。”我们的目标是生成融化地图，这将使我们能够评估其对更广泛的地球系统的影响，为地幔底部的动力学和结构提供见解，并探索深度融化的起源。

Melting is a critically important component of the Earth system. The chemistry of the crust and upper mantle is controlled by melting and crystallisation at mid-ocean ridges, hotspots and island arcs. Plate tectonics, volcanos and heat flow, and many other Earth processes are all affected by shallow melting. However, melting may be equally important at the other 'end' of the mantle - the core-mantle boundary (CMB). This boundary is, objectively speaking, the most significant in the Earth system as it represents a huge contrast in most physical properties (e.g., temperature, density, chemistry, viscosity).The presence of a melt phase in the lowermost mantle would significantly affect whole mantle thermodynamics and chemistry. The presence of melt would alter the viscosity at the base of the mantle and could affect the generation of plumes. It could provide a persistent hidden reservoir for primordial chemical components 'missing' from the upper mantle. These include incompatible radiogenic elements which might raise the temperature at the base of the mantle, altering the heat-flow out of the core. This would have consequences for the energy available to power the geodynamo: the rapid convection of liquid iron in the Earth's core which generates its magnetic field. It could also potentially sequester volatile phases like water and CO2, altering our picture of the evolution of the abundance of these near the surface through deep time. Experiments have provided plausible candidates for such a melt phase. These include the melting of basalts that have formed at mid-ocean ridges (MORB) which have descended to the core mantle boundary in subducting slabs. Another possibility is that melt is left over from the time when the entire Earth was molten (billions of years ago). While these have been shown experimentally to be possible, we would like to be able to observe their presence directly - a long standing challenge in the Earth Sciences. Seismology provides the only direct probe of the deepest parts of the Earth. The lowermost mantle shows a range of interesting seismic features, including a strong signature of seismic anisotropy (the variation of seismic wavespeed with direction). This is generally ascribed to the deformation of lower mantle minerals but can also be caused by the preferred alignment of an included melt phase. In order to distinguish between these two mechanisms, we propose a new technique which includes measurements of another parameter: seismic attenuation. We have a large dataset of seismic waveforms which image the lowermost mantle across the world, to which we will apply our new methodology. This will allow us to test for the presence of melt across a broad swath of D''. The map of melt we aim to generate will allow us to assess its effect on the broader Earth system, provide insights into the dynamics and structure of the base of the mantle, and probe the origins of deep melting.