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受洋中脊、热点和岛弧的熔融和结晶作用控制。板块构造、火山和热流以及许多其他地球过程都受到浅层熔融的影响。然而，熔融在地幔的另一端--核幔边界（CMB）可能同样重要。客观地说，这个边界是地球系统中最重要的，因为它代表了大多数物理性质的巨大对比（例如，在最低地幔中熔融相的存在将显著地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，并可能影响羽流的生成。它可以为上地幔“缺失”的原始化学成分提供一个持久的隐藏库。这些因素包括不相容的放射性元素，它们可能会提高地幔底部的温度，改变核心的热流。这将对地球发电机的可用能量产生影响：地球核心中液态铁的快速对流产生了磁场。它还可能隔离水和二氧化碳等挥发性相，改变我们对这些物质在地表附近的丰度在深部时间的演变的看法。实验已经为这种熔融相提供了合理的候选者。这些包括在洋中脊（MORB）形成的玄武岩的熔融，这些玄武岩在俯冲板块中下降到核幔边界。另一种可能性是，熔融物是整个地球熔融时（数十亿年前）留下的。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-这是地球科学中的一个长期挑战。地震学提供了对地球最深处的唯一直接探测。最低地幔显示出一系列有趣的地震特征，包括强烈的地震各向异性特征（地震波速度随方向的变化）。这通常归因于下地幔矿物的变形，但也可能是由所包含的熔体相的优选排列引起的。为了区分这两种机制，我们提出了一种新的技术，其中包括测量的另一个参数：地震衰减。我们有一个庞大的地震波形数据集，可以对世界各地的最低地幔进行成像，我们将应用我们的新方法。这将使我们能够测试在D“的一个广泛的条带上是否存在熔体。我们旨在生成的熔融地图将使我们能够评估其对更广泛的地球系统的影响，深入了解地幔底部的动力学和结构，并探索深熔的起源。

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.