Detecting melt in the deep mantle with seismic anisotropy and attenuation

利用地震各向异性和衰减探测深部地幔中的熔体

基本信息

批准号：
NE/S010203/1
负责人：
James Wookey
金额：
$ 51.99万
依托单位：
University of Bristol
依托单位国家：
英国
项目类别：
Research Grant
财政年份：
2019
资助国家：
英国
起止时间：
2019 至无数据
项目状态：
已结题

来源：
https://gtr.ukri.org/projects?ref=NE%2FS010203%2F1
关键词：
Detecting melt deep mantle seismic

项目摘要

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.

融化是地球系统极其重要的组成部分。地壳和上地幔的化学成分受到洋中脊、热点和岛弧的熔化和结晶的控制。板块构造、火山和热流以及许多其他地球过程都受到浅层融化的影响。然而，熔化在地幔的另一“端”——核心-地幔边界（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特性（例如温度、密度、化学、粘度）的巨大反差。最下地幔中熔融相的存在将显着影响整个地球系统。地幔热力学和化学。熔体的存在会改变地幔底部的粘度，并可能影响羽流的产生。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储存库。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，从而改变从地核流出的热流。这将对地球发电机的可用能量产生影响：地核中液态铁的快速对流产生磁场。它还可能封存水和二氧化碳等挥发性相，改变我们对地表附近这些挥发性相在深层时间演化的看法。实验为这种熔化相提供了可能的候选物。其中包括在大洋中脊（MORB）形成的玄武岩的熔化，这些玄武岩已经下降到俯冲板片中的核心地幔边界。另一种可能性是，熔化物是整个地球熔化时（数十亿年前）留下的。虽然这些已经通过实验证明是可能的，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，但也可能是由所含熔体相的优先排列引起的。为了区分这两种机制，我们提出了一种新技术，其中包括另一个参数的测量：地震衰减。我们拥有一个大型地震波形数据集，可以对世界上最下地幔进行成像，我们将对其应用我们的新方法。这将使我们能够测试大范围的 D'' 区域是否存在融化。我们旨在生成的熔化图将使我们能够评估其对更广泛的地球系统的影响，提供对地幔底部的动力学和结构的见解，并探讨深层熔化的起源。