Building a tectonic plate: Water, magma and faults in the oceanic lithosphere

在大洋中脊，构造板块被拉开，地幔缓慢上升，部分熔融，产生岩浆，岩浆上升并凝固，形成新的地壳，即地球的外层低密度层。然而，在一些岩浆供应不足的山脊部分，地幔熔融产生岩浆的效率较低，新地壳的形成跟不上构造板块的拉伸。这里的断层作用将地幔岩石带到地表，并产生了异常的“非火山”海床，其中包含来自地幔的岩石。这些地幔岩石由于与海水接触而发生化学变化，海水会穿透裂缝和裂缝。循环水被岩石的结构吸收，改变了组成它的矿物质。这个过程通常会产生一种叫做蛇纹岩的矿物，因此被称为蛇纹岩化。与此同时，地幔岩石将热量和化学物质传递给热液流体，热液流体被输送到海底，并在热液喷口处逸入海洋。在喷口附近释放的化学物质可能包括贵金属和微量元素，并产生有价值的矿藏。高压、高温和高酸度的极端物理条件，维持着独特的生物群落，这些生物群落被认为是当今最接近导致地球早期生命发展的条件的模拟物。非火山地壳中的热液过程是固体地球与海洋之间能量和化学交换的重要通道，但其深层结构和形成机制尚不清楚。非火山地壳的成分是什么？它在世界海洋中的分布有多广？它能吸收多少水？本项目旨在建立缓慢扩张脊处海洋岩石圈吸积和水化的综合模型，并描述岩浆、断层和热液流体之间的相互作用。我的研究将集中在大西洋中脊的彩虹区域，这是一个构造伸展和岩浆输入在空间上变化迅速的山脊部分，提供了全球海洋中脊系统遇到的不同条件的完整图景。我将使用全波形地震层析成像，这是一种地球物理成像技术，它使用地震振荡的全部记录和联合地球物理反演，来重建海底下岩石属性的详细和完整的表示。我将把这些约束与岩石物理学和机器学习辅助下的自动岩石分类结合起来，以估计成分、孔隙度、熔体含量和水化作用。我的工作将对固体地球和海洋之间的能量和化学交换，以及地球深处化学物质的循环利用产生影响。

At mid-ocean ridges, tectonic plates are pulled apart and the Earth's mantle slowly rises and is partially molten, producing magma that rises and solidifies to form new crust, the outer low-density layer of the Earth. However, on some ridge sections where magma supply is low, the production of magma by mantle melting is less efficient and creation of new crust cannot keep up with the stretching of the tectonic plates. Here faulting brings mantle rocks to the surface and produces anomalous "non-volcanic" seabed, containing rocks from the Earth's mantle. These mantle rocks are chemically modified by contact with sea water penetrating cracks and fractures. Circulating water becomes assimilated in the structure of the rocks, modifying the minerals that compose it. This process often produces a family of minerals called serpentinites and is thus called serpentinization. At the same time the mantle rocks transfer heat and chemicals to the hydrothermal fluids, which are transported to the seabed and escape into the ocean at hydrothermal vent sites.The chemicals released near the vents can include precious metals and trace elements and give rise to valuable mineral deposits. The extreme physical conditions of high pressure, high temperature and high acidity, sustain unique biological communities that are thought to represent the closest present day analogue of the conditions that led to the development of early life on Earth. Hydrothermal processes in non-volcanic crust represent an important gateway for energy and chemical exchange between the solid Earth and the oceans, but its deep structure and formation mechanisms are still poorly understood. What is the composition of non-volcanic crust? How widespread is it in the World's oceans? How much water does it assimilate?This project aims to produce an integrated model of accretion and hydration of the oceanic lithosphere at slow-spreading ridges and to characterize the interaction between magma, faults, and hydrothermal fluids. My study will focus on the Rainbow area of the Mid-Atlantic Ridge, a ridge section where the tectonic stretching and magmatic input vary rapidly in space, providing a complete picture of the different conditions encountered along the global mid-ocean ridge system. I will use full-waveform seismic tomography, a geophysical imaging technique which uses the entire record of the seismic oscillations, and joint geophysical inversion, to reconstruct a detailed and complete representation of the rock properties beneath the seabed. I will combine these constraints with rock physics and automated rock classification aided by machine learning to estimate composition, porosity, melt content and hydration. My work will have implications for the energy and chemical exchange between the solid Earth and the oceans, and for the recycling of chemicals in the deep Earth.