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

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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.