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

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.

The main impact goals of this project are to:
- Develop and publish robust methodologies and associated software and documentation to estimate important geological parameters from geophysical models, including porosity, composition, hydration and temperature of rocks in the subsurface.
- Advance the state of the art in the geophysical imaging and characterization of fluids at depth. The study of fluids in the solid Earth system is of great importance for understanding and estimating geohazards and georesources. The study of hydrothermal fluid flow through fracture networks has direct bearing on the exploitation of geothermal energy and on the identification and extraction of hydrocarbons, therefore my research will impact on the UK's capability to identify and exploit alternative energy sources and keep up with energy demand in a changing World.
- Develop a conceptual model of the accretion, composition and hydration of non-volcanic oceanic lithosphere. The presence of water can drastically affect the mechanical properties of the lithosphere and can have important consequences on the seismogenic behaviour and the potential for large earthquakes, with direct societal and economic implications.
- Advance our understanding of what governs the location, size, vigour and longevity of submarine hydrothermal systems. This is particularly important at present, because of the growing interest in the exploitation of associated sulphide deposits for commercial extraction of valuable metals such as copper, gold, silver and zinc. My research will have implications on the global distribution of hydrothermal vent fields and therefore on the distribution of mineral deposits and on the environments in which they form. Hydrothermal vent fields are also important because they host unique biological communities which have evolved to scrape a living by extracting energy from chemical reactions in extreme temperature and pressure conditions. The study of organisms found on black smoker vent fields has led to the discovery of new chemical compounds with interesting and useful properties for medical applications. By providing new constraints on the evolution and driving mechanism of vent fields my research will help understand the conditions under which these ecosystems become established.

The main non-academic stakeholders who will benefit from my research are:
- Government organizations and research institutions interested in seabed exploration and resources, including the British Geological Survey (BGS) and the Marine Management Organization (MMO).
- Policy stakeholders including the International Seabed Authority, which governs the exploitation of offshore mineral resources.
- The seabed mining industry. This nascent sector includes companies involved in the exploration of seabed mineral resources and developing commercially viable extraction projects. Ensuring that this is done in a way that minimizes the impact on the environment is of great importance.
- The geophysical exploration industry. The ICL FullWave group has strong links with several major companies in the sector.
- The geothermal energy sector.
- Other companies that operate in the World's oceans including related to offshore construction and seabed cable installation;
- Companies offering submersible dives to paying customers. Hydrothermal vents on mid-ocean ridges are a popular destination for deep-sea tourism.
- Associations and organizations interested in the preservation of seabed ecosystems.
- Publics interested in deep-sea ecosystems.