Multi-Phase Flow in High-Temperature Hydrothermal Systems at Oceanic Spreading Centres — A Case Study at the East Pacific Rise

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Hydrothermal systems at oceanic spreading centres are a key component in the Earth’s mass and energy cycles. Hot springs at the seafloor form when seawater enters the oceanic crust and is heated up by magmatic processes. Hot hydrothermal fluids are highly reactive and facilitate a thermal and chemical exchange between oceanic crust and deep sea environment. This exchange provides the nutrition for biological communities, forms economically interesting ore deposits, and contributes to the global carbon cycle.Although submarine hydrothermal systems are investigated for more than four decades, are many of their characteristics still ambiguous. When saltwater is strongly heated, it separates into a high-density, high-salinity brine phase and low-density, low-salinity vapour phase. Measured salinity variations in fluids exiting at hydrothermal vents, especially after heat input from magmatic activity, are the main indicator for these phase transitions and phase separations. The pathways and flow dynamics of these phases within the oceanic crust have not been investigated in a realistic, three-dimensional (3-D) numerical model. Of particular interest is the flow of saline brines, because interactions between fluid and rock depend on salinity (dissolution and precipitation of quartz, transport of metals). It is also unclear, which processes control the location of hydrothermal vent fields at fast spreading centres.To shed light on the hydrothermal processes hidden within the oceanic crust, this project will combine seismic data analysis, numerical modelling, and time series of vent fluid measurements. The central point of the project is a detailed case study targeting the extraordinary well-studied section of the East Pacific Rise at 9º 50’ N, where a high-resolution seismic data set reveals structural details of the oceanic crust (e.g. high-porosity zones that may be hydrothermal fluid pathways). The seismic data also shows characteristics of the underlying magmatic system such as variations in depth and melt fraction along the ridge axis. These seismic information will be considered in a new 3-D numerical model for saltwater hydrothermal systems, which will be developed in the first part of the project. The numerical results will be compared against observations and measurements at the seafloor. The project aims at establishing causal links between vent field distributions, vent fluid temperature and salinity, structure of the underlying oceanic crust, and characteristics of the magmatic system. The expected results will not only reveal the dynamics of submarine hydrothermal systems but also their interaction with magmatic processes and hydrological structures within the oceanic crust.