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Probing the strange properties of hydrothermal water

Deep beneath our feet lies a hidden world where water is transformed into a superheated fluid that strips the very living rock of its chemicals and minerals. These hot, mineral-rich liquids are known as hydrothermal fluids and they are involved in many of the planet’s geological processes. A team of scientists using ISIS Neutron and Muon Source (ISIS) are investigating these fluids in the hope that as well as shedding light on the mechanisms that power the planet, we might learn how to exploit them in the fight against climate change.

Hydrothermal water can come from several sources. It can be released from molten rock (magma) as it cools and forms rocks undergoing metamorphism (changing from one type of rock to another due to pressure, temperature or chemical influence). Seawater can be sucked underground at geological faults, such as subduction zones, where the ocean floor is dragged into the Earth’s interior, or it can percolate down from the surface as rainwater, flowing through cracks and fissures in the rock.

Whatever its origin, once it is deep enough and has been subjected to high enough levels of heat and pressure, the benign substance we are familiar with here on the surface is transformed into an aggressive, chemically-reactive fluid that can dissolve crystalline rocks (which are insoluble at surface pressure) and turn into a sort of super-hot chemical soup.

At super-high temperatures, liquids such as water would usually become a gas (and simply boil away), but the super-high pressure present as such depth prevents this from happening. As such, the water is transformed into something called a ‘supercritical’ fluid that enjoys all the freedom of movement of gas, but still dissolves materials like a fluid.

These supercritical properties mean that hydrothermal fluids can travel through porous rocks and through the narrowest of fissures as if they weren’t there, which makes them excellent transport mechanisms for minerals. As they travel upwards towards the surface, they cool and the pressure decreases until the minerals they are transporting come out of solution and are deposited as veins of crystalline rock that are often chemically different to the rock that was dissolved originally.

On the surface, the results of these mineral relocations can sometimes be seen as veins of minerals or metals running through rocks.

Although the existence of hydrothermal fluids has been known about for decades, there is not much understanding of the chemical processes and mechanisms that occur as the fluid transitions through scavenging elements from the rock to become a solution and then depositing them. After all, you can’t make measurements in-situ because they take place tens or even hundreds of kilometres underground, which means there is an awful lot of rock in the way.

Therefore they can only be studied in the lab, but to do so you must first replicate the conditions of extreme heat and pressure present underground. Recent developments in high pressure and high-temperature experimental techniques at ISIS Neutron and Muon Source (ISIS) are finally making this possible through the use of the PEARL instrument, which can simulate the heat and pressure present at depth.

The hurdle to overcome is actually seeing what is going on – you can’t just pop open the lid to have a look because you’ll immediately release the heat and pressure and thus change the outcome, which is where ISIS’ neutrons come in. Because neutrons don’t carry an electric charge, they are able to pass through the experiment set-up, interact with the atoms within and measure the reactions, and then pass out to sensors and record any changes as they happen.

Previous experiments have really only been able to look at what hydrodynamic fluids look like before heating and after cooling, but this hasn’t revealed much about the structural changes that take place between the two phases. There has been a lot of thermodynamic information and computer modelling but no direct observation.

The experiments taking place at ISIS are the result of a collaboration, led by Dr Martin Wilding (now at the University of Manchester), between the beamline scientists at the PEARL instrument and Sheffield Hallam University. Their goal is to use neutrons to see what is happening to the structure at the various phase transition points – before solubility, during (while in solution), as it comes out of solution, and the final structure of the deposited material.

The team designed an assembly into which is placed a small capsule of ‘starter’ material made up of quartz (a mineral composed of silicon and oxygen atoms) and a type of ‘heavy water’ containing deuterium instead of hydrogen. Heavy water (D2O) is used in place of H2O because neutrons interact more incoherently with the hydrogen atoms in normal water, which would create too much signal ‘noise’ to see what is going on. The sample is welded within a capsule of non-reactive noble metals – the weld is necessary because the supercritical fluid will exploit even the tiniest gap and escape, and the capsule must be non-reactive to prevent any chemical reaction with the highly aggressive fluid. The capsule is then subjected to high pressure and heat while neutrons probe the resulting chemical reactions.

The experiment, which took place over a period of 48 hours, simulates the conditions that are present at a depth of around 30-50km, which, in geological terms, isn’t very deep, but is still within the range where many important geochemical processes take place.

During the course of the experiment, neutrons are fed into the sample and, as they pass out, they create a diffraction pattern that allows the team to watch the solution change in real-time. Before the melt, there will be peaks in the diffraction pattern that represent the quartz crystalline structure. Then, as it melts and the quartz passes into solution, the peaks disappear. Finally, as it cools and comes out of solution, the diffraction pattern will be replaced by new peaks that represent the new crystalline element. This allows them to not only see what goes in and what comes out but also at what pressure and at what temperature the various changes take place.

After a recent successful run of the experiment, the team now has to extract the data – a process made difficult because, during the liquid phase, there is a great of background noise that must be filtered out before the relevant peaks can be revealed. Once these lengthy processes are complete, the data can then be interpreted.

The team hopes that the data will allow the development of structural computer models that, until now have been based on simulations and not experimental data. The goal is to create a robust model of how water and silicates mix at temperatures and pressures, which will form a basis for understanding how hydrodynamic fluids interact at a chemical and atomic level.

In future experiments, the team will aim to see what happens when the solution is cooled at different rates to see whether faster or slower cooling affects the structure of the deposited crystal.

It might seem like an awful lot of work just to figure out something that happens so far out of sight and mind, but the experiments could have some real-world applications in areas such as carbon sequestration. Because geothermal water can carry extremely high concentrations of carbon dioxide, it could be used to transport the greenhouse gas deep underground into rock strata where it will be deposited as carbonates and carbon-bearing minerals – effectively locking it harmlessly away and preventing it from reaching the atmosphere.

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Last updated: 12 August 2019


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