Dr I-Ming Chou – Building Containers for Extreme Geological Fluids

Earth’s crust plays host to many different fluids, which are often pressurised and heated to extreme degrees by the geological processes taking place around them. Harnessing his previous experience at the US Geological Survey, Dr I-Ming Chou at the Chinese Academy of Sciences designs vessels suitable for containing these fluids, while also enabling researchers to easily measure them in the lab using advanced optical techniques. Through their work, Dr Chou’s team has presented cutting-edge designs based on fused silica capillary tubes. Their designs could soon transform geologists’ understanding of the chemical processes taking place far below us.

Underground Fluids

Deep underneath the Earth’s surface, immense geological forces can elevate fluids trapped in the crust to extreme temperatures and pressures. Under the right conditions, we can see these superheated fluids as they are ejected all the way to the surface, in dramatic natural features such as geysers and hydrothermal vents on the seafloor.

By studying the unique properties and compositions of these fluids, geologists can learn much about the physical characteristics of the crust, and the chemical reactions that shape its composition. However, while these samples can be studied in their natural state by drilling holes deep into Earth’s crust, this is a costly and time-consuming process.

Instead, researchers today are aiming to recreate geological fluids in the lab directly. This poses a significant challenge: without highly advanced equipment capable of containing substances under such extreme pressure-temperature conditions, they will rapidly cool down and lose pressure. Dr I-Ming Chou at the Chinese Academy of Sciences aims to overcome this problem by developing cutting-edge new containers for measuring geological fluids. His team’s designs are now offering realistic new glimpses of the processes playing out far beneath our feet.

Investigations with Raman Spectroscopy

A key feature of geological fluids is the presence of both organic molecules such as methane (CH₄), and inorganic substances such as hydrogen sulphide (H₂S), contained within them. At extreme temperatures and pressures, these substances can dissolve and diffuse through water in characteristic ways, which are thought to play important roles in geological processes, as well as the chemical composition of Earth’s crust.

Researchers observe and study these unique processes through a technique named ‘Raman spectroscopy’. This method relies on the fact that the relative positions of a molecule’s constituent atoms are never static; rather, depending on the molecule’s composition, they will vibrate and rotate in characteristic ways.

When the molecule is exposed to the uniform photons produced by a laser, they will initially interact with these vibrations, either losing or gaining energy in the process. Afterwards, the photons are re-emitted with new energies and directions, in an overall process named ‘Raman scattering’. Crucially, the shifts in energy experienced by the photons are unique to the molecule they were scattered from.

So, by detecting these re-emitted photons, researchers can identify molecules by their specific Raman scattering fingerprints. However, even for the most advanced Raman spectroscopy techniques for studying geological fluids at elevated pressures and temperatures, suitable containers are still needed.

Earth’s interior structure, showing pockets of geological fluids.

Building Optical Cells

Until now, lab-based studies have largely relied on devices named ‘diamond anvil’ cells, which exploit the extreme hardness of diamond to compress samples to immense pressures. For fluids, however, this technique faces several shortcomings: including small sample sizes, an inability to directly measure or change sample pressures, and difficulties with loading gas samples onto the cells. In addition, diamond can reduce and interfere with any Raman signals passing through.

To perform Raman spectroscopy effectively, cells containing the fluids must be exceptionally robust – but must also incorporate transparent windows to let laser light in, and scattered photons out. Without extreme precautions, these windows would simply weaken the cells, allowing the pressurised, superheated liquids inside to break out.

In their earlier research, Dr Chou and his colleagues addressed the problem using a glass-like material named fused silica. Containing silicon dioxide (SiO₂) arranged in irregular patterns, this material is both incredibly pure and transparent, and able to withstand considerable temperatures and pressures. Through subsequent work, the researchers have now developed two types of cells that incorporate fused silica, named the ‘high-pressure optical cell’ (HPOC) and the ‘fused silica capillary capsule’ (FSCC), finally allowing them to carry out Raman spectroscopy on geological fluids.

The High-pressure Optical Cell

The first device developed by Dr Chou’s team contained flexible capillary tubes with either square or round cross sections, each made from fused silica. The use of the square tube minimised any optical distortion, enabling better visual observations. Measuring 300 micrometres across, the tubes were sealed at one end, and connected to a high-pressure valve at the other.

To strengthen the tubes, the manufacturers coated them with a thin layer of heat-resistant polymer, a small section of which was removed by the researchers to provide a clear optical window. In previous studies, high-pressure capillary tube setups were inserted into a temperature-controlling device, with a glass cover for both temperature control and protection. In contrast, the team’s new structures were enclosed in a larger capillary tube of stainless steel, with a small part ground away to expose the optical window. The resulting HPOC has numerous advantages over any predecessors, including the diamond anvil cell.

Firstly, the HPOC allowed the researchers to directly load their sample fluids, and continuously monitor their temperatures and pressures in extended experiments. It also enabled them to perform Raman spectroscopy with very little attenuation or interference in the light passing through the transparent window – allowing for precise measurements of their energy shifts. The small volumes of the cells made them suitable for studying rarer samples, which are in limited supply, as well as poisonous samples, such as hydrogen sulphide and carbon monoxide.

Furthermore, pressures inside the capillary tube could be elevated to as high as 160 MPa – a thousand times larger than the pressure exerted by the atmosphere at Earth’s surface. Finally, the fused silica walls of the tube meant that the device as a whole was inexpensive, inert to sulphur, and highly permeable to hydrogen – paving the way for some advanced experimental capabilities, such as controlling the redox state of the sample by imposing hydrogen pressure externally through diffusion.

In their subsequent experiments, Dr Chou’s team prepared a sample of room-temperature water in the capillary tube, and then introduced a further sample of highly pressurised methane gas. In the hours that followed, the HPOC allowed them to directly observe how methane molecules passed across the gas-water interface, and diffused towards the end of the tube over time. Using Raman spectroscopy, they could then determine the diffusion rate of methane in water by measuring the concentrations of dissolved methane at different sample positions at various time. Further experiments explored other aspects of the interaction between water and organic molecules – including the solubility of methane hydrate.

From these results, Dr Chou and his colleagues showed how the capabilities of the HPOC could be extended to study processes involving larger organic molecules, including ethane, propane and butane – as well as salt solutions instead of pure water. In addition, they showed how Raman spectra could be collected across wide ranges of fixed pressures and temperatures. The successes of these early experiments have now set the stage for more complex analysis – involving fluids and mixtures that more closely resemble real geological environments.

Diagram showing Dr Chou’s experimental setup. The lower part shows a prepared HPOC containing sample solution, mercury, and water as a pressure medium.

The Fused Silica Capillary Capsule

Alongside the HPOC, the second type of cell developed by Dr Chou’s team is designed for studying ‘fluid inclusions’. These microscopic bubbles of gas and liquid trapped inside crystals contain a variety of both organic and inorganic materials. They are now thought to play a key role in many geological processes – including the healing of fractured minerals, which requires fluid inclusions to be trapped in water. Previously, they could only be studied by synthesising fluid inclusions within fractures in minerals, such as quartz, garnet, fluorite, calcite, and halite. However, any subsequent experiments involving Raman spectroscopy were difficult to carry out at the pressures and temperatures required during synthesis.

In the researchers’ FSCC device, samples can be studied by first loading them into a fused silica capillary tube, which is 6 centimetres long and has one end sealed. Then the tube is centrifuged, forcing the samples to its closed end. Afterwards, the open end of the tube is connected to a vacuum pressure line, through which any air in the sample is evacuated. Gaseous samples are then added by immersing the sealed end of the tube in ultra-cold liquid nitrogen, and switching the pressure tube from a vacuum to a sample gas – which is allowed to either condense or freeze over the next several minutes. Finally, the tube is evacuated, and its open end is sealed while frozen and under vacuum.

This setup had several unique advantages for studying fluid inclusions, including the formation of large and uniform gas bubbles, which are still able to tolerate high internal pressures. In addition, since hydrogen can readily pass through the disordered molecular structure of fused silica, researchers can readily initiate useful redox reactions from outside the capsule, without otherwise disturbing the sample.

The further capabilities enabled by the team’s FSCC now open up a whole new range of intriguing geological experiments. Raman spectroscopy of fluid inclusions in the lab will be particularly useful for studying reactions between individual hydrocarbons, crude oils containing mixtures of hydrocarbons, and inorganic gases. One such process is ‘cracking’, where organic molecules containing long chains of carbon atoms break down into smaller chains when under thermal stress, altering the composition of organic material in Earth’s crust.

In the case of inorganic samples, FSCCs could be used to study the formation of ions containing uranium, within solutions of lithium chloride. This process is known to take place at extreme pressures in the crust, forming the ores that we use to source uranium for nuclear power stations – as studied by a team led by Jean Dubessy at the University of Lorraine in France. In addition, research headed by Dr Chou showed how hydrothermal circulation within the newly-formed oceanic crust is influenced by the separation of sulphate-bearing seawater – with important consequences for the transport of heat, water, and chemicals throughout Earth’s oceans and atmosphere.

Elsewhere, Dr Chou’s team showed that their capsules could be applied in calibration for thermocouples, which are a key element of the heating and cooling stages required for microthermometric and Raman spectroscopic analyses of fluid inclusions.

Improving Understanding of Geological Fluids

Over the past three decades, Raman spectroscopy has already contributed significantly to our understanding of the Earth’s composition, and the materials involved in its geological processes. Through the use of HPOCs and FSCCs, the capabilities of the technique could soon be enhanced further still, through better analysis of minerals, organic materials, silicate melts, and water-based solutions in the crust.

With transparent windows that are resilient to extreme temperatures and pressures, stable in highly acidic conditions, and inert to reactive substances such as sulphur, Dr Chou’s team hopes that their approach will soon enable experiments involving many different chemical systems, which more closely simulate the conditions present deep within Earth’s crust. With further improvements, it could even lead to cutting-edge simulations of extraterrestrial ocean waters, such as those on Europa or Enceladus.