The liquid-liquid phase transition (LLPT) of water - influence on mineral solubility

To understand the chemical and physical processes that occur in subduction zones, as well as during diagenesis and metamorphism in sediments and the deeper crust, an understanding of the composition and behaviour the fluids present is essential. These fluids are dominated by water, the properties of which are dependent upon pressure and temperature, amongst many other factors. Recent findings indicate that a liquid-liquid transition, from a low- to a high-density structure, takes place in water. The pressure and temperature dependency of this transition is uncertain, but appears to be relevant to pore fluids in sediments, hydrothermal fluids in the crust, and may also take place at depth within a subduction zone. This prevents simple extrapolation of properties such as solubility (which determines fluid chemistry) from low to high pressure and temperature conditions. Quantitative observations in situ at high pressure and temperatures must be made. Here, such experimental observations are proposed, using diamond anvil cell techniques coupled with Raman and X-ray spectroscopy, in order to determine the influence of the liquid-liquid transition of water on the properties of water in Earth’s interior. As the LLPT of water is likely to occur at high pressure and high temperature, its influence in hydrous systems towards supercritical conditions such as in metamorphic and melting processes in the Earth's crust and mantle may play an important role in volcanic processes, changing the physical characteristics of the subsurface. For instance, the transport of carbon in subduction zones is driven by fluids thought to be H2O-rich relative to other volatiles. These aqueous fluids liberated during metamorphic devolatilization in subducting lithosphere act as agents of transport of solutes into the overlying wedge of Earth's mantle inducing physico-chemical modifications of slab and wedge. As the most abundant volatile and a very effective solvent, water controls the properties of subduction zones fluids. These are the very fluids that result in the destructive volcanism of subduction arcs such as the Indonesian and Philippine islands. The solvent properties of water depend on the density, structural order, hydrogen bond strength and the dissociation character of H2O molecules which, in turn, is strongly related to the pressure and temperature environmental conditions. As a consequence, the modifications of the physico-chemical properties of water under pressure and temperature conditions relevant to the Earth's lower crust and upper mantle should affect the mechanisms of carbon transport in Earths subduction zones. The knowledge of the solubility of carbonate minerals, especially the most abundant one in sediments, and the speciation of aqueous solutes under pressure and temperature conditions where the LLPT of water has been reported to occur is critical to provide an understanding of the effect of aqueous fluids on the geological processes occurring in the Earth's mantle and overall on the Earth's carbon cycle and help to better constrain the modelling of the Earth's carbon cycle. The proposed research aims to investigate and characterise the influence of the liquid-liquid phase transition of water on the solubility and the speciation of minerals at pressure and temperature conditions relevant to cold subduction zones such as that driving arc volcanism near Singapore. Recent findings indicate that a liquid-liquid transition, from a low- to a high-density structure, takes place in water. The pressure and temperature dependency of this transition is uncertain, but appears to be relevant to pore fluids in sediments, hydrothermal fluids in the crust, and may also take place at depth within a subduction zone. This prevents simple extrapolation of properties such as solubility (which determines fluid chemistry) from low to high pressure and temperature conditions. The proposed research aims to investigate and characterise the influence of the liquid-liquid phase transition of water on the solubility and the speciation of minerals at pressure and temperature conditions relevant to cold subduction zones such as that driving arc volcanism near Singapore. Here, we test the hypothesis that the liquid–liquid phase transition (LLPT) of water influences water–mineral interactions in the near- and sub-surface of Earth. Also, that it prevents simple extrapolation of thermodynamic properties from low- to high-P and T. Using diamond anvil cell techniques, Raman and X-ray spectroscopy, we will determine the influence of this transition on geofluids and the consequent stability of minerals in Earth’s interior. Quantitative observations in situ at high pressure and temperatures will be made. Diamond anvil cell techniques will be coupled with Raman and X-ray spectroscopy, in order to determine the influence of the liquid-liquid transition of water on the properties of water in Earth’s interior. The solvent properties of water depend on the density, structural order, hydrogen bond strength and the dissociation character of H2O molecules which, in turn, is strongly related to the pressure and temperature environmental conditions. As a consequence, the modifications of the physico-chemical properties of water under pressure and temperature conditions relevant to the Earth's lower crust and upper mantle should affect the mechanisms of carbon transport in Earths subduction zones. The knowledge of the solubility of carbonate minerals, especially the most abundant one in sediments, and the speciation of aqueous solutes under pressure and temperature conditions where the LLPT of water has been reported to occur is critical to provide an understanding of the effect of aqueous fluids on the geological processes occurring in the Earth's mantle and overall on the Earth's carbon cycle and help to better constrain the modelling of the Earth's carbon cycle.