Water in the Earth’s interior

Phase diagram for water substance. image – craigssenseofwonder.wordpress.com

Water at supercritical conditions is a strange beast and has some remarkable chemistry. It is a fluid with properties that are a blend of gas and liquid properties. Steam at supercritical conditions (around 220 – 250 bar and about 600 °C)  is in common use in large power plants since it can be expanded in steam turbines for power generation. It has gas-like properties such that – as an Oxygen carrier – it could even support combustion/oxidation processes. It has liquid like properties and can be used as a solvent.

It would seem that if water is contained in the interior of the earth’s crust it could be at pressures above 22 MPa (220 bar) and temperatures above 374°C, beyond the critical point, and its properties as a very aggressive solvent  could be controlling the behavior of magma. So perhaps plate tectonics is all down to water?

I am a little skeptical since I observe – in passing – that the behaviour of supercritical steam does not seem to dissolve away steam turbine blades or casings when used in power generation!

A new paper on the 

Microscopic structure of water at elevated pressures and temperatures  by C. J. Sahle, C. Sternemann, C. Schmidt, S. Lehtola, S. Jahn, L. Simonelli, S. Huotari, M. Hakala, T. Pylkkanen, A. Nyrow, K. Mende, M. Tolan, K. Hamalainen and M. Wilke.

 Proceedings of the National Academy of Sciences, 2013; DOI: 10.1073/pnas.1220301110

From the press release from the Helmholtz Centre, Potsdam

13.03.2013 | Potsdam: Earth is the only known planet that holds water in massive quantities and in all three phase states. But the earthly, omnipresent compound water has very unusual properties that become particularly evident when subjected to high pressure and high temperatures. In the latest issue of the Proceedings of the National Academy of Science (PNAS), a German-Finnish-French team published what happens when water is subjected to pressure and temperature conditions such as those found in the deep Earth. At pressures above 22 MPa and temperatures above 374°C, beyond the critical point, water turns into a very aggressive solvent, a fact that is crucial for the physical chemistry of Earth’s mantle and crust.

“Without water in Earth’s interior there would be no material cycles and no tectonics. But how the water affects processes in the upper mantle and crust is still subject of intense research”, said Dr. Max Wilke from the GFZ German Research Centre for Geosciences, who carried out the experiments along with his colleague Dr. Christian Schmidt and a team from the TU Dortmund. To this end, the research team brought the water to the laboratory. First, the microscopic structure of water as a function of pressure and temperature was studied by means of X-ray Raman scattering. For that purpose, diamond anvil cells of the GFZ were used at the European Synchrotron Radiation Facility ESRF in Grenoble. Inside the cell, a very small sample of water samples was enclosed, heated and brought to high temperatures and pressures. The data analysis was based on molecular dynamics simulations by the GFZ scientist Dr. Sandro Jahn.

“The study shows that the structure of water continuously develops from an ordered, polymerized structure to a disordered, marginally polymerized structure at supercritical conditions,” explains Max Wilke. “The knowledge of these structural properties of water in the deep earth is an important basis for the understanding of chemical distribution processes during metamorphic and magmatic processes.” This study provides an improved estimate of the behavior of water under extreme conditions during geochemical and geological processes. It is believed that the unique properties of supercritical water also control the behavior of magma.

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