The First Steps When Water Meets Rock

A new study uses computer simulations and spectroscopic techniques to look at how water molecules interact with the mantle mineral olivine.

When water encounters certain magnesium- and iron-rich mantle minerals, exciting things can happen. This process, called serpentinization, creates hydrogen and, at the seafloor, can lead to the formation of hydrocarbons and small organic compounds. The products of serpentinization support hydrothermal vent ecosystems and may have created conditions permissive for the origin of life. Naturally, scientists are interested in learning the finer details of how this process occurs to get closer to defining when and how life began.   

DCO members Tingting Liu, Siddharth Gautam, David Cole (all at Ohio State University, USA), and Eugene Mamantov (Oak Ridge National Laboratory, USA) performed computer simulations and spectroscopic analyses of fine-scale interactions between water and a type of olivine, a major rock-forming mineral in the upper mantle. In a new paper in Physical Chemistry Chemical Physics [1], they report that water forms three distinct layers when it comes in contact with the olivine. The simulations and measurements complement each other, improving our understanding of the structure and dynamics of water-olivine interactions, and lay a foundation for future research on serpentinization.

“Olivine is one of the most widespread silicates on Earth. Its interaction with water in nature is ubiquitous,” said Liu. “This work is the first step to understand the mechanism of adsorption and water dynamics on the surface of olivine. It describes the first step in the serpentinization process.” 

The researchers used a computer simulation method called molecular dynamics to model how water first interacts with a form of olivine called forsterite. Molecular dynamics provides a way to study the physical movements of molecules in a system. 

Liu et al
Researchers used a combination of QENS experiments and molecular dynamics simulations to quantify the structure and dynamics of water on olivine’s surface. Water molecules (green) form three distinct layers on the surface of olivine (red, gold, and blue). Credit: Reproduced from Liu et al., with permission from the Royal Society of Chemistry.

The simulations predicted that the water forms three distinct layers as it encounters the forsterite surface. The water molecules closest to the forsterite surface stick, creating an extremely dense and almost immobile layer. The second and third layers of water molecules are less dense, and molecules can move between the two layers. They found that the top layer contributes the major mobility of the water-olivine interface.  

For these analyses, the researchers used computing resources provided by the Deep Carbon Observatory, hosted by the Data Science Team (PI: Peter Fox) at Rensselaer Polytechnic Institute. “This study would not have happened at all without them,” said Liu.

To detect the mobility of water molecules on the forsterite surface, the researchers employed quasi-elastic neutron scattering (QENS). In QENS, researchers shoot a beam of neutrons at a sample and then detect the neutrons scattered by the sample. The scattering event encodes information about the sample. They used nanopowdered forsterite synthesized by colleagues at Oak Ridge National Laboratory and hydrated with water vapor. The researchers repeated this experiment on three different neutron scattering instruments, so that they could detect the water’s dynamics over timescales of 1 picosecond to 1 nanosecond.

This work is part of Liu's doctoral dissertation to provide “a complete picture of water-olivine interaction,” and to inform future research on serpentinization. The current study is especially relevant to understanding the capacity of basalt minerals at the seafloor to sequester carbon dioxide from the surface and the formation of hydrocarbons through chemical reactions independent of life, in submarine environments. Delineating the structure and dynamics of water-rock interactions is vital for understanding these and other chemical reactions and transport processes that shape Earth’s subsurface.
 

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