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Metamorphic reactions govern the evolution of the Earth’s crust through a range of tectonic movements, most of which are influenced by thermodynamic contributions. However, there is evidence that disequilibrium textures on the sub-millimeter scale in crustal rocks leads to a kinetic hindrance, and often full impedance, which prevents the thermodynamic equilibrium from being reached. Now a team of Researchers from the UK have employed time-resolved (4D) synchrotron X-ray microtomography to image a complete metamorphic reaction in an effort to show how chemical transport evolves during the course of a reaction.
Thermodynamic processes govern how the Earth’s tectonic systems work, with the systems possessing a tendency towards a state of minimum energy and equilibrium within their environments. As an example, the process of hydrous mineral dehydration in a subducting plate, can produce an overpressure in the slab fluid and trigger an earthquake.
Another scenario involves the mechanisms of chemical transport, including water expulsion, that dictate the rate of transformation and the presence of physical properties, such as fluid pressure. These are a couple of examples where metamorphic reactions have an impact, and until now, the direct observation of these (and other) metamorphic processes has not been possible, be it in nature or in a controlled experiment. Such uncertainties in important processes have limited Scientists' understanding of the involved reaction pathways, in which the kinetic controls of a reaction are dependent upon.
As such, the reaction pathway knowledge to date has been limited to disequilibrium textures, and only because these textures are still observable in these processes after the reaction has occurred. Such textures include mineral zoning, coexisting polymorphs and reaction rims, all of which occur on the sub-millimeter scale.
The team of UK Researchers have now used 4D synchrotron X-ray microtomography to image to show how chemical transport evolves during reaction in real-time. The reaction of choice was the dehydration of gypsum to form bassanite and water.
This approach allowed for the direct investigation of the metamorphism by allowing microstructural and mineralogical information to be gathered on the micron scale as the reaction proceeded.
The Researchers undertook a confined heating experiment to investigate the conversion of gypsum to bassanite. The Researchers who performed the research used the microtomography beamline 2BM at the Advanced Photon Source (USA), using an X-ray transparent hydrothermal cell, whilst documenting the 3D X-ray microtomographic time series data, i.e. 4D data. A Cooke pco.edge sCMOS camera with 2560 × 2160 pixels (pixel size 6. 5× 6.5 μm2) was used to image the metamorphic reactions.
The gypsum reaction was chosen as it is a suitable fit for the beamline, allowing it to be completed in hours, rather than years which some silicate experiments would take. The reaction was also performed at 100 °C, much lower than would be required for stronger minerals, but for the weaker gypsum structure it provided the ideal environment.
The 3D microtomographic datasets of the entire sample were acquired at 15 minute intervals and possessed a voxel size of 1.3 µm. This was found to be sufficient to image, in detail, the growth of the grains. The absorption contrast allowed for the growing pore spaces to be segmented from the solid phase(s).
In a similar manner to most dehydration reactions, the process produced a solid molar volume reduction which led to the formation of pores – 29% porosity when fully dehydrated. The pores surrounded the newly formed bassanite grains, which in turn also produced fluid-filled moats. Across these moats, the transport and diffusion of dissolved ions was found to be the cause of grain growth, and as the moat grew in size, both the diffusion and reactions rates slowed down.
However, the reaction also produced a net volume increase due to the formation of water molecules in the reaction, and produced a greater volume than was available in the pore space. To counteract this, water removal was required, but this caused the fluid pressure to increase and slowed the reaction down. This showed that there was a relationship between the reaction mechanisms and the fluid expulsion.
Another approach using 2 x 5 mm cylindrical samples were also tested. The samples were exposed to 9 MPa of confining pressure, 4 MPa pore fluid pressure at 115 °C for 9 hours. The pressure difference was found to be small with no microscopical differential stresses present. However, anisotropic grain-scale stresses did arise when the two pressures were not equal, but the low effective pressure in the reaction prevented pore collapse through compaction.
The experiment has shown an end member condition, where in a natural setting, compaction could occur if the excess fluid is allowed to drain, thus, creating an increase in the effective pressure of the reacting rocks.
In terms of real-world implications, the Researchers have shown that solid volume changes occur due to the dehydration process, which in turn creates the pathway for the facilitation of both hydrous and dissolved molecules through mass transfer processes. The created pores allow the water molecules to escape early on in the reaction, whilst generating a diffusion gradient in which the dissolved solute molecules migrate towards the growing grains.
The Researchers have shown that the reaction rate can be controlled by the fluid pressure and the diffusion of dissolved solute molecules, which is determined by the hydraulic properties of the rock. Also, the research has deduced that if a transient porosity is maintained throughout the reaction, then the early removal of fluids is likely to induce seismic activity.
Overall, the Researchers have provided a new route to understanding the chemical transport mechanism in these reactions, with the potential of future work providing a fundamental understanding of how the hydraulic and chemical evolution of a natural dehydrating system occurs.
Source:
“A 4D view on the evolution of metamorphic dehydration reactions”- Bedford J., et al, Scientific Reports, 2017, DOI:10.1038/s41598-017-07160-5
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Semnic/Shutterstock.com
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