The first observation of a super-hydrated phase of the clay mineral kaolinite could improve the understanding of processes that lead to volcanism and affect earthquakes.
In high-pressure and high-temperature X-ray measurements, Lawrence Livermore National Laboratory scientist Hyunchae Cynn and colleagues from Yonsei University in the Republic of Korea, Deutsches Elektronen-Synchrotron (DESY) in Germany, Carnegie Institution of Washington, the George Washington University, SLAC National Accelerator Laboratory and the University of South Carolina created conditions similar to those in so-called subduction zones, where an oceanic plate dives under the continental crust. The transport and release of water during subduction causes strong volcanic activity.
The research appears in Nature Geoscience.
In a subduction zone, a heavy oceanic plate meets a second, lighter continental plate and moves under it and into Earth’s mantle. With the oceanic plate, water enters the earth as it is trapped in minerals of the oceanic crust or overlaying sediments. These minerals slowly sink deeper into the mantle over millions of years. With increasing depth, temperature and pressure, the minerals become unstable, break down and transform into new compounds.
During these transformations, water is released and rises into the surrounding, hotter mantle where it decreases the melting temperature of the mantle rock. "When the mantle rocks melt, magma is generated. This can lead to volcanic activity when the magma rises to the surface," said Yongjae Lee, from Yonsei University, who led the study. "While we know that the water cycle in subduction zones influences volcanism and possibly seismicity, we don’t know much about the processes that form this cycle."
Since these processes take place many kilometers under Earth’s surface, it is impossible to observe them directly. One way to learn more about the transformations in greater depths of subduction zones is to create similar conditions in the laboratory. High-pressure and high-temperature measurements allow scientists to take a close look at the structural changes in the different minerals that form the crust and sediments.
One of these minerals is kaolinite, a clay mineral containing aluminium that is an important part of oceanic sediments. The scientists observeed the formation of a new phase of the mineral, so-called super-hydrated kaolinite. They examined a sample of kaolinite in the presence of water at pressures and temperatures corresponding to those at different depths in subduction zones. With X-ray diffraction and infrared spectra measurements, structural and chemical changes were characterized. The super-hydrated kaolinite contains more water than any other known aluminosilicate mineral in the mantle. When pressure and temperature sink back to ambient conditions, the structure reverts to its original form.
The observation of the formation and breakdown of the super-hydrated kaolinite bears important information about the processes that occur over a depth range of about 75-480 kilometers in subduction zones. The release of water that takes place when the super-hydrated kaolinite breaks down could be an important part of the water cycle that causes volcanism along subduction zones. The breakdown probably happens below a depth of about 200 kilometers, the released water could then contribute to the formation of magma. The study could improve the understanding of the geochemical processes in subduction zones of Earth.
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