Clay mineral waters Earth's mantle from the inside - Dr. Yongjae Lee

High-pressure X-ray measurements provide new insights into processes that cause seismicity and volcanism

NOVEMBER 21, 2017

The first observation of a super-hydrated phase of the clay mineral kaolinite could improve our understanding of processes that lead to volcanism and affect earthquakes. In high-pressure and high-temperature X-ray measurements that were jointly conducted at SSRF in China, SSRL and APS in USA, DESY in Germany, and PAL in Korea, scientists 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. An international team led by Dr. Yongjae Lee, staff scientist at HPSTAR in China and professor at Yonsei University in the Republic of Korea and, presents the results in the scientific journal “Nature Geoscience”.

In a subduction zone, a heavy oceanic plate meets a second, lighter continental plate and moves under it and into the 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 in the subduction zone increase and the minerals become instable, break down and transform into new compounds. During the 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.”, explains Yongyae Lee 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 kilometres under the Earth’s surface, it is impossible to observe them directly. One way to learn more about the transformations in subduction zones is to create similar conditions in the laboratory. High-pressure and high-temperature experiments 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 the oceanic sediments. The scientists were now able to observe 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 synchrotron X-ray diffraction and infrared spectra measurements, structural and chemical changes were characterized.

At a pressure of 2.7 Giga-Pascal (GPa), more than 25,000 times the average pressure at sea level, and a temperature of 200 degrees Celsius, the super-hydrated phase was observed. These conditions are present at a depth of about 75 kilometres in subduction zones, which coincides with the boundary between shallow and intermediate earthquakes. In the new phase, water molecules are enclosed between the layers of the mineral. The super-hydrated kaolinite contains more water than any other known aluminosilicate mineral in the mantle. “During the formation of the super-hydrated kaolinite, significant amount of water will be removed at the interface between subducting and overriding slabs“ explains Dr. Ho-Kwang Mao, the Director of HPSTAR who supported this work as one of the co-authors. “This will affect the frictional properties of the slabs and hence the mechanism how earthquakes occur along the subduction zones will change“.

Caption: Ordinary kaolinite under an electron microscope. Credit: Yongyae Lee

Lee’s team spent about 2 years to map the stability field of the super-hydrated kaolinite under various subduction zone conditions using five different synchrotron radiation facilities all around the world. This includes BL14B1 at SSRF in Shanghai, China. “We plan to develop a collaboration program to extend the pressure and temperature conditions further deep into the Earth“ says Dr. Wen Wen of SSRF who supported part of this high-pressure synchrotron experiments. The super-hydrated kaolinite broke down at 5 Giga-Pascal and 500 degrees, and two additional transformations happened at higher pressures and temperatures upto 19 Giga-Pascal and 800 degrees Celsius. During these transformations, the water that was intercalated in the kaolinite is released.

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 kilometres to 480 kilometres in subduction zones. Formation of the super-hydrated kaolinite could influence seismicity and mechanism of the earthquakes. Additionally, the release of water that happens 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 kilometres, the released water could then contribute to the formation of magma. The scientists assume that other minerals in the sediment or crust could undergo similar transformations. Thus, the study could improve the understanding of the geochemical processes in subduction zones.