5.4 LEE Yongjae

A role for subducted super-hydrated kaolinite in the Earth’s deep water cycle

Huijeong Hwang1, Donghoon Seoung1,2, Yongjae Lee1,3,*, Zhenxian Liu4, Hanns-Peter Liermann5, Hyunchae Cynn6, Thomas Vogt7, Chi-Chang Kao2, Ho-Kwang Mao3,8

1Department of Earth System Sciences, Yonsei University, Seoul 03722, Republic of Korea; 2Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA; 3Center for High Pressure Science & Technology Advanced Research (HPSTAR), Shanghai 201203, China; 4Department of Civil and Environmental Engineering, The George Washington University, Washington, DC 20052, USA; 5Photon Sciences, Deutsches Elektronen-Synchrotron (DESY), Notkestraße 85, 22607 Hamburg, Germany; 6High-Pressure Physics Group, Physics and Life Sciences, Lawrence Livermore National Laboratory, Livermore, CA 94550 USA; 7NanoCenter & Department of Chemistry and Biochemistry, University of South Carolina, SC 29208, USA 8Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015, USA; (Present affiliation of D. S. : Department of Earth System and Environmental Sciences, Chonnam National University, Gwangju 61186, Republic of Korea)

*e-mail: yongjaelee@yonsei.ac.kr

Water is the most abundant volatile component in the Earth and influences both physical and chemical properties of the Earth materials.  It continuously enters the Earth through subduction zones where it reduces the melting temperature of rocks to generate magmas while the rest travels further deep into the Earth.  Our understanding of the water cycle in the Earth has emphasized dehydration processes along the subduction zones.  Here we show that the formation and subsequent breakdown of super-hydrated kaolinite have important implications for water transport, volcanism, and possibly seismicity along the subduction zones.  We measured in-situ and time-resolved high-pressure/high-temperature synchrotron X-ray diffraction and infrared spectra to characterize structural and chemical changes of kaolinite at conditions corresponding to those found in subduction zones.  Synchrotron X-ray powder diffraction patterns of kaolinite at 2.7(1) GPa after heating to 200 °C in the presence of water, a condition corresponding to a depth of about 75km in cold slabs, show the appearance of a reflection with a d-spacing near 10Å which arises from pressure-induced insertion of water.  This new super-hydrated phase of kaolinite has a ~31 % larger unit cell volume and a ~ 8.4% lower density than the original kaolinite and has, with 29 weight-% H2O, the highest water content of any known aluminosilicate mineral in the Earth.  As pressure and temperature approach 19 GPa and ca. 800 ˚C, we observe the sequential breakdown of super-hydrated kaolinite to phase-Pi, diaspore, and topaz-OH along with the formations of coesite and stishovite.  Breakdown of super-hydrated kaolinite in cold slabs subducted below 200 km then leads to the release of water that may further affect seismicity and help fuel arc volcanism at the surface.

H. Hwang, D. Seoung, Y. Lee*, Z. Liu, H.-P. Liermann, H. Cynn, T. Vogt, C.-C. Kao, H.-K. Mao, Nature Geoscience, Vol.10, pp.947-953, 2017.