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Hydration and Dehydration Breakdown of Pyrophyllite in the Subducting Sediments
Yoonah Bang,Huijeong Hwang,Hanns-Peter Liermann,Tae-Yeol Jeon,DuckYoung Kim,Yu He,Yongjae Lee 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
The sedimentary layer in a subducting slab consists of ca. 60 percent of water and ca. 40 percent of sediments, mainly composed of alumina and silica, especially in the form of hydrous clay minerals. The behavior of clay minerals under such water-rich subduction zone conditions is therefore important to understand the water transport into the deep Earth and related geochemical and geophysical phenomena. Here, we show sequential hydration and dehydration breakdown reactions of pyrophyllite (Al₂Si₄O10(OH)₂) under water saturated conditions along cold subduction geotherms down to the depth of mantle transition zone. At conditions corresponding to a depth of ca. 145 km (ca. 4.7 GPa and 435°C), we observe hydration breakdown of pyrophyllite to form a phase assemblage including a further hydrated mineral, gibbsite, which would remove ca. 14 volume % of water from the surroundings. As the pressure and temperature increase up to a depth of ca. 600 km (ca. 19 GPa and 700-900°C), we observe sequential dehydration breakdowns and formations of phase assemblages including diaspore, topaz, phase egg, and δ-AlOOH, which would release water into the region progressively. In addition, total energy calculations based on density functional theory showed an excellent agreement in terms of phase stability with our experiments, explaining the underlying mechanism for these sequential breakdown reactions. Our results thus provide a new mechanism of deep water cycle along the cold subduction interface that is potentially related to the origin of double seismic zone activities.
Atomic-scale mixing between MgO and H₂O in the deep interiors of water-rich planets
Taehyun Kim,Stella Chariton,Vitali Prakapenka,Anna Pakhomova,Hanns-Peter Liermann,Zhenxian Liu,Sergio Speziale,Sang-Heon Shim,Yongjae Lee 대한지질학회 2021 대한지질학회 학술대회 Vol.2021 No.10
Water-rich planets exist in our Solar System (Uranus and Neptune) and are found to be common in the extrasolar systems (some of the sub-Neptunes). In conventional models of these planets a thick water-rich layer is underlain by a separate rocky interior. Here we report experimental results on two rock-forming minerals, olivine ((Mg,Fe)₂SiO₄) and ferropericlase ((Mg,Fe)O), in water at the pressure and temperature conditions expected for the water-rich planets. Our data indicate a selective leaching of MgO, which peaks between 20 and 40 GPa and above 1,500 K. For water-rich planets with 1–6 Earth masses (>50 wt% H₂O), the chemical reaction at the deep water–rock interface would lead to high concentrations of MgO in the H₂O layer. For Uranus and Neptune, the top ~3% of the H₂O layer would have a large storage capacity for MgO. If an early dynamic process enables the rock–H₂O reaction, the topmost H₂O layer may be rich in MgO, possibly affecting the thermal history of the planet.
A role for subducted super-hydrated kaolinite in Earth’s deep water cycle
Hwang, Huijeong,Seoung, Donghoon,Lee, Yongjae,Liu, Zhenxian,Liermann, Hanns-Peter,Cynn, Hyunchae,Vogt, Thomas,Kao, Chi-Chang,Mao, Ho-Kwang Nature Publishing Group UK 2017 Nature geoscience Vol.10 No.12
Water is the most abundant volatile component in the Earth. It continuously enters the mantle through subduction zones, where it reduces the melting temperature of rocks to generate magmas. The dehydration process in subduction zones, which determines whether water is released from the slab or transported into the deeper mantle, is an essential component of the deep water cycle. Here we use in situ and time-resolved high-pressure/high-temperature synchrotron X-ray diffraction and infrared spectra to characterize the structural and chemical changes of the clay mineral kaolinite. At conditions corresponding to a depth of about 75 km in a cold subducting slab (2.7 GPa and 200 °C), and in the presence of water, we observe the pressure-induced insertion of water into kaolinite. This super-hydrated phase has a unit cell volume that is about 31% larger, a density that is about 8.4% lower than the original kaolinite and, with 29 wt% H<SUB>2</SUB>O, the highest water content of any known aluminosilicate mineral in the Earth. As pressure and temperature approach 19 GPa and about 800 °C, we observe the sequential breakdown of super-hydrated kaolinite. The formation and subsequent breakdown of super-hydrated kaolinite in cold slabs subducted below 200 km leads to the release of water that may affect seismicity and help fuel arc volcanism at the surface.