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% H2O, 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.
Bibliographical noteFunding Information:
This work was supported by the Global Research Laboratory (NRF-2009-00408) and National Research Laboratory (NRF-2015R1A2A1A01007227) programs of the Korean Ministry of Science, ICT and Planning (MSIP). H.-K.M. was supported by NSF Grants EAR-1345112 and EAR-1447438. We also received surpport from NRF grants NRF-2016K1A4A3914691 and NRF-2016K1A3A7A09005244. Experiments using the synchrotrons were supported by the Collaborative Access Program of SSRL and general user programs of PAL, SSRF, ALS and APS. HPCAT operations are supported by DOE-NNSA under Award No. DE-NA0001974 and DOE-BES under Award No. DE-FG02-99ER45775, with partial instrumentation funding by NSF. The APS is supported by DOE-BES under Contract No. DE-AC02-06CH11357. The operation of the Infrared Lab of the NSLS-II at BNL was supported by COMPRES (EAR 1606856) and CDAC (DE-NA-0002006). NSLS-II is supported by DOE-BES under Contract No. DE-SC0012704. The ALS is support by DOE-BES under Contract No. DE-AC02-05CH11231. Part of this work was performed under the auspices of the US DOE by LLNL under Contract DE-AC52-07NA27344. Parts of this research were carried out at PETRA III at DESY, a member of the Helmholtz Association (HGF).
© 2017 The Author(s).
All Science Journal Classification (ASJC) codes
- Earth and Planetary Sciences(all)