A Study Of Physical And Chemical Properties Of Water-Bearing Phases Under Extreme High Pressure And Temperature Conditions

The rapid development of high-pressure technology in the past hundred years has provided us with powerful methods for investigating materials under extreme high pressure.The interior of the Earth is under extreme high temperature and high pressure,with pressures of 13 5 GPa and temperatures of 4000 K at the bottom of the lower mantle.Water has a significant influence on the physical and chemical properties of minerals in the lower mantle,and the water cycle in the deep Earth is an important part of the Earth’s internal materials cycle.Therefore,studying minerals which serve as water-carriers in the lower mantle can provide us suitable explanation for the complex dynamical procedure of the lower mantle and deepen our understanding of the material cycle inside the Earth.In this paper,to investigate the structure,phase transition,chemical composition,stability,and phase relationship of water-bearing minerals in the lower mantle,we performed high-pressure high-temperature experiments in laser-heated diamond anvil cells,combining synchrotron radiation X-ray diffraction experiments,high-pressure multigrain analysis,ex-situ transmission electron microscopy analysis,high pressure infrared absorption spectrum measurement and other high-pressure methods.We have performed high-temperature and high-pressure experiments in Fe-O-H and Al-Fe-O-H system,under the pressure-temperature range of 87-98 GPa,16002400 K,and found that the iron-rich δ-FeOOH-AlOOH is unstable along the lower mantle geotherm,decomposed into an iron-depleted δ-AlOOH-FeOOH and a new ironrich hexagonal phase,which is named HH1-phase.The crystal structure of the HH1phase was determined at 79 GPa using the multigrain crystallography method.The chemical formula was obtained as Fe12.76O18Hx(x～4.5,containing 4 wt.%water)in the Fe-O-H system.The hydrous Fe-rich HH1-phase,if included in the material of the upwelling plumes,will decompose on its rise to the upper part of the lower mantle and release water.Our results should provide constraints on water storage in the deep lower mantle and have implications for deep mantle dynamics.We conducted high-pressure and high-temperature experiments in model hydrated basalts in the pressure-temperature range of 84-113 GPa and 1800-2200 K.And we found the formation of an Al-rich Nt-phase(～24.4-32.4 wt%Al2O3),coexisting with Al-depleted bridgmanite(～6.4-7.6 wt%Al2O3),δ-phase,and iron-rich phase.Infrared spectroscopy of a pure synthetic Al-rich Nt-phase shows OH bending and stretching vibrations at high pressures,indicative of its hydrous nature.This study suggests that Al-rich Nt-phase can serve as a potential water carrier in subducted oceanic crust in the deep lower mantle.we conducted high-pressure and high-temperature experiments in hydrous Mgrich and Si-rich mantle related components,under the pressure-temperature range of 84-103 GPa and 1800-2300 K.Our results indicate that the HH-phase serves as major water storage in a pyrolitic composition,whereas the Al-rich CaCl2-type δ-phase and SiO2 phase are major water storage phases in a SiO2-rich basaltic composition.Based on our experimental results,we have discussed the significance of the formation and decomposition of HH-phase for the dynamics of the lower mantle.We also propose a potential mechanism of water transport to the deep earth through subduction plates,in which Nt-phase is served as a water carrier.And the phase equilibrium experiments in the multi lower mantle composition can provide us with new sightseeing in lower mantle water storage and deep-water cycle.