High-T And High-P Spectroscopy Study Of Olivine And Humite Minerals And Its Geological Impication

The concentration and distribution of water in Earth’s interior affects the evolution and dynamics processes of the Earth and planets.Slab subduction is the only possible geodynamic process which can transport water in the form of hydrous minerals,from the surface reservoir down into the earth’s interior.Among various hydrous minerals,the serpentine-group would break down at pressure-temperature conditions of the upper mantle to generate a series of dense hydrous magnesium silicate（DHMS）phases,such as the humite-group minerals.These DHMS phases have been proposed as potential carriers of water into the upper mantle and transition zone.Olivine is the most abundant and important nominally anhydrous mineral in the upper mantle,and humite group minerals are representives for the DHMS phases in the subduction slabs,whose compositions can be treated as mixtures of olivine and brucite We conducted systematic high-pressure hightemperature infrared（IR）and Raman spectra on olivine and humite group minerals（including clinohumite,humite and norbergite）,using Diamond anvil cells to study the hydrogen in deep mantle and the influence fromcomponent,temperature,pressure on the thermodynamic properties.Hydrous forsterite has similar OH modes with humite group minerals,and the OH-stretching modes have similar behaviors at high temperature.The OH bands above3450 cm^-1shift to lower frequencies at elevated temperature,while the ones below 3450cm^-1shift to higher frequencies.The hydrogen positions becomed disordered at elevated temperature.The distances of the oxygen-oxygen edges in Mg O₆octahedra for protonation tend to become similar to each other at elevated temperatures,which may provide a microscopic explanation for the electrical conductivities of DHMS phases at high temperatures.On the other hand,,at high pressures,the OH-stretching modes for humite group minerals above 3450 cm^-1,systematically show positive pressure dependences,which are controlled by the neighboring H-H repulsion,while the ones below 3450 cm^-1shift to lower frequencies at elevated pressure,due to the alleviation local H-H repulsion effect by either F^-or Ti⁴⁺substitutions.The new hydrogen bonds created by Ti substitution are likely to participate in the hydrogen bond symmetrization at high pressure,which possibly increasing the stability of humite group minerals and storing water to deeper mantle.Iron and hydrogen incorporation in olivine increase the intrinsic anharmonic contribution to thermodynamic properties.The magnitude of this anharmonic contribution decreases slightly with increasing pressure,while the pressure effect on the external anharmonicity（thermal expansivity）is much more significant.As compared pressure effect,temperature is much more important.For the case of pure OH-clinohumite with very small anharmonic effect.The anharmonic contributionbecome larger in magnitude with increasing F content in clinohumite,while such anharmonic contributions are even more significant in the humite and norbergite samples with higher fluorine concentrations.Therefore,when modeling the high-temperature thermodynamic properties,like phase equilibria and isotopic fractionation factors,the compositional（or maybe even pressure）effect should not be ignored.The thermodynamic Grüneisen parameter of olivine is also modeled as a function of both pressure and temperature.The value of the Grüneisen parameter decreases with increasing pressure,while become almost insensitive to temperature above 600 K.The derived Grüneisen parameter with anharmonic correction is consistent with the reported values from P-V-T EOS fitttings,while the anharmonic correction would decrease the?parameter by approximately 4%at high temperatures.The thermodynamic Grüneisen parameter is applied to the modeling of adiabatic geothermal gradients and wave velocities in geodynamics.The intrinsic anharmonicity has no effect on the bulk wave velocity.The adiabatic temperature（T_S）profile for olivine is computed（0<P<14 GPa,300<T<2000 K）,which can be treated as a linear function of P.Our calculation can be adopted as a useful tool for modeling the radial temperature distribution in planetary mantles and setting up connection between isothermal and adiabatic compressions of minerals,which would further provide important parameters for modeling the adiabatic dynamics processes in the Earth and planetary interiors.