Experimental Study On The Behavior Of Hydrogen In Minerals From The Crust And Mantle Of Earth At High Temperatures

Volatile elements,including hydrogen,carbon,nitrogen,sulfur,halogen,are inextricably linked to the origin of life,habitability,environmental change,natural disaster and mineralization.Abundance of volatiles in Earth's interior may exceed their storage in Earth's surface.In particular,volatiles in Earth's interior are the key factor to regulate Earth system energy and environment.Minerals are the basic unit of solid Earth and important carriers of volatiles in the deep Earth.Thus,investigation of volatiles in minerals provides fundamental understanding to volatile cycle in the deep Earth and their effects on resource and environment.Among volatiles,hydrogen can be incorporated into the structure of deep Earth minerals as various chemical components such asOH^-,H₂O,NH₄⁺.Previous knowledge of hydrogen point defects in minerals in the Earth's interior is mainly based on the characterization done at ambient conditions.However,the behaviors and properties of hydrogen-bearing components in the deep Earth's minerals at high temperatures and pressures may be different from those at ambient conditions,which impede further investigation of water cycle in the deep Earth.In this study,considering the high mobility of hydrogen at high temperatures,Fourier transform infrared spectroscopy(FTIR),Raman,X-ray diffraction,electron microprobe analyses were used to further decipher the mechanisms controlling the incorporation and mobility of hydrogen.We selected typical minerals from Earth's crust and mantle,such as feldspar,forsterite,wadsleyite,ringwoodite and phengite,to further investigate and quantify the behaviors of hydrogen in minerals at high temperatures.(1)The behaviors of different hydrous species in feldspars at high temperaturesSeveral hydrogen species can be incorporated in feldspars,including structural H₂O(type ? and ?)and OH(type ?,?a,and ?b).Using in situ high temperature FTIR,different temperature dependence of several hydrogen species in feldspars were observed.With increasing temperature,structural H₂O demonstrated weak stability,which partly dehydrated at 400?and totally dehydrated at 600?.In contrast,structuralOH remained stable and showed negligible loss up to800?.Furthermore,Type ?a OH changed sites of incorporation,i.e.,shifting from the stronger hydrogen bonds to the weaker hydrogen bonds at about 200?,which was not observed for type ?bOH.Thus,the results show different behaviors of hydrogen in feldspars at different temperatures.which may contribute to the H/D fractionation between feldspars and water vapor in Earth's interior.Thus,temperature dependence of hydrogen should be considered when analyzing data related to hydrogen isotope in natural feldspars.Besides,the redistribution of H atoms in the structure at high temperatures may affect the stability of lattice in feldspar.(2)The effect of hydrogen on the displacive phase transition in feldsparTo further investigate the effect of hydrogen redistribution on the lattice at high temperatures,the effect of hydrogen on displacive phase transition in feldspar was investigated.We selected three anorthoclases with different hydrogen concentrations originated from same region with similar chemical composition and Al-Si disordering.During the heating process to 800?,in situ high temperature Raman and X-ray diffraction results all displayed the reversible occurrence of displacive phase transition in three anorthoclases,but differed in temperature.Displacive phase transition temperature was around 500-600?for samples with lower hydrogen concentration.In comparison,temperatures of samples with higher concentration showed lower transition temperature at 200-300?,corresponding to the temperature of hydrogen redistribution.Thus,the redistribution of hydrogen may be responsible for decreasing the displacive phase transition temperature.To conclude,hydrogen impacts the displacive phase transition temperature in feldspar,which may provide new insights for understanding displacive phase transition.Due to the intimate link between the elastic properties of feldspar and displacive phase transition,the results can be an alternative explanation for the phenomenon related to elastic properties(e.g.,low-velocity anomaly,deformation characterization in the Earth's crust).(3)Storage and mobility of hydrogen in forsterite and its high-pressure polymorphs at high temperaturesAfter the preliminary research on the behaviors and effects of hydrogen in the crust minerals at high temperatures,then the mantle minerals were investigated.The main minerals in the upper mantle and mantle transition zone are olivine and its high-pressure polymorphs(wadsleyite and ringwoodite),respectively.Under the upper mantle conditions,hydrogen can be easily incorporated in the structure of minerals at the ppm concentration level.However,hydrogen diffusivities in these minerals are not well constrained and require further assessment to predict its fate in the deep Earth.In this study,iron-free forsterite,wadsleyite and ringwoodite were synthesized with starting mixtures of MgO,SiO₂ and brucite powder.Synthesis experiments were performed using 1500-ton multi-anvil press at University Clermont Auvergne,France.Synthesis conditions were 1200-1800?,15-22 GPa and run durations varied between 1 hour to 3 hours.The behaviors of hydrogen at high temperatures were investigated using in situ high temperature FTIR spectroscopy.The results showed that the absorption coefficients in forsterite at high temperatures strongly decreased,while the frequencies of hydroxyl band had negligible changes.The redistributions of hydrogen in wadsleyite at high temperatures were observed,i.e.,the main band changed from stronger bond to weaker bond.Furthermore,we witnessed a very rapid dehydration at 750?in wadsleyite.The frequency of the mainOH band in ringwoodite shifted from 3109 cm^-1to 3612 cm^-1and 3350 cm^-1.Hydrogen may rearrange to more stable configuration.The hydrogen diffusivities in forsterite and wadsleyite were obtained after annealing at high temperatures(500-750?)with different durations.Hydrogen in Si site is more stable than that in the Mg site in forsterite.The activation energy of hydrogen at 3324 cm^-1,3358 cm^-1and 3578cm^-1in wadsleyite were 206±63 kJ/mol,231±59 kJ/mol and 278±65 kJ/mol,respectively.The diffusion of hydrogen in[Si]site in forsterite could be used to constrain the magma ascent rates.This study demonstrates that hydrogen at higher wavenumbers diffuses faster in wadsleyite at mantle transition zone conditions.According to the relationship betweenOH frequency and D/H fractionation in mantle minerals,?D can be enriched in wadsleyite.Therefore,the mantle transition zone may display marked isotopic effects with time.In addition,when the mantle transition zone is affected by the high temperature from the a rising mantle plume,hydrogen located at different positions in the structure of wadsleyite may show different behaviors in dehydration.(4)The effect of nitrogen on the behavior of hydrogen in phengiteBased on the research of hydrogen in nominally anhydrous minerals at high temperatures,the effect of coexisting volatile(nitrogen)on dehydrogenation of hydrous minerals was also investigated in this study.Hydrogen and nitrogen,in form of hydroxyl and ammonium,incorporated in the structure of phengite,which can carry hydrogen and nitrogen deep into 300km.Up to now,hydrogen and nitrogen preservation and cycling between Earth's surface and interior are still not well understood.Using FTIR,we conducted kinetics quantification in ammonium-bearing and ammonium-free phengite at high temperatures from 720?to 850?,to explore the efficiency of ammonium loss,dehydration and their mutual influence.The results indicate that ammonium loss is faster than dehydration,and dehydration of ammonium-bearing phengite is one order of magnitude faster than dehydration of ammonium-free phengite.The activation energies of dehydration,ammonium loss in ammonium-bearing phengite,and dehydration in ammonium-free phengite are 150±88 kJ/mol,535±110 kJ/mol and 549±126kJ/mol,respectively.Our study,for the first time,shows that nitrogen can promote the dehydration in phengite.The results indicate that the presence of nitrogen can promote hydrogen diffusion in phengite.Thus,considering coexisting volatiles in Earth's interior,water may be released at depth locus shallower than theoretical prediction based on the breakdown of phengite(i.e.<300 km).This study provides new constraints on water cycle in subduction zones.Overall,in this study,the behaviors of hydrogen under high temperatures in minerals from Earth's crust and mantle were systematically explored,which cannot be observed at ambient conditions.The new results broaden the knowledge of volatiles in minerals in Earth's interior,further constrain volatile cycles in the Earth system,and shed light on their effects from atomic scale to macro scale.