Quantitative Analysis Of H-species In NAMs By Unpolarized Infrared Spectroscopy

Water occurs as various forms inside the Earth,including molecular water(H2O),hydroxyl groups(OH),molecular hydrogen(H2),methane(CH4),etc.The carriers of water in the deep Earth are fluids,melts,hydrous minerals,and nominally anhydrous minerals(NAMs).NAMs are a special group of minerals whose ideal chemical formula contain no hydrogen,but trace amounts of hydrogen can be detected as H-related point defects in the crystal structure.The presence of hydrogen in NAMs is usually by neutral charge balance or by occupying interstitial sites,e.g.,as OH,H2O,H2 or NH4+.The dominant minerals of the silicate Earth(i.e.the silicate crust and mantle),such as quartz,feldspar,clinopyroxene,orthopyroxene,olivine,garnet,wadsleyite,ringwoodite and bridgmanite,are all NAMs.As such,NAMs are the main water reservoir in the mantle.Water in NAMs has received increasing attention in the past decades.The content of water in NAMs(expressed commonly in the community as the equivalent form of H2O)is usually not significant,e.g.,ranging from a few to thousands of ppm H2O but could be more than 10000 ppm H2O in some minerals such as wasleyite and ringwoodite.Although of very small amounts,water in NAMs has disproportionate effects on many physicochemical properties of the host minerals,which in turn affect various chemical/physical aspects and dynamics of the Earth.Water in NAMs has strong influences on many geochemical,geophysical and geodynamical processes of the Earth,such as ion diffusion,partial melting,electrical conductivity,seismic velocity,rheological strength,plate tectonics and exchange of water between Earth's interior and exterior.The effect of water on these various aspects,as well as the partitioning,distribution and storage of water inside the Earth,is closely related to its concentration levels,the types of its occurrence and the way it gets incorporated.Consequently,knowledge of the species and amount accurately of hydrogen in nominally anhydrous minerals is important prerequisite for study of many physicochemical properties and effects in the Earth's interior.There are many methods that have been developed for quantifying the amount of water in NAMs,including Fourier-transform infrared(FTIR)spectroscopy,secondary ion mass spectrometry(SIMS),elastic recoil detection analysis(ERDA),nuclear magnetic resonance(NMR),etc.FTIR spectroscopy was the first method established for determining water contents in NAMs,and remains the most widely used one.The main advantages of FTIR analysis include:(a)it is very sensitive to minor hydrogen,with very low detection limit down to ppb levels,(b)it distinguishes easily different H-species,(c)it is non-destructive,and the analysis is easy and can be in situ carried out very small samples,and(d)it can provide important information about the orientations of H in the lattice.Attempts have been made to use unpolarized infrared analyses on unoriented anisotropic crystals of nominally anhydrous minerals to determine H contents,rather than using the more demanding polarized techniques that are more accurate(given that a reliable calibration is available).In this context,different approaches have been either empirically or theoretically proposed for the quantification;however,the involved accuracy has not been systematically documented by experimental work of both polarized and unpolarized analyses.In this study,we present a careful evaluation of experimentally grown,gem-quality OH-bearing olivine,clinopyroxene,and orthopyroxene single crystals.The samples were prepared for polarized and unpolarized infrared analyses,and the obtained spectra were used to estimate the H2O contents.We show that,regardless of the applied protocol,a single unpolarized determination is inadequate for quantitative analysis and the uncertainty could be up to?80%.The unpolarized method of Paterson(1982),by considering the linear absorbance intensity either through a single analysis or by averaging the data from multi-grain analyses,commonly underestimates the H2O content,by a factor of up to-6.The other unpolarized calibration method by using the averages of integrated absorbances of unoriented grains is in general of good accuracy,mostly within ±25%even for analyses on 2 grains(with perpendicular indicatrix sections),and the accuracy is even better if as many as 10 gains of random orientations are involved,e.g.,within ±10%.Therefore,the latter method may be safely applied to quantify H in anisotropic minerals if a reasonable number of randomly oriented grains are chosen for the analyses.However,the uncertainty is non-systematic,and both underestimates and overestimates of H are possible depending upon orientation.These results provide a basis for quantifying H-species in anisotropic minerals and for documenting the quantitative effect of H on the physical properties of the host phases.