Calibration Of Infrared Absorption Coefficient Of Structural Hydroxyl In Garnet And Influence Of Composition

As one of the main rock-forming minerals,garnet provides an important storage place for hydrogen in the subduction zone and mantle.H in garnet has an important influence on the physicochemical properties of minerals,such as electrical conductivity,mechanical strength,melting behavior,viscosity,cation diffusion rate,heat conduction and rheological properties.It will further affect the physical and chemical properties of rocks in the earth,and eventually lead to the occurrence of geological processes such as earthquakes and volcanism.Therefore,accurate determination of water content in garnet is of great significance to the understanding of related physical and chemical processes in the earth,and it is a key problem in geoscience.FTIR(Fourier transform infrared spectroscopy)is one of the most commonly used methods to measure the water content of garnet.It has the advantages of in-situ analysis,water type discrimination and fast measurement speed.In order to calculate the water content accurately by FTIR,the absorption coefficient of garnet must be calibrated.At present,the most commonly used absorption coefficient of garnet is 1.39 ppm-1·cm-2(Bell et al.1995).The result is obtained by hydrogen manometry method for pyrope,the absorption coefficients of different garnet endmembers were not meeasured.Later,some scholars reported the absorption coefficients of other garnet endmembers.However,due to its large sample consumption(hundreds of mg),relatively high detection limit(tens of ppm)and no comparison with the general calibration method,it has not been widely used.In addition,whether the composition difference of different garnet endmembers has influence on its infrared absorption coefficient have not been fully evaluated.TC/EA-MS(an online method combing thermal conversion elemental analyzer combining with isotope ratio spectrometry)can be used to measure the real water content of garnet.It has the advantages of low sample consumption and relatively low detection limit,meanwhile it can measure hydrogen isotopes simultaneously.In this work,the FTIR single-crystal spectra of end-member or approaching end-member composition(Spessartine and Grossular)and solid solution(Rhodolite)are measured.Microscopic observation,backscattering imaging,X-ray fluorescence mapping and Fourier transform infrared spectra show that these gem-quality garnets are homogeneous in major elements and H concentration.We determined the hydrogen contents of 8 natural garnet with gem quality by TC/EA-MS and obtained water contents of 11-2291ppm.The results show a good linear relationship with the total integral area of OH absorption peak measured by FTIR.Combining the results obtained from FTIR and TC/EA-MS,the infrared absorption coefficients were calculated to be 1.92±0.12 ppm-1·cm-2 for spessartine and 1.63±0.20 ppm-1·cm-2 for grossular,which are 38%and 17%higher than the calibration results of Bell et al.(1995)(1.39 ppm-1·cm-2),respectively.These differences of absorption coefficients are caused by the differences of mineral compositions and infrared absorption spectra.The infrared absorption coefficient for rhodolite is 0.4±14 ppm-1·cm-2,which is consistent with the absorption coefficient(0.75±0.44 ppm-1·cm-2)reported by Maldener et al.(2003).These coefficients can be used to determine the H content of natural spessartine,grossular and rhodolite.It should be noted that when using these infrared absorption coefficients of garnet,samples with similar composition should be chosen.In addition,we also noticed the effect of Fe2+and MgO content on the infrared absorbance of hydroxyl in NAMs and the positive correlation between TiO₂ and absorption coefficient.