A Systematic Study On The Water Content Of Nominally Anhydrous Minerals In Eclogite From Jinheqiao In The Dabie Orogen

Most hydrous minerals are unstable when the continental crust was subducted to subarc depths of>80km.Thuss nominally anhydrous minerals?NAMs?in ultrahigh-pressure?UHP?metamorphic rocks become the main carrier to transport water into the suabarc mantle,a crucial means to trace the UHP metamorphic fluids.The present study has utilized the Fourier transform infrared spectroscopy?FTIR?to investigate the water contents of garnet and omphacite in the UHP eclogites at Jinheqiao in the Dabie orogen.FTIR analyses were made on garnet and omphacite,respectrively.The FTIR spectra of garnet grains indicate that the OH-absorption bands can be divided into four groups:3400-3435cm^-1?Type M?,3540-3580cm^-1?Type ??,3600-3630 cm'1?Type ??and 3635-3655cm^-1?Type ??.The first group is ascribed to the molecular H₂O in submicroscopic fluid inclusions.The others are caused by the structural OH.The intensity of the Type I is positively correlated with the intensity of the Type M,while Type ? is positively correlated with the intensity of the Type M in most grains.The FTIR spectra of omphacite grains show that OH-absorption bands can be divided into three groups:3435-3460cm^-1?Type ??,3520-3550cm^-1?Type ??and 3600-3630cm^-1?Type ??.All the three groups are arisen from structure OH.It is worth noting that the intensity of Type ? is positively correlated with the intensity of Type ? in omphacite.The water contents of garnet fall in a wide range,with total water of<1 to 3293 ppm,molecular water of<1 to 1195 ppm and hydroxyl?OH?of<1 to 1917 ppm,respectively.It is also the case for omphacite,varying from 48 to 625 ppm.The average water contents of garnets and omphacites vary from 749 to 1164 ppm and 132 to 324 ppm,respectively.This implying that garnets have similar or even higher capacity for water storage as omphacite,thus an important media for subducted slab to transport water into deep mantle.The average hydroxyl contents are correlated to Si,Ca,Na,Ti,Zr and Pr for garnets in most samples,indicating that the incorporation of OH in garnet is dominated by hydrogarnet substitution.The average water contents of omphacites are correlated to Al,Ca and Na for most samples,suggesting that the incorporation of OH in omphacite is controlled by both vacancies at M2 and substitution of A1 for Si.No correlation has been observed between type? band and other elements in omphacite,implying that the type ? band was caused by multiple complex substitution mechanisms.Different distributions of water contents from core to rim were observed in garnet grains,suggesting that molecular water in garnet would either be caused by fluid addition during exhumation,or be primary fluid inclusions or transformed from primary hydroxyl.The average contents of molecular water are correlated to garnet compositions for most Jinheqiao eclogite samples,implying that the molecular water was internal origin,mainly transformed from decompression exsolution of structural hydroxyl during exhumation.In this regard,water can be saturated in peak metamorphic garnet.The variable water contents in garnet was affected by several factors such as protolith nature,fluid availability,pressure and temperature,but dominated by decompression dehydration during exhumation.Due to the water loss during exhumation,the highest water content is corresponding to the garnet's capacity for water storage under peak UHP metamorphism.Therefore,the distribution of water contents in the NAMs may provide important means to trace the evolution of metamorphic fluids in the continental subduction channel with respect to the subduction and exhumation processes of the continental crust.