Hydroxyl Research Of NAMS In Ultrahigh-Pressure Eclogites From Dabie-Sulu Orogen

Investigation of fluid activity during the UHP metamorphism was one of the most importanttopics in studying geodynamics of orogenic belt, but little was known up to the present. Whenthe continental crust subducted to certain mantle depth (＞100 km), even if no hydrous mineralsexisted in the plate, the nominally anhydrous minerals (NAMs, namely not H-beating in the idealchemical formula) in UHP rocks could contain trace amount of water, therefore become the maincarders to transport water into the mantle, and new ways to trace the UHP metamorphic fluidactivity. Form of high-pressure eclogites was an important step in crust-mantle substance cycle incontinental crust subduction belt. Research of water in high-pressure eclogites could providedimportant evidence for dynamics model of continent crust subduction and exhumation.In this thesis, 21 fresh eclogite samples were collected from several representative localitiesin the Dabie-Sulu UHP metamorphic belt: Shuanghe, Bixiling, Wumiao, Maobei, bore core ofChinese Continental Sciences Drilling(CCSD). Used Fourier transform infrared spectroscopy(FTIR), we particular researched structural water content and distributing characteristic in garnetand omphacite in UHP eclogites. Used electron probe microanalysis (EMPA) we gained majorelement content of minerals. Used transmission electron microscopy (TEM) bright fieldtechnique we particular researched mineral ultramicrostructural such as shape, defect, dislocationand discussed relativity of structural water content, distributing characteristic and mineralultramicrostructural in garnet and omphacite.Omphacite and garnet in 21 UHP eclogites from Dabie-Sulu UHP metamorphic belt havebeen investigated by fourier transform infrared spectroscopy in this work. Omphacite and garnetanalysis spots had two or more absorb peaks in typical OH vibration infrared absorbregion(3000～3800 cm^-1). Garnet had four groups of OH absorbance bands at 3400～3450 cm^-1,3555～3575 cm^-1, 3610～3635 cm^-1, 3640～3655 cm^-1. The first group ascribed to H₂O insubmicroscopic fluid inclusions, but the three latter groups arose from structural OH. Omphacitehad three groups of OH absorbance bands at 3440～3455cm^-1, 3520～3535cm^-1,3620～3630cm^-1.The result demonstrates that all of garnet and omphacite grains from Dabie UHPmetamorphic belt contain certain structural water with the content scope from 67μg/g to 3288 μg/g and from 200μg/g to 2813μg/g, respectively, the contents of structural water in whole rockof eclogite range from 300μg/g to 750μg/g. Omphacite and garnet grains from Sulu UHPmetamorphic belt contain certain structural water with the content scope from 68μg/g to 529μg/g and from 20μg/g to 875μg/g, respectively, the contents of structural water in whole rock ofeclogite range from 150μg/g to 300μg/g. The trace amount of water would drastically changethe physical and chemical properties of the whole metamorphic system.Not only heterogeneous distribution of water content of garnet, omphacite occured indifferent sample, but also in the same sample. It can be observed that there are two types of waterdistribution characteristics within single garnet and omphacite grain, one is homogeneous and theother is unhomogeneous. The unhomogeneous phenomena show two facts that the water contentsincrease from the core, the middle to the rim within the grains, and both the core and the rim islow but the middle high. Thereby implying that eclogitic rocks formed during continentalsubduction have the potential to recycle (at least) several hundreds ppm water into mantle depths,even the continental slab subducted to mantle depth beyond the stability fields of hydrousminerals.Chemical compositions at different spots were same in the single mineral by electron probemicroanalysis. We had not found that the unhomogeneous distribution of water contents haddirect relation to the chemical compositions of mineral. Used transmission electron microscopyon eight samples with homogeneous water distribution characteristics, we could discovered thatstructural water content and distributing characteristic had relation to dislocation density ofmineral grain. Garent sample Shx2-2-g-4, Wma-g-1, Shx7-1, Shx7-2, corresponding watercontent 427μg/g, 593μg/g, 3101μg/g, 3219μg/g, corresponding dislocation density 0.25×10⁷cm^-2, 0.83×10⁷ cm^-2, 2.51×10⁷ cm^-2, 3.48×10⁷cm^-2. While water content rised from 500 to3000, grain had higher dislocation density. Structural water content had positive relation todislocation density.