Water In Ultrahigh-Pressure Eclogites From Dabie Orogen

Investigation of fluid activity during the UHP metamorphism is one of the most important topics in studying geodynamics of orogenic belt, but little is known up to the present. When the continental crust subducted to certain mantle depth (>100 km), even if no hydrous minerals existed in the plate, the nominally anhydrous minerals (NAMs, namely not H-bearing in the ideal chemical formula, such as garnet, pyroxene, rutile, quartz/coesite) in UHP rocks can contain trace amount of water, therefore become the main carriers to transport water into the mantle, and new ways to trace the UHP metamorphic fluid activity.In this thesis, ～70 fresh eclogite samples were collected from several representative localities in the Dabie orogen: Bixiling, Shuanghe, Wumiao, Maowu, Xindian and Sanzusi in South Dabie; Huangwei and Baizhangya in North Dabie; Huangzhen in the low-temperature metamorphic unit. Garnet, omphacite and rutile in UHP eclogites were analyzed by Micro-FTIR (Fourier Transform Infrared Spectroscopy) and EMP (Electron Microprobe) analysis, aimed to investigating the occurrence and content of structural water on different scales. The results of ～2000 IR analyzed points demonstrate that these NAMs contain trace/minor amount of structural water occurring as OH in point defects. The trace amount of water would drastically change the physical and chemical properties of the whole metamorphic system.In the typical OH vibration region (3000-3800 cm^-1), three group of absorbance bands at 3610-3630 cm^-1, 3560-3580 cm^-1 and 3400-3440 cm^-1 appear in the IR spectra of garnet. Group â…  and â…¡ bands arose from structural OH, and group â…¢ was ascribed to H₂O in submicroscopic fluid inclusions. Water contents (wt. H₂O) range from 14 to 1915 ppm, with most being between 200 and 800 ppm, and independent on chemical compositions. The range of water contents in South Dabie garnets is identical with that in North Dabie garnets. Comparison with the IR spectra of hydrogrossular and hydroandradite suggests that group I and II bands in the Dabie garnets are caused by structural OH, and that the SiO₄^4-<->(O₄H₄)^4- substitution is possibly the main substitution mechanism. Three bands at ～3450 cm^-1, ～3520 cm^-1, ～3620 cm^-1 in IR spectra of omphacite were all ascribed to structural OH absorbance with contents varying from 115 to 695 ppm, and water contents in Huangzhen omphacites are slightly higher than that in South Dabie omphacites. A rough positive correlation between water contents and vacancies at M2 sites indicate that the possible substitution in omphacite includes OH occupying the 02 position and points to the M2 vacancy. The main OH absorbance bands of rutile appear at 3280 cm^-1 and 3295 cm^-1, sometimes with a weak and wide band at ～3360 cm^-1 which were all caused by structural OH with content varying from 161 to 854 ppm. Comparison with the experimental results suggests that substitutions of Ti³⁺+H⁺<->Ti⁴ and Fe³⁺+H⁺<->Ti⁴⁺ are important. The estimated whole-rock water content based on mineral water contents and their proportions is between 260 and 750 ppm, thereby implying that eclogitic rocks formed during continental subduction 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 hydrous minerals.Heterogeneous distribution of water content of garnet, omphacite and rutile occurs at hand specimen scale with various water content of garnet within individual samples. The preserved chemical differences likely indicate that very limited fluid mobility occurred during UHP metamorphism and the eclogitic rocks resided at mantle conditions for a limited time span, and imply that they were exhumed shortly after...