Microstructures And Plastic Deformation Mechanism Of Garnet From The UHP Eclogites

Garnet is an important constituent mineral in eclogite,garnet-peridotite/granulite and other crustal or upper mantle rocks,which has a higher creep strength and brittle-ductile temperature than its paragenetic silicate minerals.Therefore,the rheological properties of garnet could play a critical role in the geodynamical processes of plate subduction and exhumation.While the research on garnet's microstructures is essential to acquire the rheological information of these geodynamical processes.In this dissertation,the samples were collected from the typical UHP eclogites and orogenic garnet-peridotites outcrops at the Dabie-Sulu metamorphic belt.Firstly,based on the research achievements of oxidation decorating dislocation in olivine and premelting phenomenon in crystal defects,the premelting decoration method for revealing dislocations in garnet are established and used to calibrate its dislocation density differential stress meter.Secondly,the premelting decoration method and other microstructural analysis technique,e.g.electron microprobe analyzer?EMPA?,Fourier transform infrared spectroscopy?FTIR?,Micro-CT,electron back-scattered diffraction?EBSD?and transmission electron microscope?TEM?,were used to investigate the correlations between grain morphology,dislocation and its substructures with chemical composition,differential stress and water content in the deformed garnets.The main achievements in this study are summarized as follows:?1?The dislocation structures premelting decoration experiments on garnets from the eclogites and garnet-peridotites show that their free dislocation and subgrain boundary structures could be revealed by the temperatures of 940-1020°C heat treatment,which exhibit a positive correlation with pyrope content.Under the reflected light mode of optical microscopy,the decorated free dislocations and subgrain boundaries such as dislocation array,network and wall are exhibited as dark short line,spot,circle-or semicircle-ring shape as well as their combined groups,which are similar with the typical dislocation patterns under the bright field of TEM.Based on the secondary electron image and atomic force microscope?AFM?analysis results,it can be confirmed that the shape of decorated lines is bulging out of the polished surface,which could change the path of reflected light and lead to a dark color line under the reflected light mode.The chemical composition and structural analysis results based on the backscattered electron image,Raman spectroscopy and TEM show that the decorated lines are formed by partial melting at dislocations and subgrain boundaries during the high temperature heat treatment.Since the high oxygen diffusivity at the polished surface of garnet,the new products generated by premelting reaction are hematite+magnetite+melt.In contrast,the new phases in the interior of garnet are changed to Al-magnetite+melt due to the low oxygen diffusivity.Therefore,the dislocation decoration mechanism of garnet should be assigned to premelting reaction,which is different from that of oxidation decoration in olivine.This method of revealing dislocation can be termed as premelting decoration?PMD?,which provides a new approach to study the garnet dislocation patterns and its geological significance.?2?The dislocation structures of garnets and olivines in garnet-peridotite samples from Bixiling,Nanshanling and Xugou regions were investigated by premelting decoration and oxidation decoration,respectively.The results show that the dislocation density of olivine and garnet is 5.00?12.10×10⁶ cm^-2 and 1.46?5.22×10⁵ cm^-2,that means the value of former is about 10?20 times that of the latter.The differential stress of these three regions by using the dislocation density differential stress meter of olivine is24.87?38.69 MPa.Based on this study and previous data,it is found that when the material coefficient of garnet is revised to 2.8,the ratio of differential stresses between garnet and olivine is closer to the 1:1 isoline.That means the differential stress value would be closer to the true state when using this revised material coefficient in the dislocation density differential stress meter of garnet.?3?The water incorporation and dislocation structures in garnet from Shuanghe UHP eclogites were investigated by FTIR and TEM,respectively.The results show that the water content in garnet grains are 66?2256 ppm,which are mainly incorporated by the hydrogarnet substitution mechanism.The peaks of 3600-3655 cm^-1and 3540-3580 cm^-1can be assigned to the different size of hydrogarnet clusters in grossular and andradite respectively,which indicates that the distribution of water has micro-and nano-size heterogeneity in garnet grains.The dislocation density and subgrain boundary density of these grains is 1.22?15.55×10⁶ cm^-2 and 1.61?24.34×10⁵ cm^-2 respectively.The differential stress recorded by dislocation densities is 65?234 MPa.According to compare these data with its corresponding water content,it can be found that the former is independent with water content and controlled by the differential stress,while the latter exhibit a positive correlation with water content.Because the subgrain boundaries are generally known as a result of free dislocation sliding and/or climbing motion,their relationships with water content suggest that the water could enhance the velocity and multiplication of dislocation,which in turn could promote the formation of subgrain boundaries and provoke the deformation mechanism changed from dislocation creep into dynamic recovery and recrystallization.?4?The macro-and micro-deformation structures,chemical composition,water incorporation,and their relationships in garnet from the typical UHP eclogites that exposed within gneiss,jadeite-quartzite,marble,and together with garnet-peridotite were studied by using the premelting decoration and other microstructural analysis technique.The results show that the deformation extent of garnet in UHP eclogite is controlled by the type of country rock,chemical composition,grain size,differential stress and deformation stage.Their relationship can be illustrated by the case of eclogites exposed within gneiss since their similar deformation conditions.In these samples,the deformation extent of garnet exhibits a negative correlation tendency with grain size,dislocation density,and Flinn coefficient,but shows a positive correlation with subgrain development.This correlation indicates that the plastic deformation of garnets is dominated by recovery-and recrystallization-accommodated dislocation creep mechanism.Based on the shape and dislocation patterns of garnet,the deformation process can be divided into 4 stages:early dislocation creep stage,dislocation creep stage,dynamic recovery stage,and dynamic recrystallization stage.It can be inferred that the differential stress?50?60 MPa?recorded by the garnet from weaker-medium deformed eclogites should be able to represent the stress state of peak metamorphism stage.