Deformation Experimental Investigation On Rheological Properties Of Hydrous Garnet At High Temperatures And Pressures

Although garnet is a relatively minor component of Earth’s crust and upper mantle, it may significantly affect the deformation of the lower continental crust, of ultra-high pressure rocks associated with subduction, such as eclogites, and of the upper mantle. Especially, as garnet is an important constituent mineral in subducted slabs and in the transition zone, knowledge of rheological properties of garnet is essential to understanding the geodynamic response of these important regions of Earth’s interior. However, relatively little is currently known about the creep strength of garnet at high pressure. Although several studies have been made to constrain the rheological properties of garnet or garnet analogs, experimental results mainly focused on deformation micro structures or mechanical data at lower pressures. Quantitative experimental investigations of the creep strength of garnet at mantle conditions have been hindered due to technical limitations. To date, few deformation experiments of garnet at high temperatures and pressures have been carried out, especially studies that quantify the effect of water on the creep behavior of garnet are very limited. As a consequence, the deformation behavior of subducted slabs and its related geodynamics processes (e.g., delamination, deep subduction) in a water-rich enviroment remain poorly understood.To explore experimentally the influence of water on the rheological properties of garnet at high temperatures and pressures, deformation experiments of garnet under hydrous conditions at at high temperatures and pressures have been carried out in this thesis. The garnet samples were fabricated from crushed eclogite rocks collected from Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt of China. The deformation apparatus is deformation-DIA (D-DIA) coupled with the synchrotron X-ray source available at beam line X17B2of the National Synchrotron Light Source (NSLS), Brookhaven National Laboratory, USA. The research work in this thesis mainly includes triaxial compressive creep experiments of hydrous garnet at high temperatures and high pressures, measurements of water content of deformed garnet samples and preliminary scanning electronic microscope (SEM) and electron back-scattered diffraction (EBSD) studies on their deformation microstructures.(1) Thirteen deformation experiments were carried out on seven samples at pressures of1.6to5.6GPa and temperatures of1223to1423K using a deformation-DIA apparatus coupled with synchrotron X-rays. The garnet samples were deformed at constant strain rates ranging from0.64to3.7×10-5s-1to total axial strains of6to30%. Dehydration of a talc sleeve in the sample assembly provided a water source for establishing a hydrous environment during the experiment. Synchrotron X-rays provided the means of in situ determining the strain (and strain rate), stress and pressure of the sample.(2) Infrared spectra were obtained for the doubly polished samples after deformation experiments by a Bruker Tensor37Fourier transform infrared (FTIR) spectrometer equipped with a Hyperion2000super-microscope. Water concentrations in the samples were determined based on Paterson’s calibration.For garnet samples, the absorption peak that occurs at～3600cm-1shifts from3570to3625cm-1with increasing pressure. The band intensity of infrared absorption for garnet samples increases with increasing pressure at lower pressures (1.6-2.9GPa), and then decreases at higher pressures (4.9-5.6GPa). The shape of the infrared spectra for each sample is similar from one region to the next, although the height of the absorption peak varies from one point to the next wihin a sample.The water concentrations calculated from Paterson’s calibration for garnet indicate that, the water content in each garnet sample varies from one point to another within a same sample. Therefore, for garnet, we report the range of water content and the average value. Average water content was determined from at least5FTIR spectra for each sample. From a comparison of water content in our samples, it is clear that the average water content in garnet increases from7440to21000H/106Si as pressure increases from1.6to2.9GPa, but then decreases to～3000H/106Si as pressure increases to4.9and5.6GPa.To measure the degree of water saturation in each experiment, a San Carlos olivine sample was stacked with a garnet sample during the preparation of each sample assembly. Water solubility for olivine as a function of temperature, pressure and water fugacity has previously been experimentally determined, while little is known about water solubility in garnets of our composition. For our olivine samples, the approximately linear increase in water solubility with increasing water fugacity is in good agreement with that reported for single crystals. Based on this systematic dependence of water solubility on water fugacity (and pressure) in our olivine samples, we conclude that our deformation experiments were carried out under water-saturated conditions. Water fugacity was calculated based on the equation of state for water at experimental P-T conditions.More importantly, the water content in garnet increases with increasing water fugacity as pressure increases from1.6to2.9GPa, but then decreases even with higher water fugacity at pressures of4.9and5.6GPa. This result is very similar to that of previous study on the water solubility of the synthetic pyrope samples.Both SEM and EBSD techniques were used for microstructural and crystallographic analyses of garnet samples after deformation experiments. From the SEM images, grain boundaries are distinctly serrated, indicating that grain boundary migration was active during deformation. A population of smaller grains surrounding larger grains suggests that dynamic recrystallization was active during deformation. Furthermore, it is evident that individual grains shortened in the direction of the compression. Our observations of these deformation microstructures suggest that the dominant deformation mechanism of our garnet samples is dislocation glide assisted by dynamic recrystallization. Finally, based on electron backscattered diffraction analyses, we note that our samples exhibit a weak crystallographic preferred orientation (CPO). Because garnet has a cubic crystal structure with66possible slip systems available for plastic deformation, garnets deformed by dislocation creep in nature and in the laboratory typically exhibit similarly weak CPOs.A fit of our experimental results on the creep of garnet samples to a power law yields where ε is the axial strain rate in s-1, σ is the differential stress in MPa,fH2O is the water fugacity in MPa, R is the gas constant, and P and T are pressure in MPa and temperature in K, respectively. This flow law demonstrates strong dependencies of strain rate on water fugacity with a water fugacity exponent of r=1and on pressure with an activation volume of V*=28×10-6m3/mol. A comparison of our creep results with those from previous studies on the dislocation creep of garnet at high pressure was made. At a pressure of2GPa and temperature of1473K, for a given stress under water-saturated conditions, it is clear that garnet creeps～2orders of magnitude faster under hydrous conditions than under anhydrous conditions, demonstrating the significant effect of water on the creep strength of garnet aggregates in the dislocation creep regime. Based on the results of our present study, relative to other garnet-bearing rocks, the lower creep strength of garnet in some naturally deformed garnet-rich rocks from the shallow upper mantle reported in previous studies can be explained by the role of water weakening. Importantly, the flow law for garnet samples deformed in the dislocation creep regime under hydrous condition in our study provides an important constraint on the viscosity of the garnet-rich layer of the subducted oceanic lithosphere in a subduction zone and in the transition zone, where rocks are likely enriched in water. In a water-rich environment, the viscosity of a garnet-rich layer in a subducting slab is lowered by the presence of water in the shallow upper mantle, because garnet becomes weaker with increasing water content for the increasing depth. However, garnet then becomes stronger with further increase in depth. This transition occurs at about the pressure at which water solubility begins to decrease with increasing pressure. Furthermore, at greater depths, the viscosity will be high because the direct effect of pressure on the creep through thermally activated processes outweighs the indirect effect on the creep through water solubility. Thus, in the deeper part of the upper mantle, the process of delamination of the subducting oceanic crust from the decending slabs should occur because of the high viscosity contrast, even if the the subducted crust is in the water-enriched mantle transition zone. The delamination and stagnation of the subducted oceanic crust around the mantle transition zone may provide a clue to understanding the observed feature that410-km discontinuity is split into two discontinuities in some subduction zones and the deflection of the slow velocity anomalies imaged by seismic tomography at the660km discontinuity in some locations that close to the subducted plates.In addition, preliminary results from deformation experiments at lower pressures in a Paterson apparatus and higher pressures in a D-DIA apparatus without a synchrotron radiation source have been briefly included in this thesis. Firstly, we attempted to carry out two hot-pressing runs and four triaxial compressive experiments and one compression-extension test on hydrous andradite using the gas-medium (Paterson) apparatus at the rock deformation laboratory in the University of Minnesota. From this work, we reached two main points:a) the oxygen buffer is important for the stability of andradite; b) the mechanical data from each deformation experiment yield a stress exponent n≈3, suggesting that dislocation creep is the dominant deformation mechanism for the andradite samples. Secondly, under anhydrous conditions, two simple shear deformation experiments on garnet samples and San Carlos olivine samples were completed with a D-DIA apparatus without X-ray beam at the rock deformation laboratory in the Uinversity of Minnesota. The strain markers in the deformed samples, rotated farther in the olivine sample than in the associated garnet sample, indicating the creep strength of garnet aggregates is larger than that of the olivine aggregates under anhydrous conditions.