Water Contents And Fabrics Of Peridotite Xenoliths From The Muskox Kimberlite In The Slave Craton(Canada)

Olivine is the major mineral of the upper mantle.Water concentration of olivine has critical influence on ductile deformation and physical properties of the upper mantle.Due to the fast eruption of kimberlites,hydrogen diffusion has a slight effect on the water content of olivine.Hence peridotite xenoltihs brought by kimberlites are ideal samples to study distribution of in-situ water content in the upper mantle.The Muskox kimberlite pipe in the northern Slave craton(Canada)erupted with plenty of peridotite xenoliths in the middle Jurassic.To investigate water distribution and rheology of the stable cratonic lithospheric mantle,this thesis studied water contents and fabrics of olivine from Muskox peridotite xenoliths by using the Fourier transform infrared spectroscopy(FTIR)and electron backscatter diffraction(EBSD)analysis.Water concentration in nominally anhydrous minerals is mainly determined by FTIR spectroscopy based on the absorption coefficients of infrared light,which vary along different crystallographic directions in anisotropic minerals.Ideally,water contents in olivine should be analyzed on oriented grains using the polarized infrared light.However,this method is very time consuming because it requires mineral separation and orientation.Previous studies usually used unpolarized infrared light and calculated the average water content of many olivine grains in a thin section by Paterson(1982)calibration.However,application of unpolarized light assumes random orientation of mineral grains,which may underestimate water contents in olivine.Olivine in the upper mantle often develops lattice preferred orientation.In order to obtain water contents in olivine in a more effective and accurate way,this thesis presents a new method to measure in situ water contents of olivine in a thin section using the combined EBSD and FTIR analysis.Based on results of Bell et al.(2003),an ellipsoid of the water absorption coefficient(W)is established using W values along three crystallographic directions[100],[010]and[001].Then based on the EBSD-derived orientation of an olivine grain,the specific water absorption coefficient along a certain direction(Ws)in the ellipsoid is determined.Finally,water content of the olivine grain is calculated using the Beer-Lambert equation.This method is applied to measure water concentration of olivine from garnet peridotite xenoliths in Muskox.The results demonstrate that this method allows us to determine in situ water distribution of olivine in microscale and investigate the relationship between water contents and fabrics of olivine.The average water content of olivine from 7 peridotite xenoliths is 52±30 ppm which can represent the mantle in situ water content because of the ignorable effect of hydrogen diffusion.Four garnet peridotite samples show porphyroclastic texture and contain phlogopite.Olivine from garnet peridotite samples has the average Mg#of 90.5 and average water content of 82±5 ppm,and develops the B-type fabric due to dominance of the[001](010)slip system.Three spinel peridotites have granoblastic texture and contain amphibole.Olivine from spinel peridotites shows the average Mg#of 92 and average water content of 33±6 ppm,and the E-type fabric due to dominant activation of[100](001)slip system.According to thermometers and barometers of minerals,garnet peridotites reached equilibrium temperature of 1210±20 ? and pressure of 6±0.3 GPa,whereas spinel peridotites were subjected to equilibrium temperature of 920±60 ? and pressure of 4 GPa.Therefore,water concentration and fabrics of the Slave cratonic mantle lithosphere has a two-layered structure.Although both garnet peridotites and spinel peridotites have experienced mantle metasomatism,spinel peridotites at shallow depths remain characterized by low water contents and high Mg#values after large-scale partial melting in the early stage,and formed the E-type fabric.In contrast,garnet peridotites at great depths have higher water contents,which leads to strain localization and the B-type fabric in a ductile shear zone near the lithosphere-astheonophere boundary.Compared with other cratons,the lithospheric mantle of the Slave craton is relatively water poor.Mantle metasomatism can significantly increase water concentration in olivine and lead to fabric transition.Beucase fabrics of olivine control seismic anisotropy of peridotites,interpretation of seismic anisotropy in cratons requires a layered model.