Water Contents And Fabrics Of Peridotite Xenoliths From The Udachnaya Kimberlite In The Siberian Craton

Water can be incorporated into lattice defects of normally anhydrous minerals of the upper mantle as hydroxyl.Water can significantly affect physical and chemical properties of mantle minerals,as well as deformation mechanisms and rheology of the upper mantle.Previous studies have revealed wide variations in water contents in peridotite xenoliths and olivine megacrysts,implying heterogeneous water distribution in the upper mantle.The Udachnaya kimberlite erupted at?360 Ma and brought large amounts of fresh peridotites,eclogites and granulites,which provide a window to investigate deformation and water distribution in the lithospheric mantle beneath a stable craton.In this study,Fourier transform infrared spectroscopy(FTIR)and electron backscattered diffraction(EBSD)were used to analyze water content and fabrics of 9 garnet lherzolite xenoliths from the Udachnaya kimberlite pipe.The major elements and trace elements of the peridotite samples and major minerals were presented.Nine peridotite xenoliths can be divided into three groups:group 1 are peridotite mylonite,group 2 are peridotite protomylonite,and group 3 is coarse-grained peridotite with granoblastic texture.The Mg#in olivine,orthopyroxene and clinopyroxene ranges from 90.4-91.6,91.6-92.6 and 89.9-93.0,respectively.The trace elements of clinopyroxene reveal mantle metasomatism by both silicate melts and carbonatitic melts.The calculated equilibrium temperature of the peridotite xenoliths varies from 846 to 1276?,and the calculated pressure ranges from 4.0 GPa to 5.2 GPa.The P-T condition of coarse-grained peridotite follows the geothermal profile with the surface heat flow of 35 mW/m2,whereas the others show higher geothermal gradients.The water contents in olivine,orthopyroxene,clinopyroxene and garnet from the garnet lherzolite xenoliths are 34-166 ppm,32-126 ppm,83-166 ppm and 7-46 ppm,respectively.Nine peridotite xenoliths develop different fabrics.Recrystallized olivine in peridotite mylonites develops[100](001)slip system,i.e.,the E-type fabric.In contrast,olivine in most peridotite protomylonites and coarse-grained peridotite develop[100](010)slip system,i.e.,the A-type fabric.Calculated seismic properties of peridotites show that olivine fabrics control to the seismic anisotropy of peridotites.For all the samples,the fastest P-wave velocity(Vpmax)parallel to the lineation.Vpmax is 8.47-8,84 km/s and P-wave anisotropy is 3.4%-10.7%in mylonite peridotite;Vpmax is 8.4-8.59 km/s and P-wave anisotropy is 2.2%-7.5%in protomylonite peridotite;whereas Vpmax is 8.7 km/s and P-wave anisotropy is 8.2%in coarse peridotite.The eruption of the Siberian Traps occurred at?250 Ma.Combined with previous water content measurements,the lithospheric mantle of the Siberian craton at a depth of 130-220 km has been hydrated by mantle metasomatism at?360 Ma,which caused strain localization and development of mantle shear zone characterized by the E-type olivine fabric.Because water can significantly decrease the melting point of peridotites,when the mantle plume arrived at the lithosphere-asthenosphere boundary of the Siberian craton,the relatively hydrous lithospheric mantle should be subjected to a high degree of partial melting and contribute to the formation of the Siberian Traps,the largest igneous province in the world.