Fabrics And Water Contents Of Peridotites From The Luobusa Ophiolite In Tibet

As an important indicator of a suture zone,ophiolites represent the remnant of the subducted oceanic lithosphere.Peridotites in ophiolites include ultramafic plutonic rocks and serpentinized mantle peridotites.Their fabrics reflect the thermodynamic processes from the mid-ocean ridge to a subduction zone.The most common fabric of olivine in ophiolitic peridotites is the A-type fabric,characterized by the[100]axis parallel to the lineation and the(010)plane parallel to the foliation.However,both naturally deformed peridotites and olivine aggregates in deformation experiments found various fabric types in olivine.The Luobusa ophiolite,located in the Yarlung-Tsangbo suture zone,are the remnant of the subducted Neo-Tethys oceanic lithosphere.Recent discovery of ultrahigh-pressure and super-reduced minerals such as in situ diamond,coesite with stishovite pseudomorph,SiC,natural iron in the Luobusa peridotites suggests that the Luobusa peridotites may come from a great depth of the upper mantle,even from the mantle transition zone.In order to explore the complex evolution history of the Luobusha ophiolite,here we studied peridotite samples from two boreholes of the Luobusa Scientific Drilling Project.Water contents of olivine,orthopyroxene(Opx)and clinopyroxene(Cpx)were obtained using the Fourier transform infrared spectroscopy(FTIR),the lattice-preferred orientations of olivine and orthopyroxene were measured using electron backscattered diffraction(EBSD),and dislocations in olivine were observed using the transmission electron microscope(TEM).Combined with major and trace elements in minerals,we discussed the deformation process the Luobusa peridotites.The average water contents of olivine,orthopyroxene and clinopyroxene from 24 peridotite samples are 16±10 ppm,90±50 ppm and 490±110 ppm,respectively.Olivine is poor in water,whereas Cpx has extremely high water contents relative to olivine and Opx.The partition coefficient between Cpx and Opx(DHCpx/Opx)is 5.37.Trace elements in clinopyroxene show that the Luobusa peridotites did not experience remarkable mantle metasomatism.The high water content in Cpx may result from the following reasons.First,DHCpx/Opx in peridotites increases with pressure,hence the high value of DHCpx/Opx implies a deep origin of the Luobusa peridotites.Second,the diffusion rate of H in olivine and Opx is much faster than that in Cpx.Olivine and Opx may have experienced water loss during the exhumation,while Cpx still preserved in situ water content.To determine fabric types of olivine,we analyzed lattice preferred orientations of olivine and Opx,as well as dislocations of olivine from 9 peridotite samples.EBSD results indicate that olivine in the Luobusa peridotites developed A-,B-and E-type fabrics.TEM observations show that the dominant slip system in olivine include[100](010),[001](010),[100](001)and[001](100).The water contents of olivine in all samples are very low,hence the different fabrics of olivine are not related to water contents.Compared with previous deformation experiments,the A-and E-type fabrics of olivine in the Luobusa peridotites were formed in the shallow upper mantle,whereas the B-type fabric of olivine may be formed in the upper mantle deeper than 200 km and be rapidly carried up to the oceanic lithosphere by mantle plume.Therefore,the Luobusa peridotites contain peridotites formed in both the shallow and deep upper mantle.These peridotites were moved and subducted with the Neo-Tethys plate,and obducted on land.