High-pressure polymorphs of olivine (wadsleyite and ringwoodite) are major minerals in the mantle transition zone (MTZ).Phase transformations in olivine are important for a series of geodynamic problems such as the mineralogical and evolutionary history of the mantle,mantle convection patterns,and deep focus earthquakes in subduction zones.In this study,we examine phase transformations in olivine with two compositions,namely Mg 2 SiO 4 (Fo 100) and (Mg 0.9 Fe 0.1) 2 SiO 4 (Fo 90),at pressures between 14.1 and 20 GPa and a constant temperature of 1400°C,using the newly installed multi-anvil system at the Laboratory for Studies of the Earth's Deep Interior (SEDI),China University of Geosciences (Wuhan).At 14.1 GPa,Fo 90 transformed completely into the wadsleyite structure (β),while Fo 100 remained as olivine (α).Between 14.8 and 15.6 GPa,both Fo 100 and Fo 90 transformed into the wadsleyite structure.Wadsleyite crystals were identified by two characteristic Raman peaks between 722 and 723 and 917 and 919 cm 1.They exhibit a bimodal grain size distribution:large-crystals with average grain sizes greater than 100 μm and microcrystals less than 10 μm.The population of microcrystals increased with pressure,apparently due to the increase in over-pressure (the difference between the experimental pressure condition and the equilibrium transformation pressure at 1400°C),which promotes nucleation and retards grain growth.All run charges contained large numbers of wadsleyite microcrystals,because of the low activation energy of the nucleation process.The experimentally observed microstructure may shed light on the morphology of wadsleyite observed in shocked meteorites.At 19.5 GPa,wadsleyite coexisted with ringwoodite (γ) in Fo 100,but was absent in Fo 90.At 20 GPa,both samples transformed completely into ringwoodite,which was characterized by the 798 and 840 cm 1 Raman lines.Ringwoodite crystals are euhedral grains (average grain size 10-20 μm),with well-developed triple junctions.The complex upper mantle structure
Water content in nominally anhydrous minerals （NAMs） of the high-pressure （HP） metamorphic rocks controls the thermal structure, rheology and partial melting of orogenic belts. This paper conducts a systematic analysis of water in NAMs of the HP granulites from the Greater Himalayan Sequence （GHS）, representing the thickened lower crust of the eastern Himalayan Orogen. The present result shows that the garnet, clinopyroxene, feldspar, quartz and kyanite contain 188 ppm-432 ppm, 193 ppm-547 ppm, 335 ppm-1 053 ppm, 125 ppm-185 ppm and 89 ppm H2O, respectively, and indicates that the thickened lower crust of the Himalayan Orogen is relatively wet rather than dry. The considerable concentrations of water in the HP granulites are expected to promote the rheological weakening of the metamorphic core of the Himalayan Orogen, providing a favorable evidence for the channel flow model of the exhumation of thickened lower crust.
