An experimental study of phase transformations in olivine under pressure and temperature conditions corresponding to the mantle transition zone  

作　　者：WU Yao WANG YanBin ZHANG YanFei JIN ZhenMin WANG Chao ZHOU ChunYin 

机构地区：[1]State Key Laboratory of Geological Processes and Mineral Resources,China University of Geosciences,Wuhan 430074,China

出　　处：《Chinese Science Bulletin》2012年第8期894-901,共8页

基　　金：supported by SinoProbe-Deep Exploration in China (SinoProbe-0801);the National Natural Science Foundation of China (41174076 and 41002068)

摘　　要：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 Highpressure 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 trans formations in olivine with two compositions, namely Mg2SiO4 （Fol00） and （Mg0.9Feo.1）2SiO4 （Fo9o）, at pressures between 14.1 and 20 GPa and a constant temperature of 1400°C, using the newly installed multianvil system at the Laboratory for Studies of the Earth＇s Deep Interior （SEDI）, China University of Geosciences （Wuhan）. At 14.1 GPa, Fo90 transformed completely into the wadsleyite structure （13）, while Fol00 remained as olivine （ct）. Between 14.8 and 15.6 GPa, both Folo0 and Fo90 transformed into the wadsleyite structure. Wadsleyite crystals were identified by two characteristic Raman peaks between 722 and 723 and 917 and 919 cm1. They exhibit a bimodal grain size distribution： largecrystals with average grain sizes greater than 100 μm and micro crystals less than 10 pm. The population of microcrystals increased with pressure, apparently due to the increase in overpressure （the difference between the experimental pressure condition and the equilibrium transformation pressure at 1400°C）, which pro motes 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 （7） in Fol0o, but was absent in Fo90. At 20 GPa, both samples transformed completely into ringwoodite, which was characterized by the 798 and 840 cm1 Ra man lines. Ringwoodite crystals are euhedral grains （average grain size 1020 pm）, with welldev

关 键 词：地幔过渡带 压力条件 橄榄石 实验室 温度条件 相变 尖晶石结构 Mg2SiO4 

分 类 号：P542[天文地球—构造地质学] O642.1[天文地球—地质学]