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

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 Mg2SiO4 (Fo100) and (Mg0.9Fe0.1)2SiO4 (Fo90), 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, Fo90 transformed completely into the wadsleyite structure (β), while Fo100 remained as olivine (α). Between 14.8 and 15.6 GPa, both Fo100 and Fo90 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 Fo100, but was absent in Fo90. 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 in eastern China determined from seismological studies cannot be explained by the simple transformation sequence of the olivine system alone. Phase transformations in other pyroxene-normative components (including pyroxenes and garnets) and the interaction of these components with olivine may be responsible for the complex structure. High-pressure and high-temperature experimental studies on complex systems (e.g. olivine-pyroxene), combined with data from geophysical exploration, may help in establishing a more realistic geological-petrological model for eastern China and further our understanding of the possible physical mechanisms that are responsible for the complex structure. Such studies will have profound implications for understanding the dynamic processes in the deep Earth interior.