5:15 PM - 7:15 PM
[SIT20-P12] Disproportionation of wadsleyite splits the 520-km seismic discontinuity
Keywords:520-km discontinuity, disproportionation of wadsleyite, phase transition, high-pressure and high-temperature
Abstract:
The seismic discontinuities, which define the layered structure of the solid Earth, are the most important features in the deep interior and play a critical role in geodynamics. They are generally caused either by compositional jumps (such as the Moho and Gutenberg discontinuities defining the Earth’s mantle) or by mineral phase transformations (e.g., the discontinuities at 410 and 660-km depths defining the mantle transition zone). In contrast, the 520-km discontinuity, separating the upper and lower parts of the mantle transition zone caused by the wadsleyite to ringwoodite phase transition, shows a splitting at around 500 and 560 km depths, while the origin of the splitting is unclear because a simple wadsleyite to ringwoodite transition should produce a single seismic discontinuity. Here, based on high-pressure and high-temperature experiments under mantle transition zone conditions, we found the disproportionation of wadsleyite to (Mg0.92Fe0.08)2SiO4 and (Mg0.89Fe0.11)2SiO4 compositions. Since iron affects the phase transition pressure between wadsleyite and ringwoodite strongly, the two types of wadsleyite should transfer to ringwoodite at different depths, leading to the splitting of the 520-km discontinuity. The disproportionation of wadsleyite may also account for the seismic anisotropy observed in the upper part of the mantle transition zone and the relatively weak seismic velocity jump of the 520-km discontinuity compared to that interpreted from mineral physics data.
The seismic discontinuities, which define the layered structure of the solid Earth, are the most important features in the deep interior and play a critical role in geodynamics. They are generally caused either by compositional jumps (such as the Moho and Gutenberg discontinuities defining the Earth’s mantle) or by mineral phase transformations (e.g., the discontinuities at 410 and 660-km depths defining the mantle transition zone). In contrast, the 520-km discontinuity, separating the upper and lower parts of the mantle transition zone caused by the wadsleyite to ringwoodite phase transition, shows a splitting at around 500 and 560 km depths, while the origin of the splitting is unclear because a simple wadsleyite to ringwoodite transition should produce a single seismic discontinuity. Here, based on high-pressure and high-temperature experiments under mantle transition zone conditions, we found the disproportionation of wadsleyite to (Mg0.92Fe0.08)2SiO4 and (Mg0.89Fe0.11)2SiO4 compositions. Since iron affects the phase transition pressure between wadsleyite and ringwoodite strongly, the two types of wadsleyite should transfer to ringwoodite at different depths, leading to the splitting of the 520-km discontinuity. The disproportionation of wadsleyite may also account for the seismic anisotropy observed in the upper part of the mantle transition zone and the relatively weak seismic velocity jump of the 520-km discontinuity compared to that interpreted from mineral physics data.