The Elastic Anisotropy Change Near the 410‐km Discontinuity: Predictions From Single‐Crystal Elasticity Measurements of Olivine and Wadsleyite. Issue 4 (6th April 2018) - Zhang, Jin S.
Abstract: We present a model of the single‐crystal elasticity of olivine with ~9–10 mol% Fe based on all available single‐crystal velocity measurements. A set of finite strain equations of state that account for thermoelastic effects is used for fitting the individual elastic moduli at various pressure‐temperature conditions. The same analysis is applied to reanalyze the experimentally determined single‐crystal elastic moduli of hydrous and anhydrous Fe‐bearing wadsleyite. Based on the obtained thermoelastic parameters of individual elastic moduli, we then calculated various elastic anisotropy indices of olivine and wadsleyite and extrapolated them to the pressure‐temperature conditions expected in the Earth's mantle. The results suggest that the elastic anisotropy in the Earth's upper transition zone (410–520 km depth) is likely below the seismic detection limit if it originates primarily from the lattice preferred orientation of wadsleyite crystals. On the other hand, the elastic anisotropy of olivine is still relatively high (~19–25%) at 410 km depth. Thus, if a mantle flow field induces lattice preferred orientation of olivine and wadsleyite near 410 km depth, a reduction of elastic anisotropy is expected across the 410‐km discontinuity. Key Points: The elastic anisotropy of Fe‐bearing olivine remains nearly constant down to ~180 km depth, then decreases slightly with depth to ~410 km The elastic anisotropy of Fe‐bearing wadsleyite at transition zone pressures is likely less than half the value at ambient pressure In addition to a strong isotropic velocity increase, the 410‐km discontinuity could be characterized by a strong decrease of anisotropy