First principles determination of the structure and elasticity of hydrous ringwoodite

Abstract

The mechanism by which hydrogen is incorporated in ringwoodite affects the density, composition, and elasticity of the mineral, with implications on the seismic velocities of Earth's transition zone. This study uses density functional theory to calculate the structure, elasticity, and energetics of each of three possible defect mechanisms for the incorporation of hydrogen in ringwoodite. Relative energetics of the defects predict that the three defects exist in ratios of 64:10:25 for the VMg″ + 2H**, MgSi″ + 2H**, and VSi″″ + 4H**** defects, respectively. Results of a least squares fit to existing experimental data on hydrous ringwoodite produce a best fit model, including each of the three defects in ratios of 75:11:14. Therefore 51–66% of the hydrogen in hydrous ringwoodite is associated with VMg″ defects, 8–10% is associated with MgSi″ defects, and 24–41% is associated with VSi″″ defects. One wt % H2O decreases seismic wave velocities by an amount equivalent to ∼200–350 K temperature increase. The seismic parameter dlnvs/dlnvp owing to hydration is twice that owing to temperature and reduces the impedance contrast at the 520 km boundary, allowing for the separation of the effects of water and temperature in the lower transition zone.