Cation disorder in ringwoodite and its effects on wave speeds in the Earth's transition zone

Abstract

Inclusion of the effects of point defects on ringwoodite's elasticity helps reconcile mineralogical models of the transition zone with seismic observations of the impedance contrast at 520‐km depth. This study uses density functional theory to calculate the disorder and its effects on the elasticity of Mg2SiO4‐spinel (ringwoodite) in the Earth's transition zone. Spinel‐structured Mg2SiO4, ringwoodite, also can contain significant amounts of thermally activated coupled point defects between Mg sites and Si sites. The energetics of Mg2SiO4‐spinel, inverse spinel, and disordered structures show ∼4% Mg‐Si disorder in the mantle's transition zone, where 100‐K lateral temperature variations can cause approximately 1.2% change in disorder. Lateral temperature variations in the transition zone lead to wave speed changes because of the minerals' intrinsic temperature‐related elasticity changes. Independent of temperature, 1% cation disorder causes an additional decrease in c11 and c44 of 0.8% and 0.4%, respectively. Since the structure and stability of the Mg2SiO4 polymorphs play a key role in the density and impedance contrasts across the boundaries in the Earth's transition zone, the effect of defects has to be taken into account.