Influence of iron on the elastic properties of wadsleyite and ringwoodite

摘要

We investigate by first‐principles the influence of iron on the elastic properties of the β–phase (wadsleyite) and γ–phase (ringwoodite), polymorphs of olivine, the most abundant minerals of the upper and lower parts of the transition zone, respectively. Our study aims to complement experiments to understand details of the 410 km and 520 km discontinuities. The full elastic‐tensor C_(ij), bulk (K), and shear (G) moduli are determined under static conditions for β–γ–(Mg_(1–x)Fe_x)_2SiO_4 with x = 0.125. Wave propagation anisotropy in single crystals and polarization anisotropy in aggregates with preferred orientation are investigated and compared with those of iron‐free wadsleyite and ringwoodite for a thorough understanding of the effect of iron. We examine the effect of iron on velocity contrasts due to phase changes and conclude that iron enhances ΔV_P and ΔV_S across the α → β transition but suppresses them across the β → γ transition. The latter might contribute to suppress locally the 520 km discontinuity if this has a significant contribution from the β → γ transition. We show that lateral variation of iron, δx, produces lateral velocity heterogeneity ratios similar to those produced by lateral variations of temperature, δT, both producing ratios comparable to values extracted from seismic tomography studies. However, in contrast with δT, δx produces negative values for density to longitudinal and shear wave velocity ratios. This might be considered the fingerprint of lateral variations of iron concentration. These negative ratios appear similar to results inferred from geodynamical models compiled by Karato and Karki (2001) for the upper mantle and transition zone.