First-principles Studies Of The Elastic Properties Of Hydrous And Iron-bearing Forsterite

Olivine, wadsleyite and ringwoodite are considered to be the major components of the upper mantle and the mantle transition zone. Based on density functional theory, the first-principles simulation is conducted to study the crystal structures of hydrous, iron-bearing, hydrous iron-bearing wadsleyite and ringwoodite. We discuss the effect of water content, iron and water on the elastic modulus (K, G) and wave velocities (Vp, Vs) of olivine, wadsleyite and ringwoodite from0to30GPa. The major results in this theses are summarized as followed:1. The elastic modulus and the wave velocities of olivine, wadsleyite and ringwoodite decrease with water contents by0,1.65,3.3wt.%, respectively. The shear wave velocities and the compressional wave velocities linearly decrease with the increasing of water contents.2. The elastic modulus of0,1.65wt.%and3.3wt.%olivine, wadsleyite, ringwoodite linearly increase with pressure (0-30GPa). However, compared with the variation of bulk modulus, the change of shear modulus is smaller.3. Incorporation of water reduces the bulk sound velocity contrasts from olivine to wadsleyite and from wadsleyite to ringwoodite. Furthermore, the bulk velocity contrasts generally decrease with pressure (0-30GPa).4. The stable crystal structures of6.25mol%wadsleyite, ringwoodite (Mg1.875Fe0.125SiO4) and hydrous iron-bearing wadsleyite, ringwoodite (Mg1.75Fe0.125sH0.25SiO4) were constructed.5. We discuss the effect of iron, water on the elastic modulus and the wave velocities from0to30GPa. The iron incorporation reduces the shear modulus and the wave velocities of wadsleyite and ringwoodite. The elastic modulus and the elastic wave velocities of hydrous iron-bearing wadsleyite, ringwoodite were reduced, because of the integrated effects of iron and water. With increasing pressure, this trend is gradually weakened.