Elasticity Of Spinel At High Pressures And Temperatures:Implications For The Hales Discontinuity

Many discontinuities have been discovered in the Earth,such as Moho,410 km discontinuities,and 660 km discontinuities.Grasping the causes of these seismic discontinuities is very important to understand the evolution of the Earth.Besides the obvious ones,many secondary discontinuities,such as the Hales discontinuities,have also been extensively investigated recently.The Hales discontinuity was first observed in the central United States,showing a-4%shear wave velocity jump at a depth of 75 km.It was proposed that this discontinuity was caused by the mineral transition from spinel to garnet.To further understand the Hales discontinuity,we calculate the thermoelasticity of spinel by the first-principles calculations based on density functional theory.The thermodynamic and elastic properties of spinel at high pressures and temperatures up to 15 GPa and 1800 K are calculated based on density functional theory within the local density approximation.Our calculated results are consistent well with the available experimental data.C₁2 and K_s of spinel depend linearly on pressure,while C₁1,C₄4,and G exhibit slightly nonlinear dependence on pressure.Pressure effects on V_p and V_s are nonlinear mainly due to the nonlinear relation between G and pressure.We do not observe the 727cm^-1 and 492cm^-1 Raman peaks and the distinct softening in K_s of spinel at?5 GPa and ambient temperature.The maximum contribution of spinel to A_s at top upper mantle is just 2%,which can be negligible at top upper mantle.Spinel has the greatest sound velocities and V_p/V_s ratio up to the depth of 400 km among the major minerals in the upper mantle.The jump of V_s caused by the spinel to garnet transition is far less than a?4%jump in V_s in observed in the Hales discontinuity.Therefore,the Hales discontinuity may not be caused by this transition.