Elasticity of Majorite and a Majorite‐Pyrope solid solution to high pressure: Implications for the Transition Zone

Abstract

[1 ] High seismic velocity gradients in the transition zone have been attributed to large pressure derivatives of the elastic moduli of transition zone minerals, such as majorite-garnet. Here we present Brillouin scattering measurements of the elasticity of polycrystalline Mg-majorite (Mj 100 , Mg 4 Si 4 O 12 ) and a Mj 50 Py 50 majorite -pyrope solid solution to pressures similar to those at the top of the transition zone (15.1(1) GPa and 12.8(1) GPa, respectively). The pressure derivatives of the adiabatic bulk (K S ) and shear (m ) moduli of both Mj 50 Py 50 and Mj 100 are 4.2(3) and 1.4(2), respectively, and are equal to those of pyrope within the experimental uncertainties. We conclude that neither the adiabatic compression of majorite-garnet nor any other likely transition zone phase is able to produce the high velocity gradients observed seismically. High velocity gradients are consistent with gradual transformations between minerals with very different elastic properties. The transformation of $ 25% or more of clinopyroxene into majorite above 520 km, and the formation of Ca silicate perovskite below 520 km could produce the observed seismic gradients.

Mechanical properties of ettringite and thaumasite—DFT and experimental study

“…The values of the elastic constants of thaumasite are systematically higher-this is due to the fact that the Si(OH) 6 2 − octahedral units are stiffer than Al(OH) 6 3 − similarly to what was observed in the case of SiO 6 8 − in comparison with AlO 6 9 − in several oxides and silicates [44][45][46][47][48]. On the contrary, the elastic anisotropy of thaumasite is smaller than in ettringite.…”

Pressure and temperature dependences of elastic properties of grossular garnet up to 17 GPa and 1 650 K

“…Although our ∂G/∂T value (-1.1×10 -2 GPa/K) is markedly lower than that of Isaak et al (1992), this value is still higher than those of pyrope and pyrolitic garnets. It has been reported that Mg end-member garnets (Mg 3 Al 2 Si 3 O 12 pyrope and MgSiO 3 majorite) showed similar temperature dependence of K S and G (Sinogeikin and Bass, 2002), and that the aluminum content does not influence the temperature dependence of K S and G. In contrast, the data of this study and Isaak et al (1992) suggest that the ∂G/∂T value depends on the calcium content, although ∂K S /∂T value is not significantly influenced by the calcium content. The data imply that Ca-rich garnet such as garnet in MORB composition should have stronger temperature dependence of V S than that of peridotite composition.…”

Calculation of the energetics of water incorporation in majorite garnet

“…Indeed variations in seismic velocities, expressed as dlnv s /dlnv p , have been used to suggest a strategy to disentangle the effects of temperature and water content for ringwoodite and wadsleyite (Panero 2010;Li et al 2011). The temperature and pressure dependence of majorite sound velocities have been established experimentally (Irifune et al 2008;Sinogeiken and Bass 2002). Constraining the effect of water on majorite elasticity is critical to the accurate interpretation of seismic velocities in Earth's transition zone.…”