Search

Young's modulus and sound velocities of partially sintered alumina, zirconia, ATZ, and mullite ceramics with a broad range of porosities determined via the impulse excitation technique (IET) and the ultrasonic wave propagation pulse‐echo technique (UPT) are found to be in good agreement and exhibit sigmoidal curves as a function of the sintering temperature. The porosity dependence of the relative values of Young's modulus and sound velocities is either close to or below (but never above) the prediction based on an exponential relation, and none of them is significantly below the prediction based on a percolation relation or a recently proposed numerical benchmark relation for overlapping monosized spheres. Values are lower when concave pores or irregular anisometric grains are dominating. The correlation of the normalized longitudinal wave velocities and relative transverse wave velocities, with VL/VT ratios ranging from 0.50 to 1.91, is well predicted by models for Poisson's ratios between 0.35 and 0.2. [ABSTRACT FROM AUTHOR]

Using first-principles density functional theory (DFT), we model the thermoelastic properties of hydrated ringwoodite (γ-(Fe, Mg) 2 SiO 4), a potential water reservoir in the Earth's mantle transition zone (MTZ). Our calculations indicate that hydration, in general, leads to a reduction in the sound wave velocity of ringwoodite. However, increased pressure tends to suppress this reduction. For our 1.56 wt% H 2 O containing ringwoodite model, we find that the suppression is so large that at pressure and temperature (P-T) conditions relevant to the lower part of the MTZ the sound wave velocities for the hydrous (1.56 wt% H 2 O) ringwoodite become very similar to that of the anhydrous ringwoodite. However, when the water concentration is increased to 3.3 wt%, the pressure-induced suppression of the velocity reduction due to hydration is not so significant. We have given a plausible explanation for the same on the basis of the electronic structure of the hydrous ringwoodite models. We conclude that in the lower part of the MTZ, seismic wave data is sufficiently robust to detect regions of very high-water content (∼3.3 wt%). However, if the water concentration is less than 1.56 wt%, sound wave velocities may not be able to precisely detect the state of hydration of the MTZ. [Display omitted] • Our study provides important insights into the defect chemistry of Fe containing hydrous ringwoodite. • Though hydration leads to a reduction of the sound wave velocities, increased pressure tends to suppress this reduction. • Seismic waves may fail to detect regions of low H 2 O concentration (< 1.56 wt%) in lower part of the mantle transition zone. • However, they may be robust enough to detect regions of very high-water content (∼3.3 wt%). [ABSTRACT FROM AUTHOR]

The low-velocity zone (LVZ) is a persistent seismic feature in a broad range of geological contexts. It coincides in depth with the asthenosphere, a mantle region of lowered viscosity that may be essential to enabling plate motions. The LVZ has been proposed to originate from either partial melting or a change in the rheological properties of solid mantle minerals. The two scenarios imply drastically distinct physical and geochemical states, leading to fundamentally different conclusions on the dynamics of plate tectonics. We report in situ ultrasonic velocity measurements on a series of partially molten samples, composed of mixtures of olivine plus 0.1 to 4.0 volume % of basalt, under conditions relevant to the LVZ. Our measurements provide direct compressional (V P) and shear (V S) wave velocities and constrain attenuation as a function of melt fraction. Mantle partial melting appears to be a viable origin for the LVZ, for melt fractions as low as ~0.2%. In contrast, the presence of volatile elements appears necessary to explaining the extremely high V P/V S values observed in some local areas. The presence of melt in LVZ could play a major role in the dynamics of plate tectonics, favoring the decoupling of the plate relative to the asthenosphere.

The pressure dependence of the sound velocities, single-crystal elastic constants, and shear and adiabatic bulk moduli of a natural gahnite (ZnAl2O4) spinel have been determined to ∼9 GPa by gigahertz ultrasonic interferometry in a diamond anvil cell. The elastic constants of gahnite are (in GPa): C11 = 290(3), C12 = 169(4), and C44 = 146(2). The elastic constants C11 and C12 have similar pressure derivatives of 4.48(10) and 5.0(8), while the pressure derivative of C44 is 1.47(3). In contrast to magnetite, gahnite does not exhibit C44 mode softening over the experimental pressure range. The adiabatic bulk modulus KS0 is 209(5) GPa, with pressure derivative Ks' = 4.8(3), and the shear modulus G0 = 104(3) GPa, with G'= 0.5(2). Gahnite, along with chromite (FeCr2O4) and hercynite (FeAl2O4) are the least compressible of the naturally occurring oxide spinels. Evaluation of Birch's Law for isostructural minerals indicates that spinels containing transition metals on both the [4]A and [6]B sites follow a trend about five times more negative than oxide and silicate-spinel phases without any, or only one transition metal. [ABSTRACT FROM AUTHOR]