Single-crystal elasticity of grossular- and almandine-rich garnets to 11 GPa by Brillouin scattering

The high-pressure elasticity of grossular-rich Grs₈₇And ₉Pyp₂Alm₂ and almandine-rich Alm ₇₂Pyp₂₀Sps₃Grs₃And ₂ natural garnet single crystals were determined by Brillouin scattering to 11 GPa in a diamond anvil cell. The experiments were carried out using a 16:3:1 methanol-ethanol water mixture as pressure medium. The aggregate moduli as well as their pressure derivatives were obtained by fitting the data to Eulerian finite strain equations. The inversion yields K_S0 = 165.0 ± 0.9 GPa, G₀ = 104.2 ± 0.3 GPa, (∂K_S/∂P) _T0 = 3.8 ± 0.2, and (∂G/∂P) ₀ = 1.1 ± 0.1 for the grossular-rich composition and K_S0 = 174.9 ± 1.6 GPa, G₀ = 95.6 ± 0.5 GPa, (∂K_S/∂P)_T0 = 4.7 ± 0.3, and (∂G/∂P)₀ = 1.4 ± 0.1 for the almandine-rich garnet. Both individual and aggregate elastic moduli of the two garnets define nearly linear modulus pressure trends. The elastic anisotropy of the garnets increases weakly in magnitude with compression. Isothermal compression curves derived from our results are generally consistent with static compression data under hydrostatic conditions, and the effects of nonhydrostaticity on previous diffraction data can be identified. The pressure derivatives obtained here are generally lower than those reported in high-pressure polycrystalline ultrasonic elasticity studies. In combination with earlier Brillouin scattering data for pyrope, our results allow us to constrain the effect on elastic moduli of Fe²⁺-Mg²⁺ substitution in pyrope-almandine, Ca²⁺-Mg²⁺ in pyrope-grossular, and Fe³⁺-Al³⁺ substitution in andradite-grossular at high pressures. This new data set thus allows us to place improved constraints on the compositional dependence of seismic velocities in the rocks of the upper mantle.