Elastic wave velocities at Mg₃Al₂Si₃O₁₃-pyrope garnet to 10 GPa

Abstract

Elastic wave velocities of Mg3Al2Si3O12 pyrope garnet were measured to 10 GPa at ambient temperature, using ultrasonic interferometry in a 1000 ton split-cylinder, multi- anvil apparatus (USCA-1000). The sample used in the ultrasonic measurements was a polycrystalline specimen hot-pressed at 5 GPa and 1350 °C in a 2000 ton uniaxial split- sphere apparatus (USSA-2000) from a homogeneous glass of pyrope composition. Special P-T paths used during synthesis minimized effects of decompressing and thermal cracking; the bulk density of the sample was indistinguishable from the X-ray density. The elastic wave velocities measured at the ambient condition agree with the Hashin-Shtrikman av­erages of the single crystal values within the mutual uncertainties. The high-pressure ex­periments yielded the elastic moduli and their pressure derivatives (finite strain fit) for the shear modulus G0 = 92 ± 1 GPa, G′0 = (∂G/∂P)T = 1.6 ± 0.2 and for the longitudinal modulus L0 = 294 ± 1 GPa, L′0 = (∂L/∂P)T = 1A ± 0.5, (L = Ks + 4/3G), from which the bulk modulus K0 = 171 ±2 GPa, K′0 = (∂K/∂P)t = 5.3 ± 0.4 was calculated. The pressure derivative for the shear modulus of pyrope did not differ from those of natural pyrope-almandine-grossular garnets. For the bulk modulus, the pressure derivative for py­rope agreed with that for pyrope-almandine but was substantially higher (25%) than that for the Ca-bearing garnet. In the pyrope-majorite series, K′0 remained constant, whereas G′0 increased by 25 for 38% majorite content.