Elastic wave velocities at Mg ₃ Al ₂ Si ₃ O ₁₃ -pyrope garnet to 10 GPa

State University of New York at Stony Brook, Mineral Physics Institute, Stony Brook, NY, United States

Elastic wave velocities of Mg ₃ Al ₂ Si ₃ O ₁₂ 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 degrees 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 G ₀ = 92+ or -1 GPa. G' ₀ = (theta G/theta P)T = 1.6+ or -0.2 and for the longitudinal modulus L ₀ = 294+ or -1 GPa, L' ₀ = (theta L/theta P)T = 7.4 + or - 0.5, (L = K _s +4/3G), from which the bulk modulus K ₀ = 171+ or -2 GPa, K' ₀ = (theta K _s /theta P)T = 5.3+ or -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.

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