Elastic wave velocities of pyrope-majorite garnets (Py(62)Mj(38) and Py(50)Mj(50)) to 9 GPa

Polycrystalline specimens of two pyrope-majorite garnets(2) (Py(62)Mj(38) a nd Py(62)Mj(38)) were synthesized in a 2000-ton uniaxial split-sphere appar atus (USSA-2000) at pressures of 16 GPa and temperatures of 1670 K for run durations of about 2 h using homogeneous glasses as starting materials. Ult rasonic interferometric measurements on these specimens were conducted at p ressures up to 9 GPa at room temperature using a 1000-ton uniaxial split-cy linder apparatus (USCA-1000) with Bi as an internal pressure marker. From t he measurements of the travel times for compressional (P) and shear (S) wav es, the velocities and elastic moduli are calculated using length/density c hanges determined from the travel-time data. Third-order Eulerian finite st rain analysis of these data yield the longitudinal (L) and shear (G) moduli and their pressure derivatives [M-0' = (partial derivative M/partial deriv ative P)(T)]: for Py(62)Mj(38), L-0 = 291 +/- 0.6 GPa and L-0' = 9.1 +/- 0. 6, G(0) = 90 +/- 1 GPa and Gb = 1.9 +/- 0.2, from which we calculate the va lues for the adiabatic bulk modulus (K = L - 4/3 G) to be K-0 = 171 +/- 5 G Pa and K-0' = 6.2 +/- 0.5. These new data for the shear modulus and its pre ssure derivative are in agreement with the previous values for the same com position (Rigden, S.M., Gwanmesia, G.D., Liebermann, R.C., 1994. Elastic wa ve velocities of a pyrope-majorite garnet to 3 GPa. Phys. Earth Planet. Int er. 86, 35-44.), but the P wave velocity, the bulk modulus and its pressure derivative are somewhat higher. For the Py(50)Mj(50) composition, these va lues are L-0 = 289 +/- 6 GPa, L-0' = 9.2 +/- 0.7, G(0) = 89 +/- 1 GPa, G(0) ' = 2.1 +/- 0.2, K-0 = 170 +/- 5 GPa, and K-0' = 6.4 + 0.5. The high-pressu re derivatives of K and G for these Py-Mj garnets may be important in expla ining the high velocity gradients observed in seismic models of the transit ion zone of the Earth's mantle. (C) 2000 Elsevier Science B.V. All rights r eserved.