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

• Published in: Physics of the earth and planetary interiors 2000-06, Vol.120 (1), p.153-163
• Main Authors: Liu, Jun, Chen, Ganglin, Gwanmesia, Gabriel D, Liebermann, Robert C
• Format: Article
• Language: English
• Subjects: Elastic wave; Majorite; Pyrope
• ISSN: 0031-9201; 1872-7395
• DOI: 10.1016/S0031-9201(00)00152-7

Polycrystalline specimens of two pyrope–majorite garnets 2 2 Compositions Mg 2Al 3Si 2O 12–MgSiO 3 form a continuous solid-solution at high pressures and temperatures. The end members are: Mg 2Al 3Si 2O 12-pyrope and MgSiO 3-majorite (abbreviated here and throughout our paper as Py and Mj). The crystal structure of pyrope is cubic ( Ia3d) and that of majorite is tetragonal ( I 4 1/ a). As demonstrated by Parise et al. (1996), the symmetry changes from cubic to tetragonal near compositions with 75 mol% majorite. Both of the specimens under study in this paper are, therefore, of cubic symmetry with the garnet structure and represent solid-solutions between Mg 2Al 3Si 2O 12 and MgSiO 3. Thus, we believe that it is appropriate to term these specimens, and others in our studies, “pyrope–majorite garnets”. (Py 62Mj 38 and Py 62Mj 38) were synthesized in a 2000-ton uniaxial split-sphere apparatus (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. Ultrasonic interferometric measurements on these specimens were conducted at pressures up to 9 GPa at room temperature using a 1000-ton uniaxial split-cylinder apparatus (USCA-1000) with Bi as an internal pressure marker. From the measurements of the travel times for compressional (P) and shear (S) waves, the velocities and elastic moduli are calculated using length/density changes determined from the travel-time data. Third-order Eulerian finite strain analysis of these data yield the longitudinal ( L) and shear ( G) moduli and their pressure derivatives [ M 0′=(∂ M/∂ P) T ]: for Py 62Mj 38, L 0=291±6 GPa and L 0′=9.1±0.6, G 0=90±1 GPa and G 0′=1.9±0.2, from which we calculate the values for the adiabatic bulk modulus ( K= L−4/3 G) to be K 0=171±5 GPa and K 0′=6.2±0.5. These new data for the shear modulus and its pressure derivative are in agreement with the previous values for the same composition (Rigden, S.M., Gwanmesia, G.D., Liebermann, R.C., 1994. Elastic wave velocities of a pyrope–majorite garnet to 3 GPa. Phys. Earth Planet. Inter. 86, 35–44.), but the P wave velocity, the bulk modulus and its pressure derivative are somewhat higher. For the Py 50Mj 50 composition, these values 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-pressure derivatives of K and G for these Py–Mj garnets may be important in explaining the high velocity gradients observed in seismic models of