Elastic wave velocity of polycrystalline Mj80Py20 garnet to 21 GPa and 2,000 K

The elastic wave velocities of polycrystalline Mj 80 Py 20 garnet along the majorite–pyrope system have been measured at pressures up to 21 GPa and temperatures up to 2,000 K using ultrasonic interferometry in conjunction with in situ X-ray diffraction techniques in a Kawai-type multi-anvil apparatus. The elastic moduli of Mj 80 Py 20 garnet and their pressure and temperature derivatives are determined by a two-dimensional linear fitting of the present experimental data, yielding: K S = 161.5 (7) GPa , ∂K S / ∂P = 4.42 (4) , ∂K S / ∂T = −0.0154 (2) GPa/K , G = 86.2 (2) GPa , ∂G / ∂P = 1.28 (1) , ∂G / ∂T = −0.0096 (5) GPa/K. The present results together with those of the studies on the majorite–pyrope solid solutions suggest the pressure and temperature derivatives of elastic moduli are insensitive to the majorite content in the majorite–pyrope system. The velocity gradients of the majoritic garnets in the majorite–pyrope system are 3 ~ 6 times lower than those required to account for the high seismic velocity gradients observed in the mantle transition zone.