Sound velocities of Fe3Al2Si3O12 almandine up to 19 GPa and 1700 K

• Velocities of almandine have been determined under mantle transition P-T conditions.
• KS’ and G’ of almandine are substantially lower than those of the earlier study.
• Velocities of pyrolitic majorite are well reproduced by those of end-member garnets.

Elastic wave velocities of synthetic Fe3Al2Si3O12 almandine have been determined at simultaneous high pressure and temperature up to 19 GPa and 1700 K by the ultrasonic technique in conjunction with in situ synchrotron X-ray diffraction in a multi-anvil apparatus. Velocities of almandine are found substantially lower than those of other major end-member garnets such as pyrope, grossular, and MgSiO3 majorite, while their pressure and temperature derivatives are comparable to those of the latter garnets. The observed density, and compressional (VP) and shear (VS) velocities were combined and fitted to functions of the Eulerian strain EoS, yielding a adiabatic bulk modulus KS0 = 174.2 (12) GPa and a shear modulus G0 = 94.9 (7) GPa, and their pressure and temperature derivatives ∂KS/∂P = 4.61 (14), ∂G/∂P = 1.06 (6), ∂KS/∂T = −2.67 (7) × 10−2 GPa K−1, and ∂G/∂T = −1.31 (8) × 10−2 GPa K−1. The pressure derivative of the bulk modulus of almandine is similar to those of other garnet end-members, which is in contrast to the substantially higher value (∂KS/∂P = 6.2 (5)) reported for pure almandine in an earlier study based on experiments up to 3 GPa. The present new results combined with those of pyrope, grossular, and MgSiO3 majorite are successfully used to reproduce the sound velocities of majoritic garnet in the pyrolite composition.