Sound velocities of aluminum-bearing stishovite in the mantle transition zone

摘要

The elasticity of Al-bearing stishovite with 1.0, 3.3, and 4.5wt% Al2O3 was investigated in the multianvil apparatus at high pressures and temperatures up to 21GPa and 1700 K, by ultrasonic interferometry in conjunction with in situ X-ray techniques. The moduli K-S and G are found to decrease with increasing Al2O3 content, while their pressure and temperature derivatives do not change in a significant manner for 1.0 and 3.3 wt% Al2O3. The temperature derivatives for 4.5 wt% Al2O3, however, are larger, which may result from a change in the Al substitution mechanism at high Al2O3 content. It is shown that acoustic velocities of any mid-ocean ridge basalt are lower by -0.4% than those calculated from pure stishovite data. Velocity perturbations up to -3.4% (V-P) and -4.2% (VS) in subducted slabs are explained by the combination of the thermal equilibration (Delta T similar to 600 K) of the slab and Al enrichment in stishovite.