Thermal equation of state of Al‐ and Fe‐bearing phase D

Abstract

Pressure‐volume‐temperature relations have been measured to 20.6 GPa and 1273 K for Fe‐ and Al‐bearing phase D (Mg0.99Fe0.12Al0.09Si1.75H2.51O6) using synchrotron X‐ray diffraction with SPEED‐MkII multianvil press at SPring‐8 facility. The analysis of room temperature data fitted to a third‐order Birch‐Murnaghan equation of state (EOS) yields V0 = 85.32 ± 0.02 Å3; K0 = 141.5 ± 3 GPa and K' = 6.2 ± 0.4. The pressure was calibrated using the Au EOS by Anderson et al. (1989). Fixing K' to 4.0, gives K0 = 155.3 ± 0.8 GPa. These values are consistent with thermal EOS analysis as well as previous estimations for Fe‐Al‐bearing and Fe‐Al‐free phase D. A fit to high‐temperature P‐V‐T data using Birch‐Murnaghan EOS yields V0 = 85.32 ± 0.02 Å3; K0 = 139.6 ± 3.0 GPa; K' = 6.6 ± 0.4; (∂KT/∂T)P = ‐ 0.023 (8) GPa K−1 and zero‐pressure thermal expansion α = a0 + a1T with a0 = 3.4 (2) × 10−5 K−1 and a1 = 0.4 (6) × 10−8 K−1. The estimated Anderson‐Grüneisen parameter is δT = 4.9. Lattice dynamical approach using the Mie‐Grüneisen‐Debye EOS yields Grüneisen parameter γ0 = 1.09 ± 0.09 and q = 0.42 ± 0.97, if Debye temperature θ0 fixed at 920 K, as calculated from sound velocities. The EOS from this study enables the accurate estimation of the density of phase D in a pyrolitic composition under deep mantle conditions. The density reduction of hydrated subducting slab (∼1 wt.% H2O) in the lower mantle due to the presence of ∼7% of phase D would be 1.0%.