Accommodation of volume changes in phase transition zones: Macroscopic scale

Abstract

The volume changes associated with a phase transition need to be accommodated by creep. Otherwise, a pressure anomaly impeding the transformation is induced. Phase changes in mantle flows are therefore delayed by this process, which occurs not only on a microscopic scale but also, as shown in the present paper, on a macroscopic (i.e., kilometric) scale. The mechanism is illustrated first with the help of a simple analytical “pipe flow” model. The broadening and average deflection of the phase transition vary with the vertical velocity of the flow V, the viscosity in the zone of phase transition η, the density jump Δρ, and the pressure interval over which the phase change occurs Δ P. They are characterized by a nondimensional number: α = (4/3)ηgV (Δρ/ΔP2). For small α, the deflection of the phase transition is equal to α times the width of the transition loop. A numerical, one‐dimensional stationary model is then used to quantify how the flow is hampered by mass anomalies associated with the phase transition deflections. The response functions for harmonic loads in a self‐gravitating mantle are computed. Transition deflection is shown to have a significant influence in the case of a thin and viscous discontinuity. Conversely, it may be negligible if the material within transition zones is weakened. In an estimate where we link the viscosity in the transition zone to the level of deviatoric stress, we show that sizable (≈7 km) deflections of the 670 km discontinuity can be expected as a consequence of these macroscopic volume changes associated with phase transformations.