Accommodation of volume changes in phase transition zones:Macroscopic scale

摘要

The volume changes associated with a phase transition need to be accommodated bycreep. Otherwise, a pressure anomaly impeding the transformation is induced. Phasechanges in mantle flows are therefore delayed by this process, which occurs not only on amicroscopic 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 transitionvary with the vertical velocity of the flow V, the viscosity in the zone of phasetransition the density jump p, and the pressure interval over which the phase changeoccurs Δ P. They are characterized by a nondimensional number: α = (4/3)gV (Δ_p/ΔP~2).For small a, the deflection of the phase transition is equal to a times the width of thetransition loop. A numerical, one-dimensional stationary model is then used to quantifyhow the flow is hampered by mass anomalies associated with the phase transitiondeflections. The response functions for harmonic loads in a self-gravitating mantle arecomputed. Transition deflection is shown to have a significant influence in the case of athin and viscous discontinuity. Conversely, it may be negligible if the material withintransition zones is weakened. In an estimate where we link the viscosity in the transitionzone to the level of deviatoric stress, we show that sizable (≈7 km) deflections of the 670 kmdiscontinuity can be expected as a consequence of these macroscopic volume changesassociated with phase transformations.