Viscosity-depth profile of the Earth's mantle: Effects of polymorphic phase transitions

Abstract

We study the changes in rheological parameters and effective viscosity across mantle polymorphic phase transitions and the variations of these quantities with temperature and pressure throughout the earth's mantle. The intrinsic activation energy for oxygen ion diffusion in oxides E0* is shown to be systematically related to oxygen ion packing by E0*(kcal/mol) = (187±16) ‐ (3.8±0.8) Vo=(Å3), where Vo= is the volume per oxygen ion at zero pressure and 25°C. This relation allows the change in activation energy δE0* across a polymorphic phase transition to be estimated from the associated change in density. Under the assumptions that the activation volume V* remains constant or decreases across a phase transition and that the activation energy for subsolidus creep is equal to the activation energy for O= diffusion, we use δE0* in a general non‐Newtonian flow law to estimate the increase in effective viscosity η and the decrease in activation volume V* across a phase transition. By modeling V* and the activation energy by using thermodynamical and mechanical relations for elastic continua and data from seismically derived earth models for relevant elastic parameters, we are able to estimate better the increases in viscosity across polymorphic phase transitions and throughout an adiabatic mantle. Our best models of mantle viscosity have (1) η increasing by less than an order of magnitude across any phase transition, (2) η essentially constant throughout the lower mantle, and (3) η in the lower mantle no more than 2 orders of magnitude greater than η in the upper mantle.

Constraints on Seismic Models from Other Disciplines - Constraints on 3-D Seismic Models from Global Geodynamic Observables: Implications for the Global Mantle Convective Flow

“…We will consider the latter first since the surface boundary conditions can be obtained as a special case of the internal matching conditions. These density jumps have long been interpreted as manifestations of phase-change boundaries (e.g., Jeanloz and Thompson, 1983), which presumably also affect the depth variation of viscosity at these depths (e.g., Sammis et al, 1977). The PREM density profile is characterized by two major jumps at depths of 400 and 670 km.…”

High‐resolution, two‐dimensional vertical tomography of the central Pacific mantle using ScS reverberations and frequency‐dependent travel times

“…This viscosity profile also includes a low-viscosity asthenospheric channel with subasthenosphere viscosity, low enough to allow flow, and a high-viscosity lower mantle, which are all required in order to fit the global geoid observations [Hager, 1991]. Placing these viscosity jumps near the phase transitions at 400 and 670 km seems to be geophysically plausible [Sammis et al, 1977]. Since these elastic thicknesses are 3-5 times larger than those expected for the Tonga-Hawaii corridor [Cahnant, 1987], these particular models are ruled out.…”

On lateral viscosity contrast in the mantle and the rheology of low-frequency geodynamics

“…For creep controlled by dislocation motion, the effective (stress-averaged) viscosity is (2.4) qen = (Q' -"/A) ~X P CG*(p, T)/R 77 , where A is a constant, n is a power-law exponent ( x 3), G* is the Gibbs free activation energy, and R the gas constant with pressure P and temperature T (Sammis et al 1977;Karato 1989). Exponential dependence occurs in all mantle-creep processes wherein ionic diffusion controls rate.…”