Diffusion in minerals of the Earth's lower mantle: constraining rheology from first principles (original) (raw)
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Viscosity-depth profile of the Earth's mantle: Effects of polymorphic phase transitions
Journal of Geophysical Research, 1977
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 Eo* is shown to be systematically related to oxygen ion packing by Eo*(kcal/mol) = (187 ñ 16)-(3.8 ñ 0.8)Vo-(•a), where Vo = is the volume per oxygen ion at zero pressure and 25øC. This relation allows the change in activation energy/SE0* 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 •E0* in a general non-Newtonian flow law to estimate the increase in effective viscosity r/ 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) r/increasing by less than an order of magnitude across any phase transition, (2) r/ essentially constant throughout the lower mantle, and (3) r/in the lower mantle no more than 2 orders of magnitude greater than r/ in the upper mantle.
Earth and Planetary Science Letters, 2018
Lower mantle tomography models consistently feature an increase in the ratio of shear-wave velocity (V S) to compressional-wave velocity (V P) variations and a negative correlation between shear-wave and bulk-sound velocity (V C) variations. These seismic characteristics, also observed in the recent SP12RTS model, have been interpreted to be indicative of large-scale chemical variations. Other explanations, such as the lower mantle post-perovskite (pPv) phase, which would not require chemical heterogeneity, have been explored less. Constraining the origin of these seismic features is important, as geodynamic simulations predict a fundamentally different style of mantle convection under both scenarios. Here, we investigate to what extent the presence of pPv explains the observed high V S /V P ratios and negative V S-V C
Efficacy of the post-perovskite phase as an explanation for lowermost-mantle seismic properties
Nature, 2005
Constraining the chemical, rheological and electromagnetic properties of the lowermost mantle (D 00 ) is important to understand the formation and dynamics of the Earth's mantle and core. To explain the origin of the variety of characteristics of this layer observed with seismology, a number of theories have been proposed 1 , including core-mantle interaction, the presence of remnants of subducted material and that D 00 is the site of a mineral phase transformation. This final possibility has been rejuvenated by recent evidence for a phase change in MgSiO 3 perovskite (thought to be the most prevalent phase in the lower mantle 2 ) at near core-mantle boundary temperature and pressure conditions 3 . Here we explore the efficacy of this 'post-perovskite' phase to explain the seismic properties of the lowermost mantle through coupled ab initio and seismic modelling of perovskite and post-perovskite polymorphs of MgSiO 3 , performed at lowermostmantle temperatures and pressures. We show that a post-perovskite model can explain the topography and location of the D 00 discontinuity, apparent differences in compressional-and shearwave models 1 and the observation of a deeper, weaker discontinuity 4,5 . Furthermore, our calculations show that the regional variations in lower-mantle shear-wave anisotropy are consistent with the proposed phase change in MgSiO 3 perovskite.
Mantle Dynamics in Super-Earths: Post-Perovskite Rheology and Self-Regulation of Viscosity
2012
Abstract: Simple scalings suggest that super-Earths are more likely than an equivalent Earth-sized planet to be undergoing plate tectonics. Generally, viscosity and thermal conductivity increase with pressure while thermal expansivity decreases, resulting in lower convective vigor in the deep mantle. According to conventional thinking, this might result in no convection in a super-Earth's deep mantle. Here we evaluate this.
Journal of Geophysical Research, 2010
The pressure effect of silicon self-diffusion of MgSiO 3 perovskite was investigated by molecular dynamics (MD). The viscosity variation of MgSiO 3 perovskite in the lower mantle was derived by the Nabarro-Herring (Herring-Nabarro) model. For the MD calculation, the spontaneous jumping of atoms by self-diffusion was reproduced without using artificial forces, and the consistency of migration enthalpy (H m *) with experimental data was improved. The results showed that migration enthalpy increases monotonically with increasing pressure. The viscosity of MgSiO 3 perovskite in the lower mantle increases monotonically with increasing depth. The obtained depth profile is distinguishable from that of MgO periclase and can be utilized to determine which mineral dominates the lower mantle rheology. Depending upon the assumed shape of the depth profile for lower mantle viscosity, we considered the dominant mineral as (1) MgSiO 3 perovskite for the monotonic shape case or (2) MgO periclase for the hill shape case that has the highest-viscosity zone in the middle of the lower mantle.