Effects of multiple phase transitions in a three‐dimensional spherical model of convection in Earth's mantle

Abstract

Abstract. Numerical models of mantle convection that incorporate the major mantle phase changes of the transition zone reveal an inherently three-dimensional flow pattern, with cylindrical features and linear features that behave differently in their ability to penetrate the 670-kin discontinuity. The dynamics are dominated by accumulation of cold linear downwellings into rounded pools above the endothermic phase change at 670 km depth, resulting in frequent "avalanches" of upper mantle material into the lower mantle. The effect of the exothermic phase transition at 400 km depth is to reduce the overall degree of layering by pushing material through the 670-kin phase change, resulting in smaller and more frequent avalanches, and a wider range of morphologies. Large quantifies of avalanched cold material accumulate above the coremantle boundary (CMB), restfiring in a region of strongly depressed mean temperature at the base of the mantle. The 670-kin phase change has a strong effect on the temperature field, with three distinct regions being visible: (1) the upper mantle, containing linear downwellings and pools of cold material in the transition zone and characterized by a high amplitude long-wavelength spectrum; (2) the midmantle, containing quasi-cylindrical avalanche conduits and characterized by a low amplitude, broad spectrum; and (3) the deep mantle, containing large pools of cold, avalanched material and characterized by a high amplitude, ultra-red (i.e., long wavelength) spectrum. The effect on the velocity field is very different. Flow penetration across the 670-kin phase change is strongly wavelength-dependent, with easy penetration at long wavelengths but strong inhibition at short wavelengths. Thus, when comparing numerical models with longwavelength seismic tomography, diagnostics based on the density field, such as the radial correlation function, are much more sensitive to the effects of phase transitions than those based on the velocity field. The amplitude of the geoid is not significantly affected by the partial layering, because the contribution from the strong heterogeneity in the transition zone is almost perfectly balanced by the contribution from deflection of the 670-km discontinuity. Avalanches are associated with geoid lows. However, a more complex viscosity structure is required to correctly match the sign of the geoid over slabs in Earth.

Influence of the Ringwoodite-Perovskite transition on mantle convection in spherical geometry as a function of Clapeyron slope and Rayleigh number

“…The approximate Ra and P values would suggest that we are today either in the transitional regime or just in the whole mantle convection regime and very unlikely to be in the layered regime. Seismological evidence seems to strongly support this, with observations of subducting slabs descending from the upper to the lower mantle (Grand et al, 1997;van der Hilst et al, 1997;Creager and Jordan, 1984) and also observations of stagnant slabs, which might reflect some resistance at this boundary (Fukao et al, 1992) …”

“…While there has been notable work including this phase change in spherical geometry (Tackley et al, 1993(Tackley et al, , 1994Machetel et al, 1995;Bunge et al, 1997), there has only been limited work in spherical geometry to characterise the influence of the value of the slope of the phase change and the vigour of convection on the behaviour. Elements of our work in spherical geometry have been presented previously (Wolstencroft and Davies, 2008a,b).…”

Thermo-Chemical Structure of the Lower Mantle: Seismological Evidence and Consequences for Geodynamics

“…Again, the density contrast and quantity of sinking material should not be too large to allow the development of instabilities. Slabs could temporarily stack in the mid-mantle, and reach the bottom of the mantle in occasional avalanches (Tackley et al, 1994). There are indeed some indications from classical tomography that significant amounts of slabs are deflected in the top of the lower mantle, around 1000 km (Fukao et al, 2001).…”

Partial advection of equidensity surfaces: A solution for the dynamic topography problem?

“…However, because continents are carried by plate tectonics at a presumably faster velocity than that at which the sluggish lower mantle evolves, the relative topography variations should be indicative of the maximum amplitude of the absolute dynamic topogra- The whole or layered mantle models offer a simplified view of mantle dynamics and neither is totally satisfactory. Models where a layered convection is punctuated by events (avalanches) of whole mantle flows have also been proposed [Machetel and Weber, 1991;Peltier and Solheim, 1992;Tackley et al, 1994]. However, from the point of view of computing an instantaneous dynamic topography, they are akin to whole mantle models during avalanches and to layered mantle models otherwise.…”