Mesoscale structures in the transition zone: Dynamical consequences of boundary layer activities

Abstract

Recent geophysical evidence from seismology, mineral physics, viscosity inversion shows that the mantle between 400 and 1000 km is extremely complicated, with intermediate scale structures present regionally as seismic reflectors under the 660 km discontinuity and bent plume-like structures under the transition zone. We have studied the dynamics of the transition zone with two models, an axisymmetric spherical-shell (2-D) model with a horizontally averaged temperature-and pressure-dependent viscosity and a 3-D Cartesian model with a depth-dependent viscosity. Two mantle phase transitions have been employed in both models. Results of the 2-D axisymmetric model show that the interaction of the lower mantle plumes with the transition zone can result in a horizontal channel flow right underneath the 660 km and in the birth of secondary plume some distance away from the lower mantle plume. The strength of the secondary plume increases in strength with larger viscosity contrast across the 660 km discontinuity. In the 3-D model we have found that with the presence of a second low viscosity zone somewhere between 660 and 1000 km, many secondary instabilities are developed in the second asthenosphere and the mesoscale thermal structure developed can become quite complex. Many small-scale plumes can emanate from the transition zone. Occasionally a very large plume burst, with a near-surface radius exceeding 1000 km, can develop from the hot lower-mantle material trapped in the second asthenosphere. Both the viscosity and the phase transition structure between 660 km and 1000 km can exert a significant influence on the plume distribution and cause singular plume eruption events in the upper mantle. Plume instabilities originating below the 660 km discontinuity in the western Pacific might have launched a large hot upwelling into the upper mantle, thus precipitating the massive flood basalt volcanism in the Ontong-Java region.

Mantle transition zone topography and structure beneath the Yellowstone hotspot

“…A dynamical model to explain this observation is that convective cells confined to the upper mantle are dominating the 410 thermal structure [ Gilbert et al , 2003]. If a lower mantle origin for the hotspot is preferred, then intermittent heat and mass transfer across the 660 [ Yuen et al , 1998] could explain our discontinuity topography observations.…”

Critical phenomena in thermal conductivity: Implications for lower mantle dynamics

“…The existence of thermo-chemical plumes (Yuen et al, 1993;Ishii and Tromp, 1999) engendered by iron enrichment further suggests both chemical differentiation of the lower mantle, and a density stratification favorable to inducing layered convection (discussed above). Formation of a second low viscosity zone from lower-mantle upwellings (Kido and Cadek, 1997;Yuen et al, 1998) would promote stratification.…”

Dynamics and Evolution of Venus’ Mantle Through Time

“…Phase transitions also alter mantle flow by either absorbing or releasing latent heat. Secondary plumes (e.g., Yuen et al 1998;Yoshida 2004) are likely to form when hot upwellings penetrate an endothermic phase change like that from ringwoodite to perovskite at a pressure of 23 GPa or equivalent depth of 730-740 km inside Venus (Ishii et al 2018;Trønnes et al 2019).…”