The influence of a chemical boundary layer on the fixity, spacing and lifetime of mantle plumes

Abstract

Seismological observations provide evidence that the lowermost mantle contains superposed thermal and compositional boundary layers that are laterally heterogeneous. Whereas the thermal boundary layer forms as a consequence of the heat flux from the Earth's outer core, the origin of an (intrinsically dense) chemical boundary layer remains uncertain. Observed zones of 'ultra-low' seismic velocity suggest that this dense layer may contain metals or partial melt, and thus it is reasonable to expect the dense layer to have a relatively low viscosity. Also, it is thought that instabilities in the thermal boundary layer could lead to the intermittent formation and rise of mantle plumes. Flow into ascending plumes can deform the dense layer, leading, in turn, to its gradual entrainment. Here we use analogue experiments to show that the presence of a dense layer at the bottom of the mantle induces lateral variations in temperature and viscosity that, in turn, determine the location and dynamics of mantle plumes. A dense layer causes mantle plumes to become spatially fixed, and the entrainment of low-viscosity fluid enables plumes to persist within the Earth for hundreds of millions of years.

Constraints on thermochemical convection of the mantle from plume heat flux, plume excess temperature, and upper mantle temperature

“…However, owing to the temperature dependent viscosity, the top layer in the current study is significantly (a factor of $20) more viscous than the bottom layer ( Figure 5b). The entrainment should eventually mix the two components and destroy a layered system [Sleep, 1988;Davaille, 1999a;Gonnermann et al, 2002;Jellinek and Manga, 2002;McNamara and Zhong, 2004;Zhong and Hager, 2003]. However, none of our models has evolved into that stage, while reaching a steady state (Figure 5a).…”

“…[3] Mantle compositional structure (i.e., isochemical and whole mantle convection versus thermochemical and lay-ered mantle convection) has significant effects on the dynamics of the mantle, as demonstrated by studies on heat transfer and mantle structure for a compositionally heterogeneous or layered mantle [Davaille, 1999a[Davaille, , 1999bTackley, 1998Tackley, , 2002Kellogg et al, 1999;Jellinek and Manga, 2002;Zhong, 2004, 2005]. Although these studies provide important insights into the dynamics of a layered mantle, little attempt has been made to examine the consequences of a layered mantle to surface observations that may be used to constrain layered mantle models.…”

Deep mantle structure and the postperovskite phase transition

“…This number largely determines the stability and morphology of a dense layer embedded at the base of the mantle and is commonly used in both numerical and dynamical experiments (22,(24)(25)(26). A moderately thick chemical layer can Fig.…”

On the location of plumes and lateral movement of thermochemical structures with high bulk modulus in the 3-D compressible mantle

“…The stability of the dense chemical structures is mainly controlled by the balance between net buoyancy (thermal and chemical effects combined) and plume-induced entrainment that tends to erode the chemical structures over time [Sleep et al, 1988;Davaille, 1999;Jellinek and Manga, 2002;Gonnermann et al, 2002;Zhong and Hager, 2003;Nakagawa and Tackley, 2004;Huang, 2008]. A net negative buoyancy of ∼1% for the dense chemical structures may be needed for the structure to survive for 4.5 Gyr of the Earth's history, if the chemical density anomaly is constant with depth [Zhong and Hager, 2003].…”