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Effect of thermal diffusion on the stability of strongly tilted mantle plume tails

Ross Kerr1, John Lister2 and Catherine Mériaux3

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 Institute of Theoretical Geophysics, Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge CB3 0WA, United Kingdom
3 School of Mathematical Sciences and MC2, Monash University, Victoria 3800,

Numerical calculations showing the rise and eventual instability of a cylinder of buoyant fluid, as a function of dimensionless time. The cylinder has an initial radius a, and a Rayleigh number of 80. The side views show the distribution of tracers (left) and buoyancy (right).

 

Mantle plumes are produced by heat conducted into the Earth's mantle from the underlying core. This heating forms a thermal boundary layer of hot, low viscosity fluid, which focuses into narrow plumes that rise through the mantle. At the Earth's surface, partial melting of the plumes produces flood basalts from plume heads and volcanic island chains from plume tails.

As plume tails rise through the mantle, they are deflected by large-scale convection driven by the subduction of cold lithospheric plates. Last year, we reported a series of laboratory experiments that investigated the effect of thermal diffusion on the gravitational stability of these plume tails when they have been strongly tilted. This year, we examined this instability using a series of numerical calculations (Figure 1).

At large viscosity ratios, we conclude that the instability is unaffected by thermal diffusion when the Rayleigh number Ra is greater than about 300. When Ra is less than 300, thermal diffusion significantly increases the time for instability, as the rising fluid region needs to grow substantially by entrainment before it becomes unstable. When Ra is less than about 140, and the rise height available is less than about 40 times the cylinder radius, the rising region of fluid is unable to grow sufficiently and instability is prevented. When our results are applied to the Earth, we predict that thermal diffusion will stabilize plume tails in both the upper and lower mantle (Kerr, Mériaux and Lister 2008). We also predict that some of the buoyancy flux in mantle plumes is lost during ascent to form downstream thermal wakes in any larger scale mantle flow.


Kerr RC, Mériaux C, Lister JR (2008) Effect of thermal diffusion on the stability of strongly tilted mantle plume tails. Journal of Geophysical Research 113: B09401, doi:10.1029/2007JB005510