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The stability of strongly tilted mantle plume tails

Ross Kerr1 and Catherine Mériaux2

1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia
2 School of Mathematical Sciences and MC2, Monash University, Victoria 3800, Australia.

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. The behaviour of these plume tails was first investigated in the laboratory by shearing compositional plumes (e.g. Whitehead, 1982; Richards and Griffiths, 1988). In these studies, the plume tails became gravitationally unstable if their angle to the horizontal became less than a critical angle. However, in subsequent laboratory experiments with sheared thermal plumes (Richards and Griffiths, 1989; Kerr and Mériaux, 2004), this gravitational instability has never been seen, even when the plumes were deflected to almost horizontal orientations.

To examine whether this contrasting behaviour is due to diffusion, we have investigated theoretically and experimentally the gravitational stability of a horizontal cylindrical region of buoyant fluid. At low Reynolds numbers and large viscosity ratios, the convective flow depends on only one dimensionless parameter: the Péclet number Pe. We find that the flow is stable at small Pe (Figure 1), and unstable to gravitational instability at large Pe (Figure 2). The critical Péclet number is about 140. These results explain the stability of the sheared thermal plume experiments of Richards and Griffiths (1989) and Kerr and Mériaux (2004). The results also predict that sheared thermal plume tails are gravitationally stable in both the upper and lower mantle.

Figure 1.  A stable rising cylinder of buoyant fluid, at Pe = 82.5


Figure 2.  Gravitational instability of a rising cylinder of buoyant fluid, at Pe = 533.


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Whitehead JA (1982) Instabilities of fluid conduits in a flowing earth - are plates lubricated by the asthenosphere? Geophysical Journal of the Royal astronomical Society. 70: 415-433