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Published December 1993 | public
Journal Article

Entrainment from a bed of particles by thermal convection

Abstract

Differentiation in magma chambers, in the Earth's core and in the partially molten early Earth is a competitive process between sedimentation and re-entrainment of crystals in the presence of convection. Previous studies show that the particles suspended in convective layers eventually settle and do so almost as fast as in the absence of convection. However, the nature and magnitude of the competing entrainment has remained unclear. Here we provide a simple theory and experimental evidence showing that entrainment occurs at the crests of dunes created in the particle bed at the base of a convecting fluid. In both laminar and turbulent regimes, the dune formation and entrainment are driven by viscous stresses produced by thermal plumes. At sufficiently high Rayleigh numbers the particles are probably entrained by Reynolds stresses. Entrainment in the Earth's core is hardly possible because it requires unreasonably small crystals. Entrainment of 10^(−2)–10^(−1) cm diameter crystals is very likely in magma oceans. For magma chambers entrainment requires large viscosities (> 10^6 P) and even when it occurs, the total amount of the suspended solid fraction is very small.

Additional Information

© 1993 Elsevier Science Publishers B.V. Received August 2, 1993; revised and accepted October 10, 1993. The authors wish to thank Charles Campbell and Stewart Turner for discussion and comments, the reviewers for their time and efforts in evaluating the manuscript and Michael Baker and Moritz Heimpel for the help in preparing the experiment. This work was supported by a NSF grant EAR-9205821.

Additional details

Created:
August 22, 2023
Modified:
October 24, 2023