The Upward Migration of Self-Convecting Magma Bodies

Abstract

A magma body existing in an upper mantle containing a small percentage of partial melt should be subject to internal convection. The presence of partial melt means that the matrix follows closely the temperature gradient versus pressure, or depth, of the mantle solidus. This gradient is several times the adiabatic gradient of a mixed magma body, and the difference is available to drive convection. A magma body rising by Stokes drift in a viscous upper mantle at 0.1 m y^-1 would contain 11 km³. If thermal convection were dominant, the body would elongate vertically and migrate upward at 18 m y^-1 by melting material off the top and crystallising it at the bottom. However, in a chemically inhomogeneous system, the process of dissolving an unlike solid off the top is limited by chemical diffusion and the convection behaves as chemically driven. For olivine dissolving into dry basalt the upward velocity drops to 0.16 m y^-1, taking 0.7 My to traverse 100 km.

Larger bodies move more rapidly than smaller ones and would tend to sweep them up, as well as concentrating incompatible elements into the melt. The difference between ridge-crest upwelling, where mantle matrix rises to near the surface as well as does the magma, and island ‘hot spot’ volcanism, where the crystals are irreversibly left behind, can account for the differences in basalt composition. The peculiar sequence and timing of eruptions on the Hawaiian Islands is readily explained by magma self-convection and thermal diffusion in the lower lithosphere. While the mantle may be quite heterogeneous, there is no need to postulate complex sequential melting events, or different geochemical reservoirs that persist over geologic time.