SUMMARY: The most important factors which control the accumulation of primary magma in the mantle are:
(1) The degree of partial melting required for permeability, which is suggested as10–20%.
(2) The creep properties of the partially molten mantle, the type of creep probably being Nabarro-Herring creep.
(3) The type of flow of the ascending mantle material; the flow may be parallel or divergent dependent on the depth.
(4) The depth of extension of the feeder dykes depending on the tensional stress distribution with depth and the rheological properties of the mantle. It is shown that the partially molten mantle becomes stratified into layers of magma and residuum, the layers of residuum being one or several metres thick. The stratified mantle constitutes the primary source of magma. A dispersed source system is formed in ascending convection currents, while a large magma chamber is formed in ascending plumes.
The small diffusion rates imply that the melting is of a fractional type rather than batch melting.