THERMAL AND PETROLOGICAL CONSEQUENCES OF MELT MIGRATION WITHIN MANTLE PLUMES

The high temperatures and high degree of melting expected in the core of mantle plumes have virtually no expression in the eruption temperatures of hotspot lavas, nor in the composition of their glasses, which is restricted in the basaltic field. A solution to this paradox is looked for in the melt migration processes within the melting region of mantle plumes. Three dimensional convective calculations at Rayleigh number of 10(6) allow estimates of the Possible temperature, melt fraction and stress fields within a plume. Two regions with different melt migration patterns can be distinguished. A lower zone ranging in depth from the base of the melting region (150 km) to around 80-100 km where the first melt fraction is redistributed in a sub-horizontal vein network and convects in response to the steep horizontal temperature gradient. This process is able to homogenize the temperature within the melting region very efficiently. The high (300-degrees-C) temperature contrast between the centre of the plume and the surrounding mantle can be reduced to a few tens of degrees at the top of this zone. Fractional crystallization of high pressure phases will strongly modify the composition of the melt as it circulates toward the periphery of the melting region. A second upper zone, where the sub-vertical vein orientation will make possible rapid melt migration toward the surface, extends to the base of the lithosphere. Due to the buffering of the plume temperature around a value close to the mean upper mantle temperature, the degree of adiabatic melting within this upper zone will not greatly exceed that beneath normal spreading centres, even in the case of on-ridge hotspots. The lavas erupted at hotspots are likely to result from the mixing in various proportions of these low pressure melts (basalts) with the highly evolved liquids (possibly with kimberlitic to alkalic affinities) resulting from fractional crystallization of the high-pressure melt fractions produced at the base of the melting region. This scenario could account for the low eruption temperatures and Mg contents of hotspot lavas, in spite of a complex high pressure, and thus high temperature, history evidenced by some geochemical trends.