Some remarks on the movement of small melt fractions in the mantle

Details Some remarks on the movement of small melt fractions in the mantle Some remarks on the movement of small melt fractions in the mantle

Oct 1989

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1989e%26psl..95...53m&link_type=abstract

Earth and Planetary Science Letters, Volume 95, Issue 1-2, p. 53-72.

Computer Science

156

Scientific paper

Small melt fractions very rich in potassium are likely to be present beneath the lithosphere, and will separate from the matrix when there is more than 10-3% melt present if their viscosity is 0.1 Pa s. Such melt movement cannot transport heat, so the melt will solidify before it reaches the surface. Where it does so depends on the temperature within the lithosphere, and the mineralogy of mantle nodules is used to show that the thermal structure beneath the Kaapvaal Craton can be described by a plate model if the thickness of the mechanical boundary layer is 165 km, instead of being 100 km as is typical of oceanic areas. In a plate whose temperature changes little with time melt solidifies to produce a metasomatic layer. This layer is likely to be best developed in Archaean lithosphere, but is probably present in some form even within old oceanic plates. Because the melt fractions are so small, strong fractionation of Rb from Sr, and Sm from Nd, occurs in their source regions. The decay of 87Rb and 147Sm in the metasomatic layer can produce large isotopic anomalies relative to the convecting upper mantle in 1 Ga. Remelting of this layer occurs when it is heated by heat conduction from a plume, or decompressed by extension. Such melting requires a smaller perturbation to the temperature of the lithosphere than that which is necessary to produce dry melting, and can account for the isotopic ratios and K-rich composition of melts produced near plumes and in the early stages of continental stretching. Though it is more difficult to detect, a similar, though probably less enriched, layer may be present beneath the oceans.

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