Conditions for melting and metasomatism in the Earth's mantle

Abstract

If we know the compositions of mantle rocks at various depths, and the geotherm in different tectonic environments, then the conditions for melting are defined by experimentally determined solidus curves. The term "metasomatism" in crustal processes is defined as reaction by solution or vapors, not by melts or magmas, and the same definition should apply to mantle processes (reaction with magmas is "hybridization"). H_2O, CO_2, or both are made available for metasomatic reactions at deep dissociation fronts, or by solidification of volatile-charged magmas. Regions eligible for metasomatism are limited by solidus curves above which the melts dissolve volatile components. Beneath the lithosphere, there can be no metasomatism between about 120 and 260 km, because melting intervenes. Solidification of kimberlitic magmas at the base of continent al lithosphere is a source of metasomatic fluids. Mantle metasomatism is expected in several regions above subducted oceanic lithosphere, interspersed with magmatic events. Major differentiation of the earth is accomplished by melting, but meta somatism may cause significant redistribution of some elements.