Continental Rifting and Mantle Degassing

Abstract

Various lines of evidence have previously revealed the necessity for a vapour phase as an active ingredient of continental rift valley magmatism. Further constraints are now placed on the magmatology by the consistently low H₂O contents of natural peralkaline felsic glasses. The H₂O data confirm that these “highly evolved” liquids cannot be the products of closed system fractional melting or crystallization. They show, too, that the vapour coexisting with rift valley magmas must be deficient in H₂O. The vapour must be strongly reducing, with high concentrations of carbon and halogen gases, and probably nitrogen. Rift valley magmas result from the fluxing action of this vapour as it escapes from the deep mantle and passes through the lithosphere segment below the rift. Initially, the vapour produces metasomatic carbonates, and halogen bearing silicates in the upper mantle and lower crust: subsequently it buffers the activities of Na, Fe, Si, F and CI, and trace elements in the ensuing magmas. The physical expression of the activity is, first, metasomatic and thermal uplift: followed by devolatilization, melting, eruption and subsidence as the flux cycle climbs through the lithosphere.