A review of experimental evidence on the origin of basaltic and nephelinitic magmas

Abstract

Geological and petrological arguments are used to select those basaltic compositions which have been directly derived from the upper mantle with minimum opportunity for crystal fractionation or for contamination within the crust. The experimental study of the crystallization of these magmas at high pressures, under both anhydrous and hydrous conditions provides evidence of genetic links between magma types. These studies also provide data necessary to evaluate a major constraint on “primary” magmas, i.e. that magmas derived by direct partial melting of the upper mantle must have olivine and enstatite among their liquidus phases at the P, T conditions of magma segregation. The characteristic magma derived from either direct partial melting (∼ 20–25%) of pyrolite or fractional crystallization of olivine-rich tholeiite at depths of 15–35 km is high alumina olivine tholeiite. At 35–70 km partial melting of anhydrous pyrolite or fractional crystallization of olivine-rich tholeiite or tholeiitic picrite produces a series of liquids from olivine-rich tholeiite through olivine basalt and alkali olivine basalt to basanite (∼25% Ol, ∼5% Ne). With availability of water in the pyrolite source or during crystal fractionation, the pyrolite solidus is depressed. This produces changes in the subsolidus mineralogy and in the nature of liquids formed by low degrees of partial melting or by extensive crystal fractionation of less undersaturated magmas. Olivine nephelinites and basanites may be generated by small degrees of melting of water-bearing (0.1–0.2% H 2O) pyrolite at ∼50–70 km and olivine melilite nephelinites, olivine-rich nephelinites and olivine-rich basanites form in a similar way, as hydrous magmas at 70–100 km depth.