V. Petrogenesis and discussion

Abstract

Basaltic magmas are formed by partial melting of a source rock of peridotitic composition (pyrolite) under upper mantle conditions. Experimental studies of the mineralogy of pyrolite and the melting relations of various basaltic magmas under high-pressure conditions are integrated in an attempt to present an internally consistent model of source composition, derived liquid compositions and residual mantle compositions. The role of a small (0.1 %) content of water in the upper mantle is treated in some detail. The presence of the low velocity zone in the upper mantle is attributed to a small (< 5 %) degree of melting of pyrolite containing approximately 0.1% water. The small liquid fraction present in the low-velocity zone is highly undersaturated olivine nephelinite or olivine melilite nephelinite. Other magma types of direct upper mantle derivation ranging from olivine trachybasalt to olivine melilitite and to tholeiitic picrite are assigned to a genetic grid expressing the depth (pressure) of magma segregation, the degree of partial melting of the source pyrolite, the water content and approximate temperature of the magma. While this genetic model can account for variations in major element abundances and normative mineralogy among basalts, there are variations in abundances of the incompatible elements, particularly K, Rb, Ba, and the rare earths, which are inconsistent with a model invoking a constant source composition for all mantle-derived basalts. Additional factors producing source inhomogeneity, particularly in incompatible element abundances, include the possibility of two-stage melting and of chemical zoning within the low-velocity zone. It is suggested that vertical migration of a fluid or incipient melt phase, enriched in H 2 O, CO 2 and incompatible elements, occurs within the low-velocity zone. The evolution of continental and oceanic rift systems and of the Hawaiian volcanic province is discussed in relation to the depths and conditions of magma genesis derived from the models of magma genesis.