PHASE-EQUILIBRIUM CONSTRAINTS ON THE ORIGIN OF BASALTS, PICRITES, ANDKOMATIITES

Experimental phase equilibrium studies at pressures ranging from 1 atm to 10 GPa are sufficient to constrain the origin of igneous rocks for med along oceanic ridges and in hotspots. The major element geochemist ry of MORE is dominated by partial crystallization at low pressures in the oceanic crust and uppermost mantle, forcing compliance with liqui d compositions in low-pressure cotectic equilibrium with olivine, plag ioclase and often augite too; parental magmas to MORE formed by partia l melting, mixing, and pooling have not survived these effects. Simila rly, picrites and komatiites can transform to basals by partial crysta llization in the crust and lithosphere. However, parental picrites and komatiites that were successful in erupting to the surface typically have compositions that can be matched to experimentally-observed anhyd rous primary magmas in equilibrium with harzburgite [L + Ol + Opx] at 3.0 to 4.5 GPa. This pressure is likely to represent an average for po oled magmas that collected at the top of a plume head as it flattened below the lithosphere. There is substantial uniformity in the normativ e olivine content of primary magmas at all depths in a plume melt colu mn, and this results in pooled komatiitic magmas that are equally unif orm in normative olivine. However, the imposition of pressure above 3 GPa produces picrites and komatiites with variations in normative enst atite and Al2O3 that reveal plume potential temperature and depths of initial melting. Hotter plumes begin to melt deeper than cooler plumes , yielding picrites and komatiites that are enriched in normative enst atite and depleted in Al2O3 because of a deeper column within which or thopyroxene can dissolve during decompression. Pressures of initial me lting span the 4 to 10 GPa range, increasing in the following order: I celand, Hawaii, Gorgona, Belingwe, Barberton. Parental komatiites and picrites from a single plume also exhibit internal variability in norm ative enstatite and Al2O3, indicating either a poorly mixed partial me lt aggregation process in the plume or the imposition of partial cryst allization of olivine-orthopyroxenite on a well-mixed parental magma. Plume shape and thermal structure can also influence the petrology and geochemistry of picrites and komatiites. Liquids extracted from harzb urgite residues [L + OI + Opx] will dominate magmatism in a plume head , and can erupt to form komatiites in oceanic plateaus. Liquids extrac ted from garnet peridotite residues in a plume axis will gain in impor tance when the plume head partially solidifies and is removed from the hotspot by a moving lithosphere, as is the case for Hawaii. The parad oxical involvement of garnet indicated by the heavy rare earth element s in picrites that otherwise have a harzburgite signature in Hawaii ca n be explained by the mixing and collection of magmas from the plume a xis. Volcanic rocks from Hawaii and Gorgona and xenoliths from cratoni c mantle provide evidence for the importance of partial crystallizatio n of plume magmas when they encounter a cold lithosphere. Harzburgite residua and olivine-orthopyroxene cumulates formed in plumes can yield compositionally distinct lithospheric mantle which is buoyant, and th is could have provided an important foundation for the stabilization o f the first continents. (C) 1998 Elsevier Science B.V. All rights rese rved.