Origin of some magmas in oceanic and circum-oceanic regions

Abstract

Partial melting experiments on spinel-lherzolite, a rock which probably occurs in relatively shallow parts of the oceanic upper mantle, demonstrate that alkali basaltic melt is formed at depths of at least 20 kbar whereas tholeiitic melt is formed at lower pressures (< 15 kbar) under anhydrous conditions. The specimen studied was a relatively iron-rich natural spinel-lherzolite (Fe/Mg+Fe=0.15) and the melts produced have ratios comparable to those obtained in basalts. Slight increase of degree of partial melting produces picritic melt over a wide pressure range. Under hydrous (water-excess) conditions, andesitic melt is produced by partial melting of the same natural spinel-lherzolite and a synthetic lherzolite. The melting experiments on two different abyssal tholeiites from the Mid-Atlantic Ridge suggest that the derivation of olivine tholeiite from a more mafic magma or a mantle peridotite (lherzolite) is possible, but is limited to depths shallower than 25 km under essentially anhydrous conditions, whereas plagioclase tholeiite may have been formed by fractional crystallization at depths of about 20 km in the presence of a small amount (∼ 2 wt.%) of water. It is suggested that under mid-ocean ridges, partial melting of spinel-lherzolite at depths shallower than 60 km would produce olivine-tholeiitic magma, which differentiates at shallower levels (20–25 km) under either essentially anhydrous or hydrous (but vapor-absent) conditions to produce abyssal tholeiites of olivine-tholeiite type or plagioclase-tholeiite type. It may be also possible that the former olivine-tholeiite is generated by direct partial melting of plagioclase-lherzolite. Alkali basalts in the oceanic region may be generated at depths greater than 50 km by relatively small degree of partial melting. Along island arcs and continental margins, where the subduction zones probably exist, partial melting of lherzolite would take place in the presence of water that may be supplied by breakdown of hydrous minerals in the subducted oceanic crust, thereby producing andesitic magmas. High-alumina basalt magma could be produced by partial melting of the dehydrated oceanic crust in the subduction zone at depths between 40 and 60 km, where garnet is unstable above the solidus.