Effect of Axial Magma Chambers Beneath Spreading Centers on the Compositions of Basaltic Rocks

Ferrobasalts are among a range of basalt types erupted over large, shallow, axial magma chambers beneath the East Pacific Rise crest near 9°N, and the Galapagos Rift near 86°W.Geological and geochemical evidence indicates that they evolve in small, isolated magma bodies either in conduit systems over the chambers, or in the solidifying walls of the magma chamber flanks away from the loci of fissure eruptions.The chambers act mainly as buffers in which efficient mixing ensures that a uniform average, moderately fractionated parent is supplied to the range of magma types erupted at the sea floor.Formation of ferrobasalts in isolated magma bodies prevents them from mixing directly with primitive olivine tholeiite supplied to the base of the East Pacific Rise magma chamber at 9°N.Rises with abundant ferrobasalts may have magma chambers with broad, flat tops over which eruptions occur in wide, probably structurally unstable rift zones.Rises without ferrobasalts may have magma chambers with tapered tops which persistently constrain eruptions to narrow fissure zones, preventing formation of isolated shallow magma bodies where ferrobasalts can evolve.Formation of a flat-topped, rather than a tapered, magma chamber may reflect a high rate of magma supply relative to spreading rate.Steady-state compositions can be approached in magma chambers, probably approximating the least-fractionated typical lavas erupted above them.On this basis, the bulk magma composition in the chamber beneath the Galapagos Rift has become more iron-rich through time, a consequence of magma migration eastward down the rift as it shoaled to the west under the influence of the Galapagos Island hot spot.Ferrobasalts rarely erupt at the slow-spreading Mid-Atlantic Ridge.Instead, geochemical evidence indicates that they can mix with significant proportions (50-70%) of primitive olivine tholeiite both deep in the crust prior to ascent, and at shallow levels in the crust during ascent.They are brought to the surface usually only after having been entrained in, mixed with, and diluted by primitive olivine tholeiite.Mixing evidently occurs in small magma bodies infrequently supplied with magma.Lavas of contrasting geochemistry and histories of fractionation and mixing have been supplied to the same DSDP sites.This implies that separate, unlinked reservoir-conduit systems exist, rather than the shallow, interconnected, steadystate axial magma chambers of faster spreading rises.