Compositional variation in normal MORB from 22^o-25^oN: Mid- Atlantic ridge and Kane Fracture Zone

Basalts collected by dredging between 23oN and 25oN include samples from the active spreading ridges, median valley walls, and older walls of the Kane Fracture Zone transform fault at about 24oN. There is no evidence of present or past volcanic activity within the transform zone. Petrogenesis of the basalts, is deduced from both petrographic and geochemical evidence. It is shown that a basalt-type common to the area is a logical choice as a mantle-derived primary magma; it is close to the composition of average normal MORB. We outline a comprehensive hypothesis for melting, fractionation, and mixing which we believe may be adequate to explain compositional variation in these and other suites of 'normal MORB'. An important consequence of this model is that MORB presently at a given position on a low-pressure, multiply saturated cotectic may have reached that point by a variety of paths; some may be nearly direct mantle derivatives, while others may have experienced a complex history of polybaric and low-pressure fractionation and/or mixing.-Authors