Primary magmas and the origin of basalts

Abstract

Basaltic rocks have chemical compositions and behave under experimental conditions in a way which suggests that extrusive basalt magmas are, in the main, the residual liquids of well advanced crystal fractionation processes operating at relatively low pressures. Once this is admitted, there is no reason to deny that such magmas have undergone appreciable crystal fractionation at all depths during their movement to the surface. The compositions of extrusive basalts should, therefore, be regarded as the end products of a continuous series of evolutionary changes whose precise effects will depend upon the place where partial melting occurred, and the rate at which the liquids move towards the surface relative to the rate of cooling. An attempt is made to construct a petrogenetic scheme for the basic magmas based on this view of igneous activity.