Magma mixing at mid-ocean ridges: Evidence from basalts drilled near 22° N on the Mid-Atlantic Ridge

Abstract

This paper summarizes the results of a detailed petrological, geochemical and experimental study of basalts recovered during Legs 45 and 46 of the Deep Sea Drilling Project, and presents evidence that magma mixing has played an important role, along with crystal fractionation, in the evolution of these and other sea-floor basalts. Three major lines of evidence are considered, including phenocryst mineralogy, melt inclusion compositions, and whole rock major and trace element chemistry. Taken together, this evidence indicates that “primitive” magmas and their attendant phenocrysts are episodically injected into fractionating magma chambers containing more evolved cogenetic magmas. The dominant products of this steady-state process of repeated magma injection, mixing and fractionation are moderately evolved ocean floor basalts. Compositional extremes, including “primitive”, mantle-derived primary magmas and highly evolved differentiation products, will be rare. This model accounts for several enigmatic petrological characteristics of ocean-floor basalts previously inadequately explained by partial melting and crystal fractionation processes. Consequently, it is suggested that magma mixing is a fundamental process that is intrinsically interrelated with crystal fractionation in ocean-floor basalt petrogenesis.