Magma mixing at mid-ocean ridges - Evidence from legs 45 and 46-DSDP

An integrated petrologic and geochemical study of basalts recovered in Legs 45 and 46 (DSDP) has indicated, on the basis of disequilibrium mineralogy, that these moderately evolved basalts are mixtures of primitive mantle-derived tholeiites with more evolved magmas. Plagioclase phenocrysts are characterized by substantial diversity in composition and zoning pattern. Many olivine and plagioclase phenocrysts are too refractory to be in equilibrium with liquids of the host basalt composition but possess a composition consistent with crystallization from a primitive mantle-derived basalt liquid. On the basis of melt inclusions trapped in the olivine phenocrysts, features of the primitive melt are estimated. It is suggested that subvolcanic magma chambers beneath midocean ridges receive periodic injections of this primitive melt and its attendant phenocrysts which mix with fractionated chamber-bound magmas, resulting in observed moderately evolved lavas.