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

Abstract

An integrated petrologic and geochemical study of basalts recovered in Legs 45 and 46 (DSDP) has yielded evidence that these moderately evolved basalts are mixtures of primitive, mantle‐derived tholeiites with more evolved magmas. The hybrid nature of these rocks is recognized on the basis of disequilibrium mineralogy. Plagioclase phenocrysts exhibit substantial diversity in composition and zoning patterns including both normally and reversely zoned grains and abundant textural evidence of resorption. Many olivine and plagioclase phenocrysts are too refractory (e.g., Fo90−88) to be in equilibrium with liquids of the host basalt composition and are in fact of a composition consistent with crystallization from a primitive, mantle‐derived basalt liquid. An estimate of this primitive melt, presumed to be parental to the observed lavas, has been derived from melt inclusions trapped in the olivine phenocrysts. These are characterized by high Ca/Al and low TiO2, two chemical features typical of primitive ocean floor basalts recovered elsewhere in the Atlantic.We suggest that subvolcanic magma chambers beneath mid‐ocean 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. Chemical modeling of ocean floor basalt differentiation assuming a combination of magma mixing and crystal fractionation alleviates some apparent anomalies encountered in previous studies (Rhodes et al., 1978).