The 226Ra enrichment in oceanic basalts: Evidence for melt‐cumulate diffusive interaction processes within the oceanic lithosphere

Abstract

U‐Th decay chains in young volcanic rocks have become a unique geochemical tool with the potential to constrain the physical processes involved in the generation and transport of magmas. Among the different isotopes of the decay series, the measured disequilibrium between 226Ra and 230Th isotopes in basaltic melts has been a crucial player in the interpretation of the U‐series data as a whole. However, the key assumption proposed in all previous models, that the origin of 226Ra‐230Th disequilibrium in oceanic basalts is only produced during mantle melting processes, has remained largely unchallenged. Here we present an alternative process for the origin of the 226Ra‐230Th disequilibrium in oceanic basalts. We argue that diffusive interaction of magmas with cumulates formed in the oceanic crust beneath mid ocean ridges or oceanic islands may be responsible for the observed 226Ra‐230Th disequilibrium in oceanic basalts. Hence the inferences drawn from (226Ra)/(230Th) activity ratios about mantle melting and melt transport processes beneath mid‐ocean ridges and ocean islands are at best equivocal.