Cerium isotope geochemistry of ocean island basalts

Abstract

The source of ocean island basalts is one of geology's major problems, and many different models have been proposed. One of these involves recycling of continental and oceanic crustal material into the mantle source of ocean island basalt magmas^1,2. Island arc basalts are an important link in the chain of evidence that supports this model, as they display isotope signatures in common with both ocean-floor sediment^2,4 and ocean island basalts^1,5. But hafnium isotope data⁶ appear either to place severe restrictions on the quantity of sedimentary material recycled into the mantle, or to demand very thorough homogenization of this material during the subduction process. The lanthanum–cerium isotope method⁷ offers a prospect of applying similar constraints to the recycling of different sediment types into the mantle, based on cerium and neodymium isotope systematics. Here I report cerium isotope ratios for eight ocean island basalts. When plotted against published neodymium isotope data these form a 'mantle array' of compositions which indicates coherent behaviour of the two elements during mantle differentiation. This precludes the recycling into the deep mantle of ocean-floor sediments with a large net cerium anomaly.