Molybdenum isotope evidence for subduction-modified, recycled mafic oceanic crust in the mantle sources of ocean island basalts from La Palma and Hawaii

Abstract

The recycling of mafic oceanic crust in subduction zones represents an important geological process to explain the geochemical diversity of the Earth's mantle. Based on radiogenic isotope and trace element data it has been proposed that recycled crust may form part of the deep-rooted mantle sources of ocean island basalts. Here we provide Mo isotope data for basalts from the ocean islands of La Palma and Hawaii to further address this issue. Samples have Mo isotope ratios (δ98/95Mo from −0.51 to −0.26 ± 0.06‰; 2σ; for La Palma and −0.50 to −0.11 ± 0.06‰; 2σ; for Hawaii) that extend to values significantly lower, and Ce/Mo ratios (112 to 32 for La Palma and 86 to 9 for Hawaii) that are significantly higher than those for the depleted mantle and bulk silicate Earth. Experimental data and model calculations show that neither magmatic processes (partial melting, residual sulphide) nor assimilation of marine sediments readily explain these observations as both processes result in δ98/95Mo values that are either too high, or Ce/Mo that are too low compared to the measured data for basalts from La Palma and Hawaii. Rather, their Mo isotope and Ce/Mo characteristics are uniformly matched by eclogite-facies, MORB-type meta-basalts that once formed part of oceanic lithosphere subducted to mantle depths. Our observations therefore provide strong evidence that subduction-modified, mafic oceanic crust is present in the mantle source of La Palma and Hawaii. As such, combined Mo isotopes and Ce/Mo ratios in ocean island basalts may provide an additional distinctive and robust tracer of recycled lithosphere in the mantle.