Molybdenum isotopic heterogeneity for intraplate basalts and its origin

Abstract

The molybdenum (Mo) isotope system is a powerful tool for tracing recycled crustal components in volcanic rocks. Existing Mo isotope data show that intraplate basalts have highly variable Mo isotopes. However, the heterogeneity of Mo isotopes in intraplate basalts was interpreted to be inherited from recycling of different slab components, and its origin remains unclear. Here we synthesize Mo isotope and other geochemical data for global intraplate basalts, which provide insights into Mo recycling in Earth's deep mantle. Global intraplate basalts display highly variable δ98Mo values of −0.84‰ to 0.10‰, suggesting mantle Mo isotopic heterogeneity. Most of the basalts have low δ98Mo values and high Ce/Mo ratios that are similar to those of the oceanic eclogites, indicating that recycled dehydrated oceanic crustal components were involved in their mantle sources. However, some intraplate basalts have heavy Mo isotopes. This reveals that slab components at shallow depths were dragged by the downgoing slab and have carried heavy Mo isotopes into the deep mantle. Significantly, high δ98Mo values of these basalts are also associated with low Ce/Mo ratios, high fluid-mobile element contents, and depleted radiogenic SrNd isotopes. Given that recycled basaltic oceanic crust and sediment are generally efficiently dehydrated during subduction and sequester light Mo isotopes, subducted serpentinised lithospheric mantle is a plausible candidate to transfer heavy Mo isotopes into the deep mantle. Thus, Mo isotope systematics of intraplate basalts record complex geochemical fluxes from subducting slab to the deep mantle, in which subducted serpentinites play an important role in the Mo isotope recycling. Our study highlights the effectiveness of Mo isotopes as indicators for geochemical heterogeneity of Earth's deep mantle.