DO WE REALLY NEED MANTLE COMPONENTS TO DEFINE MANTLE COMPOSITION?

Abstract

We discuss the concept of components in the Earth's mantle starting from a petrological and geochemical approach, but adopting a new method of projection of geochemical and isotopic data. This allows the compositional variability of magmatic associations to be evaluated in multi-dimensional space, thus simultaneously accounting for a large number of compositional variables. We demonstrate that ocean island basalts (OIB) and mid-ocean ridge basalts (MORB) are derived from a marble-cake mantle, in which different degrees of partial melting of recycled lithosphere, which are heterogeneous in age and composition, contribute to the magma genesis. This view is supported by the variability in the geochemical and isotopic signatures of OIB that are observed on the scale of a single ocean island as well as on that of an ocean, mostly varying between two extreme compositions, that are not strictly related to the commonly accepted mantle components (DMM, EMI, EMII, HIMU). Rather they are a distinctive feature of the mantle source sampled at each ocean island and are strongly dependent on the Pb isotope system. We recommend a change in perspective in studies of MORB–OIB geochemistry from one based on physically distinct mantle components to a model based on the existence of a marble-cake-like upper mantle. Although resembling the statistical upper mantle, this model implies that geochemical homogenization can be attained only within the limits of local mantle composition, so that a world-wide uniform depleted reservoir cannot be sampled by simply extending the volume of the region undergoing partial melting.