Isotopic and chemical effects produced in a continuously differentiating convecting Earth mantle

Abstract

Trace element variations have established the present concept of chemical heterogeneity of the Earth’s mantle. A continuous range of variations is observed for the mantle source of basalts, from mantle depleted in incompatible trace elements to mantle enriched in such elements, in particular light rare earth elements. This heterogeneity is supported by the study of orogenic lherzolites and ultramafic nodules from volcanoes and kimberlite pipes. Radiogenic isotopes of Sr, Pb and Nd confirm this heterogeneity and show that it results from ancient chemical fractionations. This trace element and radiogenic isotope heterogeneity is identifiable in the Precambrian mantle. Pb isotopes are the clearest tracers of this past heterogeneity. Correlations are observed between variations in incompatible element ratios, radiogenic isotope ratios and radiogenic isotope and trace element ratios. These correlations define large domains (source of alkali basalts, source of mid-oceanic ridge basalts) and more restricted domains (e.g. the Canary Islands). Thus, they show that the mantle is heterogeneous on very large scales (ocean scales) as well as in more limited areas. Radiogenic data on ultramafic nodules also show that these mantle materials have isotopic heterogeneities at the mineral scale. These data also allow an estimation of the time scale of creation of these heterogeneities: they are of the order of some 10 9 years (= 1 Ga). The mechanisms involved in the chemical evolution of the mantle are discussed. Heterogeneity is created by chemical fractionation during petrogenetic processes; essentially oceanic lithosphere formation, storage into the continental crust and also recycling of sediments during lithospheric plate subduction. This heterogeneity tends to be erased in the mantle by convection and diffusion. The former efficiently mixes the mantle at large scales. The latter is very inefficient in solid mantle conditions, but can homogenize the radiogenic isotopes during partial melting. All these processes have been continuously active through geological time. A mathematical model is proposed which describes the chemical evolution of a continuously differentiating convecting mantle. The correlations, and in particular mantle isochrons, appear as artefacts without time significance. Evolution of both trace element ratios and isotopic compositions can be described simultaneously if isotopic homogenization is easier than chemical mixing. Variations of lead isotopes tend to indicate an ancient (initial) heterogeneity of the mantle which can be possibly attributed to loss of lead from mantle to core. The rate of chemical fractionation of the mantle cannot have been constant with time but was faster during the Archaean than at present.