Applications of radiogenic tracers

Abstract

The earth can be simply divided into several broad geochemical reservoirs. At the first order, the earth is composed of a metallic core, the convecting silicate earth (including the asthenosphere), and the lithosphere. The lithosphere comprises a crust and corresponding lithospheric mantle, which does not convect. Continental crust and subcontinental lithospheric mantle (SCLM) are quite heterogeneous and compositionally distinct compared with the oceanic crust and oceanic lithospheric mantle. Since each reservoir is chemically distinct, they will evolve to correspondingly distinct isotopic ratios, potentially allowing the tracking of magmas and fluids from and between them. The relevance of this in the minerals industry can be varied, from requiring/desiring magmas sourced from the mantle for exploration models, or needing crustal contamination to drive sulphide saturation, through to monitoring the effects of fluids of differing compositions in a hydrothermal system. Differentiation within the crust and crustal growth Nd model ages (T DM ) and crustal evolution Nd isotopes offer an effective way of placing first-order constraints on processes pertaining to the formation and subsequent stabilisation and recycling of continental crustal material. In principle, when new continental crust is formed (e.g. on the modern earth at plate margins), the crustal material will start out with an isotopic signature in equilibrium with the depleted mantle (DMM) from which it was derived (Figure 11.1a). However, because continental crustal material contains relatively low Sm/Nd ratios with respect to the mantle, this new material will only ingrow new (additional) 143 Nd relatively slowly. Hence it will evolve to negative e Nd values (Figure 11.1a), and if unmodified will preserve a T DM which is the same as its magmatic age. Hence we would be able to measure its Nd isotopic signature today and calculate a T DM which would reflect precisely the age of formation of that portion of the continental crust, and hence constrain the time of that particular episode of crustal addition. However, more generally, the continental crust undergoes processes of continued internal differentiation. For example, remelting of this crust during orogenesis or granite formation will segregate crustal material into a more differentiated granitic component, with even lower Sm/Nd (∝ 147 Sm/ 144 Nd) ratios, and a (relatively) more mafic residue with higher Sm/Nd ratios than the protolith. Over time these two products will evolve to distinctly different measured values today (Figure 11.1b).