Constraints from loess on the Hf–Nd isotopic composition of the upper continental crust

Abstract

Knowledge of the average composition of the upper continental crust is crucial to establish not only how it formed but also when. While well constrained averages have been suggested for its major and trace element composition, no values exist for its Nd and Hf isotopic compositions even though radiogenic isotopic systems provide valuable information on its average model age.Here we present Nd and Hf isotopic data determined on a large number of loess deposits from several continents. We demonstrate that these deposits have very uniform Nd and Hf isotopic compositions. We obtain an average Nd isotopic composition that is similar to previous estimates for the upper continental crust (εNd=−10.3±1.2(1σ)) and we establish a new Hf average value at εHf=−13.2±2(1σ). This average falls on the “Terrestrial Array”, demonstrating that the two parent–daughter ratios are not decoupled during crust formation. Trace element data acquired on the same set of samples allow us to calculate an average 147Sm/144Nd ratio for the upper continental crust: 0.1193±0.0026, a value slightly higher than previous estimates. Based on the relationship between Sm/Nd and Nd isotopes, we estimate the average Nd extraction age of upper continental crust from the depleted mantle at TDM(Nd)=1.82±0.07 Ga. This model age is entirely consistent with previous suggestions made for example by Goldstein et al. (1984).Assuming that for each individual sample, the Hf model age cannot be younger than the Nd model age, our new Nd–Hf isotopic data provide a value for the very poorly known 176Lu/177Hf ratio of the upper crust. Our estimate is 176Lu/177Hf = 0.0125 ± 0.0018, a value significantly lower than commonly used values (0.0150–0.0159; Griffin et al., 2002; Goodge and Vervoort, 2006; Hawkesworth et al., 2010) but higher than Rudnick and Gaoʼs (2003) estimate of 0.0083. The impact of our new 176Lu/177Hf ratio on crustal model ages of zircon populations is not simple to evaluate but the Hf model ages calculated with this new Lu/Hf ratio could be younger by up to 500 Ma.