Refining the average µ182W and µ142Nd compositions of the Archean to Proterozoic upper crust

Abstract

The chemical composition of siliciclastic rocks provided key insights into the chemical composition of the upper continental crust and was suggested to record the onset of modern-style plate tectonics by ∼3.0 Ga. Recent studies aiming to establish the crustal µ 182 W and µ 142 Nd isotope compositions, which record processes within the first 60 and 500 million years after Earth’s formation, yielded contradictory results. Previously reported µ 182 W and µ 142 Nd data for sedimentary rocks yielded deficits, excesses, and Bulk Silicate Earth-like compositions. This complicates interpretations of the formation of Earth’s crust and the homogenisation history of Hadean mantle components. To constrain these processes and obtain a first-order estimate of the evolution of µ 182 W and µ 142 Nd in the upper crust, we provide new µ 182 W and µ 142 Nd data for Archean to Proterozoic siliciclastic rocks from the Kaapvaal and Yangtze cratons, including complementary trace-element, 176 Lu- 176 Hf, and 147 Sm- 143 Nd data. The absence of µ 182 W and µ 142 Nd anomalies in the investigated siliciclastic rocks indicates that crustal units with anomalous µ 182 W and µ 142 Nd compositions were only minor contributors. To reconcile these findings with previous studies, we suggest a model where crustal and mantle domains with anomalous short-lived isotope compositions were relatively small and isolated. Efficient pooling of detrital material during sediment transport can explain the striking absence of anomalous µ 182 W and µ 142 Nd values in the investigated sedimentary rocks. The available data for sedimentary rocks can be reproduced by simple geochemical modelling assuming silicate differentiation and subsequent homogenization of Earth’s mantle, which reduced the size of anomalous domains over time.