Sm-Nd Isotopic Compositions Research Articles

Nd Isotope Systematics of Coarse- and Fine-Grained Sediments: Examples from the Middle Proterozoic Belt-Purcell Supergroup

Sm-Nd isotopic compositions are reported for 20 fine- and 5 coarse-grained samples of the Belt-Purcell Supergroup of western North America. $$\varepsilon_{Nd}$$ and Sm/Nd ratios for fine-grained Belt-Purcell sediments are remarkably uniform, indicating crustal residence ages ranging only from 1.6 to 2.1 Ga. $$\varepsilon_{Nd}$$ and Sm/Nd ratios for coarse-grained sediments, however, are much more varied and contain evidence of Sm-Nd fractionation during sedimentary recycling. Assuming that this REE fractionation occurred between the time source areas were eroded and when sediment was deposited, a new method of calculating crustal residence age intervals is derived for the coarse-grained samples, which serves to place constraints on the average time since source materials were extracted from the mantle. The Nd data from both sample groups provide an average age estimate of $$2.0 \pm 0.3$$ Ga for the Middle Proterozoic crust from which most of the Belt-Purcell Supergroup was derived. The isotopic data, toge...