U-Pb and Lu-Hf isotope systematics of zircons from the Mississippi River sand: Implications for reworking and growth of continental crust

We carried out in situ U-Pb and Lu-Hf isotope analyses of detrital zircons from the Mississippi River in order to understand crustal reworking and continental growth rates. The U-Pb analyses for 416 zircons reveal three major peaks of crust formation at 2.8-2.6 Ga, 1.8-0.9 Ga, and after 0.2 Ga. Initial Hf isotope ratios were obtained for 402 of the dated zircons, and only 8% of the zircons have ε_Hf(T)DM values less negative than -2.5. These data correspond to a crustal residence time of <120 m.y. This finding indicates that crustal reworking was a very important process in continental crust formation. The ε_Hf(T)DM population demonstrates that reworking was predominant at 2.5-2.0 Ga and after 0.9 Ga, whereas juvenile crust formation dominated between 2.0 and 1.6 Ga. We calculated the mantle-extraction model ages to estimate the continental growth rate. Approximately half of the grains have model ages between 2.0 and 1.3 Ga, indicating rapid crustal growth during this time. The continental growth rate suggests that 15% and 78% crust in the source region of the zircons formed by 2.5 and 1.3 Ga, respectively.