Excess hafnium-176 in meteorites and the early Earth zircon record

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Thrane, K.
Borg, L.E.
(2012)
Geochemistry, Geophysics, Geosystems, 13, Q03002

The long-lived 176Lu-to-176Hf decay system is a powerful tool to understand ancient chemical fractionation events associated with planetary differentiation. Detrital Hadean zircons (>3.8 Gyr) from the Jack Hills metasedimentary belt of Western Australia record extremely enriched Hf-isotope signals suggesting early extraction of a continental crust (>4.5 Gyr) but fail to identify a prevalent complementary depleted mantle reservoir, suggesting that crust formation processes in the early Earth were fundamentally distinct from today. However, this conclusion assumes that the Hf-isotope composition of bulk chondrite meteorites can be used to estimate the composition of Earth prior to its differentiation into major silicate reservoirs, namely the bulk silicate Earth (BSE). We report a 176Lu-176Hf internal mineral isochron age of 4869 ± 34 Myr for the pristine SAH99555 angrite meteorite. This age is ∼300 Myr older than the age of the Solar System, confirming the existence of an energetic process yielding excess 176Hf in affected early formed Solar System objects through the production of the 176Lu isomer (t1/2 ∼3.9 hours). Thus, chondrite meteorites contain excess 176Hf and their present-day composition cannot be used to infer the Lu-Hf parameters of BSE. Using a revised BSE estimate based on the SAH99555 isochron, we show that Earth's oldest zircons preserve a record of coexisting enriched and depleted hafnium reservoirs as early as ∼4.3 Gyr in Earth's history, with little evidence for the existence of continental crust prior to ∼4.4 Gyr. This new view suggests continuous juvenile crustal growth and recycling throughout the Hadean and Archean eras, perhaps analogous to modern plate tectonics.