Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga

摘要

The Long-favored paradigm for the development of continental crust is one of progressive growth beginning at ～4 billion years ago (Ga). To test this hypothesis, we measured initial ~(176)Hf/~(177)Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. ε_(Hf) (deviations of ~(176)Hf/~(177)Hf from bulk Earth in parts per 10~4) values show large positive and negative deviations from those of the bulk Earth. Negative values indicate the development of a Lu/Hf reservoir that is consistent with the formation of continental crust (Lu/ Hf ≈ 0.01), perhaps as early as 4.5 Ga. Positive ε_(Hf) deviations require early and likely widespread depletion of the upper mantle. These results support the view that continental crust had formed by 4.4 to 4.5 Ga and was rapidly recycled into the mantle.