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Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga
T. M. Harrison,1,2*J. Blichert-Toft,3W. Müller,1,4F. Albarede,3P. Holden,1S. J. Mojzsis5
The long-favored paradigm for the development of continentalcrust is one of progressive growth beginning at 4 billion yearsago (Ga). To test this hypothesis, we measured initial 176Hf/177Hfvalues of 4.01- to 4.37-Ga detrital zircons from Jack Hills,Western Australia. Hf (deviations of 176Hf/177Hf from bulk Earthin parts per 104) values show large positive and negative deviationsfrom those of the bulk Earth. Negative values indicate the developmentof a Lu/Hf reservoir that is consistent with the formation ofcontinental crust (Lu/Hf 0.01), perhaps as early as 4.5 Ga.Positive Hf deviations require early and likely widespread depletionof the upper mantle. These results support the view that continentalcrust had formed by 4.4 to 4.5 Ga and was rapidly recycled intothe mantle.
1 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia. 2 Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California at Los Angeles, Los Angeles, CA 90095, USA. 3 Ecole Normale Supérieure, CNRS Unité Mixte de Recherche 5570, 69364 Lyon Cedex 7, France. 4 Department of Geology, Royal Holloway University of London, Egham, TW20 0EX, UK. 5 Department of Geological Sciences, University of Colorado, Boulder, CO 80309, USA.
* To whom correspondence should be addressed. E-mail: mark.harrison{at}anu.edu.au
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TECHNICAL COMMENTS
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Temperature spectra of zircon crystallization in plutonic rocks.
4 billion years ago (Ga). To test this hypothesis, we measured initial 176Hf/177Hf values of 4.01- to 4.37-Ga detrital zircons from Jack Hills, Western Australia. 
Hf (deviations of 176Hf/177Hf from bulk Earth in parts per 104) 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 
