Early history of Earth's crust–mantle system inferred from hafnium isotopes in chondrites

Martin Bizzarro (), Joel A. Baker, Henning Haack, David Ulfbeck and Minik Rosing
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Martin Bizzarro: Geological Museum
Joel A. Baker: Danish Lithosphere Center
Henning Haack: Geological Museum
David Ulfbeck: Danish Lithosphere Center
Minik Rosing: Geological Museum

Nature, 2003, vol. 421, issue 6926, 931-933

Abstract: Abstract The 176Lu to 176Hf decay series has been widely used to understand the nature of Earth's early crust–mantle system1,2,3,4,5,6. The interpretation, however, of Lu–Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu ( λ 176 Lu ), as well as bulk-Earth reference parameters. A recent calibration of the λ 176 Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9 Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3 Gyr ago7, but does not provide evidence for a complementary depleted mantle reservoir. Here we report Lu–Hf isotope measurements of different Solar System objects including chondrites and basaltic eucrites. The chondrites define a Lu–Hf isochron with an initial 176Hf/177Hf ratio of 0.279628 ± 0.000047, corresponding to λ 176 Lu = 1.983 ± 0.033 × 10-11 yr-1 using an age of 4.56 Gyr for the chondrite-forming event. This λ 176 Lu value indicates that Earth's oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment7. The depletion event must have occurred no later than 320 Myr after planetary accretion, consistent with timing inferred from extinct radionuclides8,9,10.

Date: 2003
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DOI: 10.1038/nature01421

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