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

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

Feb 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003natur.421..931b&link_type=abstract

Nature, Volume 421, Issue 6926, pp. 931-933 (2003).

Computer Science

43

Scientific paper

The 176Lu to 176Hf decay series has been widely used to understand the nature of Earth's early crust-mantle system. The interpretation, however, of Lu-Hf isotope data requires accurate knowledge of the radioactive decay constant of 176Lu (λ176Lu), as well as bulk-Earth reference parameters. A recent calibration of the λ176Lu value calls for the presence of highly unradiogenic hafnium in terrestrial zircons with ages greater than 3.9Gyr, implying widespread continental crust extraction from an isotopically enriched mantle source more than 4.3Gyr ago, 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 λ176Lu = 1.983 +/- 0.033 × 10-11yr-1 using an age of 4.56Gyr for the chondrite-forming event. This λ176Lu value indicates that Earth's oldest minerals were derived from melts of a mantle source with a time-integrated history of depletion rather than enrichment. The depletion event must have occurred no later than 320Myr after planetary accretion, consistent with timing inferred from extinct radionuclides.

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