Physics
Scientific paper
Dec 2001
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001agufm.v52b..10s&link_type=abstract
American Geophysical Union, Fall Meeting 2001, abstract #V52B-10
Physics
8125 Evolution Of The Earth
Scientific paper
The Lu decay constant recently determined by Scherer et al. 2001 (i.e., 1.865 x 10-11 yr-1) agrees with the results of the two latest physical counting experiments (1.86 x 10-11 yr-1; Dalmasso et al 1992, Nir-El and Lavi 1998), but is ca. 4 percent lower than the decay constants that have been used throughout the Hf isotope literature (e.g., 1.94 x 10-11, Tatsumoto et al., 1981; 1.93 x 10-11 Sguigna et al, 1982). In addition to making Lu-Hf ages older by ca. 4 percent, the revised decay constant also shifts the calculated initial epsilon Hf values of early Archean and Hadean rocks and zircons that are used to constrain crust-mantle differentiation in the early Earth. The initial epsilon Hf values for low-Lu/Hf samples such as zircons and evolved felsic rocks shift downward by 2-4 epsilon units, primarily due to the shift in the position of the CHUR evolution curve rather than that of the samples themselves. Mafic rocks, such as komatiites have higher Lu/Hf ratios that are closer to that of CHUR and therefore their initial epsilon Hf values do not shift as much (up to 1.3 epsilon units lower or 0.4 epsilon units higher). Using the old decay constant, some early Archean rocks (e.g., Amitsoq gneisses; Vervoort et al., 1996, Vervoort and Blichert-Toft, 1999) seemed to have very high initial epsilon Hf values (up to +6), implying that the upper mantle was moderately depleted in the early Archean and that a substantial volume of crust was produced in the Hadean. However, when recalculated with the new decay constant, the data suggest that the mantle was only slightly depleted, requiring less early crust extraction, and allowing a later date for the onset of significant crust production. In contrast, the extremely low recalculated epsilon Hf values of Earth's oldest zircons (Amelin et al., 1999, Amelin et al., 2000) indicate that Earth's first crust formed at or before 4.3 Ga, and that this crust remained intact long enough (>200 million years) to evolve to such low epsilon Hf values. Either the volume of crust that was extracted in the Hadean was too small to cause moderate mantle depletion, or much of that early crust was recycled back into the mantle, thus effectively erasing evidence of early depletion. Importantly, the validity of these conclusions depends on the CHUR parameters (176Lu/177Hf, 176Hf/177Hf) that are used to calculate initial epsilon Hf values. References: E. Scherer, C. Munker, K. Mezger, Science 293, 683 (2001). J. Dalmasso, G. Barci-Funel, G.J. Ardisson, Appl. Radiat. Isotopes 43, 69 (1992). Y. Nir-El, N. Lavi, Appl. Radiat. Isotopes 49, 1653 (1998). M. Tatsumoto, D. M.. Unruh, P.J. Patchett, Mem. Natl. Inst. Polar Res., Special Issue no 20, Proceedings of the Sixth Symposium on Antarctic Meteorites, p. 237 (1981). A.P. Sguigna, A.J. Larabee, J.C. Waddington, Can. J. Phys. 60, 631 (1982). J.D. Vervoort, P.J. Patchett, G.E. Gehrels, A.P. Nutman, Nature 379, 624 (1996). J.D. Vervoort, J. Blichert-Toft, Gecohim. Cosmochim. Acta. 63, 533 (1999). Y. Amelin, D.-C. Lee, A.N. Halliday, R.T. Pidgeon, Nature 399, 252 (1999). Y. Amelin, D.-C. Lee, A.N. Halliday, Geochim Cosmochim Acta 64, 4205 (2000).
Mezger Klaus
Münker Carsten
Scherer Erik E.
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