Lu-Hf isotopic systematics

Blichert-Toft J., Albarede F., and Kornprobst J. (1999) Lu-Hf isotope systematics of garnet pyroxenites from Beni Bousera, Morocco implications for basalt origin. Science 283, 1303-1306. 

Jacob D. E., Bizinis M., and Salters V. J. M. (2002) Lu-Hf isotope systematics of subducted ancient oceanic crust Roberts Victor eclogites. Geochim. Cosmochim. Acta Spec. Suppl.) 66S1, A360. 

Schmidberger S. S., Simonetti A., Francis D., and Gariepy C. (2002) Probing Archean lithosphere using the Lu—Hf isotope systematics of peridotite xenohths from Somerset Island kimberlites, Canada. Earth Planet. Sci. Lett. 197, 245-259. 

Lu-Hf isotope systematics provide an important complement to Sm-Nd in the study of the cmst and mantle (e.g., Patchett et al., 1981 Salters and Hart, 1991 Vervoort and Blichert-Toft, 1999). In the crustal context, Lu-Hf is extremely important because of the —1% hafnium content of zircon, and the consequent ability to isotopically characterize the hafnium within grains that have been U-Pb dated (Patchett et al., 1981 Corfu and Stott, 1996 Vervoort et al., 1996 Amelin et al., 1999). However, the Lu-Hf isotopic system is currently overshadowed by a controversy over the decay constant. For many years, a value for the Lu decay constant of 1.94 X 10 yr, based on the eucrite meteorite isochron of Patchett and Tatsumoto (1980) and Tatsumoto et al. (1981) was used. More recent physical determinations reviewed by Begemann et al. (2001) have high dispersion, but do not seem to corroborate the 1.94 X 10 value. At the present time, there is a discrepancy between values based on U-Pb-dated terrestrial Precambrian REE-rich minerals, such... 

The robustness of the Lu-Hf isotope system in some mantle environments is demonstrated by the precise Lu-Hf isochron of 1,413 67 Myr dehned by clinopyroxene separates from the Beni Bousera peridotite massif (Pearson and Nowell, 2003). This age probably dates the time of melt extraction from these rocks and is considerably more precise than the Sm-Nd isochron or the scattered Re-Os isotope systematics of these rocks. This indicates the potential power of this system in dating mantle rocks. The initial results from the Lu-Hf isotope system indicate that of the incompatible element isotope systems, it is the more robust to metasomatic effects, with signatures frequently recording the time-integrated response to melt depletion. 

Figure 21 Stratigraphic age versus Nd- and Hf-crustal residence ages. Model ages were calculated using hnear E evolution from 0 to - -10 for Nd and 0 to - -16 for Hf, from 4.56 Gyr to present. The similarity of the model age systematics underscores the overall coherent behavior of the Sm-Nd and Lu-Hf isotopic systems in the sedimentary environment (after Vervoort et al., 1999).

Figure 1 Systematics of Nd- and Hf-isotopic evolution in the bulk Earth, continental crust, and mantle. Daughter elements Nd and Hf are more incompatible during mantle melting (more likely to go into a partial melt of mantle rock) than Sm and Lu, respectively. As a result, the continental crust has a lower Sm/Nd and Lu/Hf ratio than the mantle, and lower Nd- and Hf-isotope ratios. Young continental crust has isotope ratios similar to the mantle, and the older the continental terrain, the lower the Nd- and Hf-isotope ratios. Rb-Sr behaves in the opposite sense, such that the parent element Rb is more incompatible than the daughter element Sr. (a) Schematic example of the evolution of Nd- and Hf-isotope ratios of a melt and the melt residue from a melting event around the middle of Earth history from a source with the composition of the bulk Earth, (b) The same scenario as in (a), but with the isotope ratios plotted as e d and snf. The bulk Earth value throughout geological time is defined as e d and SHf = 0> and e-value of a sample is the parts per 10 deviation from the bulk Earth value.

Since a number of isotope systematics (U, Th/Pb, Lu/Hf, Sm/Nd) in clastic sediments is essentially controlled by the heavy mineral fraction, such considerations are of considerable importance for any geological interpretations. 

The discussion of the chemistry of clastic sediments suggested an overall mafic to felsic evolution of global sediments, and presumably of UCC, their ultimate source, in the course of geologic history, with a major evolutionary step across the Archean/Proterozoic transition. The response of isotopes to this evolutionary scenario can best be gauged by consideration of the REE isotope systematics, such as the Sm/Nd and Lu/Hf. 

Soderlund, U., Patchett, J.P., Vervoort, J.D., and Isachsen, C.E., 2004. The 176Lu decay constant determined by Lu-Hf and U-Pb isotope systematics of Precambrian mafic intrusions. Earth Planet. Sci. Lett., 219, 311-24. 

Blichert-Toft and Albarede [77] carried out the first published cosmochemical Hf isotopic study. They investigated the Lu-Hf systematics of chondrites, which featured only imprecise published TIMS data, due to the relatively low Hf contents of these meteorites. Their results yielded an initial solar system ratio of Hf/ Hf= 0.27974 + 3. This was later revised by Blichert-Toft et al. [70] to... 

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