Lu-Hf isotope system

Patchett J. P. (1983). Importance of the Lu-Hf isotopic system in studies of planetary chronology and chemical evolution. Geochim. Cosmochim. Acta, 47 81-91. 

It has been recognized that the Lu-Hf isotopic system in zircon is a powerful tool for deciphering the evolution of the earth s crust and mantle. " Zircon normally contains 0.5-2 wt % Hf, which results in an extremely low Lu/Hf ratio (" Lu/" Hf < 0.002) and consequently a negligible radiogenic growth of Hf due to the decay of Lu. Therefore, the Hf/" Hf ratio of zircon can be regarded as the initial value at the time when it crystaUized. LA-ICP-MS with a multiple ion collector system has also been employed to study the hafnium isotopic composition of zircon and baddeleyite standards in U-Pb geochronology. °°... 

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. 

The Lu- Hf isotopic system (half-life —37 Gyr) is, in many ways, chemically similar to Sm- Nd. In both isotopic schemes the parent and daughter elements are refractory lithophile elements, such that their relative abundances in the Earth were probably not modified during accretion, nor did they participate in core formation. Thus, as for the Sm-Nd system, the compositions of chondritic meteorites can, in principle, be used to establish bulk silicate Earth isotopic compositions and Lu/Hf ratios directly. The potential, therefore, exists for establishing a precise isotopic baseline to use for recognizing fine-scale deviations in isotopic compositions, which can then be used to reveal... 

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... 

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).

Blichert-Toft J, Alberede F (1997) The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crast system. Earth Planet Sci Lett 148 243-258... 

There are few pubhshed Lu-Hf isotope studies of mantle xenohths because of difficulties in efficient ionization of hafnium by thermal ionization mass spectrometers. Multicollector plasma mass spectrometers are a solution to this problem and data are emerging that promise to be a more revealing tool in mantle environments than neodymium isotopes. The variety of Lu/Hf fractionation displayed by mantle minerals (Figure 42) indicates that, as with other isotope systems, isotopic variation should be considerable and initial results are confirming this. Salters and Zindler (1995) found very radiogenic Hf/ Hf at relatively unradiogenic neodymium isotope compositions in spinel peridotites from Salt Lake Crater, Hawaii. Radiogenic Hf/ Hf also characterizes low-T circum-cratonic... 

One of the maj or difficulties in making high-precision measurements involves accurate correction for spectral and isobaric interferences. In MC-ICP-MS, making interfering element corrections (lECs) for these interferences is not completely straightforward and introduces an additional component of error that must be propagated into the final uncertainty. In the Hf isotope system Lu and Yb isobarically interfere with Hf and this provides a good example of the mechanism of lECs. 

Figure 3 Crust-mantle differentiation patterns for the decay systems Rb-Sr, Sm-Nd, Lu-Hf, and Re-Os. The diagram illustrates the depletion-enrichment relationships of the parent-daughter pairs, which lead to the isotopic differences between continental crust and the residual mantle. For example, the Sm/Nd ratio is increased, whereas the Rb/Sr ratio is decreased in the residual mantle. This leads to the isotopic correlation in mantle-derived rocks plotted in Figure 4(a). The construction is similar to that used in Figure 2, but D values have been adjusted slightly for greater clarity.

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