Hf-W isotopes

Lee DC, Halliday AN (1996) Hf-W Isotopic evidence for rapid accretion and differentiation in the early solar system. Science 274 1876-1879... 

Halliday, A. N., Rehkamper, M., Lee, D.-C. and Yi, W. (1996) Early evolution of the Earth and Moon new constraints from Hf-W isotope geochemistry. Earth and Planetary Science Letters, 142, 75-89. 

Kleine, T., Mezger, K., Mtinker, C., Palme, H. andBischoff, A. (2004) Hf- W isotope systematics of chondrites, eucrites, and Martian meteorites chronology of core formation and early mantle differentiation in Vesta and Mars. Geochimica et Cosmochimica Acta, 68, 2935-2946. 

Lee D-C. and Halliday A. N. (2000a) Accretion of primitive planetesimals Hf-W isotopic evidence from enstatite chondrites. Science 288, 1629—1631. 

Lee D.-C. and Halliday A. N. (2000b) Hf-W isotopic systematics of ordinary chondrites and the initial i82Hf/i80Hf of the solar system. Chem. Geol. 169, 35-43. Lee D.-C., Halliday A. N., Davies G. R., Essene E. L, Eitton J. G., and Temdjim R. (1996) Melt enrichment of shallow depleted mantle a detailed petrological, trace element and isotopic study of mantle derived xenohths and megacrysts from the Cameroon line. J. Petrol. 37, 415-441. 

Although not strictly related to mantle depletion and crust formation, Hf- W isotopic compositions do provide clear evidence for early planetary differentiation of the Earth, Moon, and Mars related to core formation. The results (Kleine et al., 2002 Yin et al., 2002 Schoenberg et al., 2002) from this short half-life t — 8 Myr) system provide convincing evidence that metal... 

The timing of core formation Although a number of isotopic systems are potentially useful as chronometers of core formation the Hf-W isotope system is the one which is regarded as the most robust (Jacobsen, 2005). The merits of the Hf-W chronometer in estabiishing the time of core formation are that the isotope 182Hf decays to 182W with a half-life of 9 Ma, so that after... 

Ma the Hf-W chronometer is dead. This means that the Hf-W isotopic system is very useful in dating events that took place within the first 50 Ma of the life of the solar system. In addition the elements Hf and W display very different geochemical behavior making this isotope system particularly useful for dating core formation, for Hf is a lithophile element, with a tendency to remain in the silicate mantle whereas W is a siderophile element under highly reducing conditions, with a preference to concentrate in the Earth s metallic core (see Text Box 2.3). 

Leya I, Wider R, Halliday AN (2000) Cosmic-ray production of tungsten isotopes in lunar samples and meteorites and its implications for Hf-W cosmochemistry. Earth Planet Sci Lett 175 1-12 Loss RD, Lugmair GW (1990) Zinc isotope anomalies in Allende meteorite inclusions. Astrophys J 360 L59-L62... 

Another example is provided by the chemical fractionation of tungsten into planetary cores. Tungsten has a short-lived radioactive isotope, W, which decays into Hf. Tungsten is siderophile and hafnium is lithophile. Consequently, the daughter isotope, 182Hf, will be found either in the core or the mantle depending on how quickly metal fractionation (core formation) occurred relative to the rate of decay. The Hf- W system is used to date core formation on planetary bodies. We will discuss the details of using radioactive isotopes as chronometers in Chapters 8 and 9. 

The W isotopic compositions of various terrestrial samples, chondrites, iron meteorites, basaltic achondrites, lunar samples, and Martian meteorites are expressed as deviations in parts per 104 from the value for the silicate earth (such as the W in a drill bit or chisel), which are the same as those of average solar system materials, represented by carbonaceous chondrites. These values are summarized in Fig. 8.9, from which it can be seen that early segregated metals such as the iron meteorites and metals from ordinary chondrites have only unradiogenic W because they formed early with low Hf/W. The time differences between metal objects segregated from parents with chondritic Hf/W are revealed by the differences in W isotopic compositions between each of the metal objects and chondrites. The Hf-W model ages of all these metals indicate that all of their parent bodies formed within a few million years, implying rapid accretion in the early history of the solar system. 

Fig. 17. Elution curve for short-lived W isotopes modeling the seaborgium separation [52J in ARCA II using a solution of 0,1 M HNO3/510 4 M HF with a flow rate of 1 mL/min. The 1.6x8 mm columns are filled with the cation-exchange resin Aminex A6. Reproduced from [52] with the permission of Oldenbourg Verlag.

Snyder G. A., LeeD.-C., Ruzicka A. M., Taylor L. A., HaUiday A. N., and Prinz M. (1998) Evidence of late impact fractionation and mixing of silicates on iron meteorite parent bodies Hf—W, Sm—Nd, and Rb—Sr isotopic studies of sificate inclusions in HE irons. In Lunar Planet. Sci. XXIX, 1142. The Lunar and Planetary Institute, Houston (CD-ROM). 

While lead isotopes have been useful, the i 2jjf i82- chronometer (Ti/2 = 9 Myr) has been at least as effective for defining rates of accretion (Halliday, 2000 Halliday and Lee, 1999 Harper and Jacobsen, 1996b Jacobsen and Harper, 1996 Lee and Halliday, 1996, 1997 Yin et al, 2002). Like U-Pb, the Hf-W system has been used more for defining a model age of core formation (Kramers, 1998 Horan et al. 

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