Extinct radionuclide

A number of extinct radionuclides (7B, 10Be, 14C, 22Na, 26A1, 36C1, 35S, 37Ar, 41Ca) have been identified in meteorites or lunar materials.1 These radionuclides can be also used in isotope... 

The dating methods discussed up to now have been based on the use of long-lived radionuclides that are present in nature. Dating is also possible using extinct radionuclides, that is, nuclei whose half-lives are so short that if they existed at the time of formation of our solar system, they would have decayed away essentially completely by now. The nuclides 129I t /2 = 1.57 x 107 y) and 244Pu t /2 = 8.08 x 107 y) are noteworthy examples of this type of nuclide. 

The decay of extinct radionuclides is measured by measuring anomalies in the isotopic abundance of their stable daughters. For example, 129I decays to 129Xe and its decay will lead to an anomalously high concentration of 129Xe in the mass spectrum of Xe isotopes found in a rock system. What is dated is the formation... 

Jacobsen, S. B., and Harper, C. L., Jr. (1996). Accretion and early differentiation history of the Earth based on extinct radionuclides. In Earth Processes Reading the Isotope Code, A. Basu and S. Hart, eds. Pp. 47-74. American Geophysical Union, Washington D.C. 

McKeegan K. D., Chaussidon M., Krot A. N., Robert F., Goswami J. N., and Hutcheon I. D. (2001) Extinct radionuclide abundances in Ca, Al-rich inclusions from the CV chondrites AUende and Efremovka a search for synchronicity. In Lunar Planet. Sci. XXXII, 2175. The Eunar and Planetary Institute, Houston (CD-ROM). 

Podosek F. A. and Swindle T. D. (1988) Extinct radionuclides. In Meteorites and the Early Solar System (eds. J. F. Kerridge and M. S. Matthews). University of Arizona Press, Tucson, pp. 1093-1113. 

Swindle T. D. (1993) Extinct radionuclides and evolutionary timescales. In Protostars and Planets III (eds. E. H. Levy and J. I. Lunine). University of Arirona Press, Tucson, pp. 867-881. 

Meteorites yield a variety of ages, each reflecting a specific episode in their histories. Some of these "ages" are indicated in Figure 1 the end of nucleosynthesis in a star (i) the first formation of solids in the Solar system (7) melt crystallization in parent bodies (75) excavation of meteoroids from these bodies and the meteorite s fall to Earth. Other events, like volcanism or metamorphism on parent objects can be established by gas retention as can formation intervals (based on extinct radionuclides) measuring the time between the last production of new nucleosynthetic material and mineral formation in early Solar System materials 14). In the following sections we discuss some of these ages and the information that they convey. 

Extinct radionuclides Chronology of solar system formation... 

Extinct radionuclides have a huge advantage in precision over long-lived radionuclides. For example, a change in a factor of two in the abundance of requires only one half-life, 15.7 Ma, while the same length of time will cause a change of less than 1% in the abundance of since it is only about 1% of a half-life. On the other hand, since the extinct radionuclide is, by definition, completely decayed away, it is necessary to determine its abundance at some particular time to determine ages, a problem for all extinct radionuclides. 

The first extinct radionuclide studied, and by far the most commonly studied of the extinct radionuclides, is Most chondrite... 

Gilmour JD (2000) The extinct radionuclide timescale of the Solar System. Space Sci Rev 192 123-132. Gilmour JD, Saxton JM (2001) A time-scale of formation of the first solids. Phil Trans R Soc London A 359 2037-2048... 

Sometimes, cosmic-ray-induced isotope shifts are also a nuisance, because they may compromise the precise determinations of isotopic compositions of elements of interest. This is often a major problem in the case of trapped noble gases (Ott 2002, this volume), but may also require attention when e.g., small excesses or deficits of daughter isotopes of an extinct radionuclide present in the early solar system are to be determined (e g., Leya et al. 2000c). 

Lewis RS (1975) Rare gases in separated whitlockite from the St. Severin chondrite xenon and krypton from fission of extinct Geochim Cosmochim Acta 39 417-432 Leya I, Wider R (1999) Nucleogenic production of Ne isotopes in the Earth s cmst and upper mantle induced by alpha particles from the decay of U and Th. J Geophys Res 104 15439-15450 Lin WJ, Manuel OK (1987) Xenon decay products of extinct radionuclides in the Navajo, New Mexico well gas. Geochem J 21 197-207... 

Figure 10.2 Radioactive decay diagram to illustrate the difference between long-lived and extinct radionuclides. Shown are the decay of long-lived (T 4 = 4.47 Ga) and extinct T4 = 103 Ma), which are assigned arbitrary initial abundances of 2 and 1, respectively. The age of the Earth ( 4.47 Ga) and the solar system (4.57 Ga) are marked by the shaded field. Today, about 50% of the that was present at the start of the solar system (at t = 0) is still alive, whereas essentially all " Sm has decayed to " Nd.

Some short-lived radionuclides were sufficiently abundant at the start of the solar system to produce variations in the abundance of their daughter isotopes in early-formed objects (Table 10.2). The half-lives of these nuclides are between about 0.1 and 100 Ma (Table 10.2). Hence, the parent isotopes are no longer present today, but they were synthesized in stars shortly before solar system formation and therefore they were present in the early solar nebula. The isotopic record of these nuclides provides information about stellar nucleosynthetic sites active shortly before the birth of the solar system and the time scales over which the early solar system formed and first differentiated. Depending on half-life and chemical affinities of parent and daughter isotopes, extinct radionuclide systems can be used to date processes as diverse as the formation of CAIs and chondrules, volatile element depletion and planetary difierentiation (e.g., core segregation and differentiation of early silicate reservoirs). In particular, they are powerful tools to study the Earth s accretion and core formation [90-92],... 

In particular, the technique of MC-ICP-MS has significantly furthered the routine application of a number of extinct radionuclide systems, such as Al— Mg, and ° Pd— Ag, for a wide range of extraterrestrial samples, enabled routine analysis of the long-lived chronometer, and... 

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