Extinct nuclides

Early studies of Mg isotope ratios in geological materials used the notation A Mg to mean per mil deviations from a standard as expressed in Equation (1) above, a convention that persists today (e.g., Elsu et al. 2000). The values assigned to A "Mg in those studies represent the level of mass-dependent isotopic fractionation relative to the standard. The same convention defined fi Mg as the per mil deviation from the standard after correction for the mass fractionation evidenced by A "Mg. In this system of nomenclature, A values refer to mass dependentfractionations while 5 values refer to deviations from mass-dependent fractionation (i.e., the S Mg defines excesses in Mg relative to mass fractionation attributable to decay of the extinct nuclide Al). In some cases A "Mg has been replaced by the symbol Fn,g (Kennedy et al. 1997) where the F refers to fractionation. ... 

Another coupled system is the Sm-Nd system, with two Sm isotopes ( Sm and Sm) undergoing ot-decay to become two Nd isotopes ( Nd and Nd). The half-life of Sm is 106 billion years and that of is 103 million years. In principle, the concepts for the U-Pb system (such as concordia and discordia, Nd-Nd isochron) can also be applied to the Sm-Nd system. However, the Sm-Nd coupled system has not found many applications. One reason is that the half-life of " Sm is so short that it is an extinct nuclide. Secondly, the half-lives of Sm and " Sm are very different, by a factor of 1000 (in contrast, the half-lives of and 235 differ only by a factor of 6.3). Hence, the coupled system has found only limited applications to very old rocks, such as meteorites and very old terrestrial rocks. 

Dating method 4 Extinct nuclides for relative ages... 

Start from the equation for the growth of the daughter D = Dq + Po(1 - If the parent is an extinct nuclide, it would have completely decayed to the daughter. That is, for extinct nuclide,... 

Solution Because the Initial Al/ Al ratio in meteorite 1 is greater, meteorite 1 formed earlier. That is, meteorite 1 has an older age. Because the initial ratio in meteorite 2 is half of that in meteorite 1, the age difference is the half-life, which is 0.71 million years. That is, meteorite 1 is older than meteorite 2 by 0.71 Ma. If the age of meteorite 1 is 4562 Ma, then the age of meteorite 2 is 4561.3 Ma. This example shows that using extinct nuclides can distinguish small age differences in the early evolution history of the solar system. 

Next, we examine whether or not the formation time interval suggested from 29I/127I and (136Xe)Pu/238U is quantitatively consistent. We will also examine how these conclusions from extinct nuclides are affected by other constraints, especially that of missing Xe. 

Jacobsen S. B. and Yin Q. Z. (2001) Core formation models and extinct nuclides. In Lunar Planet. Sci. XXXII, 1961. The Lunar and Planetary Institute, Houston (CD-ROM). 

Figure 5 Plot of variations in (which arise from the decay of the extinct nuclide versus time expressed...

Figure 5 Plot of variations in (which arise from the decay of the extinct nuclide versus time expressed as ppm differences from present-day compositions. An initial Stn/ Sm = 0.007 is assumed (Prinzhofer et al., 1992). If an LREE-depleted mantle (high Sm/Nd) formed within the first 200-300Ma of Earth history, then with the current 5-10 ppm analytical precision, a positive Nd deviation would be expected to be observed in any sample derived from this source. Lunar samples (Nyquist et al, 1995) and martian meteorites (Harper et al., 1995) show clear evidence of Nd variations reflecting very early differentiation on these bodies. Nd anomalies have now been suggested for some 3.7-3.S Ga terrestrial samples from southwest Greenland (Harper and Jacobsen, 1992 Boyet et al, 2002 Caro et al., 2003). Most of the 3.8 Ga gneisses that have been measured do not show these effects and recent re-measurement of the original Harper and Jacobsen sample by Papanastassiou et al. (2003) has not confirmed the original result (see text). The different/curves are for different degrees of depletion (20% and 40%) of...

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