Now extinct radionuclides

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 quantitative comparison of various shortlived radionuclide systems with each other and with Pb-Pb chronology has only been made possible by new data obtained during the last decade, or in many cases, the last few years. Over this same time period, evidence for the decay of several important new short-lived isotopes in the early solar system has been discovered. The record of now-extinct isotopes in early solar system materials is becoming sufficiently well defined to allow construction of a plausible timeline and scenario for solar system origin. 

There is now observational evidence for the existence of isotopic anomalies involving the p-isotopes of Kr, Sr, Mo, Xe, Ba and Sm in various meteoritic materials [32]. These anomalies manifest themselves as excesses or deficits of the abundances of the p-nuclides with respect to the more neutron-rich isotopes, when comparison is made with the bulk SoS mix. In addition, two isotopic anomalies are attributable to the now extinct neutron-deficient radionuclides 92Nbg and 146Sm which have decayed in the meteoritic material where excesses of 92Zr and 142Nd are observed. 

A number of short-lived radionuchdes also existed at the time that the Sun and the rocky bits of the solar system were forming (Table 1). These nuclides are sufficiently long-lived that they could exist in appreciable quantities in the earhest solar system rocks, but their mean fives are short enough that they are now completely decayed from their primordial abundances. In this sense they are referred to as extinct nuchdes. Although less familiar than the still-extant radionuclides, these short-lived isotopes potentially play similar roles their relative abundances can, in principle, form the basis of various chronometers that constrain the timing of early chemical fractionations, and the more abundant radioisotopes can possibly provide sufficient heat to drive differentiation (i.e., melting) of early accreted planetesimals. The very rapid rate of decay of the short-lived isotopes, however, means that inferred isotopic differences translate... 

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