Extinct radioactive decay systems

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], 

Iron meteorites exhibit variations in the ratio, which were originally 

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Extinct radioactive decay systems are based on short-lived parent isotopes that were alive during the early history of the solar system. These radioactive parents are no longer present today, because they have essentially decayed away completely, as they feature short half-lives of only about 0.1-100 Ma (Figure 10.2, Table 10.2). Their former presence can be inferred from meteorites, based on abundance variations for the daughter isotopes that we can measure today. Differences in the isotopic compositions of the daughter elements can then provide very precise temporal constraints on processes that occurred in the earliest solar system and information on the kinds of stars that last added material to the molecular cloud from which the solar system formed. 

The coupled Pb and U- Pb decay systems can provide age information based on Pb/ ° Pb and Pb/ Pb only. This is advantageous as Pb isotopic compositions can be determined with higher precision than the elemental parent/daughter ratio that must be applied in conventional chronological studies. This technique of Pb-Pb dating can provide the most precise absolute age data for solids formed early in the solar system and is the lynchpin for the calibration of extinct radioactive decay systems. Analytical improvements that have been achieved... 

A third source for radionuclide generator systems is the recovery of radioactive parents from extinct radioactive decay processes, such as Th, which is recovered from decay products. The Th represents a convenient, long-lived iT i = 7,340 years) source from which Ac is recovered, which is the parent of the Ac/ Bi generator system. 

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.

The radioactive isotope 26Al decays to 26Mg with a very short half-life of 7.2 X 105 years, and so 26Al becomes extinct within a few million years of its formation. Some have likened this isotope system to the second-hand on the clock of the Universe. 26Al was produced by explosions in supernovae early in the history of the Universe. The discovery of the excesses of the daughter isotope 26Mg in very early solar system objects such as CAIs indicates that they contained 26Al at the time of their formation and so must have formed within a very short time of the supernova explosion (see Section 2.3.3.2). 

In addition to stable elements, radioactive elements are also produced in stars. The unstable but relatively long-lived isotopes °K, Th, and are the internal heat source that drives volcanic activity and processes related to internal convection in the terrestrial planets. The short-lived transuranium elements such as Rn and Ra that are found on the Earth are all products of U and Th decay. These isotopes are sometimes used as tracers of natural terrestrial processes and cycles. Long-lived isotopes, such as Rb and Sm, are used for the precise dating of geological samples. When the solar system formed, it also contained several short-lived isotopes that have since decayed and are now extinct in natural wstems. These include A1, Pu, Pd, and 1. Al, with a half-life of less than 1 Ma, is particularly important because it is a potentially powerful heat source for planetary bodies and because its existence in the early solar system places tight constraints on the early solar system chronology. 

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