Radioactive chronometers

Cosmochemistry is the study of the chemical compositions of various solar system materials. Chondrites are the most abundant primitive samples. They are essentially sedimentary rocks composed of mechanical mixtures of materials with different origins (chondrules, refractory inclusions, metal, sulfide, matrix), which we will call components. Chondrites formed by the accretion of solid particles within the solar nebula or onto the surfaces of growing planetesimals. They are very old (>4.5 billion years, as measured by radioactive chronometers) and contain some of the earliest formed objects in the solar system. Chondrites have bulk chemical compositions very similar to the solar photosphere, except... 

Much of what we learned about the U, U and Th decay chain nuclides as chronometers and process indicators we owe to these seminal studies based on the measurement of radioactivity. 

Because radioactive decay is a nuclear process, the rate of radioactive decay is totally unaffected by any external factors. Unlike chemical reactions, therefore, there is no dependency on temperature, or pressure, or any of the other environmental factors which affect the rate at which normal chemical reactions occur. This is the reason why radioactive decay chronometers, such as 14C, Ar-Ar, and U-series methods, are so important in geology and archaeology - they provide an absolute clock . 

As for other extinct radioactivities, to provide absolute ages, the Mn/ Cr chronometer has to be anchored to a sample for which the absolute age is known from a traditional chronometer, usually U-Pb because of its high precision. There is debate going on how to do this with Mn and how to compare the Mn- Cr data to other short-lived and long-lived radiometric systems (Lugmair and Shukolyukov 1998 Birck et al. 1999 Quitte 2001). Further high precision work is still required to settle the debate. 

This was the first extinct radioactivity detected (Jeffrey and Reynolds 1961) and was made possible by the early high sensitivity of rare gas measurements and the low abrmdance of Xe in rocks. I has only one stable isotope at mass 127. Its abundance is measured as Xe after exposing a sample to an adequate neutron flux. The correlation between Xe and Xe observed in a stepwise degassing of a sample demonstrates that the excess Xe results from decay (Fig. 9h). Results in primitive meteorites and inclusions show that i29j/i2tj j.jose to 10 . Chronometry with I- Xe has been widely used in meteorite work (Reynolds 1963 Hohenberg 1967) but occasionally has some diflflculties to agree with the other chronometers due to the sensitivity of I to secondary processes and water alteration (Pravdivtseva et al. 2003 Busfleld et al. 2004 see also Swindle and Podosek (1988) for an extensive review). ... 

In order to use radioactive decay of an element for a chronometer, the decay constant must not have changed with time. How do we know that this is true ... 

In addition to the processes of stellar nucleosynthesis, there are two other ways in which isotopes are produced. One is radioactive decay. Many of the nuclides produced by explosive nucleosynthesis are unstable and decay to stable nuclei with timescales ranging from a fraction of a second to billions of years. Those with very short half-lives decayed completely into their stable daughter isotopes before any evidence of their existence was recorded in objects from our solar system. However, radioactive nuclei from stellar nucleosynthesis that have half-lives of >100 000 years left a record in solar system materials. For those with half-lives of more than 50 million years some of the original nuclei from the earliest epoch are still present in the solar system today. The ultimate fate of all radioactive nuclides is to decay to their stable daughter nuclides. Thus, the only real distinction between isotopes produced by stellar nucleosynthesis and those produced by decay of radioactive nuclides produced by stellar nucleosynthesis is the time scale of their decay. We choose to make a distinction, however, because radioactive nuclides are extremely useful to cosmo-chemists. They provide us with chronometers with which to constmct the sequence of events that led to the solar system we live in, and they provide us with probes of stellar nucleosynthesis and the environment in which our solar system formed. These topics appear throughout this book and will be discussed in detail in Chapters 8, 9, and 14. 

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. 

Samarium has seven naturally occurring isotopes, two of which, 147Sm and 148Sm are radioactive (Table 4.2). The -decay of long-lived 147Sm to 143Nd (t1/2 =1.06 x 1011 years ka =6.54 x 10 12 yr 1) is a widely used chronometer. In contrast, the half-life of... 

The laws of radioactive decay are the basis of chronology by nuclear methods. From the variation of the number of atoms with time due to radioactive decay, time differences can be calculated rather exactly. This possibility was realized quite soon after the elucidation of the natural decay series of uranium and thorium. Rutherford was the first to stress the possibility of determining the age of uranium minerals from the amount of helium formed by radioactive decay. Dating by nuclear methods is applied with great success in many fields of science, but mainly in archaeology, geology and mineralogy, and various kinds of chronometers are available. 

The ability of short-lived radioisotopes to function as chronometers for the early solar system is critically dependent on there having been an initially uniform distribution of the radioactivity throughout the nebula, or at least in those regions from which meteoritic components are derived. Only in this circumstance can differences in initial abundances of a radionuclide compared to a stable counterpart, as inferred by the excesses of the respective daughter isotope, be interpreted as due to radioactive decay from the initial inventory. The homogeneity of the distribution of radionuclides in the solar nebula depends, in turn, on the processes that created those isotopes some time before the formation of early solar system materials. For the longer-lived isotopes listed in Table 1 (e.g., Hf, 129 92, ... 

Aside from anoxic or suboxic basins, the other marine environment suitable for radioactive geochronometry is salt-marsh deposits. As sea level has risen over the past 100 years, salt marshes have kept up by vertical growth of a vegetated framework that supports sediment accumulation. In addition, since high salt marshes are inundated by seawater only —5% of the year, the surface becomes an accumulator of atmospherically derived species including °Pb. The radioactive decay of °Pb can then be used to determine the age of levels in the salt marsh and thereby the accumulation rate of the salt marsh and its components. Since the salt-marsh vertical growth depends on the rise in sea level, the °Pb chronometer becomes a proxy for the rate of rise... 

A unique answer to this question is not possible since an "age" is a time interval between two events and an age may depend on the specific chronometer used. An accurate chronometer must involve a mechanism operating on a predictable, but not necessarily constant rate. The "clock" starts by an event beginning the time interval and its end must be clearly and sharply recorded. Chronometers used in modem geo- and cosmo-chronology usually involve long-lived, naturally occurring radioactive isotopes such as the U-isotopes, Rb or... 

Radioactive decay allows calculation of an age if the concentrations of both parent and daughter nuclide are known, the beginning of the time interval is defined and the system is not disturbed (i.e. it is a "closed system") during the time interval. Some chronometers involve production of particular stable or radioactive nuclides, or decay of the latter. Typically, the chronometer half-life should be comparable with the time interval being measured. 

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