The nuclear family decay

Radiogenic Isotope Geology
February 7, 1997

Isotopes of the elements come in two forms: those that are radioactive and those that, so far as we are aware, are not and so are termed stable. A radioactive parent isotope is transmuted by the process of nuclear decay into a daughter isotope, of another element, which of course may itself be stable or radioactive. (This nomenclature has, at least so far, escaped the encroachment of political correctness.) Such daughter isotopes are said to be radiogenic. Variations in the natural abundance ratios of isotopes of a given element may then arise in two fundamentally different ways. First, the mass difference between the isotopes may result in fractionation because of slightly different physical, chemical and biological behaviour, and because of "hot atom" effects. Second, radioactive decay of a parent may change the abundance of a radiogenic daughter. The latter style of process is of inestimable value in the geosciences, and there are two major types of application. On the one hand, radioactive decay provides the chronometer in whose tempo we have the most faith (I exclude here the views of some creationists who subscribe to a supposedly higher faith: they have been addressed in G. Brent Dalrymple's admirable The Age of the Earth.) Much geochronology thus relies on the precise measurement of radiogenic isotopes. On the other hand, the ratio of a radiogenic daughter to a stable isotope of the same element is often a very useful geochemical tracer, because it can elucidate the provenance of the element and can be affected by the timing of separation or fractionation of parent from daughter. In Radiogenic Isotope Geology, Alan Dickin presents a masterly overview of the history, principles, experimental techniques and practical applications of this wide-ranging and burgeoning field.

The book is arranged into 16 chapters with relevant references at the end of each chapter. As an appendix, there is a rudimentary yet serviceable chart of the nuclides, and there is a comprehensive and useful index, including both subjects and the first author of cited references. In the preface, Dickin explains that the approach adopted is deliberately historical. On reading this, certain misgivings immediately arose in my mind because it is a matter of fine judgement to preserve a balance between appropriate details of historical development (and the potentially thorny issues of priority), and yet still convey the excitement and scientific import of more recent findings. Such apprehension was misplaced, for the author's judgement is continuously shrewd and insightful so that one comes to trust it in areas outside one's direct experience: in this regard the good single-authored volume scores immeasurably over the edited contributions of a team of specialists who may lack a coherent and consistent philosophy of approach.

The first two chapters, "Nucleosynthesis and nuclear decay" and "Experimental techniques") provide the required theoretical and technical background for much that follows. The account is full and lucid with a citation policy that allows original/early contributions, important reviews and interesting recent (ie up to 1994) work all to be mentioned. The appropriateness of Dickin's treatment, in terms both of the material presented and the degree of detail, is here established. I doubt that many physicists consider the invariance of decay constants over the past 4.6 billion years to be an issue of Hutton's uniformitarianism (after all catastrophism would not necessarily imply a new model of physics, and secular changes of rates can be accommodated within the Huttonian viewpoint) but the mere fact that the issue of invariance is discussed is impressive. The techniques chapter provides a most useful starting point for neophytes, and introduces important concepts such as isotope dilution and the differences between isochrons and errorchrons.

Each of chapters three to five is devoted to a specific radiogenic isotope method (constituting the "three pillars of lithophile isotope geology" in the author's felicitous phrase). The Rb-Sr system starts with an unfortunate typographical error in that the atomic number of strontium is 38, not 37, but thereafter the progress is sure-footed through such diverse topics as meteorite chronology, dating sedimentary rocks and the isotopic evolution of seawater strontium. Chapter four, which introduces the Sm-Nd method, includes a careful description of the various model ages calculable from this decay scheme. The historical approach is here seen to advantage in that Fig 4.21 displays for 1985-92 the changing citation rates of papers which introduced different depleted mantle evolution models for Nd: the reader can see at a glance the judgement of "science". The next chapter, on lead isotopes, is a clear account of the significant advances in geochronology achieved by the modern uranium-lead methods pioneered by T. Krogh, and thereafter guides the reader through the intricacies of terrestrial lead isotope geochemistry.

In the first third of the book, Dickin thus provides the foundation of much modern radiogenic isotope geochemistry. As if to demonstrate the utility and versatility of these methods, two chapters are then devoted to thematic applications, the isotope geochemistry of oceanic volcanic rocks (chapter six) and of continental rocks (chapter seven). These are of intrinsic value both didactically and in showing how the isotopic dimension has been central to many recent advances in understanding of the lithosphere. However, the chapters also perform a second function whereby, in a natural way and an appropriate setting, they allow the introduction, description and elucidation of generalisable tools, eg the Nd-Sr diagram in chapter six, and two component mixing models and inversion modelling in chapter seven. Trace element and light element stable isotope data are aptly pressed into service as required. Each of the remaining chapters deals with a set of somewhat more specialised methods, which carries the benefit that the potential readership is accordingly wider.

Chapters eight and nine deal with the parent-daughter pairs Re-Os, and Lu-Hf, La-Ce, La-Ba, K-Ca respectively. They contain crisp accounts of both geochronological and isotope geochemical applications. The noble gases are covered in chapters ten and 11. The first focuses on dating methods with careful descriptions of the potassium-argon and argon-argon techniques, together with their respective merits and some prominent (or at least popular, vide K-feldspar thermochronometry) areas of application.

The succeeding chapter aims at an altogether different target and gives a concise and thoughtful introduction to the noble gas geochemistry of helium, argon, xenon and neon (in that order: krypton does not feature in the index) with particular emphasis on their role in models of the degassing of the earth.

The motif of couplet chapters wherein the first addresses geochronology and the second isotope geochemistry is maintained for 12 and 13, the topic now being the natural uranium series. Both counting and mass spectrometric analytical techniques are described, and dating methods are conveniently grouped under daughter-excess and daughter-deficiency headings. The illustrative applications in chapter 13 are again high temperature (or "hard rock" in the argot of geologists) including melting models and the evolution of the mantle. The final three chapters each stand alone. Chapter 14 handles the cosmogenic nuclides - those formed by interaction of terrestrial target nuclei and galactic cosmic rays. The systems dealt with are 14C, 10Be, 36Ce, 129I, 26Al (chapter 11 having included discussion of 3He and 21Ne): these sections will be of wide interest to the growing numbers of geomorphologists who are becoming aware of the great potential of cosmogenic exposure age dating. The extinct radionuclides are covered in the brief Chapter 15, and a particularly clear account of fission track dating in Chapter 16 brings the text to a close.

The book will be of very widespread interest and will be routinely used by practising academic and industrial geologists and geochemists. Senior undergraduates would all benefit from their personal copy, but this probably requires that a softback version appear.

Anthony E. Fallick is reader in isotope geosciences, Scottish Universities Research and Reactor Centre.

Radiogenic Isotope Geology

Author - Alan P. Dickin
ISBN - 0 521 43151 4
Publisher - Cambridge University Press
Price - £65.00
Pages - 452

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