Journal of Petrology Volume 41 Number 12 Pages 1823-1824 2000
© Oxford University Press 2000
BOOK REVIEW |
Geochronology and Thermochronology by the 40Ar/39Ar Method, 2nd edn, by Ian McDougall and T. Mark Harrison. Oxford University Press, Oxford, 1999. 269 pp. ISBN 0195109201. £65
When the first edition of McDougall and Harrison appeared in 1988, it summarized the state of knowledge in Ar–Ar geochronology, and provided every graduate student and academic with essential details and knowledge of the technique. It was not an easy read, but went far beyond the summary chapters on Ar–Ar geochronology in general isotope geology texts. It had some foibles, such as the infamous errors in the diffusion equations in Table 5.1 (included, it was rumoured, to catch the unwary Ph.D. student). It was, and remains the essential reference book found in every Ar–Ar laboratory throughout the world. My edition was very well thumbed and continued to be one of the books most often pulled from the bookcase behind my desk. Geochronology and Thermochronology by the 40Ar/39Ar Method is the only dedicated book expounding 40Ar/39Ar dating techniques, theory and interpretation, and as such has had a huge influence on the Ar–Ar community.The book is now over 10 years old and has been in need of a second edition for several years. So the question is, if you have the first edition, should you use up those very limited funds for books, to buy the second edition? The short answer is yes. As the authors state in their preface, there have been several improvements in the techniques, automation and lasers, in particular, but there have also been several advances in interpretation. I would go further and say that the techniques you will be using now and the application papers you will want to find, are described and referenced in this edition and not in the first edition. Now does that mean it is perfect ... every Argonaut’s dream? Not quite, there are many good things about the new edition and a few not so good.
The first three chapters retain their original titles and retain the flavour of the first edition, chiefly by retaining many of the diagrams. However, the early chapters have been fully updated and I found excellent new sections on the use of lasers, including discussions of single grain analysis, dating young volcanic rocks and high spatial resolution techniques, which I think have been the main technical advances of the last decade. These new techniques have been important in the increase in popularity of Ar–Ar geochronology and growth in number of Ar–Ar laboratories. The sections on materials suitable for dating have also received comprehensive updates, including the newer literature examples and newly introduced mineral systems such as the manganese oxides. There are also particularly important updates to error calculation, the nuclear reactors and international standards. The question of intercalibration of standards, a particular area of controversy for Ar–Ar dating in recent years, is handled in detail.
The later chapters depart more completely from the original book, starting with a clear exposition of data presentation and interpretation. My first complaint, however, is that the anion vacancy model for excess argon diffusion at different rates has made it into the second edition. I thought most workers now attributed the release of excess argon at high temperatures to melt inclusions as shown by Esser et al. (1997), which is referenced elsewhere in the book. The rest of the section on excess argon is dominated by use of duplicate steps to correct K-feldspar cycle heating experiments and misses the opportunity for a general discussion of excess Ar in solid and fluid inclusions. The chapter on Ar diffusion theory and measurements is still the only complete text on the subject, and as such it is worth a book by itself, but, oh dear, Table 5.1 seems to have made it through to the second edition unchanged! The chapter contains the most up-to-date work on K-feldspars derived from cycle heating experiments. For those of you who have been asleep for the last decade, K-feldspar thermochronology is a technique developed chiefly by the UCLA group led by Mark Harrison, which can reveal continuous thermal histories from plutonic K-feldspars. All aspects of the technique are explained and discussed in the thermochronology chapter, including some aspects so new they have not even appeared in print yet! The applications and case histories chapter covers stratigraphic dating of igneous events and thermochronology, both of which are new and give real insight into the techniques. The stratigraphic geochronology covers the K/T, for some reason denoted the K/P by the authors, and a section on dating young tuffs associated with hominid evolution, one of Ian McDougall’s special interests. The section on thermal histories of continental crust details the contribution of K-feldspar thermochronology to thermal history studies, Mark Harrison’s area of special interest. In fact, K-feldspar thermochronology permeates many new areas of the book, and its successes are applauded, as you might expect. Its seems, however, that the authors ran out of steam at the end, and two application sections, paleomagnetism and lunar geochronology, from the original book are repeated. Neither is particularly current and I would rather have seen some applications in the areas of current attention such as perhaps the huge increase in the use of plagioclase for dating extremely young volcanic rocks, discriminating against contaminating older grains in tuffs, dating manganese minerals or direct Ar–Ar dating of deformation.
Finally, is the second edition of McDougall and Harrison value for money as an Ar–Ar source book (because it is not cheap)? Do the readers of Journal of Petrology need this second edition? The answer is yes; it is easy to grumble if some aspects of the subject are not covered in detail, but in fact it is a remarkably difficult task to write such a source book and keep it up to date. The authors have succeeded in updating their original work and making it relevant to modern Ar–Ar dating.
Simon P. Kelley
Department of Earth Sciences, Open University
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