An attempt was made to use CO2laser step-heating method to date the late Pleistocene basaltic groundmass (DF-2) from Tengchong volcanic field. Among the. Book review: Advances in 40Ar/39Ar dating: From archaeology to planetary sciences handbook Geochronology and Thermochronology by the 40Ar/39Ar Method (McDougall and Harrison ), a new Publication Subtype, Journal Article. Article Navigation. Research Article|November 09, Bridging the gap: 40Ar/ 39Ar dating of volcanic eruptions from the 'Age of Discovery'. Katie Preece.
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.
Argon–argon dating - Wikipedia
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.
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!
Argon Geochronology Methods
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.
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.
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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? Argon loss and excess argon are two common problems that may cause erroneous ages to be determined.
Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma. Both techniques rely on the measurement of a daughter isotope 40Ar and a parent isotope. Because the relative abundances of the potassium isotopes are known, the 39ArK produced from 39K by a fast neutron reaction can be used as a proxy for potassium.
Instead, the ratios of the different argon isotopes are measured, yielding more precise and accurate results. The amount of 39ArK produced in any given irradiation will be dependant on the amount of 39K present initially, the length of the irradiation, the neutron flux density and the neutron capture cross section for 39K.
However, because each of these parameters is difficult to determine independantly, a mineral standard, or monitor, of known age is irradiated with the samples of unknown age.
The monitor flux can then be extrapolated to the samples, thereby determining their flux. This flux is known as the 'J' and can be determined by the following equation: In addition to 39Ar production from 39K, several other 'interference' reactions occur during irradiation of the samples.
Other isotopes of argon are produced from potassium, calcium, argon and chlorine. As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample.
These reactor produced isotopes of argon must be corrected for in order to determine an accurate age. The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses. For example, to determine the amount of reactor produced 40Ar from 40K, potassium-rich glass is irradiated with the samples.
The desirable production of 38Ar from 37Cl allows us to determine how much chlorine is present in our samples. Multiple argon extractions can be performed on a sample in several ways.
Step-heating is the most common way and involves either a furnace or a laser to uniformily heat the sample to evolve argon. The individual ages from each heating step are then graphically plotted on an age spectrum or an isochron.
Mechanical crushing is also a technique capable of releasing argon from a single sample in multiple steps. Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control.
For example, laser spot sizes of microns or less allow a user to extract multiple argon samples from across a small mica or feldspar grain.
The results from a laser probe can be plotted in several graphical ways, including a map of a grain showing lateral argon distribution. Total fusion is performed using a laser and results are commonly plotted on probability distribution diagrams or ideograms.
For the J to be determined, a standard of known age must be irradiated with the samples of unknown age.