Dating methods

From the radioactive decay constants and measurement of the amount of argon in a rock sample, the length of time since formation of the rock can be estimated. Essentially, the dating method requires fusion of a rock sample under high vacuum to release the argon gas that has collected through radioactive decay of potassium. The amount of argon is determined mass spectrometrically,... 

Speciman Preparation for Transmission Electron Microscopy of Materials (J. C. Brauman, R. M. Anderson, and M. L. McDonald, eds.) MRS Symp. Proc vol. 115, Materials Research Society, Pittsburg, 1988. This conference proceedings contains many up-to-date methods as well as references to books on various aspects of specimen preparation. 

J. M. Michels, Dating Methods in Archeology, Seminar Press, New York, 1973, 230 pp., S. Fleming, Dating in Archeology A Guide to Scientific Techniques Dent, London, 1976, 272 pp. 

The Juan Fernandez Islands, Chile, he east of the South American mainland at approximately 33°30 S, 80°W (Fig. 6.8). The archipelago consists of two main islands, Masatierra, which lies about 660 km from the mainland, and Masafuera, which lies 150km further west. A third, small island, Santa Clara, lies off the western end of Masatierra and probably was attached to Masatierra at one time. The ages of the islands have been determined using potassium-argon dating methods to be 5.8 2.1 million years for Santa Clara 4.23 0.16 million years for Masatierra and 2.44 0.14 million years for Masafuera (Stuessy et ah, 1984). 

Data on mineral s arates in present day volcanic rocks. Since every dating method (including the K-Ar or C systems) can be affected by several geochemical perturbations which may lead to erroneous ages, the best test for the °Th- U mineral isochrons consists in the analysis of presently erupted lavas or historic lavas of well known eruption dates. Rather surprisingly the data obtained on such samples are not so numerous (some examples are illustrated in Fig. 10). Early data showed that, in some cases, there were interlaboratory analytical discrepancies, especially in Th isotope ratios measured on mineral separates extracted from the same lava flows (this was the case for the 1971 lava from Mt. Etna and 1944 lava from Mt. Vesuvius Capaldi and Pece 1981 Hemond and Condomines 1985 Capaldi et al. 1985). This emphasizes the fact that °Th- U mineral analyses... 

Th- U dating of samples from subaerial volcanoes. When several volcanic rocks covering a significant period of the eruptive activity of a volcano can be dated (either by mineral isochrons or by other dating methods), the evolution through time of the ( °Th/ Th)o or ( °Th/ U)o initial ratios will be revealed (see section 3.4). If these ratios remain nearly constant, then they may be used to calculate the ages of other lavas... 

Figure 22. Comparison of the ages obtained by the and dating methods on MORE...

Capaldi G, Pece R (1981) On the reliability of the °Th- U dating method apphed to yotmg volcanic rocks. J Volcanol Geotherm Res 11 367-372... 

Hemond Ch, Condomines M (1985) On the reliabihty of the dating method applied to yonng... 

Agreement is found for ages determined on the same material by independent dating methods... 

Schwarcz HP, Harmon RS, Thompson P, Ford DC (1976) Stable isotope studies of fluid inclusions in speleothems and their paleochmatic significance. Geochim Cosmochim Acta 40 657-665 Schwarcz HP, Rink WJ (2001) Dating methods for sediments of caves and rock shelters. Geoarchaeology 16 355-372... 

Shen GJ (1996) Th/ °Th dating method Methodology and application to Chinese speleothem sample. Quat Sci Rev 15 699-707... 

Figure 1. Dating methods employed by archaeologists (e.g., for an up-to-date overview of dating methods applied to human evolution see Griin et al., in press). Also shown is the radimentary chronology of human evolution for the last million years.

The discovery of the decay chain, of course, started with the seminal work of Marie Curie in identifying and separating Ra. Through the work of the Curies and others, all the members of the decay chain were identified. An important milestone for geochronometrists was the discovery of °Th (called Ionium) by Bertram Boltwood, the Yale scientist who also made the first age determinations on minerals using the U-Pb dating method (Boltwood in 1906 established the antiquity of rocks and even identified a mineral from Sri Lanka-then Ceylon as having an age of 2.1 billion years )... 

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