Isotopic dating of geological samples using the K/Ar method and its variant, the 40Ar/39Ar dating technique, provides ages that in favourable circumstances are precise and accurate to within 1%. Limiting factors include the accuracy of the decay constants for 40K and the age of neutron fluence monitor minerals. For rapidly cooled igneous rocks, a K/Ar or 40Ar/39Ar age normally will give a good estimate of the age of eruption, but for slowly cooled igneous rocks or metamorphic rocks an age measured on a sample is likely to be a cooling age. As 40Ar*/39ArK ratios can now be determined often to better than 0.1%, the main limitation on accuracy relates to how well the 40K decay constants are known. Better determination of the β− and γ decays of 40K, the basis for the decay constants, is suggested at the present time, rather than adoption of new decay constants linked to another decay scheme. Until improved independently measured decay constants for 40K become available, there may be some circumstances where the newly proposed decay constants need to be used, especially when comparing ages on volcanic igneous rocks with those measured in another system.
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Advances in 40Ar/39Ar Dating: From Archaeology to Planetary Sciences
Decoding the complete history of Earth and our solar system requires the placing of the scattered pages of Earth history in a precise chronological order, and the 40Ar/39Ar dating technique is one of the most trusted dating techniques to do that. The 40Ar/39Ar method has been in use for more than 40 years, and has constantly evolved since then. The steady improvement of the technique is largely due to a better understanding of the K/Ar system, an appreciation of the subtleties of geological material and a continuous refinement of the analytical tools used for isotope extraction and counting. The 40Ar/39Ar method is also one of the most versatile techniques with countless applications in archaeology, tectonics, structural geology, orogenic processes and provenance studies, ore and petroleum genesis, volcanology, weathering processes and climate, and planetary geology. This volume is the first of its kind and covers methodological developments, modelling, data handling, and direct applications of the 40Ar/39Ar technique.