Abstract
The 40K/40Ar and derived 40Ar/39Ar isotopic dating methods are used extensively for age determination of a wide variety of rock types. 40K/40Ar is primarily used in terrestrial samples that have experienced relatively simple geological histories. 40Ar/39Ar is used for both terrestrial and extraterrestrial samples, the latter including meteorites and those collected by planetary missions. The versatility of 40Ar/39Ar geochronology lies in the range of gas extraction techniques used (lasers, furnaces, and crushing release) and the number of different Ar components (i.e., of different origins) present. Often, the Ar present in a sample is a heterogeneous mix of two or more of these components making it more challenging to disentangle the 40K-derived radiogenic 40Ar. Moreover, the application of this method to lunar and martian meteorites can provide information about the planetary surface including surface exposure, impact, and alteration. This chapter describes the wealth of Ar components present in extraterrestrial samples, terrestrial basalts, and tektites. Selected examples are used to demonstrate how a combination of different Ar extraction techniques and graphical approaches, supported by textural and chemical data, are used to deconvolve some if not all of the Ar components that may be present in a sample.
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