The 40Ar-39Ar dating technique: principles, developments, and applications in cosmochronology

During the last ten years a new variant of the K-Ar dating method, the 40Ar-39Ar technique has been established. It is the aim of this review to demonstrate that the 40Ar-39Ar technique, even in cases of a not totally closed K-Ar-system, provides a chronologically significant age and also in addition further information on processes and events during the history of the rock studied. Firstly a comprehensive presentation of the technique is given followed by a review of recent methodical developments. It is shown that, e.g. in lunar surface samples radiogenic 40Ar can be distinguished from excess 40Ar. It is also demonstrated that intermediate loss of 40Ar as well as the presence of inherited 40Ar can be corrected. As examples 40Ar-39Ar results obtained from breccias from the Noerdlinger Ries crater and from the lunar highlands are presented. Finally the 40Ar-39Ar chronology of the carbonaceous chondrite Allende and the unusually high ages of some of its white inclusions are discussed in detail. (author)