40Ar-39Ar dating by laserprobe: implications for the interpretation of K-Ar ages.

Dating geological events is the basis for any reconstruction of the evolution of the earth's crust. Chronological data may provide estimates of the age, length and rate of a geological event and enable comparison with theoretical modelling. Numerous dating techniques commonly applied to the earth sciences are based on radioactive decay. One of the most important methods is based on the radioactive decay of 40K to 40Ar. The K-Ar method (and the 40Ar-39Ar variant) represents one of the most versatile and powerful geochronological tools, which can be applied to a wide range of geological problems. Potassium is a major element in the upper crust and in most common rock-forming minerals (e.g., feldspars, micas, amphiboles). The half-life of 40K of 1.25 Ga and the highly incompatible behaviour of argon, which in most cases only allows the incorporation of negligible amounts of ambient argon in minerals, allows the K-Ar method of dating to be applied to rocks ranging in age from a few thousand years to the oldest rocks available. In the last decade, there has been increasing interest in the 40Ar-39Ar variant of the K-Ar method due to the development of laser extraction techniques. The argon laserprobe technique has become an important and powerful dating method, which permits 40Ar-39Ar analysis of very small samples (down to micrograms) and has opened new fields of research. The greatest advantage of the laser extraction method is that it enables in-situ dating at high spatial resolutions, thereby allowing intragrain and intergrain age mapping of rock chips, with the great potential of linking petrology to argon isotope records. This note first provides a brief description of the general principles of the 40Ar-39Ar method and argon laserprobe dating, and then focuses on the importance of linking textural-chemical information to argon age records, and on the implications for the interpretation of K-Ar ages.