Radiogenic Argon in a Cooling Dike

Abstract

High precision geochronological studies require understanding of relations between apparent (measured) K–Ar ages and various episodes in a cooling history of magmatic bodies: intrusion of liquid magma, chemical closure of a magmatic system (decreasing temperature below a solidus), and closure of a parent–daughter isotope system. There arises a number of problems connected not only with distortions of true ages due to repeated thermal or tectonic influence on rocks after crystallization, but also due to radiogenic argon accumulation in the process of post-crystallization sub-solidus decrease of temperatures. For definition of an instant for liquid intrusion, chemical closing of a system should be taken into account in connection with influence of the latent heat of crystallization. The account of the latter is feasible, though it complicates calculations. So far, it is important to show the principal approach to time definition of intruding magma.