The application of isochron diagrams in40Ar-39Ar dating: A discussion

The presence of a non-radiogenic argon component within rocks and minerals at the time of their crystallization (termed initial argon) is now widely recognized. Present data indicate that the³⁶Ar concentration of this initial argon is in the order of 10^?9 cm³/g STP in a wide range of geological materials. In⁴⁰Ar-³⁹Ar analyses the preference for small samples and division of the gas evolved into a number of temperature steps means that the amount of initial³⁶Ar analyzed is often less than the argon extraction line blank. Thus if blank corrections are not made an isochron plot will represent mixing lines between atmospheric argon and the argon derived from the sample. In this case the isochron parameters will probably be in error. Secondary alteration of samples adds atmospheric argon not related to the initial argon present in a rock at the time of its formation and must be eliminated or avoided if the isochron technique is to be used. The inability to obtain isochrons for samples with excess radiogenic argon may be related to significant extraction system contamination. At present the application of the two-error regression technique is being misapplied in determining the quality of fit and in some cases underestimating the error limits assigned to the isochron parameters.