Rare-gas dating,III. Evaluation of a double-spiking procedure for potassium-argon dating

A double-spike method for KAr dating is described. Use of long-lived³⁹Ar in addition to the usual³⁸Ar in the spike permits argon isotopic discrimination occurring after insertion of the spike to be monitored at the same time as ages are determined. A “bootstrap” (self-contained) method of calibrating the spike for isotopic composition is described which, when used appropriately, gives final ages which are independent of the published isotopic composition of the terrestrial atmospheric argon used as a comparison gas. The method calls for a mass spectrometer which resolves adjacent isotopes to a high degree. We have tested the method nevertheless with a mass spectrometer of relatively low resolving power, the AEI MS-10, and found it to be successful in part and without any disadvantages with respect to conventional techniques in those instances where the method fails because of insufficient resolution. That is, we find the discrimination factor inferred in runs on young rocks and blanks to be valid and useful. That factor changes by as much as 1.3% over the life of a filament in the ion source of the MS-10, proving that close control of the discrimination factor is essential for analytical precision with the instrument. These assertions are documented graphically with a compilation of data which represents more than two years experience with the method. We also report measurements of the mass discrimination of the MS-10 and its linearity with mass which indicate that the latter quantity is acceptable when the repeller voltage for the ion source is close to +1 V and is unacceptable otherwise. The spike material is available, upon request, to those laboratories which decide firmly to undertake further testing of the method.