The Application of Neutron Activation to the Determination of Ages of Young Terrestrial Rocks.

The [39]Ar-[40]Ar dating method has been developed over the past decade and applied with great success to Lunar samples and meteorites. The potential of the method as applied to terrestrial samples has still to be fully realized because of the greater complexity of terrestrial rocks. One promising area of research is. the application to dating very young rocks. where the 39Ar-40Ar method should be more sensitive than the conventional K-Ar dating. The work described in this thesis was an attempt to investigate the area of young rock dating and involved both the development of the necessary techniques and the application to a specific geological problem. On the technical side, a static mass spectrometer and associated argon extraction and purification system was constructed capable of detecting 5x10[-13] ccSTP of argon. In parallel with this, the appropriate techniques for fast neutron irradiation of large (20g) samples with low fluences, were developed. The application of the technique involved the investigation of suitable young rock neutron monitors. Two geological samples of known age were studied for this purpose; Bishop Tuff sanidine (0.7 x 10[6] years) and Bern muscovite (18.7 x 10[6] years). Finally, the methods developed were successfully applied to a suite of East African volcanic rocks with ages in the range (0.1-2.3)x10 6 years. The significance of this study is twofold in that the ages relate to;(a) the elucidation of the pluvial (glacial) cycles in East Africa and (b) the rate of petrogenesis of lavas from a single magma body