Argonbestimmungen an Kaliummineralien—IX: Messungen zu den verschiedenen Arten der Argondiffusion

Abstract

By using the radioactive isotopes Ar 37 and Ar 39 it was possible to measure the diffusion of argon in natural samples of sanidine, phonolithe and augite, as well as in a Bohemian tectite (moldavite) and in synthetic sylvine. The measurements of moldavite and sanidine resulted in a straight line when log D/ a 2 was plotted against 1/ T; the results of the other material in the same graph were two straight lines cutting each other. The boundary layer theory applied by Wrage to solve this problem made it possible to show that the straight line resulting from the moldavite and sanidine measurements and one of the two straight lines resulting from the other material examined represent the volume diffusion. The second straight line in the results of phonolithe and augite however, does not obey these laws of volume diffusion. In this case another mechanism has to be assumed, probably based on diffusion from lattice imperfections. For all substances studied the values of D/ a 2 for the volume diffusion at room temperature are <5.10 −24 (sec −1). Thus the argon loss caused by volume diffusion is so small that the potassium-argon ages on these materials need not be corrected. For the diffusion from lattice imperfections however, the values for D/ a 2 at room temperature would be between 6.10 −18 and 2.10 −28 (sec −1). This means that argon losses which are not based on volume diffusion may have considerable losses. The investigations proved again that for potassium-argon dating sanidine and mica are the best suitable material, whereas other feldspars and rocks are less suited and salts may be used in exceptional cases only.