An attempt to date some of the diamonds by a K-Ar isochron method

For examining the abundance of rare gas and the isotope composition in the earth mantle, it will do to analyze the rare gas captured in the substances came from the mantle, but in most cases, the mantle substances have undergone the action of weathering and alteration. On the contrary, diamond crystals, which may be called physical fossil in the mantle, are very stable under high temperature and high pressure, and are hardly damaged by chemical substances, therefore, the possibility that those crystals retain the rare gas having captured during the crystal growth in the mantle as it is high. From such viewpoint, the authors have used the diamond samples of about one carat, separated their rare gas, and measured the isotope ratio over the last five years, thus the interesting results have been obtained. Especially, the result of having precisely measured the helium isotope ratio He-3/He-4 with a gaseous mass spectrometer of high resolution was higher than the value of carbonaceous chondrite. It was attempted to determine the age of diamond formation by K-Ar method. The principle of K-Ar isochron method, the determination of K by activation analysis and with a tandem accelerator, and the mass analysis of Ar are reported. But the obtained result of about 6 b.y. was older than the age of the earth 4.54 b.y., therefore, it is doubtful. (Kako, I.)