The Stability of Lead Isotopes from Thorium

Abstract

SINCE my recent letter on the subject of thorium lead (NATURE, February 15, p. 469) I have had some correspondence with Dr. Arthur Holmes, who, in agreement with Boltwood, had previously concluded from geological evidence that lead could not be the end product of thorium, because thorium minerals often contain so little lead in comparison with what is to be expected from their age. He pointed out that the age of Ceylon thorite as determined from the ratio of lead to thorium was curiously anomalous. Taking, as preferable, Rutherford's values for the periods of uranium and thorium, 0.72 and 1.9 (×10¹⁰ years) respectively (in the ratio of 1 to 2.6, instead of 3.2, the figure used in the previous letter), the proportion of the thorite lead derived from the thorium would be 95.5 per cent., and from the uranium 4.5 per cent. The quantity of thorium lead per gram of thorium would be 0.0062. The rate of growth would be 4.72 ×10^-11 gram of lead per gram of thorium per year, and the age of the mineral 131 million years. A Ceylon pitchblende (U = 72.88 per cent., Pb = 4.65 per cent.) has a ratio of lead to uranium of 0.064, giving the age as 512 million years, and Dr. Holmes considers that this is likely to be of the same geological age as the thorite, and to be, of all the Ceylon results, the most trustworthy for age measurements.