Holmes[141] has further extended this work. He examined a number of rare earth and allied minerals from the Christiania district, which Brögger considers to be of approximately Lower Devonian age, and found the ratio of lead to uranium to approximate quite closely, for almost all the minerals examined, to 0·045. Representing the change in the usual way as
| U | → | 8He | + | Pb |
| 238·5 | → | 31·92 | + | 207·1 |
and using the data calculated by Rutherford and others for the rates of decay, he gives the age of Lower Devonian strata as about 370 million years. This figure is about twice as great as that deduced by palæontologists from the flora and fauna, and greater still than the times based on physical data, e.g. rates of cooling, precession and nutation, etc. His figures for pre-Cambrian rocks, based on the same ratio, range between 1000 and 1640 million years, the later being deduced from a thorianite from the Archæan rocks of Ceylon. Strutt’s figure for Archæan rocks is about 700 million years; this was derived from work on the helium ratio, which must now be considered.[142]
[141] Proc. Roy. Soc. 1911, A, 85, 248.
[142] See Strutt, Proc. Roy. Soc. 1908, A, 82, 166; 1909, 83, 96; 1909, 83, 298; 1910, 84, 194.
In 1898 Travers[143] had examined the effect of heat on cleveite and fergusonite, and found that about half the total helium, together with hydrogen, is given off at a bright red heat. He considered it likely that the helium was combined with a metal (though he recognised no distinction between occlusion and combination) and remarked: ‘The results of such experiments cannot therefore serve as a basis for speculation as to the origin or history of the substances in question.’ The chemical inactivity of helium, however, as well as the experiments of Moss and Gray, who showed that helium was evolved on grinding the materials,[144] indicate that the gas is mechanically bound only. This, however, introduces the difficulty, if an attempt be made to use the helium-uranium ratio to calculate the age of minerals, that the gas would be expected to escape from a porous material, so that its amount is never so great as it should be. Strutt himself found that helium escapes rapidly from powdered monazite, whilst even the solid mineral was found to evolve helium at a rate much in excess of the probable rate of production by radioactive changes. Similar results were found with thorianite, and the only conclusion, since helium is found in the minerals, is that under the conditions under which these minerals exist in the earth’s crust, this escape is checked or altogether prevented. It follows, however, that any age determined from the helium ratio must be a minimum age, since there is always the chance of loss; this of course is not the case—except where the minerals have suffered chemical changes—with the lead ratio, and may account for the discrepancies observed.
[143] Proc. Roy. Soc. 1898-99, 64, 140.
[144] Vide Gray, Proc. Roy. Soc. 1908, A, 82, 306.
Strutt’s earlier work on the helium ratio was made with phosphate minerals (coprolites and fossil bones) of known ages. The ratios found were not in order of age, the minerals being very permeable, so that helium had probably been lost. He next turned his attention to igneous rocks, and selected zircon for the work. Here he obtained some sort of regularity in the order of age and the order given by the ratio, and assumed that if helium were lost at all, it must be lost in roughly proportional amounts by reason of the similarity in conditions. Geological criticism tends to lessen the trustworthiness of the conclusions; it is pointed out that the age of a specimen of zircon is not necessarily that of the rock in which it occurs, for zircon is an extremely stable mineral, and might survive unchanged several fusions and re-crystallisations of the magma. Strutt replies to this that at the temperature of fusion of a rock, zircon would certainly give up its accumulated helium, so that the age determined from the helium content would be that of the last fusion, i.e. the age as given by geological data. On the other hand, our ignorance of the real mechanism of the crystallisation of a magma, and especially of the amount and effect of the pressures obtaining, robs this reply of its force, and the objection must be counted valid.
In still later work Strutt used sphene and thorianite, and his results agree as well as can be expected. The sphenes used were all from Archæan rocks, except one, which was from a Tertiary volcanic deposit of the Laacher See, near Coblenz (the lake is in the crater of an extinct volcano). In this case the helium ratio was very much smaller (about 1⁄4000 of the values for Archæan rocks) indicating the (comparatively) extremely recent formation of the deposit.