MEASURING GEOLOGIC TIME BY THE STRONTIUM METHOD

Abstract

The strontium method for measuring geological age has been examined in each of its aspects, and the general conclusion is made that evidence is now sufficient to show this method as capable of providing reliable age measurements. Of the total strontium in the earth9s crust, 0.5–1.0% is considered to be radiogenic, almost all of which is concentrated in potassium and cesium minerals. Lepidolite contains the highest proportion of radiogenic strontium, in which mineral it usually predominates completely. Amazonite, pollucite, hydrothermal pegmatitic microcline, zinnwaldite, some specimens of lithium-rich muscovite, and probably also rhodizite and lithium-rich biotite commonly contain a high proportion of radiogenic strontium; age measurements may be made on most of these minerals, if not all. Age determinations on other minerals are also possible, and the strontium method could probably be extended to granite biotites, which would increase its scope very considerably. With lepidolite, a reasonably reliable age can be obtained in most instances without an isotope analysis, although one is desirable. An isotope analysis is imperative for age determinations on all other minerals. For a quantitative isotope analysis of strontium, the mass spectrograph is superior in sensitivity and accuracy to other methods; a quantitative analysis can be made on as little as 0.3 mg. of strontium salt, and a conveniently low proportion of radiogenic strontium can be determined in the presence of an excess of common strontium. In all, 32 strontium age determinations are known, 30 of which have been made according to a spectrochemical procedure outlined in this paper. The procedure is rapid—10–15 determinations (without isotope analyses) may be made in quadruplicate in 3 days—but lacks precision, since age reproducibility is usually only within about ±10–15%, even when carried out in quadruplicate. A more accurate procedure for determining Sr/Rb ratios is urgently needed to increase the usefulness of the strontium method; investigation of other spectrochemical methods is recommended. Where comparisons were possible, strontium ages tally reasonably well with lead and helium (magnetite) ages, with few exceptions. The strontium method should be of particular value in pre-Cambrian time and is probably superior to the lead and helium methods for dating very ancient rocks. The extreme antiquity of pegmatites from southeast Manitoba has been established beyond reasonable doubt; their age (2100 × 10 6 years) is considered to be greater than any other region on the earth9s crust on which sufficient data are available. The method is, however, apparently not so suitable for dating relatively young specimens, and even under the most favourable circumstances, that is, if lepidolite is available, it seems unlikely that an age of less than 50 × 10 8 years could be measured successfully. Lead, helium, and strontium methods have been compared.