The Problem of Contamination in Obtaining Accurate Dates of Young Geologic Rocks

Abstract

There now exist at least two methods of radiometric dating covering most parts of the geologic time-scale from essentially 0 to 4.5 × 10⁹ years, which fact puts geochronology on a reasonably sound basis. Almost all systems of natural radioactive decay are now being used to obtain more geologic information than just radiometric dates, although radiometric dates in themselves throw light on a great number of geologic problems. The K/Ar system of dating does not have the diversity of geologic applications the Rb/Sr and U/Pb methods have owing to the chemical inertness of argon, but it is this very property of argon coupled with its large size and the ubiquity of its parent, potassium, which has given the method an almost unique position in dating geologic events with high precision in the range of 50,000 to 50,000,000 years, here termed “geologically young”. Older rocks, of course, can be dated by K/Ar but other decay schemes may be applied equally well. New investigations of the U/He method, of intermediate decay members of the two uranium series, and fission track counting give promise of ending the unique position of the K/Ar method in dating young rocks, but each of these schemes so far has had either limitations of application more severe than obtain for the K/Ar method or has lacked the precision obtainable with the K/A scheme.