The radioactive methods of geologic age determination involve the four basic requirements: (1) a known, systematic rate of disintegration of the radioactive elements; (2) accurate measurement and sampling; (3) absence of disintegration product as a primary constituent; (4) no addition or subtraction of the disintegration product or its source during the history of the material. A critical review is presented of our knowledge of the fulfillment of these requirements by the various radioactive methods. Emphasis is placed on the helium method because of its possible wide applicability, because no adequate review has heretofore appeared, and because considerable confusion exists concerning the present status of this method.
Following a discussion of the necessity for completely discarding all of Urry’s helium age measurements, a summary of the remaining data shows that (1) with only a few exceptions the helium age ratios lie in the proper geologic sequence when the results for acidic rocks and for basic rocks are considered separately, (2) the age ratios for acidic rocks are consistently lower than the age ratios for corresponding basic rocks, and (3) the helium age ratios, even for basic rocks, are substantially lower than corresponding lead ages on radioactive minerals. These general inconsistencies stimulated the present reinvestigation of fundamental requirements underlying the radioactive methods. The retentivity of rocks for helium has been found to be the major source of uncertainty in the present application of this method. A review of the lead method indicates that only comparatively few ages can be considered as possible fixed points in the lead time scale. The uncertainty in the isotopic constitution of ordinary lead is seldom an important source of error, but the need for isotopic analysis in all lead age measurements is emphasized by the relative insensitivity of atomic weight determinations. Further work needs to be done on the effects of leaching and weathering of radioactive minerals. Brief reviews of the isotopic ratio, rubidium-strontium, and possible potassium methods of age determination are included for the sake of completeness.