30 MEASURING TIME IN GEOLOGY 



deposited from vapours or solutions in the vein in the mine in which 

 it was found. At that date in ideal cases, radioactive elements are 

 incorporated in the minerals of igneous rocks and ore veins, and 

 from that date the decay products of the radioactive elements are, 

 again under ideal circumstances, imprisoned and preserved within 

 the rock or the vein. So, by measuring the amount of parent element 

 left, together with the amount of daughter element produced, and 

 by applying the half-life formula characteristic for this decay process, 

 we arrive at the 'age' of the rock or the ore vein. 



Sediments, on the other hand, might contain any amount of decay 

 products from earlier radioactive cycles, washed into the sediment 

 at its formation. They are only suitable for radioactive dating when 

 they contain minerals w^hich formed during the process of sedimenta- 

 tion. Moreover, those 'authigenic' or newly formed minerals must 

 have radioactive elements incorporated. An example, at present the 

 only one used, is the mineral glauconite. This is a complex silicate, 

 containing potassium. Its normal form is greenish grains which form 

 on the bottom of warm, shallow seas, presumably under some bio- 

 chemical control which is as yet not well-known. Glauconite dating 

 is, however, possible for younger sediments only, less than half a 

 billion years of age. It does not apply to the earlier periods of the 

 earth's history in which the origin of life on earth is situated. 



For these older periods, we consequently may state that all absolute 

 dating is done on igneous rocks and ore veins, to the exclusion of 

 sediments. 



Radioactive decay series in geology 



There are now four radioactive decay processes in use for the 

 dating of older rocks. They have been summarized in Table I. In 

 order of ascending atom number of the parent element, they are the 

 potassium-argon method, the rubidium-strontium method, and the 

 thorium-lead and uranium-lead methods. The latter, although starting 

 from three different radioactive parent elements, thorium and two 

 different uranium isotopes, and although their decay processes are 

 quite different, are always used together, because they have the same 

 stable daughter element, lead. There is a difference, however, in 

 that all three produce a different isotope of lead, as shown in Table I. 

 On this fact the modem refinement of this method of absolute 

 dating is based. 



