AGE DETERMINATION Estimates of the age of limestones can be made 



OF LIMESTONES from the intensity of thermoluminescence and 



the alpha-ray activity (Zeller, 1954; Zeller, 

 and others, 1957). The glow curves of natural thermoluminescence, without lab- 

 oratory exposure to gamma rays, are determined, and the areas under high tem- 

 perature peaks from 250 to 375C are measured with a planimeter. The low-tem- 

 perature peaks are not used because the long standing at earth tempera- 

 ture anneals out a good deal of the thermoluminescence. The number of 

 alpha particles per hour per square centimeter emitted from a sample of the 

 powdered material is measured with a special scintilometer (Ockermann and 

 Daniels, 1954) . The intensity of thermoluminescence light depends on the num- 

 ber of trapped electrons that are released, and the number of trapped electrons 

 depends on the number of alpha particles and the length of time that they have 

 been bombarding the crystal. If these three were the only variables, it would be 

 fairly simple to determine the age from the alpha counts and the intensity of 

 light. However, the thermoluminescence sensitivity of a rock to radioactivity 

 varies greatly with the crystal structure and the impurities present. Accordingly, 

 after the thermoluminescence glow curve has been determined, the sample is 

 heated further to drain out all the accumulated, trapped electrons produced by 

 natural radioactivity, and then it is exposed to gamma radiations from cobalt 00 

 ranging from 72,000 to 288,000 roentgens. The original intensity of the natural 

 thermoluminescence at the high-temperature peaks is matched by a measured 

 amount of gamma radiation, R a , as read from the calibration curve. When these 

 values of R a are divided by the alpha counts, a number is obtained which is di- 

 rectly proportional to the geological age for the older limestones. The propor- 

 tionality constant obtained from calibration curves permits an estimate of age 

 since the last crystallization. 



A complication was found in the corals and stalactites (Zeller, and others, 

 1957) and in the limestones of Recent and Tertiary ages. Their thermolumines- 

 cence was much too bright for the age and the alpha particle activity. Then it 

 was found that fresh laboratory-precipitated crystals of calcium carbonate ex- 

 hibit thermoluminescence even without exposure to X-rays or gamma rays (Zel- 

 ler, and others, 1955), and this effect is increased greatly by pressure. Appar- 

 ently pressure and crystallization can occasionally set up electrical charges which 

 are sufficient to release electrons in a crystal lattice, just as radioactivity does, 

 and some of these electrons become trapped for later emission of light when the 

 crystal is heated. Zeller found that high pressures of several tons per square 

 inch could be used to distinguish between the two kinds of thermoluminescence. 

 High pressures increase the intensity of thermoluminescence of fresh crystals 

 and geologically young limestones, whereas they decrease the thermolumines- 

 cence of geologically old limestones. They increase the crystallization-induced 

 thermoluminescence, but decrease the radiation-induced thermoluminescence. In 



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