THE LONG-LIVED CLOCKS
All other practical age-determination schemes are based on a few long-lived isotopes, with half-lives relatively near the age of the earth (4.5 [AEONS]). They are:
| Table III | |||
|---|---|---|---|
| Isotope | Emits | Decays to | Half-life |
| Uranium-238 | 8 [ALPHA PARTICLES][10] | Lead-206 | 4.51 aeons |
| Uranium-238 | Spontaneous fission | 2 Fragments | 10 million aeons[11] |
| Uranium-235 | 7 Alpha particles | Lead-207 | 0.713 aeons |
| Thorium-232 | 6 Alpha particles | Lead-208 | 14.1 aeons |
| Rubidium-87 | Beta particle | Strontium-87 | 4.7 aeons |
| Potassium-40 | Electron capture | Argon-40 | 1.3 aeons |
| ...... | ...... | ...... | ...... |
| Rhenium-187[12] | Beta particle | Osmium-187 | 40 aeons |
It is apparent that Table II on [page 6], showing the long-lived radioactive nuclides, is much longer than the list of the seven shown here that are actually useful in practice. Some of the nuclides that are theoretically available are useless on a practical basis, because they are so rare in nature. Many others cannot be used for reasons that are fundamental to the whole process of nuclear age determination by “whole hourglass” (that is, parent-daughter) methods. Let’s look at these reasons.
These methods are based on closed systems in which the daughter products of the radioactive decay are locked with the parent material from the beginning of the system, and nothing is added or removed thereafter. To state it in terms of our analogy, the hourglass must be in perfect working order—no leaks or cracks permitted.
There is another fundamental requirement: At the beginning, the bottom part of the hourglass must be empty. If some sand were already in the bottom at the start, we would mistakenly be led to conclude that the time elapsed was longer than it actually was. That necessity places a severe limitation on the type of system we can use.
Consider, for example, the decay of potassium-40 into calcium-40. Measuring this process is perfectly suitable from the point of view of half-life, but the daughter product is identical with the most common isotope of ordinary calcium. And calcium is present everywhere in nature! Even the purest mineral of potassium, sylvite (the salt, potassium chloride), contains so much calcium impurity that the [RADIOGENIC] daughter calcium, produced by the decay of potassium in geologic time, is negligible in comparison. We can say that the bottom of this potassium-40 hourglass has been stuffed with so much sand from the very beginning that the few grains that fall through the waist are lost in the overall mass. This demonstrates that schemes involving the decay of a relatively rare nuclide into a relatively common one are not usable. Natural geochemical separations of elements are never perfect, anyway.
Similarly, the decay of any of the [RARE EARTH] elements into other rare earth elements is not particularly helpful, because the rare earths are so similar chemically they tend to travel together when they move in nature.[13] Wherever the parent isotope goes, the daughter tags along.