Th and U also give rise to radioactive series -- different series from that of U, containing different nuclides and ending in different nuclides of lead. Chemists can apply similar techniques to all three, resulting in three different dates for the same rock sample. Uranium and thorium have similar chemical behavior, so all three of these nuclides frequently occur in the same ores.
If all three dates agree within the margin of error, the date can be accepted as confirmed beyond a reasonable doubt. Since all three of these nuclides have substantially different half-lives, for all three to agree indicates the technique being used is sound. But even so, radioactive-series dating could be open to question.
The rock being dated must remain a closed system with respect to uranium, thorium, and their daughter nuclides for the method to work properly. Both the uranium and thorium series include nuclides of radon, an inert gas that can migrate through rock fairly easily even in the few days it lasts.
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To have a radiometric dating method that is unquestionably accurate, we need a radioactive nuclide for which we can get absolutely reliable measurements of the original quantity and the current quantity. Is there any such nuclide to be found in nature? The answer is yes. Which brings us to the third method of radiometric dating. The element potassium has three nuclides, K39, K40, and K Only K40 is radioactive; the other two are stable. K40 is unusual among radioactive nuclides in that it can break down two different ways.
It can emit a beta particle to become Ca40 calcium , or it can absorb an electron to become Ar40 argon Argon is a very special element. Argon is a gas at Earth-normal temperatures, and in any state it exists only as single atoms. By contrast, potassium and calcium are two of the most active elements in nature.
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They both form compounds readily and hold onto other atoms tenaciously. What does this mean? It means that before a mineral crystallizes, argon can escape from it easily. It also means that when an atom of argon forms from an atom of potassium inside the mineral, the argon is trapped in the mineral. So any Ar40 we find deep inside a rock sample must be there as a result of K40 decay. That and some simple calculations produce a figure for how long the K40 has been decaying in our rock sample.
What happens if our mineral sample has not remained a closed system? What if argon has escaped from the mineral? What if argon has found its way into the mineral from some other source? If some of the radiogenic argon has escaped, then more K40 must have decayed than we think -- enough to produce what we did find plus what escaped. In other words, a mineral that has lost argon will be older than the result we get says it is. In the other direction, if excess argon has gotten into the mineral, it will be younger than the result we get says it is.
An isochron dating method isochron dating is described in the next section can also be applied to potassium-argon dating under certain very specific circumstances. When isochron dating can be used, the result is a much more accurate date. Yet a fourth method, rubidium-strontium dating, is even better than potassium-argon dating for old rocks. The nuclide rubidium Rb87 decays to strontium Sr87 with a half-life of 47 billion years.
Strontium occurs naturally as a mixture of several nuclides. If three minerals form at the same time in different regions of a magma chamber, they will have identical ratios of the different strontium nuclides. The total amount of strontium might be different in the different minerals, but the ratios will be the same.
Now, suppose that one mineral has a lot of Rb87, another has very little, and the third has an in-between amount. That means that when the minerals crystallize there is a fixed ratio of Rb As time goes on, atoms of Rb87 decay to Sr, resulting in a change in the Rb Sr87 ratio, and also in a change in the ratio of Sr87 to other nuclides of strontium.
The decrease in the Rb Sr87 ratio is exactly matched by the gain of Sr87 in the strontium-nuclide ratio. It has to be -- the two sides of the equation must balance. If we plot the change in the two ratios for these three minerals, the resulting graph comes out as a straight line with an ascending slope. This line is called an isochron. When every one of four or five different minerals from the same igneous formation matches the isochron perfectly, it can safely be said that the isochron is correct beyond a reasonable doubt.
There are numerous other radiometric dating methods: A full cite for this book is given in the bibliography. Now, why is all this relevant to the creation-vs. Every method of radiometric dating ever used points to an ancient age for the Earth. For creationists to destroy the old-Earth theory, they must destroy the credibility of radiometric dating. They have two ways to do this. They can criticize the science that radiometric dating is based on, or they can claim sloppy technique and experimental error in the laboratory analyses of radioactivity levels and nuclide ratios.
Is there any way to criticize the theory of radiometric dating? Well, look back at the axioms of radiometric dating methods. Are any of those open to question.source site
What are the assumptions used with radiometric dating?
Or at least, they seem to be. Do we know, for a fact, that half-lives are constant axiom 1? Do we know for a fact that nuclide ratios are constant axiom 2? Regarding the first question: However, if all we had were theoretical reasons for believing axiom 1, we would be right to be suspicious of it.
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Do we have observational evidence? On several occasions, astronomers have been able to analyze the radiation produced by supernovas. In a supernova, the vast amount of energy released creates every known nuclide via atomic fusion and fission. Some of these nuclides are radioactive. We can also detect the characteristic radiation signatures of radioactive decay in those nuclides.
We can use that information to calculate the half-lives of those nuclides. In every case where this has been done, the measured radiation intensity and the calculated half-life of the nuclide from the supernova matches extremely well with measurements of that nuclide made here on Earth. And when we look at a supernova in the Andromeda Galaxy, 2,, years old, we see nuclides with the exact same half-lives as we see here on Earth. Not just one or two nuclides, but many.
For these measurements to all be consistently wrong in exactly the same way, most scientists feel, is beyond the realm of possibility.
What about nuclide ratios? Are they indeed constant? Minerals form by recognized chemical processes that depend on the chemical activity of the elements involved. The chemical behavior of an element depends on its size and the number of electrons in its outer shell. This is the foundation of the periodic table of the elements, a basic part of chemistry that has stood without challenge for a hundred and fifty years.
The shell structure depends only on the number of electrons the nuclide has, which is the same as the number of protons in its nucleus. K39 is chemically identical to K40; the only way we can distinguish between them is to use a nonchemical technique like mass spectrometry.
Water molecules containing oxygen are lighter and therefore evaporate faster than water molecules with oxygen However, as far as is known such fractionation occurs only with light nuclides: Sr86 atoms and Sr87 atoms behave identically when they bond with other atoms to form a mineral molecule. If there are ten Sr86 atoms for every Sr87 atom in the original magma melt, there will be ten Sr86 atoms for every Sr87 atom in the minerals that crystallize from that melt. The only other possible source of error is in laboratory technique. To translate theory into useful measurements, the lab procedures must be accurate.
A contaminated rock sample is useless for dating. A sample that is taken from the surface, where atoms could get in and out easily, is also useless. Samples must be taken by coring, from deep within a rock mass.
Assumptions of Radioactive Dating
To date a rock, chemists must break it down into its component elements using any of several methods, then analyze nuclide ratios using a mass spectrometer. But we can try to minimize error. And when we do, the dates produced can be accepted as accurate. When samples taken from different parts of a given igneous rock formation are dated by different people at different labs over many years, the possibility that all those measurements could be wrong is vanishingly small.
Some may well be wrong.
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If nine analyses agree, and a tenth produces radically different results, the odd-man-out is usually considered a result of some kind of error and discarded. And some radiometric techniques have a much better success ratio than that. Creationist objections to radiometric dating techniques basically fall into three categories: The assumptions that are used in radiometric dating techniques are perfectly justified given current physics.
Creationist geologist John Woodmorappe is the best known of the creationists who attempt this approach. Based on this, he claims that radiometric dating methods don't produce consistent results, that geologists conceal radiometric dates which don't match what's expected, and that therefore the whole methodology of radiometric dating is worthless. In an article for the creationist journal Creation Science Research Quarterly , Woodmorappe listed odd aberrant dates, and claimed that there are many, many more.