Difference between revisions of "Radiometric dating"

Latest revision as of 06:00, November 2, 2019

See also Counterexamples to an Old Earth.

Radiometric dating is a method of determining the age of an artifact by assuming that on average decay rates have been constant (see below for the flaws in that assumption) and measuring the amount of radioactive decay that has occurred.^[1] Radiometric dating is mostly used to determine the age of rocks, though a particular form of radiometric dating—called Radiocarbon dating—can date wood, cloth, skeletons, and other organic material.

Because radiometric dating fails to satisfy standards of testability and falsifiability, claims based on radiometric dating may fail to qualify under the Daubert standard for court-admissible scientific evidence. It is more accurate for shorter time periods (e.g., hundreds of years) during which control variables are less likely to change.

Basic mechanism

Radiometric dating proceeds from the fact that certain substances (radioactive isotopes) decay, with near-clockwork accuracy, into other elements, and that the old elements and the new elements can be chemically distinguished and can be quantitatively measured. Even the individual isotopes of an element can be accurately measured, though they can't be chemically separated. The radioactive decay of a given isotope proceeds by a well-known exponential decay function involving a "half-life" for that isotope:

where H is the half-life. So, as a totally implausible and fictional oversimplification, if we had compelling reason to believe that some sample consisted of 100% Uranium-238 at some era that we are interested in, like the Permian-Triassic boundary, and it now consists of 96.2% Uranium and 3.8% Lead-206, we could conclude that the event took place 250 million years ago. (It did, but that is not the reason we know this.)

There are other radioactive decays that are used in radiometric dating, involving Carbon, Nitrogen, Potassium, Argon, Thorium, Rubidium, Strontium, and others. In practice, unlike the oversimplified scenario described above, there are several "radiometric clocks" ticking simultaneously, and it takes a lot of painstaking work for scientists to pick out the various decays. Also, analysis of the isotopes of the decay products can help determine what series the product came from. The Uranium-Lead series produces Lead-206, whereas the Thorium-Lead series produces Lead-208. And primordial Lead, that is, away from any radioactive decay, has a characteristic isotope mix of 204, 206, 207, and 208. Analysis of this will show which series the Lead came from.

Assumptions and pitfalls

Needless to say, radiometric dating is not easy. It requires very careful measurement and analysis technique, as well as careful analysis of the assumptions, about such things as the purity and composition of the samples, to do this effectively. There are also assumptions about the accuracy of the equipment involved, and, of course, assumptions about the fundamental physics involved. The findings of the scientific community, on such things as the age of the Earth and the dates of various epochs (e.g. Permian, Triassic, Cretaceous, Paleogene) are the result of decades of measurement and analysis by hundreds of people.

All scientific investigations involve three general types of assumptions:

1. The scientists doing the investigation know how to perform the investigation—in short, they know what they are doing. If one is investigating temperature patterns to draw conclusions and predictions about weather, this means that the investigators understand the subject matter, and understand the connection between temperature and weather patterns.
2. The equipment works, is properly maintained, and is properly calibrated. That is, the thermometers work properly.
3. The fundamental scientific principles are understood. This means that it is known how the coefficient of thermal expansion works.

In the case of radiometric dating, the first problem is by far the hardest. It takes years of careful observation and reasoning, by many scientists, to reach a consensus about what actually happened and when.

First assumption: initial quantities

One key assumption is that the initial quantity of the parent element can be determined. With uranium-lead dating, for example, the process assumes the original proportion of uranium in the sample. One assumption that can be made is that all the lead in the sample was once uranium, but if there was lead there to start with, this assumption is not valid, and any date based on that assumption will be incorrect (too old). Fortunately, isotopic analysis of the lead can mitigate the uncertainty, as can measurements of many samples in many contexts.

In the case of carbon dating, it is not the initial quantity that is important, but the initial ratio of C¹⁴ to C¹², but the same principle otherwise applies.

Recognizing this problem, scientists try to focus on rocks that do not contain the decay product originally. For example, in uranium-lead dating, they use rocks containing zircon (ZrSiO₄), though it can be used on other materials, such as baddeleyite.^[2] Zircon and baddeleyite incorporate uranium atoms into their crystalline structure as substitutes for zirconium, but strongly reject lead. Zircon has a very high closure temperature, is very chemically inert, and is resistant to mechanical weathering. For these reasons, if a rock strata contains zircon, running a uranium-lead test on a zircon sample will produce a radiometric dating result that is less dependent on the initial quantity problem.

Second assumption: proper calibration

This is fairly easy. How to calibrate radiation detectors is a problem that has been worked on since the days of Pierre and Marie Curie.

Third assumption: understanding of the rate of decay

This one is also easy. The assumption is that the rates of decay of the isotopes involved are reasonably accurately known, and that they are constant over long periods of time. Fortunately, this is the case. Radioactivity has been studied in great detail for over a century. The physical constants (nucleon masses, fine structure constant, etc.) involved in radioactive decay are well characterized, and the processes are well understood. Careful astronomical observations show that the constants have not changed significantly in billions of years—spectral lines from distant galaxies would have shifted perceptibly if these constants had changed. In some cases radioactive decay itself can be observed and measured in distant galaxies when a supernova explodes and ejects unstable nuclei.

There are a few effects that can alter radioactive half-lives, but they are mostly well understood, and in any case would not materially affect the radiometric dating results. That is, the analysis of the isotopic and chemical composition of the sample has far greater uncertainty than any uncertainty in the decay rate itself.

The major reason that decay rates can change is that the electric field, from the atom's electron cloud, can change due to chemical changes. That is, electrons can move closer to or farther away from the nucleus depending on the chemical bonds. This affects the coulomb barrier involved in Alpha decay, and therefore changes the height and width of the barrier through which the alpha particle must tunnel. The effect of this on alpha decay, which is the most common decay mode in radiometric dating, is utterly insignificant.

There is another effect that takes place in the "electron capture" type of Beta decay. This is an example of the Weak force, and is fairly rare. Electron capture requires that there be an electron in the vicinity of the nucleus, so its activity depends strongly on the configuration of the electron cloud, which depends on the chemical state. In fact, it is possible to shut down electron capture completely—simply ionize the substance so that there are no electrons nearby.

There is a fairly well-known example of chemical state affecting electron capture activity. The ⁷Be nucleus (Beryllium-7) is an electron capturer with a half-life of about 53 days, turning into Lithium-7. The variation is about 1.5%. While this half-life is way too short to be useful for radiometric dating, the effect of the chemical state is noticeable. The reason is that, because the atomic number is only four, the 2s valence electrons are very close to the 1s electrons involved in capture.^[3] See Systematics of beta decay for more details.

There is good reason to expect that the rate of decay of a radioactive material is largely constant. Nevertheless, some creationists, perhaps with a goal of calling into question the reliability of radiometric dating for the purpose of advancing a "young Earth" doctrine, raise objections about the science.^{[4][5][6][7][8][9][10]} One common objection relates to Beryllium-7, explained above. The discrepancy is very small. Another involves a very large discrepancy, that can be provoked in a particle accelerator, by radically altering the electron cloud. The conditions for this to happen do not occur in nature. There are other tiny discrepancies, apparently related to the rotation of the Sun. One general claim that creationists make is that "Radioactive decay rates were almost certainly not constant near the creation or beginning of the universe" without giving any reason for this belief. But even if decay rates changed wildly in the earliest days of the universe, if they have stayed essentially constant for 13 billion years, the universe must be at least 13 billion years old.

For a fairly technical explanation of the radioactivity process, see the radioactivity page.

Outside influences

It is important that the sample not have had any outside influences. One example of this can be found in metamorphic rocks.^[11] This does not mean that all rock samples are unreliable, but it is possible to account for a process which throws off the data for metamorphic rocks.

For example, with Uranium-lead dating with the crystallization of magma, this remains a closed system until the uranium decays. As it decays, it disrupts the crystal and allows the lead atom to move. Likewise, heating the rock such as granite forms gneiss or basalt forms schist. This can also disrupt the ratios of lead and uranium in the sample.

Calibration

In order to calibrate radiometric dating methods, the methods need to be checked for accuracy against items with independently-known dates.

Carbon dating, with its much lower maximum theoretical range, is often used for dating items only hundreds and thousands of years old, so can be calibrated in its lower ranges by comparing results with artifacts who's ages are known from historical records.

Scientists have also attempted to extend the calibration range by comparing results to timber which has its age calculated by dendrochronology, but this has also been questioned because carbon dating is used to assist with working out dendrochronological ages.

Otherwise, calibration consists of comparing results with ages determined by other radiometric dating methods.

However, tests of radiometric dating methods have often shown that they do not agree with known ages of rocks that have been seen to form from volcanic eruptions in recent and historic times, and there are also examples of radiometric dating methods not agreeing with each other.

Young earth creationists therefore claim that radiometric dating methods are not reliable and can therefore not be used to disprove Biblical chronology.

Acceptance and reliability

Although radiometric dating methods are widely quoted by scientists, they are inappropriate for aging the entire universe due to likely variations in decay rates. Scientists insist that Earth is 4.6 billion years old^[12] while the Bible (the infallible word of God^[13]) suggests that the world to be around 6-10 thousand years old.^[14] Because atheists commonly hold the positions of naturalism and uniformitarianism, many atheists are particularly vocal in claiming that the earth is 4.6 billion years old.

C14 dating was being discussed at a symposium on the prehistory of the Nile Valley. A famous American colleague, Professor Brew, briefly summarized a common attitude among archaeologists towards it, as follows:
"If a C14 date supports our theories, we put it in the main text. If it does not entirely contradict them, we put it in a footnote. And if it is completely 'out of date', we just drop it."
Few archaeologists who have concerned themselves with absolute chronology are innocent of having sometimes applied this method...^[15]

A geological guidebook published by the Queensland government acknowledges that the dates are not absolute, but must be interpreted:

Also, the relative ages [of the radiometric dating results] must always be consistent with the geological evidence. ... if a contradiction occurs, then the cause of the error needs to be established or the radiometric results are unacceptable^[16]

One example of scientists not accepting radiometric dates is that of Mungo Man, a human fossil from New South Wales. When originally found, it was dated by radiocarbon dating at around 30,000 years old. This was later revised to 40,000 years. Another scientist later used other methods to derive a date of 62,000 years. The original discoverer, unconvinced by this result, used a different method again, and again came up with a date of 40,000 years.

The fallibility of dating methods is also illustrated by the fact that dating laboratories are known to improve the likelihood of getting a "correct" date by asking for the expected date of the item. For example, the Sample Record Sheet for the University of Waikato Radiocarbon Dating Laboratory asks for the estimated age, the basis for the estimate, and the maximum and minimum acceptable ages.^[17]

Some major methods of radiometric dating

There are several major types of radiometric dating in use:^[18][19]

1. Radiocarbon dating, also called carbon dating
2. Potassium-argon dating
3. Uranium-lead dating
4. Uranium-thorium
5. Rubidium-strontium dating

Antiquated notions

As early as of 1673, John Ray, an English naturalist, wrote that

"in the primitive times and soon after the Creation the earth suffered far more concussions and mutations in its superficial part than afterward".^[20]

This was hundreds of years before radioactivity was discovered. While we don't know for sure just what "concussions" and "mutations" would mean in modern terminology, if we assume they meant meteor strikes and radioactive decay, respectively, his remarks are rather striking.

Bibliography

• Lamb, Andrew, Carbon dating into the future 24 March 2007 (Creation Ministries International).
• Faure, G. and Mensing, T., Isotopes: Principles and Applications, Wiley & Sons, 2005.
• Walker, M., Quarternary Dating Methods, Wiley & Sons, 2005.
• Walker, Tas., Radioactive decay rate depends on chemical environment, Journal of Creation 14(1):4–5, April 2000.
• Walker, Tas., The way it really is: little-known facts about radiometric dating, Creation 24(4):20–23, September 2002.

Notes and references

See also

• Radioactivity
• Alpha decay
• Beta decay
• Gamma decay
• Quantitative Analysis of Alpha Decay
• Systematics of beta decay
• Essay:Radiometric dating - a non-Creationist Perspective
• Literalist Bible chronology cites examples of radiometric dating results of artifacts which support a literalist dating of Biblical events such as the Exodus in 1577 B.C.