Radiometric dating - Wikipedia
Radiocarbon dating is a method for determining the age of an object containing organic C atoms in the sample and not just the few that happen to decay during the measurements; it can therefore be .. C had immediately been spread across the entire carbon exchange reservoir, it would have led to an increase in the Radiometric dating is the determination of the date at which materials were material when it was created and which presumably have not diffused out. important to historians and archeologists as a method of determining the age of human artifacts. Compute the number of carbon nuclei in a 25 gram sample ( the gram. This paper describes in relatively simple terms how a number of the .. The next few pages cover a broad overview of radiometric dating techniques, radioactive decay effect called hole diffusion, which is an electrical effect in a crystal.
We will also get a distribution of averaged values for samples in each period. Now, some claim is being made about these distributions. It is undoubtedly being claimed that the mean values ascend as one goes up the geologic column. It is also being claimed that the standard deviations are not too large. It is also being claimed that the different methods have distributions that are similar to one another on a given geologic period.
The only correlation I know about that has been studied is between K-Ar and Rb-Sr dating on precambrian rock. And even for this one, the results were not very good.
This was a reference by Hurley and Rand, cited in Woodmorappe's paper. As far as I know, no study has been done to determine how different methods correlate on the geologic column excluding precambrian rock.
The reason for my request is that a correlation is not implied by the fact that there are only 10 percent anomalies, or whatever. I showed that the fact that the great majority of dates come from one method K-Ar and the fact that many igneous bodies have very wide biostratigraphic limits, where many dates are acceptable, makes the percentage of anomalies irrelevant to the question I am asking.
And since this agreement is the strongest argument for the reliability of radiometric dating, such an assumption of agreement appears to be without support so far. The question of whether different methods correlate on the geologic column is not an easy one to answer for additional reasons.
Since the bulk of K-Ar dates are generally accepted as correct, one may say that certain minerals are reliable if they tend to give similar dates, and unreliable otherwise. We can also say that certain formations tend to give reliable dates and others do not, depending on whether the dates agree with K-Ar dates.
Thus we can get an apparent correlation of different methods without much of a real correlation in nature. It's also possible for other matter to be incorporated into lava as it rises, without being thoroughly melted, and this matter may inherit all of its old correlated radiometric dates.
Coffin mentions that fission tracks can survive transport through lava, for example. It may also be that lava is produced by melting the bottom of continents and successively different layers are melted with time, or there could be a tendency for lighter isotopes to come to the top of magma chambers, making the lava there appear older. But anyway, I think it is important really to know what patterns appear in the data to try to understand if there is a correlation and what could be causing it.
Not knowing if anomalies are always published makes this harder. It is often mentioned that different methods agree on the K-T boundary, dated at about 65 million years ago.
This is when the dinosaurs are assumed to have become extinct. This agreement of different methods is taken as evidence for a correlation between methods on the geologic column. One study found some correlated dates from bentonite that are used to estimate the date of the K-T boundary. I looked up some information on bentonite. It is composed of little glass beads that come from volcanic ash.
This is formed when lava is sticky and bubbles of gas in it explode. So these small particles of lava cool very fast. The rapid cooling might mean that any enclosed argon is retained, but if not, the fact that this cooling occurs near the volcano, with a lot of argon coming out, should guarantee that these beads would have excess argon.
As the gas bubble explodes, its enclosed argon will be rushing outward along with these tiny bubbles as they cool. This will cause them to retain argon and appear too old.
In addition, the rapid cooling and the process of formation means that these beads would have Rb, Sr, U, and Pb concentrations the same as the lava they came from, since there is no chance for crystals to form with such rapid cooling. So to assume that the K-Ar dates, Rb-Sr dates, and U-Pb dates all reflect the age of the lava, one would have to assume that this lava had no Sr, no Pb, and that all the argon escaped when the beads formed. Since the magma generally has old radiometric ages, I don't see how we could have magma without Pb or Sr.
So to me it seems to be certain that these ages must be in error.
- Radiocarbon dating
Furthermore, the question arises whether bentonite always gives correlated ages, and whether these ages always agree with the accepted ages for their geologic period. I believe that bentonite occurs in a number of formations of different geologic periods, so this could be checked. If bentonite does not always give correlate and correct ages, this calls into question its use for dating the K-T boundary. Back to top Note that if there are small pockets in crystals where both parent and daughter product can accumulate from the lava, then one can inherit correlated ages from the lava into minerals.
Thus even the existence of correlations is not conclusive evidence that a date is correct. Back to top If a date does not agree with the expected age of its geologic period, and no plausible explanation can be found, then the date is called anomalous.
But if we really understand what is going on, then we should be able to detect discrepant dates as they are being measured, and not just due to their divergence from other dates. Geologists often say that the percentage of anomalies is low. But there are quite a number of rather outstanding anomalies in radiometric dating that creationists have collected.
These anomalies are reported in the scientific literature. For example, one isochron yielded a date of 10 billion years. A Rb-Sr isochron yielded a date of 34 billion years. K-Ar dates of 7 to 15 billion years have been recorded. It's also not uncommon for two methods to agree and for the date to be discarded anyway. Samples with flat plateaus which should mean no added argon can give wrong dates. Samples giving no evidence of being disturbed can give wrong dates.
Samples that give evidence of being disturbed can give correct dates. The number of dates that disagree with the expected ages is not insignificant. I don't know what the exact percentage is. Many dates give values near the accepted ones. But even these often differ from one another by 10 or 20 percent. And quite a few other dates are often much, much farther off.
Whatever is making some of these dates inaccurate could be making all of them inaccurate. It's interesting to note that in a few cases, old radiometric dates are above young ones. The fact that different methods often give different dates is noted by geologists. Here are some quotes from http: Age estimates on a given geological stratum by different radiometric methods are often quite different sometimes by hundreds of millions of years.
There is not absolutely reliable long-term radiological "clock". The uncertainties inherent in radiometric dating are disturbing to geologists and evolutionists One example is the rocks from the Kaupelehu Flow, Hualalai Volcano in Hawaii which was known to have erupted in These rocks were dated by a variety of different methods. Of 12 dates reported the youngest was million years and the oldest was 2. The dates average 1. Geologists explain the Kaupelehu date by the lava being cooled rapidly in deep ocean water and not being able to get rid of its enclosed argon.
Instead, the uncertainty grows as more and more data is accumulated In addition, Woodmorappe gives over sets of dates "that are in gross conflict with one another and with expected values for their indicated paleontological positions. This does not include dates from minerals that are thought to yield bad dates, or from igneous bodies with wide biostrategraphic ranges, where many dates are acceptable.
He states that the number of dates within range are less than the number of anomalies, except for the Cenozoic and Cretaceous. When one adds in the fact that many anomalies are unreported, which he gives evidence for, the true distribution is anyone's guess. There have been criticisms of John Woodmorappe's study, but no one has given any figures from the literature for the true percentage of anomalies, with a definition of an anomaly, or the degree of correlation between methods.
Steven Schimmrich's review of this study often concerns itself with John W's presentation of geologists explanation for anomalies, and not with the percentage of anomalies; the later is my main concern. Here are a couple of more quotes about anomalies: The carbon age of the buried trees is only years, but some of the overlying volcanic material has a ,year potassium-argon age.
Still another evidence for problems with radiometric dating was given in a recent talk I attended by a man who had been an evolutionist and taken a course in radiometric dating.
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The teacher gave 14 assumptions of radiometric dating and said something like "If creationists got a hold of these, they could cut radiometric dating to pieces. Many sedimentary uranium ores are not. On another point, if we can detect minerals that were not molten with the lava, as has been claimed, then this is one more reason why there should be no anomalies, and radiometric dating should be a completely solved problem.
But that does not appear to be the case, at least especially on the geologic column. I'm not claiming that anomalous results are being hidden, just that the agreement of a mass of results, none of which has much claim to reliability, does not necessarily mean much. Picking out a few cases where radiometric dates appear to be well-behaved reminds me of evolutionary biologists focusing on a few cases where there may be transitional sequences.
It does not answer the overall question. And as I said above, I'm also interested to know how much of the fossil-bearing geologic column can be dated by isochrons, and how the dates so obtained compare to others. Concerning K-Ar anomalies, here is a quote from Woodmorappe's paper cited above, p.
Gerling et al called attention to some chlorites yielding K-Ar dates of 7 to 15 b. It had been noted that some minerals which yield such dates as beryl, cordierite, etc. They also pointed out that for the anomalies to be accounted for by excess argon, unreasonably high partial pressures of Ar during crystallization would have to be required. They concluded by suggesting some unknown nuclear process which no longer operates to have generated the Ar. Here is another quote from Woodmorappe about isochrons, since some people think that mixing scenarios or other age-altering scenarios are unlikely: If this condition does not hold, invalid ages and intercepts are obtained.
Models yield isochron ages that are too high, too low, or in the future, sometimes by orders of magnitude. The fact that the only "valid" K-Ar isochrons are those for which the concentration of non-radiogenic argon Ar36 is constant, seems very unusual.
This suggests that what is occuring is some kind of a mixing phenomenon, and not an isochron reflecting a true age. The following quote is from http: We have analyzed several devitrified glasses of known age, and all have yielded ages that are too young.
Some gave virtually zero ages, although the geologic evidence suggested that devitrification took place shortly after the formation of a deposit. Back to top One of the main arguments in favor of radiometric dating is that so many dates agree with each other, that is, with the date expected for their geologic period.
But it's not evident how much support this gives to radiometric dating. If a rock dates too old, one can say that the clock did not get reset. If it dates too young, one can invoke a later heating event.
Neither date would necessarily be seen as anomalous. If lava intrudes upon geologic period X, then any date for the lava of X or later will not be seen as anomalous. And even if the date is one or two geologic periods earlier, it may well be close enough to be accepted as non-spurious. If one does not know the geologic period of a rock by other means, then of course one is likely to date it to find out, and then of course the date agrees with the geologic period and this will not be seen as anomalous.
So it is difficult to know what would be a reasonable test for whether radiometric dating is reliable or not. The percentage of published dates that are considered as anomalous has little bearing on the question. Back to top The issue about igneous bodies may need additional clarification. If a lava flow lies above geologic period A and below B, then allowable ages are anything at least as large as A and no larger than B.
This is called the biostratigraphic limit of the flow. Now, according to Woodmorappe's citations, many lava flows have no such limits at all, and most of them have large limits. For example, a flow lying on precambrian rock with nothing on top would have no limits on its dates.
And such flows often have a large internal scatter of dates, but these dates are not considered as anomalies because of the unrestricted biostratigraphic limit. Other flows with wide biostratigraphic limits have weak restrictions on allowable dates. This is one reason why just reporting the percentage of anomalies has little meaning. Thus these ages, though they generally have a considerable scatter, are not considered as anomalies. He cites another reference that most igneous bodies have wide biostrategraphic limits.
Thus just by chance, many dates will be considered within the acceptable ranges. Again, the percentage of anomalies means nothing for the reliability of radiometric dating. Now, igneous bodies can be of two types, extrusive and intrusive. Extrusive bodies are lava that is deposited on the surface. These cool quickly and have small crystals and form basalt. Intrusive bodies are deposited in the spaces between other rocks. These cool more slowly and have larger crystals, often forming granite.
Both of these tend on the average to have wide biostrategraphic limits, meaning that a large spread of ages will be regarded as non-anomalous. And if we recall that most radiometric dating is done of igneous bodies, one sees that the percentage of anomalies is meaningless. Thus we really need some evidence that the different methods agree with each other.
To make the case even stronger, "Many discrepant results from intrusives are rationalized away immediately by accepting the dates but reinterpreting the biostrategraphic bracket," according to John Woodmorappe.
This of course means that the result is no longer anomalous, because the geologic period has been modified to fit the date. Finally, the fact that the great majority of dates are from one method means that the general but not universal agreement of K-Ar dating with itself is sufficient to explain the small percentange of anomalies if it is small. Back to top Now, the point about agreement is that whatever figure is given about how often ages agree with the expected age, is consistent with the fact that there is no agreement at all between K-Ar and other methods, since so many measurements are done using K-Ar dating.
And one of the strongest arguments for the validity of radiometric dating is that the methods agree. So when one combines all of the above figures, the statement that there are only 10 percent anomalies or 5 percent or whatever, does not have any meaning any more. This statement is made so often as evidence for the reliability of radiometric dating, that the simple evidence that it has no meaning, is astounding to me. I don't object to having some hard evidence that there are real agreements between different methods on the geologic column, if someone can provide it.
The precambrian rock is less interesting because it could have a radiometric age older than life, but this is less likely for the rest of the geologic column. It's not surprising that K-Ar dates often agree with the assumed dates of their geological periods, since the dates of the geological periods were largely inferred from K-Ar dating.
By the way, Ar-Ar dating and K-Ar dating are essentially the same method, so between the two of them we obtain a large fraction of the dates being used. Some information from an article by Robert H. History of the Radioisotope based Geologic Time Scale Before the discovery of radioactivity in the late nineteenth century, a geological time scale had been developed on the basis of estimates for the rates of geological processes such as erosion and sedimentation, with the assumption that these rates had always been essentially uniform.
On the basis of being unacceptably old, many geologists of the time rejected these early twentieth century determinations of rock age from the ratio of daughter to radioactive parent large. Byincreased confidence in radioisotope dating techniques and the demands of evolution theory for vast amounts of time led to the establishment of an expanded geological time scale.
The construction of this time scale was based on about radioisotope ages that were selected because of their agreement with the presumed fossil and geological sequences found in the rocks.
Igneous rocks are particularly suited to K-Ar dating. The crucial determiners are therefore volcanic extrusive igneous rocks that are interbedded with sediments, and intrusive igneous rocks that penetrate sediments. This verifies what I said about almost all of the dates used to define correct ages for geologic periods being K-Ar dates. Also, the uncertainty in the branching ratio of potassium decay might mean that there is a fudge factor in K-Ar ages of up to a third, and that the occasional agreements between K-Ar ages and other ages are open to question.
So the point is that there is now no reason to believe that radiometric dating is valid on the geologic column. Back to top Another issue is that sometimes the geologic periods of rocks are revised to agree with the ages computed. This also makes data about percentages of anomalies less meaningful. It sometimes seems that reasons can always be found for bad dates, especially on the geologic column.
If a rock gives a too old date, one says there is excess argon. If it gives a too young date, one says that it was heated recently, or cannot hold its argon. How do we know that maybe all the rocks have excess argon? It looks like geologists are taking the "majority view" of K-Ar dating, but there is no necessary reason why the majority of rocks should give the right date. The following quote is from the article by Robert H. What is a Radioisotope Age? The relationship of a radioisotope age with real-time must be based on an interpretation.
A discussion of rubidium-strontium ages in the Isotope Geoscience Section of the journal, Chemical Geology, specifically states that a radioisotope age determination "does not certainly define a valid age information for a geological system.
Any interpretation will reflect the interpreters presuppositions bias. Back to top Concerning the need for a double blind test, it would seem that there are many places where human judgment could influence the distribution of measured radiometric dates. It could increase the percentage of anomalies, if they were regarded as more interesting.
It could decrease them, if they were regarded as flukes. Human judgment could determine whether points were collinear enough to form an isochron. It could determine whether a point can justifiably be tossed out and the remaining points used as an isochron. It could determine whether one should accept simple parent-to-daughter K-Ar ratios or whether some treatment needs to be applied first to get better ages. It could influence whether a spectrum is considered as flat, whether a rock is considered to have undergone leaching or heating, whether a rock is porous or not, or whether a sample has been disturbed in some way.
Since one of the main reasons for accepting radiometric dates at least I keep hearing it is that they agree with each other, I think that geologists have an obligation to show that they do agree, specifically on the geologic column. Since we do not know whether or how much human judgment is influencing radiometric dating, a double blind study is most reasonable. And it should not be restricted to just one or two well-behaved places, but should be as comprehensive as possible.
Back to top The following information was sent to me by e-mail: Radiometric dating is predicated on the assumption that throughout the earth's history radioactive decay rates of the various elements have remained constant. Is this a warranted assumption? Has every radioactive nuclide proceeded on a rigid course of decay at a constant rate? This has been challenged by studies involving Carbon C This transformation may be accomplished in a number of different ways, including alpha decay emission of alpha particles and beta decay electron emission, positron emission, or electron capture.
Another possibility is spontaneous fission into two or more nuclides. While the moment in time at which a particular nucleus decays is unpredictable, a collection of atoms of a radioactive nuclide decays exponentially at a rate described by a parameter known as the half-lifeusually given in units of years when discussing dating techniques. After one half-life has elapsed, one half of the atoms of the nuclide in question will have decayed into a "daughter" nuclide or decay product.
In many cases, the daughter nuclide itself is radioactive, resulting in a decay chaineventually ending with the formation of a stable nonradioactive daughter nuclide; each step in such a chain is characterized by a distinct half-life.
In these cases, usually the half-life of interest in radiometric dating is the longest one in the chain, which is the rate-limiting factor in the ultimate transformation of the radioactive nuclide into its stable daughter. Isotopic systems that have been exploited for radiometric dating have half-lives ranging from only about 10 years e.
It is not affected by external factors such as temperaturepressurechemical environment, or presence of a magnetic or electric field. For all other nuclides, the proportion of the original nuclide to its decay products changes in a predictable way as the original nuclide decays over time. This predictability allows the relative abundances of related nuclides to be used as a clock to measure the time from the incorporation of the original nuclides into a material to the present.
Accuracy of radiometric dating[ edit ] Thermal ionization mass spectrometer used in radiometric dating. The basic equation of radiometric dating requires that neither the parent nuclide nor the daughter product can enter or leave the material after its formation. The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created.
It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron. This can reduce the problem of contamination.
In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3.
The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate. This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established. On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades.
Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero. The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.
As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes. This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature.
The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis.
The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No.
The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.
This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth.
In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization.
On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams. Uranium—lead dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe.
This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event.
This can be seen in the concordia diagram, where the samples plot along an errorchron straight line which intersects the concordia curve at the age of the sample. Samarium—neodymium dating method[ edit ] Main article: Samarium—neodymium dating This involves the alpha decay of Sm to Nd with a half-life of 1.
Accuracy levels of within twenty million years in ages of two-and-a-half billion years are achievable. Potassium—argon dating This involves electron capture or positron decay of potassium to argon