More Bad News for Radiometric Dating
This would seem to imply that the problem of radiometric dating has my web site bearing on the question of the age of the geologic column. Of course, there are many problems with such dating methods, such as Radiometric dating is largely done on rock that has formed from solidified lava. the published dates on the flows in the Nevada Atomic Bomb Test site where one of. More on radioactive dating problems. A further response to Reasonable Faith Adelaide. Published: 20 June (GMT+10). thebluetones.info
Under favourable circumstances the isochron method may be helpful, but tests by other techniques may be required. For example, the rubidium-strontium method would give a valid isotopic age of the biotite sample with inherited argon.
For example, different kinds of quartz have different colors due to various impurities that are included but not part of the repetitive unit of the quartz crystal. So even if the crystal excludes the daughter element, it could be present in impurities. Thus crystals, as they form, may have tiny imperfections that accept parent and daughter products in the same ratios as they occur in the lava, so one can inherit ages from the lava into minerals in this way.
It is also possible that parent and daughter elements could be present in boundaries between regular crystal domains. I don't know how we can be sure that a crystal will exclude argon or other daughter substances except by growing it in the laboratory under many conditions. There can also be argon or other daughter products added from the air or from other rocks. One could say that we can detect whether the daughter is embedded in the crystal structure or not.
But this would require an atom by atom analysis, which I do not believe is practical. Why K-Ar dating is inaccurate Since K-Ar potassium-argon dating is one of the most prevalent techniques, some special commentary about it is in order. Potassium is about 2. Argon is about 3. This is about one ten millionth of the mass of the rock, a very tiny percentage. And yet, with a large amount of argon in the air and also filtering up from rocks below, and with excess argon in lava, with argon and potassium water soluble, and argon mobile in rock, we are still expecting this wisp of argon to tell us how old the rock is!
The percentage of Ar40 is even less for younger rocks.
Scientist Realizes Important Flaw in Radioactive Dating
For example, it would be about one in million for rocks in the vicinity of 57 million years old. To get one part in 10 million of argon in a rock in a thousand years, we would only need to get one part in 10 billion entering the rock each year.
This would be less than one part in a trillion entering the rock each day, on the average. This would suffice to give a rock having an average concentration of potassium, a computed potassium-argon age of over million years!
We can also consider the average abundance of argon in the crust. This implies a radiometric age of over 4 billion years. So a rock can get a very old radiometric age just by having average amounts of potassium and argon.
It seems reasonable to me that the large radiometric ages are simply a consequence of mixing, and not related to ages at all, at least not necessarily the ages of the rocks themselves. The fact that not all of the argon is retained would account for smaller amounts of argon near the surface, as I will explain below. This could happen because of properties of the magma chambers, or because of argon being given off by some rocks and absorbed by others. I don't see how one can possibly know that there are no tiny cracks in rocks that would permit water and gas to circulate.
The rates of exchange that would mess up the dates are very tiny. It seems to me to be a certainty that water and gas will enter rocks through tiny cracks and invalidate almost all radiometric ages. Let me illustrate the circulation patterns of argon in the earth's crust. So argon is being produced throughout the earth's crust, and in the magma, all the time. In fact, it probably rises to the top of the magma, artificially increasing its concentration there.
Now, some rocks in the crust are believed not to hold their argon, so this argon will enter the spaces between the rocks. Leaching also occurs, releasing argon from rocks.
Heating of rocks can also release argon. Argon is released from lava as it cools, and probably filters up into the crust from the magma below, along with helium and other radioactive decay products. All of this argon is being produced and entering the air and water in between the rocks, and gradually filtering up to the atmosphere. But we know that rocks absorb argon, because correction factors are applied for this when using K-Ar dating.
So this argon that is being produced will leave some rocks and enter others. The partial pressure of argon should be largest deepest in the earth, and decrease towards the surface. This would result in larger K-Ar ages lower down, but lower ages nearer the surface.
As for K-Ar dating, here is a quote given above: Now, argon is very soluble in magma, which can hold a lot of it: After the material was quenched, the researchers measured up to 0. They noted, 'The solubility of Ar in the minerals is surprisingly high'. This is from a paper by Austin available at ICR. This paper also discusses Mount St. Helens K-Ar dating, and historic lava flows and their excess argon.
So magma holds tremendous amounts of argon. Now, consider an intrusive flow, which cools within the earth. All its argon will either remain inside and give an old age to the flow, or will travel through surrounding rock, where it can be absorbed by other rocks. So magma should have at least 20 times as much argon as a rock million years old by K-Ar dating. In fact, the argon in the magma may well be even higher, as it may concentrate near the top. This amount of argon is enough to raise 20 times the volume of magma to a K-Ar age of million years, and probably times the volume of the magam to an age of 57 million years.
So one sees that there is a tremendous potential for age increases in this way. It is not necessary for this increase in age to happen all at once; many events of this nature can gradually increase the K-Ar ages of rocks. In general, older rocks should have more argon because they have been subject to more exposure to such argon, but their true age is not necessarily related to their K-Ar radiometric age. We can also consider that most volcanoes and earthquakes occur at boundaries between plates, so if the lava has flowed before, it is likely to flow again nearby, gradually increasing the age.
I suppose earthquakes could also allow the release of argon from the magma. Other mechanisms include dissolving of rock, releasing its argon, fracturing of rock, with release of argon, argon from cooling lava under water entering the water and entering other rocks, and argon from cooling lave entering subterranean water and being transported to other rock. There are so many mechanisms that it is hard to know what pattern to expect, and one does not need to rely on any one of them such as more argon in the magma in the past to account for problems in K-Ar dating.
Since even rocks with old K-Ar dates still absorb more argon from the atmosphere in short time periods, it follows that rocks should absorb quite a bit of argon over long time periods, especially at higher pressures. In fact, if a rock can absorb only a ten millionth part of argon, that should be enough to raise its K-Ar age to over million years, assuming an average amounts of potassium.
It wouldn't require many internal cracks to allow a ten millionth part of argon to enter. Also, as the rock deforms under pressure, more cracks are likely to form and old ones are likely to close up, providing more opportunity for argon and other gases to enter. I mentioned a number of possibilities that could cause K-Ar dates to be much older than the true ages of the rocks.
Here is another way that K-Ar dates can be too old: If we assume the earth went through a catastrophe recently, then the crustal plates might have been agitated, permitting lava and argon to escape from the magma. Thus a lot of argon would be filtering up through the crust. As intrusive flows of lava cooled inside the crust, they would have been in an environment highly enriched in argon, and thus would not have gotten rid of much of their argon.
Thus they would have hardened with a lot of argon inside. This would make them appear old. The same goes for extrusive flows on the surface, since argon would be filtering up through the earth and through the lava as it cooled. The following was sent to me by a friend: In areas where tremendous tectonic activity has taken place, highly discordant values for the ages are obtained. The difficulties associated are numerous and listed as follows: There seems to be a great deal of question regarding the branching ratio for K40 into Ar40 and Ca But the value is not really known.
The observed value is between 0. However, this doesn't remedy the situation and the ages are still too high [low? The geochronologists credit this to "argon leakage". There is far too much Ar40 in the earth for more than a small fraction of it to have been formed by radioactive decay of K This is true even if the earth really is 4.
In the atmosphere of the earth, Ar40 constitutes This is around times the amount that would be generated by radioactive decay over the age of 4. Certainly this is not produced by an influx from outer space. Thus, a large amount of Ar40 was present in the beginning.
Since geochronologists assume that errors due to presence of initial Ar40 are small, their results are highly questionable. Argon diffuses from mineral to mineral with great ease. It leaks out of rocks very readily and can move from down deep in the earth, where the pressure is large, and accumulate in an abnormally large amount in the surface where rock samples for dating are found.
They would all have excess argon due to this movement. This makes them appear older. Rocks from deeper in the crust would show this to a lesser degree. Also, since some rocks hold the Ar40 stronger than others, some rocks will have a large apparent age, others smaller ages, though they may actually be the same age.
If you were to measure Ar40 concentration as function of depth, you would no doubt find more of it near the surface than at deeper points because it migrates more easily from deep in the earth than it does from the earth into the atmosphere.
It is easy to see how the huge ages are being obtained by the KAr40 radiometric clock, since surface and near-surface samples will contain argon due to this diffusion effect. Some geochronologists believe that a possible cause of excess argon is that argon diffuses into mineral progressively with time. Significant quantities of argon may be introduced into a mineral even at pressures as low as one bar. If such [excessive] ages as mentioned above are obtained for pillow lavas, how are those from deep-sea drilling out in the Atlantic where sea-floor spreading is supposed to be occurring?
Potassium is found to be very mobile under leaching conditions. This could move the "ages" to tremendously high values. Ground-water and erosional water movements could produce this effect naturally.
Rocks in areas having a complex geological history have many large discordances. In a single rock there may be mutually contaminating, potassium- bearing minerals. There is some difficulty in determining the decay constants for the KAr40 system.
Geochronologists use the branching ratio as a semi-emperical, adjustable constant which they manipulate instead of using an accurate half-life for K A number of recent lava flows within the past few hundred years yield potassium-argon ages in the hundreds of thousands of years range. This indicates that some excess argon is present. Where is it coming from? And how do we know that it could not be a much larger quantity in other cases? If more excess argon were present, then we could get much older ages.
It is true that an age difference in the hundreds of thousands of years is much too small to account for the observed K-Ar ages. But excess argon is commonly invoked by geologists to explain dates that are too old, so I'm not inventing anything new. Second, there may have been a lot more more argon in the magma in the past, and with each eruption, the amount decreased. So there would have been a lot more excess argon in the past, leading to older ages. For rocks that are being dated, contamination with atmospheric argon is a persistent problem that is mentioned a number of times.
Thus it is clear that argon enters rock easily. It is claimed that we can know if a rock has added argon by its spectrum when heated; different temperatures yield different fractions of argon. It is claimed that the argon that enters from the atmosphere or other rocks, is less tightly bound to the crystal lattice, and will leave the rock at a lower temperature. But how do we know what happens over thousands of years? It could be that this argon which is initially loosely bound if it is so initially gradually becomes more tightly bound by random thermal vibrations, until it becomes undetectable by the spectrum technique.
The fact that rock is often under high pressure might influence this process, as well. The branching ratio problem We now consider in more detail one of the problems with potassium-argon dating, namely, the branching ratio problem. Here is some relevant information that was e-mailed to me. There are some very serious objections to using the potassium-argon decay family as a radiometric clock.
The geochronologist considers the Ca40 of little practical use in radiometric dating since common calcium is such an abundant element and the radiogenic Ca40 has the same atomic mass as common calcium. Here the actual observed branching ratio is not used, but rather a small ratio is arbitrarily chosen in an effort to match dates obtained method with U-Th-Pb dates.
The branching ratio that is often used is 0. Thus we have another source of error for K-Ar dating. The Branching Ratio Dr. Henke criticized some statements in my article taken from Slusher about the branching ratio for potassium.
Slusher asserted that the best known value of the branching ratio was not always used in computing K-Ar radiometric ages. Unfortunately, Dalrymple says nothing about the calculation of the branching ratio. He simply gives the correct value for the K-Ar system. The issue is not just how well this was known in the past, but which value was actually used, and whether dates published in the past have been computed with the most recent value.
Often values for constants are standardized, so that the values actually used may not be the most accurate known. All that Dalrymple says is that his ages were all recomputed using the most accurate values of the constants. This implies that some of them were originally computed using less accurate values, which is similar to Slusher's point. He admits that Slusher's statements about it would have been true in the 's and early 's, but are no longer true.
But he didn't say when the correct value for the branching ratio began to be used. Even some figures from Faure, Principles of Isotope Geology, are based on another constant that is 2 or 3 percent too low, according to Dalrymple, and so there may be many ages in the literature that need revision by small amounts.
However, Harland et al imply that nearly the correct value for the branching ratio has been known and used since the mid-fifties. We now consider whether they can explain the observed dates.
In general, the dates that are obtained by radiometric methods are in the hundreds of millions of years range. One can understand this by the fact that the clock did not get reset if one accepts the fact that the magma "looks" old, for whatever reason.
That is, we can get both parent and daughter elements from the magma inherited into minerals that crystallize out of lava, making these minerals look old. Since the magma has old radiometric dates, depending on how much the clock gets reset, the crust can end up with a variety of younger dates just by partially inheriting the dates of the magma. Thus any method based on simple parent to daughter ratios such as Rb-Sr dating is bound to be unreliable, since there would have to be a lot of the daughter product in the magma already.
And Harold Coffin's book Creation by Design lists a study showing that Rb-Sr dates are often inherited from the magma. Even the initial ratios of parent and daughter elements in the earth do not necessarily indicate an age as old as 4.
Radioactive decay would be faster in the bodies of stars, which is where scientists assume the heavy elements formed. Imagine a uranium nucleus forming by the fusion of smaller nucleii.
At the moment of formation, as two nucleii collide, the uranium nucleus will be somewhat unstable, and thus very likely to decay into its daughter element. The same applies to all nucleii, implying that one could get the appearance of age quickly. Of course, the thermonuclear reactions in the star would also speed up radioactive decay. But isochrons might be able to account for pre-existing daughter elements. Furthermore, some elements in the earth are too abundant to be explained by radioactive decay in 4.
Some are too scarce such as helium. So it's not clear to me how one can be sure of the 4. Why older dates would be found lower in the geologic column especially for K-Ar dating In general, potassium-argon dates appear to be older the deeper one goes in the crust of the earth. We now consider possible explanations for this. There are at least a couple of mechanisms to account for this. In volcano eruptions, a considerable amount of gas is released with the lava.
This gas undoubtedly contains a significant amount of argon Volcanos typically have magma chambers under them, from which the eruptions occur. It seems reasonable that gas would collect at the top of these chambers, causing artificially high K-Ar radiometric ages there.
In addition, with each successive eruption, some gas would escape, reducing the pressure of the gas and reducing the apparent K-Ar radiometric age.
Thus the decreasing K-Ar ages would represent the passage of time, but not necessarily related to their absolute radiometric ages. As a result, lava found in deeper layers, having erupted earlier, would generally appear much older and lava found in higher layers, having erupted later, would appear much younger.
This could account for the observed distribution of potassium-argon dates, even if the great sedimantary layers were laid down very recently. In addition, lava emerging later will tend to be hotter, coming from deeper in the earth and through channels that have already been warmed up. This lava will take longer to cool down, giving more opportunity for enclosed argon to escape and leading to younger radiometric ages.
Another factor is that rocks absorb argon from the air. It is true that this can be accounted for by the fact that argon in the air has Ar36 and Ar40, whereas only Ar40 is produced by K-Ar decay. But for rocks deep in the earth, the mixture of argon in their environment is probably much higher in Ar40, since only Ar40 is produced by radioactive decay.
As these rocks absorb argon, their radiometric ages would increase. This would probably have a larger effect lower down, where the pressure of argon would be higher. Or it could be that such a distribution of argon pressures in the rocks occurred at some time in the past.
This would also make deeper rocks tend to have older radiometric ages. Recent lava flows often yield K-Ar ages of aboutyears. This shows that they contain some excess argon, and not all of it is escaping.
If they contained a hundred times more excess argon, their K-Ar ages would be a hundred times greater, I suppose. And faster cooling could increase the ages by further large factors. I also read of a case where a rock was K-Ar dated at 50 million years, and still susceptible to absorbing argon from the air.
This shows that one might get radiometric ages of at least 50 million years in this way by absorbing Ar40 deep in the earth without much Ar36 or Ar38 present.
If the pressure of Ar40 were greater, one could obtain even greater ages. Yet another mechanism that can lead to decreasing K-Ar ages with time is the following, in a flood model: One can assume that at the beginning of the flood, many volcanoes erupted and the waters became enriched in Ar Then any lava under water would appear older because its enclosed Ar40 would have more trouble escaping.
As time passed, this Ar40 would gradually pass into the atmosphere, reducing this effect and making rocks appear younger. In addition, this would cause a gradient of Ar40 concentrations in the air, with higher concentrations near the ground. This also could make flows on the land appear older than they are, since their Ar40 would also have a harder time escaping. Cross-examination The Mobility of Argon Dr. Henke criticizes my concern that argon can move in and out of minerals: Plaisted wants to give his readers the impression that argon can readily move in and out of minerals and, therefore, the gas is too volatile for radiometric dating.
Specifically, he quotes one of his anonymous friends that claims that argon easily diffuses from minerals p. Of course, these statements are inaccurate generalizations.
Geochronologists are aware that excess argon may accumulate on mineral surfaces and the surface argon would be removed before analysis. However, Henke admits that this can happen in some cases. He states that geologists are aware of this problem, and make allowances for it. But it is more difficult to remove argon that has deposited on cracks in the mineral, which can be difficult to see.
Henke referenced Davis A. Young frequently, but I was not able to find Young referenced in any of the other sources I examined except Dalrymple Henke states that hornblendes retain argon very well, but then later says that they can easily absorb excess argon.
Scientist Realizes Important Flaw in Radioactive Dating – Proslogion
Geologists also recognize that heating causes argon to leave minerals, and that dissolved argon in a mineral that does not escape will become incorporated into it, artificially increasing its K-Ar age. I will comment more on this below, but a few comments now are appropriate. For a temperature of K 27 degrees Cthere is no significant argon loss from biotite. At K degrees Cthere is a slow but significant diffusion rate.
At K degrees Closs of argon is quite rapid. To lose one percent in one year requires a temperature of nearly degrees centigrade. Thus the temperature does not have to be very high for argon to move through rock.
This also justifies Slusher's statements about argon moving in and out of rocks with ease. However, it does not seem likely that sedimentary rocks would be this hot very often, except near lava or magma flows.
But argon does not need to move through all rock in order to influence radiometric dates, it only has to reach ancient lava flows. This it can do by following the path of the ancient lava flow itself, coming up along the path of the magma. As the magma or lava cools, this path will consist entirely of hot magma or lava, and so the argon will have a free path, and will continue to enter the magma as it cools. Thus in many cases, the lava or magma will never completely degas, and extra argon will end up trapped in the cooled rock.
This will result in artificially increased K-Ar ages. Many ancient lava flows are relatively flat, in contrast to modern ones. Also, they appear to have been covered over quickly. The flatness means that the lava is a contiguous mass, and can still be reached from the hot magma by a continuous path of hot rock. The fact that they soon are covered over means that the argon has a hard time escaping vertically from the lava, so argon coming up from the mantle will tend to enter the cooling rock.
Both facts will tend to produce artificially high K-Ar ages in these flows which will not be seen in modern lava flows in the same manner. Modern lava flows often come down the sides of volcanoes, and thus become separated from their source by large distances. Also, they do not get quickly buried by additional sediment. Thus modern lava flows are not subject to the same mechanism of artificial increases in their K-Ar ages as are ancient ones.
Also, it is reasonable to assume that as argon leaves the mantle in successive eruptions, the amount of argon remaining is reduced, so that later lava flows are less susceptible to such artificial increases in age.
The path of magma also becomes longer for later flows, and the magma probably also is a little cooler, inhibiting argon flow. Thus later lava flows give younger K-Ar ages.
Another point to note is that even after it cools, the lava or magma may still have many cracks in it, permitting argon to flow. This argon will tend to deposit on the surface of minerals, but with the passage of time it will tend to diffuse into the interior, even if only a very small distance. This is especially true as the lava is cooling. This will make it more difficult to detect this added argon by the spectrum test described below.
Also, the diffusion of argon in cracks and channels of a mineral is likely much less temperature-dependent than diffusion through unbroken regions of the mineral, since diffusion through cracks and channels simply involves jumps through the air. By a combination of diffusion through cracks and channels, and short passages through unbroken regions of the mineral, argon may be able to reach a considerable distance into the mineral.
At low temperatures, this may become the dominant means by which argon diffuses into a mineral, but the effect of this kind of diffusion at low temperatures may not be evident until many years have passed. Thus it may take experiments lasting 50 or years at low temperatures to detect the effects of this kind of diffusion of argon, which however could be significantly increasing the K-Ar ages of minerals over long time periods.
Dickin Radiogenic Isotope Geology,p. It has been claimed that this can be accomplished by preheating samples under vacuum or by leaching them briefly with hydroflouric acid, or both However Armstrong has questioned whether atmospheric argon, that has been acquired by minerals over a long interval of time, can be removed by this method.
Added atmospheric argon can be detected, because the ratio of argon 40 to argon 36 for atmospheric argon is But argon 40 coming up from the mantle and diffusing into a mineral would not be detectable in this way, because it has a higher ratio of argon 40 to argon This shows that rocks can adsorb a large amount of argon relative to the argon needed to give them old K-Ar ages, and also suggests that old K-Ar ages can be produced by external argon from the mantle.
Over a long period of time, adsorbed argon will tend to diffuse into the rock, and thus it will be possible for even more argon to be deposited on the surface, increasing K-Ar ages even more. Concerning excess argon, Faurep.
Generally, excess 40Ar is observed in minerals that have been exposed to a high partial pressure of argon during regional metamorphism, in pegmatites The argon that may either diffuse into the minerals or may be occluded within them is derived by outgassing of K-bearing minerals in the crust and mantle of the Earth.
The presence of excess 40Ar increases K-Ar dates and may lead to overestimates of the ages of minerals dated by this method.
Do different methods agree with each other on the geologic column? Let us consider the question of how much different dating methods agree on the geologic column, and how many measurements are anomalous, since these points are often mentioned as evidences of the reliability of radiometric dating.
It takes a long time to penetrate the confusion and find out what is the hard evidence in this area. In the first place, I am not primarily concerned with dating meteorites, or precambrian rocks.
What I am more interested in is the fossil-bearing geologic column of Cambrian and later age. Now, several factors need to be considered when evaluating how often methods give expected ages on the geologic column. Some of these are taken from John Woodmoreappe's article on the subject, but only when I have reason to believe the statements are also generally believed.
First, many igneous formations span many periods, and so have little constraint on what period they could belong to. The same applies to intrusions. In addition, some kinds of rocks are not considered as suitable for radiometric dating, so these are typically not considered. Furthermore, it is at least possible that anomalies are under-reported in the literature.
Finally, the overwhelming majority of measurements on the fossil bearing geologic column are all done using one method, the K-Ar method. And let me recall that both potassium and argon are water soluble, and argon is mobile in rock.
Thus the agreement found between many dates does not necessarily reflect an agreement between different methods, but rather the agreement of the K-Ar method with itself. For example, if 80 percent of the measurements were done using K-Ar dating, and the other 20 percent gave random results, we still might be able to say that most of the measurements on a given strata agree with one another reasonably well.
So to me it seems quite conceivable that there is no correlation at all between the results of different methods on the geologic column, and that they have a purely random relationship to each other. Let us consider again the claim that radiometric dates for a given geologic period agree with each other. I would like to know what is the exact or approximate information content of this assertion, and whether it could be or has been tested statistically.
It's not as easy as it might sound. Let's suppose that we have geologic periods G Let's only include rocks whose membership in the geologic period can be discerned independent of radiometric dating methods. Let's also only include rocks which are considered datable by at least one method, since some rocks I believe limestone are considered not to hold argon, for example.
Now, we can take a random rock from Gi. We will have to restrict ourselves to places where Gi is exposed, to avoid having to dig deep within the earth. Let's apply all known dating methods to Gi that are thought to apply to this kind of rock, and obtain ages from each one. Then we can average them to get an average age for this rock. We can also compute how much they differ from one another. Now we have to be careful about lava flows -- which geologic period do they belong to?
What about rocks that are thought not to have their clock reset, or to have undergone later heating episodes? Just to make the test unbiased, we will assign altitude limits to each geologic period at each point on the earth's surface at least in principle and include all rocks within these altitude limits within Gi, subject to the condition that they are datable. The measurements should be done in a double-blind manner to insure lack of unconscious bias.
For each geologic period and each dating method, we will get a distribution of values. 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.
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 Potassium has a half-life of 1. Rubidium—strontium dating method[ edit ] Main article: Rubidium—strontium dating This is based on the beta decay of rubidium to strontiumwith a half-life of 50 billion years.
This scheme is used to date old igneous and metamorphic rocksand has also been used to date lunar samples. Closure temperatures are so high that they are not a concern.
Rubidium-strontium dating is not as precise as the uranium-lead method, with errors of 30 to 50 million years for a 3-billion-year-old sample.