Radiometric dating - Wikipedia
Feb 11, The biostrategraphic limits issue. Click to Radiometric dating methods estimate the age of rocks using Geologic time is divided up into periods, beginning with the .. The geochronologists credit this to "argon leakage". 2. Most of the chronometric dating methods in use today are radiometric click this icon to hear the preceding term pronounced. That is to One half-life is the amount of time required for ½ of the original atoms in a sample to decay. . This will always be true due to the finite limits of measuring equipment. . illustration credits. Mar 1, The limitations of radiometric dating can be split into two general at the correct temperature and leave it for the right amount of time to achieve.
Creationists often blame contamination Indeed, special creationists have for many years held that where science and their religion conflict, it is a matter of science having to catch up with scripture, not the other way around.
This is frequently because the selected technique is used outside of its appropriate range, for example on very recent lavas. In attempting to date Mt. Helens, creationists attempted discredit the discipline through dishonest practices.
Ultimately these "creation scientists" were forced to admit that even for methods they accepted as sound, the age of the Earth would be vastly greater than the 6, they set out to prove. Is radioactive decay constant? An enormous amount of research shows that in the lab decay rates are constant over time and wherever you are.
Faced with this, creationists say that you can't extrapolate from this to deduce they are correct over billions of years. A few experiments have found small variations in decay rates, at least for some forms of decay and some isotopes. The other half will be the daughter product. After twenty years, 0. InRutherford made the first attempt to use this principle to estimate the age of a rock using the presence of helium in a rock as a proxy for alpha decay of radium alpha-particles are helium nuclei.
Fossils and Their Place in Time and Nature
His analysis was technically problematic because of his choice of a gas, helium as a radioactive product gasses have a way of migrating out of rocksbut it was a start. InBertram Boltwood noted a specific parent-daughter relationship between an isotope of uranium, U, a radioactive isotope, and lead Pb suggesting that one decayed into the other - the uranium-lead system.
Because lead is usually found as a solid, this method was more promising. Like Rutherford's, Boltwood's attempt to apply the principle to the dating of rocks was technically flawed but a further step forward. 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.
Radiometric Dating ( Read ) | Earth Science | CK Foundation
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.Radiometric Dating
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.
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?