Chaffin, eds, problems with radiometric dating. This fact has profound implications for radiometric dating. He comes closest to recognizing the fact that the Sr concentration is a third or confounding variable in the isochron simple linear regression. Uranium-Lead U-Pb Radioisotope Dating Method Problems First Problem: Common Lead by Troy Lacey on January 23, Featured in Answers in Depth. However, there are some problems with it. Assumption three, that no daughter element problems with radiometric dating at the beginning, simply cannot be granted. In fact, U and Th both have isotopes of radium in their decay chains with half lives of a week or two, and 6.
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As someone who has studied problems with radiometric dating in detail, I have always been a bit amused by the assertion that radioactive dating is a precise way to determine the age of an object. This false notion is often promoted when radioactive dates are listed with utterly unrealistic error bars. In this reportfor example, we are told that using one radioactive dating technique, a lunar rock sample is 4, problems with radiometric dating, million years old, plus or minus 23 million years old, problems with radiometric dating.
Of course, that error estimate is complete nonsense. It refers to one specific source of error — the uncertainty in the measurement of the amounts of various atoms used in the analysis. Most likely, that is the least important source of error. If those rocks really have been sitting around on the moon for billions of years, I suspect that the the wide range of physical and chemical processes which occurred over that time period had a much more profound effect on the uncertainty of the age determination.
This is best illustrated by the radioactive age of a sample of diamonds from Zaire. Their age was measured to be 6. Do you see the problem? Those who are committed to an ancient age for the earth currently believe that it is 4.
Obviously, then, the minimum error in that measurement is 1. Such uncertainties are usually glossed over, especially when radioactive dates are communicated to the public and, more importantly, to students. Generally, we are told that scientists have ways to analyze the object they are dating so as to eliminate the problems with radiometric dating due to unknown processes that occurred in the past.
One way this is done in many radioactive dating techniques is to use an isochron. However, a recent paper by Dr. Robert B. Hayes has pointed out a problem with isochrons that has, until now, not been considered. The elements rubidium and strontium are found in many rocks. One form of rubidium Rb is radioactive. As illustrated above, a neutron in a Rb atom can eject an electron often called a beta particleproblems with radiometric dating, which has a negative charge.
Since a neutron has no charge, it must become positively charged after emitting an electron. In fact, it becomes a proton. This changes the chemical identity of the atom.
It is no longer Rb; it is strontium Sr Sr is not radioactive, so the change is permanent. We know how long it takes Rb to turn into Sr, so in principle, if we analyze the amount of Rb and Sr in a rock, we should be able to tell how long the decay has been occurring. Of course, there are all sorts of uncertainties involved. How much Sr was in the rock when it first formed? Was Rb or Sr added to the rock by some unknown process?
Was one of them removed from the rock by some unknown process? The isochron is supposed to take care of such issues, problems with radiometric dating. Essentially, rather than looking at the amounts of Rb and Sr, we look at their ratios compared to Sr The ratio of Sr to Sr is graphed versus the ratio of Rb to Sr for several different parts of the rock. How does that help? Thus, it provides an independent analysis of the rock that does not depend on the radioactive decay that is being studied.
The amount problems with radiometric dating Sr that was already in the rock when it formed, for example, should be proportional to the amount of Sr that is currently there. Since the data are divided by the amount of Sr, the initial amount of Sr is cancelled out in the analysis. He says that there is one process that has been overlooked in all these isochron analyses: diffusion. Atoms problems with radiometric dating molecules naturally move around, and they do so in such as way as to even out their concentrations.
A helium balloon, for example, will deflate over time, because the helium atoms diffuse through the balloon and into the surrounding air. Well, diffusion depends on the mass of the thing that is diffusing. Sr diffuses more quickly than Sr, and that has never been taken into account when isochrons are analyzed. No problem. Now that Dr. Hayes has brought it up, we can take it into account, right?
If the effects of diffusion can be taken into account, it will require an elaborate model problems with radiometric dating will most certainly require elaborate assumptions. Hayes suggests a couple of other approaches that might work, but its not clear how well. So what does this mean? If you believe the earth is very old, then most likely, all of the radioactive dates based on isochrons are probably overestimates. How bad are the overestimates? Most likely, the effect will be dependent on the age.
I would think that the problems with radiometric dating the sample, problems with radiometric dating, the larger the overestimate. As a young-earth creationist, I look at this issue in a different way. Certainly not enough to justify the incredibly unscientific extrapolation necessary in an old-earth framework. This newly-pointed-out flaw in the isochron method is a stark reminder of that. A good isochron was supposed to be rock-solid evidence pun intended problems with radiometric dating the radioactive date is reliable.
We now know that it is not. Wile, I was waiting for you to comment on this, problems with radiometric dating, because I wanted to ask if you think this problem can be extrapolated to other isotopes such as lead and argon. If so, it seems to be a pretty big deal. As I said, carbon dating is an exception, but most other modern radiometric dates are produced using an isochron.
Are the samples we see in the RATE study, for example, just anomalies, existing on the ends of the bell curve, or are these indicative of an endemic misunderstanding of the process? Are there any theories that could account for the accelerated decay rate or how the daughters could have gotten in to the samples?
Thus, any significant amount of daughter product will produce a very old date. In my view, if two different dating schemes give significantly different answers, then either one of them is wrong or both of them are wrong.
Scientists exclude what we think are anomalous data all the time. Unfortunately, that discarded data might be what gives us real insight. Young-earth creationists have a hard time explaining the general results of long-lived isotopes and their daughter products being present. On the other side, problems with radiometric dating, old-earthers have a hard time explaining all the discordance.
If radioactive dating is so reliable, problems with radiometric dating, why do different methods yield different results? Why are some of those differences really, really large? As is often the case, there are problems on both sides.
The side you end up coming down on often depends on which problems you are most comfortable trying to deal with. Physicists already theorize that dark matter would affect nuclear decay rates; what if the leftover energy went to the dark matter?
The heat problem occurs everywhere there are radioactive isotopes, so throughout the crust and mantle of the earth, problems with radiometric dating, for example.
The dark matter would have to be there in order to take the heat. You can think of dark matter here as a lot like the luminiferous ether: physicists actually picture it as part of giant galactic flows — so that the right scale for the size of a flow would be on the order of light-years. Since its interaction with normal matter is incredibly weak, it can very easily pass through the earth, problems with radiometric dating.
Or something. Not to mention that different models of dark matter would lead to different interactions. Are we able to calculate the mass of the earth from our knowledge of its contents, and not just the gravitational force we detect? I think if there were much dark matter in the earth, it would be noticeable. We also know the overall composition of the crust and mantle from samples. Thus, the only real unknown is the composition of the core.
Using the mass and all those other problems with radiometric dating, we deduce that the core is mostly iron with some nickel.
I fear it is more a matter of philosophy rather than hard science: to posit gradual change in fossil record is only itself possible if the phyla being examined is similar in appearance, but apparently better adapted to its environment than earlier assumed examples.
The problem with that, is that, in the first case, there appear to be no transitional fossils when problems with radiometric dating should be millionsand to make the assumption previously herein stated, evolutionary conclusions are more akin to a combination of wishful thinking combined with a sympathetic magic mindset, than to observable examples.
Evolution is taught as established fact, and scientific enquiry is severely trammelled by those who prefer a status quo. Every fossil between organisms alive now and abiogenesis is a transitional fossil, Tony. There are also transitional fossils and organisms in the misguided definition of the word you are using. I admire your faith, Cromwell, problems with radiometric dating.
Yet you state it as fact. Then, you claim that all fossils are a transition between that unrealistic event and the life we see now. Thanks for writing an informative article. Error bars have their place, but you are correct in pointing out that they are often misunderstood not only by the general public, but by scientists who are not savvy in radiometric dating. I am not convinced that differential diffusion of isotopes will be all that significant.
After all, problems with radiometric dating, fractionation of light elements, such as oxygen, provides us with all sorts of insights into geologic processes because the mass difference between O and O is rather significant, whereas the mass difference between Sr and Sr is not all that great, in terms of ratios.
The differences are even less significant for more massive isotopes such as in samarium-neodymium dating Nd and Nd If fractionation problems with radiometric dating turn out to be important for isochrons, one would expect that there would be a trend, with lighter nuclides e.
jeff tarbox dating
In addition, the radioactive decay rates have not been constant. From the protective garment of skin to the engineering of our bones and new discoveries about our brain, this issue is packed with testimony to the Master Designer. You're almost done! Please follow the instructions we emailed you in order to finish subscribing. Answers in Genesis is an apologetics ministry , dedicated to helping Christians defend their faith and proclaim the good news of Jesus Christ.
redeem Biggest Matching-Gift Offer Yet! Donate Now. View Cart. Radiometric Dating: Problems with the Assumptions by Dr. Andrew A. Snelling on October 1, ; last featured August 4, Featured in Answers Magazine. Audio Version. Share: Email Using: Gmail Yahoo! Outlook Other. Radiometric Dating PART 1: Back to Basics PART 2: Problems with the Assumptions PART 3: Making Sense of the Patterns This three-part series will help you properly understand radiometric dating, the assumptions that lead to inaccurate dates, and the clues about what really happened in the past.
Prev ious Article Bones Next Article Creation on Display. Answers Magazine October — December Browse Issue Subscribe. Footnotes A. Vardiman, A.
Snelling, and E. Chaffin, eds. El Cajon, California: Institute for Creation Research; St. Joseph, Missouri: Creation Research Society, , pp. Walsh Pittsburgh: Creation Science Fellowship, , pp. Chaffin El Cajon, California: Institute for Creation Research; Chino Valley, Arizona: Creation Research Society, , pp.
Not Billions Green Forest, Arkansas: Master Books, , pp. Austin, ed. Faure and T. Mensing, Isotopes: Principles and Applications , 3rd ed.
Dickin, Radiogenic Isotope Geology , 2nd ed. UK: Cambridge University Press, Ivey, Jr. Pittsburgh: Creation Science Fellowship, , pp. DeYoung, Thousands. Not Billions Green Forest, Arkansas: Master Books, Science What Is Science? Astronomy Biology Chemistry Environmental Science Fossils Genetics Geology Human Body Mathematics Physics. Newsletter Get the latest answers emailed to you. I agree to the current Privacy Policy. Thank You! Thank you for signing up to receive email newsletters from Answers in Genesis.
You can also sign up for our free print newsletter US only. Finish your subscription You're almost done! Your newsletter signup did not work out. As the ocean floor sinks, it encounters increasing pressures and temperatures within the crust. Ultimately, the pressures and temperatures are so high that the rocks in the subducted oceanic crust melt. Once the rocks melt, a plume of molten material begins to rise in the crust. As the plume rises it melts and incorporates other crustal rocks.
This rising body of magma is an open system with respect to the surrounding crustal rocks. Volatiles e. It is possible that these physical processes have an impact on the determined radiometric age of the rock as it cools and crystallizes.
Time is not a direct measurement. The actual data are the ratios of parent and daughter isotopes present in the sample. Time is one of the values that can be determined from the slope of the line representing the distribution of the isotopes. Isotope distributions are determined by the chemical and physical factors governing a given magma chamber. Some granites in New Hampshire, Arizona, Washington State, Colorado, and Wyoming range from ppm U. Rhyolites in Yellowstone N. average about 7 ppm U.
Most genetic models for uranium deposits in sandstones in the U. require a granitic or silicic volcanic source rock to provide the uranium. Most of the uranium deposits in Wyoming are formed from uraniferous groundwaters derived from Precambrian granitic terranes.
Uranium in the major uranium deposits in the San Juan basin of New Mexico is believed to have been derived from silicic volcanic ash from Jurassic island arcs at the edge of the continent. From the above sources, we see that another factor influencing radiometric dates is the proportion of the magma that comes from subducted oceanic plates and the proportion that comes from crustal rock.
Initially, we would expect most of it to come from subducted oceanic plates, which are uranium and thorium poor and maybe lead rich. Later, more of the crustal rock would be incorporated by melting into the magma, and thus the magma would be richer in uranium and thorium and poorer in lead. So this factor would also make the age appear to become younger with time. There are two kinds of magma, and the crustal material which is enriched in uranium also tends to be lighter.
For our topic on radiometric dating and fractional crystallization, there is nothing that would prevent uranium and thorium ores from crystallizing within the upper, lighter portion of the magma chamber and descending to the lower boundaries of the sialic portion.
The same kind of fractional crystallization would be true of non-granitic melts. I think we can build a strong case for fictitious ages in magmatic rocks as a result of fractional cystallization and geochemical processes. As we have seen, we cannot ignore geochemical effects while we consider geophysical effects.
Sialic granitic and mafic basaltic magma are separated from each other, with uranium and thorium chemically predestined to reside mainly in sialic magma and less in mafic rock. Here is yet another mechanism that can cause trouble for radiometric dating: As lava rises through the crust, it will heat up surrounding rock.
Lead has a low melting point, so it will melt early and enter the magma. This will cause an apparent large age. Uranium has a much higher melting point. It will enter later, probably due to melting of materials in which it is embedded. This will tend to lower the ages. Mechanisms that can create isochrons giving meaningless ages: Geologists attempt to estimate the initial concentration of daughter product by a clever device called an isochron.
Let me make some general comments about isochrons. The idea of isochrons is that one has a parent element, P, a daughter element, D, and another isotope, N, of the daughter that is not generated by decay. One would assume that initially, the concentration of N and D in different locations are proportional, since their chemical properties are very similar.
Note that this assumption implies a thorough mixing and melting of the magma, which would also mix in the parent substances as well.
Then we require some process to preferentially concentrate the parent substances in certain places. Radioactive decay would generate a concentration of D proportional to P. By taking enough measurements of the concentrations of P, D, and N, we can solve for c1 and c2, and from c1 we can determine the radiometric age of the sample.
Otherwise, the system is degenerate. Thus we need to have an uneven distribution of D relative to N at the start. If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them. The bigger c1 is, the older the rock is.
That is, the more daughter product relative to parent product, the greater the age. Thus we have the same general situation as with simiple parent-to-daughter computations, more daughter product implies an older age. This is a very clever idea.
However, there are some problems with it. First, in order to have a meaningful isochron, it is necessary to have an unusual chain of events. Initially, one has to have a uniform ratio of lead isotopes in the magma. Usually the concentration of uranium and thorium varies in different places in rock. This will, over the assumed millions of years, produce uneven concentrations of lead isotopes.
To even this out, one has to have a thorough mixing of the magma. Even this is problematical, unless the magma is very hot, and no external material enters. Now, after the magma is thoroughly mixed, the uranium and thorium will also be thoroughly mixed.
What has to happen next to get an isochron is that the uranium or thorium has to concentrate relative to the lead isotopes, more in some places than others. So this implies some kind of chemical fractionation. Then the system has to remain closed for a long time. This chemical fractionation will most likely arise by some minerals incorporating more or less uranium or thorium relative to lead. Anyway, to me it seems unlikely that this chain of events would occur.
Another problem with isochrons is that they can occur by mixing and other processes that result in isochrons yielding meaningless ages. Sometimes, according to Faure, what seems to be an isochron is actually a mixing line, a leftover from differentiation in the magma.
Fractionation followed by mixing can create isochrons giving too old ages, without any fractionation of daughter isotopes taking place. To get an isochron with a false age, all you need is 1 too much daughter element, due to some kind of fractionation and 2 mixing of this with something else that fractionated differently. Since fractionation and mixing are so common, we should expect to find isochrons often.
How they correlate with the expected ages of their geologic period is an interesting question. There are at least some outstanding anomalies. Faure states that chemical fractionation produces "fictitious isochrons whose slopes have no time significance.
As an example, he uses Pliocene to Recent lava flows and from lava flows in historical times to illustrate the problem. He says, these flows should have slopes approaching zero less than 1 million years , but they instead appear to be much older million years. Steve Austin has found lava rocks on the Uinkeret Plateau at Grand Canyon with fictitious isochrons dating at 1. Suppose sample B has no P or D but the same concentration of N as A.
Then a mixing of A and B will have the same fixed concentration of N everywhere, but the amount of D will be proportional to the amount of P. This produces an isochron yielding the same age as sample A. This is a reasonable scenario, since N is a non-radiogenic isotope not produced by decay such as lead , and it can be assumed to have similar concentrations in many magmas.
Magma from the ocean floor has little U and little U and probably little lead byproducts lead and lead Magma from melted continental material probably has more of both U and U and lead and lead Thus we can get an isochron by mixing, that has the age of the younger-looking continental crust.
The age will not even depend on how much crust is incorporated, as long as it is non-zero. However, if the crust is enriched in lead or impoverished in uranium before the mixing, then the age of the isochron will be increased. If the reverse happens before mixing, the age of the isochron will be decreased. Any process that enriches or impoverishes part of the magma in lead or uranium before such a mixing will have a similar effect. So all of the scenarios given before can also yield spurious isochrons.
I hope that this discussion will dispel the idea that there is something magical about isochrons that prevents spurious dates from being obtained by enrichment or depletion of parent or daughter elements as one would expect by common sense reasoning.
So all the mechanisms mentioned earlier are capable of producing isochrons with ages that are too old, or that decrease rapidly with time. The conclusion is the same, radiometric dating is in trouble. I now describe this mixing in more detail. Suppose P p is the concentration of parent at a point p in a rock. The point p specifies x,y, and z co-ordinates. Let D p be the concentration of daughter at the point p.
Let N p be the concentration of some non-radiogenic not generated by radioactive decay isotope of D at point p. For U Pb dating, P would be U and D would be Pb and N would be Pb Suppose this rock is obtained by mixing of two other rocks, A and B.
Suppose that A has a for the sake of argument, uniform concentration of P1 of parent, D1 of daughter, and N1 of non-radiogenic isotope of the daughter. Thus P1, D1, and N1 are numbers between 0 and 1 whose sum adds to less than 1.
Suppose B has concentrations P2, D2, and N2. Let r p be the fraction of A at any given point p in the mixture. So the usual methods for augmenting and depleting parent and daughter substances still work to influence the age of this isochron. More daughter product means an older age, and less daughter product relative to parent means a younger age.
In fact, more is true. Any isochron whatever with a positive age and a constant concentration of N can be constructed by such a mixing. It is only necessary to choose r p and P1, N1, and N2 so as to make P p and D p agree with the observed values, and there is enough freedom to do this. Anyway, to sum up, there are many processes that can produce a rock or magma A having a spurious parent-to-daughter ratio.
Then from mixing, one can produce an isochron having a spurious age. This shows that computed radiometric ages, even isochrons, do not have any necessary relation to true geologic ages. Mixing can produce isochrons giving false ages. But anyway, let's suppose we only consider isochrons for which mixing cannot be detected. How do their ages agree with the assumed ages of their geologic periods?
As far as I know, it's anyone's guess, but I'd appreciate more information on this. I believe that the same considerations apply to concordia and discordia, but am not as familiar with them. It's interesting that isochrons depend on chemical fractionation for their validity. They assume that initially the magma was well mixed to assure an even concentration of lead isotopes, but that uranium or thorium were unevenly distributed initially.
So this assumes at the start that chemical fractionation is operating. But these same chemical fractionation processes call radiometric dating into question. The relative concentrations of lead isotopes are measured in the vicinity of a rock. The amount of radiogenic lead is measured by seeing how the lead in the rock differs in isotope composition from the lead around the rock.
This is actually a good argument. But, is this test always done? How often is it done? And what does one mean by the vicinity of the rock? How big is a vicinity? One could say that some of the radiogenic lead has diffused into neighboring rocks, too. Some of the neighboring rocks may have uranium and thorium as well although this can be factored in in an isochron-type manner.
Furthermore, I believe that mixing can also invalidate this test, since it is essentially an isochron. Finally, if one only considers U-Pb and Th-Pb dates for which this test is done, and for which mixing cannot be detected. how do they correlate with other dates and with conventional ages? The above two-source mixing scenario is limited, because it can only produce isochrons having a fixed concentration of N p. To produce isochrons having a variable N p , a mixing of three sources would suffice.
This could produce an arbitrary isochron, so this mixing could not be detected. Also, it seems unrealistic to say that a geologist would discard any isochron with a constant value of N p , as it seems to be a very natural condition at least for whole rock isochrons , and not necessarily to indicate mixing. I now show that the mixing of three sources can produce an isochron that could not be detected by the mixing test.
First let me note that there is a lot more going on than just mixing. There can also be fractionation that might treat the parent and daughter products identically, and thus preserve the isochron, while changing the concentrations so as to cause the mixing test to fail. It is not even necessary for the fractionation to treat parent and daughter equally, as long as it has the same preference for one over the other in all minerals examined; this will also preserve the isochron. Now, suppose we have an arbitrary isochron with concentrations of parent, daughter, and non-radiogenic isotope of the daughter as P p , D p , and N p at point p.
Suppose that the rock is then diluted with another source which does not contain any of D, P, or N. Then these concentrations would be reduced by a factor of say r' p at point p, and so the new concentrations would be P p r' p , D p r' p , and N p r' p at point p. Now, earlier I stated that an arbitrary isochron with a fixed concentration of N p could be obtained by mixing of two sources, both having a fixed concentration of N p. With mixing from a third source as indicated above, we obtain an isochron with a variable concentration of N p , and in fact an arbitrary isochron can be obtained in this manner.
So we see that it is actually not much harder to get an isochron yielding a given age than it is to get a single rock yielding a given age. This can happen by mixing scenarios as indicated above.
Thus all of our scenarios for producing spurious parent-to-daughter ratios can be extended to yield spurious isochrons. The condition that one of the sources have no P, D, or N is fairly natural, I think, because of the various fractionations that can produce very different kinds of magma, and because of crustal materials of various kinds melting and entering the magma.
In fact, considering all of the processes going on in magma, it would seem that such mixing processes and pseudo-isochrons would be guaranteed to occur. Even if one of the sources has only tiny amounts of P, D, and N, it would still produce a reasonably good isochron as indicated above, and this isochron could not be detected by the mixing test.
I now give a more natural three-source mixing scenario that can produce an arbitrary isochron, which could not be detected by a mixing test. P2 and P3 are small, since some rocks will have little parent substance. Suppose also that N2 and N3 differ significantly. Such mixings can produce arbitrary isochrons, so these cannot be detected by any mixing test.
Also, if P1 is reduced by fractionation prior to mixing, this will make the age larger. If P1 is increased, it will make the age smaller. If P1 is not changed, the age will at least have geological significance. But it could be measuring the apparent age of the ocean floor or crustal material rather than the time of the lava flow. I believe that the above shows the 3 source mixing to be natural and likely.
We now show in more detail that we can get an arbitrary isochron by a mixing of three sources. Thus such mixings cannot be detected by a mixing test. Assume D3, P3, and N3 in source 3, all zero. One can get this mixing to work with smaller concentrations, too. All the rest of the mixing comes from source 3. Thus we produce the desired isochron. So this is a valid mixing, and we are done.
We can get more realistic mixings of three sources with the same result by choosing the sources to be linear combinations of sources 1, 2, and 3 above, with more natural concentrations of D, P, and N. The rest of the mixing comes from source 3. This mixing is more realistic because P1, N1, D2, and N2 are not so large. I did see in one reference the statement that some parent-to-daughter ratio yielded more accurate dates than isochrons.
To me, this suggests the possibility that geologists themselves recognize the problems with isochrons, and are looking for a better method. The impression I have is that geologists are continually looking for new methods, hoping to find something that will avoid problems with existing methods.
But then problems also arise with the new methods, and so the search goes on. Furthermore, here is a brief excerpt from a recent article which also indicates that isochrons often have severe problems.
If all of these isochrons indicated mixing, one would think that this would have been mentioned: The geological literature is filled with references to Rb-Sr isochron ages that are questionable, and even impossible. Woodmorappe , pp. Faure , pp. Zheng , pp.
Zheng pp. He comes closest to recognizing the fact that the Sr concentration is a third or confounding variable in the isochron simple linear regression. Snelling discusses numerous false ages in the U-Pb system where isochrons are also used.
However, the U-Th-Pb method uses a different procedure that I have not examined and for which I have no data. Many of the above authors attempt to explain these "fictitious" ages by resorting to the mixing of several sources of magma containing different amounts of Rb, Sr, and Sr immediately before the formation hardens.
Akridge , Armstrong , Arndts , Brown , , Helmick and Baumann all discuss this factor in detail. Anyway, if isochrons producing meaningless ages can be produced by mixing, and this mixing cannot be detected if three or maybe even two, with fractionation sources are involved, and if mixing frequently occurs, and if simple parent-to-daughter dating also has severe problems, as mentioned earlier, then I would conclude that the reliability of radiometric dating is open to serious question.
The many acknowledged anomalies in radiometric dating only add weight to this argument. I would also mention that there are some parent-to-daughter ratios and some isochrons that yield ages in the thousands of years for the geologic column, as one would expect if it is in fact very young.
One might question why we do not have more isochrons with negative slopes if so many isochrons were caused by mixing. This depends on the nature of the samples that mix. It is not necessarily true that one will get the same number of negative as positive slopes.
If I have a rock X with lots of uranium and lead daughter isotope, and rock Y with less of both relative to non-radiogenic lead , then one will get an isochron with a positive slope. If rock X has lots of uranium and little daughter product, and rock Y has little uranium and lots of lead daughter product relative to non-radiogenic lead , then one will get a negative slope.
This last case may be very rare because of the relative concentrations of uranium and lead in crustal material and subducted oceanic plates. Another interesting fact is that isochrons can be inherited from magma into minerals.
Earlier, I indicated how crystals can have defects or imperfections in which small amounts of magma can be trapped. This can result in dates being inherited from magma into minerals. This can also result in isochrons being inherited in the same way. So the isochron can be measuring an older age than the time at which the magma solidified. This can happen also if the magma is not thoroughly mixed when it erupts.
If this happens, the isochron can be measuring an age older than the date of the eruption. This is how geologists explain away the old isochron at the top of the Grand Canyon. From my reading, isochrons are generally not done, as they are expensive. Isochrons require more measurements than single parent-to-daughter ratios, so most dates are based on parent-to-daughter ratios. So all of the scenarios given apply to this large class of dates. Another thing to keep in mind is that it is not always possible to do an isochron.
Often one does not get a straight line for the values. This is taken to imply re-melting after the initial solidification, or some other disturbing event. Anyway, this also reduces the number of data points obtained from isochrons. Anyway, suppose we throw out all isochrons for which mixing seems to be a possibility.
Due to some published anomalies, I don't think we know that they have any clear relationship to the assumed dates. It is also interesting that the points for isochrons are sometimes selected so as to obtain the isochron property, according to John Woodmorappe's paper.
Do the various methods correlate with one another? We have been trying to give mechanisms that explain how the different dating methods can give dates that agree with one another, if the geologic column is young.
But if there is a variation, such effects could help to explain it. It's not only a matter of incorporation in minerals either, as one sometimes does whole rock isochrons and I suppose parent-daughter ratios of whole rock, which would reflect the composition of the magma and not the incorporation into minerals.
We all seem to have this image in our mind of the various dating methods agreeing with each other and also with the accepted age of their geologic periods. So we are investing a lot of time and energy to explain how this marvelous agreement of the various methods can arise in a creationist framework.
The really funny thing to me is that it is very possible that we are trying to explain a phantom of our imagination. The real radiomatric dating methods are often very badly behaved, and often disagree with one another as well as with the assumed ages of their geological periods. It would really be nice if geologists would just do a double blind study sometime to find out what the distributions of the ages are. In practice, geologists carefully select what rocks they will date, and have many explanations for discordant dates, so it's not clear how such a study could be done, but it might be a good project for creationists.
There is also evidence that many anomalies are never reported. Concerning the geologic time scale, Brown writes: "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. Maybe only 15 in all. Why is this? It is possible that the reason is that uranium-lead dates so rarely agree with the correct dates. So there may not be anything to explain.
For example, it's not clear to me that we need to worry about isochrons or whether U and U dates etc. agree with each other. I'd like to know how often this happens, in any case, especially on the geologic column of Cambrian and above. People should read John Woodmorappe's articles on radiometric dating to see some of the anomalies. One might say that if there were problems, then geologists wouldn't use these methods.
I think we need something more solid than that. John W. did have an example of a correlation study for K-Ar and Rb-Sr dating in precambrian rocks. The correlation was not very good. I assume he would have mentioned if any others had been done.
Maybe since then? What we really need is the raw data on how these dates correlate, especially on the geologic column of Cambrian and above. We need to see the data to know if there is really any need to explain anything away. Many anomalies never get published, according to John Woodmorappe's references; other quotes indicate that the various methods typically disagree with each other. A few years ago I took a course in the "Evolution of Desert Environments".
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