More U is found in carbonate rocks, while Th has a very strong preference for granites in comparison. I saw a reference that uranium reacts strongly, and is never found pure in nature.
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So the question is what the melting points of its oxides or salts would be, I suppose. I also saw a statement that uranium is abundant in the crust, but never found in high concentrations. To me this indicates a high melting point for its minerals, as those with a low melting point might be expected to concentrate in the magma remaining after others crystallized out. Such a high melting point would imply fractionation in the magma.
Thorium is close to uranium in the periodic table, so it may have similar properties, and similar remarks may apply to it. It turns out that uranium in magma is typically found in the form of uranium dioxide, with a melting point of degrees centrigrade. This high melting point suggests that uranium would crystallize and fall to the bottom of magma chambers.
Geologists are aware of the problem of initial concentration of daughter elements, and attempt to take it into account. U-Pb dating attempts to get around the lack of information about initial daughter concentrations by the choice of minerals that are dated. For example, zircons are thought to accept little lead but much uranium.
Thus geologists assume that the lead in zircons resulted from radioactive decay. But I don't know how they can be sure how much lead zircons accept, and even they admit that zircons accept some lead. Lead could easily reside in impurities and imperfections in the crystal structure. Also, John Woodmorappe's paper has some examples of anomalies involving zircons. It is known that the crystal structure of zircons does not accept much lead.
However, it is unrealistic to expect a pure crystal to form in nature. Perfect crystals are very rare. In reality, I would expect that crystal growth would be blocked locally by various things, possibly particles in the way. Then the surrounding crystal surface would continue to grow and close up the gap, incorporating a tiny amount of magma.
I even read something about geologists trying to choose crystals without impurities by visual examination when doing radiometric dating. Thus we can assume that zircons would incorporate some lead in their impurities, potentially invalidating uranium-lead dates obtained from zircons. Chemical fractionation, as we have seen, calls radiometric dates into question. But this cannot explain the distribution of lead isotopes.
There are actually several isotopes of lead that are produced by different parent substances uranium , uranium , and thorium. One would not expect there to be much difference in the concentration of lead isotopes due to fractionation, since isotopes have properties that are very similar. So one could argue that any variations in Pb ratios would have to result from radioactive decay.
However, the composition of lead isotopes between magma chambers could still differ, and lead could be incorporated into lava as it traveled to the surface from surrounding materials. I also recall reading that geologists assume the initial Pb isotope ratios vary from place to place anyway. Later we will see that mixing of two kinds of magma, with different proportions of lead isotopes, could also lead to differences in concentrations. Mechanism of uranium crystallization and falling through the magma We now consider in more detail the process of fractionation that can cause uranium to be depleted at the top of magma chambers.
Uranium and thorium have high melting points and as magma cools, these elements crystallize out of solution and fall to the magma chamber's depths and remelt. This process is known as fractional crystallization. What this does is deplete the upper parts of the chamber of uranium and thorium, leaving the radiogenic lead. As this material leaves, that which is first out will be high in lead and low in parent isotopes. This will date oldest.
Magma escaping later will date younger because it is enriched in U and Th. There will be a concordance or agreement in dates obtained by these seemingly very different dating methods. This mechanism was suggested by Jon Covey. Tarbuck and Lutgens carefully explain the process of fractional crystallization in The Earth: An Introduction to Physical Geology.
They show clear drawings of crystallized minerals falling through the magma and explain that the crystallized minerals do indeed fall through the magma chamber. Further, most minerals of uranium and thorium are denser than other minerals, especially when those minerals are in the liquid phase.
Crystalline solids tend to be denser than liquids from which they came. But the degree to which they are incorporated in other minerals with high melting points might have a greater influence, since the concentrations of uranium and thorium are so low. Now another issue is simply the atomic weight of uranium and thorium, which is high. Any compound containing them is also likely to be heavy and sink to the bottom relative to others, even in a liquid form. If there is significant convection in the magma, this would be minimized, however. At any rate, there will be some effects of this nature that will produce some kinds of changes in concentration of uranium and thorium relative to lead from the top to the bottom of a magma chamber.
Some of the patterns that are produced may appear to give valid radiometric dates. The latter may be explained away due to various mechanisms. Let us consider processes that could cause uranium and thorium to be incorporated into minerals with a high melting point. I read that zircons absorb uranium, but not much lead. Thus they are used for U-Pb dating. But many minerals take in a lot of uranium. It is also known that uranium is highly reactive. To me this suggests that it is eager to give up its 2 outer electrons.
This would tend to produce compounds with a high dipole moment, with a positive charge on uranium and a negative charge on the other elements. This would in turn tend to produce a high melting point, since the atoms would attract one another electrostatically. I'm guessing a little bit here.
There are a number of uranium compounds with different melting points, and in general it seems that the ones with the highest melting points are more stable. I would suppose that in magma, due to reactions, most of the uranium would end up in the most stable compounds with the highest melting points.
These would also tend to have high dipole moments. Now, this would also help the uranium to be incorporated into other minerals. The electric charge distribution would create an attraction between the uranium compound and a crystallizing mineral, enabling uranium to be incorporated. But this would be less so for lead, which reacts less strongly, and probably is not incorporated so easily into minerals. So in the minerals crystallizing at the top of the magma, uranium would be taken in more than lead. These minerals would then fall to the bottom of the magma chamber and thus uranium at the top would be depleted.
It doesn't matter if these minerals are relatively lighter than others. The point is that they are heavier than the magma. Two kinds of magma and implications for radiometric dating It turns out that magma has two sources, ocean plates and material from the continents crustal rock.
This fact has profound implications for radiometric dating. Mantle material is very low in uranium and thorium, having only 0. The source of magma for volcanic activity is subducted oceanic plates. Subduction means that these plates are pushed under the continents by motions of the earth's crust. While oceanic plates are basaltic mafic originating from the mid-oceanic ridges due to partial melting of mantle rock, the material that is magma is a combination of oceanic plate material and continental sediments.
Subducted oceanic plates begin to melt when they reach depths of about kilometers See Tarbuck, The Earth, p. In other words, mantle is not the direct source of magma.
Further, Faure explains that uraninite UO sub2 is a component of igneous rocks Faure, p. Uraninite is also known as pitchblende. According to plate tectonic theory, continental crust overrides oceanic crust when these plates collide because the continental crust is less dense than the ocean floor. 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. 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. Rhyolites in Yellowstone N.
Most genetic models for uranium deposits in sandstones in the U. Most of the uranium deposits in Wyoming are formed from uraniferous groundwaters derived from Precambrian granitic terranes.
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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 upper portion of the sialic magma would be cooler since its in contact with continental rock, and the high melting point of UO sub 2 uranium dioxide, the common form in granite: 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.
Radiocarbon Dating - Chemistry LibreTexts
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. 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.
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. 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. The carbon isotope would vanish from Earth's atmosphere in less than a million years were it not for the constant influx of cosmic rays interacting with molecules of nitrogen N 2 and single nitrogen atoms N in the stratosphere.
Both processes of formation and decay of carbon are shown in Figure 1. Diagram of the formation of carbon forward , the decay of carbon reverse. Carbon is constantly be generated in the atmosphere and cycled through the carbon and nitrogen cycles. Once an organism is decoupled from these cycles i.
When plants fix atmospheric carbon dioxide CO 2 into organic compounds during photosynthesis, the resulting fraction of the isotope 14 C in the plant tissue will match the fraction of the isotope in the atmosphere and biosphere since they are coupled. After a plants die, the incorporation of all carbon isotopes, including 14 C, stops and the concentration of 14 C declines due to the radioactive decay of 14 C following. This follows first-order kinetics. The currently accepted value for the half-life of 14 C is 5, years. This means that after 5, years, only half of the initial 14 C will remain; a quarter will remain after 11, years; an eighth after 17, years; and so on.
The equation relating rate constant to half-life for first order kinetics is. In samples of the Dead Sea Scrolls were analyzed by carbon dating. From the measurement performed in the Dead Sea Scrolls were determined to be years old giving them a date of 53 BC, and confirming their authenticity. Carbon dating has shown that the cloth was made between and AD. Thus, the Turin Shroud was made over a thousand years after the death of Jesus. Describes radioactive half life and how to do some simple calculations using half life.
The technique of radiocarbon dating was developed by Willard Libby and his colleagues at the University of Chicago in Libby estimated that the steady-state radioactivity concentration of exchangeable carbon would be about 14 disintegrations per minute dpm per gram. In , Libby was awarded the Nobel Prize in chemistry for this work. He demonstrated the accuracy of radiocarbon dating by accurately estimating the age of wood from a series of samples for which the age was known, including an ancient Egyptian royal barge dating from BCE.
Before Radiocarbon dating was able to be discovered, someone had to find the existence of the 14 C isotope. They found a form, isotope, of Carbon that contained 8 neutrons and 6 protons. Using this finding Willard Libby and his team at the University of Chicago proposed that Carbon was unstable and underwent a total of 14 disintegrations per minute per gram. Using this hypothesis, the initial half-life he determined was give or take 30 years. Although it may be seen as outdated, many labs still use Libby's half-life in order to stay consistent in publications and calculations within the laboratory.