Retrocausality - Wikipedia
Some of these minerals (e.g. sanidine, biotite, zircon) are amenable to radiometric dating. Since the Z-coal lies just above the Hell Creek formation, the Z-coal. Synonyms for radiometric dating at uzveli.info with free online thesaurus, antonyms, and definitions. Find descriptive alternatives for radiometric dating. Radiometric dating is largely done on rock that has formed from solidified lava. is open to question, this can potentially be explained by processes occurring in.
When iron was chemically removed from the vessels by treatment with chelating agents, the response of the vessel tissues to specific protein antibodies increased dramatically. This is another indication of the association of residual iron with these preserved proteins.
Some ostrich vessels were incubated in a solution of hemoglobin. This hemoglobin had been extracted from the red blood cells of chicken and ostrich blood, and then re-diluted to its original concentration in the avian blood. The vessels sitting in hemoglobin solution have shown no signs of degradation for more than two years. In contrast, the ostrich vessels in plain water or phosphate buffered saline PBS showed significant degradation within three days, i.
These results dramatically demonstrate the efficacy of iron-based tissue preservation, which has not generally been taken into account in earlier estimates of how long proteins can survive.
Ostrich blood vessels incubated for 30 days at room temperature under oxygenated conditions, with B or without D added hemoglobin in the incubation solution. From Figure S5 of Schweitzer et al. Initial infrared spectroscopy suggested the presence of highly crosslinked collagen within dinosaur fossil tissue. Both of these reactions depend on the oxidation potential of iron. To test the roles of these mechanisms, they incubated fresh, demineralized chicken bone using corresponding treatments to induce collagen crosslinking.
Analytical results showed that these treatments did indeed induce the type of crosslinking which is expected to make the collagen more resistant to decomposition. Also, the preserved dinosaur tissues were found to be sufficiently crosslinked to withstand a chemical which cleaves lightly-crosslinked molecules: Fossil vessels treated with the reducing agent [NaBH4, which can cleave low-order intermolecular crosslinks] yielded no significant changes in FTIR analysis, suggesting that the non-enzymatic crosslinks formed in this tissue are irreducible.
Such bond formations occur between three or more peptide strands, and as such, tend to be highly resistant to reductive cleavage. Assessment of Evidence That Soft Tissue Can Persist for 70 Million Years As might be expected, young earth creationists have taken these observations of soft tissue from dinosaur bones as evidence that these fossils cannot be more than a few thousand years old — and therefore, conventional geological methods like radioactive dating must be terribly flawed, since these methods show the rock layers entombing these fossils as being about 70 million years old.
I cited a couple of these young earth articles at the beginning of this article. Some of these sites misrepresent the facts, stating that actual red blood cells have been found. As noted above, that is not the case: The actual organic remains are highly crosslinked remnants of a several proteins which are known to have stable structures.
These remnants retain the shape of the original soft tissue, which is not surprising, since they were confined within tiny pores in the dinosaur bones. The main attack by young earth creationists on the antiquity of these finds is an argument from incredulity, based on ignorance: Yes, experiments on protein degradation in test tubes indicate that proteins would break down completely within about a million years. But lots of examples show that the rate of protein degradation varies wildly, depending on the conditions, so no one can say with certainty how long some fragments of protein can last, preserved with iron and sealed in mineral pores.
It is difficult to devise definitive experiments to mimic that timespan. The ostrich vessels discussed above had their lifetime before degradation extended from three days to more than two years, a factor of over We can even see widely differing decomposition rates in our food, depending on how it is treated. If you leave a jug of milk on the counter for two weeks, lots of biochemistry will take place mainly lactose fermentation to acids but also including protein degradation which may render it unfit for human consumption.
However, if the milk is contacted with the right bacteria and other materials, the milk proteins can be preserved in the form of cheese which can sit stably on the shelf for years. Similarly, if beef is ground to hamburger and left in a package on the counter for a week, it will rot.
However, if the beef is sliced thin and dried to jerky, it will last for months. Same milk and same beef, but with a different set of conditions they can retain proteins for ten or a hundred times longer, depending on their circumstances. From studies like there, we know that the flesh and skin can decay off a human in about a month under humid conditions. Now, consider this individual: Thus, it is absurd to say that because proteins disappear in a million years under one set of conditions, therefore protein remnants could not endure for more than million years under some other conditions.
Since that claim in one form or another is at the heart of the young earth interpretation of these fossil tissues, the young earth case here collapses. Invariance of Radioactive Decay Rates We may contrast the uncertainties regarding biological decay, to certainties regarding the physical decay of radioactive elements.
These decay rates have been measured in many laboratories in many ways for many years, and they are essentially invariant. There are some particular circumstances where radioactive decay rates can be accelerated, but these are well-understood within the framework of physics. For instance, in a nuclear reactor or bomb, an artificially high density of uranium means that the neutrons from one splitting nucleus have a high probability of striking another nucleus and causing it to split.
However, these effects are understood and predictable. They advance various objections, but these objections have been refuted over and over again by practicing scientists. For instance, see these resources: What evidence is there for the earth being billions of years old? Radiometric Dating Does Work! Delves into several detailed examples, including the Hell Creek formation.
For my part, I have documented the errors of young earth arguments regarding the dating of Grand Canyon rocksand the dating of some recent lava flows. I have also compiled some other physical evidences e. It would be tedious to retrace all those prior discussions. Thousands and thousands of radioactive dating measurements have been made, so naturally there will be a few that give anomalous results.
For many of these cases, it can be seen why the results were odd — for instance, the rock sample may have been re-heated after it initially solidified, which partially re-set the atomic clock. However, the RATE scientists did not find any actual evidence of faster radioactive decay; they simply asserted that there MUST have been accelerated radioactive decay, in order to meet their existing young earth model.
They presented no valid physical support for this. They rehashed four arguments against conventional old-earth dating, but these arguments have been thoroughly debunked by practicing scientists . To overturn the last hundred years of physics would merit a legitimized platform to tell the world at large that old-earth dating methods are unreliable.
It might even merit a Nobel Prize. The standard young earth creationist excuse for not doing this is: Young earth geologists have been allowed to present a number of papers and to lead field trips at recent meetings of the secular Geological Society of America, so it is in fact possible to get a hearing among geologists for an unusual perspective, as long as there is solid supporting data.
This change in fauna has been obvious for over a century. Many independent radiometric measurements have been made on these tektites, consistently showing them to be around million years old.
Although the exact mechanism of the catastrophe is not clear, it is widely thought that this meteorite strike is tied to the mass extinctions of some three-quarters of plant and animal species on Earth, including all non-avian dinosaurs, which occurred at that time.
The dark band in this photo indicated by the white arrow is the Z-coal, marking the top of the Hell Creek formation in Montana. Museum of Paleontology, U. Some of these minerals e.
Since the Z-coal lies just above the Hell Creek formation, the Z-coal was deposited somewhat later. Thus, the dinosaur fossils in the Hell Creek rocks must be older than the Z-coal. Here are some radioactive datings of ash-containing layers at the Z-coal bed, from a table compiled by U. Geological Survey radiometric dating expert Brent Dalrymple. Moreover, these dates are very close to the dates million years determined elsewhere for the tektites that mark the K-T boundary.
This uniformity of nuclear radioactive decay stands in stark contrast to the enormous and unpredictable variations in the rate of the biological decay of soft tissues discussed above.
Thus, it is irrational to use the unexpected perseverance of flexible tissues in dinosaur bones as ground to reject the radioactive dating of the rock layers in which these bones were found. But that is what the young earth creationists are trying to do. The Dinosaur-Bird Connection Dinosaurs were clearly reptiles, and so the normal expectation would be that the protein sequences recovered from dinosaur fossils would more closely match extant reptiles than any other class of living animals.
However, mainstream scientists, guided by the hypothesis of evolution and the common ancestry of all animals, have a different expectation. They would predict that the proteins from dinosaurs would be closer to birds than to any living reptiles. They share so many characteristics  that extinct and living birds are usually classified as lying within the theropod group. It should be noted that birds have continued to diverge from their reptilian origins over the past million years.
Radioactive dating geology definition
Most modern birds no longer display features like teeth or claws on their wings, which were more prominent in earlier fossil birds, so birds now look very unlike reptiles. Cladogram of Archosaurs, a branch of reptiles. Nearly all of these groups are extinct, and thus are known only from fossils. I put red boxes around the only two groups that have living representatives. Other extant reptiles, such as lizards, snakes, and turtles, lie on entirely different branches of the evolutionary family tree, and thus are considered more distant from dinosaurs.
I underlined two key dinosaur types Tyrannosaurus and Hadrosaurs or duckbills which figure in the academic work discussed above. According to this cladistic scheme, dinosaurs are more closely related to birds than to any living reptile, and so the proteins extracted from the dinosaur bones should resemble bird proteins more than alligator or crocodile proteins, even though dinosaurs are reptiles like crocodiles.
And that turns out to be the case: Also, a key observation by Mary Schweitzer inpublished in , was that the T. As noted above, medullary bone temporarily appears within female bird bones, where it serves as a reservoir of calcium for producing the shells of eggs. In Lee and Werning  reported medullary bone in fossils of two other dinosaurs, the theropod Allosaurus and the ornithopod Tenontosaurus.
Thus, as happens again and again, predictions based on evolution were borne out upon experimental investigation. Side comment on the dinosaur-bird connection: Sunspots have absolutely nothing to do with the rate of C decay, which defines the half-life of that radioactive element. Hovind has confused two completely different concepts. Quantum mechanics, that stout pillar of modern physics, which has been verified in so many different ways that I couldn't begin to list them all even if I had them at hand, gives us no theoretical reason for believing that the C rate of decay has changed or can be significantly affected by any reasonable process.
We also have direct observation: That radiocarbon ages agree so closely with tree-ring counts over at least years, when the observed magnetic effect upon the production rate of C is taken into account, suggests that the decay constant itself can be assumed to be reliable.
We also have laboratory studies which support the constancy of all the decay rates used in radiometric dating. A great many experiments have been done in attempts to change radioactive decay rates, but these experiments have invariably failed to produce any significant changes. It has been found, for example, that decay constants are the same at a temperature of degrees C or at a temperature of degrees C and are the same in a vacuum or under a pressure of several thousand atmospheres.
Measurements of decay rates under differing gravitational and magnetic fields also have yielded negative results. Although changes in alpha and beta decay rates are theoretically possible, theory also predicts that such changes would be very small [ Emery, ] and thus would not affect dating methods. There is a fourth type of decay that can be affected by physical and chemical conditions, though only very slightly.
This type of decay is electron capture e. Because this type of decay involves a particle outside the nucleus, the decay rate may be affected by variations in the electron density near the nucleus of the atom.
For example, the decay constant of Be-7 in different beryllium chemical compounds varies by as much as 0. The only isotope of geologic interest that undergoes e.
Measurements of the decay rate of K in different substances under various conditions indicate that variations in the chemical and physical environment have no detectable effect on its e. Dalrymple,p. Harold Slusher, a prominent member of the Institute for Creation Research, claimed that "Experiments have shown that the decay rates of cesium and iron 57 vary, hence there may be similar variations in other radioactive decay rates.
This statement merely reveals Slusher's ignorance of nuclear physics. Gamma decay of an excited state of iron 57 has been studied, but this has nothing to do with the kinds of decays used in radiometric dating. Brush,p. These changes are irrelevant to radiometric dating methods. They will switch tracks faster than you can say "tiddlywinks.
Dinosaur Soft Tissue
Morris claimed that free neutrons might change the decay rates. However, Henry Morris, that icon of creationism, only demonstrated that he knew no more about radiometric dating than does Dr.
Free neutrons might change one element into another, but the decay rates all remain true to their elements. Another attempt by Morris invokes neutrinos. Morris [ ] also suggests that neutrinos might change decay rates, citing a column by Jueneman 72 in Industrial Research.
The subtitle of Jueneman's columns, which appear regularly, is, appropriately, "Scientific Speculation. Jueneman describes a highly speculative hypothesis that would account for radioactive decay by interaction with neutrinos rather than by spontaneous decay, and he notes that an event that temporarily increased the neutrino flux might "reset" the clocks.
Jueneman, however, does not propose that decay rates would be changed, nor does he state how the clocks would be reset; in addition, there is no evidence to support his speculation. Those mysterious neutrinos seem to be a hot topic!
Slusher and Rybka also propose that neutrinos can change decay rates, citing an hypothesis by Dudley 40 that decay is triggered by neutrinos in a "neutrino sea" and that changes in the neutrino flux might affect decay rates. This argument has been refuted by Brush 20who points out that Dudley's hypothesis not only requires rejection of both relativity and quantum mechanics, two of the most spectacularly successful theories in modern science, but is disproved by recent experiments.
Dudley himself rejects the conclusions drawn from his hypothesis by Slusher and Rybkanoting that the observed changes in decay rates are insufficient to change the age of the Earth by more than a few percent Dudley, personal communication,quoted in 20, p.
Thus, even if Slusher and Rybka were correct--which they are not--the measured age of the Earth would still exceed 4 billion years. Judging from the above, it is easy to see that creationists are indulging in wild fishing expeditions. Compare their flighty arguments to the solid support provided by theoretical work, laboratory testing, and, for the shorter half-lives, actual observation, and add to that the statistical consistency of the dates obtained, including numerous cross-checks between different "clocks," and only one conclusion is left.
The radiometric decay rates used in dating are totally reliable. They are one of the safest bets in all of science. The initial C content cannot be known. Various living samples give very different ratios. With at least one notable exception on the books, plants and animals get their carbon from the atmosphere. Plants take it in directly, and animals eat the plants.
Thus, it gets passed up the food chain. It is not surprising, therefore, to find that the carbon in living plants and animals is in reasonable equilibrium with the atmospheric carbon Some creationists, however, have claimed that certain plants can reject carbon in favor of carbon Because of the chemical similarity of carbon and carbon, it is unlikely that such plants could deviate much from the ratio of C to C found in the atmosphere.
Neither freak cases nor small deviations pose much of a problem for radiocarbon dating, which, after all, works well with a wide variety of plant and animal species.
Hence, we only have to worry about the initial concentration of C in the atmosphere. Topic R1 shows that the level of C in the atmosphere has not varied appreciably over tens of thousands of years. Therefore, the initial C content is known for any reasonable sample! The notable exception involves certain mollusks, which get much of their carbon from dissolved limestone. Since limestone is very old it contains very little carbon Thus, in getting some of their carbon from limestone, these mollusks "inherit" some of the limestone's old age!
That is, the limestone carbon skews the normal ratio between C and C found in living things. If one dates such mollusks, one must be extra careful in interpreting the data. Not every mollusk shell presents such problems, and the dating of other material might yield a cross-check.
Further study might even allow correction tables. The discovery has strengthened the carbon method, not weakened it! By the way, shouldn't the creationist be worried over the old, carbon age of the limestone? Why is it that limestone has so little C in it? Partial contamination, say of a block of wood, may affect its different parts to different degrees. Insect burrows, cracks, and partial decay may allow contamination later on to affect those portions of the sample unequally.
However, there are laboratory techniques, often ingenious, for dealing with such problems. If the sample shows evidence of being hopelessly contaminated it is pitched. Some samples, such as a section of a tree trunk, may well contain material of considerably different ages. The interior portion of a tree trunk could easily be several hundred years older than the outer portions. In summing up this point, we do know within good limits what the initial C was for any reasonable sample.
A sample will not have different ratios of carbon unless it has been contaminated or reflects a genuine range of ages. It is very difficult or impossible to prove that a given sample has not been contaminated. Parent or daughter products could have leached in or out of the sample. In the case of carbon dating, the daughter product is ordinary nitrogen and plays no role in the dating process.
We are only interested in tallying the original C still present in the sample, the surviving "parent" isotope. The C that is incorporated in the carbon structure of cellulose and the other structural materials of living plants and animals is not going to do much migrating after burial. If structural carbon migrated easily there soon wouldn't be any cellulose, lignin, chitin or other structural carbon compounds left in the soil!
A piece of wood, for example, would soon turn into a formless cloud of graphite or soot in the soil, with perhaps a little ash marking the original shape! Clearly, that is not something which normally happens. Residues or solutions which do migrate can usually be washed out of the structural matrix of the sample with various chemicals. To put it another way, we might imagine a piece of buried wood as being something like a sponge. Any carbon-containing liquid originally possessed by that sponge might well leak over time and be replaced by something else.
However, unless the sponge itself disintegrates, the carbon which holds its fibers together must stay put. Thus, by choosing a sample that is structurally intact, one may rule out any significant loss of C In addition, the magma chamber would be expected to be cooler all around its borders, both at the top and the bottom as well as in the horizontal extremities, and these effects must also be taken into account.
For example, heavier substances will tend to sink to the bottom of a magma chamber. Also, substances with a higher melting point will tend to crystallize out at the top of a magma chamber and fall, since it will be cooler at the top.
These substances will then fall to the lower portion of the magma chamber, where it is hotter, and remelt. This will make the composition of the magma different at the top and bottom of the chamber. This could influence radiometric dates.
This mechanism was suggested by Jon Covey and others. The solubility of various substances in the magma also could be a function of temperature, and have an influence on the composition of the magma at the top and bottom of the magma chamber. Finally, minerals that crystallize at the top of the chamber and fall may tend to incorporate other substances, and so these other substances will also tend to have a change in concentration from the top to the bottom of the magma chamber.
There are quite a number of mechanisms in operation in a magma chamber. I count at least three so far -- sorting by density, sorting by melting point, and sorting by how easily something is incorporated into minerals that form at the top of a magma chamber.
Radiometric dating calculator | British Archive of Country Music
Then you have to remember that sometimes one has repeated melting and solidification, introducing more complications.
There is also a fourth mechanism -- differences in solubilities. How anyone can keep track of this all is a mystery to me, especially with the difficulties encountered in exploring magma chambers. These will be definite factors that will change relative concentrations of parent and daughter isotopes in some way, and call into question the reliability of radiometric dating.
In fact, I think this is a very telling argument against radiometric dating. Another possibility to keep in mind is that lead becomes gaseous at low temperatures, and would be gaseous in magma if it were not for the extreme pressures deep in the earth. It also becomes very mobile when hot. These processes could influence the distribution of lead in magma chambers. Let me suggest how these processes could influence uranium-lead and thorium-lead dates: The following is a quote from The Earth: The magnesium and iron rich minerals come from the mantle subducted oceanic plateswhile granite comes from continental sediments crustal rock.
The mantle part solidifies first, and is rich in magnesium, iron, and calcium. So it is reasonable to expect that initially, the magma is rich in iron, magnesium, and calcium and poor in uranium, thorium, sodium, and potassium. Later on the magma is poor in iron, magnesium, and calcium and rich in uranium, thorium, sodium, and potassium. It doesn't say which class lead is in.
But lead is a metal, and to me it looks more likely that lead would concentrate along with the iron. If this is so, the magma would initially be poor in thorium and uranium and rich in lead, and as it cooled it would become rich in thorium and uranium and poor in lead.
Thus its radiometric age would tend to decrease rapidly with time, and lava emitted later would tend to look younger. Another point is that of time. Suppose that the uranium does come to the top by whatever reason. Perhaps magma that is uranium rich tends to be lighter than other magma. Or maybe the uranium poor rocks crystallize out first and the remaining magma is enriched in uranium. Would this cause trouble for our explanation?
It depends how fast it happened. Some information from the book Uranium Geochemistry, Mineralogy, Geology provided by Jon Covey gives us evidence that fractionation processes are making radiometric dates much, much too old. The half life of U is 4. Thus radium is decaying 3 million times as fast as U At equilibrium, which should be attained inyears for this decay series, we should expect to have 3 million times as much U as radium to equalize the amount of daughter produced.
Cortini says geologists discovered that ten times more Ra than the equilibrium value was present in rocks from Vesuvius. They found similar excess radium at Mount St.
Helens, Vulcanello, and Lipari and other volcanic sites. The only place where radioactive equilibrium of the U series exists in zero age lavas is in Hawiian rocks. We need to consider the implications of this for radiometric dating.
How is this excess of radium being produced? This radium cannot be the result of decay of uranium, since there is far too much of it. Either it is the result of an unknown decay process, or it is the result of fractionation which is greatly increasing the concentration of radium or greatly decreasing the concentration of uranium.
Thus only a small fraction of the radium present in the lava at most 10 percent is the result of decay of the uranium in the lava. This is interesting because both radium and lead are daughter products of uranium.
If similar fractionation processes are operating for lead, this would mean that only a small fraction of the lead is the result of decay from the parent uranium, implying that the U-Pb radiometric dates are much, much too old.
Cortini, in an article appearing in the Journal of Volcanology and Geothermal Research also suggests this possibility. By analogy with the behaviour of Ra, Th and U it can be suggested that Pb, owing to its large mobility, was also fed to the magma by fluids.
This can and must be tested. The open-system behaviour of Pb, if true, would have dramatic consequences On the other hand, even if such a process is not operating for lead, the extra radium will decay rapidly to lead, and so in either case we have much too much lead in the lava and radiometric dates that are much, much too ancient!
It is also a convincing proof that some kind of drastic fractionation is taking place, or else an unknown process is responsible. He says this is inexplicable in a closed-system framework and certainly invalidates the Th dating method. And it is also possible that something similar is happening in the U decay chain, invalidating U based radiometric dates as well. In fact, U and Th both have isotopes of radium in their decay chains with half lives of a week or two, and 6. Any process that is concentrating one isotope of radium will probably concentrate the others as well and invalidate these dating methods, too.
Radium has a low melting point degrees K which may account for its concentration at the top of magma chambers. What radiometric dating needs to do to show its reliability is to demonstrate that no such fractionation could take place. Can this be done? With so many unknowns I don't think so. How Uranium and Thorium are preferentially incorporated in various minerals I now give evidences that uranium and thorium are incorporated into some minerals more than others.
This is not necessarily a problem for radiometric dating, because it can be taken into account. But as we saw above, processes that take place within magma chambers involving crystallization could result in a different concentration of uranium and thorium at the top of a magma chamber than at the bottom. This can happen because different minerals incorporate different amounts of uranium and thorium, and these different minerals also have different melting points and different densities.
If minerals that crystallize at the top of a magma chamber and fall, tend to incorporate a lot of uranium, this will tend to deplete uranium at the top of the magma chamber, and make the magma there look older. Concerning the distribution of parent and daughter isotopes in various substances, there are appreciable differences. Faure shows that in granite U is 4. Some process is causing the differences in the ratios of these magmatic rocks.
Depending on their oxidation state, according to Faure, uranium minerals can be very soluble in water while thorium compounds are, generally, very insoluble. These elements also show preferences for the minerals in which they are incorporated, so that they will tend to be "dissolved" in certain mineral "solutions" preferentially to one another.
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. 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 uraniumuraniumand 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. Uranium is believed to be able to incorporate itself as a trace material in many other minerals of low density, and so be relatively highly concentrated in the crust.
A lower mantle concentration of uranium is inferred because if the mantle contained the same uranium concentration as the crust, then the uranium's heat of radiactive decay would keep the crust molten. 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. 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.