Clocks in the Rocks
The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. In fact, I think this is a very telling argument against radiometric dating. This radium cannot be the result of decay of uranium, since there is far too much of it. 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.
Of course, the isotopic abundances of decay products, especially lead, would follow a 1 — U curve. The level has since dropped, as this bomb pulse or "bomb carbon" as it is sometimes called percolates into the rest of the reservoir. Age of the Moon Our best clues to the age of the Moon are the radiometric dates of the oldest Moon rocks, those from the lunar highlands. From about until , when atmospheric nuclear testing was banned, it is estimated that several tonnes of 14 C were created. This process will generate an igneous rock of yet another composition.
The following is a quote from The Earth: The magnesium and iron rich minerals come from the mantle subducted oceanic plates , while 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 in , years 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 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 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.
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.
To determine this, a blank sample of old, or dead, carbon is measured, and a sample of known activity is measured. The additional samples allow errors such as background radiation and systematic errors in the laboratory setup to be detected and corrected for.
The results from AMS testing are in the form of ratios of 12 C , 13 C , and 14 C , which are used to calculate Fm, the "fraction modern". Both beta counting and AMS results have to be corrected for fractionation. The calculation uses Libby's half-life of 5, years, not the more accurate modern value of 5, years. The reliability of the results can be improved by lengthening the testing time.
Radiocarbon dating is generally limited to dating samples no more than 50, years old, as samples older than that have insufficient 14 C to be measurable. Older dates have been obtained by using special sample preparation techniques, large samples, and very long measurement times. These techniques can allow measurement of dates up to 60, and in some cases up to 75, years before the present.
This was demonstrated in by an experiment run by the British Museum radiocarbon laboratory, in which weekly measurements were taken on the same sample for six months.
The measurements included one with a range from about to about years ago, and another with a range from about to about Errors in procedure can also lead to errors in the results. The calculations given above produce dates in radiocarbon years: To produce a curve that can be used to relate calendar years to radiocarbon years, a sequence of securely dated samples is needed which can be tested to determine their radiocarbon age.
The study of tree rings led to the first such sequence: These factors affect all trees in an area, so examining tree-ring sequences from old wood allows the identification of overlapping sequences.
In this way, an uninterrupted sequence of tree rings can be extended far into the past. The first such published sequence, based on bristlecone pine tree rings, was created by Wesley Ferguson. Suess said he drew the line showing the wiggles by "cosmic schwung ", by which he meant that the variations were caused by extraterrestrial forces. It was unclear for some time whether the wiggles were real or not, but they are now well-established.
A calibration curve is used by taking the radiocarbon date reported by a laboratory, and reading across from that date on the vertical axis of the graph.
The point where this horizontal line intersects the curve will give the calendar age of the sample on the horizontal axis. This is the reverse of the way the curve is constructed: Over the next thirty years many calibration curves were published using a variety of methods and statistical approaches. The improvements to these curves are based on new data gathered from tree rings, varves , coral , plant macrofossils , speleothems , and foraminifera. The INTCAL13 data includes separate curves for the northern and southern hemispheres, as they differ systematically because of the hemisphere effect; there is also a separate marine calibration curve.
The resulting curve can then be matched to the actual calibration curve by identifying where, in the range suggested by the radiocarbon dates, the wiggles in the calibration curve best match the wiggles in the curve of sample dates. This "wiggle-matching" technique can lead to more precise dating than is possible with individual radiocarbon dates. Bayesian statistical techniques can be applied when there are several radiocarbon dates to be calibrated. For example, if a series of radiocarbon dates is taken from different levels in a given stratigraphic sequence, Bayesian analysis can help determine if some of the dates should be discarded as anomalies, and can use the information to improve the output probability distributions.
Several formats for citing radiocarbon results have been used since the first samples were dated. As of , the standard format required by the journal Radiocarbon is as follows. For example, the uncalibrated date "UtC Related forms are sometimes used: Calibrated dates should also identify any programs, such as OxCal, used to perform the calibration. A key concept in interpreting radiocarbon dates is archaeological association: It frequently happens that a sample for radiocarbon dating can be taken directly from the object of interest, but there are also many cases where this is not possible.
Metal grave goods, for example, cannot be radiocarbon dated, but they may be found in a grave with a coffin, charcoal, or other material which can be assumed to have been deposited at the same time. In these cases a date for the coffin or charcoal is indicative of the date of deposition of the grave goods, because of the direct functional relationship between the two. There are also cases where there is no functional relationship, but the association is reasonably strong: Contamination is of particular concern when dating very old material obtained from archaeological excavations and great care is needed in the specimen selection and preparation.
In , Thomas Higham and co-workers suggested that many of the dates published for Neanderthal artefacts are too recent because of contamination by "young carbon". As a tree grows, only the outermost tree ring exchanges carbon with its environment, so the age measured for a wood sample depends on where the sample is taken from. This means that radiocarbon dates on wood samples can be older than the date at which the tree was felled.
In addition, if a piece of wood is used for multiple purposes, there may be a significant delay between the felling of the tree and the final use in the context in which it is found.
Another example is driftwood, which may be used as construction material. It is not always possible to recognize re-use. Other materials can present the same problem: A separate issue, related to re-use, is that of lengthy use, or delayed deposition. For example, a wooden object that remains in use for a lengthy period will have an apparent age greater than the actual age of the context in which it is deposited.
The Pleistocene is a geological epoch that began about 2. The Holocene , the current geological epoch, begins about 11, years ago, when the Pleistocene ends. Before the advent of radiocarbon dating, the fossilized trees had been dated by correlating sequences of annually deposited layers of sediment at Two Creeks with sequences in Scandinavia.
This led to estimates that the trees were between 24, and 19, years old,  and hence this was taken to be the date of the last advance of the Wisconsin glaciation before its final retreat marked the end of the Pleistocene in North America. This result was uncalibrated, as the need for calibration of radiocarbon ages was not yet understood. Further results over the next decade supported an average date of 11, BP, with the results thought to be most accurate averaging 11, BP.
There was initial resistance to these results on the part of Ernst Antevs , the palaeobotanist who had worked on the Scandinavian varve series, but his objections were eventually discounted by other geologists. In the s samples were tested with AMS, yielding uncalibrated dates ranging from 11, BP to 11, BP, both with a standard error of years. Subsequently, a sample from the fossil forest was used in an interlaboratory test, with results provided by over 70 laboratories. In , scrolls were discovered in caves near the Dead Sea that proved to contain writing in Hebrew and Aramaic , most of which are thought to have been produced by the Essenes , a small Jewish sect.
These scrolls are of great significance in the study of Biblical texts because many of them contain the earliest known version of books of the Hebrew bible.
The results ranged in age from the early 4th century BC to the mid 4th century AD. In many cases the scrolls were determined to be older than the palaeographically determined age. Subsequently, these dates were criticized on the grounds that before the scrolls were tested, they had been treated with modern castor oil in order to make the writing easier to read; it was argued that failure to remove the castor oil sufficiently would have caused the dates to be too young.
Multiple papers have been published both supporting and opposing the criticism. Soon after the publication of Libby's paper in Science , universities around the world began establishing radiocarbon-dating laboratories, and by the end of the s there were more than 20 active 14 C research laboratories. It quickly became apparent that the principles of radiocarbon dating were valid, despite certain discrepancies, the causes of which then remained unknown.
Taylor, " 14 C data made a world prehistory possible by contributing a time scale that transcends local, regional and continental boundaries". It provides more accurate dating within sites than previous methods, which usually derived either from stratigraphy or from typologies e.
The advent of radiocarbon dating may even have led to better field methods in archaeology, since better data recording leads to firmer association of objects with the samples to be tested. These improved field methods were sometimes motivated by attempts to prove that a 14 C date was incorrect.
Taylor also suggests that the availability of definite date information freed archaeologists from the need to focus so much of their energy on determining the dates of their finds, and led to an expansion of the questions archaeologists were willing to research. For example, from the s questions about the evolution of human behaviour were much more frequently seen in archaeology.
The dating framework provided by radiocarbon led to a change in the prevailing view of how innovations spread through prehistoric Europe. Researchers had previously thought that many ideas spread by diffusion through the continent, or by invasions of peoples bringing new cultural ideas with them. As radiocarbon dates began to prove these ideas wrong in many instances, it became apparent that these innovations must sometimes have arisen locally.
This has been described as a "second radiocarbon revolution", and with regard to British prehistory, archaeologist Richard Atkinson has characterized the impact of radiocarbon dating as "radical Similar results are also obtained from the study of spontaneous fission events from uranium and plutonium One of the standard references for modeling the age of the Earth is G.
From such data, and from estimates of how long it would take to produce the quantities of various lead isotopes now found on the earth, geochronologists feel that the 4. The ages of rocks returned to Earth from the Apollo missions range from 3. The older age determinations are derived from rocks collected on the lunar highland, which may represent the original lunar crust. Our best clues to the age of the Moon are the radiometric dates of the oldest Moon rocks, those from the lunar highlands.
Dalrymple reports that thirteen samples from the lunar highlands gave the oldest ages. These were collected by Apollo 15, 16, 17 and Luna The radiometric dates range from 3. If we take the oldest ages to be the age of the Moon, then we place that age at about 4. Clocks in the Rocks The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes: Lead isochrons are also an important radioactive dating process. Some of the decays which are useful for dating, with their half-lives and decay constants are: Uranium-Lead Dating Ages determined by radioactive decay are always subject to assumptions about original concentrations of the isotopes.
Clocks in the Rocks. Index Beta decay concepts. Potassium-Argon Method Potassium-Argon dating has the advantage that the argon is an inert gas that does not react chemically and would not be expected to be included in the solidification of a rock, so any found inside a rock is very likely the result of radioactive decay of potassium.
More detail on Potassium-Argon dating. Rubidium-Strontium The rubidium-strontium dating method is often used in geologic studies.
Age of the Earth To the intriguing question "How old is the Earth? Index Reference Krane Sec 6. Meteorite Dating "Meteorites, which many consider to be remnants of a disrupted planet that oriaginally formed at about the same time as the earth, have provided uranium-lead and rubidium-strontium ages of about 4. Index Reference Dalrymple Ch 5. Age of the Moon Our best clues to the age of the Moon are the radiometric dates of the oldest Moon rocks, those from the lunar highlands. A sample of the kind of data that leads to such a projected age is the rubidium-strontium isochron of lunar sample which yields a time to last melting of 4.
This isochron was discussed in Dalrymple and credited to Papanastassiou and Wassenburg, Data from Papanastassiou, D.
Imsges: problems with radioactive isotope dating
Other corrections must be made to account for the proportion of 14 C in different types of organisms fractionation , and the varying levels of 14 C throughout the biosphere reservoir effects. If P1 is not changed, the age will at least have geological significance. Contamination with modern carbon causes a sample to appear to be younger than it really is:
It's largest mineral constituent is olivine and the actual form is called dunite. 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. The discordia is often interpreted by extrapolating both ends to intersect the Concordia.
Here I want to concentrate on another source of error, namely, processes that take place within magma chambers. Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene. The initial ratio has particular importance for problems with radioactive isotope dating the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. One of these is the philosophy of naturalism, the impact of which I discussed in my article. Then these concentrations would be reduced by a factor of say dating fotos p at point p, and so the new concentrations would be P p r' pD p r' pand N p r' p at point p. Data from Papanastassiou, D. A key concept problems with radioactive isotope dating interpreting radiocarbon dates is archaeological association:
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