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- Radiometric dating - Wikipedia
- Radiometric dating
By anyone's standards, 50 billion years is a long time. In fact, this form of dating has been used to date the age of rocks brought back to Earth from the moon. So, we see there are a number of different methods for dating rocks and other non-living things, but what if our sample is organic in nature? For example, how do we know that the Iceman, whose frozen body was chipped out of glacial ice in , is 5, years old? Well, we know this because samples of his bones and hair and even his grass boots and leather belongings were subjected to radiocarbon dating.
Radiocarbon dating , also known as carbon dating or simply carbon dating, is a method used to determine the age of organic material by measuring the radioactivity of its carbon content. So, radiocarbon dating can be used to find the age of things that were once alive, like the Iceman. And this would also include things like trees and plants, which give us paper and cloth. So, radiocarbon dating is also useful for determining the age of relics, such the Dead Sea Scrolls and the Shroud of Turin.
With radiocarbon dating, the amount of the radioactive isotope carbon is measured. Compared to some of the other radioactive isotopes we have discussed, carbon's half-life of 5, years is considerably shorter, as it decays into nitrogen Carbon is continually being created in the atmosphere due to the action of cosmic rays on nitrogen in the air.
Carbon combines with oxygen to create carbon dioxide. Because plants use carbon dioxide for photosynthesis, this isotope ends up inside the plant, and because animals eat plants, they get some as well. When a plant or an animal dies, it stops taking in carbon The existing carbon within the organism starts to decay back into nitrogen, and this starts our clock for radiocarbon dating.
A scientist can take a sample of an organic material when it is discovered and evaluate the proportion of carbon left in the relic to determine its age. Radiometric dating is a method used to date rocks and other objects based on the known decay rate of radioactive isotopes. The decay rate is referring to radioactive decay , which is the process by which an unstable atomic nucleus loses energy by releasing radiation.
Each radioactive isotope decays at its own fixed rate, which is expressed in terms of its half-life or, in other words, the time required for a quantity to fall to half of its starting value. There are different methods of radiometric dating. Uranium-lead dating can be used to find the age of a uranium-containing mineral.
Uranium decays to lead, and uranium decays to lead The two uranium isotopes decay at different rates, and this helps make uranium-lead dating one of the most reliable methods because it provides a built-in cross-check. Additional methods of radiometric dating, such as potassium-argon dating and rubidium-strontium dating , exist based on the decay of those isotopes. Radiocarbon dating is a method used to determine the age of organic material by measuring the radioactivity of its carbon content. With radiocarbon dating, we see that carbon decays to nitrogen and has a half-life of 5, years.
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Want to watch this again later? What is Radioactive Dating? Principles of Radiometric Dating.
Our ancestors measured the passing of time with water clocks or hourglasses. Nature has none of our modern watches. It measures time -like our ancestors - by using hourglasses provided by radioactivity. In the radioactivity hourglass upper part, that gradually empties, are decaying nuclei. At the bottom part, slowly filling up, are the nuclei resulting from these decays. So a rock can get a very old radiometric age just by having average amounts of potassium and argon.
It seems reasonable to me that the large radiometric ages are simply a consequence of mixing, and not related to ages at all, at least not necessarily the ages of the rocks themselves. It seems to me to be a certainty that water and gas will enter most, if not all, volcanic type rocks through tiny openings and invalidate almost all K-Ar ages.
Rocks are not sealed off from the environment. This contamination would seem to be more and more of a problem the older the rock became. Let me illustrate the circulation patterns of argon in the earth's crust. So argon is being produced throughout the earth's crust, and in the magma, all the time. In fact, it probably rises to the top of the magma, artificially increasing its concentration there. Now, some rocks in the crust are believed not to hold their argon, so this argon will enter the spaces between the rocks.
Leaching also occurs, releasing argon from rocks. Heating of rocks can also release argon. Argon is released from lava as it cools, and probably filters up into the crust from the magma below, along with helium and other radioactive decay products. All of this argon is being produced and entering the air and water in between the rocks, and gradually filtering up to the atmosphere. So this argon that is being produced will leave some rocks and enter others. Different Dating Methods Agree. It is often said that a great many dating methods, used on a single specimen, will agree with each other, thus establishing the accuracy of the date given.
In reality, the overwhelming majority of measurements on the fossil bearing geologic column are all done using one method, the K-Ar method Recall that both potassium and argon are water soluble, and argon a gas is mobile in rock. Thus the agreement found between many dates does not necessarily reflect an agreement between different methods, but rather the agreement of the K-Ar method with itself Especially noting that Dalrymple suggested that only K-Ar dating methods were at all trust worthy.
I have seen no good double-blinded research studies that say otherwise. One would think that if this were a good science, then such studies would be done and published, but they are strangely lacking. Also, specific differences are known and have been known to exist between different dating methods. For example, Isotopic studies of the Cardenas Basalt and associated Proterozoic diabase sills and dikes have produced a geologic mystery.
Using the conventional assumptions of radioisotope dating, the Rb-Sr and K-Ar systems should give concordant "ages". However, it has been known for over 20 years that the two systems give discordant "ages", the K-Ar "age" being significantly younger than the Rb-Sr "age". The "argon reset model" was the first explanation proposed for the discordance. A metamorphic event is supposed to have expelled significant argon from these rocks. The reset model is unable to reconcile the new data, leading to a metamorphic event which is excessively young and inconsistent with the conventional stratigraphic interpretation.
The "argon leakage model" also attempts to explain why these rocks have about half the argon which seems to be required by the Rb-Sr system. The leakage model supposes an incredible improbability. Both the old and new data imply that the rocks leaked argon in nearly exact proportion to the abundance of potassium producing a "leakage isochron", an explanation not supported by a quantity of an appropriate mineral or mesostasis phase. Strong negative correlation between K-Ar model age and K 2 O in the upper portion of the Cardenas Basalt is not easily explained in a consistent manner.
Furthermore, reset and leakage models have difficulty explaining the abundance of initial 36 Ar in the rocks, especially the abundance of 36 Ar in those rocks which supposedly leaked the most 40 Ar. Three alternatives are suggested to the two argon loss models. The "argon inheritance model" and "argon mixing model" simply propose that argon is positively correlated with potassium from its magma source or produced by a mixing process, and that the linear relationship on a plot of 40 Ar versus 40 K is an artifact of the magma, not produced by radioisotope decay within these rocks.
The inheritance of argon seems to be a better model than is the mixing model. All three explanations offered as alternatives to the argon loss models invalidate using the K-Ar system as conventional geochronology would assume. The word "isochron" basically means "same age".
Isochron dating is based on the ability to draw a straight line between data points that are thought to have formed at the same time. The slope of this line is used to calculate an age of the sample in isochron radiometric dating. The isochron method of dating is perhaps the most logically sound of all the dating methods - at first approximation. This method seems to have internal measures to weed out those specimens that are not adequate for radiometric evaluation.
Also, the various isochron dating systems seem to eliminate the problem of not knowing how much daughter element was present when the rock formed. Isochron dating is unique in that it goes beyond measurements of parent and daughter isotopes to calculate the age of the sample based on a simple ratio of parent to daughter isotopes and a decay rate constant - plus one other key measurement.
What is needed is a measurement of a second isotope of the same element as the daughter isotope. Also, several different measurements are needed from various locations and materials within the specimen. This is different from the normal single point test used with the other "generic" methods. To make the straight line needed for isochron dating each group of measurements parent - P, daughter - D, daughter isotope - Di is plotted as a data point on a graph.
The X-axis on the graph is the ratio of P to Di. For example, consider the following isochron graph: Obviously, if a line were drawn between these data points on the graph, there would be a very nice straight line with a positive slope. Such a straight line would seem to indicate a strong correlation between the amount of P in each sample and the extent to which the sample is enriched in D relative to Di.
Obviously one would expect an increase in the ratio of D as compared with Di over time because P is constantly decaying into D, but not into Di. So, Di stays the same while D increases over time. But, what if the original rock was homogenous when it was made? What if all the minerals were evenly distributed throughout, atom for atom?
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What would an isochron of this rock look like? It would look like a single dot on the graph. Because, any testing of any portion of the object would give the same results. The funny thing is, as rocks cool, different minerals within the rock attract certain atoms more than others. Because of this, certain mineral crystals within a rock will incorporate different elements into their structure based on their chemical differences. However, since isotopes of the same element have the same chemical properties, there will be no preference in the inclusion of any one isotope over any other in any particular crystalline mineral as it forms.
So, when put on an isochron graph, each mineral will have the same Y-value. Since a perfectly horizontal line is likely obtained from a rock as soon as it solidifies, such a horizontal line is consistent with a "zero age. Time might still be able to be determined based on changes in the slope of this horizontal line. As time passes, P decays into D in each sample. That means that P decreases while D increases. This results in a movement of the data points. Each data point moves to the left decrease in P and upwards increase in D.
Since radioactive decay proceeds in a proportional manner, the data points with the most P will move the most in a given amount of time. Thus, the data points maintain their linear arrangement over time as the slope between them increases. The degree of slope can then be used to calculate the time since the line was horizontal or "newly formed". The slope created by these points is the age and the intercept is the initial daughter ratio.
The scheme is mathematically sound. The nice thing about isochrons is that they would seem to be able to detect any sort of contamination of the specimen over time. If any data point became contaminated by outside material, it would no longer find itself in such a nice linear pattern. Thus, isochrons do indeed seem to contain somewhat of an internal indicator or control for contamination that indicates the general suitability or unsuitability of a specimen for dating. So, it is starting to look like isochron dating has solved some of the major problems of other dating methods.
However, isochron dating is still based on certain assumptions. All areas of a given specimen formed at the same time. The specimen was entirely homogenous when it formed not layered or incompletely mixed. Limited Contamination contamination can form straight lines that are misleading. Isochrons that are based on intra-specimen crystals can be extrapolated to date the whole specimen. Given these assumptions and the above discussion on isochron dating, some interesting problems arise as one considers certain published isochron dates.
So, what exactly is a whole-rock isochron? Whole-rock isochrons are isochrons that are based, not on intra-rock crystals, but on variations in the non-crystalline portions of a given rock. In other words, sample variations in P are found in different parts of the same rock without being involved with crystalline matrix uptake. This is a problem because the basis of isochron dating is founded on the assumption of original homogeny.
If the rock, when it formed, was originally homogenous, then the P element would be equally distributed throughout. Over time, this homogeny would not change. Thus, any such whole-rock variations in P at some later time would mean that the original rock was never homogenous when it formed. Because of this problem, whole-rock isochrons are invalid, representing the original incomplete mixing of two or more sources. Interestingly enough, whole rock isochrons can be used as a test to see if the sample shows evidence of mixing. If there is a variation in the P values of a whole rock isochron, then any isochron obtained via crystal based studies will be automatically invalid.
The P values of various whole-rock samples must all be the same, falling on a single point on the graph. If such whole-rock samples are identical as far as their P values, mixing would still not be ruled out completely, but at least all available tests to detect mixing would have been satisfied. And yet, such whole-rock isochrons are commonly published. For example, many isochrons used to date meteorites are most probably the result of mixing since they are based on whole-rock analysis, not on crystalline analysis.
There are also methods used to detect the presence of mixing with crystalline isochron analysis. If a certain correlation is present, the isochron may be caused by a mixing. However, even if the correlation is present, it does not mean the isochron is caused by a mixing, and even if the correlation is absent, the isochron could still be caused by a more complex mixing Woodmorappe, , pp. Therefore such tests are of questionable value. Interestingly, mainstream scientists are also starting to question the validity of isochron dating. The determination of accurate and precise isochron ages for igneous rocks requires that the initial isotope ratios of the analyzed minerals are identical at the time of eruption or emplacement.
Studies of young volcanic rocks at the mineral scale have shown this assumption to be invalid in many instances. Variations in initial isotope ratios can result in erroneous or imprecise ages. Nevertheless, it is possible for initial isotope ratio variation to be obscured in a statistically acceptable isochron. Independent age determinations and critical appraisal of petrography are needed to evaluate isotope data.
If initial isotope ratio variability can be demonstrated, however, it can be used to constrain petrogenetic pathways. But then,] The cooling history will depend on the volume of magma involved and its starting temperature, which in turn is a function of its composition. If the initial variation is systematic e.
In short, isochron dating is not the independent dating method that it was once thought. As with the other dating methods discussed already, isochron dating is also dependent upon "independent age determinations". Isochrons have been touted by the uniformitarians as a fail-safe method for dating rocks, because the data points are supposed to be self-checking Kenneth Miller used this argument in a debate against Henry Morris years ago.
Now, these geologists, publishing in the premiere geological journal in the world, are telling us that isochrons can look perfect on paper yet give meaningless ages, by orders of magnitude, if the initial conditions are not known, or if the rocks were open systems at some time in the past?! That sounds like what young earth creationists have been complaining about all along. But then, these geologists put a happy face on the situation. The problem is that it is starting to get really difficult to find a truly independent dating method out of all the various dating methods available.
Furthermore, because most upper crustal rocks cooled below annealing temperatures long after their formation, early formed lead rich in Pb is locked in annealed sites so that the leachable component is enriched in recently formed Pb The isotopic composition of the leachable lead component then depends more on the cooling history and annealing temperatures of each host mineral than on their geological age; and the axiom that Pb isotopes cannot be fractionated in the natural environment, is invalid.
Although these experiments are based on a strong Hf attack on zircons, we believe, given the widespread U anomalies of several hundred percent observed in groundwater Osmond and Cowart , that they apply to the differential mobility of radiogenic Pb isotopes on a local and global scale. Also, consider the following excerpt concerning ancient zirons from the Gabbro-Peridotite Complex of the Mar: Zircon age calculations on the base of Upb systematics have been complicated by high share of common Pb and uncertainty of its isotope composition.
Common lead was captured in the process of zircon crystallization, perhaps, by mineral and fluid inclusions. But there is a small share of inherited zircon substance with the age of 3. Thus, the discordia itself obtained by us is interpreted as a result of mixture of newly formed young zircon with some share of Archean zircon presented in each studied crystal.
Also, if errors for individual zircon tests are too large, these values are simply discarded. This enhances the mobility of U and especially Pb. So, how confident can one be in zircon dates who's published Pb levels range from very high to very low? It seems to me that quite often published U-Pb and Pb-Pb dates do in fact involve fairly significant Pb levels.
Radiometric dating - Wikipedia
Of course, if the level of Pb is too high, the data obtained is not calibrated, but is simply discarded. Doesn't this mess up the idea that all lead in zircons must be the result of radioactive decay? It is also of interest in regard to radiometric dating that Robert Gentry claims to have found "squashed" polonium haloes as well as embryonic uranium radiohaloes in coal deposits from many geological layers claimed to be hundreds of millions of years old. These haloes represent particles of polonium and uranium, which penetrated into the coal at some point and produced a halo by radioactive decay.
The fact that they are squashed indicates that part of the decay process began before the material was compressed, so the polonium had to be present before compression. Since coal is relatively incompressible, Gentry concludes that these particles of uranium and polonium must have entered the deposit before it turned to coal. However, there is only a very small amount of lead with the uranium; if the uranium had entered hundreds of millions of years ago, then there should be much more lead. However, it's just hard to believe, according to conventional geological time scales, that this coal was compressed any time within the past several thousand or even hundred million years.
Some have argued that "radon that results from uranium decay is an inert gas and may have escaped, resulting in little lead being deposited. This would make the observed haloes consistent with an old age for the coal. In addition, not all of the radon would be on the surface of the particles of uranium. That which was inside or bordering on coal would likely not be able to escape. Since radon has a half-life of about 4 days, it would not have much time to escape, in any event.
What happens when something is dated as being very old, but shows little or no physical signs of relative aging? This basalt group is rather large covering an area of , square kilometers and fills a volume of , cubic kilometers. The vast extent and sheer volume of such individual flows are orders of magnitude larger than anything ever recorded in known human history. Within this group are around individual lava flows each of rather uniform thickness over many kilometers with several extending up to kilometers from their origin. Now, the problem with the idea that these flows span a period of over 11 million years of deposition is that there is significant physical evidence that the CRBG flows were deposited relatively rapidly with respect to each other and with themselves.
The average time between each flow works out to around 36, years, but where is the erosion to the individual layers of basalt that one would expect to see after 36, years of exposure? The very fact that these flows cover such great distances indicate that the individual flows traveled at a high rate of speed in order to avoid solidification before they covered such huge areas as they did. Also, there are several examples where two or three different flows within the CRBG mix with each other.
This suggests that some of the individual flows did not have enough time to solidify before the next flow s occurred. If some 36, years of time are supposed to separate each of the individual flows where is the evidence of erosion in the form of valleys or gullies cutting into the individual lava flows to be filled in by the next lava flow? There are no beds of basalt boulders that would would expect to be formed over such spans of time between individual flows. However, a recent real time study by Riebe et.
Over the course of 36, years this works out to between 6 to 7 meters 19 to 23 feet of vertical erosion. This is significant erosion and there should be evidence of this sort of erosion if the time gap between flow was really 36, years. So, where is this evidence?
For several other such flows in the United States and elsewhere around the world the time intervals between flows are thought to be even longer - and yet still there is little evidence of the erosion that would be expected after such passages of time. For example, the Lincoln Porphyry of Colorado was originally thought to be a single unit because of the geographic proximity of the outcrops and the mineralogical and chemical similarities throughout the formation. Later, this idea was revised after radiometric dating placed various layers of the Lincoln Porphyry almost 30 million years apart in time.
But how can such layers which show little if any evidence of interim erosion have been laid down thousands much less millions of years apart in time? Other examples, such as the Garrawilla Lavas of New South Wales, Australia, are found between the Upper Triassic and Jurassic layers and yet these lavas, over a very large area, grade imperceptibly into lavas which overlie Lower Tertiary sedimentary rock supposedly laid down over million years later.
The Napperby depositional sequence represents the upper limit of the Gunnedah Basin sequence, with a regional unconformity existing between the Triassic and overlying Jurassic sediments of the Surat Basin north of the Liverpool Ranges. The Gunnedah Basin sequence includes a number of basic intrusions of Mesozoic and Tertiary rocks. These are associated with massive extrusions of the Garrawilla Volcanic complex and the Liverpool, Warrumbungle and Nandewar Ranges. Also, throughout the CRBG and elsewhere are found "pillow lava" and palagonite formations - especially near the periphery of the lava flows.
There are a few outcrops where tens of meters of vertical outcrop and hundreds of meters of horizontal outcrop consist entirely of pillow structures. Also, palagonite, with a greenish-yellow appearance produced via the reaction of hot lava coming in contact with water, is found throughout. These features are suggestive of lava flow formation in a very wet or even underwater environment.
Certainly pillow lavas indicate underwater deposition, but note that lavas can be extruded subaqeously without the production of pillow structures. The potential to form pillow lava decreases as the volume of extruded lava increases. Thus, the effective contact area between lava and water where pillow formations can potentially form becomes proportionately smaller as the volume of lava extruded becomes larger.
Other evidences of underwater formation include the finding of fresh water fossils such as sponge spicules, diatoms, and dinoflagellates between individual lava flows. Consider some interesting conclusions about these findings by Barnett and Fisk in a paper published in the journal, Northwest Science: The Palouse Falls palynoflora reflects reasonably well the regional climatic conditions as evidence by the related floras of the Columbia Plateau.
The presence of planktonic forms, aquatic macrophytes, and marsh plants indicates that deposition of the sediments took place in a body of water, probably a pond or lake. This interpretation is supported by the presence of abundant diatoms. The general decrease in aquatic plants and increase in forest elements upward in the section suggest a shallowing or infilling of the pond or lake, perhaps due to increased volcanic activity and erosion of ash from the surrounding region. Supporting this view is the presence of thin bands of lignite near the top of the section, with a cm coal layer just underlying the capping basalt.
Now, what is interesting here is that these "forest elements" to include large lenses of fossilized wood are widely divergent in the type of preserved wood found. It is interesting that hundreds of species are found all mixed up together ranging from temperate birch and spruce to subtropical Eucalyptus and bald cypress. The petrified logs have been stripped of limbs and bark and are generally found in the pillow complexes of the basaltic flows, implying that water preserved the wood from being completely destroyed by the intense heat of the lava as it buried them.
For Barnett and Fisk to suggest that the finding of such fossil remains suggest the presence of a small pond or lake being filled in by successive flows just doesn't seem to add up. How are such ecologically divergent trees going to get concentrated around an infilling pond or lake? Also, how is a 10cm layer of coal going to be able to form under the "capping basalt"?
It is supposed to take very long periods of time, great pressure, heat, and moisture to produce coal. How did this very thin layer of coal form and then be preserved without evidence of any sort of uneven erosion by a relatively thin layer of capping basalt? Also, numerous well-rounded quartzite gravel, cobbles, and boulders locally interbedded within and above the basalt flows. Does this make any sense? It seems more likely that huge shortly spaced watery catastrophes were involved in formation of many of these features - concentrating and transporting mats of widely divergent vegetation and inorganic rocks over long distances before they were buried by shortly spaced lava flows traveling rapidly over huge areas.
Lava traveling rapidly under water would experience rapid surface cooling and fracturing of this surface "skin". As it turns out, entablatures and colonnades are a common structural feature of basalts. These features are named by analogy to the respective horizontal and vertical architectural structures. Some have hypothesized that as water cools the outer "skin" of the molten lava a thin crust is rapidly formed. Then, the large temperature gradient between the crust above and the molten lava below creates tensional stresses that crack the crust which allow water to percolate through these cracks to come in contact with more molten lava and form another crust, which then cracks.
In the end, this rapid cyclical cooling process produces a thick slab of rock with columnar jointing. One other evidence of fairly rapid cooling is the finding that these basalts contain relatively small crystals. When magma cools, crystals form because the solution is super-saturated with respect to some minerals.