Radioactive dating methods


Potassium - Argon and Argon - Argon dating are based on the current understanding that radioactive Potassium decays to the stable form, Argon with a half-life of approximately 1. The same principle holds true for the other isotope dating methods. Radioactive decay occurs at a constant exponential or geometric rate. The rate of decay is proportional to the number of parent atoms present. There are some circumstances that can affect this rate such as magnetic fluctuations etc But in general, this rate is felt by the vast majority of mainstream scientists to be a fundamental constant.

That was until August of Jenkins et. In other words, the decay rates show annual changes that closely reflect the Earth's distance from the Sun see illustration. If magnetic fluxuations or other influencing forces are strong enough, radiometric decay rates could be much more significantly effected. In short, the assumption that decay rates are immune to outside influences isn't as solid as it once appeared to be.

For example, if there are equal amounts of parent and daughter isotopes, then one half-life has passed. If there are three times as many daughter isotopes as parent, then two half-lives have passed, and so on. Most minerals, which contain radioactive isotopes, are in igneous rocks. The majority of scientists today assume that the dates they give indicate the time the magma cooled. This also assumes that there was no initial daughter isotopes contained in the magma at the time of cooling.

The assumption is that at least a great majority of the isotope present was the parent isotope. This parent isotope then degraded to the daughter isotope over time. Consider the following statement by Dalrymple, a well-known geologist: This is because 40 Ar is an inert gas that does not combine chemically with any other element and so escapes easily from rocks when they are heated.

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Thus, while a rock is molten, the 40 Ar formed by the decay of 40 K escapes from the liquid. So, according to Dalrymple, K-Ar or Ar-Ar are the only methods that have little or no concern for the presence of initial daughter isotopes. This means that all the other radioisotope-dating methods excepting isochron methods are brought into serious question. The reason for this is because unless the initial ratio of parent to daughter isotope is known, the current ratio would be worthless as a means of determining elapsed time. A rock cannot be said to be millions or billions of years old if there is no way of knowing what the original composition of the rock was at the time that it was formed.

The assumption for the K-Ar method is that all argon escapes at the time of rock formation because argon is a gas while potassium is not. Likewise, the other non-isochron dating methods, such as uranium-lead, also fall short because who is to say wh en the "zero date" was when there was only parent isotope and no daughter?

Because of this problem, it might be a significant error to simply assume that all original isotopes present in a given rock were parent isotopes. This assumption has been shown to be faulty. Lets now consider how fossils are dated with many of these methods, such as the potassium-argon method. The mineralized fossils themselves are not directly datable by radiometric techniques. The sedimentary rock that buried them is also not datable. It is assumed then that the fossil is as old as the igneous rock fragment that it is buried with. Aside from the zero-date problems noted above, one might consider the possibility that the fossil might not be as old as the sediment that buried it in the first place.

For example, lets say that my pet dog dies. I decide to bury it in the back yard. Is the dog as old as the dirt that I buried it in? Likewise, who is to say that some fossils were not buried in sedimentary material that was weathered from significantly more ancient formations?

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Potassium-Argon and Argon-Argon Dating. Since Potassium-Argon and Argon-Argon dating techniques are the most common and are considered, even by geologists, to be among the most accurate of all the radioisotope dating methods, lets consider these in particular detail. Argon is a noble gas. The main isotopes of argon in terrestrial systems are 40 Ar Naturally occurring 40 K decays to stable 40 Ar Minerals are dated by measurement of the concentration of potassium, and the amount of radiogenic 40 Ar that has accumulated. For example, if 40 Ar is lost by diffusion while the rock cooled, the age-dates represent the time elapsed since the rock cooled sufficiently for diffusive losses to be insignificant.

Or, if excess 40 Ar is present in the rock, the calculated age-dates are too old. Radioactive 39 Ar decays back to 39 K by beta emission with a half-life of years, but the decay is slow compared to the analysis time and can be ignored Faure, How is this calibrated? Also, even if the argon-argon dating method does eliminate the "contamination" problem, it does not solve the problem of original argon.

Did the clock get reset to zero when the volcano erupted? Or, was there some argon trapped in the rocks originally? It must be first calibrated against a sample of "known age". Recent testing of volcanic material from Mt. Calibration Against Pliny the Younger was written by P. Renne tested Ar-Ar dating by checking it against the 79 A.

Radiometric Dating

The true age was years. The test was off only 7 years. The conclusions of Renne and his team read as follows: Of note however is that this test was not double blinded, and the number of such tests is not statistically significant as far as scientific analysis is concerned. Although interesting, it is basically a case study report, and as such it has very little scientific weight as far as statistical predictability. In the first place, I am not primarily concerned with dating meteorites, or Precambrian rocks. I will begin this section with a short discussion from Andrew Snelling, an associate professor of geology in El Cajon, California.

However, it is well established that volcanic rocks e. If so, then the K-Ar and Ar-Ar "dating" of crustal rocks would be similarly questionable. In other experiments muscovite was synthesized from a colloidal gel under similar temperatures and Ar pressures, the resultant muscovite retaining up to 0. This is approximately 2, times as much Ar as is found in natural muscovite. Thus under certain conditions Ar can be incorporated into minerals which are supposed to exclude Ar when they crystallize. Thus all K-Ar and Ar-Ar "dates" of crustal rocks are questionable, as well as fossil "dates" calibrated by them.

In summary, many scientists assume that since argon is a gas, all of it should have escaped from the lava before it cooled. Therefore, all the 40 Ar in the rock should be the result of decay from potassium. Based on the measured potassium, argon, and the decay rate, they calculate an age. That is why it does not matter how long the magma was in the volcano before it erupted. They believe that when the volcano erupts, all the 40 Ar escapes, and the atomic clock gets reset to zero.

If all the argon escaped from hot lava of volcanoes that erupted long ago, then all the argon should escape from the hot lava of volcanoes that erupt in modern times too. But modern lava does have 40 Ar in it. This is known as the "excess argon problem". Scientists are well aware of this problem and use various calibration methods to "correct" for this problem. However, how are these calibration methods established? Upon what basis are they validated? Calibration of the Argon-Argon Dating Method. Let me emphasize again that this dating method is a relative dating method. In other words, it must be calibrated relative to a different dating method before it can be used to date materials relative to that other dating method.

This same problem exists for all other relative radiometric dating techniques. Fission track dating is a radioisotopic dating method that depends on the tendency of uranium Uranium to undergo spontaneous fission as well as the usual decay process. The large amount of energy released in the fission process ejects the two nuclear fragments into the surrounding material, causing damage paths called fission tracks. These tracks can be made visible under light microscopy by etching with an acid solution so they can then be counted. The usefulness of this as a dating technique stems from the tendency of some materials to lose their fission-track records when heated, thus producing samples that contain fission-tracks produced since they last cooled down.

The useful age range of this technique is thought to range from years to million years before present BP , although error estimates are difficult to assess and rarely given. Generally it is thought to be most useful for dating in the window between 30, and , years BP. A problem with fission-track dating is that the rates of spontaneous fission are very slow, requiring the presence of a significant amount of uranium in a sample to produce useful numbers of tracks over time.

Additionally, variations in uranium content within a sample can lead to large variations in fission track counts in different sections of the same sample. Because of such potential errors, most forms of fission track dating use a form of calibration or "comparison of spontaneous and induced fission track density against a standard of known age.

The principle involved is no different from that used in many methods of analytical chemistry, where comparison to a standard eliminates some of the more poorly controlled variables. In the zeta method, the dose, cross section, and spontaneous fission decay constant, and uranium isotope ratio are combined into a single constant. Of course, this means that the fission track dating method is not an independent method of radiometric dating, but is dependent upon the reliability of other dating methods.

The reason for this is also at least partly due to the fact that the actual rate of fission track production.

Some experts suggest using a rate constant of 6. Wagner, Letters to Nature , June 16, In other words, the actual rate of fission track production isn't really known, nor is it known if this rate can be affected by various concentrations of U or other physical factors.

For example, all fission reactions produce neutrons. What happens if fission from some other radioactive element, like U or some other radioisotope, produces tracks? Might not these trackways be easily confused with those created by fission of U ? The human element is also important here. Fission trackways have to be manually counted. This is problematic since interpreting what is and what is not a true trackway isn't easy. Geologists themselves recognize the problem of mistaking non-trackway imperfections as fission tracks.

For example, it is recommended that one choose samples with as few vesicles and microlites as possible. But, how is one to do this if they are so easily confused with true trackways? Fortunately, there are a few other "hints". True tracks are straight, never curved. They also tend to show characteristic ends that demonstrate "younging" of the etched track.

True tracks are thought to form randomly and have a random orientation. Therefore, trackways that show a distribution pattern tend not to be trusted as being "true". Certain color and size patterns within a certain range are also used as helpful hints. This is yet another reason why calibration with other dating techniques is used in fission track dating. It just isn't very reliable or accurate by itself.

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And, it gets even worse. Fairly recently, Raymond Jonckheere and Gunther Wagner American Minerologist, published results showing that there are two kinds of real fission trackways that had "not been identified previously. As it turns out, the "stable tracks do not shorten significantly even when heated to temperatures well above those normally sufficient for complete annealing of fission tracks. The tracks through fluid are also interesting. They are "excessively long". This is because a fission fragment traveling through a fluid inclusion does so without appreciable energy loss.

Such features, if undetected, "can distort the temperature-time paths constructed on the basis of confined fission-track-length measurements. These problems have resulted in several interesting contradictions, despite calibration. For example, Naeser and Fleischer Harvard University showed that, depending upon the calibration method chosen, the calculated age of a given rock from Cerro de Mercado, Mexico in this case could be different from each other by a factor of " sixty or more " - - "which give geologically unreasonable ages.

In addition, published data concerning the length of fission tracks and the annealing of minerals imply that the basic assumptions used in an alternative procedure, the length reduction-correction method, are also invalid for many crystal types and must be approached with caution unless individually justified for a particular mineral. No wonder the authors recommend only going with results that do not provide "geologically unreasonable ages". Another example of this sort of aberrancy comes in the form of glass globs known as "tektites". Tektites are thought to be produced when a meteor impacts the Earth.

When the massive impact creates a lot of heat, which melts the rocks of the Earth and send them hurtling through the atmosphere at incredible speed. As these fragments travel through the atmosphere, they become superheated and malleable as they melt to a read-hot glow, and are formed and shaped as they fly along.

It is thought that the date of the impact can be dated by using various radiometric dating methods to date the tektites. For example, Australian tektites known as australites show K-Ar and fission track ages clustering around , years. The problem is that their stratigraphic ages show a far different picture. Edmund Gill, of the National Museum of Victoria, Melbourne, while working the Port Campbell area of western Victoria uncovered 14 australite samples in situ above the hardpan soil zone.

This zone had been previously dated by the radiocarbon method at seven locales, the oldest dating at only 7, radiocarbon years Gill Charcoal from the same level as that containing specimen 9 yielded a radiocarbon age of 5, years. The possibility of transport from an older source area was investigated and ruled out. Since the "Port Campbell australites include the best preserved tektites in the world Aboriginal implements have been discovered in association with the australites.

A fission-track age of , years and a K-Ar age of , years for these same australites unavoidably clashes with the obvious stratigraphic and archaeological interpretation of just a few thousand years. Commenting on the above findings by Lovering and his associates, the editors of the book, Tektites, state that, "in this paper they have built an incontrovertible case for the geologically young age of australite arrival on earth" Barnes and Barnes , p. The argument that various radiometric dating methods agree with each other isn't necessarily true. Here we have the K-Ar and fission track dating methods agreeing with each other, but disagreeing dramatically with the radiocarbon and historical dating methods.

These findings suggest that, at least as far as tektites are concerned, the complete loss of 40 Ar and therefore the resetting of the radiometric clock may not be valid Clark et al. It has also been shown that different parts of the same tektite have significantly different K-Ar ages McDougall and Lovering, This finding suggests a real disconnect when it comes to the reliability of at least two of the most commonly used radiometric dating techniques.

In short, it seems like fission track dating is tenuous a best - even when given every benefit of the doubt. It is just too subjective and too open to pitfalls in interpretation to be used as any sort of independent measure of estimating elapsed time. There is a methodological problem connected with the manner in which geologists infer the argon-retention abilities of different minerals. Concerning the suitability of different minerals for K-Ar dating, Faure , p. By comparing the K-Ar dates yielded by such minerals with the expected ones.

Thus the correctness of the geologic time scale is assumed in deciding which minerals are suitable for dating. For example, concerning the use of glauconies for K-Ar dating, Faure , p. Therefore, K-Ar dates of 'glauconite' have often been regarded as minimum dates that underestimate the depositional age of their host. It is also interesting that Faure , pp. However, if these "known" ages are incorrect, then fission track dating that is based on these ages is also incorrect. Thus fission track dating is not an independent test that helps to verify the accuracy of other tests.

The result is that radiometric dating in general is in danger of being based on circular reasoning.

Radiometric dating

Inconsistencies and other Problems with various Radiometric Dating Techniques. Raul Esperante teamed up with Dr. This formation is approximately meters thick and consists of many layers of sedimentary rock. Yet, within essentially all of these layers are hundreds of very well preserved fossil whales. In fact, many of them are so well preserved that their baleen is still intact and attached in the usual position that baleen is attached in living whales. Usually baleen detaches within a few days or even hours after death.

Some of the fossilized whales and dolphins also have preserved remains of skin outlines around the fossilized bones. The skeletons themselves are generally well articulated and show no evidence of scavenging or significant decay. The fossil whales must have died and been completely buried by diatomaceous sediment within a very short time of death no scavenging, decay, significant disarticulation, or loss of baleen.

The layers are very smooth without significant erosion or unevenness to suggest the passage of time between layers. There is no significant bioturbation very few tunnels or evidence of trace fossils or digging within the sedimentary layers that would be expected given long periods of time between the formation of subsequent layers. There are finely preserved shards of volcanic glass within all of the layers that have very sharp edges without the usual rounding that would be expected due to the relatively rapid ability of water to dissolve silica if long periods of time took place during the build up of these sedimentary layers.

These layers were deposited in shallow seas with evidence of flowing currents, which works against the potential counter-hypothesis that these layers were formed under anoxic conditions. Cosmogenic nuclides are isotopes that are produced by interaction of cosmic rays with the nucleus of the atom. The various isotopes produced have different half lives see table. Cosmogenic dating using these isotopes are becoming a popular way to date the time of surface exposure of rocks and minerals to cosmic radiation.

While the idea is fairly straightforward, there are just a few problems with this dating method. To illustrate this problem, consider that 3 H dating has been used to establish the theory that the driest desert on Earth, Coastal Range of the Atacama desert in northern Chile which is 20 time drier than Death Valley has been without any rain or significant moisture of any kind for around 25 million years. The only problem with this theory is that recently investigators have discovered fairly extensive deposits of very well preserved animal droppings associated with grasses as well as human-produced artifacts like arrowheads and the like.

Radiocarbon dating of these finding indicate very active life in at least semiarid conditions within the past 11, years - a far cry from 25 million years. As it turns out, cosmogenic isotope dating has a host of problems. The production rate is a huge issue. Production rates depend upon several factors to include "latitude, altitude, surface erosion rates, sample composition, depth of sample, variations of cosmic and solar ray flux, inclusion of other radioactive elements and their contribution to target nucleotide production, variations in the geomagnetic field, muon capture reactions, various shielding effects, and, of course, the reliability of the calibration methods used.

So many variables become somewhat problematic. This problem has been highlighted by certain studies that have evaluated the published production rates of certain isotopes which have been published by different groups of scientists. At least regarding 36 Cl in particular, there has been "no consistent pattern of variance seen between each respective research group's production rates.

In short, "different analytical approaches at different localities were used to work out 36 Cl production rates, which are discordant. So, what are the possible explanations for this "discordance"? Uncertainty in the independent chronology used to determine the age of surfaces used to calibrate a Cl production rate ex. There are 3 different latitude-altitude scaling systems in use worked out by different researchers. Whole rock analysis vs. It seems that the whole rock analysis method and the resulting optimization problem may underestimate the significance of other production pathways, i.

Fe and Ti spallation? Doesn't give one a great deal of confidence in the unbiased reliability of cosmogenic isotopic dating techniques - does it? Different Methods for Dating the Himalayan Mountains. The Himalayan mountains are said by most modern scientists to have started their uplift or orogeny some 50 million years ago. However, recently in Yang Wang et. Dalrymple's work early work on 26 historic lava flows showed that many of them had excess argon and were not set to zero at the eruption of the volcano. The following is the data from these tests: If the present data are representative, argon of slightly anomalous composition can be expected in approximately one out of three volcanic rocks.

Dalrymple may have a point. It seems like rocks dating within one or two million years cannot be accurately dated by K-Ar techniques just because of the relatively wide ranges of error. However, can rocks that are tens or hundreds of millions of years be more accurately dated? Perhaps, if these rocks were in fact closed systems and were not subject to contamination by external argon. Investigators also have found that excess 40 Ar is trapped in the minerals within lava flows. The obvious conclusion most investigators have reached is that the excess 40 Ar had to be present in the molten lavas when extruded, which then did not completely degas as they cooled, the excess 40 Ar becoming trapped in constituent minerals and the rock fabrics themselves.

However, from whence comes the excess 40 Ar, that is, 40 Ar which cannot be attributed to atmospheric argon or in situ radioactive decay of 40 K? It is not simply "magmatic" argon? Funkhouser and Naughton found that the excess 40 Ar in the Hualalai flow, Hawaii, resided in fluid and gaseous inclusions in olivine, plagioclase, and pyroxene in ultramafic xenoliths in the basalt, and was sufficient to yield "ages" of 2. Many recent studies confirm the mantle source of excess 40 Ar.

Hawaiian volcanism is typically cited as resulting from a mantle plume, most investigators now conceding that excess 40 Ar in the lavas, including those from the active Loihi and Kilauea volcanoes, is indicative of the mantle source area from which the magmas came. Considerable excess 40 Ar measured in ultramafic mantle xenoliths from Kerguelen Archipelago in the southern Indian Ocean likewise is regarded as the mantle source signature of hotspot volcanism. Further confirmation comes from diamonds, which form in the mantle and are carried by explosive volcanism into the upper crust and to the surface.

When Zashu et al. The conventional K-Ar dating method was applied to the dacite flow from the new lava dome at Mount St. Porphyritic dacite which solidified on the surface of the lava dome in gives a whole rock K-Ar 'age' of 0. Mineral concentrates from the dacite which formed in give K-Ar 'ages 'from 0. These dates are, of course, preposterous.

The fundamental dating assumption no radiogenic argon was present when the rock formed is brought into question. Instead, data from the Mount St. Helens dacite argue that significant "excess" argon was present when the lava solidified in Phenocrysts of orthopyroxene, hornblende and plagioclase are interpreted to have occluded argon within their mineral structures deep in the magma chamber and to have retained this argon after emplacement and solidification of the dacite.

Orthopyroxene retains the most argon, followed by hornblende, and finally, plagioclase. The lava dome at Mount St. Helens dates very much older than its true age because phenocryst minerals inherit argon from the magma. The study of this Mount St. Helens dacite brings yet another question to mind: How accurate are K-Ar "ages" from the many other phenocryst-containing lava flows world-wide? Potassium is about 2. Argon is about 3. We can assume then that the magma is probably about 2. Now, Lets say we are trying to date a one billion year old rock. How much of it would be 40 K? This would leave us with a 0.

This gives about 0. This is about one ten millionth of the mass of the rock, a very tiny fraction. If the rock weighed one gram, the Ar in the rock would weight one ten millionth of a gram. And yet, with a relatively large amount of argon in the air, argon filtering up from rocks below, excess argon in lava, the fact that argon and potassium are water soluble, and the fact that argon is mobile in rock and is a gas, we are still expecting this wisp of argon gas to tell us how old the rock is? The percentage of 40 Ar is even less for younger rocks. For example, it would be about one part in million for rocks in the vicinity of million years old.

However, to get just one part in 10 million of argon in a rock in a thousand years, we would only need to get one part in 10 billion entering the rock each year. This would be less than one part in a trillion entering the rock each day, on the average. This would suffice to give a rock an average computed potassium-argon age of over a billion years.

Some geochronologists believe that a possible cause of excess argon is that argon diffuses into certain minerals progressively with time and pressure. Significant quantities of argon may be introduced into a mineral even at pressures as low as one bar. We can also consider the average abundance of argon in the crust.

This implies a radiometric age of over 4 billion years. 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.

Radiometric Dating: Methods, Uses & the Significance of Half-Life

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. Two isotopes of Uranium and one isotope of Th are radioactive and decay to produce various isotopes of Pb.

The decay schemes are as follows. Note that the present ratio of. If these two independent dates are the same, we say they are concordant. We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates. The Concordia curve can be calculated by defining the following:. Zircon has a high hardness 7. Zircon can also survive metamorphism. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced. Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.

Discordant dates will not fall on the Concordia curve. Sometimes, however, numerous discordant dates from the same rock will plot along a line representing a chord on the Concordia diagram. Such a chord is called a discordia. We can also define what are called Pb-Pb Isochrons by combining the two isochron equations 7 and 8. Since we know that the , and assuming that the Pb and Pb dates are the same, then equation 11 is the equation for a family of lines that have a slope.

The answer is about 6 billion years. This argument tells when the elements were formed that make up the Earth, but does not really give us the age of the Earth. It does, however, give a maximum age of the Earth. Is this the age of the Earth? Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Modern Oceanic Pb - i. Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon.

Thus, our best estimate of the age of the Earth is 4.

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The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. Since K is one of the 10 most abundant elements in the Earth's crust, the decay of 40 K is important in dating rocks. But this scheme is not used because 40 Ca can be present as both radiogenic and non-radiogenic Ca. Since Ar is a noble gas, it can escape from a magma or liquid easily, and it is thus assumed that no 40 Ar is present initially.

Note that this is not always true. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped. If this happens, then the date obtained will be older than the date at which the magma erupted. For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar. Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years. The dating equation used for K-Ar is: Some of the problems associated with K-Ar dating are Excess argon.

This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Minerals should not contain any excess Ar because Ar should not enter the crystal structure of a mineral when it crystallizes. Thus, it always better to date minerals that have high K contents, such as sanidine or biotite. If these are not present, Plagioclase or hornblende.

If none of these are present, then the only alternative is to date whole rocks. Some 40 Ar could be absorbed onto the sample surface. This can be corrected for. Most minerals will lose Ar on heating above o C - thus metamorphism can cause a loss of Ar or a partial loss of Ar which will reset the atomic clock. If only partial loss of Ar occurs then the age determined will be in between the age of crystallization and the age of metamorphism. If complete loss of Ar occurs during metamorphism, then the date is that of the metamorphic event.

The problem is that there is no way of knowing whether or not partial or complete loss of Ar has occurred. Examples of questions on this material that could be asked on an exam. Prior to the best and most accepted age of the Earth was that proposed by Lord Kelvin based on the amount of time necessary for the Earth to cool to its present temperature from a completely liquid state. Principles of Radiometric Dating Radioactive decay is described in terms of the probability that a constituent particle of the nucleus of an atom will escape through the potential Energy barrier which bonds them to the nucleus.

Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 half-life there will be 0. Some examples of isotope systems used to date geologic materials. To see how we actually use this information to date rocks, consider the following: To account for this, we first note that there is an isotope of Sr, 86 Sr, that is: If we divide equation 4 through by the amount of 86 Sr, then we get: Note also that equation 5 has the form of a linear equation, i.

How can we use this? In nature, however, each mineral in the rock is likely to have a different amount of 87 Rb. Thus, once the rock has cooled to the point where diffusion of elements does not occur, the 87 Rb in each mineral will decay to 87 Sr, and each mineral will have a different 87 Rb and 87 Sr after passage of time. The Concordia curve can be calculated by defining the following: The discordia is often interpreted by extrapolating both ends to intersect the Concordia.

Pb leakage is the most likely cause of discordant dates, since Pb will be occupying a site in the crystal that has suffered radiation damage as a result of U decay. U would have been stable in the crystallographic site, but the site is now occupied by by Pb.