Radiocarbon dating - Wikipedia
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The possible confounding effects of contamination of parent and daughter isotopes have to be considered, as do the effects of any loss or gain of such isotopes since the sample was created. It is therefore essential to have as much information as possible about the material being dated and to check for possible signs of alteration. Alternatively, if several different minerals can be dated from the same sample and are assumed to be formed by the same event and were in equilibrium with the reservoir when they formed, they should form an isochron.
This can reduce the problem of contamination. In uranium—lead datingthe concordia diagram is used which also decreases the problem of nuclide loss. Finally, correlation between different isotopic dating methods may be required to confirm the age of a sample. For example, the age of the Amitsoq gneisses from western Greenland was determined to be 3. The procedures used to isolate and analyze the parent and daughter nuclides must be precise and accurate.
This normally involves isotope-ratio mass spectrometry. For instance, carbon has a half-life of 5, years. After an organism has been dead for 60, years, so little carbon is left that accurate dating cannot be established.
On the other hand, the concentration of carbon falls off so steeply that the age of relatively young remains can be determined precisely to within a few decades. Closure temperature If a material that selectively rejects the daughter nuclide is heated, any daughter nuclides that have been accumulated over time will be lost through diffusionsetting the isotopic "clock" to zero.
The temperature at which this happens is known as the closure temperature or blocking temperature and is specific to a particular material and isotopic system. These temperatures are experimentally determined in the lab by artificially resetting sample minerals using a high-temperature furnace.
As the mineral cools, the crystal structure begins to form and diffusion of isotopes is less easy. At a certain temperature, the crystal structure has formed sufficiently to prevent diffusion of isotopes.
This temperature is what is known as closure temperature and represents the temperature below which the mineral is a closed system to isotopes. Thus an igneous or metamorphic rock or melt, which is slowly cooling, does not begin to exhibit measurable radioactive decay until it cools below the closure temperature. The age that can be calculated by radiometric dating is thus the time at which the rock or mineral cooled to closure temperature.
This field is known as thermochronology or thermochronometry. The age is calculated from the slope of the isochron line and the original composition from the intercept of the isochron with the y-axis. The equation is most conveniently expressed in terms of the measured quantity N t rather than the constant initial value No. The above equation makes use of information on the composition of parent and daughter isotopes at the time the material being tested cooled below its closure temperature.
This is well-established for most isotopic systems. Plotting an isochron is used to solve the age equation graphically and calculate the age of the sample and the original composition. Modern dating methods[ edit ] Radiometric dating has been carried out since when it was invented by Ernest Rutherford as a method by which one might determine the age of the Earth.
In the century since then the techniques have been greatly improved and expanded. The mass spectrometer was invented in the s and began to be used in radiometric dating in the s. It operates by generating a beam of ionized atoms from the sample under test. The ions then travel through a magnetic field, which diverts them into different sampling sensors, known as " Faraday cups ", depending on their mass and level of ionization. On impact in the cups, the ions set up a very weak current that can be measured to determine the rate of impacts and the relative concentrations of different atoms in the beams.
Uranium—lead dating method[ edit ] Main article: Uranium—lead dating A concordia diagram as used in uranium—lead datingwith data from the Pfunze BeltZimbabwe.
Is Carbon Dating Accurate?
This scheme has been refined to the point that the error margin in dates of rocks can be as low as less than two million years in two-and-a-half billion years. Zircon has a very high closure temperature, is resistant to mechanical weathering and is very chemically inert. Now, I can only speak for myself here, but for me the weight of evidence and credibility remains firmly on the side of the Bible.
All radiometric dating methods are based on one basic concept. That is, radioactive elements decay at a constant rate into other elements — like a very steady and reliable clock. However, from what is known so far, the degree of variation caused by these factors appears to be fairly minimal.
So, the ticking of the clock itself still remains fairly predictable and therefore useful as a clock. Of course, in order to know how long a clock has been ticking, one has to know when it started ticking.
Also, even if the actual ticking of the clock is reliable, one has to know if any outside influence has been able to move the hands of the clock beyond what the mechanism of the clock itself can achieve. Why do I start with the potassium-argon K-Ar dating method? The Most Common Method: Because of this feature where only the parent product starts off in a solidifying rock, and because of its relative abundance within the rocks of the Earth, the K-Ar dating method is by far the most popular in use today.
So, how is this special feature achieved? So, how does the lava, once it cools off and solidifies, get rid of all of the daughter product?
Well, as it turns out, 40Ar just so happens to be an gas. So, when the lava flows out onto the surface of the ground, all of the 40Ar gas bubbles out and leaves behind only the parent product or 40K in the crystals forming within the solidifying lava. So, all one has to do to determine the age of a volcanic rock is heat up the crystals within the rock and then measure the amount of 40Ar gas that is released compared to the amount of 40K that remains.
The ratio of the parent to the daughter elements is then used to calculate the age of the rock based on the known half-life decay rate of 40K into 40Ar — which is around 1. In any case, the logic for calculating elapsed time here appears to be both simple and straightforward see formula for the calculation. Basically, when half of the 40K is used up, million years have passed — simple!
When Some Argon gets Trapped: Experimental studies done on numerous modern volcanoes with historically known eruption times have been evaluated. Overall, however, such errors are relatively minor compared to the ages of rocks usually evaluated by K-Ar dating on the order of tens to hundreds of millions of years.Carbon Dating Flaws
Well, if even a small amount of argon gas can be trapped, on occasion, within lava flows that happen to cool a little faster than usual, what happens when lava flows are cooled at an even faster rate? Or, what happens when lava is cooled and formed into rock under pressure? Function of Pressure and Rates of Cooling: This same rather significant problem is also true for helium-based dating decay of uranium and thorium produces 4He.
Ages calculated from these measurements increase with sample depth up to 22 million years for lavas deduced to be recent. Of course, this only makes sense. How is a gas, like argon, going to completely escape from a molten rock if the rock hardens too fast?
As ofthis paradox has been explained as the result of a disequilibrium of open-system degassing of the erupting magma. For example, when the granitic rocks of the western Alps where compressed and uplifted into mountains the extreme pressure exerted on these rocks forced excess 40Ar into the pre-formed crystals contained within these rocks.
Now, the excess 40Ar gas that got incorporated into these pressurized rocks had to come from somewhere — right? So, where did it come from? For whatever reason, what is clear is that 40Ar gas was present in fairly significant quantities within the rock surrounding the crystals in question.
Another example is from a study on granitic rocks in northeastern Japan which showed ages up to 16 billion years Gafar greater than the assumed 4. If the K-Ar levels cannot be trusted, what other clock is more reliable? However, in Yang Wang et. In an interview with Science Daily inWang argued: The new evidence calls into question the validity of methods commonly used by scientists to reconstruct the past elevations of the region.
So far, my research colleagues and I have only worked in two basins in Tibet, representing a very small fraction of the Plateau, but it is very exciting that our work to-date has yielded surprising results that are inconsistent with the popular view of Tibetan uplift.
In any case, this finding of Ma for lack sediments still contrasts sharply with mainstream thinking that these regions should be around 50 million years old. Too Old or Too Young: Sometimes, the K-Ar ages is far to old. And, sometimes, the K-Ar age is far too young. For example, isotopic studies of the Cardenas Basalt and associated Proterozoic diabase sills and dikes have produced a geologic mystery. The reason for this, as cited by the New Mexico Research Lab, is that the basic assumptions behind K-Ar dating cannot be known with confidence over long periods of time: Argon loss and excess argon are two common problems that may cause erroneous ages to be determined.
Excess argon may be derived from the mantle, as bubbles trapped in a melt, in the case of a magma. Different kinds of rocks and crystals absorb or retain argon at different rates.
For example, concerning the use of glauconites for K-Ar dating, Faurep. It seems to me that this is just a bit subjective and circular. Most of the time volcanic lavas release all or almost all of the 40Ar gas as are left with essentially pure 40K as a staring point.
Lavas that cool more rapidly than usual retain some 40Ar gas and therefore show a small increase in apparent age which is fairly insignificant relatively speaking usually less than 1 Ma.
However, 40Ar degassing is inversely related to the rate of cooling and the degree of hydrostatic pressure in the surrounding environment. This feature is being discovered to be a fairly common problem.
The Argon-Argon dating method is also not an independent dating method, but must first be calibrated against other dating methods: At best, then it is a relative dating method. Of the various radiometric methods, uranium-thorium-lead U-Th-Pb was the first used and it is still widely employed today, particularly when zircons are present in the rocks to be dated.
Through a long series of intermediate isotopes, radioactive uranium eventually decays into lead, which is stable, not radioactive, and therefore does not decay into anything else. Right off the bat, however, things are a little less straightforward as compared to K-Ar dating. How then can anyone know when the clock started ticking?
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.
Using Radiocarbon Dating to Establish the Age of Iron-Based Artifacts
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. 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. It is key to point out that the usefulness of the method of dating carbon in iron-based materials relies on the source of the carbon found in the materials see sidebar.
For the case of iron-based materials, van der Merwe and Stuiver 2 first demonstrated that it was feasible to extract the carbon from different iron-based materials and use it to establish their age using radiocarbon dating. A total of 15 samples of iron-based materials were dated by beta counting at Yale University 23 using a dependable method to extract carbon from iron utilizing flow-through combustion in oxygen with cryogenic trapping of CO2.
These studies showed that in a wide range of cases, the carbon in iron-based materials could be extracted and reliably radiocarbon dated. The Yale beta counter, however, required significant amounts of carbon compared to the amounts that were usually available from artifacts without consuming or damaging them.
The amount of carbon required was 1g, equivalent to 50 g of a 2. In the late s, radiocarbon dating by accelerator mass spectrometry AMS became common. This new methodology required only 1 mg instead of 1 g of carbon. Inthe present authors published 9 a new carbon-extraction method for iron based on a sealed-tube combustion with CuO in quartz.
This greatly simplified the previous technique and required only materials readily available in the standard AMS graphite-preparation laboratory: Unlike the previous techniques, no exotic gas-trapping equipment is required. Thus, over the years, the sample-size requirement has been greatly reduced and the carbon-extraction procedure has been simplified.
Using Radiocarbon Dating to Establish the Age of Iron-Based Artifacts
However, as has been mentioned, for a radiocarbon date on iron to be meaningful, the carbon extracted from the iron-based material must be from biomass contemporaneous with original manufacture. In addition to fossil fuels such as coal and coke, other carbon sources such as geological carbonates e. Complications arising from the recycling of artifacts must also be considered.
These limitations of the dating technique have been well summarized by van der Merwe 3 and Cresswell. If rust can be dated reliably, it opens up a large number of possibilities for dating iron artifacts.
Investigators will not need to cut into valuable artifacts for clean metal, but will be able to use surface corrosion products. This potentially opens the way for dating precious samples such as the iron plate found in the Great Pyramid at Gizeh, 1011 now at the British Museum.
It may also be possible to date completely rusted artifacts, commonly found in waterlogged early Iron-Age sites in Europe and in underwater shipwrecks. Previous investigators had been careful to remove rust from iron prior to dating for fear that it adds contamination.
A key issue though, is whether any of the original carbon remains within the matrix of rust and other corrosion products. If not, rust and similar materials are clearly of no interest for radiocarbon dating and should probably be removed since, at best, they can do no good.