As the table above illustrates, several "undesirable" reactions occur on isotopes present within every geologic sample.
These reactor produced isotopes of argon must be corrected for in order to determine an accurate age. The monitoring of the interfering reactions is performed through the use of laboratory salts and glasses. For example, to determine the amount of reactor produced 40 Ar from 40 K, potassium-rich glass is irradiated with the samples. The desirable production of 38 Ar from 37Cl allows us to determine how much chlorine is present in our samples. Multiple argon extractions can be performed on a sample in several ways.
Step-heating is the most common way and involves either a furnace or a laser to uniformily heat the sample to evolve argon. The individual ages from each heating step are then graphically plotted on an age spectrum or an isochron.
Mechanical crushing is also a technique capable of releasing argon from a single sample in multiple steps. Laser probes also allow multiple ages to be determined on a single sample aliquot, but do so using accurate and precise spatial control. For example, laser spot sizes of microns or less allow a user to extract multiple argon samples from across a small mica or feldspar grain. The results from a laser probe can be plotted in several graphical ways, including a map of a grain showing lateral argon distribution.
Total fusion is performed using a laser and results are commonly plotted on probability distribution diagrams or ideograms. For the J to be determined, a standard of known age must be irradiated with the samples of unknown age.
Traditionally, this primary standard has been a hornblende from the McClure Mountains, Colorado a. Some of these include other isotopic dating techniques e. This uncertainty results from 1 the branched decay scheme of 40 K and 2 the long half-life of 40 K 1. Because the J value is extrapolated from a standard to an unknown, the accuracy and precision on that J value is critical.
The other important advantage of Ar-Ar dating is the extra data gained from step heating: What's the point of this? Well, different minerals within the rock will give up their argon at different temperatures, so each step will give us a ratio of 40 Ar to 39 Ar which we can use in the equation to calculate a date. Now, recall that we said that when the rock was first formed, the 39 K and 40 K from which these are derived must have appeared in the same ratio in each mineral, because both isotopes of potassium have the same chemical properties.
This means that if the rock cooled rapidly enough that all the minerals in it have the same date, and if there has been no argon loss, and if there is no excess argon added to the system, then the dates we calculate at each step of the heating will be the same date. For example, if the date increases at each step, then we are quite possibly looking at a slow-cooling igneous rock in which different minerals crystallized out of the magma at different times, a possibility we can investigate further. And if the dates we get are all over the place, then we are probably looking at excess argon.
Now the bad news is that there is no way we can somehow manipulate this data to give us a correct date for the sample. But the good news is that we do know that there's a problem; whereas if we'd analyzed the same rock using the K-Ar method, then it would have supplied us with a date and there'd have been no sign in the K-Ar data of anything wrong with it. For these reasons Ar-Ar dating has largely superseded K-Ar dating , although the simpler method is still employed in some cases where it is known to be unproblematic or where Ar-Ar is unsuitable for some technical reason.
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Policies and guidelines Contact us. Proper statistical evaluation confirms this. Note that in Figure 2c the last four or five steps may be taken to define a plateau.
Historical Geology/Ar-Ar dating - Wikibooks, open books for an open world
The two sets of plateau ages see Figure 2c overlap statistically. Examples of acceptable left and unacceptable right age spectra where relevant details of the plateau sections are obscured by use of inappropriate apparent age scales top panels. The former and latter derive gas from low and high argon rentention sites respectively. Thus low-temperature steps normally exhibit ages lower than, or equal to, those of the higher temerature steps. The second explanation involves 39 Ar recoil within the sample.
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Like the recoil of a gun when a bullet is fired, the 39 Ar formed by the reaction, recoils. Examples of both phenomena excess 40 Ar and 39 Ar recoil are given in Baksi and references cited therein. Age spectra are shown in Figure 3. The first sample shows a descending-staircase type of age spectrum. This is by definition untenable. Similar two-step plateau ages of None of these specimens yields an age plateau or isochron sufficiently accurate to be utilized for tracing hotspot tracks.
Age spectra for disturbed samples from the Tristan da Cunha hotspot track the Walvis ridge. Age spectra showing older apparent ages for low-temperature steps than for high-temperature steps. Age spectra are shown in Figure 4. These descending staircase age spectra may indicate the presence of excess 40 Ar.
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For this reason see Introduction above the corresponding isochron plots need to be critically examined. Baksi showed that the MSWD values for these straight lines are 36, 5. Age spectra are shown in Figure 5. For AII, the probability value is 0. However, the isochron is derived from the first four steps, two of which show very high atmospheric argon 36 Ar contents. The gas is derived from altered sites and hence the corresponding age is suspect Baksi , A plateau age of The corresponding isochron Baksi , yields a low precision age of This rock could be said to yield a somewhat imprecise but usable age for evaluating the hotspot track trace.
Age spectra yielding unacceptable ages from the Tristan da Cunha hotspot track. Age spectrum yielding an imprecise but usable age for evaluating the Tristan da Cunha hotspot track. Based on statistical evaluation of the isochron plots Baksi showed that each of these was untenable. Elsewhere it will be shown that this conclusion is fully supported by critical examination of the individual age spectra.
The plate motion model derived by Muller et al. Only a very few of these purported ages can be used to date time of crystallization. The plate motion model of Muller et al. Furthermore, a cursory glance at the age spectra drawn to an appropriate scale , shows few if any acceptable ages were recovered Figures 7 — 9. I examine their results for rocks from the Isle of Mull. All whole-rock samples; see Figure 7.