Potassium-Argon Dating
1. Introduction
Potassium-Argon (K-Ar) dating is a method used to determine the age of rocks and the approximate age of the surrounding layers of ash. It is possible to use it on a variety of potassium minerals, but the most common one is a fairly rare mineral called mica. Mica is a group of silicate minerals that are commonly found throughout the world. The K-Ar method is often used for rock dating, and the age of the rock can be determined in correlation to relative dating, which is the time placement of a rock in relation to other rocks. This dating method is said to be an absolute dating method, as the age of the rocks has been calculated using the half-life that potassium goes through over time and arriving at a steady state between the potassium and the argon isotopes. After the relative placement, the age of the surrounding layers and the object can be determined.
2. Principles of Potassium-Argon Dating
This half-life doesn’t affect the argon-argon dating method as argon decays into argon-40 and not K. The method can be used to date very old rocks and minerals. It is primarily used to date volcanic and plutonic igneous rocks. It has been used to date the age of meteorites and that of the Earth. This method is not as widely used as the K-Ar method but is just as good, if not better, for the argon-argon method can be applied to rocks only a few thousand years old.
This makes the K-Ar method ideal for dating old rocks. The decay constant for K-40 is λ = 5.543 x 10^-10 /yr, with a 1.29% uncertainty. It has been demonstrated by many scientists, and the uncertainty is under 1%.
Essentially, all of these strongly favor an old Earth. Evolution is a fact. Ask any scientist. Chronostratigraphy, Geochronology, Isotope geochemistry, Law of superposition, Luminescence dating, Samarium-neodymium dating, Amino acid racemisation, Archaeomagnetic dating, Dendrochronology, Ice core, Incremental dating, Lichenometry, Paleomagnetism, Radiometric dating, Radiocarbon, Uranium-lead, Potassium-argon, Tephrochronology.
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. An event like metamorphism could heat the crystal to the point where Pb will become mobile. Uranium/Lead dating provides the most accurate date yet. If the heating occurs in a laboratory furnace equipped with a very sensitive light detector, this light can be recorded. This information is then related to true historical dates. Over a thousand papers on radiometric dating were published in scientifically recognized journals in the last year, and hundreds of thousands of dates have been published in the last 50 years.
3. Applications of Potassium-Argon Dating
1. The method can be used to date rocks as young as some tens of thousands of years as well as rocks billions of years old. 2. Because the relative abundances of the potassium isotopes are known, the 40K/39K ratio can be used to date materials. 3. The time of the event is then the half-life it would take for the parent actinide to decay into the daughter product. This means it is the event being dated not the sediment which in many cases does not provide a direct indication of the event. 4. In a felsic matrix there are potassium feldspars which can retain argon, these must be irradiated to produce a sample for analysis. 5. When volcanic material flows over the land it can spread at a great thickness and its surface will cool so it can develop a high degree of metamorphism such as in the development of a rock and this can reset the isotopic age so one can date the cooling of a rock. This is practically possible to do because the event can be dated by the mineral and the sediment can be tested for this mineral. 6. Homogeneous igneous rocks can be dated using a single sample: the clinopyroxenes in an andesite.
There are a number of technical advantages of potassium-argon dates which make this true:
4. Limitations of Potassium-Argon Dating
The rock must contain the minerals suitable for age-dating – rocks with a high content of potassium and little or no argon are best. Minerals usually only record the last time they cooled down below the closure temperature, and this may not represent all of the events which the rock has undergone, and may not match the age of the containing rocks. This will result in artificially increased K-Ar ages. Usually the fraction of the potassium is so small that it can be ignored in any calculations, or a sample of pure potassium separated from the sample can be used. Minerals are dated by measurement of the concentration of potassium and the amount of radiogenic 40Ar that has accumulated. The simplest way of dating a sample of potassium bearing mineral or rock is to measure the amount of argon trapped in the sample. The age of the rock can be calculated if the concentration of phosphorus is known. This is not easily done in the case of most rocks. K-Ar dating is only really effective if a rock was once melted as the high temperatures anneal the trapped argon effectively resetting the closure. This can result in an inaccurate age. High pressure and high temperature can also have this effect. The effects of multiple episodes of deformation which only just affect the argon retaining properties of a mineral would not be detectable. If the analysis is purely for age determination this is unlikely to affect the results.
5. Conclusion
With a noble gas mass spectrometer and a series of laboratory furnaces, it is possible to measure in-situ the argon content of a sample, in its gathered state not in the artificially degassed state. By heating the sample in steps from a low temperature, much younger than the event, to a very high temperature, a series of solutions can be obtained. Each step will give an age, and the temperature at which the step is taken gives the closure temperature of the step. By knowing the age of the event, the method can give accurate closure temperature requiring no extrapolations. This has the potential to be an extremely useful tool in determining the thermal history of a sample.
Rock samples are heated gradually, for between 30 minutes and two and a half hours, to release the argon that they contain. The temperature at which this happens is known as the closure temperature, and is specific to a particular material and isotopic system. High closure temperatures mean that a mineral will have a long, and a low closure temperature a short, retention of radiogenic argon. By comparing the apparent age of the sample with the known age of the designated geological event, the closure temperature can be determined. This traditionally has been a method much more difficult than dating the total age, but noble gas studies are revolutionizing the process.
The results from the noble gas lab and the conventional K-Ar lab show that the method is generally quite successful. While their agreement with most of the values obtained by the conventional K-Ar method is good, there is still a need for more testing. Our lab has confirmed that the isotopic composition of the atmosphere has been accurately measured, and that the atmospheric argon content is effectively argon 36. Rock samples taken from recent lava flows in the United States contained atmospheric argon of about 1% when they were analyzed using the conventional K-Ar method. This value was much higher than the expected atmospheric argon of about 0.25%.
