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The potassium-argon K-Ar isotopic dating method is especially useful for determining the age of lavas. Developed in the s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale. Potassium occurs in two stable isotopes 41 K and 39 K and one radioactive isotope 40 K. Potassium decays with a half-life of million years, meaning that half of the 40 K atoms are gone after that span of time. Its decay yields argon and calcium in a ratio of 11 to The K-Ar method works by counting these radiogenic 40 Ar atoms trapped inside minerals.
The ratio of K to Ar is plotted.
Note that time is expressed in millions of years on this graph, as opposed to thousands of years in the C graph. Click on the "Show Movie" button below to view this animation. K-Ar Decay Profile.
Jan 31, The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas. Developed in the s, it was important in developing the theory of plate tectonics and in calibrating the geologic time shawchapman.com: Andrew Alden. Potassium-Argon Dating Potassium-Argon dating is the only viable technique for dating very old archaeological materials. Geologists have used this method to date rocks as much as 4 billion years old. It is based on the fact that some of the radioactive isotope of Potassium, Potassium (K),decays to the gas Argon as Argon (Ar).
How are Samples Processed? Clicking on the "Show Movie" button below will bring up an animation that illustrates how a K-Ar sample is processed and the calculations involved in arriving at a date.
This is actually a mini-simulator, in that it processes a different sample each time and generates different dates. K-Ar Processing. Limitations on K-Ar Dating The Potassium-Argon dating method is an invaluable tool for those archaeologists and paleoanthropologists studying the earliest evidence for human evolution. As with any dating technique, there are some significant limitations. Contents 2. Introduction 3. Superposition 4. Stratigraphy 5. Cross Dating 6.
Dendrochronology 8. Radiocarbon Dating 9.
Potassium Argon Dating Obsidian Hydration Dating The target mineral is separated using heavy liquids, then hand-picked under the microscope for the purest possible sample. This mineral sample is then baked gently overnight in a vacuum furnace.
These steps help remove as much atmospheric 40 Ar from the sample as possible before making the measurement. A precise amount of argon is added to the gas as a "spike" to help calibrate the measurement, and the gas sample is collected onto activated charcoal cooled by liquid nitrogen.
Then the gas sample is cleaned of all unwanted gasses such as H 2 O, CO 2SO 2nitrogen and so on until all that remains are the inert gassesargon among them. Three argon isotopes are measured: 36 Ar, 38 Ar, and 40 Ar.
If the data from this step is clean, the abundance of atmospheric argon can be determined and then subtracted to yield the radiogenic 40 Ar content.
This "air correction" relies on the level of argon, which comes only from the air and is not created by any nuclear decay reaction. It is subtracted, and a proportional amount of the 38 Ar and 40 Ar are also subtracted. The remaining 38 Ar is from the spike, and the remaining 40 Ar is radiogenic.
Potassium-argon dating Meaning
Because the spike is precisely known, the 40 Ar is determined by comparison to it. Variations in this data may point to errors anywhere in the process, which is why all the steps of preparation are recorded in detail.
Potassium argon dating
A variant of the K-Ar method gives better data by making the overall measurement process simpler. The key is to put the mineral sample in a neutron beam, which converts potassium into argon Because 39 Ar has a very short half-life, it is guaranteed to be absent in the sample beforehand, so it's a clear indicator of the potassium content.
The advantage is that all the information needed for dating the sample comes from the same argon measurement. Accuracy is greater and errors are lower. This method is commonly called "argon-argon dating. The physical procedure for 40 Ar- 39 Ar dating is the same except for three differences:. These effects must be corrected, and the process is intricate enough to require computers.
The Ar-Ar method is considered superior, but some of its problems are avoided in the older K-Ar method. Also, the cheaper K-Ar method can be used for screening or reconnaissance purposes, saving Ar-Ar for the most demanding or interesting problems.
These dating methods have been under constant improvement for more than 50 years. The learning curve has been long and is far from over today.
With each increment in quality, more subtle sources of error have been found and taken into account. Good materials and skilled hands can yield ages that are certain to within 1 percent, even in rocks only 10, years old, in which quantities of 40 Ar are vanishingly small.
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Andrew Alden is a geologist based in Oakland, California. He works as a research guide for the U. Geological Survey.
Potassium-argon dating definition, a method for estimating the age of a mineral or rock, based on measurement of the rate of decay of radioactive potassium into argon. See more. The potassium-argon dating method has been used to measure a wide variety of ages. The potassium-argon age of some meteorites is as old as 4, years, and volcanic rocks as young as 20, years old have been measured by this method. Argonargon dating. Radiometric dating. Additional Information. External Websites. How potassium-argon dating works Published: 24 June (GMT+10) Photo Wikipedia by Tas Walker. One of the most widely used dating methods is the potassium-argon method, which has been applied to 'dating' rocks for decades, especially igneous rocks that have solidified from molten magma.
ated January 31, The method relies on satisfying some important assumptions:.