What are the limits of potassium argon dating
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.
The attraction of the method lies in the fact that one of the daughter elements is argon which is an inert gas. This means that the geologist can plausibly assume that all argon gas escapes from the molten magma while it is still liquid. He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes the initial argon content is zero.
He assumes that any argon that he measures in his rock sample must have been produced by the radioactive decay of potassium since the time the rock solidified. He imagines that his radioactive hour glass sealed when the rock solidified, and his radioactive clock started running. And he hopes the rock has remained sealed until the time he collected his sample. With these assumptions the geologist only needs to measure the relative amounts of potassium and argon in the rock at the present time to be able to calculate an age for the rock.
Although it is a simple calculation the big question is whether his assumptions about the rock were correct. If the rock actually contained some argon when it solidified then the calculated age would be too old. On the other hand, if the rock was later disturbed by a geological upheaval and lost argon the age would be too young. How can the geologist know? What he does is check his calculated age with the ages produced by other dating methods.
In other words, he checks to see if his calculated result falls into the range where he expects it to fall, given the geological situation of where he found his rock. He always does this check because no dating method can be trusted on its own. What happens if the results conflict? By this he means that argon gas in his rock has come from the melting of some older rocks deep underground and contaminated his sample with a higher concentration of argon, which is why its age is too old.
This is a standard explanation and is essentially a new story about the past, different from the original story that explained how potassium-argon dating works. We could ask ourselves which of the details of this story have been observed. It is a story about older rocks, melted rocks, solidified rocks and argon gas. It explains what each of these were doing deep inside the earth millions of years ago.
Too old compared with what? With the true age of the rock. The problem is that although radiogenic argon and excess argon have different names they are exactly the same isotope—argon It is impossible to distinguish between them experimentally. So, how do we work out how much excess argon we have? We can only calculate the amount of excess argon if we know the true age of the rock.
What happens when the age is too young? In this case the method is again salvaged by changing his assumptions about the past. Often a heating event is invoked to liberate the argon from the solid rock, although other assumptions are made as well. What happens if the age falls into the range he expected? In this case the geologist assumes that everything went well, and he publishes his result as the crystallization age of the rock. So although the potassium-argon method has been used for dating rocks for decades, the results it has produced have tended to reinforce the geological framework that already existed.
At most it may have modified the framework a little. The scores of dates that have been produced have had a life like hens in a chicken coop. Whenever a new date is introduced it has to find its pecking order within the geological community. Some dates are accepted, some are rejected, some are overturned and some are modified until everything is in its place, and order reigns again. We have supplied this link to an article on an external website in good faith.
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Potassium–argon dating, abbreviated K–Ar dating, is a radiometric dating method used in These factors introduce error limits on the upper and lower bounds of dating, so that final determination of age is reliant on the environmental factors. Potassium-argon dating, method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock.
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
Most people envision radiometric dating by analogy to sand grains in an hourglass: In principle, the potassium-argon K-Ar decay system is no different. Of the naturally occurring isotopes of potassium, 40K is radioactive and decays into 40Ar at a precisely known rate, so that the ratio of 40K to 40Ar in minerals is always proportional to the time elapsed since the mineral formed [ Note:
Potassium-Argon Dating Methods
The attraction of the method lies in the fact that one of the daughter elements is argon which is an inert gas. This means that the geologist can plausibly assume that all argon gas escapes from the molten magma while it is still liquid. He thinks this solves his problem of not knowing the initial quantity of the daughter element in the past and not being able to go back in time and make measurements. He assumes the initial argon content is zero. He assumes that any argon that he measures in his rock sample must have been produced by the radioactive decay of potassium since the time the rock solidified.
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Potassium-argon dating , method of determining the time of origin of rocks by measuring the ratio of radioactive argon to radioactive potassium in the rock. This dating method is based upon the decay of radioactive potassium to radioactive argon in minerals and rocks; potassium also decays to calcium
Potassium Argon Dating
Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present. Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or mineral through alteration processes. Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs. However, the 40 K isotope is radioactive and therefore will be reduced in quantity over time. But, for the purposes of the KAr dating system, the relative abundance of 40 K is so small and its half-life is so long that its ratios with the other Potassium isotopes are considered constant. Argon, a noble gas, constitutes approximately 0. Because it is present within the atmosphere, every rock and mineral will have some quantity of Argon. Argon can mobilized into or out of a rock or mineral through alteration and thermal processes. Like Potassium, Argon cannot be significantly fractionated in nature. However, 40 Ar is the decay product of 40 K and therefore will increase in quantity over time.
Potassium—argon dating , abbreviated K—Ar dating , is a radiometric dating method used in geochronology and archaeology. It is based on measurement of the product of the radioactive decay of an isotope of potassium K into argon Ar. Potassium is a common element found in many materials, such as micas , clay minerals , tephra , and evaporites. In these materials, the decay product 40 Ar is able to escape the liquid molten rock, but starts to accumulate when the rock solidifies recrystallizes. The amount of argon sublimation that occurs is a function of the purity of the sample, the composition of the mother material, and a number of other factors. Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar accumulated to the amount of 40 K remaining.
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