Potassium-Argon Dating Methods
Potassium—Argon dating or 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 , tephra, and evaporites.
In these materials, the decay product 40 Ar is able to escape the liquid molten rock, but starts to build up when the rock solidifies re crystallises. Time since recrystallization is calculated by measuring the ratio of the amount of 40 Ar to the amount of 40 K remaining.
A modified version of this problem with the recent age of ar. Although initially measured as a new k/ar dating in the mid-bosnian schist mts. Therefore, our argon.
Skip to search form Skip to main content You are currently offline. Some features of the site may not work correctly. DOI: The aim of this chapter is to present the K-Ar and Ar-Ar dating techniques in the context of noble gas studies, since there are already several recent texts on K-Ar and Ar-Ar dating Dickin ; McDougall and Harrison View PDF.
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Ar-Ar Geochronology Laboratory
Ar-Ar dating: principles Ar-Ar dating is the workhorse in geochronology and allows dating of samples that range in age from the origin of the solar system up to a few hundred thousand years. The basic principle of this dating method is accumulation of radiogenic 40 Ar from 40 K by an electron-capture decay. The method is thus a modified K-Ar dating method and allows dating of all types of samples that contain reasonable amounts of potassium.
Particularly usefull are K-rich minerals such as K-feldspar, micas and hornblende. The half-life of 40 K is 1.
samples and Ar/Ar or K/Ar ages are available for these two minerals in We are considering saving the mica remains from the Ar/Ar dating for Pb isotope.
Time is a fundamental parameter in the Earth Sciences whose knowledge is essential for estimating the length and rate of geological processes. The 40 Ar- 39 Ar method, variant of the K-Ar method, is based on the radioactive decay of the naturally occurring parent 40 K half-life 1. The 40 Ar- 39 Ar method, applied to K-bearing systems minerals or glass , represents one of the most powerful geochronological tools currently available to constrain the timing of geological processes. It can be applied to a wide range of geological problems and to rocks ranging in age from a few thousand years to the oldest rocks available.
The development of the laser extraction technique has expanded fields of application, including among others:. Gianfranco di Vincenzo Ph.
Potassium-argon (K-Ar) dating
Kelley, Simon Excess argon in K-Ar and Ar-Ar geochronology. Chemical Geology , pp. The K—Ar and Ar—Ar dating techniques occasionally produce anomalously old ages attributed to excess argon, and such data is often rejected as not offering any insight into the age, thermal history or geochemistry of the rock.
However, improvements in the quantification of argon geochemistry now provide a framework to model excess argon in both open and closed systems.
The aim of this chapter is to present the K-Ar and Ar-Ar dating techniques in the context of noble gas studies, since there are already several recent texts on K-Ar.
Potassium has three naturally occurring isotopes: 39 K, 40 K and 41 K. The positron emission mechanism mentioned in Chapter 2. In addition to 40 Ar, argon has two more stable isotopes: 36 Ar and 38 Ar. Because K an alkali metal and Ar a noble gas cannot be measured on the same analytical equipment, they must be analysed separately on two different aliquots of the same sample.
The idea is to subject the sample to neutron irradiation and convert a small fraction of the 39 K to synthetic 39 Ar, which has a half life of years. The age equation can then be rewritten as follows: 6. The J-value can be determined by analysing a standard of known age t s which was co-irradiated with the sample: 6. The great advantage of equation 6. This is done by degassing the sample under ultra-high vacuum conditions in a resistance furnace.
At low temperatures, the weakly bound Ar is released, whereas the strongly bound Ar is released from the crystal lattice at high temperatures until the sample eventually melts.
Ar–Ar and K–Ar Dating
The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace.
Heating causes the crystal structure of the mineral or minerals to degrade, and, as the sample melts, trapped gases are released.
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 into argon.
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. What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. Argon makes up 1 percent of the atmosphere. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.
That is, a fresh mineral grain has its K-Ar “clock” set at zero. The method relies on satisfying some important assumptions:. Given careful work in the field and in the lab, these assumptions can be met. The rock sample to be dated must be chosen very carefully. Any alteration or fracturing means that the potassium or the argon or both have been disturbed.
Historical Geology/K-Ar dating
In the diagram below I have drawn 2 different age spectra. The bottom, green spectrum is what we would expect to see if we had an ideal sample that has no excess-Ar, and the top, blue spectrum is what we might expect if the sample contained excess-Ar in fluid inclusions. The data for each of those 7 steps is represented by one of the 7 boxes on the diagram.
In this article we shall examine the basis of the K-Ar dating method, how it works, and what can go wrong with it. It is possible to measure the proportion in which 40 K decays, and to say that about Potassium is chemically incorporated into common minerals, notably hornblende , biotite and potassium feldspar , which are component minerals of igneous rocks. Argon, on the other hand, is an inert gas; it cannot combine chemically with anything.
As a result under most circumstances we don’t expect to find much argon in igneous rocks just after they’ve formed. However, see the section below on the limitations of the method. This suggests an obvious method of dating igneous rocks. If we are right in thinking that there was no argon in the rock originally, then all the argon in it now must have been produced by the decay of 40 K. So all we’d have to do is measure the amount of 40 K and 40 Ar in the rock, and since we know the decay rate of 40 K, we can calculate how long ago the rock was formed.
From the equation describing radioactive decay , we can derive the following equation:. There are a number of problems with the method. One is that if the rocks are recent, the amount of 40 Ar in them will be so small that it is below the ability of our instruments to measure, and a rock formed yesterday will look no different from a rock formed fifty thousand years ago. The severity of this problem decreases as the accuracy of our instruments increases.
Still, as a general rule, the proportional error in K-Ar dating will be greatest in the youngest rocks.
K–Ar dating facts for kids
Raw data of the argon isotopes have been uploaded as the electronic supplementary material. Fluid inclusions in hydrothermal quartz in the 2. To constrain the origin of the fluid and the quartz precipitation age, we conducted Ar—Ar dating for the quartz via a stepwise crushing method.
With the 40Ar/39Ar dating method the samples are first irradiated with fast neutrons in a nuclear reactor to transform a proportion of 39K to 39Ar. The amount of
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 Thus, the ratio of argon and potassium and radiogenic calcium to potassium in a mineral or rock is a measure of the age of the sample.
The calcium-potassium age method is seldom used, however, because of the great abundance of nonradiogenic calcium in minerals or rocks, which masks the presence of radiogenic calcium. On the other hand, the abundance of argon in the Earth is relatively small because of its escape to the atmosphere during processes associated with volcanism. 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.
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