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.
Ar–Ar and K–Ar Dating
Potassium—argon dating. An absolute dating method based on the natural radioactive decay of 40 K to 40 Ar used to determine the ages of rocks and minerals on geological time scales. Argon—argon dating. A variant of the K—Ar dating method fundamentally based on the natural radioactive decay of 40 K to 40 Ar, but which uses an artificially generated isotope of argon 39 Ar produced through the neutron irradiation of naturally occurring 39 K as a proxy for 40 K.
For this reason, the K—Ar method is one of the few radiometric dating techniques in which the parent Skip to main content Skip to table of contents.
But, for the purposes of the KAr dating system, the relative abundance of 40K is a technique capable of releasing argon from a single sample in multiple steps.
Some updates to this article are now available. The sections on the branching ratio and dating meteorites need updating. Radiometric dating methods estimate the age of rocks using calculations based on the decay rates of radioactive elements such as uranium, strontium, and potassium. On the surface, radiometric dating methods appear to give powerful support to the statement that life has existed on the earth for hundreds of millions, even billions, of years.
We are told that these methods are accurate to a few percent, and that there are many different methods. We are told that of all the radiometric dates that are measured, only a few percent are anomalous. This gives us the impression that all but a small percentage of the dates computed by radiometric methods agree with the assumed ages of the rocks in which they are found, and that all of these various methods almost always give ages that agree with each other to within a few percentage points.
Since there doesn’t seem to be any systematic error that could cause so many methods to agree with each other so often, it seems that there is no other rational conclusion than to accept these dates as accurate. However, this causes a problem for those who believe based on the Bible that life has only existed on the earth for a few thousand years, since fossils are found in rocks that are dated to be over million years old by radiometric methods, and some fossils are found in rocks that are dated to be billions of years old.
If these dates are correct, this calls the Biblical account of a recent creation of life into question. After study and discussion of this question, I now believe that the claimed accuracy of radiometric dating methods is a result of a great misunderstanding of the data, and that the various methods hardly ever agree with each other, and often do not agree with the assumed ages of the rocks in which they are found. I believe that there is a great need for this information to be made known, so I am making this article available in the hopes that it will enlighten others who are considering these questions.
Even the creationist accounts that I have read do not adequately treat these issues. At the start, let me clarify that my main concern is not the age of the earth, the moon, or the solar system, but rather the age of life, that is, how long has life existed on earth.
K-Ar dating calculation
Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks. Over naturally-occurring isotopes are known.
Potassium-Argon (K-Ar) Dating. The isotope 40K is one of 3 isotopes of Potassium (39K, 40K and 41K) and is about % of the natural potassium found in.
Geochronology involves understanding time in relation to geological events and processes. Geochronological investigations examine rocks, minerals, fossils and sediments. Absolute and relative dating approaches complement each other. Relative age determinations involve paleomagnetism and stable isotope ratio calculations, as well as stratigraphy. Speak to a specialist.
Geoscientists can learn about the absolute timing of geological events as well as rates of geological processes using radioisotopic dating methods. These methods rely on the known rate of natural decay of a radioactive parent nuclide into a radiogenic daughter nuclide. Over time, the daughter nuclide accumulates in certain minerals.
Different isotopic systems can be used to date a range of geological materials from a few million to billions of years old.
Potassium-Argon and Argon-Argon Dating of Crustal Rocks and the Problem of Excess Argon
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 By comparing the proportion of K to Ar in a sample of volcanic rock, and knowing the decay rate of K, the date that the rock formed can be determined.
For the argon dating methods the daughter product is a gas, and can escape from each step, are often not enumerated; further, the K content of the material is.
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. More complex e.
Argon-argon dating works because potassium decays to argon with a known decay constant. However, potassium also decays to 40 Ca much more often than it decays to 40 Ar. This necessitates the inclusion of a branching ratio 9. This led to the formerly-popular potassium-argon dating method. However, scientists discovered that it was possible to turn a known proportion of the potassium into argon by irradiating the sample, thereby allowing scientists to measure both the parent and the daughter in the gas phase.
There are several steps that one must take to obtain an argon-argon date: First, the desired mineral phase s must be separated from the others.
Posts about K-Ar dating written by The Noble Gasbag. samples and situations where this K-Ar dating technique works really well, it isn’t perfect. we can use this information to identify which heating steps can be used to calculate an age.
The technique uses a few key assumptions that are not always true. These assumptions are:. Assumption 2 can cause problems when analysing certain minerals, especially a mineral called sanidine. This is a kind of K-rich feldspar that forms at high temperatures and has a very disordered crystal lattice. This disordered crystal lattice makes it more difficult for Ar to diffuse out of the sample during analysis, and the high melting temperature makes it difficult to completely melt the sample to release the all of the gas.
Assumption 3 can be a problem in various situations. This J-value is then used to help calculate the age of our samples. This new technique dealt with any problems associated with assumption 1 of the K-Ar technique. Being able to measure both the parent and daughter isotope at the same time also opened up a whole new level of gas-release technique that helped to address any problems associated with assumption 3.
Most people envision radiometric dating by analogy to sand grains in an hourglass: the grains fall at a known rate, so that the ratio of grains between top and bottom is always proportional to the time elapsed. 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: 40K is a potassium atom with an atomic mass of 40 units; 40Ar is an argon atom with an atomic mass of 40 units].
In theory, therefore, we can estimate the age of the mineral simply by measuring the relative abundances of each isotope. Over the past 60 years, potassium-argon dating has been extremely successful, particularly in dating the ocean floor and volcanic eruptions. K-Ar ages increase away from spreading ridges, just as we might expect, and recent volcanic eruptions yield very young dates, while older volcanic rocks yield very old dates.
Finally—and perhaps most importantly—the K-Ar dating method Consequently, the first step of Ar-Ar analysis would yield an age that was too.
We report a combined geochronology and palaeomagnetic study of Cretaceous igneous rocks from Shovon K—Ar dating based on seven rock samples, with two independent measurements for each sample, allows us to propose an age of Stepwise thermal and AF demagnetization generally isolated a high temperature component HTC of magnetization for both Shovon and Arts-Bogds basalts, eventually following a low temperature component LTC in some samples.
Rock magnetic analysis identifies fine-grained pseudo-single domain PSD magnetite and titanomagnetite as primary carriers of the remanence. Because of their similar ages, we combine data from Shovon and data previously obtained from Khurmen Uul These poles are consistent with those from the European apparent polar wander path APWP at 90, and Ma, and other published pole from the Mongol-Okhotsk suture zone, Amuria and North China blocks. This confirms the lack of a discernable latitudinal motion between Amuria and Siberia since their final accretion by the Late Jurassic—Early Cretaceous, and reinforces the idea that Europe APWP can be used as a reference for Siberia by the mid-Cretaceous.
Central Asia is a fascinating place for testing palaeomagnetic tools that provide for tectonic constraints. This deformation is accommodated by two main components of 1 east and southeastward extrusions of continental lithospheric units Fig. Enkin et al. Palaeomagnetism is sensitive to inclination, therefore, it is a powerful tool to describe these northward versus southward palaeolatitude movements between different blocks.
For this reason, numerous palaeomagnetic studies have been undertaken all-over Asia in the last 25 yr.