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.
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Learn More in these related Britannica articles: This is possible in potassium-argon K-Ar dating, for example, because most minerals do not take argon into their structures initially. In rubidium-strontium dating, micas exclude strontium when they form but accept much rubidium. In uranium-lead U-Pb dating of zircon, the zircon is found to exclude initial lead….
The radioactive decay scheme involving the breakdown of potassium of mass 40 40 K to argon gas of mass 40 40 Ar formed the basis of the first widely used isotopic dating method. Since radiogenic argon was first detected in by the American geophysicist….
Potassium—argon dating has made it possible to establish that the earliest remains of man and his artifacts in East Africa go back at least 2,, years, and probably further. Potassium-argon dating , for instance, can provide the age of a specimen by clocking the rate at which radioactive isotopes of these elements have decayed.
When radiometric methods cannot be applied, investigators may still ascribe a relative age to a fossil by relating it to the…. More About Potassium-argon dating 5 references found in Britannica articles Assorted References major reference In dating: Analysis of separated minerals In dating: Potassium—argon methods age determination of tektites In tektite: Chemistry and petrography of tektites archaeology In archaeology: Dating Homo erectus In Homo erectus: With 18 protons and 22 neutrons, the atom has become Argon Ar , an inert gas.
For every K atoms that decay, 11 become Ar How is the Atomic Clock Set? When rocks are heated to the melting point, any Ar contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again.
As the K in the rock decays into Ar, the gas is trapped in the rock. The Decay Profile In this simulation, a unit of molten rock cools and crystallizes. 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. 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. The long half-life of 40 K allows the method to be used to calculate the absolute age of samples older than a few thousand years.
Potassium argon dating range
The quickly cooled lavas that make nearly ideal samples for K—Ar dating also preserve a record of the direction and intensity of the local magnetic field as the sample cooled past the Curie temperature of iron. The geomagnetic polarity time scale was calibrated largely using K—Ar dating.What Can Potassium Argon Dating Be Used For?
Potassium naturally occurs in 3 isotopes: Conversion to stable 40 Ca occurs via electron emission beta decay in Conversion to stable 40 Ar occurs via electron capture in the remaining Argon, being a noble gas , is a minor component of most rock samples of geochronological interest: When 40 K decays to 40 Ar argon , the atom typically remains trapped within the lattice because it is larger than the spaces between the other atoms in a mineral crystal.
Entrained argon—diffused argon that fails to escape from the magma—may again become trapped in crystals when magma cools to become solid rock again.
After the recrystallization of magma, more 40 K will decay and 40 Ar will again accumulate, along with the entrained argon atoms, trapped in the mineral crystals. Measurement of the quantity of 40 Ar atoms is used to compute the amount of time that has passed since a rock sample has solidified. Despite 40 Ca being the favored daughter nuclide, it is rarely useful in dating because calcium is so common in the crust, with 40 Ca being the most abundant isotope.