K–Ar dating

Potassium-Argon dating (K–Ar dating) is a radiometric dating method used in geochronology and archaeology. It is based on the 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 ^40Ar is able to escape the liquid (molten) rock, but starts to accumulate when the rock solidifies (recrystallizes).

The K–Ar dating method relies on the assumption that there is no argon from previous events present in the rock to be dated, that the decay of ^40K is constant over time, and that the rock has remained a closed system to ^40K and ^40Ar since it cooled. The method is used to date volcanic rocks and is also widely used to date lunar samples and meteorites.

Principles

The radioactive decay of ^40K to ^40Ar is the basis of the K–Ar dating method. Potassium-40 (^40K) is a radioactive isotope of potassium which has a very long half-life of 1.25 billion years. ^40K decays by two processes: it breaks down into calcium-40 (^40Ca) through beta decay (89.1% of the time) and into argon-40 (^40Ar) through electron capture (10.9% of the time).

The ratio of ^40K to ^40Ar is measured to determine the age of the sample. This requires the precise measurement of both the total potassium present in the sample and the amount of ^40Ar that has accumulated as a result of the radioactive decay of ^40K.

Procedure

The sample is first crushed and then melted in the presence of high temperatures in a vacuum or an inert gas atmosphere to release the argon. The argon gas is then collected and its isotopic composition is measured using mass spectrometry. The amount of ^40Ar is compared to the amount of ^40K remaining in the rock. From this, the age of the rock can be calculated.

Applications

K–Ar dating has been crucial in the development of the geologic time scale, and has been used to date samples as old as 4 billion years. It is particularly useful for dating volcanic rocks and ash layers within sedimentary sequences, which can then be used to date the sedimentary rocks themselves. K–Ar dating is also used in the study of planetary science to date meteorites and lunar rocks.

Limitations

The accuracy of K–Ar dating relies on several assumptions. If argon has escaped from the rock after it formed, the dates obtained will be too young. Conversely, if argon has been trapped from the atmosphere during the formation of the rock, the dates obtained will be too old. Additionally, the rock must have remained a closed system to ^40K and ^40Ar since its formation. If either of these conditions is not met, the calculated age may not be accurate.

See also

• Radiocarbon dating
• Uranium-lead dating
• Rubidium-strontium dating