Isotopes of argon

The naturally occurring ⁴⁰K, with a half-life of 1.248×10⁹ years, decays to stable ⁴⁰Ar by electron capture (10.72%) and by positron emission (0.001%), and also to stable ⁴⁰Ca via beta decay (89.28%). These properties and ratios are used to determine the age of rocks through potassium–argon dating.

Despite the trapping of ⁴⁰Ar in many rocks, it can be released by melting, grinding, and diffusion. Almost all argon in the Earth's atmosphere is the product of ⁴⁰K decay, since 99.6% of Earth's atmospheric argon is ⁴⁰Ar, whereas in the Sun and presumably in primordial star-forming clouds, argon consists of ~85% ³⁶Ar, ~15% ³⁸Ar and only trace ⁴⁰Ar. Similarly, the ratio of the isotopes ³⁶Ar:³⁸Ar:⁴⁰Ar in the atmospheres of the outer planets is measured to be 8400:1600:1.

In the Earth's atmosphere, radioactive ³⁹Ar (and to a lesser extent ³⁷Ar) is made by cosmic ray activity, primarily from ⁴⁰Ar. In the subsurface environment, ³⁹Ar is also produced through neutron capture by ³⁹K or ⁴²Ca, with proton or alpha emission respectively; ³⁷Ar was created in subsurface nuclear explosions similarly from ⁴⁰Ca. The content of ³⁹Ar in natural argon is measured to be of (8.6±0.4)×10⁻¹⁶ g/g, or (0.964±0.024) Bq/kg weight.

The content of ⁴²Ar (half-life 33 years) in the Earth's atmosphere, though it had previously been reported as a cosmogenic isotope, is lower than 6×10⁻²¹ of the element. Many endeavors require argon depleted in the cosmogenic isotopes, known as depleted argon and this may be obtained from underground sources that have been isolated from the atmosphere long enough for these isotopes to decay.

³⁶Ar, in the form of argon hydride, was detected in the Crab Nebula supernova remnant during 2013. This was the first time a noble molecule was detected in outer space.

List of isotopes