K–Ar

Naturally occurring K has three isotopes, 39K (93.2581 per cent), 40K (0.0117 per cent)and 41K (6.7302 per cent), of which 40K is radioactive with a half life of 1.248 × 109 years (λ = 0.581 × 10−10 yr−1). Unlike most other decay schemes in geochemistry, however, the decay of 40K is via a branched decay scheme, in which 89.1 per cent of the decay events produce 40Ca via beta decay and the remaining 10.9 per cent produce 40Ar through electron capture. Both of these decay products are the most abundant isotopes of the daughter element in question. Therefore, particularly in the case of 40Ca, the radiogenic contribution is very hard to detect as Ca is a highly abundant element, and >96 per cent of all Ca is 40Ca. In contrast, although >99 per cent of all Ar is 40Ar, because there is so little Ar present on earth, the contribution to the total Ar budget through the radioactive decay of K is significant and can be measured readily enough – despite a half life of 1.248 Gyr and only ~10 per cent of those decays producing Ar.

Ar is of particular interest for geochronology, as it is a noble gas and hence does not form any molecular bonds in minerals or bind with the crystal lattice in any way. Hence, when minerals grow, they do not incorporate Ar as a fundamental part of the crystal itself, and hence Ar (and indeed all noble gases) are the most incompatible of elements in geochemistry. However, when K within a crystal decays to form Ar, the Ar remains trapped within the crystal lattice as it has a larger ionic radius than the parent 40K atom, and hence is typically larger than other gaps in mineral lattices as well. Therefore, to a first order it can often be considered that all 40Ar measured within a mineral is the result of radioactive decay since the mineral will not contain any when it grows.

However, changes in pressure and/or temperature may warp the lattice of the mineral sufficiently to allow the accumulated Ar to diffuse and escape, thus effectively resetting the system. Because Ar does not chemically bind with the lattice of the minerals, the closure temperature of Ar is relatively low in most silicate minerals.