The radioactive parent K is an alkali metal (as with Rb). The radioactive isotope 40K decays to 2 daughter isotopes; the daughter Ca is an alkali Earth (as with Sr) whereas Ar is a noble gas.
K is enriched in micas, amphiboles and clays relative to Ca, whereas Ar, being an inert gas, may be lost to the atmosphere during disturbance events.
The radioactive isotope 40K presently makes up only 0.012% of K.
- branched decay: 1 parent decays to 2 daughter elements (40Ca and 40Ar)
- branching ratio (b-decay/total) is c. 0.89, so 89% of decays are to 40Ca
- 11% or decays are to the rare gas 40Ar
- combined half-life (both branches) of 40K is 1.25 Ga
- Parent 40K not abundant relative to total K
- daughter 40Ca is the most abundant isotope of Ca, with 40Ca comprising 97% of Ca (i.e. 40Ca/42Ca ratio ~151).
Therefore, until recent with improvements in mass spectrometry instrumentation, it has been difficult to measure the abundance of 40Ca precisely enough to detect changes in 40Ca due to decay of 40K. The 42Ca/44Ca ratio is commonly used to correct for instrumental mass fractionation.
Ar is an inert noble gas, in low abundance in the mantle and in igneous systems and readily lost initially, so little Ar is incorporated into igneous minerals when they first form and no initial Ar correction required. However, radiogenic Ar may be redistributed amongst metamorphic minerals during metamorphism or diagenesis. Ar is also readily lost during disturbance or alteration, so minerals commonly display evidence of open-system behaviour.
To obtain a 40Ar/39Ar date, a few mg high-purity sample of a suitable high-K mineral (such as biotite, hornblende or tourmaline, for example) is placed in a nuclear reactor and the 39K is converted to 39Ar by neutron capture. Then it is only necessary to measure the 40Ar/39Ar ratio of the irradiated mineral, using a mass spectrometer, to obtain date. A flux monitor is irradiated with the sample in the reactor to determine the neutron flux density and capture cross-section and thus, the K content of mineral.
Step heating or laserprobe melting of the irradiated minerals in a vacuum is undertaken to extract Ar for isotopic analysis. The minerals are placed in slots in a vacuum and a laser or induction furnace is used to progressively heat up or fuse the mineral and release trapped Ar for isotopic analysis using a mass spectrometer.