On the Earth, 40 Ar makes up The longest-lived radioactive isotopes are 39 Ar with a half-life of years, 42 Ar with a half-life of All other isotopes have half-lives of less than two hours, and most less than one minute. The naturally occurring 40 K , with a half-life of 1.
Argonargon dating | geochronology | vermontdairyproducers.com
Potassium, an alkali metal, the Earth's eighth most abundant element is common in many rocks and rock-forming minerals. The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present. Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral. Potassium can be mobilized into or out of a rock or mineral through alteration processes.
The older method required splitting samples into two for separate potassium and argon measurements, while the newer method requires only one rock fragment or mineral grain and uses a single measurement of argon isotopes. The sample is generally crushed and single crystals of a mineral or fragments of rock hand-selected for analysis. These are then irradiated to produce 39 Ar from 39 K. The sample is then degassed in a high-vacuum mass spectrometer via a laser or resistance furnace.