This is the science that deals with measuring past time and assigns geologic events to their proper dates based on geologic data.
The 40Ar/39Ar isotopic dating technique is a variant of the conventional K–Ar method and is based on the formulation of 39Ar during irradiation of potassium-bearing samples. It is used to date terrestrial rocks and minerals as well as meteorites and lunar samples ranging in age from approximately 2,000 years to the age of the Solar System (4.5 billion years). The method is derived from the natural occurrence of the radioactive isotope of potassium, 40K, which has a dual decay to 40Ca and 40Ar and a half–life of 1.25 billion years. Radiogenic 40Ar ideally accumulates in a mineral over geologic time. So by irradiating a sample of unknown age with a standard of known age and then measuring the abundances of argon isotopes, we can determine an 40Ar/39Ar date, that is, the sample's geologic age. As a result of irradiation, 39Ar serves as a proxy for potassium since it is produced from 39K by fast neutron bombardment. After irradiation, argon isotopes are extracted from samples and standards and separately measured on a gas–source mass spectrometer. Apparent ages for the samples then are calculated by comparing the sample's 40Ar/39Ar ratio with that of the standard. Isotopic measurements on modern mass spectrometers are highly sensitive and precise. Thus, very small amounts of material, ranging in size from a single mineral grain to a few millimeters, are analyzed commonly with small associated analytical errors (less than 0.25% absolute).
40Ar/39Ar geochronology has evolved rapidly over the past 25 years into one of the most commonly used isotopic dating techniques because of its applicability to a broad range of geologic problems. At the U.S Geological Survey, 40Ar/39Ar geochronology is used primarily for mission–related scientific research funded through congressionally–mandated programs. Currently, the method is applied to geologic studies on the origin and thermal histories of mineral deposits; emplacement, cooling, and uplift history of plutonic rocks; formation of metamorphic belts; development of volcanic terranes, formation and amalgamation of the Earth's crust; age and development of the landscape; and the timing of catastrophic events in earth history, such as the K/T boundary event.