Isokronpassning

Isokronpassning, often shortened to isochron dating, is a radiometric dating technique used to determine the age of rocks and minerals. The name "isochron" comes from the Greek words "isos" (equal) and "chronos" (time), reflecting the method's principle of measuring isotopic ratios that are equal at the time of mineral formation.

This technique relies on the radioactive decay of parent isotopes into daughter isotopes. For isochron dating, a system is chosen where the parent and daughter isotopes are present in the mineral at the time of its formation, and there is no initial difference in the isotopic ratio of the daughter isotope between different mineral grains or phases within a rock sample. As time passes, the parent isotope decays into the daughter isotope, increasing its concentration.

By analyzing multiple samples from the same rock or mineral suite, each with potentially different initial amounts of the parent and daughter isotopes, a plot can be constructed. This plot graphs the ratio of the daughter isotope to a stable, non-radiogenic isotope of the daughter element against the ratio of the parent isotope to the same stable isotope. If the samples all formed at the same time from a common source and have remained closed systems (meaning no isotopes have been added or removed), they will fall on a straight line, known as an isochron. The slope of this isochron is directly proportional to the age of the rock. The y-intercept of the isochron provides information about the initial isotopic ratio of the daughter element.

Commonly used isotopic systems for isochron dating include rubidium-strontium (Rb-Sr), samarium-neodymium (Sm-Nd), and uranium-lead (U-Pb). Isokronpassning is a powerful tool in geology for understanding the timing of geological events, the formation of igneous rocks, and the history of metamorphic processes.