isokronimenetelmät

Isokronimenetelmät, also known as isochron dating, is a radiometric dating technique used in geology and archaeology. It is a variation of the uranium-lead dating method that utilizes the decay of uranium isotopes to lead isotopes. The key principle behind isochron dating is the assumption that a mineral or rock sample initially contained no radiogenic lead isotopes, meaning all the lead present was primordial.

The method involves analyzing the ratios of parent uranium isotopes (238U and 235U) to their respective stable lead daughter isotopes (206Pb and 207Pb) in multiple samples from the same geological unit. These samples are plotted on a graph where the y-axis represents the ratio of a daughter isotope to a non-radiogenic lead isotope (e.g., 206Pb/204Pb) and the x-axis represents the ratio of a parent isotope to the same non-radiogenic lead isotope (e.g., 238U/204Pb). If the initial assumption holds true, and all samples have the same initial lead isotopic composition, they will align on a straight line.

This line is called an isochron. The slope of the isochron is directly proportional to the age of the sample, while the y-intercept indicates the initial lead isotopic ratio. By measuring the slope, scientists can calculate the age of the rock or mineral. The isochron method has the advantage of not requiring knowledge of the initial amount of radiogenic lead, making it more robust than simple parent-daughter ratio dating. It is widely used to date igneous rocks, metamorphic rocks, and meteorites, providing crucial information about Earth's history and the formation of the solar system.