Parent daughter ratio radiometric dating
Thus, at the moment of crystallization, the ratio of the concentration of the radiogenic isotope of the daughter element to that of the non-radiogenic isotope is some value independent of the concentration of the parent.
As time goes on, some amount of the parent decays into the radiogenic isotope of the daughter, increasing the ratio of the concentration of the radiogenic isotope to that of the daughter.
Whole rock isochron dating uses the same ideas but instead of different minerals obtained from one rock uses different types of rocks that are derived from a common reservoir; e.g. It is possible to date the differentiation of the precursor melt which then cooled and crystallized into the different types of rocks.
One of the best known isotopic systems for isochron dating is the rubidium–strontium system.
Spectroscopy is used to determine what the minerals are, and how much there is (you saw how to recognize the element in a gas in the spectroscopy lab - with a better spectroscope you can also determine the density and from that the number of particles.) The half life of isotopes is determined by theory, and confirmed with controlled experiments.The greater the initial concentration of the parent, the greater the concentration of the radiogenic daughter isotope will be at some particular time.Thus, the ratio of the daughter to non-radiogenic isotope will become larger with time, while the ratio of parent to daughter will become smaller.However, methods using extinct radionuclides give only relative ages and have to be calibrated with radiometric dating techniques based on long-living radionuclides like Pb-Pb-dating to give absolute ages.
Isochron dating is useful in the determination of the age of igneous rocks, which have their initial origin in the cooling of liquid magma.
Radiometric dating--the process of determining the age of rocks from the decay of their radioactive elements--has been in widespread use for over half a century.