Lead—lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite. For most dating requirements it has been superseded by uranium—lead dating U—Pb dating , but in certain specialized situations such as dating meteorites and the age of the Earth it is more important than U—Pb dating. There are three stable "daughter" Pb isotopes that result from the radioactive decay of uranium and thorium in nature; they are Pb, Pb, and Pb. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from U, U and Th respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate.
Uranium–lead dating - Wikipedia
The passage of time can be measured in many ways. For humans, the steady movement of the hands on a clock marks off the seconds and the hours. In nature, the constant decay of radioactive isotopes records the march of years. Scientists can use the clocklike behavior of these isotopes to determine the age of rocks, fossils, and even some long-lived organisms.
It is normally assumed that in any small part of the Earth's crust and underlying mantle which, at the time of formation, contained primordial lead together with uranium and thorium, no radiogenic lead could have been present. With the passage of time atoms of radiogenic Pb, Pb, and Pb gradually replaced uranium and thorium atoms. If, at one instant in time, all the lead in the area under discussion was removed in solution and deposited as a lead ore, then this would preserve a record of the isotopic balance of lead at the time.
What was missing from the early geologic time scale? While the order of events was given, the dates at which the events happened were not. With the discovery of radioactivity in the late s, scientists were able to measure the absolute age , or the exact age of some rocks in years. Absolute dating allows scientists to assign numbers to the breaks in the geologic time scale.