Radiometric dating uses what isotopes
All rocks and minerals contain long-lived radioactive elements that were incorporated into Earth when the Solar System formed.These radioactive elements constitute independent clocks that allow geologists to determine the age of the rocks in which they occur.The atoms in some chemical elements have different forms, called isotopes.These isotopes break down at a constant rate over time through radioactive decay.
Typically commonly occurring fossils that had a widespread geographic distribution such as brachiopods, trilobites, and ammonites work best as index fossils.
This is well-established for most isotopic systems.
However, construction of an isochron does not require information on the original compositions, using merely the present ratios of the parent and daughter isotopes to a standard isotope.
The number of parent atoms originally present is simply the number present now plus the number of daughter atoms formed by the decay, both of which are quantities that can be measured.
Samples for dating are selected carefully to avoid those that are altered, contaminated, or disturbed by later heating or chemical events.