Isotope sediment dating
For a single element, these atoms are called isotopes.
Because isotopes differ in mass, their relative abundance can be determined if the masses are separated in a mass spectrometer (see below Use of mass spectrometers).
The situation is analogous to the death rate among human populations insured by an insurance company.
Even though it is impossible to predict when a given policyholder will die, the company can count on paying off a certain number of beneficiaries every month.
Fortunately for geochronology the study of radioactivity has been the subject of extensive theoretical and laboratory investigation by physicists for almost a century.
The rock or mineral must have remained closed to the addition or escape of parent and daughter atoms since the time that the rock or mineral (system) formed. It must be possible to correct for other atoms identical to daughter atoms already present when the rock or mineral formed. In isochron methods that make use of the rubidium–strontium or samarium–neodymium decay schemes (see below), a series of rocks or minerals are chosen that can be assumed to have the same age and identical abundances of their initial isotopic ratios.
In other words, it is the obligation of geochronologists to try to prove themselves wrong by including a series of cross-checks in their measurements before they publish a result.
Such checks include dating a series of ancient units with closely spaced but known relative ages and replicate analysis of different parts of the same rock body with samples collected at widely spaced localities.
Proportion 1 becomes: Stated in words, this equation says that the rate at which a certain radioisotope disintegrates depends not only on how many atoms of that isotope are present but also on an intrinsic property of that isotope represented by λ, the so-called decay constant.
Values of λ vary widely—from 10 is the time elapsed since time zero.