Unraveling such complications which, depending on their maximum lead-retention temperature, can also exist within other minerals generally requires in situ micro-beam analysis via, say, ion microprobe SIMS or laser ICP-MS. Under conditions where no lead loss or gain from the outside environment has occurred, the age of the zircon can be calculated by assuming exponential decay of Uranium. Thus, once the rock has cooled to the point where diffusion of elements does not occur, the 87 Rb in each mineral will decay to 87 Sr, and each mineral will have a different 87 Rb and 87 Sr after passage of time. For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. Elements like K, U, Th, and Rb occur in quantities large enough to release a substantial amount of heat through radioactive decay.
We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates.
This gives us only a minimum age of the Earth. Measuring the amount of 14 C in this dead material thus enables the determination of the time elapsed since the organism died. Finally, ages can also be determined from the U—Pb system by analysis of Pb isotope ratios alone. From the Pb-Pb isochron equation 11 we can make some arguments about meteorites. This is only a problem when dating very young rocks or in dating whole rocks instead of mineral separates. When an organism dies, the 14 C decays back to 14 N, with a half-life of 5, years.