Kallioperän ikä

Koska esim. jotkin uraanin isotoopit muuttuvat toisiksi isotoopeiksi ajan kuluessa, ns. puoliintumisiän puitteissa, niin voidaan sen perusteella arvioida näytteen ikää. Menetelmä on kuitenkin vaativa, koska erilaisia isotooppeja esiintyy runsaasti.

http://fi.wikipedia.org/wiki/Iänmääritysmenetelmä

http://fi.wikipedia.org/wiki/Iänmääritysmenetelmä#Uraani-lyijy-ajoitus

"The long half-life of the isotope uranium-238 (4.51×109 years) makes it well-suited for use in estimating the age of the earliest igneous rocks and for other types of radiometric dating (including uranium-thorium dating and uranium-lead dating)."

http://en.wikipedia.org/wiki/Half-life

http://en.wikipedia.org/wiki/Uranium

"Dating rocks by these radioactive timekeepers is simple in theory, but the laboratory procedures are complex. The numbers of parent and daughter isotopes in each specimen are determined by various kinds of analytical methods. The principal difficulty lies in measuring precisely very small amounts of isotopes."

http://pubs.usgs.gov/gip/geotime/radiometric.html

"The uranium-lead radiometric dating scheme is one of the oldest available, as well as one of the most highly respected. It has been refined to the point that the error in dates of rocks about three billion years old is no more than two million years.

Uranium-lead dating is usually performed on the mineral "zircon" (ZrSiO4), though it can be used on other materials. Zircon incorporates uranium atoms into its crystalline structure as substitutes for zirconium, but strongly rejects lead. It has a very high blocking temperature, is resistant to mechanical weathering and is very chemically inert. Zircon also forms multiple crystal layers during metamorphic events, which each may record an isotopic age of the event. In situ micro-beam analysis can be achieved via laser ICP-MS or SIMS techniques [6] .

One of its great advantages is that any sample provides two clocks, one based on uranium-235's decay to lead-207 with a half-life of about 700 million years, and one based on uranium-238's decay to lead-206 with a half-life of about 4.5 billion years, providing a built-in crosscheck that allows accurate determination of the age of the sample even if some of the lead has been lost."

http://en.wikipedia.org/wiki/Radiometric_dating