Radiometric dating geologic time and the age of the earth
And even though various radioactive elements have been used to "date" these rocks, for the most part, the methods are basically the same. This means that if you had some pure uranium-238 with no lead in it, 4.5 billion years later one half of it would have decayed into its stable daughter product (lead-206). Where these alpha particles finally stop, crystal deformation occurs (and) shows up as a discolouration or a darkening of the crystals.They consist of measuring the amount of radioactive (mother) element and comparing it to the amount of stable (daughter) element. Uranium is radioactive, which means it is in the process of changing from an unstable element into a stable one. And after 9 billion years there would be 75% lead and 25% uranium, and so on. (an) episode of drastically accelerated decay has ... When the crystal is looked at under a microscope, these discolourations appear as dark ringshence the name "pleochroic halo".
If an igneous or other rock is metamorphosed, its radiometric clock is reset, and potassium-argon measurements can be used to tell the number of years that has passed since metamorphism.
Few people realize it but all radiometric dating methods require making at least three assumptions. Now the magnitude of the radius of a pleochroic halo in a particular mineral depends on the amount of energy that the alpha particle has ... depends on the half-life of the particular decay responsible for this alpha particle emission.
In other words, the magnitude of the radius of a pleochroic halo in a particular crystal depends on the half-life of the decay responsible for the alpha particle emission. the radii of pleochroic haloes corresponding to a definite decay in a particular mineral are ...
For example, if a rock sample was below the water table at any time, leaching would take place.
For Uranium/Lead dating this means that some of the uranium that was initially present would be "leached" out of the rock.