Rubidium 87 radiometric dating
For example lavas dated by KAr that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.Such trapped Ar is not problematical when the age of the rock is in hundreds of millions of years.The only problem is that we only know the number of daughter atoms now present, and some of those may have been present prior to the start of our clock. The reason for this is that Rb has become distributed unequally through the Earth over time.We can see how do deal with this if we take a particular case. For example the amount of Rb in mantle rocks is generally low, i.e. The mantle thus has a low If these two independent dates are the same, we say they are concordant.Since the halflife of carbon14 is 5730 years, scientists can measure the age of a sample by determining how many times its original carbon14 amount has been cut in half since the death of the organism.For example, an object with a quarter of its original amount (2x1/2) should be roughly 11,460 years old.
By definition, D* = N1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.In all radiometric procedures there is a specific age range for when a technique can be used.If there is too much daughter product(in this case nitrogen14), age is hard to determine since the halflife does not make up a significant percentage of the material's age.Thus, if we start out with 1 gram of the parent isotope, after the passage of 1 halflife there will be 0.5 gram of the parent isotope left.
Rubidium 87 radiometric dating comments 

Clocks in the Rocks  HyperPhysics Concepts
paulette60