Half life and radioactive dating
Nd ratios on several minerals with a mass spectrometer and then from the slope determine the age of the rock. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped.
The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. If this happens, then the date obtained will be older than the date at which the magma erupted.
By definition, D* = N-1) (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.The decay process takes time and there is value in being able to express the rate at which a process occurs.A useful concept is half-life, which is the time required for half of the starting material to change or decay.Zircon has a high hardness (7.5) which makes it resistant to mechanical weathering, and it is also very resistant to chemical weathering. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.Other minerals that also show these properties, but are less commonly used in radiometric dating are Apatite and sphene.