Variations in light stable isotopes (such as hydrogen, carbon, nitrogen, oxygen, sulphur, chlorine and bromine) are caused by processes that are called isotope fractionation. Fractionation is the process where differences in physical or chemical properties, mostly the result from differences in the weight of the isotopes result in different enrichments of one of the isotopes of an element. Variations in the isotope composition of an element can also have its origin from radioactive decay of mother isotopes which may be present in different quantities in the original environment. In this case these daughter isotopes are called radiogenic. The isotope geochemistry of strontium is a good example of an element where isotope variations are mainly caused by radioactive decay of a mother isotope, in this case the β-decay of 87Rb to stable 87Sr. Chlorine and bromine do not have any radiogenic isotopes and all terrestrial isotope variations observed for chlorine and bromine are caused by fractionation processes on the stable isotopes.