Hafnium

Some ¹⁷⁶Hf is radiogenic as a result of it being formed as a product of beta decay of radioactive ¹⁷⁶Lu (half-life of 3.73×10¹⁰ years) [301]. Thus, relations between the isotope-amount ratiosn(¹⁷⁶Hf)/n(¹⁷⁷Hf) and n(¹⁷⁶Hf)/n(¹⁷⁶Lu) have been used to determine the ages of minerals and rocks. Because of the long half-life of ¹⁷⁶Lu, these ratios have been used in geochronology studies that document some of the oldest rocks in the Solar System and on Earth (Fig. IUPAC.72.1). Hafnium isotopic compositions of terrestrial materials evolved differently depending on the relative rates of ¹⁷⁶Hf production. Geologists can use calculated lutetium-hafnium ages and the initial isotope-amount ratio n(¹⁷⁶Hf)/n(¹⁷⁷Hf) along with other isotopic data from the oldest rocks in the Earth to infer that the Earth’s crust differentiated within the first few hundred million years after condensation of the oldest solid matter in the Solar System [502]. Radioactive ¹⁸²Hf decays to ¹⁸²W with a half-life of 8.9×10⁶ years, which is much less than the age of meteorites and the Earth. Therefore, measurements of the amounts of hafnium and tungsten isotopes in meteorites and terrestrial samples reveal the earlier presence of ¹⁸²Hf. As a result, this provides information about chemical differentiation and evolution of the early Solar System [503], [504].