F3 Geochronology

• • Atoms are made of electrons, protons and neutrons
  • Number of protons = atomic number
  • Number of protons + number of neutrons = atomic mass
  • Same atomic number, different atomic mass = isotope
  • Parent = decaying radioactive isotope
• • Over time, as parent atoms decay, there is an increase in the ratio of daughter atoms to parent atoms
  • Half life - the time it takes for half of the parent atoms to decay to daughter atoms
  • For accuracy we choose radioactive atoms with long half lives
  • As the number of half lives increase, the amount of parent atoms approaches zero but never reaches it
• Potassium - Argon dating

  • Most widely used method of radiometric dating
  • Potassium present in many common rock forming minterals. 11% of potassium decays to make argon. Potassium is the main element in orthoclase feldspar. Orthoclase itself can NOT be used as daughter element argon is a gas which leaks out of the feldspar
• Samarium and Neodymium dating

  • Rare earth metals found in silicates
  • Ultramafic and mafic rocks contain a higher amount of samarium than neodymium, which remains in the magma
  • Fractional crystallisation - neodymium found in felsic minerals, used to follow how magmas develop as well as their age.
  • Isochron shows decay - the steeper the slope, the greater the number of half lives and therefore the older the rock
• Requirements for radiometric dating

  • Radioactive minerals need to be present
  • Daughter products must not have escaped
  • No weathering or metamorphism - closed system
• Limitations of radiometric dating

  • Closure temperature is the temperature at which a system has cooled - no diffusion of isotopes out of the system
  • Radiometric dating of metamorphic rocks is less likely to produce accurate dates because metamorphism may cause loss or gain of isotopes