Geochronology

Geochronology is the science of providing ages of events in the history of the Earth and extraterrestrial material and of determining the temporal rates of geological processes by using a number of different dating methods.
The ages can be absolute (e.g. radiometric ages) or relative (e.g. stratigraphic ages).

Absolute age dating (numerical dating)

Most absolute dating methods rely on the analysis of radioactive isotopes and their radiogenic decay products. By measuring the amount of radiogenic "daughter" isotopes produced by radioactive decay from a “mother” isotope with a known half-life, Earth scientists can establish the “absolute age”, i.e. the age in absolute units of time (Ma, ka, etc.) of the parent material. A number of radioactive isotopes from different elements, such as uranium, thorium, rhenium, samarium, lutetium, rubidium and potassium are used for this purpose. Techniques exist to date practically all geological materials, from billions of years in age to historical records. For instance:

  • The U-Pb (decay of 238U -> 206Pb, 235U -> 207Pb) and Th-Pb (232Th -> 208Pb) geochronology of zircon and monazite is used for determining the age of emplacement of igneous rocks and the evolution of magma chambers in the crust. U-Th-Pb ages of metamorphic minerals, such as zircon, titanite, rutile, monazite and xenotime can be used to date high temperature thermal events and to determine the petrochronology, i.e. the temperature-pressure-time evolution, of rocks.
  • Sm-Nd (147Sm -> 143Nd) ages of garnet, pyroxene and amphibole are most useful to constrain metamorphic high-pressure events reflecting for instance eclogitisation of oceanic and continental crust and to date the formation of basic intrusive rocks such as gabbros.
  • Rb-Sr (87Rb -> 87Sr), Ar-Ar (40K -> 40Ar), Fission track, and Uranium-Thorium-Helium (U/Th)-He mineral ages are all well suited to date the cooling of a rock in range of ca. 500 °C to 70 °C.
  • Additionally, 143Nd/144Nd, 176Hf/177Hf, 87Sr/86Sr, and 187Os/188Os isotope ratios provide important geochemical information. For instance, they help to distinguish different mantle and crustal reservoirs and allow the reconstruction of the evolution of these reservoirs.

 

From the large number of different geochronological methods we are currently apply the following:

  • Rb-Sr mineral dating and Sr isotope geochemistry by Thermal Ionization Mass Spectrometry (TIMS)
  • Sm-Nd mineral dating and Nd isotope geochemistry by Thermal Ionization Mass Spectrometry (TIMS)
  • U-Pb and Th-Pb accessory mineral dating by in-situ methods, i.e. Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Secondary Ion Mass Spectrometry (SIMS)
  • U-Pb and Th-Pb accessory mineral dating and mineral chemistry by Laser-Ablation Inductively-Coupled Plasma Mass Spectrometry (LA-ICP-MS) and Laser-Ablation Split-Stream Inductively-Coupled Plasma Mass Spectrometry (LASS-ICP-MS)
  • Hf isotope systematics of zircon by Laser-Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS)
  • Chemical U+Th-Pb monazite dating by Electron Microprobe (EMP)
  • 182W signatures of terrestrial rocks

 

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