Radiometric dating - internal clocks in rocks

• Basics of radioactivity.
• What event is being dated (closed versus open systems)?
• Carbon-14 dating.
• What geologic materials can be dated?
• Accuracy, error and testing the technique.
• Other dating techniques.

Basics of radioactive decay and isotopic dating

Radioactive decay occurs at an exponential rate, meaning that it can be described in terms of a half life. After one half live, half of the original radioactive isotope material in the system under consideration decays. Another half life and half of the remaining material decays, and so on. This is for unforced decay. Forced decay is when the isotopic material is packed densely enough that a decay in one unstable atom sends out a particle that hits another atom and causes it to decay. If it is packed too densely there is a run away reaction and one of those unpopular mushroom clouds or meltdowns. Normal concentrations of radioactive material on earth are well below the levels where forced decay occurs so we can use the relatively simple mathematics of exponential decay to describe the process. the rate of decay is constant for earthly P-T conditions.

A major assumption is that the rock or mineral being dated has been a closed system so that no parent isotope or daughter product has escaped or been added. This assumption can be tested for.

Systems commonly used for radiometric dating, with half lives.

• uranium 238 --> lead 206, 4,510 Ma (Ma= Millions of years).
• rubidium 87 --> strontium 86, 47,000 Ma.
• potassium 40 --> argon 40, 1,300 Ma.
• carbon 14 --> nitrogen 14, 5,730 years (note this is much shorter).
• 
• Major istopes used for dating purposes. Diagram courtesy of USGS.

For more information on radioactivity and geochronology - USGS site - http://geology.cr.usgs.gov/capabilities/gronemtrac/geochron/geochron.html .

What event is being dated?

What event sets the clock, or more succinctly, when is the system closed?

• significant thermal control on diffusion rates and hence on system closure.
• age of crystallization one possibility:
  • zircons (type of mineral common in granites) and U-Pb dating.
  • easier for volcanic rocks at surface sinice cool quickly (have a simpler thermal history).
• time of cooling through closure temperature:
  • muscovite, K-Ar dating, and time of end of metamorphism.
  • more complicated because metamorphic rocks can have more complicated thermal histories.
• time since exposed to cosmogenic radiation (time since last exposure):
  • Carbon-14 system is a good example of this. time since death and communication with atmosphere.
  • can also date time since burial or time since exposed using other cosmogenic radiation.
• using different systems on the same rock can get a thermal history.
• different systems and different materials can date different types of geologic events.

Carbon-14 dating

• with such a short half-life negligible primordial C-14 left, but it is around, so where does it come from?
• semi-constant production by cosmic ray bombardment so semi-constant C-12 vs. C-14 ratio in atmosphere. From other sources they have worked out history of production and resulting ratio.
• when alive an organism is in communication with the atmosphere (part of the short-term carbon cycle) and maintains this ratio of C-12 to C-14.
• when it dies the C-14 decays and the ratio changes so that by measuring the ratio you can calculate time since death.

Diagram focusing on some short-lived radioactive isotopes, including carbon-14. Some of these other isotope systems are also used for dating purposes. Diagram from USGS website: http://sofia.usgs.gov/publications/fs/73-98/

What geologic materials can be dated?

This depends on what system is being used, which determines what type of event is being dated.

• what would the date from a conglomerate mean?
• igneous rocks generally work well, depending on composition. Volcanic rocks, because of their quick and simple cooling history are the simplest to date.
• whole rock vs. mineral separates dates.

Accuracy, error and testing the technique

Some thoughts on accuracy and error:

• measurement or analytic error.
• contamination (violation of assumptions) error.

Testing the technique:

• Are the results repeatable?
• Are different radiometric clocks consistent with each other?
• Are the results consistent with geologic relationships?
• Do materials of a known age (e.g. archeological) give the right answer?
• Are the results consistent with other independent rates of measurement?
  • Devonian volcanics, Devonian corals, tidal retardation, and years in a day. Rate of retardation is about 2 seconds every 1000 years.
  • GPS measurements of present day plate motions.
• technique has been thoroughly tested and 'passes' all the above tests.

Other dating techniques:

• dendrochronology: use of unique history of annual tree rings.
  • continuous history of tree rings back > 11,000 years, intermittant back to a 20,000 year length.
  • used in archeology and geomorphology studies, provides paleoclimate record.
• fission track densities and fission track ages.
• 
• The scars of age take many forms. In this particular case radioactive material distributed throughout certain minerals leave a scar as an individual atom decays and a particle goes shooting off through the surrounding lattice structure, as portrayed schematically in the above diagram. Apatite and zircon are two common minerals used because they naturally come with a fair bit of radioactive material. The density of fission tracks is a function of age since cooling and of the concentration of the decaying isotope. The latter can be measured for the given specimen and then the age can be solved for. This has been used for uplift and cooling histories.
• Image from USGS site ( http://energy.cr.usgs.gov/other/uranium/ )of fission tracks around a U rich particle (microscope view).
• crater densities used to relatively date age of a planetary surface. If can assume or work out a historic crater flux can assign numbers.

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