Geological Time

| Geologic Time Scale | Plate Tectonics | Radiometric Dating | Deep Time | Geological History of New Zealand |

Radiometric Dating

Radiometric measurements of time

Since the early twentieth century scientists have found ways to accurately measure geological time. The discovery of radioactivity in uranium by the French physicist, Henri Becquerel, in 1896 paved the way of measuring absolute time. Shortly after Becquerel's find, Marie Curie, a French chemist, isolated another highly radioactive element, radium. The realisation that radioactive materials emit rays indicated a constant change of those materials from one element to another. The New Zealand physicist Ernest Rutherford, suggested in 1905 that the exact age of a rock could be measured by means of radioactivity. For the first time he was able to exactly measure the age of a uranium mineral. When Rutherford announced his findings it soon became clear that Earth is millions of years old.

These scientists and many more after them discovered that atoms of uranium, radium and several other radioactive materials are unstable and disintegrate spontaneously and consistently forming atoms of different elements and emitting radiation, a form of energy in the process. The original atom is referred to as the parent and the following decay products are referred to as the daughter. For example: after the neutron of a rubidium-87atom ejects an electron, it changes into a strontium-87 atom, leaving an additional proton.

Major radioactive elements used for radiometric dating.

• Radiocarbon Dating
• Potassium-Argon Dating
• Uranium-Lead Dating
• Fission track analysis

Radiocarbon Dating

Carbon is a very special element. In combination with hydrogen it forms a component of all organic compounds and is therefore fundamental to life. Willard F. Libby of the University of Chicago predicted the existence of carbon-14 before it was actually detected and formulated a hypothesis that radiocarbon might exist in living matter.

Willard Libby and his colleague Ernest Anderson showed that methane collected from sewage works had measurable radiocarbon activity whereas methane produced from petroleum did not. Perseverance over three years of secret research to develop the radiocarbon method came into fruition and in 1960 Libby received the Nobel Prize for chemistry for turning his vision into an invaluable tool.

The basic principle

Carbon has three naturally occurring isotopes, with atoms of the same atomic number but different atomic weights. They are ¹²C, ¹³C and ¹⁴C. C being the symbol for carbon and the isotopes having atomic weights 12, 13 and 14. The three isotopes don't occur equally either, 98.89% of carbon is ¹²C, 1.11% is ¹³C - and only one ¹⁴C atom exists in nature for every 1,000,000,000,000 ¹²C atoms in living material. ¹²C and ¹³C are both stable whereas ¹⁴C is unstable or radioactive.

The radiocarbon dating method is based on the rate of decay of the radioactive or unstable ¹⁴C which is formed in the upper atmosphere through the effect of cosmic ray neutrons upon nitrogen 14.

The reaction is as follows: ¹⁴n + n => ¹⁴C + p

(n is a neutron and p is a proton)

After formation the three carbon isotopes combine with oxygen to form carbon dioxide. The carbon dioxide mixes throughout the atmosphere, dissolves in the oceans, and via photosynthesis enters the food chain to become part of all plants and animals. In principle the uptake rate of ¹⁴C by animals is in equilibrium with the atmosphere.

As soon as a plant or animal dies, they stop the metabolic function of carbon uptake and with no replenishment of radioactive carbon, the amount of ¹⁴C in their tissues starts to reduce as the ¹⁴C atoms decay.

The half-life of ¹⁴C

Libby and his colleagues first discovered that this decay occurs at a constant rate. They found that after 5568 years, half the ¹⁴C in the original sample will have decayed and after another 5568 years, half of that remaining material will have decayed, and so on. This half-life (t 1/2) is the name given to this value which Libby measured at 5568±30 years. This became known as the Libby half-life. After 10 half-lives, there is a very small amount of radioactive carbon present in a sample. At about 50 000 to 60 000 years, the limit of the technique is reached (beyond this time, other radiometric techniques must be used for dating). By measuring the ¹⁴C concentration or residual radioactivity of a sample whose age is not known, it is possible to obtain the number of decay events per gram of Carbon. By comparing this with modern levels of activity (1890 wood corrected for decay to 1950 AD) and using the measured half-life it becomes possible to calculate a date for the death of the sample.

1950 AD is the marker date used in this technique. As a result of atomic bomb usage, ¹⁴C was added to the atmosphere artificially. This affects the ¹⁴C ages of objects younger than 1950.

Any material which is composed of carbon may be dated. Herein lies the true advantage of the radiocarbon method.

Here are some of the materials that can be successfully dated using this method:

Potassium-Argon Dating

Potassium-Argon (K-Ar) dating is the most widely applied technique of radiometric dating. Potassium is a component in many common minerals and can be used to determine the ages of igneous and metamorphic rocks. The Potassium-Argon dating method is the measurement of the accumulation of Argon in a mineral. It is based on the occurrence of a small fixed amount of the radioisotope ⁴⁰K in natural potassium that decays to the stable Argon isotope ⁴⁰Ar with a half-life of about 1,300 million years. In contrast to a method such as Radiocarbon dating, which measures the disappearance of a substance, K-Ar dating measures the accumulation of Argon in a substance from the decomposition of potassium.

Argon, being an inert gas, usually does not leech out of a mineral and is easy to measure in small samples. This method dates the formation or time of crystallisation of the mineral that is being dated; it does not tell when the elements themselves were formed. It is best used with rocks that contain minerals that crystallised over a very short period, possibly at the same time the rock was formed. This method should also be applied only to minerals that remained in a closed system with no loss or gain of the parent or daughter isotope.

Uranium-Lead Dating

Uranium-Lead (U-Pb) dating is the most reliable method for dating Quaternary sedimentary carbonate and silica, and fossils particulary outside the range of radiocarbon. Quaternary geology provides a record of climate change and geologically recent changes in environment. U-Pb geochronology of zircon, baddelyite, and monazite is used for determining the age of emplacement of igneous rocks of all compositions, ranging in age from Tertiary to Early Archean. U-Pb ages of metamorphic minerals, such as zircon or monazite are used to date thermal events, including terrestrial meteoritic impacts. U-Pb ages of zircon in sediments are used to determine the provenance of the sediments.

Fission track analysis

The Fission track analysis is based on radiation damage (tracks) due to the spontaneous fission of ²³⁸U. Fission-tracks are preserved in minerals that contain small amounts of uranium, such as apatite and zircon.

Fission-track analysis is useful in determining the thermal history of a sample or region. By determining the number of tracks present on a polished surface of a grain and the amount of uranium present in the grain, it is possible to calculate how long it took to produce the number of tracks preserved. As long as the mineral has remained cool, near the earth surface, the tracks will accumulate. If the rock containing these minerals is heated, the tracks will begin to disappear. If the rock is heated high enough, >120°C for apatite, all tracks will disappear. Zircons will loose their tracks at higher temperatures of 200°. The tracks will then begin to accumulate when the rock begins to cool. If a rock cools quickly as in the case of a volcanic rock or a shallow igneous intrusion, the fission-track ages will date this initial cooling. If the mineral formed at depth or was deeply buried after formation, the fission-track age will reflect this later heating and cooling. Fission-track analysis has been successfully applied to many diverse areas of the earth sciences: volcanology, mineral deposits, stratigraphy, basin analysis, tectonics, and impact of extraterrestrial bodies.

  Reference Websites

The Waikato laboratory at The University of Waikato, Hamilton, New Zealand is a national radiocarbon facility undertaking both Standard Radiometric Dating and Accelerator Mass Spectrometry Dating (AMS). On their site go to Radiocarbon WEB Info to find information presented jointly with Oxford University on the development of the radiocarbon method:

http://www.radiocarbondating.com/
http://www.c14dating.com/int.html

  Reference Books

Bowman, S. (1990). Radiocarbon Dating. Berkeley and Los Angeles: University of California Press. http://www.radiocarbondating.com/

Hamblin, W. K. and Christiansen, E. H. (1998). Earth's dynamic systems. New Jersey: Prentice-Hall Inc.