Radioactive dating

Radioactive dating is a method of dating rocks and minerals using radioactive isotopes. This method is useful for igneous and metamorphic rocks, which cannot be dated by the stratigraphic correlation method used for sedimentary rocks.

Over 300 naturally-occurring isotopes are known. Some do not change with time and form stable isotopes (i.e. those that form during chemical reactions without breaking down). The unstable or more commonly known radioactive isotopes break down by radioactive decay into other isotopes.

Radioactive decay is a natural process and comes from the atomic nucleus becoming unstable and releasing bits and pieces. These are released as radioactive particles (there are many types). This decay process leads to a more balanced nucleus and when the number of protons and neutrons balance, the atom becomes stable.

This radioactivity can be used for dating, since a radioactive 'parent' element decays into a stable 'daughter' element at a constant rate. The rate of decay (given the symbol λ) is the fraction of the 'parent' atoms that decay in unit time. For geological purposes, this is taken as one year. Another way of expressing this is the half-life period (given the symbol T). The half-life is the time it takes for half of the parent atoms to decay. The relationship between the two is: T = 0.693 / λ

How is this radioactivity measured?

Many different radioactive isotopes and techniques are used for dating. All rely on the fact that certain elements (particularly uranium and potassium) contain a number of different isotopes whose half-life is exactly known and therefore the relative concentrations of these isotopes within a rock or mineral can measure the age. For an element to be useful for geochronology (measuring geological time), the isotope must be reasonably abundant and produce daughter isotopes at a good rate.

Either a whole rock or a single mineral grain can be dated. Some techniques place the sample in a nuclear reactor first to excite the isotopes present, then measure these isotopes using a mass spectrometer (such as in the argon-argon scheme). Others place mineral grains under a special microscope, firing a laser beam at the grains which ionises the mineral and releases the isotopes. The isotopes are then measured within the same machine by an attached mass spectrometer (an example of this is SIMS analysis).

What dating methods are there?

Radiocarbon (14C) dating

This is a common dating method mainly used by archaeologists, as it can only date geologically recent organic materials, usually charcoal, but also bone and antlers.

All living organisms take up carbon from their environment including a small proportion of the radioactive isotope 14C (formed from nitrogen-14 as a result of cosmic ray bombardment). The amount of carbon isotopes within living organisms reaches an equilibrium value, on death no more is taken up, and the 14C present starts to decay at a known rate. The amount of 14C present and the known rate of decay of 14C and the equilibrium value gives the length of time elapsed since the death of the organism.

This method faces problems because the cosmic ray flux has changed over time, but a calibration factor is applied to take this into account. Radiocarbon dating is normally suitable for organic materials less than 50 000 years old because beyond that time the amount of 14C becomes too small to be accurately measured.

Rubidium-Strontium dating (Rb-Sr)

This scheme was developed in 1937 but became more useful when mass spectrometers were improved in the late 1950s and early 1960s. This technique is used on ferromagnesian (iron/magnesium-containing) minerals such as micas and amphiboles or on limestones which also contain abundant strontium. However, both Rb and Sr easily follow fluids that move through rocks or escape during some types of metamorphism. This technique is less used now.

Potassium-Argon dating (K-Ar)

The dual decay of potassium (K) to 40Ar (argon) and 40Ca (calcium) was worked out between 1921 and 1942. This technique has become more widely used since the late 1950s. Its great advantage is that most rocks contain potassium, usually locked up in feldspars, clays and amphiboles. However, potassium is very mobile during metamorphism and alteration, and so this technique is not used much for old rocks, but is useful for rocks of the Mesozoic and Cenozoic Eras, particularly unaltered igneous rocks.
Argon-Argon dating (39Ar-40Ar)

This technique developed in the late 1960s but came into vogue in the early 1980s, through step-wise release of the isotopes. This technique uses the same minerals and rocks as for K-Ar dating but restricts measurements to the argon isotopic system which is not so affected by metamorphic and alteration events. It is used for very old to very young rocks.

Samarium-Neodymium (Sm-Nd)

The decay of 147Sm to 143Nd for dating rocks began in the mid-1970s and was widespread by the early 1980s. It is useful for dating very old igneous and metamorphic rocks and also meteorites and other cosmic fragments. However, there is a limited range in Sm-Nd isotopes in many igneous rocks, although metamorphic rocks that contain the mineral garnet are useful as this mineral has a large range in Sm-Nd isotopes. This technique also helps in determining the composition and evolution of the Earth's mantle and bodies in the universe.

Rhenium-Osmium (Re-Os) system

The Re-Os isotopic system was first developed in the early 1960s, but recently has been improved for accurate age determinations. The main limitation is that it only works on certain igneous rocks as most rocks have insufficient Re and Os or lack evolution of the isotopes. This technique is good for iron meteorites and the mineral molybdenite.

Uranium-Lead (U-Pb) system

This system is highly favoured for accurate dating of igneous and metamorphic rocks, through many different techniques. It was used by the beginning of the 1900s, but took until the early 1950s to produce accurate ages of rocks. The great advantage is that almost all igneous and metamorphic rocks contain sufficient U and Pb for this dating. It can be used on powdered whole rocks, mineral concentrates (isotope dilution technique) or single grains (SHRIMP technique).

The SHRIMP technique

The SHRIMP (Sensitive High Resolution Ion MicroProbe) technique was developed at the Research School of Earth Sciences, Australian National University, Canberra in the early 1980s. It has revolutionised age dating using the U-Pb isotopic system. Using the SHRIMP, selected areas of growth on single grains of zircon, baddeleyite, sphene, rutile and monazite can be accurately dated (to less than 100 000 years in some cases). This technique not only dates older mineral cores (what we call inherited cores), but also later magmatic and/or metamorphic overgrowths so that it unravels the entire geological history of a single mineral grain. It can even date nonradioactive minerals when they contain inclusions of zircons and monazite, as in sapphire grains. The SHRIMP technology has now been exported to many countries such as the USA, France, Norway, Russia, Japan and China. It can help fix the maximum age of sedimentary rocks when they contain enough accessory zircon grains (usually need about 100 grains).

Because of advancements in geochronology for over 50 years, accurate formation ages are now known for many rock sequences on Earth and even in space. The oldest accurately dated rocks on Earth are metamorphosed felsic volcanic rocks from north-west Western Australia. These were dated at about 4.5 billion years old using single zircon grains on the SHRIMP.

Fission track dating

Several minerals incorporate tiny amounts of uranium into their structure when they crystallise. The radioactive decay from the uranium releases energy and particles (this strips away electrons leading to disorder in the mineral structure). The travel of these particles through the mineral leaves scars of damage about one thousandth of a millimetre in length. These 'fission tracks' are formed by the spontaneous fission of 238U and are only preserved within insulating materials where the free movement of electrons is restricted. Because the radioactive decay occurs at a known rate, the density of fission tracks for the amount of uranium within a mineral grain can be used to determine its age.

To see the fission tracks, the mineral surface is polished, etched with acids, and examined with an electron microscope. An effective way to measure the uranium concentration is to irradiate the sample in a nuclear reactor and produce comparative artificial tracks by the induced fission of 235U.

Fission track dating is commonly used on apatite, zircon and monazite. It helps to determine the rates of uplift (for geomorphology studies), subsidence rates (for petroleum exploration and sedimentary basin studies), and the age of volcanic eruptions (this is because fission tracks reset after the eruption). However, care is needed as some samples have fission tracks reset during bushfires, giving far too young ages. Fission track dating is mostly used on Cretaceous and Cenozoic rocks.

Terms
 
  • The atomic number of an element is given by the number of protons present within the element's nucleus, and this helps determine the chemical properties of that element.
  • The atomic mass of an element combines the number of protons and neutrons within its nucleus.
  • The atomic weight of an element is the average relative weight (mass) of atoms and can vary to give different isotopic members of the element.
  • Isotopes are atoms with the same atomic number (i.e. protons) and have different atomic masses (i.e. number of neutrons). For example, the element Potassium (represented by the symbol K) has three isotopes: Isotope 39K, 40K, 41K (Relative abundance in nature 93.1%, 0.01%, 6.9%). The numbers 39, 40, and 41 are the mass numbers. As all three isotopes have 19 protons, they all have the chemical properties of Potassium, but the number of neutrons differs: 20 in 39K, 21 in 40K, and 22 in 41K. Potassium has an atomic weight of 39.102, close to the mass (39) of its most abundant isotope in nature (39K).


Ian Graham
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Tags radioactive, dating, carbon, archaeology, palaeontology, geosciences, geology, geological time, stratigraphy,