Uranium-thorium dating
Encyclopedia
Uranium-thorium dating, also called thorium-230 dating, uranium-series disequilibrium dating or uranium-series dating, is a radiometric dating
technique commonly used to determine the age of calcium carbonate
materials such as speleothem
or coral
. Unlike other commonly used radiometric dating
techniques such as rubidium-strontium
or uranium-lead
dating, the uranium-thorium technique does not measure accumulation of a stable end-member decay product
, instead calculating an age from the degree to which equilibrium
has been restored between the radioactive isotope
thorium
-230 and its radioactive parent uranium
-234 within a sample.
Because uranium
is soluble to some extent in all natural
water
s, any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight. In contrast, thorium
is not soluble in natural waters under conditions found at or near the surface of the earth and so materials grown in or from these waters do not usually contain thorium. As time passes after the formation of such a material, the uranium-234 in the sample decays to thorium-230, with a half-life
of 245,000 years. The thorium-230 is itself radioactive with a half-life
of 75,000 years and so instead of accumulating indefinitely (as for instance is the case for the uranium-lead
system) it instead approaches secular equilibrium
with its parent isotope. At equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of uranium-234 decays per year in the same sample.
Uranium-thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, the precision
with which we can measure the thorium-230/uranium-234 ratio in a sample, and the accuracy to which we know the half-lives of thorium-230 and uranium-234. Note that to calculate an age using this technique the ratio of uranium-234 to its parent isotope uranium-238 must also be measured.
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
technique commonly used to determine the age of calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
materials such as speleothem
Speleothem
A speleothem , commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems are typically formed in limestone or dolostone solutional caves.-Origin and composition:...
or coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
. Unlike other commonly used radiometric dating
Radiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
techniques such as rubidium-strontium
Rubidium-strontium dating
The rubidium-strontium dating method is a radiometric dating technique that geologists use to determine the age of rocks.Development of this process was aided by Fritz Strassmann, who later went on to discover nuclear fission with Otto Hahn and Lise Meitner....
or uranium-lead
Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
dating, the uranium-thorium technique does not measure accumulation of a stable end-member decay product
Decay product
In nuclear physics, a decay product is the remaining nuclide left over from radioactive decay. Radioactive decay often involves a sequence of steps...
, instead calculating an age from the degree to which equilibrium
Secular equilibrium
In nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate is equal to its decay rate.-Secular equilibrium in radioactive decay:...
has been restored between the radioactive isotope
Isotope
Isotopes are variants of atoms of a particular chemical element, which have differing numbers of neutrons. Atoms of a particular element by definition must contain the same number of protons but may have a distinct number of neutrons which differs from atom to atom, without changing the designation...
thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
-230 and its radioactive parent uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
-234 within a sample.
Because uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
is soluble to some extent in all natural
Nature
Nature, in the broadest sense, is equivalent to the natural world, physical world, or material world. "Nature" refers to the phenomena of the physical world, and also to life in general...
water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
s, any material that precipitates or is grown from such waters also contains trace uranium, typically at levels of between a few parts per billion and few parts per million by weight. In contrast, thorium
Thorium
Thorium is a natural radioactive chemical element with the symbol Th and atomic number 90. It was discovered in 1828 and named after Thor, the Norse god of thunder....
is not soluble in natural waters under conditions found at or near the surface of the earth and so materials grown in or from these waters do not usually contain thorium. As time passes after the formation of such a material, the uranium-234 in the sample decays to thorium-230, with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 245,000 years. The thorium-230 is itself radioactive with a half-life
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
of 75,000 years and so instead of accumulating indefinitely (as for instance is the case for the uranium-lead
Uranium-lead dating
Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...
system) it instead approaches secular equilibrium
Secular equilibrium
In nuclear physics, secular equilibrium is a situation in which the quantity of a radioactive isotope remains constant because its production rate is equal to its decay rate.-Secular equilibrium in radioactive decay:...
with its parent isotope. At equilibrium, the number of thorium-230 decays per year within a sample is equal to the number of uranium-234 decays per year in the same sample.
Uranium-thorium dating has an upper age limit of somewhat over 500,000 years, defined by the half-life of thorium-230, the precision
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
with which we can measure the thorium-230/uranium-234 ratio in a sample, and the accuracy to which we know the half-lives of thorium-230 and uranium-234. Note that to calculate an age using this technique the ratio of uranium-234 to its parent isotope uranium-238 must also be measured.