Uranium-uranium dating
Encyclopedia
Uranium-uranium dating is a radiometric dating
technique which compares two isotope
s of uranium
(U) in a sample: 234U and 238U. 234U/238U dating is one of several radiometric dating techniques exploiting the uranium radioactive decay series, in which 238U undergoes 14 alpha
and beta decay
events while decaying to the stable isotope 206Pb
. Other dating techniques using this decay series include uranium-thorium
(using 230Th/238U) and uranium-lead dating
.
238U, with a half-life
of about 4.5 billion years, decays to 234U through emission of an alpha particle
to an isotope of thorium
(234Th), which is comparatively unstable with a half-life of just 24 days. 234Th then decays through beta particle
emission to an isotope of protactinium
, 234Pa. 234Pa decays with a half-life of 6.7 hours, again through emission of a beta particle, to 234U. This isotope has a half-life of about 245,000 years. The next decay product
, 230Th, has a half-life of about 75,000 years and is used for the related 230Th/238U
technique. Although analytically simpler than 230Th/238U dating, in practice 234U/238U dating is almost never used as unlike 230Th/238U dating it requires prior knowledge of the 234U/238U ratio at the time the material under study was formed. For those materials (principally marine carbonates) for which the initial ratio is known, 230Th/238U
remains a superior technique. This restricts the application of 234U/238U to extremely rare cases where the initial 234U/238U is well-constrained and the sample is also beyond the ca. 450,000 year upper limit of the 230Th/238U
technique.
Unlike other radiometric dating techniques, those using the uranium decay series (except for those using the stable final isotopes 206Pb and 207Pb) compare the ratios of two radioactive unstable isotopes. This complicates calculations as both the parent and daughter isotopes decay over time into other isotopes.
In theory, the 234U/238U technique can be useful in dating samples between about 10,000 and 2 million years Before Present
(BP), or up to about eight times the half-life of 234U. As such, it provides a useful bridge in radiometric dating techniques between the ranges of 230Th/238U
(accurate up to ca. 450,000 years) and U-Pb dating
(accurate up to the age of the solar system, but problematic on samples younger than about 2 million years).
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 which compares two 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...
s of 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...
(U) in a sample: 234U and 238U. 234U/238U dating is one of several radiometric dating techniques exploiting the uranium radioactive decay series, in which 238U undergoes 14 alpha
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
and beta decay
Beta decay
In nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
events while decaying to the stable isotope 206Pb
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
. Other dating techniques using this decay series include uranium-thorium
Uranium-thorium dating
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...
(using 230Th/238U) and uranium-lead dating
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...
.
238U, 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 about 4.5 billion years, decays to 234U through emission of an alpha particle
Alpha particle
Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus, which is classically produced in the process of alpha decay, but may be produced also in other ways and given the same name...
to an isotope of 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....
(234Th), which is comparatively unstable with a half-life of just 24 days. 234Th then decays through beta particle
Beta particle
Beta particles are high-energy, high-speed electrons or positrons emitted by certain types of radioactive nuclei such as potassium-40. The beta particles emitted are a form of ionizing radiation also known as beta rays. The production of beta particles is termed beta decay...
emission to an isotope of protactinium
Protactinium
Protactinium is a chemical element with the symbol Pa and atomic number 91. It is a dense, silvery-gray metal which readily reacts with oxygen, water vapor and inorganic acids. It forms various chemical compounds where protactinium is usually present in the oxidation state +5, but can also assume...
, 234Pa. 234Pa decays with a half-life of 6.7 hours, again through emission of a beta particle, to 234U. This isotope has a half-life of about 245,000 years. The next 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...
, 230Th, has a half-life of about 75,000 years and is used for the related 230Th/238U
Uranium-thorium dating
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...
technique. Although analytically simpler than 230Th/238U dating, in practice 234U/238U dating is almost never used as unlike 230Th/238U dating it requires prior knowledge of the 234U/238U ratio at the time the material under study was formed. For those materials (principally marine carbonates) for which the initial ratio is known, 230Th/238U
Uranium-thorium dating
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...
remains a superior technique. This restricts the application of 234U/238U to extremely rare cases where the initial 234U/238U is well-constrained and the sample is also beyond the ca. 450,000 year upper limit of the 230Th/238U
Uranium-thorium dating
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...
technique.
Unlike other radiometric dating techniques, those using the uranium decay series (except for those using the stable final isotopes 206Pb and 207Pb) compare the ratios of two radioactive unstable isotopes. This complicates calculations as both the parent and daughter isotopes decay over time into other isotopes.
In theory, the 234U/238U technique can be useful in dating samples between about 10,000 and 2 million years Before Present
Before Present
Before Present years is a time scale used in archaeology, geology, and other scientific disciplines to specify when events in the past occurred. Because the "present" time changes, standard practice is to use AD 1950 as the origin of the age scale, reflecting the fact that radiocarbon...
(BP), or up to about eight times the half-life of 234U. As such, it provides a useful bridge in radiometric dating techniques between the ranges of 230Th/238U
Uranium-thorium dating
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...
(accurate up to ca. 450,000 years) and U-Pb dating
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...
(accurate up to the age of the solar system, but problematic on samples younger than about 2 million years).