Helium dating (abbreviated (U–Th)/He dating) refers to a variety of He diffusion methods that utilize the mobility of radiogenic He atoms to determine the thermal history of a rock. Helium diffusion experiments are often used to help interpret information retrieved from U–Th/He thermochronometric experiments. Kinematic parameters derived from He diffusion is done through estimating He diffusion over a range of temperatures. The use of
density functional theory
Density functional theory (DFT) is a computational quantum mechanical modelling method used in physics, chemistry and materials science to investigate the electronic structure (or nuclear structure) (principally the ground state) of many-body ...
helps in estimating energy barriers for He to overcome as it diffuses across various crystallographic directions. Discrepancies, however, between observed and predicted He diffusion rates is still a problem and likely stem from unresolved problems in crystal defects and radiation damage in natural grains as opposed to theoretical grains. Depending on the mineral analyzed there are different assumptions to be made on He mobility. For example, He diffusion in minerals such as
zircon
Zircon () is a mineral belonging to the group of nesosilicates and is a source of the metal zirconium. Its chemical name is zirconium(IV) silicate, and its corresponding chemical formula is Zr SiO4. An empirical formula showing some of th ...
,
rutile
Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite.
Rutile has one of the highest refractive indices at vis ...
, and
monazite have been shown to be strongly
anisotropic
Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ver ...
.
A relatively new dating method, ''tritium–helium dating'' has been developed for determining rates of oxygen utilization in the ocean.
4He/3He Thermochronometry
Traditional U–Th/He thermochronometry determines the temperature T
c that the analyzed sample had at a time in the past corresponding to the age given by its content of parent and daughter
nuclides
Nuclides (or nucleides, from nucleus, also known as nuclear species) are a class of atoms characterized by their number of protons, ''Z'', their number of neutrons, ''N'', and their nuclear energy state.
The word ''nuclide'' was coined by the Am ...
. More information, however, can be concluded about a mineral's thermal history if an analysis of the He distribution in-situ is performed. Similar to the
argon–argon dating
Argon–argon (or 40Ar/39Ar) dating is a radiometric dating method invented to supersede Potassium-argon dating, potassiumargon (K/Ar) dating in accuracy. The older method required splitting samples into two for separate potassium and argon measur ...
(which uses
40Ar and
39Ar isotopes) where
39Ar is a second non-radiogenically produced isotope derived from
39K, each step-heating release of
39Ar can be directly associated with a date. With
Helium-3
Helium-3 (3He see also helion) is a light, stable isotope of helium with two protons and one neutron. (In contrast, the most common isotope, helium-4, has two protons and two neutrons.) Helium-3 and hydrogen-1 are the only stable nuclides with ...
(
3He) production the
4He/
3He evolution is interpreted to provide an intragranular
Helium-4
Helium-4 () is a stable isotope of the element helium. It is by far the more abundant of the two naturally occurring isotopes of helium, making up about 99.99986% of the helium on Earth. Its nucleus is identical to an alpha particle, and consi ...
(
4He) distribution. This method is superior in two ways: diffusion kinetics for
4He can be precisely determined and the
4He distribution provides a continuous path in a time-temperature history s opposed to a single point in a bulk-grain date.
More specifically, the
4He distribution in a grain is a function of the time-integrated internal production from parent nuclides, minus diffusion loss and alpha ejection. This is done in conjunction with the assumption that the model is a spherical grain and calculations correlate with a radial position within that sphere. These calculations also assume that diffusion is
isotropic
In physics and geometry, isotropy () is uniformity in all orientations. Precise definitions depend on the subject area. Exceptions, or inequalities, are frequently indicated by the prefix ' or ', hence '' anisotropy''. ''Anisotropy'' is also ...
.
Use to support creationism
In 1997, the
Institute of Creation Research began a research project, named "RATE" (Radioisotopes and the Age of The Earth), which aimed at determining the validity of scientifically accepted
radiometric dating
Radiometric dating, radioactive dating or radioisotope dating is a technique which is used to Chronological dating, date materials such as Rock (geology), rocks or carbon, in which trace radioactive impurity, impurities were selectively incorporat ...
. One paper published from this research project describes the perceived issues of uniformitarian (U–Th)/He dating.
The assumptions made in the creationist arguments neglect the sensitivity that He diffusion methods have in regard to temperature fluctuations over time – especially since the
granodiorite
Granodiorite ( ) is a coarse-grained (phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar.
The term banatite is sometimes used informally for various rocks ranging from gra ...
analyzed in the study has very complex geologic and thermal history.
References
{{Reflist
Radiometric dating