Mass-independent

isotope fractionation Isotope fractionation describes fractionation processes that affect the relative abundance of isotopes, phenomena which are taken advantage of in isotope geochemistry and other fields. Normally, the focus is on stable isotopes of the same element. ...

or Non-mass-dependent fractionation (NMD), refers to any

chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...

physical process Physical changes are changes affecting the form of a chemical substance, but not its chemical composition. Physical changes are used to separate mixtures into their component compounds, but can not usually be used to separate compounds into chem ...

that acts to separate

isotope Isotopes are two or more types of atoms that have the same atomic number (number of protons in their nuclei) and position in the periodic table (and hence belong to the same chemical element), and that differ in nucleon numbers (mass numbers) ...

s, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic

fractionation Fractionation is a separation process in which a certain quantity of a mixture (of gases, solids, liquids, enzymes, or isotopes, or a suspension) is divided during a phase transition, into a number of smaller quantities (fractions) in which the ...

s (including typical

kinetic fractionation Kinetic fractionation is an isotopic fractionation process that separates stable isotopes from each other by their mass during unidirectional processes. Biological processes are generally unidirectional and are very good examples of "kinetic" isot ...

s and

equilibrium fractionation Equilibrium isotope fractionation is the partial separation of isotopes between two or more substances in chemical equilibrium. Equilibrium fractionation is strongest at low temperatures, and (along with kinetic isotope effects) forms the basis of ...

s) are caused by the effects of the mass of an isotope on atomic or molecular velocities, diffusivities or bond strengths. Mass-independent fractionation processes are less common, occurring mainly in

photochemical Photochemistry is the branch of chemistry concerned with the chemical effects of light. Generally, this term is used to describe a chemical reaction caused by absorption of ultraviolet (wavelength from 100 to 400 nm), visible light (400–7 ...

and

spin-forbidden reactions In chemistry, the selection rule (also known as the transition rule) formally restricts certain chemical reaction, reactions, known as spin-forbidden reactions, from occurring due to a required change between two differing quantum states. When a r ...

. Observation of mass-independently fractionated materials can therefore be used to trace these types of reactions in nature and in laboratory experiments.

Mass-independent fractionation in nature

The most notable examples of mass-independent fractionation in nature are found in the isotopes of

oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...

and

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

. The first example was discovered by

Robert N. Clayton Robert Norman Clayton (March 20, 1930 – December 30, 2017) was a Canadian-American chemist and academic. He was the Enrico Fermi Distinguished Service Professor Emeritus of Chemistry at the University of Chicago. Clayton studied cosmochemistry ...

Toshiko Mayeda Toshiko K. Mayeda (née Kuki) (1923–13 February 2004) was a Japanese American chemist who worked at the Enrico Fermi Institute in the University of Chicago. She worked on climate science and meteorites from 1958 to 2004. Early life and educat ...

, and Lawrence Grossman in 1973, in the oxygen isotopic composition of

refractory In materials science, a refractory material or refractory is a material that is resistant to decomposition by heat, pressure, or chemical attack, and retains strength and form at high temperatures. Refractories are polycrystalline, polyphase, ...

calcium-aluminium-rich inclusions in the

Allende meteorite The Allende meteorite is the largest carbonaceous chondrite ever found on Earth. The fireball was witnessed at 01:05 on February 8, 1969, falling over the Mexican state of Chihuahua. After it broke up in the atmosphere, an extensive search for ...

. The inclusions, thought to be among the oldest solid materials in the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

, show a pattern of low ¹⁸O/¹⁶O and ¹⁷O/¹⁶O relative to samples from the Earth and

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

. Both ratios vary by the same amount in the inclusions, although the mass difference between ¹⁸O and ¹⁶O is almost twice as large as the difference between ¹⁷O and ¹⁶O. Originally this was interpreted as evidence of incomplete mixing of ¹⁶O-rich material (created and distributed by a large star in a

supernova A supernova is a powerful and luminous explosion of a star. It has the plural form supernovae or supernovas, and is abbreviated SN or SNe. This transient astronomical event occurs during the last evolutionary stages of a massive star or when ...

) into the

Solar nebula The formation of the Solar System began about 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. Most of the collapsing mass collected in the center, forming the Sun, while the rest flattened into a ...

. However, recent measurement of the oxygen-isotope composition of the

Solar wind The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...

, using samples collected by the Genesis spacecraft, shows that the most ¹⁶O-rich inclusions are close to the bulk composition of the solar system. This implies that Earth, the Moon, Mars, and asteroids all formed from ¹⁸O- and ¹⁷O-enriched material.

Photodissociation Photodissociation, photolysis, photodecomposition, or photofragmentation is a chemical reaction in which molecules of a chemical compound are broken down by photons. It is defined as the interaction of one or more photons with one target molecule. ...

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

in the Solar nebula has been proposed to explain this isotope fractionation. Mass-independent fractionation also has been observed in

ozone Ozone (), or trioxygen, is an inorganic molecule with the chemical formula . It is a pale blue gas with a distinctively pungent smell. It is an allotrope of oxygen that is much less stable than the diatomic allotrope , breaking down in the lo ...

. Large, 1:1 enrichments of ¹⁸O/¹⁶O and ¹⁷O/¹⁶O in ozone were discovered in laboratory synthesis experiments by Mark Thiemens and John Heidenreich in 1983, and later found in stratospheric air samples measured by Konrad Mauersberger. These enrichments were eventually traced to the three-body ozone formation reaction. :O + O₂ → O₃* + M → O₃ + M* Theoretical calculations by

Rudolph Marcus Rudolph Arthur Marcus (born July 21, 1923) is a Canadian-born chemist who received the 1992 Nobel Prize in Chemistry "for his contributions to the theory of electron transfer reactions in chemical systems". Marcus theory, named after him, provid ...

and others suggest that the enrichments are the result of a combination of mass-dependent and mass-independent

kinetic isotope effect In physical organic chemistry, a kinetic isotope effect (KIE) is the change in the reaction rate of a chemical reaction when one of the atoms in the reactants is replaced by one of its isotopes. Formally, it is the ratio of rate constants for th ...

s (KIE) involving the

excited state In quantum mechanics, an excited state of a system (such as an atom, molecule or nucleus) is any quantum state of the system that has a higher energy than the ground state (that is, more energy than the absolute minimum). Excitation refers to a ...

O₃* intermediate related to some unusual

symmetry Symmetry (from grc, συμμετρία "agreement in dimensions, due proportion, arrangement") in everyday language refers to a sense of harmonious and beautiful proportion and balance. In mathematics, "symmetry" has a more precise definit ...

properties. The mass-dependent isotope effect occurs in asymmetric species, and arises from the difference in

zero-point energy Zero-point energy (ZPE) is the lowest possible energy that a quantum mechanical system may have. Unlike in classical mechanics, quantum systems constantly Quantum fluctuation, fluctuate in their lowest energy state as described by the Heisen ...

of the two formation channels available (e.g., ¹⁸O¹⁶O + ¹⁶O vs ¹⁸O + ¹⁶O¹⁶O for formation of ¹⁸O¹⁶O¹⁶O.) These mass-dependent zero-point energy effects cancel one another out and do not affect the enrichment in heavy isotopes observed in ozone. The mass-independent enrichment in ozone is still not fully understood, but may be due to isotopically symmetric O₃* having a shorter lifetime than asymmetric O₃*, thus not allowing a

statistical Statistics (from German: ''Statistik'', "description of a state, a country") is the discipline that concerns the collection, organization, analysis, interpretation, and presentation of data. In applying statistics to a scientific, industria ...

distribution of energy throughout all the

degrees of freedom Degrees of freedom (often abbreviated df or DOF) refers to the number of independent variables or parameters of a thermodynamic system. In various scientific fields, the word "freedom" is used to describe the limits to which physical movement or ...

, resulting in a mass-independent distribution of isotopes.

Mass-independent carbon dioxide fractionation

The mass-independent distribution of isotopes in stratospheric ozone can be transferred to carbon dioxide (CO₂). This anomalous isotopic composition in CO₂ can be used to quantify

gross primary production In ecology, primary production is the synthesis of organic compounds from atmospheric or aqueous carbon dioxide. It principally occurs through the process of photosynthesis, which uses light as its source of energy, but it also occurs through c ...

, the uptake of CO₂ by vegetation through

photosynthesis Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored i ...

. This effect of terrestrial vegetation on the isotopic signature of atmospheric CO₂ was simulated with a global model and confirmed experimentally.

Mass-independent sulfur fractionation

Mass-independent fractionation of sulfur can be observed in ancient sediments, where it preserves a signal of the prevailing environmental conditions. The creation and transfer of the mass-independent signature into minerals would be unlikely in an atmosphere containing abundant oxygen, constraining the

Great Oxygenation Event The Great Oxidation Event (GOE), also called the Great Oxygenation Event, the Oxygen Catastrophe, the Oxygen Revolution, the Oxygen Crisis, or the Oxygen Holocaust, was a time interval during the Paleoproterozoic era when the Earth's atmosphere ...

to some time after . Prior to this time, the MIS record implies that sulfate-reducing bacteria did not play a significant role in the global sulfur cycle, and that the MIS signal is due primarily to changes in volcanic activity.

References

{{DEFAULTSORT:Mass-Independent Fractionation Isotopes Geochemistry Fractionation

Mass-independent fractionation in nature

Mass-independent carbon dioxide fractionation

Mass-independent sulfur fractionation

See also

References