Mass-independent fractionation
Encyclopedia
Mass-independent fractionation refers to any chemical
or physical process
that acts to separate isotope
s, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic fractionation
s (including typical kinetic fractionation
s and equilibrium fractionation
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
and spin-forbidden reactions
. Observation of mass-independently fractionated materials can therefore be used to trace these types of reactions in nature and in laboratory experiments.
and sulfur
. The first example was discovered by Robert N. Clayton, Toshiko Mayeda, and Lawrence Grossman in 1973, in the oxygen isotopic composition of refractory
calcium-aluminium-rich inclusions in the Allende meteorite
. The inclusions, thought to be among the oldest solid materials in the Solar System
, show a pattern of low 18O/16O and 17O/16O relative to samples from the Earth and Moon
. 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
) into the Solar nebula
. However, recent measurement of the oxygen-isotope composition of the Solar wind
, 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. Photochemical dissociation of carbon monoxide
in the Solar nebula has been proposed to explain this isotope fractionation.
Mass-independent fractionation also has been observed in ozone
. Large, 1:1 enrichments of 18O/16O and 17O/16O in ozone were discovered in laboratory synthesis experiments by John Heidenreich and Mark Thiemens 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.
Theoretical calculations by Rudolph Marcus and others suggest that the enrichments are the result of a combination of mass-dependent and mass-independent kinetic isotope effect
s (KIE) involving the excited state
O3* intermediate
related to some unusual symmetry
properties. For formation of ozone substituted with a heavy oxygen atom at the terminal position of the molecule, a highly zero-point energy
difference sensitive KIE creates large enrichments for the differences in mass between 18O, 17O, and 16O. For formation of ozone substituted with a heavy oxygen atom at the central position of the molecule (or unsubstituted), the relatively short lifetime the O3* intermediate does not allow a statistical
distribution of energy throughout all the degrees of freedom
, resulting in a mass-independent distribution of isotopes.
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.
Chemical change
Chemical changes occur when a substance combines with another to form a new substance or, alternatively, decomposes into two or more different substances. These processes are called chemical reactions and , in general, are not reversible except by further chemical reactions...
or physical process
Physical change
Physical changes are changes affecting the form of a chemical substance, but do not change the chemical composition of that substance. Physical changes are used to separate mixtures into their component compounds, but can not usually be used to separate compounds into chemical elements or simpler...
that acts to separate 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, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic fractionation
Fractionation
See also: Fractionated spacecraftFractionation is a separation process in which a certain quantity of a mixture is divided up in a number of smaller quantities in which the composition changes according to a gradient. Fractions are collected based on differences in a specific property of the...
s (including typical kinetic fractionation
Kinetic fractionation
Kinetic fractionation is a process that separates stable isotopes from each other by their mass during unidirectional processes.One naturally occurring example of kinetic fractionation is the evaporation of seawater to form clouds...
s and equilibrium fractionation
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 forms the basis of the most widely used isotopic paleothermometers : D/H and 18O/16O records from ice...
s) are caused by the effects of the mass of an isotope on atomic or molecular velocities, diffusivities
Diffusion
Molecular diffusion, often called simply diffusion, is the thermal motion of all particles at temperatures above absolute zero. The rate of this movement is a function of temperature, viscosity of the fluid and the size of the particles...
or bond strengths. Mass-independent fractionation processes are less common, occurring mainly in photochemical
Photochemistry
Photochemistry, a sub-discipline of chemistry, is the study of chemical reactions that proceed with the absorption of light by atoms or molecules.. Everyday examples include photosynthesis, the degradation of plastics and the formation of vitamin D with sunlight.-Principles:Light is a type of...
and spin-forbidden reactions
Selection rule
In physics and chemistry a selection rule, or transition rule, formally constrains the possible transitions of a system from one state to another. Selection rules have been derived for electronic, vibrational, and rotational transitions...
. 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 oxygenIsotopes of oxygen
There are three stable isotopes of oxygen that lead to oxygen having a standard atomic mass of 15.9994 u. 17 radioactive isotopes have also been characterized, with mass numbers from 12O to 28O, all short-lived, with the longest-lived being 15O with a half-life of 122.24 seconds...
and sulfur
Isotopes of sulfur
Sulfur has 25 known isotopes with mass numbers ranging from 26 to 49, four of which are stable: 32S , 33S , 34S , and 36S...
. The first example was discovered by Robert N. Clayton, Toshiko Mayeda, and Lawrence Grossman in 1973, in the oxygen isotopic composition of refractory
Refractory
A refractory material is one that retains its strength at high temperatures. ASTM C71 defines refractories as "non-metallic materials having those chemical and physical properties that make them applicable for structures, or as components of systems, that are exposed to environments above...
calcium-aluminium-rich inclusions in the Allende meteorite
Allende meteorite
The Allende meteorite is the largest carbonaceous chondrite ever found on Earth. The fireball was witnessed at 1:05 a.m. on February 8, 1969, falling over the Mexican state of Chihuahua. After breaking up in the atmosphere, an extensive search for pieces was conducted and it is often described as...
. The inclusions, thought to be among the oldest solid materials in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
, show a pattern of low 18O/16O and 17O/16O relative to samples from the Earth and Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
. 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
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
) into the Solar nebula
Solar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
. However, recent measurement of the oxygen-isotope composition of the Solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, using samples collected by the Genesis spacecraft
Genesis (spacecraft)
The Genesis spacecraft was a NASA sample return probe which collected a sample of solar wind and returned it to Earth for analysis. It was the first NASA sample return mission to return material since the Apollo Program, and the first to return material from beyond the orbit of the Moon...
, 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. Photochemical dissociation of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
in the Solar nebula has been proposed to explain this isotope fractionation.
Mass-independent fractionation also has been observed in ozone
Ozone
Ozone , or trioxygen, is a triatomic molecule, consisting of three oxygen atoms. It is an allotrope of oxygen that is much less stable than the diatomic allotrope...
. Large, 1:1 enrichments of 18O/16O and 17O/16O in ozone were discovered in laboratory synthesis experiments by John Heidenreich and Mark Thiemens in 1983, and later found in stratospheric
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
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 and others suggest that the enrichments are the result of a combination of mass-dependent and mass-independent kinetic isotope effect
Kinetic isotope effect
The kinetic isotope effect is the ratio of reaction rates of two different isotopically labeled molecules in a chemical reaction. It is also called "isotope fractionation," although this term is somewhat broader in meaning...
s (KIE) involving the excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
O3* intermediate
Reaction intermediate
A reaction intermediate or an intermediate is a molecular entity that is formed from the reactants and reacts further to give the directly observed products of a chemical reaction. Most chemical reactions are stepwise, that is they take more than one elementary step to complete...
related to some unusual symmetry
Molecular symmetry
Molecular symmetry in chemistry describes the symmetry present in molecules and the classification of molecules according to their symmetry. Molecular symmetry is a fundamental concept in chemistry, as it can predict or explain many of a molecule's chemical properties, such as its dipole moment...
properties. For formation of ozone substituted with a heavy oxygen atom at the terminal position of the molecule, a highly zero-point energy
Zero-point energy
Zero-point energy is the lowest possible energy that a quantum mechanical physical system may have; it is the energy of its ground state. All quantum mechanical systems undergo fluctuations even in their ground state and have an associated zero-point energy, a consequence of their wave-like nature...
difference sensitive KIE creates large enrichments for the differences in mass between 18O, 17O, and 16O. For formation of ozone substituted with a heavy oxygen atom at the central position of the molecule (or unsubstituted), the relatively short lifetime the O3* intermediate does not allow a statistical
Statistical mechanics
Statistical mechanics or statistical thermodynamicsThe terms statistical mechanics and statistical thermodynamics are used interchangeably...
distribution of energy throughout all the degrees of freedom
Degrees of freedom (physics and chemistry)
A degree of freedom is an independent physical parameter, often called a dimension, in the formal description of the state of a physical system...
, resulting in a mass-independent distribution of isotopes.
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 EventGreat Oxygenation Event
The Great Oxygenation Event , also called the Oxygen Catastrophe or Oxygen Crisis or Great Oxidation, was the biologically induced appearance of free oxygen in Earth's atmosphere. This major environmental change happened around 2.4 billion years ago.Photosynthesis was producing oxygen both before...
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.