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Mass-independent (isotope) fractionation refers to any chemical or physical process that acts to separate isotopes, where the amount of separation does not scale in proportion with the difference in the masses of the isotopes. Most isotopic fractionations (including typical kinetic fractionations and equilibrium fractionations) 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.
Mass-independent fractionation in nature
The most notable examples of mass-independent fractionation in nature are found in the isotopes of oxygen 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.
Another important mass-independent fractionation is found in ozone in the stratosphere. 1:1 variation of 18O/16O and 17O/16O in ozone was discovered in laboratory synthesis experiments by John Heidenreich and Mark Thiemens in 1983, and later found in stratospheric air samples measured by Konrad Mauersberger. Theoretical calculations by Rudolph Marcus and others suggest that 18O- and 17O enrichment in ozone is caused by the effects of 17O and 18O on molecular symmetry, and on the lifetimes of intermediate, excited states in the synthesis reaction.
Mass-independent fractionation has also recently been discovered in sulfur from ancient geological samples, particularly those formed more than 2,450 million years ago, by James Farquhar, Huiming Bao, and Mark Thiemens. Although the details of the fractionation process are not yet known, it appears most likely to be caused by photochemical reactions involving sulfur-bearing molecules in the early atmosphere. The creation and transfer of the mass-independent signature into minerals would be unlikely in an atmosphere containing abundant oxygen, indicating that the atmosphere was anoxic during the Archean eon, before 2,450 million years ago.
See also
References
Clayton R. N., Grossman L., Mayeda T. K. (1973) A component of primitive nuclear composition in carbonaceous meteorites. Science, v. 182, p. 485-488.
Farquhar J., Bao H. and Thiemens M. (2000) Atmospheric influence of the Earth's earliest sulfur cycle. Science, v. 289, p. 756-758.
Gao Y. Q. and Marcus R. A. (2001) Strange and unconventional isotope effects in ozone formation. Science, v. 293, p. 259-263.
Heidenreich J. E. III and Thiemens M. H. (1983) A non-mass-dependent isotope effect in the production of ozone from molecular oxygen. Journal of Chemical Physics, v. 78, p. 892-895.
Mauersberger K. (1987) Ozone isotope measurements in the stratosphere. Geophysical Research Letters, v. 14, p. 80-83.
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