Mass-independent fractionation

Mass-independent isotope fractionation or Non-mass-dependent fractionation (NMD), 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.

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.

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 effects (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.

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 to some time after 2,450 million years ago. 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.

See also

• Equilibrium fractionation
• Kinetic fractionation
• Isotope geochemistry