Mass-independent fractionation
Mass-independent fractionation is isotope separation that does not follow simple mass rules, as seen in oxygen isotopes of meteorites.
Mass-independent fractionation describes isotope variations that do not follow the usual rule linking the size of an effect to the difference in mass. On an oxygen isotope graph, such variations move samples off the terrestrial fractionation line.
Meteorites show large mass-independent oxygen variations. CAIs are enriched in oxygen-16 relative to Earth, plotting along a line of slope close to 1, while different meteorite groups occupy separate positions. The quantity Δ17O expresses each sample's distance from the terrestrial line and is used to tell groups apart.
Why these variations exist is a long-standing problem. In 1983, chemists showed that ozone formation in the laboratory produces mass-independent effects, so chemistry can create them. A leading idea for the Solar System is self-shielding: ultraviolet light from the young Sun or nearby stars broke apart carbon monoxide molecules containing rarer oxygen isotopes more readily, releasing them to form water that later reacted with dust.
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