Mass-dependent fractionation
Mass-dependent fractionation is the sorting of isotopes by mass during processes such as evaporation and diffusion.
Heavier isotopes of an element move and react slightly more slowly than lighter ones. During evaporation, condensation, diffusion or chemical reactions, this causes the isotopes to separate a little, in proportion to their mass differences. This is mass-dependent fractionation.
For oxygen, the shift in oxygen-17 is roughly half the shift in oxygen-18, because the mass difference from oxygen-16 is half as big. On a graph of the two ratios, all Earth materials therefore lie along a line with a slope of about 0.52, called the terrestrial fractionation line.
Mass-dependent fractionation records physical processes. Heavy isotope enrichment in some CAIs and chondrules shows they lost part of their mass by evaporation when molten. Zinc and potassium isotopes in lunar rocks suggest volatile loss during the Moon's formation. In contrast, effects that do not follow this mass rule, called mass-independent, point to different source reservoirs or unusual chemistry.
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