Intro.

Two simplified precesses of volatile element depletion:

1. partial condensation from a cooling gas;

2. partial evaporation from condensed dust or melt.

For elements with more than one isotope, a  physicall kinetic or fractional  equilibrium stable isotope effect during evaporation can be a measure of the extent of partial evaporation.

Moderately volatile isotopes such as S, Cl, K and Zn with well-determined abundance depletions and no-significant isotopic fractionations during planetary differentiation, which make them very useful for assessing kinetic isotope effect.

As the difference in vibrational energy of bonds formed by the different isotopes at equilibrium or under kinetic conditions, when any exchange reacion occurs there are always mass-dependant stable isotopic fractionations. Druing the reactions, heavy isotopes tend to enrich in the compounds where the element forms the stiffest bonds. 

As a result, during evaporation or condensation processes, the lightest isotopes are more likely to be enricned in the vopor phaes while the heaviest ones  will  be enriched in the condensed phase. This effect can be inferred by the mass difference between the isotopes.