On the mass independent fractionations of O, Hg, Si, Mg and Cd during open-system evaporation or thermal decomposition

Many experiments in which an element or a mineral is evaporated or thermally decomposed under vacuum are known to consistently display unexpected behaviors. These include too low rates of evaporation, smaller (i.e. closer to 1) than predicted fractionation factors, and an inconsistent behavior of the stable isotope ratios of a given element (i.e. massindependent fractionation). This applies to many elements including O, Hg, Si, Mg and Cd. We present interpretations for a series of earlier observations, including experiments by Miller et al. (2002) in which mass-independent O isotope fractionations are produced during thermal decomposition of carbonates [1], and the finding of Estrade et al. (2009) showing an unexpected slope in a plot of &199Hg vs &201Hg (close to 1.2 instead of 2.4) during open-system evaporation of Hg [2]. These and related results can be explained if a fraction (usually a few to several tens of percent) of the evaporated compounds actually forms (or re-equilibrate) under conditions of isotope equilibrium, the remaining fraction obeying kinetic fractionation of its stable isotopes. This is the mixing of the that results in the appearance of mass-independence, rather than the action of a novel isotope effect having non-cannonical mass law.