Evaporation of moderately volatile elements from silicate melts: experiments and theory

Moderately volatile elements (MVEs) are sensitive tracers of vaporisation in geological and cosmochemical processes owing to their balanced partitioning between vapour and condensed phases. Differences in their volatilities allow the thermodynamic conditions, particularly temperature and oxygen fugacity (fO₂), at which vaporisation occurred to be quantified. However, this exercise is hindered by a lack of experimental data relevant to the evaporation of MVEs from silicate melts. We report a series of experiments in which silicate liquids are evaporated in one-atmosphere (1-atm) gas-mixing furnaces under controlled fO₂, from the Fe-“FeO” buffer (iron-wüstite, IW) to air (10^−0.68 bar), bracketing the range of most magmatic rocks. Time- (t) and temperature (T) series were conducted from 15 to 930 min and 1300–1550 °C, at or above the liquidus for a synthetic ferrobasalt, to which 20 elements, each at 1000 ppm, were added. Refractory elements (e.g., Ca, Sc, V, Zr, REE) are quantitatively retained in the melt under all conditions. The MVEs show highly redox-dependent volatilities, where the extent of element loss as a function of fO₂ depends on the stoichiometry of the evaporation reaction(s), each of which has the general form M^{x + n}O_{( x + n )/2} (l) = M^xO_{x /2} (g) + n/4O₂. Where n is positive (as in most cases), the oxidation state of the element in the gas is more reduced than in the liquid, meaning lower oxygen fugacity promotes evaporation. We develop a general framework, by integrating element vaporisation stoichiometries with Hertz-Knudsen-Langmuir (HKL) theory, to quantify evaporative loss as a function of t, T and fO₂. Element volatilities from silicate melts differ from those during solar nebular condensation, and can thus constrain the conditions of volatile loss in post-nebular processes. Evaporation in a single event strongly discriminates between MVEs, producing a step-like abundance pattern in the residuum, similar to that observed in the Moon or Vesta. Contrastingly, the gradual depletion of MVEs according to their volatility in the Earth is inconsistent with their loss in a single evaporation event, and instead likely reflects accretion from many smaller bodies that had each experienced different degrees of volatilisation.