Exploration of quantitative kinetic models for the evaporation of silicatemelts in vacuum and in hydrogen

Two basic approaches (pure component reference (PCR) and equilibrium refere nce (EQR)) to modeling silicate melt evaporation are explored. The PCR mode l calculates the maximum possible evaporation rates of the pure oxides from their equilibrium vapor pressures and rescales these rates according to th e activities of the oxides in the silicate melts and the melt densities. Th e EQR model calculates the maximum possible evaporation rates based on the equilibrium vapor pressures of the melts. Differences between the calculate d and experimentally determined evaporation rates are accounted for with ev aporation (alpha (evap)) coefficients that are only dependent on temperatur e. Two versions of the PCR model, Cases 1 and 2, are explored to try to res olve apparently contradictory conclusions about the composition of the evap orating species based on Mg and Si isotope fractionation during evaporation (species are not in thermodynamic equilibrium proportions) and direct meas urements of gas species in Langmuir experiments (species are in roughly equ ilibrium proportions). The Case 2 and EQR models cannot explain the observe d isotope fractionations unless evaporation occurred under non-Rayleigh con ditions, either because there was significant recondensation during the exp eriments or because diffusion was playing a limiting role. Whether or not the role of diffusion is included, the PCR and EQR models ar e able to reproduce the elemental results of evaporation experiments of "ch ondritic" melts from temperatures of 1700 to 2000 degreesC, and up to mass losses of about 95%. However, the models underestimate absolute evaporation rates in very Ca- and Al-rich melts. This may reflect errors in the model used to estimate oxide activities. The EQR model can only reproduce the obs erved evaporation behavior of Na if, unlike the other oxides, its alpha (ev ap) coefficient is close to unity. Based on available diffusion data, diffusion is not slow enough in "chondri tic" or forsteritic melts to explain the isotopic fractionations of Mg and O in the evaporation experiments, but it may play a role in limiting Si iso tope fractionation. Provided recondensation was not a significant factor in the experiments, at present PCR Case 1 appears to be the best model if bot h the Langmuir and the isotopic fractionation experiments are to be explain ed.