A Thermodynamic Basis for Extensive Intermediate Temperature Vapor Phase Transport on Meteorite Parent Bodies

Introduction: Compelling observational evidence for extensive vapor phase transport on the Allende parent body has been in the literature for many years [1]. This evidence includes 1) widespread observations of disequilibrium Fe-rich olivine on surfaces and in internal cracks in MgSiO4, 2) widespread occurrences of isolated grains of low temperature pentlandite in the matrix, and 3) vesicle-like largely empty pores where metal/sulfide droplets had initially been in chondrules. It was suggested [1] that this vapor transport took place on a body that initially incorporated significant amounts of ices and was subsequently heated on a short time scale by extinct radionucleides. The possible gas phase species involved in this transport have not discussed until now. This work remedies that omission, and shows that at least Fe, Si, Mg, and Ni are indeed highly mobile as hydroxide molecules. Thermodynamic Approach: All calculations have been done using the Gibbs Solver in the commercial HSC5 thermodynamics program from Outokumpu Research. The internal database contains all relevant data from the JANAF [2] and Glushko [3] compilations. This has been augmented with theoretical data on Cr containing molecules from Ebbinghaus [4] and on Si molecules from Allendorf et al [5]. This combination provides enough data to do meaningful calculations even though there are undoubtedly relevant molecules omitted because of lack of data. Since by definition the solids in an unequilibrated meteorite are not in equilibrium with each other, the gas phase cannot be in global equilibrium with the solids. To estimate possible vapor transport rates we instead calculate the gas phase species in local equilibrium with selected solids or groups of solids. To these solids we add 0.9 moles of Ar and 0.1 mole of H2O vapor and include in the system all gas phase species containing these and elements from the solids. We then calculate the composition that minimizes the Gibbs free energy of this system as a function of temperature at one atmosphere total pressure. The resulting mole fractions of vapor species correspond directly with their vapor pressures as long as these are low. Results: Consider first the vapor species above a region containing only MgSiO3 and Mg2SiO4. The pressures of the dominant Si and Mg molecules are shown in the plot below. 100 300 500 700 90