Condensation of major and trace elements in dust-rich environments

Recent astronomical observations have shown that dust can get locally concentrated in protoplanetary disks, forming ring structures. The thermal processing of such regions could lead to dust evaporation and local enrichment of the solar gas in condensable elements. Previous studies focusing on major element behavior have shown that condensation of such dust-enriched gas could lead to the formation of a silicate melt with compositions resembling that of chondrules. However, previous studies focusing on dust-enriched environments were restricted to a limited set of elements. To study the mineralogical and chemical composition of condensates in these conditions, we have performed equilibrium calculations using the FactSage™ software for a dust-enriched solar gas. The calculations were done with dust-enrichment factors of 1 (solar composition), 10 and 100 at pressures ranging between 10⁻⁶ bar and 10⁻³ bar, for a CI-chondrite dust and a H-chondrite dust. The trace element condensation was accurately modelled with newly calculated activity coefficients in different solid and melt solutions. The available gas phase database was completed with new trace element species that are important to consider in oxidized conditions. The mineralogical sequence, melt composition and condensation temperature for all condensable elements were then quantified.

Our calculations show that the iron contents of olivine in equilibrium with a gas that is x100 enriched in CI-dust is consistent with that of amoeboid olivine aggregates and chondrules. Furthermore, our estimated temperature at which fayalite can form in these conditions is higher than what was previously proposed, enabling diffusion and homogenization of iron in olivine. The calculated composition of refractory metals for a x10 and x100 CI-dust enriched gas at 10⁻⁴ bar is consistent with the measured compositions of refractory metal nuggets. The possibility for these grains to have formed in an H₂O ice-enriched gas can be ruled out as the calculated fractionation patterns in this case did not match the observed compositions.