Gas-melt interactions and their bearing on chondrule formation

Abstract

Interactions between nebular gas and molten silictes or oxides coul have played a major role in the formation and differentiation of the first solids formed in the solar system. In order to simulate such interactions, we set up a new experimental device in which isothermal condensation experiments have been conducted. Partially molten chondrule-like samples have been exposed to high SiO(g) partial pressures, for intervals between 80 and 300 s and at temperatures ranging from 1600 to 1750 K. Results show that silica entering from the gas phase could be responsible for several textural and mineralogical featues observed in natural chondrules. For instance, these experiments reproduce not only the mineralogical zonation of porphyritic olivine-pyroxene chondrules with the peripheral location of pyroxenes, but also olivine resorption textures and the common poikilitical enclosure of olivines in pyroxenes. In the light of these similarities, we advocate that gas-melt interactions through condensation are viable mechanisms for chondrule formation and hence may place severe constrains on the history of these primitive objects. In the nebula, high SiO(g) partial pressures could have been established by the volatization of regions with high dust/gas ratio. A possible scenario for this stochastic thermal activity is the intense activity of the protosun in its young stellar object phase.