Chondrule formation by open-system melting of nebular condensates.

摘要

The formation of chondrules is believed to have occurred in the solar nebula, but the precise mechanism of formation remains unknown. Proposed origins vary from direct condensates of liquids in the solar nebula to evaporation residues of dust agglomerates. In this study we examined the possibility that chondrule precursors experienced open-system melting. We studied the effects of continuous evaporation on a silicate starting-material and compared our results with properties of natural chondrules. We conclude that evaporation and reduction could have played an important role during chondrule formation. It is possible that these processes were responsible for the chemical fractionation observed in the various objects of the Solar System.; Evaporation of a CI-like starting composition at different pressures produced residues, which exhibited trends matching those in natural chondrules, especially those of Type IA and IIA. The formation of silica-rich chondrules (Type IB and IIB) by evaporation requires precursors with lower ratios of olivine to pyroxene.; In the recovered charges, metallic iron was present only in the 1-atm evaporation runs of an H₂-CO₂ mixture and was completely absent in the low-pressure runs. Metallic iron in chondrules is possibly the result of desulfurization of troilite and reduction of iron-rich olivines in environments of elevated pressures relative to the canonical nebula. In addition, some metallic iron may have resulted from recondensation during cooling.; Porphyritic textures were readily formed (via Ostwald ripening) in the residual melts of open system melting. The conditions in which these textures formed (at temperatures close to 200°C below their liquidus) contradict existing formation models, which require heating the melt to just about 50°C below its liquidus. Formation of porphyritic textures by evaporation removes the need for a mysterious but sophisticated heating mechanism, which heated each chondrule to just the right temperature, dictated by its composition, in order to form this texture.; We demonstrated that by evaporating a silicate composition in a semi-sealed container we induced back reaction of potassium and thereby partially suppressed isotopic fractionation. We propose that back reaction during partial evaporation of chondrule was an important contributor to the lack of isotopic fractionation in these objects.; We conclude that chondrules were formed in regions containing chondrules, dust and gas in which heating times, pressures and cooling rates varied geographically, giving way to the different types of chondrules.