Experimental Constraints on Chondrule Formation

摘要: Chondrule textures depend on the extent of melting chondrule precursor material when cooling starts. If viable nuclei remain in melt, crystallization begins immediately, producing crystals with shapes that approach equilibrium. not, does not occur until melt is supersaturated, resulting more rapid growth rates and formation skeletal or dendritic crystals. A texture thus indicates whether were destroyed, which implies a temperature above liquidus for its particular composition. The presence absence chondrules can be used to constrain their peak temperatures, range from 1400-1850oC. Heating times less than second result aggregates starting materials coated glass, resembling agglutinates rather objects typical textures, suggesting heating are longer. duplicated very wide rates, but if olivine zoning matched rate should within 101000°C/hr. size overgrowths relict grains cannot infer rates. Chondrules melted canonical nebular gas lose sulfur alkalis minutes, while iron loss silicate continues over many hours. Mass isotopic fractionation suppressed partial pressures species interest high enough ambient gas. bulk mineral composition arrays reproduced large by evaporation. However, condensation SiO into simulate zonation some chondrules, pyroxene silica polymorph near rims. equilibration noncanonical would require time periods 1. General Introduction Chondrules, tiny spherules containing olivine, pyroxene, metal, sulfide igneous most abundant components chondritic meteorites, accounting up 80% rock (e.g., Zanda 2004; Jones et al. this volume). Chondritic bodies turn throughout main asteroid belt. therefore document widespread early inner solar system. central question mechanism was an astrophysical process planetary still totally resolved, though shock waves protoplanetary disk Desch & Connolly 2002; volume) currently one favored mechanisms. might have been formed (Varela 2002), agglomeration mist microdroplets dust (Wood 1996), dustballs There evidence small cryptocrystalline CB CH chondrites direct liquids (Krot 2001), Mg-rich melts 2003; Libourel 2003). In case solids, precursors widely assumed condensates, i.e. finegrained dust, either fractionated CI composition, such easily converted Hewins Fox Some larger probably present, relicts fragmented different annealed condensates refrac-