Chondrule synthesis using fine-grained precursors

High temperature petrologic experiments have been used in order to reproduce the textures of chondrules, which are rounded to irregularly shaped ferromagnesion silicate objects. Such experiments shed light on the conditions that existed and mechanisms that operated in the early solar nebula, as natural chondrules are believed to have formed there due to some type of heating event. The exact nature of this heating event and the conditions that existed at the time of the formation of the solar nebula are not completely understood. Chondrules, which are believed to be composed of some of the oldest remnants of the solar system, nebular condensates, are the basic components of chondrites. Chondrites comprise ∼82% of all meteorites. Despite years of petrographic examination and experimental petrology, the thermal history of chondrules still remains uncertain. Natural chondrules exhibit a variety of different textures ranging from glassy, barred, porphyritic, microporphyritc to protoporphyritc. Petrologic experiments in a muffle tube furnace under controlled fugacity conditions using type IAB bulk composition analogs have been successful in reproducing each of these textures in the laboratory. Charges are prepared, heated, water quenched, mounted, polished and photographed using back-scattered electron imagery. Subsequent analysis provides numerical data, which can then be used to calculate the nominal grain size of the olivine crystals in each charge. Porphyritic chondrules are the most abundant in nature by far and any model for chondrule formation must be capable of producing porphyritic textures. To reproduce this texture in the laboratory, however, seems to require a very narrow range of maximum temperature and soak time parameters even when using a variety of different types of fine-grained and agglomerated olivine precursor material. Experiments undertaken in this study bring into question some of the basic assumptions of various classical models of chondrule formation, which call for fine-grained precursors (nebular condensates) being processed by a single heating event. In light of these findings, models for chondrule formation that incorporate mechanisms for the recycling of fine-grained precursor material appear to be more favorable.