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Paper: Kinetic Condensation of Silicate Melt and Its Role in the Chemical Diversity of Chondrules
Volume: 341, Chondrites and the Protoplanetary Disk
Page: 456
Authors: Nagahara, H.; Ozawa, K.; Tomomura, S.
Abstract: Chemical compositions of chondrules are characterized by large fractionations in major and minor elements without isotopic mass fractionation. To avoid selection bias, bulk chemical compositions have been obtained for all chondrules in thin sections of two primitive ordinary chondrites, Krymka (LL3.1) and Bishunpur (LL3.1). The observed range of chondrule compositions is much smaller than that previously reported, and is characterized by large variations in MgO/SiO2 ratios and FeO abundances with nearly constant refractory component abundances. These variations cannot be explained by a widely accepted model of chondrule formation - melting of heterogeneous precursor materials. Because chondrules formed in an open system during cooling, condensation of liquid should be examined. We have developed a kinetic condensation model that has a cooling rate as a critical parameter, and quantitatively evaluated the role of cooling rate, degree of supersaturation, and initial gas composition. The compositional trajectories of condensing liquid and coexisting gas during progressive condensation have been quantitatively examined. Comparing the model results with the chondrule compositions, we show that the diversity of chondrule compositions (except for Al-rich ones) can be reproduced if chondrules are either liquid condensates or quenched products of coexisting residual gas, and that the initial gas from which chondrules condensed must have been depleted in refractory elements by ~30% and Fe by ~80% relative to CI in a dust-enriched system. The plausible cooling rate of the system is 0.01-1°C/hour at Ptot = 10−4 bar with a dust enrichment of 500 times solar and supersaturation ratio > 1.05. We propose the following scenario for chondrule formation. Dust, previously depleted in refractory elements and Fe metal, was enriched by several hundred times relative to solar abundances. During chondrule formation, the dust was almost totally vaporized; the obtained gas cooled rapidly and became supersaturated. Liquid droplets condensed out of this gas; the residual gas quenched to form liquid droplets with fractionated compositions. This scenario has the advantage that all the chondrules can be formed from a common precursor through a common process.
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