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| Paper: |
Experimental Constraints on Chondrule Formation |
| Volume: |
341, Chondrites and the Protoplanetary Disk |
| Page: |
286 |
| Authors: |
Hewins, R.H.; Connolly, H.C., Jr.; Lofgren, G.E.; Libourel, G. |
| Abstract: |
Chondrule textures depend on the extent of melting of the chondrule precursor material when cooling starts. If viable nuclei remain in the melt, crystallization begins immediately, producing crystals with shapes that approach equilibrium. If not, crystallization does not occur until the melt is supersaturated, resulting in more rapid growth rates and the formation of skeletal or dendritic crystals. A chondrule texture thus indicates whether nuclei were destroyed, which implies a melting temperature above the liquidus temperature for its particular composition. The presence or absence of skeletal or dendritic crystals in chondrules can be used to constrain their peak temperatures, which range from 1400-1850°C. Heating times of less than a second result in aggregates of starting materials coated with glass, resembling agglutinates rather than objects with typical chondrule textures, suggesting that heating times are longer. Chondrule textures can be duplicated with a very wide range of cooling rates, but if olivine zoning is to be matched the cooling rate should be within the range 10- 1000°C/hr. The size of overgrowths on relict grains cannot be used to infer cooling rates. Chondrules melted in a canonical nebular gas lose sulfur and alkalis in minutes, while iron loss from the silicate melt continues over many hours. Mass loss and isotopic fractionation can be suppressed if the partial pressures of the species of interest are high enough in the ambient gas. Chondrule bulk and mineral composition arrays can be reproduced to a large extent by evaporation. However, condensation of SiO into the melt can simulate the zonation in some chondrules, with pyroxene and a silica polymorph near the rims. The partial equilibration of chondrule melt with noncanonical nebular gas would require heating for time periods of hours.
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