| Abstract: | Abstract— The liquidus temperatures of chondrules range from about 1200 °C to almost 1900 °C, based on the calculation of Herzberg (1979). Dynamic melting and crystallization experiments with no external seeding suggest that some chondrule textures formed with initial temperatures below the liquidus (e.g., porphyritic, granular) and some were completely melted (e.g., excentroradial, glassy). Type I and III chondrules in carbonaceous chondrites in this interpretation consist of incompletely melted magnesian chondrules, completely melted silica-rich chondrules and intermediate composition chondrules with both porphyritic and nonporphyritic textures. A similar pattern for ordinary chondrites, with data also for Type II porphyritic and barred olivine chondrules, suggests that few chondrules with liquidus temperatures over 1750 °C were completely melted and few with under 1400 °C were incompletely melted. The range of liquidus temperatures for barred olivine chondrules, for which initial temperatures appear to have been essentially at the liquidus, is similar. Most chondrules may therefore have been heated to temperatures of 1400–1750 °C and, because of a peak in the distribution of barred olivine chondrule temperatures at 1500–1550 °C, the temperatures appear normally distributed within this range. Given a narrow range of temperatures, bulk composition is at least as important as initial temperature in controlling chondrule textures. Truly granular (not microporphyritic) Type I and truly glassy Type II and III chondrules appear under-represented in nature according to this model, based on internal nucleation experiments. External heterogeneous nucleation, or seeding due to droplet-dust collisions, is likely to occur in a dusty nebula and has been shown to reproduce chondrule textures experimentally. Generally high initial temperatures (1600–1800 °C), coupled with dust-seeding of superheated droplets of less refractory composition is an alternative explanation of chondrule textures. Cooling rates of 100–1000 °C/hr are required for chondrules, which must have been mass produced in clouds with sufficient particle density to buffer cooling rate and perhaps also initial temperature. Melting precursor particles in a thick clump and/or the nebular mid-plane would provide evaporation and thus explain the high oxidation state and volatile content of chondrules, relative to the bulk hydrogen-rich nebula, as well as the nature of the cooling. |
