CHONDRULES AS PRODUCTS OF DUST COLLISIONS WITH TOTALLY MOLTEN DROPLETS WITHIN A DUST-RICH NEBULAR ENVIRONMENT - AN EXPERIMENTAL INVESTIGATION

Chondrules are usually interpreted as the products of incomplete melting of mineral aggregates in the solar nebula and have textures controlled by the number of residual nuclei present after melting. If barred olivine (BO) chondrules formed by nearly total melting at their respective liquidi, i.e., over a wide temperature range, it is surprising that there are so few totally glassy chondrules. The occurrence of fine-grained, accretionary rims on chondrules suggests mineral dust in the chondrule-forming environment which might have collided with totally molten droplets to produce chondrule textures. We have, therefore, conducted experiments in which mineral dusts encounter molten droplets during their cooling at 500 degrees C/h and cause nucleation. Our experiments reproduce the majority of common chondrule textures. The production of textures in our experiments is a function of the temperature at which dust is encountered, the number of dust grains encountered and, to a lesser extent, the mineralogy of the encountered mineral gains. Crystal growth rates increase with lower dust encounter temperatures giving a textural range with increasingly skeletal or elongated crystals for lower temperature encounters. We have produced some less commonly discussed types of chondrule textures, e.g., barred-olivine-porphyritic-olivine pyroxene (BO/POP) by dust seeding that have not been reproduced by the incomplete melting of starting materials. The reproduction of BO/POP and excentroradial textures by dust collisions, and the scarcity of natural totally glassy chondrules, suggest that formation by seeding of total melts was common for chondrules with the lowest liquidus temperatures, e.g., FeO-rich chondrules. From these seeding experiments we suggest that the maximum melting temperature chondrules experienced is limited only to the temperature at which evaporation of chondrule liquid occurs. However, we have reproduced the very fine-grained microporphyritic textures characteristic of Type IA chondrules only by incomplete melting. Such chondrules could not have reached average internal temperatures above their liquidus temperatures. The conclusion, that at least some chondrules formed from total melts that collided with mineral dust during cooling, indicates that chondrule formation occurred within a region of the nebula rich in mineral dust.