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 melti ng of mineral aggregates in the solar nebula and have textures control led by the number of residual nuclei present after melting. If barred olivine (BO) chondrules formed by nearly total melting at their respec tive liquidi, i.e., over a wide temperature range, it is surprising th at 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 m olten droplets to produce chondrule textures. We have, therefore, cond ucted experiments in which mineral dusts encounter molten droplets dur ing their cooling at 500 degrees C/h and cause nucleation. Our experim ents reproduce the majority of common chondrule textures. The producti on 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. C rystal growth rates increase with lower dust encounter temperatures gi ving 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-porphyrit ic-olivine pyroxene (BO/POP) by dust seeding that have not been reprod uced by the incomplete melting of starting materials. The reproduction of BO/POP and excentroradial textures by dust collisions, and the sca rcity of natural totally glassy chondrules, suggest that formation by seeding of total melts was common for chondrules with the lowest liqui dus temperatures, e.g., FeO-rich chondrules. From these seeding experi ments we suggest that the maximum melting temperature chondrules exper ienced is limited only to the temperature at which evaporation of chon drule liquid occurs. However, we have reproduced the very fine-grained microporphyritic textures characteristic of Type IA chondrules only b y incomplete melting. Such chondrules could not have reached average i nternal 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 o ccurred within a region of the nebula rich in mineral dust.