Evaporation Experiments on Planetary Materials.

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It has been believed that evaporation and condensation processes playd important roles in differentiation of materials during the formation of the solar system. The following results were obtained in the present project in relation to the above topics.(1) Experimental techniques for exactly obtaining time variation in the amounts of evaporation and condensation by using ultra-high temperature thermal balance to discuss their kinetics strictly were developed. First of all, dehydration experiments on hydrous silicate minerals and glass were carried out, and it was confirmed that kinetics of dehydration, wthich is regarded as incongruent evaporation of water, can be discussed by time variation in the amouts of dehydration. Evaporation experiments of alkalies (Na and K) on silicate melts were carried out at low pressures, and the evaporation rates were successfully obtained.(2) A new model on evaporation of Na from silicate melts and origin of chondrules was proposed. It was shown that chondrule formation is possible not only by open-system flash heating mechanism which has been proposed previously but by closed-system gradual heating. This gave a new viewpoint on the chondrule formation issue.(3) Evaporation experiments on Fe-S minerals (metallic iron and troilite) were done, and their kinetics (modes and rates) were obtained. Evaporation rates of these minerals in the primordial solar nebula were estimated based on the present results, and the Fe/S fractionation in the primordial solar nebula was discussed by combining a model for formation of a protoplanetary disk with the evaporation data.