** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.
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Research
Space and planetary sciences
202009202009
Bubbles to Chondrites - I. Evaporation and condensation experiments, and formation of chondrules
Nakano Y, Hashimoto A
Evaporation, Condensation, Experiment, Chondrule, Chemical fractionation, Boiling, Jet-droplet, Chondrule fromation, Protoplanetary disk, Solar system
Snapshots of jet-droplet ejection from the boiling sample. Run #83 (200 W). The video camera speed is 30 fm s-1, and its exposure time is 1/60 s. Experiments with high laser power (200 W) produce jet droplets in all directions because the entire samples are kept above boiling temperatures. The bright ball in the center is an effect of halation, which obscures the sample. The actual sample diameter is about one half of the ball. The field of the view is ~35 mm across.
We propose a simple model of chondrule formation that is supported by our new experiments. With a laser-heating and inert-gas-cooling technique, we obtained evaporation and condensation pathways starting with chondritic compositions till ends, and extracted ‘relative volatilities’ of elements from them. Above boiling points, we observed numerous silicate droplets being ejected from collapsed cavities of vapor bubbles on the surface of molten sample, known as jet-droplets. We postulate jet-droplets as origin of chondrules. The formation mechanism of jet-droplets requires a dense and large solid body (>3 cm across), named ‘duston’, for chondrule precursors. Our chondrule formation model presumes dustons having CI-like composition. Upon boiling, a duston ejects jet-droplets from its molten surface and simultaneously forms an adiabatically expanding vapor cloud around it. The jet-droplets supercool and incorporate the supersaturated vapor and fine condensates while they travel through the cloud, thus completing their makeup as chondrules. The compositions and the mixing ratio of the three components (jet-droplet, vapor and condensate) can be exactly predicted by using relative volatilities of elements, given the chondrule composition to be fitted and the conditions: vaporization degree (VD) and redox state (fs) of the duston. We attempt to reproduce bulk compositions of chondrules in total of 600. About 75% chondrules are successfully matched with specific combinations of VD and fs for each chondrule. The model altogether explains 3.5 features of chondrules: maximum size and size-frequency distribution; chemical variety; and textural variety.
