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    Space and planetary sciences


    Bubbles to Chondrites-II. Chemical fractionations in chondrites

    Hashimoto A, Nakano Y

    Chondrites, Chondrules, Chemical fractionations, Protoplanetary disk, CAIs, D/H ratio, Lighting discharge, Planetesimal, 26Al, Redox state, Evaporation, Condensation, Vortex, Snow line

    An image of nebular lightning-discharge inside a large vortex near the snowline. a Oppositely charged two layers, one consisting of ultra-fine icy particles with negative charges and the other consisting of large dust aggregates (dustons: Nakano & Hashimoto, 2020) with positive charges, create a large potential field in-between, 1-100 MeV. Nebular electric discharge propagates with a wide cross section, 1-10 km across, due to large mean free paths of nebular gas molecules (H2), but converges into a narrow stream once it hits an unlucky duston because vaporized volatiles create a dense vapor cloud around the duston (with diameter ~1000 times as large as duston). b The converged discharge heats the surface of the duston above its boiling temperature and numerous jet droplets are released into the space - origin of chondrules (Nakano & Hashimoto, 2020). Since the lateral dimension of the negatively charged layer may be thousands of kilometers, one discharge event could neutralize thousands of positively charged dustons, causing a discharge cascade as shown in a. The discharge path creates a large magnetic field around it, which exerts a pinching effect onto it. Once the discharge ceases, however, plasma expands supersonically and destroys the vapor cloud. Some chondrules acquire natural remanent magnetization.

    We attempt to develop a possible theory of chemical fractionations in chondrites, that is consistent with various features of chondritic components and current observation of protoplanetary disks (PPD). Combining the 3+2 component fitting calculation that simulates chondrule formation process proposed in paper (I) with additional mixing procedures, we investigate essential causes that made various types of chondrites evolve from the uniform solar system composition, the CI-chondritic composition. Seven chemical types of chondrites (CM, CV, CO, E, LL, L and H) are examined, for which reliable chemical compositions for both bulk chondrites and chondrules therein are known. High vaporization degree of the primordial dust aggregates (dustons) required by the calculation vindicates that the chondrule formation was the driving force for the chemical fractionations in all chondrites examined. Various initial redox states in dustons and different timings of CAIs’ invasion to the chondrule formation zone are identified for different chondrite types. These results, together with a good correlation with the D/H ratios of chondrites measured previously, lead us to the notion that PPD evolved from reducing to oxidizing. We explore the heating mechanism for the chondrule formation and the place it occurred. Only heat source being consistent with our chondrule formation model is lightning discharge. We postulate that large vortices encompassing the snow-line are ideal places for large charge separation to occur between dustons and small ice particles, and that direct strikes on dustons should make them boil for ten seconds and longer and allow a swarm of chondrules released from their surfaces. Chemical fractionations are completed by an aerodynamic separation of dustons from chondrules inside the vortex, in such a way that the dustons fall fast into the vortex center and form a planetesimal immediately, while chondrules with dust mantles fall slow and form a thin veneer on the planetesimal surface. During collisional episodes, the veneers are preferentially fragmented and reassemble themselves by a weak self-gravity to form a rubble-piled chondritic asteroid, i.e. chondrite.