Abstract
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.
Highlights
1 Introduction In pursuit of fundamental rules of chemical fractionations caused by evaporation and condensation of primordial dust aggregates, we have discovered an alternative and compelling route in evolution of chondritic materials in the solar system protoplanetary disk
This paper starts as a sequel to paper (I), in that basically the same model and calculation procedure are applied to the bulk chemistry of chondrites, but having gained an entirely new perspective on the origin of chondrites, we expand our view to the evolution of protoplanetary disks (PPD) materials from dust to planetesimals
This paper started as a sequel to paper (I), in that basically the same model and calculation procedure are applied to the bulk chemistry of chondrites, but having seen entirely new results on the origin of chondrites, we have expanded our view to the evolution of PPD materials to planetesimals
Summary
Recent theories and observations indicate much colder protoplanetary disks (PPD) with much smaller masses, as Williams and Cieza (2011) in their review paper described, Hashimoto and Nakano Progress in Earth and Planetary Science “Initially, disks rapidly funnel material onto the star but, as the surrounding molecular core is used up or otherwise disperses, the accretion rate decreases and a small amount of material persists. That these discs can be considered protoplanetary is apparent through the geometry of the Solar System and the high detection rate of exoplanets”. The equilibrium condensation model of chondritic materials has lost its basis and we are in need of a new mechanism to explain the chemical fractionations in chondrites
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