Other
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29r.475h&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 475
Other
Chondrites, Chondrule, Gravitational Effects, Meteoritic Composition, Regolith, Abundance, Isotopes, Oxidation-Reduction Reactions, Particle Size Distribution, Volatility
Scientific paper
H was proposed that chondrules formed as the products of energetic impacts in a very thick dynamic dust layer of an accreting asteroid-sized object and that the various chondrule groups, and thus chondrite classes, formed by variations in the number and intensity of impacts. We here argue that in such a dust layer there was probably a steady flow of volatiles and that on occasion conditions may have resembled those of a fluidized bed in which density and size sorting produced the metal-silicate fractionation and chondrule size distributions observed among the chondrites. The existence of a temporary atmosphere is suggested by the elemental and isotopic abundance patterns observed in chondrules. The atmosphere may have been permanent, but was probably transient, consisting of water and other volatiles from the parent body most probably produced during accretion and chondrule formation. It seems unlikely that such an atmosphere would be cosmic in composition and there are experimental reasons for suspecting that the H/O ratio was many orders of magnitude below cosmic and the P(Na) was much higher than expected for gases of cosmic composition. The requirements for minimal fluidization are determined by equating the upward drag force of the escaping volatiles and the downward gravitational force on the particles. We calculate that most asteroids smaller than few hundred kilometers should be capable of producing a sufficiently high flow rate of volatiles to produce fluidization. The extent of presumed fluidization seems to decrease with present volatile contents of the classes, consistent with the loss of volatiles during fluidization from parents of generally similar original composition.
Benoit Paul H.
Huang Shizhen
Sears Derek W. G.
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