Computer Science
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.456w&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 456
Computer Science
Chondrules, Compound, Origin, Nebula, Evolution
Scientific paper
We studied compound chondrules in 79 cm2 of ordinary chondrite (OC) thin sections. Compound chondrules consist of a primary that solidified first and one or more secondaries attached to the primary. Sibling compound chondrules have very similar textures and compositions; most, perhaps all, seem to consist of chondrules melted in the same heating event. About 1.4% of all chondrules are the primaries of sibling compound chondrules. A smaller fraction, 1.0%, of all chondrules are the primaries of independent chondrules, the members of which were melted in separate heating events. Independent chondrules show appreciable differences in texture and/or composition. We propose that sibling chondrules originated when numerous chondrules were created from one large, more-or-less homogeneous, precursor assemblage that was flash-melted to produce a large set (perhaps 100-1000) of chondrules; some of these collided while molten, probably within several centimeters of the production site. We envision that small radial velocities were imparted to the members of the set, with small differences in velocity causing collisions among those few in intersecting trajectories. If all chondrules were produced this way, the collision efficiency was 1.4%; if only 10% were produced in this fashion, the efficiency rises to 14%. The original Gooding-Keil model of independent compound chondrule formation calls for random collisions to occur while the secondaries were molten. This appears improbable because the mean period between collisions in the dusty midplane of the nebula is estimated to be hours (or days), orders of magnitude longer than the period during which chondrules could have retained low viscosities following a flash-heating event in a cool (<700 K) nebula. We suggest that most independent compound chondrules formed by the mechanism that accounts for chondrules with relict grains and for chondrules with coarse- grained rims: the primary chondrule was embedded in a porous dust assemblage at the time of the second heating event; it experienced minimal melting because melting efficiency increases with increasing surface/volume ratio. There is a minor tendency for the FeO/(FeO+MgO) ratio in independent secondaries to be higher than in primaries, as expected if this ratio increased with time in the nebular dust. However, Monte Carlo calculations confirm that the compositions of independent secondaries are not randomly distributed, but related to those of primaries. Some exchange probably occurred during the fusion of the two chondrules, but this mechanism seems unable to account for the general similarity of independent primary/secondary compositions. This suggests that, in the environment where, at any one time, chondrules were forming (perhaps the interface between the gaseous nebula and the dusty midplane), the dust composition was more uniform than it was in the central midplane at a later time when agglomeration occurred.
Krot Alexander N.
Rubin Alan E.
Wasson John T.
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