Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.439s&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 439
Other
Acfer 059, Chondrites, Chondrules, Metal-Silicate Fractionation, Solar Nebula
Scientific paper
It has long been recognized that chondrules in individual chondrites have populations with restricted size ranges. Dodd's landmark study [1] quantified this observation for silicate and metal chondrules in ordinary chondrites and argued for an aerodynamic sorting mechanism. Later studies, e.g., [2] and others cited therein, have confirmed and extended these observations. Our work has added a consideration of chondrule shape [3] and extended the data on metal vs. silicate chondrules [4]. These observations lead to conclusions regarding radial transport of material in the solar nebula. Ordinary chondrites contain an intimate mixture of nearly spherical droplet chondrules and angular clastic chondrules that define a single size-sorted population within a given chondrite [3]. Many clastic chondrules preserve an arcuate face that suggests they were once part of a much larger droplet chondrule, indicating that droplet chondrules were formed in a larger range of sizes than are now observed in these chondrites, and that droplet chondrules were broken up in the solar nebula to yield the clastic objects now observed [3]. Chondrites represent restricted "size-bins" of chondrules sampled during accretion of the parent bodies [5], probably by aerodynamic processes in the nebula. The particular "size-bins" we observe sampled a very restricted portion of the range of sizes that once existed in the nebula. It seems unlikely that all the larger chondrules would have been destroyed. Thus the rarity of their appearance in known chondrites suggests that large chondrules were deposited (accreted) into other "size-bins" at heliocentric distances not represented by the known chondrites, and that sorting processes in the solar nebula must have included a radial component. A similar conclusion was reached in the study of an unusual CR2 chondrite, Acfer 059, in which metal chondrules are preserved with their original rounded shapes. Separate size distributions of metal and silicate chondrules reveal that these two populations with contrasting densities have similar size distributions (but differing size ranges) with mean diameters of 0.74 mm and 1.44 mm, respectively [4]. These mean sizes have almost identical ratios of mass to cross-section area and are thus aerodynamically equivalent. Relative to primitive CI compositions, Acfer 059 and essentially all other chondrites have iron-depleted compositions. The similarity of aerodynamic characteristics and the similarity in size distributions of these two chondrule populations strongly suggest that the iron (and other siderophile) depletions in chondrites are directly related to the aerodynamic process that produced the sorting. The extreme rarity of metal-enriched chondrites (relative to CI compositions) implies that this process had a radial component. The excess metal must have concentrated into a region of the solar nebula not generally sampled by known meteorites. Chondrules of all types in primitive ordinary and carbonaceous chondrites acquired fine-grained rims in the nebula prior to accretion into parent bodies. Broken and droplet surfaces alike are coated by rim material indicating that rims were added after fragmentation of chondrules. Thus chondrules experienced a variety of environments in the nebula: first, the hot environment in which they formed; next, a violent environment in which cool, brittle chondrules were fragmented; then an environment in which rims were acquired (some cold and some hot?); and finally a quiescent environment of accretion into planetesimals where breakage and abrasion of rim material was minimal. It is likely that chondrules and other components of chondrites were sorted during each of these stages. References: [1] Dodd R. T. (1976) EPSL, 30, 281-291. [2] Rubin A. E. (1989) Meteoritics, 24, 179-189. [3] Leenhouts J. M. and Skinner W. R. (1991) Meteoritics, 26, 363. [4] Skinner W. R. and Leenhouts J. M. (1993) LPSC XXIV, 1315-1316. [5] Skinner W. R. and Leenhouts J. M. (1991) Meteoritics, 26, 396.
Leenhouts James M.
Skinner Wilbert R.
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