Computer Science
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30q.569r&link_type=abstract
Meteoritics, vol. 30, no. 5, page 569
Computer Science
3
Chondrites, Carbonaceous, Enstatite, Formation, Ordinary, R, Meteorites, Carlisle Lakes, Ningqiang
Scientific paper
There are 12 well-established chondrite groups distinguishable by significant compositional hiatus among their compositional and petrographic characteristics. Because each group represents a different parent asteroid, formed in a distinct nebular region (and/or at a particular time), it is plausible that chondrite properties varied in a smooth fashion with heliocentric distance (HD). Oxidation state. Thermodynamic calculations indicate that the equilibrium FeO/(FeO+MgO) ratio increases with decreasing nebular temperature. Because the nebular temperature gradient decreased with HD, at all times inner nebular regions had a lower oxidation state than more distant regions. If the time of agglomeration increased with HD, outer-solar-system materials generally would have acquired more ferroan compositions. By these criteria we infer that enstatite chondrites formed closer to the Sun, OC at intermediate HD, and R chondrites and carbonaceous chondrites still farther from the Sun. Oxygen isotopic composition. The nearer the Sun, the higher the nebular temperature and the larger the fraction of infalling interstellar material that evaporated; this resulted in greater equilibration with nebular gas and greater isotopic homogeneity. Because the Earth, Moon and EH-EL chondrites lie on the terrestrial fractionation (TF) line on the standard three O-isotope diagram, and martian meteorites (Delta^(17)O=0.36 per mil) and eucrites (Delta^(17)O=-0.40 per mil) lie close to this line, we infer that the mean nebular (i.e., solar) O-isotopic composition was on or near the TF line. At >1 AU the absolute value of Delta^(17)O increased. We infer that EH and EL chondrites formed at a HD <1 AU; H, L and LL chondrites (mean Delta^(17)O = 0.73, 1.07 and 1.26 per mil) formed appreciably beyond Mars' distance of 1.5 AU (probably near 2.5 AU at the 3:1 Jupiter-period resonance); R chondrites (Delta^(17)O ~2.9 per mil) and CR, CM, CO, CV and CK chondrites (Delta^(17)O ~ -1.6, -2.3, -4.5, -3.4 and -4.5 per mil) formed at still greater HD. Episodic accretion of 16O-poor and 16O-rich materials to the nebula may account for positive Delta^(17)O values for OC and R chondrites and negative values for carbonaceous chondrites. Refractory lithophile abundance. It is plausible that chondrite refractory lithophile abundances (RLA) reflect the efficiency which refractory precursor grains settled to the nebular midplane. High temperatures in the inner nebula caused most presolar refractory grains to evaporate; fine refractory condensates formed during subsequent nebular cooling. At greater HD, refractory residues survived, perhaps reaching mm-to-cm sizes. The resultant gradient in RLA is reflected in the modal abundance of CAIs. By these criteria, EH and EL chondrites (which have CI- and Mg-normalized mean RLA of 0.88 and 0.78, and CAI abundances of <=0.1 vol.%) formed closest to the Sun, OC (~0.90; ~0.1%) formed farther away, R chondrites (0.95; <0.1%) formed farther still, and CR, CM, CO, CV and CK chondrites (>=1.00 ~1-5%) formed at the greatest HD.Matrix/chondrule modal abundance ratio. Because matrix material in type-3 chondrites contains appreciable nebular dust (a probable chondrule precursor component), chondrites with high matrix/chondrule modal abundance ratios probably agglomerated in nebular regions where the efficiency of chondrule production was low. Plausible chondrule-formation mechanisms (e.g., lightning) decrease in efficiency with increasing HD; hence, chondrites with high matrix/chondrule ratios formed at greater HD than chondrites with low ratios. Most of the interchondrule silicates, kamacite and sulfides in EH3 chondrites may represent reprocessed chondrule fragments. If so, the actual chondrule abundance of EH3 chondrules is ~90 vol.%. Chondrite groups can thus be ranked in order of increasing matrix/chondrule ratio: EH: 0.1; OC: 0.2; R (Carlisle Lakes): 0.9; CO: 0.9; CR: 0.9; CV: 1.0; CK (Ningqiang): 2.3; CM: 4.Degree of chondrule melting. Droplet chondrules (BO, RP, C) formed by the complete melting of precursor materials; in contrast, surviving relict nuclei in porphyritic chondrules indicate incomplete melting. Chondrite groups ranked in order of decreasing abundance of droplet chondrules probably reflect increasing HD and a probable decrease in the efficiency of chondrule melting: EH (17%), OC (16%), R (~8%), CV (6%), CM (~5%), CO (4%), CK (<1%), CR (<1%). Because chondrite groups are ranked in the same basic order for variations in five distinct properties, an underlying factor such as varying HD is probably responsible.
Rubin Alan E.
Wasson John T.
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