Nebula Formation of the H, L, and LL Parent Bodies from a Single Batch of Chondritic Materials

Details Nebula Formation of the H, L, and LL Parent Bodies from a Single Batch of Chondritic Materials Nebula Formation of the H, L, and LL Parent Bodies from a Single Batch of Chondritic Materials

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28..358h&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 358

Computer Science

7

Chondrules, Nebula, Ordinary Chondrites, Oxygen Isotopes, Roosevelt County 075, Semarkona

Scientific paper

Introduction: Studies of ordinary chondrites have established that a) components in H chondrites are chemically and isotopically indistinguishable from those in L and LL chondrites; b) the chondrules show similar but larger variations than the chondrite groups; and c) mean chondrule sizes increase from H through L to LL. Correlations between chondrule size and oxygen isotopes [1], FeO [2], and metal spherule size in CR chondrites [3] suggest that size sorting of chondritic ingredients from a single reservoir could be responsible for all of the differences between H, L, and LL. Oxygen isotopes: Clayton et al. [1] found that the smallest chondrules in Dhajala (H3.8) and Weston (H4) plot closer to the terrestrial fractionation line than the large chondrules. Since H-chondrites have smaller chondrules and oxygen isotope ratios closer to the terrestrial fractionation line, size sorting of a single population of chondrules could produce the observed variation through the equilibrated ordinary chondrites. Differences in FeO: Two populations of chondrules with distinctly different metal and FeO contents have been identified among the chondrules in unequilibrated ordinary chondrites: type I and type II [2,4] with type I being more reduced than type II chondrules. Type I and type II chondrules found in H chondrites are vitually identical in mineral composition to type I and II chondrules in LL chondrites [5,6]. The differences in FeO between equilibrated H, L, and LL chondrites can be explained solely in terms of different proportions of type I and II if the type I/type II ratio is higher in H than in LL chondrites. We have studied Semarkona (LL3.0) and Roosevelt County 075, which has been classified as H3.2 [5]. We find that: a) type I chondrules are smaller than type II chondrules b) type I chondrules are relatively more abundant in RC075. Size sorting of chondrules is therefore consistent with our observations and tends to make H chondrites more reduced than L and LL chondrites. Metal: Metal is more abundant in H than L and LL. This is partly due to a higher proportion of metal-rich type I chondrules but is also due to a higher abundance of individual metal particles and spherules in H-chondrites. Because the aerodynamic processes must have operated on the metal spherules as well this would imply that the mean size of metal spherules in the source region was much smaller than the mean size of the silicate chondrules. Since metal spherules are probably ejected from chondrules, this is consistent with expectations. Conclusions: We find that size sorting of chondrules from a common source region can explain the major differences between ordinary chondrites. This would require the chondritic components found in the ordinary chondrites to have formed in the same region of the nebula, during a limited period of time. Aerodynamic sorting of the chondrules seems to be the most viable mechanism to produce size-sorting of chondrules. Since small chondrules take longer to spiral inwards through the nebula we would predict that the H parent body was farthest from the Sun, contrary to [6]. References: [1] Clayton R. N. et al. (1991) GCA, 55, 2317-2337. [2] Scott E. R. D and Taylor G. J. (1983) JGR, 88, B275-B286. [3] Skinner W. R. and Leenhouts J. M. (1993) LPS XXIV, 1315-1316. [4] McSween H. Y (1985) Meteoritics, 20, 523-540. [5] McCoy T. J. et al (1993) Meteoritics (submitted). [6] Jones R. H. (1990) GCA, 54, 1785-1802. [7] Wasson J. T. (1985) Meteorites, 267, Freeman.

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