Elemental abundances in meteoritic chondrules Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/08612s31d

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  • High-precision instrumental neutron activation analysis has been used to determine Al, Na, Mn, Sc, Cr, Fe, Co, and Ni in a suite of more than 500 chondrules from 26 distinct meteorites. These meteorite specimens represent the H, L, LL and C chemical classes and most of the petrologic types. In addition radiochemical activation analysis has been used to determine K, Rb and Cs in a suite of chondrules from the LL chemical class. To a limited extent trace element data on individual chondrules were correlated with petrographic observations on the same chondrules. I. In general it was found that the siderophilic elements Fe, Co, Ni and Ir are depleted in chondrules compared to the whole chondrite. The lithophilic elements Na, Mn, Sc, Cr and Al are generally enriched in chondrules compared to the whole chondrite. In contrast to the other lithophilic elements, Rb and Cs are generally depleted in the chondrules. Both major and trace elements may exhibit multimodal population distributions for the chondrules sets. II. Petrographic observations of the same chondrules for which trace element contents were determined by INAA suggest that the trace element distribution may be consistent with the mineral assemblages except for a positive Ir-Al and Al-Sc correlations which occur in many chondrule sets. Correlations between chondrule mass and Al, Na, Sc, Co, Lr and Cu contents were observed for certain chondrule sets. III. Chondrules from the H and LL groups appear to exhibit consistent compositional variations in going from low petrographic grades to high petrographic grades. The variations are observed most readily for an increasing Al-Na correlation coefficients and decreasing Mn-Na correlation coefficients with increasing petrologic types. A decrease in the dispersion of Mn and Na was also observed with increasing petrologic types. Na contents in the chondrules increase as a function of petrologic type. These observations are interpreted as indications of increasing equilibration of the chondrules with their matrices. IV. It appears that there are slight chemical differences between the Vigarano and Ornans subgroups as defined by Van Schmus (1969). This work supports the conclusions of Van Schmus and Wood (1967) and Van Schmus (1969) that the C2 and C3 groups are not generally related to one another by thermal equilibration processes while Karoonda may be a product of thermal equilibration of material similar to the Ornans subgroup. V. The Ni/Co ratio was found to be variable within chondrules from the same meteorite; for example, the range in Ochansk chondrules is from 10 to 60. VI. Theories concerning the origin of chondrules are discussed in the context of the elemental abundances and correlations observed in this study. a. Some chondrules may be produced by volcanism or impact on a homogeneous magma but they are not believed to be the main mechanism of production due to the chondrule inhomogeneity and the Al-Sc and Al-Ir correlations. b. The constrained equilibrium theory appears to be inconsistent with the positive Al-Ir and Al-Sc correlations and with the mass element correlations observed for Cu, Al, Sc and Ir. c. Some chondrules may have been produced by impact onto solid rock but this mechanism does not appear to be able to produce the necessary chemical fractionation observed in some chondrules. d. The chemical data is consistent with the remelting of preexisting dust in the solar nebula. The remelting appears to have occurred during terminal stages of the metal silicate fractionation or by a preferential melting of silicate material. The chondrules produced in any one event were apparently mixed with chondrules from other events and finally incorporated into the parent body. Additional chondrules may have been produced by impact during the terminal stages of accretion. The chemical evidence then suggests that some of the chondrules equilibrated with the matrix material of their parent body.
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