A combined chemical-petrological study of separated chondrules from the Richardton meteorite

Lamont-Doherty Geological Observatory of Columbia University, Palisades, NY 10964 U.S.A.
Earth and Planetary Science Letters (Impact Factor: 4.73). 03/1979; 42(2):223-236. DOI: 10.1016/0012-821X(79)90028-1


Individual chondrules have been separated from the H5 chondrite Richardton and subjected to a detailed chemical-petrological study. A portion of each chondrule has been examined petrographically and phase chemistry determined by electron microprobe analysis. Of the remaining portion an aliquot was taken for measurement of major element abundances by microprobe using a microfusion technique. Rb, Sr,87Sr/86Sr and REE were determined by mass spectrometric isotope dilution.The chondrules define a Rb-Sr isochron age of 4.39 ± 0.03Ga(λ = 1.42 × 10−11 a−1) and an initial ratio of 0.7003 ± 7. The age is interpreted as a metamorphic age and indicates that Sr isotope equilibration occurred in the Richardton parent body for some 100 Ma or more after condensation of the solar system. Metamorphism had little effect on chondrule textures but effected Fe/Mg exchange to produce highly uniform olivine and pyroxene compositions, and may have caused some redistribution of REE.The major element compositions of Richardton chondrules are mostly constant and close to reported averages for Tieschitz, Bishunpur and Chainpur. They contain high-temperature condensate elements in close to cosmic proportions, but are deficient in Fe. Theories of chondrule origin are briefly reviewed, and while it is difficult to distinguish between direct condensation and dust fusion by impacting, it is postulated that iron was fractionated from silicate prior to or during chondrule formation.

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    ABSTRACT: A precise87Rb-87Sr whole-rock isochron for H chondrites and an internal isochron for Tieschitz (H3) have been determined. The age and87Sr/86Sr initial ratio of the whole rocks are4.52 ± 0.05 b.y. and0.69876 ± 0.00040(λ(87Rb) = 1.42 × 10−11yr−1). For Tieschitz, whereas handpicked separates plot on a well-defined line, heavy liquid separates scatter in the87Rb/86Sr vs.87Sr/86Sr diagram. Leaching experiments by heavy liquids indicate that they might have a sizeable effect on Tieschitz minerals. The age and87Sr/86Sr initial ratio as determined by handpicked separates are4.53 ± 0.06 b.y. and0.69880 ± 0.00020, indistinguishable from the whole-rock isochron.These results are interpreted as “primitive isochrons” dating the condensation of chondrites from the solar nebula. The best value of this event is given by joining both isochrons together at4.518 ± 0.026 b.y. and87Sr/86Sr= 0.69881 ± 0.00016. The near identity of this initial ratio with the one of Allende white inclusions argues in favor of a sharp isochronism of condensation from a87Sr/86Sr homogeneous nebula. Data from Guaren˜a [11] and Richardton [48] are interpreted as secondary internal isochrons, 100 m.y. after the condensation of the whole rocks.The data are then used to constrain a thermal evolution model of the H chondrite parent body. This body might have a 150–175 km radius, and might have been heated by26Al. An26Al/27Al ratio of 4–6 × 10−6 is enough for heating such a body. Further tests for this model are proposed.
    No preview · Article · Mar 1979 · Earth and Planetary Science Letters
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    ABSTRACT: A study of volatile element concentrations in individual chondrules in the Chainpur LL3 chondrite is presented. Volatile elements differing in geochemical behavior tended to vary randomly and correlated variations were observed only for those elements inferred to occupy similar mineral sites. Variations in Co and Ni in kamacite show that metal in chondrule interiors has not approached equilibrium, and that postformational element distribution has mainly altered siderophile concentrations in surface metal grains, and has probably left ionic species unaltered. Etching to remove 3 to 5% of the chondrule dissolved siderophiles, enhanced Zn and Na volatiles relative to nonvolatile Cr in the etch, and produced a large Cd enhancement. If metamorphic redistribution of nonsiderophile volatiles was neglible, their high concentrations are inconsistent with chondrule formation by direct condensation, and models involving sudden melting of preexisting solids can explain the observations but require rapid cooling to prevent volatile loss.
    No preview · Article · Aug 1979 · Geophysical Research Letters
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    ABSTRACT: 207Pb/204Pb-206Pb/204Pb whole meteorite isochrons for Richardton (H5) and Farmington (L5) are presented and give Pb-Pb ages of 4.545 ± 0.010 and 4.620 ± 0.010 Ga respectively (errors ± 2σ). The Pb-Pb isochron for Farmington passes below the Can˜on Diablo troilite composition, which may therefore not be the initial Pb composition for this meteorite.All samples show an apparent excess radiogenic lead for single-stage (closed-system) evolution when Can˜on Diablo troilite is used for the initial lead composition. Evidence is presented to show that the apparent excess Pb cannot be explained by terrestrial contamination. There is no unique isotopic composition for initial lead that yields concordant ages at 4.55 Ga for all samples for either meteorite. The data likewise cannot be reconciled to Can˜on Diablo initial lead through any of the conventional two- and three-stage evolution models.The apparent excess Pb, with respect to a Can˜on Diablo troilite composition and a single-stage closed-system history, and the apparent inhomogeneous initial Pb isotopic compositions, appear to be real. This may be an indication that the U-Pb systems in these meteorites are disturbed, but this disturbance cannot be described consistently by any of the conventional episodic evolution models.
    No preview · Article · Feb 1980 · Earth and Planetary Science Letters
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