A combined chemical-petrological study of separated chondrules from the Richardton meteorite
ABSTRACT 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: Abstract–Nineteen nonporphyritic pyroxene and pyroxene ⁄ olivine chondrules, chondrule fragments, and irregular objects were studied from two equilibrated chondrites, the ordinary (L ⁄ LL5) Knyahinya chondrite and the Rumuruti type (R4) Ouzina chondrite. Major element contents for almost all objects in the chondrites are disturbed from their chondritic ratios, most probably during metamorphic re-equilibration. However, the volatile elements (Na2O + K2O) in Ouzina scatter around the CI line, probably the result of being generated and ⁄ or processed in different environments as compared with those for Knyahinya. All studied objects from Knyahinya and Ouzina possess systematically fractionated trace element abundances. Depletion of LREE with respect to HREE and ultra-refractory HFSE documents variable degrees of LREE transport into an external mineral sink and restricted mobility of most of the HREE and HFSE. Moderately volatile elements preserve volatilitycontrolled abundances. Strongly fractionated Rb⁄ Cs ratios (up to 10· CI) in all studied objects suggest restricted mobility of the large Cs ion. All studied objects sampled and preserved Y and Ho in solar proportions, a feature that they share with the nonporphyritic chondrules of unequilibrated ordinary chondrites.Meteoritics & planetary science 01/2012; · 2.80 Impact Factor
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ABSTRACT: Abstract— We have studied the I-Xe system in chondrules and clasts from ordinary chondrites. Cristobalite-bearing clasts from Parnallee (LL3.6) closed to Xe loss 1–4 Ma after Bjurböle. Feline (a feldspar- and nepheline-rich clast also from Parnallee) closed at 7.04 ± 0.15 Ma. Two out of three chondrules from Parnallee that yielded well-defined initial I ratios gave ages identical to Bjurböle's within error. A clast from Barwell (L6) has a well-defined initial I ratio corresponding to closure 3.62 ± 0.60 Ma before Bjurböle.Partial disturbance and complete obliteration of the I-Xe system by shock are revealed in clasts from Julesburg (L3.6) and Quenggouk (H4), respectively. Partial disturbance by shock is capable of generating anomalously high initial I ratios. In some cases, these could be misinterpreted, yielding erroneous ages. A macrochondrule from Isoulane-n-Amahar contains concentrations of I similar to “ordinary” chondrules but, unlike most ordinary chondrules, contains no radiogenic 129Xe. This requires resetting 50 Ma or more later than most chondrules.The earliest chondrule ages in the I-Xe, Mn-Cr, and Al-Mg systems are in reasonable agreement. This, and the frequent lack of evidence for metamorphism capable of resetting the I-Xe chronometer, leads us to conclude that (at least) the earliest chondrule I-Xe ages represent formation. If so, chondrule formation took place at a time when sizeable parent bodies were present in the solar system.Meteoritics & Planetary Science. 02/2010; 35(3):445 - 455.
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ABSTRACT: There is a good consensus about the age of the most primitive materials in the Solar System represented by carbonaceous chondrites. From these, a canonical age of the Solar System of 4567 Ma has been defined. Nevertheless, the age of the subsequent events that modified the accreted material that formed meteorites has been only partially constrained by geochronological methods. In this work, we report KAr and PbPb ages for individual chondrules from eight Mexican chondrites: three H5 (Cosina, Nuevo Mercurio and Aldama), one LL5 (Tuxtuac), one L4 (Zapotitlán Salinas), one L5 (El Pozo), one L6 (Pácula), and one carbonaceous CV3 (Allende). Analysis were performed by using combined isotope dilution thermal ionization mass spectrometry and noble gas mass spectrometry. The purpose was to obtain some understanding of the thermal history of these meteorites and their parent bodies. The analyzed chondrules and matrix show a comprehensive KAr age range from the Solar System formation to 442 Ma, demonstrating that ordinary chondrites preserve an extensive record of disruption and accretion of their parent bodies.Revista mexicana de ciencias geológicas 04/2010; 27(1):123-133. · 0.51 Impact Factor