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Chondrules, matrix and coarse-grained chondrule rims in the Allende meteorite: Origin, interrelationships and possible precursor components

1987; Elsevier BV; Volume: 51; Issue: 7 Linguagem: Inglês

10.1016/0016-7037(87)90182-7

1872-9533

Alan E. Rubin, J. T. Wasson,

Planetary Science and Exploration

Bulk compositions of 20 chondrules, 13 coarse-grained chondrule rims and two composite samples of matrix material from CV3 Allende were determined by instrumental neutron activation analysis. Three rare nonporphyritic CV chondrules (from Allende, Leoville and Vigarano) were analyzed by broad-beam electron microprobe analysis. Chondrule precursor components deduced from chondrule compositions are characterized by: (1) refractory and semi-refractory lithophiles with low FeO, (2) common siderophiles and chalcophiles, (3) common to slightly volatile lithophiles (Si, Cr, Mn), and modal pyroxene with moderately high FeO(FeO + MgO), (4) moderately volatile siderophiles and (5) alkalies. These precursor components can be explained by plausible processes occurring in the solar nebula. The precursor components of coarsegrained rims are apparently the same as those of chondrules. These components differ from those inferred previously for OC, EH or CM-CO chondrules. In general, the compositions of coarse-grained rims are more similar to the mean chondrule composition than to those of the chondrules they enclose. Rim siderophile contents are generally 2–5X greater than those in the chondrules and 0.5 to 2X those in bulk Allende. Rims as a group are more homogeneous than chondrules, suggesting that rims formed from finer-grained precursors. Several chondrules and rims have fractionated rare-earth patterns and may have been affected by metamorphism. The enclosure of some coarse-grained rims by fine-grained matrix-like rims (that are almost certainly of nebular origin) indicates that coarse-grained rims were also formed in the nebula. Two matrix samples are similar in composition to some coarse-grained rims, suggesting that matrix was derived from similar compositional reservoirs of nebular dust. Chondrules with coarse-grained rims were reheated in space following the accretion of dusty coatings. This restricts plausible chondrule-forming mechanisms to those that allow multiple nebular heating events.