Abstract
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.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
