Coarse-grained rims on magnesium-rich and magnesium-poor chondrules in ordinary chondrites

Details Coarse-grained rims on magnesium-rich and magnesium-poor chondrules in ordinary chondrites Coarse-grained rims on magnesium-rich and magnesium-poor chondrules in ordinary chondrites

Jul 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29q.487k&link_type=abstract

Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 487

Computer Science

Chondrites, Chondrule, Magnesium, Meteoritic Composition, Meteoritic Microstructures, Rims, Electron Probes, Mineralogy, Petrography, Scanning Electron Microscopy, Solar Corona, Temperature Effects

Scientific paper

Chondrules with igneous rims and enveloping compound chondrules in ordinary chondrites (OC) preserve the record of solids that were present at different times or in different regions of the solar nebula during chondrule formation. These objects demonstrate that OC chondrules experienced multiple episodes of chondrule formation. This conclusion is consistent with the presence of relict grains in chondrules that were probably produced by disaggregation of chondrules of a previous generation, small range in OC chondrule O isotope composition, and interelement correlations in bulk chondrule data that can be interpreted as the random sampling of a previous generation of chondrules. Rims around chondrules can be divided into two major categories: fine-grained rims (FGR), typically opaque and Fe rich and relatively coarse-grained rims. Thirteen CGR on Mg-rich chondrules (type I, Fa/Fs less than 10 mol%) and nine rims on Mg-poor chondrules (type II, Fa/Fs greater than 10 mol%) were studied petrographically, by electron microprobe analysis, and scanning electron microscopy. Many of the CGR on type I chondrules show evidence of significant and, in many cases, complete melting. Similar Fa and Fs contents in mafic minerals of OC igneous rims and their type I chondrule hosts indicate that many OC chondrules experienced multiple heating events during a time short compared to the time necessary for appreciable evolution in the mean Fa or Fs of the nebular solids, and were than withdrawn from the chondrule-forming region. Type II chondrules and their CGR formed from more oxidized material mixed with fragments of type I chondrules and were heated to lower temperatures than type I chondrules and their CGR. Type I and type II chondrules may have formed in different OC nebular subregions or at different times and were mixed together before or during agglomeration to form chondrites.

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