Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.384k&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 384
Other
Chondrules, Ordinary Chondrites, Oxidation, Silica
Scientific paper
Most ordinary chondrite (OC) chondrules have compositions similar to those of bulk OC in terms of lithophile-element abundances. There are only a few rare chondrule classes that deviate significantly from OC-like compositions; these include Al-rich chondrules, chromitic and chromite-bearing silicate chondrules, and silica-rich chondrules. We studied 41 thin sections of unequilibrated OC and found 82 silica-bearing chondrules that can be divided into two major categories: silica-pyroxene chondrules and silica-fayalite- pyroxene chondrules. These chondrules are more common in H (>3/cm^2) than in L and LL chondrites (<1/cm^2). Silica-pyroxene chondrules consist mainly of low-Ca pyroxene and silica and have radial and porphyritic textures. Silica-bearing radial pyroxene (RP) chondrules contain 5-10 vol% silica grains; the low-Ca pyroxene is uniform in individual chondrules but varies from one chondrule to another (Fs(sub)10.2- Fs(sub)31.5). Silica-bearing porphyritic pyroxene (PP) chondrules contain 15- 40 vol% silica; the low-Ca pyroxene varies in composition within individual PP chondrules and tends to be more magnesian than in the silica-bearing RP chondrules (Fs(sub)5.0-Fs(sub)21.1). Petrographic observations suggest that some PP chondrules were not completely molten; they appear to have cooled more slowly than the silica-bearing RP chondrules. Silica-fayalite-pyroxene chondrules consist of silica, fayalite, and low-Ca pyroxene; accessory high-Ca pyroxene, plagioclase mesostasis, troilite, and metallic Fe-Ni are also present. Based on texture and the modal abundances of pyroxene and silica these chondrules can be divided into two types: (1) radial or porphyritic silica-fayalite-pyroxene chondrules containing 5-40 vol% silica and (2) granular silica-fayalite-pyroxene chondrules consisting almost entirely (90-95 vol%) of silica. Silica-fayalite-bearing pyroxene chondrules are texturally and compositionally similar to the silica-bearing pyroxene chondrules described above; the principal difference between them is the presence of fayalite-forming veins within or rims around the silica grains. The continuum between these chondrule categories implies that they are genetically related: We infer that the fayalite veins and rims formed by nebular alteration of the silica grains. Fayalite forms veins along the silica grain boundaries in granular silica-fayalite-bearing chondrules. Fragments of granular silica chondrules occur as relict clasts within two pyroxene chondrules in Sharps. These fragments were altered after chondrule solidification. Conclusions: (1) Silica-bearing chondrules have similar textures to common mafic silicate chondrules and were formed by melting silica-rich precursor material that possibly formed by nonequilibrium condensation. (2) The higher abundance of silica-bearing chondrules in H than in L and LL chondrites may indicate a greater degree of silica condensation in the H-formation region. (3) Silica-fayalite-bearing chondrules formed by alteration of silica-bearing chondrules. The common occurrence of both categories within the same chondrite suggests that oxidation and fayalite formation by nebular gas was an inefficient process.
Krot Alexander N.
Wasson John T.
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