Computer Science
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29..255r&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 2, p. 255-264
Computer Science
24
Breccia, Chondrites, Meteoritic Composition, Oxygen Isotopes, Abundance, Geochemistry, Iron, Olivine, Petrography, Meteorites, Antarctic Meteorites, Pca Meteorites, Pca 91002, Chondrites, Samples, Meteorite, Stony Meteorites, R Chondrites, Composition, Data, Description, Mineralogy, Laboratory Studies, Neutron Activation Analysis, Inaa, Comparison, Geochemistry, Petrography, Classification, Oxidation, Chondrules, Refractory Material, Lithophiles, Weathering, Clasts, Recrystallization, Shock Effects, Parent
Scientific paper
Pecora Escarpment (PCA)91002 is a light/dark-structured chondrite breccia related to Carlisle Lakes and Rumuruti; the meteorite contains approximately 10-20 vol% equilibriated (type-5 and -6) clasts within a clastic groundmass, much of which was metamorphosed to type-3.8 levels. The olivine compositional distribution forms a tight cluster that peaks at Fa(38-40); by contrast, low-Ca pyroxene compositions are highly variable. Opaque phases indentified in PCA91002 and its paired specimen, PCA91241, include pyrrhotite, pentlandite, pyrite, chromite, ilmenite, metallic Cu and magnetite. The majority of the rock is of shock stage S3-S4; there are numerous sulfide rich shock veins and 50-microns plagioclase melt pockets. Instrumental neutron activation analysis shows that, unlike Carlisle Lakes and ALH85151, PCA91002 exhibits no Ca enrichment or Au depletion; because PCA91002 is relatively unweathered, it seems probable that the Ca and Au fractionations in Carlisle Lakes and ALH85151 were caused by terrestrial alteration. The Rumuruti-like (formerly Carlisle- Lakes-like) chondrites now include eight separate meteorites. Their geochemical and petrographic similarities suggest that they constitute a distinct chondrite group characterized by unfractionated refractory lithophile abundances (0.95 +/- 0.05x CI), high bulk Delta O-17, a low chondrule/groundmass modal abundance ratio, mean chondrule diameters in the 400 +/- 100 micron range, abundant NiO-bearing ferroan olivine, sodic plagioclase, titanian chromite, abundant pyrrhotite and pentlandite and negligible metallic Fe-Ni. We propose that this group be called R chondrites after Rumuruti, only the fall. The abundant NiO-bearing ferroan olivine grains, the occurrence of Cu-bearing sulfide, and the paucity of metallic Fe-Ni indicate that R chondrites are highly oxidized. It is unlikely that appreciable oxidation took place on the parent body because of the essential lack of plausible oxidizing agents (e.g., magnetite or hydrated silicates). Therefore, oxidation of R chondrite material must have occurred in the nebula. A few type-I porphyritic olivine chondrules containing olivine grains with cores of Fa(3-4) composition occur in PCA91002; these chondrules probably formed initially as metallic-Fe-Ni-bearing objects at high nebular temperatures. As temperatures decreased and more metallic Fe was oxidized, these chondrules accreted small amounts of oxidized material and were remelted. The ferroan compositions of the greater than 5 microns olivine grains in the R chondrites reflect equilibration with fine-grained FeO-rich matrix material during parent body metamorphism.
Kallemeyn Gregory W.
Rubin Alan E.
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