Computer Science
Scientific paper
Jul 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994metic..29s.450b&link_type=abstract
Meteoritics (ISSN 0026-1114), vol. 29, no. 4, p. 450-451
Computer Science
Abundance, Breccia, Chemical Composition, Meteoritic Composition, Pyroxenes, Regolith, Grain Size, Iron Compounds, Lava, Magnesium, Metamorphism (Geology), Mineralogy, Temperature Effects
Scientific paper
The polymict eucrite Petersburg is a regolith breccia containing significant proportions of matrix pyroxenes with compositions intermediate between those found in diogenites and noncumulate eucrites. Possible sources of these intermediate-composition pyroxenes are given. Abundant eucritic clasts in Petersburg allow examination of material from which matrix minerals were derived. The majority of Petersburg lithic clasts represent Fe-rich, equilibrated eucrites similar to Juvinas. Clast pyroxenes within 20-30 microns of clast edges have significantly higher Mg# than interior pyroxenes; this is apparently the result of subsolidus alteration after clast formation. Based on matrix textures, Hewins 13,41 has inferred thermal metamorphism of the Petersburg breccia under a hot breccia blanket or lava flow. The uniform width and ubiquity of Petersburg alteration rims suggests formation during a single thermal event after final breccia accumulation. We have analyzed Petersburg clast A, a medium-grained (average grain size about 1 mm), subophitic clast predominantly composed of lath-shaped plagioclase (44%) and elongate to equant pyroxene (44%) with mesotasis (10%) and minor opaques (2%). Pyroxenes are unzoned with respect to Mg/Fe and are dominantly composed of low-Ca clinopyroxene with fine (less than 1 micron thick) augite exsolution lamellae that causes electron microprobe analyses to range in apparent composition from Wo(2.0)En(46.5) to Wo(32.7)En(33.7). Plagioclase composition ranges from An(77.4) to An(88.7). Rare Earth Element (REE) abundances are about 26x chondrite with a significant negative Eu anomaly. Electron microprobe analyses of fused beads indicate that bulk Mg# of this clast is about 49. Given the narrow width of alteration rims on Petersburg eucrite clasts, the large size of clast A (greater than 0.6 g), and the fact that Neutron Activation Analysis INAA and fused bead samples were obtained from the interior of clast A, bulk clast compositions were unaffected by the metamorphic redistribution of Fe/Mg described above. Such an interpretation is difficult to reconcile with data for clast A from Petersburg, since bulk composition is more REE rich and yet more magnesian than that of Pomozdino. Assuming Binda-like cumulate material, modeling of this composition requires mixture with liquid that is more REE rich than any eucritic material yet analyzed. Clast A may represent a very-low-proportion partial melt of a source that has chondritic REE abundances. Intermediate-composition matrix pyroxenes in Petersburg are derived at least in part from materials that have trace-element compositions similar to Stannern, but are more magnesian. Some smaller (less than 60 microns) matrix pyroxene grains probably have compositions that are largely the result of redistribution of Fe/Mg during metamorphism after breccia accumulation.
Buchanan Paul C.
Reid Michael A.
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