Lithic Clasts in the Polymict Eucrite Petersburg
Abstract
The polymict eucrite Petersburg is a regolith breccia containing significant proportions of matrix pyroxenes with compositions interrnediate between those found in diogenites and noncumulate eucrites. Possible sources of these intennediate composition pyroxenes include cumulate eucrites, noncumulate eucrites more magnesian than "main group" eucrites, cores of zoned pyroxenes from unequilibrated eucntes, and eucritic materials affected by metamorphic redistribution of Fe/Mg. Abundant eucritic clasts in Petersburg allow examination of materials 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 micrometers of clast edges have significantly higher Mg# than interior pyroxenes; this is apparently the result of subsolidus alteration after clast formation. Fe-rich alteration rims of similar thickness are present on matrix pyroxenes of diogenitic composition. Fe/Mg exchange during lithification of howardites and polymict eucrites has been reported previously [l,2]. Based on matrix textures, Hewins [3,4] 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. Less common among Petersburg eucrite clasts are more magnesian compositions. Mittlefehldt [5] reported data from Petersburg clast RC03 with bulk Mg# of 46 and REE abundances of ~13x chondrite. We have analyzed Petersburg clast A, a medium-grained (average grain size ~l mm), subophitic clast predominantly composed of lath-shaped plagioclase (44%) and elongate to equant pyroxene (44%) with mesostasis (10%) and minor opaques (2%). Pyroxenes are unzoned with respect to Mg/Fe and are dominantly composed of low-Ca clinopyroxene with fine (<1 micrometer thick) augite exsolution lamellae that causes electron microprobe analyses to range in apparent composition from Wo(sub)2.0En(sub)46.5 to Wo(sub)32.7En(sub)33 7. These compositions are distinctly more magnesian than pyroxene in "main group" eucrites [6]. Plagioclase composition ranges from An77.4 to An88.7. REE abundances are ~26x chondrite with a signifilcant negative Eu anomaly. Electron microprobe analyses of fused beads indicate that bulk Mg# of this clast is ~49. Given the narrow width of alteration rims on Petersburg eucrite clasts, the large size of clast A (>0.6 g), and the fact that 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. Stolper [7] suggested that eucrites Stannern and Sioux County represented increasing degrees of partial melting of similar source areas; other eucrites (e.g., Nuevo Laredo) represented fractionation of Sioux County-like liquids (Fig. l). Later studies demonstrated the presence of eucritic materials that could not easily be explained by this interpretation. Clast RC03 from Petersburg [5] and clast CF3 from Kapoeta [8] were interpreted as derived from different source regions than other noncumulate eucrites, based on major and trace element compositions. Hewins [4] came to a similar conclusion for eucritic clasts in Petersburg based on anomalously high Ni/Co ratio of clast metal. Warren et al.[9], however, interpreted the REE-rich -21x chondrite), but magnesian (Mg# ~45) eucrite Pomozdino as a partial cumulate, a mixture of Stannern-like melt and cumulate material. 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 matenal yet analyzed. Clast A may represent a very low proportion partial melt of a source that has chondritic REE abundances, but is significantly more magnesian than the source from which the "main group" eucrites were derived. 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 (<60 micrometers) matrix pyroxene grains probably have compositions that are largely the result of redistribution of Fe/Mg during metamorphism after breccia accumulation. There is no evidence that any of the intermediate-composition pyroxenes in the matrix of Petersburg were derived from zoned pyroxenes similar to those in Pasamonte. References: [l] Labotka T. C. and Papike J. J. (1980) Proc. LPSC 11th, 1103-1130. [2] Fuhrman M. and Papike J. J. (1981) Proc. LPSC 12th, 1257-1279. [3] Hewins R. H. (1979) LPS X, 543-545. [4] Hewins R. H. (1979) GCA, 43, 1663-1673. [5] Mittlefehldt D. W. (1979) GCA, 43, 1917-1935. [6] BVSP (1981) Basaltic Volcanism on the Terrestrial Planets, Pergamon. [7] Stolper E. (1977) GCA, 41, 587-611. [8] Smith M. R. (1982) Ph.D dissertation, Oregon State University. [9] Warren P. H. et al. (1990) Proc. LPSC 20th, 281-297. [10] Christophe Michel-Levy M. et al. (1987) Bull. Mineral., 110, 449-458. [11] Hamet J. et al. (1978) Proc. LPSC, 9th, 1115-1136. [12] Jerome D. Y. (1970) Ph.D. dissertation, University of Oregon. [13] McCarthy T. S. et al. (1973) EPSL, 18, 433-442. [14] Palme H. et al. (1978) Proc. LPSC, 9th, 25-57. [15] Wanke H. et al. (1974) Proc. LSC, 3rd, 1251-1268. [16] Warren P. H. and Jerde E. A. (1987) GCA, 51, 713-725.
- Publication:
-
Meteoritics
- Pub Date:
- July 1994
- Bibcode:
- 1994Metic..29S.450B
- Keywords:
-
- Abundance;
- Breccia;
- Chemical Composition;
- Meteoritic Composition;
- Pyroxenes;
- Regolith;
- Grain Size;
- Iron Compounds;
- Lava;
- Magnesium;
- Metamorphism (Geology);
- Mineralogy;
- Temperature Effects;
- Lunar and Planetary Exploration;
- BINDA; BOUVANTE; EUCRITE;
- MAIN GROUP; EUCRITE;
- NON-CUMULATE; EUCRITE;
- POLYMICT; JOHNSTOWN; JUVINAS; KAPOETA; LITHIC CLASTS; NUEVO LAREDO; PASAMONTE; PETERSBURG; POMOZDINO; SIOUX COUNTY; STANNERN; THERMAL METAMORPHISM