Unusual Dark Clasts in the Vigarano CV3 Chondrite: Record of Parent Body Process

Details Unusual Dark Clasts in the Vigarano CV3 Chondrite: Record of Parent Body Process Unusual Dark Clasts in the Vigarano CV3 Chondrite: Record of Parent Body Process

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.383k&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 383

Other

Aqueous Alteration, Clasts, Cv Chondrites, Metamorphism, Parent Bodies

Scientific paper

A variety of dark lithic clasts have been reported from CV3 chondrites and are commonly called "dark inclusions" (DIs). The DIs widely range in texture from chondritic with chondrules and Ca-Al-rich inclusions (CAIs) embedded in a matrix (similar to host meteorites), to fine-grained aggregates of Fe-rich olivine free of coarse-grained components [1,2]. The DIs have been interpreted to represent (1) primary aggregates of materials in the solar nebula [3-5] and (2) materials that were affected by thermal metamorphism on their parent bodies [6]. We present the results of petrographic and scanning electron microscope (SEM) studies of two unusual clasts found in the Vigarano CV3 chondrite. The two unusual clasts, which we will call CL1 and CL2, are approximately 1.2 x 1.0 mm^2 (CLl) and 0.8 x 0.6 mm^2 (CL2) and occur within one of the large clasts (2.8 x 1.0 mm^2). CL1 and CL2 have very similar mineralogies and textures; they contain irregular to oval-shaped inclusions consisting mostly of fine grains of Fe-rich olivine embedded in the matrix of the clasts, and are free of distinct chondrules, CAIs, and coarse mineral fragments. Thus, they resemble the fine-grained variety of DIs. Under the optical microscope, most inclusions resemble chondrules or chondrule fragments in shape and size. However, they are brownish-translucent in transmitted light and are clearly distinct from chondrules in the Vigarano host. The inclusions are characteristically flattened in direction, exhibiting apparent foliation. Our SEM observations reveal the following unusual characteristics: (1) the inclusions are not mere random aggregates of olivine grains but have peculiar internal textures, i.e., assemblies of round or oval-shaped outlines, which are suggestive of pseudomorphs after porphyritic or granular olivine chondrules; (2) one of the thick inclusion rims contains a network of vein-like strings of elongated olivine grains, (3) an Fe-Ni metal aggregate in CL1 has an Fe-, Ni-, S-rich halo, suggesting a reaction between its precursor and the surrounding matrix; and (4) olivine in the clasts commonly shows a swirly, fibrous texture similar to that of phyllosilicate. These characteristics are not reconciled with a primary (unprocessed) origin for the clasts but suggest that they were involved in a secondary process such as aqueous alteration. The fine grains of olivine in these clasts were presumably produced by thermal transformation of phyllosilicate. Serpentine is known to start transforming to olivine at >300 degrees C [7]. The inclusions in CL1 and CL2 resemble chondrules: Some of them are surrounded by distinctive rims similar to chondrule rims commonly seen in the carbonaceous chondrites. Therefore, we believe that the precursor material of the clasts would have been related to some types of chondrites, possibly the Vigarano host itself, and that aqueous alteration and thermal metamorphism occurred in the meteorite parent body. We suggest that some of the dark inclusions and clasts previously reported from CV3 chondrites and other types of meteorites may have origins common to these clasts in Vigarano. References: [1] Fruland R. M. et al. (1978) Proc. LPSC 9th, 1305-1329. [2] Johnson C. A. et al. (1990) GCA, 54, 819-830. [3] Bischoff A. et al. (1988) LPSC XIX, 88-89. [4] Kurat G. (1989) Z. Naturforsch., 44a, 988-1004. [5] Palme H. (1989) Z. Naturforsch., 44a, 1005-1014. [6] Bunch T. E. and Chang S. (1983) LPSC XIV, 75-76. [7] Akai J. (1992) Proc. NIPR Symp. Antarct. Meteorites, 5, 120-135.

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