Mathematics – Logic
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28q.408n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 408
Mathematics
Logic
1
Allende, Carbonaceous Chondrites, Cv Chondrites, Impact Craters, Leoville, Shock
Scientific paper
Introduction: It has been found that shock effects are recorded in many carbonaceous chondrites [e.g., 1,2], suggesting that impact events are a common process in the early evolution of planetesimals. In order to understand textural and mineralogical characteristics of carbonaceous chondrites affected by multiple impacts and also by impacts under high temperature, we carried out high-temperature, multiple shock experiments using the Allende CV3 chondrite. One of our goals is to reproduce the texture of the Leoville CV3 chondrite, which shows a strong preferred orientation of the flattened chondrules with high aspect ratios (average of 1.9) [3]. Leoville shows much evidence of deformation by moderate shock pressures [1]. However, it remains uncertain whether such moderate shock pressures can flatten round chondrules to such high aspect ratios. There was an unsuccessful attempt to reproduce the texture of Leoville by a single shock [4]. Results: Shocked once (21.7 GPa) at room temperature. Chondrules are flattened with an average axial ratio (long axis/short axis) of 1.4 and show a moderate preferred orientation nearly perpendicular to the compacting axis. Almost all olivine and low-Ca pyroxene in chondrules exhibit numerous fractures and show wavy extinction. The matrix is strongly compacted, but each olivine grain is distinguishable in SEM images. Metal and sulfide grains in the matrix are considerably deformed, but do not appear to have melted. Shocked twice (21.3 and 21.7 GPa) at room temperature. Chondrules are clearly more flattened with an average axial ratio of 1.9 and show a more pronounced preferred orientation. Olivine and low-Ca pyroxene in chondrules show extremely fine fractures and exhibit wavy extinction. The matrix is so strongly compacted that each olivine grain can hardly be distinguished in SEM images. Metal and sulfide grains in the matrix are strongly elongated along surfaces of chondrules. Shocked once (21.3 GPa) at high temperature (607 degrees C). Chondrules are flattened with an average axial ratio of 1.8 and show a strong preferred orientation. Olivine grains in the matrix look very smooth in SEM images. Many Si- and Ca-rich glassy grains ranging 10-50 micrometers in diameter occur in the matrix. Metal and sulfide are apparently melted to form network-like veins in the matrix. In places, the Fe-Ni-S melt is segregated in areas ~400 micrometers across. Conclusions: The doubly shocked Allende is most similar to Leoville in chondrule and matrix texture. The singly shocked Allende at room temperature and 607 degrees C also shows chondrule flattening and preferred orientation, but differs considerably in matrix texture from Leoville. This appears to indicate that the deformation texture of Leoville resulted from repeated impacts under low temperature. The doubly shocked experiment reveals that repeated impacts by relatively mild shock pressures apparently facilitate mechanical compaction of chondrite without intensive heat generation. The effects of shock at high temperature (607 degrees C) are quite different from those at room temperature. The flattening of chondrules and the compaction of matrix are clearly more pronounced at 607 degrees C than at room temperature. This can be explained by an increase of the plasticity of components constituting chondrules and matrix at the high temperature. Melting of metal, sulfides, and silicates are also significantly facilitated at the high temperature. References: [1] Nakamura T. et al. (1992) EPSL, 114, 159-170. [2] Scott E. R. D. et al. (1991) LPS XXII, 1205-1206. [3] Cain P. M. et al. (1986) EPSL, 77, 165-175. [4] King E. A. et al. (1978) Meteoritics, 13, 549.
Nakamura Takashi
Sekine Tomoyuki
Takeda Hidenori
Tomeoka Kazushige
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