The Effects of Shock Metamorphism on Metal Textures and Metallographic Cooling Rates in L-Group Ordinary Chondrites

Details The Effects of Shock Metamorphism on Metal Textures and Metallographic Cooling Rates in L-Group Ordinary Chondrites The Effects of Shock Metamorphism on Metal Textures and Metallographic Cooling Rates in L-Group Ordinary Chondrites

Jul 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.322b&link_type=abstract

Meteoritics, vol. 28, no. 3, volume 28, page 322

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1

Cooling Rates, L Chondrites, Metal, Shock

Scientific paper

Postshock reheating and rapid cooling have severely disturbed geothermometers and coolingrate indicators in L chondrites. This has generated an ongoing debate on the interpretation of the early thermal history of an 'onion-shell' [1] or a 'rubble-pile' [2] L-chondrite parent body. In order to access the degree that reheating has perturbed metallographic cooling rates (disrupting this early thermal history), we have undertaken a study of textures derived from shock metamorphism and subsequent fast cooling of sulfides and Fe-Ni metals. Classification of shock stage by petrographic analysis [3] was performed on 70 L- chondrite thin sections, 55 Antarctic and 15 non-Antarctic, in order to place each into their appropriate shock stage. Fe-Ni metals from representative samples of each shock stage were studied optically and analyzed chemically with a Cameca SX-50 electron microprobe to obtain textural and chemical data. Meteorites of shock stage S3. Meteorites of S4 shock stage contain features similar to lower-shocked S3s, except that they exhibit more abundant melt pockets with melt droplets larger by an order of magnitude (~10 micrometers in diameter). Samples of shock grade S5 have numerous melt droplets of troilite adjacent to troilite grains. They also contain small metal-silicate melt veins and patchy fizzed troilite [4]. These textures indicate reheating to an average temperature of 600-800 degrees C at 45-55 GPa [3]. At S6 stage large melt pockets and veins are readily visible and all original metal textures are totally disrupted. Metallographic cooling rate data were obtained four moderately- to highly-shocked samples. The lack of coherency for these samples is indicative of heterogeneous cooling following shock and confirms the statement by numerous workers that extreme care must be taken when selecting samples of L chondrites for cooling-rate studies. Six low-shock samples were also analyzed for metallographic cooling rates. Incoherency in most of the samples may be due to the slight degree of shock reheating present in even the 'least-shocked' L chondrites, however great care was taken during the analyses to avoid those regions where shock-related features were noted. These samples do, however, display a general trend in cooling rates. The lowest metamorphic grade yielded the slowest cooling rates and an increase in grade followed an increase in cooling rate. This is opposite the relationship predicted by the onion-shell model and suggests that the L parent body may have broken up while still hot and then gravitationally reaccreted as a rubble-pile [6]. Further study of metal and sulfide textures in L chondrites will provide additional information that can be used to better quantify the effect of shock metamorphism on L chondrites. This additional information will, in turn, furnish the necessary low-shock samples for use in determining the thermal history and, hence, the internal structure of the L-chondrite parent body. References: [1] Pellas P. and Storzer D. (1981) Proc. R. Soc. Lond., A374, 253-270. [2] Taylor G. J. et al. (1987) Icarus, 69, 1-13. [3] Stoffler D. et al. (l991) GCActa, 55, 3845-3867. [4] Scott E. R. D. et al. (1991) Meteoritics, 26, 393. [5] Raikes S. and Ahrens T. J. (1979) Geophys. J. R. Astron. Soc., 58, 717-747. [6] Grimm R. E. (1985) JGR, 90, 2022-2028.

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