The unique Antarctic carbonaceous chondrites, Belgica (B)-7904,Yamato (Y)-86720,Y-82162,Y-793321 are thermally metamorphosed. However, the heat source of the thermal metamorphism is not known. Two strong possibilities are shock-induced heating and heating on the parent body. The explosive impact method was used to check the possibility of heating of phyllosilicates by shock compression. Examining the shocked specimens from the Murchison meteorite and terrestrial lizardite, the following were found : (1) Phyllosilicates in the shocked (>32.1GPa) specimens changed to nearly amorphous substances; (2) the phyllosilicates in specimens shocked at lower pressures were still crystalline and undamaged; (3) some void-like (bubble) textures were widely observed in the amorphous substances; (4) the other minerals such as pyroxenes and olivines which did not change to glass phases seem to be little affected by shock. These facts do not suggest that the unique Antarctic chondrites experienced significant shock

Akai,Junji

Sekine,Toshimori

National Institute of Polar Research Repository

Proc. NIPR Symp. Antarct. Meteorites, 7, 101-109, 1994 SHOCK EFFECTS EXPERIMENTS ON SERPENTINE AND THERMAL METAMORPHIC CONDITIONS IN ANTARCTIC CARBONACEOUS CHONDRITE Junji AKAI 1 and Toshimori SEKINE2 1 Department of Geology and Mineralogy, Faculty of Science, Niigata University, 8050, lkarashi 2-nocho, Niigata 950-21 2 National Institute for Research in Inorganic Materials, Namiki, Tsukuba 305 Abstract: The unique Antarctic carbonaceous chondrites, Belgica (B)-7904, Yamato (Y)-86720, Y-82162, Y-793321 are thermally metamorphosed. How­ever, the heat source of the thermal metamorphism is not known. Two strong possibilities are shock-induced heating and heating on the parent body. The explosive impact method was used to check the possibility of heating of phyllosilicates by shock compression. Examining the shocked specimens from the Murchison meteorite and terrestrial lizardite, the following were found: ( 1) Phyllosilicates in the shocked ( > 32.1 GPa) specimens changed to nearly amor­phous substances; (2) the phyllosilicates in specimens shocked at lower pres­sures were still crystalline and undamaged; (3) some void-like (bubble) textures were widely observed in the amorphous substances; ( 4) the other minerals such as pyroxenes and olivines which did not change to glass phases seem to be little affected by shock. These facts do not suggest that the unique Antarctic chon­drites experienced significant shock. 1. Introduction Antarctic carbonaceous chondrites, Belgica (B)-7904, Yamato (Y)-86720 and Yamato (Y)-82162 are unique in their petrographic features (ToMEOKA et al., 1989a, b; ZOLENSKY et al., 1989; MAYEDA and CLAYTON, 1990; IKEDA, 1991, 1992). These three have characteristics of both CI and CM groups or intermediate features between the two: their oxygen isotopic nature is similar to that of CI, but Y-86720 and B-7904 are similar to CM in mineralogy and chemistry. Y-82162 is similar to Cl but has many characteristic clasts. Furthermore, these three and Yamato (Y)-793321 carbonaceous chondrites experienced characteristic thermal metamorphism (AKAI, 1984, 1988, 1990) which has not been found from non­Antarctic carbonaceous chondrites except for equilibrated carbonaceous chondrites. The following characteristics of the thermal metamorphism can be sum­marized: ( 1) dehydration and subsequent transformation of serpentine type phyllo­silicates to olivine through an intermediate structure; (2) formation of void struc­tures ( AKAI, 1992b). The origin of the void structure is not fully clear but will be discussed in another paper. T-T-T (Time-Temperature-Transformation) diagrams were obtained by heat-101 102 J. AKAi and T. SEKINE ing experiments for estimation of the thermal metamorphism observed in meteorites (AKAi, 1992a). Comparing these experimental results to the observed data on the unusual carbonaceous chondrites, the degree of thermal metamorphism experienced by them was estimated to be in the following order: B-7904 :Z Y-86720 > Y-82162 > Y-793321. However, the causes of metamorphism (heating) or its equivalent events are not yet clear. MIYAMOTO ( 1991) pointed out that the suggested metamorphic temperature for the unique carbonaceous chondrites is theoretically obtainable through internal heating by decay of extinct radionuclides. On the other hand, heating by radiation from a fixed star ( for example, the T-Tauri stage of the sun) may also be possible. KIMURA and IKEDA (1992) suggested that a shock event ( <45 GPa) may be the most plausible mechanism for the heating of constituent minerals in B-7904. Shock-recovery experiments on phyllosilicates have been carried out (BosLOUGH et al., 1980; LANGE et al., 1985; TYBURCZY et al., 1986; TYBURCZY, 1991). Other shock experiments have been carried out on quartz by TATTEVIN et al. (1990). KIMURA and IKEDA (1992) referred to the results of LANGE et al. (1985) in which antigorite shocked up to 45 GPa lacks petrographically observable shock indicators, although a significant amount of H20 was driven out of the antigorite. These results seem to be consistent with the results of observation of thermally metamorphosed Antarctic carbonaceous chondrite. However, there are no direct observations confirming such changes for shock metamorphosed phyllosilicates by HR TEM yet. Shock events are believed to have occurred widely in the early solar system and to be closely related to carbonaceous chondritic materials. So, shock recovery experiments could establish systematic changes in various minerals by increasing shock pressures. Thus, the purposes of this study are: ( 1) to test the effect of shock-induced heating, (2) to summarize the shock effects in phyllosilicates over the pressure range 12. 7 to 57.6 GPa, and (3) to evaluate shock effects in the constituent mineral grains and also in the textures. 2. Shock Experiments The impact method accelerated by a propellant gun which is reviewed by GOTO and SYONO ( 1984) has been used for the shock recovery experiments reported here. In this study, specimens from the Murchison meteorite (run No. #304, #316, #320, #321) and lizardite (#302, #303, #314, #315) from Lizard, Cornwall, England were used although the composition of the latter material is not always similar to those in meteorites. Lizardite contains much Mg, on the other hand, serpentine in carbona­ceous chondrite is Fe rich in general (BARBER, 1981; Akai, 1988). The experiments were carried out at the National Institute for Research in Inorganic Materials by one of the authors (T. S. ). The shock wave reflects within specimen of which shock impedance is lower than that of the metal container if the specimen is thin enough relative to the flyer Shock Experiments of Serpentine Table I. Shock experiment conditions. 103 Run No. Impact velocity First pressure* The last equilibrium pressure (km/s) (GPa) (GPa) #302 1.22 13.2 26.3 #303 I. 71 19.5 39.6 #304 1.44 32.1 #314 0.86 5.2 10.0 #315 1.31 8.3 16.1 #316 1.06 12.7 #320 1.78 57.6 #321 1.08 23.0 * First pressures can be calculated only when Hugoniot of the specimen is known. plate. The first shock state is established by the impact velocity and the Hugoniot of the specimen. During multiple shock reflections the pressure increases up to the final equilibrium pressure which is established by the impact velocity and the Hugoniot of the metal container (Table 1 ). Specimens were prepared as follows: specimen disks were mounted in stainless steel rings of 6 mm in inner diameter. Both sides of the ring were sandwiched by stainless steel disks. Planar shock waves were transmitted to the specimen through this design. The Hugoniot of serpentine by TYBURCZY (1991) was used to calculate the first shock state of lizardite. Flyers were an Al-alloy stainless steel and tungsten. Pressures were estimated by the impedance match method using the measured impact velocity of the flyer plates. Table 1 shows the shock conditions. 3. Results and Discussions Examination of the shocked specimens revealed the following: The phyllosilicates in shocked specimens #316, and #314 & #315 were still crystalline and not damaged by the shock pressures below 16 G Pa (peak pressure), although some heterogeneous pressure effects are present. In TEM images of #316, crystalline serpentine with minor damage was found (Fig. 1). Such results may be due to heterogeneities of pressure transmission into the specimens. The other minerals such as pyroxenes, olivines and so on seem to have been little affected by the relatively weak shock pressures investigated here. The phyllosilicates in shocked specimens of #321 were also crystalline (Fig. 2). Phyllosilicates in the specimens of #304, #302 and #303 shocked above 26 GPa (peak pressure) were changed to an almost amorphous state, although these changes were inhomogeneous probably because of heterogeniety in shock effects on the serpentine. Figure 3 shows the results for run product #304. Figure 3b is a TEM image of run #304, showing a shocked serpentine grain exhibiting only a few remnant lattice fringes. The phyllosilicates in #320 subjected to the highest shock pressure in the present experiments were changed to an amorphous state with 104 J. AKAi and T. SEKINE Fig. I. EM images of phyllosilicate in the run products of #316. (a) 7 A Layer structures are evident. (b) A slightly damaged serpentine structure. This is the only example found in the current study that is similar to the inter­mediate structure in transformation from serpentine to olivine observed for ther­mally affected carbonaceous chondrites. AEM analysis shows additional S, Mn and Ca peaks. (c) Damaged phyllosilicate structure. occasional presence of void-like (bubble) textures (Fig. 4). No surviving phyllosil­icates were found in this run product. However, the other typical textural features resulting from shock, for example, shock lamelae, planar features, fragmentation Shock Experiments of Serpentine Fig. 2. EM images of the run product of #321. (a) Layer structure of 7 A phyllosilicates (serpentines). (b) Nearly amorphous state formed by shock, with only faintly recognizable layer structure. 1000A Fig. 3. EM images of nearly amorphous materials transformed from phy/losi/icate in the Murchison meteorite (product of run #304). a, b: Nearly amorphous substances and irregular voids (bubbles) are found. 105 106 J. AKAi and T. SEKINE 1000A Fig. 4. EM images of amorphous substances found in the products of run No. #320. b a, b: Glass phases produced by shock. Bubble or void structures are predominant. r. ,.,__,_u_Lu ..... .....L........LLL,.,,,L.._._1...t..L,w..L..u.d .. LJ.  ..L1!.>..JH .. L .. u ...1. Mg S; Figs. 5a, b. EM images of shocked terrestrial lizardite ( a: #315; b: #303 ). and so on, were not evident. AEM analyses were carried out on phyllosilicates and amorphous substances derived probably from the phyllosilicates. AEM spectra are shown in Figs. 1, 2, 4 Shock Experiments of Serpentine Table 2. Results observed in shock experiments. Run No. Impact velocity (km/s) The last equilibrium pressure -- --��ser������- l_ts __ (GPa) Phyllosilicates in Murchison meteorite #316 1 .06 12.7 #321 1 .08 23.0 #304 1 .44 32.1 #320 1.78 57.6 Terrestrial lizardite #31 4 0.86 10.0 #315 1.31 16.1 #302 1.22 26.3 #303 1.71 39.6 0: present, X : absent, 6: identification is not certain. Glass: amorphous materials. Phyllosilicate Glass 06 XL', 06 6 XI'\ 0 X 0 0 X 0 06 01\ 06 X 0 107 and 5. Compositions of phyllosilicates in the Murchison meteorite vary to a considerable degree (BARBER, 1981; AKAi, 1988). In general, amorphous sub­stances formed have compositions which are fundamentally similar to those of unshocked serpentine but sometimes with additional small amounts of S, Ca and/or Mn. Tochilinite and other sulfides may be damaged and offer S to amorphous materials although direct evidence for this is not yet confirmed. The results of this study are summarized in Table 2. Transitional structures in the transformation from serpentine to an olivine structure were not clearly found but the amorphous state was widely observed (Fig. 5) in serpentines shocked in excess of 32 GPa. It has been revealed by LANGE et al. (1985) that shock-recovered antigorite serpentine showed changes in its infrared spectra and in textures, such as partial melting with gas bubbles, and also showed shock-induced water loss. TYBURCZY et al. ( 1986) also studied shock-induced volatile loss from a carbonaceous chondrite in order to interpret the distribution of volatiles within an accreting planet. They examined the Murchison meteorite and showed that devolatilization started at about 11 GPa and was almost completed at about 30 GPa. However, detailed observations of such changes by TEM have not been carried out. The present study revealed fine structures and fine textures in changes of phyllosilicate by shock effects. The present result that serpentine shocked at above 30 GPa is in a nearly amorphous state is consistent with that of TYBURCZY et al. ( 1986). The formation of the amorphous state from serpentine was suggested in this study although the process is not clear. 4. Summary In examining the shocked specimens the following results were obtained: ( 1) Phyllosilicates shocked at � 32 GPa were changed to a nearly amorphous 108 J. AKAi and T. SEKINE state. (2) On the other hand, phyllosilicates shocked to pressures < 23 GPa were still crystalline and not apparently damaged. (3) Some void-like (bubble) textures were widely observed in the amorphous, shocked phyllosilicates. ( 4) However, the other textural features of shock were not observed. Other minerals, for example pyroxenes and olivines seem to be little affected by shock. (5) Furthermore, transitional structures intermediate between serpentine and olivine structure were not clearly observed. (6) Phyllosilicates are very susceptible to pressure, becoming amorphous at pres­sures � 32 GPa in our experiments. (7) These facts suggest that the unique Antarctic carbonaceous chondrites did not experience significant impact pressures. In particular, amorphous (glass) sub­stances are not observed in the Antarctic carbonaceous chondrites. Void-like (bubble) structures are observed exclusively in olivine grains in the latter meteorites. Acknowledgments The authors thank two anonymous reviewers for their kind and constructive comments. References AKAi, J. (1984): Mineralogical characterization of matrix materials in carbonaceous chondrites, Yamato-7933211 and Belgica-7904 by HREM. Papers Presented to the 9th Symposium on Antarctic Meteorite, March 22-24, 1984. Tokyo, Natl Inst. Polar Res., 59-61. AKAi, J. (1988): Incompletely transformed serpentine-type phyllosilicates in the matrix of Antarctic CM chondrites. Geochim. Cosmochim. Acta, 52, 11593-11599. AKAi, J. (1990): Mineralogical evidence of heating events in Antarctic carbonaceous chondrites, Y-86720 and Y-82162. Proc. NIPR Symp. Antarct. Meteorites, 3, 55-68. AKAi, J. ( 1992a): T-T-T diagram of serpentine and saponite, and estimation of metamorphic heating degree of Antarctic carbonaceous chondrites. Proc. NIPR Symp. Antarct. Meteorites, 5, 120-135. AKAi, J. (1992b): Void structure in some constituent minerals in Antarctic carbonaceous chondrites. Papers Presented to the 17th Symposium on Antarctic Meteorites, August 19-21, 1992. Tokyo, Natl Inst. Polar Res., 11-14. BARBER, D. J. ( 1981): Matrix phyllosilicates and associated minerals in CM2 carbonaceous chondrites. Geochim. Cosmochim. Acta, 45, 945-970. BOSLOUGH, M. B., WELDON, R. J. and AHRENS, T. J. (1980): Impact-induced water loss from serpentine, nontronite, and kernite. Proc. Lunar Planet. Sci. Conf., 11th, 2145-2158. GOTO, T. and SvoNo, Y. (1984): Technical aspect of shock compression experiments using the gun method. Materials Science of the Earth's Interior, ed. by I. SuNAGA WA. Tokyo, Terra, 605-619. IKEDA, Y. (1991): Petrology and mineralogy of the Yamato-82162 chondrite (Cl). Proc. NIPR Symp. Antarct. Meteorites, 4, 187-225. IKEDA, Y. ( 1992): An overview of the research consotium, "Antarctic carbonaceous chondrites with CI affinities, Y amato-86720, Y amato-82162 and Belgica-7904". Proc. NIPR Symp. Antarct. Shock Experiments of Serpentine 109 Meteorites, 5, 49-73. IKEDA, Y., NOGUCHI, T. and KIMURA, M. (1992): Petrology and mineralogy of the Yamato-86720 carbonaceous chondrite. Proc. NIPR Symp. Antarct. Meteorites, 5, 136-154. KIMURA, M. and IKEDA, Y. (1992): Mineralogy and petrology of an unusual Belgica-7904 carbona­ceous chondrite: Genetic relationships among the components. Proc. NIPR Symp. Antarct. Meteorites, 5, 74-119. LANGE, M. A., LAMBERT, P. and AHRENS, J. T. (1985): Shock effects on hydrous minerals and implications for carbonaceous meteorites. Geochim. Cosmochim. Acta, 49, 1715-1726. MAYEDA, T. K. and CLAYTON, R. N. (1990): Oxygen isotopic compositions of several Antarctic meteorites. Papers Presented to the 15th Symposium on Antarctic Meteorites, May 30--June 1, 1990. Tokyo, Natl Inst. Polar Res., 196-197. MIYAMOTO, M. (1991): Thermal metamorphism of CI and CM carbonaceous chondrites: An internal heating model. Meteoritics, 26, 111-115. TATTEVIN, H., SYONO, Y. and KIKUCHI, M. (1990): Shock deformation of alpha quartz: Laboratory experiments and TEM investigation. Eur. J. Mineral., 2, 227-234. TOMEOKA, K., KOJIMA, H. and YANAI, K. (1989a): Yamato-82162: A new kind of CI carbonaceous chondrite found in Antarctica. Proc. NIPR Symp. Antarct. Meteorites, 2, 36-54. TOMEOKA, K., KOJIMA, H. and Y ANAi, K. ( 1989b): Yamato-86720: A CM carbonaceous chondrite having experienced extensive aqueous alteration and thermal metamorphism. Proc. NIPR Symp. Antarct. Meteorites, 2, 55-74. TYBURCZY, J. A. (1991): Shock wave equation of state of serpentine to 150 GPa: Implications for the occurrence of water in the Earth's lower mantle. J. Geophys. Res., 96, 18011-18027. TYBURCZY, J. A., FRISCH, B. and AHRENS, T. A. (1986): Shock-induced volatile loss from a carbonaceous chondrite: Implication for planetary accretion. Earth Planet. Sci. Lett., 80, 201-207. ZOLENSKY, M. E., BARRET, R. and PRINZ, M. (1989): Mineralogy and petrology of Yamato-86720 and Belgica-7904. Papers Presented to the 14th Symposium on Antarctic Meteorites, June 6-8, 1989. Tokyo, Natl Inst. Polar Res., 24-26. (Received July 15, 1993; Revised manuscript received December 13, 1993) 

Shock effects experiments on serpentine and thermal metamorphic conditions in Antarctic carbonaceous chondrite

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Shock effects experiments on serpentine and thermal metamorphic conditions in Antarctic carbonaceous chondrite

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