The presence and intensity of absorption band near 3μm in the reflectance spectra are due to the presence and abundance of hydrates and/or hydroxylions. Previously, we have shown that Renazzo (CR2 chondrite) is an excellent spectral match to 2 Pallas (B-type asteroid) in the wavelength region near 3μm. In this paper, we have measured infrared diffuse reflectance spectra of eight Antarctic CR2 chondrites and have compared reflectance spectra of CR2 chondrites with those of eight asteroids which belong to the C, G, B and F taxonomic classes. The spectral absorption near 3μm of Antarctic CR2 chondrites are deepened and rounded compared to that of Renazzo (fell 1824). This difference implies the presence of corrosion products probably due to terrestrial weathering of the Antarctic CR2 chondrites. This is the reason why no asteroids has been found that has an Antarctic CR2 meteorite counterpart. In addition to the close similarity between Pallas and Renazzo, the overall spectral profile of 511 Davida also exhibit similarity to the Renazzo spectrum

Miyamoto,Masamichi

Sato,Kimiyasu

P(論文)The presence and intensity of absorption band near 3μm in the reflectance spectra are due to the presence and abundance of hydrates and/or hydroxylions. Previously, we have shown that Renazzo (CR2 chondrite) is an excellent spectral match to 2 Pallas (B-type asteroid) in the wavelength region near 3μm. In this paper, we have measured infrared diffuse reflectance spectra of eight Antarctic CR2 chondrites and have compared reflectance spectra of CR2 chondrites with those of eight asteroids which belong to the C, G, B and F taxonomic classes. The spectral absorption near 3μm of Antarctic CR2 chondrites are deepened and rounded compared to that of Renazzo (fell 1824). This difference implies the presence of corrosion products probably due to terrestrial weathering of the Antarctic CR2 chondrites. This is the reason why no asteroids has been found that has an Antarctic CR2 meteorite counterpart. In addition to the close similarity between Pallas and Renazzo, the overall spectral profile of 511 Davida also exhibit similarity to the Renazzo spectrum.departmental bulletin pape

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Antarct. Meteorite Res., 11, 155-162, 1998 INFRARED DIFFUSE REFLECTANCE SPECTRA OF WEATHERED AND UNWEATHERED CR2 CHONDRITES: COMPARISON WITH ASTEROIDS Kimiyasu SATO and Masamichi MIYAMOTO Mineralogical Institute, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033 Abstract: The presence and intensity of absorption band near 3 µm in the reflec­tance spectra are due to the presence and abundance of hydrates and/or hydroxyl ions. Previously, we have shown that Renazzo (CR2 chondrite) is an excellent spectral match to 2 Pallas (B-type asteroid) in the wavelength region near 3 µm. In this paper, we have measured infrared diffuse reflectance spectra of eight Antarctic CR2 chondrites and have compared reflectance spectra of CR2 chondrites with those of eight asteroids which belong to the C, G, Band F taxonomic classes. The spectral absorption near 3 µm of Antarctic CR2 chondrites are deepened and rounded compared to that of Renazzo (fell 1824). This difference implies the presence of corrosion products probably due to terrestrial weathering of the Antarctic CR2 chondrites. This is the reason why no aster­oids has been found that has an Antarctic CR2 meteorite counterpart. In addition to the close similarity between Pallas and Renazzo, the overall spectral profile of 511 Davida also exhibit similarity to the Renazzo spectrum. 1. Introduction Absorption bands near 3 µm in diffuse reflectance spectra of the silicates are due to the presence of hydrates and/or hydroxyl ions and the intensity of the 3 µm band is related to the amount of hydrous minerals in a silicate sample (e.g., MIYAMOTO, 1991). Molecular water in minerals shows broad absorption bands near 3400 cm- 1 (2.94 µm) and hydroxyls typically show sharp absorption bands near 3650 cm- 1 (2.74 µm) (e.g., RYSKIN, 1974; SALISBURY et al., 1988). Therefore, the absorption bands near 3 µm are the spectral signatures of hydrated silicates included in meteorites and surface materials of solar system objects. Aqueous alteration processes on asteroids would have been possible if liquid water, derived from the originally accreted water ice, were available (ZoLENSKY and McSwEEN, 1988). Liquid water could have been produced by any or all of the heating processes of asteroids, such as decay of radionuclides, accretional bombardment and induction heat­ing (ScoTT et al., 1989). Thus, the 3 µm absorption bands of asteroids are thought to be results of the aqueous alteration processes. We have previously compared the reflectance spectra near 3 µm of asteroid 1 Ceres and 2 Pallas ( commonly classified into G and B type, respectively) with those of various carbonaceous chondrites, including thermally metamorphosed ones. We have shown that Renazzo ( CR2 chondrite) is an excellent spectral match to Pallas (SATO et al., 1997). In this paper, we measured infrared diffuse reflectance spectra of eight CR2 chondrites 155 156 K. SATO and M. MIYAMOTO from Antarctica and examined the spectral profiles of the absorption bands near 3 µm. The spectral features of the 3 µm absorption bands are compared with those of asteroids which are members of the C, G, B and F taxonomic classes. The C, G, B and F type asteroids have been thought to be compositionally similar to carbonaceous chondrites because of their low albedos and neutral colors in the visible-near infrared wavelength region. 2. Samples and Experimental Techniques CR2 chondrites studied in this paper are summarized in Table 1. Samples of these CR2 chondrites weighing approximately 50 mg were ground in a corundum mortar and passed through a 100 µm sieve to obtain powder samples for spectral measurements. Each powder sample was dried in a desiccator at room temperature for at least 48 hours in order to remove any adsorbed water from the grain surface. A specimen weighing approximately 20 mg was taken from each powder sample and used for the spectral measurements. Diffuse reflectance spectra were measured in dry-air surroundings by the use of a Fourier transform infrared spectrometer (JASCO, FT/IR-300E) equipped with a diffuse reflectance attachment at Mineralogical Institute, University of Tokyo. Each pow­der sample was placed in the hollow space of a sample holder 1.5 mm in depth. After setting the sample in the spectrophotometer, the sample was left in dry-air surroundings for at least 1 hour before measuring diffuse reflectance spectra. Dry air was passed into the spectrophotometer throughout the reflectance measurement. Spectra were taken over the range from 7900 cm- 1 (1.27 µm) to 400 cm- 1 (25 µm) at a resolution of 4 cm- 1• An aluminum-coated mirror was used as standard. Scans were integrated 1000 times to en­hance the signal-to-noise ratio. The ratio of meteorite reflectance to standard spectrum displays only mineralogical features. Diffuse reflectance spectrum of Renazzo was mea­sured under similar conditions and was taken over the range from 3950 cm-1 (2.53 µm) to 400 cm- 1 (25 µm) at a resolution of 4 cm- 1• Table 1. Summary of CR2 chondrites studied and the integrated intensity of absorp­tion band near 3 µm. Meteorites Integrated intensity# Renazzo 124 Y-790112 197 Y-793495 254 Y-8449 266 A-881595 192 EET 87747 257 EET 87770 185 EET 92042 244 EET 92052 276 # The unit is normalized reflectancexcm-1• Weathering B B!C B B B!C Remarks Fell 1824 Antarctic Antarctic Antarctic Antarctic Antarctic Antarctic Antarctic Antarctic 0.4 4000 Reflectance Spectra of Antarctic CR2 Chondrites 3500 3000 WAVENUMBER (cm-1) 2500 Fig. 1. Infrared diffuse reflectance spectrum near 3 µm of CR2 chondrite, Asuka-881595, with a background line obtained by least squares fitting. The reflectance spectrum is scaled to 1 at 2500 cm-1• 157 We calculated the integrated intensity of absorption bands near 3 µm in order to study the integrated intensity of carbonaceous chondrites. The integrated intensity of absorption bands near 3 µm was calculated as follows: Fig. 1 shows an example of the normalized reflectance spectrum of Asuka-881595 with the background line obtained by the least squares fitting. A continuum is approximated using a straight line fit to the spectrum using reflectances from 3950 to 3800 cm- 1 and from 2600 to 2500 cm- 1 • After normalizing reflectance data at 2500 cm-1, the integrated intensity was calculated by numerically integrating the spectra from 3800 to 2600 cm- 1• The unit of the integrated intensity is normalized reflectancexcm- 1 • 3. Results and Discussion 3.1. Reflectance spectra of CR2 chondrites Infrared diffuse reflectance spectra of nine CR2 chondrites are shown in Fig. 2. They exhibit various intensities of absorption bands near 3 µm which are caused by hydrous minerals. The values of the integrated intensity of the absorption bands are summarized in Table 1. Antarctic CR2 chondrites show more intensive hydration bands than Renazzo, and their values of the integrated intensity are 1.4-2.2 times larger than that of Renazzo. The shape of their 3 µm bands is more round than that of Renazzo which exhibits a triangular shape. As was mentioned above, the 3 µm hydrated silicate bands are composed of a sharp 3650 cm- 1 feature dominantly due to hydroxyl ions and a much broader 3400 cm- 1 absorption dominantly due to interlayer molecular water. The 3 µm absorption feature in the reflectance spectrum of Renazzo is due to the presence of hydroxyl ions in the intergrown phyllosilicates contained in the matrix (NELEN et al., 158 K. SATO and M. MIYAMOTO 1975; ZoLENSKY et al., 1993; WEISBERG et al., 1995). Goethite, reported as a major corro­sion product of Fe-Ni alloys in Antarctic meteorites by B ucHWALD and CLARKE (1989), shows broad absorption bands near 3 µm. The position of reflectance minima of goethite is about 3150 cm-1 (3.17 µm). For severely weathered Antarctic meteorites, the absorp­tion bands near 3150 cm- 1 becomes noticeable because of the presence of corrosion products (MIYAMOTO, 1991). The round shape of the 3 µm bands of Antarctic CR2 chon­drites may be due to some corrosion products. Among 8 Antarctic CR2 chondrites, 4 meteorites which belong to EET series are paired with each other (GROSSMAN, 1994; GROSSMAN and ScoRE, 1996). Spectral profiles of EET 87747, EET 92042, and EET 92052 are almost the same and this result is consistent with the consideration that they are paired with each other. Spectral profile of EET 87770 is similar to those of other 3 EET meteorites but shows weaker absorption bands near 3 µm. This probably indicates that EET 87770 is terrestrially less weathered than other meteorites of EET series, although we cannot exclude the possibility of sample heterogeneity. 2.5 1 � WAVELENGTH (µm) 3 4 U 1 � � 1 0 � � 1 A 881595 4000 3500 3000 2500 WAVENUMBER (cm-1) WAVELENGTH (µm) 2.5 3 4 1 1 1 1 4000 3500 3000 2500 WAVENUMBER (cm-1) Fig. 2. Infrared diffuse reflectance spectra of CR2 chondrites. All spectra are scaled to 1 at 2500 cm-1and offset for clarity. The scale bar for 10% is displayed on the lower right side of figures. Reflectance Spectra of Antarctic CR2 Chondrites 159 3.2. Reflectance spectra of asteroids A majority of dark asteroids which have relatively flat visible and near infrared reflectance spectra are classified as the C, G, B and F asteroids. The C, G, B, and F asteroids have been considered to be related to carbonaceous chondrites, on the basis of their low albedos and featureless reflectance spectra in the visible-NIR wavelength re­gion. Telescopic reflectance spectra of low-albedo asteroids near 3 µm were reported by JONES et al. (1990) and we utilized their spectral data. Eight asteroids selected based on Table 2. Summary of selected eight asteroids compared with CR2 chondrites. Asteroids 1 Ceres 2 Pallas 10 Hygiea 13 Egeria 130 Elektra 148 Gallia 409 Aspasia 511 Davida 1 '\i D D D D�D 1 Ceres (G) 2 Pallas (B) Class Date of observation G 04/25/1987 B 04/24/1987 C 02/01/1988 G 10/03/1987 G 02/02/1988 GU 02/01/1988 ex 02/03/1988 C 04/21/1987 148 Gallia (GU) 1 ,  409 Aspasia ( CX) Sll Davida (C) I 2.4 2.6 2.8 3 3.2 3.4 3.6 2.4 2.6 2.8 3 3.2 3.4 3.6 WAVELENGTH (µm) WAVELENGTH (µm) Fig. 3. Telescopic reflectance spectra of selected eight asteroids. All spectra are scaled to 1 at 2.53 µm and offset for clarity. The scale bar for 10% is displayed on the lower right side of figures. 160 K. SATO and M. MIYAMOTO higher quality of their spectral data are summarized in Table 2. Telescopic reflectance spectra of these asteroids are shown in Fig. 3 and their reflectance spectra were com­pared with those of CR2 chondrites. The taxonomic classifications of asteroids are based on THOLEN and BARucc1 (1989). Because of obscuration by the telluric atmosphere, the telescopic reflectance observations do not include the region around 2. 7 µm. This is the important region where minerals containing structural or adsorbed water can be distin­guished. 3.3. Comparison between CR2 chondrites and asteroids We have compared reflectance spectra of newly studied CR2 chondrites with those of 8 selected asteroids. Shown in Fig. 4 are the closest spectral matches. As was reported by SATO et al. (1997), spectral shapes of asteroid 2 Pallas and Renazzo are in excellent agreement in the wavelength region near 3 µm. In addition, although the quality of tele­scopic spectral data for 511 Davida is lower than that of Pallas, the overall spectral profile of 511 Davida also exhibits similarity to the Renazzo spectrum. 2.4 2 Pallas and Renazzo 511 Davida and Renazzo 2.6 2.8 3 3.2 Wavelength (µm) I 3.4 3.6 Fig. 4. Asteroid-meteorite counterparts that have similar reflectance profiles. Open squares: Telescopic reflectance spectra of asteroids. Solid curves: Diffuse reflectance spec­trum of Renazzo. All spectra are scaled to 1 at 2.53 µm and offset for clarity. The scale bar for 10% is displayed on the lower right side of the figure. Reflectance Spectra of Antarctic CR2 Chondrites 161 In this study, no asteroid has been found that has an Antarctic CR2 meteorite coun­terpart. CR2 chondrites from Antarctica show the 3 µm hydration bands which have been deepened and rounded probably by terrestrial weathering. Therefore, it is difficult to compare reflectance spectra near 3 µm of weathered meteorites with those of aster­oids directly. In the wavelength region from 2.4 to 3.6 µm, Pallas and Davida show spectral simi­larity. But their visible-near infrared reflectance spectra are different each other. As to the proper elements of their orbits, semimajor axis, eccentricity and inclination, these asteroids are separated by two secular resonances. Therefore, Pallas and Davida cannot be considered to be genetically related each other. 4. Conclusions We examined the spectral profiles of the absorption bands near 3 µm of the Antarc­tic CR2 chondrites, Renazzo and the asteroids which belong to C, G, B and F taxonomic classes. The result is as follows: (1) The spectral absorption near 3 µm of the Antarctic CR2 chondrites are more intensive and round compared to that of Renazzo. The differ­ence in spectral profiles between Antarctic CR2 chondrites and Renazzo implies the presence of corrosion products probably due to terrestrial weathering of the Antarctic CR2 chondrites. (2)In addition to an excellent agreement between asteroid 2 Pallas and Renazzo, the overall spectral profile of asteroid 511 Davida also exhibits similarity to the Renazzo spectrum. Acknowledgments We thank National Institute of Polar Research and Meteorite Working Group for providing Antarctic meteorite samples. Asteroidal reflectance spectra were taken from the Planetary Data System (PDS) Small Bodies Node (SBN), located in the Astronomy Department of University of Maryland. We also thank Dr. T. Hmm for his useful com­ments, and Dr. C.M. PIETERS and Dr. H. TAKEDA for constructive reviews. References BucttWALD, V.F. and CLARKE, R. S., Jr. (1989): Corrosion of Fe-Ni alloys by Cl-containing akaganeite (/3-FeOOH): The Antarctic meteorite case. Am. Mineral., 74, 656-667. GROSSMAN, J.N. (1994): The meteoritical bulletin, No. 76, 1994 January: The U.S. Antarctic Meteorite Col­lection. Meteoritics, 29, 100-143. GROSSMAN, J.N. and ScoRE, R. (1996): The meteoritical bulletin, No. 79, 1996 July: Recently classified speci­mens in the United States Antarctic Collection (1994-1996). Meteorit. Planet. Sci., 31, A161-Al 74. JONES, T.D., LEBOFSKY, L.A., LEWIS, J.S. and MARLEY, M.S. (1990): The composition and origin of the C, P, and D asteroids: Water as a tracer of thermal evolution in the outer belt. Icarus, 88, 172-192. MIYAMOTO, M. (1991): Differences in the degree of weathering between Antarctic and non-Antarctic meteor­ites inferred from infrared diffuse reflectance spectra. Geochim. Cosmochim. Acta, 55, 89-98. NELEN, J., KuRAT, G. and FREDRIKssoN, K. (1975): The Renazzo chondrite-A reevaluation (abstract). Meteor­itics, 10, 464-465. RYSKIN, YA.I. (1974): The vibrations of protons in minerals: Hydroxyl, water and ammonium. The Infrared 162 K. SATO and M. MIYAMOTO Spectra of Minerals, ed. by V.C. FARMER. London, Mineralogical Society, 137-181. SALISBURY, J.W., WALTER, L.S. and D'ARIA, D. (1988): Mid-infrared (2.5 to 13.5 µm) Spectra of Igneous Rocks. Open-File Report 87-263. Reston, U. S. Geological Survey. SATO, K., MIYAMOTO, M. and ZOLENSKY, M.E. (1997): Absorption bands near three micrometers in diffuse reflectance spectra of carbonaceous chondrites: Comparison with asteroids. Meteorit. Planet. Sci., 32, 503-507. ScoTT, E.R.D., TAYLOR, G.J., NEWSOM, H.E., HERBERT, F., ZoLENSKY, M.E. and KERRIDGE, J.F. (1989): Chemi­cal, thermal and impact processing of asteroids. Asteroids II, ed. by R. P. BINZEL et al. Tucson, Univ. Arizona Press, 701-739. THOLEN, D.J. and BARucc1, M.A. (1989): Asteroid taxonomy. Asteroids II, ed. by R. P. BINZEL et al. Tucson, Univ. Arizona Press, 298-315. WEISBERG, M.K., PRINTZ, M. , CLAYTON, R.N., MAYEDA, T. K. , GRADY, M.M. and PILLINGER, C.T. (1995): The CR chondrite clan. Proc. NIPR Symp. Antarct. Meteorites, 8, 11-32. ZoLENSKY, M.E. and McSwEEN, H.Y., Jr. (1988): Aqueous alteration. Meteorites and the Early Solar System, ed. by J.F. KERRIDGE and M.S. MATTHEWS. Tucson, Univ. Arizona Press, 114-143. ZoLENSKY, M.E., BARRET, R. and BROWNING, L. (1993): Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites. Geochim. Cosmochim. Acta, 57, 3123-3148. (Received November 4, 1997; Revised manuscript accepted January 5, 1998) 

Infrared diffuse reflectance spectra of weathered and unweathered CR2 chondrites: Comparison with asteroids

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Infrared diffuse reflectance spectra of weathered and unweathered CR2 chondrites: Comparison with asteroids

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