Infrared spectroscopic taxonomy for carbonaceous chondrites from speciation of hydrous components

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1 Meteoritics & Planetary Science 40, Nr 1, (2005) Abstract available online at Infrared spectroscopic taxonomy for carbonaceous chondrites from speciation of hydrous components Takahito OSAWA 1*, Hiroyuki KAGI 1, Tomoki NAKAMURA 2, and Takaaki NOGUCHI 3 1 Laboratory for Earthquake Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Tokyo , Japan 2 Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Hakozaki, Fukuoka , Japan 3 Department of Materials and Biological Sciences, Ibaraki University, Bunkyo 2 1 1, Mito , Japan * Corresponding author. osawa@eqchem.s.u-tokyo.ac.jp (Received 15 March 2004; revision accepted 24 November 2004) Abstract Mid-infrared absorption spectra for all types of carbonaceous chondrites were obtained in this study to establish a versatile method for spectroscopic classification of carbonaceous chondrites. Infrared spectra were measured using a conventional KBr pellet method and diamond press method. Spectra of hydrous carbonaceous chondrites exhibit intense O-H stretching vibrations. CI chondrites are identifiable by a characteristic sharp absorption band appearing at 3685 cm 1, which is mainly attributable to serpentine. X-ray diffraction analysis showed the presence of serpentine. However, Yamato (Y-) (C1) does not have the band at 3685 cm 1 because of its thermal metamorphism. CM and CR chondrites have an intense absorption band at approximately 3600 cm 1. This absorption tends to appear in CM chondrites more strongly than CR chondrites because the intensity ratios of an OH stretching mode at 3520 cm 1 compared to 3400 cm 1 for CM chondrites are in the range of , which is systematically higher than those of CR chondrites ( ). Therefore, the two types of chondrites are distinguishable by their respective infrared spectra. The spectrum feature of the Tagish Lake meteorite is attributable to neither CI nor CM chondrites. CO chondrites are characterized by weak and broad absorption at 3400 cm 1. CV chondrites have weak or negligible absorption of water. CK chondrites also have no water-induced absorption. CH and CB chondrites have a sharp absorption at 3692 cm 1 indicating the presence of chrysotile, which is also supported by observations of X-ray diffraction and TEM. The combination of spectroscopic classification and the diamond press method allows classification of carbonaceous chondrites of very valuable samples with small quantities. As one example, carbonaceous chondrite clasts in brecciated meteorites were classified using our technique. Infrared spectra for a fragment of carbonaceous clasts (<1 µg) separated from Willard (b) and Tsukuba were measured. The 3685 cm 1 band found in CI chondrites was clearly detected in the clasts, indicating that they are CI-like clasts. INTRODUCTION Fourier transform infrared (FTIR) spectroscopy is a useful technique to analyze the molecular structures of various materials. Particularly, this method is a fundamental analytical technique to determine the structure of organic molecules because organic function groups (C-H, C=O, C=C bonds, etc.) can be identified from a fingerprint absorption appearing on IR spectra. Hydrous components can also can be detected sensitively because of the large dipole moment of the O-H bond. Direct information on the location of hydrogen atoms is unobtainable from X-ray diffraction analysis or transmission electron microscopy (TEM). For that reason, IR spectroscopy is a powerful means for speciation of the hydrous composition in minerals. Furthermore, FTIR spectroscopy can sensitively detect silicate minerals by showing strong absorption bands that are induced by vibrations of the O-Si-O bond. The method is widely used in geological and mineralogical fields. For example, IR spectroscopic analysis can be applied to minerals to analyze hydrous microphases (e.g., Nakashima et al. 1989; Kagi and Takahashi 1998). Reflective infrared spectra for meteorites are used to compare optical properties of meteorites with those of celestial objects to identify candidate parent bodies for different types of meteorites (e.g., Burbine and Binzel 2002; Bus and Binzel 2002; Kanno et al. 2003). Thermal infrared emission spectra of martian meteorites were applied for comparison with martian infrared data (Hamilton et al. 71 The Meteoritical Society, Printed in USA.

2 72 T. Osawa et al. 1997, 2001, 2003). Mid-infrared spectra for many kinds of meteorites have been reported so far (e.g., Sandford 1984, 1993; Salisbury et al. 1991; Jones et al. 2004). Infrared diffusive reflectance spectra of carbonaceous chondrites have also been measured to identify hydrous minerals (e.g., Miyamoto 1992; Miyamoto and Zolensky 1994). Our previous work obtained mid-infrared spectra for some Antarctic micrometeorites and carbonaceous chondrites using FTIR spectroscopic analyses by the diamond press method (Osawa et al. 2001). Infrared spectra for extremely small samples (<1 µg) were obtained by that method. Results of analyses showed that Orgueil (CI1), Murchison (CM2), Y (CR2), and Y (CR2) have intense absorption bands at 1000 and 3400 cm 1 that are attributable to Si-O and O-H stretching vibrations, respectively. We suggested that the shape of infrared spectra may be used to classify them. This study developed a spectroscopic classification method for carbonaceous chondrites from speciation of their hydrous components. All carbonaceous chondrites have intense and sharp absorption at the region of cm 1 arising from the O-Si-O asymmetric stretching vibration. Each silicate mineral has a unique and specific pattern of spectral features in the wave number region reflecting their chemical compositions, crystal structures, or both (e.g., Salisbury et al. 1992). The absorption band of Si-O vibration is a common spectral feature in carbonaceous chondrites, but the main absorption is not discussed here because the present study specifically addresses the water-induced band. O-H stretching vibrations that appear in the region from 3000 to 3700 cm 1 provide useful information regarding characteristics of each type of carbonaceous chondrite because of the variety of hydrous components present in these meteorites. In addition, infrared spectra in the O-H stretching vibrations are useful for comparisons between astronomical observations of asteroids and the laboratory spectra of minerals and meteorites (Rivkin 2003, 2004). For that reason, we mainly discuss the O-H stretching band here. Although H-O-H bending vibration appears at ~1630 cm 1, the band is not discussed here because the combination modes of silicate minerals appear at 1640 cm 1 and overlap with the bending mode of H-O-H (e.g., Benesi and Jones 1959; Lenza and Vasconcelos 2001). Here we present mid-infrared absorption spectra for Antarctic and non-antarctic carbonaceous chondrites and carbonaceous chondrite clasts in brecciated meteorites using the diamond press and conventional KBr methods. In addition, X-ray diffraction analyses and TEM observations are performed for some carbonaceous chondrites. The objectives of the present work are the development of spectroscopic taxonomy of carbonaceous chondrites and clarification of the condition of water in them by assigning the water-induced absorption band. In addition, we examine the connection between spectral shapes and mineral composition. EXPERIMENTAL PROCEDURES This work adopted both the diamond press method and KBr pellet method for measuring infrared absorption spectra. Small chips of Antarctic carbonaceous chondrites that were used in the present study are listed in Table 1. The KBr method was applied to six non-antarctic meteorites because sufficient amounts of samples were available for analyses. KBr powder for preparing pellets was kept in a desiccator because of its hygroscopic nature. Salisbury et al. (1987) observed a strong water band near 3400 cm 1 in the infrared spectra of calcite and quartz, which is attributable to water in the hygroscopic KBr. In the present work, applying spectra of blank KBr pellets as a reference alleviated the problem of moisture in KBr pellets. The typical KBr/meteorite ratio is For the diamond press method, a small chip of carbonaceous chondrites (~100 µm, 1 µg) was pressed between a pair of type IIa diamonds (Diamond express; Sumitomo Electric Industries Ltd.) and thinned such that samples were translucent to infrared light (Fig. 1). A piece of matrix material with a diameter of less than 100 µm was selected from fragments of carbonaceous chondrites and placed between the diamonds. Infrared spectra were recorded using an FTIR spectrometer (Spectra 2000; Perkin-Elmer Inc.) equipped with an IR microscope. We used globar light as an IR incident light source, as well as a liquid-nitrogen-cooled HgCdTe (MCT) detector and a KBr beam splitter. Infrared spectra in the transmission mode were obtained through a square optical aperture (60 60 µm 2 ) with a spectral resolution of 4 cm 1 (the width of an unresolved line) and data sampling every 1cm 1. All the spectra shown in this paper represent the average of several hundred individual scans. Dry air was pumped constantly into the microscope to eliminate water vapor. Interference by the strong C=O asymmetric stretching mode of carbon dioxide near 2350 cm 1 was removed, to the greatest extent possible, by software. Reference spectra were obtained on the sample-free region on the diamond before measuring the samples (Fig. 1). For six non-antarctic meteorites, samples were powdered and mixed with KBr in an agate mortar and made into pellets. A reference spectrum was obtained from a blank KBr pellet without sample powder. Standard samples of phyllosilicates (saponite, lizardite) were measured by the diamond press method. The validity of the diamond press method must be verified because an infrared spectrum obtained by this method may be different from those obtained by the conventional KBr method. Figure 2 shows infrared spectra of Mighei measured independently using these two analytical methods. No significant difference was observed between the two infrared absorption spectra; therefore, we inferred that the two data series can be discussed equally. The new analytical method provides infrared spectra with various advantages compared to those obtained by the conventional KBr method. For example, only an extremely small amount of sample

3 Infrared spectroscopic taxonomy for carbonaceous chondrites 73 Fig. 1. Schematic illustration of the diamond press method. A small chip of carbonaceous chondrite is set on a type IIa diamond plate (1) and is sandwiched between two diamonds (2). After the meteorite sample is pressed and flattened, the upper diamond is removed (3); then the infrared transmission spectrum is measured (4). Table 1. Carbonaceous chondrites in the present study. Sample Class Find or fall Shock stage WG a Method Non-Antarctic meteorites Acfer 207 CH Find S2 W2 KBr Dhofar 015 CK3 Find S3 W1 Diamond Gujba CB Fall 1984 KBr Ivuna CI1 Find Diamond Karoonda CK4 Fall 1930 S1 Diamond Maralinga CK4 Find S2 Diamond Mighei CM2 Fall 1889 S1 KBr Murchison CM2 Fall 1969 S1 2 Diamond Murray CM2 Fall 1950 S1 Diamond NWA 470 CH Find S1 W1 KBr NWA 760 CV3 Find S2 W2 KBr NWA 765 (Fezna) CK4 5 Find S2 W1 Diamond Orgueil CI1 Fall 1864 Diamond Sayama CM2 Fall 1986 Diamond Tagish Lake C2 Fall 2000 KBr Antarctic meteorites Asuka CR2 Find Diamond Yamato CR2 Find B Diamond Yamato CM2 Find Diamond Yamato CO3 Find S1 Diamond Yamato CR2 Find Diamond Yamato CO3.3 Find S1 Diamond Yamato C1 Find Diamond Yamato CV3 Find Diamond Yamato CV3 Find Diamond Breccias b Tsukuba H5 6 Fall 1996 Diamond Willard (b) H Find Diamond a Weathering grade. Grades A C are used for Antarctic meteorites. b Carbonaceous chondrite clasts are measured.

4 74 T. Osawa et al. Fig. 2. Comparison of KBr and diamond press methods. Spectra are displayed for Mighei. No large difference between results of the two analytical methods is evident. The weak absorption band near 2350 cm 1 in the spectrum obtained by the diamond press method is caused by residual absorption from atmospheric carbon dioxide. (<1 µg) is required in the new method, indicating that the method is suitable to measure a valuable sample. Moreover, sample preparation is easier than that by the KBr method and more samples can be measured. To examine the consistency between infrared spectra and mineral composition, X-ray diffraction analysis was performed using synchrotron radiation at beam line 3A of the Photon Factory, Institute of Material Science, High Energy Accelerator Research Organization (Tsukuba, Japan). A small piece of a sample (100 µm in diameter) was picked from matrix of individual meteorites, mounted on a thin glass fiber of 5 µm in diameter, and exposed to X-rays monochromated at ± Å in a Gandolfi camera. The ultrahigh intensity and well-monochromated synchrotron X-ray device allowed us to obtain a clear X-ray powder diffraction pattern of each small lump with a short exposure duration of approximately 30 min (Nakamura et al. 2003a). During this study, we recognized that the matrix and clasts in Northwest Africa (NWA) 470 (CH) contain abundant phyllosilicates. We performed TEM observations of these samples. The samples were embedded in epoxy resin and ultramicrotomed (~70 nm thick) using a Leitz-Reichert Super Nova ultramicrotome. The samples were observed using a Fig. 3. Infrared transmission spectra for seven non-antarctic carbonaceous chondrites (four CM2, two CI1, and C2). All meteorites have Si-O stretching vibration near 1000 cm 1, O-H stretching vibration at ~3400 cm 1, and H-O-H bending vibration at ~1640 cm 1. The two CI1 chondrites show a sharp absorption band at 3685 cm 1. TEM (JEM-2000 FX II TEM; JEOL) equipped with Philips DX4 energy dispersive X-ray analysis system at the Center for Instrumental Analysis, Ibaraki University.

5 Infrared spectroscopic taxonomy for carbonaceous chondrites 75 Fig. 4. X-ray diffraction patterns of Orgueil (CI) and Y (CM). Orgueil contains serpentine (Ser) and saponite (Sp). RESULTS AND DISCUSSION CI, CM, and CR Chondrites Figure 3 shows the infrared absorption spectra in the transmission mode for small fragments of matrices of seven non-antarctic meteorites. Only C1 and C2 chondrites are shown in this figure. Orgueil (CI1) and Ivuna (CI1) have a broad absorption band at about 3400 cm 1 and a sharp peak at 3685 cm 1 showing the presence of hydrous components in these meteorites. Miyamoto and Zolensky (1994) and Sandford (1984) also reported sharp absorption at the same wave number in the reflectance spectrum of Orgueil. Sharp absorption is caused by hydroxyl ions that are bound structurally in phyllosilicates because freely vibrating hydroxyl groups lacking hydrogen bond interactions with surrounding oxygen atoms are featured by sharp IR bands appearing at a higher wave number than those of hydrogenbonded hydroxyl groups (e.g., Hair 1975). Synchrotron X-ray diffraction analysis also indicates that hydrous phases such as saponite and serpentine are enriched in Orgueil (Fig. 4). Bland et al. (2004) also observed 64.2 wt% of saponiteserpentine phases in a bulk sample of Orgueil. The four CM2 chondrites studied here share a common spectral feature. The O-H stretching band of CM2 chondrites is broad and asymmetrical because it comprises two individual bands: a band at ~3600 cm 1 and a broad band at ~3400 cm 1 (Osawa et al. 2001). The 3600 cm 1 band can be assigned to hydrogen bonded Si-OH and 3400 cm 1 band indicates molecular water hydrogen bonded to each other and to Si-OH groups (McDonald 1958; Hair 1975; Orcel et al. 1986). The present study shows that the spectral feature found in Murchison commonly appears in the four CM2 chondrites. Among these CM2 chondrites, Sayama is a Japanese carbonaceous chondrite (recently identified as an extraterrestrial material by a noble gas measurement), which demonstrates the presence of primordial trapped noble gases and a very short cosmic-ray exposure age of less than 1 Ma (Yoneda et al. 2001). Although the black meteorite was expected to be a CI chondrite from its appearance, we had

6 76 T. Osawa et al. Fig. 5. Infrared absorption spectra for nine Antarctic carbonaceous chondrites. Y has no sharp absorption at 3685 cm 1 because of thermal metamorphism. Y (CV3) has no water-induced absorption band, indicating its anhydrous composition. initially suggested that the meteorite would be classified to CM2 chondrite from spectral characteristics because of the disappearance of sharp absorption at 3685 cm 1 found in Ivuna and Orgueil. Eventually, the chondrite was classified into CM chondrite based on its oxygen isotopic composition and mineralogical evidence (Nakamura et al. 2003b). An infrared absorption spectrum of the Tagish Lake meteorite belongs to neither the CI1 nor CM2 groups (Fig. 3), corresponding to the result of mineralogical observation (Nakamura et al. 2003a). The O-H stretching band has a characteristic triangle shape and the top of the peak is at 3430 cm 1. A weak absorption band is observed at about 3680 cm 1, but this peak is not sharp and differs from those observed in CI chondrites. Tagish Lake is an ungrouped type 2 carbonaceous chondrite. The meteorite bears similarities to CI1 and CM chondrites, but is distinct from both in terms of mineralogical characteristics (Zolensky et al. 2002; Nakamura et al. 2003a). Tagish Lake is thought as a candidate of a D-type asteroid because of the similarity in these reflectance spectra (Hiroi et al. 2001). This meteorite has also another importance its parent body has been suggested as the source of some micrometeorites. Noguchi et al. (2002) estimated that a major mineral assemblage of some saponite-rich Antarctic micrometeorites before atmospheric entry is very similar to that of Tagish Lake lithology (Nakamura et al. 2003a). Infrared spectra of nine Antarctic carbonaceous chondrites are shown in Fig. 5. Y (C1) has no sharp absorption band at 3685 cm 1 of the type observed in other CI1 meteorites like Ivuna and Orgueil (Osawa et al. 2001). The disappearance of the band at 3685 cm 1 may be caused by dehydration that occurs from thermal metamorphism because the texture of this meteorite suggests that matrix phyllosilicates were dehydrated and altered to olivine by heating, in contrast with non-antarctic CI chondrites (Tomeoka et al. 1989). Osawa et al. (2001) reported that the absorption band at 3685 cm 1 of Orgueil was weakened preferentially by heating at 500 C for 30 sec. The result is consistent with the infrared spectrum of Y A sharp band was not found in the reflectance spectrum of the meteorite (Bishop and Pieters 1991; Miyamoto 1992; Hiroi et al. 1996; Sato et al. 1997). Y displayed post-alteration heating, at possibly as high as 400 C, but most CI chondrites experienced temperatures of C (Zolensky et al. 1993). Hiroi et al. (1996) also deduced that the meteorite had been heated to C from the 3 µm (~3300 cm 1 ) band strength of heated Murchison samples. Therefore, the 3685 cm 1 band cannot be used as a classification index for CI chondrites that have experienced significant thermal metamorphism. The IR spectrum of Y (CM2) is very similar to those of non-antarctic CM2 chondrites, as Fig. 5 shows. The O-H stretching band comprises two absorption components. A reflectance spectrum of this meteorite also shows the same spectral features (Miyamoto 1992). X-ray diffraction of the matrix of this meteorite (Fig. 4) shows that it consists mainly of serpentine, tochilinite, and mix-layered minerals of serpentine and tochilinite, all of which are O-H bearing phases. Three CR2 chondrites have common spectral

7 Infrared spectroscopic taxonomy for carbonaceous chondrites 77 characteristics that are similar to CM2 chondrites. The O-H stretching band comprises two spectral components at 3400 cm 1 and 3600 cm 1 as in the case of CM2 chondrites (Fig. 5). The latter band has relatively weak intensity relative to its associated silicate bands when compared with intensities of CM2 chondrites. Miyamoto and Zolensky (1994) reported a reflectance spectrum of Renazzo (CR2) and concluded that the amount of hydrous minerals in Renazzo is smaller than those in the CM or CI chondrites from its spectral features around 3400 cm 1. If CR2 chondrites generally have low amounts of hydrous minerals compared with CM2 chondrites, CR2 chondrites may be distinguished from CM2 chondrites by their infrared spectra. However, we infer that the qualitative classification between CR and CM chondrites is not perfect because some CM chondrites have CR-like spectra. For example, the infrared spectrum of Mighei (CM2) is very similar to that of Y (CR2) (Figs. 3 and 5). In general, however, the absorption band at 3600 cm 1 in CM2 chondrites is more intense than the 3400 cm 1 band, which is a distinctive spectral feature that is found exclusively in CM2 chondrites. A quantitative discussion about the spectral difference between CM and CR chondrites is presented later. CO, CV, and CK Chondrites Two CO3 chondrites, Y and Y-81020, have a broad and smooth absorption band at about 3400 cm 1 (Fig. 5). Their infrared absorption spectra are similar to that of Kainsaz (CO3) (Osawa et al. 2001). Their O-H stretching bands are composed of a single component at 3400 cm 1. Unlike CM2 and CR2 chondrites, no band is found at 3600 cm 1. The band at 3400 cm 1 is clearly weaker than those of CM2 and CR2 chondrites, indicating a lower concentration of water. For that reason, CO3 chondrites are easily distinguishable from other chondrites. Y (CV3) has no absorption bands that are attributable to O-H stretching vibration, although it had been confined for a long period in Antarctic ice, presuming that hydrous components in other Antarctic carbonaceous chondrites are not derived from Antarctic ice. The spectrum shape is similar to that of Allende (CV3) (Osawa et al. 2001). Y (CV3) exhibits not only a Si-O stretching band, but also a very weak O-H stretching band at 3400 cm 1. Miyamoto and Zolensky (1994) also report such a weak absorption band in Vigarano (CV3). Presence of minor saponite is reported in Vigarano (Zolensky et al. 1993), but our spectrum does not show the characteristic saponite absorption band at 3679 cm 1. Y also has no absorption band at 3679 cm 1. Presumably, the band at 3679 cm 1 does not exist because of the low abundance of saponite: CV3 chondrite comprises mainly olivine (e.g., Bland et al. 2004) and phyllosilicates are minor phases. Tomeoka and Tanimura (2000) reported that about 5% of chondrules in Vigarano have a phyllosilicate-rich rim. Generally, CV3 chondrites show a weak O-H stretching band or none at all. Therefore, within the Fig. 6. Infrared transmission spectra for CV, CK, CB, and CH chondrites. CB and CH chondrites show distinctive absorption bands at 3692 cm 1. general class of carbonaceous chondrites, this meteorite class is characterized by its very weak OH features (provided they are not badly weathered, see below).

8 78 T. Osawa et al. Table 2. Spectral characteristics of each type of carbonaceous chondrite. Class ~2900 cm 1a 3400 cm cm cm cm 1 Relative Relative peak height peak area 3520 cm 1 / 3400 cm cm 1 / 3685 cm 1 O-H/Si-O b CI c Very intense Intense d d d Tagish Lake Very intense CM Very intense Very intense CR Very intense Intense CO Intense CV Weak or absent 0.79 e 0.88 e 0.3 e CK n.d. n.d. 0.0 CH Detectable Intense Intense CB Detectable Intense Intense a A pair of two peaks caused by C-H stretching vibration. b Relative peak area of O-H stretching band near 3400 cm 1 to Si-O stretching band near 1000 cm 1. c No absorption. d These values include the data of CI chondrite clasts in brecciated ordinary chondrites. e Data of NWA 760. n.d. = not determined. NWA 760 (CV3) has a weak absorption band at 3400 cm 1 (Fig. 6), which may show the effect of terrestrial weathering because the weathering grade of the meteorite is W2 (Table 1). Severely weathered ordinary chondrites generally show intense absorption because of O-H stretching vibration (Salisbury and Hunt 1974; Miyamoto 1988; Salisbury et al. 1991; Miyamoto and Zolensky 1994). Meteorites collected in the desert have been weathered and are generally not fresh. This may be the reason why NWA 760 (CV3) has a relatively intense absorption band at 3400 cm 1 among CV chondrites. Therefore, strongly weathered CV3 chondrites are indistinguishable from CO3 chondrites using mid-infrared spectra. Two desert CK chondrites do not show signs of hydrous components: Dhofar 015 (CK3) and NWA 765 (CK4 5). Their weathering grades are W1. Maralinga (CK4) and Karoonda (CK4) also have no O-H stretching band. These CK chondrites have very flat spectra from 1300 to 4000 cm 1 and have no water-induced absorption. Their spectral features are similar to Allende (Osawa et al. 2001) and Y (CV3). Therefore, carbonaceous chondrites without O-H stretching band at 3400 cm 1 can be classified into CV or CK, but these two classes cannot be distinguished using the 3400 cm 1 band. Infrared spectra of the two classes clearly show their dry conditions, indicating that these chondrites were derived from parent bodies differently than the chondrites deriving from the hydrated parent bodies. The dried parent bodies may be located in the inner region of the solar system compared with hydrous carbonaceous parent bodies. CB and CH Chondrites Gujba belongs to the class of CB chondrites (B means bencubbinite), which is a new grouplet of primitive metal-rich chondrites (Weisberg et al. 2001). CB chondrites have marked similarities to the CR and CH chondrites in their mineral compositions (e.g., Weisberg et al. 1995). One class of carbonaceous chondrite designated by Bischoff et al. (1993) is CH, which shows high metal abundance and high Fe concentration. Gujba (CB) and two desert CH chondrites, NWA 470 and Acfer 207, show several unique spectral characteristics (Fig. 6). Their spectral shapes are not similar to those of CR chondrites. Gujba, NWA 470, and Acfer 207 have four common absorption bands at 2854, 2924, ~3400, and 3692 cm 1 (Fig. 6). A pair of bands at 2854 and 2924 cm 1 arise from symmetric and asymmetric C-H stretching vibrations of aliphatic organics, respectively (e.g., Sandford et al. 1991). If these peaks are not the result of terrestrial contamination, their presence indicates extraterrestrial aliphatic organic components. Intense absorption near 3400 cm 1 clearly implies the presence of hydrous components. Shapes of their O-H stretching bands are similar to those of CI chondrites rather than CM2 or CR2. The most remarkable peak is detected at 3692 cm 1, which is distinguishable from the peak of 3685 cm 1 observed in CI chondrites. The sharp absorption band can be attributed to free O-H stretching vibrations of a phyllosilicate. Therefore, the band can be used as an indicator of CB and CH chondrites. Characteristics and assignments of the band are presented later. Spectroscopic Taxonomy and Correspondence with Phyllosilicates The present study developed a spectroscopic classification technique of carbonaceous chondrites based on hydrous component speciation. Table 2 summarizes the spectroscopic taxonomy. Hydrous carbonaceous chondrites are classified easily using the criteria, but CB chondrite cannot be distinguished from CH chondrite by spectroscopic taxonomy. We believe that the convenient categorization

9 Infrared spectroscopic taxonomy for carbonaceous chondrites 79 Fig. 8. Infrared transmission spectra for CI chondrites and carbonaceous chondrites clasts of Tsukuba and Willard (b) breccias. The spectra have absorption bands at 3685 cm 1. Fig. 7. Relative peak heights of 3520 and 3600 cm 1 normalized to those of 3400 cm 1 (a) and those of 3670 and 3692 cm 1 to 3685 cm 1 (b). Peak heights are estimated after baseline correction. CR and CM chondrites are distinguishable from the other chondrite classes. technique is sufficient for practical use. However, to establish a classification method more precisely and quantitatively, peak heights at 3400, 3520, 3600, 3670, and 3692 cm 1 are estimated after the baseline correction for each infrared spectrum without curve fitting. Figure 7 shows the relative peak heights of 3520 and 3600 cm 1 normalized to those of 3400 cm 1 (a), and those of 3670 and 3692 cm 1 normalized to 3685 cm 1 (b). In the upper diagrams (a), CM and CR chondrites are clustered in each characteristic area and are readily distinguishable from the other carbonaceous chondrite classes. It is relatively difficult to discriminate among some CM chondrites and CR chondrites by their spectral shape, but these classes are separated in the diagram. This implies that CM chondrites have more intense absorption bands (at 3600 and 3520 cm 1 ) than the bands of CR chondrites. In this illustration, we arbitrarily set a boundary between the two classes at 0.92 on the vertical axis, but that value is somewhat subjective. In the lower diagram (b), CB and CH chondrites are distributed in the area above CI chondrites, reflecting the sharp absorption band at 3692 cm 1. Thereby, they can be distinguished from CI chondrites using a line of demarcation set to 1.0 in the diagram. If a meteorite is judged to be a carbonaceous chondrite from the appearance and/or interior texture, the

10 80 T. Osawa et al. Fig. 9. Comparison among spectra of CB, CH, CI, and phyllosilicates. The saponite measured here is a source clay mineral from The Clay Minerals Society. The lizardite is from Okaya, Nagano, Japan. The spectral data of chrysotile is based on Salisbury et al. (1992). The characteristic band of CI chondrites at 3685 cm 1 corresponds to the band of lizardite rather than that of saponite (3679 cm 1 ). On the other hand, the wave number of the sharp band found in the CB and CH chondrites is systematically higher than those of CI chondrites and corresponds to that of chrysotile. meteorite class will be roughly identifiable from the shape of mid-infrared spectrum. When we cannot judge whether it is CM or CR from the spectral shape, its class can be determined from the relative peak height diagram. The relative peak area of the O-H stretching band at cm 1 to Si-O stretching band at cm 1 is also calculated after baseline correction (Table 2). The abundance of water can be estimated from these stretching band values. CO and CV chondrites have markedly lower values than CI, CM, and CR chondrites. Although the values of CB and CH chondrites are relatively higher than those of CO and CV chondrites, they are systematically lower than those for CI, CM, and CR chondrites. The relative peak areas are useful for helping identify the type of carbonaceous chondrites, even though the accurate concentration of water cannot be calculated from them in the present stage. Spectroscopic taxonomy can be applied to carbonaceous chondrite clasts that are rarely found in brecciated meteorites. Although the carbonaceous chondrite clasts are very small, infrared spectra can be obtained by the diamond press method. In this study, we measured chips of carbonaceous chondrite clasts smaller than 1 µg found in two brecciated ordinary chondrites: Tsukuba and Willard (b). Infrared spectra for Ivuna, Orgueil, and these clasts are shown in Fig. 8. Both clasts clearly display a sharp absorption band at 3685 cm 1 similar to those produced by Ivuna and Orgueil, indicating that these clasts can be spectrally classified as CI chondrite materials. Nakashima et al. (2003) shows that the mineral composition of Tsukuba resembles that of CI chondrite. Willard (b) also has the CI-like mineral composition. Presence of the absorption band at 3685 cm 1 indicates that these brecciated meteorites had not been severely heated during cultivation on their parent bodies because the band can disappear by a high temperature condition. Indeed, the O-H stretching band at 3685 cm 1 is not observed in Y (C1) because of thermal metamorphism. That fact is consistent with the high concentration of solar noble gases (Nakashima et al. 2003) and the results of heating experiment for Orgueil (Osawa et al. 2001), which indicate mild heating of these breccias. Infrared spectra of carbonaceous chondrites are fundamentally explained by their mineral compositions. However, it is remarkable that the mineral compositions of CM chondrites cannot completely predict their infrared spectra. This discrepancy has already been noted by some researchers (e.g., Miyamoto and Zolensky 1994). The enigmatic discrepancy may relate with the temperature of aqueous alteration on their parent bodies. Kermarec et al. (1994) reported the infrared spectra of nickel talc and nepouite hydrothermally synthesized at C: nickel phyllosilicates synthesized at 25 C have a broad O-H stretching band around 3440 cm 1, whereas those synthesized at 150 and 250 C have extra O-H stretching bands at 3627 (talc) and 3645 cm 1 (nepouite). They also reported a similar

11 Infrared spectroscopic taxonomy for carbonaceous chondrites 81 Fig. 10. X-ray diffraction patterns of NWA 470 (CH). NWA 470 is comprised of six major minerals: saponite, serpentine, magnetite, troilite, calcite, and kamacite. The serpentine is enriched in chrysotile. trend for poorly-crystallized and well-crystallized nickel hydroxide. These phenomena are analogous to the relationship between infrared spectra of CI and CM chondrites. However, the spectral difference between CI and CM cannot be explained by crystallinity because CM chondrites do not contain poorly-crystallized phyllosilicates whereas CI chondrites do, as shown in Fig. 4. A 001 reflection of serpentine in Orgueil (CI) appearing at 2θ = ~18 is markedly broader than that of Y (CM), indicating the low crystallinity of serpentine. Nevertheless, CI chondrites have a sharp absorption band at 3685 cm 1 that is not detected in CM chondrites. Thus, the profiles of the infrared spectra of CM chondrites have yet to be fully explained. Although the 3600 cm 1 band found in CM and CR chondrites can be assigned to serpentine based on X-ray diffraction analyses indicating that these chondrites have plenty of serpentine (see Fig. 4), the reason why CM chondrites have no sharp band of serpentine remains unknown. The 3600 cm 1 band cannot be attributed to tochilinite, which has a sharp absorption at 2.7 µm (~3700 cm 1 ) (Moroz et al. 1997). Sharp absorption at 3685 cm 1 might disappear in the spectra for CM chondrites because the intense absorption at about 3600 cm 1 is the result of several spectral components; those components have not been separated and assigned in the present stage. The differences in infrared spectra among hydrous carbonaceous chondrites presumably reflect their evolutional history of the parent body in the early solar system. Characteristics of phyllosilicates in hydrous carbonaceous chondrites are controlled by conditions of aqueous alteration, which depend on temperature. The temperature of aqueous activity on the CM parent body is estimated to have been 0 25 C (Clayton and Mayeda 1999; Rosenberg et al. 2001). In contrast, CI chondrites experienced slightly warmer temperatures, but still below 50 C (Leshin et al. 1997). The slight difference in alteration temperature is the presumed cause of the difference in the spectral features, as in the case of the experiments by Kermarec et al. (1994). The free O-Hinduced sharp band of 3685 cm 1 is invisible because hydroxyl ions in phyllosilicates are bonded to other hydroxyl ions or molecular waters in low temperature condition. CI, CB, and CH chondrites have sharp absorption bands at high wave numbers (near 3690 cm 1 ), indicating the presence of hydroxyl ions in phyllosilicates. Figure 9 shows a comparison of infrared absorption spectra of CI, CB, and CH chondrites and three phyllosilicates in the range from 3500 to 3850 cm 1. Lizardite is a kind of serpentine group mineral that is generally found in CI and CM chondrites (e.g., Barber 1981). Serpentine-group minerals exist in other types of carbonaceous chondrites such as CH chondrites (Greshake et al. 2002); they have been discovered even in micrometeorites (e.g., Noguchi et al. 2002). Saponite, a trioctahedral type smectite, is another general phyllosilicate that is found in carbonaceous chondrites (e.g., Brearley 1995). Therefore, saponite and serpentine are plausible candidates of phyllosilicates causing intense O-H stretching absorption in spectra of the hydrous carbonaceous chondrites. Lizardite has a sharp infrared absorption at 3685 cm 1 ; several bands are adjacent to the peak at lower wave numbers. Saponite has a clear sharp band at 3679 cm 1 and a broad band near 3630 cm 1, the latter of which presumably comprises several bands. The peak position of Ivuna (CI) agrees with that of lizardite rather than that of saponite (3679 cm 1 ); the relatively broad band at ~3630 cm 1 observed for the saponite spectrum was not found in Ivuna, but the result is not consistent with detection of saponite in CI chondrite in diffraction experiments. Spectral features in the peak at 3679 cm 1 found in saponite are not identical with IR spectra

12 82 T. Osawa et al. Fig. 11. A low-magnification bright field TEM image of the matrix of NWA 470: a) the matrix is mostly composed of phyllosilicate. A large amount of magnetite (mt), fragmented into narrow parallel strips during the sectioning process, is observed. In a dark band that runs from the lower left to the upper right is a fold of the ultra thin section. The honeycomb-like structure is a perforated plastic supporting film; b) an enlarged image of a phyllosilicate-dominant matrix. Many fibrous phyllosilicates are nm thick. of CI chondrites because the peak overlaps the sharp absorption band of lizardite at 3685 cm 1 and/or is weak compared with the sharp band of lizardite. On the other hand, sharp absorption bands found in CB and CH chondrites appear systematically at higher wave numbers than those of CI chondrites (Fig. 9). The peak position of the sharp absorption for CB and CH chondrites is 3692 cm 1, corresponding to that of chrysotile rather than to saponite or lizardite. The main absorption of chrysotile appears at wave numbers that are higher than that of lizardite (e.g., Bishop et al. 2002). Another peak that is found in chrysotile at 3645 cm 1 is also detected in NWA 470 (CH), but the absorption is relatively weak and the presence of the band is not clear in the spectra of Gujba (CB) and Acfer 207 (CH). Fig. 12. TEM images of the matrix of a heavily hydrated clast in NWA 470: a) the clast matrix is composed mainly of nm thick phyllosilicates; b) a framboidal aggregate of magnetite comprised of euhedral magnetite crystals of ~400 nm in width. Therefore, the intense sharp absorption of CH and CB chondrites observed at 3692 cm 1 should be assigned to chrysotile. X-ray diffraction analysis also supports that inference. Figure 10 shows the X-ray diffraction pattern of NWA 470 (CH). The meteorite comprises six major minerals: saponite, serpentine, magnetite, troilite, calcite, and kamacite. If lizardite were abundant in this meteorite, the 11l reflection would appear intensely at ~51 and ~60. The diffraction pattern of Y clearly shows the presence of abundant lizardite (Fig. 4). An intense peak at 60 shows the characteristic 112 reflection of lizardite. On the other hand, chrysotile has a very weak 204 reflection at the same position. Therefore, chrysotile is enriched in NWA 470 based on the result of X-ray diffraction analysis. This result suggests that the temperatures of aqueous activity in the parent bodies of CH and CB chondrites are higher than those of CI and CM chondrites.

13 Infrared spectroscopic taxonomy for carbonaceous chondrites 83 Fig. 13. High-resolution TEM images of typical morphology of phyllosilicates in the host matrix (a c) and the matrix of the clast (d f). The phyllosilicates are mainly serpentine with small amounts of phyllosilicates with lattice fringes of nm and nm in width. The serpentine has straight and curved sharp lattice fringes.

14 84 T. Osawa et al. TEM Observation of Matrix and a Clast of NWA 470 Based on infrared spectra and synchrotron radiation X- ray diffraction of a matrix fragment and a clast from NWA 470 (CH), the meteorite contains abundant serpentine (chrysotile). To confirm the presence of phyllosilicate in this chondrite, ultramicrotomed samples were prepared for TEM observation. Figure 11 shows low-magnification bright-field images of the host matrix of NWA 470 that is composed mainly of phyllosilicate and contains minor amounts of magnetite and Fe sulfide. This is the first case of CH chondrite that contains abundant phyllosilicate in the matrix, although heavily hydrated clasts have been reported in CH chondrites (Greshake et al. 2002). One clast, which is black under a stereomicroscope with a sharp boundary from its surroundings, also shows heavy aqueous alteration (Fig. 12). It is composed mainly of phyllosilicate with relatively small amounts of magnetite, Fe sulfide, olivine, and low-ca pyroxene. Some framboidal aggregates of magnetite were visible (Fig. 12b). High-resolution TEM images of phyllosilicates in the matrix and the clast show that the phyllosilicate consists mainly of serpentine (0.70 nm lattice fringes) with small amounts of phyllosilicates with nm and nm lattice fringes (Fig. 13). The latter phyllosilicates may be chlorite and saponite, respectively. Serpentine in this sample has straight and curved lattice fringes. The occurrence of serpentine is very similar to that seen in a heavily hydrated lithic clast in PAT CH chondrite (Greshake et al. 2002). Although serpentine with a similar occurrence in the clast in PAT is not interpreted as chrysotile, the serpentine in this sample is chrysotile, based on infrared spectra and synchrotron radiation X-ray diffraction data. Chrysotile is common among CM chondrites (Barber 1981; Tomeoka and Buseck 1985) even though the absorption band assigned to chrysotile was not observed in infrared spectra for CM chondrites used in this work. Therefore, we must combine information obtained independently from TEM observation, X-ray diffraction, and infrared spectra because they are complementary. CONCLUSIONS Infrared absorption spectra of Antarctic and non- Antarctic carbonaceous chondrites were obtained by FTIR analyses using the KBr pellet and diamond press methods. Hydrous carbonaceous chondrites have an intense absorption band at around 3400 cm 1 that is attributable to the O-H stretching vibration. The present study developed a spectral classification of carbonaceous chondrites using the O-H stretching band. Only CI chondrites have a very sharp absorption band at 3685 cm 1. It is mainly attributable to free O-H ions that originate from serpentine rather than saponite. CM and CR chondrites have distinctive bands at 3600 cm 1, which we assign to serpentine. The spectral difference between CI and CM is presumably attributable to differences in the temperature of aqueous activity on their parent bodies. CM chondrites tend to show absorption that is more intense than that of CR chondrites. The spectrum of Tagish Lake is similar to neither CI nor CM chondrites. CO chondrites have only a broad band at 3400 cm 1. CV chondrites have a weak or negligible O-H stretching band because of their anhydrous condition. CK chondrites also have no water-induced absorption. CH and CB chondrites have distinctive absorption bands at 3692 cm 1 and 3400 cm 1 that indicate the presence of chrysotile. X-ray diffraction analysis and TEM observation of NWA 470 (CH) also support that idea. This study developed a practical technique for spectroscopic classification of carbonaceous chondrites. This technique is applicable to carbonaceous chondrite clast trapped in brecciated meteorites. By measuring IR spectra for carbonaceous chondrite clasts in Willard (b) and Tsukuba, these samples were classified into CI. Thereby, we have established the feasibility of this method. Acknowledgments Small chips of Antarctic carbonaceous chondrites were provided by the National Institute of Polar Research. Powder of the Sayama meteorite was supplied from National Science Museum. D. Nakashima provided the Tsukuba sample. This study was supported by a Grant-in-aid for Scientific Research ( , , ) from JSPS and by a Grant-in-aid for the 21st century COE program for Frontiers in Fundamental Chemistry from MEXT. Constructive reviews by V. E. Hamilton, an anonymous reviewer, and S. A. Sandford greatly helped to improve the manuscript. Editorial Handling Dr. Scott Sandford REFERENCES Barber D. J Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites. 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