Infrared diffuse reflectance spectra of carbonaceous chondrites: Amount of hydrous minerals

Abstract— Infrared diffuse reflectance spectra (2.53–25 μm) of some carbonaceous (C) chondrites were measured. The integrated intensity of the absorption bands near 3 μm caused by hydrous minerals were compared with the modal content of hydrous minerals for the meteorites. The CM and CI chondrites show larger values of the integrated intensity than those of the unique C chondrites Y82162, Y86720 and B7904, suggesting that the amount of hydrous minerals in the CM and CI chondrites is larger, which supports the contention that hydrous minerals were dehydrated by thermal metamorphism in the unique chondrites. Orgueil (CI) has the largest value of the integrated intensity among the C chondrites we measured and shows a sharp absorption band at 3685 cm^?1 (2.71 μm) that is not seen in the spectra of the CM chondrites. There is an excellent correlation between the observed hydrogen content in C chondrites and the integrated intensity. The CM chondrites show a wide variation in the strength of absorption bands at 1470 cm^?1 (6.8 μm), despite the similarity in absorption features near 3 μm for all CM chondrites. The 1470 cm^?1 band could be due to the presence of some hydrocarbons but may also be a result of terrestrial alteration processes.     

Remote spectroscopic identification of carbonaceous chondrite mineralogies: Applications to Ceres and Pallas

High-resolution spectroscopic observations of asteroids Ceres and Pallas have been obtained in the 1.0- to 2.6-μm region. Combined with previous spectralmeasurements at other wavelengths, this work presents the broadband spectral reflectances of these asteroids over the 0.4 to 3.6-um region. This extended coverage permits new analyses of the surface mineralogies of these objects. Using laboratory comparison spectra of meteorites and mixtures of terrestrial minerals, the surfaces of Ceres and Pallas are consistent with mixtures of opaques and hydrated silicates, such as are found in types C1 and C2 meteorites. This research emphasizes the importance of the 3-um spectral region for studying by remote methods the relationship of carbonaceous chondrite mineralogies to asteroid surfaces.     

The composition of asteroid 2 Pallas and its relation to primitive meteorites

High resolution spectroscopic observations of asteroid 2 Pallas from 1.7-3.5 μm are reported. These data are combined with previous measurements from 0.4-1.7 μm to interpret Pallas' surface mineralogy. Evidence is found for low-Fe²⁺ hydrated silicates, opaque components, and low-Fe²⁺ anhydrous silicates. This assemblage is very similar to carbonaceous chondrite matrix material such as is found in type CI and CM meteorites, but it has been subjected to substantial aqueous alteration and there is a major extraneous anhydrous silicate component. This composition is compared to that of asteroid 1 Ceres. Although there are substantial differences in their broad band spectral reflectances, it appears that both asteroids are genetically related to known carbonaceous chondrites.     

Material composition assessment and discovering sublimation activity on asteroids 145 Adeona, 704 Interamnia, 779 Nina,and 1474 Beira

Spectrophotometric observations of 145 Adeona, 704 Interamnia, 779 Nina, and 1474 Beira—asteroids of close primitive types—allowed us to detect similar mineralogical absorption bands in their reflectance spectra centered in the range 0.35 to 0.92 μm; the bands are at 0.38, 0.44, and 0.67–0.71 μm. On the same asteroids, the spectral signs of simultaneous sublimation activity were found for the first time. Namely, there are maxima at ～0.35–0.60 μm in the reflectance spectra of Adeona, Interamnia, and Nina and at ～0.55–075 μm in the spectra of Beira. We connect this activity with small heliocentric distances of the asteroids and, consequently, with a high insolation at their surfaces. Examination of the samples of probable analogues allowed us to identify Fe³⁺ and Fe²⁺ in the material of these asteroids through the mentioned absorption bands. For analogues, we took powdered samples of carbonaceous chondrites Orgueil (CI), Mighei (CM2), Murchison (CM2), and Boriskino (CM2), as well as hydrosilicates of the serpentine group. Laboratory spectral reflectance study of the samples of low-iron Mg serpentines (<2 wt % FeO) showed that the equivalent width of the absorption band centered at 0.44–0.46 μm strongly correlates with the content of Fe³⁺ in octahedral and tetrahedral coordinations. Our conclusion is that this absorption band can be used as a qualitative indicator of Fe³⁺ in the surface matter of asteroids and other solid celestial bodies. The comparison of the listed analog samples and the asteroids by parameters of the spectral features suggests that the silicate component of the asteroids' surface material is a mixture of hydrated and oxidized compounds, including oxides and hydroxides of bivalent and trivalent iron and carbonaceous-chondritic material. At the same time, the sublimation activity of Adeona, Interamnia, Nina, and Beira at high surface temperatures points to a substantial content of water ice in their material. This contradicts the previously existing notions on the C-type and similar asteroids as bodies containing water only in the bound state. Moreover, since the sublimation process simultaneously occurs in four primitive-type bodies at small heliocentric distances, we may suppose that this phenomenon is common for the main-belt asteroids.     

Thermal metamorphism of the C,G, B,and F asteroids seen from the 0.7 μm, 3 μm,and UV absorption strengths in comparison with carbonaceous chondrites

Abstract Thermal metamorphism study of the C, G, B, and F asteroids has been revisited using their UV, visible, NIR, and 3 μm reflectance spectra. High-quality reflectance spectra of seven selected C, G, B, and F asteroids have been compared with spectra for 29 carbonaceous chondrites, including thermally-metamorphosed CI/CM meteorites. There are three sets of spectral counterparts, among which 511 Davida and B-7904 are the most similar to each other in terms of both spectral shape and brightness. By comparing the 0.7 μm and 3 μm absorption strengths of 21 C, G, B, and F asteroids and heated Murchison samples, these asteroids have been grouped into three heating-temperature ranges. These correspond to (1) <400 °C: phyllosilicate-rich; (2) 400–600 °C: phyllosilicates transformed to anhydrous silicates; and (3) >600 °C: fully anhydrous. A good correlation between the UV and 3 μm absorption strengths has been confirmed for the C, G, B, and F asteroids and the CI, CM, and CR meteorites. A plot of the UV absorption strength vs. the IRAS diameter for 142 C, G, B, and F asteroids shows that the maximum UV absorption strength decreases as the diameter increases for the asteroids >60 km, with a notable exception, Ceres. These relationships suggest that some of the larger asteroids may be the heated inner portions of once larger bodies and that common CI/CM meteorites may have come from the lost outer portions, which escaped extensive late-stage heating events.     

Thermal metamorphism of CI and CM carbonaceous chondrites: An internal heating model

Abstract— Infrared diffuse reflectance spectra were measured for several thermally metamorphosed carbonaceous chondrites with CI-CM affinities which were recently found from Antarctica. Compared with other CI or CM carbonaceous chondrites, these Antarctic carbonaceous chondrites show weaker absorption bands near 3 μm due to hydrous minerals, and weaker absorption bands near 6.9 μm due to carbonates, interpreted as thermal metamorphic features. These absorption bands also disappear in the spectra of samples of the Murchison (CM) carbonaceous chondrite heated above 500 °C, implying that the metamorphic temperatures of the Antarctic carbonaceous chondrites considered here were higher than about 500 °C. Model calculations were performed to study thermal metamorphism of carbonaceous chondrites in a parent body internally heated by the decay of the extinct nuclide ²⁶Al. The maximum temperature of the interior of a body more than 20 km in radius is 500–700 °C for the bulk Al contents of CI and CM carbonaceous chondrites, assuming a ratio of ²⁶Al/²⁷Al = 5 × 10^?6 which has been previously proposed for an ordinary-chondrite parent body. The metamorphic temperatures experienced by the Antarctic carbonaceous chondrites considered here may be attainable by an internally heated body with an ²⁶Al/²⁷Al ratio similar to that inferred for an ordinary-chondrite parent body.     

碳质球粒陨石有机物红外光谱研究

碳质球粒陨石是太阳系中最原始的物质之一.通过对碳质球粒陨石的光谱分析,可以建立其与母体小行星之间的联系,有助于探测小行星表面物质成分、研究太阳系早期的演化历史.研究了6个CM2型碳质球粒陨石和11个煤炭样品(碳质球粒陨石所含有机质的地球类比物)可见-远红外谱段反射光谱特征,并分析了它们与有机组分的关系.结果表明,对于不...     

Could G-class asteroids be the parent bodies of the CM chondrites?

Abstract— I review the dynamical and compositional evidence for possibly linking CM chondrites and asteroids having G-class taxonomic designations. Three G asteroids have been identified through previous theoretical studies as being likely meteorite source bodies due to their locations near resonances. Two of these objects, 19 Fortuna and 13 Egeria, have spectral properties that are consistent with such a linkage with CM chondrites. Fortuna has a similar strength 0.7 μm absorption feature and near-infrared spectral slope to CM chondrites but a weaker ultraviolet feature. Egeria also has the characteristic 0.7 μm feature of CM chondrite spectra but does not match as well in the near-infrared. However, since the 0.7 μm feature is apparent in the spectra of approximately one-half of measured C-type asteroids, no definitive statement about any linkages can be made. Ceres is spectrally different from known meteorites in the 3 μm wavelength region and cannot be convincingly linked with any meteorite group.     

Infrared diffuse reflectances (2.5–25 μm) of some meteorites

We measured emissionless infrared diffuse reflectances of some meteorites by using a Fourier transform infrared spectrophotometer to obtain additional information on identification of asteroidal surface materials. Although the spectral features of diffuse reflectances are apparently different from those of transmission spectra, the absorption bands of constituent minerals can be detected. C2 carbonaceous chondrite materials can be distinguished from C3 materials by the depth of their hydration bands around 3 and 6 μm. These hydration bands do not lose contrast by pulverization of the sample. Acid dissolution experiment shows that the 6.8-μm band in the spectra of the Murchison (C2) meteorite is probably due to carbonates. The enstatite meteorite examined (Norton County) shows absorption bands around 10 μm caused by pyroxene unlike the iron meteorite examined (Mundrabilla). Because these latter two meteorite types give similar spectra without strong absorption bands in the UV-Visible-near IR region, middle infrared spectra should be helpful in interpreting asteroidal surface materials when combined with the UV-Visible-near IR spectra.     

Ceres’ spectral link to carbonaceous chondrites—Analysis of the dark background materials

Ceres’ surface has commonly been linked with carbonaceous chondrites (CCs) by ground‐based telescopic observations, because of its low albedo, flat to red‐sloped spectra in the visible and near‐infrared (VIS/NIR) wavelength region, and the absence of distinct absorption bands, though no currently known meteorites provide complete spectral matches to Ceres. Spatially resolved data of the Dawn Framing Camera (FC) reveal a generally dark surface covered with bright spots exhibiting reflectance values several times higher than Ceres’ background. In this work, we investigated FC data from High Altitude Mapping Orbit (HAMO) and Ceres eXtended Juling (CXJ) orbit (~140 m/pixel) for global spectral variations. We found that the cerean surface mainly differs by spectral slope over the whole FC wavelength region (0.4–1.0 μm). Areas exhibiting slopes ?1 constitute only ~3% of the cerean surface and mainly occur in the bright material in and around young craters, whereas slopes ≥?10% μm^?1 occur on more than 90% of the cerean surface; the latter being denoted as Ceres’ background material in this work. FC and Visible and Infrared Spectrometer (VIR) spectra of this background material were compared to the suite of CCs spectrally investigated so far regarding their VIS/NIR region and 2.7 μm absorption, as well as their reflectance at 0.653 μm. This resulted in a good match to heated CI Ivuna (heated to 200–300 °C) and a better match for CM1 meteorites, especially Moapa Valley. This possibly indicates that the alteration of CM2 to CM1 took place on Ceres.     

Importance of space weathering simulation products in compositional modeling of asteroids: 349 Dembowska and 446 Aeternitas as examples

Abstract— Based on recent progress in simulating space weathering on asteroids using pulse‐laser irradiation onto olivine and orthopyroxene samples, detailed analyses of two of the A and R type asteroid reflectance spectra have been performed using reflectance spectra of laser‐treated samples. The visible‐near‐infrared spectrum of olivine is more altered than that of pyroxene at the same pulse‐laser energy, suggesting that olivine weathers more rapidly than orthopyroxene in space. The same trend can be detected from reflectance spectra of the asteroids, where the more olivine an asteroid has, the redder its 1 μm band continuum can become. Comparison of the 1 μm band continuum slope and the 2/1 μm band area ratio between the asteroids and olivine and pyroxene samples (including the laser‐treated ones) suggests that asteroids may be limited in the degree of space weathering they can exhibit, possibly due to the short life of their surface regolith. Their pyroxenes may also have a limited chemical composition range. Fitting the visible continuum shape and other parts of the spectra (especially the 2μm part) has been impossible with any combination of common rock‐forming minerals such as silicates and metallic irons. However, this study shows, for the first time, excellent fits of reflectance spectra of an A asteroid (Aeternitas) and an R asteroid (Dembowska), including their visible spectral curves, band depths and shapes, and overall continuum shapes. Our results provide estimates that Aeternitas consists of 2% fresh olivine, 93% space‐weathered olivine, 1% space‐weathered orthopyroxene, and 4% chromite, and that Dembowska consists of 1% fresh olivine, 55% space‐weathered olivine, and 44% space‐weathered orthopyroxene. These results suggest that space weathering effects maybe important to the interpretation of asteroid reflectance spectra, even those with deep silicate absorption bands. Modified Gaussian model deconvolutions of the laser‐irradiated olivine samples show that their identity as olivine remained. The most recent submicroscopic mineralogical analyses have revealed that the laser‐irradiated olivine samples contain nanophase iron particles similar to those in space‐weathered lunar samples.     

Assessing the spatial variability of the ~3 μm OH/H2O absorption feature in CM2 carbonaceous chondrites

H₂O and OH are readily detected in hydrated minerals in CM chondrites via reflectance spectroscopy due to their characteristic vibration absorptions at infrared wavelengths. Previous spectroscopic work on bulk powdered CM chondrites has shown that spectral parameters, like the wavelength position of the “3 μm absorption feature,” vary systematically with the extent to which the samples have been aqueously altered. However, it is yet unclear how these spectral features may vary across an intact meteorite chip when measured at spatial scales smaller than that of the individual components of the meteorite. Here, we explore the spatial variability of this spectral feature and others on intact CM2 chips which, unlike powders, retain their petrologic and textural characteristics. We also model the modal mineralogy of the bulk meteorite powders and correlate this with key spectral features, demonstrating that microscope Fourier transform infrared spectroscopic mapping provides a powerful, rapid, and non-destructive technique for assessing compositional diversity and variations in water–rock interactions in chondritic planetary materials. In all CM2 chondrites studied here, we find that variations in the position, shape, and strength of the 3 μm absorption feature reveal a single chondrite can exhibit as much spectral variation as the entire suite of CM2 chondrites. The observed variations in the position and shape of the 3 μm feature within individual CM2 chondrite chips suggest a range of alteration products (e.g., Mg-rich to Fe-rich phyllosilicates) are present and record sub-mm scale variations in the amount and/or chemistry of the altering fluids. The samples having experienced the most progressive aqueous alteration show the least amount of variability in features like the 3 μm absorption band minimum position, whereas the least altered samples exhibit the most variability. We also find that the bulk spectral signatures in the least altered samples appear to be biased toward the spectral signatures of clasts versus matrix. By extension, asteroid reflectance spectra exhibiting 3 μm absorption features consistent with those measured here may be interpreted in a similar framework in which the spectrum of what may appear to be the least altered asteroids represents an average that belies the true diversity of mineralogy and chemistry of the body.     

Nature and degree of aqueous alteration in CM and CI carbonaceous chondrites

We investigated the petrologic, geochemical, and spectral parameters that relate to the type and degree of aqueous alteration in nine CM chondrites and one CI (Ivuna) carbonaceous chondrite. Our underlying hypothesis is that the position and shape of the 3 μm band is diagnostic of phyllosilicate mineralogy. We measured reflectance spectra of the chondrites under dry conditions (elevated temperatures) and vacuum (10^?8 to 10^?7 torr) to minimize adsorbed water and mimic the space environment, for subsequent comparison with reflectance spectra of asteroids. We have identified three spectral CM groups in addition to Ivuna. “Group 1,” the least altered group as determined from various alteration indices, is characterized by 3 μm band centers at longer wavelengths, and is consistent with cronstedtite (Fe‐serpentine). “Group 3,” the most altered group, is characterized by 3 μm band centers at shorter wavelengths and is consistent with antigorite (serpentine). “Group 2” is an intermediate group between group 1 and 3. Ivuna exhibits a unique spectrum that is distinct from the CM meteorites and is consistent with lizardite and chrysotile (serpentine). The petrologic and geochemical parameters, which were determined using electron microprobe analyses and microscopic observations, are found to be consistent with the three spectral groups. These results indicate that the distinct parent body aqueous alteration environments experienced by these carbonaceous chondrites can be distinguished using reflectance spectroscopy. High‐quality ground‐based telescopic observations of Main Belt asteroids can be expected to reveal not just whether an asteroid is hydrated, but also details of the alteration state.     

Spectral reflectance properties of carbonaceous chondrites: 2. CM chondrites

We have examined the spectral reflectance properties and available modal mineralogies of 39 CM carbonaceous chondrites to determine their range of spectral variability and to diagnose their spectral features. We have also reviewed the published literature on CM mineralogy and subclassification, surveyed the published spectral literature and added new measurements of CM chondrites and relevant end members and mineral mixtures, and measured 11 parameters and searched pair-wise for correlations between all quantities. CM spectra are characterized by overall slopes that can range from modestly blue-sloped to red-sloped, with brighter spectra being generally more red-sloped. Spectral slopes, as measured by the 2.4:0.56 μm and 2.4 μm:visible region peak reflectance ratios, range from 0.90 to 2.32, and 0.81 to 2.24, respectively, with values <1 indicating blue-sloped spectra. Matrix-enriched CM spectra can be even more blue-sloped than bulk samples, with ratios as low as 0.85. There is no apparent correlation between spectral slope and grain size for CM chondrite spectra - both fine-grained powders and chips can exhibit blue-sloped spectra. Maximum reflectance across the 0.3-2.5 μm interval ranges from 2.9% to 20.0%, and from 2.8% to 14.0% at 0.56 μm. Matrix-enriched CM spectra can be darker than bulk samples, with maximum reflectance as low as 2.1%. CM spectra exhibit nearly ubiquitous absorption bands near 0.7, 0.9, and 1.1 μm, with depths up to 12%, and, less commonly, absorption bands in other wavelength regions (e.g., 0.4-0.5, 0.65, 2.2 μm). The depths of the 0.7, 0.9, and 1.1 μm absorption features vary largely in tandem, suggesting a single cause, specifically serpentine-group phyllosilicates. The generally high Fe content, high phyllosilicate abundance relative to mafic silicates, and dual Fe valence state in CM phyllosilicates, all suggest that the phyllosilicates will exhibit strong absorption bands in the 0.7 μm region (due to Fe³⁺-Fe²⁺ charge transfers), and the 0.9-1.2 μm region (due to Fe²⁺ crystal field transitions), and generally dominate over mafic silicates. CM petrologic subtypes exhibit a positive correlation between degree of aqueous alteration and depth of the 0.7 μm absorption band. This is consistent with the decrease in fine-grained opaques that accompanies aqueous alteration. There is no consistent relationship between degree of aqueous alteration and evidence for a 0.65 μm region saponite-group phyllosilicate absorption band. Spectra of different subsamples of a single CM can show large variations in absolute reflectance and overall slope. This is probably due to petrologic variations that likely exist within a single CM chondrite, as duplicate spectra for a single subsample show much less spectral variability. When the full suite of available CM spectra is considered, few clear spectral-compositional trends emerge. This indicates that multiple compositional and physical factors affect absolute reflectance, absorption band depths, and absorption band wavelength positions. Asteroids with reflectance spectra that exhibit absorption features consistent with CM spectra (i.e., absorption bands near 0.7 and 0.9 μm) include members from multiple taxonomic groups. This suggests that on CM parent bodies, aqueous alteration resulted in the consistent production of serpentine-group phyllosilicates, however resulting absolute reflectances and spectral shapes seen in CM reflectance spectra are highly variable, accounting for the presence of phyllosilicate features in reflectance spectra of asteroids across diverse taxonomic groups.     

Search for relations among a sample of 460 asteroids with featureless spectra

We present an analysis of 460 featureless asteroid spectra in the range 0.5-0.92 μm obtained in the Small Solar System Objects Spectroscopic Survey. The spectra are described in terms of the continuum steepness (cSlope), its concavity (RRE), and the blue wing of drop in the UV reflectance (BD). Comparison with meteorite spectra confirms the link between CM meteorites and asteroids with asteroids with 0.7 μm band. Also, it is found that asteroids with extreme negative slope values may be related to CK chondrites and that asteroids with pronounced concave-down curvature are related to CO chondrites. An analysis of the distribution of the spectral parameters with semimajor axis, diameter, and albedo is performed.     

A plausible link between the asteroid 21 Lutetia and CH carbonaceous chondrites

A crucial topic in planetology research is establishing links between primitive meteorites and their parent asteroids. In this study, we investigate the feasibility of a connection between asteroids similar to 21 Lutetia, encountered by the Rosetta mission in July 2010, and the CH3 carbonaceous chondrite Pecora Escarpment 91467 (PCA 91467). Several spectra of this meteorite were acquired in the ultraviolet to near‐infrared (0.3–2.2 μm) and in the midinfrared to thermal infrared (2.5–30.0 μm or 4000 to ~333 cm⁻¹), and they are compared here to spectra from the asteroid 21 Lutetia. There are several similarities in absorption bands and overall spectral behavior between this CH3 meteorite and 21 Lutetia. Considering also that the bulk density of Lutetia is similar to that of CH chondrites, we suggest that this asteroid could be similar, or related to, the parent body of these meteorites, if not the parent body itself. However, the apparent surface diversity of Lutetia pointed out in previous studies indicates that it could simultaneously be related to other types of chondrites. Future discovery of additional unweathered CH chondrites could provide deeper insight in the possible connection between this family of metal‐rich carbonaceous chondrites and 21 Lutetia or other featureless, possibly hydrated high‐albedo asteroids.     

Hydrogen concentrations on C‐class asteroids derived from remote sensing

Abstract— We present spectroscopic observations of 16 asteroids from 1.9‐3.6 μm collected from the United Kingdom Infrared Telescope (UKIRT) from 1996–2000. Of these 16 asteroids, 11 show some evidence of a 3 μm hydrated mineral absorption feature greater than 2s? at 2.9 μm. Using relations first recognized for carbonaceous chondrite powders by Miyamoto and Zolensky (1994) and Sato et al. (1997), we have determined the hydrogen to silicon ratio for these asteroids and calculated their equivalent water contents, assuming all the hydrogen was in water. The asteroids split into 2 groups, roughly defined as equivalent water contents greater than ?7% (8 asteroids, all with 3 μm band depths greater than ?20%) and less than ?3% for the remaining 8 asteroids. This latter group includes some asteroids for which a weak but statistically significant 3 μm band of non‐zero depth exists. The G‐class asteroids in the survey have higher water contents, consistent with CM chondrites. This strengthens the connection between CM chondrites and G asteroids that was proposed by Burbine (1998). We find that the 0.7 μm and 3 μm band depths are correlated for the population of target objects.     

Analysis of faint absorption bands in the reflectance spectra of A-type asteroids

Faint absorption bands detected in the visible range of the reflectance spectra of A-type asteroids suggest a various mineralogical composition of their surface. From the analysis of these bands, we conclude that, on some asteroids of this optical type, both olivine and pyroxene are present, while mostly clinopyroxene with minor admixtures of Cr-containing minerals, presumably chromites, is on the surface of others. A new estimate of the forsterite content in the olivine of the asteroids 289 Nenetta and 446 Aeternitas (Fo ～ 50–60%) made by the absorption band near 500 nm in their spectra agrees with the estimate we obtained previously from the modeling of the reflectance spectra of the asteroid Aeternitas.     

The nature of c-class asteroids from 3-μm spectrophotometry

We present narrowband spectrophotometry between 2.3 and 3.5 μm for 14 main-belt C asteroids greater than 100 km in diameter. Absorption features at 3 μm due to water of hydration are present in the spectra of 9 of the asteroids, with intensities ranging from 6 to 23%. The other 5 asteroids have no 3-μm absorption greater than 2% in intensity. The strength of the “water” feature in the spectra correlates positively with the strength of the UV absorption feature shortward of 0.4 μm, and negatively with the slope of the continuum between 1.2 and 2.2 μm. These correlations are the same as those seen in laboratory spectra of carbonaceous chondrites, whose silicate compositions range from hydrated phyllosilicates to anhydrous olivine. We find no correlation between composition and semimajor axis for C asteroids as a class. The present C-asteroid population may be fragments of larger parent bodies with anhydrous C3-like cores and hydrated C11- or C2M-like mantles.     

Synchrotron‐based infrared microspectroscopy as a useful tool to study hydration states of meteorite constituents

Abstract— We present the results of the infrared (IR) microscopic study of the anomalous carbonaceous chondrites Dhofar (Dho) 225 and Dhofar 735 in comparison to typical CM2 chondrites Cold Bokkeveld, Murray, and Mighei. The Fourier transform infrared (FTIR) 2.5–14 μm reflectance measurements were performed on conventional polished sections using an infrared microscope with a synchrotron radiation source. We demonstrate that the synchrotron‐based IR microspectroscopy is a useful, nondestructive tool for studying hydration states of meteorite constituents in situ. Our results show that the matrices of Dho 225 and Dho 735 are dehydrated compared to the matrices of typical CM2 chondrites. The spectra of the Dho 225 and Dho 735 matrices lack the 2.7–2.8 μm absorption feature present in the spectra of Cold Bokkeveld, Murray, and Mighei. Spectral signatures caused by Si‐O vibrations in fine‐grained, Fe‐rich olivines dominate the 10 μm spectral region in the spectra of Dho 225 and Dho 735 matrices, while the spectra of normal CM2 chondrites are dominated by spectral signatures due to Si‐O vibrations in phyllosilicates. We did not detect any hydrated phases in the spectra of Dho 225 and Dho 735 polished sections. In addition, the near‐infrared reflectance spectra of Dho 225 and Dho 735 bulk powders show spectral similarities to the Antarctic metamorphosed carbonaceous chondrites. We confirm the results of previous mineralogical, chemical, and isotopic studies indicating that the two meteorites from Oman are the first non‐Antarctic metamorphosed carbonaceous chondrites.