Carbonaceous chondrites record processes of aqueous alteration in the presence of hydrated and hydroxylated minerals, which could have provided a source of water in the inner solar system (Alexander ...et al., 2012, 2013). In this study, thermogravimetric analysis (TGA) was performed on 26 CM chondrites, which cover a range of degree of aqueous alteration from 2.0, such as Meteorite Hills (MET) 01070, to 2.6, such as Queen Alexandra Range (QUE) 97990, in order to quantify their water content. In addition, by measuring the release of volatile elements as a function of temperature, we obtained information on the mineralogy of water-bearing phases and provide indicators of aqueous alteration based on water released by phyllosilicates. These analyses are combined with infrared spectroscopy (IR) made on meteorite pellets heated up to 300°C. The infrared features (–OH band at 3-μm and SiO4 around 10-μm) revealed a correlation with TGA. The two techniques are in agreement with the scheme of aqueous alteration proposed by Rubin et al. (2007) and Alexander et al. (2013) based on phyllosilicate abundance.
The low temperature (200–400°C) mass loss observed in TGA is attributed to Fe-oxy-hydroxydes (ferrihydrite, goethite). However, the proportion of these minerals formed by terrestrial alteration remains unknown. TGA also revealed two anomalous CM chondrites, Pecora Escarpment (PCA) 02012 and PCA 02010. Their TGA curves are significantly different from those of “regular” CMs with little mass loss, which can be related to the dehydration history of these meteorites in response to a heating event (Raman measurements also point toward a thermal event, Quirico et al., 2013). In the case of more mildly heated chondrites, such as with Wisconsin Range (WIS) 91600, the TGA curve presents similar mass loss to the other CMs.
Seven bulk measurements of CR chondrites and 3 measurements of matrix-enriched parts of CR meteorites were also studied by TGA, and confirm the low hydration level of chondrules and a significant alteration of the matrix. The water content of the matrix of the CM 2.6 QUE 97990 was estimated and compared to TGA of the matrix enriched portion of the CR2 EET 92159 and that of Orgueil.
Results suggest a similar aqueous alteration degree between Orgueil and the matrix of CMs (around 25wt.%) and a lower alteration of the CR2 matrix (11wt.% of H2O).
•We have determined MIR absorption properties of 40 carbonaceous chondrites (CC).•Variability in the shape of the silicate features and intensity of 3-μm band are found.•These variations are related ...to aqueous alteration extent and thermal metamorphism.•Terrestrial serpentine fail to reproduce the silicate features of altered CCs.
In this work, infrared transmission spectra (2–25μm range, 5000–400cm−1) of 40 carbonaceous chondrites were analyzed (21 CMs, 5 CVs, 6 CRs, 3 CKs, 3 C2s and 2 CIs). All these meteorite groups are known to have experienced significant aqueous alteration (except the CKs). These IR measurements provide information about the parent body processes experienced, as well as spectra for comparison with observations of Solar System small bodies and possibly with astronomical observations of accretion and debris disks.
This study reveals that each meteorite group appears to have specific signatures in the measured IR spectral range. In the case of the CI and CM groups, results show a variability in the shape of the silicate features that can be related to the evolution of the mineralogy with increasing extent of aqueous alteration extent as described by several authors with other techniques. This evolution of the silicate feature can be seen in the variation in the relative intensities of olivine and phyllosilicate IR features. The variability in the silicate features is correlated with the intensity of an –OH related absorption at 3-μm, which can be used for the classification of the meteorites according to the level of hydration. Interestingly, in the case of CM chondrites, for which the mineralogy is expected to be dominated by phyllosilicates (serpentine mostly), the shape of the silicate absorption resembles that of an amorphous silicate, with a broad and symmetric 10-μm band, unlike terrestrial phyllosilicates.
The CV and CK groups have IR spectra that are dominated by olivine absorption. From this feature, it is possible to determine average Mg numbers for the olivine. For the CVs, the olivine Mg numbers appear to decrease in the order Kaba–Grosnaja–Vigarano–Mokoia–Allende. This trend is likely related to the long duration of metamorphism experienced by these samples and the chemical re-equilibration between chondritic components. In the case of CK chondrites, the inferred bulk Mg# of olivine is 67 (±1), and no variation is observed between the three studied samples, which is likely related to their high degree of equilibration.
The 6 CR chondrites show the most variability in their IR spectra, from CM-like spectra in the case of the CR1 Grosvenor Mountains (GRO) 95577 to CV-like spectra for Roberts Massif (RBT) 04133 and Graves Nunataks (GRA) 06100 (one of them being most probably misclassified). Spectra of the remaining CRs show mixtures of various silicate component.
Finally, these spectra can be used for comparison with emission spectra of fine-grained asteroid surfaces and dust-rich cometary tails. In the case of Tempel 1, the only group of CC that matches the observed feature around 10-μm region is the CR group. The spectral comparison shows some striking similarities between CRs and Tempel 1 dust. A genetic link between CR2 and comets is not proven, but mineralogical similarities are suggested from the IR spectra.
Here we report a comparison between reflectance spectroscopy of meteorites under asteroidal environment (high vacuum and temperature) and Main Belt and Near Earth Asteroids spectra. Focusing on the ...–OH absorption feature around 3 μm, we show that the asteroidal environment induces a reduction of depth and width of the band, as well as a shift of the reflectance minimum. We then decompose the –OH feature into several components with a new model using Exponentially Modified Gaussians. Unlike previous studies, we confirme the link between these components, the aqueous alteration history and the amount of water molecules inside of the sample, using the shape of this spectral feature only. We then apply this deconvolution model to asteroids spectra which were obtained with a space-borne telescope and two space probes, and find a strong similarity with the components detected on meteorites, and among asteroids from a same type. Based on the conclusions drawn from our meteorites experiment, we suggest to use the 3-μm band as a tracer of the alteration history of the small bodies. Using the 3-μm band only, we show that Ryugu has been heavily altered by water, which is consistent with its parent body being covered with water ice, then went through a high temperature sequence, over 400 °C. We also point out that the 3-μm band of Bennu shows signs of its newly discovered surface activity.
•Exponentially Modified Gaussians are used to model the shape of the 3-μm band.•The components of the band depend on the aqueous alteration and relative humidity.•Similar components are detected on the asteroids spectra.•The components are used to retrieve the alteration history of the asteroids.•Signs of activity on Bennu and two Cgh-types are detected through the –OH band.
Analysis of the propagation of the Comet Nucleus Sounding Experiment by Radiowave Transmission (CONSERT) signal throughout the small lobe of the 67P/CG nucleus has permitted us to deduce the real ...part of the permittivity, at a value of 1.27 ± 0.05. The first interpretation of this value, using the dielectric properties of mixtures of ices (H2O, CO2), refractories (i.e. dust) and porosity, led to the conclusion that the comet porosity lies in the range 75–85 per cent. In addition, the dust-to-ice ratio was found to range between 0.4 and 2.6 and the permittivity of dust (including 30 per cent porosity) was determined to be lower than 2.9. This last value corresponds to a permittivity lower than 4 for a material without any porosity. This article is intended to refine the dust permittivity estimate by taking into account updated values of the nucleus densities and dust/ice ratio and to provide further insights into the nature of the constituents of comet 67P/CG. We adopted a systematic approach: determination of the dust permittivity as a function of the volume fraction of ice, dust and vacuum (i.e. porosity) and comparison with the permittivity of meteoritic, mineral and organic materials from literature and laboratory measurements. Then different composition models of the nuclei corresponding to cosmochemical end members of 67P/CG dust are tested. For each of these models, the location in the ice/dust/vacuum ternary diagram is calculated based on available dielectric measurements and confronted to the locus of 67P/CG. The number of compliant models is small and the cosmochemical implications of each of them is discussed, to conclude regarding a preferred model.
This paper reports the first reliable quantitative determination of the thermal metamorphism grade of a series of nine CV3 chondrites: Allende, Axtell, Bali, Mokoia, Grosnaja, Efremovka, Vigarano, ...Leoville, and Kaba. The maturity of the organic matter in matrix, determined by Raman spectroscopy, has been used as a powerful metamorphic tracer, independent of the mineralogical context and extent of aqueous alteration. This tracer has been used along with other metamorphic tracers such as Fe zoning in type-I chondrules of olivine phenocrysts, presolar grain abundance and noble gas abundance (bulk and P3 component). The study shows that the petrologic types determined earlier by Induced ThermoLuminescence were underestimated and suggests the following values: PT (Allende-Axtell) >3.6; PT (Bali-Mokoia-Grosnaja) ∼3.6; PT (Efremovka-Leoville-Vigarano)
=
3.1–3.4; PT (Kaba) ∼3.1. The most commonly studied CV3, Allende, is also the most metamorphosed. Bali is a breccia containing clasts of different petrologic types. The attribution suggested by this study is that of clasts of the highest petrologic types, as pointed out by IOM maturity and noble gas bulk abundance. CV3 chondrites have complex asteroidal backgrounds, with various degrees of aqueous alteration and/or thermal metamorphism leading to complex mineralogical and petrologic patterns. (Fe,Mg) chemical zoning in olivine phenocrysts, on the borders of type I chondrules of porphyritic olivine- and pyroxene-rich textural types, has been found to correlate with the metamorphism grade. This suggests that chemical zoning in some chondrules, often interpreted as exchanges between chondrules and nebular gas, may well have an asteroidal origin. Furthermore, the compositional range of olivine matrix is controlled both by thermal metamorphism and aqueous alteration. This does not support evidence of a nebular origin and does not necessarily mirror the metamorphism grade through (Fe,Mg) equilibration. On the other hand, it may provide clues on the degree of aqueous alteration vs. thermal metamorphism and on the timing of both processes. In particular, Mokoia experienced significant aqueous alteration after the metamorphism peak, whereas Grosnaja, which has similar metamorphism grade, did not.
Chondrites are exhumed from their parent bodies by impacts, which at the same time can result in heating and mechanical modification (compaction, deformation, fracturing, etc.). However, whether ...impacts are responsible for the occurrence of heated C2s remains controversial since radiogenic and solar heating have also been invoked to explain them. Here we report a Raman and infrared study of the composition and structure of Insoluble Organic Matter (IOM) in a series of 39 CM and C2-ungrouped chondrites. These parameters are tracers of the extent and nature of thermal metamorphism a meteorite has experienced and reflect the degree to which the thermally driven and irreversible carbonization of IOM has proceeded. We propose a carbon-based classification of heated C2 chondrites that reveals a high occurrence frequency of thermally processed C2 chondrites (>36%). This classification is in agreement with the mineralogical classification scheme of Nakamura (2005). Strongly heated C2 chondrites (PCA 02012, PCA 91008, Y 96720) display an IOM structural evolution that is dissimilar to that of type 3 chondrites that experienced long duration radiogenic thermal metamorphism. These differences almost certainly reflect kinetic constraints on IOM modification during short duration heating events. QUE 93005 is a weakly heated chondrite that experienced a retrograde aqueous alteration. Its very aliphatic-rich IOM points to a parent body hydrogenation through interactions with water. The closed-system conditions required by this mechanism could be satisfied by a kinetic confinement during a very short duration impact. MET 01072, a heavily compacted and uni-axially deformed chondrite, did not experience post-accretional heating. In this case, the deformation features probably reflect a low-velocity impact. In contrast, the weakly metamorphosed chondrite EET 96029 experienced one or several low pressure impacts that triggered mild heating and partial dehydration without deformation features. The study of a series of lithologies from the Tagish Lake C2-ungrouped chondrite confirms the coexistence of various degrees of post-accretional alteration, the most altered lithologies having experienced a moderate degree of heating. Overall, the high prevalence of heating in C2 chondrites, the evidence of short-duration heating in the most heated C2s and the ability of low velocity collisions to trigger heating favor impacts (against solar heating), as the dominant heating mechanism. Finally, our set of data does not support the action of a low temperature oxidation process that would control the aliphatic abundance in unheated primitive C2s.
•We measured bidirectional reflectance spectra of different group of carbonaceous chondrites.•We propose a method to quantify the water on asteroid surfaces.•We present a diagram, separating ...accretionary properties from parent body processes.
In this study, we measured bidirectional reflectance spectra (0.5–4.0μm) of 24 CMs, five CRs, one CI, one CV, and one C2 carbonaceous chondrites. These meteorites are known to have experienced an important variability in their relative degrees of aqueous alteration degree (Rubin et al. 2007. Geochim. Cosmochim. Acta 71, 2361–2382; Howard et al. 2009. Geochim. Cosmochim. Acta 73, 4576–4589; Howard et al. 2011. Geochim. Cosmochim. Acta 75, 2735–2751; Alexander et al. 2013. Geochim. Cosmochim. Acta 123, 244–260). These measurements were performed on meteorite powders inside an environmental cell under a primary vacuum and heated at 60°C in order to minimize adsorbed terrestrial water. This protocol allows controlling of atmospheric conditions (i.e. humidity) in order to avoid contamination by terrestrial water. We discuss various spectral metrics (e.g. reflectance, band depth, single-scattering albedo, …) in the light of recent bulk composition characterization (Howard et al. 2009. Geochim. Cosmochim. Acta 73, 4576–4589; Howard et al. 2015. Geochim. Cosmochim. Acta 149, 206–222; Alexander et al. 2012. Science 337, 721; Beck et al. 2014. Icarus 229, 263–277; Garenne et al. 2014. Geochim. Cosmochim. Acta 137, 93–112). This study reveals variability of reflectance among meteorite groups. The reflectance is not correlated with carbon or hydrogen abundance neither with measured grain size distribution. We suggest that it is rather controlled by the nature of accreted components, in particular the initial matrix/chondrule proportion. Band depth, integrated band depth, mean optical path length, normalized optical path length, effective single-particle absorption thickness were calculated on the so called 3-μm band for reflectance spectra and for single scattering albedo spectra. They were compared with hydrated phase proportions from previous study on the same meteorites by thermogravimetric analyses and infrared spectroscopy in transmission. We find that normalized optical path length (NOPL) is the most appropriate to quantify water abundance, with an absolute error of about 5wt.%. These datasets also reveal a variability of the band shape between 2.8 and 2.9μm, which is interpreted as reflecting variation in the chemical composition and structure of phyllosilicates. This chemical variation could also be used to quantify the aqueous alteration degree between meteorite groups. The combination of reflectance at 2μm and the depth of 3-μm band can be combined, to classify carbonaceous chondrites in reflectance in term of primary composition (e.g. matrix/chondrule ratio, carbon content) and secondary processes (e.g. aqueous alteration, thermal metamorphism). This could be used to decipher the nature of aqueous alteration in C-complex asteroids.
Geothermometers based on Raman spectrometry of carbonaceous matter and covering a wide range of temperatures (100–650 °C) have been developed over recent years. While Raman data have been largely ...interpreted in terms of temperature, they are also the fingerprint of certain metamorphic conditions, especially in the low temperature range relevant to poorly ordered carbonaceous matter. This study investigates the Raman spectra of two series of chondritic carbonaceous matter and coal samples formed from different precursors and under different metamorphic conditions.
The Raman spectra of Polyaromatic Carbonaceous Matter (PCM) from 42 chondrites and 27 coal samples, measured with visible (514 nm) and ultra-violet (244 nm) excitation wavelengths, are analyzed. The Raman spectra of low rank coals and chondrites of petrologic types 1 and 2, which contain the more disordered PCM, reflect the distinct carbon structures of their precursors. The 514 nm Raman spectra of high rank coals and chondrites of petrologic type 3 exhibit continuous and systematic spectral differences reflecting different carbon structures present during the metamorphism event. They result from differences in the chemical structures of the precursors concerning for instance the reticulation of polyaromatic units or an abundance of ether functional groups, or possibly from a lack of carbonization processes to efficiently expel oxygen heteroatoms, due to weak lithostatic pressure and confinement. These results suggest that the use of low temperature carbon thermometers should be restricted to a given geological context. At the same time, the sensitivity of Raman spectra to precursors and certain metamorphic conditions could be used to obtain information other than temperature.
The analysis also provides evidence of the accretion of
relatively homogeneous PCM precursors among ordinary CO and CV carbonaceous chondrite parent bodies, given that the 514 nm Raman spectra of PCM efficiently trace the metamorphism grades. Looking closer, however, the 514 nm Raman data are more scattered in chondrites than in coals and the maturity tracers are less sensitive and miscorrelate with the atomic H/C ratio, suggesting slight compositional and structural differences among the PCM precursors accreted.
We present here several laboratory analyses performed on the freshly fallen Mukundpura CM chondrite. Results of infrared transmission spectroscopy, thermogravimetry analysis, and reflectance ...spectroscopy show that Mukundpura is mainly composed of phyllosilicates. The rare earth trace elements composition and ultrahigh‐resolution mass spectrometry of the soluble organic matter give results consistent with CM chondrites. Finally, Raman spectroscopy shows no signs of thermal alteration of the meteorite. All the results agree that Mukundpura has been strongly altered by water on its parent body. Comparison of the results obtained on the meteorite with those of other chondrites of known petrologic types leads to the conclusion that Mukundpura is similar to CM1 chondrites, which differ from its original classification as a CM2.
•The surface of comet 67P/CG has been imaged by the VIRTIS instrument aboard ROSETTA.•Refractory polyaromatic organics mixed with opaque minerals account for the low albedo.•Semi-volatiles organics ...(solid at 220K) induce a broad band centered at 3.2µm.•Laboratory photolytic/thermal residues formed from interstellar ice analogs are fair analogs.•No hydrated minerals are detected, suggesting no link with carbonaceous chondrites.
The VIRTIS (Visible, Infrared and Thermal Imaging Spectrometer) instrument aboard the Rosetta spacecraft has performed extensive spectral mapping of the surface of comet 67P/Churyumov-Gerasimenko in the range 0.3–5µm. The reflectance spectra collected across the surface display a low reflectance factor over the whole spectral range, two spectral slopes in the visible and near-infrared ranges and a broad absorption band centered at 3.2µm. The first two of these characteristics are typical of dark small bodies of the Solar System and are difficult to interpret in terms of composition. Moreover, solar wind irradiation may modify the structure and composition of surface materials and there is no unequivocal interpretation of these spectra devoid of vibrational bands. To circumvent these problems, we consider the composition of cometary grains analyzed in the laboratory to constrain the nature of the cometary materials and consider results on surface rejuvenation and solar wind processing provided by the OSIRIS and ROSINA instruments, respectively. Our results lead to five main conclusions: (i) The low albedo of comet 67P/CG is accounted for by a dark refractory polyaromatic carbonaceous component mixed with opaque minerals. VIRTIS data do not provide direct insights into the nature of these opaque minerals. However, according to the composition of cometary grains analyzed in the laboratory, we infer that they consist of Fe-Ni alloys and FeS sulfides. (ii) A semi-volatile component, consisting of a complex mix of low weight molecular species not volatilized at T∼220K, is likely a major carrier of the 3.2µm band. Water ice contributes significantly to this feature in the neck region but not in other regions of the comet. COOH in carboxylic acids is the only chemical group that encompasses the broad width of this feature. It appears as a highly plausible candidate along with the NH4+ ion. (iii) Photolytic/thermal residues, produced in the laboratory from interstellar ice analogs, are potentially good spectral analogs. (iv) No hydrated minerals were identified and our data support the lack of genetic links with the CI, CR and CM primitive chondrites. This concerns in particular the Orgueil chondrite, previously suspected to have been of cometary origin. (v) The comparison between fresh and aged terrains revealed no effect of solar wind irradiation on the 3.2µm band. This is consistent with the presence of efficient resurfacing processes such as dust transport from the interior to the surface, as revealed by the OSIRIS camera.
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