Recently published space-based observations of main-belt asteroids with the AKARI telescope provide a full description of the 3 μm band, related to the presence of OH bearing minerals. Here, we use ...laboratory spectra of carbonaceous chondrites obtained under controlled atmosphere (CI,CM,CO,CV,CR Tagish Lake) to derive spectral metrics related to the water content in the samples. After testing several spectral metrics, we use a combination of band depth at 2.75 μm and 2.80 μm that shows a correlation with H2O in the sample determined by TGA, though with a high uncertainty (4 wt% H2O). This relation is used to determine water content at the surface of large C-complex main-belt asteroids and discuss the origin of the variability found. On average C-complex Main-Belt Asteroids (MBA) have water contents of 4.5 wt% (volume average, (1) Ceres excluded), significantly lower than average CM chondrites. The estimated water content for the most hydrated asteroids are lower than those of the most hydrated meteorites, a difference that could be attributed to space-weathering. An anti-correlation is also present between water content and overall spectral slope, which is opposite to expectation from laboratory simulations of space weathering on dark carbonaceous chondrites. This suggests that part of the variability in the surface hydration among the different C-complex asteroids is not due to space-weathering, but to the composition of surface material. When applied to Ceres, the hygrometer presented in this work enables us to estimate that at least 1.22 wt% of the hydrogen is present in the form of organics. This richness in organics strengthens the connection between Ceres and cometary materials.
•Quantification of equivalent water in CC from reflectance is discussed.•Equivalent water is quantified at the surface of for large C-complex asteroids.•Equatorial region on Ceres are expected to contain 1.22 wt% of organic H.
This paper focuses on the post-accretion history of CV3 chondrites, through a combination of petrographic and mineralogical characterization, magnetic measurements, spectral (Raman and Infrared) and ...thermo-gravimetric analysis of 31 meteorites (including 7 falls, 21 Antarctic and 3 non-Antarctic finds) spanning a wide metamorphic range.
We classify the 21 Antarctic chondrites and the Bukhara fall into the CVRed, CVOxA, and CVOxB subgroups. We establish quantitative parameters relevant for this sub-classification. In comparison to CVOx, CVRed chondrites are characterized by (i) a lower abundance of matrix, (ii) a higher abundance of metal, (iii) the presence of Ni-poor sulfides. In comparison to CVOxB, CVOxA are characterized by (i) similar matrix abundance, (ii) a higher abundance of metal, (iii) the presence of metal almost exclusively under the form of awaruite, (iv) lower Ni content of sulfides, (v) lower magnetic susceptibility and saturation remanence.
Both CVOx (CVOxA and CVOxB) and CVRed experienced aqueous alteration, and contain oxyhydroxides and phyllosilicates. We show that the abundance of these hydrated secondary minerals observed today in individual CV chondrites decreases with their peak metamorphic temperature. This is interpreted either as partial dehydration of these secondary minerals or limited hydration due to the rapid exhaustion of the water reservoir during parent body thermal metamorphism. Moreover, the lower abundance of oxyhydroxides (that have a lower thermal stability than phyllosilicates and may in large part postdate the peak of thermal metamorphism) in more metamorphosed CV chondrites is interpreted as lower availability of aqueous fluids during retrograde metamorphism in these meteorites.
Lastly, we show that in comparison to CVOxB, CVOxA are systematically (i) more metamorphosed, (ii) less hydrated, (iii) depleted in ferromagnetic minerals, (iv) but enriched in metal in the form of secondary awaruite. CVOxA may be thermally metamorphosed CVOxB. CVRed are significantly different from CVOx (matrix abundances, alteration products, opaque minerals), but span the same wide metamorphic range. This could be indicative of a laterally heterogeneous CV parent body, or suggest the existence of distinct parent bodies for CVOx and CVRed chondrites.
The Piancaldoli ordinary chondrite fell in northern Italy on August 10, 1968. Preliminary studies led to its classification as an LL3.4 unequilibrated ordinary chondrite. However, recent developments ...in classification procedures have prompted us to re‐examine its mineralogical, petrographic, spectroscopic, chemical, and isotopic features in a multi‐technique study. Raman spectra and magnetic properties indicate that Piancaldoli experienced minimal thermal metamorphism, consistent with its high bulk hydrogen content and the Cr contents of ferroan olivines in its type II chondrules. In combination with findings of previous studies, our data thus confirm the variability of Cr contents in ferroan olivines in type II chondrules as a proxy of thermal metamorphism. Furthermore, our results reveal that Piancaldoli is less altered than previously reported and should be reclassified as an LL3.10 unequilibrated ordinary chondrite. Our results also imply that the bulk deuterium enrichment, as observed in Piancaldoli (LL3.10), Bishunpur (LL3.15), and Semarkona (LL3.00), is a specific signature of the most primitive unequilibrated ordinary chondrites. Based on our results, we propose that, to date, Piancaldoli is the second least‐altered unequilibrated ordinary chondrite fall after Semarkona. This work reiterates the importance of meteorite collections worldwide as fundamental resources for studying the formation conditions and evolution of our solar system.
The search for asteroidal parent bodies of chondrites through various techniques is an ongoing endeavor. A link between ordinary chondrites (OCs) and S-type asteroids has previously been established ...by the sample return of the Hayabusa space mission. OCs are the class with the most abundant samples in our meteorite collection. We present an in-depth study of the reflectance spectra of 39 equilibrated and 41 unequilibrated ordinary chondrites (EOCs and UOCs). We demonstrate that consistent measuring conditions are vital for the direct comparison of spectral features between chondrites, otherwise hampering any conclusions. We include a comparison with a total of 466 S-type asteroid reflectance spectra from various databases. We analyze (i) if a difference between EOCs and UOCs as well as between H, L and LL can be seen, (ii) if it is possible to identify unequilibrated and equilibrated S-type asteroid surfaces and (iii) if we can further constrain the match between OCs and S-type asteroids all based on reflectance spectra.
As a first step, we checked the classification of the 31 Antarctic UOCs analyzed in the present work, using petrography and magnetic measurements, and evidenced that 74% of them were misclassified. Reflectance spectra were compared between EOCs and UOCs as well as between H, L and LL chondrites using a set of spectral features including band depths and positions, peak reflectance values, spectral slopes and the Ol/(Ol + Px) ratio. UOCs and EOCs reflectance spectra show no clear-cut dichotomy, but a continuum with some EOCs showing stronger absorption bands and peak reflectance values, while others are comparable to UOCs. Moreover, we show by the example of 6 EOCs that their band depths decrease with decreasing grain size. Based on reflectance spectra alone, it is thus highly challenging to objectively identify an unequilibrated from an equilibrated S-type surface. There is no clear distinction of the chemical groups: only LL EOCs of petrographic type >4 can be distinguished from H and L through less deep 2000 nm band depths and 1000 nm band positions at longer wavelengths. No dichotomy of S-type asteroids can be seen based on the Ol/(Ol + Px) ratio. Their average Ol/(Ol + Px) ratio matches EOCs better than UOCs. A principal component analysis (PCA) was performed illustrating that both the unknown degree of space weathering and the unknown regolith grain size on asteroid surfaces hinder the distinction between equilibrated and unequilibrated surfaces. Lastly, an anti-correlation between the diameter of the asteroids and their 1000 nm band depth is found indicating that larger sized S-type asteroids show finer grained surfaces.
•Reflectance spectroscopy•Unequilibrated Ordinary Chondrites (UOC)•Equilibrated Ordinary Chondrites (EOC)•S-type Asteroids•Asteroid parent bodies
Miller Range (MIL) 07687 is a peculiar carbonaceous chondrite officially classified as a CO3. However, it has been found to display unique petrographic properties that are atypical of this group. ...Moreover, Raman spectra of its polyaromatic carbonaceous matter do not reflect a structural order consistent with the metamorphic history of a type 3 chondrite. As a result, it has been suggested to be an ungrouped C2 chondrite with CO affinities, although it has not been fully excluded as a CO chondrite. The ambiguity of the meteorite’s classification is the motivation behind the present study. We conclude that MIL 07687 is a unique carbonaceous chondrite with possible affinities to CO, CM, and/or some ungrouped carbonaceous chondrites. The difficulty in classifying this meteorite stems from (1) its heavily weathered nature, which interferes with the interpretation of our oxygen (O‐)isotopic measurements; (2) the overlap in the petrographic and O‐isotopic descriptions of various COs, CMs, and ungrouped meteorites in the Meteoritical Society Database. Optical and infrared spectra are consistent with the meteorite’s unequilibrated nature and indicate that it is probably mildly aqueously altered. Despite traces of aqueous alteration having previously been described in MIL 07687, this is the first time that the presence of hydrated amorphous silicates is reported. In fact, our results show that its present hydration is beyond that of most CO3s, less than most CM2s, and comparable to primitive CR2s. Consequently, we support the meteorite’s C2‐ung label, although a CO2 or CM2 classification cannot be fully excluded.
Renazzo‐type (CR) chondrites are a relatively rare group of carbonaceous chondrites with the vast majority having escaped thermal alteration. This means that CRs are composed of relatively ...unprocessed material, depending on the extent of aqueous alteration they have experienced. Hydration in CRs ranges from incipient alteration of matrix glass, up to nearly complete replacement of the rock by hydration products. The extent of secondary processes is often difficult to assess in these meteorites, due to their heterogeneity and diversity of alteration products. Yet, this is crucial in order to understand the extent of geological processing that occurred on the primary parent body. Additionally, the parent asteroids of CRs remain a mystery, mainly because terrestrial oxyhydroxide signatures dominate the reflectance spectra of CRs. In this work, we have conducted optical and IR reflectance and transmission spectra of 25 CR chondrites in order to (i) better evaluate the extent of aqueous alteration that occurred on the CR parent body, and (ii) find possible parent body candidates. Terrestrial oxyhydroxides were removed from 12 samples, as these tend to interfere with the optical‐IR spectra of CRs. Our results suggest, among other, that (i) aqueous alteration in most of our CRs was limited to the matrix and (ii) most CRs may stem from a continuum of X‐to‐C complex asteroids, depending on their extent of aqueous alteration. More specifically, the endmembers being Xk/Xn types and Cgh/Ch types. This has strong implication in regard to what we can expect from the Psyche mission.
K- and L-type asteroids are considered to be the parent bodies of CV and CO chondrites. Spectral models of L-types invoke an enrichment in CAI with respect to the chondrites in the meteorite ...collection. Barbarian asteroids are associated to L-type asteroids yet the relationship between these populations is still not clear. We aim to investigate the link between the K- and L-type and Barbarian asteroids and the CV and CO chondrites by means of spectral matching of a large number of reflectance spectra of objects from either population. We seek to identify matches based on observed rather than modelled spectral features. We employ a matching criterion that accounts for the residuals and the correlation of the spectral features. The only free parameter in the comparison is the degree of alteration of the asteroids with respect to the meteorites expressed via an exponential model. We derive an absolute scale of similarity between the spectra using laboratory data from irradiation experiments. CVOxA chondrites are the best match to the asteroids, in particular to K-type (7 out of 11 asteroids matched) and Barbarians (11 out of 16). CO chondrites provide convincing matches for K-types (5 out of 11) and Barbarians (7 out of 16) as well. A single non-Barbarian L-type is matched to a meteorite. Only a few asteroids are matched to CVOxB and CVRed chondrites. Barbarian asteroids are represented among CO and CVOxA chondrites without requiring an enrichment of CAI in the asteroids. Four candidate Barbarian asteroids are identified, three of which are classified as K-types. These asteroids are favourable targets for polarimetric observations. The discrepancy between L-type asteroids and CV and CO chondrites is likely related to the ambiguity of the asteroid class itself. An extension of the taxonomy to include polarimetric properties is required.
Although spectral surveys and spacecraft missions provide information on small bodies, many important analyses can only be performed in terrestrial laboratories. For now, the total number of parent ...bodies represented in our meteorites collection is estimated to about 150 parent bodies, of which 50 parent bodies represented by the poorly studied ungrouped chondrites. Linking ungrouped meteorites to their parent bodies is thus crucial to significantly increase our knowledge of asteroids. To this end, the petrography of 25 ungrouped chondrites and rare meteorite groups was studied, allowing grouping into 6 petrographic groups based on texture, mineralogy, and aqueous and thermal parent body processing. Then, we acquired visible-near-infrared reflectance spectroscopy data, in order to compare them to ground-based telescopic observations of asteroids. The reflectance spectra of meteorites were obtained on powdered samples, raw samples and polished sections. Our results showed that sample preparation influences the shape of the spectra, and thus asteroid spectral matching, especially for carbonaceous chondrites. Overall, the petrographic groups defined initially coincide with reflectance spectral groups. We define links between some of the studied ungrouped chondrites and asteroid types that had no meteorite connection proposed before, such as some very primitive type 3.00 ungrouped chondrites to B-type or Cg-type asteroids. We also matched metamorphosed ungrouped carbonaceous chondrites to S-complex asteroids, suggesting that this complex is not only composed of ordinary chondrites or primitive achondrites, as previously established, but may also host carbonaceous chondrites. Conversely, some ungrouped chondrites could not be matched to any known asteroid type, showing that those are potential samples from yet unidentified asteroid types.
Miller Range (MIL) 07687 is a peculiar carbonaceous chondrite officially classified as a CO3. However, it has been found to display unique petrographic properties that are atypical of this group. ...Moreover, Raman spectra of its polyaromatic carbonaceous matter does not reflect a structural order consistent with the metamorphic history of a type 3 chondrite. As a result, it has been suggested to be an ungrouped C2 chondrite with CO affinities, although it has not been fully excluded as a CO chondrite. The ambiguity of the meteorite classification is the motivation behind the present study. We conclude that MIL 07687 is a unique carbonaceous chondrite with possible affinities to CO, CM and/or some ungrouped carbonaceous chondrites. The difficulty in classifying this meteorite stems from (i) its heavily weathered nature, which interferes with the interpretation of our oxygen (O-)isotopic measurements (ii) the overlap in the petrographic and O-isotopic descriptions of various COs, CMs and ungrouped meteorites in the Meteoritical society database. Optical and infrared spectra are consistent with the meteorite unequilibrated nature and indicate that it is probably mildly aqueously altered. Despite traces of aqueous alteration having previously been described in MIL 07687, this is the first time that the presence of hydrated amorphous silicates is reported. In fact, our results show that its present hydration is beyond that of most CO3s, less than most CM2s, and comparable to primitive CR2s. Consequently, we support the meteorite s C2 ung label, although a CO2 or CM2 classification cannot be fully excluded.
Recently published space-based observations of main-belt asteroids with the AKARI telescope provide a full description of the 3 {\mu}m band, related to the presence of OH bearing minerals. Here, we ...use laboratory spectra of carbonaceous chondrites obtained under controlled atmosphere (CI,CM,CO,CV,CR Tagish Lake) to derive spectral metrics related to the water content in the samples. After testing several spectral metrics, we use a combination of band depth at 2.75 {\mu}m and 2.80 {\mu}m that shows a correlation with H2O in the sample determined by TGA, though with a high uncertainty (4 wt% H2O). This relation is used to determine water content at the surface of large C-complex main-belt asteroids and discuss the origin of the variability found. On average C-complex Main-Belt Asteroids (MBA) have water contents of 4.5 wt% (volume average, (1) Ceres excluded), significantly lower than average CM chondrites. The estimated water content for the most hydrated asteroids are lower than those of the most hydrated meteorites, a difference that could be attributed to space-weathering. An anti-correlation is also present between water content and overall spectral slope, which is opposite to expectation from laboratory simulations of space weathering on dark carbonaceous chondrites. This suggests that part of the variability in the surface hydration among the different C-complex asteroids is not due to space-weathering, but to the composition of surface material. When applied to Ceres, the hygrometer presented in this work enables us to estimate that at least 1.22 wt% of the hydrogen is present in the form of organics. This richness in organics strengthens the connection between Ceres and cometary materials.
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