Bidirectional reflectance spectroscopy of carbonaceous chondrites: Implications for water quantification and primary composition
Introduction
Asteroids contain refractory materials that are expected to have only been marginally affected by chemical processes since the formation of the Solar System. They can be considered as geological antiques that are expected to preserve precious clues to the birth of our planetary system. The main-belt is spectroscopically diverse suggesting that a variety of mineralogical compositions is present. Large spectroscopic surveys combined with dynamical studies have led to the classification of asteroid families (taxons), some of which have related meteorite groups (Tholen, 1984, Bus and Binzel, 2002, Gaffey et al., 1993, Vilas, 1994, Burbine, 1998, McSween et al., 2002, DeMeo and Carry, 2014). However, in the case of the darkest objects (the C-complex, and the D-types from DeMeo et al. (2009)) associations to specific meteorite groups can be difficult, because of the lack of diagnostic absorptions in the visible and near-infrared (VNIR).
Major spectroscopic surveys are limited to the VNIR and typically lack the 3-μm feature, which is the most diagnostic for –OH/H2O bearing phases (water–ice, hydrated hydroxylated minerals, alcohol function). Such “water” related absorptions have been observed on asteroidal surfaces (Lebofsky et al., 1981, Larson et al., 1983, Jones et al., 1990, Rivkin et al., 2002), but observations in this region are usually limited to large objects and the effort of building a taxonomy based on this spectral range is ongoing (Takir and Emery, 2012). At present, most observations at 3-μm have been interpreted by the presence of –OH bearing mineral phase, and a few of them by the presence of water ice (mostly outer main belt objects, Campins et al., 2010, Rivkin and Emery, 2010, Takir and Emery, 2012; see also Beck et al. (2011) for an alternative – goethite). Across the main belt, 3-μm absorption bands have been found for different asteroid classes with variations in band shape and band depth (Takir and Emery, 2012, Rivkin et al., 2003). Some other asteroid classes do not present detectable 3-μm absorption features.
Some meteorite groups also show clear evidence of hydration in the form of secondary minerals formed under low-temperature, by precipitation from an aqueous fluid. This is particularly evident for some carbonaceous chondrite classes, including the CI group. The CI group is an extreme case of aqueously altered meteorites since almost all primary minerals were transformed to secondary phase, including phyllosilicates (a mixture of clays and serpentine; Tomeoka and Buseck, 1988). This aqueous alteration event was heterogeneous across the different carbonaceous chondrite families and within a given group. From the nature and amounts of secondary mineral phases, alteration sequences have been discussed (e.g. McSween, 1979, Bunch and Chang, 1980, Tomeoka and Buseck, 1985, Zolensky and McSween, 1988, Takir et al., 2013). Recently, aqueous alteration scales have been constructed based on petrography and crystal chemistry (Rubin et al., 2007, Harju and Rubin, 2013), phyllosilicate abundance (Howard et al., 2009, Howard et al., 2011, Howard et al., 2015, Beck et al., 2014, Garenne et al., 2014) and C and H isotopic analyses (Alexander et al., 2012, Alexander et al., 2013).
Our objective here is to compare the spectral metrics of aqueously altered carbonaceous chondrites in reflectance, with an emphasis on the 3-μm region for comparison with remote observations of small bodies. The reflectance spectra were measured using a set of 23 CMs, 5 CRs from Antarctica and 4 chondrite falls (Orgueil (CI), Murchison (CM), Allende (CV) and Tagish Lake (C2)). Spectra were obtained under vacuum and moderate temperature to minimize contamination by adsorbed water, and with a high photometric accuracy (<0.25%). These same specific samples of each meteorite were previously studied by infrared spectroscopy in transmission (Beck et al., 2014) and with thermogravimetric analyses (Garenne et al., 2014) to evaluate modal mineralogy and water abundance. Many of these meteorites also had modal abundances determined by X-ray diffraction, although on different powder aliquots. Based on these two studies, we calculate various spectral metrics related to the 3-μm band and try to identify the most reliable way to quantify hydrogen on these dark meteorites and improve our understanding of the variability of the 3-μm band on low albedo asteroids.
Section snippets
Meteorite samples studied
Reflectance spectroscopy was performed on 24 CMs, 5 CRs, 1 CI and 1 CV and one C2 chondrites (Table 1). Here we focused in particular on CM, CR and CI chondrites since they have been described in several articles as being significantly altered meteorites groups. These three groups have very distinct petrographical properties, different matrix proportion, carbon content, abundances of opaque phases, metal, secondary mineral phases, and water content (Brearley, 2006, Weisberg and Huber, 2007,
Reflectance spectra of Antarctic CMs
All spectra acquired for CM chondrites (excluding the fall Murchison) are presented in Fig. 1. CM reflectance spectra are characterized by five typical criteria (Cloutis et al., 2011b): (1) A generally low reflectance (R); (2) A deep 3-μm band due to hydration (hydroxylated minerals, H2O-bearing minerals); (3) a silicate band at 0.7 μm due to Fe3+–Fe2+ charge transfer; (4) a silicate band around 0.9–1.2 μm due to Fe2+ crystal field transitions; (5) a band around 3.4–3.5 μm which can be assigned to
Grain size?
Grain size can have a first order effect on the reflectance spectra of pure minerals. In general, decreasing grains size tends to increase the number of scatterings and leads to an increase in reflectance. When the grain size approaches that of the incident wavelength, a change of scattering regime occurs leading to a decrease in reflectance (Mustard and Hays, 1997). In the case of carbonaceous chondrites, which are fine-scale mixture of components with contrasted optical properties, the grain
Conclusion
In this work, reflectance spectra of 32 carbonaceous chondrites (24 CMs, 5 CRs, 1 CI, 1 CV, Tagish Lake) were measured with high photometric accuracy (<0.0025 in reflectance) in the 0.5–4 μm spectral range. From these data, it is possible to draw the following conclusions regarding the spectral properties of these materials and the capability to infer parent body processes on low albedo asteroids from reflectance spectroscopy.
The reflectance (taken at 2 μm in this work) is roughly correlated with
Acknowledgments
The Meteorite Working Group and the Antarctic Meteorite Research Program are acknowledged for providing the samples. Funding and support from CNES, the Programme National de Planétologie as well as Grant ANR-10-JCJC-0505-01 from the Agence Nationale de la Recherche are acknowledged. KTH was supported by NASA Cosmochemistry Grant NNX14AG27G.
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