Transmission infrared spectra (2–25 μm) of carbonaceous chondrites (CI, CM, CV–CK, CR, C2 ungrouped): Mineralogy, water, and asteroidal processes
Introduction
The mineralogy of chondrites is an integrated record of presolar, nebular and asteroidal histories. The complexity of their petrology is reflected in the menagerie of mineral phases that they contain (Brearley and Jones, 1998, Rubin, 1997). Chondrites are composed of 15 groups out of which 7 belong to the so-called carbonaceous chondrites (CCs) class. CCs share specific compositional characteristics, in particular within the O and Cr isotopes spaces (Clayton and Mayeda, 1999, Warren, 2011a). They are considered as primitive Solar System samples for the similarity of their chemistry with the solar photosphere (Barrat et al., 2012). Nevertheless, they all experienced often extensive parent body processing.
The observations of chemical re-equilibration between mineral phases, of re-crystallization and growth of new minerals, as well as thermo-luminescence studies indicate that thermal metamorphism occurred in the parent bodies of all chondrites groups, including CCs (Dodd et al., 1967, McSween, 1977, Guimon et al., 1995). Structural and chemical modifications in the organic matter of CCs have also been observed by Raman and infrared (IR) spectroscopy and attributed to long and short duration metamorphisms (Bouvier et al., 2007, Bonal et al., 2006, Busemann et al., 2007, Cody et al., 2008, Quirico et al., 2011, Orthous-Daunay et al., 2013). The heat source required for metamorphism can be either radiogenic heating by short-lived radionuclides (Grimm and McSween, 1989, Grimm and McSween, 1993), heat deposited during impact and subsequent burial within ejecta blankets (Warren, 2011b), or possibly solar heating for asteroids whose orbital perihelia is close enough to the Sun (Chaumard et al., 2012).
CCs have also interacted with water-rich fluids in their parent body, leading to the hydrolysis of primary constituents and subsequent crystallization of secondary minerals. This process, usually referred to as aqueous alteration, has been described for most CCs groups, including CV, CR, CI and CM (see the review by Brearley (2006) and reference therein). Within these groups, there is variability in the abundances and the nature of the secondary minerals, leading to scales of aqueous alteration (McSween, 1979, Zolensky et al., 1993, Rubin et al., 2007, Rubin and Harju, 2012, Howard et al., 2009, Howard et al., 2010, Howard et al., 2011, Alexander et al., 2013, Takir et al., 2013).
The water responsible for the alteration can be ‘preserved’ within hydrous minerals and can be retained by these minerals when heated to several hundreds of degrees centigrade. The contribution of CCs as a source for the Earth water has been much debated, for instance from the perspective of hydrogen isotopes (Deloule et al., 1998, Alexander et al., 2012, Bonal et al., 2013).
In this paper, we use IR spectroscopy to try to characterize the amount of water in CCs and to understand the factors controlling the observed diversity in the extent and mineralogy of alteration. This method can relatively rapidly probe both abundances and speciation of water, and provide insights into the mineralogy of the samples (e.g., Osawa et al., 2005, Beck et al., 2010). These transmission spectra are fairly comparable with observational spectra measured from thermally radiating sub-micrometer sized dust particles in cometary or asteroidal environments (Crovisier et al., 1997, Emery et al., 2006, Vernazza et al., 2010, Licandro et al., 2011). They might also provide points of comparison to accretion and debris disks (Morlok et al., 2012, Olofsson et al., 2012).
Section snippets
Samples
In this study, chondrites from the CI, CM, CR, CV–CK groups were analyzed. These four groups of meteorites were selected because they show evidence of aqueous alteration as described extensively in the literature. In addition, samples that are classified as C2 ungrouped were also studied. The sample list is given in Table 1.
Two CI chondrites were analyzed, Orgueil and Ivuna. The CM group is the largest CC group, and 21 samples were selected for the present study. Most of them are from the
The IR spectra of silicates in the 2–25 μm range
The 2–25 μm range is dominated by SiO4 related vibrations, whether as stretching modes of Si–O within tetrahedron around 10 μm, or as bending modes at higher wavelength. In the case of OH-bearing minerals, additional absorptions are present around 3 and 6 μm due to –OH stretching, X–OH bending and combinations of the two, and at higher wavelengths due to –OH libration (in the 12–16 μm range). Characteristic transmission spectra of terrestrial silicate minerals are presented in Fig. 1. From the
Quantifying the level of hydration
Because a given secondary mineralogy is controlled by a multiplicity of factors (P, T, fO2, pH, pE, rock/fluid ratio, etc.) that potentially vary between and within parent bodies, ordering meteorites according to their degree of aqueous alteration is a difficult task. The approach followed here is to quantify aqueous alteration by the amount of H2O and –OH contained by a sample, following Howard et al., 2009, Howard et al., 2010, Howard et al., 2011 and Howard and Alexander (2013) who measured
CM and CI chondrites: which phyllosilicates?
CM chondrites are the most abundant CCs and an extensive literature exists on the composition of their phyllosilicates. According to numerous studies, the dominant phases belong to the serpentine group, typically a mixture of Fe-rich cronstedtite and Mg-rich serpentine (see Brearley, 2006). However, from our observations, none of the standard serpentine phases have an IR spectra that matches that of CM chondrites.
The chemical composition of matrix phyllosilicates has often been inferred from
Conclusions
This work demonstrates the potential for the classification of CCs based on IR spectroscopy. This method, which is inexpensive in term of the amount of time and material required, is able to unraveling the mineralogical diversity of these meteorites. In this study, the diversity observed can be understood in the light of parent body processes. The following conclusions can be drawn:
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It is essential to characterize meteorites with respect to mineral standards to understand their spectroscopic
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. M. Hadjikrikorian is deeply thanked for her help in the sample preparation. The authorship of Gicquel et al. (2012) is also thanked for sharing the Spitzer data of Tempel 1 dust. We are very grateful to C.M.O’D.
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