# Organic and mineralogic heterogeneity of the Paris meteorite followed by FTIR hyperspectral imaging

Z. DIONNET<sup>ID 1*</sup>, A. ALEON-TOPPANI<sup>1</sup>, D. BAKLOUTI<sup>1</sup>, F. BORONDICS<sup>2</sup>, F. BRISSET<sup>3</sup>, Z. DJOUADI<sup>1</sup>, C. SANDT<sup>2</sup>, and R. BRUNETTO<sup>1</sup>

*(Received 06 February 2018; revision accepted 27 June 2018)*

**Abstract**—Significant compositional and structural heterogeneity at nm to mm scales is an important characteristic of primitive extraterrestrial materials. Here, we report the analysis of high-resolution Fourier transform IR hyperspectral imaging analytical measurements at the micron scale on a fragment of the Paris carbonaceous chondrite, supported by Raman and SEM-EDS measurements. The fragment is crushed in a diamond compression cell. The micro-FTIR analyses are performed in transmission with two setups, an imaging microscope with a matrix detector using a thermal source and a system using a single point detector coupled with the synchrotron source at the SOLEIL synchrotron facility. We obtain the spatial distribution of chemical/mineralogical components. We confirm at a larger scale (10 µm) the presence of hydrated amorphous silicates observed at a smaller scale (1 µm). Based on the relative abundance of different minerals (hydrated amorphous silicate, olivine, diopside, and serpentine), we propose a sequence of aqueous alteration. Considering the spatial correlation of minerals with organic matter, we discuss the effects of aqueous alteration on the organic matter in bulk. In particular, we detect an increase in the CH₂/CH₃ ratio in the altered zone and present the possible scenarios that led to the observed chain length shortening/cracking of hydrocarbons.

## INTRODUCTION

Primitive extraterrestrial materials are characterized by large mineralogical and compositional heterogeneities at different scales: from nm to mm (Rietmeijer 1998). This heterogeneity is the result of a complex set of pre- and postaccretional processes (Alexander et al. 2007). For instance, aqueous alteration (Brearley 2006) and thermal metamorphism (Huss et al. 2006) can modify the matter inside the parent body. In the matrix of primitive meteorites, different phases are observed close to each other (e.g., amorphous and crystalline phases, minerals and carbonaceous materials; Le Guillou et al. 2014; Leroux et al. 2015), with very different formation and evolution histories. The compositional heterogeneity of extraterrestrial materials has been investigated in the

last decades with different techniques, such as scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), nanoscale secondary ion mass spectrometry (NanoSIMS), synchrotron-based X-ray techniques (XANES, STXM), micro-Raman spectroscopy, and infrared microspectroscopy and imaging (see e.g., Westphal et al. 2016).

Among these techniques, IR microspectroscopy is a powerful tool as it is nondestructive, able to characterize molecular vibrations and phonons, and in particular to detect both the mineral and the carbonaceous phases. It also allows hyperspectral imaging studies, useful to visualize the spatial distribution of different components and their assembly and thus to deduce constraints about their formation and their evolution in the young solar

<footnote n="1">¹Institut d'Astrophysique Spatiale, CNRS, Université Paris-Sud, Université Paris-Saclay, Bât. 121, 91405 Orsay Cedex, France</footnote>

<footnote n="2">²Synchrotron SOLEIL, l'Orme des Merisiers, 91192 Saint-Aubin, France</footnote>

<footnote n="3">³Institut de Chimie Moléculaire et des Matériaux d'Orsay, Université Paris Sud, Université Paris-Saclay, 91405 Orsay Cedex, France</footnote>

<footnote>*Corresponding author. E-mail: zelia.dionnet@u-psud.fr</footnote>
system. However, the analysis of different forms of extraterrestrial organic materials is challenging, because of their relatively low abundance with respect to other components (Le Guillou et al. 2014).

Carbonaceous chondrites and more specifically CM chondrites have accreted the primordial components of our solar system. Since their formation, they have undergone different degrees of aqueous alteration (and low metamorphism); the study of such primitive objects is key to understanding the composition of primitive solid matter. The evolution of the organic matter with hydrothermal action is still a matter of debate (Brearley 2006; Le Guillou et al. 2014) and infrared analyses could bring information about hydration of the component. Thanks to infrared analyses, we can study the organic matter knowing the mineral environment and deduce some information about its evolution under aqueous alteration on the parent body.

So far, IR microspectroscopy has been performed, at diffraction limited spatial resolution (2.5–15 µm in the mid-IR) using synchrotron radiation sources (e.g., Brunetto et al. 2011) or using focal plan array (FPA) imager for rapid acquisition. FPA imaging systems allow high-throughput analysis of large samples with resolutions close to the diffraction limit while synchrotron radiation microspectroscopy performed in confocal mode is better suited for obtaining information at the diffraction limit at specific locations. The use of FPA matrix detectors on IR microspectroscopy (see Yesiltas et al. 2014) for an application to meteorites) has triggered the development of FTIR microtomography (Martin et al. 2013; Quaroni et al. 2015).

The data obtained in the laboratory with infrared microspectroscopy are also comparable to the astronomical observations of the interstellar medium (ISM) and the primitive solar system small bodies (asteroids, comets, TNOs) (Bradley et al. 1999; Brunetto et al. 2011; Merouane et al. 2012; Vernazza et al. 2015). Infrared hyperspectral imaging is also widely used in planetary science. Imaging remote sensing spectrometers have been developed in the last decades, and these instruments have been successfully used onboard different exploration missions across the solar system (e.g., De Sanctis et al. 2017; for a recent example on dwarf planet Ceres). Developments of miniaturized IR hyperspectral imaging instruments for in situ exploration of planetary surfaces have achieved tens of microns spatial resolution (Pilorget and Bibring 2013).

The possibility to perform nondestructive laboratory-based micro-FTIR hyperspectral imaging will be very important in the coming years. Sample return missions Hayabusa 2 (JAXA) and OSIRIS-REx (NASA) will collect precious samples on primitive asteroids rich in carbonaceous materials (and probably

in water) and will bring them back to Earth to be studied in laboratory settings combining various analytical techniques. Similarly, the MMX (JAXA) mission will collect samples from Phobos. In the sequence of analyses, micro-FTIR hyperspectral imaging can provide a first, powerful nondestructive characterization of whole fragments, in order to identify areas of interest and provide useful information before subsequent destructive analyses.

In this paper, we present a series of micro-FTIR hyperspectral imaging measurements obtained on a fragment of the Paris meteorite. We study the spatial distribution and correlation of the different chemical and mineralogical components. The methods developed here can then be applied to other kinds of extraterrestrial materials to analyze their composition and heterogeneity.

## MATERIALS AND METHODS

The Paris meteorite is a CM 2.7–2.8 carbonaceous chondrite exceptionally well preserved from terrestrial alteration (Bourot-Denise et al. 2010; Zanda et al. 2010; Hewins et al. 2014; Leroux et al. 2015). It contains peculiar inclusions of organic matter with IR signatures similar to those of some interstellar carbons (Merouane et al. 2012). The heterogeneity of the organic matter has been recently described in Vinogradoff et al. (2017), where they observed two types of organic matter: submicrometric individual organic particles and diffuse organic matter finely distributed in the matrix. The Paris meteorite has two lithologies, which have undergone different degrees of aqueous alteration (Leroux et al. 2015). Less altered regions are rich in amorphous silicate with nanoinclusions of iron and sulfide-type GEMS (glass with embedded metal and sulfides). The matrix in the more altered regions is a mixture of amorphous silicates and Fe-rich poorly crystalline phyllosilicates containing fewer metal inclusions. The presence of amorphous silicates and metal inclusions in the two lithologies shows that the Paris meteorite has remained quite pristine (Hewins et al. 2014; Marrocchi et al. 2014).

## Sample Preparation

We analyzed a small fragment from the matrix of the Paris meteorite. The sample was provided by B. Zanda of the MNHN (Muséum National d'Histoire Naturelle de Paris). Unfortunately, the information on the lithology it originated from was not recorded at the time of extraction.

The sample was crushed (see Fig. 1) in a diamond compression cell, which is transparent in the IR range (similar to that described by Brunetto et al. 2011). After
<figure><img src="image_1.png" /><figcaption>Fig. 1. Optical image (obtained by reflectance) of the Paris fragment pressed in a diamond compression cell. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

crushing, we obtained a sample of 50 × 40 µm² and 3.0 ± 0.5 µm in thickness. During measurements the cell was opened and the sample was exposed to air.

## Sample Analyses

We used two apparatus based on IR microspec-
troscopy to obtain hyperspectral maps in transmission,
combined with Raman measurements. All these measure-
ments were performed at the SMIS (Spectroscopie et
Microscopie dans l'Infrarouge utilisant le Synchrotron)
beamline of the French synchrotron SOLEIL (Source
Optimisée de Lumière d'Energie Intermédiaire du
LURE). We describe below the specifications used for
each analytical technique. The spectral resolution was
4 cm⁻¹ for all the measurements and the spatial resolu-
tion, depending on the wavelength is given by Abbe's
equation (Hackett et al. 2015).

The first series of infrared transmission
measurements was performed with an Agilent (model
Cary 670/620) microspectrometer, with its internal source
(Globar), a 25× objective (numerical aperture 0.81)
amplified by a high magnification system (providing
additional magnification of 5×) placed in front of a
128 × 128 pixel FPA detector. We obtained a projected
pixel size of 0.66 × 0.66 µm and a field of view of
84 × 84 µm on the focal plane. The spectral range of
these measurements was 850–4000 cm⁻¹. We obtained a
spatial resolution between 3.3 µm (for a wavelength of
2.7 µm) and 12 µm (for a wavelength of 10 µm). Note
that the spatial resolution of our analysis is diffraction
limited regardless of the projected pixel size; however,
the dense sampling (0.7 µm) allows the detection of
structures smaller than the wavelength, as will be
shown in Fig. 4 (panels e and f) with the detection of
2- to 3-µm-sized carbonate and sulfate grains.

The second series of infrared maps (in transmission
mode) was collected using a Thermo Scientific Con-
tinµum XL microscope equipped with a 32× objective
(numerical aperture 0.65) coupled with a Nicolet 5700
FTIR spectrometer and illuminated by the synchrotron
IR source. We used an aperture of 6 µm × 6 µm and a
single element MCT detector operating in the 650–
4000 cm⁻¹ spectral range. In this case, point-to-point
mapping was performed to cover a chosen area on the
sample. In our experiment, mapping over an area of
80 × 80 µm with 1024 scans per spectrum lasted about
200 min. In the case of the FPA measurement, the whole
image of the FPA matrix was projected on the focal
plane, providing a field of view of 84 × 84 µm². All spec-
tra are thus collected simultaneously without moving the
X–Y stage of the microscope and therefore we can
directly image the sample with shorter acquisition time
(typically about 40 min for 1024 scans). Using these two
detectors, we studied the localization and spatial correla-
tion of different components in the fragment. In both
cases, the spectra were divided by reference spectra
acquired on a clean area of the diamond window.

We also performed Raman measurements to
confirm the identification of some minerals in the
meteorite and their accurate localization. We used a
confocal DXR Raman microspectrometer at the SMIS
beamline in SOLEIL with 532 nm excitation and
0.3 mW power. The sample was illuminated through an
Olympus UIS2 100X/0.90 NA LWD objective providing
a spot size of about 0.6 µm diameter.

Finally, some analyses were also performed with
SEM and we have mapped the repartition of magnesium,
silicon, calcium, and iron. SEM acquisitions were done
at 20 kV under 20 Pa nitrogen pressure on a Zeiss
Supra 55 VP FEG fitted with an EDAX EDS analytical
system.

## Data Analysis Procedure

First, we applied the principal components analysis
denoising method to obtain higher signal-to-noise ratio.
With this method, we decomposed the signal in principal
components (Caussinus 1986), then reconstructed the
dataset keeping 30 main components to significantly
reduce the noise. We first isolated the spectra containing
meteorological signatures (5729 spectra for Agilent dataset),
eliminating all the points of the substrate around the
sample for subsequent analyses.

Given the very large number of acquired spectra,
individual analysis of each spectrum would be extremely
time-consuming. Therefore, the following data analysis
protocol was established to capture the information in
the dataset. The chemical and mineral components of
the samples were thus evaluated in three steps:
1. A first inspection of the IR vibrational bands observed in the average spectrum of the whole sample revealed the main components such as hydrated amorphous silicate, carbonate, serpentine, and organic matter (identified through its aliphatic C-H moieties). These components were then mapped to investigate their distribution in the sample, revealing its heterogeneity. Several regions with different chemical compositions were thus determined from these maps.

2. For each region found in the previous step, we identified minor bands that were present in selected spectra of these regions.

3. Finally, we computed the second derivative for each spectrum in the dataset, and we studied the spatial distribution of the minima of the previously identified chemical component absorbance bands. This method allowed us to easily remove the spectral continuum and to better detect minor components. For instance, we can better separate the contribution of CH₂ and CH₃ aliphatic stretching modes, as shown in the Analysis of the CH2/CH3 Ratio section.

Knowing the major and minor IR components present in our sample, we have extracted, from each spectrum, the abundance of each component, which is proportional to the area under the corresponding band. For this, after subtracting a local linear continuum, we calculated the area under the band of one specific vibration mode for each component. Finally, the band strengths A (see Table 1) provided in the literature (Brunetto et al. [2011] and references therein) were used to deduce their column density N (molecules cm⁻²) thanks to the following formula: N = Area/A. We thus obtained spatial distributions of the main component abundance. For OH and CH bands, we normalized their areas to the silicate band area peaked at about 1035 cm⁻¹ (Matrajt et al. 2004) which can be used as a tracer of the quantity of matter (Yesiltas and Kebukawa 2016).

We also studied the spatial correlations by reporting the abundance of one component with respect to the abundance of another one for each spectrum. The dispersion of the scatter plot indicated the correlation between the chosen components (see Figs. 7 and 8).

## RESULTS

In this section we first list the components present in the sample, explore their spatial distribution, and focus on results from a few peculiar points useful for the discussion, such as the CH₂/CH₃ ratio or the spatial correlation between OH and CH stretching vibrations.

<table><caption>Table 1. Band strength for major IR bands measured according to Matrajt et al. (2005), Dartois et al. (2004), Jäger et al. (1998), and Brunetto et al. (2011).</caption><thead><tr><th>Component</th><th>Band strength<br>(×10<sup>-18</sup> cm/group)</th></tr></thead><tbody><tr><td>Silicates</td><td>200</td></tr><tr><td>Carbonates</td><td>1000</td></tr><tr><td>CH<sub>2</sub> symmetric stretching</td><td>2.4</td></tr><tr><td>CH<sub>3</sub> symmetric stretching</td><td>2</td></tr><tr><td>CH<sub>2</sub> asymmetric stretching</td><td>8.4</td></tr><tr><td>CH<sub>3</sub> asymmetric stretching</td><td>12.5</td></tr><tr><td>OH bending</td><td>210</td></tr></tbody></table>

## Average Spectra and Fragment Composition

In Fig. 2, we report the average spectra for the whole sample and the average of 10 spectra extracted from the three different regions of interest described below and measured with the FPA connected to the Globar source.

Our assignment is presented in Table 2 and is based on previous studies of meteorites or interstellar dust analogs (Matrajt et al. 2004; Dartois et al. 2005; Merouane et al. 2012; Yesiltas and Kebukawa 2016). The description of the corresponding modes is presented in these papers. The assignation of specific silicates is described below.

The main feature was the SiO silicate band around 1035 cm⁻¹, visible almost everywhere in our sample with variable intensities and structures. For the majority of the spectra and also for the average spectrum (see black spectrum in Fig. 2) we observed this feature as a broad peak, which is characteristic of a disordered structure, that is an amorphous structure (Tamanai et al. 2009) or at least a poorly crystalline silicate. We assign this almost ubiquitous large band to an amorphous or poorly crystalline silicate dominating the Paris sample. Indeed, the shape of the observed band and the position of the maximum is very similar to the one observed by Demyk et al. (2017), which synthetized Mg and Fe-rich amorphous silicates. In that paper, the degree of amorphization was confirmed by XRD diffraction and comparison with crystalline silicates was led (see also Boudet et al. 2005; Coupeaud et al. 2011). In Fig. 3, we compare the spectrum of synthesized amorphous silicate (pink dashed line) with the spectrum of the amorphous-rich region observed in our sample (red solid line). A similar band was also observed in chondritic smooth IDPs with a hydrated amorphous silicate by Bradley (2003).

Moreover, we observed some differences in the shape and peak position of the SiO band for some
<figure><img src="image_2.png" /><figcaption>Fig. 2. Spectra (a) between 2600 and 3720 cm⁻¹ and (b) between 850 and 1250 cm⁻¹ for different regions of the samples. We distinguished three different regions A, B, and C, corresponding to different composition (see Fig. 4). From bottom to top: the average spectrum of the whole sample (black), an average of 10 spectra extracted from the serpentine-rich region (extracted from region B, green), from the region rich in aliphatic CH (region C, blue), and from the amorphous silicate and olivine-rich region (region A, red). For clarity, all curves are spaced by 0.15 in the absorbance scale. (Color figure can be viewed at wileyonlinelibrary.wiley.com.)</figcaption></figure>

specific locations in our sample: the maxima of the band can be shifted by few tens of wavenumbers, the full-width half-maximum (FWHM) varies by ±10 cm⁻¹, and in some spots of the sample a peculiar structure of the peak appears. We used the term “amorphous silicate” loosely to refer to the spectra exhibiting a single broad SiO band (the structure could be poorly crystalline) and on the contrary, the detection of small structures inside this band indicates the presence of more organized silicates contributing to the SiO feature (see Fig. 3). This structuration is indicative of the local presence of diopside, serpentine, and olivine. We observed three bands at 1086, 1014, and 960 cm⁻¹ that can be assigned to serpentine (more specifically to chrysotile according to Salisbury et al. 1991; Beck et al. 2014 [see grey spectrum in Fig. 3]). In some spectra of this region, we have also observed a weak shoulder in the large O-H stretching band at 3660 cm⁻¹ that corresponds to a narrow band assigned to structural OH of a phyllosilicate phase. Raman analyses confirmed the detection of chrysotile in the same region

as will be shown in Fig. 5c (using band positions reported by Rinaudo et al. 2003). In another part of the sample, we observed peaks at 1069, 960, and 917 cm⁻¹ characteristic of diopside (Omori 1971) in the infrared spectra. The attribution of the diopside was confirmed by Raman analyses of this peculiar region and the detection of a band at 1010 cm⁻¹ (Huang et al. 2000) in the Raman spectra of the same region (see Fig. 5c). Finally, we observed the main band of the olivine in FTIR at 883 cm⁻¹, also detected the typical olivine doublet in the 818–846 cm⁻¹ spectral region in the Raman spectrum (see Fig. 5b). In both cases, the positions of the band(s) indicated a ratio Mg/(Mg+Fe) around 54%, according to Hamilton (2010) for the FTIR and Kuebler et al. (2006) for the Raman measurements. The olivine was hard to detect and we could detect it only in a few Raman spectra.

The aliphatic CH stretching bands around 2900 cm⁻¹ were also observed with slightly varying intensities throughout the sample. The detection of the bending (1500–1750 cm⁻¹) and the stretching (3000–
<table><caption>Table 2. Peak positions of the spectral bands observed in the sample.</caption><thead><tr><th>Position (cm<sup>-1</sup>)</th><th>Assignment</th></tr></thead><tbody><tr><td>Large band between 850 and 1140 with a maximum at 1035</td><td>"Amorphous" SiO stretching</td></tr><tr><td>880</td><td>Olivine SiO stretching (B3)</td></tr><tr><td>921</td><td>Diopside SiO stretching</td></tr><tr><td>960</td><td>Diopside/serpentine SiO stretching</td></tr><tr><td>1014</td><td>Serpentine SiO stretching</td></tr><tr><td>1069</td><td>Diopside SiO stretching</td></tr><tr><td>1086</td><td>Serpentine SiO stretching</td></tr><tr><td>1150–1180</td><td>Sulfate SO stretching</td></tr><tr><td>1256</td><td>CC aromatic skeleton</td></tr><tr><td>1326</td><td>CH<sub>3</sub> bending</td></tr><tr><td>1413</td><td>CH<sub>2</sub> bending</td></tr><tr><td>1410–1480</td><td>Carbonate CO stretching</td></tr><tr><td>1633</td><td>OH bending</td></tr><tr><td>2854</td><td>CH<sub>2</sub> symmetric stretching</td></tr><tr><td>2864</td><td>CH<sub>3</sub> symmetric stretching</td></tr><tr><td>2926</td><td>CH<sub>2</sub> asymmetric stretching</td></tr><tr><td>2958</td><td>CH<sub>3</sub> asymmetric stretching</td></tr><tr><td>3000–3750</td><td>OH stretching</td></tr><tr><td>3660</td><td>OH stretching mode of the hydroxyl groups in hydrated silicates</td></tr></tbody></table>

3750 cm⁻¹) OH modes indicated the presence of water in our sample.

Carbonate contribution was also visible around 1400 cm⁻¹. Thanks to the Raman spectra (see Fig. 5a) exhibiting peaks at 1085 and 281 cm⁻¹, we identified the calcium carbonate seen in IR as calcite (Rutt and Nicola 1974). The elementary composition provided by SEM-EDX analyses in the spots identified as calcite indicates that they are indeed enriched in calcium, which supports the calcium carbonate identification. Finally, the IR band at 1165 cm⁻¹ was attributed to sulfates (Salisbury et al. 1991; Yesiltas et al. 2014), more precisely to gypsum, an anhydrous calcium sulfate (Lane 2007). This attribution was confirmed by the Raman analyses through the detection of bands at 1015, 490, and 415 cm⁻¹ (Buzgar et al. 2009).

## Spatial Distribution of the Components Studied by Mid-Infrared

We mapped the spatial distribution of the different functional groups using the procedure described in the Data Analysis Procedure section by studying the area under the corresponding bands.

First, we studied silicate distribution. We identified an amorphous or poorly crystalline silicate phase in the majority of the spectra. Using the second derivative, we isolated the corresponding band from the contribution

<figure><img src="image_3.png" /><figcaption>Fig. 3. Spectrum from the amorphous region in red compared to amorphous spectrum from Demyk et al. (2017) in dashed pink and spectra from the serpentine region in black compared to the one from a pur serpentine in gray (Beck et al. 2014). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

of the other silicates and we obtained the distribution of the amorphous silicate shown in Fig. 4a. In all these spectra, we also observed the broad band of O-H stretching, indicating that the amorphous silicate is hydrated (this fact will be further supported below). Olivine is mainly located at the left of the sample (see Fig. 4b). We thus observed on the left of the sample a region rich in amorphous silicate and olivine (region A) and on the right of the sample a region (region C) rich in amorphous silicate but with lower olivine content. SEM analyses (see Fig. 4i) showed that these two regions are rich in iron compared to magnesium. Indeed, we measured a Mg/(Mg+Fe) ratio of 0.28 for region A and of 0.25 for region C.

Between these two regions, we observed a region with significantly different spectra (region B). We distinguished a structure inside the silicate band which indicates a crystalline structuration of the mineral phase (see green spectrum in Fig. 2). The map of the structural OH was obtained with synchrotron radiation. This suggested the presence of phyllosilicates in this region. Moreover, this region analyzed with the SEM showed enrichment in Si and Mg and depletion in Fe.
A ratio Mg/(Mg+Fe) of 0.66 in this region confirms the depletion of iron in this region. The detection of a Mg-rich region confirmed the identification of chrysotile (the magnesium pole of the serpentine). Inside region B, we identified a spot containing pyroxene (diopside) at the top of the sample.

Carbonates (see Fig. 4e) were also very localized with two very small and concentrated spots (2 × 3 µm) at the top of region B. Simultaneously, we have detected a spot of calcium sulfate, i.e., gypsum, nearby (see Fig. 4f).

The OH band was observed in almost every spectrum with different abundances across the sample. The distribution did not seem to be exactly the same for the OH stretching and bending modes. This may be due to the fact that the OH bending spectral region contains contributions from the C=C and C=N absorptions, besides the H₂O signatures. The OH stretching was less present in the crystalline silicate-rich region. Indeed, in the serpentine, the OH was present as structural OH.

Figure 4l shows the heterogeneous distribution of the aliphatic CH bands. Aliphatic CH stretching bands were intense in particular in one area: in a spot at the right of the sample (region C). The concentration in this area was at least three times higher than the average aliphatic C-H concentration in the whole sample whereas we found some spectra around the center of the sample in region A and on top of region B with a much lower content in aliphatic moieties. Olivine (see Fig. 4b) was colocated with the organic matter poor in aliphatic moieties in region A, and chrysotile (serpentine) is colocated with the one in region B.

Finally, we also studied the distribution of organic polyaromatic matter with Raman microspectroscopy (see Fig. 4h) since it provides complementary information to infrared and specific information on the polyaromatic carbons. We easily detected the D and G bands of polyaromatic carbons, similar to that measured by Merouane et al. (2012) in the bulk of the Paris meteorite, and we compared our observed peak intensity with the abundance of aliphatic CH detected with FTIR. It should be noted that absolute intensity values of the D and G bands do not necessarily correlate with actual concentrations of polyaromatic carbons, since Raman intensities are not quantitative, but it still gives an indication on the presence or absence of polyaromatic carbons.

We found that the aromatic C=C was present in the entire sample but more presented at the bottom of the sample (Fig. 4h; see Fig. 5d). Moreover, we did not detect significant variations in the position or FWHM of the G and D bands inside the sample. We also looked for possible spatial correlations between aliphatic CH and polyaromatic carbons, and found that the distribution of polyaromatic carbons was different

from the distribution of aliphatic CH observed in FTIR data. For instance, the aliphatic CH-rich spot did not correspond to a region rich in aromatic matter.

## Analysis of the CH₂/CH₃ Ratio

It was particularly interesting to focus on the CH₂/CH₃ ratio in the IR spectra to evaluate aliphatic chain lengths and branching levels (a lower CH₂/CH₃ ratio corresponds to shorter aliphatic chain lengths and/or higher branching levels, especially if we assume chemically similar initial organic compounds or mixtures which would have evolved separately or differently). We first determined this ratio from the average black spectrum shown in Fig. 2. For this, we fitted the aliphatic bands with five Gaussian profiles to better estimate the contribution of each vibration. The CH₂/CH₃ ratio (calculated after the correction by band strength) was derived from the asymmetrical bands (2926 cm⁻¹ for the CH₂ band and 2958 cm⁻¹ for the CH₃ band, see Merouane et al. [2012] and references therein). From Fig. 6b we found a value of 2.6 ± 0.2 (1.75 ± 0.1 before correction by band strength).

We checked for the variation of this ratio in our sample. For this, we used an analytical procedure to estimate the CH₂/CH₃ ratio for each of the 5729 selected spectra. A local baseline was used in the asymmetric-stretching region and a band area ratio was derived from each analyzed spectrum. We thus obtained the distribution shown in Fig. 6a. This figure shows varying values of the CH₂/CH₃ ratio and exhibits some hotspots, located in the top of region B, close to the carbonates, diopside, and serpentine grains. A double-check of the intensity scale provided in Fig. 6a was performed on some spectra, including the hotspots. We used the 5 Gaussian fitting procedure for the selected spectra and calculated the CH₂/CH₃ ratio. The maximum value found was 3.5 ± 0.2 (2.35 ± 0.1 before correction by band strength) in some spectra of region B, the most altered region in the sample. In region A, we obtained values around 1.75 ± 0.25 (see Fig. 6c) (1.18 ± 0.1 before correction by band strength).

## Correlation OH/CH

An interesting correlation diagram can be plotted between the abundance of water and aliphatic CH, as shown in Fig. 7a. We observed a distribution along a line, showing a correlation between water and aliphatic moieties signal with correlation coefficient value *r* = 0.86. The distribution of aliphatic carbon normalized to the quantity of matter was close to an asymmetric Gaussian profile (see Fig. 7b). From this distribution, we evaluated the median quantity *m* = 0.27
<figure><img src="image_4.png" /><figcaption>b) Olivine</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>c) Diospyside</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>d) Serpentine</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>e) Carbonates</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>f) Sulfates</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>g) Structural OH</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>h) Raman aromatics</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>i) Ca, Fe and Mg distributions</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>j) OH stretching</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>k) OH bending + C=C</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>l) Aliphatic matter</figcaption></figure>

<figure><img src="image_15.png" /></figure>
Fig. 4. Spatial distribution of (a) the amorphous silicates; (b) olivine; (c) pyroxene; (d) serpentine; (e) carbonate; (f) sulfate; (g) structural OH obtained with synchrotron radiation; (h) the polyaromatic carbons obtained with the Raman spectrometer; (i) distributions of abundance of calcium (red), iron (blue), and magnesium (green) obtained with the SEM; (j) OH stretching vibration; (k) OH bending vibration; and (l) the aliphatic CH groups. Except the distributions (g), (h), and (i), all the distributions were obtained with the FPA. The distributions (a), (b), and (c) (purple box) were obtained by the method of the second derivative. Please note that some artifacts are present in map (g) at the border of the meteorite, due to a difficult removal of the spectral continuum. All the distribution presented in the blue box were obtained with raw spectra by integration of the area under the corresponding band. The rings visible in panels (e) and (f) are due to diffraction limit. The black rectangle in panel (h) represents unavailable data. Finally distributions in the brown box were normalized to the distribution of silicates. The dashed white lines delimit the three main regions A, B, and C defined in the studied sample. (Color figure can be viewed at wile yonlinelibrary.com.)

of CH/SiO oscillators and the standard deviation, σ, calculated after the correction by band strength. We considered spots as particularly rich in CH, when CH/SiO values were 1σ higher than the median value, i.e., higher than 0.4 CH/SiO oscillators. Then we plotted the spatial distribution of these singular points (see Fig. 7c), and we found that almost all these spectra were located in region C, an OH-rich region. This confirmed the existence of a co-localization of water and aliphatic-rich organic matter in the sample. We also checked for the spatial distribution of spectra with high CH₂/CH₃ ratio (red dots in Fig. 7), and we did not observe any significant correlation with water or the total aliphatic carbon abundance. We stress that the points with high CH₂/CH₃ ratio are relatively poor in aliphatic bonds (see Fig. 7) but their spectra still have a signal/noise ratio high enough to evaluate the CH₂/CH₃ ratio correctly (see Fig. 6c).

The mean value of CH oscillators normalized to the number of SiO oscillators was around 0.25 (see Fig. 7b). By using the atomic percent of carbon measured in the matrix of the Paris meteorite by Noun et al. (2013), we estimated the total ratio of carbon with respect to silicon. Considering that our sample was large enough compared to the typical size of organic particles (Vinogradoff et al. 2017) to be representative of the matrix, we deduced that between one-third and half of the carbon present in the matrix was contained in CH₂ or CH₃ bonds, the rest being included in environments such as C=C, C=O, C=N, C≡C, C≡N.

DISCUSSION

Silicates

In our sample we detected regions mainly composed of amorphous silicates (regions A and C). With TEM analyses, Leroux et al. (2015) have also observed amorphous silicates on the Paris meteorite at a small scale (a few hundreds of nm) in the less altered lithology. Our observations confirm the presence of amorphous silicates at a bigger scale (the region rich in amorphous silicate is as large as 40 µm). This part of

our fragment probably came from the less altered lithology.

Figure 4 shows that the OH stretching and HOH bending modes were colocalized with the amorphous silicates. We plotted in Fig. 8 the correlation between the OH and SiO stretching bands in the amorphous silicates (following the example of Yesiltas and Kebukawa 2016). We observed a linear relationship with a small offset, which means that the water and the silicate abundances were correlated (r = 0.89). This suggests that water and/or hydroxyl groups are most probably contained in the amorphous silicate phase. Hydration of amorphous primitive chondrites has already been observed by Le Guillou et al. (2014). The offset of the linear correlation was 4.25, which represented only 5% of the average area of the OH band. This small offset corresponded to the contribution of water adsorbed on the sample surface as the linear regression presented here was done removing diopside- and serpentine-rich areas. Indeed, the presence of a diopside-rich area could also increase the offset: a second linear regression with all the points gave an offset of 6.75.

We also noticed that the diospide-rich spectra contained less water and were significantly below the correlation curves. Due to the pixel size, we did not observe a pure diopside spectra; the diopside was always mixed with some amorphous silicates. The small OH stretching contribution, seen in the diospide-rich spectra, came from the amorphous silicate phase. Besides, Fig. 8 shows that the average OH/SiO ratio in the amorphous silicates spectra was higher than the same ratio in the serpentine-rich spectra. Compared to the serpentine, the amorphous silicate phase contained on average more water and hydroxyl groups.

Finally, the IR spectra indicated that our sample was dominated by hydrated amorphous silicates. This result confirms the suggestion made by Leroux et al. (2015) about the probable hydration of the amorphous silicates they observed in the matrix of the Paris meteorite.

In addition to amorphous silicates, region A contained olivine (Fo₅₅). IR spectra indicated that the mixture between the amorphous phase and the olivine
<figure><img src="image_16.png" /></figure>

<figure><img src="image_17.png" /><figcaption>Fig. 5. Optical image showing in red the spots that have been explored by Raman spectroscopy and micro-Raman spectra showing (a) calcium carbonate and sulfate, (b) olivine, (c) diopside, and (d) D and G band polyaromatic bands. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

was intimate at a scale smaller than the size of the pixel (0.7 µm). This is in agreement with the TEM analyses performed by Leroux et al. (2015), where they observed small grains of forsterite at a scale of hundreds of nanometers embedded in amorphous structures. However, the ratio Mg/(Mg+Fe) measured in our sample is not in agreement with the one measured by Leroux et al. (2015). Moreover, studies of CM and C2 chondrites with MIR analyses have shown that olivine

is progressively replaced by OH-bearing phases through aqueous alteration (Beck et al. 2014). The olivine-poor region had a concentration of OH oscillators twice more significant than the concentration in olivine-rich region (region A). Note, that the iron-rich olivines in region A should have easily disappeared under aqueous alteration (Wogelius and Walther 1992). These observations corroborate the idea that regions B and C with a depletion in olivine underwent aqueous
<figure><img src="image_18.png" /><figcaption>Fig. 6. a) Distribution of the CH₂/CH₃ ratio. Points outside the meteorite were forced to zero. The distribution was spatially smoothed by a binning of 4 pixels. b) Fit of 5 Gaussian profiles obtained for the average spectrum, we obtained a ratio of 2.6 ± 0.2. c) Fit of 5 Gaussian profiles obtained for a spectrum in the north of the sample, we obtained a ratio of 3.5 ± 0.2. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

alteration. The olivine-rich amorphous region A showed
limited signs of alteration compared to region C that
appeared depleted in olivine and enriched in water.

Located between these amorphous-rich regions
(regions A and C), region B was rich in crystalline
minerals, such as serpentine, diopside, carbonate, and
sulfate. Leroux et al. (2015) detected some serpentine in
the part of the Paris meteorite which underwent
aqueous alteration. Vinogradoff et al. (2017) also
performed TEM studies of sections extracted from the
matrix of Paris and confirmed the observations made by
Leroux et al. (2015). In particular, both studies showed
evidence that amorphous silicates have been
progressively replaced by crystalline nano-phyllosilicates
in the matrix. Secondary alteration phases included
micron-sized carbonates, as those observed in region
B (see Fig. 4e). The detection of crystalline phases in
region B shows that this region underwent
more hydrothermal alteration compared with regions A
and C.

In summary, the study of the mineral components
in our sample in the context of what was measured by
Leroux et al. (2015) and Vinogradoff et al. (2017)
indicates that region A was amorphous-rich and olivine-
rich with little signs of alteration, region C underwent
moderate degrees of aqueous alteration leading to the
disparition of olivine and enrichment of OH-bearing

phases, and region B included phyllosilicates and
carbonates pointing to stronger degrees of aqueous
alteration. Such local heterogeneities were probably due
to different initial conditions (temperature, porosity,
pH, mineralogy, and composition) of alteration at a
micro-size scale (Brearley 2006).

Organic Matter

Paris, as a very primitive meteorite (Zanda et al.
2010; Hewins et al. 2014), has undergone limited
thermal metamorphism, thus providing an interesting
opportunity to study the link between very early
aqueous alteration and meteoritic organic matter. Many
studies have already been conducted on the soluble
organic matter (SOM; see Schmitt-Kopplin et al. [2010];
Martins et al. [2015] and references therein) and the
insoluble organic matter (IOM) of carbonaceous
chondrites (e.g., Kebukawa et al. 2011; Quirico et al.
2014). However, they often involved the destruction of
the sample and aggressive processes to separate the two
components (hydrolysis, harsh acid hydrolysis, solvent
extraction). In our case, we worked on the bulk organic
matter without extracting or separating the SOM or
IOM, and our nondestructive analysis provided
information on the diversity of the organic matter
preserving the mineral context. Here, we discuss
<figure><img src="image_19.png" /><figcaption>Fig. 7. a) Correlation diagram OH-CH: OH stretching/SiO ratio band ratio versus CH/SiO band ratio. b) Histogram distribution of the CH band area normalized by the abundance of SiO. c) Inset showing the spatial distribution of pixels (black points) with CH/SiO band area ratio 1σ higher than the median value. Red points in all panels correspond to pixels with CH₂/CH₃ values higher than 3.2. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

detection of spatial and compositional heterogeneities of
the organic matter.

The Aromatic Component

The organic matter of the Paris meteorite has been
recently studied by Vinogradoff et al. (2017). By

<figure><img src="image_20.png" /><figcaption>Fig. 8. Correlation diagrams between OH stretching and silicates. The blue distribution represents the amorphous silicate-rich pixels, the red one represents the serpentine-rich pixels, and the dark one represents the diopside-rich pixels. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

combining different techniques, they detected spatial
and compositional heterogeneities of the organics in the
matrix of Paris. They observed that organic particles are
embedded within the silicate matrix and frequently are
in contact with nanosulfides. More specifically, through
XANES and STXM studies they were able to detect a
spatial and compositional heterogeneity defining two
populations: submicrometric discrete organic-rich
particles and diffuse organics closely embedded within
silicates. The discrete particles were richer in C=C and
poorer in carboxylic groups than the diffuse organics.
Similar evidences of the existence of two populations of
organic materials have been reported by Le Guillou
et al. (2014) in the matrix of Renazzo (CR2),
Murchison (CM2), and Orgueil (CI). The size of the
discrete particles observed by Vinogradoff et al. (2017)
was a few hundreds of nm, which is unfortunately
smaller than the resolution of our microspectroscopic
analysis. Thus, it was not possible to clearly separate
the two populations in our hyperspectral maps.

However, Vinogradoff et al. (2017) noted that the
XANES signatures of the discrete particles within the
matrix were closer to the XANES signatures of the
extracted IOM of Paris than the diffuse organics. In
addition, Raman spectra of the Paris matrix reported
here and by Merouane et al. (2012) are essentially
indistinguishable from the Raman spectra of the Paris
IOM reported by Vinogradoff et al. (2017). In all these
studies, typical Raman G-band parameters were 1585–
1590 cm⁻¹ for peak position and 90–100 cm⁻¹ for the
FWHM, both in the IOM and in the bulk fragments of
Paris. All measurements agreed within typical error bars
of the chosen fitting functions. The Raman G and D
bands are known to be particularly sensitive to the
structure and degree of order of the polyaromatic component in complex carbons of meteorites (see e.g., Quirico et al. 2014). Based on this evidence, we suggest that the kind of discrete particles identified by Vinogradoff et al. (2017) were probably the main carriers of the Raman D and G bands we observed in the matrix of Paris (see Fig. 4h).

## Spatial Heterogeneity of the Aliphatics

In Figs. 4 and 6, we reported evidence of the detection of organic materials rich in aliphatic CH₂ and CH₃ bonds. This component was observed everywhere in the sample but its spatial distribution was not uniform. In particular, in region C we detected a hotspot of aliphatic carbon with a concentration between three and four times the average concentration in the whole sample. By using the band strength (Brunetto et al. 2011), we estimated that the ratio between aliphatics and silicates was 5% in mass on the average spectra (whole sample) and reached 12% in mass in region C, shown in Fig. 4 (panel l). The average value was consistent with the ones measured in other CMs (Anders and Zinner 1993). The hotspot of aliphatic organic matter observed in region C was also rich in amorphous silicates, in water (about two times richer than the average of region A), and in iron (see Fig. 4i).

## Compositional Heterogeneities of the Aliphatics

In addition to a nonuniform spatial distribution of the aliphatics, one can also study their compositional heterogeneity by evaluating the CH₂/CH₃ ratio. A study of the heterogeneity of the CH₂/CH₃ ratio was presented by Kebukawa et al. (2010) in the meteorite Bell (CM2) using near-field IR spectroscopy. They mapped the distribution of the CH₂ and the CH₃ vibrations at micron scale for a region of ~8 µm in size, then they extracted the ratio CH₂/CH₃, but they did not observe local heterogeneities.

First, we evaluated the average ratio CH₂/CH₃ in our sample, which was found to be 2.6 ± 0.2, a value in agreement with the one measured by Merouane et al. (2012) on the Paris meteorite. A ratio of 2.5 ± 0.4 was measured on bulk matrix fragments of Murchison (CM2) by Matrajt et al. (2005). This value is similar to the one measured in grains in the ISM. However, it is higher than the one of CM Murchison bulk (R = 1.0) and Bell bulk (R = 1.4) powders reported by Kebukawa et al. (2010). The estimated value of the CH₂/CH₃ ratio may slightly depend on the chosen fitting functions: our calculation on data provided by Kebukawa et al. (2010) with our method provided a value of about 1.4 ± 0.2.

Subsequently, we studied the local variations of the CH₂/CH₃ ratio. In region A of our sample (the least aqueously altered), the ratio CH₂/CH₃ decreased

to about 1.75 ± 0.25 (see Fig. 6a), while in region B (the most aqueously altered) it increased up to about 3.5 ± 0.2. Note that our study presents for the first time a heterogeneous CH₂/CH₃ ratio detected spectroscopically in the matrix of the Paris meteorite.

Some processes, such as the breaking of bonds by energetic particles or the aromatization and/or “polymerization” of aliphatic molecules by heating, can affect the chain length and branching level of aliphatics. For instance, some laboratory experiments on IDPs have shown that ion irradiation can decrease chain lengths and give lower CH₂/CH₃ ratios (see e.g., Muñoz-Caro et al. 2006). The effects of hydrothermal alteration on CH₂/CH₃ ratios are less clear. Kebukawa et al. (2011) proposed a classification of meteorites as a function of the ratio CH₂/CH₃ in the extracted IOM. They defined a group, which included the majority of the primitive IOM type 1 and 2 chondrites, characterized by low CH₂/CH₃ and high H/C ratios. Another spectral group, which included type 3.0 chondrites, was characterized by a slightly higher CH₂/CH₃ ratio and a lower aliphatic C–H intensity. Moreover, they noticed lower abundance of aliphatic moieties and lower abundance of O-bearing phase in more altered meteorites. They suggested this could be a signature of partial oxidation of aliphatic moieties during a mild parent body alteration. Orthous-Daunay et al. (2013) studied the variations of the IOM composition in meteorites showing different degrees of hydrothermal alteration. They concluded that the more altered is the meteorite, the higher is the CH₂/CH₃ ratio, and the more the IOM has an aromatization. There does not seem to be a clear similar tendency on bulk (Quirico, personal communication). A low CH₂/CH₃ ratio could be due to a partial oxidation of aliphatic during the mild parent body alteration (Kebukawa et al. 2011). We do not have evidence to go further.

In our sample, a peculiarly high CH₂/CH₃ ratio of the aliphatic carbon was observed in region B close to phyllosilicates and carbonates. As discussed above, based on the evolution of the silicates proposed by Leroux et al. (2015), we have inferred that this region is the most hydrothermally altered in our sample. Region B was also relatively poor in carbons with respect to region C. Higher CH₂/CH₃ ratios imply higher elongation of the aliphatic chains or lower degrees of branching and/or cross-linking. We conclude that longer aliphatic chains and/or a lower degree of ramification in region B were a result of the hydrothermal alteration.

## CONCLUSION

We have used the Paris meteorite to show the possibility to investigate the heterogeneity of primitive extraterrestrial materials via the combination of full field
IR imaging, IR microspectroscopy with synchrotron source, and Raman microscopy. Taking advantage of the high signal-to-noise ratio of the synchrotron source, the speed of analysis and high definition of the FPA images, as well as the high resolution and specificity of Raman microscopy, we have obtained a more comprehensive description of the composition of organic and inorganic matter in our sample.

We have detected several micro-environments corresponding to different degrees of aqueous alteration. Based on the study of the mineral phases, we have proposed a scenario which describes the effect of variable aqueous alteration inside our sample. The more primitive part of our fragment was mainly composed of small grains of olivine embedded in hydrated amorphous silicates. The aqueous alteration led, in the first stage, to the disappearance of olivine and enrichment of OH-bearing amorphous silicate. Finally, a higher degree of hydrothermal alteration led to the formation of phyllosilicates and carbonates.

Based on this information about the evolution inside the sample, we have characterized the effect of aqueous alteration on the organic matter. We have shown that the advantage of FTIR spectroscopy is the possibility to characterize all organic matter at once without chemical modification due to extraction, thus preserving the mineral context. We have observed an accumulation of aliphatic-rich organic matter in the amorphous silicate and water-rich region, that was also rich in iron. The most altered region was characterized by a lower abundance of aliphatics and a higher elongation of the aliphatic chains or lower degrees of branching and/or cross-linking.

*Acknowledgments*—This work is supported by the “IDI 2015” project funded by the IDEX Paris-Saclay (Grant ANR-11-IDEX-0003-02), and funded by the French national program “Programme National de Planetologie” (PNP) and by the ANR project RAHIIA SSOM (Grant ANR-16-CE29-0015-01) of the French Agence Nationale de la Recherche. The microspectroscopy activities are supported by grants from Région Ile-de-France (DIM-ACAV) and SOLEIL. We thank L. d’Hendecourt, E. Quirico, and B. Zanda for fruitful discussions, and P. Beck and Y. Kebukawa for providing IR data of meteorites. We thank two anonymous referees for constructive comments on this manuscript.

*Editorial Handling*—Dr. Scott Sandford

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