# The Paris CM chondrite: Secondary minerals and asteroidal processing

Yves Marrocchi, Matthieu Gounelle, Ingrid Blanchard, Florent Caste, Anton

T. Kearsley

► To cite this version:

Yves Marrocchi, Matthieu Gounelle, Ingrid Blanchard, Florent Caste, Anton T. Kearsley. The Paris CM chondrite: Secondary minerals and asteroidal processing. Meteoritics and Planetary Science, 2014, 49 (7), pp.1232-1249. 10.1111/maps.12329. hal-01053040

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# The Paris CM chondrite: Secondary minerals and asteroidal processing

Yves MARROCCHI¹,²*, Matthieu GOUNELLE³,⁴, Ingrid BLANCHARD³,⁵,
Florent CASTE³,⁶, and Anton T. KEARSLEY⁷

¹Université de Lorraine, CRPG, UMR 7358, Vandoeuvre les Nancy, F-54501, France

²CNRS, CRPG UMR 7358, Vandoeuvre les Nancy, F-54501, France

³Laboratoire de Minéralogie et de Cosmochimie du Muséum, MNHN and CNRS, UMR 7202, 75005 Paris, France

⁴Institut Universitaire de France, Maison des Universités, 103 boulevard Saint-Michel, 75005 Paris, France

⁵Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot,
UMR 7154 CNRS, F-75005 Paris, France

⁶Institut de Minéralogie et de Physique des Milieux Condensés, UMR 7590,
CNRS-UPMC, Campus Jussieu, 4 place Jussieu, 75005 Paris, France

⁷Imaging and Analysis Centre, Department of Science Facilities, Natural History Museum, London SW7 5BD, UK

*Corresponding author. E-mail: yvesm@crpg.cnrs-nancy.fr

*(Received 02 December 2013; revision accepted 06 May 2014)*

**Abstract**–We report a petrographic and mineralogical survey of Paris, a new CM chondrite considered to be the least-altered CM identified so far (Hewins et al. 2014). Compared to other CMs, Paris exhibits (1) a higher concentration of Fe-Ni metal beads, with nickel contents in the range 4.1–8.1 wt%; (2) the systematic presence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains; and (3) ubiquitous tochilinite/cronstedtite associations with higher FeO/SiO₂ and S/SiO₂ ratios. In addition, Paris shows the highest concentration of trapped ³⁶Ar reported so far for a CM chondrite (Hewins et al. 2014). In combination with the findings of previous studies, our data confirm the reliability of (1) the alteration sequence based on the chemical composition of tochilinite/cronstedtite associations to quantify the fluid alteration processes and (2) the use of Cr content variability in type II ferroan chondrule olivine as a proxy of thermal metamorphism. In contrast, the scales based on (1) the Fe³⁺ content of serpentine in the matrix to estimate the degree of aqueous alteration and (2) the chemical composition of Fe-Ni metal beads for quantifying the intensity of the thermal metamorphism are not supported by the characteristics of Paris. It also appears that the amount of trapped ³⁶Ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites, for both aqueous alteration and thermal metamorphism. Considering Paris, our data suggest that this chondrite should be classified as type 2.7 as it suffered limited but significant fluid alteration and only mild thermal metamorphism. These results point out that two separated scales should be used to quantify the degree of the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of CM chondrites.

## INTRODUCTION

Carbonaceous chondrites are composed of four main components formed in the accretion disk 4.56 Gyr ago: refractory inclusions (CAIs: Ca-Al rich inclusions), chondrules, Fe-Ni metal, and fine-grained matrix. These materials sampled diverse environments in the early solar system and appear to offer insights into the high-temperature primary processes that took place during

the evolution of the accretion disk. Chondrules, CAIs, metal, and matrix were subsequently accreted together to form asteroidal parent bodies that represent the source of the unequilibrated meteorites. Thus, chondrites can be regarded as clastic sedimentary rocks. Despite their complexity, chondrites represent our only witnesses of the formation of the solar system. However, most chondrites experienced secondary alteration, resulting in changes to their primary texture,
mineralogy, and chemical and isotopic compositions (Brearley 2006; Huss et al. 2006). These secondary features form the basis of meteorite classification (Van Schmus and Wood 1969), with a scale (from 1 to 6) describing the progressive roles of (1) low temperature aqueous alteration (from type 3 to 1) and (2) thermal metamorphism (from type 3 to 6). Despite being commonly used, this scale is limited as it does not distinguish between two unrelated processes that could have affected chondrites, simultaneously or at different times, during the geological evolution of asteroids. Thus, two separate scales are probably required to properly describe the respective role of aqueous alteration and thermal metamorphism in establishing the characteristics of chondrites.

Among the carbonaceous chondrites, CM chondrites are the most abundant group, representing around 30% of falls and finds on Earth. CM-like matter also constitutes an important fraction of xenoliths discovered in meteorites (Gounelle et al. 2003; Briani et al. 2012), attesting to the importance of these types of meteorite in the formation and the evolution of the solar system. CM chondrites exhibit fairly homogeneous bulk chemical compositions characterized by (1) moderate enrichment of refractory elements relative to CI chondrites; (2) depletion of elements more volatile than Mg relative to CI chondrites; (3) specific Ca/Mg, Fe/Mg, Cr/Mg and Mn/Mg; and (4) flat REE abundance patterns (Wasson and Kalllemeyn 1988). In addition to these chemical features, CM chondrites have clearly recorded the action of an aqueous fluid at some point in their history, as revealed by the presence of a variety of hydrous minerals such as serpentine and clays. The CM chondrites represent the largest group of hydrated meteorites and span a wide range of conditions, from less altered (i.e., CM2) down to heavily altered (i.e., CM 1) (Zolensky et al. 1997). In the past decades, several studies have attempted to establish a coherent scale for determining the degree of aqueous alteration of CM chondrites. Based on modal abundances and the Fe/Si ratios of fine-grained matrix, McSween (1979) proposed an alteration sequence subdivided into three categories: partially altered, altered, and highly altered. The scale was later extended following the identification of several mineralogical and textural properties that were interpreted as reflecting progressive CM aqueous alteration (1) the chemical compositions of hydrated minerals and (2) the degree of preservation of isolated silicates in fine-grained matrix material and mafic silicates within chondrules (Browning et al. 1996). These features were used to establish the mineralogical alteration index (hereafter MAI), which characterizes the degree of aqueous alteration of each CM chondrite (Browning et al. 1996).

More recently, Rubin et al. (2007) proposed a more comprehensive version, based mainly on the changes in chemical composition (i.e., FeO/SiO₂ and S/SiO₂) of large clumps of serpentine-tochilinite intergrowths, which allows a petrological subtype to be directly attributed to any given CM chondrite. In addition, a complementary scale has also been proposed to determine the relative classification of slightly altered CM chondrites based on the extent of kamacite corrosion by S-rich water (Palmer and Lauretta 2010).

In addition to aqueous alteration, it was rapidly recognized that some CM chondrites had also undergone secondary episodes of metamorphic heating that induced partial or complete dehydration of hydrous minerals (Nakamura 2005; Tonui et al. 2014). According to synchrotron X-ray diffraction data, Nakamura (2005) proposed classification of heated CM chondrites into four groups, with the intensity of metamorphism increasing from stages I to IV. More recently, it was suggested that the thermal history of CM chondrites could be deciphered by studying the composition and texture of Fe-Ni metal and sulfide phases (Kimura et al. 2008, 2011). The authors used these characteristic features to classify CM chondrites into three categories (1) unheated (category A), (2) mildly metamorphosed (category B) and (3) highly metamorphosed (category C).

The mineralogical and petrographic observations of CM chondrites thus suggest they are unique meteorites that may have undergone both aqueous alteration and thermal metamorphism (Kimura et al. 2011). As previously described, several features have been proposed for evaluating the intensity of these processes and to establish a coherent classification of CM chondrites. However, the absence of an unaltered CM chondrite has prevented us from anchoring these scales to a hypothetical type 3.0 and has consequently led to some uncertainty in attributing absolute petrographic subtypes to CM chondrites. Recently, a new CM chondrite, Paris, has been proposed as a type 2.9 ± 0.1 chondrite, based on its low degree of aqueous and thermal secondary alteration (Bourot-Denise et al. 2010; Hewins et al. 2014). This presents a good opportunity to address the pertinence of the different indicators of secondary alteration undergone by CM chondrites (i.e., aqueous alteration and thermal metamorphism). In order to investigate this, we performed a petrographic and mineralogical survey of the Paris CM, with the objective not of providing an extensive description of this chondrite, which has already been published (Hewins et al. 2014), but rather of focusing on a discussion of the diverse indicators of aqueous and thermal secondary modifications of primitive meteorites.
<figure><img src="image_2.png" /><figcaption>Fig. 1. a) Composite backscattered electron image of Paris-6, one of the three sections of Paris surveyed in this study. b) Composite EDX elemental map of Mg (red), Al (blue), and Ca (green) of Paris-6 revealing the distribution of chondrules, CAIs, and matrix. (see online version for color figure.)</figcaption></figure>

## ANALYTICAL PROCEDURE

Elemental distribution maps were obtained on the three entire sections at the Natural History Museum in London, using a Zeiss EVO 15LS scanning electron microscope (SEM) equipped with an Oxford Instruments XMax silicon drift energy dispersive X-ray (EDX) detector, and INCA version 18 software (Fig. 1). Backscattered electron images and EDX maps were acquired at an acceleration voltage of 20 kV and 3 nA beam current. Due to the size of the samples (each approximately 3.5 cm²), coverage at high resolution required over 200 separate maps to be collected overnight per section (e.g., 224 maps to create Fig. 1), using the Smartmap automated-stage raster procedure. For each X-ray map area, multiple frames of 1024 by

768 points were collected, with a pixel-spacing of 1.52 µm in the X and Y axes. X-ray emission spectra of 0–10 keV energy (in 2048 channels) were stored for each pixel location. Elemental X-ray maps were subsequently extracted from the data set (approximately 1.2 GB), and combined in a conventional Mg = red, Al = blue, Ca = green color scheme to reveal areas of specific interest (Fig. 1), from which spectra were extracted to identify phases. Subsequent detailed observations of the three sections were performed at CRPG (Nancy, France) using a JEOL JSM-6510 equipped with an EDX Genesis X-ray detector, using a 3 nA primary beam at 15 kV. We also performed multielement EDX mapping (Mg, Si, Fe, Ni, S, Na, Ca, and Al) of the different components present in Paris (e.g., CAIs, chondrules...). Modal abundances of CAIs, chondrules, matrix, Fe-Ni metal, sulfides, and carbonates were determined microscopically at high magnification in reflected light using an automated point counter. In each section, 333 points were performed on a surface area of 66.5 mm².

Quantitative analyses of the mineralogical compositions were made with a CAMECA SX-50 and SX-100 electron microprobe (CAMECA, Paris, France) at the University of Paris VI. A 10 nA focused beam (≈2 µm), accelerated to 15 kV potential difference, was used for spot analyses of silicates, oxides, metals, and sulfides with 20 s analysis times. For the analysis of metal and sulfides, we used Fe, Ni, Co, and Cr-metals, and pyrite and schreibersite as standards for Fe, Ni, Co, Cr, S, and P, respectively. The detection limits obtained were estimated to be 0.03 wt% for Fe, Co, and P; 0.08 wt% for Ni; and 0.006 wt% for S and Cr. Detection limits in silicates were 0.03 wt% for SiO₂, Al₂O₃, CaO, and MgO; 0.04 wt% for Na₂O, K₂O, TiO₂, and P₂O₅; and 0.07 wt% for NiO, Cr₂O₃, and FeO.

## RESULTS

No significant differences between the three sections were revealed in the detailed surveys. For example, the modal abundance determinations from elemental distribution maps presented homogeneous results (Table 1) with average distributions of 63% matrix (σ = 2.1), 28% chondrules (σ = 2.0), 4% sulfides (σ = 2.1), 2% metal (σ = 2.1), 2% carbonates (σ = 0.6), and 1% CAIs (σ = 0.3). From hereon, we will therefore discuss the three sections together.

In total, 32 CAIs were detected, ranging in mean size from 60 to 470 µm. Spinel (MgAl₂O₄) is the major mineral, occurring mainly as massive nodules (Fig. 2; Table 2). The most abundant refractory inclusions in Paris consist of spinel mantled by diopside, followed by those composed mainly of spinel, spinel-pyroxene-olivine, and spinel-hibonite (Table 2). Grains of
<figure><img src="image_3.png" /><figcaption>Fig. 2. a–d) Backscattered electron images of four representative fine-grained Ca-Al-rich inclusions observed in Paris. Most of them present important alteration features indicated by their fragmented structures and the presence of rims mainly composed of phyllosilicates.</figcaption></figure>

<table><caption>Table 1. Modal abundance (vol%) of major components observed in the three sections of Paris.</caption><thead><tr><th>section no.</th><th>5</th><th>6</th><th>7</th><th>ave.</th><th>σ</th></tr></thead><tbody><tr><td>Matrix (%)</td><td>61</td><td>65</td><td>64</td><td>63</td><td>2.1</td></tr><tr><td>Chondrules (%)</td><td>28</td><td>26</td><td>30</td><td>28</td><td>2.0</td></tr><tr><td>Fe-Ni metal (%)</td><td>2</td><td>2</td><td>0.5</td><td>2</td><td>0.9</td></tr><tr><td>Sulfides (%)</td><td>6</td><td>3</td><td>2</td><td>4</td><td>2.1</td></tr><tr><td>CAIs (%)</td><td>1</td><td>1</td><td>1.5</td><td>1</td><td>0.3</td></tr><tr><td>Carbonates (%)</td><td>2</td><td>3</td><td>2</td><td>2</td><td>0.6</td></tr></tbody></table>

perovskite were observed as an accessory mineral in several inclusions. Gehlenite, as a mantle surrounding spinel nodules, was only detected three times in all the CAIs examined (Table 2). Although some of the refractory inclusions appeared to be largely intact, most of them exhibited important alteration features as revealed by their fragmented structures (Fig. 2). For example, the most abundant CAIs had a bimodal structure characterized by a spinel-rich core with a well-developed fine-grained rim composed mainly of phyllosilicates (Figs. 2 and 3). Phyllosilicates were generally present in all refractory inclusions but in variable amounts, ranging from a few percent up to 40%. The complex mineral intergrowths observed within CAIs make it difficult to identify which phase(s) the phyllosilicates have replaced. Some CAIs also

present sodium-rich areas composed of the secondary alteration minerals, nepheline, and/or sodalite (Fig. 3e; Table 2).

Type I chondrules were generally small (<0.5 mm) and were characterized by either a porphyritic or granular texture, depending on the abundance of mesostasis. They exhibited different degrees of alteration, as indicated by the variable amounts of tochilinite/cronstedtite and magnetite relative to preserved Fe-Ni metal beads. However, compared to other CM chondrites, Paris contains more chondrules with well-preserved metal beads (Fig. 4). The chondrule mesostasis presented a systematic replacement of glass by phyllosilicates (Figs. 4d and 4f), whereas olivines and pyroxenes were generally unaltered. Type II chondrules are present in Paris, mainly as olivine-rich chondrules with a porphyritic texture (type II PO). Cr₂O₃ in ferroan olivines in type II chondrules ranges from 0.2 to 0.5 wt%, with an average of 0.37 and σ-Cr₂O₃ = 0.08 (1σ; Table 3). The matrix is fine-grained (Fig. 5) and is significantly enriched in sulfur (Table 4). Its composition corresponds to a mixture of serpentine, saponite, tochilinite, and iron sulfides. Carbonates are present only as Ca-carbonates and were found either in isolation in the matrix (Figs. 5e and 5f) or within CAIs. No dolomite or complex carbonates were observed in the three sections
<table><caption>Table 2. Chemical composition (wt%) of minerals observed in Ca-Al-rich inclusions.</caption><thead><tr><th>Mineral</th><th>No. Points</th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>K<sub>2</sub>O</th><th>CaO</th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>NiO</th><th>FeO/MgO</th></tr></thead><tbody><tr><td>Spinel</td><td>27</td><td>bdl</td><td>28.5 ± 0.8</td><td>0.9 ± 1.4</td><td>69.3 ± 2.5</td><td>bdl</td><td>1.1 ± 0.4</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>99.7</td></tr><tr><td>Diopside</td><td>14</td><td>bdl</td><td>18.5 ± 1.7</td><td>51.1 ± 1.9</td><td>3.8 ± 1.5</td><td>bdl</td><td>23.6 ± 1.8</td><td>0.4 ± 0.7</td><td>bdl</td><td>bdl</td><td>2.2 ± 1.6</td><td>bdl</td><td>99.6</td></tr><tr><td>Phyllosilicates</td><td>12</td><td>0.6 ± 0</td><td>16.9 ± 2.1</td><td>26.8 ± 2.9</td><td>4.9 ± 2.8</td><td>bdl</td><td>0.7 ± 0.3</td><td>bdl</td><td>0.5 ± 0.4</td><td>bdl</td><td>36.7 ± 11.1</td><td>0.8 ± 0.6</td><td>88.7 ± 2.1</td></tr><tr><td>Olivine</td><td>7</td><td>bdl</td><td>56.1 ± 0.5</td><td>42.4 ± 0.4</td><td>0.18 ± 0.1</td><td>bdl</td><td>0.7 ± 0.5</td><td>bdl</td><td>0.2 ± 0.2</td><td>bdl</td><td>0.6 ± 0.5</td><td>bdl</td><td>100.1</td></tr><tr><td>Hibonite</td><td>5</td><td>bdl</td><td>2.9 ± 0.3</td><td>0.3 ± 0.1</td><td>83.1 ± 1.7</td><td>bdl</td><td>8.4 ± 0.4</td><td>4.1 ± 0.8</td><td>bdl</td><td>bdl</td><td>0.4 ± 0.1</td><td>bdl</td><td>99.2</td></tr><tr><td>Perovskite</td><td>4</td><td>bdl</td><td>0.8 ± 0.2</td><td>bdl</td><td>1.9 ± 0.4</td><td>bdl</td><td>38.7 ± 0.6</td><td>58.1 ± 0.7</td><td>bdl</td><td>bdl</td><td>0.9 ± 0.5</td><td>bdl</td><td>100.4</td></tr><tr><td>Gehlenite</td><td>3</td><td>bdl</td><td>0.6 ± 0.2</td><td>22.1 ± 0.7</td><td>36.9 ± 0.6</td><td>bdl</td><td>39.8 ± 0.5</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>99.4</td></tr></tbody></table>
<figure><img src="image_4.png" /><figcaption>Fig. 3. a) Backscattered electron images of a fine-grained Ca-Al-rich inclusion. b to f) EXD elemental maps of Al, Ca, Cl, Na, and S distribution in this CAI.</figcaption></figure>

surveyed. Poorly crystallized phases (hereafter PCP) are abundant in the matrix and span a wide range of textures and compositions, with FeO/SiO₂ and S/SiO₂ ratios ranging from 2.7 to 6.1 and from 0.3 to 1.1, respectively (Table 5).

Fe-Ni metal beads are abundant in Paris (Table 1), mainly as kamacite (Fig. 6a), though rare taenite beads were also observed (Table 6). The Fe-Ni metal beads were mostly found within chondrules (Figs. 4a and 4c) but also dispersed throughout the matrix (Figs. 6a and 6b). Compared to other CM chondrites, Paris presents higher abundance of Fe-Ni metal beads but most of them showed partial signs of alteration characterized by (1) alteration rims (Figs. 6a and 6b), (2) surface pitting

(Fig 6a), and (3) massive intrusions (Fig. 6b). The secondary phases associated with Fe-Ni metal beads comprised clays, tochilinite/cronstedtite associations, or magnetite. Magnetite was frequently observed in Paris as (1) a corrosion product of Fe-Ni metal beads in the matrix (Fig. 6b) or (2) framboid spherule clusters in the matrix, in the fined-grained rims surrounding chondrules or, more rarely, within chondrules in close association with altered mesostasis (Fig. 6c). Large sulfide beads are dispersed throughout the matrix and are mainly composed of massive pyrrhotite surrounded by nickel-rich sulfides (Table 7; Figs. 6c and 6d). All the pyrrhotite grains exhibited thin lamellae and tiny blebs of pentlandite (Figs. 6d and 6e). On rare occasions,
<figure><img src="image_5.png" /><figcaption>Fig. 4. Backscattered electron images of (a) a metal-rich type I chondrule surrounded by an alteration rim, (b) a type II chondrule with forsterite relic grains, (c) a type I chondrule that presents well-preserved metal beads, (d) detail of chondrule shown in (e) illustrating alteration of the mesostasis in phyllosilicates, (f) a metal-free type I chondrule, and (g) detail of the chondrule shown in (h) illustrating alteration in/of the mesostasis.</figcaption></figure>

sulfides were also present within chondrules, either in association with Fe-Ni metal beads in the so-called opaque assemblages (Fig. 6f) (Rubin et al. 1999) or as liquid-shape blebs, composed entirely of troilite as recently reported in CV chondrites (Marrocchi and Libourel 2013).

DISCUSSION

Extent of Hydrothermal Alteration

The petrography and mineralogy of Paris is clearly
that of a CM chondrite as proposed by Bourot-Denise

et al. (2010) based on petrology, oxygen isotopic
composition, and exposure age. Although CM
chondrites are generally complex breccias, none of the
sections we studied had a brecciated nature. The modal
abundance of different components (CAIs, chondrules,
metal, sulfides, and matrix) is comparable to that of
other CM chondrites except for a high abundance of
Fe-Ni metal (Table 1) (Rubin et al. 2007). Since metal
is prone to attack by hydrothermal fluids, such a high
abundance of Fe-Ni metal can be interpreted to be a
sign of limited fluid alteration, as noted by Bourot-
Denise et al. (2010). This is confirmed by the systematic
occurrence of calcium carbonates (Figs. 5e and 5f) at
<table><caption>Table 3. Chemical composition (wt%) of type II ferroan olivines.</caption><thead><tr><th>No.</th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>K<sub>2</sub>O</th><th>CaO</th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>NiO</th><th>Total</th></tr></thead><tbody><tr><td>158</td><td>bdl</td><td>21.9</td><td>34.7</td><td>0.1</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.3</td><td>0.3</td><td>40.3</td><td>0.1</td><td>97.83</td></tr><tr><td>64</td><td>bdl</td><td>30.4</td><td>36.1</td><td>bdl</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.4</td><td>30.9</td><td>bdl</td><td>98.16</td></tr><tr><td>161</td><td>bdl</td><td>33.1</td><td>36.8</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.5</td><td>0.3</td><td>27.4</td><td>bdl</td><td>98.28</td></tr><tr><td>139</td><td>bdl</td><td>28.0</td><td>36.0</td><td>bdl</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.5</td><td>0.3</td><td>34.0</td><td>0.1</td><td>98.95</td></tr><tr><td>16</td><td>bdl</td><td>33.4</td><td>35.0</td><td>0.6</td><td>bdl</td><td>0.4</td><td>bdl</td><td>0.5</td><td>0.3</td><td>26.9</td><td>1.5</td><td>98.61</td></tr><tr><td>34</td><td>bdl</td><td>28.9</td><td>35.7</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.3</td><td>33.3</td><td>0.1</td><td>98.94</td></tr><tr><td>13</td><td>bdl</td><td>36.3</td><td>37.4</td><td>0.2</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.4</td><td>0.3</td><td>24.2</td><td>bdl</td><td>99.11</td></tr><tr><td>35</td><td>bdl</td><td>25.8</td><td>35.2</td><td>0.2</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.3</td><td>0.3</td><td>37.2</td><td>0.1</td><td>99.18</td></tr><tr><td>138</td><td>bdl</td><td>39.7</td><td>38.6</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.4</td><td>0.3</td><td>20.0</td><td>bdl</td><td>99.21</td></tr><tr><td>25</td><td>bdl</td><td>25.6</td><td>35.2</td><td>bdl</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.4</td><td>0.2</td><td>37.7</td><td>0.1</td><td>99.24</td></tr><tr><td>157</td><td>bdl</td><td>37.9</td><td>38.5</td><td>bdl</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.2</td><td>0.3</td><td>22.4</td><td>bdl</td><td>99.31</td></tr><tr><td>30</td><td>bdl</td><td>23.2</td><td>34.5</td><td>0.2</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.4</td><td>40.2</td><td>0.1</td><td>99.09</td></tr><tr><td>18</td><td>bdl</td><td>39.4</td><td>38.6</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.5</td><td>0.3</td><td>20.6</td><td>bdl</td><td>99.44</td></tr><tr><td>159</td><td>bdl</td><td>36.6</td><td>38.2</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.3</td><td>0.2</td><td>24.0</td><td>0.1</td><td>99.54</td></tr><tr><td>160</td><td>bdl</td><td>34.4</td><td>37.9</td><td>bdl</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.4</td><td>0.4</td><td>26.4</td><td>bdl</td><td>99.53</td></tr><tr><td>17</td><td>bdl</td><td>35.8</td><td>37.6</td><td>0.08</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.3</td><td>25.1</td><td>bdl</td><td>99.5</td></tr><tr><td>32</td><td>bdl</td><td>36.1</td><td>37.9</td><td>0.09</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.4</td><td>24.5</td><td>bdl</td><td>99.43</td></tr><tr><td>27</td><td>bdl</td><td>27.1</td><td>35.8</td><td>0.08</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.4</td><td>35.6</td><td>0.2</td><td>99.63</td></tr><tr><td>31</td><td>bdl</td><td>36.5</td><td>37.8</td><td>0.2</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.4</td><td>24.3</td><td>bdl</td><td>99.56</td></tr><tr><td>33</td><td>bdl</td><td>29.3</td><td>35.9</td><td>0.2</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.2</td><td>33.3</td><td>0.1</td><td>99.53</td></tr><tr><td>20</td><td>bdl</td><td>28.1</td><td>35.8</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.4</td><td>34.7</td><td>bdl</td><td>99.69</td></tr><tr><td>42</td><td>bdl</td><td>30.1</td><td>36.4</td><td>0.4</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.3</td><td>0.3</td><td>31.9</td><td>0.1</td><td>99.72</td></tr><tr><td>41</td><td>bdl</td><td>33.6</td><td>37.3</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.3</td><td>28.0</td><td>bdl</td><td>99.78</td></tr><tr><td>26</td><td>bdl</td><td>19.0</td><td>34.0</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.4</td><td>0.3</td><td>45.9</td><td>0.1</td><td>99.75</td></tr><tr><td>63</td><td>bdl</td><td>30.5</td><td>36.8</td><td>bdl</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.4</td><td>31.5</td><td>0.1</td><td>99.72</td></tr><tr><td>15</td><td>bdl</td><td>36.1</td><td>37.5</td><td>0.2</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.3</td><td>0.3</td><td>25.0</td><td>bdl</td><td>99.78</td></tr><tr><td>36</td><td>bdl</td><td>24.5</td><td>34.6</td><td>0.3</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.3</td><td>0.4</td><td>38.6</td><td>bdl</td><td>98.86</td></tr><tr><td>23</td><td>bdl</td><td>28.6</td><td>36.1</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.3</td><td>34.2</td><td>0.1</td><td>99.92</td></tr><tr><td>19</td><td>bdl</td><td>31.0</td><td>36.5</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.5</td><td>0.5</td><td>31.2</td><td>bdl</td><td>99.93</td></tr><tr><td>22</td><td>bdl</td><td>27.7</td><td>35.2</td><td>0.2</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.5</td><td>0.4</td><td>35.6</td><td>bdl</td><td>99.83</td></tr><tr><td>43</td><td>bdl</td><td>34.5</td><td>36.9</td><td>bdl</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.3</td><td>0.3</td><td>27.8</td><td>bdl</td><td>99.93</td></tr><tr><td>11</td><td>bdl</td><td>40.7</td><td>38.9</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.2</td><td>19.6</td><td>bdl</td><td>100.06</td></tr><tr><td>45</td><td>bdl</td><td>40.4</td><td>38.6</td><td>bdl</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.3</td><td>0.3</td><td>20.4</td><td>0.1</td><td>100.24</td></tr><tr><td>10</td><td>bdl</td><td>41.0</td><td>38.5</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.2</td><td>19.9</td><td>bdl</td><td>100.26</td></tr><tr><td>162</td><td>bdl</td><td>35.0</td><td>38.0</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.5</td><td>0.2</td><td>26.4</td><td>bdl</td><td>100.32</td></tr><tr><td>39</td><td>bdl</td><td>26.7</td><td>35.5</td><td>0.2</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.5</td><td>0.5</td><td>36.6</td><td>0.1</td><td>100.36</td></tr><tr><td>37</td><td>bdl</td><td>28.2</td><td>36.1</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.4</td><td>0.3</td><td>35.2</td><td>bdl</td><td>100.35</td></tr><tr><td>12</td><td>bdl</td><td>40.9</td><td>38.7</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.2</td><td>19.8</td><td>0.1</td><td>100.39</td></tr><tr><td>38</td><td>bdl</td><td>41.6</td><td>39.6</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.2</td><td>0.3</td><td>18.6</td><td>bdl</td><td>100.4</td></tr><tr><td>40</td><td>bdl</td><td>33.2</td><td>37.3</td><td>0.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.4</td><td>0.3</td><td>28.9</td><td>bdl</td><td>100.41</td></tr><tr><td>29</td><td>bdl</td><td>29.3</td><td>36.2</td><td>0.1</td><td>bdl</td><td>0.1</td><td>bdl</td><td>0.4</td><td>0.4</td><td>33.8</td><td>0.2</td><td>100.52</td></tr><tr><td>28</td><td>bdl</td><td>24.7</td><td>35.4</td><td>0.1</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.3</td><td>0.4</td><td>39.7</td><td>bdl</td><td>100.83</td></tr><tr><td>24</td><td>bdl</td><td>23.9</td><td>35.1</td><td>0.1</td><td>bdl</td><td>0.4</td><td>bdl</td><td>0.2</td><td>0.5</td><td>40.7</td><td>bdl</td><td>100.85</td></tr></tbody></table>

the expense of more complex carbonates (e.g., dolomite and breunnerite), which occur preferentially in the more-altered CM chondrites (Benedix et al. 2003; Rubin et al. 2007). A direct comparison of Paris with QUE 97990 (Table 8), the least-altered CM chondrite according to Rubin et al. (2007), highlights the relative low degree of alteration of Paris, as revealed by (1) the higher abundance of Fe-Ni metal in Paris (2% versus 1%), (2) the dispersive nature of matrix phyllosilicates in Paris relative to their ubiquitous occurrence in QUE

97990, and (3) the higher FeO/SiO₂ and S/SiO₂ ratios of Paris's PCPs (Table 5). Thus, though Paris seems less hydrothermally altered than other CM chondrites, it is not possible to ascribe it to petrographic type 3, which requires only a minor amount of hydrous minerals, unaltered glassy chondrule mesostases, and no PCP clumps (Rubin et al. 2007). On the contrary, Paris clearly exhibits significant signs of alteration indicated by the important amounts of PCP and carbonates dispersed throughout the matrix. In addition, despite
<figure><img src="image_6.png" /><figcaption>Fig. 5. a and b) Backscattered electron images of the Paris matrix showing unaltered Fe-Ni metal beads (white) and PCP clumps. c and d) Detailed backscattered electron images of PCP clumps present within the matrix of Paris. Clumps are composed of complex intergrowths of tochilinite/cronstedtite. e and f) Backscattered electron images of Ca-carbonates (CaCO₃) present within the matrix.</figcaption></figure>

their high degree of preservation relative to those found in other CMs, the Fe-Ni metal beads in Paris also show clear signs of alteration such as rims, surface pitting, and massive intrusions systematically observed at the peripheries of metallic beads (Figs. 6a and 6b) (Morlok and Libourel 2013). According to the scale proposed by Palmer and Lauretta (2010), these features share important similarities with alteration class 2, representing CM chondrites that experienced partial fluid alteration. Hence, we propose that Paris experienced extensive but not pervasive alteration that affects most of phases expecting of some of the Fe-Ni metal beads. Given that, the characteristics of Paris

suggest that it was subjected to more limited alteration than other CM chondrites. This could be the result of (1) specific alteration conditions in terms of oxygen fugacity, fluid composition, pH and/or temperature (Dyl et al. 2006); (2) a different planetesimal size and fluid/rock ratio (Young et al. 2003); (3) a shorter time scale of hydrothermal activity (Petitat et al. 2011; Dyl et al. 2012); or (4) nonuniform alteration in a convecting system within a single parent body (Palguta et al. 2010). Nevertheless, following the method proposed by Rubin et al. (2007), and according to their arbitrary classification of the former least-altered CM chondrite QUE 97990 as type 2.6, a petrologic subtype of 2.7
<table><caption>Table 4. Chemical composition (wt%) of different zones of the matrix and their related mineralogical alteration index (MAI).</caption><thead><tr><th>No.</th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>K<sub>2</sub>O</th><th>CaO</th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>NiO</th><th>Cl</th><th>s</th><th>Total</th><th>MAI</th></tr></thead><tbody><tr><td>108</td><td>0.4</td><td>20.8</td><td>28.1</td><td>2.9</td><td>0.1</td><td>0.3</td><td>0.1</td><td>0.3</td><td>30.6</td><td>1.2</td><td>0.1</td><td>3.0</td><td>88.1</td><td>0.56</td></tr><tr><td>102</td><td>0.5</td><td>12.2</td><td>21.5</td><td>2.0</td><td>bdl</td><td>0.5</td><td>0.3</td><td>0.2</td><td>43.1</td><td>1.5</td><td>0.1</td><td>3.3</td><td>85.3</td><td>0.27</td></tr><tr><td>97</td><td>0.3</td><td>16.5</td><td>26.4</td><td>2.7</td><td>0.1</td><td>0.2</td><td>0.2</td><td>0.2</td><td>33.9</td><td>1.8</td><td>0.2</td><td>2.5</td><td>85.0</td><td>0.52</td></tr><tr><td>101</td><td>0.4</td><td>12.6</td><td>24.3</td><td>2.0</td><td>0.1</td><td>2.2</td><td>0.3</td><td>0.2</td><td>36.2</td><td>1.4</td><td>0.1</td><td>2.4</td><td>83.6</td><td>0.37</td></tr><tr><td>122</td><td>0.4</td><td>19.6</td><td>27.1</td><td>2.4</td><td>bdl</td><td>0.2</td><td>0.4</td><td>0.3</td><td>28.4</td><td>1.6</td><td>0.2</td><td>2.3</td><td>82.9</td><td>0.5</td></tr><tr><td>124</td><td>0.4</td><td>14.1</td><td>26.0</td><td>2.8</td><td>bdl</td><td>0.6</td><td>0.5</td><td>0.2</td><td>34.0</td><td>1.5</td><td>0.1</td><td>2.3</td><td>82.8</td><td>0.62</td></tr><tr><td>113</td><td>0.3</td><td>12.3</td><td>23.0</td><td>2.7</td><td>bdl</td><td>0.3</td><td>0.2</td><td>0.3</td><td>37.7</td><td>1.6</td><td>0.1</td><td>2.7</td><td>81.3</td><td>0.48</td></tr><tr><td>103</td><td>0.5</td><td>13.1</td><td>24.4</td><td>2.1</td><td>0.1</td><td>1.1</td><td>0.4</td><td>0.2</td><td>35.4</td><td>1.6</td><td>0.1</td><td>2.0</td><td>81.6</td><td>0.39</td></tr><tr><td>Mean</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.46</td></tr><tr><td>St dev</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>0.11</td></tr></tbody></table>

<table><caption>Table 5. Chemical composition (wt%) of PCP and their FeO/SiO₂ and S/SiO₂.</caption><thead><tr><th>No.</th><th>Na<sub>2</sub>O</th><th>MgO</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>K<sub>2</sub>O</th><th>CaO</th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>MnO</th><th>FeO</th><th>NiO</th><th>Cl</th><th>P<sub>2</sub>O<sub>5</sub></th><th>SO<sub>2</sub></th><th>Total</th><th>FeO/SiO<sub>2</sub></th><th>S</th><th>S/SiO<sub>2</sub></th></tr></thead><tbody><tr><td>50</td><td>0.3</td><td>6.2</td><td>16.5</td><td>2.4</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>43.7</td><td>2.6</td><td>bdl</td><td>bdl</td><td>16.8</td><td>75.2</td><td>2.7</td><td>8.4</td><td>0.5</td></tr><tr><td>52</td><td>0.0</td><td>5.2</td><td>10.6</td><td>1.7</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>54.8</td><td>1.9</td><td>bdl</td><td>bdl</td><td>16.2</td><td>81.2</td><td>5.2</td><td>8.1</td><td>0.8</td></tr><tr><td>56</td><td>0.2</td><td>4.2</td><td>16.7</td><td>2.1</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>54.6</td><td>1.5</td><td>bdl</td><td>bdl</td><td>17.9</td><td>81.2</td><td>3.3</td><td>9.0</td><td>0.5</td></tr><tr><td>71</td><td>0.4</td><td>7.2</td><td>10.0</td><td>3.0</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>43.5</td><td>2.4</td><td>bdl</td><td>bdl</td><td>7.5</td><td>78.9</td><td>4.3</td><td>3.8</td><td>0.4</td></tr><tr><td>74</td><td>0.1</td><td>5.2</td><td>16.0</td><td>1.9</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>56.2</td><td>0.6</td><td>bdl</td><td>bdl</td><td>11.6</td><td>80.7</td><td>3.5</td><td>5.8</td><td>0.4</td></tr><tr><td>77</td><td>0.4</td><td>7.2</td><td>13.0</td><td>2.8</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>43.2</td><td>1.5</td><td>bdl</td><td>bdl</td><td>12.9</td><td>75.3</td><td>3.3</td><td>6.4</td><td>0.5</td></tr><tr><td>96</td><td>0.1</td><td>5.5</td><td>12.4</td><td>2.2</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>62.0</td><td>1.4</td><td>bdl</td><td>bdl</td><td>28.3</td><td>85.3</td><td>5.0</td><td>14.2</td><td>1.1</td></tr><tr><td>97</td><td>0.4</td><td>8.0</td><td>19.7</td><td>1.5</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>64.3</td><td>0.2</td><td>bdl</td><td>bdl</td><td>12.4</td><td>77.0</td><td>3.3</td><td>6.2</td><td>0.3</td></tr><tr><td>98</td><td>0.2</td><td>6.5</td><td>11.0</td><td>4.4</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>52.9</td><td>3.0</td><td>bdl</td><td>bdl</td><td>23.0</td><td>85.5</td><td>4.8</td><td>11.5</td><td>1.0</td></tr><tr><td>99</td><td>0.4</td><td>7.5</td><td>13.1</td><td>1.9</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>54.7</td><td>0.8</td><td>bdl</td><td>bdl</td><td>19.8</td><td>84.6</td><td>4.2</td><td>9.9</td><td>0.8</td></tr><tr><td>100</td><td>0.3</td><td>8.0</td><td>10.8</td><td>2.1</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>46.9</td><td>1.6</td><td>bdl</td><td>bdl</td><td>15.0</td><td>84.6</td><td>4.3</td><td>7.5</td><td>0.7</td></tr><tr><td>117</td><td>0.5</td><td>10.2</td><td>14.5</td><td>2.2</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>45.3</td><td>1.4</td><td>bdl</td><td>bdl</td><td>15.5</td><td>77.4</td><td>3.1</td><td>7.8</td><td>0.5</td></tr><tr><td>118</td><td>0.3</td><td>7.4</td><td>11.3</td><td>1.7</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>53.2</td><td>1.8</td><td>bdl</td><td>bdl</td><td>20.6</td><td>85.7</td><td>4.7</td><td>10.3</td><td>0.9</td></tr><tr><td>120</td><td>0.3</td><td>3.9</td><td>10.0</td><td>1.6</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>60.6</td><td>0.7</td><td>bdl</td><td>bdl</td><td>22.0</td><td>81.3</td><td>6.1</td><td>11.0</td><td>1.1</td></tr><tr><td>121</td><td>0.1</td><td>6.9</td><td>11.1</td><td>2.6</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>55.1</td><td>0.7</td><td>bdl</td><td>bdl</td><td>18.1</td><td>85.7</td><td>4.9</td><td>9.1</td><td>0.8</td></tr><tr><td>122</td><td>0.2</td><td>10.8</td><td>14.8</td><td>2.3</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>53.6</td><td>1.6</td><td>bdl</td><td>bdl</td><td>20.0</td><td>85.6</td><td>3.6</td><td>10.0</td><td>0.7</td></tr><tr><td>123</td><td>0.2</td><td>8.3</td><td>12.0</td><td>1.9</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>50.6</td><td>1.9</td><td>bdl</td><td>bdl</td><td>21.5</td><td>86.0</td><td>4.2</td><td>10.8</td><td>0.9</td></tr><tr><td>142</td><td>0.3</td><td>8.8</td><td>18.8</td><td>3.1</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>51.5</td><td>3.7</td><td>bdl</td><td>bdl</td><td>12.8</td><td>79.5</td><td>2.7</td><td>6.4</td><td>0.3</td></tr><tr><td>Mean</td><td>0.3</td><td>7.1</td><td>13.5</td><td>2.3</td><td>—</td><td>—</td><td>—</td><td>—</td><td>52.6</td><td>1.6</td><td>—</td><td>—</td><td>17.3</td><td></td><td>4.1</td><td>8.7</td><td>0.7</td></tr><tr><td>Stdev</td><td>0.1</td><td>1.9</td><td>3.0</td><td>0.7</td><td>—</td><td>—</td><td>—</td><td>—</td><td>6.2</td><td>0.9</td><td>—</td><td>—</td><td>5.0</td><td></td><td>0.9</td><td>2.5</td><td>0.3</td></tr></tbody></table>

would be a reasonable estimation for Paris as recently proposed by Rubin (2014) (Fig. 7).

To more quantitatively assess the extent of fluid alteration, we calculated the Mineralogical Alteration Index (hereafter MAI) of Browning et al. (1996), which consists of estimating the Fe³⁺ content of serpentine in the matrix. Because of the sulfur richness of the Paris matrix, it was necessary to select analyses with up to 3 wt% S instead of the recommended value of 2 (Table 4). The calculated MAI from 43 analyses is 0.46 (σ = 0.11), higher than any other CM chondrite with the exception of Murchison. Thus, according to this scale, Paris should be classified as type 2.5, although the high abundance of well-preserved metal in Paris compared to Murchison (2% versus 0.3%) suggests that Paris is less altered.

Considering the limited degree of aqueous alteration of Paris, an important finding is the few occurrences of gehlenite observed within CAIs (i.e., *n* = 3 in 32 CAIs). This confirms that melilite-rich refractory inclusions are very rare in CM2 chondrites, regardless of the extent of aqueous alteration (Armstrong et al. 1982; McPherson et al. 1983; Simon et al. 2006; Rubin 2007). These observations confirm the unique nature of refractory inclusions in CM chondrites compared to other carbonaceous chondrite groups that commonly present melilite-rich refractory inclusions (Simon et al. 2006; Rubin 2007). That being said, we cannot exclude the possibility that the limited but significant secondary fluid alteration undergone by Paris induced melilite breakdown due to the high susceptibility of gehlenite to alteration compared to other phases in refractory
<figure><img src="image_7.png" /><figcaption>Fig. 6. a and b) Backscattered electron images of partially altered Fe-Ni metal beads. In rare cases, metal beads are associated with iron oxides (right). c) Backscattered electron image of framboidal magnetites within a type I chondrule. d and e) Backscattered electron images of sulfide beads composed of massive pyrrhotite in association with Ni-rich pentlandite. The brightness has been deliberately decreased in order to identify potential lamellae and blebs within sulfide grains. f) Backscattered electron image of opaque assemblages located within a type I chondrule.</figcaption></figure>

inclusions (Nomura and Miyamoto 1998). Thus, there is clearly a need for specific microstructural observations of gehlenite occurrences in Paris CAIs and their relationships to minerals in association with.

## Extent of Thermal Metamorphism

Ever since the recognition of thermal metamorphic features in ordinary and carbonaceous chondrites, several indicators have been put forward to quantitatively assess the metamorphic temperatures experienced. The distribution and variability of Cr in type II ferroan chondrite olivine has been shown to

systematically change as metamorphism increases from type 3.0 to type 3.2 (Grossman and Brearley 2005). The average chromium content of 0.37 wt% and σ-Cr₂O₃ of 0.08 determined in FeO-rich olivine from Paris (Table 3) indicate its low metamorphic grade, close to the least thermally altered CO chondrites and to the pristine 3.00 chondrite, Semarkona (Fig. 8) (Grossman and Brearley 2005).

More recently, Kimura et al. (2008, 2011) proposed that textural and chemical properties of Fe-Ni metal and sulfide beads can be used as sensors of the thermal history of ordinary, CO, and CM chondrites. Based on the nickel and cobalt content of Fe-Ni metal beads and
<table><caption>Table 6. Chemical composition (atom%) of Fe-Ni metal beads observed in Paris.</caption><thead><tr><th>No.</th><th>Si</th><th>Fe</th><th>Co</th><th>Ni</th><th>P</th><th>s</th><th>Cr</th><th>Total</th></tr></thead><tbody><tr><td>1</td><td>bdl</td><td>95.8</td><td>0.1</td><td>4.2</td><td>0.4</td><td>bdl</td><td>0.5</td><td>100.5</td></tr><tr><td>2</td><td>bdl</td><td>94.8</td><td>0.5</td><td>4.1</td><td>0.4</td><td>bdl</td><td>0.3</td><td>100.2</td></tr><tr><td>3</td><td>0.5</td><td>90.7</td><td>bdl</td><td>6.1</td><td>0.3</td><td>bdl</td><td>0.8</td><td>98.5</td></tr><tr><td>4</td><td>0.5</td><td>90.1</td><td>0.2</td><td>6.0</td><td>0.4</td><td>bdl</td><td>0.7</td><td>98.0</td></tr><tr><td>5</td><td>0.6</td><td>90.3</td><td>0.1</td><td>6.1</td><td>0.4</td><td>bdl</td><td>0.7</td><td>98.2</td></tr><tr><td>6</td><td>bdl</td><td>92.1</td><td>0.3</td><td>5.7</td><td>0.3</td><td>bdl</td><td>0.5</td><td>99.0</td></tr><tr><td>7</td><td>bdl</td><td>92.7</td><td>0.3</td><td>5.8</td><td>0.3</td><td>bdl</td><td>0.5</td><td>99.6</td></tr><tr><td>8</td><td>bdl</td><td>92.2</td><td>0.3</td><td>5.6</td><td>0.3</td><td>bdl</td><td>0.5</td><td>98.9</td></tr><tr><td>9</td><td>bdl</td><td>93.4</td><td>0.3</td><td>5.5</td><td>0.4</td><td>bdl</td><td>0.4</td><td>100.1</td></tr><tr><td>10</td><td>bdl</td><td>92.8</td><td>0.2</td><td>5.2</td><td>0.4</td><td>bdl</td><td>0.4</td><td>99.1</td></tr><tr><td>11</td><td>bdl</td><td>93.9</td><td>bdl</td><td>5.2</td><td>0.4</td><td>bdl</td><td>0.4</td><td>99.9</td></tr><tr><td>12</td><td>0.2</td><td>92.0</td><td>0.3</td><td>5.4</td><td>0.2</td><td>bdl</td><td>0.8</td><td>98.9</td></tr><tr><td>13</td><td>0.5</td><td>92.4</td><td>bdl</td><td>5.4</td><td>0.3</td><td>bdl</td><td>0.9</td><td>99.5</td></tr><tr><td>14</td><td>bdl</td><td>93.3</td><td>0.1</td><td>4.3</td><td>0.4</td><td>bdl</td><td>0.4</td><td>98.6</td></tr><tr><td>15</td><td>bdl</td><td>94.0</td><td>0.2</td><td>4.3</td><td>0.3</td><td>bdl</td><td>0.3</td><td>99.1</td></tr><tr><td>16</td><td>bdl</td><td>94.1</td><td>0.2</td><td>4.2</td><td>0.3</td><td>bdl</td><td>0.3</td><td>99.2</td></tr><tr><td>17</td><td>bdl</td><td>92.0</td><td>0.3</td><td>5.9</td><td>0.3</td><td>bdl</td><td>0.5</td><td>99.0</td></tr><tr><td>18</td><td>0.8</td><td>93.0</td><td>0.2</td><td>5.2</td><td>0.3</td><td>bdl</td><td>0.8</td><td>100.3</td></tr><tr><td>19</td><td>0.7</td><td>88.8</td><td>0.1</td><td>8.1</td><td>0.2</td><td>bdl</td><td>0.9</td><td>98.9</td></tr><tr><td>20</td><td>bdl</td><td>93.8</td><td>0.1</td><td>5.5</td><td>0.4</td><td>bdl</td><td>0.4</td><td>100.1</td></tr><tr><td>21</td><td>bdl</td><td>92.1</td><td>0.3</td><td>5.5</td><td>0.4</td><td>bdl</td><td>0.3</td><td>98.6</td></tr><tr><td>22</td><td>bdl</td><td>93.3</td><td>0.2</td><td>5.1</td><td>0.3</td><td>bdl</td><td>0.5</td><td>99.4</td></tr><tr><td>23</td><td>0.1</td><td>93.0</td><td>0.1</td><td>5.4</td><td>0.3</td><td>bdl</td><td>0.9</td><td>99.8</td></tr><tr><td>24</td><td>bdl</td><td>93.7</td><td>0.3</td><td>5.4</td><td>0.3</td><td>bdl</td><td>0.6</td><td>100.4</td></tr><tr><td>25</td><td>0.1</td><td>94.0</td><td>0.2</td><td>5.1</td><td>0.2</td><td>bdl</td><td>0.7</td><td>100.5</td></tr><tr><td>26</td><td>0.2</td><td>93.1</td><td>0.3</td><td>5.3</td><td>0.3</td><td>bdl</td><td>0.8</td><td>99.8</td></tr><tr><td>27</td><td>bdl</td><td>93.8</td><td>0.5</td><td>5.5</td><td>0.4</td><td>bdl</td><td>0.0</td><td>100.3</td></tr><tr><td>28</td><td>bdl</td><td>91.4</td><td>0.6</td><td>8.1</td><td>0.2</td><td>bdl</td><td>0.0</td><td>100.3</td></tr><tr><td>29</td><td>bdl</td><td>92.0</td><td>0.3</td><td>7.2</td><td>0.3</td><td>bdl</td><td>0.7</td><td>100.6</td></tr><tr><td>30</td><td>bdl</td><td>92.9</td><td>0.3</td><td>7.6</td><td>0.1</td><td>bdl</td><td>0.2</td><td>101.2</td></tr><tr><td>31</td><td>0.8</td><td>93.0</td><td>0.2</td><td>4.4</td><td>0.2</td><td>bdl</td><td>1.3</td><td>99.9</td></tr><tr><td>32</td><td>2.2</td><td>90.2</td><td>0.1</td><td>5.1</td><td>0.2</td><td>bdl</td><td>1.1</td><td>99.0</td></tr><tr><td>33</td><td>bdl</td><td>93.3</td><td>0.3</td><td>4.9</td><td>bdl</td><td>bdl</td><td>0.1</td><td>99.1</td></tr><tr><td>34</td><td>0.0</td><td>93.0</td><td>0.2</td><td>6.7</td><td>bdl</td><td>bdl</td><td>0.0</td><td>100.1</td></tr><tr><td>35</td><td>0.0</td><td>93.5</td><td>0.2</td><td>6.0</td><td>0.2</td><td>bdl</td><td>0.1</td><td>100.2</td></tr><tr><td>36</td><td>0.0</td><td>93.5</td><td>0.3</td><td>6.1</td><td>0.2</td><td>bdl</td><td>0.2</td><td>100.3</td></tr><tr><td>37</td><td>0.4</td><td>92.3</td><td>0.2</td><td>5.4</td><td>0.3</td><td>bdl</td><td>1.0</td><td>99.7</td></tr><tr><td>38</td><td>0.4</td><td>91.7</td><td>0.2</td><td>5.6</td><td>0.3</td><td>bdl</td><td>1.1</td><td>99.4</td></tr><tr><td>39</td><td>bdl</td><td>94.0</td><td>0.3</td><td>4.9</td><td>0.4</td><td>bdl</td><td>0.3</td><td>99.8</td></tr><tr><td>40</td><td>bdl</td><td>89.4</td><td>0.3</td><td>8.0</td><td>0.2</td><td>bdl</td><td>0.4</td><td>98.3</td></tr><tr><td>41</td><td>bdl</td><td>93.0</td><td>0.3</td><td>6.3</td><td>bdl</td><td>bdl</td><td>0.1</td><td>99.7</td></tr><tr><td>42</td><td>bdl</td><td>92.5</td><td>0.2</td><td>6.3</td><td>bdl</td><td>bdl</td><td>0.0</td><td>99.1</td></tr><tr><td>43</td><td>bdl</td><td>94.7</td><td>0.2</td><td>5.3</td><td>0.3</td><td>bdl</td><td>0.6</td><td>101.1</td></tr><tr><td>44</td><td>0.3</td><td>93.6</td><td>0.2</td><td>5.2</td><td>0.3</td><td>bdl</td><td>0.8</td><td>100.3</td></tr><tr><td>45</td><td>0.3</td><td>93.4</td><td>0.1</td><td>5.2</td><td>0.3</td><td>bdl</td><td>0.7</td><td>100.0</td></tr><tr><td>46</td><td>0.2</td><td>92.5</td><td>0.2</td><td>5.3</td><td>0.3</td><td>bdl</td><td>0.8</td><td>99.4</td></tr><tr><td>47</td><td>0.1</td><td>93.7</td><td>0.2</td><td>5.4</td><td>0.3</td><td>bdl</td><td>0.9</td><td>100.6</td></tr><tr><td>48</td><td>0.1</td><td>91.2</td><td>0.4</td><td>6.6</td><td>0.3</td><td>bdl</td><td>0.7</td><td>99.4</td></tr><tr><td>49</td><td>bdl</td><td>90.4</td><td>0.3</td><td>6.8</td><td>0.6</td><td>bdl</td><td>0.6</td><td>98.7</td></tr><tr><td>50</td><td>bdl</td><td>94.1</td><td>0.2</td><td>5.4</td><td>0.2</td><td>bdl</td><td>0.1</td><td>99.9</td></tr><tr><td>51</td><td>bdl</td><td>95.0</td><td>0.3</td><td>4.4</td><td>0.3</td><td>bdl</td><td>0.5</td><td>100.6</td></tr><tr><td>52</td><td>bdl</td><td>95.0</td><td>0.2</td><td>4.3</td><td>0.3</td><td>bdl</td><td>0.5</td><td>100.4</td></tr><tr><td>53</td><td>bdl</td><td>94.5</td><td>0.1</td><td>5.5</td><td>bdl</td><td>bdl</td><td>0.0</td><td>100.3</td></tr><tr><td>54</td><td>bdl</td><td>92.5</td><td>0.3</td><td>6.5</td><td>bdl</td><td>bdl</td><td>0.0</td><td>99.4</td></tr></tbody></table>
<table><caption>Table 7. Chemical composition (atom%) of sulfides.</caption><thead><tr><th>Fe</th><th>Co</th><th>Ni</th><th>P</th><th>Cr</th><th>S</th><th>Total</th></tr></thead><tbody><tr><td>59.2</td><td>0.1</td><td>1.9</td><td>bdl</td><td>bdl</td><td>36.9</td><td>98.3</td></tr><tr><td>61.9</td><td>0.1</td><td>0.4</td><td>bdl</td><td>0.1</td><td>35.2</td><td>97.8</td></tr><tr><td>61.8</td><td>0.2</td><td>1.2</td><td>bdl</td><td>0.1</td><td>36.5</td><td>99.7</td></tr><tr><td>62.9</td><td>bdl</td><td>0.2</td><td>bdl</td><td>0.1</td><td>36.2</td><td>99.4</td></tr><tr><td>33.0</td><td>0.5</td><td>24.8</td><td>bdl</td><td>bdl</td><td>36.3</td><td>94.7</td></tr><tr><td>39.9</td><td>0.9</td><td>24.6</td><td>bdl</td><td>0.1</td><td>32.0</td><td>97.4</td></tr><tr><td>37.2</td><td>0.7</td><td>21.4</td><td>bdl</td><td>0.1</td><td>34.9</td><td>94.4</td></tr><tr><td>35.6</td><td>0.7</td><td>24.0</td><td>bdl</td><td>0.1</td><td>32.7</td><td>93.0</td></tr><tr><td>42.8</td><td>0.7</td><td>17.3</td><td>bdl</td><td>bdl</td><td>34.1</td><td>95.2</td></tr><tr><td>59.0</td><td>0.5</td><td>3.7</td><td>bdl</td><td>0.1</td><td>36.6</td><td>99.8</td></tr><tr><td>60.6</td><td>0.3</td><td>1.6</td><td>bdl</td><td>0.1</td><td>36.6</td><td>99.3</td></tr><tr><td>49.7</td><td>0.3</td><td>10.2</td><td>bdl</td><td>0.1</td><td>34.7</td><td>95.0</td></tr><tr><td>43.8</td><td>0.5</td><td>20.2</td><td>bdl</td><td>bdl</td><td>34.0</td><td>98.6</td></tr><tr><td>59.8</td><td>0.4</td><td>2.6</td><td>bdl</td><td>0.1</td><td>36.6</td><td>99.4</td></tr><tr><td>61.7</td><td>0.1</td><td>1.0</td><td>bdl</td><td>0.2</td><td>36.7</td><td>99.8</td></tr><tr><td>60.6</td><td>bdl</td><td>1.3</td><td>bdl</td><td>0.3</td><td>37.2</td><td>99.6</td></tr><tr><td>60.9</td><td>0.1</td><td>1.0</td><td>bdl</td><td>0.2</td><td>36.8</td><td>99.2</td></tr><tr><td>61.3</td><td>0.1</td><td>1.3</td><td>bdl</td><td>0.3</td><td>36.7</td><td>99.8</td></tr><tr><td>57.8</td><td>0.4</td><td>3.8</td><td>bdl</td><td>bdl</td><td>37.0</td><td>99.1</td></tr><tr><td>57.6</td><td>0.3</td><td>4.5</td><td>bdl</td><td>bdl</td><td>37.4</td><td>99.9</td></tr><tr><td>56.9</td><td>0.3</td><td>5.3</td><td>bdl</td><td>bdl</td><td>37.7</td><td>100.4</td></tr><tr><td>35.3</td><td>0.9</td><td>30.5</td><td>bdl</td><td>bdl</td><td>33.0</td><td>99.8</td></tr><tr><td>35.2</td><td>1.1</td><td>29.9</td><td>bdl</td><td>bdl</td><td>32.9</td><td>99.2</td></tr><tr><td>62.6</td><td>0.3</td><td>0.9</td><td>bdl</td><td>bdl</td><td>36.6</td><td>100.4</td></tr><tr><td>62.3</td><td>0.2</td><td>0.6</td><td>bdl</td><td>bdl</td><td>36.6</td><td>99.7</td></tr><tr><td>61.0</td><td>0.2</td><td>0.7</td><td>bdl</td><td>bdl</td><td>36.6</td><td>98.4</td></tr><tr><td>62.6</td><td>0.2</td><td>0.2</td><td>bdl</td><td>0.2</td><td>36.1</td><td>99.5</td></tr><tr><td>63.3</td><td>0.1</td><td>0.2</td><td>bdl</td><td>0.2</td><td>36.7</td><td>100.7</td></tr><tr><td>63.0</td><td>bdl</td><td>0.3</td><td>bdl</td><td>0.3</td><td>36.2</td><td>99.8</td></tr><tr><td>62.4</td><td>bdl</td><td>0.9</td><td>bdl</td><td>bdl</td><td>37.4</td><td>100.7</td></tr><tr><td>58.4</td><td>0.4</td><td>3.9</td><td>bdl</td><td>0.1</td><td>36.2</td><td>99.1</td></tr><tr><td>37.2</td><td>0.8</td><td>28.4</td><td>bdl</td><td>bdl</td><td>33.0</td><td>99.4</td></tr><tr><td>61.9</td><td>0.1</td><td>1.1</td><td>bdl</td><td>bdl</td><td>37.3</td><td>100.6</td></tr><tr><td>46.3</td><td>0.9</td><td>18.1</td><td>bdl</td><td>0.0</td><td>34.7</td><td>100.1</td></tr><tr><td>56.8</td><td>0.3</td><td>5.1</td><td>bdl</td><td>bdl</td><td>36.2</td><td>98.5</td></tr><tr><td>62.4</td><td>0.3</td><td>0.7</td><td>bdl</td><td>bdl</td><td>36.6</td><td>100.1</td></tr><tr><td>61.5</td><td>0.3</td><td>0.9</td><td>bdl</td><td>0.0</td><td>37.1</td><td>99.8</td></tr><tr><td>63.1</td><td>bdl</td><td>0.2</td><td>bdl</td><td>bdl</td><td>36.8</td><td>100.1</td></tr></tbody></table>

the textures of sulfides, CM chondrites were divided into three categories (1) unheated (category A), (2) mildly metamorphosed (category B), and (3) highly metamorphosed (category C). Category A chondrites are characterized by Fe-Ni metal beads with solar Ni/Co ratios and only rare occurrences of pentlandite blebs or lamella in pyrrhotite grains. These features share important similarities with the Fe-Ni metal and sulfide beads observed in the most pristine chondrites (Kimura et al. 2008, 2011). These authors associated category A CMs with stage I of Nakamura (2005). This stage has been proposed to be characteristic of CM chondrites that experienced thermal metamorphism below 300 °C on the basis of the presence of X-ray diffraction patterns of tochilinite, whose breakdown temperature is

<table><caption>Table 8. Comparison between QUE 97990 (the least-altered CM chondrite according to Rubin et al. 2007) and Paris.</caption><thead><tr><th></th><th>QUE 97990</th><th>Paris</th></tr></thead><tbody><tr><td>Chondrule mesostasis</td><td>Phyllosilicates</td><td>Phyllosilicates</td></tr><tr><td>Matrix phyllosilicates</td><td>Abundant</td><td>Variable</td></tr><tr><td>Metallic Fe-Ni</td><td>~1 vol%</td><td>~2 vol%</td></tr><tr><td>Mafic silicate<br>phenocrysts in<br>chondrules</td><td>Unaltered</td><td>Unaltered</td></tr><tr><td>Large PCP clumps</td><td>15-40 vol%</td><td>15-40 vol%</td></tr><tr><td>PCP composition<br>“FeO/SiO<sub>2</sub>”</td><td>2.0-3.3</td><td>4.1 ± 0.9</td></tr><tr><td>Sulfide</td><td>Mainly po +pn</td><td>Mainly po +pn</td></tr><tr><td>Carbonate</td><td>Ca carbonate</td><td>Ca carbonate</td></tr></tbody></table>

estimated at 245 °C (Fuchs et al. 1973). Fe-Ni metal beads in category B are characterized by a narrow range of nickel contents of between 4.6 and 8.5 wt% (Kimura et al. 2011). In contrast to category A, all pyrrhotite grains in category B contain blebs or lamella of pentlandite. Pyrrhotite grains contain less than 0.9 wt% Ni and 0.2 wt% Co, while pentlandite beads present higher concentrations, in the range 18.8-36.1 wt% and 0.7-1.9 wt% for Ni and Co, respectively. Kimura et al. (2011) proposed that category B CMs belong to stages II and III of Nakamura (2005). These stages are characterized by the absence of tochilinite and serpentine reflections and the partial dehydration of phyllosilicates. Based on these characteristics, Nakamura (2005) proposed that stages II and III represent CM chondrites that underwent a small degree of secondary heating in the range 300-750 °C. Category C is characterized by (1) large variations in the nickel content of Fe-Ni metal beads (0.9-57.9 wt%) and (2) the absence of blebs and lamella of pentlandite in pyrrhotite grains. Kimura et al. (2011) suggested that such CMs were subjected to the most severe thermal metamorphism at temperature higher than 750 °C. They anchored this category to the stage IV of Nakamura (2005) that is related to CM chondrites showing X-ray diffraction patterns characteristics of anhydrous minerals. For the Paris chondrite, our observations show (1) a high abundance of Fe-Ni metal beads, mainly kamacite, with nickel and cobalt contents between 4.1 and 8.1 wt%, and 0.1 and 0.5 wt%, respectively (Table 6; Fig. 9); and (2) the systematic occurrence of thin lamellae and tiny blebs of pentlandite in pyrrhotite grains (Figs. 6d and 6e). Thus, according to the scale proposed by Kimura et al. (2011), Paris falls within the category B, which has been proposed to correspond to a moderate degree of metamorphism between 300 °C and 750 °C.
<figure><img src="image_8.png" /></figure>

Fig. 7. Correlation between PCP composition ((a) S/SiO₂ and (b) FeO/SiO₂) and petrological subtype in CM chondrites (Rubin et al. 2007). The trends have been interpreted as reflecting an increase in the phyllosilicate/sulfide ratio in PCP during fluid alteration. The Paris data confirm the reliability of the scale and the pristine nature of this CM chondrite, which should be classified as type 2.7.

## Insights into Secondary Alteration Indicators

Paris is clearly a unique meteorite that challenges our ability to quantitatively assess the intensity of secondary alteration. Our results lead us to a curious situation where, depending on the indicators chosen, Paris can be considered as either (1) a highly pristine meteorite that underwent only gentle and partial fluid alteration (PCP compositions and extent of kamacite corrosion) and mild thermal metamorphism (Cr content

<figure><img src="image_9.png" /></figure>

Fig. 8. Standard deviation versus mean Cr₂O₃ contents of type II ferroan chondrule olivine in ordinary and CO chondrites (Acfer 094 also shown) (Grossman and Brearley 2005). Paris lies along the trend defined by the CO chondrites, close to the least thermally altered CO chondrites and the pristine chondrite Acfer 094.

<figure><img src="image_10.png" /></figure>

Fig. 9. Ni versus Co concentration (wt%) of Fe-Ni metal beads in Paris. With the exception few beads, all the data fall into the type B group of Kimura et al. (2008, 2011), which characterizes chondrites that experienced moderate thermal metamorphism between 300 °C and 750 °C.

of type II FeO-rich olivine), or (2) a meteorite that experienced moderate fluid alteration (MAI) followed by a relatively high degree of thermal metamorphism
(concentration of Ni and Co in metal and sulfide beads). Further assessment of the secondary alteration indicators is therefore clearly required if we are to untangle the complex relationship between fluid alteration and thermal metamorphism.

Based on the Fe³⁺ content of serpentine, Browning et al. (1996) developed the Mineralogical Alteration Index for estimating the intensity of fluid alteration experienced by CM chondrites. The MAI can be calculated as

$$Fe^{3+} = 2(2 - Si) - Al \quad (1)$$

and

$$MAI = 2 - \frac{Fe^{3+}}{2 - Si} \quad (2)$$

By substituting Equation (1) into the formula for MAI (Equation 2), the result is

$$MAI = \frac{Al}{2 - Si} \quad (3)$$

Paris presents the same MAI as Murchison (i.e., 0.46 versus 0.43) and therefore according to this scale, these meteorites have experienced similar alteration conditions. However, our results clearly indicate that Paris is less altered than QUE 97990—the least-altered CMs according to (Rubin et al. 2007)—and therefore Murchison. Furthermore, the determination of MAI is based on a complex algorithm that requires determination of the average composition of sulfur-poor matrix phyllosilicates (i.e., <2 wt% S) by electron microprobe. Paris, however, is characterized by the presence of sulfur-rich phyllosilicates (>2 wt% S; Table 4). In addition, the determination of MAI involves (1) the aluminum content of CM chondrite matrix that is not a direct function of alteration (Equation 3) (Velbel and Palmer 2011) and (2) the use of the silicon content of the matrix at the denominator (Equation 3) that might lead in division by zero when the alteration products approach pure serpentines (Palmer and Lauretta 2010). However, Browning et al. (1996) reported that the MAI correlates with different parameters of CM chondrites such as (1) the modal proportion of isolated matrix silicates, (2) the volume percent of chondrule alteration, and (3) the bulk H content. In addition, these authors also reported a strong negative correlation between the MAI and the ³⁶Ar content of CM chondrites, suggesting that noble gas contents decrease with increasing fluid alteration. Considering Paris, the amount of trapped ³⁶Ar is the highest reported so far for a CM chondrite

<figure><img src="image_11.png" /><figcaption>Fig. 10. Relationship between the amount of trapped ³⁶Ar and CM chondrite petrographic subtype as determined from PCP chemical compositions. Noble gas data of Paris are from Bourot-Denise et al. (2010) and Hewins et al. (2014). Noble gas data and petrographic subtype of other CMs are from Browning et al. (1996) and Haack et al. (2012). The amount of trapped ³⁶Ar decreases with increasing the degrees of aqueous alteration.</figcaption></figure>

(1.10 × 10⁻⁶ cc g⁻¹) (Bourot-Denise et al. 2010; Hewins et al. 2014), twice as much as Murchison (8.40 × 10⁻⁷ cc g⁻¹) (Browning et al. 1996) although, Paris has the same MAI as Murchison (i.e., 0.46 versus 0.43) and therefore should have similar trapped ³⁶Ar contents (Browning et al. 1996). Together, these observations confirm that the MAI is not fully adapted to quantitatively address the question of fluid alteration as recently recognized by several studies (Rubin et al. 2007; Howard et al. 2009; Palmer and Lauretta 2010; Velbel and Palmer 2011).

Rubin et al. (2007) proposed that the variations in tochilinite/cronstedtite (i.e., PCP) composition between different CM chondrites could be used as an indicator of the intensity of fluid alteration. The high FeO/SiO₂ and S/SiO₂ ratios determined for the Paris PCPs (Table 5) are consistent with such a scale and confirm, in agreement with other petrologic properties, the pristine nature of Paris (Fig. 7). In addition, a good positive correlation is observed between CM chondrite petrographic subtypes and their ³⁶Ar contents (Fig. 10). This confirms that fluid alteration plays an important role in reducing the abundances of noble gases in CM chondrites, as suggested in previous studies (Browning et al. 1996; Nakamura et al. 1999; Nakasyo et al. 2000). Given that Paris yields the highest ³⁶Ar content
reported so far for a CM chondrite, such a correlation confirms (1) the reliability of the alteration sequence proposed by Rubin et al. (2007) and (2) the fact that noble gases, or at least trapped argon-36, can be used as a proxy for quantitative assessment of the intensity of fluid alteration experienced by CM chondrites (Nakasyo et al. 2000).

In addition to their susceptibility to fluid alteration, noble gases have also been reported to be highly sensitive to thermal metamorphism; their abundances decreasing with increasing degrees of metamorphism (Huss 1997; Nakamura et al. 2000; Huss et al. 2003). Based on the assumption that all members of a given class of chondrite accreted the same amount of noble gases, this property has been used for their metamorphic classification (Heymann and Mazor 1968; Sears et al. 1980; Anders and Zadnik 1985). Thus, the high ³⁶Ar content of Paris supports the hypothesis that this chondrite underwent only gentle thermal metamorphism and can be tied to stage I, defined by Nakamura et al. (2000) as unheated CM chondrites. However, this conflicts with the classification of Kimura et al. (2011), based on the Ni and Co contents of metal and sulfide beads, which suggests that Paris experienced moderate metamorphism between 300 °C and 750 °C. Low-degree thermal metamorphism is further confirmed in Paris by the presence of (1) carbonates, as these minerals tend to break down when heated above 400 °C (Miyamoto 1991) and (2) tochilinite, whose breakdown temperature is estimated to be around 250 °C (Fuchs et al. 1973). Thus, our observations and the noble gas content of Paris do not support the use of Co and Ni concentrations of metal and sulfide beads as a reliable proxy for the thermal history of CM chondrites.

While our data confirm that PCP compositions and olivine Cr concentrations are powerful indicators of fluid alteration and thermal metamorphism, respectively, the abundance of trapped ³⁶Ar represents an additional proxy for probing these secondary modifications. Based on a compilation of literature data (Browning et al. 1996; Nakamura et al. 2000; Bourot-Denise et al. 2010), it appears that the abundance of trapped ³⁶Ar is affected by both fluid alteration (Fig. 10) and thermal metamorphism (Nakamura et al. 2000). This suggests destruction of the primary noble gas carrier phase (Marrocchi et al. 2005, 2011; Marrocchi and Marty 2013) during the course of secondary alteration. Thus, the amount of trapped ³⁶Ar, in association with petrographic and mineralogical observations, can be used to quantify the secondary alteration modifications experienced by chondrites. When applied to Paris, this indicator confirms that it should be considered the least-altered CM chondrite known so far, characterized by

intensive but not pervasive fluid alteration and mild thermal metamorphism.

The classification of Paris as a CM 2.7 (or 2.9 ± 0.1 according to Hewins et al. (2014) indicates an important limitation of the meteorite classification inherited from Van Schmus and Wood (1969). According to this scale, Paris is really close to the pristine chondrites 3.00 even though the two types of meteorite clearly differ as revealed by the important sign of aqueous alteration experienced by Paris. This poses a problem: the space left for potential CM chondrites less altered than Paris starts to shrink dangerously, which would likely lead to the use (and misuse) of extra digits for classifying CM chondrites. The existence of potential less altered CMs is suggested by the actual gap in oxygen isotopic along the trend defines by CO and CM chondrites (Hewins et al. 2014), assuming that the differences in the oxygen isotopic composition between CMs and COs is due to hydrothermal alteration, which has not been unambiguously demonstrated yet. In addition, there is growing evidence that chondrites could have undergone both secondary processes during their geological evolutions (Tonui et al. 2014) while the actual classification allow only one secondary process to be attributed to any given chondrite. This situation clearly calls for a reassessment of the meteorite classification in order to (1) give more flexibility to the aqueous alteration scale, which is currently concentrated along only two subdivisions (i.e., 3 to 1) and (2) take into account that a chondrite could have experienced aqueous alteration and thermal metamorphism.

## CONCLUDING REMARKS

Thanks to the unique nature of the Paris CM it has been possible to test the different indicators of secondary alteration experienced by CM chondrites. Our data confirm the reliability of (1) the alteration sequence proposed by Rubin et al. (2007) to quantify fluid alteration processes and (2) the use of Cr content variability in type II ferroan chondrule olivine as a proxy of thermal metamorphism. In addition, it appears that the amount of trapped ³⁶Ar is a sensitive indicator of the secondary alteration modifications experienced by chondrites, both for aqueous alteration and thermal metamorphism. Our data suggest that the Paris CM chondrite suffered partial but significant fluid alteration and mild thermal metamorphism and should be classified as type 2.7 according to the actual classification. However, this result highlights the challenge ahead to classify potential less altered CMs whose existence is suggested by the CO-CM correlation for oxygen isotopes.
*Acknowledgments*—Michel Fialin and Frédéric Couffignal are thanked for assistance during sample preparation and analysis. Jérôme Lavé is warmly thanked for his undeniable passion for the meteorite classification. Emmanuel Jacquet and Laurette Piani are thanked for helpful discussions. We thank Alan Rubin and an anonymous reviewer for constructive comments and associate editor Michael Zolensky for careful editing. This study was supported by grants from the *Programme National de Planétologie (INSU-CNRS)* and the *Région Lorraine*. This is CRPG-CNRS contribution #2313.

*Editorial Handling*—Dr. Michael Zolensky

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