L Krämer Ruggiu
email: lisa.kramer.ruggiu@vub.be

B Devouard

J Gattacceca

L Bonal

H Leroux

J Eschrig

D Borschneck

A J King

P Beck

Y Marrocchi

V Debaille

R D Hanna

O Grauby

DETECTION OF INCIPIENT AQUEOUS ALTERATION IN CARBONACEOUS CHONDRITES

Keywords: carbonaceous chondrites, ungrouped chondrites, aqueous alteration

We discuss if the detection of aqueous alteration depends on the techniques that are used.

We apply different methods to estimate the extent of aqueous alteration on four ungrouped carbonaceous chondrites showing limited aqueous alteration and thermal metamorphism: Chwichiya 002, El Médano (EM) 200, Northwest Africa (NWA) 12957 and NWA 11750, classified as C3 or C3.00-ung. The aim is to propose a reliable methodology to identify the most primitive chondrites. Chwichiya 002, NWA 11750 and NWA 12957 display very primitive matrices and could be amongst the most primitive chondrites currently known, similar to the least altered lithologies of the CM chondrites Paris (CM2.9) and Asuka (A) 12085 (CM2.8), A 12236 (CM2.9) and A 12169 (CM3.0). The structure of organic matter and Cr 2 O 3 in ferroan olivines show that the four meteorites have been less heated than the least metamorphosed standard/reference type 3 chondrite, Semarkona (LL3.00), with Chwichiya 002, NWA 12957 and NWA 11750 similar to the CO3.0s, Acfer 094 (C2-ung) and Paris meteorites. Chwichiya 002 and NWA 12957 show similar alteration phases and degree of alteration, with high abundances of amorphous material with embedded metal and sulfide, resembling Glass with Embedded Metal and Sulfide (GEMS)-like materials, and tochilinite-cronstedtite intergrowths (TCIs) as the major alteration phases. The matrix in NWA 11750 contains aggregates of nanoscale olivine crystals and abundant carbonates, observed as micrometer-sized carbonate veins surrounding chondrules, and as nanoscale carbonates mixed with the fine-grained materials. It also contains abundant grains of metal and a low abundance of phyllosilicates. El Medano 200 shows a high abundance of magnetite (~ 10 vol%), nanoscale phyllosilicates, troilite, and organic matter. The variability of the secondary alteration phases in the meteorites suggests different alteration mechanisms, likely depending on both the starting composition of the meteorites and the composition of the fluids of alteration.

Scanning and transmission electron microscopy (SEM and TEM) allow the identification of primitive phases, the composition and spatial distribution of the secondary phases. X-ray diffraction (XRD) can detect alteration products, including some amorphous phases, although this is limited by the small coherence domains of small TCIs and other phyllosilicates.

Transmission infrared (IR) spectroscopy can detect phyllosilicate and carbonate, but is ineffective for the detection of amorphous phases, metal, or sulfide. Both matrix defocused electron microprobe analyses (EMPA) and thermogravimetric analysis (TGA) allow detection of hydrated minerals, such as phyllosilicates and carbonates, but are strongly influenced by the presence of organic matter and do not reflect the overall alteration state of a meteorite. We conclude that the assessment of the primitivity of a chondrite is highly technique dependent. We propose a combination of XRD and the Cr 2 O 3 in ferroan olivines or Raman spectroscopy for a rapid characterization of the alteration state of a chondrite, and the detection of the most primitive meteorites. Finally, the combination of XRD and TEM allows for the detection of all primary and secondary phases and represents an ideal methodology for the characterization and detailed study of primitive chondrites and the different types of incipient aqueous alteration.

Introduction

The knowledge of the spatial distribution and composition of small Solar System bodies provides insight into the formation and dynamical evolution of the Solar System. Chondrites represent an invaluable source of samples from these bodies, allowing us to assess the detailed composition of the materials that accreted in the early Solar System. However, most chondrites underwent secondary processes: aqueous alteration, and thermal and shock metamorphism, which modified their primary mineralogy, textures, and isotopic compositions (e.g., [START_REF] Brearley | The action of water[END_REF][START_REF] Huss | Thermal metamorphism in chondrites[END_REF]. Even though they blur the information about the building blocks of the Solar System, these secondary features give precious information about the geologic activity of the chondrites' parent bodies.

Although chondrites are nearly all affected by secondary processes (e.g., [START_REF] Barber | Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites[END_REF][START_REF] Zolensky | Aqueous alteration[END_REF][START_REF] Stöffler | Shock effects in meteorites. Meteorites and the early Solar System[END_REF][START_REF] Scott | Shock metamorphism of carbonaceous chondrites[END_REF][START_REF] Buseck | Matrices of Carbonaceous Chondrite Meteorites[END_REF][START_REF] Brearley | The action of water[END_REF]Marrocchi et al., 2021), the search for the least processed samples has been a long-standing one in meteoritics, as such material would provide access to the physico-chemical conditions in the solar nebula. From this perspective, it is necessary to constrain secondary processes based on robust qualitative and quantitative parameters. Thermal metamorphism leads to known modifications in some carbonaceous chondrites, such as partial or complete dehydration of hydrous minerals [START_REF] Nakamura | Post-hydration thermal metamorphism of carbonaceous chondrites[END_REF], changes in composition and/or texture of Fe-Ni metal and sulfide phases [START_REF] Kimura | Fe-Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites[END_REF][START_REF] Kimura | Fe-Ni metal and sulfide minerals in CM chondrites: An indicator for thermal history[END_REF], and changes of the structure of organic matter [START_REF] Bonal | Thermal history of type 3 chondrites from the Antarctic meteorite collection determined by Raman spectroscopy of their polyaromatic carbonaceous matter[END_REF][START_REF] Bonal | Water and heat: New constraints on the evolution of the CV chondrite parent body[END_REF]. Those modifications can be evidenced by using numerous proxies, such as petrographic characteristics (e.g., [START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF], chemical and isotopic compositions, as well as infrared (IR) and Raman signatures [START_REF] Tonui | Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites[END_REF][START_REF] Bonal | Thermal history of type 3 chondrites from the Antarctic meteorite collection determined by Raman spectroscopy of their polyaromatic carbonaceous matter[END_REF][START_REF] Hanna | Distinguishing relative aqueous alteration and heating among CM chondrites with IR spectroscopy[END_REF]. The latter indicates a temperature range of thermal metamorphism in some carbonaceous chondrites from < 250°C to > 750°C [START_REF] Nakamura | Post-hydration thermal metamorphism of carbonaceous chondrites[END_REF][START_REF] Tonui | Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites[END_REF].

The source and timing of thermal metamorphism through decay of the short-lived radionuclide 26 Al and/or impact is hard to identify in carbonaceous chondrites [START_REF] Zolensky | Aqueous alteration[END_REF][START_REF] Grimm | Water and the thermal evolution of carbonaceous chondrite parent bodies[END_REF][START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF][START_REF] Huss | Thermal metamorphism in chondrites[END_REF][START_REF] Tonui | Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites[END_REF][START_REF] Vacher | Collisional and alteration history of the CM parent body[END_REF].

Besides long term thermal metamorphism through 26 Al decay, some carbonaceous chondrites show evidence of shock metamorphism, which can be observed in deformation and/or transformation of minerals inside chondrites and creates localized melting, metal-troilite mixtures, and changes of abundances of rare gases and volatile trace elements (Bischoff and Stöffler, 1992).

In addition to thermal and shock metamorphism, assessing the extent and nature of aqueous alteration in carbonaceous chondrites is necessary to understand the secondary processes in primitive asteroids. Aqueous alteration of chondrites may have taken place before final accretion [START_REF] Barshay | Chemistry of Primitive Solar Material[END_REF][START_REF] Grossman | Early chemical history of the Solar System[END_REF][START_REF] Lewis | Low temperature condensation from the solar nebula[END_REF][START_REF] Prinn | Solar nebula chemistry: origins of planetary, satellite and cometary volatiles[END_REF]Metzler et al., 1992, Bischoff 1998;[START_REF] Ciesla | A nebular origin for chondritic fine-grained phyllosilicates[END_REF][START_REF] Toppani | A "dry" condensation origin for circumstellar carbonates[END_REF] on temporary protoplanetary bodies, subsequently broken by collisions, scattering the altered materials into the nebula and mixing them with unaltered, anhydrous materials; or after accretion on a parent body formed by unaltered nebular materials [START_REF] Brown | The fall, recovery, orbit, and composition of the Tagish Lake meteorite: A new type of carbonaceous chondrite[END_REF][START_REF] Browning | Correlated alteration effects in CM carbonaceous chondrites[END_REF][START_REF] Bunch | Carbonaceous chondrites-II. Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface conditions[END_REF]Hanowski andBrearley, 2000, 2001;McSween, 1979a;[START_REF] Tomeoka | Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni[END_REF][START_REF] Trigo-Rodriguez | Non-nebular origin of dark mantles around chondrules and inclusions in CM chondrites[END_REF][START_REF] Zolensky | Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites[END_REF]. The temperature at which aqueous alteration took place is still debated and could range from -10°C to 250°C [START_REF] Baker | The oxygen isotopic composition of water from Tagish Lake: Its relationship to low-temperature phases and to other carbonaceous chondrites[END_REF][START_REF] Benedix | Carbonates in CM2 chondrites: Constraints on alteration conditions from oxygen isotopic compositions and petrographic observations[END_REF]Clayton and Mayeda, 1984;[START_REF] Guo | Temperatures of aqueous alteration and evidence for methane generation on the parent bodies of the CM chondrites[END_REF][START_REF] Verdier-Paoletti | Oxygen isotope constraints on the alteration temperatures of CM chondrites[END_REF]Vacher et al., 2019a;[START_REF] Zolensky | Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites[END_REF]. Moreover, different mechanisms, such as direct precipitation [START_REF] Morse | Influences of temperature and Mg:Ca ratio on CaCO 3 precipitates from seawater[END_REF] or interface dissolution-precipitation [START_REF] Morse | Influences of temperature and Mg:Ca ratio on CaCO 3 precipitates from seawater[END_REF][START_REF] Morse | Calcium carbonate formation and dissolution[END_REF][START_REF] Putnis | The mechanism of reequilibration of solids in the presence of a fluid phase[END_REF][START_REF] Ruiz-Agudo | Coupled dissolution and precipitation at mineral-fluid interfaces[END_REF]) could be at the origin of aqueous alteration products. These reactions can produce a variety of secondary products [START_REF] Krot | Infiltration metasomatism of the Allende coarse-grained calcium-aluminum-rich inclusions[END_REF]: carbonates [START_REF] Horita | Oxygen and carbon isotope fractionation in the system dolomite-water-CO_2 to elevated temperatures[END_REF][START_REF] Morse | Influences of temperature and Mg:Ca ratio on CaCO 3 precipitates from seawater[END_REF][START_REF] Morse | Calcium carbonate formation and dissolution[END_REF], phyllosilicates, tochilinitecronstedtite-intergrowths (TCIs), sulfides (and sulfates), magnetite [START_REF] Pignatelli | Multiple precursors of secondary mineralogical assemblages in CM chondrites[END_REF][START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF], fayalite [START_REF] Doyle | Early aqueous activity on the carbonaceous and ordinary chondrite parent asteroids recorded by secondary fayalite[END_REF] and kirschteinite [START_REF] Macpherson | 53 Mn-53 Cr chronology of Ca-Fe silicates in CV3 chondrites[END_REF].

In addition, deciphering the composition of the fluids responsible for the aqueous alteration is not straight-forward. Indeed, temperature, oxygen fugacity (fO 2 ), pH, water/rock ratio, as well as the chemical composition of the primary phases, can have an impact on the kinetics of the reactions and the secondary products. For example, formation of tochilinite in CM chondrites requires alkaline conditions, low fO 2 and low temperature [START_REF] Zolensky | Aqueous alteration[END_REF]Vacher et al., 2019b). Indeed, localized variations in geochemical conditions, particularly in pH, are present in CM chondrites, called "microchemical environments" [START_REF] Brearley | The action of water[END_REF], as for example, the alteration fluids in the fine-grained rims and matrix of CM chondrites are thought to be more alkaline than those in the interior of chondrules (e.g., [START_REF] Chizmadia | Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002[END_REF]Brearley, 2008, Brearley, 2006). Most CM chondrites are also considered to be complex breccias, comprising multiple lithologies that preserve differing hydration conditions (e.g., [START_REF] Lentfort | Classification of CM chondrite breccias-implications for the evaluation of samples from the OSIRIS-REx and Hayabusa 2 missions[END_REF], making classification of their alteration state challenging.

Efforts to quantify the degree of secondary processing in chondrites have resulted in the definition of a scale, called petrologic type. In 1967, Van Schmuss and Wood proposed a secondary classification of chondrites based on mineral chemistry (olivine and pyroxene homogeneity, water and carbon content, Ni in sulfides), petrography (low-Ca pyroxenes crystallinity, feldspar modal abundance) as well as matrix and chondrule textures. This classification splits chondrites into 6 petrographic groups, scaling from 1 to 6, with type 3 indicating the most primitive chondrites, based on their aqueous alteration and thermal metamorphism. This scale is independent of shock metamorphism, which is classified with its own scale [START_REF] Stöffler | Shock effects in meteorites. Meteorites and the early Solar System[END_REF][START_REF] Scott | Shock metamorphism of carbonaceous chondrites[END_REF]. Types from 3 to 1 present increasing degrees of aqueous alteration. Types from >3 to 6 show increasing degree of thermal metamorphism. This single digit scale is the accepted classification framework for petrologic types used to date, as attribution of a type for meteorites requires necessary simplification. Nonetheless, this classification has the disadvantage of separating thermal metamorphism and aqueous alteration, whereas numerous studies prove they can both occur within a single chondrite [START_REF] Buseck | Matrices of Carbonaceous Chondrite Meteorites[END_REF][START_REF] Kerridge | Aqueous activity on asteroids: evidence from carbonaceous meteorites[END_REF][START_REF] Krot | Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration[END_REF][START_REF] Tomeoka | Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni[END_REF][START_REF] Zolensky | Aqueous alteration[END_REF][START_REF] Zolensky | CM chondrites exhibit the complete petrologic range from type 2 to 1[END_REF][START_REF] Nakamura | Post-hydration thermal metamorphism of carbonaceous chondrites[END_REF][START_REF] Tonui | Petrographic, chemical and spectroscopic evidence for thermal metamorphism in carbonaceous chondrites I: CI and CM chondrites[END_REF]. Other classifications based on various characteristics, such as petrography, textures and chemistry [START_REF] Browning | Correlated alteration effects in CM carbonaceous chondrites[END_REF][START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF], bulk element composition (H, C and N) (Alexander et al., 2013), and/or phyllosilicate content [START_REF] Howard | Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments[END_REF] can also be used to classify chondrites, but are usually not established for all chondrite groups. Subtypes of the type 3 chondrites have been defined by [START_REF] Sears | Measuring metamorphic history of unequilibrated ordinary chondrites[END_REF] using thermoluminescence parameters. These subtypes can also be separated using the Cr 2 O 3 content of ferroan olivine [START_REF] Grossman | The onset of metamorphism in ordinary and carbonaceous chondrites[END_REF], or Raman spectral properties of the fine-grained matrix (Bonal et al., 2006[START_REF] Bonal | Thermal history of type 3 chondrites from the Antarctic meteorite collection determined by Raman spectroscopy of their polyaromatic carbonaceous matter[END_REF]. Subtypes between 2 and 3 have also been established for CM chondrites based on petrologic criteria [START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF][START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF][START_REF] Kimura | .8: Their characteristic features and classification[END_REF].

Meteorites with subtype 3.0 to ≥3.7 show evidence of both metamorphism and aqueous alteration [START_REF] Sears | Overview and classification of meteorites[END_REF][START_REF] Sears | Metamorphism and aqueous alteration in low petrographic type ordinary chondrites[END_REF][START_REF] Bonal | Water and heat: New constraints on the evolution of the CV chondrite parent body[END_REF]. One ambiguity of the current classification is that chondrites considered to be the most primitive are found between types 2.9 and 3.00 [START_REF] Hewins | The Paris meteorite, the least altered CM chondrite so far[END_REF][START_REF] Kimura | .8: Their characteristic features and classification[END_REF]. Also, detecting aqueous alteration is the intrinsic proprieties of the meteorites (e.g., porosity, permeability) and may thus be found only in specific parts of a meteorite [START_REF] Bland | Why aqueous alteration in asteroids was isochemical: High porosity ≠ high permeability[END_REF]. In addition, the nature and composition of the fluid can vary (e.g., water, calcium rich fluids, partial pressure of CO 2 (pCO 2 ) abundance, supercritical fluids), and thus induce a variety of alteration products, making the use of a transgroup aqueous alteration scale difficult. As it depends on the variety of effects of thermal metamorphism and/or aqueous alteration, identifying the most primitive chondrites turns out to be a challenge. In addition, many previously classified chondrites have not been studied in detail according to the most recent classification criteria, implying the existence of potential missclassified meteorites (e.g., [START_REF] Marrocchi | The Piancaldoli meteorite: A forgotten primitive LL3. 10 ordinary chondrite[END_REF]. The study of aqueous alteration in meteorites is also of interest in relation to the Hayabusa2 and Origins, Spectral Interpretation, Resource Identification, and Security-Regolith Explorer (OSIRIS-REx) spacecraft to asteroids (162173) Ryugu and (101955) Bennu, respectively. Both missions showed that surface materials from those asteroids were hydrated [START_REF] Hamilton | Evidence for widespread hydrated minerals on asteroid (101955) Bennu[END_REF][START_REF] Kitazato | The surface composition of asteroid 162173 Ryugu from Hayabusa2 nearinfrared spectroscopy[END_REF][START_REF] Lauretta | The unexpected surface of asteroid (101955) Bennu[END_REF]Pilorget et al., 2021).

Beyond the limitations of classification scales, the definition of aqueous alteration in chondrites remains vague. There are no widely accepted unique criteria to estimate if a chondrite has suffered aqueous alteration or not. In this paper, we review different methods used to estimate the extent of aqueous alteration in chondrites and apply them to the study of four ungrouped carbonaceous chondrites showing very little aqueous alteration and thermal metamorphism: Chwichiya 002, El Medano (EM) 200, Northwest Africa (NWA) 12957 and NWA 11750. We discuss whether the detection of aqueous alteration in these chondrites depends on the technique used or the investigation scale. We then propose the most reliable methodology to identify chondrites with no trace of metamorphism and low degree of aqueous alteration. We also compare the chondrites of this study to the chondrites long considered as the least aqueously altered and metamorphosed and show that three of the four chondrites in this study could be part of the most primitive material in the current meteorite collection.

Methodology

Samples

In order to investigate the ambiguity of the subtype 3.0-2.9-2.8, we chose to study four ungrouped carbonaceous chondrites whose official classification indicates no effects of metamorphism nor aqueous alteration; El Medano 200 (C3), Chwichiya 002, NWA 11750 and NWA 12957 (all C3.00-ung) [START_REF] Ruzicka | The Meteoritical Bulletin, No. 102[END_REF][START_REF] Gattacceca | The Meteoritical Bulletin, No. 107[END_REF]2021).

Petrography

Optical maps of the four meteorites were made with a Leica DM2500P petrographic microscope, using reflected light on thick polished sections. Backscattered Electron (BSE) images were taken with a scanning electron microscope (SEM) for all meteorites with a Hitachi S3000-N at the Centre Européen de Recherche et d'Enseignement en Géoscience de l'Environnement (CEREGE). Higher-resolution SEM images and element maps were made with a Field Emission Gun (FEG) microscope Jeol JSM 7900F equipped with an EDS (Energy-dispersive X-ray spectroscopy) XFlash 6/60 detector at Centre Interdisciplinaire de Nanoscience de Marseille (CINaM) laboratory.

Elemental maps were done using a probe current of 10 nA, an acceleration voltage of 5 kV and 20 min counting time.

Focused Ion Beam (FIB) sections were extracted from Chwichiya 002 at IEMN, University of Lille, using a dual-beam Thermo-Fisher Strata DB235 workstation; and NWA 11750 and EM 200 FIB sections were extracted at CP2M (Centre Pluridisciplinaire de Microscopie Électronique et de Microanalyse) Marseille using a dual-beam FIB (FEI Helios 600 Nano Lab), with a Gallium (Ga + ) ion beam, starting with 30 keV current. To limit the damage and amorphization of the slide faces under the ion beam, the current is reduced to 5 keV for thinning and then 1 keV for final cleaning. TEM observations of the FIB sections were made on a Thermo-Fisher Titan Themis S/TEM at the microscopy platform of the University of Lille for HAADF imaging and EDS mapping. Elemental maps were obtained using a four-quadrant, windowless, super-X SDD EDS detector with a probe current of 300 pA, an accelerating voltage of 300 keV, and a counting time of ~20 min. The uncertainties are of the order of 3% for major elements in silicates (O, Si, Mg and Fe-, and ~20% for the minor elements (Al, Ca, Mn, Cr).

High-resolution TEM (HR-TEM), and selected area electron diffraction (SAED) patterns were made on a TEM Jeol JEM-2010 at CINaM laboratory.

Modal abundances

The modal abundances of chondrite components (chondrules, matrix, opaques) were evaluated by point counting on optical maps, with 95% confidence interval around the modal abundance computed after Howarth 1998. Chondrule sizes were measured by contouring chondrules' outlines on optical maps, followed by image analysis, using the ImageJ software. Modal abundances of minor and alteration phases were measured on both FEG-SEM and STEM-EDS composite elemental maps, using pixel counting with the ImageJ software.

Mineral major and minor elements

Electron defocused beam (10 μm) microprobe analyses of the matrix were done using an electron microprobe (EMP) Cameca SX100 at UPMC CAMPARIS facility. The operating conditions were 15 kV accelerating voltage with a current of 10 nA and a counting time of 30 s. Both natural and synthetic standards were used for calibration: albite for Na; anorthite for Al; apatite for P; diopside for Mg, Si, Ca; orthoclase for K; pyrite for S; MnTiO 3 for Mn and Ti; Cr 2 O 3 for Cr; Fe 2 O 3 for Fe; and NiO for Ni. The detection limits are (in ppm): 445 for Na, 466 for Al, 714 for Mg, 669 for Si, 605 for Ca, 602 for K, 392 for S, 1044 for Mn, 419 for Ti, 496 for Cr, 1271 for Fe, and 1331 for Ni.

Defocused EMP analytical totals in the matrix are reduced by the presence of elements such as H and C, from hydrated minerals such as phyllosilicates (interlayered H 2 O and -OH groups), or hydration in the amorphous materials, as well as organic matter (C, H) and carbonates (C) not measured during matrix EMP analyses. Porosity does not lower the analytical total unless it is filled with epoxy resin (C-, H-and O-rich) [START_REF] Prêt | A new method for quantitative petrography based on image processing of chemical element maps: Part II. Semi-quantitative porosity maps superimposed on mineral maps[END_REF]. The samples used in this study were not embedded in epoxy under vacuum, thus only a negligible part of the porosity is filled with epoxy resin and should not influence the EMPA totals. Major elements' composition of minerals and alteration phases were obtained by EDS microanalyses on the Thermo-Fisher Titan Themis S/TEM of the University of Lille.

Bulk major and trace elemental analyses

Around ~50 mg of powdered samples were divided in 3 fractions to evaluate the possible chemical heterogeneity of the samples. Each subfraction was mixed with ~0.3 g of a 4:1 ultrapure metaborate-tetraborate mixture. The mixture was heated in a graphite crucible at 1000° C for 10 minutes. After cooling down, the bead was dissolved in 50 ml of 2N HNO 3 with a stirring magnet for 5 hours. Major element oxide concentrations were determined by measuring solutions on an ThermoScientific iCAP 7000 Series inductively coupled optical emission mass spectrometer (ICP-OES) at Laboratoire G-Time at ULB. Yttrium was used as internal standard and the calibration curves were obtained on artificial multi-element standards. Natural standards from USGS (BHVO-2, RGM-2) were used for quality control. The external reproducibility expressed as relative standard deviation (RSD) for individual oxide, based on 5 replicates of BHVO-2 and RGM-2 was better than 2%, except for K2O (<10%) ad P 2 O 5 (<13%).

Trace elements measurements were performed on an Agilent 7700 ICP-MS at Laboratoire G-Time at ULB. Indium was used as internal standard for trace elements analyses, and the calibration curves were also obtained on artificial multi-element standards. The USGS BHVO-2 natural standard was used for quality control. The RSD is better than 5% (based on 6 replicates of BHVO-2) for trace elements (except Rb and Lu, 7 and 10% respectively). It is better than 10% (based on 5 replicates of the BHVO-2), except Pb, better than 28%.

Infrared (IR) spectroscopy

IR reflectance spectroscopy was performed at the Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), using the SHADOWS instrument [START_REF] Potin | SHADOWS: a spectro-gonio radiometer for bidirectional reflectance studies of dark meteorites and terrestrial analogs: design, calibrations and performances on challenging surfaces[END_REF]. The instrument was used in normal standard mode as well as in a μ-beam mode enabling measurements of IR reflectance spectra of areas of ~3 mm². Spectra were measured under nadir incidence and with an observation angle of 30°. Spectra were normalized to infragold TM and spectralon TM . Each spectrum was measured from 0.3 µm to 2.6 µm, with a step of 20 nm.

Infrared transmitted spectra were obtained at IPAG at Grenoble France, using a Bruker Hyperion 3000 infrared microscope and a Vertex 70V Fourier transform infrared spectroscopy (FTIR) spectrometer. The infrared beam was focused through an 15x objective, creating a spot on the sample of 50x50 µm². Spectra were measured at 4 cm -1 spectral resolution over a range of 4000-650 cm -1 . Matrix pieces were chosen under binocular microscope (~50 µm) and transferred onto a 30.5 mm type IIa synthetic diamond window. The pieces were then crushed by another diamond window. Spectra of the four chondrites were also acquired from powdered samples. The window containing the most crushed pieces with optical shapes was analyzed in an environmental cell enabling exposure of the sample to a vacuum (10 -6 mbars). Infrared transmission spectra were acquired under vacuum at 20°C, at 100°C and 250°C or 300°C, in order to progressively remove the effect of adsorbed ambient water and study the meteorites under "dry conditions" [START_REF] Beck | Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids[END_REF].

Raman spectroscopy

Raman spectroscopy was performed at Laboratoire de Géologie de Lyon, Université Claude Bernard, Ecole Normale Supérieur de Lyon in France. Data were collected using a Labram HR800 spectrometer equipped with an Nd ion laser providing 532 nm excitation. Spectra were recorded in the 490-2230 cm -1 range.

Magnetic analyses

Petrophysical parameters (magnetic properties) were also used for a better characterization of the studied meteorites. Magnetic susceptibility was measured with a MFK1 or a KLY2 instrument, depending on the sample size. Saturation magnetization was determined from hysteresis loops measured with a Princeton model 2900 Micromag vibrating sample magnetometer (see [START_REF] Hewins | The Paris meteorite, the least altered CM chondrite so far[END_REF] for a comprehensive description). All magnetic measurements were performed at CEREGE.

X-ray Diffraction (XRD)

XRD analyses were conducted on both a polished section and bulk powder for NWA 11750 and NWA 12957, in order to see differences between the two sample preparation methods. For EM 200, analyses were made on a polished section, and for Chwichiya 002 on a powdered sample.

Classical XRD analyses were carried out with an X'Pert Pro MPD diffractometer (Panalytical) equipped with a cobalt tube (λ = 1.79 Å) operating at 40 kV and 40 mA, with a θ -2θ goniometer, at CEREGE. A rotation of the samples (powders and sections) in the horizontal plane at 15 rpm was used in order to improve the counting statistics. The powders were placed on Si supports (nondiffracting), using a few drops of ethanol, in order to obtain a thin and homogeneous layer of crystals, and the polished sections were fixed on circular supports. Analytical conditions were for powders, counting from 5° to 75° (2θ) in steps of 0.033°, for a total counting time of 4 hours; for the polished sections, from 4° to 75° (2θ) in steps of 0.033°, and a total counting time of 5 hours. Mineral identification was done using the X'Pert Highscore Plus (Panalytical) software and the ICDD's PDF2 database.

Additional PSD-XRD analyses were conducted on powder samples for all four meteorites at the Natural History Museum, London. For each meteorite, ~50 mg of powder was packed into an aluminum sample holder and analyzed using an Enraf-Nonius PDS120 X-ray diffractometer with an INEL curved 120° position-sensitive-detector (PSD-XRD). Cu-Kα 1 radiation was selected with a Ge <111> monochromator, and the size of the primary beam on the sample was restricted to 0.24×2.00 mm using a set of post-monochromator slits. Samples were rotated throughout the analyses and XRD patterns were collected from each meteorite for 16 hours, and from terrestrial mineral standards under the same analytical conditions for 30 minutes. Mineral abundances in the meteorite powders were determined using a profile-stripping method that has previously been described in detail [START_REF] Bland | Modal mineralogy of carbonaceous chondrites by X-ray diffraction and Mössbauer spectroscopy[END_REF][START_REF] Howard | Modal mineralogy of CM2 chondrites by X-ray diffraction (PSD-XRD). Part 1: Total phyllosilicate abundance and the degree of aqueous alteration[END_REF][START_REF] King | Modal mineralogy of CI and CI-like chondrites by X-ray diffraction[END_REF]. Briefly, the XRD pattern of a mineral standard was scaled to the same measurement time as the meteorite analysis (i.e., ×32) and then reduced by a factor in order to match its intensity in the pattern of the meteorite. The standard XRD pattern was then subtracted, leaving a residual meteorite pattern. This process was repeated for all phases identified in the meteorite until there were zero counts remaining in the residual and the sum of the fit factors was one. Fit factors were corrected for relative differences in X-ray absorption to give a final volume fraction in the meteorite [START_REF] Cressey | Rapid whole-pattern profile-stripping method for the quantification of multiphase samples[END_REF]. Uncertainties in the mineral abundances are typically <1% [START_REF] King | Modal mineralogy of CI and CI-like chondrites by X-ray diffraction[END_REF].

Thermogravimetric analysis (TGA-DSC)

Thermogravimetric and Differential Calorimetry analysis (TGA-DSC) were performed using a Mettler-Toledo TGA-DSC3+ instrument at the Institute des Sciences de la Terre (ISTerre, Grenoble, France). 50 mg of each sample was manually crushed with an agate mortar in order to obtain a fine powder. 35mg of this powder was then measured in a 70 μL alumina crucible under a 50mL/min inert N 2 atmosphere. The temperature was increased from 25°C to 1075°C at a heating rate of 10°C/min.

FTIR spectra of the degassed species were measured every 23 s over a period of 120 min. From the resulting TGA mass loss curve, the first derivative (DTG) curve can be calculated, which allows for a better visualization of temperature regions associated with strong mass losses. This, coupled with the IR spectra measured of the degassed phases, provides information about the quantity and type of hydrated mineral in the sample. The IR spectra are interpreted by integrating over wavelength ranges of specific chemical compounds of interest and looking at the evolution of these peak areas over time.

The wavelength ranges of interest used here are CO (2240-2040 cm -1 ), CO 2 (2450-2240 cm -1 ), CH (3000-2800 cm -1 ), H 2 O (4200-3000 cm -1 ) and SO 2 /SO 3 (1420-1330 cm -1 ) . For correction of background noise, several measurements of empty crucibles were performed and later subtracted from the TGA curves of the samples. To get boundaries on the evolution of the chemical compound peak areas in the IR spectra, the same wavelength ranges as mentioned above are assessed for the empty crucible, and the average and standard deviation are determined for each compound. This is done for all empty crucible measurements, and an average standard deviation is determined. The boundaries are shown as green dotted lines in the plots (Fig. S4-7).

X-ray computed tomography

Two chips of NWA 11750 were imaged on a Zeiss Versa 620 XRM at 100 kV and 12 W at the University of Texas High Resolution X-ray CT (UTCT) facility. For the smaller chip, 3001 views were collected on the 0.4 objective with an acquisition time of 5s per view. A LE2 filter was used and a beam hardening correction of 0.3 was applied during reconstruction and the final voxel size was 3.00 µm. The larger chip was imaged using the flat panel detector with 9001 views and a 0.05 s acquisition time per view. A LE3 filter was used and a beam hardening correction of 0.25 was applied during reconstruction with a final voxel size of 4.50 µm.

2.12

Oxygen isotopes

Carbonate oxygen isotopic compositions were analysed using a CAMECA IMS 1270 E7 at the Centre de Recherches Pétrographiques et Géochimiques (CRPG) laboratory in Nancy, France. A Cs+ primary ion beam (~1510 µm spot area) with a current of ~5 nA was used in order to collect 16 O -, 17 O -and 18 O -secondary ions in multi-collection mode using three Faraday cups (L'2, FC2 and H1). Charge compensation was applied using a normal-incidence electron gun. Mass Resolving

Power (MRP = M/ΔM) was adjusted to ~7000 to resolve interference from 16 OH -on the 17 O -peak and achieve maximum flatness on the top of the 16 O -and 18 O -peaks (entrance and exit slits of FC2

were adjusted to ~70 µm and ~170 µm, respectively). 16 O -and 18 O -secondary ions were collected on L'2 and H1, respectively (slit 1, MRP ≈ 2500). Pre-sputter on a large area (~2020 µm) was applied before each measurement during 60 s, to remove carbon coating at the surface of the calcite grains.

Acquisition time was set to ~5s, and measurements were repeated over 30 cycles to achieve counting

Results

Petrographic description

The petrographic characteristics of the studied chondrites are reported in Table 1.

Chwichiya 002, NWA 11750, and NWA 12957 show overall CM-like petrography (Fig. 1), with high matrix abundance (63.3-74.0 vol%), low chondrule abundance (12.9-26.0 vol%), and small chondrule apparent diameter (average in the range 240-480 µm) (Table 1), but differ from CM chondrites in their bulk oxygen isotopic compositions and lack of evidence for significant aqueous alteration [START_REF] Gattacceca | The Meteoritical Bulletin, No. 107[END_REF]. No amoeboid olivine aggregate (AOA) or calciumaluminum rich inclusions (CAIs) were observed in those meteorites. They were approved as type 3.00, based on the absence of detectable phyllosilicates in XRD patterns and IR transmission spectra, as well as Raman spectroscopy of the fine-grained matrix polyaromatic carbonaceous matter reflecting a peak metamorphism temperature lower than that for Semarkona (LL3.00) [START_REF] Bonal | Thermal history of type 3 chondrites from the Antarctic meteorite collection determined by Raman spectroscopy of their polyaromatic carbonaceous matter[END_REF]. They show a high FeO content in their matrices: 34.86 wt% for NWA 12957, 34.30 wt% for NWA 11750 and 39.39 wt% for Chwichiya 002 (Table 1). A distinguishing feature of NWA 11750 is the presence of large carbonate veins cutting through the matrix and as cracks in the fine-grained rims all around chondrules (Fig. S1). The position of the carbonate veins around the chondrules suggests hexagonal shapes, resembling desiccation patterns, an observation confirmed by CT data (Fig. S2). Carbonates in NWA 11750 have δ 18 O values of ~30-40 ‰, with slightly negative Δ 17 O (average -0.63 ‰) (Fig. 2). Although the carbonate O-isotopic compositions plot within errors on the TFL, they show do not show  18 O and  17 O values close to 0‰ as commonly observed for carbonates resulting from terrestrial weathering [START_REF] Tyra | Terrestrial alteration of carbonate in a suite of Antarctic CM chondrites: Evidence from oxygen and carbon isotopes[END_REF]. Due to the low permeability of CM chondrites [START_REF] Bland | Why aqueous alteration in asteroids was isochemical: High porosity ≠ high permeability[END_REF], contamination by the terrestrial water occurs only in fractures, and can be associated with evaporite minerals. The veins in NWA 11750 contain only calcite, with no gypsum. The oxygen isotopic composition of the calcite at the locations analyzed suggests that it is extraterrestrial in origin. EM 200 (C3 with affinities with CO3) [START_REF] Ruzicka | The Meteoritical Bulletin, No. 102[END_REF] has a smaller amount of matrix (44.1 vol%), a high proportion of magnetite (10.7 vol%) and a small chondrule size (130±80 µm) (Table 1). It is classified as a type 3 based on similar characteristics as Chwichiya 002, NWA 11750 and NWA 12957 and shows a lower concentration of FeO in its matrix (23.79 wt%) (Table 1). Because these four chondrites show a similar primitive petrologic type as the ungrouped chondrite Acfer 094, with limited secondary alteration, we followed the protocol in [START_REF] Abe | Occurrences, abundances, and compositional variations of cosmic symplectites in the Acfer 094 ungrouped carbonaceous chondrite[END_REF] to look for cosmic-symplectites (COS). We used a polished section of Acfer 094 to verify our method enabled us to identify COS using Fe-S-O EDS mapping. We were able to identify COS in Acfer 094, but no COS was observed in the four chondrites of this study.

Metamorphic state

The Raman parameters of the structural order of the polyaromatic matter in the matrices show that the studied chondrites appear to be less heated than the least heated type 3 ordinary chondrite Semarkona (LL3.00). Comparison of the Raman spectral parameters with the data from [START_REF] Quirico | Prevalence and nature of heating processes in CM and C2-ungrouped chondrites as revealed by insoluble organic matter[END_REF] show that the four studied meteorites do not show evidence for protracted metamorphic heating due to 26 Al decay observed in type 3 chondrites, but are rather similar to type 2 meteorites such as Murchison with only minor short-term heating probably due to shock (Fig. 3). EM 200 shows a larger variability of heating compared to the other three meteorites, with potentially slightly more heated zones. The ferroan olivine show Cr 2 O 3 content of 0.27 ± 0.12 wt% (n=31) for NWA 11750, 0.32 ± 0.10 wt% (n=16) for NWA 12957, 0.36 ± 0.10 wt% (n=42) for Chwichiya 002 and 0.36 ± 0.24 wt% (n= 30) for EM 200. The mean and standard deviations for Chwichiya 002 and NWA 12957 are close to the data for Acfer 094 (C2-ung) (Newton et al., 1994), while the data for NWA 11750 are close to the Y-81020 (CO 3.0), Colony (CO 3.0), and Paris (CM2) meteorites (Fig. 4) [START_REF] Grossman | The onset of metamorphism in ordinary and carbonaceous chondrites[END_REF][START_REF] Hewins | The Paris meteorite, the least altered CM chondrite so far[END_REF]. EM 200 has a higher standard deviation for Cr 2 O 3 content of ferroan olivine, setting it apart from the carbonaceous chondrites trend, and close to type 3.1 ordinary chondrites (Fig. 4). [START_REF] Pourkhorsandi | Modification of REE distribution of ordinary chondrites from Atacama (Chile) and Lut (Iran) hot deserts: Insights into the chemical weathering of meteorites[END_REF]. EM 200 and NWA 11750 show a slight depletion of Na, and enrichment of K, which is not related to the metamorphic or weathering state and could come from element redistribution on the parent body [START_REF] Braukmüller | The chemical composition of carbonaceous chondrites: Implications for volatile element depletion, complementarity and alteration[END_REF].

Bulk mineralogy

Bulk compositions

The high Ca content of Chwichiya 002, NWA 11750 and NWA 12957 is linked to a bulk high abundance of carbonates.

Visible near infrared reflectance

The visible-near-infrared (VIS-NIR) reflectance spectra show no strong absorption bands, with only very broad and shallow absorptions around 1 µm (Fig. 6). They do not display 2 µm absorption bands indicative of olivine and pyroxene and have a flat spectral VIS-NIR slope. They all show a reflectance maximum around 600 nm, with a slight positive slope from 1 µm to longer wavelengths. These dark and featureless spectra can be attributed to the high abundance of matrix (44.1, 63.0, 73.4 and 74.0 vol%) rich in phases producing no spectral features (carbonaceous matter, opaques) that can mask silicate absorption bands (Table 1). They are also consistent with the low abundance of chondrules (12.9 -35.2 vol%), rich in silicates, that would produce deep absorption bands around 1 µm and 2 µm (Table 1). The absorption bands of phyllosilicates (combination of the Fe 2+ and Fe 3+ absorptions at 0.7, 0.9, 1.1 μm, H 2 O absorption at 1.9 μm, and 2.3 μm absorption of -OH) are absent from the IR reflectance spectra of the studied samples.

Thus, no significant aqueous alteration products can be detected by VIS-NIR reflectance spectroscopy in those meteorites, agreeing with a type 3 classification for all meteorites.

Although the absence of large absorption bands is also found in CM chondrite spectra, the overall spectra of the studied meteorites are very different from spectra of CM chondrites (Fig. 6).

X-ray diffraction

Semi-quantitative modal abundances were calculated from the XRD and PSD-XRD patterns (Table 3). Errors are around 5 vol% for XRD and 1 vol% for PSD-XRD, depending on the phase and crystallinity. Only modal abundances of crystalline phases could be extracted from XRD patterns and all abundances are normalized to a total of 100 wt%. However, even when amorphous phases could not be quantified in the modal abundances, some were clearly detected on the XRD patterns of Chwichiya 002 and NWA 12957, leading to overestimation of the abundances of crystalline phases. The PSD-XRD measurements enabled the estimation of amorphous phase modal abundances by using Cu K-α1 radiation. Iron-bearing components influence the fluorescent background, and thus after removing other crystalline phases from the XRD patterns, the contribution of any Fe-bearing amorphous phases can be approximated [START_REF] Howard | Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments[END_REF][START_REF] King | Modal mineralogy of CI and CI-like chondrites by X-ray diffraction[END_REF]. For all of the meteorites, XRD patterns of polished sections and powdered samples showed similar minerals phases. Only the semi-quantitative abundances slightly vary, by no more than 5 vol%, depending on the sample preparation. This is within the error due to quantitative analysis of the patterns, as these abundances are only semi- (2 vol%). Pentlandite was not detected by XRD in any of the samples.

Thermogravimetric analysis

Table 4 shows the mass loss in each meteorite for the different temperature ranges usually used in the literature for TGA analyses [START_REF] Garenne | The abundance and stability of "water" in type 1 and 2 carbonaceous chondrites (CI, CM and CR)[END_REF]. Those temperature ranges were chosen as carbonaceous chondrite alteration phases release volatiles at different temperatures [START_REF] Garenne | The abundance and stability of "water" in type 1 and 2 carbonaceous chondrites (CI, CM and CR)[END_REF]. Table 4 shows the associated IR signature of gasses lost during heating at the different temperature ranges. Indeed, IR signatures of the gas should allow the identification of the phases at the origin of the mass losses in TGA and permit the refining of the phases associated with the temperature ranges in [START_REF] Garenne | The abundance and stability of "water" in type 1 and 2 carbonaceous chondrites (CI, CM and CR)[END_REF] (Table 4).

Between 0-200°C, all meteorites show loss of adsorbed and loosely bound water that are commonly attributed to terrestrial contamination but could also originate from interlayer water in smectite. As XRD patterns did not detect any smectite in the studied meteorites, we believe this temperature range is only associated with terrestrial contamination. Surprisingly, no meteorites showed IR signature of H 2 O at <200°C, which will be discussed in section 4.1.1. The second part of the TGA between 200-400°C is supposedly due to release of H 2 O and OH from (oxy)hydroxide minerals, usually linked to terrestrial weathering, although organic matter in the matrix can also increase the mass loss in this temperature range [START_REF] Gilmour | Water abundance in the Tagish Lake meteorite from TGA and IR spectroscopy: Evaluation of aqueous alteration[END_REF]. EM 200

shows a total mass loss of 13.43 wt%, with a signature of CH gases on the IR spectra, probably due to a high concentration of organic matter in the matrix. In this temperature range, NWA 12957 shows a mass loss of 2.96 wt%, with an IR signature of CO 2 , which can also be attributed to organic matter. Chwichiya 002 and NWA 11750 show a mass loss of around 3.24 wt% and 1.50 wt% respectively, with no signature on IR, probably due to loss of hydroxyl groups (-OH) from (oxy)hydroxides, according to their more weathered condition. The 400-770°C temperature range is directly correlated to abundance of phyllosilicates, due to the release of hydroxyl groups (-OH) [START_REF] Garenne | The abundance and stability of "water" in type 1 and 2 carbonaceous chondrites (CI, CM and CR)[END_REF], but SO 2 from sulfides, as well as CO 2 and CH from organics in the matrix increase the mass loss. NWA 11750 shows a significant mass loss of 8.55 wt%, with a signature of CO 2 /CO and CH, from organics; EM 200 has a mass loss of 7.56 wt% with a signature of CO 2 /CO and SO 2 , due to sulfides and abundant organics; and Chwichiya 002 and NWA 12957 show a mass loss of 6.81 wt% and 4.85 wt% with an IR signature of CO 2 /CO from organics. Finally, the temperature range 770-900°C is assigned to the CO 2 /CO release from carbonates [START_REF] Garenne | The abundance and stability of "water" in type 1 and 2 carbonaceous chondrites (CI, CM and CR)[END_REF]. EM 200 shows a mass loss of 5.68 wt% at this temperature and the other three meteorites show minimal mass losses < 1.0 wt%.

Matrix petrography

Electron microscopy

A combination of FEG-SEM, STEM, HR-TEM and SAED was used to identify phases, as well as to document textures and to determine modal abundances in the matrices of the four chondrites. SEM-EDS maps of the four meteorites have been acquired in order to document modal abundances in the matrix at a larger scale (crystals > 100 nm) (Fig. 7). A total of three SEM-EDS maps of ~225 μm 2 were studied for EM 200, Chwichiya 002 and NWA 12957, and five maps of ~225 μm 2 for NWA 11750. As all meteorites show heterogeneity in their matrices, we chose the most representative zone for each meteorite to extract the FIB sections used for TEM studies (Fig. S3). For Chwichiya 002, EM 200 and NWA 11750, one FIB section of ~60 μm² for each meteorite was extracted from the matrices in order to conduct High-angle annular dark-field imaging (HAADF) observations and STEM-EDS mapping. Modal abundance calculated from SEM-EDS and STEM-EDS maps are reported in Table 5.

On SEM-EDS images, Chwichiya 002 and NWA 12957 show broadly similar textures with micrometric forsterite and enstatite fragments, troilite, pentlandite, magnetite, and abundant micrometric TCIs aggregates, all set in a fine nanoscale grained matrix, named here "fine-grained mix" (Table 5, Fig. 7a and Fig. 7c). The difference in secondary phases between the two meteorites is the modal abundance of TCIs with 22.0 vol% and 5.3 vol% for Chwichiya 002 and NWA 12957; and sulfides with 4 vol% and 13.0 vol% for Chwichiya 002 and NWA 12957, respectively, with both Fe-Ni dominant sulfides. In addition, a minor abundance of metal, mostly kamacite, is observed in both meteorites. Chwichiya 002 shows a small amount of phosphates and brucite is observed in NWA 12957. Using STEM-EDS, the observation of the "fine-grained mix" in Chwichiya 002 allows the distinction of the different phases unresolved by SEM. It is composed of amorphous materials, TCIs, and a low abundance of forsterite, enstatite, metal, Feoxide, and pentlandite (Fig. 8a, Fig. 9a). Nanosized crystals of troilite are present in low abundance. The presence of amorphous material is confirmed using SAED that shows patterns typical of an amorphous material with characteristic diffuse rings (Fig. 10 A3). These amorphous phases contain abundant metal and sulfide inclusions (Fig. 10 A1,A2). The elemental composition of the amorphous component in Chwichiya 002 is on average O (63 at%), Si (16 at%), Fe (9.2 at%), Mg (7.3 at%), Al (1.9 at%) and Ca (1.3 at%) (n=5). The assemblage of amorphous material with inclusions of metal and sulfide resembles Glass with Embedded Metal and Sulfide (GEMS)-like material. The modal abundances are similar to the ones found in SEM-EDS maps, except that the STEM-EDS maps show higher porosity in the matrix of Chwichiya

On SEM-EDS maps, NWA 11750 shows micrometric grains of silicates (Mg-rich olivine and enstatite), troilite, kamacite, and tetrataenite grains and micrometric diffuse patches of carbonates and serpentine (Fig. 7d, Fig. S3b). Some larger silicate grains, that may well represent chondrules fragments, can be found with sizes from 10 μm to 1 mm. These grains are set in a large abundance of nanoscale fine-grained mix (Table 5). Pentlandite is found as traces in some matrix zones, although too small to be analyzed. STEM-EDS maps reveals that the "fine-grained mix" of NWA 11750 is composed of carbonates finely intermixed with abundant olivine, a few enstatite grains, Mg-rich serpentine, and subhedral to euhedral taenite metal grains (Fig. 8b, Fig. 9b). The studied FIB section STEM-EDS map shows two types of olivine: small nanoscale olivines (Fa ~25 ) forming aggregates with an engrained texture (Fig. 10 B1,B3), and a porous aspect with nanometric sulfides inclusions; and larger micrometric to millimetric grains of olivine (Fa ~10 ) (bottom of the section Fig. 8b). Traces of troilite, sulfates and Ca-phosphates are also observed in STEM-EDS maps. A difference between SEM-EDS and STEM-EDS abundances in NWA 11750 is the modal abundance of carbonates. This phase is well intermixed with olivine and pyroxene, as observed on STEM-EDS maps (32.6 vol%). Therefore, on SEM-EDS maps, only the larger veins of carbonates are observed (3.66 vol%), while the small, intermixed carbonates are "lost" in the "fine-grained mix".

SEM-EDS images of the matrix of EM 200 show micrometric silicates and magnetite, with submicrometric troilite, pentlandite, and metal (taenite) grains, all set in a nanosized "finegrained mix". No TCIs were found on FEG-SEM images (Fig. 7b, Fig. S3d). On STEM-EDS maps, the "fine-grained mix" is composed of olivine, pyroxene, metal, magnetite, troilite and pentlandite, embedded in a nanocrystalline phyllosilicates and organic matter assemblage (Fig. 8c, Fig. 9c). We observed Mg-rich olivine (Fe ~10-35 ) as well as Ca-rich and Ca-poor pyroxene.

Most of the FIB section shows recrystallization in fine lamellar nanoscale phyllosilicates and organic matter (Fig. 10 C1,C2), but silicate crystal borders do not show traces of alteration. EM 200 TEM maps also show thin and straight lamellae of TCIs, with dominant cronstedtite (Fig. 10 C1, C2), and rare grains of phosphates and chromite. Rare amakinite Fe(OH) 2 is observed interlayered inside one exceptionally large TCI (Fig. S8). The large modal abundance of magnetite identified from the bulk analyses (Table 1) is not observed at the scale of the matrix FIB section. This shows that magnetite is distributed heterogeneously in micrometric crystals rather than in nanometric grains in the matrix. The modal abundances between SEM and TEM

were not different within errors (Table 5), except for the porosity estimated abundance, higher on STEM, showing a nano-sized porosity. 6).

Electron microprobe (EMP) total in the matrix

Infrared Transmission Spectroscopy

The analyses were performed on several fragments of matrix from each meteorite to be as representative as possible. In Figure 12, we show the most representative spectra for each meteorite at 20°C, and 250°C (or 300°C for Chwichiya 002). Those two temperature steps were chosen to demonstrate the difference of hydration and reduce the effect of adsorbed terrestrial water. The 3000 cm -1 absorbance is due to interlayer H 2 O and -OH groups in phyllosilicates [START_REF] Tuddenham | Relation of infrared spectra and chemical analysis for some chlorites and related minerals[END_REF][START_REF] Oinuma | Infrared study of mixed-layer Clay Miner[END_REF][START_REF] Bishop | The influence of structural Fe, Al and Mg on the infrared OH bands in spectra of dioctahedral smectites[END_REF], or in the water from amorphous material. Indeed, recent studies showed that amorphous silicates in Acfer 094, Paris or CMs can contain considerable water (Le Guillou et al., 2018;[START_REF] Ohtaki | Search for meteoritic GEMS I: Comparison of amorphous silicates in Paris and Acfer 094 chondrite matrices and in anhydrous chondritic interplanetary dust particles[END_REF][START_REF] Villalon | Search for meteoritic GEMS II: Comparison of inclusions in amorphous silicates from the Paris chondrite and from anhydrous chondritic interplanetary dust particles[END_REF]. The position and intensity of those absorptions vary with the structure and chemical composition of the phyllosilicates. The low abundance of phyllosilicates in the studied samples does not create a sharp spectral feature from phyllosilicates in the 3000 cm -1 region, making it difficult to make a clear mineral identification. The 1000 cm -1 spectral feature is due to the absorption by Si-O groups (stretching) and reflects the silicate compositions, including secondary phases such as phyllosilicates. Extraterrestrial phyllosilicates can be recognized by a broad symmetric band around 1000 cm -1 [START_REF] Beck | Transmission infrared spectra, (2-25 μm) of carbonaceous chondrites (CI, CM, CV-CK, CR, C2 ungrouped): Mineralogy, water, and asteroidal processes[END_REF]. At 250°C, NWA 11750 matrix shows no hydration around the 3000 cm -1 absorption band. It shows a mixture of olivine and pyroxene (with a dominance of olivine), with no clear signs of phyllosilicates on the spectra in the 1000 cm -1 region. The presence of carbonates can be inferred from a band around 1440 cm -1 , but mineral identification was not possible since the carbonate mode around 880 cm -1 , most diagnostic of composition, could not be identified with enough accuracy. Chwichiya 002 shows a weakly hydrated matrix with a slight signature around 3000 cm -1 at 300°C. Silicate bands show no sign of phyllosilicates absorptions. NWA 12957 has a slightly more hydrated matrix than NWA 11750 and Chwichiya 002, with a feature at around 3000 cm -1 even at 250°C, and weak signatures of phyllosilicates in addition to olivine and pyroxene in the 1000 cm -1 region. Finally, EM 200 spectra show low hydration around 3000 cm -1 and low amounts of phyllosilicates in the 1000 cm -1 region, although higher than the other meteorites studied.

Discussion

Techniques for evaluating the alteration extent

Secondary phases can be generally found throughout meteorites. During the chondrite accretion process, water-ice grains are mainly accreted alongside the fine-grained matrix [START_REF] Matsumoto | Discovery of fossil asteroidal ice in primitive meteorite Acfer 094[END_REF], implying that the incipient aqueous alteration would first affect chondrite matrices. In addition, the fine-grained texture of the matrix is also prone to a faster alteration than larger silicates in chondrules. Therefore, most of the common alteration phases in chondrites are observed in the matrix, such as TCIs, carbonates, phyllosilicates, and sulfides (e.g., [START_REF] Brearley | The action of water[END_REF].

Bulk studies (TGA, XRD in this study) have the benefit of being more representative of a whole meteorite compared to studies of matrices only, but the signal of the secondary phases is "diluted" by the presence of large silicates from the chondrules (Tables 2 and6). Microanalyses of separated matrix pieces (transmitted IR, EMPA total in the matrix in this study) increases the ratio of the alteration phases compared to anhydrous silicates, facilitating their detection (Tables 4 and6). In addition, in situ analyses of the matrix (SEM, TEM) allow observation of the incipient aqueous alteration. Here we discuss the detection of secondary alteration phases in slightly altered chondrites. We conclude by proposing an ideal combination of methods to identify the most primitive chondrites through the characterization of the extent of aqueous alteration.

Bulk studies

Vis-NIR reflectance spectroscopy

Vis-NIR reflectance spectroscopy can be used to detect -OH, and H 2 O absorptions in meteorite samples, as well as on asteroid surfaces. In this work, we did not detect any hydration in the studied chondrites using Vis-NIR reflectance. The reflectance spectra are influenced by many factors other than the hydration state of the meteorites. Indeed, temperature, texture, grain size, opaque minerals, organic phases and terrestrial weathering can change the slope of the spectra [START_REF] Beck | Hydrous mineralogy of CM and CI chondrites from infrared spectroscopy and their relationship with low albedo asteroids[END_REF][START_REF] Garenne | Bidirectional reflectance spectroscopy of carbonaceous chondrites: Implications for water quantification and primary composition[END_REF][START_REF] Johnson | Optical properties of carbonaceous chondrites and their relationship to asteroids[END_REF][START_REF] Singer | Effects of temperature on remotely sensed mineral absorption features[END_REF][START_REF] Takir | 3-μm reflectance spectroscopy of carbonaceous chondrites under asteroid-like conditions[END_REF][START_REF] Hapke | Space weathering from Mercury to the asteroid belt[END_REF][START_REF] Cloutis | Spectral reflectance properties of carbonaceous chondrites 4: aqueously altered and thermally metamorphosed meteorites[END_REF]. A high abundance of phyllosilicates can also influence the spectral slope, in addition to having weak absorption bands around 0.7 μm, 0.9 μm, 1.1 μm and 2.3 μm, such as those detected in CM chondrites (Fig. 6). For weakly altered samples, phyllosilicates are not present in large enough amounts (Table 3) to be detected by IR reflectance spectroscopy as observed in the four studied chondrites (Fig. 6). This method is useful to find potential parent bodies of altered chondrites, through a comparison of IR reflectance spectra of meteorites and asteroid surfaces (e.g., [START_REF] Ruggiu | Visible-Infrared spectroscopy of ungrouped and rare meteorites brings further constraints on meteorite-asteroid connections[END_REF], but this technique is not efficient to detect minor degrees of aqueous alteration.

TGA

In the literature, TGA data are interpreted using temperature ranges that are associated with different gases released from minerals phases [START_REF] Garenne | The abundance and stability of "water" in type 1 and 2 carbonaceous chondrites (CI, CM and CR)[END_REF] (Table 4). However, in those temperature ranges, gases from other phases, such as organics or sulfides (including tochilinite from TCIs), can also be released (Table 4). The gases released at those temperature ranges show different signatures in the FTIR spectra than expected: CH and CO at 200-400°C;

CH, CO, and CO 2 at 400-770°C (Table 4, Fig. S4-S7). Therefore, the mass loss abundances represent only a "maximum" of oxy-hydroxides from terrestrial alteration (200-400°C), phyllosilicates (400-770°C) and carbonate (770-900°C), all without taking into consideration the potential contribution of sulfides, TCIs, or organic matter (Table 4). Indeed, the high proportion of organic matter may explain the significant mass losses for EM 200 at 200-400°C (13.43 wt%)

and at 700-900°C (5.68 wt%), as only small terrestrial alteration is observed, and no carbonates were detected with other instruments (Table 5, Table 7). In addition, the different temperature ranges do not allow for the distinction of different solid solutions of phyllosilicates, which are important in the characterization of secondary alteration phases: TCIs (with Fe-serpentine cronstedtite) and Mg-serpentine from olivine alteration. Surprisingly, none of the studied samples showed an IR signature of H 2 O at <200°C, which we tentatively attribute to water abundances below the detection limit of the instrument. Also, Chwichiya 002, NWA 11750 and NWA 12957 all show mass losses 1 wt% in the carbonate range (400-770°C), while showing bulk abundances of carbonates of 1-6 vol% on XRD (Table 3). Consequently, TGA associated with IR transmitted spectroscopy could represent a promising technique to measure the abundance of aqueous alteration phases, if some parameters (such as sulfides and organic matter content) are considered beforehand, and additional work on standards for the study of meteorites is made. Temperature ranges should be refined in order to precisely define the mass loss temperature for different phases. Furthermore, the organic matter content of each meteorite should be measured in order to calculate its influence in the mass loss.

XRD

The detection limits of our XRD analyses for quantitative modal abundances were ~ <5 vol% for most phases (Table 3). The detection limits of XRD are largely related to the composition and the crystallinity of the phases. For example, as the XRD pattern of calcite has one very intense peak, making it easily detected even in low abundances (<1 vol%), it could easily be detected in all meteorites (~2 vol%) except for EM 200, which also showed no carbonates with all other instruments (Table 7). Metal was also detected by XRD in the matrix (~1 vol%) of NWA 12957 and NWA 11750. Low abundances of magnetite were also detected (down to 2 vol% for NWA 12957) (Table 3). Sulfide detection via XRD is more complicated and depends on chemistry. Indeed, Fe-sulfides are hardly differentiated from one another because troilite and pyrrhotite have the same crystalline structure, creating only a subtle shift in the position of the peaks in XRD patterns, and could consequently be mixed or inverted. Fe-Nisulfides (pentlandite) were not detected by XRD in the studied chondrites because of low abundances and peak overlap with major silicate phases. XRD patterns showed small phyllosilicate (serpentine) peaks only in Chwichiya 002 (2 vol%). Phyllosilicates had been overlooked in the analyses performed for the registration of the meteorite by the Meteoritical Society [START_REF] Ebert | Northwest Africa 11024-A heated and dehydrated unique carbonaceous (CM) chondrite[END_REF]. PSD-XRD identified poorly crystalline phyllosilicates (serpentine) in all samples (2-6 vol%). XRD and PSD-XRD "phyllosilicate" detection include the detection of both Mg-and Fe-serpentine phases, also including cronstedtite in large TCIs (with a large enough coherence domain).

Poorly crystalline phases with low intensity peaks, such as small TCIs or phyllosilicates, are harder to detect than crystalline materials. The interstratified tochilinite and cronstedtite in TCIs results in small coherence domains, making detection by XRD challenging. PSD-XRD analysis found 2 vol% phyllosilicates in EM 200 (Table 3), although TEM analyses showed a high proportion of nanoscale phyllosilicate (54 vol% of the FIB section) (Fig. 10 C1,C2).

Nanoscale phyllosilicates in EM 200 have too small coherence domains to be detected in XRD patterns. Abundances of amorphous material can be deduced from PSD-XRD patterns and are important to assess the potential presence of Glass with Embedded Metal and Sulfide (GEMS)like amorphous material in primitive chondrites (Table 7). Although we suspect most amorphous phases are GEMS-like materials in Chwichiya 002 and NWA 12957, like those we observed in STEM in Chwichiya 002 (Fig. 9a), other phases such as extremely fine-grained phyllosilicates with small coherency domains also create broad and weak diffraction peaks, increasing the estimated amorphous phase abundance. Therefore, the amorphous abundances estimated from PSD-XRD provide an upper limit of GEMS-like phases and could also include other poorly crystallized alteration phases. In conclusion, XRD provides a useful bulk technique for detecting the diversity of primary and secondary phases in the studied chondrites. XRD can detect all primary phases, as well as secondary phases (phyllosilicates, carbonates, TCIs, secondary sulfides, magnetite).

Matrix analyses

IR transmitted spectroscopy

Chwichiya 002, EM 200 and NWA 12957 show small absorptions around 3000 cm -1 at high temperature (> 250°C) (Fig. 12), suggesting low abundances of phyllosilicates, including cronstedtite, in the TCIs. The hydration signal could also reflect the hydration of amorphous silicates, observed in Chwichiya 002 by HR-TEM. Weak phyllosilicate absorption bands in the 1000 cm -1 region are also detected for EM 200, for which abundant nanoscale phyllosilicates were observed in the matrix by TEM-EDS (54 vol%) (Table 5). This demonstrates that IR transmission spectroscopy is sensitive to small bulk abundances (<2 vol%) of phyllosilicates, including the abundance of TCIs in the matrices of the meteorites. IR transmission analyses also allow the detection of carbonates around 1500-800 cm -1 . NWA 11750 shows low IR absorption of carbonates (Fig. 12), while its matrix shows high abundances of carbonates up to 32.6 vol% using FEG-SEM/TEM (Table 5). IR transmitted spectroscopy is a useful tool to detect phyllosilicate as well as the cronstedtite in TCIs but is limited for the detection of carbonate. It is also unable to detect magnetite and sulfides (troilite, pentlandite, pyrrhotite), which are common products of aqueous alteration in carbonaceous chondrites.

Defocused EMP analyses in the matrix

This technique allows for an initial characterization of the general alteration state and hydration of the meteorite groups, CV being the most metamorphosed, and CM/CI showing the highest abundance of secondary hydrated phases. With the four studied meteorites plotting close to CI (EM 200), CM and CO (Chwichiya 002 and NWA 12957), as well as CV chondrites (NWA 11750) (Fig. 11), this method is not adequate to directly characterize the aqueous alteration state of chondrites, because it is too highly influenced by other parameters. EMPA totals show deviations compared to bulk analytical methods due to the influence of the different densities of the various phases in the matrix on the quantification results (e.g., Zanda et al., 2018), and a density correction should be added to improve the results [START_REF] Zanetta | Modal abundance, density and chemistry of micrometer-sized assemblages by advanced electron microscopy: Application to chondrites[END_REF]. Defocused EMPA totals can be lowered by the content of magnetite (Fe 3 O 4 ) falsely reported as FeO, and the presence of organic matter (high in the studied chondrites, even if no quantification has been done on carbon). An additional complexity is the presence of metal counted as FeO, which will artificially increase the total. For Chwichiya 002, EM 200 and NWA 12957, this factor is negligible as they possess a low abundance of metal (≤ 1 vol%) but it could increase NWA 11750

EMPA totals. The totals can be corrected by using the approximative metal and magnetite compositions and abundances, although these methods do not seem adequate for the assessment of the aqueous alteration state of chondrites. In addition, sulfides (troilite, pyrrhotite, pentlandite)

do not influence the EMPA totals but are very sensitive indicators of secondary alteration (e.g., [START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF]. In conclusion, the alteration stages defined by EMPA totals cannot reflect the overall alteration stage, as too many factors other than secondary alteration influence the totals, while major secondary products (sulfides) do not.

Electron microscopy

SEM and TEM are the only instruments that enable observation of the relationships between the various phases and their locations in situ, and EDS mapping gives the chemical compositions of all different primary and secondary phases (Fig. 9). These techniques allow identification of metal grains, silicates, and amorphous GEMS-like material as well as incipient crystallization of known secondary phases, including TCIs, carbonates, secondary sulfides, and magnetite. The analyses using FEG-SEM and EDS are rapid and allow the study of multiple zones of the matrix.

Nonetheless, the modal abundances obtained from SEM-and TEM-EDS maps are directly correlated to the spatial resolution of the techniques. The study of matrix using FEG-SEM is limited to the observation of ca. >100 nm crystals (at 5 kV acceleration voltage). Thus, the nanoscale fine-grained matrix is not accessible through FEG-SEM mapping. FEG-SEM is used to determine the most representative area of the alteration state of the chondrite but cannot identify the nano-scale secondary phases. Thus, the modal abundances obtained from FEG-SEM maps should be taken as the modal abundances of micron-sized and larger materials with an unresolved "fine-grained mix". The TEM resolution can go down to atomic resolution providing highdefinition imaging that allows the identification of the nano-sized grains in the fine-grained matrix, with the additional application of EDS and SAED techniques. It can be used to refine the results of the larger-scale SEM EDS mapping, such as the "fine-grained mix" (Fig. S3). This "fine-grained mix" was identified as mainly GEMS-like material and TCIs, with smaller abundance of magnetite, pentlandite, pyroxene, olivine and metal in Chwichiya 002; mainly carbonates and olivine crystals, with smaller abundance of phyllosilicates, sulfides, magnetite, metal and pyroxene in NWA 11750; and as mainly nanophyllosilicates and organic matter, with smaller abundance of sulfides, magnetite, metal, pyroxene and olivine in EM 200.

The use of SAED in TEM is crucial as it is the only technique allowing the clear identification of GEMS-like material, whereas the detection of amorphous material by XRD can include weakly crystalline phases, and nanoscale intermixed crystals (small TCIs or phyllosilicates). HR-TEM can also be used to look for incipient alteration of the margins of the silicate grains as evidence of incipient aqueous alteration. The limitation of TEM studies for alteration in chondrites is the area of observation related to FIB preparation techniques: typical section prepared by FIB methods are ca. 60 μm² and are rarely representative of the matrix variability present in a meteorite. Careful selection of location of the FIB sections using SEM mapping can limit this bias. However, the zones of FIB section extraction are usually chosen away from large porosities to avoid breaking during the FIB section extraction, or away from large veins or crystals, or even away from zones with unusually monomineral-rich zones, which creates a sampling bias in the modal abundances estimated from TEM observations. Thus, other bulk analyses can identify some phases missing from TEM observation, due to an absence of those phases in the extracted region of the FIB section (e.g., large magnetite grains in EM 200; large carbonates veins in NWA 11750; carbonates in NWA 12957 and Chwichiya 002) (Table 7).

In conclusion, TEM allows the identification of all primary and secondary phases from incipient aqueous alteration, and thus the characterization of primitive chondrites, but should be used in combination with larger scale instruments such as SEM and/or XRD.

4.2

The alteration state of the studied chondrites

Metamorphism

Using Raman spectroscopy, Chwichiya 002, NWA 11750 and NWA 12957 show a petrologic type <3.00. Raman parameters are closer to those of type 2 chondrites having experienced short time heating, probably due to shock, equivalent to R1 (highly primitive with no structural modifications by heating) and R3 (slight structural modifications induced by a weak heating) from [START_REF] Quirico | Prevalence and nature of heating processes in CM and C2-ungrouped chondrites as revealed by insoluble organic matter[END_REF] (Fig. 3). Meteorites from R2 group (strongly heated chondrites) are described as potentially dehydrated, while R1 and R3 groups are generally not dehydrated. Thus, even if the studied meteorites have undergone short time heating, most likely triggered by shocks or impacts, their Raman spectra parameters do not show evidence of longterm metamorphism necessary to dehydrate CM chondrites.

The Cr 2 O 3 content in ferroan olivine of Chwichiya 002, NWA 11750 and NWA 12957 are close to unmetamorphosed meteorites such as Acfer 094 (C2-ung), Y-81020 and Colony (both CO3.0 chondrites), and Paris (CM2.7-2.9) (Fig. 4). EM 200 shows higher standard deviation of the Cr 2 O 3 contents, which is consistent with its Raman spectra parameters that show a larger variability of short-time heating (Fig. 3). [START_REF] Ebert | Northwest Africa 11024-A heated and dehydrated unique carbonaceous (CM) chondrite[END_REF] show that a long-term peak temperature of 400-600°C is required to dehydrate CM chondrites.

In addition, heated CMs show no hydration absorption on infrared spectroscopy, but the IR spectra (transmission in matrix) of EM 200 shows a visible 3 μm band, Chwichiya 002 and NWA 12957 show a slight 3 μm band, but NWA 11750 shows no obvious 3 μm band from hydration (Fig. 12). Finally, the 10 μm absorption band does not reflects a high abundance of phyllosilicates, strengthening the non-dehydrated by heating hypothesis.

Aqueous alteration

The chondrites in this study are classified as type 3 for EM 200 and type 3.00 for the others, which all show a CO-CM-like petrography. The evolution of aqueous alteration in CM or CM-like chondrites has been studied extensively (e.g., [START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF]Suttle et al., 2021 and references therein). The composition of the alteration products is defined by the primary composition, but is also influenced by the fluid composition [START_REF] Velbel | Replacement of olivine by serpentine in the Queen Alexandra Range 93005 carbonaceous chondrite (CM2): Reactant-product compositional relations, and isovolumetric constraints on reaction stoichiometry and elemental mobility during aqueous alteration[END_REF]. The four meteorites in this study show variation in both the degree of alteration and their alteration phases, suggesting different alteration conditions and/or fluid as well as primary compositions.

Chwichiya 002 and NWA 12957 show similar petrography as well as similar alteration, which strengthens their potential pairing. Their potential pairing is yet to be verified, however, because their oxygen isotopic compositions (Δ 17 O=-4.59 for NWA 12957 and Δ 17 O=-3.88 for Chwichiya 002) and Raman parameters show differences (Fig. 3). They show alteration with micron-sized aggregates of TCIs (Fig. 8a, Fig. 9a), and low abundances of carbonates, magnetite, and secondary sulfides (pyrrhotite, pentlandite) (Table 5). They show abundant amorphous material on PSD-XRD analyses, with GEMS-like materials identified using SAED in Chwichiya 002, and extensive estimated porosity on TEM observations (Fig. 10, A1-A3). In addition, they have a low abundance of metal preserved from alteration (≤ 1 vol%), probably dissolved or transformed into magnetite (Table 5). These meteorites suggest aqueous alteration by a fluid dissolving metal grains and troilite, while not completely altering amorphous materials to phyllosilicates, and crystalizing a low abundance of TCIs, carbonates, magnetite, and secondary sulfides (pentlandite, pyrrhotite). Small error bars of Raman parameters (Fig. 3) and small RSD of bulk major composition (Table 2) suggest homogenous alteration of the meteorites.

NWA 11750 shows abundant unaltered metal grains (3 vol% in the matrix) and primary troilite and pyrrhotite sulfides (~3 vol%), with a low abundance of phyllosilicates (serpentine), and no TCIs or magnetite (Table 5). NWA 11750 contains abundant carbonate in its matrix (Fig. 2). It does not display GEMS-like materials (Table 5) and does not show a hydration signature in IR spectra (Fig. 12). All these characteristics attest to a minimally altered matrix, except for the absence of GEMS-like materials. Besides carbonates, NWA 11750 matrix is largely composed of nanocrystals of olivine (Fig. 10 B1,B3). NWA 11750 matrix composition can be interpreted as an alteration from CO 2 -rich aqueous fluid, dissolving any GEMS-like material that might have been present and/or converting it to phyllosilicates, while preserving metal and sulfides (troilite), and crystallizing abundant carbonates, similar to that observed in some CM chondrites [START_REF] Tsuchiyama | Discovery of primitive CO2-bearing fluid in an aqueously altered carbonaceous chondrite[END_REF]. NWA 11750 in an exceptional chondrite in view of its high amount of extraterrestrial carbonates in the matrix that contrasts with an overall very subtle aqueous alteration, making it a sample of choice to study CO 2 and Ca-rich aqueous fluid-assisted alteration, that could also give insight into the carbonate formation on CM-like asteroids such as Bennu [START_REF] Kaplan | Bright carbonate veins on asteroid (101955) Bennu: Implications for aqueous alteration history[END_REF]. Both Raman parameters (Fig. 3) and bulk major composition (Table 2) suggest homogenous alteration of NWA 11750.

EM 200 shows a different alteration style, with abundant preserved primary sulfides (troilite), and no amorphous phases, nor TCIs and carbonates. It shows a high abundance of magnetite, with 10.7 vol% by magnetic analyses (Table 1). TGA and defocused EMPA totals both indicate abundant organic matter in the matrix, which is also observed by TEM associated with abundant nanophyllosilicates (Fig. 10 C1,C2). EM 200 is the least primitive of the four studied meteorites, with a higher degree of metamorphism and aqueous alteration. Both Raman parameters (Fig. 3) and bulk compositions (Table 2) also suggest a more heterogeneous alteration compared to the other three meteorites. It was altered by a fluid dissolving any primary amorphous phases that might have been present, while preserving primary troilite, and crystallizing abundant magnetite, with a low abundance of serpentine and carbonates.

Aqueous alteration in CM-like chondrites

GEMS-like assemblage

Glass with Embedded Metal and Sulfide (GEMS) are oxygen-rich amorphous materials, of ~50-500 nm diameter, found in interplanetary dust particles IDPs. They are dominated by a

Mg-rich and Fe-poor amorphous silicate with nanoscale sulfides (pyrrhotite) and metal (kamacite) inclusions, typically ~a few to 10 nm diameter, with often admixed carbon [START_REF] Bradley | Chemically anomalous, preaccretionally irradiated grains in interplanetary dust from comets[END_REF][START_REF] Dobrică | Transmission electron microscopy of CONCORDIA ultracarbonaceous Antarctic micrometeorites (UCAMMs): mineralogical properties[END_REF]. So far, GEMS are identified in cometary-type (anhydrous chondritic porous) IDPs and ultracarbonaceous micrometeorites (UCAMMs) [START_REF] Bradley | Chemically anomalous, preaccretionally irradiated grains in interplanetary dust from comets[END_REF](Bradley , 2013;;[START_REF] Dobrică | Transmission electron microscopy of CONCORDIA ultracarbonaceous Antarctic micrometeorites (UCAMMs): mineralogical properties[END_REF][START_REF] Ishii | Multiple generations of grain aggregation in different environments preceded solar system body formation[END_REF]. GEMS are thought to be radiation-processed presolar dust condensed prior to the presolar molecular cloud [START_REF] Bradley | Chemically anomalous, preaccretionally irradiated grains in interplanetary dust from comets[END_REF], and/or highly processed presolar interstellar dust condensed from the proto-solar nebula [START_REF] Ishii | Multiple generations of grain aggregation in different environments preceded solar system body formation[END_REF], and/or condensed in the cold presolar molecular cloud and/or in the outer protoplanetary disk [START_REF] Keller | On the origins of GEMS grains[END_REF].

Large abundance of amorphous silicates has been found in the matrices of primitive CR chondrites [START_REF] Brearley | Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307: Origins and evidence for diverse, primitive nebular dust components[END_REF][START_REF] Abreu | Early solar system processes recorded in the matrices of two highly pristine CR3 carbonaceous chondrites, MET 00426 and QUE 99177[END_REF]Le Guillou et al., 2015b;[START_REF] Guillou | Relationships between organics, water and early stages of aqueous alteration in the pristine CR3. 0 chondrite MET 00426[END_REF][START_REF] Vollmer | Amorphous silicates as a record of solar nebular and parent body processes-A transmission electron microscope study of fine-grained rims and matrix in three Antarctic CR chondrites[END_REF][START_REF] Zanetta | Processes and temperatures of FGR formation in chondrites[END_REF], as well as in several other carbonaceous chondrites [START_REF] Brearley | Matrix and fine-grained rims in the unequilibrated CO3 chondrite, ALHA77307: Origins and evidence for diverse, primitive nebular dust components[END_REF]Vollmer et al., 2009;[START_REF] Nguyen | Coordinated analyses of presolar grains in the Allan Hills 77307 and Queen Elizabeth Range 99177 meteorites[END_REF]Nittler et al., 2013). Some of the least altered, most primitive carbonaceous chondrites show extremely finegrained amorphous silicate with embedded metal and sulfides, resembling GEMS assemblages [START_REF] Bradley | Chemically anomalous, preaccretionally irradiated grains in interplanetary dust from comets[END_REF][START_REF] Greshake | The primitive matrix components of the unique carbonaceous chondrite Acfer 094: A TEM study[END_REF][START_REF] Noguchi | Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites[END_REF][START_REF] Hopp | Chemical composition and iron oxidation state of amorphous matrix silicates in the carbonaceous chondrite Acfer 094[END_REF][START_REF] Chizmadia | Mineralogy, aqueous alteration, and primitive textural characteristics of fine-grained rims in the Y-791198 CM2 carbonaceous chondrite: TEM observations and comparison to ALHA81002[END_REF], often referred to amorphous or GEMS-like materials [START_REF] Barber | Matrix phyllosilicates and associated minerals in C2M carbonaceous chondrites[END_REF][START_REF] Hewins | The Paris meteorite, the least altered CM chondrite so far[END_REF][START_REF] Marrocchi | The Paris CM chondrite: Secondary minerals and asteroidal processing[END_REF][START_REF] Leroux | GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites[END_REF][START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF][START_REF] Vollmer | Amorphous silicates as a record of solar nebular and parent body processes-A transmission electron microscope study of fine-grained rims and matrix in three Antarctic CR chondrites[END_REF][START_REF] Zanetta | Nebular thermal processing of accretionary fine-grained rims in the Paris CM chondrite[END_REF][START_REF] Ohtaki | Search for meteoritic GEMS I: Comparison of amorphous silicates in Paris and Acfer 094 chondrite matrices and in anhydrous chondritic interplanetary dust particles[END_REF][START_REF] Villalon | Search for meteoritic GEMS II: Comparison of inclusions in amorphous silicates from the Paris chondrite and from anhydrous chondritic interplanetary dust particles[END_REF]. Those GEMS-like phases found in chondrites are comparable to the GEMS assemblages found in IDPs, i.e. they consist of an amorphous silicate with embedded metal and sulfide inclusions. GEMS-like phases identified in Chwichiya 002 are similar to GEMS-like phases found in the most primitive meteorites but differ in their higher Fe-content of the amorphous matrix, larger troilite and metal inclusions, and show larger sizes than IDPs GEMS phases.

Those GEMS-like materials could be the primary component of the chondritic matrix as they are observed in the most primitive micrometeorites, and in the most primitive CM, CR, CO and ungrouped chondrites such as Acfer 094 and ordinary chondrites. GEMS-like phases are thought to be the first phases to be dissolved by aqueous alteration, through leaching and oxidization, and are recrystallized in nanophase assemblages of hydrated phyllosilicates and secondary sulfides (e.g., Le Guillou et al., 2015;[START_REF] Noguchi | Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites[END_REF]Vacher et al., 2019b).

Their presence in Chwichiya 002 attests than the meteorite is relatively well preserved from aqueous alteration and thermal metamorphism. One of the least altered CM chondrites, Paris, contains a primitive lithology, classified as type 2.9 that contains abundant GEMS-like materials [START_REF] Marrocchi | The Paris CM chondrite: Secondary minerals and asteroidal processing[END_REF][START_REF] Hewins | The Paris meteorite, the least altered CM chondrite so far[END_REF][START_REF] Leroux | GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites[END_REF][START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF] (Table 8).

Recently, [START_REF] Kimura | .8: Their characteristic features and classification[END_REF] proposed three meteorites as the closest ones to theoretical CM3.00: Asuka (A) 12085 (CM2.8), A 12236 (CM2.9) and A 12169 (CM3.0). The Asuka meteorites contain abundant amorphous materials with sulfides and enstatite whisker inclusions in the matrix observed using TEM, suggesting a lower degree of alteration than the most preserved lithology of Paris [START_REF] Kimura | .8: Their characteristic features and classification[END_REF]. High abundances of amorphous phases are detected in Chwichiya 002 and NWA 12957 by PSD-XRD (12-23 vol%) (Table 3). GEMS-like materials are unambiguously identified in Chwichiya 002 and show amorphous structure on SAED (Fig. 10 A3), showing less recrystallization with aqueous alteration, as attested by more diffuse rings, than the most preserved lithology in Paris [START_REF] Leroux | GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites[END_REF], being similar to A 12169 [START_REF] Kimura | .8: Their characteristic features and classification[END_REF]. The high Ca content of the amorphous phases of Chwichiya 002 can be created by terrestrial reactions, as those meteorites show a slight terrestrial weathering, potentially crystallizing small abundance of terrestrial carbonates.

Metal

Most type 3.00 carbonaceous chondrites show abundant metal grains (>2 vol%) as observed in some C-ung meteorites [START_REF] Davidson | Mineralogy and petrology of Dominion Range 08006: A very primitive CO3 carbonaceous chondrite[END_REF][START_REF] Greshake | The primitive matrix components of the unique carbonaceous chondrite Acfer 094: A TEM study[END_REF][START_REF] Van Kooten | Isotope record of mineralogical changes in a spectrum of aqueously altered CM chondrites[END_REF][START_REF] Noguchi | Variation of mineralogy and organic material during the early stages of aqueous activity recorded in Antarctic micrometeorites[END_REF][START_REF] Rubin | Collisional facilitation of aqueous alteration of CM and CV carbonaceous chondrites[END_REF][START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF][START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF][START_REF] Scott | Thermal processing of silicate dust in the solar nebula: clues from primitive chondrite matrices[END_REF][START_REF] Singerling | Primary iron sulfides in CM and CR carbonaceous chondrites: Insights into nebular processes[END_REF], A 12169 (> 2 vol%), A 12236 and A 12085 (1-2 vol%) [START_REF] Kimura | .8: Their characteristic features and classification[END_REF], or

Paris (1.2-3 vol%) [START_REF] Hewins | The Paris meteorite, the least altered CM chondrite so far[END_REF][START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF]. In general, metal abundance decreases with aqueous alteration, and is thus thought to be a primary constituent of chondrites [START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF], although tetrataenite or awaruite could be produced by secondary processes in CVs [START_REF] Bonal | Water and heat: New constraints on the evolution of the CV chondrite parent body[END_REF]. The meteorites in this study have variable metal abundances: NWA 11750

shows 3 vol% by XRD, close to the most preserved lithology in the Paris meteorite, and higher than A 12169; Chwichiya 002 and NWA 12957 both contain 1 vol% by XRD, similar to the abundance of metal in A 12085 and the least preserved lithology in Paris; EM 200 contains <1 vol% by XRD (Table 8). Point counting of metal is efficient for the estimation of the metal abundance as it is direct observations for grains <1 µm. For the studied meteorites, point counting of the metal gave similar results as the XRD data (Table 1). The use of the metal content as an indicator of aqueous alteration could be questioned in view of the observations in the matrix of NWA 11750. The metal grains are subhedral to euhedral (Fig. 11b), with relatively large sizes (10-200 nm) compared to surrounding matrix crystals. These two characteristics point to secondary rather than primary constituents of the matrix, although those grains show mainly kamacite and tetrataenite composition.

Porosity of the matrices

The high porosity observed in the highly primitive meteorite Acfer 094 suggests that primitive chondrites may have contained a lot of water-ice-filled pores during accretion [START_REF] Matsumoto | Discovery of fossil asteroidal ice in primitive meteorite Acfer 094[END_REF]. The matrices of the least altered chondrites consist mostly of GEMSlike or amorphous materials with high porosity [START_REF] Leroux | GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites[END_REF][START_REF] Kimura | .8: Their characteristic features and classification[END_REF][START_REF] Suttle | The aqueous alteration of CM chondrites, a review[END_REF]. During aqueous alteration, this porosity is filled with fine-grained to fibrous phyllosilicates or small TCIs [START_REF] Leroux | GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites[END_REF]. Following Figure 15 in [START_REF] Leroux | GEMS-like material in the matrix of the Paris meteorite and the early stages of alteration of CM chondrites[END_REF], describing the filling of the porosity around GEMS-like materials by TCIs or fine-grained phyllosilicates, Chwichiya 002 FIB section observations correspond to the second stage of incipient alteration, with fine grained TCIs forming in the porosity around the amorphous phases, with remaining porosity between the GEMS-like materials (Fig. 8a). Nevertheless, estimated porosity measurements using microscopy are highly scale dependent, which has to be taken into consideration when estimating porosity using SEM and STEM. The choice of the FIB sections for the TEM studies are usually made away from large porosity to avoid breaking during the FIB section extraction, which creates a sampling bias in the TEM porosity estimation.

TCIs

Depending on the environment and the fluid composition, the dissolution of GEMS-like materials associated with the alteration of metal grains can be expected to form sulfides (tochilinite), phyllosilicates (cronstedtite) and/or magnetite [START_REF] Palmer | Aqueous alteration of kamacite in CM chondrites[END_REF][START_REF] Pignatelli | Multiple precursors of secondary mineralogical assemblages in CM chondrites[END_REF][START_REF] Pignatelli | Mineralogical, crystallographic and redox features of the earliest stages of fluid alteration in CM chondrites[END_REF]. Those phyllosilicates and sulfides form finely intermixed materials (TCIs) that replaced most of the primary matrix [START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF]Vacher et al., 2019b;[START_REF] Zolensky | Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites[END_REF][START_REF] Suttle | The aqueous alteration of CM chondrites, a review[END_REF]. TCIs are used as proxies for the determination of CM chondrite subtypes [START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF]. The Paris type 2.9 lithology, A 12169 and A 12236 do not show TCIs, while A 12085 has minor TCIs, as is also observed in NWA 11750 and EM 200. Chwichiya 002 and NWA 12957 show abundant TCIs (5.3-22 vol%), resembling the Paris least preserved lithology type 2.7 (12 ± 3 vol%) (Table 8).

Sulfides

Whether the origin of the different sulfides in carbonaceous chondrites is primary or secondary is still uncertain. Indeed, pyrrhotite and pentlandite in CM can represent primary hightemperature phases (Schrader et al., 2016;[START_REF] Singerling | Primary iron sulfides in CM and CR carbonaceous chondrites: Insights into nebular processes[END_REF]. In addition, GEMS, by definition, contain sulfides, present only as pyrrhotite. Conversely, the ratio of troilite compared to pentlandite and pyrrhotite is a criterion for defining the alteration subtype of chondrites, with major troilite for the least altered chondrites, and secondary euhedral pyrrhotite grains characteristic of secondary processes (McSween, 1979b;[START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF]. The A 12169, A 12236 and A 12085 chondrites contain troilite, but Paris contains mainly pyrrhotite and pentlandite (Table 8). Thus, in this study, when possible, we distinguished troilite as a primary phase and pentlandite/pyrrhotite as secondary phases, but this separation remains disputable. For techniques that did not allow distinction between sulfides (troilite, pyrrhotite, pentlandite), we postulate that sulfides count as secondary phases, as used as a proxy of secondary alteration [START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF]. EM 200 contains troilite, with no trace of pentlandite nor pyrrhotite, and abundances similar to A 12236 and A 12085; NWA 11750 has a similar abundance of troilite and pyrrhotite, but no pentlandite; and Chwichiya 002 and NWA 12957 contain rare troilite with dominant pentlandite, with abundance closer to the Paris meteorite (Table 8).

Phyllosilicates

With increasing aqueous alteration, TCIs are recrystallized into larger phyllosilicate aggregates of 20-60 µm, associated with larger secondary euhedral pyrrhotite grains (McSween, 1979b;[START_REF] Rubin | Progressive aqueous alteration of CM carbonaceous chondrites[END_REF]. Afterward, pseudomorphic phyllosilicate crystals start to replace the anhydrous silicates (pyroxene and olivine). Ferroan olivines are altered faster than magnesian olivine [START_REF] Wogelius | Olivine dissolution kinetics at near-surface conditions[END_REF][START_REF] Hanowski | Aqueous alteration of chondrules in the CM carbonaceous chondrite, Allan Hills 81002: implications for parent body alteration[END_REF][START_REF] Zolensky | Mineralogy and composition of matrix and chondrule rims in carbonaceous chondrites[END_REF][START_REF] Pignatelli | Multiple precursors of secondary mineralogical assemblages in CM chondrites[END_REF], as ferroan olivines are altered from margin inwards, in contrast to magnesian olivine which suffer meshwork replacement, forming Fe to Mg-rich serpentine [START_REF] Velbel | Formation of protective surface layers during silicate-mineral weathering under well-leached, oxidizing conditions[END_REF][START_REF] Velbel | Replacement of olivine by serpentine in the Queen Alexandra Range 93005 carbonaceous chondrite (CM2): Reactant-product compositional relations, and isovolumetric constraints on reaction stoichiometry and elemental mobility during aqueous alteration[END_REF][START_REF] Wicks | An idealized model for serpentine textures after olivine[END_REF]. Therefore, phyllosilicates are a major proxy for identification of chondrite subtypes [START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF].

In the Asuka chondrites discussed above, only A 12085 (type 2.8) shows small serpentine detection with XRD, and in the Paris meteorite, the type 2.7 lithology shows abundant serpentine (Table 8). The four studied chondrite meteorites show minor serpentine detected by XRD ( [START_REF] Noguchi | Mineralogy of fine-grained matrix, fine-grained rim, chondrule rim, and altered mesostasis of a chondrule in Asuka 12169, one of the least altered CM chondrites[END_REF]. Finer grained phyllosilicates (serpentine and cronstedtite) might not be detected by XRD, but are visible by TEM, even in type 3.0 meteorites, as seen for A 12169 and the four meteorites of this study.

Carbonates

Carbonates crystallize during the earliest stages of alteration, generally in the form of calcite (e.g., [START_REF] Lee | Aragonite, breunnerite, calcite and dolomite in the CM carbonaceous chondrites: High fidelity recorders of progressive parent body aqueous alteration[END_REF][START_REF] Howard | Classification of hydrous meteorites (CR, CM and C2 ungrouped) by phyllosilicate fraction: PSD-XRD modal mineralogy and planetesimal environments[END_REF]. First generation carbonates fill void spaces in the meteorite, whereas later carbonates replace primary anhydrous silicates and grow on already existing secondary phases. Carbonates are found in altered subtypes <2.7 chondrites [START_REF] Rubin | An American on Paris: Extent of aqueous alteration of a CM chondrite and the petrography of its refractory and amoeboid olivine inclusions[END_REF]. A 12169, A 12236 and A 12085 and the most primitive Paris lithology contain rare to no carbonate, while the more altered Paris lithology contains 1-3 vol% carbonate (Table 8). EM 200

shows no traces of carbonates, similar to the referenced Asuka meteorites and the Paris primitive lithology (Table 8). (Fig. 2), thus isotopic analysis of the carbonates in NWA 11750 showed that their origin is extraterrestrial (Fig. 2). We are therefore confident that the majority of the carbonates in NWA 11750 were not formed by terrestrial alteration. Although, small terrestrial weathering observed in NWA 11760 could have produced small abundance of terrestrial carbonates.

Other primordial and secondary phases

A hypothetical CM3.00 would contain submicrometric primary silicates such as forsterite and enstatite crystals. We observed nanometric to submicrometric crystals of forsterite and enstatite in the four meteorites of this study (Fig. 7 and Fig. 8). Hypothetical CM3.00 matrices would contain micron scale Ca-Al-rich inclusions (CAIs) and abundant presolar grains [START_REF] Bland | A cornucopia of presolar and early Solar System materials at the micrometer size range in primitive chondrite matrix[END_REF], although those phases were not studied here. Iron-rich olivine, fayalite (Fa>90), could result from aqueous alteration at low temperature (<300°C), as observed in CV and CO chondrites [START_REF] Krot | Progressive alteration in CV3 chondrites: More evidence for asteroidal alteration[END_REF]. Even though, NWA 11750 shows abundant olivine in the matrix, their Fa content is <30, suggesting no relation to fluid-assisted alteration, or a different fluid from CV alteration. The abundant NWA 11750 olivine could be a primary nebular phase, but its occurrence as polycrystalline, engrained aggregates could also suggest crystallization during a secondary process, whose origin and nature remains undetermined.

In this study, we investigate the most representative matrix of each of the four studied meteorites. Of course, chondrites are also considered to be complex breccias, implying multiple lithologies with different alteration conditions and degrees (e.g., [START_REF] Lentfort | Classification of CM chondrite breccias-implications for the evaluation of samples from the OSIRIS-REx and Hayabusa 2 missions[END_REF]. Therefore, a detailed investigation of potential different lithologies and the determination of presolar grain abundances in the matrices of Chwichiya 002, NWA 12957, NWA 11750 and EM 200 should be performed in order to compare all possible lithologies to the several lithologies of Paris, A 12169, A 12236 and A 12085, to confirm of their comparison.

Conclusion

We report on four ungrouped carbonaceous chondrites, Chwichiya 002, NWA 12957, NWA 11750, and EM 200, that show variations in both the degree and products of alteration, illustrating different alteration conditions and/or primary constituents. We find that Chwichiya System. Its matrix contains no amorphous material, but abundant preserved nanocrystals of olivine and metal, with low abundance of magnetite, phyllosilicates and no TCIs, and we would suggest a classification as a 2.9 subtype rather than the 3.00 subtype currently used in the Meteoritical Bulletin database. EM 200 has a more altered matrix with a high abundance of magnetite and nanoscale phyllosilicates, although primary troilite is preserved. Its matrix shows no carbonate, no TCIs, and would thus suggest a classification as a 2.8 subtype rather than the 3.00 subtype currently used in the Meteoritical Bulletin database. The variety of alteration styles among the studied samples reflect the variety of primary constituents of chondrites, aqueous alteration conditions, and alteration fluid compositions.

In addition, we demonstrate that the characterization of the alteration state of chondrites is highly technique dependent. Indeed, some phases that are key to study the incipient aqueous alteration of chondrites, such as small TCIs, nanoscale phyllosilicates (serpentine), or amorphous materials, are challenging to characterize with a number of techniques (e.g., FEG-SEM, IR spectroscopy, EMPA or TGA). We show that the search for most primitive chondrites should utilize multiple complementary techniques, in order to identify the widest variety of preserved primary phases and secondary products, independently of the style of alteration. Our analyses indicate that, as a first step, the combination of EMPA (Cr 2 O 3 content in ferroan olivine) or Raman spectroscopy with XRD, is an efficient approach to identify the least altered chondrites and characterize their alteration state. Although TEM remains a complex technique, it is the only technique allowing nano-scale observation of fine-grained matrices, and a precise localization of the start of aqueous alteration (GEMS-like material dissolution and recrystallization into TCIs, fine phyllosilicate crystallization, silicate alteration). TEM, being limited by the small-scale of FIB sections, should be used in combination with larger scale techniques, such as XRD or SEM, to draw conclusions about the bulk meteorite. XRD, and especially PSD-XRD, allows the detection of all primordial phases, as well as all various secondary alteration phases with low detection limits. We propose that a combined PSD-XRD and TEM approach represents the ideal methodology to study the incipient aqueous alteration in primitive chondrites at both the bulk and nanometer scale, independently of the large variety of aqueous alteration conditions.

Acknowledgement

This work was funded the European Research Council under the H2020 framework program/ERC grant agreement no. 771691 (Solarys). We thank Damien Chaudanson from CINaM for the help with the TEM and FEG-SEM analyses. We thank Julien Longerey for the help with SEM analyses and the polished sections. We thank David Troadec for the FIB sections, prepared at IEMN, University of Lille. HL thanks funding by I-SITE ULNE and the MEL (Métropole européenne de Lille) as well as the electron microscope facility at the University of Lille with the support of the Chevreul Institute, the European FEDER and Région Hauts-de-France. AJK was funded by UK Research and Innovation (UKRI) grant MR/T020261/1. We thank Hope Ishii as associate editor, Sasha Krot as reviewer and an anonymous reviewer for their comments that helped improve the quality of the manuscript.

Supplementary materials

  statistics ~0.2‰ (1σ) for δ 18 O and ~0.3‰ for δ 17 O. Isotope ratios ( 17 O/ 16 O and 18 O/ 16 O) are presented in per mil (‰) relative to Standard Mean Ocean Water (SMOW): (1) where x represents 17 O or 18 O and SMOW the ratios of the SMOW standard. We used three terrestrial standard materials (San Carlos olivine, diopside and calcite) to define the instrumental mass fractionation line for the three oxygen isotopes. Instrumental Mass Fractionation (IMF) for calcite matrix was determined from our in-house Mexican calcite standard at the beginning and end of each analytical session. IMF values for each sample analysis were then calculated by accounting for the linear deviation over the time of the IMF values. Typical measurement errors (2σ), accounting for errors on each measurement and the external reproducibility of the standard, were estimated to be ~0.5‰ for δ 18 O, ~0.6‰ for δ 17 O, and ~0.7‰ for Δ 17 O (i.e., Δ 17 O = δ 17 O -0.52 × δ 18 O), where Δ 17 O represents the departure from the TFL.

Figure 11 and

 11 Figure11and Table6show the analytical total of the electron microprobe (EMP) analyses

  NWA 12957 and Chwichiya 002 contain 1-3 vol% carbonates (estimated by XRD), comparable with the most altered Paris lithology. TEM observations of NWA 11750 contain extremely abundant fine-grained carbonates filling the porosity in between olivine along with fine grained phyllosilicates (Fig. 8b, Fig. 9b, Fig. 10 B2). Of course, carbonates can be produced by terrestrial fluid circulation, and as shown by Tyra et al. (2007), the O-isotopic composition of carbonates precipitated from terrestrial fluids plot along the mass-dependent terrestrial fractionation line (slope 0.52) with δ 18 O and δ 17 O values close to 0‰. Large carbonates veins in NWA 11750 show δ 18 O around 30-40 ‰, with slightly negative Δ 17 O (average -0.63 ‰)

  002 and NWA 12957 are among the least altered chondrites, with abundant amorphous phases, observed as GEMS-like assemblages in Chwichiya 002, and TCIs and sulfides as secondary alteration phases. They could represent primitive CM-like chondrites, similar to the A 12169, A 12269, A 12085 meteorites (CM2.8-3.0), with only incipient H 2 O-rich fluid alteration. NWA 11750 contains a high abundance of extraterrestrial carbonates in the matrix and is an exceptional meteorite to study incipient CO 2 -and Ca-rich aqueous fluid assisted alteration in the early Solar

Figure S1 :

 S1 Figure S1: RX tomography mapping of carbonates veins around a chondrule of NWA 11750.

Figure S2 :

 S2 Figure S2: NWA 12957 TGA analyses

Table 3 )

 3 , Chwichiya 002 and NWA 12957 having 2-6 vol% of phyllosilicates; and NWA 11750 and EM 200 2 vol% (Table 8), closer to A 12169, A 12236, and A 12085. Even if TCIs are counted separately in modal abundances, the abundance of serpentine by XRD comprises both serpentine from olivine alteration and cronstedtite from the TCIs, explaining the high phyllosilicate abundances for Chwichiya 002 and NWA 12957. Although no detection of phyllosilicates is made in A 12169 (3.0) by XRD, TEM observations show small phyllosilicates in the matrix