The multiplication of decimal petrologic schemes for different or the same chondrite groups evinces a lack of unified guiding principle in the secondary classification of type 13 chondrites. We show that the current OC R and CO classifications can be a posteriori unified with only minor reclassifications if the decimal part of the subtype is defined as the ratio mFaIFaII of the mean fayalite contents of type I and type II chondrules rounded to the nearest tenth with adaptations from Cr systematics for the lowest subtypes. This parameter is more efficiently evaluable than the oftused relative standard deviations of fayalite contents and defines a general metamorphic scale from M0.0 to M1 where the suffixed number is the rounded m. Type 3 chondrites thus span the range M0.0M0.9 and M1 designates type 4. Corresponding applications are then proposed for other chondrite groups. Known type 1 and 2 chondrites are at M0.0 i.e. the metamorphic grade of type 3.0 chondrites. Independently we define an aqueous alteration scale from A0.0 to A1.0 where the suffixed number is the rounded phyllosilicate fraction PSF. For CM and CR chondrites the subtypes can be characterized in terms of the thinsectionbased criteria of previous schemes which are thus incorporated in the present framework. The rounding of the PSF to the in principle nearest tenth makes the proposed taxonomy somewhat coarser than those schemes but hereby more robust and more likely to be generalized in future meteorite declarations. We propose the corresponding petrologic subtype to be 3PSF rounded to the nearest tenth so that type 1 would correspond to subtypes 2.0 and 2.1. At the level of precision chosen nonzero alteration and metamorphic degrees remain mutually exclusive so that a single petrologic subtype approx 3mPSF indeed remains a good descriptor of secondary processes.
['Astrophysics - Earth and Planetary Astrophysics', 'Physics - Geophysics']
The secondary classification of unequilibrated chondrites
{'The secondary classification of unequilibrated chondrites': "Emmanuel Jacquet 1 , Béatrice Doisneau 1  \n1 Institut de Minéralogie, de Physique des Matériaux et de Cosmochimie (IMPMC), Muséum national d'Histoire naturelle, Sorbonne Université, CNRS; CP52, 57 rue Cuvier, 75005 Paris, France.", 'Abstract': "The multiplication of decimal petrologic schemes for different or the same chondrite groups evinces a lack of unified guiding principle in the secondary classification of type 1-3 chondrites. We show that the current OC, R and CO classifications can be a posteriori unified, with only minor reclassifications, if the decimal part of the subtype is defined as the ratio m =FaI/FaII of the mean fayalite contents of type I and type II chondrules rounded to the nearest tenth (with adaptations from Cr systematics for the lowest subtypes following Grossman and Brearley 2005). This parameter is more efficiently evaluable than the oft-used relative standard deviations of fayalite contents and defines a general metamorphic scale from M0.0 to M1 (where the suffixed number is the rounded m ). Type 3 chondrites thus span the range M0.0-M0.9 (i.e. 3.0-3.9) and M1 designates type 4. Corresponding applications are then proposed for other chondrite groups (with, e.g., CV secondary classification reduced to essentially three grades from M0.0 to M0.2, that is, subtypes 3.0-3.2). Known type 1 and 2 chondrites are at M0.0 (i.e. the metamorphic grade of type 3.0 chondrites), even so-called 'CY' chondrites, since our metamorphic scale is insensitive to brief heating. Independently, we define an aqueous alteration scale from A0.0 to A1.0, where the suffixed number is the (rounded) phyllosilicate fraction (PSF) introduced by Howard et al. (2015). For CM and CR chondrites, the subtypes can be characterized in terms of the thin-section-based criteria of previous schemes (Rubin et al. 2007; Harju et al. 2014) which are thus incorporated in the present framework. The rounding of the PSF to the (in principle) nearest tenth makes the proposed taxonomy somewhat coarser than those schemes, but hereby more robust and more likely to be generalized in future meteorite declarations. We propose the corresponding petrologic subtype to be 3PSF , rounded to the nearest tenth (so that type 1 would correspond to subtypes 2.0 and 2.1). At the level of precision chosen, nonzero alteration and metamorphic degrees remain  \nmutually exclusive, so that a single petrologic subtype ≈ 3+ m -PSF indeed remains a good descriptor of secondary processes (obviating the explicit mention of our separate scales, unless finer subdivisions are adopted for the most primitive chondrites). While proposing a unified way of delineating subtypes, our framework is no substitute for multi-technique studies leading to confidently interpreted multi-parameter schemes for specific groups.", '1. Introduction': "Half a century ago, Van Schmus and Wood (1967) put chondrite taxonomy on its modern foundations by decoupling primary and secondary classifications in a two-dimensional scheme. The primary classification was based on chemical groups presumably representing different primary parent bodies (or families thereof). The secondary classification distinguished petrologic types arranged on a scale from 1 to 6 which characterized parent body processes such as thermal metamorphism, which could vary in degree in the same parent body.  \nCertainly, the understanding of those processes has evolved. It is no longer believed that the whole ordered sequence from 1 to 6 represents 'increasing equilibration' (Van Schmus and Wood 1967), or that chondrules may have formed in situ out of type 1 (e.g. CI) material (Mason 1962). Rather, type 1, 2, 3 carbonaceous chondrites-whose labels echoed Wiik (1956)'s types I, II and III-, represent decreasing degrees of aqueous alteration , and thermal metamorphism increases from type 3 to 6 (e.g. Krot et al. 2014). Although there is thus no global ordinal significance in the Van Schmus and Wood (1967) petrologic types, their descriptive value has stood the test of time (Sears 2016) and no meteoriticist can conceivably dispense with them.  \nWith the recognition that type 3 chondrites were the most pristine, it was also quickly realized that they still exhibited considerable diversity (e.g. Dodd et al. 1967; McSween 1977a), and the search for the least processed material warranted further refinement of the classification. Sears et al. (1980) hence subdivided type 3 ordinary chondrites in decimal subtypes ranging from 3.0 to 3.9, a scheme that was later refined by Grossman and Brearley (2005) for the interval 3.00-3.2. Such decimal subtypes (ranging from 3.0 to 3.8) were also  \nproposed for CO carbonaceous chondrites by various authors (Scott and Jones 1990; Sears et al. 1991; Chizmadia et al. 2002; Bonal et al. 2007; Sears 2016). The Meteoritical Bulletin Database (MetBullDB) also lists metamorphic subtypes for Rumuruti chondrites (e.g. Bischoff 2000) and CL chondrites (Metzler et al. 2021). Subtypes have also been proposed for CV chondrites (Guimon et al. 1995; Bonal et al. 2006) and enstatite chondrites (Quirico et al. 2011) but have not gained wide currency.  \nType 2 chondrites have also undergone subclassifications. For CM chondrites, Rubin et al. (2007) proposed subtypes from 2.0 (as they renamed type 1) to 2.6, later extended to 2.7 (for Paris; Marrocchi et al. 2014; Rubin 2015) and 2.8-3.0 (Kimura et al. 2020). Harju et al. (2014) proposed subtypes from 2.0 to 2.8 for CR chondrites. Alexander et al. (2013) and Howard et al. (2015) proposed other sets of subtypes, theoretically spanning the range 1.0-3.0, based on the H and phyllosilicate contents, respectively, for these two chondrite groups and some ungrouped carbonaceous chondrites. None of these aqueous alteration schemes has entered the MetBullDB yet (notwithstanding two 'CM2.0' (that is, CM1), NWA 11638 and NWA 11732).  \nThis multiplication of subtype definitions developed over the past four decades, sometimes for the same chemical groups, is problematic for the meteoritics student and the classifying community at large. Of course, a classification scheme is a matter of convention and must involve some arbitrariness, but a consensus is difficult to achieve if the delimitation of each subtype obeys no global logic except that of a ranking (as commented by Sears 2016), with divisions sometimes so fine as to make assignation quite uncertain. Scales developed by the same research group for different chondrite groups then stand little chance to be comparable, as Harju et al. (2014) observe when comparing their work on CR with the Rubin et al. (2007) CM classification. For example, a type CM2.5 chondrite according to the scale of Rubin et al. (2007) is more than three times more hydrated than a CR2.5 according to the scale of Harju et al. (2014) (see Fig. 5 of Howard et al. 2015).  \nCertainly, secondary processing on different parent bodies cannot be wholly described by the same single set of numbers (Sears 2016). Each parent body had its own set of time-temperature histories, water/rock ratios, grain size distribution etc. and different secondary effects will not be correlated the same way, if at all. Thus secondary classification must be ultimately relative (Brearley and Jones 1998; Sears 2016). Yet it would bring welcome clarity if, as for  \ninteger Van Schmus and Wood (1967) types, each subtype were defined, even approximately, by some simple common empirical property valid across all chondrite groups, warranting a common name (e.g. Howard et al. 2015; Bonal et al. 2006, 2007). Subtypes would then be robust against reclassification to different groups, and could be meaningfully attributed to ungrouped chondrites.  \nThis does not amount to basing secondary classification on one single criterion, especially given the complexity of the secondary processes or perhaps stochastic variability in the protoliths that prevent a simple one-dimensional picture even in single parent bodies. In fact, it is the very correlation between several parameters for different members of a group which substantiates their collective interpretation as metamorphic or aqueous alteration effects. A good classification system should take into account different parameters (Sears 2016), thus allowing at times to overrule the indications of single parameters. Although thus the assignation of samples to subtypes should rely on several parameters, the definition of the subtypes (and the allowed intervals of variation of said parameters) should be ideally tailored, mutatis mutandis , after one conventional, well-chosen defining criterion .  \nA difficulty in finding 'universal' defining criteria for secondary classification is that, at the level of precision of the modern decimal schemes, thermal metamorphism and aqueous alteration are no longer mutually exclusive (e.g. Schrader et al. 2015; Suttle et al. 2021a; Howard et al. 2010). If, say, a chondrite has been aqueously altered to type 2.5 (according to some scale) and has undergone metamorphism comparable to some type 3.1 chondrite, should the net petrologic type be 2.5 + 0.1 = 2.6? Obviously not. It thus seems inescapable to define two independent scales for metamorphism and aqueous alteration, with criteria robustly appropriate for each.  \nThe purpose of this paper is thus to propose and justify such a two-dimensional secondary classification for unequilibrated chondrites, with as few arbitrary decisions as possible, building upon the previous literature and the previous classification systems which alone justify the interpretation of secondary effects. Our criteria will be designed to be as simple as possible, so as to ideally appear as natural options to the 21 st century meteoritics student not yet familiar with the existing literature on the Van Schmus and Wood (1967) scale. Yet our purpose is not to supersede the Van Schmus and Wood scale. In most cases, the essential information will still be capturable by one petrological Van Schmus-Wood (sub)type, redefined as a simple function of the metamorphic and alteration  \ndegrees, and close to existing practice. So our metamorphic and alteration scales are essentially meant to adjust the definitions of the familiar petrological types. They should not be considered as a complete secondary classification system but rather as a flexible framework which has to be adapted to each group, as we will illustrate for those with existing subclassifications.", '2. Methods': "Although our work is largely based on the literature and mathematical investigations (deferred to the supplements for clarity), it was necessary to measure the olivine compositions of type I and type II porphyritic chondrules in several sections (borrowed from the Muséum national d'Histoire naturelle, Paris) of ordinary and CO chondrites listed in Table 1. The sections were mapped in back-scattered electron (BSE) by a Tescan Clara FEG-dual EDS SEM. Minor and major element concentrations of olivine were obtained with a Cameca SX-5 electron microprobe (EMP) at the Centre de Microanalyse de Paris VI (CAMPARIS). Analytical conditions were 15 kV voltage and 10 nA beam current. Standards were albite (Na), orthose (K, Al), pyrite (Fe, S), MnTiO2 (Mn, Ti), olivine (Si, Mg), Cr2O3 (Cr), NiO (Ni), apatite (P), vanadinite (Cl). Typical detection limits (see Electronic Annex) were Na: 0.04-0.06 wt%; K: 0.05-0.07 wt%; Fe: 0.1-0.2 wt%; Si: 0.06-0.08 wt%; Ti: 0.04-0.05 wt%; Mg: 0.08-0.1 wt%; Ca: 0.05-0.07 wt%; Mn: 0.08-0.11 wt%; Al: 0.04-0.06 wt%; Cr: 0.04-0.05 wt%; Ni: 0.1-0.2 wt%; P: 0.06-0.07 wt%; Cl: 0.04-0.06 wt%; S: 0.050.08 wt%.  \nWe define type I chondrules as those chondrules whose silicates had Mg# = (Mg/(Mg+Fe) <10 mol% upon accretion and type II chondrules as those more oxidized than this. Although diffusion during metamorphism progressively blurs this distinction for the olivine, it is worth remembering that olivine-rich type I and II chondrules can still be readily distinguished texturally (e.g. Scott and Taylor 1983; McCoy et al. 1991), with type I chondrules showing more rounded olivine, pyroxene (still close to pure enstatite because of slower Fe diffusion than in olivine) concentrated near the margin, opaques dominated by metal rather than sulphides, the possible presence of dusty olivine etc. (Fig. 1).  \nTable 1 : Summary of olivine and modal analyses in the samples studied. Arrows in the subtype column indicate suggested reclassifications (the 3.7 classification for Parnallee follows Bonal et al. (2006) and Sears et al. (1991)). For Dhajala, point counting was performed on the (larger) section 3146sp1; the Krymka point counting occurred on (likewise larger) section 3440sp1. The italicized olivine microprobe data of Semarkona and Ornans are from McCoy et al. (1991) and Scott and Jones (1990), respectively.  \n<!-- image -->  \n<!-- image -->  \nFigure 1: Chondrules in the Chainpur (hitherto LL3.4, reclassified LL3.2 herein) meteorite, in back-scattered electron. (a) Prototypical type IA and type IIA chondrules. Little effect of metamorphism is visible. (b) An uncommon type IB chondrule with fayalite-enriched olivine (Fa30-31) and more magnesian pyroxene (Fs9-15). This chondrule has undergone metamorphic equilibration (analyses are listed in the Electronic Annex).  \n<!-- image -->  \nThe difficulty is rather to obtain a reproducible average olivine composition in these chondrules, which vary in granulometry, the presence of fractures easing diffusion etc., with of course each individual crystal being zoned and subject to various sectioning geometries. The works of Scott and Jones (1990) and McCoy et al. (1991), on CO and LL chondrites, respectively, which inspired our measurements, did not specify the selection criteria of their chondrules and one may thus suspect a bias in favour of large chondrules with large crystals which diffusion would take longer to equilibrate. To obtain a representative olivine volume-weighted (that is, area-weighted on a representative section) average, a random sampling, uniformly distributed on the surface of the section, would be  \nthe cleanest way to go (with labelling each point as either type I or type II) in principle. For instance, Dodd et al. (1967) selected points in 500 µm steps in traverses 1000 µm apart. So one could envision applying this method and identify the chondrule type analysed. Yet, even allowing for a 20 µm leeway to find neighbouring silicate grains (Dodd et al. 1967), such a method would be inefficient to find olivine (rather than pyroxene or even other phases), and a fortiori olivine in sufficient number for each chondrule type (e.g. if one of them is relatively rare). For example, in ordinary chondrites, type I olivine (most critical for classification) would be reached in less than a tenth of the time, according to our point counting estimates (discussed in supplement E).  \nSo we chose instead to generalize, mutatis mutandis , the method introduced by Grossman and Brearley (2005) to estimate the first moments of the Cr2O3 contents in type II chondrule olivine, that is, analyze the core of one 'typical olivine' per randomly chosen chondrules. Supplement A.1 shows the statistical advantage of performing only one analysis per chondrule for a given total number of analyses.  \nWe performed the 'random selection' by making 1000 µm steps (the exact step being immaterial so long it is predefined and larger than typical chondrules) and choosing the closest suitable chondrule of the quota of interest (porphyritic type I, and then type II). As a crude way to introduce weighing by olivine amount, we imposed that olivine should dominate over pyroxene. This is necessary because pyroxene-rich type I (IBA, IB) chondrules may originally contain distinctly more ferroan olivine than type IA chondrules (e.g. Jones 1994; Jacquet et al. 2021) and the smaller size of the olivine chadacrysts would further facilitate the diffusion of Fe during metamorphism. So we avoided additional variability from a likely minor contributor in terms of total olivine volume. Also, the pyroxene-rich type I and type II chondrules may also be less easily texturally distinguishable in ordinary chondrites.  \nTo make the concept of 'typical olivine' more objective, we defined it by mentally reordering all apparent (unbroken) olivine crystals of the chondrules in order of decreasing apparent area and taking the grain for which the cumulative olivine area reaches half the total area. If this grain contained an apparent relict (e.g. a forsteritic grain inside a type II chondrule or a dusty olivine in a type I chondrule), another one closest in size was chosen. If the typical grain is the largest olivine grain of the chondrule (that is, if that olivine makes more than 50 % of the total apparent area of the chondrule olivine), the whole chondrule was  \ndeemed unsuitable and the next nearest one was sought. This excludes isolated olivine grains (Jacquet et al. 2021) or any unrepresentative chondrule fragment where the typical grain as defined above cannot be trusted to be typical of the unbroken chondrule. This removes any ambiguity of definition with respect to the matrix and mitigates the effect of fragmentation (either post- or preaccretion), unless it affected preferentially one subpopulation of type I or type II chondrules 1 . Of course, the identification of the typical olivine during a microprobe session is, in practice, subject to error (for we obviously do not actually measure the surface of all olivine grains on the fly), but keeping our definition in mind does reduce the freedom of choice (and therefore the resulting contribution to the standard deviation, see Supplement A).  \nStrictly speaking, the average olivine composition of a chondrule type calculated this way is not the olivine-volume-weighted average composition of olivine in this chondrule type. Although the 'typical grain' may yield a fairly accurate estimate of the average olivine composition of a given chondrule (as discussed mathematically in Supplement A.2), the chances of a given chondrule to be chosen, which are proportional to the area of its Voronoi domain (i.e. the set of points where it is the closest suitable chondrule), are not exactly proportional to its amount of olivine. Yet barring a strong correlation between typical Fa content and chondrule size, our calculated average should not be far from the olivine volume-weighted one, and as such, the method is quite simple to implement and merely regulates the already standard initiative of analysing chondrule silicates during the classification of a chondrite. By generalizing the Grossman and Brearley (2005) method, it allows all metamorphic subtypes of type 3 chondrites to be determined with the same kind of dataset, with only the required statistics of each chondrule type varying with the suspected levelclearly, we need more type II chondrule data for lower than for higher subtypes, where their olivine is nearly equilibrated. We suggest at least 20-30 type I olivine analyses for putative type 3.2-4 samples. In this study, we measured 2339 data points per section.", '3. Overview': "In the same way Stöffler et al. (1991) called their S hock stages 'S1', 'S2', etc. and Wlotzka (1993) his W eathering grades 'W0', 'W1', etc., we will introduce  \na prefix 'A' for A queous alteration grades and a prefix 'M' for M etamorphic grades. The prefixes at once prevent confusion with any other scale in the literature.  \nThe aqueous alteration degrees will run from A0.0 to A1.0, with A0.0 corresponding to type 3 chondrites, and A0.9 and A1.0 to petrologic type 1. Type 2 chondrites encompass the intermediate alteration grades from A0.1 to A0.8. For type 1 and 2 samples, the number after the prefix is intended as the number to be subtracted from 3 to obtain the Van Schmus-Wood petrologic subtype.  \nThe metamorphic grades will run from M0.0 to M1, with M1 standing for petrologic type 4 (and higher). For type 3 samples, the number after the prefix M is intended as the decimal part of a redefined petrologic type (e.g. M0.2 should correspond to 3.2). In the discussion to follow though, petrologic types will refer to past literature, unless otherwise noted, again to prevent confusion.  \nThe purpose of the succeeding sections is to provide general definitions for the aqueous alteration and metamorphic grades. More precisely, we wish to define real numbers a and m (which we may call aqueous alteration and metamorphic degrees ) whose rounding to a neighbouring tenth will yield the numbers after the prefixes A and M, respectively (i.e. we have A r(a) and M r(m) with r the rounding function). We will find that a = PSF (phyllosilicate fraction) and m = FaI/FaII (ratio of average Fa contents of type I and II chondrules) are generally good options. By default, the rounding should be to the nearest tenth (e.g. if m = 0.44, we shall have M0.4), but it can be adapted for each group (e.g. to mark petrographically significant transitions). For a particular sample, the rounding may be further amended if other assignation criteria overrule it. Again, choosing a defining criterion does not mean that assignation criteria (i.e. those used, in practice, to assign a specimen to a grade) should be restricted to it (Sears 2016). Kimura et al. (2024), for example, proposed 20 criteria to delineate the 3.0 subtype, which will largely fit our own M0.0/M0.1 and A0.0/A0.1 boundaries. The defining criterion, as we put it, merely constrains the boundaries of the different assignation criteria for the defined grades, and the more diverse such assignation criteria are, the more robust the secondary classification is.", '4. The aqueous alteration scale': "We start with the aqueous alteration scale, because it is on the whole most straightforward. Indeed Howard et al. (2015) made a convincing case that the phyllosilicate fraction (PSF) allowed cross-calibration of aqueous alteration scales for different chondrite groups. They did not include other putative secondary minerals (like sulfides, oxides, carbonates, fayalite…) which may be controversial; their modal contribution would be anyway comparatively minor. The PSF is the (volume) ratio of (hydrous) phyllosilicates to all silicates. To be sure, the PSF is affected by the chondrule/matrix ratio, since the matrix is more easily hydrated (Abreu 2016), but so long the alteration grades are sufficiently wide to overcome effects of such fluctuations within a given chemical group, the PSF should do as a relative scale within it.  \nWe thus adopt a = PSF as the defining criterion for our alteration grades. Russell et al. (2021)'s definition of type 1 carbonaceous chondrites by PSF > 0.9 is conformable to our convention of type 1 corresponding to A0.9 and A1.0 (although our convention may be somewhat wider if PSF ≥ 0.85 warrants a rounding to A0.9). Conversely, the restriction of type 3 to A0.0 keeps all CV, CO, UOC at type 3 and most CRs at type 2. The type 2/type 3 transition would roughly correspond to the detectability of phyllosilicates by X-ray diffraction and infrared spectroscopy (5 and 2 vol% respectively; Krämer-Ruggiu et al. 2022). It may be noted that the Van Schmus and Wood (1967) definitions of types 1 and 2 were appropriate for CI and CM chondrites, respectively, and are now obsolete given the increasing diversity of aqueously altered chondrites known.  \nWhile the PSF is thus the defining criterion for the aqueous alteration degree, it certainly does not provide a full classification system alone. Howard et al. (2015) acknowledged that the Position Sensitive Detector X-Ray Diffraction (PSD-XRD) which allows to measure it (and whose data will be used here, unless otherwise noted) is not routine. Other methods exist (conventional XRD, which is less sensitive, possibly thermal gravimetric analysis combined with infrared spectroscopy; Krämer-Ruggiu et al. 2022) which are not either. Thus more operational, thin section-applicable assignation criteria must be sought, and in fact have been offered in the literature. These we now undertake to integrate for different chemical groups. We note that the general Kimura et al. (2024) criteria for the 3.0/2.9 boundary, on alteration of chondrule feldspar and glass, CAI melilite and matrix amorphous phases should fit our A0.0/A0.1  \nboundary, although only that on the destruction of O-anomalous presolar grain (dropping below 100 ppm) is quantitative.", '4.1 CM chondrites': 'Aqueous alteration has been best studied in CM chondrites. Rubin et al. (2007) and Rubin (2015) devised a petrographic classification scheme based on chondrule silicate alteration, metal abundance, matrix and tochilinitecronstedtite intergrowths (TCI) compositions, and the nature of sulfides and carbonates. This classification was found to correlate well with the PSF (Howard et al. 2015; Fig. 2). So, essentially, the matter for us is simply to redraw the boundaries with the same diagnostic criteria.  \nFigure 2 : PSF (Howard et al. 2015) as a function of the subtypes proposed by Rubin et al. (2007) and Harju et al. (2014) for CM and CR chondrites, respectively. We also show the boundaries chosen for the alteration grades A0.5-A0.9 (only tentative for A0.5/A0.6) for CM and A0.0-A0.2 for CR. Very altered (e.g. Marrocchi et al. 2023) or shock-heated (Briani et al. 2013) CRs are explicitly named.  \n<!-- image -->  \nOur freedom is limited to adjustments of the rounding of the PSF. We tie it further by making our alteration grades conservative with respect to the Rubin et al. classification, in that each of them corresponds to one or two Rubin (2015) subtypes, so as to make conversion straightforward. We note that PSF ~ 0.8 corresponds to an important transition, namely the beginning of significant replacement of mafic phenocrysts in chondrules, first affecting mostly pyroxene (most rapidly replaced; e.g. Hanowski and Brearley (2001); see also Fig. 7 of Howard et al. (2015)). This transition means that the value PSF=0.8 should be a boundary rather than the center of an alteration grade interval. Thus we elect to have A0.8 correspond to PSF between 0.8 and 0.85 (rather than the interval [0.75; 0.85[), encompassing Rubin et al. (2007)\'s subtypes 2.3 and 2.2, whereas A0.7 corresponds to their 2.4 and 2.5.  \nTable 2 lays out all our alteration grades adjusted for CM and the relationship with the Rubin classification is illustrated in Fig. 2. This effects a synthesis between the Rubin et al. (2007) and Howard et al. (2015) classifications. We have added criteria of Velbel et al. (2015) on chondrule enstatite and ferroan olivine, and adjusted the percentage limits for the chondrule mafic phenocrysts replacement for A0.8 (10 and 50 % rather than 2 and 85 %) to make them more readily evaluable and in line with the suggested PSF limits (this does not change the classification of the specific chondrites studied by Rubin et al. (2007)) 2 . Our table does not include the weakly altered \'CM2.8\', \'CM2.9\' and \'CM3.0\' chondrites of Kimura et al. (2020) since their PSF is unknown and their affiliation to CM may be uncertain, given the large difference in alteration degree. It is also premature to list classification criteria for subtypes populated by only one member; in fact such is already the case for our A0.5 (Paris) whose properties are here only tentatively taken from Rubin (2015)\'s subtype 2.7. At the other extreme, the grade A1.0 is unrepresented in CMs since their highest PSF measured by Howard et al. (2015) was 0.93 3 . CM1 would be restricted to grade A0.9 chondrites (which the MetBullDB variously refers to as \'CM1\', \'CM1/2\' or even \'CM2\' currently).  \nTable 2 : Aqueous alteration scale for CM chondrites. Parenthesized PSF value under A0.5 is that of its sole member, Paris (see Howard and Zanda 2019), making any delineation of the grade premature. po = pyrrhotite, pn = pentlandite, int = intermediate sulfide. Most petrographic criteria from Rubin (2015), with MgO/"FeO" from Kimura et al. (2020) and replacement of enstatite and ferroan olivine after Velbel et al. (2015).  \nWe commented above that our classification is coarser than Rubin et al.\'s but it is hereby more robust and practical. Most CM chondrites are genomict breccias, prone to significant variations in apparent alteration degree distributions from one thin section to another (e.g. Lentfort et al. 2021). So intrinsic variations in PSF are superimposed on analytical errors (5 vol%; Howard et al. 2015). It is thus pointless to base a classification on PSF variations below 0.05. Now, variations beyond this scale are meaningful; there is no way, for example, that a A0.9 (i.e. with chondrule phenocrysts mostly replaced) can be mistaken for a A0.7 (where these phenocrysts are largely intact, and TCI are BSE-brighter; Lentfort et al. 2021), or vice-versa, unless the studied thin section mainly samples one unrepresentative clast. Thus in our scheme, uncertainty cannot be more than one subtype (for borderline samples). We note that the PSF of a genomict breccia still makes sense in representing the silicate-weighted average  \nof the PSF of its constituent lithologies; one could thus display both the bulk and range of alteration degrees (for lithologies representing at least 10 vol%; Jacquet 2022) of a CM chondrite, e.g. in the form \'A0.7 (0.7-0.8)\'. We note that PSDXRD measurements such as those of Howard et al. (2015) (on 0.2 g aliquots of homogenized >>1 g samples) \'only\' bear on the whole-rock PSF, but Rubin et al. (2007)\'s thin section criteria embodied in Table 2 would allow estimates at the lithology scale (and thus of the range of subtypes represented).  \nBesides, our taxonomic simplification is only for \'first-look\' classification purposes (upon declaration to NomCom). Nothing here prevents any researcher interested in subtler insights on alteration from plotting any property against any underlying alteration metric (e.g. TCI composition, PSF…) taken as a continuous variable, rather than the discrete subtypes.', '4.2 CR chondrites': "The CR chondrites were subclassified based on petrographic properties by Harju et al. (2014). The PSF sets apart the same grouplet of heavily aqueously altered CRs (Al Rais and GRO 95577) which they ranked at their lowest subtypes (2.3 and 2.0, respectively). Weisberg and Huber (2007) did not report anhydrous silicates in GRO 95577, although Howard et al. (2015) measured PSF = 0.89 (sufficient for a CR1 classification though), but remnants of olivine phenocrysts unreplaced by phyllosilicates might be discernable in their Fig. 2b. It would be premature to delineate the one-member alteration grades corresponding to these meteorites, not to mention the uncertainty on their genetic link with mainstream CRs (Marrocchi et al. 2023).  \nAs to the other, 'mainstream' CRs, the Harju et al. (2014) sequence is not always consistent with water content-based classification (Alexander et al. 2013) or TEM examination of matrices (Abreu 2016). Many of the Harju et al. (2014) subtypes differ only in subjectively defined degrees (e.g. mafic phenocryst alteration may be 'none', 'incipient', 'moderate', 'altered'), which may be also liable to sample heterogeneities (Abreu 2016). In fact, from a PSF perspective, there is not much variation after all (as held from a textural perspective by Le Guillou et al. 2015). Indeed, when excluding Al Rais and GRO 95577, the measured PSF do not exceed 0.25 (Dhofar 1432; Howard et al. 2015). So there is only room for three alteration grades for mainstream CRs, viz. A0.0, A0.1 and A0.2.  \nAlteration grade A0.0 would be carved out of Harju et al. (2014)'s subtype 2.8. After deducting amorphous phases, Abreu and Brearley (2010)'s CR3 MET 00426 and QUE 99177 show PSF=0.01 and 0.02, respectively. Certainly, these meteorites were not exempt from all aqueous alteration (e.g. the 'smooth rims' after silica-rich igneous rims; Harju et al. 2014; Martinez and Brearley 2022), but no known carbonaceous chondrite is (even Acfer 094, with its 'cosmic symplectites'; e.g. Sakamoto et al. 2007), and their presolar grain abundances bear witness to their pristinity (e.g. Haenecour et al. 2018; Leitner et al. 2015), using Kimura et al. (2024)'s O-anomalous grain criterion. While Harju et al. (2014) consider the mostly crystalline state of chondrule mesostases as an alteration effect, it may be primary, e.g. as a result of slow cooling (e.g. Jacquet et al. 2021), similar to that proposed by Wick and Jones (2012) for the plagioclase-bearing chondrules in CO chondrites. We thus concur with their literature classification as CR3 (Abreu and Brearley 2018).  \nIt may be noted that Howard et al. (2015) eventually lumped amorphous silicates with phyllosilicates in the calculation of the PSF. Since the latter originally grow at the expenses of the former, this would hide the relative pristinity of the CR3 above (all CR would then have PSF > 0.1; see Fig. 4 of Howard et al. 2015), as also observed by Abreu (2016). We thus hold that, true to its etymology, the PSF numerator should not extend to amorphous phases, regardless of whether they were hydrated prior to accretion or at the very onset of parent body alteration (e.g. Le Guillou et al. 2015).  \nMost CRs would be A0.1, which encompasses a wide range of Harju et al. (2014) subtypes. A0.2 would essentially correspond to the alteration degree of Renazzo, i.e. Harju et al. (2014)'s subtype 2.4, with their subtype CR2.5 GRA 06100 having undergone brief reheating (Briani et al. 2013) and thence partial dehydration (Prestgard et al. 2023). There is, however, no PSF datum for meteorites classified at Harju et al. (2014)'s subtype 2.4. Yet phyllosilicates in those studied by Abreu (2016) account for somewhat over half of their matrix, which in turn have modal abundances 25.8-37.6 vol% so, with only moderate chondrule alteration, this should yield a PSF around 0.2. Direct measurements would be worthwhile.  \nTable 3 lists our proposed alteration scale for CRs. It, again, can be seen as a synthesis between the Harju et al. (2014) and the Howard et al. (2015) scales (see Fig. 2), though not as conservative as our CM classification as it divides Harju et al. (2014)'s subtype 2.8. The grades A0.0, A0.1, A0.2 also correspond  \nto Prestgard et al. (2023)'s degrees (i), (ii) and (iii), respectively, and would respect (if simplify) the ranking of Abreu (2016), notwithstanding the contradictions on (possibly heterogeneous) LAP 04516 in the literature, with Harju et al. (2014) and Prestgard et al. (2023) having seen less hydrated samples than Abreu (2016).  \nTable 3 : Aqueous alteration scale for CR chondrites. Chondrule criteria from Harju et al. (2014); matrix silicate mineralogy after Abreu (2016); PSF after Howard et al. (2015); presolar grain abundances after Haenecour et al. (2018); Leitner et al. (2015), adopting the Kimura et al. (2024) 3.0/2.9 threshold as our A0.0/A0.1 boundary. Parentheses indicate that the alteration grades in question are only represented by one member so cannot be meaningfully delineated.", '4.3 Other carbonaceous chondrites': "Phyllosilicates are detected in a few CO chondrites (Alexander et al. 2018), with the highest PSF value of 0.05 for CO3.0 ALH A77307. We thus deem them all A0.0.  \nAmong CV chondrites, phyllosilicates are reported in the so-called oxidized Bali-type subgroup (CVoxB; as opposed to the oxidized Allende-type CVoxA; e.g. Weisberg et al. 1997). With a PSF = 0.04 (Howard et al. 2010), this warrants again a type A0.0. One might consider subdividing A0.0 in a A0.00 and A0.05 (for PSF lower and higher than 2.5 %, respectively), with the latter representing CVoxB which thus would no longer need a specific designation among CVox (see also Gattacceca et al. 2020).  \nAll CI chondrites studied by King et al. (2015) are A1.0, as anhydrous silicates, likely chondrule and AOA debris (e.g. Morin et al. 2022), are very minor.  \nThe so-called 'CY chondrites' (e.g. Ikeda 1992; King et al. 2019, 2021; Suttle et al. 2021a, 2023) pose a problem of convention. Indeed, they presently contain no detectable hydrous phyllosilicate but this may be the result of a brief  \n(possibly impact-related) heating, judging from Raman properties and olivine zoning (Suttle et al. 2023; Nakato et al. 2008). Their previous PSF may have reached from 0.6 to 1.0 (i.e. CM- to CI-like; Suttle et al. 2021a). If we sticked to the present-day PSF, all CY should be A0.0, that is type 3, but this would falsely convey a pristine state to most meteoriticists. Suttle et al. (2021a) rather attributed petrologic types according to the inferred pre-heating state, distinguishing 'CY1' and 'CY2' chondrites. This corresponds to the usage in the MetBullDB where 'CY1' are listed as 'CI1' or 'CI1/2' and 'CY2' as 'C2ung'. So the relevant PSF could be the maximum one inferable in preterrestrial alteration history. This has the drawback of being more model-dependent, but the inferred thermal reheating may be explicitly indicated, e.g. in the form of the suffix 'T' proposed by Tonui et al. (2014), e.g. 'A1.0T' or 'CY1T' 4 (which we may supplement with a dedicated thermal classification, to which we will return in the section The metamorphic scale). We comment that the 'CY1T' and 'CY2T' may not be genetically related (to the same preaccretionnary protolith) as the former show distinctly higher 'plateau' volatile element abundances, in line with their higher matrix proportion (see Fig. 13 of King et al. (2019) 5 ). So the chondrule-free CY1T probably never contained many chondrules (similar to CI; Morin et al. 2022), and may form in due course a distinct (primary) group, if not related to CI themselves.", '4.4 Noncarbonaceous chondrites': "Aqueous alteration effects, and in particular phyllosilicates, are detected in unequilibrated ordinary chondrites (e.g. Hutchison et al. 1987; Brearley 2014). For Krymka and Bishunpur, Grant et al. (2023) found 12 and 16 wt%, respectively, of an 'X-ray amorphous' component, which they tentatively identified as phyllosilicates, but these cannot make the whole of it as (i) their specific diffractions peaks were missing (ii) chondrule mesostasis must also be present, at modal abundances presumably comparable to the plagioclase modes of equilibrated ordinary chondrites (8.9-9.8 wt%; Dunn et al. 2010) (iii) their estimated matrix-normalized water abundance (4.8-8.2 wt%) is far below their values for pure phyllosilicates (15-20 wt%) which must thus make a  \ncorresponding fraction only of the matrix (itself 10-15 wt% of OCs). Thus bulk phyllosilicate abundances must be a few wt% for the least metamorphosed ordinary chondrites, similar to CVs, and all OCs should thus be A0.0.  \nSame for most R chondrites. Yet R5 chondrite LAP 04840 contains 13.4 vol% ferri-magnesiohornblende and 0.4 vol% phlogopite, which may result from high-temperature hydration (McCanta et al. 2008). Similarly, R6 chondrite MIL 11207 contains 13 vol% OH-bearing minerals (Gross et al. 2017). Although amphiboles are not phyllosilicates, and notwithstanding our focus on type 1-3 chondrites, we could extend the numerator of the PSF (perhaps to be renamed 'Nominally Hydrous Silicate Fraction') to such nominally hydrous silicates. In this case, those equilibrated chondrites would thus warrant a A0.1 grade.", '5. The metamorphic scale': "Since the Sears et al. (1980) scheme for ordinary chondrites is widely used, with, in fact, no recent rival to our knowledge (since the Grossman and Brearley (2005) refinements were conservative), any redefinition of the metamorphic degrees would do well to closely stick to their subdivisions for ordinary chondrites. Yet, what defining criterion should we abstract from the OC subtypes? Sears et al. (1980) originally calibrated their subclassification (and the range of variation of their assignation criteria) after the increase of the induced thermoluminescence (TL) sensitivity, as a result of the crystallization of secondary feldspar out of chondrule mesostases. Specifically, their petrologic types PT may be given by:  \n𝑃𝑇 =  3.8 + 0.1 × floor(3 log 10 𝑇𝐿 Dhajala )  \nWith 'floor' the floor function 6 and TLDhajala the induced TL sensitivity at the ~130 °C peak, normalized to the Dhajala H3.8 chondrite.  \nHowever, the same formula applied to CO chondrites would make ALH 77307 (CO3.0) a type 3.3 (Scott and Jones 1989). This may be due to the presence of (feldspar-bearing) refractory inclusions or chondrules rich in primary anorthite (Wick and Jones 2012), so Sears et al. (1991a) in effect replaced the factor 3 of the above formula with a factor 4 for CO chondrites. Conversely, CK chondrites, which are certainly metamorphosed, show no detectable induced TL, for as yet unclarified reasons (Guimon et al. 1995). For enstatite chondrites, TL  \nsensitivity does not bear a simple relationship to metamorphism either (Zhang et al. 1996). Clearly, TL is not solely a function of thermal metamorphism (when it is) and although powerful to resolve subtypes within given chemical groups, including COs (Sears et al. 1980; Sears 2016), it was not claimed by its developers to provide the universal scale we are seeking (e.g. Sears 2016).  \nBonal et al. (2006, 2007) sought a universal sensor of the peak metamorphic temperature in the maturity of organic matter (previously studied in UOCs by Quirico et al. 2003) but, however relevant as a relative scale, this was criticized by Sears (2016), in particular as to the uncertain premise that the precursor organic matter was the same across all chondrite groups. At times, because of its sensitivity to peak temperatures, it may measure rather shock events than the protracted metamorphism of interest here (e.g. Briani et al. 2013). The Bonal et al. (2007) CO subtypes would also jump from 3.1 to 3.6, hinting at an overprecise classification in between, only evaluable with Raman. Also, no more than TL sensitivity does Raman spectroscopy lend itself to a 'natural' expression of m without arbitrary numerical parameters. Finally, beyond subtype ~3.6, organic matter maturity may provide little discrimination (see e.g. Fig. 2 of Bonal et al. 2016). This, again, does no prejudice to its important classifying power for less metamorphosed chondrites (it may be the handiest tool for some groups such as CVs, as discussed in their own subsection later).  \nOlivine phenocrysts in chondrules have a better claim as 'universal' sensor of metamorphism, in addition to being, in practice, the criterion most used by OC and CO classifiers. As noted by Grossman and Brearley (2005), olivine is the phase most resistant to aqueous alteration, and so long overgrowths possibly precipitated from fluids (e.g. Han et al. 2022) or serpentinized areas are avoided, its properties are independent of aqueous alteration. Their mineral chemistry should be likewise independent of shock effects (again for whatever portion of olivine escaped phase transitions such as melting) given the short diffusion lengths then (e.g. a few microns in shocked L chondrites; Ciocco et al. 2022).  \nIn fact, the very type 3/type 4 transition was defined by Van Schmus and Wood (1967) in terms of the homogenization (at the 5 percent mean deviation (PMD) 7 of FeO level) of olivine-which olivine, in ordinary chondrites, mainly meant chondrule olivine, because of the low abundance of matrix and the technical difficulty in analyzing small grains (Grossman 2011). The progress toward this  \nboundary could thus as well be measured by the degree of homogenization of olivine. In their table 4, Sears et al. (1982) showed that their subtypes could be correlated with the fayalite relative standard deviation RSD(Fa), i.e. the standard deviation of the fayalite content divided by the mean (often called the 'coefficient of variation'), which we can subsume in the following equation:  \n𝑃𝑇 =  3.9 - 0.1 × floor(10 RSD(Fa))  \nIf generalized, this would suggest a simple m ≈ 1 RSD(Fa). This, in fact, is the formula commonly used for R chondrites (Bischoff 2000). Yet this does not quite do, for (i) this formula actually overestimates the petrological types assigned to ordinary chondrites in the literature below subtype 3.6 (e.g. Fig 3; Table 5 of Sears et al. 1982; Table 4 of Sears et al. 1991b), warranting revision, and (ii) the RSD(Fa) actually stalls around 50 % for OCs below subtype 3.5 (e.g. Sears et al. 1982). At this stage, the relative standard deviation is essentially indistinguishable from that of the original assemblage, which ultimately depends on the type I/type II chondrule ratio (see Supplement C). In CO chondrites, RSD(Fa) reaches 125 % for the least metamorphosed sample ALH 77307 (Scott and Jones 1990). Also RSD(Fa) depends on what olivine grain size can be selected (e.g. in the matrix) and the spatial resolution of analyses; it is as such most subject to inter-operator bias. Wherever type II chondrules are rare, as in CV chondrites, the PMD (e.g. McSween 1977b) and a fortiori the RSD(Fa) may be strongly affected by any fluctuation in the number of type II olivine analyses selected.  \nFigure 3 : RSD(Fa) as a function of petrologic type (data from Scott and Jones (1990) and MetBullDB for CO; Sears and Hasan (1987), Scott (1984), Dodd et al. (1967) for UOC). We also show the RSD(Fa) values proposed by Sears et al. (1982) for UOC, and Sears (2016) for CO.  \n<!-- image -->  \nNow, the stability of the relative standard deviation below 3.5 for ordinary chondrites does not mean that the fayalite distribution does not change before. The Fe enrichment of type I chondrule olivine is in fact already appreciable from subtype 3.2 onward (e.g. McCoy et al. 1991; Huss et al. 2006). In fact, with Fa contents of type II chondrules showing smaller changes during metamorphism (e.g. McCoy et al. 1991; Scott and Jones 1990), the main control on RSD(Fa) is type I chondrule composition. Thus type I chondrule olivine composition should be essentially the metric of metamorphism. It was incidentally the basis for the calibration of the Scott and Jones (1990) scheme for CO chondrites (matching Warrenton and Parnallee as their then-current subtype 3.6).  \nThe simplest function of the mean 8 fayalite content of type I chondrule olivine (henceforth FaI) going from 0 to 1 as metamorphism advances to type 4 would be a linear function (for a given chemical group), ideally:  \n𝑚 = Fa I -Fa I,0 Fa limit -Fa I,0  \nWith FaI,0 the value before metamorphism, and Falimit the value of an hypothetical analog carried past olivine equilibration (i.e. type 4).  \nThis will be our guiding idea. However, the above expression is not yet an operational formula, for FaI,0 and Falimit would have to be inferred for any single chondrite, which is somewhat model-dependent (e.g. as to taxonomic assignations), especially for an ungrouped chondrite. Falimit may not be welldefined if no part of the parent body reached type 4, and even if it did, there may be a finite range of values therein (e.g. Rubin 1990).  \nYet, whenever type II chondrules exist in significant amounts (that is all but the most reduced chondrites), FaI,0 << Falimit ~ FaII with FaII the mean type II chondrule composition. Indeed (i) the one order-of-magnitude difference in Fa contents between type I and type II chondrules in the least metamorphosed chondrites may be seen e.g. in the plots by Ushikubo and Kimura (2021). (ii) As may be seen in the Fa histograms compiled by Brearley and Jones (1998), type II chondrule olivines equilibrate more rapidly with matrix olivine, because they were compositionally closer to it originally and ferroan olivine has larger diffusion coefficients (Chakraborty 2010). As such, with matrix+type II chondrules typically dominating the olivine budget together (chiefly matrix for carbonaceous chondrites and chiefly type II chondrules for ordinary chondrites), FaII rapidly approaches the whole-rock value.  \nFor such type II chondrule-bearing chondrites, we may then supersede the ideal formula with the better defined (if slightly larger) expression:  \n𝑚 = Fa I Fa II  \n(Yet we will have to return to the ideal formula for reduced chondrites)  \nFollowing our default rounding rule, the transition to type 4 (i.e. M1), i.e. olivine homogenization, could be redefined as FaI/FaII ≥ 0.95. We note that this ratio would be relatively robust to interoperator (or electron microprobe) bias as any bias in favor e.g. of large olivines would in general lower both the numerator and the denominator (given normal zoning, whether igneous or diffusion-induced). Hence the limited differences (comparable to statistical errors) between literature values and ours for matched meteorites (except for the Scott and Jones (1990) data for CO3.8 Isna; Fig. 4). Supplement B shows that for a given number of analyses, evaluation of m is a more precise proxy of metamorphism than a RSD(Fa) estimated from a Dodd et al. (1967)-style data collection, simply because the former allows to preferentially sample type I chondrule olivines which are chiefly affected by equilibration.  \nIt is worth cautioning that a given value of m does not correspond to a fixed time-temperature history for different parent bodies. Indeed, in the same way the PSF underlying the aqueous grades are affected by the abundance of finegrained matrix, the equilibration of chondrule olivine depends on the crystal size distribution in chondrules. For a given time-temperature history, chondrites with smaller chondrules will thus acquire higher metamorphic grades. Metamorphic grades rather relate to the ratio between the diffusion length and a typical chondrule phenocryst size (see Supplement D).  \nFigure 4 : Comparison of FaI/FaII measured in this study and literature values for matched meteorites (Scott and Jones 1990; McCoy et al. 1991; Noonan 1975; Noonan et al. 1977 (identifying in the latter Saint Mary's County 'metal-rich' and 'metal-poor' chondrules to type I and II, respectively)). Error bars are 1 sigma.  \n<!-- image -->  \nAlthough any FaI/FaII value above 0.1 is the result of metamorphism and thus can serve as a measure thereof, the value for the least metamorphosed chondrites is not zero but depends on the pre-metamorphic chondrule populations. In CO chondrites, this is 0.03 (for ALH 77307; Scott and Jones 1990) and for UOCs 0.06 (for Semarkona; McCoy et al. 1991). We obviously cannot blindly apply the default rounding to the nearest tenth there, otherwise the OCs would start at M0.1. Also, even for CO, any interoperator bias in the measurement of FaI would unduly affect the assessment of pristinity of the lowest-subtype samples, so fayalite content may not be the best assignation criterion for them.  \nFor these lowest subtypes, however, chondrule olivine notoriously retains a sensitive tracer of metamorphism in the Cr content of the type II chondrule olivine (Fig. 5). Cr indeed is lost early from type II chondrule olivine, presumably to join chromite (Bunch et al. 1967), and this Grossman and Brearley (2005) used to refine the lowest subtypes of UOCs. Rubin and Li (2019) later proposed specific boundaries for CO chondrites. So we should adapt  \nthe rounding of m according to the mean Cr2O3 content of the type II chondrules, following these precedents, which have hitherto shown boundaries only in graphical form.  \nFigure 5 : Cr2O3 average vs. standard deviation in type II chondrule olivine in unequilibrated chondrites. Data from Grossman and Brearley (2005), Schrader and Davidson (2017), Hewins et al. (2014), Schrader et al. (2015). Dashed lines represent the 3.05/3.1 and 3.15/3.2 boundaries proposed for UOC and CO by Grossman and Brearley (2005) and Rubin and Li (2019), respectively, and solid lines are our revisions thereof.  \n<!-- image -->  \nSince the starting Cr2O3 contents in type II chondrules was not the same in different chondrite groups, we cannot merely specify boundaries as a function of the absolute Cr2O3 content (as in Kimura et al. 2024) but we may reason in terms of its relative decrease since the onset of metamorphism. In fact, the 3.1/3.2 boundaries proposed by Grossman and Brearley (2005) for UOC and Rubin and Li (2019) for CO happen to correspond both to Cr2O3 at one third the highest measured (presumably pre-metamorphic) value Cr2O3(0). Since this corresponds to m ~ 0.10-0.18 (Fig. 6), we may likewise accept Cr2O3/Cr2O3(0) = 1/3 as the boundary between M0.1 and M0.2. (Supplement D also explores why, theoretically, there should be a (rough) link between Cr2O3/Cr2O3(0) and m ).  In  \nother words, M0.0 and M0.1 correspond to the loss of two thirds of type II chondrule olivine Cr. We then propose the M0.0/M0.1 boundary to mark the loss of the first third, i.e. a critical Cr2O3/Cr2O3(0) = 2/3 (this coincides with the 3.0/3.1 boundary of Kimura et al. (2024) in the case of CO chondrites). This would reproduce all Grossman and Brearley (2005) and most Rubin and Li (2019) classifications. Still, their graphs would put their 3.0/3.1 boundaries somewhat earlier (around Cr2O3/Cr2O3(0) = 0.8) 9 but these would increase the risk of really pristine chondrites being misclassified as M0.1 because of statistical or systematic errors in mean composition estimates 10 . Similar to Sears (2016), we a fortiori refrain from proposing finer subdivisions (e.g. the subtypes 3.00 and 3.05 of Grossman and Brearley (2005) or Rubin and Li (2019)), although one might base some on other criteria for specific groups (e.g. sulfide redistribution in matrix; Grossman and Brearley 2005). However finely defined, each subtype, including M0.0, must represent a finite range of metamorphic alterations rather than a discrete step.  \nFigure 6 : FaI/FaII as a function of type II chondrule olivine Cr2O3 normalized to the pre-metamorphic value in UOC and CO chondrites. Vertical lines indicate our boundaries for M0.0, M0.1 and M0.2 (i.e. subtypes 3.0, 3.1 and 3.2 for type 3 chondrites). Cr data from Grossman and Brearley (2005), MetBullDB (for CO) and this study; literature Fa data from McCoy et al. (1991) for Semarkona and Jacquet et al. (2015) for Bishunpur. Error bars are 1 sigma.  \n<!-- image -->  \nNow, this redefinition of the lowest subtypes presupposes knowledge of the initial value Cr2O3(0), which would again depend on the identification of the chemical group. Yet MnO seems to provide a suitable proxy, in that the most unmetamorphosed chondrites generally show Cr2O3/MnO ≈ 1.2 in their type II chondrule olivine (Fig. 7). Why should that be? Bulk type II chondrules have nearly chondritic Cr2O3/MnO ratios (e.g. Gordon 2009) given the close volatilities of Mn and Cr (half-condensation temperatures of 1158 vs. 1296 K; Lodders 2003) and the limited devolatilization of such chondrules (Gordon 2009). Alexander and Ebel (2012) report an average type II bulk ratio of 1.3 (compared to 1.5 for CI; Lodders 2003). Since the olivine/melt partition coefficients are close for both elements (Jing et al. 2024), the fractional crystallization of olivine should not incur much change in that ratio in olivine. This is supported by the relatively parallel zoning profiles of Mn and Cr in  \nolivine (notwithstanding the final drop in Cr2O3 due to incipient chromite precipitation; Jones 1990) and the overall correlation between Cr2O3 and MnO in both Semarkona (Jones 1994) and ALH 77307 (Jones 1992) type II chondrule olivine. Whatever the value of this explanation, we may thus set threshold Cr2O3/MnO values of 1.2×2/3 = 0.8 and 1.2/3 = 0.4 for the M0.0/M0.1 and M0.1/M0.2 boundaries, respectively.  \nFigure 7 : Relative standard deviation of Cr2O3 as a function of Cr2O3/MnO (ratio of the means) in type II chondrule olivine. Same data sources as Fig. 5. Vertical lines indicate our proposed boundaries for M0.0, M0.1, M0.2.  \n<!-- image -->  \nHowever lengthy the justification above, the resulting classification algorithm (for type II chondrule-bearing unequilibrated chondrites) is simple and may thus be recapitulated as follows:  \n- 1° Measure type I and type II chondrule olivine compositions (one typical grain of the closest olivine-dominated chondrule per random point; see the Method section)\n- 2° Calculate mean type II olivine composition:\n- If Cr2O3/MnO ≥ 0.8, metamorphic grade is M0.0.  \nIf 0.4 ≤ Cr2O3/MnO < 0.8, metamorphic grade is M0.1.  \n- 3° Else, calculate mean type I Fa composition and the ratio m = FaI/FaII. The metamorphic grade is M r ( m ) where r ( m ) is m rounded to the nearest tenth ≥ 0.2.\n- 4° Barring significant aqueous alteration, the petrologic subtype is 3+ r(m).  \nAgain, this only governs the subtype indicated by olivine composition. Other assignation criteria may overrule it, depending on the adaptations made for each chondrite group. These we now move on to consider.", '5.1 Ordinary chondrites': 'We announced as our touchstone that our redefinition of subtypes should match closely those of Sears et al. (1980) for ordinary chondrites. We have yet to show that our proposed formula for m fulfills this requirement.  \nBy construction, this is already the case for all subtypes 3.0-3.1 (M0.0-M0.1) as recalibrated by Grossman and Brearley (2005). Strictly speaking, the reformulation in terms of Cr2O3/MnO fails for H3.10 chondrite RC 075, which shows a Cr2O3/MnO = 0.89 > 0.8, but this is due to an anomalously low mean MnO of 0.38 wt% (±0.21). If the highest Cr2O3 of the Grossman and Brearley (2005) study is taken as a reference for the initial Cr2O3 (0.52 ± 0.15 wt% for QUE 97008) the M0.0/M0.1 and M0.1/M0.2 boundaries in terms of Cr2O3 can be set at ~0.35 and 0.17 wt% Cr2O3, respectively, matching the Grossman and Brearley (2005) classifications without exception (Fig. 5).  \nFor more metamorphosed UOCs, we see from Fig. 8, that m generally matches the decimal part of the literature subtype within errors (~one decimal subtype), but there are clear outliers which we discuss now.  \nFigure 8 : FaI/FaII as a function of petrologic subtype for UOC and CO chondrites. For UOCs, data scatter along the diagonal line corresponding to m equaling the decimal part of the petrologic type (PT), with named outliers discussed in the text. For CO, as will be seen in their dedicated subsection, the interval 3.3-3.6, which deviates most clearly from the 1:1 line, is hitherto illdefined and requires revision. Literature data from McCoy et al. (1991), Jacquet et al. (2015), Baeza (2017), DeHart (1989), Noonan et al. (1977), Scott and Jones (1990), MetBullDB (for CO).  \n<!-- image -->  \nMezö-Madaras is classified as L3.7 (e.g. Sears et al. 1980) but is here ranked as M0.4. The discrepancy is a clear consequence of its genomict character, as equilibrated L4-5 clasts have long been described in this meteorite (e.g. Binns 1968; Fig. 9). Since TL sensitivity gains one order of magnitude every three subtypes, ~10 % of equilibrated clasts in an otherwise low-subtype host suffices to bring a bulk TL sensitivity at subtype 3.7 level. The same explanation seems to hold for Chainpur (hitherto LL3.4 but M0.2): here, only ~1 % of equilibrated material would be needed, and this is borne out by the occasional observation of quite equilibrated isolated chondrules (not part of larger clasts): two (out of 114 chondrules examined) with crystalline feldspar and olivine Fa consistent with LL4 chondrites interpreted by Ruzicka (1990) to have been annealed before  \nagglomeration, or two type I chondrules (#4 and 5 in fraction 4) out of 24 studied by Keil et al. (1964) with likewise LL4-like olivine Fa contents but Fs around 10 mol%. We also observed such chondrules in our section, one of which is depicted in Fig. 1b.  \nFigure 9 : Back-scattered electron map of section 4416sp1 of ordinary chondrite Mezö-Madaras (hitherto L3.7 but reclassified here as L3.4). An equilibrated clast is visible on the middle top.  \n<!-- image -->  \nThus, the strong dependence of TL sensitivity on metamorphism, a strength of the Sears et al. (1980) classification, also presents a drawback in incurring a strong sensitivity to equilibrated xenoliths. This is nonnegligible as most type 3 ordinary chondrites are brecciated at thin section scale (in particular 86 % of LL3s; Bischoff et al. 2018), although only a fraction of those breccias may be genomict. In this case, the bulk TL value may correspond to no lithology (as in Mezö-Madaras and Chainpur), let alone the host (which would give the maximum metamorphic level of the meteorite as an ensemble). In contrast, FaI/FaII of a genomict ensemble would roughly correspond to the volumeweighted average of the FaI/FaII of the different lithologies 11 . Thus the effect of  \nminor lithologies would be minimal, and can of course be further mitigated by avoiding them in the point selection (as we have done here for Mezö-Madaras). Lithologies comprising at least 10 vol% (Jacquet 2022) can have their metamorphic degrees evaluated independently so as to display the full range of petrologic types in the hyphenated way customary of MetBull classifications. So Mezö-Madaras can be described as L3.4-4 or simply L3.4 if only the host subtype is meant 12 ; the full information could be shown by writing \'L3.4 (3.44)\'.  \nAnother outlier is Tieschitz, hitherto classified as type 3.6 (Sears et al. 1980) but ranked as M0.3 here. Here the TL sensitivity has been likely increased by the hydrothermal deposition of \'white matrix\' (Christophe Michel-Levy 1976) which Dobrica and Brearley (2014) showed to be polycrystalline albitic plagioclase. Its classification as 3.6 previously conflicted with limited matrix recrystallization or FeO depletion (Sears et al. 1982). Still, the low Co variability in kamacite, low bulk C or 36 Ar contents seemed consistent with higher subtypes (3.8, 3.7 and 3.6 respectively; Sears et al. 1982), but from the plots of Sears et al. (1980), Co variability actually has little discriminatory power above subtype 3.2 (and at the time was only evaluated for LLs below), C likewise little below 3.8, and 36 Ar merely sets Tieschitz beyond Krymka (LL3.2), Chainpur, Mezö-Madaras, and before Manych (LL3.4; M0.3), and Parnallee (LL3.7; M0.7), i.e. mostly \'low-subtype\' UOCs.  \nA final outlier, Clovis (n° 1), classified as H3.6 by Sears et al. (1982) appears here more metamorphosed (M0.9) than its TL data suggested. Data by Noonan (1975) also indicate m = 0.86 if his \'distinct chondrules\' and \'angular and vesicular fragments\' are interpreted as type I and II chondrules respectively judging from his photomicrographs. In fact, Sears et al. (1980) themselves had originally predicted a 3.9 classification based on olivine PMD. The lack of Fe 2+ disorder in pyroxene, consistent with equilibrated chondrites (Dundan and Walter 1967), and the olivine-spinel closure temperature of 710±40 °C, just above H3.8 chondrite Dhajala (Wight et al. 1994) are also consistent with such a high subtype. One may surmise that shock amorphized feldspar, as \'glass\' is reported by Noonan (1975), but we do not know the shock classification of  \nClovis (n° 1). Only 3 out of 111 H3 chondrites studied by Bischoff et al. (2019) reached the shock stage S4 corresponding to partial maskelynitization, and only one H3 studied by Miyahara et al. (2020) had shock-induced melting textures. So although definitely a possibility here, shock amorphization is probably a minor disturbance for TL-based UOC classification in general.  \nThus, the m -based subtypes seem to capture the essence of the Sears et al. (1980) subtypes (in a manner more robust against impact-related or other disturbances). Table 4 keeps essentially all Sears et al. (1980) criteria unchanged 13 but replaces their olivine column by the m criterion. We stress again that revision of the Sears et al. (1982) olivine compositional criteria was necessary (Fig. 3) independently of our goal of intergroup harmonization.  \nTable 4 : Metamorphic scale for UOCs. Most rows are copied verbatim from Sears et al. (1980), with "F/FM" is the matrix FeO/(FeO+MgO) normalized to the whole-rock value and TLDhajala the induced thermoluminescence sensitivity normalized to Dhakala. We have however replaced the olivine criteria and added Raman spectroscopic criteria following the works of Quirico et al. (2003) and Bonal et al. (2006, 2007), with FWHM-D being the full width at half maximum of the D band and ID,G the peak intensity of the D, G bands.  \nHowever limited our modification of the Sears et al. (1980) classification is then, wherever m clearly changes (beyond analytical error) the balance of the criteria (especially if contradicting ones can be explained away), the meteorite ought to be reclassified accordingly. All our above outliers should thus be  \nreclassified as shown in Table 1. The reclassifications of Chainpur and Tieschitz are of some moment for the Raman-based classification, since they were Quirico et al. (2003) and Bonal et al. (2006, 2007)\'s reference samples for subtypes 3.4 and 3.6, respectively. So the Raman subtype assignation boundaries ought to be redefined-or rather defined, as in fact the aforementioned works refrained from setting explicit boundaries (although Righter et al. (2022) drew some in their Fig. 2). Fig. 10-11 leave open the possibility of a fair inter-group synchronization of m - and Raman-based classifications at early stages. We have thus proposed the same M0.0/M0.1 and M0.1/M0.2 Raman boundaries for OCs and CCs (Table 4-6), allowing a simultaneous use of organic matter maturity as a universal sensor of metamorphism at least until M0.2.  \nFigure 10 : FaI/FaII as a function of Full Width at Half Maximum of Band D (FWHM-D) of the Raman spectrum of organic matter, with proposed boundaries for M0.0, M0.1 and M0.2. Raman data from Quirico et al. (2003), Bonal et al. (2006, 2007); same Fa data sources as Fig. 8, plus Simon et al. (1995) for Axtell and Allende, Karcher et al. (1985) for Leoville, Hutchison & Symes (1972) for Vigarano, Van Schmus (1969) for Grosnaja and Kaba, Wood (1967) for Mokoia. For all these CVs, FaI was taken to be the average of the data below Fa20 (that is, the hiatus preceding type II values).  \n<!-- image -->  \nFigure 11 : FaI/FaII as a function of the ratio of the peak intensities of the D and G bands of the Raman spectrum of organic matter. Same data sources as Fig. 10.  \n<!-- image -->', '5.2 Rumuruti chondrites': 'Bischoff (2000) inaugurated the practice of assigning subtypes to R3 chondrites using the RSD(Fa) formula of Sears et al. (1982). The relationship between m and RSD(Fa) depends on xI . To estimate it, we note that, in weakly metamorphosed R chondrites, Friend et al. (2017) analyzed 227 low-Ca pyroxenes below Fs10 and 106 beyond, hence a pyroxene type I/type II ratio of xI,Px /(1xI,Px ) = 227/106 = 2.1 (where xI,Px is the fraction of chondrule pyroxene hosted by type I chondrules), which should be representative of the premetamorphic ratio given the slow Fe-Mg interdiffusion in pyroxene compared to olivine.  If, owing to the likely genetic link with ordinary chondrite chondrules (e.g. Regnault et al. 2022), we apply the same 𝜃 defined in Supplement E.2, we can infer xI = 0.6.  \nFrom Fig. 12, such a value would constrain m to be close to 1RSD(Fa), i.e. the decimal part of the Bischoff et al. (2000) subtype. Thus our scheme would likely not incur marked changes to the current practice. Yet a dedicated study on R  \nchondrite metamorphic classification, including direct measurements of m and other parameters, would be worthwhile.  \nFigure 12 : Theoretical relationship (shaded areas) between m and RSD(Fa) for different values of xI (for m ≥ 0.2). See Supplement C.2 for justification. Also overplotted are the relationships between petrologic subtype and RSD(Fa) for different literature schemes, as in Fig. 3.  \n<!-- image -->', '5.3 CO chondrites': "The current MetBullDB CO subtypes mainly follow Chizmadia et al. (2002), who proposed a classification of CO chondrites based on the introduction of Fe in amoeboid olivine aggregates (AOA). The interval 3.0-3.2 was further refined by Rubin and Li (2019). Other schemes include Scott and Jones (1990) and Sears (2016). How does our metamorphic scale compare with the literature?  \nBy design, the Cr2O3-based definitions of M0.0 and M0.1 match the literature usage for CO3.0 and CO3.1. From MetBullDB data, only NWA 6701, NWA 6447 and Los Vientos 123 would have to be reclassified from CO3.1 to CO3.0 (=M0.0); and NWA 8345 (currently CO3.2) would (narrowly) become a CO3.1. We note that in refraining from refining subtypes as in Rubin and Li (2019), we avoid assigning different samples from the same strewnfield to three different subtypes. Indeed Righter et al. (2024) have their dominant Transantarctic mountain pairing groups both straddle Rubin and Li (2019)'s subtypes 3.053.15, while they fit entirely in our M0.0.  \nNominally, the higher subtypes also match with the literature. Although only Scott and Jones (1990) provided indicative values for FaI, they and Sears (2016) gave criteria in terms of the average Fa content 𝐹𝑎 ̅̅̅̅ . Fig. 13 shows that they closely overlap with ours. The RSD(Fa) thresholds should also roughly match with our classification (Fig. 12).  \nFigure 13 : Average fayalite content of CO as a function of petrologic subtype. Overplotted are the values for the Scott and Jones (1990) and Sears (2016) subtypes, as well as ours if one assumes FaII = 40 mol% (the average of our four samples) beyond 3.2 and xI = 0.74 (see Supplement C.1).  \n<!-- image -->  \nNow, in practice, MetBullDB data scatter significantly away from the nominal curves of either scheme, and it is often unclear which criterion was used for the classification. Yet most MetBullDB entries which give a reason for a subtype do invoke olivine composition (with Raman spectra also used for the lowest subtypes). At any rate, the 'average' Fa depends on the selection thresholds for olivine grains (e.g. when would an isolated grain in the matrix count? etc.) and is thus of questionable reliability.  \nThe disagreement is especially strong for literature subtypes in the range 3.33.6. In particular, the 'classical' CO chondrites making McSween (1977)'s stages I and II-Kainsaz, Felix, Ornans, Lancé-are found here to belong all to the M0.2 grade. We find no significant difference in terms of the Fa content of the type I chondrules between the extremes Kainsaz and Lancé (CO3.2 and CO3.5 in MetBullDB respectively). To be sure, the oft-cited Van Schmus (1969) dataset reported 𝐹𝑎 ̅̅̅̅ = 11.8 mol% for Kainsaz contrasting with 𝐹𝑎 ̅̅̅̅ = 18.4 21.2 mol% for Felix, Ornans and Lancé. However, the associated histograms (see also Crabb et al. (1982)) make plain that the mean Fa for Fa<20 olivine (essentially the type I population, given the observed hiatuses around Fa20) in Kainsaz and Lancé are indistinguishable (6±4 vs. 5±5 mol%; 1σ) and close to our FaI values. The difference in the overall mean Fa values lies in the lower abundance of more ferroan olivine grains selected in the Van Schmus (1969) Kainsaz section (12 % vs. 38 % in Ornans and Lancé in the same work, again assuming the Fa20 cutoff).  \nWhile Bonal et al. (2007) already ranked all these chondrites at the same subtype (their 3.6), Sears (2016) still set Kainsaz (which made all McSween (1977)'s stage I) apart (at 3.2 vs. 3.4). Now, disregarding olivine, Kainsaz still appears more 'primitive' than all McSween (1977) stage II chondrites for 5 out of 10 Sears et al. (2016) criteria (kamacite composition, 36 Ar content, organic matter maturity) even though its TL sensitivity is higher than all of them. Kainsaz may thus still be less metamorphosed in some sense than other M0.2 CO chondrites, but this does not necessarily warrant a distinct subtype. For one, as we just saw, not all criteria give the same order so further refinement below the scale of fluctuations (analytical or real) off an assumed monotonous trend may be excessive. Also, setting Kainsaz apart could make sense at the time of McSween (1977) where it appeared the most primitive CO known but since  \nmany CO3.0 and CO3.1 have been recognized afterward, any possibly useful refinement should be restricted to them (e.g. Rubin and Li 2019).  \nTable 5 lists our metamorphic grades as adapted for CO chondrites. We did not retain among our criteria the thickness of ferroan olivine veins in AOAs reported by Chizmadia et al. (2002) because they may have precipitated from fluids (e.g. Han et al. 2022) and thus not directly trace metamorphism after all.  Given that many 'classical' CO chondrites are lumped together as M0.2, while the McSween (1977) 'stage III' chondrites Warrenton and Isna retain their literature subtype (even though we measured M0.9 for Isna, but within errors of its 3.8 classification), the hereby depopulated interval M0.3-M0.6 suffers from a paucity of well-characterized chondrites. Yet there is no real hiatus, as suggested by Fig. 13. We assigned ALH 77003 to M0.4 (a moderate correction to Scott and Jones (1990)'s subtype 3.5) given the ratio FaI/FaII = 0.44 found by Scott and Jones (1990) and independently confirmed by the formula of supplement C.1 applied on their randomized histogram, using xI =0.61 (extrapolated from their ALH 77307 and ALH 82101 data) and their FaII value (this yields FaI/FaII = 0.42). This grade and other 'intermediate' metamorphic grades cannot at present be better characterized for non-olivine criteria. A dedicated study of the more metamorphosed CO would be worthwhile.  \nAmong chondrites currently classified as CO, a matrix-poor 'CA' grouplet has been set apart by Kimura et al. (2022), with an assigned subtype of 3.2. Cr systematics of ferroan olivine and Raman properties are indeed consistent with a M0.2 classification, but may marginally suggest M0.1 for Asuka-9003 and Asuka-09535.  \nTable 5 : Metamorphic scale for CO chondrites. Raman parameters as in Table 4 (Bonal et al. 2006), TL after Sears (2016), kamacite from Scott and Jones (1990).", '5.4 CV chondrites': "Many CVs show BSE-visible evidence of influx of Fe in type I chondrule olivine (e.g. Bonal et al. 2007), but calculated m values do not exceed 0.3 (e.g. Fig. 10-11) for mainstream CVs. We set apart the C3 chondrite NWA 8418 which MacPherson et al. (2023) classified as CV4. Since its matrix (probably close to type II chondrules) averages Fa38 while chondrule olivines (which are type I in great majority) show Fa20±14 (if the histogram in MacPherson et al. (2023)'s Fig. 15 is representative), it would belong to metamorphic grade M0.5 and be a CV3.5.  \nWith most CVs not exceeding M0.2, type II chondrule olivine Cr systematics should be decisive in principle, yet type II chondrules are too rare in CV chondrites for this to be practical (Grossman and Brearley 2005). To reliably estimate m in Fig. 10-11, we have even had to adopt a constant FaII = 30.3 mol%, the average of all Grossman and Brearley (2005) CV analyses, but with higher degrees of metamorphism, Fe-Mg exchange with more ferroan matrix should increase it somewhat (as in NWA 8418).  \nRaman properties of organic matter may then be the handiest tool for CV classification. Bonal et al. (2007) classified CVs according to the Raman spectra of their carbonaceous matter in three broad OC-inspired subtype ranges, i.e.  \n'3.1', '3.1-3.4' and '3.6' (which we lump with their '>3.6'). They roughly correspond to m ~ 0.05, 0.1 and 0.2 respectively (Fig. 10-11), so can be received as M0.0, M0.1 and M0.2, respectively 14 . Table 6 recapitulates the resulting metamorphic classification for CV chondrites. The subtypes are considerably coarser, and thus simplified, relative to Bonal et al. (2007) or Righter et al. (2024).  \nOur Bonal et al. (2007)-inspired (if simplified) subtypes correlate little with the TL-based ones (affected by CAIs?) of Guimon et al. (1995), even though the latter roughly spanned our suggested range of subtypes (3.0-3.3). So we can propose neither TL criteria nor meaningful correspondences with those subtypes.  \nTable 6 : Metamorphic scale for CV chondrites. Raman parameters defined as in Table 4 and after data of Bonal et al. (2007); NWA 8418 data after MacPherson et al. (2023)", '5.5 Other carbonaceous chondrites': "While most CK are equilibrated (types 4-6), some of them are of type 3. Although no subtype is officially recognized in the MetBullDB and no scheme has been proposed in the literature, the consensus is that those are >3.5, because of the paucity of refractory inclusions (Greenwood et al. 2010) and the limited heterogeneity of matrix olivine (Dunn et al. 2018; see also Chaumard and Devouard 2016).  NWA 5343, which Dunn et al. (2018) deem the least  \nmetamorphosed CK, shows a mean chondrule Fa content of 15.9 mol% and a mean matrix olivine Fa content of 36 mol%. If these approximate the (dominant) type I chondrules and type II chondrules respectively, we may estimate a metamorphic grade around M0.5, which would not be too far from Dunn et al. (2018)'s suggested (but uncalibrated) subtype 3.6-3.7. So in our scale too, CK3 would be definitely more metamorphosed than (most) CV3 (Dunn et al. 2018).  \nAcfer 094 (C3-an) has Cr2O3/MnO = 1.0 (Grossman and Brearley 2005) consistent with its very pristine character and a M0.0 grade. For CI chondrites, the 10 ferroan olivine analyses in the dataset of Endress (1994) also yield this value (in agreement with Fig. 3 of Frank et al. 2014) and a M0.0 classification. Likewise, all CM would also be M0.0 according to the data of Schrader and Davidson (2017) and Hewins et al. (2014) plotted on Fig. 8.  \nSimilarly, all CRs are M0.0, in that the type II olivine Cr2O3 never goes below two thirds of the largest value measured by Schrader et al. (2015), i.e. 0.42±0.07 wt% for EET 96259 (within error of the 0.38 wt% shown by the CR3 chondrites MET 00426 and QUE 99177). GRA 06100 and Y-793495 are marginally below the indicative M0.0/M0.1 threshold Cr2O3/MnO value of 0.8 (at 0.73 for both) but this ratio may have been somewhat lower pre-accretion in CRs than for other chondrites (since it exceeds unity only for the anomalous member Al Rais, at 1.1). We nonetheless note that GRA 06100 has undergone some short duration thermal metamorphism (Briani et al. 2013) so the relatively low Cr of its olivine might have significance.  \nIf we combine 13 ferroan olivine analyses of CY chondrite Belgica-7904 from Tomeoka (1990), Kimura and Ikeda (1992), Skirius et al. (1986), we obtain: 0.45±0.20 wt% Cr2O3 and 0.35±0.15 wt% MnO, hence a Cr2O3/MnO = 1.3±0.2. This is consistent with the undisturbed type II olivine-chromite igneous partitioning reported by Johnson and Prinz (1991), and the FaI/FaII=0.03 calculable from the histograms of Bischoff and Metzler (1991). So CY would be M0.0 too, despite the brief heating event which led to their dehydration (especially for Belgica-7904). Evidently, whether in CY or CR, olivine cannot be held a sensitive recorder of short-lived thermal events, with low integrated diffusion length (although appreciable at submicron scale near crystal boundaries; Nakato et al. 2008). As such, our metamorphic scale probes protracted heating (e.g. that due to 26 Al decay), independently of such brief heating events. Thus a separate thermal classification bearing on the brief heating events must be used for them. The Kimura et al. (2011) opaque-based  \nclassification in categories A (unheated), B (exsolution of pentlandite), C (exsolution of Ni-rich metal and Co diffusion into kamacite) may be preferable to the more numerous stages I-IV (+ 'unheated') of Nakamura (2005) also because they are applicable independently of the preheating hydration state (so e.g. applicable to CR chondrites). Yet, for CY, the Nakamura (2005) XRD criteria should be of course added to the opaque petrographical ones with which they are correlated (A=unheated-stage I; B = II-III; C = III-IV; Kimura et al. 2011), possibly along with Raman properties of organic matter (Briani et al. 2013). So one could classify e.g. Belgica 7904 as M0.0, A0.7TC (or 'A0.7TIV' if one sticks to Nakamura (2005)'s heating stages). We note that the opaque (and Raman) criteria for categories B-C may also be fulfilled by protracted metamorphism to 3.1 and beyond (Kimura et al. 2008, 2024), so that by themselves they do not distinguish between brief and protracted heating. This is not an objection to their use for petrologic subtype determination in other chondrite groups (Kimura et al. 2024), since such a disturbance is uncommon, so long the possibility remains to overrule those criteria.  \nWild 2 ferroan olivine shows MnO contents comparable to CI and CR olivine (around 0.4 wt%) and the Cr2O3 values cluster around 0.1 wt% (Frank et al. 2014). This indicates a metamorphic grade of M0.2 for these olivines, that is, if they can be considered representative of chondrules (of which they would be but fragments) when metamorphism occurred (probably not in Wild 2 itself; Frank et al. 2014). This does not mandate a time-temperature history similar to e.g. type 3.2 UOC for the grain size may have been smaller, judging from the few microporphyritic chondrules found (Frank et al. 2014). If the purported parent chondrules were already in fragmented form during metamorphism, the applicability of our chondrule-based classification would be questionable (our own measurement procedure excludes monomineralic fragments), and we might then calibrate it instead on similar-size matrix grains (thus in fine on the absolute diffusion length). Comparing with matrix olivine in UOCs, Frank et al. (2014) suggested a subtype of 3.05-3.15.  \nIn Ryugu, Mikouchi et al. (2022) reported olivines clustering around Fa1 and some type II olivine compositions spanning Fa10-56 (see also Liu et al. 2022; Kawasaki et al. 2022). This also suggests low metamorphism (M0.0).", '5.5 Reduced chondrites': "The assumptions underlying the operational expression m = FaI/FaII fail in reduced chondrites because they lack bona fide type II chondrules. We must  \nreturn to the first expression in terms of the initial type I and whole-rock olivine composition.  \nIn Kakangari, Berlin (2009) did recognize a 'type II trend' among chondrules, with alkali-rich mesostases and sometimes type II-PO-like textures, but neither the Fa (3.5 ± 0.9 mol%) nor the Fs (9.9 ± 3.5 mol%) differ appreciably from those of their 'type I trend' (3.6 ± 0.6 mol% and 8.7 ± 4.4 mol%, respectively). The (type I or II) chondrule Fa does differ from matrix olivine (1.5 mol%; Brearley 1989) and the chondrules have incompletely equilibrated with it, with their Fs values presumably approximating FaI,0 because of slower Fe-Mg interdiffusion in pyroxene. If we also equate Falimit with the bulk composition of more equilibrated K chondrites (e.g. NWA 10085, Fa2.97±0.16) 15 , we obtain m = 0.89, hence Kakangari would be a K3.9. We note that application of the Sears et al. (1982) formula using the RSD(Fa) (of chondrule olivine) would put Kakangari at subtype 3.5 (e.g. Berlin 2009) and make the distinction of subtypes dependent on order 0.2 mol% precision in the standard deviation of Fa given the low normalizing average Fa; our convention thus circumvents this.  \nEnstatite chondrites have no type II chondrule altogether, although some olivine analyses reach up to Fa25 (Lusby et al. 1987). However, in EH chondrites, average olivine fayalite contents still show a variation, decreasing from Fa1-2 to Fa0.5, correlated with the dissolution of olivine (McKibbin et al. 2024), so an olivine compositional definition of m would be nominally applicable. The Fa0.5 value is reached around 1 vol% olivine, which Zhang et al. (1995) had already proposed a threshold value between type 3 and 4 16 , so it could indeed play the role of Falimit. Yet, uncertainties in the evaluation of FaI itself from so minor a EH component phase make it an unpractical assignation criterion, only to be formally correlated to more operational ones, e.g. olivine modal abundance . It is still uncertain whether any known enstatite chondrite should count as a M0.0. The organic matter maturity of Sahara 97096-the least advanced reported by Quirico et al. (2011)-is intermediate between type 3.0 and 3.1 ordinary chondrites, although, again, the precursor need not have been the same. The low matrix-normalized abundances of presolar silicates in enstatite chondrites (Ebata et al. 2006) likewise leave room for even less  \nmetamorphosed samples. Possibly though their major silicate mineralogy and chemistry may not be too different from those observed in Sahara 97096 (~10 vol% olivine; McKibbin et al. 2024). The Fs histograms of Lusby et al. (1987) suggest that pyroxene (less sensitive to diffusion) of UEC has a mean Mg# around 98-99 comparable to the olivine in those meteorites, so olivine was likely not more oxidized or abundant than this upon accretion. So one might define a robust olivine mode-based classification, e.g. something like m ≈1-( x Ol/10 vol%), with x Ol the olivine fractional abundance, which would be rounded up to 1 for x Ol below 0.5 vol%, i.e. about the type 3/type 4 transition upheld above. Perhaps the olivine/pyroxene ratio could be more robust to fluctuations of the abundance of opaque nodules. Unfortunately, there is little overlap between the samples analyzed by Quirico et al. (2011) and McKibbin et al. (2024), so a metamorphic subclassification of EH3, with different correlated assignation parameters (Raman properties, olivine composition and abundance, presence of ferroan pyroxene, presolar grain abundances…), is not yet at hand. The same a fortiori can be said of EL3 chondrites.", '6. Outlook and perspectives': "Our metamorphic and aqueous alteration scales produce the two-dimensional classification shown in Fig. 14. When the coordinates are understood as continuous variables, chondrites are never off by more than ~0.05 from either the x or the y axis. This means that they are always either A0.0 or M0.0. Clearly, this apparent exclusivity of metamorphism and aqueous alteration is a matter of the chosen precision of the taxonomy (and its metrics). The very heating which caused melting of ice and aqueous alteration also caused incipient diffusion and recrystallization in the primary phases such as olivine. However, aqueous alteration in CM chondrites, for example, took place at temperatures variously estimated at 0 to 300°C (Suttle et al. 2021b), which would not allow Fe-Mg diffusion lengths in olivine longer than 0.01 µm over a Ma timescale (Schwinger et al. 2016). Yet we mentioned earlier the possibility of measurable Cr depletion in type II chondrule olivine in some CR chondrites (Fig. 5). As to noncarbonaceous or CV chondrites, the low amount of fine-grained matrix and water limited the amount of phyllosilicates produced but these are present up to M0.2 in OC and CV chondrites. Higher subtypes may have lost phyllosilicates by dehydration.  \nFigure 14 : Recapitulative biplot of m versus the PSF. Data from this study or literature cited in previous figures, with the following additions: Howard et al. (2010) for CV PSF values, King et al. (2015) for CI's, and Grant et al. (2023) for UOC's (we crudely calculated a maximum phyllosilicate abundance by assuming their bulk water content measured to be entirely in the form of phyllosilicates with 20 wt% water). A PSF value of zero has been assigned to CO (save for the exceptions with detectable phyllosilicates in Alexander et al. 2018) and Acfer 187/El Djouf 001 and Renazzo are assigned rounded PSF values of 0.1 and 0.2, respectively, following Table 3. Additional Fa data are from Endress (1994) for CI chondrites, Bischoff et al. (1993) for Acfer 187 and El Djouf 001, Hewins et al. (2014) for Paris, Hutchison and Symes (1972) for Murchison, Wood (1967) for other CM and CR chondrites.  \n<!-- image -->  \nThis apparent exclusivity of (taxonomically notable) aqueous and metamorphic alteration leaves the taxonomy de facto univariant (notwithstanding a possible brief heating descriptor as discussed for CY chondrites). Hence, a single Van Schmus and Wood (1967) petrologic subtype  \ncan still convey the essential information as in present MetBullDB usage. For type 3 chondrites (A0.0), the subtype can be 3+r( m ) (with, again, 'r' meaning rounding to the relevant tenth), without ambiguity. Indeed, on the whole, our redefinition is a minor correction to existing schemes, especially for ordinary chondrites (Sears et al. 1980; Grossman and Brearley 2005).  \nAs to type 1 and 2 chondrites, it would be tempting to follow Howard et al. (2015) and define petrologic subtypes as 3-2 PSF rounded to the nearest tenth, as a transition between 3 and 1. However, unless one artificially restricts subtypes to even tenths (3.0, 2.8, 2.6, 2.4 etc.), this would motivate an overprecise, and thus less robust classification (equivalent to distinguishing e.g. A0.75, A0.80, A0.85…) which we have argued against in the section The aqueous alteration scale. We prefer to compress the scale to the [2.0;3] interval, similar to Rubin et al. (2007) and Harju et al. (2014), and define petrologic subtypes as 3-r( PSF ). So type 1 would encompass subtypes 2.1 and 2.0. It must be cautioned though that while we adopt the [2.0;3] interval of Rubin et al. (2007) and Harju et al. (2014), our PSF-defined subtypes coincide with theirs only near types 1 and 3 (Tables 2-3), even though the underlying assignation criteria embody their work. So one should be mindful of possible confusions.  \n<!-- image -->  \nFigure 15 : Schematic plot of the conversion of our bidimensional subclassification in Van Schmus-Wood subtypes. At the level of precision chosen, the chondrites indeed form a one-dimensional line that can be essentially parameterized by one decimal subtype. Dashed lines show boundaries of Van Schmus and Wood (1967) integer petrologic types 1-4.  \nIn summary then, the general petrologic subtype would be 3+r( m )-r( PSF ). This is depicted in Fig. 15. This does not prejudge the possibility of more subdivisions in the alteration or metamorphic scales, e.g. A0.0 in A0.00 and A0.05; or M0.0 in M0.00 and M0.05 (e.g. the type 3.00 and 3.05 of Grossman and Brearley (2005) or Rubin and Li (2019)).  These would no longer be capturable in the single petrologic subtype above, which warrants the formal development of the separate scales. The extra information could be added parenthetically, e.g. the Bali-type oxidized CV chondrite Grosnaja would be a 'CVox3.2 (A0.05)'.  \nWe have already commented for both scales that genomict breccias can be indicated with hyphens between the represented metamorphic and aqueous alteration grades (so long they account for >10 vol%) as well as the Van Schmus and Wood (1967) types. For both scales, a bulk value roughly means a volumeweighted average of the values of the individual lithologies. So this bulk value (with possibly the range) makes sense even for them. Thus our adopted alteration and metamorphic degrees play a fully symmetric role.  \nOur purpose has been to provide a unified, yet flexible framework for the secondary classification of chondrites, molded after existing practice grafted on the Van Schmus and Wood (1967) scale. Precise boundaries between alteration or metamorphic grades and the relevant parameters must be adapted to chondrite groups of interest. We have proposed such adaptations for several groups mostly based on existing literature; in fact our classifications may be merely seen as modifications of existing ones recast in our new framework. Other such classifications can be devised for other groups (e.g. R, E, CK chondrites…) by dedicated studies and none of the existing ones should be of course exempt from revision. Yet if the present framework is adopted, assigned grades would be robust and unlikely to change by more than 0.1.  \nIn this purely taxonomic work, we have not attempted to gain new insights on secondary processes. The metrics chosen (olivine equilibration, abundance of nominally hydrous silicates) are the most obvious and safest ones possible, whatever the exact context of the processes responsible. Yet we hope that our proposed rationalization of secondary taxonomy will prove a practical tool to better unravel the postaccretionary history of chondrite parent bodies.", 'Supporting information:': "EA\\_olivine: Individual olivine data from this study.  \nSupplements A-E: Supplementary text.  \nAcknowledgments : We are grateful to Kevin Righter, Makoto Kimura, and an anonymous reviewer for their constructive comments. We thank Jeffrey Grossman, Roger Hewins, Rhian Jones, Tim McCoy, Derek Sears, Martin Suttle for providing access to literature data. This paper is dedicated to the memory of the lead author's beau-grand-père Georges Illy.", 'References': "Abreu N. M. (2016). Why is it so difficult to classify Renazzo-type (CR) carbonaceous chondrites? - Implications from TEM observations of matrices for the sequences of aqueous alteration. Geochimica et Cosmochimica Acta 194:91122.  \nAbreu N. M. Brearley A. J. (2010). 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