# From whom Bells tolls: Reclassifying Bells among CR chondrites and implications for the formation conditions of CR parent bodies

Yves Marrocchi, Emmanuel Jacquet, Julia Neukampf, Johan Villeneuve, Michael Zolensky

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## To cite this version:

Yves Marrocchi, Emmanuel Jacquet, Julia Neukampf, Johan Villeneuve, Michael Zolensky. From whom Bells tolls: Reclassifying Bells among CR chondrites and implications for the formation conditions of CR parent bodies. Meteoritics and Planetary Science, 2023, 58 (2), pp.195-206. 10.1111/maps.13947. insu-04198261

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# From whom Bells tolls: Reclassifying Bells among CR chondrites and implications for the formation conditions of CR parent bodies

Yves MARROCCHI<sup>ID 1*</sup>, Emmanuel JACQUET<sup>ID 2</sup>, Julia NEUKAMPF<sup>1</sup>, Johan VILLENEUVE<sup>ID 1</sup>, and Michael E. ZOLENSKY<sup>ID 3</sup>

*(Received 10 October 2022; revision accepted 04 December 2022)*

**Abstract**–The ungrouped carbonaceous chondrite (CC) Bells has long been considered petrographically similar to CM chondrites based on its matrix abundance and degree of aqueous alteration, but also shows significant isotopic affinities to CR chondrites. Its taxonomic status is thus important for clarifying the relationship of the CRHB (formerly “CR”) clan with other CCs. In this study, we measured the oxygen isotopic compositions of olivines in type I chondrules and isolated olivine grains in Bells. Bells olivines mostly have $\Delta^{17}O > -4‰$, similar to CR chondrites but unlike other CCs that are rich in refractory inclusions, in which chondrules are generally richer in $^{16}O$. Therefore, Bells is a CR chondrite (albeit an anomalous one), most similar to the rare, matrix-rich CRs like Al Rais. These chondrites (i) may not necessarily derive from the same primary parent body as mainstream CRs, (ii) bear witness to significant variations of the matrix/chondrule ratio within the CRHB clan, and (iii) may be a good analog for samples retrieved by the space mission OSIRIS-REx.

## INTRODUCTION

Nontraditional isotopic systematics have been used to partition meteorites into two superclans: carbonaceous (C) and noncarbonaceous (NC) meteorites (e.g., Kleine et al., 2020). Carbonaceous chondrites (CCs) are widely considered to represent the outer regions of the solar protoplanetary disk due to their (i) spectroscopic affinities to C, P, and D asteroids (e.g., DeMeo et al., 2022) and known cometary samples and interplanetary dust particles (e.g., Zolensky et al., 2008) and (ii) high concentrations of volatile elements (Alexander et al., 2012; Marrocchi et al., 2021; Vacher et al., 2020). Although planetary accretion and celestial mechanics have biased meteorite delivery to the Earth (e.g., Meibom & Clark, 1999), and thus limited our sampling of the C superclan planetesimals, some structure can still be discerned:

i. Most CCs (e.g., CM, CO, CV, CK, CL) contain >1 vol% refractory inclusions (RIs), and thus define an “RI-rich” chondrite class (Jacquet, 2022).

ii. CI chondrites are nearly 100 vol% matrix, although isolated olivine and pyroxene fragments isotopically reminiscent of RIs and chondrules are known to occur (Leshin et al., 1997; Morin et al., 2022; Piralla et al., 2020). Interestingly, the fact that Ryugu samples are nearly identical to CIs (Yokoyama et al., 2022) indicates that their prevalence in the solar system is greater than suggested by their rarity in terrestrial collections.

iii. CR, CH, and CB chondrites define a distinctive clan, classically known as the “CR clan” (Weisberg et al., 1995), but recently proposed to be termed the “CRHB clan” to avoid confusion with their eponymous chemical group (Jacquet, 2022); we will follow this convention herein. Compared to their RI-rich counterparts, CRHB chondrites have distinctive isotopic properties: isotopically heavy hydrogen and nitrogen compositions (Aleon, 2010; Piani et al., 2021), a deficit in $^{26}$Mg (derived from $^{26}$Al decay; Schrader et al., 2017; Tenner et al., 2019; van Kooten et al., 2016), and fewer $^{16}$O-rich chondrules

<footnote n="1">¹Université de Lorraine, CNRS, CRPG, UMR 7358, Vandœuvre-lès-Nancy 54500, France</footnote>

<footnote n="2">²Institut 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</footnote>

<footnote n="3">³X12 Astromaterials Research and Exploration Science, NASA Johnson Space Center, Houston, Texas 77058, USA</footnote>

<footnote>*Corresponding author. E-mail: yvesm@crpg.cnrs-nancy.fr</footnote>
(Marrocchi et al., 2022; Schrader et al., 2013; Tenner et al., 2015). Petrographically, they are depleted in RIs, but are generally fairly to very metal rich, with CH and CB chondrites having unusually abundant nonporphyritic chondrules that may have formed in impact plumes (e.g., Krot et al., 2005).

The relationships between the CRHB clan and other
CCs remain unclear. Ferromagnesian silicates in comet
Wild 2 are isotopically similar to CR chondrules (Defouilloy
et al., 2017), and some isotopic signatures of CR chondrules
may have been inherited from pristine molecular cloud
material (van Kooten et al., 2016). Based on these lines of
evidence, the CRHB clan has long been interpreted to
represent the outermost regions of the protoplanetary disk
sampled by meteorites, perhaps beyond Saturn's orbit (van
Kooten et al., 2020). However, although most CR
chondrules have $\Delta^{17}\text{O} > -4\text{‰}$ (termed "type I CR
chondrules" by Marrocchi et al., 2022), relatively $^{16}\text{O}$-rich
chondrules ($\Delta^{17}\text{O} < -4\text{‰}$; "type I CO chondrules")
isotopically comparable to their counterparts in RI-rich
CCs also occur in CRs, either intact or as relict grains
within type I CR chondrules. It has therefore been proposed
that type I CR chondrules are a later generation of
chondrules produced in a reservoir originally composed of
type I CO chondrules and CI-like dust (Marrocchi
et al., 2022), which would have largely resembled the most
matrix- and RI-rich CCs. Consequently, the peculiar
characteristics of CRHB chondrites may largely result from
the late-stage evolution of common CC materials.

Better constraining the origins of CRHB chondrites
requires a solid grasp of their entire diversity. The CR group
already includes a few members (e.g., Al Rais, Grosvenor
Mountain [GRO] 95577) that are distinctly more matrix
rich and/or aqueously altered than common CR2s, and that
might sample the CR reservoir prior to the formation of the
last generation of chondrules (type I CR chondrules;
Marrocchi et al., 2022). However, such meteorites
superficially resemble CM chondrites, which have
historically been equated with C2 chondrites (e.g., Van
Schmus & Hayes, 1974). Thus, similar meteorites may have
been erroneously classified as CMs or hitherto overlooked.

The Bells meteorite may be such a case. Mason (1963)
originally associated it with Wiik's (1956) "type II" CCs
(i.e., C2 in Van Schmus & Hayes, 1974), but was long listed
using modern nomenclature as a CM in the Meteoritical
Bulletin database. This was supported by the CM-like
oxygen isotopic compositions of Bells' magnetite and
matrix (Rowe et al., 1994), its noble gas signatures
(Zadnik, 1985), and bulk chemical compositions
(Mittlefehldt, 2002). However, Bells shows peculiar
characteristics including (i) a significant amount of Fe-Ni
metal beads despite pervasive aqueous alteration
(Brearley, 1995), (ii) an unusually high abundance of

magnetite (11–13 vol%) disseminated throughout the fine-
grained matrix (Brearley, 1995), and (iii) isotopically heavy
hydrogen and deuterium (e.g., Kerridge, 1985).
Kallemeyn (1995) voiced skepticism on the CM
classification, and Bells was reclassified as C2-ungrouped in
2006. Nonetheless, Bells is still occasionally referred to as a
CM chondrite in the literature. van Kooten et al. (2020)
maintained that Bells’ oxygen isotopic and chemical
proximity to CMs is indicative of its origin on the CM
parent body. Although the $^{54}$Cr signature they measured is
more comparable to CR chondrites, they suggested that the
CM parent body accreted a late contribution of CRHB
material, and that the spatial barrier between CRHB and
RI-rich chondrites was not fully impermeable. In addition,
the unusually high abundance of organic nanoglobules and
other organics in Bells is similar to the characteristics of
CRs (De Gregorio et al., 2013). Therefore, the taxonomic
status of Bells is key to understanding the link between
CRHB and RI-rich chondrites.

Although Bells is aqueously altered and heavily
brecciated, olivine crystals in type I chondrules and
isolated olivine grains (IOGs) that presumably became
detached from chondrules (Jacquet et al., 2021) are largely
unaltered (Brearley, 1995). They should thus retain an
isotopic record of their link to type I CR or type I CO
chondrules, a topic that has not yet been investigated.
Building on our oxygen isotopic measurements of type I
chondrules in RI-rich CCs (Marrocchi et al., 2018, 2019)
and in CR chondrites (Marrocchi et al., 2022), we here
report oxygen isotopic measurements of chondrules in
Bells. Based on our results, we discuss implications for
classifying Bells and, more broadly, for the taxonomy and
structure of the CRHB clan and the formation conditions
of CR parent bodies.

MATERIALS AND METHODS

We surveyed two thin sections of Bells (sections no.
9309-A[1] and 9309, which are derived from material from
Arizona State University) provided by the NASA Johnson
Space Center (Houston, Texas, USA). Importantly, this
material was collected soon after the Bells fall and before
any rainfall (C. Moore, personal communication, 1986).
These sections contain both chondrules and IOGs. We
imaged the sections by backscatter electron (BSE) imaging
and obtained elemental maps by energy-dispersive X-ray
(EDX) measurements using a JEOL JSM-6510 scanning
electron microscope (SEM) equipped with a Genesis EDX
detector at the Centre de Recherches Pétrographiques et
Géochimiques (CRPG-CNRS, Nancy, France) operating
with a 3 nA beam accelerated at 15 kV. Chondrules and
IOGs were examined microscopically in transmitted and
reflected light. We selected 12 type I chondrules and 17
IOGs for further analyses. Major element compositions of
the olivine crystals were determined using a JEOL JXA-8223 electron microprobe at the Institute of Geochemistry and Petrology, ETH Zurich, operating with a focused 10 nA beam accelerated at 15 kV. To minimize alkali migration during measurement, mobile elements like Na and K were measured first. High-resolution cathodoluminescence (CL) maps were obtained at the CRPG using a JEOL JXA-8230 microprobe with an acceleration voltage of 20 kV, a beam current of 100 nA, step sizes of 0.3–0.8 µm, and a dwell time of 10 ms following the method described in Regnault et al. (2022).

We measured the oxygen isotopic compositions of type I chondrule olivines and IOGs by secondary ion mass spectrometry (SIMS) using the CAMECA IMS 1270 E7 at the CRPG. $^{16}$O$^{-}$, $^{17}$O$^{-}$, and $^{18}$O$^{-}$ ions produced by a Cs$^{+}$ primary ion beam (~10 µm, ~2–2.5 nA, rastered over 5 × 5 µm$^2$ areas) were measured in multicollection mode using three Faraday cups (FCs). To remove interference from $^{16}$OH$^{-}$ on the $^{17}$O$^{-}$ peak and to maximize the flatness atop the $^{16}$O$^{-}$ and $^{18}$O$^{-}$ peaks, the entrance and exit slits of the central FC were adjusted to obtain a mass resolution power (MRP = M/ΔM) of ~7000 for $^{17}$O$^{-}$. As an additional safeguard against $^{16}$OH$^{-}$ interference, a liquid N$_2$ trap was used to reduce the pressure in the analytical chamber to <1 × 10$^{-8}$ mbar. The two off-axis multicollection FCs, L'2 and H1, were set on exit slit 1 (MRP = 2500) and, respectively, used to measure $^{16}$O$^{-}$ and $^{18}$O$^{-}$. We used 10$^{10}$ Ω, 10$^{12}$ Ω, and 10$^{11}$ Ω resistors for the L'2 ($^{16}$O), central ($^{17}$O), and H1 ($^{18}$O) FCs, respectively (for further details, see Bouden et al., 2021; Piralla et al., 2021). A normal incidence electron gun was used for charge compensation. Total measurement times were 300 s, comprising 120 s of pre-sputtering and 180 s of analysis. Two terrestrial standards, San Carlos olivine and JV1 diopside, were used to define the mass-dependent fractionation line. Instrumental mass fractionation of chondrule olivine grains was corrected using San Carlos olivine. To monitor any instrumental drift and to achieve good precision, the San Carlos olivine was reanalyzed after every 15–20 sample analyses. Typical count rates obtained on San Carlos olivine were 1.0 × 10$^9$ cps, 4.0 × 10$^5$ cps, and 2.1 × 10$^6$ cps for $^{16}$O, $^{17}$O, and $^{18}$O, respectively. All SIMS analytical spots were checked thoroughly by SEM, and any spots near fractures, in the mesostasis, or not completely within olivine grains were excluded from the data set. Oxygen isotopic compositions are expressed in δ notation as $δ^{17,18}O = ([^{17,18}O/^{16}O]_{sample}/[^{17,18}O/^{16}O]_{V-SMOW} - 1) \times 1000‰$, where V-SMOW is the Vienna Standard Mean Ocean Water value. Samples related by mass fractionation to the V-SMOW composition plot along a line with a slope of 0.52, defining the terrestrial fractionation line (TFL), whereas mass-independent variations are described by

<figure><img src="image_3.png" /><figcaption>Fig. 1. a) Composite backscattered electron image of Bells 9309-A(1), one of the two sections of Bells surveyed in this study. b) Composite EDX elemental map of Mg (red), Al (blue), and Ca (green) in the same section, revealing the distribution of chondrules, calcium-aluminum-rich inclusions, and matrix. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

$Δ^{17}O = δ^{17}O - 0.52 × δ^{18}O$, representing vertical deviations from the TFL in a triple oxygen isotope diagram. Typical $2σ$ measurement errors, accounting for internal errors on each measurement and the external reproducibility of the standards, were estimated to be ~0.4‰ for $δ^{18}O$, ~0.7‰ for $δ^{17}O$, and ~0.7‰ for $Δ^{17}O$.

## RESULTS

Our observations of Bells revealed an abundant fine-grained matrix containing porphyritic chondrules and IOGs (Figs. 1 and 2). A few calcium-aluminum-rich inclusions are also present in both sections (Fig. 1). Fe-Ni metal beads were commonly observed dispersed throughout the fine-grained matrix and within chondrules (Figs. 1 and 2). Some IOGs show microscale dendritic-like textures (Fig. 3), sometimes in contact with smooth olivine crystals (Fig. 3d); these may be local shock textures on pre-existing olivines, which may echo the heavily brecciated character of Bells (Brearley, 1995). All selected olivines are forsterites with Mg# (=Mg/[Mg + Fe] × 100) ranging from 96.4 to 99.5 (Table S1). Al$_2$O$_3$, TiO$_2$, CaO, MnO, and NiO concentrations ranged 0.05–0.98 wt%, 0.05–0.18 wt%, 0.05–0.82 wt%, 0.04–0.39 wt%, and 0.05–0.5 wt%, respectively (Table S1).

The oxygen isotopic compositions of Mg-rich chondrules and IOGs define a linear correlation (Fig. 4a; $δ^{17}O = [0.99 ± 0.13] × δ^{18}O - [2.62 ± 0.05‰]$, $r^2 = 0.96$, mean weighted standard deviation = 3.7) and plot along the primitive chondrule mineral (PCM) line (Ushikubo
<figure><img src="image_4.png" /><figcaption>Fig. 2. Backscattered electron images of (a, b) porphyritic type I porphyritic olivine–pyroxene chondrules and (c, d) isolated Mg-rich olivine grains in Bells.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 3. Backscattered electron images of dendritic-like textures in isolated olivine grains.</figcaption></figure>

et al., 2012), with δ¹⁸O and δ¹⁷O values ranging from −9.8 to +4.8‰ and from −12.4 to +1.4‰, respectively (Fig. 4a; Table S1). Δ¹⁷O values range from −7.3 to +0.7‰ (Fig. 4b). Like most CCs, and CR chondrites in particular (e.g., Marrocchi et al., 2022; Schrader et al., 2013; Tenner et al., 2015), Δ¹⁷O values are negatively correlated with

Mg#: all olivines with Mg# > 99.0 have Δ¹⁷O < −4‰, corresponding to type I CO chondrules (Marrocchi et al., 2022), whereas those with Mg# < 99.0 have Δ¹⁷O > −4‰, corresponding to type I CR chondrules; the five exceptions are olivines near the Fo₉₉ or Δ¹⁷O = −4‰ thresholds (Fig. 4). The type I CR chondrule olivines are
<figure><img src="image_6.png" /><figcaption>Fig. 4. a) Oxygen isotopic compositions and (b) $\Delta^{17}\text{O}$ values versus Mg# in chondrule olivines and isolated olivine grains in Bells. Although the majority of CM-CO-CV chondrule olivine grains have $\Delta^{17}\text{O} < -4\text{‰}$, rare chondrules with $\Delta^{17}\text{O} > -4\text{‰}$ have been reported (Marrocchi et al., 2018, 2019; Marrocchi & Chaussidon, 2015; Tenner et al., 2013, 2015, 2018). In contrast, CR chondrules show a broader range of oxygen isotopic compositions, with $\Delta^{17}\text{O}$ ranging from -6 to 0‰; among those, chondrules with $\Delta^{17}\text{O} > -4\text{‰}$ are more abundant than in CM-CO-CV chondrites. The primitive chondrule minerals line (PCM; Ushikubo et al., 2012) and terrestrial fractionation line (TFL) are shown for reference. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

more numerous, consistent with the microprobe data of Brearley (1995) showing the prevalence of $\text{Fa}_{2-4}$ compositions (see their figs. 5 and 6). The dendritic-like olivines all have type I CR compositions but show significant mass-dependent fractionations, with $\delta^{18}\text{O}$ values spanning from -13.6 to +1.5‰.

# DISCUSSION

## Reclassifying Bells as CR2-an

Since the fall of Bells in 1961, geochemistry has made great strides; a major change is the now-common use of nucleosynthetic anomalies by studying nontraditional isotopic systems (e.g., Ti, Cr, Mo; Burkhardt et al., 2021). In addition to providing key information on the structure of the circumsolar disk (e.g., Kruijer et al., 2017; Spitzer et al., 2020; Trinquier et al., 2007, 2009) and the conditions of chondrule formation (e.g., Schneider et al., 2020; Zhu (朱柯) et al., 2019), they allow potential genetic links between meteorites to be tested and established (e.g., Torrano et al., 2021). When applied to Bells, such studies have revealed that it has $^{54}\text{Cr}$ and Mg isotopic signatures similar to those reported for CR chondrites (Fig. 5; Olsen et al., 2016; van Kooten et al., 2016). Nonetheless, van Kooten et al. (2020) maintained a link between Bells and CMs by proposing the late accretion of CR material onto the CM parent body. However, the isotopic similarities between Bells and CR chondrites indicate that any CM contribution must have been minor at best. This view is reinforced by other isotopic systems:

1. Our analyses of the oxygen isotopic compositions of chondrule olivines and IOGs in Bells show large variations (Fig. 4a), and olivines with Mg# < 98 have $^{16}\text{O}$-poor compositions (i.e., $\Delta^{17}\text{O} > -2\text{‰}$) typical of CR chondrules (Fig. 4b; Marrocchi et al., 2022; Schrader et al., 2013; Tenner et al., 2013, 2015, 2018). Importantly, such $^{16}\text{O}$-poor and Mg-rich olivines are rare in other CCs (Fig. 4b; Chaumard et al., 2018, 2021; Hertwig et al., 2018; Jacquet et al., 2021; Marrocchi et al., 2018, 2019, 2021; Rudraswami et al., 2011; Schnuriger et al., 2022; Tenner et al., 2013, 2018; Ushikubo et al., 2012). Most $\Delta^{17}\text{O}$ values of Bells olivines are therefore similar to those in CR chondrites (Fig. 4b) and thus not particularly diagnostic of a CM origin as argued by van Kooten et al. (2020).

2. CR chondrites are characterized by large D and $^{15}\text{N}$ enrichments in both bulk samples and extracted insoluble organic matter (IOM; Alexander et al., 2007, 2010, 2012; Kerridge, 1985; Marrocchi et al., 2021; Vacher et al., 2020). Bulk samples and extracted IOM from Bells show similar D and $^{15}\text{N}$
<figure><img src="image_7.png" /><figcaption>Fig. 5. a) $\Delta^{17}$O and (b) $\epsilon^{54}$Cr as functions of the modal matrix abundance (vol%) for all carbonaceous chondrites. Reconstructed CRs are based on a mixture of 30% CO-like chondrules and 70% CI-like dust (see details in Marrocchi et al., 2022). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

enrichments that plot far away from CMs but near CRs (Figs. 6 and 7).

Therefore, the similarity between Bells and CM chondrites seems to be merely textural, that is, matrix abundance and the degree of aqueous alteration. However, CR chondrites were also affected by fluid circulation and, like CM chondrites, span the full range of aqueous alteration. Type 3.0 CRs (e.g., Queen Alexandra Range 99177) contain abundant Fe-Ni metal beads, glass-bearing chondrules, no magnetite, and few phyllosilicates (Abreu & Brearley, 2010). CR chondrites having experienced intermediate degrees of alteration (e.g., CR2.4 Renazzo) are characterized by the partial alteration of Fe-Ni metal beads and anhydrous silicates into phyllosilicates (Harju et al., 2014). The rare heavily altered samples (e.g., GRO 95577, classified as CR1) are matrix rich and show abundant magnetite and other oxide phases, but very few intact Fe-Ni metal beads and anhydrous silicates (Weisberg & Huber, 2007). Although the bulk chemical composition of Bells is, to the first order, less similar to CRs than CMs, this only holds true for pristine to moderately altered CRs (Figs. 8a and 9). Indeed, Bells' chemical composition shows strong

<figure><img src="image_8.png" /><figcaption>Fig. 6. Bulk hydrogen and nitrogen isotopic compositions of Bells, Tagish Lake (TTL), CM, CI, and matrix-poor and matrix-rich CR chondrites (CR2 and CR2-an, respectively). Bells shows D and $^{15}$N enrichments comparable to those reported in CR chondrites and at odds with the D- and $^{15}$N-poor isotopic compositions of CM chondrites (data from Alexander et al., 2012; Kerridge, 1985; Vacher et al., 2020). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

similarities to heavily altered CR chondrites (Fig. 8b), especially for taxonomically key elemental ratios (i.e., Zn/Co versus Se/Co; Fig. 9). This is because the abundance of moderately volatile elements is controlled by the matrix abundance (e.g., Braukmüller et al., 2018; Bryson & Brennecka, 2021; Marrocchi et al., 2022). Notably, Bells lacks the excess refractory element concentrations (when normalized to the “main component”; e.g., Cr) of equally matrix-rich CMs, as reflected by its paucity in refractory inclusions (Brearley, 1995), which is again more consistent with the CRHB clan (Fig. 8).

Thus, although Bells is similar in some aspects to CM chondrites (e.g., Figs. 8a and 9), when considering several isotopic systems, it appears that Bells rather belongs to the CRHB clan (Figs. 3–6). In addition, its chemical composition is close to CR, having experienced a significant degree of aqueous alteration, similar to Al Rais, GRO 95577, and Miller Range 090292 (Figs. 8 and 9). Altogether, this strongly suggests that Bells corresponds to matrix-rich, aqueously altered CR chondrites. These latter materials could correspond to the samples retrieved by the space mission OSIRIS-Rex (Hamilton et al., 2022). However, it must be acknowledged that there is a significant hiatus in aqueous alteration degree and matrix abundance between these meteorites and the “normal” CR chondrites (Figs. 8 and 9). It would be consequently safest, for the time being, to classify all
<figure><img src="image_9.png" /><figcaption>Fig. 7. a) Hydrogen and nitrogen isotopic compositions of insoluble organic matter (IOM) in Bells, Tagish Lake, CM, CI, and matrix-poor and matrix-rich CR chondrites (CR2 and CR2-an, respectively). b) Comparison between the hydrogen isotopic compositions of bulk chondrites and those of their IOMs (data from Alexander et al., 2007, 2010, 2012; Kerridge, 1985; Vacher et al., 2020). These data confirm that Bells falls isotopically distinct from CM chondrites and is more similar to matrix-rich CR chondrites. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

the former as anomalous CR, as in fact Al Rais and Northwest Africa (NWA) 14674 are already in the Meteoritical Bulletin Database (as CR2-an). We thus submit that Bells should be reclassified as a CR2-an. We note that Bells was one of the magnetite-rich, ¹⁶O-poor ungrouped CCs that Hewins et al. (2021) called “C2-ung1.” It is then conceivable that at least some of these particular meteorites will turn out to be CR-an as

<figure><img src="image_10.png" /><figcaption>Fig. 8. a) CI- and Cr-normalized concentrations of different elements as a function of their half-condensation temperatures (Lodders, 2003) for bulk Bells and average CM and matrix-poor CR chondrites (data from Braukmüller et al., 2018; Mittlefehldt, 2002; Wasson & Kallemeyn, 1988). b) CI- and Cr-normalized concentrations of Bells and the matrix-rich CR chondrites Al-Rais and GRO 95577 (data from Mittlefehldt, 2002; Wasson & Kallemeyn, 1988; Weisberg & Huber, 2007). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

well. The next section discusses the significance of this grouplet of matrix-rich CRs relative to normal CR.

## Implications for the Variable Formation Conditions of CR Chondrites

Although the empirical concept of a chondrite “group” does not mandate a single primary parent body (Jacquet, 2022), it is worthwhile to investigate whether matrix-rich CRs derived from the same type of parent body as the more common matrix-poor CRs. On its own, the range of the degree of aqueous alteration across the whole CR group (i.e., from type 3 to 1) does not preclude the existence of a single primary parent body. However, if we assume an onion shell structure, the most heated and
<figure><img src="image_11.png" /><figcaption>Fig. 9. CI-normalized bulk Zn/Co and Se/Co ratios of Bells, Tagish Lake, CM, matrix-poor and matrix-rich chondrites (CR2 and CR2-an, respectively). Bells and matrix-rich CR chondrites have compositions similar to those of CM chondrites. Data from Braukmüller et al. (2018), Mittlefehldt (2002), Wasson and Kallemeyn (1988), and Weisberg and Huber (2007). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

aqueously altered (i.e., central) parts should account for a significant fraction of the parent body, whereas the four matrix-rich, aqueously altered CRs are largely outnumbered by the 204 normal CRs listed in the Meteoritical Bulletin Database (as of September 14, 2022). Alternatively, one could envision more localized alteration events, for example, impacts onto a porous carbonaceous chondritic target, but this scenario is only plausible for a more matrix-rich, and perhaps originally hydrous, lithology (Marrocchi et al., 2016; Schrader et al., 2014).

How could the matrix fraction in the accreted material change by a factor of 2 during the formation of a chondritic parent body? In principle, a protracted accretion episode could allow for later accreted materials to differ markedly from earlier accreted materials (i.e., heterogeneous accretion). However, the empirical data indicate consistent isotopic and chemical compositions within each chondrite group (Johansen et al., 2015), which may indicate the fairly short time scales of planetesimal accretion (e.g., hundreds of orbits for the streaming instability; Johansen et al., 2015). Still, even if the parental cloud was originally homogeneous, one would expect larger grains (i.e., those with longer stopping times) to settle first, which could explain the positive correlation between chondrule size and metamorphic subtypes in CO chondrites (e.g., Pinto et al., 2021). Similarly, submicrometer-sized matrix monomers accreted last as dark inclusions or fine-grained rims around chondrules (e.g., Metzler et al., 1992; Pinto et al., 2022; Zanetta et al., 2021), which was confirmed

by the lack of correlation between the matrix fraction in CO chondrites and their petrologic subtypes (McSween, 1977). Although McSween (1979) reported an ~20 vol% increase in matrix fraction from the least to the most altered CMs, the modal matrix abundances in the recently described weakly altered CMs are similar to those in common CMs (Hewins et al., 2014; Kimura et al., 2020; Marrocchi et al., 2014). Nonetheless, these single parent body scenarios cannot explain the lack of CR chondrites with intermediate matrix fractions.

Thus, matrix-rich CR chondrites likely derived from one or more parent bodies that were distinct from those of common CRs. It is therefore conceivable that at some point in the future, those chondrites could be formally separated into one or several independent groups within the CRHB clan that are distinct from the CR chemical group. Historically, the matrix-rich meteorite Al Rais was the only known CRHB besides Renazzo (i.e., while CBs were described as mesosiderites; e.g., Mason, 1963), so it was logical that McSween (1979) grouped them together as CRs. However, Kallemeyn (1993) noted that most CRs classified since then have clustered around Renazzo rather than Al Rais, whose kinship they deemed to be no closer than that between L and LL ordinary chondrites. That said, absent any well-characterized grouplet exceeding the five-member threshold for establishing a new group (Jacquet, 2022) and pending any evolution of the community definition of a CR, it is best to conservatively classify them as CR-an for the time being.

Can we further specify the link between these matrix-rich “CRs” and the normal CRs in the CRHB clan? Marrocchi et al. (2022) proposed that CR chondrites present two generations of chondrules, and that, before the final population formed (that of the ¹⁶O-poor type I CR chondrules), the CR reservoir was dominated by CI-like dust (about 70%) with a minor population of type I CO chondrules. When considering Δ¹⁷O and ε⁵⁴Cr values as functions of the vol% of matrix, Bells falls along the trend defined by all CCs (Fig. 5) and near the “original” CR composition reconstructed by Marrocchi et al. (2022) for the time preceding the formation of the type I CR chondrules. Of note, the matrix-rich, aqueously altered CR chondrite Al Rais plots near Bells on the Δ¹⁷O versus matrix abundance diagram (Fig. 5a). It is thus tempting to equate Bells and other matrix-rich CRs with this “original” CR reservoir, as entertained by Marrocchi et al. (2022). However, whereas we would expect the chondrules in this reservoir to have been dominated by type I CO chondrules, with only a few type I CR chondrules, our observations show that the chondrule population of Bells is dominated by type I CR chondrules (e.g., Fig. 4). Consequently, Bells must have postdated the formation of type I CR chondrules but suffered a further admixture of CI-like dust compared to normal
CR chondrites. This may indicate its later accretion, or accretion further from the region of type I CR chondrule production (whether overlapping with those feeding normal CRs). Thus, the spatiotemporal region of the disk sampled by the CRHB clan spanned quite variable chondrule/dust ratios, even when disregarding the effects of the impacts witnessed by CB and CH chondrites (e.g., Krot et al., 2005).

## CONCLUDING REMARKS

We studied the Bells chondrite via SEM, microprobe analysis, and SIMS. The oxygen isotopic compositions of chondrule olivines and IOGs show mass-independent variations; most olivines have $\Delta^{17}O > -4‰$, as observed in CR chondrites but unlike other CCs, including CMs. Our and literature data suggest that Bells belongs to the CR chondrites, especially the rare matrix-rich CRs such as Al Rais. This implies that (i) the CR accretion region was characterized by varying matrix/chondrule ratios and (ii) matrix-rich and matrix-poor CR chondrites likely originated from different parent bodies.

*Acknowledgments*—Original data from this study are available on the Ordar database: 10.24396/ORDAR-105. This work was funded by l'Agence Nationale de la Recherche through grant CASSYSS ANR-18-CE31-0010-01 (PI Johan Villeneuve). This is CRPG contribution #2829.

*Data Availability Statement*—Original data from this study are available on the Ordar database https://doi.org/10.24396/ORDAR-105.

*Editorial Handling*—Dr. A. J. Timothy Jull

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**Table S1.** Chemical and oxygen isotopic compositions of chondrule and isolated olivine grains.