# Oxygen isotope systematics of chondrules in Rumuruti chondrites: Formation conditions and genetic link with ordinary chondrites

Maxence Regnault, Yves Marrocchi, Maxime Piralla, Johan Villeneuve, Valentina Batanova, Nicolas Schnuriger, Emmanuel Jacquet

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## To cite this version:

Maxence Regnault, Yves Marrocchi, Maxime Piralla, Johan Villeneuve, Valentina Batanova, et al. "Oxygen isotope systematics of chondrules in Rumuruti chondrites: Formation conditions and genetic link with ordinary chondrites." Meteoritics and Planetary Science, 2022, 57 (1), pp.122-135. 10.1111/maps.13778. insu-03712872

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# Oxygen isotope systematics of chondrules in Rumuruti chondrites: Formation conditions and genetic link with ordinary chondrites

Maxence REGNAULT<fnref n="1" />, Yves MARROCCHI ID<fnref n="1" />*, Maxime PIRALLA ID<fnref n="1" />, Johan VILLENEUVE ID<fnref n="1" />, Valentina BATANOVA<fnref n="2" />, Nicolas SCHNURIGER ID<fnref n="1" />, and Emmanuel JACQUET ID<fnref n="3" />

**Abstract**—Rumurutiites (R chondrites) are rare, highly oxidized chondrites belonging to the noncarbonaceous superclan and characterized by low chondrule abundances. Although textural and chemical features of Rumurutiite chondrules resemble those of ordinary chondrites (OCs), their formation conditions and potential genetic link remain debated. Here, we report high-resolution elemental X-ray mapping analyses and in situ O isotopic measurements of olivine grains from five chondrules and eight isolated olivine grains (IOGs) in the NWA 12482 R3 chondrite. The chondrules show chemical zonings similar to their counterparts in ordinary and carbonaceous chondrites (CCs), implying that gas-melt interaction processes between chondrule precursors and SiO- and Mg-rich gas were operative throughout the circumsolar disk. Our isotopic data show that R chondrules are isotopically similar to ordinary chondrules, although differences in their abundances of relict olivine grains and chondrule textural characteristics suggest different formation environments, with R chondrules being formed from ¹⁶O-poorer precursors. As with chondrules in OCs, the O isotopic characteristics of R chondrules and IOGs suggest limited transport between CC and noncarbonaceous reservoirs.

## INTRODUCTION

The advent of nontraditional isotopic systems (e.g., Ti, Cr, Mo) revealed that chondrites—the most primitive witnesses of the early solar system—display a fundamental isotopic dichotomy, with carbonaceous chondrites (CCs) showing large nucleosynthetic anomalies relative to terrestrial standards (e.g., in ⁴⁸Ca, ⁵⁰Ti, ⁵⁴Cr, and Mo and Ni isotopes; Burkhardt et al., 2019; Dauphas et al., 2014; Kruijer et al., 2017, 2020; Nanne et al., 2019; Schiller et al., 2018; Trinquier et al., 2007, 2009; Warren, 2011; Zhu et al., 2021) that are not observed in noncarbonaceous chondrites (NCs). NC and CC reservoirs may thus represent the primordial inner and outer parts of the solar system, respectively.

NCs include ordinary chondrites (OCs), enstatite chondrites (ECs), and the Rumuruti (R) chondrite group. R chondrites are rare meteorites characterized by a high abundance of matrix compared to the other NC chondrites (40–50 vol%), high FeO contents in olivine (Fa₃₅–₄₂), and a paucity of Ca-Al-rich inclusions (CAIs, <0.1 vol%; Binns & Pooley, 1979; Bischoff et al., 2011; Weisberg et al., 1991). Based on the scarcity of Fe-Ni metal beads, the high Ni content of olivine, and the ubiquitous presence of iron (III)-bearing oxides in R chondrites (Bischoff et al., 1994, 2011; Rubin & Kallemeyn, 1989, 1994), they represent the most oxidized chondrite group, and thus the oxidized counterpart of reduced ECs in the NC superclan (e.g., Jacquet et al., 2018; Piani et al., 2012, 2016, 2020; Weisberg et al., 1991, 2021).

Considering bulk chemical compositions, R chondrites show propinquity with OCs with similar

**Unified Astronomy Thesaurus:** Chondrites (228), Chondrules (229), Isotopic abundances (867), Protoplanetary disks (1300).

<footnote n="1">¹CRPG, UMR 7358, Université de Lorraine, CNRS, Vandœuvre-lès-Nancy 54501, France</footnote>

<footnote n="2">²ISTerre, UMR 5275, CNRS, Université Grenoble Alpes, Grenoble 38000, France</footnote>

<footnote n="3">³IMPMC, UMR 7590, CNRS & Muséum national d'Histoire naturelle, CP52, 57 rue Cuvier, Paris 75005, France</footnote>

<footnote>*Corresponding author. E-mail: yvesm@crpg.cnrs-nancy.fr</footnote>

<footnote>*(Received 30 September 2021; revision accepted 06 December 2021)</footnote>
refractory lithophile and siderophile abundances (in particular total iron content; Bischoff et al. [2011] and references therein). However, R chondrites also show significant differences compared to OCs with (i) $ ^{16}O-$poorer oxygen isotopic compositions (with $\Delta^{17}O = [\delta^{17}O] - 0.52 \times \delta^{18}O$ of $2.7 \pm 0.3\%$ versus $0.55-1.65\%$ for OCs; Greenwood et al., 2020; Marrocchi et al., 2020; Weisberg et al., 1991) and (ii) lower abundance of chondrules that provide an affinity to CCs (35–50 vol% versus 60–80 vol% for OCs; Kallemeyn et al., 1996; Schulze et al., 1994; Scott & Krot, 2014; Weisberg et al., 1991).

The most abundant chondrules in R chondrites have porphyritic olivine (PO) and porphyritic olivine- pyroxene textures, with mean apparent diameter of ~400 µm (Bischoff et al. [2011] and references therein). R chondrules have textures and chemical compositions resembling those of OC chondrules, with mineralogically zoned chondrules composed of low-Ca pyroxene surrounding olivine-rich core (Barosch et al., 2020; Bischoff et al., 2011; Greenwood et al., 2000; Isa et al., 2012; Kita et al., 2010, 2015; Weisberg et al., 1991). The O isotopic compositions of R chondrules mostly plot between the terrestrial fractionation line (TFL; $\Delta^{17}O = 0\%$ by definition) and the R mass fractionation line (RFL; $\Delta^{17}O \approx 3\%$; defined by the R bulk O-isotopic compositions; Bischoff et al., 2011; Greenwood et al., 2000; Isa et al., 2011; Kita et al., 2015; Pack et al., 2004; Weisberg et al., 1991) and largely overlap with OC chondrules (Piralla et al., 2021). It has thus been proposed that both OC and R chondrules formed in the same isotopically homogeneous reservoir (Kita et al., 2010, 2015), with the R chondrite parent body incorporating excess matrix. This homogeneity may have resulted from isotopic exchange between solids and solar nebular gas before or during chondrule formation (Kita et al., 2010). However, a few R chondrules show $ ^{16}O-$rich olivine grains ($-4\% < \Delta^{17}O < -1\%$) plotting on both sides of the primary chondrule mineral (PCM) line (Isa et al., 2012; Kita et al., 2015; Miller et al., 2017). The latter has been defined based on chondrule silicate O isotopic compositions in Acfer 094 (Ushikubo et al., 2012) but was inherited from chondrule precursors as attested by the O features of relict olivine grains in CCs (Marrocchi et al., 2018, 2019).

To clarify the significance of these O isotopic features, and thus the relationship among R, OC, and/or CC chondrites, it is necessary to disentangle the isotopic signatures of chondrule precursors from processes that occurred during chondrule melting. Relict grains inherited from precursors have long been recognized in chondrules (Chaumard et al., 2018; Hertwig et al., 2018; Kita et al., 2010; Rudraswami

et al., 2011; Schrader et al., 2015; Tenner et al., 2013, 2015; Ushikubo et al., 2012) but are difficult to identify if they record redox conditions similar to those of their hosts. Hence, we have undertaken coupled chemical and O isotopic studies of chondrule olivines, employing (i) high-current X-ray maps and point analyses to assess minor element distributions and (ii) a large number of analyses per chondrule to achieve better statistical representativity. In our first studies on CCs (Marrocchi et al., 2018, 2019), we found that relict grains formed Al-Ti-poor cores in olivine phenocrysts, surrounded by overgrowths enriched in incompatible elements that precipitated from the melt and whose composition eventually evolved by interaction with an SiO- and Mg-rich gas. We then demonstrated that similar processes had occurred in OCs (Piralla et al., 2021).

In this study, we investigate olivine-rich type I chondrules as well as Mg-rich isolated olivine grains (IOG; i.e., not included in chondrules; Jacquet et al., 2021) in the weakly metamorphosed R3 chondrite Northwest Africa (NWA) 12482. We examine our coupled chemical and isotopic analyses in conjunction with literature data to (i) test the putative relationship between OC and R chondrules and (ii) provide insights into the dynamic of dust production in the inner solar system and the large-scale isotopic structure of the disk.

## MATERIALS AND METHODS

We surveyed type I chondrules in one thick section of NWA 12482 (R3) from the CEREGE collection (Aix-en-Provence, France). Backscattered and secondary electron observations were performed at CRPG-CNRS (Nancy, France) using a JEOL JSM-6510 scanning electron microscope (SEM) with a 3 nA primary beam at 20 kV. High-resolution cathodoluminescence (CL) and X-ray element distribution maps were performed at the Institut des Sciences de la Terre (ISTerre, Grenoble, France) with the electron microprobe analyzer (EMPA) JEOL JXA-8230 equipped with (i) five wavelength-dispersive spectrometer (WDS; Ca, Al, Ti, Cr, Mn), (ii) one silicon drift detector energy-dispersive spectrometer (Mg, Fe, Si), and (iii) panchromatic CL system. High-resolution CL maps coupled with fast major element X-ray maps (on WDS detectors) were acquired simultaneously using an acceleration voltage of 20 kV, a beam current of 100 nA, 1.5–2 µm step size, and a dwell time of 10 ms. This allows the identification of olivine, pyroxene, and glass in chondrules. High-resolution X-ray element distribution mapping (CaO, $Al_2O_3$, $TiO_2$, $Cr_2O_3$, and MnO) was performed with an acceleration voltage and beam current of 20 kV and 500 nA, respectively, with a 1.5 µm step size and dwell time of 500 ms. The limits of detection of these
elements are below 40 ppm. Under these conditions, the
detection limits for these elements are below 40 ppm.
These X-ray maps are semiquantitative because the
background was not quantified. Thereafter, large olivine
grains were quantitatively characterized with an
accelerating voltage of 20 kV, a high probe current of
900 nA, a beam diameter of 1 µm, and a total counting
time of 400 s (peak/background) to achieve very low
detection limits of ~10 ppm for Ca, Al, Ti, Cr, and Mn.
We used the ZAF method for matrix corrections;
further details of microprobe analytical conditions,
including the employed standard routine, are available
in Batanova et al. (2015).

Oxygen isotopic measurements were performed on
chemically characterized olivine grains by secondary ion
mass spectrometry (SIMS) using the CAMECA IMS
1270 E7 at CRPG-CNRS. A Cs⁺ primary beam
(~15 µm, ~2 nA) was used to produce ¹⁶O⁻, ¹⁷O⁻, and
¹⁸O⁻ ions that were measured in multicollection mode
with two off-axis Faraday cups (FCs) for ¹⁶,¹⁸O⁻ and
one axial 10¹² Ω FC for ¹⁷O⁻ (Bouden et al., 2021).
Typical total measurement times were 360 s, with 120 s
of presputtering and 240 s of analysis. To define the
TFL, we used three terrestrial standard materials: San
Carlos olivine, magnetite, and JV1 diopside.
Instrumental mass fractionation for chondrule olivines
was determined using San Carlos olivine. Typical count
rates of the San Carlos olivine standard were
7.4 × 10⁸ cps, 2.9 × 10⁵ cps, and 1.5 × 10⁶ cps for
¹⁶O⁻, ¹⁷O⁻, and ¹⁸O⁻, respectively. To maintain good
precision, we bracketed every 10–15 analyses of
chondrule olivine crystals with five standard analyses,
achieving typical uncertainties (2σ) of 0.5, 0.8, and
0.8‰ for δ¹⁸O, δ¹⁷O, and Δ¹⁷O, respectively. The
entrance and exit slits of the axial FC were adjusted to
obtain a mass resolving power of ~7000 for ¹⁷O⁻. All
SIMS spots were checked thoroughly by SEM, and any
near fractures, in the mesostasis, not completely in
olivine grains, or near Fe-Ni metal blebs were excluded
from the data set.

RESULTS

An R3 metamorphic grade has been attributed to
the NWA 12482 Rumuruti chondrite based on (i) the
dominant abundance of low-Ca pyroxene relative to
high-Ca pyroxene and (ii) the low iron content of
olivine grains (Gattacceca et al., 2020). No precise
submetamorphic grade (R 3.0–3.9) has been reported.
Nevertheless, this uncertainty does not impact our
isotopic data as (i) the oxygen diffusion within olivine
crystals is slow enough to retain their original oxygen
isotopic composition (Marrocchi & Chaussidon, 2015;
Ryerson et al., 1989) and (ii) no isotopic resetting has

been observed in chondrites having petrographic grade
>3.6 such as Allende (e.g., Rudraswami et al., 2011).

The selected olivine-rich type I porphyritic
chondrules (n = 5) are characterized by rounded to
euhedral olivine crystals generally ~20–100 µm in
diameter. The coarser grained (>100 µm) olivine
palisades observed in PO chondrules (Marrocchi et al.,
2018, 2019; Piralla et al., 2021) are not developed in
these chondrules, which contain slightly more pyroxenes
concentrated near and parallel to chondrule rims
(Figs. 1–3). Backscattered electron images and major
element compositions show chondrule olivines to be
apparently homogenous (Table S1 in the supporting
information), but high-current X-ray maps reveal
important, complex chemical zonations. Inner chondrule
olivine grains have cores depleted in CaO, Al₂O₃, TiO₂,
and Cr₂O₃, overgrowths enriched in these elements, and
sometimes a very outermost Al-Ti-depleted rim
(Figs. 1–3; Table S1). MnO is almost homogeneous in
chondrule olivines, only increasing in concentration
toward chondrule edges, and is more enriched in
pyroxene (Figs. 1–3; Table S1). While not constituting
well-formed palisades, outer chondrule olivine grains
tend to be somewhat coarser, with wider Ca-Al-Ti-rich
overgrowths in which Ca, Al, and Ti concentrations
reach their maxima among observed chondrule olivines.
Ti, Al, and Cr show oscillatory zonings with periods of
5–20 µm, ranging from 200 to 800 ppm TiO₂, 250–
3500 ppm Al₂O₃, and 400–4000 ppm Cr₂O₃ (Figs. 1–3;
Table S1). Chondrule Ch-2 contains oversized olivine
with Cr, Ti, and Al content variations and oscillatory
zoning (Fig. 1; Table S1). IOGs (not included in
chondrules) were also studied and are characterized by
subhedral to euhedral shapes with Mg-rich cores
surrounded by Fe-rich rims (Fig. 4; Table S1).

All olivine O isotopic compositions define a good
correlation (Fig. 5a; δ¹⁷O = [0.98 ± 0.04] × δ¹⁸O –
[1.19 ± 0.19]‰, r² = 0.94, mean square weighted
deviation [MSWD] = 7.9, n = 154) plotting along or
slightly above the PCM line, with δ¹⁸O and δ¹⁷O values
ranging from –11.7 to +8.8‰ and from –13.2 to
+6.7‰, respectively (Fig. 5a; Table S1). These data are
in good agreement with those reported for R chondrules
in previous studies (Fig. 5b) (Greenwood et al., 2000;
Miller et al., 2017; Pack et al., 2004), but do not
conform to the slope 0.8 “Rumuruti Mixing Line”
(RML) of Pack et al. (2004). However, the slope of that
“RML” was controlled by only two refractory forsterite
cores and was only tentatively proposed (Pack et al.,
2004). Our data demonstrate that the Δ¹⁸O values
(≡δ¹⁸O–δ¹⁷O, measuring mass-dependent fractionation,
with the PCM lying at Δ¹⁸O ≈ 2.7‰; Piralla et al.,
2021) show a spread, and are uncorrelated with Δ¹⁷O
(Fig. 6), with an average value of 1.13 ± 1.05‰. Only
<figure><img src="image_3.png" /><figcaption>Fig. 1. High-resolution chemical maps of olivine-rich chondrule Ch-2 in NWA 12482. a) Backscattered electron image; (b–f) X-ray maps of TiO₂, Al₂O₃, CaO, MnO, and Cr₂O₃ distributions, respectively, revealing complex internal structures. Color scales represent the chemical composition of olivine grains; greenish and reddish colors indicate high abundances of minor elements in other phases (e.g., mesostasis). Ol = olivine, lpx = low-Ca pyroxene, mes = mesostasis. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

Ch-14 plots consistently below the TFL. In comparison, the host olivine of all 11 chondrules analyzed by Miller et al. (2017) in R3 clasts of Mount Prestrud 95404 plotted above the TFL (Fig. 5b).

Except for Ch-14 (Figs. 5a, 6, and 7; Table S1), each chondrule is characterized by limited internal O isotopic variabilities, although chondrule δ¹⁸O and δ¹⁷O values range from -2.4 ± 2.7‰ to +6.3 ± 1.3‰ and from -2.8 ± 2.0‰ to +4.3 ± 0.9‰, respectively (Table S1). Ch-14 shows significant largely mass-dependent fractionation with δ¹⁸O varying between -5 and +3.7‰ (Fig. 5a), reminiscent of those observed in OC chondrules (Piralla et al., 2021). The highest δ¹⁸O are carried by large (CL-dark) olivine cores, with a clear hiatus from the (CL-brighter) overgrowth (Fig. 8), and

so may in fact represent a relict signature (in a textural context similar to many OC and CC chondrules; Kita et al., 2010; Marrocchi et al., 2018). Indeed, the overall Ch-14 standard deviation of Δ¹⁷O (0.9‰) is about twice the analytical error. This is similar to the situation of OC chondrule Bish-Ch2 of Piralla et al. (2021). If, however, as in our previous work, we stick to the Ushikubo et al. (2012) statistical (3σ) threshold to isotopically identify relict grains, no single olivine satisfies it in Ch-14 or our other three chondrules. Miller et al. (2017) found two relict-bearing chondrules out of their 11 R3 clast chondrules. By comparison, Piralla et al. (2021) found relicts in 5 of 11 LL chondrite chondrules using the Ushikubo et al. (2012) threshold.
<figure><img src="image_4.png" /><figcaption>Fig. 2. High-resolution chemical maps of olivine-rich chondrule Ch-7 in NWA 12482. a) Backscattered electron image; (b–f) X-ray maps of TiO₂, Al₂O₃, CaO, MnO, and Cr₂O₃ distributions, respectively, revealing complex internal structures. Color scales represent the chemical composition of olivine grains; greenish and reddish colors indicate high abundances of minor elements in other phases (e.g., mesostasis). Ol = olivine, lpx = low-Ca pyroxene, mes = mesostasis. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

IOGs have homogeneous O isotopic compositions that are generally ¹⁶O-rich compared to the chondrules (Fig. 5a; Table S1). Al₂O₃, TiO₂, CaO, and MnO concentrations and Fo contents in IOGs range from 0.01 to 0.33 wt%, 0.003–0.100 wt%, 0.028–1.650 wt%, 0.025–0.410 wt%, and 73.9–99.5 mole%, respectively

(Fig. 7; Table S1). As previously observed for R (Pack et al., 2004) and CCs (Jacquet & Marrocchi, 2017; Jacquet et al., 2021), the most Ca-rich olivines seem restricted to the most ¹⁶O-rich compositions. The Δ¹⁷O values of chondrules and IOGs range from –7.2 to +2.8‰ (Fig. 7; Table S1).
<figure><img src="image_5.png" /><figcaption>Fig. 3. High-resolution chemical maps of olivine-rich chondrule Ch-14 in NWA 12482. a) Backscattered electron image; (b–f) X-ray maps of TiO₂, Al₂O₃, CaO, MnO, and Cr₂O₃ distributions, respectively, revealing complex internal structures. Color scales represent the chemical composition of olivine grains; greenish and reddish colors indicate high abundances of minor elements in other phases (e.g., lpx, mesostasis). Ol = olivine, lpx = low-Ca pyroxene, mes = mesostasis. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

# DISCUSSION

## Conditions of Chondrule Formation

To the first order, the textural and chemical features of R chondrules are similar to those observed in CC and OC chondrules with chondrule cores being characterized by Al-Ti-poor olivine grains whereas chondrule exteriors show Al-Ti-rich olivine crystals (Figs. 1–3; Jacquet et al., 2021; Libourel & Portail, 2018; Marrocchi et al., 2018, 2019; Piralla et al., 2021). In detail, however, R chondrules show some small differences. (i) In addition to Al-Ti depletion, inner chondrule olivine grains are depleted in Ca, and (ii)

outer chondrule olivine crystals are not mantled by multilayer, Mg-rich olivine shells, except for the coarse olivine (presumably cannibalized) in Ch-2 (bottom-left in Fig. 1). Despite these minor differences, and by comparison with CC and OC chondrules, our data suggest that R chondrules experienced gas-melt interactions between chondrule precursors and a gas enriched in SiO and Mg (Jacquet et al., 2021; Libourel & Portail, 2018; Marrocchi et al., 2018, 2019; Piralla et al., 2021). As such a process has also been proposed for ECs (Jacquet et al., 2018; Piani et al., 2016; Weisberg et al., 2021), this implies that the mechanism at the origin of chondrules was similar throughout the disk and for all chondrites.
<figure><img src="image_6.png" /><figcaption>Fig. 4. Backscattered electron image of representative isolated olivine grains in NWA 12482 from which oxygen isotopic compositions have been determined in this study; (a) IOG-1, (b) IOG-2, (c) IOG-3, (d) IOG-4, (e) IOG-5, and (f) IOG-7.</figcaption></figure>

Although interior Ca-Al-Ti-poor olivine cores are reminiscent of relict grains reported in CCs (Marrocchi et al., 2018, 2019), *isotopically* identifiable relicts are but few (Miller et al., 2017; this study). The relative paucity of relict grains in OC chondrites in general has been attributed to more extensive chondrule–chondrule recycling (Piralla et al., 2021), but this seems hardly applicable to R chondrites. Indeed, their high matrix abundances (40–50 vol%; Bischoff et al., 2011) suggest limited chondrule formation and thus limited recycling, unless the matrix was added late to a pre-existing solid population that had largely been converted to chondrules.

Marrocchi et al. (2019) attributed the ¹⁶O-rich relict grains in CCs to coarse precursor olivine that escaped

equilibration with a ¹⁶O-poor gas prior to the chondrule-forming event. Such ¹⁶O-rich, coarse precursor olivine may ultimately derive from the same nebular condensation processes that produced amoeboid olivine aggregates (AOAs) and CAIs. Since the latter are very scarce in Rumuruti chondrites (<0.1 vol%; Bischoff et al., 2011), ¹⁶O-rich signatures should be rare among chondrule precursors; indeed, no closely AOA-like signatures are observed in our data, with no Δ¹⁷O below −7.2‰ (Figs. 5, 7, and 8). If R chondrule precursors had largely isotopically equilibrated with their surrounding reservoir, any relicts would be hardly distinguishable from their hosts on the basis of Δ¹⁷O, unless the isotopic evolution of the reservoir preceded chondrule formation. Such limited evolution is
<figure><img src="image_7.png" /><figcaption>Fig. 5. a) Oxygen isotopic compositions of olivine crystals within the five chondrules of NWA 12482 (Ch-2, Ch-4, Ch-7, Ch-14, and Ch-29) and the eight IOGs. The data define a good correlation with $\delta^{17}\text{O} = [0.98 \pm 0.04] \times \delta^{18}\text{O} - [1.19 \pm 0.19]^{\text{‰}}$ (MSWD = 7.9, $r^2 = 0.94$, $n = 154$). b) Oxygen isotopic composition of our data compared to literature data (Greenwood et al., 2000; Miller et al., 2017; Pack et al., 2004). The TFL and PCM of Ushikubo et al. (2012) are shown for reference. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

conceivable given the range of $\Delta^{17}\text{O}$ observed from chondrule to chondrule or IOG to IOG (Figs. 5 and 7) and may account for the relict grains of Miller et al. (2017). So it is entirely possible that many of the Al-Ti-poor olivine cores inside chondrules are relicts (e.g., of previous generations of chondrules) but cannot be

<figure><img src="image_8.png" /><figcaption>Fig. 6. $\Delta^{18}\text{O}$ versus $\Delta^{17}\text{O}$ diagram showing individual measurements of chondrule olivine grains and IOGs of NWA 12482. The TFL and PCM are shown for reference; the red solid PCM line shows a constant $\Delta^{18}\text{O} = 2.7^{\text{‰}}$, whereas the red dashed-dotted line is the true PCM. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

isotopically identified as such (or only marginally as for chondrule Ch-14; Fig. 8). We note that the Fe/Mn ratio has been previously used to identify FeO-rich relicts in type IIA chondrules (Berlin et al., 2011; Schrader & Davidson, 2017), but this is not applicable to FeO-poor relicts in type I chondrules, at least in CCs where host and relict grains show overlapping ratios (e.g., fig. 11b of Marrocchi et al., 2019).

## Comparison Among R, OC, and CC Chondrules

In oxygen isotopic space, R chondrule olivine grains overlap those in OC chondrules rather than CCs (Fig. 9a; Kita et al., 2013; Miller et al., 2017; this study). Regardless of the $\Delta^{17}\text{O}$, R and OC chondrule olivines tend to be displaced from the PCM toward the left, that is, they have lower $\Delta^{18}\text{O}$ than most CC chondrule olivine grains (Figs. 5 and 9; Piralla et al., 2021). Therefore, in accordance with the similar Cr and Ti compositions of both OCs and R chondrites (Warren, 2011; Zhu et al., 2021), this argues for OC and R chondrules belonging to the same NC superclan.

The isotopic similarity of R and OC chondrules does not mean they derive from the same reservoir. For instance, type I chondrule abundances likely differ in R than in OC chondrites. However, the type I/type II abundances in R chondrites might be biased due to later parent body processes and thus, are not reflecting
<figure><img src="image_9.png" /><figcaption>Fig. 7. $\Delta^{17}$O values as functions of incompatible element concentrations for R chondrules and IOGs: (a) Al$_2$O$_3$, (b) TiO$_2$, (c) CaO, (d) Cr$_2$O$_3$, (e) MnO, and (f) Fo content. The plots show homogeneous oxygen isotopic compositions regardless of the incompatible element concentrations. For comparison, data for CC chondrules are also reported in this diagram (data from Marrocchi et al., 2018, 2019). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 8. a) Color CL image of Ch-14 showing dark, <sup>16</sup>O-poor, inner chondrule olivine grains surrounded by bright, <sup>16</sup>O-rich, overgrowths enriched in refractory minor elements (CaO, TiO<sub>2</sub>, and Al<sub>2</sub>O<sub>3</sub>). The position of the SIMS pits and their corresponding Δ<sup>17</sup>O are indicated. b) Oxygen isotopic compositions of olivine grains from Ch-14 (color coded as a function of the CL intensity) and from a similar isotopically heterogeneous R chondrule from the R chondrite PRE 95404 (Miller et al., 2017). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

intrinsic properties of their formation environment. Indeed, metamorphism on the parent body could make a precise evaluation of their frequency difficult due to influx of Fe in type I silicates. However, insights might be gained from the pyroxene compositions in R3 clasts, as iron diffusion is slower than in olivine grains (Cherniak & Dimanov, 2010). For instance, in the three R chondrites studied by Friend et al. (2017), 60–80% of pyroxene grains show less than 10 mole% of ferrosilite, which might represent the lower limits of premetamorphism type I chondrule frequencies. In

<figure><img src="image_11.png" /><figcaption>Fig. 9. a) Oxygen isotopic compositions of olivine grains in this study compared to OC and CC chondrules. The TFL and PCM of Ushikubo et al. (2012) are also shown for reference. b) Δ<sup>18</sup>O versus Δ<sup>17</sup>O diagram showing individual measurements of chondrule olivine grains in carbonaceous, ordinary, and Rumuruti chondrites. It confirms that the Δ<sup>18</sup>O-Δ<sup>17</sup>O can be used for deciphering the noncarbonaceous versus carbonaceous origin of a given chondrule. The TFL and PCM are shown for reference; the black solid PCM line shows a constant Δ<sup>18</sup>O = 2.7‰, whereas the black dashed-dotted line is the true PCM. Errors on Δ<sup>18</sup>O are 2 SD whereas those on Δ<sup>17</sup>O are 2 SE. CC data from Marrocchi et al. (2018, 2019), Chaumard et al. (2018), and Hertwig et al. (2018). OC data from Piralla et al. (2021). R data from Greenwood et al. (2000), Pack et al. (2004), Miller et al. (2017), and this study. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>
comparison, type I compositions never account for more than 56% of chondrules in the OCs studied by Zanda et al. (2006). R chondrules also contain fewer relicts or PO textures compared to OCs (see the Results section). It would be worthwhile to measure the Cr isotopic composition of chondrules in R chondrites, as their whole-rock composition is distinct from OCs (e.g., Zhu et al., 2021), which may further support genetic differences between their chondrule populations.

In R chondrites, IOGs commonly show negative $\Delta^{17}\text{O}$ (see also Pack et al., 2004). They have long been interpreted as chondrule debris (e.g., Jones, 1992; McSween, 1977; Pack et al., 2005), likely expelled during collisions while the chondrules were still semimolten (Jacquet et al., 2021). The paucity of R chondrule olivine grains with $\Delta^{17}\text{O} <0_{\text{\textperthousand}}$ limits them as precursors of $^{16}\text{O}$-rich IOGs. This rather suggests that the $^{16}\text{O}$-rich R chondrule-forming reservoirs were more collisional than their more commonly sampled $^{16}\text{O}$-poor counterparts. Most NC-forming reservoirs may indeed have contained lower dust concentrations than their CC ones (Rubin, 2010). However, Pack et al. (2004) hypothesized that refractory forsterites from different chondrite groups formed in the same environment. The similarity of $\Delta^{17}\text{O}$ and its apparent anticorrelation with refractory element contents suggests that they experienced similar formation processes (Pack et al., 2004). However, the O isotopic compositions of R chondrule olivine grains, although variable, are generally displaced toward the left of the PCM (Fig. 9). So as for OCs (Piralla et al., 2021), even among $^{16}\text{O}$-rich material, the contribution from a CC-like reservoir may have been minor, at variance with the suggestion of a CC inheritance of $^{16}\text{O}$-rich relicts (Miller et al., 2017). This reinforces models where chondrules accreted where they formed (Schneider et al., 2020), implying limited transport within the disk before final accretion of the chondrite parent bodies (Pinto et al., 2021).

## CONCLUDING REMARKS

We performed high-resolution elemental X-ray mapping analyses coupled with in situ O isotopic measurements of olivine grains in five chondrules and eight IOGs in the R3 NWA 12482 Rumuruti chondrite. Our main results are as follows:

1. Olivine-rich R chondrules are chemically zoned at both the chondrule and grain scales. Inner chondrule olivine grains are Ca-Al-Ti-Cr-poor whereas outer chondrule olivine crystals are enriched in those elements.

2. Our O isotopic data define a good correlation with $\delta^{17}\text{O} = [0.98 \pm 0.04] \times \delta^{18}\text{O} - [1.19 \pm 0.19_{\text{\textperthousand}}]$

($r^2 = 0.94$, MSWD = 7.9, $n = 154$). All but one chondrule are characterized by limited internal O isotopic variabilities, whereas the exception shows mass-dependent fractionation. IOGs also show homogeneous O isotopic compositions, but are generally $^{16}\text{O}$-rich compared to chondrules.

From these data, we drew the following conclusions:

1. Considering CC, OC, and R chondrites together, their chondrules show similar textural and chemical characteristics, implying a common formation process controlled by gas-melt interactions between chondrule precursors and SiO- and Mg-rich gas.

2. The paucity of $^{16}\text{O}$-rich (i.e., with $\Delta^{17}\text{O} < -15_{\text{\textperthousand}}$) relicts in R chondrules implies that R chondrules could not have derived from $^{16}\text{O}$-rich precursors unless they experienced wholesale isotopic equilibration with the surrounding $^{16}\text{O}$-poor reservoir before chondrule formation.

3. Although R chondrules are isotopically similar to OC chondrules, R chondrites formed in a different environment characterized by higher abundances of Mg-rich, type-I chondrules.

4. Similar to the features of OC chondrules, the O isotopic characteristics of chondrules and IOGs in R chondrites suggest only a limited connection between the CC and NC reservoirs.

*Acknowledgments*—Nordine Bouden is thanked for technical assistance with SIMS measurements. We are grateful to Jérôme Gattacceca (CEREGE) for loaning the NWA 12482 sample. We thank Ke Zhu and an anonymous reviewer for helpful reviews and Timothy Jull for editorial handling. This research was funded by l'Agence Nationale de la Recherche (ANR) through grant CASSYSS ANR-18-CE31-0010-01 (PI Johan Villeneuve). This is CRPG contribution #2792.

*Data Availability Statement*—Original data from this study are available on the Ordar database: https://doi.org/10.24396/ORDAR-81.

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

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**Fig. S1.** a) Oxygen isotopic composition of olivine grains in chondrule Ch-2. Δ¹⁷O values as functions of incompatible element concentrations: (b) Al₂O₃, (c) TiO₂, (d) CaO, (e) MnO, and (f) Fo content. Plots show homogeneous oxygen isotopic composition regardless of the concentrations of these different elements.

**Fig. S2.** a) Oxygen isotopic composition of olivine grains in chondrule Ch-4. Δ¹⁷O values as functions of incompatible element concentrations: (b) Al₂O₃, (c) TiO₂, (d) CaO, (e) MnO, and (f) Fo content. Plots show

homogeneous oxygen isotopic composition whatever the
concentrations of these different elements.

**Fig. S3.** a) Oxygen isotopic composition of olivine grains in chondrule Ch-14. Δ¹⁷O values as functions of incompatible element concentrations: (b) Al₂O₃, (c) TiO₂, (d) CaO, (e) MnO, and (f) Fo content. Plots show homogeneous oxygen isotopic composition whatever the concentrations of these different elements.

**Fig. S4.** a) Oxygen isotopic composition of olivine grains in chondrule Ch-29. Δ¹⁷O values as functions of incompatible element concentrations: (b) Al₂O₃, (c) TiO₂, (d) CaO, (e) MnO, and (f) Fo content. Plots show homogeneous oxygen isotopic composition whatever the concentrations of these different elements.

**Table S1.** O isotopic compositions and mineral chemistry of individual olivine grains.