# Spinel in CV chondrules: Investigating precursor legacy and chondrule thermal histories

Nicolas Schnuriger, Camille Cartier, Johan Villeneuve, Maxence Regnault, Yves Marrocchi, Valentina Batanova

► To cite this version:

Nicolas Schnuriger, Camille Cartier, Johan Villeneuve, Maxence Regnault, Yves Marrocchi, et al. Spinel in CV chondrules: Investigating precursor legacy and chondrule thermal histories. Meteoritics and Planetary Science, 2022, 57 (6), pp.1018-1037. 10.1111/maps.13802. insu-03712866

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# Spinel in CV chondrules: Investigating precursor legacy and chondrule thermal histories

Nicolas SCHNURIGER ID*<fnref n="1" />, Camille CARTIER<fnref n="1" />, Johan VILLENEUVE ID<fnref n="1" />, Valentina BATANOVA<fnref n="2" />, Maxence REGNAULT<fnref n="1" />, and Yves MARROCCHI ID<fnref n="1" />

**Abstract**—In carbonaceous chondrites, Mg-spinel (MgAl₂O₄) grains are ubiquitous in refractory inclusions but rarely reported in chondrules, where they may correspond to minerals either (i) inherited from chondrule precursors or (ii) crystallized from chondrule melts. Here, we report high-current quantitative electron microprobe measurements and secondary ion mass spectrometry oxygen isotopic analyses of Mg-spinel-bearing chondrules in the CV3 carbonaceous chondrites Northwest Africa 10235 and Allende. Compared to spinels in refractory inclusions, chondrule spinels are characterized by higher Cr contents and ¹⁶O-poorer oxygen isotopic signatures (Δ¹⁷O ≡ δ¹⁷O–0.52 × δ¹⁸O, from –2 to –6‰). Because the similar Δ¹⁷O values of chondrule olivine and spinel crystals imply their comagmatic origin, we applied a geothermometer based on the Al-Cr distribution between these minerals to determine their crystallization temperatures. The calculated temperatures range from 1200 to 1640 °C (mean = 1470 °C), most being lower than the estimated liquidus temperature of porphyritic chondrules (~1600 °C). Our results suggest that chondrules experienced relatively slow cooling rates (slower than a few hundreds of °C h⁻<fnref n="1" />), which is in good agreement with models of chondrule formation invoking nonlinear or two-stage cooling rates.

## INTRODUCTION

Spinels compose a broad group of minerals defined by the general formula A<fnref n="2" />⁺B₂³⁺O₄. Depending on the dominant trivalent ion, the spinel group is subdivided into three major series: spinel (Al³⁺), chromite (Cr³⁺), and magnetite (Fe³⁺; e.g., Barnes & Roeder, 2001; Gattacceca et al., 2011). Spinel-series minerals (A<fnref n="2" />⁺Al₂O₄) are common in terrestrial igneous and metamorphic rocks and occur as detrital grains in many sedimentary rocks. They form a continuous solid solution between ferrous (FeAl₂O₄, hercynite) and magnesian endmembers (MgAl₂O₄, spinel stricto sensu), the latter being the most common of this series. Chromite-series minerals (A<fnref n="2" />⁺Cr₂O₄) are commonly observed in ultrabasic igneous rocks. Finally, the magnetite series (A<fnref n="2" />⁺Fe₂O₄), including magnetite stricto sensu (Fe<fnref n="2" />⁺Fe₂³⁺O₄), is ubiquitous in terrestrial rocks (e.g., Pilchin, 2010).

Among extraterrestrial rocks, Mg-spinel occurs as euhedral crystals in type A and B calcium-aluminum-rich inclusions (CAIs; Aléon, 2018; Williams et al., 2017) whereas it is generally observed as anhedral grains in type C CAIs and amoeboid olivine aggregates (AOAs). Mg-spinel also represents a major component of Wark–Lovering rims, which are a succession of mineral layers surrounding most CAIs (Wark & Lovering, 1977). It has been proposed that Mg-spinel in refractory inclusions resulted from condensation processes during the evolution of the solar protoplanetary disk (Grossman, 1972; Larimer, 1967). According to thermodynamical calculations, spinel should represent one of the first minerals to condense at temperatures near 1760 °C from a gas of solar composition (i.e., 10⁻³ bar), only after highly refractory components such as corundum, hibonite, and gehlenite (Ebel & Grossman, 2000).

Chondrules are the major high-temperature components in undifferentiated meteorites, but their

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

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

<footnote>*Corresponding author. E-mail: nicolas.schnuriger@univ-lorraine.fr</footnote>

<footnote>(Received 14 October 2021; revision accepted 14 February 2022)</footnote>
formation histories are still poorly understood. They
display igneous textures and mostly comprise olivine,
pyroxene, glassy or micro-crystalline mesostasis, and
metal/sulfur beads (Connolly & Jones, 2016; Marrocchi
& Libourel, 2013). The important textural and chemical
diversities observed in chondrules are likely due to the
heterogeneities of the materials they formed from, their
various thermal histories, and/or their environments of
formation. It has been thus proposed that chondrules
must have experienced at least one high-temperature
event (minutes to several days) that reached peak
temperatures near or above the liquidus (estimated to
be 1400–1700 °C depending on chondrule chemistry)
and cooled at rates ranging from a few to thousands of
degrees per hour (Connolly & Hewins, 1991, 1995;
Connolly et al., 1998; Desch et al., 2012; Hewins et al.,
2005; Jones et al., 2018; Radomsky & Hewins, 1990;
Wick & Jones, 2012). However, the thermal histories,
and therefore the heating mechanism(s) of chondrules,
remain only partially constrained. It has been proposed
that chondrules could have formed via (i) the recycling
of early condensates from the protoplanetary disk, (ii)
the recycling of prior chondrules, (iii) the reprocessing
of planetary residues interacting with gas and dust from
the disk, or (iv) impact splashes (Libourel & Krot, 2007;
Libourel & Portail, 2018; Lichtenberg et al., 2018;
Marrocchi, Euverte, et al., 2019; Marrocchi et al., 2018;
Ruzicka et al., 2007).

Although Mg-spinel is relatively uncommon in
chondrules, occurrences have been reported in
ferromagnesian barred olivines (BOs; Rudraswami
et al., 2011), Al-rich chondrules (ARCs; e.g., Guan
et al., 2006; Maruyama et al., 1999), and Na-ARC
(Ebert & Bischoff, 2016). Therefore, they could be a key
to get better insights into the chondrule-forming event
(s). There are two possible explanations for their origin:
they could correspond to (i) magmatic minerals
resulting from the crystallization of the host melts
during the chondrule-forming event, or (ii) unmelted
relict crystals inherited from refractory inclusions that
were preserved during the chondrule-forming event. The
latter scenario could be considered due to the large
stability field of Mg-spinel at high temperature (e.g.,
1325–1550 °C in calcium-magnesium-alumina-silicate
melts at 7 kbar; Presnall et al., 1978). In that case, Mg-
spinel grains should have preserved their initial oxygen
isotopic signatures because the diffusivity of oxygen is
very low at such temperatures (~0.002 µm² yr⁻¹ at
1500 °C; Tenner et al., 2018). This scenario is consistent
with the fact that several petrographic, chemical, and
isotopic evidences suggest that chondrules may partially
derive from CAIs and AOAs. For instance, rare
occurrences of compound CAI-chondrule objects have
been reported in carbonaceous chondrites (e.g., Krot,

2019; Russell et al., 2005), and especially in CH
porphyritic chondrules (Krot et al., 2017). In addition,
the rare earth element patterns, chemical features, and
Si and O isotopic compositions of olivine grains suggest
that ferromagnesian type I chondrules are derived from
AOAs (Cohen et al., 2004; Jacquet & Marrocchi, 2017;
Marrocchi, Euverte, et al., 2019; Marrocchi et al., 2018;
Russell et al., 2005; Ruzicka et al., 2012; Schrader et al.,
2018; Villeneuve et al., 2020; Yurimoto & Wasson,
2002). Furthermore, isolated chondrules from
carbonaceous chondrites show heterogeneous ⁵⁰Ti and
⁵⁴Cr nucleosynthetic anomalies suggesting that
chondrules were partially formed from precursors
isotopically similar to CAIs and AOAs (Gerber et al.,
2017; Schneider et al., 2020).

The origin of chondrule spinels can thus be
investigated by characterizing their chemistry and
oxygen isotopic compositions. Magmatic spinels should
have O isotopic compositions similar to those of
coexisting olivines and low-Ca pyroxenes (Rudraswami
et al., 2011), whereas relict spinels should be enriched in
¹⁶O (i.e., Δ¹⁷O ≡ δ¹⁷O – 0.52 × δ¹⁸O, <–20‰) relative
to coexisting phases (Aléon, 2018; Bodénan et al., 2014;
Krot et al., 2006; Maruyama et al., 1999). Both cases
have been reported within chondrules, implying that the
O isotopic composition of Mg-spinel is useful for (i)
understanding the genetic link between refractory
inclusions and chondrules, and (ii) distinguishing spinels
that can be used to constrain the temperature of
chondrule formation based on the results of studies on
olivine-spinel assemblages and trace-element distribution
as thermometers (Coogan et al., 2014; Wan et al.,
2008).

To better understand the origin of Mg-spinel in
chondrules, we here report a petrographic, mineralogical,
and O isotopic survey of type I chondrules in the CV3
chondrites Allende and Northwest Africa (NWA)
10235. We selected these specific chondrites because
Mg-spinels are common in both their refractory
inclusions and chondrules, thus allowing their putative
genetic link to be tested. Moreover, we use these
observations to provide new constraints on the thermal
history of chondrules.

MATERIALS AND METHODS

We surveyed all chondrules in three thick sections
of Allende (A1, 3, and 7) and three thick sections of
NWA 10235 (N1, 2, and 8), all from the CRPG
(Nancy, France). The Allende sections come from the
well-known chondrite that defined the oxidized Allende-
like CV subgroup. NWA 10235 is a CV3 chondrite
containing relatively large chondrules (up to 2.8 mm in
diameter), AOAs, and fine-grained CAIs (Bouvier et al.,
<figure><img src="image_2.png" /><figcaption>Fig. 1. a, c, e) Back-scattered electron (BSE) images and (b, d, f) corresponding X-ray maps of selected chondrules in NWA 10235 and Allende. In the X-ray maps, Mg is shown in red and Si in green: forsteritic olivine appears orange, rare pyroxenes on the edge appear light green, and mesostasis darker green. Spinels, due to their compositions near the Mg endmember, appear red.a, b) Porphyritic olivine chondrule A7-Ch10 (Allende, section A7). Spinel is commonly in contact with chondrule mesostasis, but sometimes enclosed in olivine.c, d) Barred olivine chondrule N1-Ch21 (NWA 10235, section N1). Spinel is always in contact with olivine bars and glassy mesostasis.e, f) Porphyritic olivine chondrule N2-Ch36 (NWA 10235, section N2). The only spinel grain present is completely enclosed in olivine. White phase in the BSE images corresponds to the gold coating that could not have been removed after the secondary ion mass spectrometry measurements. Ol = olivine; Sp = spinel; Mes = mesostasis. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

2017). We acquired multielement (Na, Mg, Al, Si, P, Cl, K, Ca, Ti, Cr, Mn, Fe, and Ni) X-ray maps of each section using a JEOL JSM-6510 scanning electron microprobe (SEM) at the CRPG operating with a 3 nA primary beam accelerated at 15 kV. False color maps of Mg (red) and Si (green) were mixed to highlight the presence of spinel (in red) against olivine (orange) and low-Ca pyroxene (green) (Figs. 1 and 2). Among the six sections surveyed in this study, we selected 17 type I

(Mg# = Mg/[Mg + Fe] >90%) porphyritic-olivine (PO) chondrules and one BO chondrule containing Mg-spinel grains. We also selected three isolated spinel grains (ISps), three AOAs, two CAIs, and one isolated olivine grain (IOG; Jacquet et al., 2021) containing Mg-spinel (Figs. 1 and 2) to compare their chemical (major and minor element) and O isotopic compositions to those of chondrule Mg-spinels (see Supplementary information for images of every object selected).
<figure><img src="image_3.png" /><figcaption>Fig. 2. a, c, e) BSE images and (b, d, f) corresponding X-ray maps of selected refractory inclusions in NWA 10235. In the X-ray maps, Mg is shown in red and Si in green; augite appears light green, high-Ca pyroxene dark green, and spinel red.a, b) Macroscopic Ca-Al-rich inclusion N1-CAI01 (NWA 10235, section N1). Numerous euhedral spinel grains are located within the CAI and within its Wark-Lovering rim.c, d) Amoeboid olivine aggregate N2-AOA03 (NWA 10235, section N2). Spinels are present as small nodules enclosed within olivine grains.e, f) Rounded isolated spinel grain N1-ISP01 (NWA 10235, section N1), which consists almost exclusively of a >50-µm-diameter spinel grain surrounded by olivine crystals (the opaque phase in the X-ray map correspond to a remnant of gold coating). Sp = spinel; Aug = augite; Hcp = high-Ca pyroxene. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

Electron probe microanalyses of spinels and coexisting olivine grains were performed using a JEOL JXA-8230 at the Institut des Sciences de la Terre (ISTerre, Grenoble, France). Spinels were analyzed at accelerating voltage 20 kV, with probe current 80 nA and a focused beam size. Most elements (Si, V, Ti, Cr, Ni, Ca, Fe, and Mn) were acquired with WDS (wavelength-dispersive spectroscopy) using long counting time on both peak and background positions. Al and Mg were acquired by EDS (energy dispersive spectroscopy) simultaneously in combined WDS/EDS mode. Total

counting times (peak plus background) were 480 s for Si Kα; 240 s for V Kα and Ti Kα; 400 s for Ca Kα; 120 s for Fe Kα; 400 s for Ni Kα and Mn Kα; and 120 s for Cr Kα. Natural minerals, pure metal, and synthetic oxides were used as primary standards and ZAF correction was applied. Spectral interference (V Kα versus Ti Kβ) was corrected using the software-calculated correction factor. Spinel sample Bar 8601-10 (Ionov & Wood, 1992) was run as unknown to monitor precision and possible drift correction. The detection limit values calculated by JEOL software using 3-sigma criterion (Batanova et al., 2018)
are below 0.005 wt% for most elements and 0.01 wt% for Cr, Al, and Mg.

Olivine was analyzed following the high-precision method for major, minor, and trace elements at an acceleration voltage of 20 kV, beam current of 900 nA, and beam diameter of 2 µm (see Batanova et al., 2015, 2018; Piralla et al. [2021] for further details). MongOl sh11-2 olivine reference material (Batanova et al., 2019) was used as primary standard for Si, Mg, Fe, and nickel, and was analyzed as unknown after each 40 analyses of olivine to verify precision and accuracy.

The oxygen isotopic compositions of Mg-spinel and associated olivine grains were measured in situ with a CAMECA IMS 1270 E7 secondary ion mass spectrometer at the CRPG (Bouden et al., 2021). Secondary ¹⁶O⁻, ¹⁷O⁻, and ¹⁸O⁻ ions produced by a Cs⁺ primary ion beam (~8 µm diameter, ~2 nA, rastered over 5 × 5 µm²) were measured in multicollection mode on two off-axis Faraday cups (FCs, ¹⁶,¹⁸O⁻) and the axial FC (¹⁷O⁻). To remove interference of ¹⁶OH⁻ on the ¹⁷O⁻ peak, the entrance and axial exit slits were adjusted to obtain a mass resolution power (MRP) of ~6200. The MRP of the off-axis FCs was set at 2500 (slit #1). A normal-incident electron gun was used for compensating the charge on the surface of the sample. Each analysis consisted of 60 s of sputtering the sample surface to remove the coating and any surface contamination, automatically centering the secondary ion beam in the transfer optic and the axial mass, and measuring oxygen isotopes for 150 s (30 cycles of 5 s). To define the instrumental mass fractionation line for oxygen isotopes and correct for the matrix effect on spinel and olivine, we regularly measured a set of terrestrial mineral standards (Burma spinel, San Carlos olivine, and JV1 diopside) between samples. Typical count rates obtained on Burma spinel were 6.7 × 10⁸ cps, 2.7 × 10⁵ cps, and 1.4 × 10⁶ cps for ¹⁶O, ¹⁷O, and ¹⁸O, respectively. Similarly, count rates for San Carlos olivine and JV1 diopside were 1.4 × 10⁹ cps, 5.6 × 10⁵ cps, and 3.0 × 10⁶ cps for ¹⁶O, ¹⁷O, and ¹⁸O, respectively. Typical two-sigma uncertainties are 0.5, 0.8, and 0.8 ‰ for δ¹⁸O, δ¹⁷O, and Δ¹⁷O, respectively. The δₓO notation (x = 17 or 18) represents the deviation from Vienna Standard Mean Ocean Water (VSMOW) in parts per thousand, calculated as δₓO = (x/O/¹⁶Oₐample - 1) × 1000. The Δ¹⁷O notation indicates the deviation from the terrestrial fractionation line (Δ¹⁷O = 0 by definition) as: Δ¹⁷O = δ¹⁷O - 0.52 × δ¹⁸O. The quadratic sum of the errors on δ¹⁸O and δ¹⁷O were used to calculate the errors on Δ¹⁷O. Every ion probe analytical spot was carefully checked on the SEM and any spots near fractures, mesostasis, or not entirely within the targeted grain were removed from the data set.

## RESULTS

In chondrules, Mg-spinel grains occur either as small euhedral grains surrounded by glassy mesostasis (Figs. 1a–d and 3a–c) or poikilitically embedded in forsteritic olivine crystals (Figs. 1e–f and 3d). Mg-spinel grain boundaries show various textures, including corroded, smooth, and angular shapes (Fig. 3). Mg-spinel commonly occurs in clusters of several grains that are in contact or close to each other (Figs. 3a–c). When Mg-spinel occurs as a single grain in olivine crystals, a small pocket of silicate melt is systematically observed in contact with the spinel (Fig. 3d). In refractory inclusions, Mg-spinel occurs as euhedral grains in type-B CAIs (Figs. 2a and 2b) or as rounded crystals in complex association with olivine in AOAs (Figs. 2c and 2d). Rarely, Mg-spinels are observed within the matrix as isolated (hereafter ISp), rounded grains surrounded by a fine-grained olivine-rich rim, possibly of condensation origin (Figs. 2e–f; Krot et al., 2012).

Within CAIs, Mg-spinels display high Al₂O₃ and MgO contents (70.2–71.2 wt% and 28.4–28.8 wt%, respectively), and low concentrations of FeO, TiO₂, and Cr₂O₃ (<0.5 wt%; Figs. 4 and 5; Table 1). Spinels in AOAs are also nearly pure Mg-spinel, with high Al₂O₃ and MgO contents (69.5–71.7 wt% and 27.8–28.6 wt%, respectively), moderate iron contents (0.75–2.5 wt%), relatively high TiO₂ concentrations (up to 1 wt%), and low amounts of Cr₂O₃ (<0.5 wt%; Figs. 4 and 5; Table 1). The ISp from Allende (A3-ISp01) displays relatively low Al₂O₃ and MgO contents (~65.8 wt% and ~16.3 wt%, respectively), but high FeO content (~17.5 wt%) and low Cr₂O₃ and TiO₂ contents (~0.23 and ~0.12 wt%, respectively; Figs. 4 and 5; Table 1); the two others (N1—ISp01 and N8—ISp08) show high Al₂O₃ and MgO contents (69.7–71.3 wt% and 27.1–28.1 wt%, respectively), moderate FeO contents (0.6–1.7 wt%), variable TiO₂ contents (0.1–0.8 wt%), and low Cr₂O₃ contents (<0.5 wt%; Figs. 4 and 5; Table 1). Spinels in type I PO chondrules display relatively high Al₂O₃ and MgO contents (61.3–70.9 wt% and 19.43–28.5 wt%, respectively) with various amounts of FeO (0.38–12.26 wt%) and moderate amounts of TiO₂ and Cr₂O₃ (0.1–0.4 wt% and 0.4–2.8 wt%, respectively; Figs. 4 and 5; Table 1). Spinel grains within the BO chondrule (N1—Ch21; Figs. 1c–d) contain variable amounts of Al₂O₃ (61.3–70.9 wt%), have moderate MgO contents (26.9–28.3 wt%) compared to other spinels, and homogeneously low FeO contents (0.93–1.33 wt%). Among all analyzed spinels, Al₂O₃ content is anticorrelated with Cr₂O₃ content (up to 10 wt%; Figs. 4 and 5; Table 1), in agreement with the spinel (Al³⁺)-chromite (Cr³⁺) substitution; spinel (Al³⁺)-magnetite (Fe³⁺) substitution is also observed (Fig. 5).
<figure><img src="image_4.png" /></figure>

Fig. 3. BSE images of euhedral spinels in (a–c) porphyritic chondrules A7-Ch10 (Allende, section A7), N2-Ch15 (NWA 10235, section N2), and N1-Ch22 (NWA 10235, section N1), respectively, and (d) isolated olivine grain N2-IO03 (NWA 10235, section N2). Spinel crystals are always in direct contact with glassy mesostasis. Sp = spinel; Ol = olivine; M = mesostasis; Au = gold contamination due to Au metallization.

Olivines in chondrules are systematically purely forsteritic grains (Mg# ≈ 97%; Table 2).

Among the 64 chemically characterized Mg-spinel grains, we determined the O isotopic compositions of 43; the eight chondrules containing the remaining spinel grains were too close to the edge of the sections to perform isotopic measurements. We also determined the O isotopic compositions of 84 olivine grains associated with Mg-spinels in chondrules and IOGs. The oxygen isotopic compositions of spinels plot along the primary chondrule minerals line (Fig. 6; Table 1; Ushikubo et al., 2012). Mg-spinels in CAIs have δ¹⁸O = -50 to -43.2‰, δ¹⁷O = -51.3 to -44.6‰, and Δ¹⁷O = -25.3 to -22.2‰. Those in AOAs have wider ranges of O isotopic values: δ¹⁸O = -48.2 to -31‰, δ¹⁷O = -50.7 to -32.7‰, and Δ¹⁷O = -25.6 to -16.6‰ (Fig. 6a; Table 2). FeO-poor ISps (N1—ISp01 and N8—ISp01) span a broad range of O isotopic compositions, though most are similar to Mg-spinels in CAIs/AOAs: -48.2 to -32.5‰ for δ¹⁸O, -50.7 to -33.6‰ for δ¹⁷O, and -25.6 to -16.7‰ for Δ¹⁷O. In contrast, the FeO-rich ISp of Allende (A3—ISp01) presents less negative isotopic compositions: δ¹⁸O = -12.7 to -11‰, δ¹⁷O = -15.8 to -14.9‰, and Δ¹⁷O = -9.2‰. Spinel grains within chondrules have δ¹⁸O = -9.7 to +1.3‰,

δ¹⁷O = -11.4 to -3.9‰, and Δ¹⁷O = -6.5 to -1.9‰ (Fig. 6a; Table 1). Olivine grains within chondrules have O isotopic compositions of δ¹⁸O = -6.7 to +7.8‰, δ¹⁷O = -9.6 to +3.1‰, and Δ¹⁷O = -6.4 to -1‰ (Table 2). Despite the similar Δ¹⁷O values of olivine and Mg-spinel within individual chondrules (Fig. 7), olivines are, on average, depleted in ¹⁶O compared to Mg-spinels, except in the BO chondrule (Fig. 6b; Tables 1 and 2). This likely results from small matrix effects affecting both olivine and spinels grains; our standards having more Cr₂O₃ and FeO than meteoritic spinel and olivine crystals (5 versus 2% of Cr and Mg# of 90 versus 95–99). The oxygen isotopic compositions of Mg-spinel and olivine in Allende chondrules are systematically ¹⁶O-rich compared to chondrules in NWA 10235 (Fig. 6b; Tables 1 and 2). All data are available in the supporting information (Tables S1–S4).

DISCUSSION

Origin of Spinel in Chondrules

Our survey of 175 chondrules in NWA 10235 and Allende reveals that Mg-spinels are relatively common in chondrules in the investigated CV chondrites, as 10%
<figure><img src="image_5.png" /><figcaption>Fig. 4. a) Al₂O₃ and (b) TiO₂ concentrations in spinel as a function of spinel Cr₂O₃ content. Symbols indicate the data source (circles, this study; crosses, literature data) and symbol colors indicate spinel origin: Ca-Al-rich inclusions (CAI, orange), amoeboid olivine aggregates (AOA, red), porphyritic olivine chondrules (PO, blue), barred olivine chondrules (BO, green), isolated olivine grains (IOGs, light green), and isolated spinel (ISp, purple). In (a), most of the data plot near 70 wt% Al₂O₃ with very little Cr₂O₃, indicating that these minerals correspond to spinel sensu stricto. However, some data show higher Cr₂O₃ contents and lower Al₂O₃ contents, indicating Cr substitution for Al in the chromite spinel series. In (b), no specific Al³⁺–Ti³⁺ substitution is observed. Literature sources: Allen et al. (1978); Bodénan et al. (2014); Caillet et al. (1993); Michel-Levy et al. (1982); Davis et al. (1991); El Goresy (1984); Fuchs et al. (1973); Grossman (1975); Ikeda (1982, 1989); Jones (1992); Kornacki and Wood (1985); Krot and Keil (2002); Kurat et al. (1975); MacPherson and Grossman (1984); MacPherson et al. (1983); Maruyama and Yurimoto (2003); McGuire and Hashimoto (1989); Murakami and Ikeda (1994); Simon et al. (1994); Ushikubo et al. (2017); Zhang et al. (2014). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

of the surveyed chondrules contain at least one Mg-
spinel grain. Furthermore, our analyses underestimate
the abundance of Mg-spinel in chondrules due to (i) the
difficulty in discerning Mg-spinel from olivine in BSE
images (Figs. 1–3), (ii) the small size of Mg-spinel
compared to other chondrule minerals (Figs. 1–3), and
(iii) sectioning effects leading a biased estimation in 2D
(e.g., Barosch et al., 2020; Cuzzi & Olson, 2017; Ebel
et al., 2009; Hezel & Kießwetter, 2010; Hezel & Palme,
2007). In the porphyritic chondrules studied here, Mg-
spinel occurs either as clusters of euhedral grains in
contact with the mesostasis (Figs. 1a–d and 3a–c) or as
isolated euhedral grains poikilitically enclosed in olivine
crystals and in contact with melt pockets (Figs. 1e, 1f,
and 3d). These textures suggest that the Mg-spinel
grains are in equilibrium with the surrounding olivines.

In contrast, Mg-spinel grains in the BO chondrule
(Figs. 1c,d) and those isolated in the matrix (Figs. 2e
and 2f) display smother edges.

Previous studies have reported evidence for the
recycling of condensed materials in chondrules.
Nucleosynthetic ε⁵⁰Ti and ε⁵⁴Cr anomalies attributed to
a dust component isotopically similar to CAIs/AOAs
have been reported in carbonaceous chondrite
chondrules (Ebert et al., 2018; Gerber et al., 2017;
Schneider et al., 2020). Furthermore, oxygen and silicon
isotopic compositions attest to the presence of olivine
relics inherited from AOA-like condensates within type
I chondrules in carbonaceous chondrites (Marrocchi
et al., 2018; Marrocchi, Euverte, et al., 2019; Marrocchi,
Villeneuve, et al., 2019; Tenner et al., 2018; Villeneuve
et al., 2020). Finally, Mg-spinel grains with ¹⁶O-rich
<figure><img src="image_6.png" /><figcaption>Fig. 5. a) Al-rich portion of the spinel ternary diagram (top, X = FeO; left, Y = Al₂O₃; right, Z = Cr₂O₃; proportions normalized to X + Y + Z = 100) and (b) zoom in the Al₂O₃ pole (red triangle in [a]). Symbols and data sources are as in Fig. 4. The majority of the spinels plot near the Al₂O₃ pole, with some deviating toward the FeO pole. In this study, only the spinels from BO chondrules plot toward the Cr₂O₃ pole, and only those from an ISp in Allende are enriched in FeO. The integrality of the spinel from CAI is enriched in Al₂O₃ while the spinel from chondrules displays a much higher dispersion. c) The MgO- and Al₂O₃-rich portion of the spinel ternary with two elements grouped at each pole: FeO + TiO₂ (top), MgO + Al₂O₃ (left), and Cr₂O₃ + V₂O₃ (right) and (d) zoom in the Al₂O₃ + MgO pole (red triangle in [c]). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

signatures similar to CAIs/AOAs have been reported in ARCs and BOs (Maruyama & Yurimoto, 2003; Maruyama et al., 1999; Tenner et al., 2017; Zhang et al., 2014). Here, two of the three isolated Mg-spinel grains (N1—ISp01 and N8—ISp01) show Δ¹⁷O values and Cr₂O₃ contents consistent with Mg-spinel grains in CAIs and AOAs (Figs. 6a and 8a; Table 1), indicating their genetic link with condensates. The third ISp (A3—ISp01) has a Δ¹⁷O value intermediate between Mg-spinels in CAIs/AOAs and chondrules, possibly representing

either the reprocessing of a prior generation of condensates interacting with a ¹⁷,¹⁸O-rich gas or a generation that formed later from an evolved (i.e., non-solar) gas.

Mg-spinel and olivine grains within porphyritic chondrules show similar Δ¹⁷O values (Figs. 6b and 7; Tables 1–3). In addition, chondrule spinels contain significantly more Cr₂O₃ than spinels in CAIs/AOAs (Fig. 8). Of note, the Cr₂O₃ content of spinels was not significantly affected by thermal metamorphism due to the low diffusion of Cr in Mg-spinels (i.e., ~1 µm Ma⁻¹
<table><caption>Table 1. Oxygen isotopic (‰) and chemical compositions (wt‰) of Mg-spinel grains in chondrules (Ch), refractory inclusions (CAI, AOA), and isolated spinel grains (ISp) of the Allende and NWA 10235 chondrites. Because NiO and MnO content were systematically below the detection limit, these two elements are not displayed.</caption><thead><tr><th>Chondrite</th><th>Object#</th><th>δ<sup>18</sup>O<br>‰</th><th>2σ</th><th>δ<sup>17</sup>O</th><th>2σ</th><th>Δ<sup>17</sup>O</th><th>2σ</th><th>SiO<sub>2</sub><br>wt%</th><th>FeO</th><th>Al<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>MgO</th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>V<sub>2</sub>O<sub>3</sub></th><th>Total</th></tr></thead><tbody><tr><td rowspan="3">Allende-01</td><td>A1-Ch02</td><td>-10.7</td><td>0.5</td><td>-11.5</td><td>0.8</td><td>-5.9</td><td>0.8</td><td>0.1</td><td>4.8</td><td>67.8</td><td>0.0</td><td>25.4</td><td>2.8</td><td>0.4</td><td>0.1</td><td>101.4</td></tr><tr><td>-11.2</td><td>0.5</td><td>-11.6</td><td>0.8</td><td>-5.8</td><td>0.9</td><td>0.1</td><td>9.4</td><td>66.7</td><td>0.0</td><td>22.2</td><td>2.2</td><td>0.4</td><td>0.1</td><td>101.2</td></tr><tr><td>-11.7</td><td>0.6</td><td>-13.1</td><td>0.6</td><td>-7.1</td><td>0.7</td><td>0.1</td><td>9.9</td><td>65.7</td><td>n.d.</td><td>20.7</td><td>2.0</td><td>0.4</td><td>0.1</td><td>99.0</td></tr><tr><td rowspan="10">Allende-03</td><td>A3-Ch01</td><td>-8.0</td><td>0.5</td><td>-10.5</td><td>0.8</td><td>-6.4</td><td>0.9</td><td>0.2</td><td>1.1</td><td>70.2</td><td>0.0</td><td>27.9</td><td>0.9</td><td>0.2</td><td>0.2</td><td>100.7</td></tr><tr><td>-8.4</td><td>0.5</td><td>-10.1</td><td>0.9</td><td>-5.7</td><td>0.9</td><td>0.2</td><td>1.0</td><td>70.3</td><td>0.0</td><td>28.1</td><td>0.9</td><td>0.2</td><td>0.3</td><td>100.9</td></tr><tr><td>-6.1</td><td>0.5</td><td>-8.7</td><td>1.0</td><td>-5.5</td><td>1.1</td><td>0.2</td><td>2.1</td><td>69.4</td><td>0.0</td><td>27.4</td><td>1.3</td><td>0.2</td><td>0.3</td><td>100.9</td></tr><tr><td>-7.0</td><td>0.5</td><td>-10.7</td><td>0.8</td><td>-7.0</td><td>0.8</td><td>0.2</td><td>5.9</td><td>67.9</td><td>n.d.</td><td>23.9</td><td>1.1</td><td>0.2</td><td>0.2</td><td>99.4</td></tr><tr><td>-7.2</td><td>0.5</td><td>-10.2</td><td>0.8</td><td>-6.5</td><td>0.8</td><td>0.2</td><td>2.9</td><td>68.6</td><td>n.d.</td><td>25.8</td><td>1.4</td><td>0.2</td><td>0.3</td><td>99.3</td></tr><tr><td>-9.6</td><td>0.5</td><td>-11.1</td><td>0.7</td><td>-6.1</td><td>0.8</td><td>0.2</td><td>6.4</td><td>67.6</td><td>n.d.</td><td>23.2</td><td>1.3</td><td>0.2</td><td>0.2</td><td>99.2</td></tr><tr><td>-8.1</td><td>0.4</td><td>-10.2</td><td>0.6</td><td>-6.0</td><td>0.7</td><td>0.1</td><td>12.3</td><td>64.8</td><td>n.d.</td><td>19.4</td><td>2.3</td><td>0.2</td><td>0.3</td><td>99.5</td></tr><tr><td>-8.2</td><td>0.6</td><td>-10.0</td><td>0.8</td><td>-5.8</td><td>0.9</td><td>2.7</td><td>2.8</td><td>67.0</td><td>n.d.</td><td>26.4</td><td>1.2</td><td>0.2</td><td>0.2</td><td>100.5</td></tr><tr><td>-11.0</td><td>0.5</td><td>-14.9</td><td>0.7</td><td>-9.2</td><td>0.8</td><td>0.0</td><td>18.0</td><td>65.5</td><td>0.0</td><td>15.8</td><td>0.2</td><td>0.1</td><td>0.1</td><td>99.9</td></tr><tr><td>-12.7</td><td>0.4</td><td>-15.8</td><td>0.7</td><td>-9.2</td><td>0.8</td><td>0.0</td><td>17.0</td><td>66.1</td><td>0.1</td><td>16.7</td><td>0.2</td><td>0.1</td><td>0.1</td><td>100.4</td></tr><tr><td rowspan="3">Allende-07</td><td>A7-Ch05</td><td>-9.6</td><td>0.5</td><td>-11.0</td><td>0.8</td><td>-6.1</td><td>0.8</td><td>0.1</td><td>1.0</td><td>69.9</td><td>0.0</td><td>27.8</td><td>1.6</td><td>0.1</td><td>0.0</td><td>100.5</td></tr><tr><td>A7-Ch10</td><td>-9.1</td><td>0.7</td><td>-10.0</td><td>0.9</td><td>-5.3</td><td>1.0</td><td>0.1</td><td>3.1</td><td>69.8</td><td>0.0</td><td>26.4</td><td>0.6</td><td>0.3</td><td>0.2</td><td>100.7</td></tr><tr><td>-10.5</td><td>0.5</td><td>-12.4</td><td>0.8</td><td>-7.0</td><td>0.8</td><td>0.1</td><td>1.6</td><td>70.4</td><td>0.0</td><td>27.6</td><td>0.6</td><td>0.3</td><td>0.4</td><td>100.9</td></tr><tr><td rowspan="3">NWA10235-01</td><td>N1-Ch03</td><td>-10.0</td><td>0.5</td><td>-12.1</td><td>0.8</td><td>-6.9</td><td>0.9</td><td>0.1</td><td>1.6</td><td>70.8</td><td>0.0</td><td>27.7</td><td>0.4</td><td>0.3</td><td>0.4</td><td>101.4</td></tr><tr><td>N1-Ch21</td><td>1.3</td><td>0.5</td><td>-3.9</td><td>1.2</td><td>-4.6</td><td>1.2</td><td>0.2</td><td>2.5</td><td>68.9</td><td>0.6</td><td>26.6</td><td>0.8</td><td>0.2</td><td>0.2</td><td>100.1</td></tr><tr><td>-3.3</td><td>0.5</td><td>-8.2</td><td>0.7</td><td>-6.5</td><td>0.8</td><td>0.1</td><td>1.0</td><td>61.3</td><td>0.1</td><td>26.9</td><td>10.5</td><td>0.2</td><td>0.2</td><td>100.4</td></tr><tr><td rowspan="6">NWA10235-02</td><td>N1-Ch22</td><td>-5.2</td><td>0.6</td><td>-4.6</td><td>0.6</td><td>-1.9</td><td>0.7</td><td>0.1</td><td>0.9</td><td>61.5</td><td>0.1</td><td>27.0</td><td>10.6</td><td>0.2</td><td>0.2</td><td>100.7</td></tr><tr><td>N1-CAI01</td><td>-3.1</td><td>0.5</td><td>-7.5</td><td>0.8</td><td>-5.8</td><td>0.8</td><td>0.2</td><td>1.3</td><td>70.9</td><td>0.1</td><td>28.3</td><td>1.0</td><td>0.1</td><td>0.0</td><td>102.0</td></tr><tr><td>-4.4</td><td>0.5</td><td>-4.2</td><td>0.7</td><td>-1.9</td><td>0.7</td><td>0.1</td><td>0.9</td><td>61.5</td><td>0.1</td><td>27.0</td><td>10.6</td><td>0.2</td><td>0.2</td><td>100.7</td></tr><tr><td>-1.4</td><td>0.4</td><td>-6.3</td><td>0.6</td><td>-5.6</td><td>0.6</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>-6.8</td><td>0.4</td><td>-9.0</td><td>0.7</td><td>-5.4</td><td>0.8</td><td>0.1</td><td>1.4</td><td>70.7</td><td>0.0</td><td>28.0</td><td>0.7</td><td>0.2</td><td>0.2</td><td>101.4</td></tr><tr><td>-48.5</td><td>0.6</td><td>-49.9</td><td>0.7</td><td>-24.7</td><td>0.8</td><td>0.1</td><td>0.1</td><td>71.1</td><td>0.2</td><td>28.5</td><td>0.3</td><td>0.5</td><td>0.3</td><td>101.2</td></tr><tr><td rowspan="3">NWA10235-02</td><td>-50.0</td><td>0.4</td><td>-51.3</td><td>1.0</td><td>-25.3</td><td>1.0</td><td>0.0</td><td>0.0</td><td>71.1</td><td>0.1</td><td>28.6</td><td>0.3</td><td>0.2</td><td>0.3</td><td>100.8</td></tr><tr><td>-43.2</td><td>0.5</td><td>-44.6</td><td>0.9</td><td>-22.2</td><td>1.0</td><td>0.0</td><td>0.1</td><td>71.0</td><td>0.1</td><td>28.5</td><td>0.2</td><td>0.2</td><td>0.4</td><td>100.6</td></tr><tr><td>-48.2</td><td>0.5</td><td>-50.7</td><td>0.9</td><td>-25.6</td><td>0.9</td><td>0.9</td><td>0.6</td><td>69.7</td><td>0.8</td><td>28.1</td><td>0.3</td><td>0.8</td><td>0.3</td><td>101.5</td></tr><tr><td rowspan="10">NWA10235-08</td><td>N2-Ch33</td><td>-7.5</td><td>0.4</td><td>-9.0</td><td>0.7</td><td>-5.1</td><td>0.7</td><td>0.2</td><td>7.4</td><td>69.1</td><td>0.0</td><td>23.8</td><td>0.4</td><td>0.3</td><td>0.1</td><td>101.3</td></tr><tr><td>N2-Ch37</td><td>-9.3</td><td>0.6</td><td>-11.4</td><td>0.7</td><td>-6.5</td><td>0.8</td><td>0.2</td><td>0.6</td><td>70.8</td><td>0.0</td><td>28.4</td><td>0.7</td><td>0.3</td><td>0.1</td><td>100.6</td></tr><tr><td>N2-IO03</td><td>-9.1</td><td>0.5</td><td>-9.3</td><td>0.7</td><td>-4.6</td><td>0.7</td><td>0.1</td><td>0.9</td><td>70.1</td><td>0.0</td><td>28.3</td><td>1.0</td><td>0.2</td><td>0.3</td><td>101.1</td></tr><tr><td>N2-AOA02</td><td>-31.0</td><td>0.4</td><td>-32.7</td><td>0.6</td><td>-16.6</td><td>0.7</td><td>1.4</td><td>2.5</td><td>69.5</td><td>n.d.</td><td>25.7</td><td>0.2</td><td>0.9</td><td>0.2</td><td>100.6</td></tr><tr><td>N2-AOA03</td><td>-32.1</td><td>0.6</td><td>-33.1</td><td>0.9</td><td>-16.4</td><td>0.9</td><td>0.2</td><td>0.8</td><td>71.4</td><td>0.1</td><td>28.4</td><td>0.1</td><td>0.1</td><td>0.3</td><td>101.5</td></tr><tr><td>-36.0</td><td>0.5</td><td>-37.1</td><td>0.6</td><td>-18.4</td><td>0.6</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td></tr><tr><td>-38.8</td><td>0.5</td><td>-40.3</td><td>0.9</td><td>-20.2</td><td>0.9</td><td>0.6</td><td>1.1</td><td>69.5</td><td>0.7</td><td>27.9</td><td>0.6</td><td>1.0</td><td>0.3</td><td>101.7</td></tr><tr><td>N2-AOA08</td><td>-5.4</td><td>0.5</td><td>-8.0</td><td>0.7</td><td>-5.2</td><td>0.7</td><td>0.1</td><td>0.7</td><td>69.8</td><td>0.0</td><td>28.1</td><td>1.4</td><td>0.2</td><td>0.1</td><td>100.5</td></tr><tr><td>N8-Ch01</td><td>-3.3</td><td>0.5</td><td>-5.5</td><td>0.8</td><td>-3.7</td><td>0.9</td><td>0.2</td><td>0.7</td><td>68.4</td><td>0.0</td><td>27.6</td><td>1.1</td><td>0.3</td><td>0.3</td><td>98.6</td></tr><tr><td rowspan="3">N8-Ch02</td><td>-4.2</td><td>0.4</td><td>-6.8</td><td>0.5</td><td>-4.7</td><td>0.5</td><td>0.2</td><td>0.8</td><td>69.6</td><td>n.d.</td><td>27.1</td><td>1.1</td><td>0.3</td><td>0.2</td><td>99.4</td></tr><tr><td>-9.7</td><td>0.4</td><td>-9.9</td><td>0.8</td><td>-4.9</td><td>0.8</td><td>0.1</td><td>0.6</td><td>70.4</td><td>0.0</td><td>28.2</td><td>0.8</td><td>0.3</td><td>0.2</td><td>100.8</td></tr><tr><td>-1.3</td><td>0.6</td><td>-5.4</td><td>0.9</td><td>-4.7</td><td>1.0</td><td>0.1</td><td>1.6</td><td>69.6</td><td>0.0</td><td>27.2</td><td>1.2</td><td>0.3</td><td>0.2</td><td>100.2</td></tr><tr><td rowspan="3">N8-Ch05</td><td>-41.9</td><td>0.5</td><td>-42.9</td><td>0.6</td><td>-21.1</td><td>0.7</td><td>0.0</td><td>1.4</td><td>71.0</td><td>0.1</td><td>27.7</td><td>0.1</td><td>0.2</td><td>0.3</td><td>100.8</td></tr><tr><td>-40.5</td><td>0.7</td><td>-42.9</td><td>0.8</td><td>-21.8</td><td>0.9</td><td>0.1</td><td>1.7</td><td>70.7</td><td>n.d.</td><td>27.7</td><td>0.2</td><td>0.1</td><td>0.2</td><td>100.8</td></tr></tbody></table>

n.d. = not determined.
<table><caption>Table 2. Oxygen isotopic (‰) and chemical compositions (wt%) of olivine grains in chondrules containing Mg-spinel grains. Because NiO and MnO content were systematically below the detection limit, these two elements are not displayed.</caption><thead><tr><th>Chondrite</th><th>Object#</th><th>δ<sup>18</sup>O<br>‰</th><th>2σ</th><th>δ<sup>17</sup>O<br>‰</th><th>2σ</th><th>Δ<sup>17</sup>O<br>‰</th><th>2σ</th><th>SiO<sub>2</sub><br>wt%</th><th>FeO<br>‰</th><th>Al<sub>2</sub>O<sub>3</sub><br>‰</th><th>CaO<br>‰</th><th>MgO<br>‰</th><th>Cr<sub>2</sub>O<sub>3</sub><br>‰</th><th>TiO<sub>2</sub><br>‰</th><th>Total<br>‰</th><th>Mg#</th></tr></thead><tbody><tr><td rowspan="10">Allende-01</td><td rowspan="10">A1-Ch02</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.0</td><td>1.9</td><td>0.1</td><td>0.4</td><td>54.4</td><td>0.2</td><td>0.2</td><td>98.2</td><td>95.62</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>40.9</td><td>6.2</td><td>0.0</td><td>0.2</td><td>52.3</td><td>0.0</td><td>0.0</td><td>99.7</td><td>86.79</td></tr><tr><td>-5.5</td><td>0.3</td><td>-8.4</td><td>0.7</td><td>-5.5</td><td>0.7</td><td>41.3</td><td>2.1</td><td>0.1</td><td>0.2</td><td>54.6</td><td>0.2</td><td>0.1</td><td>98.7</td><td>95.36</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.3</td><td>2.1</td><td>0.1</td><td>0.2</td><td>55.1</td><td>0.2</td><td>0.1</td><td>99.1</td><td>95.30</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.0</td><td>4.2</td><td>0.0</td><td>0.2</td><td>53.3</td><td>0.2</td><td>0.1</td><td>99.1</td><td>90.70</td></tr><tr><td>-5.5</td><td>0.3</td><td>-8.8</td><td>0.7</td><td>-6.0</td><td>0.7</td><td>41.3</td><td>2.6</td><td>0.1</td><td>0.2</td><td>54.7</td><td>0.1</td><td>0.1</td><td>99.3</td><td>94.14</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.1</td><td>3.6</td><td>0.2</td><td>0.5</td><td>53.1</td><td>0.1</td><td>0.2</td><td>98.7</td><td>92.03</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.2</td><td>1.8</td><td>0.0</td><td>0.2</td><td>55.2</td><td>0.1</td><td>0.1</td><td>98.7</td><td>96.02</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.4</td><td>1.4</td><td>0.0</td><td>0.2</td><td>55.1</td><td>0.3</td><td>0.1</td><td>98.5</td><td>96.92</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.1</td><td>3.1</td><td>0.0</td><td>0.2</td><td>54.0</td><td>0.1</td><td>0.1</td><td>98.7</td><td>93.05</td></tr><tr><td rowspan="10">Allende-03</td><td rowspan="10">A3-Ch01</td><td>-4.9</td><td>0.4</td><td>-8.0</td><td>0.4</td><td>-5.4</td><td>0.5</td><td>41.3</td><td>5.2</td><td>0.1</td><td>0.4</td><td>52.7</td><td>0.1</td><td>0.0</td><td>99.9</td><td>88.70</td></tr><tr><td>-5.9</td><td>0.4</td><td>-9.0</td><td>0.4</td><td>-5.9</td><td>0.5</td><td>41.8</td><td>2.2</td><td>0.2</td><td>0.4</td><td>55.1</td><td>0.2</td><td>0.1</td><td>100.0</td><td>95.01</td></tr><tr><td>-6.7</td><td>0.4</td><td>-9.4</td><td>0.4</td><td>-5.9</td><td>0.5</td><td>42.0</td><td>1.0</td><td>0.0</td><td>0.1</td><td>56.1</td><td>0.0</td><td>0.0</td><td>99.3</td><td>97.75</td></tr><tr><td>-6.1</td><td>0.4</td><td>-8.6</td><td>0.4</td><td>-5.4</td><td>0.5</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>—</td></tr><tr><td>-6.4</td><td>0.2</td><td>-9.0</td><td>0.3</td><td>-5.6</td><td>0.3</td><td>42.1</td><td>1.3</td><td>0.0</td><td>0.1</td><td>55.6</td><td>0.1</td><td>0.0</td><td>99.2</td><td>97.09</td></tr><tr><td>-6.4</td><td>0.3</td><td>-9.2</td><td>0.3</td><td>-5.9</td><td>0.3</td><td>41.8</td><td>0.6</td><td>0.3</td><td>0.4</td><td>55.6</td><td>0.1</td><td>0.0</td><td>99.2</td><td>98.56</td></tr><tr><td>-6.1</td><td>0.2</td><td>-8.7</td><td>0.3</td><td>-5.5</td><td>0.3</td><td>41.9</td><td>0.3</td><td>0.2</td><td>0.2</td><td>56.1</td><td>0.3</td><td>0.0</td><td>99.1</td><td>99.22</td></tr><tr><td>-6.7</td><td>0.3</td><td>-9.1</td><td>0.2</td><td>-5.6</td><td>0.3</td><td>42.2</td><td>0.4</td><td>0.0</td><td>0.0</td><td>56.6</td><td>0.1</td><td>0.0</td><td>99.4</td><td>99.04</td></tr><tr><td>-6.6</td><td>0.2</td><td>-8.7</td><td>0.3</td><td>-5.3</td><td>0.3</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>—</td></tr><tr><td rowspan="10">Allende-07</td><td rowspan="10">A7-Ch05</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.7</td><td>0.2</td><td>0.4</td><td>0.7</td><td>55.8</td><td>0.1</td><td>0.1</td><td>98.9</td><td>99.46</td></tr><tr><td>-8.1</td><td>0.2</td><td>-10.5</td><td>0.2</td><td>-6.3</td><td>0.3</td><td>41.7</td><td>0.2</td><td>0.4</td><td>0.7</td><td>55.7</td><td>0.1</td><td>0.1</td><td>98.6</td><td>99.47</td></tr><tr><td>-8.0</td><td>0.3</td><td>-10.9</td><td>0.3</td><td>-6.7</td><td>0.3</td><td>41.6</td><td>0.3</td><td>0.3</td><td>0.7</td><td>55.8</td><td>0.1</td><td>0.1</td><td>98.8</td><td>99.42</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.7</td><td>0.3</td><td>0.4</td><td>0.7</td><td>55.9</td><td>0.1</td><td>0.1</td><td>99.1</td><td>99.25</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.5</td><td>0.4</td><td>0.5</td><td>0.6</td><td>55.6</td><td>0.1</td><td>0.1</td><td>98.7</td><td>99.18</td></tr><tr><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.7</td><td>0.4</td><td>0.4</td><td>0.7</td><td>55.6</td><td>0.1</td><td>0.1</td><td>99.1</td><td>99.10</td></tr><tr><td>-5.5</td><td>0.5</td><td>-8.2</td><td>0.5</td><td>-5.3</td><td>0.5</td><td>41.7</td><td>0.4</td><td>0.1</td><td>0.1</td><td>56.5</td><td>0.1</td><td>0.0</td><td>98.9</td><td>99.14</td></tr><tr><td>-6.3</td><td>0.5</td><td>-8.1</td><td>0.4</td><td>-4.8</td><td>0.5</td><td>41.4</td><td>0.5</td><td>0.0</td><td>0.1</td><td>56.6</td><td>0.1</td><td>0.0</td><td>98.8</td><td>98.92</td></tr><tr><td>-5.9</td><td>0.4</td><td>-8.0</td><td>0.4</td><td>-5.0</td><td>0.5</td><td>41.4</td><td>0.7</td><td>0.0</td><td>0.2</td><td>56.2</td><td>0.3</td><td>0.1</td><td>99.1</td><td>98.35</td></tr><tr><td rowspan="10">NWA10235-01</td><td rowspan="10">N1-Ch03</td><td>7.8</td><td>0.5</td><td>3.1</td><td>0.4</td><td>-1.0</td><td>0.5</td><td>42.0</td><td>1.3</td><td>0.1</td><td>0.7</td><td>55.3</td><td>0.2</td><td>0.0</td><td>99.8</td><td>97.11</td></tr><tr><td>3.5</td><td>0.5</td><td>-0.1</td><td>0.5</td><td>-1.9</td><td>0.6</td><td>41.9</td><td>1.2</td><td>0.2</td><td>0.8</td><td>55.0</td><td>0.2</td><td>0.0</td><td>99.5</td><td>97.16</td></tr><tr><td>4.1</td><td>0.5</td><td>0.7</td><td>0.5</td><td>-1.4</td><td>0.6</td><td>42.1</td><td>1.1</td><td>0.2</td><td>0.8</td><td>55.3</td><td>0.2</td><td>0.1</td><td>99.6</td><td>97.54</td></tr><tr><td>5.2</td><td>0.5</td><td>1.4</td><td>0.5</td><td>-1.3</td><td>0.6</td><td>41.4</td><td>1.1</td><td>0.2</td><td>1.5</td><td>54.6</td><td>0.2</td><td>0.1</td><td>99.1</td><td>97.44</td></tr><tr><td>4.1</td><td>0.5</td><td>0.6</td><td>0.5</td><td>-1.5</td><td>0.6</td><td>41.1</td><td>1.3</td><td>0.2</td><td>3.1</td><td>53.6</td><td>0.2</td><td>0.1</td><td>99.6</td><td>97.05</td></tr><tr><td>5.5</td><td>0.6</td><td>-2.3</td><td>0.8</td><td>-5.1</td><td>0.9</td><td>42.2</td><td>0.6</td><td>0.2</td><td>0.7</td><td>55.9</td><td>0.1</td><td>0.1</td><td>99.9</td><td>98.58</td></tr><tr><td>2.8</td><td>0.5</td><td>-3.7</td><td>0.7</td><td>-5.2</td><td>0.7</td><td>41.9</td><td>0.7</td><td>0.2</td><td>0.6</td><td>55.5</td><td>0.2</td><td>0.1</td><td>99.2</td><td>98.42</td></tr><tr><td>2.3</td><td>0.6</td><td>-4.2</td><td>0.6</td><td>-5.3</td><td>0.7</td><td>41.4</td><td>0.6</td><td>0.6</td><td>0.7</td><td>55.1</td><td>0.2</td><td>0.2</td><td>98.8</td><td>98.53</td></tr></tbody></table>
<table><caption>Table 2. *Continued*. Oxygen isotopic (‰) and chemical compositions (wt%) of olivine grains in chondrules containing Mg-spinel grains. Because NiO and MnO content were systematically below the detection limit, these two elements are not displayed.</caption><thead><tr><th>Chondrite</th><th>Object#</th><th>δ<sup>18</sup>O<br>‰</th><th>2σ</th><th>δ<sup>17</sup>O</th><th>2σ</th><th>Δ<sup>17</sup>O</th><th>2σ</th><th>SiO<sub>2</sub><br>wt%</th><th>FeO</th><th>Al<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>MgO</th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>Total</th><th>Mg#</th></tr></thead><tbody><tr><td rowspan="7">NWA10235-02</td><td>N2-Ch33</td><td>-5.0</td><td>0.5</td><td>-6.4</td><td>0.3</td><td>-3.7</td><td>0.4</td><td>41.9</td><td>0.2</td><td>0.3</td><td>0.7</td><td>55.9</td><td>0.1</td><td>0.1</td><td>99.2</td><td>99.43</td></tr><tr><td></td><td>-4.4</td><td>0.5</td><td>-6.8</td><td>0.3</td><td>-4.6</td><td>0.4</td><td>41.8</td><td>0.3</td><td>0.3</td><td>0.8</td><td>56.1</td><td>0.1</td><td>0.1</td><td>99.6</td><td>99.29</td></tr><tr><td></td><td>-5.7</td><td>0.5</td><td>-7.1</td><td>0.3</td><td>-4.2</td><td>0.4</td><td>42.0</td><td>0.2</td><td>0.2</td><td>0.7</td><td>56.1</td><td>0.1</td><td>0.1</td><td>99.5</td><td>99.54</td></tr><tr><td>N2-Ch37</td><td>-6.2</td><td>0.5</td><td>-9.4</td><td>0.3</td><td>-6.2</td><td>0.4</td><td>41.9</td><td>0.3</td><td>0.3</td><td>0.8</td><td>55.8</td><td>0.1</td><td>0.1</td><td>99.4</td><td>99.26</td></tr><tr><td></td><td>-6.7</td><td>0.5</td><td>-9.6</td><td>0.3</td><td>-6.1</td><td>0.4</td><td>41.9</td><td>0.4</td><td>0.1</td><td>0.4</td><td>56.2</td><td>0.1</td><td>0.0</td><td>99.2</td><td>99.06</td></tr><tr><td></td><td>-5.5</td><td>0.5</td><td>-9.2</td><td>0.3</td><td>-6.4</td><td>0.4</td><td>42.0</td><td>0.2</td><td>0.4</td><td>0.6</td><td>56.4</td><td>0.1</td><td>0.1</td><td>99.9</td><td>99.46</td></tr><tr><td>N2-IO03</td><td>-2.6</td><td>0.5</td><td>-6.0</td><td>0.3</td><td>-4.7</td><td>0.4</td><td>41.7</td><td>0.7</td><td>0.3</td><td>0.6</td><td>56.0</td><td>0.1</td><td>0.1</td><td>99.5</td><td>98.49</td></tr><tr><td rowspan="7">NWA10235-08</td><td>N8-Ch01</td><td>-2.5</td><td>0.5</td><td>-5.6</td><td>0.3</td><td>-4.2</td><td>0.4</td><td>41.9</td><td>0.7</td><td>0.3</td><td>0.6</td><td>55.9</td><td>0.2</td><td>0.1</td><td>99.7</td><td>98.35</td></tr><tr><td></td><td>-2.0</td><td>0.5</td><td>-5.5</td><td>0.3</td><td>-4.4</td><td>0.4</td><td>41.7</td><td>0.7</td><td>0.3</td><td>0.6</td><td>56.1</td><td>0.1</td><td>0.1</td><td>99.6</td><td>98.44</td></tr><tr><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>42.2</td><td>1.6</td><td>0.1</td><td>0.4</td><td>55.7</td><td>0.3</td><td>0.1</td><td>100.5</td><td>96.51</td></tr><tr><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>42.2</td><td>0.7</td><td>0.1</td><td>0.4</td><td>56.0</td><td>0.3</td><td>0.1</td><td>99.9</td><td>98.44</td></tr><tr><td></td><td>-5.1</td><td>0.5</td><td>-7.1</td><td>0.3</td><td>-4.5</td><td>0.4</td><td>41.8</td><td>2.2</td><td>0.1</td><td>0.3</td><td>55.0</td><td>0.3</td><td>0.1</td><td>100.0</td><td>95.04</td></tr><tr><td></td><td>-5.0</td><td>0.5</td><td>-7.1</td><td>0.3</td><td>-4.5</td><td>0.4</td><td>42.0</td><td>0.5</td><td>0.2</td><td>0.5</td><td>56.0</td><td>0.2</td><td>0.1</td><td>99.6</td><td>98.81</td></tr><tr><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>42.1</td><td>0.6</td><td>0.1</td><td>0.3</td><td>56.0</td><td>0.3</td><td>0.0</td><td>99.6</td><td>98.72</td></tr><tr><td rowspan="4">N8-Ch02</td><td></td><td>-2.9</td><td>0.5</td><td>-6.7</td><td>0.3</td><td>-5.2</td><td>0.4</td><td>41.8</td><td>0.6</td><td>0.1</td><td>0.5</td><td>56.1</td><td>0.3</td><td>0.1</td><td>99.7</td><td>98.57</td></tr><tr><td></td><td>-2.3</td><td>0.5</td><td>-7.0</td><td>0.3</td><td>-5.8</td><td>0.4</td><td>42.0</td><td>1.0</td><td>0.2</td><td>0.3</td><td>56.0</td><td>0.5</td><td>0.1</td><td>100.2</td><td>97.69</td></tr><tr><td></td><td>-2.8</td><td>0.5</td><td>-6.9</td><td>0.3</td><td>-5.4</td><td>0.4</td><td>42.1</td><td>0.4</td><td>0.2</td><td>0.7</td><td>56.3</td><td>0.1</td><td>0.1</td><td>100.0</td><td>99.00</td></tr><tr><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.8</td><td>0.4</td><td>0.2</td><td>0.7</td><td>55.8</td><td>0.1</td><td>0.1</td><td>99.1</td><td>99.17</td></tr><tr><td rowspan="3">N8-Ch05</td><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>41.7</td><td>0.3</td><td>0.2</td><td>0.7</td><td>55.8</td><td>0.1</td><td>0.1</td><td>98.9</td><td>99.20</td></tr><tr><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>42.0</td><td>0.3</td><td>0.2</td><td>0.7</td><td>56.0</td><td>0.1</td><td>0.1</td><td>99.4</td><td>99.21</td></tr><tr><td></td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>42.0</td><td>0.3</td><td>0.2</td><td>0.7</td><td>56.0</td><td>0.1</td><td>0.1</td><td>99.4</td><td>99.21</td></tr></tbody></table>

n.a. = not analyzed.
<figure><img src="image_7.png" /></figure>

Fig. 6. a) Oxygen isotopic composition of spinel grains within different objects: CAIs (orange), AOAs (red), PO chondrules (blue), BO chondrules (dark green), IOGs (light green), and ISPs (purple). Data are from this study only. Error bars on the measurements are smaller than the symbol size. The terrestrial fractionation line (cyan) and primary chondrule minerals line (PCM, red; Ushikubo et al., 2012) are shown for reference. Spinels in CAIs are highly enriched in $^{16}$O ($\delta^{18}$O = -50 to -40‰), as is typical for CAIs, whereas those from AOAs display a wider range of values, but still plot along the PCM. In contrast, all spinel grains from chondrules in this study are relatively enriched in heavy isotopes. Two measurements in an ISP (ISP-01, NWA 10235, section N1; see Figs. 2e and 2f) unexpectedly show values depleted in $^{16}$O, similar to spinels from chondrules. b) Oxygen isotopic compositions of olivine (triangles) and spinel grains (circles) within NWA 10235 (orange and yellow, respectively) and Allende (dark and light gray, respectively). Error bars of the measurements are smaller than the symbols. Spinel and olivine grains show broadly similar oxygen isotopic compositions plotting along the PCM line, although those in NWA 10235 span a larger range of values ($\delta^{18}$O = -10 to +10 ‰) than those in Allende ($\delta^{18}$O = -12 to -5 ‰). In the same chondrite, spinel grains are systematically slightly enriched in $^{16}$O. (Color figure can be viewed at wileyonlinelibrary.com.)

at 600 °C; Posner et al., 2016). These results imply that chondrule Mg-spinels were not inherited from refractory precursors but are of magmatic origin, resulting from

<figure><img src="image_8.png" /></figure>

Fig. 7. Bulk $\Delta^{17}$O values of olivines (blue) and spinels (orange) within individual studied chondrules. Most olivines show $\Delta^{17}$O values that overlap those of coexisting spinels, indicating their common origin. Asterisks (*) indicate chondrules in which only one spinel was measured; for those chondrules, the error bars represent analytical errors instead of the standard deviation. Two separate spinel values are provided for BO chondrule N1-Ch21 because we subdivided those spinels according to their oxygen isotopic compositions (see Fig. 6; Table 1). Although we did not measure olivine in chondrule N8-Ch05, the spinel value is given for comparison to the other chondrules. (Color figure can be viewed at wileyonlinelibrary.com.)

the crystallization of the host chondrule melt. Of note, the oxygen isotopic compositions of Mg-spinel grains within the BO chondrule define two groups with $\Delta^{17}$O ≈ -6‰ and -2‰, the latter being similar to those of BO olivines (Fig. 7; Tables 1–3). Interestingly, the former, $^{16}$O-rich Mg-spinels in the BO chondrule, have O isotopic compositions similar to those of olivine and spinel in porphyritic chondrules of CV chondrites, thus suggesting that the BO chondrule could have been inherited from a previous generation of chondrules.

## Implications for the Thermal History of Chondrules

Knowing the thermal history of chondrules is fundamental to constraining the environment and conditions under which they formed. Their thermal history is characterized by three main features: heating, peak temperature, and cooling rate. The initial heating event of chondrules remains controversial, but most authors agree that a flash heating event brought the precursor materials from their ambient to peak temperature within several seconds to a few minutes (Connolly et al., 1998; Jones et al., 2018; Tachibana & Huss, 2005). In contrast, the peak temperatures experienced by chondrules are well constrained by phase liquidus temperatures and the necessity of preserving crystal nuclei during heating. Thus, peak temperatures ranging from 1200 to 2000 °C have been proposed depending on chondrule texture and chemistry (see
<figure><img src="image_9.png" /><figcaption>Fig. 8. a) $\Delta^{17}\text{O}$ values of spinel grains in CAIs (orange), AOAs (red), PO chondrules (blue), BO chondrules (dark green), IOGs (light green), and ISPs (purple) plotted against their $Cr_2O_3$ contents. Chondrule spinels are characterized by higher $Cr_2O_3$ contents but consistently $^{17,18}\text{O}$-rich isotopic compositions. AOA, ISP, and CAI spinels contain very little $Cr_2O_3$, but AOA and ISP spinels span a large range of isotopic compositions whereas CAI spinels have extremely low $\Delta^{17}\text{O}$ values. b) The distributions of Mg-spinel $Cr_2O_3$ contents in CAIs/AOAs (orange) and chondrules (blue). Mg-spinel in refractory inclusions has lower $Cr_2O_3$ contents than those in chondrules. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

reviews by Desch et al., 2012; Hewins et al., 2005; Jones et al., 2018; Lofgren, 1996). For carbonaceous chondrites containing mostly type I porphyritic chondrules, a peak temperature around 1600 °C is widely accepted. Interestingly, recent dynamic crystallization experiments mimicking chondrule formation from the melting of AOA-like assemblages showed that relict spinel is totally resorbed at temperature $\ge 1600$ °C (Whattam et al., 2021). As no inherited grain was observed within the type I chondrules we surveyed, this suggests they have experienced peak temperatures of at least 1600 °C.

Cooling rates are probably the most ambiguous among chondrules' thermal histories. Three main

cooling paths have been proposed so far: linear, nonlinear, and two-stage cooling models (see fig. 3.10 of Jones et al., 2018). Although linear cooling models ($100–1000$° C h$^{-1}$) can reproduce all chondrule textures, many chemical and petrographic observations require a more complex explanation. Nonlinear cooling models propose that fast cooling rates of $1000–10,000$ °C h$^{-1}$ at the onset of cooling slowed to $10–1000$ °C h$^{-1}$ at lower temperatures (around 1000 °C). These models are consistent with thermal histories inferred from shock wave models (Desch & Connolly, 2002; Desch et al., 2012; Morris & Desch, 2010). Whereas the crystallization of plagioclase and zoning profiles in
<figure><img src="image_10.png" /><figcaption>Fig. 9. Olivine–spinel O isotopic fractionations per chondrule. Most chondrules show positive fractionations (i.e., olivine being enriched in heavy isotopes relative to spinel) consistent with theoretical calculations (see Implications for the Thermal History of Chondrules section). However, chondrule N1-Ch03 displays a negative fractionation, and any conclusions about this specific chondrule must be taken cautiously and the calculated temperatures might be wrong. Similarly, the population of spinel grains of N1-Ch21 with isotopic signature at $\Delta^{17}O \approx -6 \, \text{‰}$, which are not at the isotopic equilibrium with the surrounding olivine dendrites, cannot be used for the thermometry calculations. These two populations of spinel grains (N1-Ch03 and part of N1-Ch21 spinel) are highlighted with triangle symbols.</figcaption></figure>

metal grains, which require slow cooling rates of tens of °C h⁻¹, are consistent with these models (Chaumard & Devouard, 2016; Wick & Jones, 2012), the presence of a glassy mesostasis in chondrules is hardly reconcilable with slow cooling rates near glass transition temperatures (~800 °C). Two-stage cooling models propose that chondrules first cooled slowly at high temperature during a limited amount of time (minutes to hours), then cooled much faster (1000–10,000° C h⁻¹) at lower temperatures. These models can explain the preservation of relict olivine grains by preventing their dissolution in the chondrule melt, and are supported by

Fe-Mg interdiffusion profiles between relict and surrounding olivines (Soulié et al., 2017; Villeneuve et al., 2015). Two-stage models are also consistent with thermal histories inferred from planetesimal impact scenarios (e.g., Dullemond et al., 2016; Johnson et al., 2018).

The olivine–Mg-spinel mineral pair is promising for investigating chondrule thermal histories because an accurate geothermometer based on Al-Cr partitioning between olivine and spinel has been developed (Coogan et al., 2014; Wan et al., 2008). Another geothermometer exists based on thermodynamical calculations of oxygen isotopic fractionation, but it is only defined for temperatures of 0–1200 °C and requires that calculations for the quartz–olivine system be combined with calculations for the quartz–Mg-spinel system to determine the olivine–Mg-spinel fractionation (Zheng, 1991, 1993). These calculations predict positive $\delta^{18}O$ fractionations between olivine and Mg-spinel over the considered temperatures, which is in line with all our isotopic fractionation data (ranging from -0.7 ± 1 to +10.3 ± 1.8‰; Fig. 9; Table 3) except for one chondrule that displays a very negative value (N1–Ch03; -7.2 ± 0.7‰). However, the crystallization temperatures we obtain with this method are as low as 530 °C, clearly inconsistent with chondrule thermal histories. We therefore exclusively use the Al-Cr geothermometer, which is based on partitioning between forsteritic olivine and Mg-spinel, thus obviating the need to analyze the mesostasis, which is often altered in chondrules. This empirical geothermometer is given by:

$$T(^{\circ}\text{C}) = \frac{10,000}{0.575(0.162) + 0.884(0.043)\text{Cr}\# - 0.897(0.025)\ln(K_{\text{d}}) - 273.15},$$

where $K_{\text{d}} = \frac{\text{Al}_2\text{O}_{\text{olivine}}}{\text{Al}_2\text{O}_{\text{spinel}}^3}$, spinel Cr# = Cr/(Cr + Al) in atomic percent between 0 and 0.69 (values of geothermometer calibration), and the values in parentheses are the standard errors for each the parameters. Coogan et al. (2014) evaluated the temperature uncertainties to be ±22 °C.

It is therefore possible to calculate olivine–Mg-spinel crystallization temperatures for our 13 chondrules and one IOG (Fig. 10; Table 4). The low crystallization temperature obtained for chondrule N1-Ch03 (1092 ± 24 °C) is probably incorrect because the Al contents in its olivines (212 ppm atom%) are the lowest measured herein and close to the detection limit (100 ppm atom%); furthermore, the observed olivine–Mg-spinel $\delta^{18}O$ fractionation for this chondrule is negative, whereas a positive fractionation is expected
<table><caption>Table 3. Bulk oxygen isotopic compositions (‰) of olivine and Mg-spinel in chondrules. Chondrules in which only one spinel was measured are labeled with an “*”; reported errors for those chondrules are analytical and not standard deviations. We identified two populations of spinel in chondrule N1-Ch21 (“Spinel-1” and “Spinel-2”) based on their isotopic compositions.</caption><thead><tr><th>Chondrite</th><th>Object#</th><th>Mineral</th><th>n</th><th>δ<sup>18</sup>O<br>‰</th><th>2σ</th><th>δ<sup>17</sup>O<br>‰</th><th>2σ</th><th>Δ<sup>17</sup>O<br>‰</th><th>2σ</th><th>δ<sup>18</sup>O<sub>Ol</sub>–δ<sup>18</sup>O<sub>Sp</sub><br>‰</th><th>2σ</th></tr></thead><tbody><tr><td rowspan="2">Allende-01</td><td rowspan="2">A1-Ch02</td><td>Spinel</td><td>3</td><td>-11.2</td><td>0.7</td><td>-12.1</td><td>1.3</td><td>-6.2</td><td>1.1</td><td rowspan="2">5.7</td><td rowspan="2">0.8</td></tr><tr><td>Olivine</td><td>2</td><td>-5.5</td><td>0.3</td><td>-8.6</td><td>0.8</td><td>-5.7</td><td>0.8</td></tr><tr><td rowspan="2">Allende-03</td><td rowspan="2">A3-Ch01</td><td>Spinel</td><td>8</td><td>-7.8</td><td>0.8</td><td>-10.2</td><td>1.3</td><td>-6.1</td><td>1.2</td><td rowspan="2">1.9</td><td rowspan="2">1.1</td></tr><tr><td>Olivine</td><td>4</td><td>-5.9</td><td>0.6</td><td>-8.8</td><td>0.8</td><td>-5.7</td><td>0.6</td></tr><tr><td rowspan="3">Allende-07</td><td rowspan="2">A7-Ch05</td><td>Spinel*</td><td>1</td><td>-9.6</td><td>0.5</td><td>-11.0</td><td>0.8</td><td>-6.1</td><td>0.8</td><td rowspan="2">3.1</td><td rowspan="3">0.6</td></tr><tr><td>Olivine</td><td>5</td><td>-6.4</td><td>0.3</td><td>-9.0</td><td>0.4</td><td>-5.6</td><td>0.4</td></tr><tr><td>A7-Ch10</td><td>3</td><td>-9.9</td><td>0.9</td><td>-11.5</td><td>1.6</td><td>-6.4</td><td>1.3</td><td>1.8</td><td>0.9</td></tr><tr><td rowspan="6">NWA10235-01</td><td rowspan="2">N1-Ch03</td><td>Spinel*</td><td>1</td><td>1.3</td><td>0.5</td><td>-3.9</td><td>1.2</td><td>-4.6</td><td>1.2</td><td rowspan="2">-7.2</td><td rowspan="6">0.8</td></tr><tr><td>Olivine</td><td>3</td><td>-5.9</td><td>0.5</td><td>-8.1</td><td>0.5</td><td>-5.0</td><td>0.6</td></tr><tr><td rowspan="2">N1-Ch21</td><td>Spinel-1</td><td>3</td><td>-2.6</td><td>1.0</td><td>-7.3</td><td>1.2</td><td>-6.0</td><td>0.9</td><td rowspan="2">7.5</td></tr><tr><td>Spinel-2</td><td>2</td><td>-4.8</td><td>0.8</td><td>-4.4</td><td>0.7</td><td>-1.9</td><td>0.7</td></tr><tr><td rowspan="2">N1-Ch22</td><td>Spinel*</td><td>1</td><td>-6.8</td><td>0.4</td><td>-9.0</td><td>0.7</td><td>-5.4</td><td>0.8</td><td rowspan="2">10.3</td></tr><tr><td>Olivine</td><td>3</td><td>3.5</td><td>1.4</td><td>-3.4</td><td>1.2</td><td>-5.2</td><td>0.9</td></tr><tr><td rowspan="6">NWA10235-02</td><td rowspan="2">N2-Ch33</td><td>Spinel</td><td>2</td><td>-7.0</td><td>0.8</td><td>-8.7</td><td>0.9</td><td>-5.0</td><td>0.8</td><td rowspan="2">2.0</td><td rowspan="6">1.0</td></tr><tr><td>Olivine</td><td>3</td><td>-5.0</td><td>0.7</td><td>-6.8</td><td>0.5</td><td>-4.2</td><td>0.6</td></tr><tr><td rowspan="2">N2-Ch37</td><td>Spinel*</td><td>1</td><td>-9.3</td><td>0.6</td><td>-11.4</td><td>0.7</td><td>-6.5</td><td>0.8</td><td rowspan="2">3.2</td></tr><tr><td>Olivine</td><td>3</td><td>-6.1</td><td>0.7</td><td>-9.4</td><td>0.3</td><td>-6.2</td><td>0.4</td></tr><tr><td rowspan="2">N2-IO03</td><td>Spinel*</td><td>1</td><td>-9.1</td><td>0.5</td><td>-9.3</td><td>0.7</td><td>-4.6</td><td>0.7</td><td rowspan="2">6.7</td></tr><tr><td>Olivine</td><td>3</td><td>-2.4</td><td>0.5</td><td>-5.7</td><td>0.5</td><td>-4.4</td><td>0.5</td></tr><tr><td rowspan="6">NWA10235-08</td><td rowspan="2">N8-Ch01</td><td>Spinel</td><td>3</td><td>-4.3</td><td>0.9</td><td>-6.8</td><td>1.5</td><td>-4.5</td><td>1.1</td><td rowspan="2">-0.7</td><td rowspan="6">1.0</td></tr><tr><td>Olivine</td><td>2</td><td>-5.0</td><td>0.5</td><td>-7.1</td><td>0.3</td><td>-4.5</td><td>0.4</td></tr><tr><td rowspan="2">N8-Ch02</td><td>Spinel*</td><td>1</td><td>-9.7</td><td>0.4</td><td>-9.9</td><td>0.8</td><td>-4.9</td><td>0.8</td><td rowspan="2">7.1</td></tr><tr><td>Olivine</td><td>3</td><td>-2.7</td><td>0.5</td><td>-6.9</td><td>0.3</td><td>-5.5</td><td>0.5</td></tr><tr><td rowspan="2">N8-Ch05</td><td>Spinel*</td><td>1</td><td>-1.3</td><td>0.6</td><td>-5.4</td><td>0.9</td><td>-4.7</td><td>1.0</td><td rowspan="2">—</td></tr><tr><td>Olivine</td><td>0</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr></tbody></table>

according to thermodynamic calculations (Zheng, 1991, 1993; Fig. 9). For the other chondrules and the IOG, we obtained olivine–Mg-spinel crystallization temperatures ranging from 1198 to 1637 °C with a distribution that peaks at ~1470 °C (Fig. 10; Table 4). This temperature range is below the liquidus estimated for type I carbonaceous chondrite chondrules (>1600 °C; Jones et al., 2018) and consistent with the notion that porphyritic chondrules were heated to subliquidus peak temperatures, thus preserving crystal nucleation sites (Desch et al., 2012; Hewins et al., 2005; Jones et al., 2018; Radomsky & Hewins, 1990). Because olivine is the most abundant constituent of type I chondrules in carbonaceous chondrites, our results suggest that these chondrules spent the majority of their high-temperature history at temperatures near 1470 °C (±130 °C, 1 SD) and cooled sufficiently slowly (slower than a few hundreds of °C h⁻¹) to form large, chemically homogeneous, and porphyritic olivine crystals. These results are consistent with nonlinear and

two-stage cooling models, provided that cooling rates are sufficiently low at 1350–1600 °C.

## CONCLUSIONS

We surveyed six sections of two CV chondrites (NWA 10235 and Allende) and found that ~10% of type I chondrules contain Mg-spinel. We combined quantitative in situ high-current electron microprobe measurements and secondary ion mass spectrometry oxygen isotopic analyses of Mg-spinel and olivine grains within these chondrules. Our main results are:

1) Chondrule spinels show euheudral textures and are either surrounded by glassy mesostasis or poikilitically enclosed in olivine crystals while in direct contact with a layer of glass (i.e., a quenched melt pocket).

2) Chondrule spinels correspond to the Mg-endmember of the spinel group (MgAl₂O₄). Although they contain various amounts of Cr₂O₃
<figure><img src="image_11.png" /><figcaption>Fig. 10. Olivine–Mg-spinel crystallization temperatures obtained using the Al-Cr distribution geothermometer (Coogan et al., 2014; Wan et al., 2008). The temperature calculated for chondrule N1-Ch03 (triangle) is not included in the probability distribution function (dashed curve) because its δ¹⁸O fractionation is opposite to that in the other chondrules and predicted by theoretical calculations (see Implications for the Thermal History of Chondrules section).</figcaption></figure>

<table><caption>Table 4. Olivine–Mg-spinel cocrystallization temperatures calculated using the Al-Cr distribution geothermometer as updated by Coogan et al. (2014).</caption><thead><tr><th rowspan="2">Object#</th><th colspan="4">Spinel</th><th colspan="6">Olivine</th></tr><tr><th>n</th><th>Al<sub>2</sub>O<sub>3</sub> (wt%)</th><th>2σ</th><th>Cr<sub>2</sub>O<sub>3</sub> (wt%)</th><th>2σ</th><th>n</th><th>Al<sub>2</sub>O<sub>3</sub> (wt%)</th><th>2σ</th><th>Cr<sub>2</sub>O<sub>3</sub> (wt%)</th><th>2σ</th><th>T (°C)</th><th>2σ</th></tr></thead><tbody><tr><td>N1-Ch03</td><td>1</td><td>68.93</td><td>0.33</td><td>0.82</td><td>0.01</td><td>3</td><td>0.04</td><td>0.01</td><td>0.19</td><td>0.03</td><td>1092</td><td>24</td></tr><tr><td>A1-Ch02</td><td>3</td><td>66.73</td><td>0.33</td><td>2.38</td><td>0.01</td><td>10</td><td>0.07</td><td>0.01</td><td>0.15</td><td>0.01</td><td>1198</td><td>28</td></tr><tr><td>A3-Ch01</td><td>8</td><td>68.21</td><td>0.33</td><td>1.32</td><td>0.01</td><td>3</td><td>0.09</td><td>0.01</td><td>0.12</td><td>0.01</td><td>1258</td><td>30</td></tr><tr><td>A7-Ch05</td><td>1</td><td>69.93</td><td>0.33</td><td>1.56</td><td>0.01</td><td>4</td><td>0.15</td><td>0.02</td><td>0.14</td><td>0.01</td><td>1356</td><td>34</td></tr><tr><td>N8-Ch01</td><td>3</td><td>69.27</td><td>0.33</td><td>1.22</td><td>0.01</td><td>5</td><td>0.16</td><td>0.02</td><td>0.27</td><td>0.03</td><td>1381</td><td>35</td></tr><tr><td>N1-Ch21</td><td>1</td><td>61.46</td><td>0.33</td><td>10.62</td><td>0.01</td><td>5</td><td>0.18</td><td>0.03</td><td>0.21</td><td>0.03</td><td>1407</td><td>36</td></tr><tr><td>N8-Ch02</td><td>1</td><td>70.42</td><td>0.33</td><td>0.80</td><td>0.01</td><td>3</td><td>0.20</td><td>0.03</td><td>0.29</td><td>0.44</td><td>1435</td><td>37</td></tr><tr><td>N8-Ch05</td><td>1</td><td>69.55</td><td>0.33</td><td>1.15</td><td>0.01</td><td>3</td><td>0.22</td><td>0.03</td><td>0.09</td><td>0.01</td><td>1468</td><td>38</td></tr><tr><td>N2-Ch33</td><td>2</td><td>69.78</td><td>0.33</td><td>0.43</td><td>0.01</td><td>3</td><td>0.24</td><td>0.03</td><td>0.07</td><td>0.01</td><td>1496</td><td>40</td></tr><tr><td>N2-Ch37</td><td>1</td><td>70.75</td><td>0.33</td><td>0.66</td><td>0.01</td><td>3</td><td>0.28</td><td>0.04</td><td>0.09</td><td>0.01</td><td>1529</td><td>41</td></tr><tr><td>N2-IO03</td><td>1</td><td>70.14</td><td>0.33</td><td>0.99</td><td>0.01</td><td>3</td><td>0.28</td><td>0.04</td><td>0.14</td><td>0.01</td><td>1530</td><td>41</td></tr><tr><td>N1-Ch22</td><td>1</td><td>70.75</td><td>0.33</td><td>0.70</td><td>0.01</td><td>3</td><td>0.33</td><td>0.05</td><td>0.17</td><td>0.01</td><td>1580</td><td>44</td></tr><tr><td>A7-Ch10</td><td>3</td><td>70.36</td><td>0.33</td><td>0.53</td><td>0.01</td><td>6</td><td>0.39</td><td>0.05</td><td>0.09</td><td>0.01</td><td>1637</td><td>46</td></tr></tbody></table>

(0.4–2.8 wt%), they are systematically enriched in $Cr_2O_3$ compared to Mg-spinels in refractory inclusions.

3) The $\Delta^{17}O$ values of chondrule Mg-spinels differ significantly from those of Mg-spinels in refractory inclusions but are similar to those of coexisting olivine.

From these observations, we conclude that Mg-spinels in chondrules are not inherited from refractory inclusions, but instead correspond to magmatic minerals that crystallized alongside chondrule olivines. Consequently, we applied the olivine-spinel

geothermometer based on the Al-Cr distribution between the two minerals. We estimate that olivine–Mg-spinel cocrystallization occurred between 1200 and 1640 °C (mean = 1470 °C), and thus below the estimated liquidus temperature of type I carbonaceous chondrite chondrules (~1600 °C). This is consistent with models in which chondrules were heated to subliquidus temperatures for long enough to produce porphyritic textures. Furthermore, this implies relatively slow cooling rates (slower than a few hundreds of °C h⁻¹), consistent with nonlinear or two-stage chondrule cooling paths.
*Acknowledgments*—Nordine Bouden is thanked for technical assistance with SIMS measurements. This research was founded by l'Agence Nationale de la Recherche (ANR) through grant CASSYSS ANR-18-CE31-0010-01 (PI Johan Villeneuve). We thank Samuel Ebert and Daisuke Nagashima for helpful reviews, and Timothy Jull for editorial handling. This is CRPG contribution #2793.

*Data availability statement*—The data that support the findings of this study are openly available on the Ordar database: https://doi.org/10.24396/ORDAR-100.

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

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## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article. Supplementary Information

Table S1. Oxygen isotopes and chemistry of Mg-spinel.

Table S2. Oxygen isotopes and chemistry of olivine.

Table S3. Bulk oxygen isotopic composition of Mg-spinel and olivine in chondrules.

Table S4. Olivine-Mg-spinel co-crystallization temperatures.