# Chromium Isotopic Evidence for an Early Formation of Chondrules from the Ornans CO Chondrite

Ke Zhu, Jia Liu, Frédéric Moynier, Liping Qin, Conel Alexander, Yongsheng He

► To cite this version:

Ke Zhu, Jia Liu, Frédéric Moynier, Liping Qin, Conel Alexander, et al.. Chromium Isotopic Evidence for an Early Formation of Chondrules from the Ornans CO Chondrite. The Astrophysical Journal, 2019, 873 (1), pp.82. 10.3847/1538-4357/aafe79. insu-02916997

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# Chromium Isotopic Evidence for an Early Formation of Chondrules from the Ornans CO Chondrite

Ke Zhu (朱柯)<sup>1,2</sup>①, Jia Liu<sup>1</sup>①, Frédéric Moynier<sup>2,3</sup>, Liping Qin<sup>1,4</sup>, Conel M. O'D. Alexander<sup>5</sup>①, and Yongsheng He<sup>4</sup>

¹ CAS Key Laboratory of Crust-Mantle Materials and Environment and CAS Center for Excellence in Comparative Planetology, School of Earth and Space Science, University of Science and Technology of China, Hefei 230026, People's Republic of China; lpqin@ustc.edu.cn

² Institut de Physique du Globe de Paris, Université Sorbonne Paris Cité, CNRS, 1 rue Jussieu, Paris F-75005, France

³ Institut Universitaire de France, Paris, France

⁴ State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Beijing), Beijing 100083, People's Republic of China

⁵ Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, Washington, DC 20015, USA

Received 2018 June 27; revised 2018 December 28; accepted 2019 January 8; published 2019 March 6

## Abstract

Chondrules are the main components of primitive meteorites and possibly the building blocks of planetary embryos and terrestrial planets. However, their ages and modes of formation are still highly debated. Here, we present high-precision Cr isotope data of nine chondrules from one of the more primitive chondrites, the CO3 chondrite Ornans. These chondrules define an external $^{53}$Mn-$^{53}$Cr isochron, with an initial $^{53}$Mn/$^{55}$Mn of (7.1 ± 1.6) × 10$^{-6}$, corresponding to an age of 4567.6 ± 1.3 Ma when anchored to the angrite D'Orbigny (U-corrected). This age is within error of the age of formation of calcium-aluminum-rich inclusions (CAIs). All chondrules show a wide range of ε$^{54}$Cr values (+0.20 to +1.22) and a positive correlation between ε$^{53}$Cr and ε$^{54}$Cr values, suggesting mixing of different isotopic sources in the protoplanetary disk. This could reflect that silicate materials from the CAI-forming region (with complementary compositions to CAIs, i.e., low Mn/Cr and ε$^{54}$Cr) were transported to the accretion region of the CO chondrite parent body and mixed with CI-like material (high-Mn/Cr and ε$^{54}$Cr) during chondrule formation. Such mixing must have occurred prior to the formation of chondrule precursors. Furthermore, chondrules from chondrites with more CAIs (CV and CO) exhibit greater variability in ε$^{54}$Cr than chondrules from chondrites formed later with fewer CAIs (e.g., CB and CR), suggesting that the accretion regions of the former received more material transported from the inner solar system than the latter. This dichotomy may indicate the CB and CR chondrites accreted at greater orbital distances than other chondrites.

**Key words:** astrochemistry – meteorites, meteors, meteoroids – nuclear reactions, nucleosynthesis, abundances

## 1. Introduction

Chondrules are millimeter to submillimeter-sized spheres with igneous textures that are believed to have formed as molten droplets during transient heating (up to 2000 K) and rapid cooling (~10–1000 K/h) events in the solar protoplanetary disk (Hewins 1997; Krot & Bizzarro 2009). Chondrules are significant components (up to 80 wt.%) of chondrites and comprise ferromagnesian silicates (olivine and pyroxene), Fe-Ni metal, and glassy or microcrystalline mesostasis (Hewins 1997; Krot & Bizzarro 2009). As they are among the oldest solids in the solar system (Connelly et al. 2012; Bollard et al. 2017), chondrules are important records of the chemical and thermal evolution of the early solar system (Mahan et al. 2018).

Long-lived and short-lived radiogenic systems have been combined to constrain the formation ages of chondrules. U-Pb chronometry of large chondrules suggests that chondrule formation started simultaneously with calcium-aluminum-rich inclusions (CAIs) and lasted for ~4 Myr (Amelin et al. 2002; Connelly et al. 2008, 2012; Connelly & Bizzarro 2009; Bollard et al. 2015, 2017). On the other hand, $^{26}$Al-$^{26}$Mg short-lived chronometry (half-life of 0.73 Myr), suggests that chondrule precursors (dated from bulk chondrules) formed about 1.5 Myr after CAIs (Luu et al. 2015), and the last melting event (dated from internal isochrons) occurred ~1–4 Ma after the formation of CAIs (Kurahashi et al. 2008; Villeneuve et al. 2009; Kita et al. 2013; Nagashima et al. 2017; Schrader et al. 2017). However, if the initial $^{26}$Al/$^{27}$Al ratio in the solar system was heterogeneously distributed (Larsen et al. 2011; Schiller et al. 2015;

Van Kooten et al. 2016), then $^{26}$Al-$^{26}$Mg chronometry becomes ambiguous. On the other hand, the good agreement between the $^{207}$Pb-$^{206}$Pb and $^{53}$Mn-$^{53}$Cr ages of Gujba CB chondrules (Yamashita et al. 2010; Bollard et al. 2015), angrites (Kleine et al. 2012), carbonaceous achondrite (Amelin et al. 2019; Sanborn et al. 2019), as well as in Allende CV chondrules (Yin et al. 2009) supports an initially homogeneous distribution of the $^{53}$Mn/$^{55}$Mn ratio in the solar system. As such, the $^{53}$Mn-$^{53}$Cr chronometer ($T_{1/2}$ = 3.7 ± 0.2 Myr, Holden 1990) is suitable and robust for dating early solar system events (Lugmair & Shukolyukov 1998; Moynier et al. 2007; Qin et al. 2010b). Since Mn is more volatile than Cr during condensation of the nebular gas (Lodders 2003), the Mn-Cr system records the age of volatile depletion processes in the solar nebula.

However, CM- and CR-type chondrites have been subjected to secondary alteration, which could have re-distributed Cr into secondary minerals, such as carbonates, that formed in the meteorite parent bodies after chondrule formation (Jilly-Rehak et al. 2017) and thereby resetting the Mn-Cr system. The CV3 and CO3 meteorites are among the least altered and metamorphosed carbonaceous chondrites and are, therefore, the most appropriate samples to date chondrules. The Mn-Cr systematics of bulk chondrules from the CV3 Allende have been reported in one conference abstract (Yin et al. 2009) and give an age of ~4567.6 Ma, similar to the oldest Pb-Pb ages of chondrules from the same meteorite (Connelly et al. 2012). However, since Allende (CV3.6) underwent hydrothermal metamorphism at ~600°C (Cody et al. 2008), it is unclear how faithfully its chondrules record their formation ages. On
the other hand, while CO3 chondrites largely escaped aqueous alteration (Krot et al. 2014), there are presently no studies of Mn–Cr isotope ratios in CO3 chondrite chondrules.

A general difficulty associated with the isotopic analysis of chondrules from CO chondrites lies in their small size, among the smallest of any chondrite group (diameter <0.15 mm, Krot et al. 2014). However, since Cr is relatively abundant in chondrules (~3000 ppm), the Mn–Cr system can be applied to a set of small chondrules. Furthermore, previous work on ordinary chondrite (OC), CB and CV chondrules have shown that the Mn/Cr ratio varies to an extent that permits definition of high-precision external isochrons (Nyquist et al. 2001; Yin et al. 2007, 2009; Yamashita et al. 2010).

The non-mass dependent variations of the $^{54}$Cr/$^{52}$Cr ratio (reported here as $\varepsilon^{54}$Cr, the deviation in parts per 10000 of the $^{54}$Cr/$^{52}$Cr ratio normalized to the $^{50}$Cr/$^{52}$Cr ratio) can be used to trace the origins of solar system materials (Trinquier et al. 2007; Qin et al. 2010a; Olsen et al. 2016; Qin & Wang 2017). This is because inner solar system materials, such as OCs and differentiated meteorites (e.g., iron meteorites, HEDs, ureilites, angrites), are $^{54}$Cr-poor compared to the Earth, the Moon, and enstatite chondrites (ECs). These three reservoirs all have compositions similar to one another (Trinquier et al. 2007; Qin et al. 2010b; Larsen et al. 2011; Warren 2011; Göpel et al. 2015; Mougel et al. 2018). On the other hand, outer solar system materials such as the CCs are $^{54}$Cr-rich (following a $\varepsilon^{54}$Cr sequence: CK $\le$ CO $\le$ CV $<$ CM $\le$ CH $\le$ CB $<$ CR $<$ CI; Shukolyukov & Lugmair 2006; Trinquier et al. 2007; Qin et al. 2010a; Van Kooten et al. 2016).

Here, we report the first high-precision Cr isotopic data for a set of nine chondrules from one of the more primitive CO chondrites, Ornans, as well as their major element contents. The chondrules define a $^{53}$Mn-$^{53}$Cr isochron giving an age similar to the oldest solar system solids. Measurements of their $^{54}$Cr anomalies are discussed in terms of dynamic of transport of material in the early solar system.

## 2. Samples and Analytical Methods

A ~6 g sample of Ornans (CO3.4) was gently crushed and sieved. The fraction that contained grains greater than 500 µm was retained, and the largest chondrules were handpicked from this fraction for analysis. Nine individual chondrules were dissolved in a mixture of HF and HNO3 (2:1) on a hot plate (170°C) for 2 days. Subsequently, the samples were dissolved in aqua regia (concentrated HCl and HNO3 mixture in 3:1 ratio) to ensure complete digestion of fluorides, and refractory phases such as chromite and spinel. Before the chemical separation of Cr, ~15% aliquots were preserved for precise determination of the $^{55}$Mn/$^{52}$Cr ratio and major element contents by Inductively Coupled Plasma Mass Spectrometry at the University of Science and Technology of China. The procedure for the determination of the $^{55}$Mn/$^{52}$Cr ratio (standard-addition method) and major element concentrations is described in Qin et al. (2010a) and Qin et al. (2011a), respectively. The external precisions are <4% for the $^{55}$Mn/$^{52}$Cr ratios and 5%–10% for major element concentrations.

The Cr purification protocol is based on the procedure described in Qin et al. (2010a) involving a two-step cation exchange column purification protocol and a total procedure yield of ~80%. The blank of ~5 ng is negligible compared to the 6–20 µg of Cr present in individual chondrules. The final Cr

solution obtained was dried down in concentrated HNO3 3–6 times to remove residual organics (i.e., from the cation exchange resin).

The Cr isotopic compositions of the samples were analyzed by Thermal Ionization Mass Spectrometer using a Thermo Scientific TRITON Plus at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing. Approximately 2–3 µg of Cr was loaded on a single degassed Re filament (each sample was loaded on two or three filaments) and the intensity of the $^{52}$Cr+ beam was typically in the range of 8–15 V for all the analyses. The intensities of the Cr isotopes and isobaric interferences, $^{49}$Ti, $^{50}$Ti + $^{50}$Cr + $^{50}$V, $^{51}$V, $^{52}$Cr, $^{53}$Cr, $^{54}$Cr + $^{54}$Fe, $^{56}$Fe were measured on Faraday detectors L3, L2, L1, C, H1, H2, and H3 with the central cup set on $^{52}$Cr. The interferences on $^{50}$Cr and $^{54}$Cr from $^{50}$Ti, $^{50}$V, and $^{54}$Fe, respectively, were corrected by monitoring $^{49}$Ti, $^{51}$V, and $^{56}$Fe, respectively. Each sample was measured 4–6 times in static mode to ensure high precision. Within each run, 14 blocks of 30 ratios (8 s integration per ratio) were obtained using amplifier rotation. The $^{53}$Cr/$^{52}$Cr and $^{54}$Cr/$^{52}$Cr ratios were normalized to a $^{50}$Cr/$^{52}$Cr ratio of 0.051859 using an exponential law (Lugmair & Shukolyukov 1998) and are expressed in the epsilon notation ($\varepsilon$):

$$ \varepsilon^i \text{Cr} = \left[ \frac{\left( \frac{i^{\prime} \text{Cr}}{52} \text{Cr}_{\text{sample}} - \frac{i^{\prime}}{52} \text{Cr}_{\text{NIST3112a}} \right)}{\frac{i^{\prime} \text{Cr}}{52} \text{Cr}_{\text{NIST3112a}}} \right] \times 10,000 \quad (1) $$

where $i = 53$ or 54. The typical internal errors are 0.05 or less on $\varepsilon^{53}$Cr and 0.1 or less on $\varepsilon^{54}$Cr. The typical external errors for 4–6 measurements are <0.07 and <0.15 on $\varepsilon^{53}$Cr and $\varepsilon^{54}$Cr, respectively, for both the standards and the samples. The long-term reproducibility for the average $\varepsilon^{53}$Cr and $\varepsilon^{54}$Cr values of NIST 3112a obtained in individual analytical sessions over a period of 8 months are 0.06 and 0.18, respectively (2 SD, $N = 8$). Different aliquots of the USGS geological standard PCC-1 were processed through column chemistry separately and measured on different analytical sessions (Table 1). The average values of $\varepsilon^{53}$Cr and $\varepsilon^{54}$Cr for PCC-1 are 0.04 ± 0.08 and 0.05 ± 0.14, respectively (2 SD, $N = 3$), which are consistent with the values of 0.03 ± 0.04 and 0.13 ± 0.08 obtained in our previous work (Qin et al. 2010a).

## 3. Results

The Cr isotopic data and the $^{55}$Mn/$^{52}$Cr ratios of the chondrules are reported in Table 1. The previously reported Cr isotopic data for three Ornans chondrules (Qin et al. 2011b) are also listed. The $\varepsilon^{53}$Cr values of Ornans chondrules correlate with their $^{55}$Mn/$^{52}$Cr ratios, yielding a slope of 0.63 ± 0.14 (2σ), calculated using the Model 1 fit of Isoplot 4.15 (Ludwig 2003; also see Figure 1). The Ornans chondrules show large variability in their $\varepsilon^{54}$Cr values, ranging from 0.20 to 1.22, with an average value of 0.81. Also, the variations in $\varepsilon^{54}$Cr are correlated with those in $\varepsilon^{53}$Cr (and $^{55}$Mn/$^{52}$Cr) (Figure 2). The major element contents of the nine chondrules are reported in Table 2.

## 4. Discussion

### 4.1. Validity of the Mn–Cr isochron.

In order to test whether the correlation line between $^{55}$Mn/$^{52}$Cr and $\varepsilon^{53}$Cr represents a fossil isochron, other
<figure><img src="image_3.png" /><figcaption>**Figure 1.** Mn–Cr isochron for nine chondrules from the Ornans (CO3.4) carbonaceous chondrite. All uncertainties are reported at the 2σ level. The uncertainty for $^{55}\text{Mn}/^{52}\text{Cr}$ is 4%. The $\varepsilon^{53}\text{Cr}$ values display a strong positive correlation with the $^{55}\text{Mn}/^{52}\text{Cr}$ ratio. The slope of the line corresponds to 4567.6 ± 1.3 Ma relative to U-corrected D’Orbigny.</figcaption></figure>

<table><caption>**Table 1**
Mn–Cr Data Set and Fundamental Information of Chondrules Extracted from CO Chondrite Ornans</caption><thead><tr><th>Sample</th><th>Weight (mg)</th><th>Mn (ppm)</th><th>Cr (ppm)</th><th><sup>55</sup>Mn/<sup>52</sup>Cr</th><th>ε<sup>53</sup>Cr</th><th>ε<sup>54</sup>Cr</th><th>N (Replicates)</th></tr></thead><tbody><tr><td>C1</td><td>1.8</td><td>1637</td><td>3484</td><td>0.53</td><td>0.24 ± 0.06</td><td>0.78 ± 0.13</td><td>6</td></tr><tr><td>C2</td><td>3.8</td><td>1424</td><td>2842</td><td>0.57</td><td>0.21 ± 0.07</td><td>1.22 ± 0.12</td><td>6</td></tr><tr><td>C3</td><td>2.2</td><td>1493</td><td>2904</td><td>0.58</td><td>0.25 ± 0.07</td><td>1.01 ± 0.12</td><td>6</td></tr><tr><td>C4</td><td>2.9</td><td>975</td><td>3378</td><td>0.33</td><td>0.09 ± 0.06</td><td>0.20 ± 0.15</td><td>6</td></tr><tr><td>C5</td><td>2.2</td><td>1767</td><td>3032</td><td>0.66</td><td>0.23 ± 0.06</td><td>0.90 ± 0.13</td><td>6</td></tr><tr><td>C6</td><td>3.4</td><td>2121</td><td>4620</td><td>0.52</td><td>0.18 ± 0.05</td><td>0.89 ± 0.12</td><td>6</td></tr><tr><td>C7</td><td>2.8</td><td>737</td><td>3395</td><td>0.24</td><td>-0.02 ± 0.06</td><td>0.44 ± 0.22</td><td>4</td></tr><tr><td>C8</td><td>2.0</td><td>1095</td><td>3528</td><td>0.35</td><td>0.09 ± 0.05</td><td>0.85 ± 0.11</td><td>4</td></tr><tr><td>C9</td><td>9.3</td><td>1602</td><td>3044</td><td>0.59</td><td>0.24 ± 0.06</td><td>1.07 ± 0.11</td><td>6</td></tr><tr><td>#1</td><td>...</td><td>...</td><td>...</td><td>...</td><td>0.06 ± 0.05</td><td>1.11 ± 0.10</td><td>...</td></tr><tr><td>#2</td><td>...</td><td>...</td><td>...</td><td>...</td><td>0.01 ± 0.04</td><td>0.72 ± 0.08</td><td>...</td></tr><tr><td>#3</td><td>...</td><td>...</td><td>...</td><td>...</td><td>0.00 ± 0.04</td><td>0.54 ± 0.21</td><td>...</td></tr><tr><td>PCC-1</td><td>...</td><td>...</td><td>...</td><td>...</td><td>0.06 ± 0.08</td><td>0.12 ± 0.16</td><td>6</td></tr><tr><td>PCC-1</td><td>...</td><td>...</td><td>...</td><td>...</td><td>0.06 ± 0.02</td><td>0.05 ± 0.19</td><td>4</td></tr><tr><td>PCC-1</td><td>...</td><td>...</td><td>...</td><td>...</td><td>-0.01 ± 0.07</td><td>-0.02 ± 0.12</td><td>4</td></tr><tr><td>PCC-1 average</td><td>...</td><td>...</td><td>...</td><td>...</td><td>0.04 ± 0.08 (2SD)</td><td>0.05 ± 0.14 (2SD)</td><td>...</td></tr></tbody></table>

**Note.** The data for chondrules #1–3 (also Ornans Chondrules) are from Qin et al. (2011b). All uncertainties are reported at the 2σ level. The uncertainties for $^{55}\text{Mn}/^{52}\text{Cr}$, Mn and Cr contents are <4%.

possible contributions to the $\varepsilon^{53}\text{Cr}$ and Mn/Cr ratio variability must be discussed and eliminated:

(1) *Isotopic anomalies.* All the chondrules studied exhibit $^{54}\text{Cr}$ nucleosynthetic anomalies, which may also affect the measured abundance of $^{53}\text{Cr}$. In addition, cosmogenic effects due to galactic cosmic rays (GCR) are a possible source of

isotopic variability. However, we will demonstrate below that the influences of both effects are unimportant in this study.

First, previous studies of meteoritic materials have found no evidence for $^{53}\text{Cr}$ isotopic anomalies that are unrelated to the decay from $^{53}\text{Mn}$. For example, the very recent search for $^{54}\text{Cr}$-carrier phases in the acid residues of Orgueil by
<table><caption>**Table 2**
Major Element Contents in the Chondrules Extracted from CO Chondrite (ppm)</caption><thead><tr><th>Sample</th><th>Fe</th><th>Mg</th><th>Al</th><th>Ca</th><th>Ni</th><th>Ti</th><th>Na</th><th>Fe/Cr</th></tr></thead><tbody><tr><td>C1</td><td>142421</td><td>87304</td><td>8056</td><td>10105</td><td>7930</td><td>744</td><td>3025</td><td>38.1</td></tr><tr><td>C2</td><td>132058</td><td>71480</td><td>7381</td><td>8505</td><td>9749</td><td>638</td><td>1930</td><td>43.3</td></tr><tr><td>C3</td><td>124000</td><td>71130</td><td>6671</td><td>9194</td><td>8394</td><td>584</td><td>1815</td><td>39.7</td></tr><tr><td>C4</td><td>88223</td><td>104183</td><td>10982</td><td>13062</td><td>9065</td><td>1028</td><td>2698</td><td>24.3</td></tr><tr><td>C5</td><td>162285</td><td>103265</td><td>8847</td><td>12182</td><td>13233</td><td>831</td><td>2411</td><td>49.8</td></tr><tr><td>C6</td><td>225338</td><td>103926</td><td>9376</td><td>11132</td><td>20568</td><td>1183</td><td>2801</td><td>45.4</td></tr><tr><td>C7</td><td>145721</td><td>150573</td><td>20965</td><td>22815</td><td>17749</td><td>3857</td><td>4544</td><td>40.0</td></tr><tr><td>C8</td><td>198788</td><td>125239</td><td>10909</td><td>13032</td><td>13529</td><td>1065</td><td>3317</td><td>52.5</td></tr><tr><td>C9</td><td>209230</td><td>117574</td><td>11652</td><td>13312</td><td>14268</td><td>1135</td><td>2986</td><td>64.0</td></tr></tbody></table>

**Note.** The uncertainty is 5%–10% (2σ), and the Fe/Cr is atom ratio.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** $\varepsilon^{53}\text{Cr}$ vs. $\varepsilon^{54}\text{Cr}$ values of the chondrules from the Ornans (CO3.4) carbonaceous chondrite. Three data of them (also Ornans chondrules) are from Qin et al. (2011b). Since the $\varepsilon^{53}\text{Cr}$ of the Ornans chondrules is well-correlated with their $^{55}\text{Mn}/^{52}\text{Cr}$, the $\varepsilon^{54}\text{Cr}$ should also correlate with $^{55}\text{Mn}/^{52}\text{Cr}$. This relationship suggests the chondrule formed by a mixing of two kinds of precursors: (1) low Mn/Cr and low $\varepsilon^{54}\text{Cr}$, and (2) high Mn/Cr and high $\varepsilon^{54}\text{Cr}$, respectively.</figcaption></figure>

NanoSIMS showed several hotspots with extremely high $^{54}\text{Cr}$ anomalies (up to 56 times the solar value), yet no resolvable $^{53}\text{Cr}$ anomalies (Qin et al. 2011b; Nittler et al. 2018). Nor has any correlation between $^{53}\text{Cr}$ and $^{54}\text{Cr}$ anomalies been observed (Dauphas et al. 2010; Qin et al. 2011b; Nittler et al. 2018). Furthermore, the $^{54}\text{Cr}$-carrier phase is mostly Cr-bearing oxide with very low Mn/Cr (Dauphas et al. 2010; Qin et al. 2011b). Compared to the extreme $^{54}\text{Cr}$ anomalies in the carrier phases, the bulk CO chondrules only possesses up to $1.2\varepsilon$ excess in $\varepsilon^{54}\text{Cr}$, so the potential nucleosynthetic contributions to the $\varepsilon^{53}\text{Cr}$ variations in Ornans chondrules are likely to be very small and can be neglected. Additionally, step-leaching experiments show that the internal variations of $\varepsilon^{53}\text{Cr}$ and $\varepsilon^{54}\text{Cr}$ are not correlated in most of the chondrites that have been studied (Trinquier et al. 2007; Petitat et al. 2011; Yamakawa & Yin 2014; Göpel et al. 2015).

Second, Ornans chondrules in this study have relatively low Fe/Cr (see Table 2) and Ornans has an exposure age of only 20.8 Myr (Scherer & Schultz 2000). Consequently, the estimated contribution from GCRs does not exceed 0.01 for

$\varepsilon^{53}\text{Cr}$ using the calculation method in Trinquier et al. (2007), which is negligible compared to the magnitude of the variations in $^{53}\text{Cr}/^{52}\text{Cr}$ observed here.

(2) *Isochron or mixing line.* For the CO chondrules, all analyzed samples plot on a single, well-defined correlation line, which could represent a mixing line between high-Mn/Cr and low Mn/Cr components with different $\varepsilon^{53}\text{Cr}$. In such a case, $\varepsilon^{53}\text{Cr}$ should correlate with the inverse of the Cr abundance (1/Cr), which is not observed (Figure 3, MSWD = 11.9). Hence, we can exclude simple binary mixing as the origin for the correlation between $\varepsilon^{53}\text{Cr}$ and Mn/Cr ratio. Therefore, the Mn–Cr systematics measured in Ornans chondrules define an isochron.

## 4.2. Formation Ages of Chondrules

### 4.2.1. Mn–Cr Age of CO Chondrules

The slope of the isochron gives an initial $^{53}\text{Mn}/^{55}\text{Mn}$ ratio of $(7.1 \pm 1.6) \times 10^{-6}$ with an intercept that gives a $(^{53}\text{Cr}/^{52}\text{Cr})_0$ of $-0.14 \pm 0.07$. The initial $^{53}\text{Mn}/^{55}\text{Mn}$ ratio corresponds to an absolute age of 4567.6 $\pm$ 1.3 Ma (2σ) when anchored to the angrite D'Orbigny which formed at 4563.37 $\pm$ 0.25 Ma, dated by $^{207}\text{Pb}–^{206}\text{Pb}$ system using corrected $^{238}\text{U}/^{235}\text{U}$ (Brennecka & Wadhwa 2012), with an initial $^{53}\text{Mn}/^{55}\text{Mn}$ of $(3.24 \pm 0.04) \times 10^{-6}$ (Glavin et al. 2004; Amelin 2008). The uncertainties in the slope of the isochron, in the half-life of $^{53}\text{Mn}$, in the U-corrected Pb–Pb age, and in the $^{53}\text{Mn}/^{55}\text{Mn}$ initial ratio of the meteorite used as a time anchor are all propagated into the age uncertainty of the Ornans chondrules. Because of the difference in volatility in the solar nebula between Mn and Cr (Lodders 2003), one or more thermal processes could be the main cause of the variability observed in the Mn/Cr ratios between chondrules. One likely thermal event would have been the one that formed the chondrule precursors. The Ornans chondrule isochron age is similar within error to the Pb–Pb age of CV CAIs 4567.30 $\pm$ 0.16 Ma (Connelly et al. 2012), which suggests that Mn/Cr fractionations that were preserved in the CO chondrule precursors were established at about the same time as the formation of the CV CAIs. The source of heat responsible for fractionating Mn from Cr in the chondrule precursors may be related to the active young Sun (Amelin & Ireland 2013) and associated with FU Orionis outbursts (Boss et al. 2012). Furthermore, the early thermal process that formed the CO chondrule precursors should have also shaped the characteristics of nucleosynthetic anomalies (e.g., Ti, Ca, and Cr) in different chondrites and planetary bodies (Trinquier et al. 2009; Schiller et al. 2018). If this is
<figure><img src="image_5.png" /><figcaption>**Figure 3.** $\varepsilon^{53}\text{Cr}$ vs. Cr content correlation line of Ornans chondrules. It is obvious that all the chondrules are not in a line, and the MSWD for this diagram is much bigger than that (1.02) for the correlation line of $^{55}\text{Mn}/^{52}\text{Cr}$ and $\varepsilon^{53}\text{Cr}$, which substantiates the validity of the Mn–Cr isochron for Ornans chondrules in this study.</figcaption></figure>

correct, the timing of heating would therefore be within a short
time frame of 4000–20000 yr (Thrane et al. 2006; Jacobsen
et al. 2008; Larsen et al. 2011).

4.2.2. Discordance of Ages of CO Chondrules Dated by $^{26}\text{Al}-^{26}\text{Mg}$ and $^{53}\text{Mn}-^{53}\text{Cr}$

Chondrules from a different CO chondrite (Yamato-81020) have internal $^{26}\text{Al}-^{26}\text{Mg}$ isochrons that give ages that are 1.7 to 3.0 Ma younger than CAIs when using a canonical $^{26}\text{Al}/^{27}\text{Al}$ ratio for CAIs of $5 \times 10^{-5}$ (Kunihiro et al. 2004; Jacobsen et al. 2008; Kurahashi et al. 2008). These ages are also younger than the bulk Mn–Cr chondrule age presented here. Chondrules are believed to have experienced fast cooling after melting, so the relatively large difference between the internal Al–Mg and external Mn–Cr ages is unlikely to reflect the time difference between melting and subsequent crystallization from the same thermal event (chondrule forming). Rather, it could reflect the time difference between the formation of chondrule precursors (dated by external Mn–Cr isochrons) and the last thermal events experienced by these individual chondrules (dated by internal Al–Mg isochrons). The CO chondrules could have experienced multiple melting events after their formation. An alternative explanation is that the initial $^{26}\text{Al}/^{27}\text{Al}$ ratio may not have been homogeneous in the solar system (Larsen et al. 2011; Makide et al. 2013; Schiller et al. 2015; Van Kooten et al. 2016; Yin et al. 2017). These authors have argued for an initially heterogeneous distribution of $^{26}\text{Al}/^{27}\text{Al}$ based on two lines of argument. (1) The *$\mu^{26}\text{Mg}$* (mass-independent compositions of Mg isotopes in ppm) correlates with $\varepsilon^{54}\text{Cr}$ for different solar system bodies (Larsen et al. 2011).

(2) Chondrules and achondrites that have been dated by both U-corrected Pb-Pb and $^{26}\text{Al}-^{26}\text{Mg}$ systems (using the canonical value of CAI: $5 \times 10^{-5}$) show inconsistent ages with a shift of ~1.5 Ma (Larsen et al. 2011; Bizzarro et al. 2014; Schiller et al. 2015). This offset is also illustrated in Figure 4. That is, there are no “old” Al–Mg internal ages (0 Ma after CAIs) from chondrules in different chondrites, a result that is inconsistent with the “old” U–Pb ages. A possible solution for the Al–Mg chronometer might be the choice of anchor. In details, if the Al–Mg ages are anchored to D'Órbigny, i.e., the initial ($^{26}\text{Al}/^{27}\text{Al}$)$_0$ of solar system is decreased to ~$1.3 \times 10^{-5}$ (Schiller et al. 2015), the Al–Mg ages of CO chondrules will be consistent with the Mn–Cr age in this study, and this will also lead to consistent Pb-Pb and Al–Mg ages in the achondrites (Bouvier et al. 2011; Koefoed et al. 2016; Wimpenny et al. 2019). Alternatively, the Al–Mg ages could have been disturbed by parent body processes. Thus, the significance of $^{26}\text{Al}-^{26}\text{Mg}$ chronometry is debated and warrants further study.

4.2.3. Overview of Chronology of Chondrules

Bulk chondrule Mn–Cr ages from other chondrites, along with chondrule ages determined using the U-corrected U–Pb, Mn–Cr, Hf–W and Al–Mg systems in the literature are compiled in Figure 4. The initial $^{53}\text{Mn}/^{55}\text{Mn}$ of (7.1 ± 1.6) × 10⁻⁶ for the chondrules in Ornans (CO3) (this study) is identical within error to those of Allende (CV3) (6.23 ± 0.82) × 10⁻⁶ (Yin et al. 2009), Chainpur (LL3.4) (5.1 ± 1.6) × 10⁻⁶ (Yin et al. 2007), and Semarkona (LL3.0) (5.8 ± 1.9) × 10⁻⁶ (Kita et al. 2005), which predates the
<figure><img src="image_6.png" /><figcaption>**Figure 4.** U-corrected U–Pb, Mn–Cr, Al–Mg and Hf–W ages of chondrules in different chondrites published up to 2018. The green areas are estimated accretion ages of chondrites (Alexander et al. 2018 and references therein). The curve in the Al–Mg internal isochron ages of all types of chondrules shows the age frequency, and the data are from Nagashima et al. (2017) and references therein, while the only Al–Mg external isochron age is observed by Luu et al. (2015). All the U-corrected U–Pb ages are obtained by internal isochrons of individual chondrules (Amelin et al. 2002; Connelly et al. 2012; Bollard et al. 2015, 2017; Brennecka et al. 2015; Schrader et al. 2017), and all the Mn–Cr ages are dated by bulk chondrules anchored to U-corrected D’Orbigny. Among them, four Hf–W ages are obtained by external isochrons of several chondrules and matrix (Yamashita et al. 2010; Becker et al. 2015; Budde et al. 2016b, 2018). The gray bar around 0 Ma is the formation time of CAIs (Connelly et al. 2012).</figcaption></figure>

formation of Gujba (CB) chondrules that have an initial ⁵³Mn/⁵⁵Mn of (3.18 ± 0.52) × 10⁻⁶ by ~4 Myr (Yamashita et al. 2010) (Figure 4). This confirms that the formation age of CB chondrule precursors is distinct from those for other CCs and OCs. A 4 Myr age disparity permits their formation by different mechanisms, in which the younger chondrules potentially formed in a vapor-melt plume produced by a giant impact between planetary embryos (Krot et al. 2005; Bollard et al. 2015).

Comparing external Mn–Cr chondrule ages with ages obtained using other dating systems has to be done carefully as they often date different events. For instance, isochrons constructed from bulk chondrules (e.g., this study; Yin et al. 2007, 2009; Luu et al. 2015) should date the formation of the chondrule precursors, while internal isochrons (i.e., in situ Al–Mg dating and U–Pb dating by acid leaching) for individual chondrules should date the last melting events that they experienced. For example, the internal U–Pb ages of some CV chondrules are as old as the ages obtained with Mn–Cr

systematics, whereas other CV chondrules exhibit younger U–Pb ages (Yin et al. 2009; Connelly et al. 2012; Brennecka et al. 2015). This apparent inconsistency between the U–Pb and ⁵³Mn–⁵³Cr ages can be explained if the Mn–Cr external isochron (bulk chondrule isochron) dates the fractionation of Mn and Cr among the chondrule precursors, while the internal U–Pb isochrons date the last thermal events that reset the U–Pb systems in the individual chondrules. Therefore, the combined Mn–Cr age dated by bulk chondrules and internal U–Pb ages of individual chondrules would further suggest that the CV chondrules record multiple melting events.

The external Hf–W and Al–Mg isochrons for bulk CV chondrules yield ages that are slightly younger than internal U–Pb and Mn–Cr external isochrons. Furthermore, the Hf–W external age and Al–Mg ages for CR chondrules are also younger than some Pb–Pb internal ages (Yin et al. 2009; Connelly et al. 2012; Luu et al. 2015; Budde et al. 2016b, 2018; Bollard et al. 2017). The possible reasons for this discrepancy include: (1) the Al–Mg external ages were reset
<figure><img src="image_7.png" /><figcaption>**Figure 5.** $\varepsilon^{54}\text{Cr}$ of chondrules (circles) and CAI contents in Vigarano (CV, green), Ornans (CO, red), NWA 801 (CR, blue), and Gujba (CB, orange) chondrites, with the $\varepsilon^{54}\text{Cr}$ of bulk chondrites (Vigarano for CV, Ornans for CO, GRA 06110 for CR, and Bencubbin for CB (black lines with uncertainty shown as gray bars). The $\varepsilon^{54}\text{Cr}$ Data for chondrules are from Olsen et al. (2016), Qin et al. (2011b), Yamashita et al. (2010), and this study, and the bulk sample data are from Qin et al. (2010a), Trinquier et al. (2007), and Bonnand et al. (2016). Data for CAI modal abundance are from Ebel et al. (2016) and Hezel et al. (2008). There is positive correlation between the CAI mode and the variation in $\varepsilon^{54}\text{Cr}$ for chondrules in a given chondrite, which is interpreted to reflect transport of part of the chondrule precursors from the CAI-forming region to CC accretion region.</figcaption></figure>

(Alexander et al. 2018); (2) there was an initial heterogeneity in the distribution of $^{26}$Al; (3) the pool of chondrules and matrix were included in the Hf–W isochron, in which case the isochron might reflect a mixing age, because the chondrule and matrix may have different origins (see session 4.3); and (4) some of the old internal Pb–Pb ages reflect early events prior to the formation of chondrule precursors.

A more detailed comparison between different dating systems in chondrules can be found in recent review papers (Alexander & Ebel 2012; Bizzarro et al. 2017; Connelly et al. 2017; Krot & Nagashima 2017; Alexander et al. 2018).

### 4.3. Constraints on the Origin of CO Chondrules

The heterogeneous distribution of $^{54}$Cr among chondrules has major implications for the origin of chondrules and planetary bodies. The bulk CCs are characterized by $\varepsilon^{54}\text{Cr}$ ranging from $0.57 \pm 0.11$ (CK) to $1.56 \pm 0.06$ (CI), with the terrestrial composition being $0.10 \pm 0.13$ (Trinquier et al. 2009; Qin et al. 2010a; Göpel et al. 2015; Mougel et al. 2018). The $\varepsilon^{54}\text{Cr}$ for bulk Ornans is $1.03 \pm 0.19$ (Bonnand et al. 2016). The fact that one of the Ornans chondrules (C4, $\varepsilon^{54}\text{Cr} = 0.20 \pm 0.15$) has a $\varepsilon^{54}\text{Cr}$ value that falls out of the $\varepsilon^{54}\text{Cr}$ range for all of the bulk CCs, implies that chondrule

precursors might not come solely from the accretion regions of CO chondrites or the other CCs. This suggests that chondrules included more or less of the $^{54}\text{Cr-carrier phase(s)}$ than the average of the associated matrix. Therefore, the $^{54}\text{Cr-carrier phase}$ was not well-mixed in the region where they and the associated matrix were formed either. Some elemental and isotopic complementarities between the chondrules and matrix have been interpreted to reflect a common reservoir for the matrix and chondrules (Hezel & Palme 2010; Palme et al. 2015; Budde et al. 2016a, 2016b). Alternatively, the CO chondrule precursors might not have had the same sources as the matrix, as proposed by Olsen et al. (2016). Because the main components in chondrites are chondrules and matrix, compared to the bulk chondrite composition, the two components must have similar or complementary chemical and isotopic compositions. The chondrule-matrix complementarity model needs further testing. In the next section, we will discuss a possible explanation for the Cr isotopic variations with a heating-transportation model (Olsen et al. 2016).

Olsen et al. (2016) proposed that chondrule precursors were transported from the inner solar system to the CC formation regions in the outer solar system (several astronomical unit from the Sun; Morbidelli et al. 2012), where they were mixed with other materials and subsequently melted to form
chondrules. The presence of high-temperature objects, such as CAI-like refractory particles in comets that accreted in regions of the solar system more distal than those in which the CCs were formed (about 20 au away from the Sun; Brownlee et al. 2006; Zolensky et al. 2006; Nakamura et al. 2008), as well as astrophysical models (Shu et al. 1997; Ciesla 2007) support large-scale transportation of material in the early solar system.

Materials that formed in the CAI-forming region, which is generally assumed to have been close to the Sun (<0.5 au; MacPherson et al. 1988; Sossi et al. 2017), would have had low Mn/Cr ratios, as is observed for CAIs that have $^{55}$Mn/$^{52}$Cr ratios lower than 0.1 (Papanastassiou 1986; Bogdanovski et al. 2002; Papanastassiou et al. 2002). Typically, CAIs have high $\varepsilon^{54}$Cr values (~6; Trinquier et al. 2009), and their Cr is mainly carried by Cr-oxides that have high solidus temperatures (Stolper & Paque 1986). The reservoir that would have been complementary to these CAIs would then have had low $\varepsilon^{54}$Cr values (mostly the silicate components other than CAIs in CAI formation region). CI chondrites, a CC group composed mostly of matrix, have a bulk $^{55}$Mn/$^{52}$Cr ratio of 0.851 and a bulk $\varepsilon^{54}$Cr value of 1.65 ± 0.07 (Orgueil; Qin et al. 2010a), which is higher than those of all the chondrules analyzed in Ornans. Assuming that CI chondrites represent the most primitive material in the CC forming region (located in the outer solar system, beyond the orbit of Jupiter; Walsh et al. 2011; Morbidelli et al. 2012), the Cr isotope variation in the Ornans chondrules is consistent with the formation of the chondrules by mixing of material that formed in the CAI-forming region (<0.5 au, low $^{55}$Mn/$^{52}$Cr ratio and $\varepsilon^{54}$Cr value) and CI-like material from the outer solar system (beyond the orbit of Jupiter at 3–5 au, high $^{55}$Mn/$^{52}$Cr ratio and $\varepsilon^{54}$Cr value) with different ratios. Considering the large quantity of chondrules existing in the early solar system, roughly $10^{24}$ g or $3 \times 10^{27}$ chondrules (Desch et al. 2012), the formation of chondrules as suggested by Cr isotopic systematics further reinforces the large-scale transportation of materials from the inner to the outer solar system in the early protoplanetary disk. It should be noted that this mixing process does not invalidate the use of $^{53}$Mn-$^{53}$Cr as a means of dating their formation ages, as the aforementioned mixing process precludes the chondrule melting events. Furthermore, as previously indicated, $\varepsilon^{53}$Cr values do not correlate with the Cr content for Ornans chondrules, which further suggests that the Mn–Cr isochron does not represent a mixing line.

In addition, chondrules from chondrites with the most CAIs (e.g., CV and CO chondrules) exhibit larger $\varepsilon^{54}$Cr variation than chondrules from those containing fewer CAIs (e.g., CB and CR chondrules; Figure 5), which suggests that CV and CO chondrite accretion regions received more material transported from the CAI-forming region. In detail, chondrules in Vigarano (CV) and Ornans (CO) chondrites, which contain 3.11 vol.% and 2.64 vol.% of CAIs, respectively (Ebel et al. 2016), have large $\varepsilon^{54}$Cr variations between chondrules (2.8$\varepsilon$ and 1$\varepsilon$, respectively; Olsen et al. 2016), while chondrules in Gujba (CB), which has less than 0.1 vol.% CAIs (Hezel et al. 2008), have almost homogeneous $\varepsilon^{54}$Cr (Yamashita et al. 2010). CR chondrites fall in between, with a range of $\varepsilon^{54}$Cr (~0.4$\varepsilon$) and 0.12 ± 0.04 vol.% CAIs (Hezel et al. 2008; Olsen et al. 2016).

This difference in $\varepsilon^{54}$Cr characteristics and CAI content between CB and CR chondrules, and CO and CV chondrules could be related to the different orbital distances for the different chondrite accretion regions. (Olsen et al. 2016;

Van Kooten et al. 2016). Perhaps CV and CO chondrites accreted nearer the Sun, compared to CB (also CR) chondrites, so that their chondrules received more material that was transported from the CAI formation region. More Cr isotopic work on chondrules from CM, OC, and EC chondrites is needed to further test this hypothesis.

## 5. Conclusion

We report high-precision bulk Cr isotopic analyses of nine chondrules from the CO chondrite Ornans. These chondrules define an external Mn–Cr isochron giving an age of 4567.6 ± 1.3 Ma (relative to U-corrected D’Orbigny), supporting the concurrent formation of CAIs and CO chondrule precursors. The Mn–Cr fractionation among the CO chondrules probably results from one or several thermal event(s) affecting the precursor materials. Additionally, the heterogeneity of $\varepsilon^{54}$Cr in CO chondrules suggests mixing between a $\varepsilon^{54}$Cr-depleted reservoir that formed in the CAI-forming region with a $\varepsilon^{54}$Cr-enriched CI-like reservoir in the outer solar system, providing support for material transportation from the inner to the outer solar system.

This work was funded by Strategic Priority Research Program (B) (No. XDB18000000) and the Key Research Program (No. XDPB11) of Chinese Academy of Sciences, National Natural Science Foundation of China (Nos. 41625013, 41473066, 41773060), and the “111” project. We greatly appreciate Jiuxing Xia and Paolo A. Sossi for their helpful discussions. We would like to thank the Natural History Museum, London for supplying the sample of Ornans used in this study. We also appreciate the comments by an anonymous reviewer, which greatly improved the quality of this paper. K.Z. acknowledges the Ph.D. fellowship from China Scholarship Council (No. 201706340161).

## ORCID iDs

Ke Zhu (朱柯) https://orcid.org/0000-0003-3613-7239

Jia Liu https://orcid.org/0000-0001-8219-1995

Conel M. O’D. Alexander https://orcid.org/0000-0002-8558-1427

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