# Metamorphosed calcium-aluminum-rich inclusions in CK carbonaceous chondrites

Noël CHAUMARD¹,²,³,*†, Bertrand DEVOUARD¹,²,³,‡, Audrey BOUVIER⁴,¶, and Meenakshi WADHWA⁴

¹Laboratoire Magmas et Volcans, Clermont Université, Université Blaise Pascal, BP 10448, F-63000 Clermont-Ferrand, France

²CNRS, UMR 6524, LMV, Clermont-Ferrand F-63038, France

³IRD, R 163, LMV, Clermont-Ferrand F-63038, France

⁴Center for Meteorite Studies, School of Earth and Space Exploration, Arizona State University, Tempe, Arizona 85287, USA

†Present address: Laboratoire de Minéralogie et de Cosmochimie du Muséum, Muséum National d'Histoire Naturelle, UMR 7202 – CNRS, 61 rue Buffon, Paris 75005, France

‡Present address: Aix-Marseille Université, CNRS, IRD, CEREGE UM34, BP 80, Aix en Provence 13545, France

¶Present address: University of Western Ontario, Department of Earth Sciences, 1151 Richmond Street N, London, Ontario N6A 3K7, Canada

*Corresponding author. E-mail: nchaumard@mnhn.fr

*(Received 22 November 2012; revision accepted 07 December 2013)

**Abstract**–CK chondrites are the only group of carbonaceous chondrites with petrologic types ranging from 3 to 6. Although CKs are described as calcium-aluminum-rich inclusion (CAI)-poor objects, the abundance of CAIs in the 18 CK3–6 we analyzed ranges from zero to approximately 16.4%. During thermal metamorphism, some of the fine-grained CAIs recrystallized as irregular assemblages of plagioclase + Ca-rich pyroxene ± olivine ± Ca-poor pyroxene ± magnetite. Coarse-grained CAIs display zoned spinel, fassaite destabilization, and secondary grossular and spinel. Secondary anorthite, grossular, Ca-rich pyroxene, and spinel derive from the destabilization of melilite, which is lacking in all CAIs investigated. The Al-Mg isotopic systematics measured in fine- and coarse-grained CAIs from Tanezrouft (Tnz) 057 was affected by Mg redistribution. The partial equilibration of Al-Mg isotopic signatures obtained in the core of a coarse-grained CAI (CG1-CAI) in Tnz 057 may indicate a lower peak temperature for Mg diffusion of approximately 540–580 °C, while grossular present in the core of this CAI indicates a higher temperature of around 800 °C for the metamorphic event on the parent body of Tnz 057. Excluding metamorphic features, the similarity in nature and abundance of CAIs in CK and CV chondrites confirms that CVs and CKs form a continuous metamorphic series from type 3 to 6.

## INTRODUCTION

Chondritic meteorites are composed of chondrules, calcium-aluminum-rich inclusions (CAIs), Fe-Ni metal, and lithic clasts (i.e., dark inclusions [DIs] in CV chondrites) embedded in variable proportions in a fine-grained matrix that can also contain mineral fragments as well as presolar grains. CAIs were formed in the solar nebula by high-temperature processes such as condensation, evaporation, and subsequent melting (e.g., MacPherson et al. 1988) and are considered to be the oldest and the most primitive objects formed in the solar system (e.g., Hewins et al. 1996). The absolute

radiometric ages of CAIs from CV3 chondrites vary between 4567.1 ± 0.1 Ma and $4568.2_{-0.4}^{+0.2}$ Ma using the U-Pb chronometer (e.g., Amelin et al. 2010; Bouvier and Wadhwa 2010; Bouvier et al. 2011a; Connelly et al. 2012).

CAIs are mainly composed of refractory phases predicted by equilibrium thermodynamic calculations to condense from a gas with solar nebular composition (e.g., Grossman 1972), such as perovskite, hibonite, spinel, fassaite, melilite, and anorthite. Consequently, these refractory objects have been intensely studied since their first description more than 40 yr ago by Christophe Michel-Lévy (1968) as witnesses of the
earliest stages of the formation of our solar system. In addition, secondary phases are commonly observed in CAIs of chondrites and are considered to be the result of alteration processes, either within the nebula (e.g., McGuire and Hashimoto 1989) or on a parent body (e.g., Meeker et al. 1983; Krot et al. 1995), or both (e.g., Brearley and Jones [1998]; and references therein; Krot et al. 2007). The principal secondary phases are nepheline, sodalite, wollastonite, hedenbergite, grossular, and andradite (MacPherson 2003). Most studies on CAIs containing secondary minerals were carried out on the CV3-oxidized subgroup and particularly in Allende, due to the high CAI abundance in CV chondrites (10 vol%; Scott et al. 1996).

The CK chondrites form the only metamorphic series (petrologic types 3–6) among the carbonaceous chondrite groups (Kallemeyn et al. 1991) and are typically described as CAI-poor objects (<4 vol%; Kallemeyn et al. 1991; Keller et al. 1992; Noguchi 1993; Scott et al. 1996; Hezel et al. 2008). However, some CKs reveal CAI abundances similar to those reported in CVs (Chaumard et al. 2009). The CK chondrites offer a unique opportunity to study CAI evolution during parent body thermal metamorphism, beyond what was described in CV3 and CO3 chondrites (e.g., Greenwood et al. 1992; Russell et al. 1998; Krot et al. 2009). Some petrographic studies of CAIs in CK chondrites describe geochemical and mineralogical changes, such as Fe-enrichment in spinels and olivines, and replacement of perovskite by ilmenite, in response to an increasing thermal metamorphism, from type 3 (Geiger et al. 1993; Ivanova et al. 2000; Zipfel et al. 2000; Brandstätter et al. 2003; Smith and Russell 2003) to higher metamorphic types (CK4/5) (MacPherson and Delaney 1985; Keller 1992; Keller et al. 1992; Greenwood et al. 2000a, 2000b; Kurat et al. 2002; Bukovanská et al. 2003). However, a comprehensive study of the textural, petrologic, and chemical modifications in CAIs, especially at the type 3–4 transition and from a large number of CK samples, was lacking.

The aim of this work is the description and chemical analysis of CAIs in a suite of CK chondrites to characterize their modifications during parent body processes and to better constrain the conditions of thermal metamorphism on the CK parent body. In addition, Allende has been considered, up to now, as the most metamorphic CV (petrologic subtype >3.6; Bonal et al. 2006), whereas CK chondrites are of higher petrologic type (from subtype 3.8 to type 6). So, we focused on CAIs from CK chondrites at the transition from petrologic type 3 to type 4 to document the first steps of the textural and chemical re-equilibration of these chondritic components and to study the possible relationship between the CV and the CK groups

(Greenwood et al. 2003, 2004, 2010; Devouard et al. 2006; Chaumard et al. 2009; Wasson et al. 2013).

## SAMPLES AND ANALYTICAL METHODS

### Samples

Over the last decade, systematic collection efforts for meteorites in the Saharan and Antarctic deserts have resulted in the recovery of significant numbers of new CKs, especially at the type 3–4 transition. This study was carried out on 18 CK and 5 CV chondrites provided mostly by the Muséum National d'Histoire Naturelle (Paris, France) and the NASA Meteorite Working Group (Houston, USA). Table 1 provides a list of samples that were studied here. Large slices of Tnz 057 were loaned by Frederic Béroud and Christophe Boucher. For Northwest Africa (NWA) 2900, we have assumed the classification published in the Meteoritical Bulletin. Although officially classified as a CV3, the samples of NWA 2900 obtained from Stefan Ralew (main mass holder) are largely re-equilibrated and display a recrystallized matrix similar to low-grade CK4s.

### Modal Abundance Measurements

We measured modal abundances of CAIs from meteorite slices (either raw from cut or polished), polished sections embedded in epoxy resin, and thin sections (Table 1). Large slices of Allende, NWA 779, NWA 2900, NWA 4425, and Tnz 057, were scanned with an optical scanner with 7–70 µm pixel⁻¹ resolution. We used these scans to observe very large surfaces, which would not have been possible using other methods of investigation. We used a JEOL JSM-5910 LV scanning electron microscope (SEM) at Laboratoire Magmas et Volcans (LMV) in Clermont-Ferrand to obtain backscattered electron (BSE) images of the entire surfaces of the thin and polished sections. BSE images obtained were merged to a single mosaic map with a resolution of 2 µm pixel⁻¹. Finally, CK samples in which no CAIs could be recognized from optical or BSE images were mapped at 28 µm pixel⁻¹ for Fe, Mg, Ca, and Al using a CAMECA SX-100 electron microprobe at LMV.

Chondrules, CAIs, matrix, and DIs were visually recognized and manually outlined on high-resolution digital images using a graphics tablet (hand-drawing method). CAIs were recognized on the basis of high Ca and/or Al concentrations in chemical mapping, texture and mineralogy in BSE images, and shape, texture, and color in high-resolution scans. However, especially in high-resolution scans, some rounded CAIs can be
<table><caption>Table 1. List of CV and CK carbonaceous chondrites studied in this study.</caption><thead><tr><th>Meteorite name</th><th>Abbreviation</th><th>Group</th><th>Collection</th><th>Number</th><th>Sample type</th></tr></thead><tbody><tr><td>Kaba</td><td>—</td><td>CV3 Ox.</td><td>MNHN</td><td>—</td><td>Polished section</td></tr><tr><td>Bali</td><td>—</td><td>CV3 Ox.</td><td>MNHN</td><td>—</td><td>Polished section</td></tr><tr><td>Allende</td><td>—</td><td>CV3 Ox.</td><td>MNHN</td><td>—</td><td>Scans, thick section</td></tr><tr><td>Northwest Africa 2900</td><td>NWA 2900</td><td>CV3<sup>a</sup></td><td>BD</td><td>—</td><td>Polished sections, slices, thin section</td></tr><tr><td>Northwest Africa 779</td><td>NWA 779</td><td>CV3</td><td>BD</td><td>—</td><td>Polished sections, slices</td></tr><tr><td>Dar al Gani 431</td><td>DaG 431</td><td>CK3-an</td><td>MNHN</td><td>—</td><td>Thin section</td></tr><tr><td>Northwest Africa 1559</td><td>NWA 1559</td><td>CK3</td><td>BD</td><td>—</td><td>Thick section</td></tr><tr><td>Northwest Africa 4724</td><td>NWA 4724</td><td>CK3.8</td><td>MNHN</td><td>(C2006-11)</td><td>Polished section</td></tr><tr><td>Northwest Africa 4425</td><td>NWA 4425</td><td>CK3.8</td><td>MNHN + BD</td><td>(C2006-19)</td><td>Scans, polished section, slices</td></tr><tr><td>Northwest Africa 4423</td><td>NWA 4423</td><td>CK3.9</td><td>MNHN</td><td>(C2006-13)</td><td>Polished section</td></tr><tr><td>Northwest Africa 4770</td><td>NWA 4770</td><td>CK4</td><td>MNHN</td><td>—</td><td>Slices</td></tr><tr><td>Northwest Africa 4422</td><td>NWA 4422</td><td>CK4</td><td>MNHN</td><td>(C2006-12)</td><td>Polished section</td></tr><tr><td>Northwest Africa 5733</td><td>NWA 5733</td><td>CK4</td><td>MNHN</td><td>—</td><td>Polished section</td></tr><tr><td>Northwest Africa 2519</td><td>NWA 2519</td><td>CK4</td><td>MNHN</td><td>—</td><td>Polished section</td></tr><tr><td>Karoonda</td><td>—</td><td>CK4</td><td>MWG/NASA</td><td>(3970-1)</td><td>Thin section</td></tr><tr><td>Kobe</td><td>—</td><td>CK4</td><td>MWG/NASA</td><td>P4-2</td><td>Thin section</td></tr><tr><td>Maralinga</td><td>—</td><td>CK4</td><td>MNHN</td><td>—</td><td>Thin section</td></tr><tr><td>Tanezrouft 057</td><td>Tnz 057</td><td>CK4</td><td>MNHN + BD</td><td>—</td><td>Polished sections, thick section, slices</td></tr><tr><td>Northwest Africa 765</td><td>NWA 765</td><td>CK4/5</td><td>MNHN</td><td>(3777 LMT28)</td><td>Polished section</td></tr><tr><td>Pecora Escarpment 82500</td><td>PCA 82500</td><td>CK4/5</td><td>MWG/NASA</td><td>—</td><td>Thin section</td></tr><tr><td>Elephant Moraine 90007</td><td>EET 90007</td><td>CK5</td><td>MWG/NASA</td><td>—</td><td>Thin section</td></tr><tr><td>Elephant Moraine 87860</td><td>EET 87860</td><td>CK5/6</td><td>MWG/NASA</td><td>—</td><td>Thin section</td></tr><tr><td>Lewis Cliff 87009</td><td>LEW 87009</td><td>CK6</td><td>MWG/NASA</td><td>—</td><td>Thin section</td></tr></tbody></table>

BD = Bertrand Devouard. MNHN = Muséum National d'Histoire Naturelle, Paris. MWG = NASA Meteorite Working Group, Houston.

mistaken as chondrules, whereas Al-rich chondrules can be incorrectly identified as CAIs. Consequently, and when it was possible, ambiguous objects identified in high-resolution scans with uncommon shapes and aspects were checked by SEM-BSE-EDS or optical reflected microscopy. Using this method, we also tried to exclude mineral and chondrule fragments from our calculations of CAI modal abundances. We were similarly able to identify amoeboid olivine aggregates (AOAs), especially in NWA 2900. Although recognizable in BSE images (i.e., irregular shapes, mineralogy dominated by olivine, grains with triple junctions), AOAs might have been mistaken for CAIs in our high-resolution scans when SEM-BSE observations were not possible. Thus, this implies that there may be some inherent biases due to the criteria used for the differentiation of chondritic components based solely on high-resolution scans. All estimated CAI modal abundances, as well as modal abundances of the different phases observed in the CAIs investigated, are given as a percentage of the total area studied here (area%).

To estimate the error in this hand-drawing method on high-resolution scans, we recognized CAIs, chondrules, and matrix on an approximately 7 cm² scan of a slice of Tnz 057 by visual identification. Then, the

same area was mapped by BSE imaging, and another identification of CAIs, chondrules, and matrix was made on this BSE mosaic. Results obtained from these two separate identifications are presented in Table 2. Modal abundances of CAIs, chondrules, and matrix calculated from the BSE mosaic and the high-resolution scans are similar and are consistent with the values reported for Tnz 057 based on analysis of a much larger surface area (Table 3). The main difference between the methods based on high-resolution scans and BSE mosaic is the number of identified objects. More objects were counted in the method utilizing the BSE mosaic. This is a consequence of the resolution being 10 times better in the case of the BSE mosaic compared with the scan, which makes it easier to recognize small CAIs and chondrules. Although more CAIs were counted in the BSE mosaic, the modal abundance calculated is slightly lower than the value estimated from the scan. This is likely caused by the fact that CAIs have irregular shapes, which are better resolved in the BSE images, such that the error in this method is less than in the method utilizing the digital scans. So, it appears that our hand-drawing method somewhat overestimates the size of CAIs on high-resolution scans, even if CAIs with sizes smaller than the pixel size were not counted.

<footnote>aSee Sample section for more details about this classification.</footnote>
<table><caption>Table 2. CAI, chondrule, and matrix modal abundances in Tnz 057 (CK4) calculated from a BSE mosaic and a macroscopic scan of the same area.</caption><thead><tr><th>Tnz 057</th><th>BSE mosaic</th><th>Digital scan</th></tr></thead><tbody><tr><td>Surface (cm<sup>2</sup>)</td><td>7.27</td><td>7.27</td></tr><tr><td>Resolution (µm pixel<sup>-1</sup>)</td><td>4.4</td><td>48.5</td></tr><tr><td>CAIs</td><td></td><td></td></tr><tr><td>Modal abund. (area%)</td><td>10.0</td><td>10.8</td></tr><tr><td>Av. size (mm)</td><td>1.05</td><td>1.26</td></tr><tr><td>Pts count. (no.)</td><td>50</td><td>34</td></tr><tr><td>CAI/matrix</td><td>0.14</td><td>0.15</td></tr><tr><td>Chondrules</td><td></td><td></td></tr><tr><td>Modal abund. (area%)</td><td>17.3</td><td>17.2</td></tr><tr><td>Av. size (mm)</td><td>0.97</td><td>1.11</td></tr><tr><td>Pts count. (no.)</td><td>137</td><td>101</td></tr><tr><td>Chondrule/matrix</td><td>0.24</td><td>0.24</td></tr><tr><td>Matrix</td><td></td><td></td></tr><tr><td>Modal abund. (area%)</td><td>72.7</td><td>72.1</td></tr></tbody></table>

Modal abund. = modal abundance; Av. size = average size; Pts count. = points counted; CAI = calcium-aluminum-rich inclusion; BSE = backscattered electron.

Chondrule abundances were calculated using their coarse-grained mantles. Average size corresponds to an equivalent diameter if the surface area of each object is assimilated to a circle.

CAIs were considered as fine- or coarse-grained following criteria described in the General Description of CAIs in CK Chondrites section. In the cases when several sections of a single sample were analyzed, we calculated the standard deviation weighted by surface area. Errors on the CAI abundances (area%; Table 3) reported in this study correspond to 1 sigma.

To compare CAI sizes in the CK and CV chondrites studied here, we calculated an “equivalent radius” $r_e$, which is essentially the radius if the surface area of a CAI was assimilated into a circular object, and which can be expressed as:

$$r_e = (A_{CAI}/\pi)^{1/2} \quad (1)$$

where $A_{CAI}$ is the surface area of a given CAI.

## Analytical Methods

Chemical analyses and WDS elemental maps of major elements were obtained using a CAMECA SX-100 electron microprobe at LMV. Silicate and sulfide analyses were performed with an accelerating voltage of 15 keV and a beam current of 15 nA, while chemical compositions of oxides were obtained at 15 keV and 30 nA. Beam diameter was constant at approximately 1 µm. Standards used for silicates were wollastonite (Si, Ca), fayalite (Fe), forsterite (Mg), synthetic Al₂O₃ (Al), pyrophanite (Ti, Mn), synthetic Cr₂O₃ (Cr), albite (Na), orthoclase (K), and synthetic NiO or San Carlos olivine

(Ni). For sulfides and oxides, Fe, S, and Mg were calibrated with pyrite and MgO, respectively. We used a TAP crystal for the analyses of Na, Mg, Al, and Si; a PET crystal for K, Ca, Ti, Cr, and S; a LIF crystal for Mn, Fe, and Ni. The CAMECA $\phi_{P(z)}$ program was used for matrix corrections. Elemental WDS maps were acquired for Ca, Al, Mg, and Fe for surfaces ranging between 12 and 6 mm² at resolutions varying from 3 to 5 µm pixel⁻¹, using operating parameters similar to those described for point chemical analyses of silicates and sulfides.

Electron microprobe measurements of Fe contents are reported as $Fe^{2+}$ (FeO). We used the method developed by Droop (1987) based on stoichiometric criteria to estimate $Fe^{2+}$ and $Fe^{3+}$ (Fe₂O₃) contents. Moreover, some phases such as fassaite can contain Ti as $Ti^{3+}$ (Brearley and Jones [1998]; and references therein). Since we analyzed Ti as $Ti^{4+}$ (TiO₂), we adapted the method of Droop (1987) to estimate $Ti^{3+}$ (Ti₂O₃) contents in fassaite.

Specific areas of CAIs were mapped at 15 keV using an EDS system coupled with the JEOL JSM-5910 LV SEM at LMV. Si, Fe, Mg, Ca, Al, Na, K, Cr, Ti, Ni, and S elemental maps were recorded, with an approximately 1 µm pixel⁻¹ resolution.

Rare earth element (REE) contents in silicates were measured at LMV with a laser ablation inductively coupled plasma mass spectrometer (LA-ICPMS). The ICPMS is an Agilent 7500cs. Since the LA-ICPMS at LMV was modified during this study to improve measurements, mineral ablation was performed with two different laser ablation systems: a 266 nm Nd:YAG laser with spot diameters of 80 µm, 8 mJ energy, and a repetition rate of 5 Hz and 7 Hz, and a M-50E Resonetics laser system operating at 193 nm with spot diameters of 20 µm, 6 mJ energy, and a repetition rate of 4 Hz. The NIST SRM-610 glass standard (values from Gagnon et al. 2008) was used for calibration of relative sensitivities. For each analysis, we used as internal standard the CaO or Al₂O₃ content of the ablated phase previously measured by electron microprobe using the analytical conditions described above. To be sure that minerals that were ablated correspond to those in which internal standard values were measured, we checked the location of each laser spot by SEM-BSE. NIST SRM-612 and BCR2-G glasses were used as external standards to check the precision and accuracy of our measurements. We measured the masses ⁷Li, ²⁷Al, ²⁹Si, ⁴³Ca, ⁴⁴Ca, ⁴⁵Sc, ⁴⁹Ti, ⁵¹V, ⁸⁵Rb, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ¹³⁷Ba, ¹³⁸Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁷⁸Hf, ¹⁸¹Ta, ²⁰⁸Pb, ²³²Th, and ²³⁸U. Data reduction was done with the GLITTER software (Van Achterberg et al. 2001).
<table><caption>Table 3. Abundances, textures, and sizes of CAIs in CV and CK carbonaceous chondrites investigated in this study.</caption><thead><tr><th rowspan="2">Type</th><th rowspan="2">Studied area (cm²)</th><th rowspan="2">Total CAIs (area%)</th><th rowspan="2">Coarse-grained CAIs (area%)</th><th colspan="2">Number of CAIs</th><th colspan="2">Equivalent radius of all CAIs (mm)</th><th colspan="2">Equivalent radius of fine-grained CAIs (mm)</th><th colspan="2">Equivalent radius of coarse-grained CAIs (mm)</th></tr><tr><th>Fine-grained</th><th>Coarse-grained</th><th>Mean</th><th>Min.</th><th>Max.</th><th>Mean</th><th>Min.</th><th>Max.</th><th>Mean</th><th>Min.</th><th>Max.</th></tr></thead><tbody><tr><td>Kaba</td><td>Mean CV<sup>a</sup></td><td>10.0</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Bali</td><td>CV3 Ox.</td><td>0.75</td><td>3.4</td><td>0.0</td><td>10</td><td>0</td><td>0.20</td><td>0.04</td><td>0.67</td><td>0.20</td><td>0.04</td><td>0.67</td><td>—</td></tr><tr><td></td><td>CV3 Ox.</td><td>1.83</td><td>8.1</td><td>0.0</td><td>61</td><td>0</td><td>0.17</td><td>0.02</td><td>1.25</td><td>0.17</td><td>0.02</td><td>1.25</td><td>—</td></tr><tr><td>Allende</td><td>CV3 Ox.</td><td>9.20</td><td>10.3</td><td>0.9</td><td>278</td><td>3</td><td>0.22</td><td>0.05</td><td>1.31</td><td>0.21</td><td>0.05</td><td>1.31</td><td>0.95</td></tr><tr><td>NWA 779</td><td>CV3 Ox.</td><td>12.15</td><td>12.2 (5.9)</td><td>0.0</td><td>311</td><td>0</td><td>0.29 (0.11)</td><td>0.03</td><td>1.97</td><td>0.29 (0.11)</td><td>0.03</td><td>1.97</td><td>—</td></tr><tr><td>NWA 2900</td><td>CV3 Ox.</td><td>25.56</td><td>13.1 (2.3)</td><td>4.7 (2.7)</td><td>223</td><td>1</td><td>0.36 (0.06)</td><td>0.06</td><td>5.35</td><td>0.36 (0.06)</td><td>0.06</td><td>2.60</td><td>5.35</td></tr><tr><td>DaG 431</td><td>CK3</td><td>2.41</td><td>4.1</td><td>3.4</td><td>29</td><td>4</td><td>0.21</td><td>0.03</td><td>1.35</td><td>0.13</td><td>0.03</td><td>0.31</td><td>0.70</td></tr><tr><td>NWA 1559</td><td>CK3</td><td>1.86</td><td>4.3</td><td>0.6</td><td>25</td><td>2</td><td>0.26</td><td>0.08</td><td>0.86</td><td>0.24</td><td>0.08</td><td>0.86</td><td>0.40</td></tr><tr><td>NWA 4724</td><td>CK3.8</td><td>2.28</td><td>12.7 (1.6)</td><td>6.4 (4.6)</td><td>30</td><td>1</td><td>0.39 (0.09)</td><td>0.14</td><td>2.42</td><td>0.32 (0.14)</td><td>0.14</td><td>1.06</td><td>2.42</td></tr><tr><td>NWA 4425</td><td>CK3.8</td><td>25.24</td><td>10.5 (0.6)</td><td>5.7 (2.3)</td><td>116</td><td>3</td><td>0.53 (0.13)</td><td>0.11</td><td>5.66</td><td>0.45 (0.09)</td><td>0.11</td><td>1.91</td><td>4.78 (2.02)</td></tr><tr><td>NWA 4423</td><td>CK3.9</td><td>1.93</td><td>16.4</td><td>0.0</td><td>46</td><td>0</td><td>0.28</td><td>0.08</td><td>2.75</td><td>0.28</td><td>0.08</td><td>2.75</td><td>—</td></tr><tr><td>NWA 4770</td><td>CK4</td><td>7.29</td><td>3.9</td><td>3.0</td><td>19</td><td>1</td><td>0.40</td><td>0.09</td><td>2.65</td><td>0.28</td><td>0.09</td><td>0.80</td><td>2.65</td></tr><tr><td>NWA 4422</td><td>CK4</td><td>2.37</td><td>1.4</td><td>0.0</td><td>9</td><td>0</td><td>0.34</td><td>0.17</td><td>0.47</td><td>0.34</td><td>0.17</td><td>0.47</td><td>—</td></tr><tr><td>NWA 5733</td><td>CK4</td><td>1.31</td><td>0.4</td><td>0.0</td><td>3</td><td>0</td><td>0.21</td><td>0.13</td><td>0.36</td><td>0.21</td><td>0.13</td><td>0.36</td><td>—</td></tr><tr><td>NWA 2519</td><td>CK4</td><td>1.41</td><td>0.0</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Karoonda</td><td>CK4</td><td>2.54</td><td>0.1</td><td>0.1</td><td>2</td><td>2</td><td>0.20</td><td>0.09</td><td>0.31</td><td>—</td><td>—</td><td>0.20</td><td>0.09</td><td>0.31</td></tr><tr><td>Kobe</td><td>CK4</td><td>0.56</td><td>0.0</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Maralinga</td><td>CK4</td><td>4.05</td><td>0.1</td><td>0.0</td><td>3</td><td>0</td><td>0.23</td><td>0.16</td><td>0.25</td><td>0.23</td><td>0.16</td><td>0.25</td><td>—</td></tr><tr><td>Tnz 057</td><td>CK4</td><td>893</td><td>9.1 (1.4)</td><td>4.7 (0.9)</td><td>3024</td><td>45</td><td>0.56 (0.03)</td><td>0.04</td><td>10.87</td><td>0.49 (0.02)</td><td>0.04</td><td>3.50</td><td>5.22 (0.72)</td></tr><tr><td>NWA 765</td><td>CK4/5</td><td>1.76</td><td>1.9</td><td>0.0</td><td>11</td><td>0</td><td>0.27</td><td>0.10</td><td>0.57</td><td>0.27</td><td>0.10</td><td>0.57</td><td>—</td></tr><tr><td>PCA 82500</td><td>CK4/5</td><td>0.42</td><td>2.0</td><td>0.0</td><td>3</td><td>0</td><td>0.26</td><td>0.14</td><td>0.47</td><td>0.26</td><td>0.14</td><td>0.47</td><td>—</td></tr><tr><td>EET 90007</td><td>CK5</td><td>1.25</td><td>0.0</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>EET 87860</td><td>CK5/6</td><td>0.27</td><td>0.9</td><td>0.0</td><td>1</td><td>0</td><td>0.25</td><td>—</td><td>—</td><td>0.25</td><td>—</td><td>—</td><td>—</td></tr><tr><td>LEW 87009</td><td>CK6</td><td>0.77</td><td>0.0</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Mean CK<sup>a</sup></td><td></td><td>4.0</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

Values in parentheses correspond to 1 sigma uncertainties obtained when several sections of the same meteorite were analyzed.

CAI = calcium-aluminum-rich inclusion.

<footnote>ªData from Scott et al. (1996).</footnote>
All REE concentrations were normalized to CI abundances of Anders and Grevesse (1989).

To place constraints on the chronology of CK CAIs, we investigated the short-lived <sup>26</sup>Al-<sup>26</sup>Mg radiochronometer (half-life approximately 0.73 Ma) in three individual (two fine-grained and one coarse-grained) CAIs of Tnz 057. The Al-Mg study of CK CAIs was carried out at the Isotope Cosmochemistry and Geochronology Laboratory (ICGL) at Arizona State University using two different analytical methods.

First, two fine-grained CAIs (FG1-CAI and FG2-CAI) (each approximately 3 mm in size) were identified in a polished section of Tnz 057. These CAIs contain mostly Ca-rich pyroxene and anorthite. Due to the difficulty of manually extracting small fine-grained CAIs from the slices, we selected a microdrilling sampling technique for these two inclusions. Sample powders were extracted from the slice using a New Wave Research MicroMill set up with a 50 µm tungsten-carbide drill bit. The drill holes had diameters of approximately 150 µm, and a typical depth of approximately 100 µm. A few tens of micrograms of powders were recovered using ultrapure water from each of the drilled pits (three from FG1-CAI and two from FG2-CAI). These powders were further processed by acid digestion and wet chemistry for Mg purification under clean laboratory conditions, and Al/Mg and Mg isotopic analyses were carried out on the Thermo-Finnigan Neptune MC-ICPMS using procedures and analytical methods similar to those detailed in Bouvier et al. (2011b). An approximately 5% solution fraction was reserved for Al/Mg ratio measurements, and approximately 10 µg equivalent of Mg was processed through column chemistry for each of the five drilled samples (three from FG1- and two from FG2-CAI) and also for an inhouse rock standard, the Allende bulk rock (homogenized powder from the Smithsonian institution fully dissolved as a stock solution prepared at ICGL). During each analytical session, we measured the Mg isotopic compositions of an unprocessed SPEX Mg solution standard and of a column processed Allende bulk rock sample, in blocks of 3–4 repeats bracketed with the DSM-3 Mg standard, both before and after analyzing the Mg fractions of the CAI samples. This was done to assess the instrument stability and reproducibility during and between the two separate analytical sessions during which the Al-Mg analyses for this study were performed. All solutions (standards and samples) were analyzed at approximately 200 ppb Mg concentration in 3% nitric acid. The average composition for the SPEX solution analyzed during these sessions (relative to the DSM-3 standard, with 2SD errors) is as follows: δ<sup>25</sup>Mg = -1.05 (±0.04)‰, δ<sup>26</sup>Mg = -2.01 (±0.07) ‰, δ<sup>26</sup>Mg* = 0.03 (±0.03)‰ (n = 7).

Although this provides an assessment of the instrument performance, this reproducibility is not necessarily representative of the actual one for samples processed through the complete column chemical procedures. As noted above, during each analytical session, we also measured the isotopic composition of an Allende bulk rock sample that was processed along with the CAI samples through the same chemical procedures to validate the accuracy of our Mg isotope measurements and to allow interlaboratory comparisons. The 2SD uncertainties on repeats of the Allende WR made during two analytical sessions (δ<sup>25</sup>Mg = -0.17 (±0.06)‰; δ<sup>26</sup>Mg = -0.32 (±0.12)‰; δ<sup>26</sup>Mg* = 0.02 (±0.04)‰, n = 12) should be taken as representative of the external reproducibility of our Mg isotope measurements for processed unknown samples (Table 11). The Mg isotope composition of each of the samples was determined based on the average of three to seven repeated standard-bracketed analyses during one or both analytical sessions. To calculate non-mass-dependent anomalies in the <sup>26</sup>Mg/<sup>24</sup>Mg ratio due to the decay of <sup>26</sup>Al (i.e., δ<sup>26</sup>Mg*), we used the measured <sup>25</sup>Mg/<sup>24</sup>Mg and <sup>26</sup>Mg/<sup>24</sup>Mg ratios and the exponential law (using the fractionation factor β = 0.512 for our inhouse standards SPEX and Allende WR and β = 0.514 for CAIs; Davis et al. 2005).

Second, to obtain Al-Mg analyses on individual mineral phases, including some with high <sup>27</sup>Al/<sup>24</sup>Mg (i.e., >5), the coarse-grained CG1-CAI from Tnz 057 was selected for analysis by in situ LA-MC-ICPMS using analytical procedures similar to those described by Janney et al. (2011). The Neptune MC-ICPMS was coupled with a Photon Machines excimer laser ablation system operating at 193 nm and 4 ns pulses. The 29 analyses on plagioclase, anorthite, and pyroxene grains were conducted with a laser beam diameter of 69 µm that gave the required intensities for isotopic analysis. All three isotopes of Mg (<sup>25</sup>Mg, <sup>25</sup>Mg, and <sup>26</sup>Mg) and <sup>27</sup>Al signals were measured simultaneously on Faraday cups with 10<sup>11</sup> ohm resistors. The measurements were made with signals ranging from 0.2 to 0.9 V for <sup>26</sup>Mg, and 1–10 V for <sup>27</sup>Al. Analyses on the samples were bracketed with analyses on synthetic Ca–Mg–Al–Si oxide glass standards of known compositions (Ak<sub>35</sub> glass with <sup>27</sup>Al/<sup>24</sup>Mg = 5.06, and Ak<sub>65</sub> glass with <sup>27</sup>Al/<sup>24</sup>Mg = 1.43), and their Mg isotopic compositions that were used to correct for matrix and instrumental mass bias effects are given in Janney et al. (2011). For isochron calculations, we used the following reproducibilities for the standard-bracketed measurements of samples during the analytical sessions: 2 and 6% (2SD) for <sup>27</sup>Al/<sup>24</sup>Mg ratios in samples bracketed by the Ak<sub>35</sub> and Ak<sub>65</sub> glasses, respectively, and errors propagated from the 2SD internal errors for
<figure><img src="image_1.png" /><figcaption>Fig. 1. Reflected light image of a slice of Tnz 057 (CK4) containing 22% of chondrules, 11% of CAIs and 67% of matrix. Fine-grained CAIs have a whitish color and irregular shapes with sizes up to approximately 1 cm. The two largest coarse-grained CAIs (at the top) have a core composed of fassaite-spinel (black) and anorthite-grossular (white) assemblages, rimmed by a coarse-grained accretionary rim that is further surrounded by a fluffy mantle containing chondrules. This slice also contains a dark inclusion (near the center).</figcaption></figure>

the bracketing standards and the sample for the Mg isotopic ratios. Based on repeated measurements of the Ak₃₅ glass, we estimated 2SD errors on bracketed isotopic ratios of 0.12 ‰ for δ²⁵Mg, 0.22‰ for δ²⁶Mg, and 0.15‰ for δ²⁶Mg* (n = 16).

RESULTS

General Description of CAIs in CK Chondrites

In the CK chondrites investigated, two main types
of CAIs can be distinguished macroscopically (Fig. 1).
CAIs with a “fluffy” aspect are scattered throughout the
whole rock. They have a whitish color and are the most
abundant. As “fluffy” type A CAIs described in CVs
(e.g., MacPherson and Grossman 1979, 1984; Kornacki
and Wood 1984; Sylvester et al. 1992), they have
irregular shapes, with sizes ranging from approximately
100 µm to approximately 1.4 cm. We defined these
CAIs as fine-grained because at least 80% of their
grains have sizes <50 µm (the remaining 20% can be
composed of minerals with sizes up to 250 µm). Two
millimeter-sized fine-grained CAIs (FG1- and FG2-CAI)
were selected from Tnz 057 for Al/Mg and Mg isotopic

analyses by solution mode MC-ICPMS. They are
mainly composed of Ca-rich pyroxene and anorthite
rounded grains with sizes ranging from approximately
10 to 20 µm and approximately 20 to 100 µm,
respectively.

We also observed some CAIs with a coarser
texture (grains with sizes up to 1.5 mm). These coarse-
grained CAIs have sizes ranging from approximately
200 µm to approximately 2 cm and are texturally
similar to the type B or “compact” type A inclusions
(Brearley and Jones [1998]; and references therein). The
largest coarse-grained CAIs display some textural
features similar to those of type B1 inclusions: they
have a fassaite-rich core and are surrounded by a
coarse-grained rim that is 1–2 mm thick (Fig. 1).
Described in CVs, these rims have been interpreted as
accretionary rims (e.g., Brearley and Jones [1998]; and
references therein). In addition, a fluffy mantle, which
can contain chondrules, surrounds some of these
coarse-grained CAIs. During this study, we studied in
detail one coarse-grained CAI in Tnz 057 (CG1-CAI,
preliminary description of which was made by
Chaumard et al. 2011), one coarse-grained CAI in
DaG 431 (CG2-CAI, grossular-rich), and two coarse-
grained CAIs in Karoonda (CG3-CAI, which is rich in fibrous pyroxene, and CG4-CAI, which is grossular-rich). One of these coarse-grained CAIs (CG1-CAI from Tnz 057) was selected for the Al/Mg and Mg isotopic analyses by LA-MC-ICPMS.

## Modal Abundances of CAIs in CVs and CKs

We measured modal abundances and sizes of CAIs for a variety of CV and CK chondrites and present these in Table 3 and Fig. 2.

### Modal Abundances of CAIs in CV Chondrites

We analyzed sections of Allende covering a total area of 9.2 cm². We determined that this CV3 chondrite has 10.3% of CAIs, of which 0.9% are coarse-grained. The abundance of CAIs measured in this study is close to the value reported by McSween (1977) (9.4%), whereas Simon and Haggerty (1979), Kornacki and Wood (1984), May et al. (1999), Hezel et al. (2008), and Ebel et al. (2008, 2009) reported values ranging from 2.5 to 7.1%. Equivalent radii of fine-grained CAIs in Allende have a mean value of 210 µm, and approximately 70% of fine-grained CAIs have equivalent radii less than 250 µm.

We found 3.4 and 8.1% of CAIs in the 0.75 cm² of Kaba and the 1.83 cm² of Bali that were investigated, respectively. No coarse-grained CAIs were observed. In comparison, McSween (1977) reported 4% of CAIs in Bali and 7.4% in Kaba.

In the 12.15 cm² of NWA 779 that was studied, we measured 12.2 ± 5.9% of CAIs. No coarse-grained CAIs were observed. The size distribution of fine-grained CAIs in NWA 779 displays a peak at radii between 125 and 250 µm followed by a monotonic decrease in abundance for larger sizes, with a mean value of 285 µm. Approximately 55% of fine-grained CAIs have equivalent radii less than 250 µm.

The 25.56 cm² area studied of NWA 2900 contains 13.1 ± 2.3% of CAIs, of which 4.7 ± 2.7% are coarse-grained. Equivalent radii of fine-grained CAIs in NWA 2900 have a mean value of 360 µm, with a peak between 250 and 375 µm. Approximately 35% of fine-grained CAIs have equivalent radii less than 250 µm.

### Modal Abundances of CAIs in CK Chondrites

We analyzed 18 different CK chondrites (Table 3). We did not observe any CAI in Kobe (0.56 cm² section), NWA 2519 (1.41 cm²), Elephant Moraine (EET) 90007 (1.25 cm²), and Lewis Cliff (LEW) 87009 (0.77 cm²). For other CKs, CAI modal abundances vary from 0.1% in Maralinga (4.05 cm²) and Karoonda (2.54 cm²), to 16.4% in NWA 4423 (1.93 cm²),

<figure><img src="image_2.png" /><figcaption>Fig. 2. Size distributions of fine-grained CAIs. Only results from Allende, Northwest Africa (NWA) 779, NWA 2900, NWA 4425, and Tnz 057 are presented because the number of CAIs in other CV and CK samples analyzed is insufficient to produce representative histograms.</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 3. Compilation of the abundance (area%) of CAIs in CV and CK samples based on this study and previous literature data.</figcaption></figure>

overlapping with values found for CV chondrites (Fig. 3). The value obtained for Karoonda (0.1%) is similar to that reported by Noguchi (1993) (0.2%), while Hezel et al. (2008) measured 5.74% CAIs in this meteorite.

Histograms of fine-grained CAI size distribution in NWA 4425 (25.24 cm²) and Tnz 057 (893 cm²) are presented in Fig. 2. Both have a log-normal distribution with a peak between 250 and 375 µm followed by a monotonic decrease in abundance for larger sizes. The mean value of fine-grained CAI radii is 445 µm in NWA 4425 and 490 µm in Tnz 057. Approximately 56.3% of fine-grained CAI equivalent radii in NWA 4425, and only 25% in Tnz 057, are less than 250 µm.

Coarse-grained CAIs are scarce and often larger than fine-grained CAIs (from 0.18 mm in Karoonda to 21.73 mm in Tnz 057, in equivalent diameter) (Table 3). Karoonda is the only CK that showed exclusively coarse-grained CAIs.

## Mineralogy and Textures of CAIs in CK Chondrites

All phases observed in CAIs in CK chondrites are described below and are presented in the order of decreasing abundance (the range of modal abundance calculated from fine- and coarse-grained CAIs is given in parentheses). The terms “primary” and “secondary” used below are interpretative and based on the textural relationship between minerals present in CAIs. High-temperature phases such as spinel and anorthite are considered as primary minerals if they have equilibrated textures (e.g., euhedral grains, triple junctions), or secondary if they display reaction textures (e.g., anhedral grains) (e.g., see euhedral and anhedral spinel grains in Fig. 4). The observation of replacement

textures at grain boundaries is another argument supporting a primary origin. Minerals in veins are interpreted as secondary, whereas crosscut grains are primary (e.g., fassaite, spinel, and rhönite crosscut by olivine and Ca-rich pyroxene-bearing veins in Figs. 5f and 5g). Grains physically separated but observed to be in optical continuity are considered as primary because they initially formed a single grain. For example, Fig. 4 shows the image in polarized light of two anorthite grains surrounded by grossular. The continuous polysynthetic twinning of both anorthite grains indicates that anorthite is a primary phase, whereas grossular is secondary.

## Plagioclase (Approximately 10–90%)

All CAIs contain plagioclase. We observed euhedral primary plagioclases in the CK3 chondrites studied here. These grains have sizes ranging between 20 and 500 µm, display neither opaque inclusions nor vesicularity, and often occur in the cores of CAIs (Figs. 4 and 5a). Some other secondary plagioclase grains in CAIs are similar to those present in the matrix. They are 30–200 µm in size, contain tiny opaque inclusions of magnetite and sulfide, and are mostly present at the outer parts of the CAIs (Figs. 5b and 5e). As previously described in CK chondrites (Scott and Taylor 1985; Geiger and Bischoff 1991; Noguchi 1993), plagioclase compositions in CAIs display considerable variations. They are within the range of An₃₅–An₁₀₀ in DaG 431, An₁₉–An₃₆ in NWA 4425, An₁₅–An₈₆ in NWA 4724, An₄₀–An₁₀₀ in Karoonda, and An₂₇–An₉₉ in Tnz 057. Polysynthetically twinned plagioclase is rare in CAIs in CK3, but is more common in CK4 chondrites. Usually, K₂O contents range from 0.0 to 0.5 wt% in CK3 and from 0.0 to 1.0 wt% in CK4 chondrites. Plagioclase also contains
<figure><img src="image_4.png" /><figcaption>Fig. 4. An area of the coarse-grained CG2-CAI in DaG 431 (CK3-an) composed of fassaite (Fas), spinel (Sp), anorthite (An), grossular (Grs), and olivine (Ol). a) Image in polarized light. A primary grain of anorthite displays polysynthetic twinning and is surrounded by secondary grossular. The dark region in the upper-right corner is the matrix. b) BSE image of secondary grossular embedded in primary anorthite, fassaite, and spinel grains. c) Higher magnification of the boxed region in (b), showing numerous secondary Fe-rich spinel grains (<10 µm in size) within the secondary grossular near the boundary with primary anorthite. The variation in contrast in the backscattered electron (BSE) image among the different spinel grains is indicative of slight variation in their Fe/Mg ratio. Grossular contains few tiny opaque grains of ilmenite and sulfur.</figcaption></figure>

some amount of MgO, up to 0.5 wt% in CK3 and up to 0.7 wt% in CK4 chondrites. In addition, this phase has significant FeO<sub>tot</sub> contents, up to 1.4 wt% in CK3 and from 0.1 to 0.8 wt% in CK4 chondrites. Representative analyses of plagioclase are given in Table 4.

The accretionary rim of the CG1-CAI in Tnz 057 contains interstitial zoned plagioclases, with compositions either close to anorthite (An<sub>97.7</sub> ± 0.5) or more intermediate compositions (An<sub>42.1</sub> ± 13.4) (Fig. 4). Grains from the inner part of the accretionary rim have more homogeneous compositions (An<sub>59.6-97.1</sub>) compared with plagioclase from the outer part (An<sub>38.0-98.0</sub>). The core of this CG1-CAI contains fine-grained plagioclases of almost pure anorthite (An<sub>99.4</sub>) (Figs. 5h and 6). Veins of anorthite (<5 µm width) crosscut rhönite and grossular (Fig. 5h).

The only difference observed between the matrix and the fluffy mantle of the CG1-CAI in Tnz 057 is the abundance of plagioclase. The fluffy mantle contains more plagioclase grains, sometimes zoned with calcic cores and sodic borders, and with compositions ranging from approximately An<sub>15</sub> to An<sub>100</sub>.

Ca-rich Pyroxene (Approximately 5–90%)

In CAIs in CKs, compositions of Ca-rich pyroxene range continuously from En<sub>40.5</sub>Fs<sub>9.0</sub>Wo<sub>50.5</sub> to En<sub>58.6</sub>Fs<sub>16.9</sub>Wo<sub>24.5</sub> and can contain up to 7.9 wt% Al<sub>2</sub>O<sub>3</sub> and 3.3 wt% TiO<sub>2</sub>. Representative analyses of Ca-rich pyroxene grains are given in Table 5.

Fine-grained CAIs contain Ca-rich pyroxene in variable proportions (approximately 10–90%) (Figs. 5a, 5b, and 5e), either as laths (2–350 µm size) or as rounded grains (5–20 µm), forming a fleecy texture (Fig. 5b). Laths of Ca-rich pyroxene occur in fine-grained CAIs in CK3 and CK4 chondrites, with MgO-rich cores and Fe-rich rims.

In the CG1-CAI in Tnz 057, Ca-rich pyroxene-bearing veins (approximately 15 µm width and up to approximately 1 mm length) crosscut primary fassaite and rhönite, and secondary grossular (Figs. 5g, 5h, and 7). Composition of Ca-rich pyroxene in these veins is En<sub>49.7</sub> ± 5.4Fs<sub>2.1</sub> ± 1.9Wo<sub>48.2</sub> ± 3.5, with significant amounts of Al<sub>2</sub>O<sub>3</sub> (3.9 ± 1.6 wt%) and TiO<sub>2</sub> (1.3 ± 0.7 wt%). The coarse-grained accretionary rim of the CG1-CAI contains Ca-rich pyroxene either with irregular shapes between 10 and 50 µm in size, or locally with rounded grains up to 300 µm (Fig. 6). By comparison with the veins present in the core of the CG1-CAI, Ca-rich pyroxene in the coarse-grained accretionary rim is enriched in FeO (En<sub>41.9</sub> ± 1.7 Fs<sub>8.1</sub> ± 1.3Wo<sub>50.0</sub> ± 0.4). The Al<sub>2</sub>O<sub>3</sub> and TiO<sub>2</sub> contents decrease from the inner to the outer part of the accretionary rim from 5.3 to 2.0 wt% and from 2.2 to 0.1 wt%, respectively. In contrast, the FeO content
<table><caption>Table 4. Chemical compositions of representative plagioclase grains from CAIs in DaG 431 (CK3-an), NWA 4425 (CK3.8), Karoonda (CK4), and Tnz 057 (CK4).</caption><thead><tr><th rowspan="3"></th><th colspan="4">Plagioclase</th><th rowspan="3">(CG1-CAI core)</th></tr><tr><th>DaG 431</th><th>NWA 4425</th><th>Karoonda</th><th>Tnz 057</th></tr><tr><th colspan="4">(primary crystal)</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>43.03</td><td>60.92</td><td>59.00</td><td>42.58</td><td>49.87</td><td>43.16</td></tr><tr><td>TiO<sub>2</sub></td><td>0.05</td><td>0.04</td><td>n.d.</td><td>0.02</td><td>n.d.</td><td>0.01</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>35.08</td><td>24.33</td><td>26.02</td><td>35.67</td><td>32.31</td><td>36.45</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>n.d.</td><td>0.01</td><td>0.03</td><td>0.01</td><td>0.09</td><td>n.d.</td></tr><tr><td>FeO</td><td>0.02</td><td>0.75</td><td>0.47</td><td>0.54</td><td>0.64</td><td>0.13</td></tr><tr><td>NiO</td><td>0.01</td><td>0.03</td><td>n.d.</td><td>0.03</td><td>0.03</td><td>n.d.</td></tr><tr><td>MgO</td><td>0.16</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.05</td><td>n.d.</td></tr><tr><td>CaO</td><td>19.80</td><td>6.12</td><td>7.98</td><td>20.32</td><td>15.07</td><td>20.31</td></tr><tr><td>NaO</td><td>0.06</td><td>7.78</td><td>6.89</td><td>n.d.</td><td>2.72</td><td>0.07</td></tr><tr><td>K<sub>2</sub>O</td><td>n.d.</td><td>0.47</td><td>0.36</td><td>n.d.</td><td>0.08</td><td>0.01</td></tr><tr><td>Total</td><td>98.21</td><td>100.45</td><td>100.75</td><td>99.17</td><td>100.87</td><td>100.14</td></tr><tr><td colspan="7">Cation number in the basis of 8 oxygens</td></tr><tr><td>Si</td><td>2.029</td><td>2.708</td><td>2.624</td><td>1.998</td><td>2.261</td><td>1.999</td></tr><tr><td>Ti</td><td>0.002</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.000</td></tr><tr><td>Al</td><td>1.949</td><td>1.275</td><td>1.364</td><td>1.972</td><td>1.727</td><td>1.990</td></tr><tr><td>Cr</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.003</td><td>0.000</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.001</td><td>0.028</td><td>0.018</td><td>0.021</td><td>0.024</td><td>0.005</td></tr><tr><td>Ni</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.001</td><td>0.000</td></tr><tr><td>Mg</td><td>0.011</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.003</td><td>0.000</td></tr><tr><td>Ca</td><td>1.000</td><td>0.292</td><td>0.380</td><td>1.022</td><td>0.732</td><td>1.008</td></tr><tr><td>Na</td><td>0.006</td><td>0.670</td><td>0.594</td><td>0.000</td><td>0.239</td><td>0.007</td></tr><tr><td>K</td><td>0.000</td><td>0.027</td><td>0.020</td><td>0.000</td><td>0.005</td><td>0.001</td></tr><tr><td>Σ cations</td><td>4.998</td><td>5.002</td><td>5.001</td><td>5.015</td><td>4.995</td><td>5.010</td></tr><tr><td colspan="7">Cationic Ca:Na:K ratio (in%)</td></tr><tr><td>An</td><td>99.4</td><td>29.5</td><td>38.2</td><td>100.0</td><td>75.0</td><td>99.2</td></tr><tr><td>Ab</td><td>0.6</td><td>67.8</td><td>59.8</td><td>0.0</td><td>24.5</td><td>0.7</td></tr><tr><td>Or</td><td>0.0</td><td>2.7</td><td>2.0</td><td>0.0</td><td>0.5</td><td>0.1</td></tr></tbody></table>

Element detection limits (in wt%): Si: 0.03; Ti: 0.06; Al: 0.03; Cr: 0.08; Fe: 0.07; Ni: 0.06; Mg: 0.03; Ca: 0.05; Na: 0.05; K: 0.05.

n.d. = not detected; CAI = calcium-aluminum-rich inclusion.

increases from 5.0 wt% in the inner part to 8.5 wt% in the outer part (Fig. 6).

### Fassaite (Approximately 5–70%)

Primary Al, Ti-rich diopside, officially defined by the term “subsilicic titanoan aluminian pyroxene” (Morimoto et al. 1988), but often referred to by the more convenient appellation “fassaite,” occurs only in coarse-grained CAIs. Fassaite grains can range up to 1.5 mm in size and often enclose primary euhedral spinels (Figs. 4, 5c, 5f, 5g, 6, and 7). In CK3 chondrites, fassaite contains 5.2–11.5 wt% TiO₂, 15.8–21.9 wt% Al₂O₃, 6.6–11.0 wt% MgO, and less than 0.1 wt% FeO (value below the detection limit). Compositions vary from one CAI to another, but are fairly constant among all grains within a single CAI. Representative analyses are given in Table 5.

Fassaite in the core of the CG1-CAI in Tnz 057 occurs as well-defined anhedral grains containing 16.5 ± 2.5 wt% TiO₂ and 18.9 ± 1.2 wt% Al₂O₃.

Elemental mapping reveals a heterogeneous distribution of TiO₂ concentrations, which is also observable under enhanced BSE contrast (Fig. 7). Fassaite grain borders are frequently destabilized to diopside and ilmenite (see the Discussion section).

Partially destabilized grains of fassaite (20 µm) occur in the CG3-CAI in Karoonda (Fig. 5d). Grains have irregular shapes and are surrounded by anorthite and Ca-poor pyroxene. They contain 6.9 ± 0.4 wt% TiO₂, 19.4 ± 0.1 wt% Al₂O₃, and 0.48 ± 0.02 wt% FeO.

### Grossular (Approximately 10–50%)

We found garnet with a composition close to the grossular endmember (Table 6) in a coarse-grained CAI in NWA 1559, in the CG1-CAI in Tnz 057, the CG2-CAI in DaG 431, and the CG4-CAI in Karoonda.

Grossular is often associated with anorthite, but two different relations between these minerals can be observed. First, irregular masses of grossular surround
<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

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

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /></figure>

<figure><img src="image_12.png" /></figure>
Fig. 5. Backscattered electron images of CAIs from various CK samples. a) Fine-grained CAI with a typical “fluffy” aspect in Northwest Africa (NWA) 1559 (CK3), composed of anorthite and Ca-rich pyroxene. b) Fine-grained CAI from NWA 4724 (CK3.8) composed of anorthite and rounded grains of Ca-rich pyroxene, which form a fleecy texture. c) Part of a coarse-grained CAI in DaG 431 (CK3-an) composed of a primary spinel-fassaite-anorthite assemblage. Spinels are chemically zoned. Some external parts of fassaite contain ilmenite and titanomagnetite grains elongated perpendicularly to the anorthite-fassaite boundary. d) Coarse-grained CG3-CAI in Karoonda (CK4) composed of a primary spinel-fassaite-anorthite-pyroxene assemblage. Spinel grains are chemically equilibrated with the matrix and have compositions close to the hercynite endmember. Fassaite occurs as two relic grains. Ca-poor pyroxene is vesiculated, has a fibrous texture, and contains abundant Ti-oxides. e) Recrystallized fine-grained CAI in Tnz 057 (CK4) composed of plagioclase, Ca-rich pyroxene, olivine, and magnetite. Olivine and plagioclase are equilibrated with the matrix. The dashed line marks the supposed initial boundary between the CAI and matrix. f) An area of the core of the coarse-grained CG1-CAI in Tnz 057 (CK4). A fine-grained secondary anorthite-grossular mixture encloses primary spinel and fassaite grains crosscut by several forsterite-bearing veins. g) Boundary of the core and the coarse-grained accretionary rim (upper-left corner) of the coarse-grained CG1-CAI in Tnz 057 (CK4). This region is mainly composed of Ca-rich pyroxene, with small amount of plagioclase and opaque grains. Ca-rich pyroxene-bearing veins crosscut the primary fassaite-spinel-rhönite assemblage. Numerous Ti-oxides occur as elongated grains perpendicular to the fassaite-Ca-rich pyroxene boundary and along secondary veins crosscuting rhönite grains. (h) Detail of the secondary anorthite-grossular fine-grained mixture in the core of the coarse-grained CG1-CAI in Tnz 057 (CK4). Ca-px = Ca-rich pyroxene; Px = Ca-poor pyroxene; An: anorthite; Plg = plagioclase; Mag = magnetite; Sp = spinel; Fas = fassaite; Ti-ox = Ti-oxide; Ol = ferromagnesian olivine; Fo = forsterite; Grs = grossular; Rh = rhönite; Sulf = sulfide.

<figure><img src="image_13.png" /></figure>

Fig. 6. Wavelength-dispersive spectrometry compositional map in RGB colors (red = Ca, green = Al, blue = Mg) of an area of the coarse-grained CG1-CAI in Tnz 057 (CK4), showing fassaite (Fas), spinel (Sp), Ca-rich pyroxene (Ca-px), anorthite (An), Na-rich plagioclase (Na-plg), grossular (Grs), olivine (Ol), and apatite (Ap). A) Core of the CAI, B) coarse-grained accretionary rim, and C) fluffy mantle. Zonation in Fe content is visible in spinel and pyroxene at the outer part of the coarse-grained accretionary rim. The primary assemblage of fassaite and euhedral spinels is surrounded by the secondary fine-grained anorthite-grossular mixture. Forsterite-bearing veins crosscut spinels, while Ca-rich pyroxene and anorthite-bearing veins crosscut fassaite and grossular grains.

primary anorthite, as in the CG2-CAI in DaG 431 (Fig. 4). In this case, grossular compositions are within the range of Gr89.4–Gr99.2, Py0–Py10.3, And0–And1.5, and Alm0–Alm1.1. Second, grossular forms a fine-grained mixture with anorthite. This secondary assemblage

surrounds primary spinel and fassaite, as in the CG4-CAI in Karoonda and in the CG1-CAI in Tnz 057 (Figs. 5h and 6). Composition of this garnet ranges from Gr72.7Py8.4 to Gr99.3Py0.3, with 2.19 wt% TiO2, and up to 4.8 wt% FeO and 4.4 wt% Fe2O3.
<table><caption>Table 5. Chemical compositions of representative pyroxene grains from CAIs in NWA 4724 (CK3.8), DaG 431 (CK3-an), NWA 4423 (CK3.9), Karoonda (CK4), and Tnz 057 (CK4).</caption><thead><tr><th rowspan="3"></th><th colspan="4">Ca-rich pyroxene</th><th colspan="4">Fassaite</th><th colspan="2">Low-Ca pyroxene</th></tr><tr><th>NWA 4724</th><th>NWA 4423</th><th colspan="2">Tnz 057</th><th>DaG 431</th><th>Karoonda</th><th>Tnz 057</th><th rowspan="2">Karoonda</th><th rowspan="2">Tnz 057</th></tr><tr><th></th><th></th><th colspan="2">(veins in CG1-CAI rim)</th><th colspan="2">(CG1-CAI core)</th><th colspan="2">(CG1-CAI core)</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>50.86</td><td>53.54</td><td>49.38</td><td>52.91</td><td>40.36</td><td>38.67</td><td>34.16</td><td>53.10</td><td>50.87</td></tr><tr><td>TiO<sub>2</sub></td><td>0.57</td><td>0.09</td><td>2.15</td><td>0.72</td><td>2.41</td><td>2.22</td><td>1.51</td><td>0.08</td><td>0.07</td></tr><tr><td>Ti<sub>2</sub>O<sub>3</sub></td><td>n.c.</td><td>n.c.</td><td>n.c.</td><td>n.c.</td><td>3.12</td><td>4.86</td><td>14.77</td><td>n.c.</td><td>n.c.</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>5.08</td><td>0.67</td><td>5.28</td><td>2.03</td><td>18.09</td><td>19.41</td><td>17.93</td><td>10.51</td><td>8.80</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.10</td><td>n.d.</td><td>0.06</td><td>0.01</td><td>0.08</td><td>0.07</td><td>0.11</td><td>0.05</td><td>0.12</td></tr><tr><td>FeO</td><td>4.34</td><td>7.14</td><td>5.04</td><td>2.28</td><td>n.d.</td><td>0.00</td><td>0.00</td><td>0.00</td><td>16.45</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>0.55</td><td>0.00</td><td>0.74</td><td>0.54</td><td>n.c.</td><td>0.47</td><td>0.20</td><td>0.05</td><td>0.08</td></tr><tr><td>NiO</td><td>0.01</td><td>n.d.</td><td>n.d.</td><td>0.02</td><td>n.d.</td><td>0.02</td><td>0.01</td><td>0.01</td><td>0.04</td></tr><tr><td>MnO</td><td>0.13</td><td>n.d.</td><td>0.03</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>n.d.</td><td>n.d.</td></tr><tr><td>MgO</td><td>13.89</td><td>13.81</td><td>12.81</td><td>16.01</td><td>10.37</td><td>9.09</td><td>7.45</td><td>8.93</td><td>25.64</td></tr><tr><td>CaO</td><td>24.91</td><td>24.59</td><td>25.27</td><td>25.80</td><td>25.17</td><td>25.02</td><td>23.88</td><td>24.52</td><td>0.28</td></tr><tr><td>NaO</td><td>0.04</td><td>0.13</td><td>0.15</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>0.09</td></tr><tr><td>K<sub>2</sub>O</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.01</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.02</td><td>0.04</td></tr><tr><td>Total</td><td>100.46</td><td>99.98</td><td>100.92</td><td>100.33</td><td>99.62</td><td>99.82</td><td>100.03</td><td>100.40</td><td>99.99</td></tr></tbody></table><p><em>Cation number in the basis of 6 oxygens</em></p><table><tbody><tr><td>Si</td><td>1.867</td><td>1.990</td><td>1.820</td><td>1.929</td><td>1.488</td><td>1.433</td><td>1.299</td><td>1.412</td><td>1.912</td><td>1.813</td></tr><tr><td>Ti<sup>4+</sup></td><td>0.016</td><td>0.003</td><td>0.060</td><td>0.020</td><td>0.067</td><td>0.062</td><td>0.043</td><td>0.039</td><td>0.002</td><td>0.002</td></tr><tr><td>Ti<sup>3+</sup></td><td>—</td><td>—</td><td>—</td><td>—</td><td>0.096</td><td>0.150</td><td>0.469</td><td>0.329</td><td>—</td><td>—</td></tr><tr><td>Al</td><td>0.220</td><td>0.029</td><td>0.229</td><td>0.087</td><td>0.786</td><td>0.848</td><td>0.804</td><td>0.743</td><td>0.180</td><td>0.370</td></tr><tr><td>Cr</td><td>0.003</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.002</td><td>0.002</td><td>0.003</td><td>0.002</td><td>0.003</td><td>0.003</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.133</td><td>0.222</td><td>0.155</td><td>0.069</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.495</td><td>0.401</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.015</td><td>0.000</td><td>0.020</td><td>0.015</td><td>0.000</td><td>0.014</td><td>0.006</td><td>0.002</td><td>0.000</td><td>0.002</td></tr><tr><td>Ni</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.000</td><td>0.003</td><td>0.001</td></tr><tr><td>Mn</td><td>0.004</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.008</td><td>0.000</td></tr><tr><td>Mg</td><td>0.760</td><td>0.765</td><td>0.704</td><td>0.870</td><td>0.570</td><td>0.502</td><td>0.422</td><td>0.499</td><td>1.376</td><td>1.401</td></tr><tr><td>Ca</td><td>0.980</td><td>0.979</td><td>0.998</td><td>1.008</td><td>0.994</td><td>0.993</td><td>0.973</td><td>0.985</td><td>0.011</td><td>0.005</td></tr><tr><td>Na</td><td>0.003</td><td>0.009</td><td>0.011</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.006</td><td>0.003</td></tr><tr><td>K</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.001</td><td>0.000</td></tr><tr><td>Σ cations</td><td>4.001</td><td>3.997</td><td>4.000</td><td>3.999</td><td>4.003</td><td>4.005</td><td>4.019</td><td>4.013</td><td>3.998</td><td>4.000</td></tr></tbody></table>

Element detection limits (in wt%): Si: 0.06; Ti: 0.12; Al: 0.06; Cr: 0.14; Fe: 0.18; Ni: 0.12; Mn: 0.15; Mg: 0.06; Ca: 0.11; Na: 0.11; K: 0.10.

n.d. = not detected; n.c. = not calculated; CAI = calcium-aluminum-rich inclusion.

## Spinel (Approximately 5–25%)

Only coarse-grained CAIs in DaG 431, NWA 1559, Karoonda, and Tnz 057 contain primary spinel. These primary grains, enclosed in plagioclase, fassaite, or grossular, are euhedral with sizes between 10 and 200 µm (Figs. 4, 5c, 5d, 5f, 5g, 6, and 7). FeO and MgO concentrations range from 4.8–21.4 to 18.1–24.4 wt%, respectively. In the core of CAIs, spinels are MgO-rich, while those situated at the boundaries are closer to the hercynite endmember. In DaG 431, primary spinels have significant quantities of TiO₂ (0.2–1.1 wt%) and Cr₂O₃ (0.2–2.6 wt%). In CK4, they contain up to 1.8 wt% TiO₂. Representative analyses of spinels are given in Table 6.

Euhedral spinels in the CG1-CAI in Tnz 057 are chemically re-equilibrated along a diffusion gradient

of Fe (Fig. 6): from the boundary to the core of the CAI, FeO contents in spinel vary from 19.5 to 4.7 wt%. In the CG3-CAI in Karoonda, euhedral spinels are enriched in FeO (24.5 ± 1.0 wt%), and all grains are chemically equilibrated with the host matrix (Fig. 5d).

Spinels with irregular shapes and containing numerous cracks and vesicles were observed in the CG4-CAI in Karoonda and the CG1-CAI in Tnz 057. These grains have sizes up to 200 µm and are crosscut by forsterite-bearing veins. Except for the FeO content, no chemical differences have been observed between euhedral and irregular spinel grains.

In the core of the CG1-CAI in Tnz 057, we additionally observed a few spinel grains (approximately 5 µm in size) within the secondary anorthite-grossular
<figure><img src="image_14.png" /></figure>

Fig. 7. a) Backscattered electron (BSE) image and b) energy dispersive spectroscopy compositional map in RGB colors (red = Ca, green = Al, blue = Mg) of an area in the core of the coarse-grained CG1-CAI in Tnz 057 (CK4) showing fassaite (Fas), spinel (Sp), Ca-rich pyroxene (Ca-px), and forsterite (Fo). Forsterite-bearing veins are in contact with spinel grains, while veins through fassaite are composed of Ca-rich pyroxene. Fassaite displays a “marble” zonation in the BSE image (a) caused by variation in the TiO₂ content.

assemblage and associated with the Ca-rich pyroxene and anorthite-bearing veins (Fig. 5h).

Numerous spinel grains, 2–10 µm in size, occur in the CG2-CAI in DaG 431 and the CG4-CAI in Karoonda. These Fe-rich grains are surrounded by secondary grossular and are located close to primary anorthite (Fig. 4).

### Ca-Poor Pyroxene (<25%)

Ca-poor pyroxene was observed in some CAIs in NWA 4425, NWA 4724, Karoonda, and Tnz 057. Grains are between approximately 3 and approximately 20 µm, often display lath-shaped aspects, and appear to be primary phases. They have rather constant

compositions with 26.9 ± 0.9 wt% MgO, 14.3 ± 1.2 wt% FeO, and 0.4 ± 0.3 wt% CaO. Representative analyses of Ca-poor pyroxene are given in Table 5.

All pyroxene grains analyzed contain significant amounts of Al₂O₃, 3.9–5.2 wt% in NWA 4425, 4.9–5.5 wt% in NWA 4724, 2.8–5.0 wt% in Karoonda, and 4.5–8.8 wt% in Tnz 057. Al-rich low-Ca pyroxenes (Al₂O₃ contents up to 16.3 wt%) described in CAIs (Fuchs 1969; Rubin 2004) are always Fe-poor, in contrast with grains analyzed in this study (Table 5) that display high FeOₜot contents (11.8–16.5 wt%, with an average of 14.2 ± 1.2 wt%) and a chemical composition similar to terrestrial pyroxenes (Rubin [2004]; and references therein). Ca-poor pyroxenes also contain small amounts of Cr₂O₃ (0.1–0.7 wt%) and MnO (0.1–0.3 wt%).

The CG3-CAI in Karoonda contains secondary Ca-poor pyroxene (Fig. 5d). These grains are vesiculated and have jagged edges. They contain 26.2 ± 1.0 wt% MgO and 15.2 ± 1.4 wt% FeO.

### Olivine (<10%)

Olivine is a minor phase in all CAIs investigated, with typical modal abundances lower than 10%. However, we observed two fine-grained CAIs in NWA 4425 and Tnz 057 containing 20 and 40% of olivine (i.e., Fig. 5e), respectively. With sizes between approximately 10 and approximately 150 µm, olivine grains contain scarce vesicles and inclusions of opaque minerals (mainly magnetites and sulfides) in comparison with matrix olivines. In all petrologic types, olivines in fine-grained CAIs have homogeneous compositions that are chemically equilibrated with the matrix (Fa₃₁.₆ ± 1.₅) and contain 0.35 ± 0.05 wt% NiO. Representative analyses of olivine are given in Table 6.

In the CG1-CAI in Tnz 057, forsteritic olivine (Fa₉.₉ ± 0.₅) occurs in fractures crosscutting primary fassaite and spinel (Figs. 7 and 6, respectively). These grains contain 0.15 ± 0.6 wt% NiO and 0.25 ± 0.13 wt % TiO₂.

### Magnetite (<5%)

We observed two types of magnetite. The first type of magnetite is composed of grains up to few hundred microns in size, sometimes with a euhedral shape and containing no inclusions. The second type is composed of tiny grains (<10 µm in size), commonly occurring at the CAI edges. There is no chemical difference between the two types. Representative analyses are given in Table 7. As previously described in CKs (Geiger and Bischoff 1995; Greenwood et al. 2010), magnetite in CAIs contains significant amounts of MgO, Al₂O₃, TiO₂, and Cr₂O₃. In CK3, magnetite grains in CAIs contain up to 2.79 wt% MgO, 3.94 wt% Al₂O₃,
<table><caption>Table 6. Chemical compositions of representative grossular, olivine, and spinel grains from CAIs in DaG 431 (CK3-an), NWA 4425 (CK3.8), Karoonda (CK4), and Tnz 057 (CK4).</caption><thead><tr><th rowspan="3"></th><th colspan="3">Grossular</th><th colspan="3">Olivine</th><th colspan="3">Spinel</th></tr><tr><th>DaG 431</th><th>Karoonda</th><th>Tnz 057</th><th>NWA 4425</th><th>Tnz 057</th><th>CG1-CAI core</th><th>DaG 431</th><th>Karoonda</th><th>Tnz 057</th></tr><tr><th colspan="3">(CG1-CAI core)</th><th colspan="3">(CG1-CAI core)</th><th colspan="3">(CG1-CAI core)</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>39.66</td><td>37.37</td><td>39.24</td><td>37.12</td><td>37.66</td><td>41.04</td><td>0.06</td><td>n.d.</td><td>n.a.</td></tr><tr><td>TiO<sub>2</sub></td><td>0.06</td><td>2.19</td><td>0.18</td><td>0.09</td><td>n.d.</td><td>0.16</td><td>0.35</td><td>0.30</td><td>0.07</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>22.79</td><td>19.96</td><td>20.90</td><td>0.37</td><td>n.d.</td><td>0.03</td><td>62.08</td><td>68.51</td><td>58.42</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>n.d.</td><td>0.01</td><td>0.04</td><td>0.07</td><td>n.a.</td><td>n.a.</td><td>0.25</td><td>0.24</td><td>0.31</td></tr><tr><td>FeO</td><td>0.00</td><td>0.95</td><td>5.55</td><td>28.28</td><td>27.23</td><td>8.98</td><td>20.50</td><td>4.83</td><td>22.20</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>0.53</td><td>3.79</td><td>1.87</td><td>0.00</td><td>0.00</td><td>0.00</td><td>1.00</td><td>0.00</td><td>3.80</td></tr><tr><td>NiO</td><td>n.d.</td><td>0.02</td><td>0.03</td><td>0.28</td><td>0.30</td><td>0.16</td><td>0.19</td><td>0.02</td><td>0.99</td></tr><tr><td>MnO</td><td>0.09</td><td>0.03</td><td>n.d.</td><td>n.d.</td><td>n.d.</td><td>0.05</td><td>0.04</td><td>n.d.</td><td>0.11</td></tr><tr><td>MgO</td><td>0.03</td><td>2.22</td><td>0.46</td><td>33.62</td><td>34.50</td><td>48.33</td><td>13.74</td><td>24.44</td><td>11.68</td></tr><tr><td>CaO</td><td>37.63</td><td>32.93</td><td>31.61</td><td>0.04</td><td>0.10</td><td>0.13</td><td>0.05</td><td>0.02</td><td>0.01</td></tr><tr><td>NaO</td><td>n.d.</td><td>n.d.</td><td>n.d.</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></tr><tr><td>K<sub>2</sub>O</td><td>n.d.</td><td>n.a.</td><td>0.03</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></tr><tr><td>Total</td><td>100.79</td><td>99.46</td><td>99.92</td><td>99.87</td><td>99.79</td><td>98.87</td><td>98.27</td><td>98.36</td><td>97.59</td></tr></tbody></table><table><thead><tr><th></th><th colspan="3">Cation number in the basis of 12 oxygens</th><th colspan="3">Cation number in the basis of 4 oxygens</th><th colspan="3">Cation number in the basis of 4 oxygens</th></tr></thead><tbody><tr><td>Si</td><td>2.961</td><td>2.856</td><td>2.997</td><td>0.995</td><td>1.005</td><td>1.012</td><td>0.002</td><td>0.000</td><td>—</td></tr><tr><td>Ti</td><td>0.003</td><td>0.126</td><td>0.011</td><td>0.002</td><td>0.000</td><td>0.003</td><td>0.007</td><td>0.006</td><td>0.002</td></tr><tr><td>Al</td><td>2.006</td><td>1.798</td><td>1.882</td><td>0.012</td><td>0.000</td><td>0.001</td><td>1.954</td><td>1.991</td><td>1.899</td></tr><tr><td>Cr</td><td>0.000</td><td>0.000</td><td>0.003</td><td>0.002</td><td>—</td><td>—</td><td>0.005</td><td>0.005</td><td>0.007</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.000</td><td>0.061</td><td>0.355</td><td>0.634</td><td>0.608</td><td>0.185</td><td>0.458</td><td>0.100</td><td>0.513</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.030</td><td>0.218</td><td>0.107</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.021</td><td>0.000</td><td>0.080</td></tr><tr><td>Ni</td><td>0.000</td><td>0.001</td><td>0.002</td><td>0.006</td><td>0.006</td><td>0.003</td><td>0.004</td><td>0.000</td><td>0.022</td></tr><tr><td>Mn</td><td>0.005</td><td>0.002</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.001</td><td>0.000</td><td>0.003</td></tr><tr><td>Mg</td><td>0.003</td><td>0.252</td><td>0.052</td><td>1.344</td><td>1.372</td><td>1.776</td><td>0.547</td><td>0.899</td><td>0.480</td></tr><tr><td>Ca</td><td>3.010</td><td>2.696</td><td>2.587</td><td>0.001</td><td>0.003</td><td>0.003</td><td>0.001</td><td>0.001</td><td>0.000</td></tr><tr><td>Na</td><td>0.000</td><td>0.000</td><td>0.000</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>K</td><td>0.000</td><td>—</td><td>0.003</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Σ cations</td><td>8.018</td><td>8.010</td><td>7.999</td><td>2.996</td><td>2.994</td><td>2.984</td><td>3.000</td><td>3.000</td><td>3.006</td></tr></tbody></table><table><thead><tr><th></th><th colspan="3">Fe/(Fe + Mg) ratio (in %)</th></tr></thead><tbody><tr><td>32.1</td><td>30.7</td><td>9.4</td></tr></tbody></table>

Element detection limits (in wt%) for (1) grossular: Si: 0.03; Ti: 0.05; Al: 0.03; Cr: 0.09; Fe: 0.08; Ni: 0.06; Mn: 0.07; Mg: 0.03; Ca: 0.06; Na: 0.05; K: 0.05, (2) olivine: Si: 0.03; Ti: 0.06; Al: 0.02; Cr: 0.05; Fe: 0.09; Ni: 0.06; Mn: 0.07; Mg: 0.04; Ca: 0.05, and (3) spinel: Si: 0.05; Ti: 0.06; Al: 0.04; Cr: 0.08; Fe: 0.08; Ni: 0.06; Mn: 0.07; Mg: 0.04; Ca: 0.05.

n.d.: not detected; n.a. = not analyzed; CAI = calcium-aluminum-rich inclusion.

2.92 wt% TiO<sub>2</sub>, and 3.88 wt% Cr<sub>2</sub>O<sub>3</sub>. As for magnetites in CAIs in CK4–6, they contain up to 0.19 wt% MgO, 1.07 wt% Al<sub>2</sub>O<sub>3</sub>, 0.20 TiO<sub>2</sub>, and 3.77 wt% Cr<sub>2</sub>O<sub>3</sub>.

### Fe-Rich Rhönite (<5%)

Rhönite is an accessory phase of the CG1-CAI in Tnz 057 and is found enclosed within fassaite. Grains are up to 150 µm (Fig. 5g) and are heterogeneous at the micrometer scale. Due to this heterogeneity, we could not obtain quantitative chemical analyses. However, EDS analyses display significant variations in CaO (approximately 9–14 wt%), Al<sub>2</sub>O<sub>3</sub> (approximately 15–23 wt%), and TiO<sub>2</sub> (approximately 14–26 wt%). The MgO contents appear to be constant at approximately 7 wt%. Rhönite grains display higher FeO contents

(approximately 18–30 wt%) than those described in CAIs in Allende (0.3–4.2 wt%) (Fuchs 1971, 1978; Allen et al. 1978).

### Minor Phases (<2%)

CAIs in CKs contain numerous minor phases either within silicates or as isolated grains at silicate grain boundaries. They are principally oxides and not Fe, Ni metal. After magnetite, the most abundant opaque phases are ilmenite and titanomagnetite, always smaller than approximately 10 µm. We observed anhedral and subhedral grains of Ti-rich oxide between silicate grains. Tiny grains of Ti-rich oxide occurred within grossular in the CG2-CAI in DaG 431 and in the CG4-CAI in Karoonda. Ti-rich oxides (usually smaller than 5 µm)
<table><caption>Table 7. Chemical compositions of representative magnetite and titanomagnetite grains from CAIs in DaG 431 (CK3-an), NWA 4425 (CK3.8) and NWA 4422 (CK4), Tnz 057 (CK4), Pecora Escarpment (PCA) 82500 (CK4/5), EET 90007 (CK5), EET 87860 (CK5/6), and LEW 87009 (CK6).</caption><thead><tr><th rowspan="2"></th><th colspan="6">Magnetite</th><th colspan="3">Titanomagnetite</th></tr><tr><th>NWA 4425</th><th>NWA 4422</th><th>PCA 82500</th><th>EET 90007</th><th>EET 87860</th><th>LEW 87009</th><th>DaG 431</th><th>NWA 4425</th><th>Tnz 057</th></tr></thead><tbody><tr><td>MgO</td><td>0.98</td><td>0.19</td><td>0.95</td><td>0.17</td><td>0.18</td><td>0.13</td><td>1.13</td><td>2.79</td><td>0.97</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>3.78</td><td>1.07</td><td>3.36</td><td>1.65</td><td>1.76</td><td>0.52</td><td>2.63</td><td>3.94</td><td>3.55</td></tr><tr><td>TiO<sub>2</sub></td><td>0.74</td><td>0.16</td><td>0.60</td><td>0.36</td><td>0.19</td><td>0.25</td><td>2.92</td><td>2.78</td><td>4.59</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>3.04</td><td>3.77</td><td>2.91</td><td>3.82</td><td>3.85</td><td>3.91</td><td>2.25</td><td>3.88</td><td>0.11</td></tr><tr><td>FeO</td><td>29.90</td><td>30.60</td><td>30.11</td><td>30.31</td><td>30.73</td><td>30.55</td><td>29.25</td><td>28.38</td><td>33.52</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>58.60</td><td>62.70</td><td>60.00</td><td>60.11</td><td>61.60</td><td>62.55</td><td>51.68</td><td>53.13</td><td>54.61</td></tr><tr><td>MnO</td><td>0.15</td><td>0.04</td><td>0.11</td><td>0.04</td><td>0.07</td><td>0.02</td><td>0.15</td><td>0.15</td><td>0.12</td></tr><tr><td>NiO</td><td>0.44</td><td>0.28</td><td>0.33</td><td>0.18</td><td>0.23</td><td>0.27</td><td>0.23</td><td>0.38</td><td>0.31</td></tr><tr><td>SiO<sub>2</sub></td><td>n.a.</td><td>n.a.</td><td>0.08</td><td>n.a.</td><td>0.02</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>CaO</td><td>n.a.</td><td>n.a.</td><td>0.07</td><td>n.a.</td><td>0.02</td><td>n.a.</td><td>n.a.</td><td>n.a.</td><td>n.a.</td></tr><tr><td>Total</td><td>92.87</td><td>97.55</td><td>98.51</td><td>96.65</td><td>98.64</td><td>98.20</td><td>90.25</td><td>95.43</td><td>97.78</td></tr><tr><td colspan="10">Cation number in the basis of 4 oxygens</td></tr><tr><td>Mg</td><td>0.056</td><td>0.011</td><td>0.054</td><td>0.010</td><td>0.011</td><td>0.008</td><td>0.070</td><td>0.160</td><td>0.055</td></tr><tr><td>Al</td><td>0.171</td><td>0.049</td><td>0.151</td><td>0.076</td><td>0.080</td><td>0.024</td><td>0.128</td><td>0.179</td><td>0.160</td></tr><tr><td>Ti</td><td>0.021</td><td>0.005</td><td>0.017</td><td>0.011</td><td>0.005</td><td>0.007</td><td>0.091</td><td>0.080</td><td>0.132</td></tr><tr><td>Cr</td><td>0.092</td><td>0.115</td><td>0.087</td><td>0.119</td><td>0.117</td><td>0.121</td><td>0.074</td><td>0.118</td><td>0.003</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.957</td><td>0.990</td><td>0.958</td><td>0.998</td><td>0.991</td><td>0.997</td><td>1.014</td><td>0.912</td><td>1.070</td></tr><tr><td>Fe<sup>3+</sup></td><td>1.688</td><td>1.822</td><td>1.717</td><td>1.780</td><td>1.787</td><td>1.836</td><td>1.612</td><td>1.537</td><td>1.569</td></tr><tr><td>Mn</td><td>0.005</td><td>0.001</td><td>0.003</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.005</td><td>0.005</td><td>0.004</td></tr><tr><td>Ni</td><td>0.013</td><td>0.009</td><td>0.010</td><td>0.006</td><td>0.007</td><td>0.008</td><td>0.008</td><td>0.012</td><td>0.009</td></tr><tr><td>Si</td><td>—</td><td>—</td><td>0.003</td><td>—</td><td>0.001</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Ca</td><td>—</td><td>—</td><td>0.003</td><td>—</td><td>0.001</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Σ cations</td><td>3.003</td><td>3.002</td><td>3.003</td><td>3.001</td><td>3.002</td><td>3.002</td><td>3.002</td><td>3.003</td><td>3.002</td></tr></tbody></table>

Element detection limits (in wt%): Mg: 0.04; Al: 0.04; Ti: 0.06; Cr: 0.08; Fe: 0.08; Mn: 0.07; Ni: 0.06; Si: 0.05; Ca: 0.05.

n.a. = not analyzed; CAI = calcium-aluminum-rich inclusion.

at the rim of destabilized fassaite grains display preferential elongation, which probably corresponds to a crystallographic direction of the former fassaite (Fig. 5c). These grains are embedded in a fine-grained mixture of anorthite, Ca-rich pyroxene, and an unidentified porous K-rich phase, forming reaction zones as depicted in Fig. 5c. Representative analyses of titanomagnetite and ilmenite are given in Tables 7 and 8. Rare grains of perovskite, up to approximately 5 µm, are present in the core of coarse-grained CAIs. The CG1-CAI in Tnz 057, the CG2-CAI in DaG 431, and the CG4-CAI in Karoonda also contain some titanite grains.

Ca-phosphate grains occur in the coarse-grained accretionary rim of CG1-CAI in Tnz 057. All grains display numerous cracks and fractures, making it impossible to obtain quantitative analyses, but EDS spectra suggest that these grains are Cl-apatite (Ca<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl).

Although sulfides are rare in CAIs in CKs, we observed pentlandite, monosulfide solid solution, pyrrhotite, and pyrite grains with sizes from 1 to 5 µm. CAIs that were investigated in NWA 4724, NWA 4425, NWA 4423, NWA 4422, and Tnz 057 also contained tiny grains (smaller than 3 µm) of exotic sulfides such as

PtS (the platinum endmember of braggite [Pt,Pd,Ni]S) and IrAsS (iridium endmember of irarsite [Ir,Ru,Rh,Pt] AsS). In association with exotic sulfides, we also observed a few grains of a PGE-rich phase, (Os,Ru,Ir) S<sub>2</sub>, a solid solution between laurite (RuS<sub>2</sub>), and ehrlichmanite (OsS<sub>2</sub>) with some Ir. These grains are often in association with magnetite, with most being micron-sized (although some have sizes up to 6 µm). Finally, we observed micrometer-sized grains of Au in CAIs from Tnz 057; these were present mostly in the fine-grained CAIs.

The core of the CG1-CAI in Tnz 057 contains grossular (approximately 35%), spinel (approximately 20%), plagioclase (approximately 20%), Ca-rich pyroxene (approximately 15%), and fassaite (approximately 10%), whereas the accretionary rim is mainly composed of Ca-rich pyroxene (approximately 75%) and plagioclase (approximately 25%). In the CG2-CAI in DaG 431, we observed fassaite (approximately 40%), grossular (approximately 30%), plagioclase (approximately 25%), and spinel (approximately 5%). In Karoonda, the CG3-CAI is composed of plagioclase (approximately 55%), Ca-poor pyroxene (approximately 30%), spinel (approximately 10%), and fassaite (approximately 5%),
<table><caption>Table 8. Chemical compositions of representative ilmenite grains from CAIs in NWA 4422 (CK4), NWA 2519 (CK4), Maralinga (CK4), Tnz 057 (CK4), NWA 765 (CK4/5), EET 90007 (CK5), and EET 87009 (CK6).</caption><thead><tr><th></th><th colspan="7">Ilmenite</th></tr><tr><th></th><th>NWA 4422</th><th>NWA 2519</th><th>Maralinga</th><th>Tnz 057</th><th>NWA 765</th><th>EET 90007</th><th>LEW 87009</th></tr></thead><tbody><tr><td>MgO</td><td>1.68</td><td>1.79</td><td>1.72</td><td>3.79</td><td>3.97</td><td>2.05</td><td>2.07</td></tr><tr><td>TiO<sub>2</sub></td><td>52.12</td><td>53.05</td><td>51.07</td><td>51.71</td><td>37.11</td><td>46.80</td><td>49.95</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.17</td><td>0.17</td><td>0.72</td><td>0.02</td><td>4.29</td><td>2.23</td><td>1.22</td></tr><tr><td>FeO</td><td>42.11</td><td>43.19</td><td>41.02</td><td>37.74</td><td>20.91</td><td>35.55</td><td>39.06</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>2.65</td><td>0.76</td><td>3.70</td><td>5.48</td><td>18.38</td><td>7.84</td><td>4.33</td></tr><tr><td>MnO</td><td>0.94</td><td>0.81</td><td>0.68</td><td>0.38</td><td>0.67</td><td>0.59</td><td>0.87</td></tr><tr><td>NiO</td><td>0.02</td><td>0.29</td><td>0.09</td><td>0.05</td><td>0.14</td><td>0.09</td><td>0.04</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>0.01</td><td>n.d.</td><td>1.07</td><td>0.08</td><td>12.59</td><td>4.83</td><td>1.48</td></tr><tr><td>Total</td><td>99.71</td><td>100.07</td><td>100.05</td><td>99.25</td><td>98.04</td><td>99.99</td><td>99.02</td></tr><tr><td colspan="8">Cation number in the basis of 3 oxygens</td></tr><tr><td>Mg</td><td>0.063</td><td>0.066</td><td>0.064</td><td>0.142</td><td>0.087</td><td>0.076</td><td>0.078</td></tr><tr><td>Ti</td><td>0.987</td><td>0.995</td><td>0.961</td><td>0.975</td><td>0.815</td><td>0.874</td><td>0.947</td></tr><tr><td>Cr</td><td>0.003</td><td>0.003</td><td>0.014</td><td>0.000</td><td>0.081</td><td>0.044</td><td>0.024</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.887</td><td>0.901</td><td>0.859</td><td>0.791</td><td>0.637</td><td>0.738</td><td>0.824</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.050</td><td>0.014</td><td>0.070</td><td>0.103</td><td>0.250</td><td>0.146</td><td>0.082</td></tr><tr><td>Mn</td><td>0.020</td><td>0.017</td><td>0.014</td><td>0.008</td><td>0.016</td><td>0.012</td><td>0.019</td></tr><tr><td>Ni</td><td>0.000</td><td>0.006</td><td>0.002</td><td>0.001</td><td>0.001</td><td>0.002</td><td>0.001</td></tr><tr><td>Al</td><td>0.000</td><td>0.000</td><td>0.031</td><td>0.002</td><td>0.172</td><td>0.141</td><td>0.044</td></tr><tr><td>Σ cations</td><td>2.010</td><td>2.002</td><td>2.015</td><td>2.022</td><td>2.059</td><td>2.033</td><td>2.019</td></tr></tbody></table>

Element detection limits (in wt%): Mg: 0.04; Ti: 0.06; Cr: 0.08; Fe: 0.08; Mn: 0.07; Ni: 0.06; Al: 0.04.

n.d. = not detected; CAI = calcium-aluminum-rich inclusion.

whereas CG4-CAI contains grossular (approximately 50%), plagioclase (approximately 25%), spinel (approximately 15%), and Ca-rich pyroxene (approximately 10%). Accessory phases such as olivine, magnetite, and sulfide were also observed in these CAIs.

## Rare Earth Elements in CAIs in CKs

We measured REE contents of Ca-rich pyroxene and anorthite in fine-grained CAIs in NWA 1559 and Tnz 057. We also measured REE abundances in fassaite, grossular, anorthite, and Ca-rich pyroxene in the core and the coarse-grained accretionary rim of the CG1-CAI in Tnz 057. Representative analyses are given in Tables 9 and 10.

Fassaite in the unequilibrated core of the CG1-CAI in Tnz 057 is enriched in REEs compared to CI chondrites and has patterns similar to those measured in fassaites from Allende (e.g., Simon et al. 1991) (Fig. 8a). Light REEs (LREEs) are systematically depleted relative to heavy REEs (HREEs) with CI-normalized La/Yb ratios of approximately 0.19, and this mineral displays a large negative Eu anomaly (Eu/Eu* ranging from 0.9 to 0.25, where Eu* is the interpolated value between CI-normalized Sm and Gd abundances) (Fig. 8a). The most REE-enriched grain has La and Lu concentrations of approximately 45 × CI and approximately 245 × CI, respectively.

Grossular is finely intergrown with plagioclase in the core of the CG1-CAI in Tnz 057 and we could only obtain one REE pattern for this mineral (Fig. 8b). This REE pattern has a negative slope from La (approximately 60 × CI) to Yb (approximately 20 × CI), followed by a slight enrichment in Lu.

Rare earth element abundances in anorthite are in the range of 1–15 × CI (Figs. 8c and 9a) and display a positive Eu anomaly (Eu/Eu* ranging from 1.45 to 3.19), typical of this mineral. The REE pattern in a fine-grained CAI in NWA 1559 has a relatively flat slope (La approximately 11 × CI and Yb approximately 9 × CI) (Fig. 9a). In Tnz 057, the lowest REE contents in anorthite were measured in a fine-grained CAI (La approximately 1 × CI, and Yb approximately 2 × CI) (Fig. 9a), while grains in the coarse-grained accretionary rim of the CG1-CAI are systematically more enriched in REE relative to CI chondrites (Fig. 8c).

Rare earth element patterns of Ca-rich pyroxene grains measured in the coarse-grained accretionary rim of the CG1-CAI in Tnz 057 (Fig. 8d) are similar to those measured in anorthite, with a slight depletion in HREEs relative to the LREEs (CI-normalized La/Yb approximately 1.42 ± 0.41) and positive Eu anomalies (Eu/Eu* ranging from 1.63 to 3.63). For the least REE-enriched pattern, there is a slight depletion in LREEs, then a negative slope from Nd to Er followed by a positive slope to Lu.
<table><caption>Table 9. Representative rare earth element abundances in plagioclase and Ca-rich pyroxene grains from fine-grained CAIs in NWA 1559 (CK3) and Tnz 057 (CK4).</caption><thead><tr><th rowspan="2"></th><th>NWA 1559</th><th colspan="2">Tnz 057</th><th colspan="2"></th></tr><tr><th>Plagioclase</th><th>Plagioclase</th><th>Ca-rich pyroxene</th><th>Plagioclase</th><th>Ca-rich pyroxene</th></tr></thead><tbody><tr><td>Spot size (µm)</td><td>80</td><td>20</td><td>20</td><td>80</td><td>80</td></tr><tr><td>Repetition rate (Hz)</td><td>7</td><td>4</td><td>4</td><td>7</td><td>7</td></tr><tr><td>La</td><td>2.61±0.09 (0.02)</td><td>0.24±0.02 (0.02)</td><td>8.54±0.29 (0.03)</td><td>4.39±0.14 (&lt;0.01)</td><td>3.19±0.11 (&lt;0.01)</td></tr><tr><td>Ce</td><td>7.18±0.23 (0.02)</td><td>0.73±0.03 (0.02)</td><td>22.05±0.77 (0.03)</td><td>13.46±0.41 (&lt;0.01)</td><td>9.97±0.31 (&lt;0.01)</td></tr><tr><td>Pr</td><td>0.98±0.04 (0.02)</td><td>0.13±0.01 (0.02)</td><td>3.71±0.16 (0.03)</td><td>2.04±0.07 (&lt;0.01)</td><td>1.65±0.05 (&lt;0.01)</td></tr><tr><td>Nd</td><td>4.97±0.22 (0.20)</td><td>0.61±0.06 (0.11)</td><td>17.39±0.82 (0.13)</td><td>11.01±0.36 (0.01)</td><td>8.74±0.30 (0.03)</td></tr><tr><td>Sm</td><td>1.40±0.08 (0.13)</td><td>0.33±0.05 (0.10)</td><td>5.66±0.31 (0.14)</td><td>4.29±0.13 (0.01)</td><td>3.08±0.10 (0.01)</td></tr><tr><td>Eu</td><td>0.75±0.03 (0.03)</td><td>0.25±0.02 (0.03)</td><td>0.33±0.03 (0.04)</td><td>0.42±0.01 (&lt;0.01)</td><td>0.70±0.02 (&lt;0.01)</td></tr><tr><td>Gd</td><td>1.72±0.08 (0.09)</td><td>0.37±0.06 (0.12)</td><td>2.47±0.15 (0.17)</td><td>5.32±0.17 (0.01)</td><td>3.93±0.13 (0.01)</td></tr><tr><td>Tb</td><td>0.25±0.01 (0.02)</td><td>0.07±0.01 (0.02)</td><td>0.42±0.03 (0.02)</td><td>0.97±0.03 (&lt;0.01)</td><td>0.69±0.02 (&lt;0.01)</td></tr><tr><td>Dy</td><td>1.91±0.08 (0.06)</td><td>0.55±0.04 (0.06)</td><td>2.44±0.13 (0.09)</td><td>7.37±0.23 (&lt;0.01)</td><td>5.04±0.16 (0.01)</td></tr><tr><td>Ho</td><td>0.41±0.02 (0.02)</td><td>0.09±0.01 (0.02)</td><td>0.14±0.01 (0.03)</td><td>1.64±0.05 (&lt;0.01)</td><td>1.09±0.04 (&lt;0.01)</td></tr><tr><td>Er</td><td>1.19±0.05 (0.06)</td><td>0.31±0.03 (0.05)</td><td>0.27±0.04 (0.07)</td><td>5.13±0.16 (&lt;0.01)</td><td>3.10±0.10 (0.01)</td></tr><tr><td>Tm</td><td>0.20±0.01 (0.02)</td><td>0.05±0.01 (0.02)</td><td>1.08±0.05 (0.03)</td><td>0.99±0.03 (&lt;0.01)</td><td>0.46±0.02 (&lt;0.01)</td></tr><tr><td>Yb</td><td>1.44±0.06 (0.05)</td><td>0.35±0.04 (0.07)</td><td>1.06±0.08 (0.13)</td><td>2.07±0.06 (&lt;0.01)</td><td>3.34±0.10 (0.01)</td></tr><tr><td>Lu</td><td>0.21±0.01 (0.02)</td><td>0.05±0.01 (0.01)</td><td>0.03±0.01 (0.03)</td><td>1.01±0.03 (&lt;0.01)</td><td>0.48±0.02 (&lt;0.01)</td></tr></tbody></table>

Concentrations are given in ppm. Uncertainties are ±1 sigma. Detection limit values (99% confidence) are given in parentheses.

<table><caption>Table 10. Representative rare earth element abundances in plagioclase, Ca-rich pyroxene, grossular, and fassaite grains from the CGI-CAI in Tnz 057 (CK4).</caption><thead><tr><th rowspan="3"></th><th colspan="5">Tnz 057 (CGI-CAI)</th></tr><tr><th colspan="2">Ca-rich pyroxene</th><th colspan="2">Grossular</th><th colspan="2">Fassaite</th></tr><tr><th>Rim</th><th>Core</th><th>Rim</th><th>Core</th><th>Rim</th><th>Core</th></tr></thead><tbody><tr><td>Spot size (µm)</td><td>20</td><td>80</td><td>80</td><td>80</td><td>20</td><td>80</td></tr><tr><td>Repetition rate (Hz)</td><td>4</td><td>5</td><td>4</td><td>5</td><td>4</td><td>4</td></tr><tr><td>La</td><td>3.37±0.11 (0.02)</td><td>1.59±0.05 (0.01)</td><td>3.56±0.12 (0.01)</td><td>13.46±0.43 (0.02)</td><td>2.90±0.10 (0.02)</td><td>2.90±0.10 (0.02)</td></tr><tr><td>Ce</td><td>7.95±0.27 (0.02)</td><td>4.21±0.13 (0.01)</td><td>8.80±0.31 (0.02)</td><td>31.65±0.97 (0.01)</td><td>10.71±0.36 (0.02)</td><td>10.71±0.36 (0.02)</td></tr><tr><td>Pr</td><td>1.09±0.04 (0.01)</td><td>0.67±0.02 (0.01)</td><td>1.28±0.05 (0.01)</td><td>4.54±0.14 (0.01)</td><td>2.10±0.08 (0.02)</td><td>2.10±0.08 (0.02)</td></tr><tr><td>Nd</td><td>5.03±0.19 (0.07)</td><td>3.90±0.15 (0.10)</td><td>6.06±0.24 (0.09)</td><td>21.77±0.77 (0.14)</td><td>13.81±0.48 (0.09)</td><td>13.81±0.48 (0.09)</td></tr><tr><td>Sm</td><td>1.50±0.08 (0.08)</td><td>1.84±0.08 (0.05)</td><td>1.56±0.09 (0.08)</td><td>5.78±0.24 (0.08)</td><td>5.91±0.25 (0.10)</td><td>5.91±0.25 (0.10)</td></tr><tr><td>Eu</td><td>1.04±0.04 (0.03)</td><td>1.12±0.04 (0.02)</td><td>1.56±0.06 (0.03)</td><td>2.07±0.08 (0.02)</td><td>0.29±0.02 (0.03)</td><td>0.29±0.02 (0.03)</td></tr><tr><td>Gd</td><td>1.86±0.09 (0.08)</td><td>2.29±0.10 (0.05)</td><td>1.35±0.09 (0.09)</td><td>6.93±0.28 (0.07)</td><td>9.10±0.34 (0.10)</td><td>9.10±0.34 (0.10)</td></tr><tr><td>Tb</td><td>0.30±0.01 (0.01)</td><td>0.41±0.02 (0.01)</td><td>0.20±0.01 (0.01)</td><td>1.14±0.04 (0.01)</td><td>1.66±0.06 (0.02)</td><td>1.66±0.06 (0.02)</td></tr><tr><td>Dy</td><td>2.02±0.08 (0.05)</td><td>2.66±0.11 (0.03)</td><td>1.51±0.07 (0.05)</td><td>7.82±0.30 (0.04)</td><td>12.61±0.42 (0.06)</td><td>12.61±0.42 (0.06)</td></tr><tr><td>Ho</td><td>0.42±0.02 (0.01)</td><td>0.55±0.02 (0.01)</td><td>0.33±0.02 (0.01)</td><td>1.51±0.06 (0.01)</td><td>2.85±0.10 (0.02)</td><td>2.85±0.10 (0.02)</td></tr><tr><td>Er</td><td>1.31±0.06 (0.03)</td><td>1.52±0.06 (0.02)</td><td>0.93±0.05 (0.04)</td><td>4.34±0.17 (0.04)</td><td>8.95±0.30 (0.05)</td><td>8.95±0.30 (0.05)</td></tr><tr><td>Tm</td><td>0.17±0.01 (0.01)</td><td>0.22±0.01 (0.01)</td><td>0.13±0.01 (0.01)</td><td>0.57±0.03 (0.01)</td><td>1.48±0.05 (0.01)</td><td>1.48±0.05 (0.01)</td></tr><tr><td>Yb</td><td>1.24±0.06 (0.06)</td><td>1.64±0.06 (0.02)</td><td>0.94±0.06 (0.06)</td><td>3.46±0.13 (0.04)</td><td>9.88±0.35 (0.08)</td><td>9.88±0.35 (0.08)</td></tr><tr><td>Lu</td><td>0.15±0.01 (0.01)</td><td>0.19±0.01 (0.01)</td><td>0.11±0.01 (0.01)</td><td>0.62±0.03 (0.01)</td><td>1.88±0.07 (0.02)</td><td>1.88±0.07 (0.02)</td></tr></tbody></table>

Concentrations are given in ppm. Uncertainties are ±1 sigma. Detection limit values (99% confidence) are given in parentheses.

Approximately 75% of fine-grained CAIs contain Ca-rich pyroxenes with a REE pattern similar to the CAI group II (Mason and Martin 1977) (Fig. 9b). This REE pattern is characterized by a relatively flat slope for the LREEs, a progressive depletion toward the HREEs, a negative anomaly in Eu and Yb, and a positive anomaly in Tm. LREE concentrations vary from approximately 10 to approximately 300 × CI. CI-normalized La/Yb ratios vary from approximately

1.5 to approximately 26 × CI. Eu/Eu* and Tm/Tm* are in the range of 0.25–0.27 and 5.3–13.4, respectively.

One fine-grained CAI in Tnz 057 contains Ca-rich pyroxene grains with a typical type III REE pattern, which is a relatively flat pattern with negative anomalies in Eu (Eu/Eu* = 0.30 ± 0.05) and Yb (Yb/Yb* = 0.5 ± 0.2) (Fig. 9c). One fine-grained CAI in NWA 1559 contains Ca-rich pyroxene grains displaying similar REE patterns with negative anomalies in Ce,
<figure><img src="image_15.png" /><figcaption>Fig. 8. Representative rare earth element (REE) abundances in a) fassaite, b) garnet, c) plagioclase, and d) Ca-rich pyroxene grains from coarse-grained CG1-CAI in Tnz 057 (CK4) normalized to CI values from Anders and Grevesse (1989). As can be seen in (c), secondary plagioclase grains in the core or fine-grained accretionary rim are slightly depleted in Light REE (LREE) except for one grain from the coarse-grained accretionary rim (filled circles), which has a LREE-enriched pattern. The shaded gray area in (d) shows the range of REE compositions of Ca-rich pyroxene in the coarse-grained accretionary rim of the CG1-CAI. The REE pattern shown within the gray-shaded area in (d) represents the average REE concentrations measured in this mineral. REE patterns for plagioclase (c) and pyroxene (d) are similar to those in group I CAIs.</figcaption></figure>

Eu, and Yb (Fig. 9d). These patterns have some affinities to ultrarefractory REE patterns reported by MacPherson et al. (1988, and references therein) and group III REE patterns with an additional negative anomaly in Ce (Ireland and Fegley 1991). However, these patterns display lower REE concentrations compared with the typical ultrarefractory patterns.

Figure 9e shows REE patterns of two recrystallized Ca-rich pyroxene grains from a fine-grained CAI in Tnz 057 (as shown in Fig. 5e). REE abundances are characterized by a relatively flat pattern slightly depleted in LREEs with a CI-normalized La/Yb ratio of 0.72 ± 0.08 and a negative anomaly in Eu (Eu/Eu* ranging from 0.61 to 0.70). These features are similar to the characteristics of fassaite REE patterns.

## $^{26}$Al-$^{26}$Mg Isotopic Systematics of CAIs from Tnz 057

The Al/Mg ratios and Mg isotopic compositions of Tnz 057 CAIs were measured in fine-grained *FG1*-CAI and *FG2*-CAI in solution mode by MC-ICPMS and in coarse-grained *CG1*-CAI by LA-MC-ICPMS at ASU. The Al-Mg isotope compositions are presented in

Table 11, and the $^{27}$Al/$^{24}$Mg ratios are plotted versus the radiogenic excesses in $^{26}$Mg (denoted as $\delta^{26}$Mg*) in Fig. 11.

Three and two drilled samples were analyzed from *FG1*- and *FG2*-CAI, respectively. The $^{27}$Al/$^{24}$Mg ratios vary from approximately 0.25 to approximately 0.48. The Mg stable isotopic compositions relative to the DSM-3 standard yield $\delta^{25}$Mg values of approximately -0.31–0.36‰ and approximately -0.48–0.55‰ for *FG1*- and *FG2*-CAIs, respectively. These compositions are slightly lighter than that of bulk Allende (e.g., $\delta^{25}$Mg = -0.17 [±0.06] ‰, ±2SD; this study). We find small radiogenic $^{26}$Mg excesses in the isotopic compositions of the five analyzed drilled fractions with $\delta^{26}$Mg* values that vary from +0.046‰ to +0.183‰ (i.e., well outside the ±0.038‰ 2SD reproducibility based on repeat analyses of the inhouse Allende WR standard).

The in situ LA-MC-ICPMS measurements were carried out in the core and accretionary rim of *CG1*-CAI (see Fig. 6 for the definition of the boundaries of the core and accretionary rim areas, and Fig. 10 for the location of the laser ablation spot analyses). The mass-dependent Mg isotope compositions in *CG1*-CAI are
<figure><img src="image_16.png" /></figure>

Fig. 9. Representative rare earth element (REE) abundances in anorthite and Ca-rich pyroxene grains in various fine-grained CAIs from Northwest Africa (NWA) 1559 (CK3) and Tnz 057 (CK4) normalized to CI values from Anders and Grevesse (1989). a) REE patterns, similar to those in group I CAIs, of a plagioclase grain from a fine-grained CAI in NWA 1559 (CK3) (filled circles) and of two different plagioclase grains from a fine-grained CAI in Tnz 057 (CK4) (filled and empty squares). b) Shaded gray area shows range of REE concentrations in Ca-rich pyroxene grains in various fine-grained CAIs in Tnz 057 (CK4). The REE pattern shown within the gray-shaded area, similar to those of group II CAIs, represents average REE concentrations measured in this mineral. REE patterns measured in different Ca-rich pyroxene grains from recrystallized fine-grained CAIs (such as shown in Fig. 5e) in Tnz 057 (CK4), which show similarity to those of group III CAIs c), or show close affinities with the ultrarefractory REE pattern d), or e) similarity to those in the fassaite shown in Fig. 8a.

similar for the core and accretionary rim, with average δ²⁵Mg values of 3.6 ‰ (±2.0 ‰) and 4.2‰ (±1.0 ‰), respectively (expressed relative to DSM-3 standard composition, ±2SD). The LA-MC-ICPMS data in the CG1-CAI core define a generally positive correlation, despite significant scatter, on a plot of the ²⁷Al/²⁴Mg ratios versus the radiogenic excesses in ²⁶Mg (δ²⁶Mg*), indicating the in situ decay of short-lived radionuclide ²⁶Al to ²⁶Mg (Fig. 11). The regression based on 17 of

19 data points acquired within the inner part of this CAI gives an errorchron (MSWD = 19) with an initial ²⁶Al/²⁷Al ratio of 4.13 (±1.20) × 10⁻⁵ and δ²⁶Mg*₀ = -0.38 (±0.28) ‰ (2SE). The 10 analyses made in the coarse-grained accretionary rim composed of Ca-rich pyroxene do not define a positive correlation of ²⁷Al/²⁴Mg versus δ²⁶Mg*, but all have excesses in ²⁶Mg (Fig. 11), ranging from 0.530 (±0.033)‰ (spot #18) to 1.459 (±0.096) ‰ (spot #17).
<figure><img src="image_17.png" /><figcaption>Fig. 10. Location of Al-Mg spot analyses by laser ablation of CG1-CAI in Tnz 057 (CK4). White numbers indicate the analysis locations corresponding to the ablation spot numbers given in Table 11. The box indicates the area that is shown in more detail in Fig. 6.</figcaption></figure>

# DISCUSSION

## Pristine Signatures of CAIs in CKs and Comparison to CVs

In this study, REE contents were measured in situ within single grains of CAIs. Thus, these REE data are not directly comparable to those obtained for bulk CAIs. However, the bulk REE contents of the fine-grained CAIs analyzed here, which are mainly composed of anorthite and Ca-rich pyroxene, are expected to be dominated by the REE abundances in their Ca-rich pyroxene (which have significantly higher REE concentrations than the anorthite). Therefore, it can be assumed that for the fine-grained CAIs studied here, the bulk CAI REE pattern mirrors that of the Ca-rich pyroxene. However, this cannot be assumed for coarse-grained CAIs as they contain numerous other phases, some of which have not been analyzed (e.g., rhönite, spinel).

Within a single fine-grained CAI, the type of REE pattern measured in its Ca-rich pyroxene grains is the same and is similar to that for group I, II, or III, as previously defined from refractory inclusions in CVs. In addition, and as described by Grossman and Ganapathy

(1976) for fine-grained CAIs in Allende, REE patterns are highly variable from one CAI to the next. These observations seem to indicate that in Tnz 057, (1) each individual fine-grained CAI is composed of Ca-rich pyroxene grains that have been formed in the same region of the protoplanetary disk, and (2) different fine-grained CAIs in this CK chondrite recorded different times or regions of formation in the protoplanetary disk and were subsequently brought together prior to parent body accretion.

The commonly occurring fine-grained CAIs observed in CKs contain (1) anorthite grains displaying unfractionated REE patterns (group I), and (2) Ca-rich pyroxene grains with fractionated REE patterns (group II). The absence of REE fractionation in anorthite may indicate production during complete condensation and not by igneous processes (Wänke et al. 1974). As noted by MacPherson et al. (1988), unfractionated REE patterns could also be characteristic of the residue from a volatilization process. Even if a volatilization origin can be invoked for the REE patterns measured in anorthite grains, the presence of both fractionated (Ca-rich pyroxene) and unfractionated (anorthite) REE patterns in fine-grained CAIs in CKs suggests large-scale turbulent events in the solar nebula contemporaneous with
<table><caption>Table 11. *Upper*: Al-Mg isotopic compositions of reference house standards (Spex Mg solution and Allende whole rock) and Tnz 057 fine-grained CAIs (microdrilled spots of *FG1*- and *FG2*-CAI) by solution mode MC-ICPMS analysis. *Lower*: Al-Mg isotopic compositions of Tnz 057 coarse-grained *CG1*-CAI measured by laser ablation MC-ICPMS.</caption><thead><tr><th>Samples</th><th>Drill spot #</th><th>Ablation spot #</th><th><sup>27</sup>Al/<sup>24</sup>Mg</th><th>δ<sup>25</sup>Mg (‰)</th><th>±2SD</th><th>δ<sup>26</sup>Mg (‰)</th><th>±2SD</th><th>δ<sup>26</sup>Mg * (‰)</th><th>±2SD</th><th>n (repeats)</th></tr></thead><tbody><tr><td colspan="11">Solution analysis MC-ICPMS (relative to DSM-3 composition)</td></tr><tr><td>Standards</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>SPEX Mg</td><td></td><td></td><td>-1.05</td><td>0.05</td><td>-2.02</td><td>0.06</td><td>0.04</td><td>0.06</td><td>8</td></tr><tr><td>Allende WR (SI)</td><td></td><td></td><td>-0.17</td><td>0.06</td><td>-0.32</td><td>0.12</td><td>0.02</td><td>0.04</td><td>12</td></tr><tr><td>TNZ 057 CAIs</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FG1-CAI</td><td></td><td></td><td>0.130</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>FG1-CAI-1</td><td></td><td></td><td>0.338</td><td>-0.35</td><td>0.03</td><td>-0.59</td><td>0.06</td><td>0.09</td><td>7</td></tr><tr><td>FG1-CAI-2</td><td></td><td></td><td>0.251</td><td>-0.36</td><td>0.01</td><td>-0.62</td><td>0.02</td><td>0.07</td><td>5</td></tr><tr><td>FG1-CAI-3</td><td></td><td></td><td>0.256</td><td>-0.31</td><td>0.03</td><td>-0.55</td><td>0.08</td><td>0.05</td><td>6</td></tr><tr><td>FG2-CAI-1</td><td></td><td></td><td>0.334</td><td>-0.55</td><td>0.04</td><td>-0.89</td><td>0.07</td><td>0.18</td><td>3</td></tr><tr><td>FG2-CAI-2</td><td></td><td></td><td>0.482</td><td>-0.48</td><td>0.03</td><td>-0.82</td><td>0.06</td><td>0.12</td><td>3</td></tr><tr><td colspan="11">Laser ablation analysis MC-ICPMS (relative to glass standard composition)</td></tr><tr><td>TNZ 057 CAI</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>(CG1-CAI)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ak35 normalized spots</td><td>1</td><td>4.48</td><td>1.40</td><td>0.21</td><td>4.40</td><td>1.24</td><td>1.67</td><td>0.15</td><td></td></tr><tr><td></td><td>2</td><td>5.66</td><td>2.38</td><td>0.23</td><td>5.99</td><td>0.82</td><td>1.35</td><td>0.15</td><td></td></tr><tr><td></td><td>3</td><td>7.96</td><td>3.01</td><td>0.27</td><td>7.96</td><td>0.74</td><td>2.09</td><td>0.34</td><td></td></tr><tr><td></td><td>4</td><td>6.33</td><td>2.95</td><td>0.22</td><td>7.66</td><td>0.84</td><td>1.91</td><td>0.19</td><td></td></tr><tr><td></td><td>5</td><td>8.40</td><td>3.11</td><td>0.25</td><td>8.14</td><td>1.18</td><td>2.07</td><td>0.19</td><td></td></tr><tr><td></td><td>6</td><td>4.75</td><td>4.32</td><td>0.28</td><td>9.88</td><td>0.93</td><td>1.44</td><td>0.13</td><td></td></tr><tr><td></td><td>7</td><td>8.16</td><td>3.76</td><td>0.26</td><td>9.73</td><td>1.23</td><td>2.38</td><td>0.25</td><td></td></tr><tr><td></td><td>8</td><td>5.75</td><td>2.58</td><td>0.17</td><td>6.21</td><td>0.59</td><td>1.18</td><td>0.12</td><td></td></tr><tr><td></td><td>9</td><td>3.47</td><td>4.45</td><td>0.26</td><td>9.38</td><td>0.87</td><td>0.71</td><td>0.05</td><td></td></tr><tr><td></td><td>10</td><td>3.06</td><td>4.39</td><td>0.21</td><td>9.00</td><td>0.54</td><td>0.44</td><td>0.03</td><td></td></tr><tr><td></td><td>11</td><td>1.65</td><td>3.86</td><td>0.25</td><td>7.85</td><td>0.82</td><td>0.33</td><td>0.04</td><td></td></tr><tr><td></td><td>12</td><td>3.93</td><td>5.24</td><td>0.50</td><td>10.83</td><td>1.84</td><td>0.61</td><td>0.06</td><td></td></tr><tr><td></td><td>13</td><td>3.35</td><td>4.51</td><td>0.23</td><td>9.51</td><td>0.66</td><td>0.71</td><td>0.05</td><td></td></tr><tr><td></td><td>14</td><td>3.38</td><td>5.00</td><td>0.25</td><td>10.11</td><td>0.68</td><td>0.35</td><td>0.02</td><td></td></tr><tr><td></td><td>15</td><td>3.16</td><td>3.12</td><td>0.22</td><td>6.48</td><td>0.67</td><td>0.40</td><td>0.03</td><td></td></tr><tr><td></td><td>16</td><td>2.61</td><td>4.27</td><td>0.38</td><td>8.81</td><td>1.19</td><td>0.49</td><td>0.03</td><td></td></tr><tr><td></td><td>17</td><td>0.30</td><td>3.29</td><td>0.16</td><td>7.88</td><td>0.62</td><td>1.46</td><td>0.10</td><td></td></tr><tr><td>Ak65 normalized spots</td><td>18</td><td>1.38</td><td>5.03</td><td>0.33</td><td>10.34</td><td>0.97</td><td>0.53</td><td>0.03</td><td></td></tr><tr><td></td><td>19</td><td>1.15</td><td>4.78</td><td>0.28</td><td>9.88</td><td>1.01</td><td>0.56</td><td>0.03</td><td></td></tr><tr><td></td><td>20</td><td>1.36</td><td>5.12</td><td>0.27</td><td>10.55</td><td>1.08</td><td>0.56</td><td>0.02</td><td></td></tr><tr><td></td><td>21</td><td>0.30</td><td>3.78</td><td>0.36</td><td>8.34</td><td>1.57</td><td>0.95</td><td>0.05</td><td></td></tr><tr><td></td><td>22</td><td>0.30</td><td>3.84</td><td>0.36</td><td>8.44</td><td>1.60</td><td>0.94</td><td>0.05</td><td></td></tr><tr><td></td><td>23</td><td>0.33</td><td>3.75</td><td>0.25</td><td>8.15</td><td>1.02</td><td>0.83</td><td>0.04</td><td></td></tr><tr><td></td><td>24</td><td>3.72</td><td>3.99</td><td>0.18</td><td>8.47</td><td>0.68</td><td>0.69</td><td>0.04</td><td></td></tr><tr><td></td><td>25</td><td>3.96</td><td>3.20</td><td>0.23</td><td>6.96</td><td>0.80</td><td>0.72</td><td>0.04</td><td></td></tr><tr><td></td><td>26</td><td>2.45</td><td>2.83</td><td>0.24</td><td>6.40</td><td>0.80</td><td>0.89</td><td>0.06</td><td></td></tr><tr><td></td><td>27</td><td>0.90</td><td>4.22</td><td>0.23</td><td>8.91</td><td>0.76</td><td>0.68</td><td>0.03</td><td></td></tr><tr><td></td><td>28</td><td>0.73</td><td>4.39</td><td>0.25</td><td>9.26</td><td>0.98</td><td>0.71</td><td>0.03</td><td></td></tr><tr><td></td><td>29</td><td>0.71</td><td>4.10</td><td>0.20</td><td>8.66</td><td>0.80</td><td>0.66</td><td>0.02</td><td></td></tr></tbody></table>
<figure><img src="image_18.png" /><figcaption>Fig. 11. Al-Mg systematics reported in a diagram of $^{27}$Al/$^{24}$Mg versus $\delta^{26}$Mg* (in per mil) for two fine-grained (FG1- and FG2-CAI indicated with square symbols) and one coarse-grained (CG1-CAI indicated with circle and diamond symbols for core and accretionary rim analysis, respectively) CAIs of Tnz 057 (CK4). The Al-Mg isotopic analyses of the CG1-CAI were made by laser ablation MC-ICPMS, while those of the FG1- and FG2-CAI were made in solution mode by MC-ICPMS. The two data points with black bull's-eyes were excluded from the regression of the slope indicated here that yields an initial $^{26}$Al/$^{27}$Al approximately 4.1 × 10⁻⁵. For comparison, the gray line shows the canonical Al-Mg isotopic evolution line for CAIs, using an initial $\delta^{26}$Mg*0 = -0.032‰ (Thrane et al. 2006) and ($^{26}$Al/$^{27}$Al)0 approximately 5 × 10⁻⁵ (MacPherson et al. 1995, 2010; McKeegan and Davis 2003; Jacobsen et al. 2008). See text for further details, and see petrographic details for these Tnz 057 CAIs in Fig. 6.</figcaption></figure>

condensation, evaporation, and igneous processes that resulted in the formation of CAIs.

The most common type of REE pattern measured is the group II pattern (Fig. 9b). Fegley and Kornacki (1984) proposed that the group II REE pattern is formed by the evaporation of “pre-solar” dust and subsequent recondensation. It was also suggested that group II inclusions condensed from a gas from which the most refractory REEs (Lu, Er, Ho, and Dy) were removed by previous condensation of phases, such as hibonite, enriched in these refractory elements (MacPherson and Davis 1994). The complementary pattern of group II, called ultrarefractory, has been commonly described in hibonite (MacPherson [2003]; and references therein). In one fine-grained CAI studied in Tnz 057, Ca-rich pyroxene grains display similarities with the ultrarefractory pattern (i.e., negative anomalies in Ce, Eu, and Yb) (Fig. 9d). This REE signature can be a primary feature or inherited from a precursor phase. This REE signature could also be explained by

the presence of submicron refractory phases with “ideal” ultrarefractory REE patterns within Ca-rich pyroxene grains. Due to the size of the laser analysis spot (approximately 20 µm), the REE patterns obtained in Ca-rich pyroxene grains could be the result of beam overlap with minute refractory inclusions, such as hibonite. This may explain the difference between REE concentrations reported here (Fig. 9d) and the “ideal” ultrarefractory REE pattern. As they correspond to the highest temperature phases, minerals with ultrarefractory REE patterns should be removed prior to group II condensation and then later accreted with the lower temperature phases (which display group II REE patterns) that formed from the gas depleted in refractory elements. As also discussed above for different minerals within inclusions having group I and II patterns, the possible association of fractionated REE patterns (group II) and their complementary (ultrarefractory) pattern in different minerals within the same inclusion suggests that turbulent events occurred in the protoplanetary disk to that gathered earlier formed ultrarefractory phases (and their host minerals) with other later formed CAI phases prior to accretion.

Two REE patterns with close affinities with those described in fassaite have been measured in Ca-rich pyroxene grains from one fine-grained CAI (Fig. 9e). These patterns result from the REE partitioning in a crystal that formed in equilibrium with a melt. So, even if most of the fine-grained CAIs display a condensation signature (see above), these REE patterns support the fact that some mineralogical components of fine-grained CAIs originally crystallized from a melt. Grains showing these REE patterns display significant Al₂O₃ and TiO₂ contents, and some amount of FeO. These observations argue that these grains correspond to ancient primary fassaite that retains a pristine REE signature despite its destabilization into Ca-rich pyroxene by the FeO enrichment during parent body metamorphism. Although fine-grained CAIs are equilibrated in major elements and sometimes recrystallized as irregular assemblages, pristine REE contents of refractory phases in these CAIs are preserved, probably due to the lower diffusion rate of REEs compared with major elements (e.g., Cherniak and Dimanov 2010). Up to type 4, REE concentrations can be used as a tracer to distinguish between metamorphosed CAIs and recrystallized matrix. As suggested by Hirota et al. (2002) and Chaumard et al. (2011), we propose that bulk REE patterns of CKs are dominated by the REE contents of CAIs present within them.

The igneous origin of the CG1-CAI from Tnz 057 is supported by REE patterns measured in the fassaite, plagioclase, and Ca-rich pyroxene grains in this refractory inclusion (Fig. 8a, 8c, and 8d). Indeed, the
REE pattern for fassaite in the CG1-CAI, which is LREE-depleted with a negative Eu anomaly, suggests formation by crystallization from a melt. The REE patterns of anorthite (with a large positive Eu anomaly) are also in agreement with an igneous origin. However, the REE pattern for garnet in this inclusion (Fig. 8b) supports a secondary origin, probably in an asteroidal setting (see discussion in the Parent Body Metamorphism section).

## Nebular Processes

The REE patterns measured in Ca-rich pyroxene from the coarse-grained accretionary rim of the CG1-CAI in Tnz 057 are unusual (positive Eu anomaly) (Fig. 8d) and similar to those obtained for melilite in CAIs from CVs (Brearley and Jones [1998]; and references therein). Based on this observation and the ubiquity of interstitial anorthite between Ca-rich pyroxene grains, this coarse-grained accretionary rim could have been mainly composed of primary melilite that was later transformed to secondary Ca-rich pyroxene and anorthite by a reaction with a Mg, Si-rich gas, as proposed by Krot et al. (2004a). This reaction can be written as:

$$ \begin{aligned} & Ca_2Al_2SiO_7 + Ca_2MgSi_2O_7 + 5SiO(g) + 2Mg(g) + 7H_2O(g) \\ & = CaAl_2Si_2O_8 + 3CaMgSi_2O_6 + 7H_2(g) \end{aligned} \qquad (2) $$

The modal abundances of Ca-rich pyroxene and anorthite estimated for the coarse-grained accretionary rim of the CG1-CAI (approximately 75 and approximately 25%, respectively) and the presence of significant Al₂O₃ contents in Ca-rich pyroxene grains support reaction (2). Different hypotheses were proposed to explain the origin of such melilite-rich fine-grained rims, such as (1) direct condensation of melilite on the core through heterogeneous nucleation, (2) late accretion of a melilite-rich rim, (3) partial melting of the core with melt evaporation and condensation, and (4) solid-to-gas evaporation and recrystallization (Krot et al. 2004a). Because metamorphism on the CK parent body has partially overprinted nebular signatures, these hypotheses are difficult to test. However, the clear contact between the core and the coarse-grained accretionary rim would be more consistent with (1) or (2). In these cases, it is necessary to substantially modify the physical conditions during the formation of the CG1-CAI to explain a second stage of melilite formation. Considering a heterogeneous solar nebula in terms of temperature, pressure, and gas fugacities, transport into the region of melilite formation is necessary to form this type of accretionary rim. This implies transportation of CAIs, possibly by large-scale

turbulent circulation in the solar nebula, prior to their accretion into parent bodies.

## Parent Body Aqueous Alteration

### Alteration Prior to Metamorphism

In spite of the obvious effects of thermal metamorphism, aqueous alteration features can also be recognized in CAIs in CKs. The fibrous texture and chemical composition of Ca-poor pyroxene in the CG3-CAI from Karoonda suggest an initial Mg, Al-rich hydrous phase, such as serpentine and/or saponite, which was later dehydrated during parent body metamorphism. Additional evidence will be necessary to distinguish between a nebular and an asteroidal origin for those hydrous phases. Even though a parent body origin may be preferred due to the large amount of fibrous Ca-poor pyroxene in this CAI, the lack of other chondritic components with evidence for aqueous alteration in proximity to the CG3-CAI is more suggestive of a pre-accretionary alteration event.

### During Metamorphism

Other evidence supporting secondary alteration processes in the CKs is the presence of forsterite and Ca-rich pyroxene-bearing veins within Mg-rich primary phases (fassaite-spinel) in the core of the CG1-CAI from Tnz 057, and Ca-rich pyroxene and anorthite-bearing veins within the grossular-anorthite assemblage Ca-rich pyroxene. These cannot be melt veins because they display no enrichment in compatible elements such as Mg and no zonation at the vein-host mineral boundaries. Instead, these veins could be the product of the interaction between the primary mineralogy of the CAI and a metasomatic fluid, with a low fluid/rock ratio on the parent body. A CO-CO₂ or S-rich gas would be expected to have crystallized carbonated minerals and sulfides, respectively, but such phases are lacking in veins and in other portions of the CG1-CAI. The metasomatic agent involved in this alteration process could be aqueous. Indeed, hydrothermal diopside, anorthite, and forsterite are known on Earth (e.g., Python et al. 2007) and are theoretically predicted by McCollom and Shock (1998) who suggested that anhydrous minerals such as pyroxene, plagioclase, and forsterite may crystallize from high-temperature fluids (>700–800 °C). Thus, we propose that the core of the CG1-CAI underwent an alteration process by an aqueous high-temperature fluid, resulting in the dissolution of primary fassaite, spinel, and melilite, followed by in situ crystallization of Ca-rich pyroxene, anorthite, and forsterite.

We did not observe any other petrologic evidence for parent body aqueous alteration events in CAIs in
Tnz 057. Despite metamorphic overprinting, it is possible that the relatively large size and compact nature of the CG1-CAI (compared with other smaller CAIs in this meteorite) allowed the preservation of an aqueous alteration signature in this CAI. Hence, it is suggested that CAIs in CK chondrites suffered intense aqueous alteration and originally contained more alteration features, but subsequent thermal metamorphism on the CK parent body erased much of the evidence for prior aqueous alteration.

## Parent Body Metamorphism

### Fine-Grained CAIs

Fine-grained CAIs in CK3-4 chondrites are chemically equilibrated in major elements with the host matrix. They are enriched in Fe and depleted in Mg due to the Fe-Mg interdiffusion between matrix olivine and CAI phases during metamorphism. Re-equilibration also caused the replacement of primary perovskite by ilmenite, as described by Kornacki and Wood (1985) and McGuire and Hashimoto (1989). In type 4, fine-grained CAIs recrystallized as irregular assemblages of plagioclase + Ca-rich pyroxene ± olivine ± Ca-poor pyroxene ± magnetite (Fig. 5e), making them difficult to be distinguished from matrix in BSE images. They could, however, be confirmed as being refractory materials because of the pristine REE signatures of Ca-rich pyroxene and plagioclase grains present in these inclusions (Fig. 9). Compared to olivine and plagioclase reported in CK matrix (e.g., Kallmeyn et al. 1991; Rubin 1992; Noguchi 1993), olivine and plagioclase grains in CAIs in CKs contain scarce vesicules and opaque inclusions. These observations are not consistent with shock as the origin of the thermal event in CKs, as suggested by Scott et al. (1992), Rubin (1992), and Tomeoka et al. (2001, 2005). If silicate darkening occurred by the mobilization of opaque phases during shock-induced melting, olivine grains in former CAIs should have been affected by this process. Radiative heating was proposed by Chaumard et al. (2012), as an alternative process for the origin of the CK thermal event.

Fine-grained CAIs studied in this study are similar to the object composed of anorthite, diopside, and olivine described by Rubin (2013) in NWA 1559. It was suggested that this object corresponded to a former amoeboid olivine inclusion (AOI) composed of diopside, anorthite, and melilite, later affected by secondary parent body processes that caused the formation of pores by the leaching out of fine-grained anorthite and melilite (Rubin 2013). In the context of our textural (i.e., the lack of triple junctions), chemical, and isotopic results on the numerous fine-grained CAIs present in

CKs, we prefer to define these objects as CAIs and not AOIs, even if both these chondritic components could be genetically linked (e.g., Ruzicka et al. 2012).

### Coarse-Grained CAIs

As previously discussed for fine-grained CAIs, parent body metamorphism also caused partial chemical equilibration of coarse-grained CAIs. Diffusion fronts defined by Fe-Mg exchange with the matrix are mostly visible in spinel and pyroxene grains. Diffusion fronts in spinels have sizes up to few millimeters, as in the CG1-CAI in Tnz 057 (Fig. 6). Consequently, coarse-grained CAIs smaller than approximately 250 µm in CK3-4 chondrites display a complete re-equilibration of spinels, with chemical compositions close to the hercynite endmember (Fig. 5d).

Two coarse-grained CAIs in DaG 431 including the CG2-CAI display the destabilization of fassaite into diopside and ilmenite (or more rarely titanomagnetite) by Fe addition and Mg subtraction:

$$ 
\begin{aligned}
\mathrm{Ca}(\mathrm{Mg}_{\mathrm{A}}, \mathrm{Ti}_{\mathrm{B}}, \mathrm{Al}_{\mathrm{C}})(\mathrm{Al}_{\mathrm{X}}, \mathrm{Si}_{\mathrm{Y}})_{2}\mathrm{O}_{6} &+ (3\mathrm{B} + \mathrm{Z})\mathrm{FeO} \\
&= \mathrm{Ca}(\mathrm{Mg}_{\mathrm{A}-\mathrm{B}-\mathrm{Z}}, \mathrm{Fe}_{2\mathrm{B}+\mathrm{Z}}, \mathrm{Al}_{\mathrm{C}})(\mathrm{Al}_{\mathrm{X}}, \mathrm{Si}_{\mathrm{Y}})_{2}\mathrm{O}_{6} \\
&+ (\mathrm{B})\mathrm{FeTiO}_{3} + (\mathrm{Z})\mathrm{MgO}
\end{aligned}
\quad (3) $$

where A + B + C = 1; X + Y = 1; and 0 < Z < (A-B). Due to the size of the CG1-CAI, this reaction (3) is only present locally around some fassaite grains in the core of this CAI.

Two types of secondary grossular were observed in coarse-grained CAIs in CKs, depending on primary assemblage. As can be seen in the CG2-CAI in DaG 431 (CK3-an) (Fig. 4), secondary grossular is a product of a reaction between anorthite and Ca-rich pyroxene (Fig. 4). On the basis of the lack of monticellite in all CAIs investigated, we propose, as suggested by Wark (1987), the following reaction:

$$ 
\begin{aligned}
2\mathrm{Ca}_{2}\mathrm{Al}_{2}\mathrm{SiO}_{7} &+ \mathrm{CaAl}_{2}\mathrm{Si}_{2}\mathrm{O}_{8} + \mathrm{CaMgSi}_{2}\mathrm{O}_{6} \\
&= 2\mathrm{Ca}_{3}\mathrm{Al}_{2}\mathrm{Si}_{3}\mathrm{O}_{12} + \mathrm{MgAl}_{2}\mathrm{O}_{4}
\end{aligned}
\quad (4) $$

This reaction implies a production of secondary spinel, as observed in the CG2-CAI in DaG 431 (CK3-an) (Fig. 4). The consumption of melilite by reaction (4) could explain the lack of this phase in CAIs where evidence for such a reaction is observed. Reaction (4) only involves the gehlenite component. Considering both akermanite and gehlenite components, reaction (4) can be express as:

$$ 
\begin{aligned}
&\mathrm{Ca}_{2}\mathrm{Al}_{2}\mathrm{SiO}_{7} + \mathrm{Ca}_{2}\mathrm{MgSi}_{2}\mathrm{O}_{7} + \mathrm{CaAl}_{2}\mathrm{Si}_{2}\mathrm{O}_{8} \\
&+ \mathrm{CaAl}(\mathrm{Al}_{0.5}\mathrm{Si}_{0.5})_{2}\mathrm{O}_{6} \\
&= 2\mathrm{Ca}_{3}\mathrm{Al}_{2}\mathrm{Si}_{3}\mathrm{O}_{12} + \mathrm{MgAl}_{2}\mathrm{O}_{4}
\end{aligned}
\quad (5) $$
In this reaction, pyroxene is kushiroite, a phase that is not observed in the CAIs studied here (in contrast to fassaite, which occurs in numerous CAIs investigated here). In addition, and contrary to Wark (1987), reactions (4) and (5) are not Fe-free due to the Fe-Mg interdiffusion between matrix and CAIs during parent body metamorphism. Spinels produced by these reactions are equilibrated with the host matrix while primary grains display a chemical zonation (Fig. 4). Thus, reaction (5) can be expressed as:

$$ 
\begin{aligned}
Ca_2Al_2SiO_7 + Ca_2MgSi_2O_7 + CaAl_2Si_2O_8 \\
+ CaMgTi(Al_{2/3}Si_{1/3})_3O_9 + (2-X)FeO \\
= 2Ca_3Al_2Si_3O_{12} + (Fe_{1-x}Mg_x)Al_2O_4 \\
+ FeTiO_3 + (2-X)MgO
\end{aligned} 
\quad (6) 
$$

where $X < 1$. The production of secondary grossular and spinel by reaction between melilite, anorthite, and fassaite during Fe-Mg exchange between matrix and CAIs is consistent with the presence of Ti-rich oxides within grossular, as observed in the CG2-CAI in DaG 431. Thus, we suggest that secondary grossular and spinel in the CG2-CAI in DaG 431 were formed by reaction (6) in an asteroidal setting rather than in the solar nebula (FeO-free environment). We propose a similar origin for grossular in the CG4-CAI in Karoonda, which coexists with pyroxene and anhedral equilibrated spinels.

Another textural occurrence of grossular was observed during this study. Grossular is the major phase in the core of the CG1-CAI in Tnz 057, while melilite is lacking. In addition to REE measurements (see discussion in the Pristine Signatures of CAIs in CKs and Comparison to CVs section), the compact texture and the rounded shape of the core of this CAI suggest that it was formed by crystallization of a molten droplet. Grossular has previously been interpreted as a secondary phase and was extensively described in numerous CAIs from CV chondrites (e.g., Hutcheon and Newton 1981; Wark 1981, 1987; Hashimoto and Grossman 1987; Krot et al. 2004b, 2007, 2009; Fagan et al. 2005, 2006, 2007). As suggested by Krot et al. (2009) in a compact type A CAI, we propose that grossular was formed from a two-stage process during fluid-assisted thermal metamorphism. Primary melilite is replaced by secondary anorthite, which reacted with the remaining melilite to form grossular. These parent body reactions are consistent with the texture of the grossular-anorthite assemblage and the lack of melilite, monticellite, and wollastonite. In CAIs in CVs, grossular is present in veins within melilite (e.g., Fagan et al. 2007), whereas we observed no melilite in CKs of petrologic subtype >3.8. If we

assume a complete destabilization of melilite to form the secondary grossular-anorthite mixture observed in the core of the CG1-CAI in Tnz 057, this reaction may have occurred in an asteroidal setting, probably during the fluid-assisted metamorphic event on the CK parent body.

Above approximately 800 °C (the upper temperature of the grossular stability field at P <1 kbar), grossular breaks down to give wollastonite, melilite, and anorthite (e.g., Yoder 1954). The occurrence of grossular without wollastonite and melilite in the core of the CG1-CAI suggests an upper limit on the temperature for fluid-assisted metamorphic event on the parent body of Tnz 057 of about 800 °C.

## Differences Between CAIs from CKs and Other Chondrite Groups Only Due to the Destabilization of Primary Melilite?

As mentioned above, melilite was not found in CAIs from the CKs investigated during this study. Another group of chondrites, the Rumuruti (R) chondrites, contain CAIs in which melilite is also lacking (e.g., Rout and Bischoff 2008), while melilite is an abundant phase in CAIs from CV and CO chondrites. By comparison with other chondrites, CK and R chondrites experienced highly oxidizing conditions. As is the case for CK chondrites, R chondrites are highly oxidized as indicated by the scarcity of metal, the high NiO contents of olivine, as well as the presence of fayalitic olivine grains (approximately $Fa_{40-60}$) (Bischoff et al. [2011]; and references therein). CAIs were observed and extensively described in R chondrites (e.g., Rout and Bischoff 2008; Rout et al. 2009; Bischoff et al. 2011). Even though CAIs in R chondrites are different in terms of their abundances, sizes, and textures compared with CAIs in CKs, they have numerous phases in common. In type 3 lithologies, CAIs in R chondrites are mainly composed of fassaite, spinel, diopside, and Na,Al-rich alteration products (nepheline and sodalite), with minor hibonite, olivine, anorthite, ilmenite, and perovskite. As in CV3 chondrites, Na,Al-rich alteration products (mainly nepheline/sodalite-bearing secondary intergrowths) were interpreted in R chondrites as deriving from the decomposition of melilite in a nebular setting (Allen et al. 1978; MacPherson et al. 1981). In CAIs observed in CKs, it appears that all primary melilite (dominated by the gehlenite component in most CAIs; e.g., Brearley and Jones [1998] and references therein) is absent.

Previous experimental and petrologic studies showed that melilite is the primary phase most susceptible to alteration (e.g., Allen et al. 1978; MacPherson et al. 1981; Wark 1981; McGuire and Hashimoto 1989; Nomura and Miyamoto 1998; Krot
et al. 2007) and that akermanite is not stable at temperatures less than approximately 700 °C at 1 atmosphere (Harker and Tuttle 1956; Yoder 1973), whereas the lower limit of the stability field of gehlenite lies in the range of 600–715 °C at low CO₂ pressure (Hoschek 1974). Thus, it is not unlikely that all melilite was consumed by the anorthite- and grossular-producing reactions discussed above.

The formation of plagioclase from the fine-grained alteration products of melilite was described in *R* chondrites (e.g., Rout and Bischoff 2008). Based on the high abundance of oligoclase and lack of Na,Al-rich alteration products in the metamorphosed lithologies of *R* chondrites, Rout and Bischoff (2008) concluded that plagioclase replaces the nepheline/sodalite phases. Due to the high metamorphic grade of our samples relative to chondrites in which Na,Al-rich alteration products of melilite alteration were observed, nepheline and/or sodalite formed by melilite decomposition could have disappeared during the CK thermal event. We have proposed that the unidentified K-rich mixture observed in a CAI in the CK3 DaG 431 could be evidence for the existence of alteration phase(s) derived from the destabilization of melilite by aqueous alteration, later replaced by plagioclase and grossular during the thermal metamorphic event of CK chondrites. In addition, CKs experienced extreme oxidation (from FMQ + 3.1 to FMQ + 5.2), whereas *R* chondrites suffered lower oxidation (from the IW to the FMQ buffer) (Righter and Neff 2007). Thus, the lack of nepheline and/or sodalite mixtures in CAIs in CKs may result from the different metamorphic grades and oxidation histories undergone by CK and *R* chondrites.

However, we cannot exclude a formation of secondary anorthite and grossular directly by the decomposition of melilite in CK CAIs during a fluid-assisted thermal metamorphic event on the parent body, without formation of Na,Al-rich intermediate phases. Aléon et al. (2002) described pyroxene-anorthite-rich CAIs in CRs. These CAIs display textural and mineralogical similarities with fluffy CAIs observed in this study and contain small amounts of melilite showing evidence for replacement by anorthite. In COs, the alteration of CAIs seems to be a function of petrologic type (Greenwood et al. 1992; Kojima et al. 1995; Russell et al. 1998; Wasson et al. 2001; Itoh et al. 2004): melilite is unaltered in CO3.0, but then progressively breaks down to secondary phases such as diopside, nepheline, monticellite, and becomes completely transformed in petrologic types 3.4. In higher petrologic types, melilite-rich inclusions are not observed because melilite is completely destabilized (Greenwood et al. 1992; Russell et al. 1998). In Al-rich inclusions in AOAs in Allende, Hashimoto and

Grossman (1987) described the decomposition of melilite in a fine-grained mixture composed of grossular, anorthite, and feldspathoids. Finally, the hydrothermal experiments on alteration of CAIs describe the decomposition of gehlenite into hydrogrossular, calcite, and amorphous Al-Si, whereas spinel and diopside escaped alteration (Nomura and Miyamoto 1998). These authors concluded that most secondary phases found in CAIs in carbonaceous chondrites were produced from gehlenite by aqueous alteration and subsequent thermal metamorphism on their parent body. The lack of melilite, associated with the observation of unaltered Ca-rich pyroxenes and spinels in CAIs analyzed in CKs, is consistent with a secondary thermal process under conditions wherein gehlenite is unstable.

Formation of secondary anorthite and grossular by reactions in the solar nebula should also be considered. Indeed, the setting for the formation of such secondary phases is still controversial. If secondary phases could be the result of low-grade metamorphic events in the parent bodies of chondrites, several authors argue for an origin by low-temperature reactions in the solar nebula prior to accretion (e.g., Wark 1981; Davis 1985; McGuire and Hashimoto 1989). As previously discussed in the context of the extent of secondary processing, which affected the pristine CAI mineralogy (complete destabilization of primary melilite producing secondary anorthite and grossular), we favor an asteroidal metamorphic event to account for the occurrence of secondary phases such as anorthite, grossular, and spinel observed in CAIs in CKs.

As corundum, melilite, and grossite are lacking in CAIs in *R* chondrites, Rout and Bischoff (2008) concluded that melilite-rich or melilite-bearing type A and B CAIs mainly observed in CV3 chondrites are absent in *R* chondrites. In terms of abundance, size, and mineralogy, CAIs in *R* chondrites may be genetically linked to CAIs in CO3 chondrites (e.g., Rout and Bischoff 2008; Bischoff et al. 2011), whereas CAIs in CKs would be, based on this study, linked to those described in CVs. Thus, if grossular and some amount of anorthite and Ca-rich pyroxene (the dominant phases in CAIs in CKs) have a secondary origin (anorthite and Ca-rich pyroxene occurring as secondary phases in melilite-rich CAIs, and especially in type A CAIs; e.g., MacPherson 2003), we can conclude that CAIs in CK chondrites are more metamorphosed varieties of CAIs in the CV chondrites. In addition, our evaluation of the modal abundances of minerals present in CAIs studied here suggests that fluffy CAIs described in CKs could be more metamorphosed varieties of fluffy type A CAIs found in CVs. Similarly, the high abundances of secondary grossular and plagioclase in the core of the
CG1-CAI (approximately 35 and approximately 20%, respectively) and CG4-CAI (approximately 50 and approximately 25%, respectively) imply an initial texture and mineralogy (i.e., abundance of melilite) close to that described in CVs for compact type A CAIs. Regarding the modal abundance of fassaite, grossular, and plagioclase (approximately 40, 30, and 25%, respectively), the CG2-CAI appears to be linked to type B CAIs found in the CV chondrites.

## Heterogeneous CAI Modal Abundances in CKs

CKs appear to be dominated by fine-grained CAIs (Table 3), even when large areas were studied (such as for Tnz 057 and NWA 4425). However, bias due to the small section areas investigated is unlikely for small fine-grained CAIs, but may be expected for large objects, especially coarse-grained CAIs. So, we cannot eliminate the possibility that our measurements could have been biased due to the fact that we did not examine significant surface areas for each meteorite. In addition, and even if it seems not be the case for fine-grained CAIs, spatial heterogeneities in the initial distribution of CAIs within these CKs cannot be excluded.

Some of the CK chondrites studied here, and all of the petrologic types ≥4, display low abundances of fine-grained CAIs (<3%) (Table 3). These CKs (including the CK4s Karoonda, Maralinga, Kobe, NWA 4422, NWA 2519, and NWA 5733) appear to contain no coarse-grained CAIs, except for Karoonda (0.1%). They have similar modal abundance of CAIs as those initially determined by Kallemeyn et al. (1991). These low values could be explained by the fact that initial CAI mineralogies, textures, and boundaries were affected by recrystallization and chemical re-equilibration with the host matrix during metamorphism.

Contrary to previous studies describing CKs as CAI-poor objects (≤4%) (e.g., Kallemeyn et al. 1991; Noguchi 1993; Hezel et al. 2008), our results show that another subgroup of CKs is composed of samples showing significant CAI modal abundances (Table 3). These relatively CAI-rich CKs, including DaG 431 (CK3-an), NWA 1559 (CK3), NWA 4724 (CK3.8), NWA 4425 (CK3.8), NWA 4423 (CK3.9), NWA 4770 (CK4), and Tnz 057 (CK4), show a significant modal abundances of fine-grained CAIs (~4–12.5%; Table 3). Except for in NWA 4423 (CK3.9), we observed 0.6–6.4% of coarse-grained CAIs in this set of CKs (Table 3). So, contrary to the CAI-poor CKs, most of which contain no coarse-grained CAIs (and Karoonda only contains 0.1%; Table 3), this set of CKs shows numerous fine- and coarse-grained CAIs. The ranges of modal abundances of CAIs in CVs and in this set of

CKs are similar. We suggest that these CKs, all from petrologic subtype 3.8 to type 4, should be considered simply as more metamorphosed grades of the CV chondrites. Consequently, the modal abundance of CAIs should not be used as a criterion for distinguishing CV from CK chondrites. In addition, the presence of dark inclusions in NWA 4425 (CK3.8) and Tnz 057 (CK4) (Chaumard et al. 2009), up to now only observed in CVs, supports a genetic link between these CKs and CVs.

We propose that the heterogeneity in the modal abundance of fine-grained CAIs in CKs is a consequence of the CK thermal metamorphic event: in CKs of petrologic types ≥4, most of CAIs were recrystallized during metamorphism (see discussion in the Parent Body Metamorphism section). This seems to be confirmed by the progressive increase in the mean radii of the fine-grained CAI from the CV chondrites Allende (210 µm) and NWA 779 (285 µm) to the CAI-rich CK chondrites NWA 4425 (450 µm) and Tnz 057 (490 µm). This increase in the mean size is likely the consequence of the progressive disappearance of the smallest CAIs during the chemical and textural re-equilibration induced by the CK thermal metamorphic event.

## Implications Based on the Al-Mg Isotopic Analyses

While the ²⁶Al-²⁶Mg systematics in the CAIs in Tnz 057 shows evidence for the presence of live ²⁶Al in these objects at the time of their formation, there are also clear indications that this isotope system has been partially equilibrated by later events (Fig. 11). As can be seen in Fig. 11, the data for the fine-grained inclusions, FG1- and FG2-CAI (indicated in green), as well as two of the data points for the CG1-CAI (indicated in red with bull's-eyes) lie along a line with a slope corresponding to the canonical ²⁶Al/²⁷Al ratio of approximately 5 × 10⁻⁵ (e.g., MacPherson et al. 1995, 2010; McKeegan and Davis 2003; Jacobsen et al. 2008) and initial δ²⁶Mg* = -0.032‰ (Thrane et al. 2006). Other Al-Mg data (17 spot analyses) obtained in the core of the CG1-CAI in Tnz 057 (indicated in red) appear to define a correlation of radiogenic ²⁶Mg (δ²⁶Mg*) with their Al/Mg ratios (Fig. 11). If a line is regressed through these data, it yields a slope corresponding to an initial ²⁶Al/²⁷Al ratio of approximately 4.13 (±1.20) × 10⁻⁵, with an initial (δ²⁶Mg*)₀ = -0.38 ± 0.28‰. While this slope is, within the uncertainty, similar to the canonical value measured in CV CAIs (e.g., MacPherson et al. 1995, 2010; McKeegan and Davis 2003; Jacobsen et al. 2008), the low initial (δ²⁶Mg*)₀ (=−0.38 ± 0.28‰), the large MSWD (=19), and the fact that these data fall to the
right of the canonical CAI Al-Mg isochron are all indicators of later disturbance and partial equilibration of the Al-Mg system. Finally, the data obtained on the accretionary rim of the *CG1*-*CAI* in Tnz 057 (indicated in blue) all lie systematically above the canonical CAI Al-Mg isochron (Fig. 11), although some lie close to the canonical CAI Al-Mg isochron. These data are likely to be the result of redistribution of Mg between this accretionary rim and adjacent phases following the decay of $^{26}$Al.

The mass-dependent Mg isotope compositions of the fine- and coarse-grained CAIs are fractionated relative to the bulk Allende value toward lighter and heavier compositions (Table 11), respectively. These compositions indicate the different histories for these CAIs in the solar nebula. The fine-grained CAIs appear to have remained relatively pristine (relative to their precursor components) in their Mg isotopic composition, while evaporation (and possibly also the subsequent alteration) processes likely fractionated Mg isotopes in the coarse-grained CAI. The Al-Mg systematics in the fine-grained CAIs (and some of the data for the coarse-grained CAI) in Tnz 057 reinforces the mineralogical and chemical evidence presented in this study and suggests that CK CAIs most likely had similar nebular histories (involving condensation, evaporation, and/or melting) as the CV CAIs, but were later affected by more intense secondary processes (thermal metamorphism and/or aqueous alteration as discussed in previous sections). Due to the metamorphic effects observed in the mineralogy and the disturbance of Al-Mg systematics in the CAIs of Tnz 057, we cannot discuss these results as a possible consequence of heterogeneous distribution of $^{26}$Al or Mg isotopes within the solar nebula (Larsen et al. 2011).

Although the Al-Mg data presented here cannot provide rigorous chronological constraints on the secondary event(s) that led to the partial isotopic re-equilibration of the coarse-grained *CG1*-*CAI* in Tnz 057, these data can provide some estimation of the maximum temperature and duration of this thermal event. Mg diffusion experiments in anorthite indicate that given similar conditions, Mg diffuses more rapidly than Ca and Sr (LaTourette and Wasserburg 1998). For grains approximately 50 µm in size, at 700 °C, only 10 kyr is required to homogenize 90% of the Mg of an anorthite grain located within a Mg-rich environment. As a correlation still exists between the Al/Mg and Mg excesses in the core of *CG1*-*CAI* (indicating an apparent $^{26}$Al/$^{27}$Al ratio approximately 4.1 × 10$^{-5}$; Fig. 11), it is indicative that for a temperature of 700 °C, the thermal peak duration had to be shorter than tens of thousands of years (given the approximations of grain size and composition). This duration is intermediate between

what is expected for parent body thermal processing caused by nuclide decay (i.e., several millions of years; e.g., Huss et al. 2006) and shock processes (seconds; e.g., Beck et al. 2005). Furthermore, shock features are not observed in Tnz 057. However, if the Al-Mg systematics of CAIs in Tnz 057 was disturbed by thermal metamorphism associated with radiative heating (Chaumard et al. 2012), and using the timescales of 2–8 Ma after CAI formation for the accretion and thermal peak of the CV chondrite parent body (e.g., Bouvier et al. 2007), the partial equilibration of Al-Mg isotopic signatures in the core of the *CG1*-*CAI* may reflect a lower thermal peak for Mg diffusion of approximately 540–580 °C (LaTourette and Wasserburg 1998).

## CONCLUSIONS

In this study, we have investigated the abundances, chemistry, mineralogy, and petrology of CAIs in CK carbonaceous chondrites. We have shown here that CAI abundances vary significantly in both CV and CK chondrites. As such, CAI abundance cannot be used as a classification criterion for distinguishing between CV and CK chondrites. We described numerous CAIs from various samples of CK carbonaceous chondrites, in particular of petrologic types 3 and 4, which provide information about the region of formation, physical conditions within the solar nebula, and chemical and mineralogical compositions prior to parent body accretion, in addition to their evolution during parent body processes (aqueous alteration followed by fluid-assisted thermal metamorphism). We showed that the differences between CAIs from CV and CK chondrites in terms of their chemistry, mineralogy, and petrology can be the result of multiple nebular and parent body processes superimposed on their pristine signatures rather than intrinsic genetic differences. Our study confirms the close relationship between CV and CK carbonaceous chondrites. Initially described in CVs, the occurrence of dark inclusions in CKs (Chaumard et al. 2009) additionally confirms the genetic relationship between CV and CK chondrites. Finally, the Al-Mg systematics in the coarse-grained *CG1*-*CAI* and fine-grained *FG1*- and *FG2*-*CAIs* of Tnz 057 indicates that live $^{26}$Al was initially present in CK CAIs, likely with an initial abundance level similar to that in the CV CAIs. However, there is significant disturbance recorded in the Mg isotope compositions of the *CG1*-*CAI*, reflecting a greater degree of thermal alteration affecting this CAI in this CK4 relative to those typically found in CV3 chondrites. Nevertheless, this thermal event only partially equilibrated the Al-Mg systematics in the coarse- and fine-grained CAIs from the Tnz 057 CK4 chondrite analyzed in this study.
Therefore, all of the petrologic, chemical, and isotopic evidence presented here suggests that CKs could be metamorphic CVs, with petrologic types ≥3.8, with both CV and CK groups forming a complete metamorphic series from type 3 to 6. Moreover, the distribution of type 3 in the CV group and type ≥4 in the CK group could be biased, such that samples with unequilibrated chondrules are systematically classified as CVs, whereas samples with recrystallized matrices are classified as CKs. The current CV and CK classification into two distinct groups should be revised to form a single group, as already suggested by Greenwood et al. (2010) and more recently by Wasson et al. (2013).

*Acknowledgments*—We gratefully thank the Muséum National d’Histoire Naturelle (Paris, France) and the NASA Meteorite Working Group (Houston, Texas) for providing us CV and CK samples. We also thank Frédéric Béroud and Christophe Boucher for the loan of slices of Tnz 057. We are indebted to Brigitte Zanda for enlightening discussions. We thank Jean-Luc Devidal and Jean-Marc Hénot for their assistance with EPMA/LA-ICPMS and SEM, respectively. We are additionally thankful to Phil Janney and Rebekah Hines for their support and assistance with the Al-Mg analyses at ASU. We are very grateful to Marian Horstmann and the AE, Edward R. D. Scott, as well as an anonymous reviewer for constructive reviews and helpful comments, which greatly improved the manuscript. This study was partly supported by NASA grant NNX11AK75G to MW.

*Editorial Handling*—Dr. Edward Scott

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