# Compositional diversity of ordinary chondrites inferred from petrology, bulk chemical, and oxygen isotopic compositions of the lowest FeO ordinary chondrite, Yamato 982717

Akira Yamaguchi, Makoto Kimura, Jean-Alix Barrat, Richard Greenwood

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Akira Yamaguchi, Makoto Kimura, Jean-Alix Barrat, Richard Greenwood. Compositional diversity of ordinary chondrites inferred from petrology, bulk chemical, and oxygen isotopic compositions of the lowest FeO ordinary chondrite, Yamato 982717. Meteoritics and Planetary Science, 2019, 54, pp.1919-1929. 10.1111/maps.13351. insu-03685655

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# Compositional diversity of ordinary chondrites inferred from petrology, bulk chemical, and oxygen isotopic compositions of the lowest FeO ordinary chondrite, Yamato 982717

Akira YAMAGUCHI<sup>1*</sup>, Makoto KIMURA<sup>1</sup>, Jean-Alix BARRAT<sup>2,3</sup>, and Richard GREENWOOD<sup>4</sup>

*(Received 11 November 2018; revision accepted 07 June 2019)*

**Abstract**–We performed a petrologic, geochemical, and oxygen isotopic study of the lowest FeO ordinary chondrite (OC), Yamato (Y) 982717. Y 982717 shows a chondritic texture composed of chondrules and chondrule fragments, and mineral fragments set in a finer grained, clastic matrix, similar to H4 chondrites. The composition of olivine (Fa<sub>11.17</sub> ± 0.48 (1σ)) and low-Ca pyroxene (Fs<sub>11.07</sub> ± 0.98 (1σ)Wo<sub>0.90</sub> ± 0.71(1σ)) is significantly more magnesian than those of typical H chondrites (Fa<sub>16.0-20</sub>, Fs<sub>14.5-18.0</sub>), as well as other known low-FeO OCs (Fa<sub>12.8-16.7</sub>; Fs<sub>13-16</sub>). However, the bulk chemical composition of Y 982717, in particular lithophile and moderately volatile elements, is within the range of OCs. The bulk siderophile element composition (Ni, Co) is within the range of H chondrites and distinguishable from L chondrites. The O-isotopic composition is also within the range of H chondrites. The lack of reduction textures indicates that the low olivine Fa content and low-Ca pyroxene Fs content are characteristics of the precursor materials, rather than the result of reduction during thermal metamorphism. We suggest that the H chondrites are more compositionally diverse than has been previously recognized.

## INTRODUCTION

Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018). They are characterized by similar bulk chemical and oxygen isotopic compositions, mineral compositions, and petrologic textures (e.g., Clayton et al. 1991; Rubin 2005), and the ratios of metallic Fe to total Fe (Brearley and Jones 1998).

Ordinary chondrites are generally classified into three chemical types, H, L, and LL groups mainly on the basis of Mg' in major minerals such as olivine and pyroxene, and bulk siderophile element abundances. However, some OCs do not fit into these groups. There are intermediate groups (Kallemeyn et al. 1989; Rubin

1990; Wittmann et al. 2011) and low or higher FeO groups such as L/LL and L/H. The low FeO groups' OCs include “low-FeO OC” (Table 1) (Wasson et al. 1993; McCoy et al. 1994; Russell et al. 1998; Troiano et al. 2011). The silicate portion in Netschaëvo (IIE iron) has been classified as an OC, but of a very low Fa type, which may be designated “HH” (Bild and Wasson 1977). It is a subject of debate that these low-FeO OCs are the most reduced member of OCs or a separate clan of chondritic meteorites (McCoy et al. 1994).

Yamato (Y) 982717 (the original mass = 10.3 g) has been classified as an anomalous H chondrite in spite of its very low Fa and Fs values (Yamaguchi et al. 2013). The Fa and Fs values are lower than those of any other known H chondrite or other low-FeO OCs. Here, we report the petrology, bulk chemical, and oxygen

<footnote n="1"><sup>1</sup>National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan</footnote>

<footnote n="2"><sup>2</sup>Université Européenne de Bretagne, Lorient, France</footnote>

<footnote n="3"><sup>3</sup>CNRS, UMR 6538 (Domaines Océaniques), U.B.O.-I.U.E.M., Place Nicolas Copernic, 29280 Plouzané Cedex, France</footnote>

<footnote n="4"><sup>4</sup>Planetary and Space Sciences, Department of Physical Sciences, The Open University, Walton Hall, Milton Keynes MK7 6AA, UK</footnote>

<footnote>*Corresponding author. E-mail: yamaguch@nipr.ac.jp</footnote>
<table><caption>Table 1. Characteristics of well-characterized low-FeO ordinary chondrites and other chondrites.</caption><thead><tr><th></th><th>Olivine<br>Fa</th><th>Low-Ca<br>pyroxene<br>Fs</th><th>Petrologic<br>type</th><th>Shock</th><th>Weathering<br>degree</th><th>Chondrule<br>diameter (µm)</th><th>Bulk<br>Ni/Mg<sub>CI</sub></th><th>Bulk<br>Fe/Mg<sub>CI</sub></th><th>Comments</th><th>References</th></tr></thead><tbody><tr><td>Y 982717</td><td>11.2</td><td>11.1</td><td>4</td><td>S2</td><td>A/W0</td><td>343</td><td>0.89</td><td>0.93</td><td></td><td>1</td></tr><tr><td>Burnwell</td><td>15.8</td><td>13.4</td><td>4</td><td>S3</td><td>(Fall)</td><td>360</td><td>1.13</td><td>1.00</td><td></td><td>2</td></tr><tr><td>Cerro los Calvos</td><td>12.8</td><td>12.4</td><td>4</td><td>S2</td><td>W3</td><td>422</td><td>1.01</td><td>0.83</td><td></td><td>3, 4</td></tr><tr><td>EET 96031</td><td>16</td><td>15</td><td>4</td><td></td><td>B/C</td><td>&lt;1000</td><td></td><td></td><td>Paired with EET 96037,<br>96040, 96047, and 96123</td><td>5</td></tr><tr><td>LAP 04757</td><td>13</td><td>13–16</td><td>&gt;4?</td><td>C</td><td></td><td></td><td></td><td></td><td>Paired with LAP 04773</td><td>6</td></tr><tr><td>Willaroy</td><td>14.8</td><td>3–35</td><td>3.5–3.8</td><td>S2</td><td>W4</td><td>320</td><td>0.95–0.97</td><td>0.87–1.04</td><td></td><td>3, 4, 7</td></tr><tr><td>Netschævo</td><td>14.1–14.3</td><td>13.6</td><td>6</td><td></td><td></td><td>500–810</td><td>1.90–2.43</td><td>1.21–1.75</td><td>IIE silicate inclusion</td><td>8–10</td></tr><tr><td>Moorabie</td><td>15.9</td><td>15.3</td><td>3.5–3.8</td><td>S4–5</td><td>W1</td><td>650</td><td>0.74–0.82</td><td>0.75–0.77</td><td>Compositionally, L</td><td>3, 4, 11</td></tr><tr><td>Suwahib (Buwah)</td><td>14.4</td><td>11.9</td><td>3.6–3.8</td><td>S5</td><td>W1</td><td>540</td><td>0.74–0.97</td><td>0.73–1.04</td><td>Compositionally, L</td><td>3, 4, 11</td></tr><tr><td>L</td><td>22–26*</td><td>19–22*</td><td></td><td></td><td></td><td>500</td><td>0.30–1.16</td><td>0.58–1.26</td><td></td><td>11–16</td></tr><tr><td>H</td><td>16–20*</td><td>14.5–18*</td><td></td><td></td><td></td><td>300</td><td>0.43–1.62</td><td>0.59–1.77</td><td></td><td>11–16</td></tr><tr><td>EL</td><td>0.4</td><td>&lt;1</td><td></td><td></td><td></td><td>600</td><td>0.69–0.98</td><td>0.73–0.96</td><td></td><td>13, 17, 18</td></tr><tr><td>EH</td><td>0.4</td><td></td><td></td><td></td><td></td><td>200</td><td>1.23–1.63</td><td>1.17–1.49</td><td></td><td>13, 17, 18</td></tr></tbody></table>

References: 1: This work; 2: Russell et al. (1998); 3: Wasson et al. (1993); 4: McCoy et al. (1994); 5: Satterwhite and Lindstrom (1998); 6: Satterwhite and Righter (2007); 7: Chalmers and Mason (1977); 8: Bunch et al. (1970); 9: Olsen and Jarosewich (1971); 10: Bild and Wasson (1977); 11: Jarosewich (1990); 12: Friedrich et al. (2015); 13: Brearly and Jones (1998); 14: Clayton et al. (1991); 15: Kalllemeyn et al. (1989); 16: Kimura et al. (2018); 17: Kalllemeyn and Wasson (1986); 18: Weisberg et al. (2006).

*Type 4–6.
isotopic composition of Y982717 and discuss its origin. A preliminary study was presented by Yamaguchi et al. (2015).

## SAMPLES AND ANALYTICAL TECHNIQUES

Y 982717 has a dark-brown surface, and the presence of fusion crust is not clear. Subsample ,20 (1.244 g) of chip was separated from the main mass using a diamond wire saw without using a fluid. The cut surface is relatively fresh and metallic phases are visible; there are little holes and cracks formed by weathering. The subsample ,20 was further separated to two chips ,91 (0.975 g) for a polished thin section (PTS) and ,71 (0.556 g) for bulk chemical analysis by a chisel. We examined the PTS of Y 982727 (,51-1) (~8 × 9 mm). A chip 0.556 g of Y 982717 was crushed for bulk chemical analysis. Part of the powder for bulk chemical analysis was used for oxygen isotopic analysis. We examined these PTSs using an optical microscope equipped with both transmitted and reflected light, and an electron microprobe analyzer (EPMA; JEOL JXA-8200) and scanning electron microscopes (JEOL 5900LV and 7100F) equipped with an energy dispersive spectrometer (Oxford AZtec Energy) at the National Institute of Polar Research, Tokyo (NIPR). Olivine, pyroxene, chromite, FeNi metals, and troilite were analyzed using a current of 30 nA and plagioclase using 10 nA and a defocused beam (approximately 5 µm in diameter) using natural and synthetic standards at 15 keV. For Mn analysis, we used a LIFH spectrometer. Compared to normal, LIF count rates were increased by a factor of 3–4. The interference from FeKβ on CoKα was mathematically corrected for. Data were reduced using a ZAF correction program. We carefully avoided hydro-oxide veins along cracks in mafic silicates which cause significant scatters of the Fe/Mg and Fe/Mn ratios. Silica mineral phases were identified with a JASCO NRS-1000 Raman spectrometer at NIPR. Mineral modes were obtained using the software ImageJ from X-ray mapping obtained with ~12 µm broad beam and 10 µm steps. We measured apparent chondrule diameters in transmitted light. The maximum apparent diameter and the greatest diameter perpendicular to the maximum diameter of each chondrule were measured and these two measurements were then averaged.

Major and trace elements were determined, respectively, by inductively coupled plasma atomic emission spectrometry (ICP-AES) and inductively coupled plasma sector field mass spectrometry (ICP-SFMS) using the same procedures as Barrat et al. (2012). The precision and the accuracy of the results were estimated using well-characterized standards and sample duplicates. For ICP-AES results, the accuracy is better than 7% for Na and P, and probably much better than 3% for the other elements.

For ICP-SFMS, the accuracy is better than 9% for W, 4% for Pb, and less than 3% for the other elements.

Oxygen isotope analysis was performed by infrared laser-assisted fluorination (Greenwood et al. 2017). Analyses were undertaken on aliquots of ~2 mg of whole rock powder, which was drawn from a larger reservoir produced by crushing and homogenizing a clean interior chip weighing ~100 mg. To remove terrestrial weathering products, one fraction of Y 982717 was leached using a solution of ethanolamine thioglycollate (EATG) (Greenwood et al. 2012, 2017).

## RESULTS

### Textures

A PTS of Y 982717 shows a chondritic texture composed of chondrules and chondrule fragments, and mineral fragments set in a finer grained, clastic matrix, similar to H4 chondrites such as Kamioka (Fig. 1). The majority of chondrules are either partially or entirely fragmented. The apparent sizes of chondrules range from 120 to 985 µm, with the average being 343 ± 168 µm (1σ) (number of analyses = 108). These values are slightly greater than the H average (450 µm), but are significantly smaller than the L average (500 µm) (Friedrich et al. 2015). Seventy-six of 108 chondrules (70%) are porphyritic types (Fig. 2a). Several pyroxenes within chondrules and fragments display polysynthetic twinning. Mesostasis glass is devitrified. Feldspar is very fine-grained (<~5 µm). Thus, this meteorite is classified as a type 4.

Larger kamacite, taenite, and troilite nodules are irregular in shape and occur pervasively in the PTS. Modes of kamacite (~8 vol%), taenite (~0.5 vol%), and troilite (~5 vol%), and troilite/metal (wt%/wt%) value (~0.3) are similar to that of a typical H chondrite (McSween et al. 1991). Silica minerals (<0.2 vol%) are tridymite or cristobalite. Tridymite occurs in pyroxene fragments and radiating and porphyritic pyroxene chondrules (Fig. 2b). Tridymite laths are mostly outlined by thin (~1–10 µm) rims or tiny grains of augite. Irregular cristobalite grains occur in the matrix. This textural association indicates that pyroxene is a primary phase in the meteorite and did not form later by reduction during parent body metamorphism. Spinel (mostly chromite) grains (~10–280 µm) sparsely distributed in the chondrite. We found one Mg-rich spinel grain (fragment?) (100 × 50 µm) in the matrix (see below). Ca-phosphates (<0.23 mm) are apatite and merrillite and occur pervasively, in some cases associated with kamacite and taenite.

Y 982717 exhibits slight shock effects. Olivine and pyroxene show minor mottled extinction, indicating a
<figure><img src="image_2.png" /><figcaption>Fig. 1. Photomicrographs of (a) Y 982717 and (b) Kamioka (H4) for comparison. These meteorites display a typical chondritic texture. (Color figure can be viewed at wileyonline library.com.)</figcaption></figure>

shock stage of S2 (Stöffler et al. 1991). Y 982717 is slightly weathered and contains ~1% of weathering products in the portion away from the outer surface of the main mass, indicating the weathering grade is A or W1 (Yamaguchi et al. 2013). However, weathering products (Fe-hydroxide) occur pervasively along cracks and fractures in mafic minerals (Fig. 2).

## Mineral Chemistry

The compositions of olivine (Fa<sub>11.17</sub> ± 0.48 (1σ)) and low-Ca pyroxene (Fs<sub>11.07</sub> ± 0.98 (1σ)Wo<sub>0.90</sub> ± 0.71(1σ)) are significantly more magnesian than those of typical H chondrites (Fa<sub>16.0-20</sub>, Fs<sub>14.5-18.0</sub>), as well as other known low-FeO OCs (Fa<sub>12.8-16.7</sub>; Fs<sub>13-16</sub>) (Fig. 3; Table 2; McCoy et al. 1994; Brearley and Jones 1998; Russell

<figure><img src="image_3.png" /><figcaption>Fig. 2. a) Backscattered electron image (BEI) of a porphyritic olivine chondrule. Olivine (Ol) phenocrysts are set in a devitrified mesostasis glass (G). b) BEI of a chondrule that contains elongated silica minerals (dark gray) outlined by diopside in pyroxene grains (middle gray).</figcaption></figure>

et al. 1998; Troiano et al. 2011). The Ni (5.9–6.8 wt%) and Co (0.35–0.45 wt%) contents in kamacite in Y 982717 overlap with, but are slightly lower than, the range of H chondrites (Ni: 6.3–7.48 wt% and Co: 0.44–0.51 wt%) (Fig. 4; Rubin 1990). Ni contents in taenite vary from 48.4 to 69.1 wt%.

Compositions of chromite grains in Y 982717 are Usp<sub>1.6-3.0</sub>Sp<sub>11.7-16.2</sub>Chm<sub>82.0-85.9</sub> and Mg# (molar Mg/[Mg+Fe] × 100) = 12.0–16.5. Molar Al/(Ar+Cr) ratios are fairly constant (11.95–14.84). The composition of Mg-rich spinel is Usp<sub>1.1</sub>Sp<sub>56.4</sub>Chm<sub>45.3</sub> (Mg# = 45.3). Chromite contains minor amounts of V<sub>2</sub>O<sub>3</sub> (0.58–0.71 wt%) and ZnO (0.28–0.38 wt%). We could not find any intragrain chemical variation.

Most feldspar grains are too small (<~5 µm) to analyze by EPMA, but the few grains that could be measured were in the range An<sup>-10-40</sup> (Table 2). Silica
<figure><img src="image_4.png" /><figcaption>Fig. 3. Chemical composition of olivine and pyroxene in Y 982717. The data for Burnwell, Netschaëvo, LAP 04757, and H chondrites are from Russell et al. (1998), Bunch et al. (1970), Bild and Wasson (1977), Rubin (1990), Troiano et al. (2011), and Brearley and Jones (1998). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

minerals contain minor elements (<0.07 wt% Al₂O₃, <0.4 wt% FeO) (Table 2). Apatite contains a significant amount of Cl (5.2–6.1 wt%) and a small amount of F (<0.3 wt%). Merrillite contains 2.5–2.7 wt% Na₂O, 0.8–3.4 wt% MgO, and 0.8–4.5 wt% FeO.

## Bulk Composition

Table 3 presents bulk chemical compositions of 48 elements in Y 981727. The compositions are within the known OC range (see below). The influence of terrestrial weathering on the bulk composition is indicated by the absence of a Ce anomaly typically observed in silicates in Antarctic meteorites (Crozaz et al. 2003).

## Oxygen Isotopes

Oxygen isotope analyses of treated and untreated fractions of Y 982717 are plotted in Fig. 5. The untreated fraction of Y 982717 (δ¹⁷O = 2.02‰, δ¹⁸O = 2.86‰, Δ¹⁷O = 0.53‰) (N = 1) is significantly shifted to lower δ¹⁸O values compared to the EATG-treated fraction (δ¹⁷O = 2.81 ± 0.26‰ [2σ], δ¹⁸O = 4.36 ± 0.39‰ [2σ], Δ¹⁷O = 0.54 ± 0.06‰ [2σ]) (N = 2) (Fig. 5). This is in contrast to the fact that Y 982717 does not appear to be

significantly weathered (A) based on optical microscopic observations. This is a common feature of metal and sulfide-rich Antarctic finds, in which a significant amount of terrestrial oxygen can become locked into the sample via the formation of various hydroxides and oxyhydroxides, such as akaganeite (Greenwood et al. 2017). These secondary minerals are formed by interaction between primary metal and sulfide and δ¹⁸O depleted Antarctic precipitation (Greenwood et al. 2012, 2017). Thus, significant oxygen isotope shifts may be present in otherwise only lightly altered meteorite finds. The EATG-treated residue, from which terrestrial weathering products have largely been removed, plots in the H chondrite field (Fig. 5).

## DISCUSSION

Chondritic meteorites are classified on the basis of textures, mineral compositions, bulk chemical, and isotopic compositions. The sizes and abundances of chondrule types are consistent with the fact that Y 982717 is an H chondrite (Rubin 2000). Texturally, Y 982717 is indistinguishable from a typical H4 (Fig. 1).

Ordinary chondrites are characterized by refractory lithophile elements CI-Mg-normalized ~0.8–0.9 times those in CI chondrites (e.g., Kallmeyn et al. 1989; Fig. 6). These data are also distinct from EL and EH chondrites. The bulk compositions of EL chondrites are lower than those of OCs and very fractionated, whereas those of EH chondrites are similar to OCs (Kallmeyn and Wasson 1986). The lithophile element abundances of Y 982717 are within the range of OCs.

The abundances of moderately volatile elements are a key diagnostic criterion, as a consequence of volatility-controlled fractionation. The differences in volatility of Zn (moderately volatile), Mn (slightly volatile), and Al (highly refractory) allow discrimination between each of the major chondrite groups. On a diagram of Zn/Mn versus Al/Mn, Y 982717 plots on the border of the OC range (Kallmeyn et al. 1989; Wasson et al. 1993; Fig. 7).

Individual OC groups have similar refractory lithophile and moderately volatile element abundances, but can be distinguished by their bulk siderophile element abundances (i.e., lithophile to siderophile elements ratios). The CI-normalized Co/Mg (Co/MgCI) of Y 972717 is 1.222, which is the range of H chondrites (Co/MgCI = 0.995–1.348; Kallmeyn et al. 1989; Jarosewich 1990). However, Ni/MgCI (=0.892) and Fe/MgCI (=0.932) are slightly lower than the ranges of H chondrites (Ni/MgCI = 0.975–1.137 and Fe/MgCI = 0.952–1.102), but are significantly higher than L chondrites (Ni/MgCI = 0.662–0.820 and Fe/MgCI = 0.711–0.797; Kallmeyn et al. 1989). The low
<table><caption>Table 2. Chemical compositions of minerals in Y 982717.</caption><thead><tr><th></th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>CaO</th><th>FeO</th><th>MgO</th><th>MnO</th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>V<sub>2</sub>O<sub>3</sub></th><th>ZnO</th><th>Total</th></tr></thead><tbody><tr><td>Olivine</td><td></td><td></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>Average (n = 144)</td><td>40.5</td><td>0.01</td><td>0.02</td><td>10.8</td><td>48.4</td><td>0.45</td><td>0.03</td><td>0.04</td><td>&lt;0.01</td><td></td><td></td><td>100.3</td><td>Fa</td><td>Fo</td></tr><tr><td>1δ</td><td>0.27</td><td>0.01</td><td>0.02</td><td>0.44</td><td>0.46</td><td>0.01</td><td>0.02</td><td>0.04</td><td></td><td></td><td></td><td></td><td>11.2</td><td>88.8</td></tr><tr><td>Low-Ca pyroxene</td><td></td><td></td><td></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>Average (n = 191)</td><td>57.0</td><td>0.24</td><td>0.57</td><td>7.53</td><td>33.6</td><td>0.50</td><td>0.19</td><td>0.12</td><td>0.02</td><td></td><td></td><td>99.8</td><td>Wo</td><td>En</td></tr><tr><td>1δ</td><td>0.62</td><td>0.26</td><td>0.79</td><td>0.67</td><td>0.87</td><td>0.04</td><td>0.20</td><td>0.08</td><td>0.04</td><td></td><td></td><td></td><td>1.1</td><td>87.9</td></tr><tr><td>Spinel</td><td></td><td></td><td></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>1</td><td>0.06</td><td>6.67</td><td>0.01</td><td>26.5</td><td>4.00</td><td>1.52</td><td>57.9</td><td>0.82</td><td></td><td></td><td></td><td>0.67</td><td>0.38</td><td>98.5</td></tr><tr><td>2</td><td>&lt;0.02</td><td>6.15</td><td>&lt;0.01</td><td>26.1</td><td>4.15</td><td>1.39</td><td>59.1</td><td>1.09</td><td></td><td></td><td></td><td>0.65</td><td>0.34</td><td>99.0</td></tr><tr><td>Mg-rich</td><td>0.12</td><td>22.7</td><td>0.03</td><td>21.0</td><td>9.70</td><td>0.83</td><td>43.8</td><td>0.48</td><td></td><td></td><td></td><td>0.19</td><td>0.39</td><td>99.1</td></tr><tr><td>Plagioclase</td><td></td><td></td><td></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>1</td><td>57.8</td><td>25.1</td><td>7.57</td><td>0.83</td><td>0.44</td><td>&lt;0.07</td><td>&lt;0.05</td><td>0.07</td><td>6.63</td><td>0.23</td><td></td><td></td><td>98.7</td><td>1.4</td><td>60.5</td></tr><tr><td>Silica minerals</td><td></td><td></td><td></td><td></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>Average (n = 10)</td><td>98.9</td><td>&lt;0.03</td><td>0.04</td><td>0.32</td><td>&lt;0.02</td><td>&lt;0.07</td><td>&lt;0.05</td><td>&lt;0.05</td><td>&lt;0.03</td><td>&lt;0.02</td><td></td><td></td><td>99.2</td><td></td><td></td></tr><tr><td>1δ</td><td>0.45</td><td></td><td>0.02</td><td>0.44</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>
<figure><img src="image_5.png" /><figcaption>Fig. 4. Kamacite Co concentration (wt%) versus olivine fayalite (Fa) content for Y 982717 and OCs. The data of Netschaëvo and other OCs are from Rubin (1990). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

Ni/Mg<sub>CI</sub> value may be due to undersampling of taenite. Terrestrial weathering would result in loss of metallic phases by oxidation and so change the siderophile element abundances (Dennison and Lipschutz 1987). However, the weathering grade of Y 982717 is low (W0 or A). Thus, the impact on the bulk composition would be negligible. Therefore, we suggest that the abundances were established either in the nebula or during accretion.

Ordinary chondrites have nearly identical abundances of refractory and moderately volatile elements (see above), but they differ in their Fe abundances. Thus, we plotted our data on a Urey-Craig diagram, which provides an indication of the oxidation state of chondrites (Brearley and Jones 1998; Fig. 8). Our bulk chemical data cannot be directly plotted in this diagram since valences of elements (especially, Fe) are not well defined. We estimated amounts of FeO, FeNi-metals, and FeS from the ICP-MS data (Table 3) combined with the modal abundances obtained from the thin section. The amounts of Fe in metals and troilite are estimated from modes of FeNi metals (kamacite and taenite) and troilite. The rest of Fe is assigned to oxides. We assume that all Fe in oxides is ferrous (Fe<sup>2+</sup>) because OCs of low weathering degrees do not contain Fe<sub>2</sub>O<sub>3</sub> in general (Kimura et al. 2018). The abundance of Si is not obtained by solution ICP-MS due to volatilization of Si during solution experiments. So, we assumed that the Si left over after these calculations occurs in the form of SiO<sub>2</sub>. The Fe and FeO abundances are 21.7 and 6.4 wt%, respectively. The SiO<sub>2</sub> abundance is calculated by subtracting all the major element data from 100%.

<table><caption>Table 3. Major and trace element abundances of Y 972717</caption><thead><tr><th></th><th></th><th></th><th>Y 982717,71</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>ICP-AES</td><td>wt%</td><td>0.12</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>ICP-AES</td><td>wt%</td><td>1.98</td></tr><tr><td>FeO</td><td>ICP-AES</td><td>wt%</td><td>34.35</td></tr><tr><td>MnO</td><td>ICP-AES</td><td>wt%</td><td>0.30</td></tr><tr><td>MgO</td><td>ICP-AES</td><td>wt%</td><td>23.56</td></tr><tr><td>CaO</td><td>ICP-AES</td><td>wt%</td><td>1.63</td></tr><tr><td>Na<sub>2</sub>O</td><td>ICP-AES</td><td>wt%</td><td>0.89</td></tr><tr><td>K<sub>2</sub>O</td><td>ICP-AES</td><td>wt%</td><td>0.08</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>ICP-AES</td><td>wt%</td><td>0.30</td></tr><tr><td>Cr</td><td>ICP-AES</td><td>ppm</td><td>3051</td></tr><tr><td>Ni</td><td>ICP-AES</td><td>ppm</td><td>1.41</td></tr><tr><td>Li</td><td>ICP-SFMS</td><td>ppm</td><td>1.98</td></tr><tr><td>Be</td><td>ICP-SFMS</td><td>ppm</td><td>0.0292</td></tr><tr><td>K</td><td>ICP-SFMS</td><td>ppm</td><td>735</td></tr><tr><td>Sc</td><td>ICP-SFMS</td><td>ppm</td><td>8.46</td></tr><tr><td>V</td><td>ICP-SFMS</td><td>ppm</td><td>66.5</td></tr><tr><td>Mn</td><td>ICP-SFMS</td><td>ppm</td><td>2352</td></tr><tr><td>Co</td><td>ICP-SFMS</td><td>ppm</td><td>881</td></tr><tr><td>Cu</td><td>ICP-SFMS</td><td>ppm</td><td>84.67</td></tr><tr><td>Zn</td><td>ICP-SFMS</td><td>ppm</td><td>32.96</td></tr><tr><td>Ga</td><td>ICP-SFMS</td><td>ppm</td><td>5.59</td></tr><tr><td>Rb</td><td>ICP-SFMS</td><td>ppm</td><td>2.67</td></tr><tr><td>Sr</td><td>ICP-SFMS</td><td>ppm</td><td>10.53</td></tr><tr><td>Y</td><td>ICP-SFMS</td><td>ppm</td><td>2.17</td></tr><tr><td>Zr</td><td>ICP-SFMS</td><td>ppm</td><td>4.74</td></tr><tr><td>Nb</td><td>ICP-SFMS</td><td>ppm</td><td>0.422</td></tr><tr><td>Cs</td><td>ICP-SFMS</td><td>ppm</td><td>0.120</td></tr><tr><td>Ba</td><td>ICP-SFMS</td><td>ppm</td><td>3.35</td></tr><tr><td>La</td><td>ICP-SFMS</td><td>ppm</td><td>0.351</td></tr><tr><td>Ce</td><td>ICP-SFMS</td><td>ppm</td><td>0.839</td></tr><tr><td>Pr</td><td>ICP-SFMS</td><td>ppm</td><td>0.127</td></tr><tr><td>Nd</td><td>ICP-SFMS</td><td>ppm</td><td>0.643</td></tr><tr><td>Sm</td><td>ICP-SFMS</td><td>ppm</td><td>0.205</td></tr><tr><td>Eu</td><td>ICP-SFMS</td><td>ppm</td><td>0.0709</td></tr><tr><td>Gd</td><td>ICP-SFMS</td><td>ppm</td><td>0.286</td></tr><tr><td>Tb</td><td>ICP-SFMS</td><td>ppm</td><td>0.0520</td></tr><tr><td>Dy</td><td>ICP-SFMS</td><td>ppm</td><td>0.359</td></tr><tr><td>Ho</td><td>ICP-SFMS</td><td>ppm</td><td>0.0760</td></tr><tr><td>Er</td><td>ICP-SFMS</td><td>ppm</td><td>0.224</td></tr><tr><td>Yb</td><td>ICP-SFMS</td><td>ppm</td><td>0.230</td></tr><tr><td>Lu</td><td>ICP-SFMS</td><td>ppm</td><td>0.0340</td></tr><tr><td>Hf</td><td>ICP-SFMS</td><td>ppm</td><td>0.137</td></tr><tr><td>Ta</td><td>ICP-SFMS</td><td>ppm</td><td>0.0204</td></tr><tr><td>W</td><td>ICP-SFMS</td><td>ppm</td><td>0.194</td></tr><tr><td>Pb</td><td>ICP-SFMS</td><td>ppm</td><td>0.0340</td></tr><tr><td>Th</td><td>ICP-SFMS</td><td>ppm</td><td>0.0374</td></tr><tr><td>U</td><td>ICP-SFMS</td><td>ppm</td><td>0.0105</td></tr><tr><td>Co/Mg<sub>CI</sub></td><td></td><td></td><td>1.22</td></tr><tr><td>Ni/Mg<sub>CI</sub></td><td></td><td></td><td>0.89</td></tr><tr><td>Fe/Mg<sub>CI</sub></td><td></td><td></td><td>0.93</td></tr></tbody></table>

In Fig. 8, the Y 981727 data plot to the left of the H range (low Fe<sub>oxidized</sub>/Si), consistent with the mineralogical data.
<figure><img src="image_6.png" /><figcaption>Fig. 5. Oxygen isotope composition of treated and untreated fractions (EATG) of Y 982717 plotted in relation to the ordinary chondrite analyses of Clayton et al. (1991). The data for low-FeO OCs are also from Clayton et al. (1991), McDermott et al. (2016), and Troiano et al. (2011). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

The oxygen isotopic composition of Y 981727 after weathering products was removed plots in the range of H chondrites. It is known that there is a correlation between olivine Fa and Δ¹⁷O in H chondrites, and a broad one in L and LL chondrites (Fig. 9; Rubin 2005). Although the correlation in Y 981727 is broad, its composition plots along the H-L-LL trend. In summary, the bulk chemical and O-isotopic composition of Y 981727 plot on the trend defined by H-L-LL, and other low-FeO OCs.

Compositional variations of olivine (Fa<sub>11.17</sub> ± 0.48 (1σ)) and low-Ca pyroxene (Fs<sub>11.04</sub> ± 0.98 (1σ)), well-defined chondrule boundaries, partial devitrification of mesostasis, and the presence of polysynthetic twinning in some low-Ca pyroxene indicate that Y 982717 is a petrologic type 4 (Figs. 1–3). The standard deviations (1σ) of pyroxene Fs are greater than that of olivine Fa. This is commonly observed as thermal metamorphism progresses, olivine becomes equilibrated by petrologic type 4, and pyroxene is equilibrated by petrologic type 5 (Kovach and Jones 2010). We used the spinel (chromite in this case)-olivine geothermometer (Wlotzka 2005) to estimate temperatures, which were in the range 648–659 °C; slightly lower than the range of H4 (690–818 °C, average 654 °C) and similar to that of H3 (619–

711 °C, average 654 °C) (Wlotzka 2005). We could not obtain two-pyroxene equilibration temperatures due to small sizes of augite (<a few µm).

Y 982717 contains a small amount of silica minerals (cristobalite and tridymite) (Fig. 2). These silica phases could be remnants of primary phases or secondary phases formed by reduction during thermal metamorphism. Silica minerals formed by the reduction of olivine are commonly observed in ureilites. These silica minerals are fine-grained and typically occur with low-Ni Fe metals (Mittlefehldt et al. 1998). Silica minerals in Y 982717 occur interstitially with orthopyroxene grains, or have tabular shapes, and are not associated with low-Ni Fe metals. This indicates that the silica minerals are indigenous. OCs in some cases contain silica phases with a primary origin (Brigham et al. 1986; Hezel et al. 2006).

In principle, low-FeO OCs could be formed either by nebular or parent body processes (McCoy et al. 1994). It has been widely suggested that OCs experienced reduction or oxidation during metamorphism. Some OCs are suggested to have experienced oxidation (Scott et al. 1986; Rubin 1990; McSween and Labotoka 1993), whereas other OCs experienced reduction during metamorphism (Brett and Sato 1984; Sears and Weeks
<figure><img src="image_7.png" /><figcaption>Fig. 6. Mg- and CI-normalized bulk compositions of Y 982717, and average compositions of groups of OCs (H, L, LL) and enstatite chondrites (EH, EL). The data of OCs and enstatite chondrites are from Kallemeyn et al. (1989) and Kallemeyn and Wasson (1986). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 7. Bulk compositions of Y 982717 plotted on a diagram of Zn/Mn versus Al/Mn. Data of various chondrites are from Kallemeyn et al. (1989), Kallemeyn and Wasson (1981, 1986), and Barrat et al. (2012). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 8. Composition of Y 982717 plotted on the Urey-Craig diagram showing relative iron contents and oxidation states of the chondritic group. The data of Netschaëvo, Burnwell, and Willaroy are from Olsen and Jarosewich (1971), Russell et al. (1998), and Chalmers and Mason (1977), respectively. The data of Suwahib (Buwah) and Moorabie are from Jarosewich (1990). Filled circles (H) and triangle (L) are from Jarosewich (1990), and open circles (H) and triangles (L) are from Kimura et al. (2018). Areas (dotted line) for H, L, and LL are from Brearley and Jones (1998). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

1986). If Y 982717 experienced reduction, the high Mg' values in olivine and pyroxene were established by reduction during metamorphism after accretion of precursors similar to H chondrite components. However, this process seems unlikely due to the lack of any petrologic evidence for reduction. Silica phases that can form by reduction seem to be of primary origin (see above). It is unlikely that any petrologic evidence for reduction would have been subsequently erased (e.g., decomposed SiO₂, high Mg' of olivine and pyroxene, Ni-free Fe metals) by metamorphism to type 4 level. Therefore, we suggest that the low-FeO nature of Y 982717 was established in the nebula and not during thermal metamorphism.

A question arises whether these low FeO OCs represent the fourth chemical group of OCs or a continuum of H chondrites. If the former is the case, there should be a clear gap between low FeO and H chondrites. Only few numbers of low FeO OCs (i.e., Fa₁₁⁻₁₆) are
<figure><img src="image_10.png" /><figcaption>Fig. 9. Fa of olivine versus Δ¹⁷O (‰). Plotted are the data of Y 982717 as well as other low-FeO OCs, Burnwell, EET 96031, LAP 04757, and Netschaëvo (Clayton et al. 1991; Troiano et al. 2011). (Color figure can be viewed at wile yonlinelibrary.com.)</figcaption></figure>

known so far (Table 1). Except Suwahib (Buwah) and Moorabie which are reduced L, all are similar to H chondrites with respect to bulk chemical and oxygen isotopic compositions (Wasson et al. 1993; Troiano et al. 2011). In the Asuka and Yamato meteorite collection at NIPR whose textures and olivine compositions are examined, only 5 of ~5900 equilibrated “H” chondrites have Fa less than 16 (olivine Fa15.3, 15.3, 15.5, 15.8, 15.9) (Meteorite bulletin database). There is no clear compositional gap between these low FeO and H chondrites with respect to olivine Fa. Thus, we conclude that low FeO chondrites are part of the H chondrite group with Y 982717 as an extreme member.

## CONCLUSIONS

We studied the petrology and geochemistry of a low-FeO OC, Y 982717. All of its characteristics, including texture, bulk composition, O-isotopic composition, with the exception of the low Fa content of olivine, are consistent with those of equilibrated H chondrites. As a consequence, there appears to be no compelling reasons to indicate that Y 982717 is a member of a distinct group to that of the H chondrites. Consequently, we conclude that Y 982717 is the lowest FeO H chondrite found to date. It is unlikely that Y 982717 experienced reduction during metamorphism on the basis of the petrologic data. We suggest that the H chondrites are more compositionally diverse than has been previously recognized.

*Acknowledgments*—We thank Jeff Grossman for discussion, Alan Rubin and Mike Weisberg for constructive reviews, and Mike Zolensky for editorial

handling. This research was partly supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science, and Technology, Japan, and NIPR, Research Project Fund, KP307. Oxygen isotope studies at the Open University are funded by a Consolidated Grant from the UK STFC agency.

*Editorial Handling*—Dr. Michael Zolensky

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