# Evolution of the angrite parent body: Implications of metamorphic coronas in NWA 3164

Bashar Baghdadi, Gaston Godard, Albert Jambon

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Bashar Baghdadi, Gaston Godard, Albert Jambon. Evolution of the angrite parent body: Implications of metamorphic coronas in NWA 3164. Meteoritics and Planetary Science, 2013, 48 (10), pp.1873-1893. 10.1111/maps.12202. hal-00943088

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# Evolution of the angrite parent body: Implications of metamorphic coronas in NWA 3164

Bashar BAGHDADI<fnref n="1" />*, Gaston GODARD<fnref n="2" />, and Albert JAMBON<fnref n="1" />

**Abstract**—Northwest Africa 3164 is a coarse-grained angrite that shows reaction coronas, a unique character among achondrites. Olivine (Fo$_{57}$; 1.2 wt% CaO), fassaitic clinopyroxene, anorthite, and spinel account for 46–47, 28–29, 8–13, and 4–8 vol%, respectively; kamacite is an accessory phase. The spinel grains in contact with clinopyroxene are bounded by discontinuous 20 µm thick coronas of anorthite and olivine, indicating the reaction Cpx + Spl → Ol + An (R1). In addition, irregular coronas of clinopyroxene and spinel developed around the primary anorthite in contact with primary olivine, during the reaction Ol + An → Cpx + Spl (R2). R2 also generated clinopyroxene and spinel films between the secondary olivine and anorthite coronas produced during R1, implying that R1 preceded R2. Both are metamorphic reactions that developed in the solid state. Finally, the coronas are cross cut by µm-thick veinlets due to a late shock. A mass-balance study shows that R2 is almost the reverse of R1. The P–T metamorphic evolution of the rock, modeled by calculating a P–T isochemical diagram, indicates an equilibrium *T* of 940 ± 120 °C at *P* < 0.9 GPa for the initial assemblage, followed by an increase of *T* up to approximately 1000–1200 °C during reaction R1 and a subsequent cooling during R2. Several causes are envisaged to account for this metamorphic evolution. Contact metamorphism due to a hot magmatic intrusion in the angrite parent body is favored, as similar metamorphic coronas are well known in metamorphic terrestrial rocks. In addition to differentiation and magmatism, there is now evidence for metamorphism in the angrite parent body, which would have been a large asteroid or a planetary-sized body.

## INTRODUCTION

Metamorphism in meteorites is generally related to shock (e.g., Stöffler et al. 1986; Chen and El Goresy 2000) or textural maturation induced by moderate thermal effect (e.g., Stöffler et al. 1991). Unlike terrestrial rocks, metamorphic coronas that developed at the contacts between two reacting minerals are very rare in meteorites, as they have only been reported in very few mesosiderites so far (Nehru et al. 1980; Ruzicka et al. 1994). Since the pioneer work of Sederholm (1916), such coronas within metamorphic terrestrial rocks are considered to form in the solid state by chemical reaction between the two minerals in contact, when these become

unstable together as a result of changing pressure and/or temperature. Therefore, the presence of such coronas in meteorites should be related to an evolution of the P–T conditions undergone by the rock in the parent body.

In this paper, we describe metamorphic coronas in the peridotitic angrite Northwest Africa (NWA) 3164, purchased from nomads in Morocco in 2004 (Bunch et al. [2006] in Connolly et al. [2006]), and discuss their significance in terms of the rock evolution. These coronas, already mentioned by Irving et al. (2005) and Bunch et al. (2006) in Connolly et al. (2006), consist of (1) olivine + anorthite at the contacts between spinel and clinopyroxene, and (2) spinel + clinopyroxene at the contacts between olivine and anorthite. Such coronas

<footnote n="1">¹UPMC Paris 6, Institut des Sciences de la Terre Paris, UMR 7193, 4 place Jussieu, 75252 Paris Cedex 05, France</footnote>

<footnote n="2">²Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Univ. Paris-Diderot, UMR 7154 CNRS, 1 rue Jussieu, 75238 Paris Cedex 05, France</footnote>

<footnote>*Corresponding author. E-mail: bashbag@hotmail.com</footnote>

<footnote>*(Received 05 November 2012; revision accepted 27 July 2013)</footnote>
are unique among angrites and achondrites. As similar microstructures exist in peridotites of the Earth's mantle (e.g., Sederholm 1916; Green and Hibberson 1970; Griffin and Heier 1973; Ronde et al. 2005), they could provide important insights into the parent body and its history.

After a short presentation of the sample and methodology, petrographical and chemical descriptions of the NWA 3164 rock are presented. Then, the chemical reactions that produced the observed coronas are balanced and the P–T evolution that could explain them is modeled. New insights into the rock history and the angrite origin are presented.

## SAMPLE AND METHODS

### Angrite NWA 3164

NWA 3164 was purchased from nomads in Morocco in August 2004. The meteorite was published by Bunch et al. (2006) in the *Meteoritical Bulletin* no. 90, 2006 September (Connolly et al. 2006), where it was described as consisting of many individual dark brown stones, and classified as angrite. It may belong to the same stock as NWA 2999 (Bunch et al. [2006] in Connolly et al. [2006]). All stones are peridotitic angrites, display an overall granular texture, and are made of olivine, clinopyroxene, spinel, and anorthite. Irving et al. (2005), Connolly et al. (2006), and Kuehner et al. (2006) also described clinopyroxene + spinel symplectites around anorthite crystals in contact with olivine, and anorthite coronas of 10–20 µm width around spinel grains adjacent to clinopyroxene.

We studied a sawn slice of a specimen of the NWA 3164 find that was bought in Rabat (Morocco) in 2004, and is now deposited at Université Pierre et Marie Curie (UPMC, Paris, France). Its features are identical to those described in Connolly et al. (2006).

### Analytical Methods

Petrographic observations and chemical analyses of the various minerals were made on a polished thick section of about 1 cm², prepared after having spared a split for bulk-rock chemical analysis.

The minerals and their relationships have been observed by reflected light microscopy and further studied with a scanning electron microscope (SEM) Zeiss ULTRA 55, equipped with a backscattered electron (BSE) detector and an energy-dispersive spectrometer (EDS). The sample was also observed under an optical microscope by optical cathodoluminescence, within a vacuum chamber with a cold cathode OPEA (15 kV and approximately 400 mA mm⁻²).

Electron microprobe analyses of minerals were performed at UPMC using a CAMECA SX-100 equipped with wavelength-dispersive spectrometers. Operating conditions were 15 kV accelerating voltage, 10 nA sample current, 10 s peak counting time, 2 × 5 s background counting time, and a spot size of approximately 1 µm for all phases. Natural silicates and synthetic oxides were used as standards for all elements: albite (Na), orthose (K), diopside (Si, Mg, Ca), anorthite (Al), MnTiO₃ (Mn, Ti), Cr₂O₃ (Cr), Fe₂O₃ (Fe), and NiO (Ni). O was calculated by stoichiometry (see captions of Tables 1–5). Several profiles of 1 µm step were performed across the various coronas, to evidence possible mineral zoning related to these microstructures. Mineral compositions for NWA 3164 are reported in Tables 1–5. Each composition presented in the tables and in the text is an average of several spot analyses with its standard deviation (±1σ). Mineral and endmember abbreviations are from Kretz (1983).

A piece of NWA 3164 (450 mg) was prepared for chemical analysis, discarding outer crust and sawn surfaces. This fragment was finely grounded, using sapphire mortar and pestle. A split (100 mg) was used to determine the major abundances by inductively coupled plasma atomic emission spectrometry, by J.-A. Barrat at Université de Bretagne Occidentale (UBO, Brest, France). The procedures are the same as those used for other achondrites by Barrat et al. (2000) and Jambon et al. (2002, 2005). The accuracies are better than 5%.

Magnetic susceptibility of the rock was measured using a ZHinstruments SM30.

## PETROGRAPHY

Olivine is the dominant phase with clinopyroxene. It forms subhedral to anhedral mm-sized grains (Ol_g in Fig. 1). Clinopyroxene grains (Cpx_g), about two times smaller, are interstitial to olivine crystals, together with medium-sized plagioclase and smaller spinel grains (An_g and Spl_g in Fig. 1). Kamacite grains (Kam), ranging from 1 µm to 0.5 mm in size, occur everywhere in the rock, inside the other phases as well as in the interstices between them (Fig. 1); they frequently define discontinuous trails within the plagioclase and olivine grains (1–2 and 3–4 in Fig. 3d). Olivine crystals also contain micrometric inclusions of sulfide, which, as well as kamacite, is highly oxidized. Finally, iron oxide and minor calcite fill numerous veins up to 10 µm in thickness (Figs. 2–5).

The metal grains are responsible for the magnetic susceptibility of the rock, whose value (log χ = 4.25 [10⁻⁹ m³ kg⁻¹]) is close to that given by Rochette et al.
<table><caption>Table 1. Representative analyses of olivine.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Ol<sub>g</sub><sup>a</sup></th><th colspan="2">Ol<sub>g</sub><sup>b</sup></th><th colspan="2">Ol<sub>g</sub></th><th colspan="2">Ol<sub>c</sub></th></tr><tr><th>av. (17)</th><th>±σ</th><th>av. (23)</th><th>±σ</th><th>av. (40)</th><th>±σ</th><th>av. (10)</th><th>±σ</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>35.90</td><td>0.17</td><td>35.93</td><td>0.39</td><td>35.92</td><td>0.35</td><td>35.84</td><td>0.21</td></tr><tr><td>TiO<sub>2</sub></td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.04</td><td>0.04</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>0.04</td><td>0.03</td><td>0.03</td><td>0.03</td><td>0.03</td><td>0.03</td><td>0.11</td><td>0.16</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.01</td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.01</td><td>0.02</td><td>0.06</td><td>0.04</td></tr><tr><td>FeO</td><td>34.54</td><td>0.38</td><td>34.56</td><td>0.41</td><td>34.67</td><td>0.40</td><td>34.13</td><td>0.22</td></tr><tr><td>MnO</td><td>0.40</td><td>0.21</td><td>0.12</td><td>0.19</td><td>0.14</td><td>0.20</td><td>0.38</td><td>0.05</td></tr><tr><td>MgO</td><td>27.81</td><td>0.18</td><td>27.97</td><td>0.22</td><td>27.93</td><td>0.22</td><td>28.20</td><td>0.15</td></tr><tr><td>CaO</td><td>1.08</td><td>0.13</td><td>1.22</td><td>0.07</td><td>1.18</td><td>0.11</td><td>0.80</td><td>0.11</td></tr><tr><td>NiO</td><td>0.01</td><td>0.02</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.02</td><td>0.03</td><td>0.03</td></tr><tr><td>Total</td><td>99.83</td><td>0.44</td><td>99.97</td><td>0.44</td><td>99.94</td><td>0.44</td><td>99.63</td><td>0.46</td></tr><tr><td>Si</td><td>0.999</td><td>0.004</td><td>0.998</td><td>0.007</td><td>0.998</td><td>0.007</td><td>0.997</td><td>0.003</td></tr><tr><td>Ti</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.000</td><td>0.001</td><td>0.001</td></tr><tr><td>Al</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.004</td><td>0.005</td></tr><tr><td>Cr</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.001</td><td>0.001</td></tr><tr><td>Fe</td><td>0.808</td><td>0.009</td><td>0.805</td><td>0.010</td><td>0.806</td><td>0.010</td><td>0.794</td><td>0.006</td></tr><tr><td>Mn</td><td>0.005</td><td>0.005</td><td>0.003</td><td>0.004</td><td>0.003</td><td>0.005</td><td>0.009</td><td>0.001</td></tr><tr><td>Mg</td><td>1.153</td><td>0.005</td><td>1.158</td><td>0.009</td><td>1.156</td><td>0.009</td><td>1.169</td><td>0.004</td></tr><tr><td>Ca</td><td>0.032</td><td>0.004</td><td>0.036</td><td>0.002</td><td>0.035</td><td>0.003</td><td>0.024</td><td>0.003</td></tr><tr><td>Ni</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></tr><tr><td>X<sub>Mg</sub></td><td>0.577</td><td>0.003</td><td>0.578</td><td>0.003</td><td>0.578</td><td>0.003</td><td>0.586</td><td>0.002</td></tr><tr><td>FeO/MnO</td><td>88.45</td><td>13.63</td><td>84.15</td><td>7.15</td><td>85.83</td><td>10.12</td><td>90.58</td><td>11.85</td></tr><tr><td>Fo (%)</td><td>57.7</td><td>0.3</td><td>57.8</td><td>0.3</td><td>57.8</td><td>0.3</td><td>58.5</td><td>0.2</td></tr><tr><td>Fa (%)</td><td>40.4</td><td>0.4</td><td>40.2</td><td>0.4</td><td>40.3</td><td>0.4</td><td>39.8</td><td>0.2</td></tr><tr><td>La (%)</td><td>1.6</td><td>0.2</td><td>1.8</td><td>0.1</td><td>1.8</td><td>0.2</td><td>1.2</td><td>0.2</td></tr></tbody></table>

Fo = forsterite $Mg_2SiO_4$; Fa = fayalite $Fe_2SiO_4$; La = larnite $Ca_2SiO_4$; $X_{Mg} = Mg/(Fe + Mn + Mg + Ni + Ca)$; av. (n) = average of n spot analyses; $Ol_g$ = olivine granular crystals in the groundmass; $Ol_c$ = olivine from the Ol + An coronas that developed during reaction R1. Oxides in wt% and cations in atoms per formula unit.

<sup>a</sup>Olivine close (lesser than approximately 15 µm) to the Cpx + Spl coronas that formed during reaction R2.

<sup>b</sup>Olivine far (greater than approximately 15 µm) from the Cpx + Spl coronas.

<figure><img src="image_2.png" /><figcaption>Fig. 1. General image of the studied section (backscattered electron image). Void = holes and cracks (black); $An_g$ = anorthite (dark gray); $Cpx_g$ = clinopyroxene (medium gray); $Spl_g$ = spinel (medium gray); $Ol_g$ = olivine (light gray); $Kam$ = kamacite (white); the white patches consist of kamacite ± sulfide, whereas the white veinlets are filled with iron oxide.</figcaption></figure>
<table><caption>Table 2. Representative analyses of clinopyroxene.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Cpx<sub>g</sub><sup>a</sup></th><th colspan="2">Cpx<sub>g</sub><sup>b</sup></th><th colspan="2">Cpx<sub>g</sub></th><th colspan="2">Cpx<sub>c</sub></th></tr><tr><th>av. (5)</th><th>±σ</th><th>av. (40)</th><th>±σ</th><th>av. (45)</th><th>±σ</th><th>av. (17)</th><th>±σ</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>47.14</td><td>0.67</td><td>47.70</td><td>0.28</td><td>47.63</td><td>0.39</td><td>47.12</td><td>2.23</td></tr><tr><td>TiO<sub>2</sub></td><td>1.37</td><td>0.09</td><td>1.37</td><td>0.09</td><td>1.38</td><td>0.09</td><td>1.28</td><td>0.39</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>7.21</td><td>0.25</td><td>7.21</td><td>0.25</td><td>7.18</td><td>0.25</td><td>8.37</td><td>2.69</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.23</td><td>0.05</td><td>0.23</td><td>0.05</td><td>0.23</td><td>0.05</td><td>0.30</td><td>0.11</td></tr><tr><td>FeO</td><td>6.31</td><td>0.15</td><td>6.31</td><td>0.15</td><td>6.43</td><td>0.61</td><td>6.59</td><td>0.58</td></tr><tr><td>MnO</td><td>0.08</td><td>0.04</td><td>0.08</td><td>0.04</td><td>0.08</td><td>0.04</td><td>0.02</td><td>0.04</td></tr><tr><td>MgO</td><td>12.10</td><td>0.13</td><td>12.10</td><td>0.13</td><td>12.08</td><td>0.18</td><td>11.75</td><td>0.80</td></tr><tr><td>NiO</td><td>0.03</td><td>0.03</td><td>0.03</td><td>0.04</td><td>0.03</td><td>0.03</td><td>0.00</td><td>0.01</td></tr><tr><td>CaO</td><td>24.31</td><td>0.19</td><td>24.31</td><td>0.19</td><td>24.26</td><td>0.30</td><td>24.27</td><td>0.80</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.00</td><td>0.00</td></tr><tr><td>K<sub>2</sub>O</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.00</td><td>0.01</td></tr><tr><td>Total</td><td>99.04</td><td>0.46</td><td>99.40</td><td>0.26</td><td>99.36</td><td>0.31</td><td>99.73</td><td>0.53</td></tr><tr><td>Si</td><td>1.775</td><td>0.018</td><td>1.779</td><td>0.005</td><td>1.784</td><td>0.012</td><td>1.757</td><td>0.078</td></tr><tr><td>Ti</td><td>0.039</td><td>0.001</td><td>0.039</td><td>0.003</td><td>0.039</td><td>0.003</td><td>0.036</td><td>0.011</td></tr><tr><td>Al</td><td>0.309</td><td>0.009</td><td>0.318</td><td>0.011</td><td>0.317</td><td>0.012</td><td>0.368</td><td>0.120</td></tr><tr><td>Cr</td><td>0.007</td><td>0.002</td><td>0.007</td><td>0.002</td><td>0.007</td><td>0.002</td><td>0.009</td><td>0.003</td></tr><tr><td>Fe</td><td>0.230</td><td>0.049</td><td>0.197</td><td>0.005</td><td>0.202</td><td>0.020</td><td>0.206</td><td>0.018</td></tr><tr><td>Mn</td><td>0.002</td><td>0.001</td><td>0.003</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.002</td><td>0.001</td></tr><tr><td>Mg</td><td>0.670</td><td>0.018</td><td>0.675</td><td>0.007</td><td>0.675</td><td>0.009</td><td>0.653</td><td>0.044</td></tr><tr><td>Ni</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.000</td><td>0.000</td></tr><tr><td>Ca</td><td>0.966</td><td>0.020</td><td>0.975</td><td>0.007</td><td>0.973</td><td>0.010</td><td>0.970</td><td>0.030</td></tr><tr><td>Na</td><td>0.001</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.000</td><td>0.000</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>0.000</td></tr><tr><td>X<sub>Mg</sub></td><td>0.358</td><td>0.060</td><td>0.365</td><td>0.014</td><td>0.364</td><td>0.016</td><td>0.357</td><td>0.030</td></tr><tr><td>FeO/MnO</td><td>181.37</td><td>127.44</td><td>96.55</td><td>52.53</td><td>107.61</td><td>70.92</td><td>141.50</td><td>93.61</td></tr><tr><td>En (%)</td><td>33.5</td><td>0.9</td><td>33.8</td><td>0.4</td><td>33.7</td><td>0.4</td><td>32.7</td><td>2.2</td></tr><tr><td>Fs (%)</td><td>11.5</td><td>2.4</td><td>9.4</td><td>0.8</td><td>8.5</td><td>0.8</td><td>10.3</td><td>0.9</td></tr><tr><td>Wo (%)</td><td>39.0</td><td>1.8</td><td>39.9</td><td>0.5</td><td>39.8</td><td>0.8</td><td>38.2</td><td>4.9</td></tr><tr><td>Ca-Al-Ts (%)</td><td>14.6</td><td>1.8</td><td>13.8</td><td>1.1</td><td>13.9</td><td>1.2</td><td>17.1</td><td>6.2</td></tr><tr><td>Ca-Ti-Ts (%)</td><td>3.9</td><td>0.2</td><td>3.9</td><td>0.3</td><td>3.9</td><td>0.3</td><td>3.6</td><td>1.1</td></tr></tbody></table>

En = enstatite Mg₂Si₂O₆; Fs = ferrosilite Fe₂Si₂O₆; Wo = wollastonite Ca₂Si₂O₆; Ca-Al-Ts = Al-Tschermak CaAl₂SiO₆; Ca-Ti-Ts = Ti-Tschermak CaTiAl₂O₆; av. (n) = average of n spot analyses; Cpxg = clinopyroxene granular crystals in the groundmass; Cpxc = clinopyroxene from the Cpx + Spl coronas that developed during reaction R2.

Oxides in wt% and cations in atoms per formula units.

<sup>a</sup>Clinopyroxene close (less than approximately 15 µm) to the Ol + An coronas that formed during reaction R1.

<sup>b</sup>Clinopyroxene far (greater than approximately 15 µm) from the Ol + An coronas.

(2009) for another specimen of NWA 3164 (log χ = 4.54).

The NWA 3164 angrite shows some unique metamorphic features that consist of various complex coronas that are made of thin vermicular crystals (typically 5–20 µm thick, noted Olc, Splc, Cpxc, Anc in figures) that have developed between larger granular crystals (typically 1 mm sized; noted Olg, Splg, Cpxg, Ang in figures). The crystals in the coronas differ from the granular crystals by their shape and their size (e.g., Anc versus Ang in Fig. 4).

Two types of coronas occur at specific interfaces between granular crystals:

1. Spinel–clinopyroxene interface (Figs. 2 and 5): An anorthite (Anc) + olivine (Olc) complex corona

occurs at the contacts between granular spinel (Splg) and clinopyroxene (Cpxg). The anorthite corona, 10–20 µm wide, lies on the spinel side, whereas the olivine corona, of less than 20 µm in thickness, is in contact with clinopyroxene. The olivine corona is discontinuous, forming strings of elongated ovoid grains (Olc in Figs. 2 and 5) partly enveloped by clinopyroxene (white circles in Figs. 2 and 5b), which suggests that it has been partly resorbed. The thin anorthite corona is girdle-shaped and almost continuous (Figs. 2 and 3); it differs clearly from the large anorthite grains in the rock groundmass that are more granular, as revealed notably by cathodoluminescence (Anc versus Ang in Fig. 4). This anorthite corona, which developed preferentially at
<table><caption>Table 3. Representative analyses of spinel.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Spl<sub>g</sub><sup>a</sup></th><th colspan="2">Spl<sub>g</sub><sup>b</sup></th><th colspan="2">Spl<sub>g</sub></th><th colspan="2">Spl<sub>c</sub></th></tr><tr><th>av. (19)</th><th>±σ</th><th>av. (96)</th><th>±σ</th><th>av. (115)</th><th>±σ</th><th>av. (14)</th><th>±σ</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>0.11</td><td>0.05</td><td>0.09</td><td>0.03</td><td>0.09</td><td>0.10</td><td>0.07</td><td>0.10</td></tr><tr><td>TiO<sub>2</sub></td><td>0.52</td><td>0.08</td><td>0.55</td><td>0.07</td><td>0.55</td><td>0.08</td><td>0.12</td><td>0.04</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>57.27</td><td>0.34</td><td>57.64</td><td>0.39</td><td>57.58</td><td>0.47</td><td>57.30</td><td>0.54</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>4.90</td><td>0.17</td><td>4.55</td><td>0.26</td><td>4.60</td><td>0.27</td><td>5.28</td><td>0.22</td></tr><tr><td>FeO</td><td>24.04</td><td>0.24</td><td>24.19</td><td>0.26</td><td>24.16</td><td>0.27</td><td>24.17</td><td>0.31</td></tr><tr><td>MnO</td><td>0.15</td><td>0.04</td><td>0.15</td><td>0.04</td><td>0.15</td><td>0.04</td><td>0.16</td><td>0.04</td></tr><tr><td>MgO</td><td>11.71</td><td>0.13</td><td>11.86</td><td>0.12</td><td>11.83</td><td>0.15</td><td>10.73</td><td>0.14</td></tr><tr><td>CaO</td><td>0.09</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.03</td><td>0.04</td><td>0.14</td><td>0.04</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.03</td></tr><tr><td>NiO</td><td>0.03</td><td>0.03</td><td>0.02</td><td>0.03</td><td>0.02</td><td>0.03</td><td>0.01</td><td>0.03</td></tr><tr><td>Total</td><td>98.83</td><td>0.11</td><td>99.08</td><td>0.12</td><td>99.02</td><td>0.15</td><td>97.99</td><td>0.15</td></tr><tr><td>Ti</td><td>0.011</td><td>0.002</td><td>0.011</td><td>0.001</td><td>0.011</td><td>0.003</td><td>0.002</td><td>0.001</td></tr><tr><td>Al</td><td>1.846</td><td>0.007</td><td>1.850</td><td>0.009</td><td>1.850</td><td>0.009</td><td>1.860</td><td>0.009</td></tr><tr><td>Cr</td><td>0.106</td><td>0.004</td><td>0.098</td><td>0.006</td><td>0.099</td><td>0.006</td><td>0.115</td><td>0.005</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.022</td><td>0.006</td><td>0.025</td><td>0.007</td><td>0.024</td><td>0.007</td><td>0.021</td><td>0.010</td></tr><tr><td>Fe<sup>2+</sup></td><td>0.528</td><td>0.005</td><td>0.526</td><td>0.005</td><td>0.527</td><td>0.006</td><td>0.556</td><td>0.006</td></tr><tr><td>Mn</td><td>0.004</td><td>0.001</td><td>0.004</td><td>0.001</td><td>0.004</td><td>0.001</td><td>0.001</td><td>0.001</td></tr><tr><td>Mg</td><td>0.477</td><td>0.005</td><td>0.481</td><td>0.004</td><td>0.481</td><td>0.005</td><td>0.441</td><td>0.005</td></tr><tr><td>Ca</td><td>0.003</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.004</td><td>0.001</td></tr><tr><td>Na</td><td>0.001</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td></tr><tr><td>Ni</td><td>0.001</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.001</td></tr><tr><td>X<sub>Mg</sub></td><td>0.465</td><td>0.004</td><td>0.466</td><td>0.003</td><td>0.466</td><td>0.004</td><td>0.433</td><td>0.007</td></tr><tr><td>Mag (%)</td><td>1.1</td><td>0.3</td><td>1.2</td><td>0.3</td><td>1.2</td><td>0.3</td><td>1.1</td><td>0.5</td></tr><tr><td>Us (%)</td><td>1.1</td><td>0.2</td><td>1.1</td><td>0.1</td><td>1.1</td><td>0.2</td><td>0.0</td><td>0.0</td></tr><tr><td>Chr (%)</td><td>5.3</td><td>0.2</td><td>4.9</td><td>0.3</td><td>5.0</td><td>0.3</td><td>5.8</td><td>0.3</td></tr><tr><td>Hc (%)</td><td>44.3</td><td>0.8</td><td>44.2</td><td>0.8</td><td>44.3</td><td>0.9</td><td>48.7</td><td>0.9</td></tr><tr><td>Spl (%)</td><td>47.7</td><td>0.5</td><td>48.2</td><td>0.4</td><td>48.1</td><td>0.5</td><td>44.1</td><td>0.5</td></tr></tbody></table>

Chr = Chromite Fe<sup>2+</sup>Cr<sub>2</sub>O<sub>4</sub>; Us = Ulvöspinel Fe<sup>2+</sup>Al<sub>2</sub>O<sub>4</sub>; Hc = Hercynite Fe<sup>2+</sup>Al<sub>2</sub>O<sub>4</sub>; Spl = Spinel MgAl<sub>2</sub>O<sub>4</sub>; Mag = Magnetite Fe<sup>2+</sup>Fe<sup>3+</sup>Al<sub>2</sub>O<sub>4</sub>; X<sub>Mg</sub> = Mg/(Fe<sup>2+</sup> + Mn + Mg + Ni); av. (n): average of n spot analyses; Spl<sub>g</sub> = spinel granular crystals in the groundmass; Spl<sub>c</sub> = spinel from the Spl + Cpx coronas that developed during reaction R2.

Fe<sup>3+</sup> and Fe<sup>2+</sup> were calculated by stoichiometry, so that Fe<sup>2+</sup> = 1 + Ti - Mg - Mn - Ni.

Oxides in wt% and cations in atoms per formula unit.

<sup>a</sup>Spinel close to the Ol + An coronas (less than approximately 15 µm) that formed during reaction R1.

<sup>b</sup>Spinel far from the Ol + An coronas (greater than approximately 15 µm).

the expense of spinel, commonly extends far from the contact between spinel (Spl<sub>g</sub>) and clinopyroxene (Cpx<sub>g</sub>), giving locally the appearance of a single anorthite corona around spinel (Figs. 2, 4 and 5a).

2. Olivine–anorthite interface (Fig. 3): An irregular clinopyroxene (Cpx<sub>c</sub>) + spinel (Spl<sub>c</sub>) corona, from 5 to 35 µm in thickness, developed at the contacts between large olivine (Ol<sub>g</sub>) and anorthite (An<sub>g</sub>) grains. In some cases, spinel crystals display a vermicular habit within a clinopyroxene corona and are elongated perpendicularly to the latter, forming a symplectite (Spl<sub>c</sub> in Figs. 3a and 3b). In other cases, isolated grains of spinel, of 20–40 µm in size, in contact with small olivine grains form peninsulas within anorthite, especially close to kamacite grains, now almost completely oxidized, that are included in anorthite (white circle in Fig. 3c); more generally,

when large kamacite grains exist close to the corona, spinel from the corona preferentially developed around or close to them (Spl<sub>c</sub> in Fig. 3d). Coronitic clinopyroxene (Cpx<sub>c</sub>) and spinel (Spl<sub>c</sub>) are also observed, locally, as very thin µm-thick films between the coronitic olivine (Ol<sub>c</sub>) and anorthite (An<sub>c</sub>) of the anorthite + olivine complex coronas described above (Fig. 5b), or at the contact between granular olivine (Ol<sub>g</sub>) and coronitic anorthite (An<sub>c</sub>) (Fig. 4b, bottom).

The clinopyroxene–olivine (Cpx<sub>g</sub>–Ol<sub>g</sub>) and clinopyroxene–anorthite (Cpx<sub>g</sub>–An<sub>g</sub>) interfaces remain unchanged.

Finally, the rock displays numerous veinlets, less than 10 µm in thickness, which are filled with iron oxide and minor calcite (Figs. 1–5). These are late cracks, which cross cut all the minerals and above coronas.
<table><caption>Table 4. Representative analyses of anorthite.</caption><thead><tr><th rowspan="2"></th><th colspan="2">An<sub>g</sub><sup>a</sup></th><th colspan="2">An<sub>g</sub><sup>b</sup></th><th colspan="2">An<sub>g</sub></th><th colspan="2">An<sub>c</sub></th></tr><tr><th>av. (6)</th><th>±σ</th><th>av. (29)</th><th>±σ</th><th>av. (35)</th><th>±σ</th><th>av. (9)</th><th>±σ</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>42.87</td><td>0.42</td><td>42.82</td><td>0.50</td><td>42.83</td><td>0.48</td><td>42.67</td><td>0.40</td></tr><tr><td>TiO<sub>2</sub></td><td>0.03</td><td>0.01</td><td>0.02</td><td>0.03</td><td>0.02</td><td>0.02</td><td>0.03</td><td>0.04</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>36.35</td><td>0.23</td><td>36.19</td><td>0.37</td><td>36.22</td><td>0.36</td><td>36.31</td><td>0.29</td></tr><tr><td>Fe<sub>2</sub>O<sub>3</sub></td><td>0.32</td><td>0.16</td><td>0.59</td><td>0.37</td><td>0.54</td><td>0.36</td><td>0.82</td><td>0.27</td></tr><tr><td>MnO</td><td>0.00</td><td>0.00</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.02</td><td>0.03</td></tr><tr><td>MgO</td><td>0.00</td><td>0.01</td><td>0.03</td><td>0.06</td><td>0.02</td><td>0.06</td><td>0.05</td><td>0.04</td></tr><tr><td>CaO</td><td>20.34</td><td>0.20</td><td>19.94</td><td>0.43</td><td>20.01</td><td>0.43</td><td>19.81</td><td>0.24</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.05</td><td>0.03</td><td>0.06</td><td>0.02</td><td>0.06</td><td>0.02</td><td>0.09</td><td>0.02</td></tr><tr><td>Total</td><td>99.95</td><td>0.49</td><td>99.62</td><td>0.82</td><td>99.68</td><td>0.78</td><td>99.92</td><td>0.57</td></tr><tr><td>Si</td><td>1.992</td><td>0.012</td><td>1.997</td><td>0.010</td><td>1.996</td><td>0.010</td><td>1.988</td><td>0.007</td></tr><tr><td>Al</td><td>1.991</td><td>0.012</td><td>1.989</td><td>0.016</td><td>1.989</td><td>0.015</td><td>1.994</td><td>0.005</td></tr><tr><td>Fe<sup>3+</sup></td><td>0.006</td><td>0.003</td><td>0.010</td><td>0.007</td><td>0.010</td><td>0.006</td><td>0.014</td><td>0.005</td></tr><tr><td>Ca</td><td>1.013</td><td>0.014</td><td>0.996</td><td>0.022</td><td>0.999</td><td>0.022</td><td>0.989</td><td>0.008</td></tr><tr><td>Na</td><td>0.005</td><td>0.002</td><td>0.005</td><td>0.002</td><td>0.005</td><td>0.002</td><td>0.008</td><td>0.002</td></tr><tr><td>An (%)</td><td>0.995</td><td>0.002</td><td>0.995</td><td>0.002</td><td>0.995</td><td>0.002</td><td>0.991</td><td>0.002</td></tr></tbody></table>

av. (n) = average of n spot analyses; An_g = anorthite granular crystals in the groundmass; An_c = anorthite from An + Ol coronas that developed during reaction R1.

Oxides in wt% and cations in atoms per formula unit.

<sup>a</sup>Anorthite close (less than approximately 15 µm) to the Cpx + Spl coronas that formed during reaction R2.

<sup>b</sup>Anorthite far (greater than approximately 15 µm) from the Cpx + Spl coronas.

<table><caption>Table 5. Representative analyses of kamacite.</caption><thead><tr><th>Analyses</th><th></th><th></th><th>Average</th><th>±σ</th></tr></thead><tbody><tr><td>Fe</td><td>93.89</td><td>94.62</td><td>92.29</td><td>94.05</td><td>1.32</td></tr><tr><td>Si</td><td>0.01</td><td>0.00</td><td>0.06</td><td>0.02</td><td>0.03</td></tr><tr><td>Mn</td><td>0.00</td><td>0.00</td><td>0.08</td><td>0.02</td><td>0.04</td></tr><tr><td>P</td><td>0.00</td><td>0.01</td><td>0.00</td><td>0.01</td><td>0.01</td></tr><tr><td>S</td><td>0.02</td><td>0.00</td><td>0.04</td><td>0.02</td><td>0.02</td></tr><tr><td>Ni</td><td>6.40</td><td>6.72</td><td>6.55</td><td>6.51</td><td>0.15</td></tr><tr><td>Cr</td><td>0.28</td><td>0.28</td><td>0.03</td><td>0.26</td><td>0.18</td></tr><tr><td>Total</td><td>100.60</td><td>101.63</td><td>99.04</td><td>100.89</td><td>1.42</td></tr></tbody></table>

Values are in wt%.

## CHEMISTRY

### Bulk-Rock Chemistry and Mode

The bulk-rock analysis, which included the veins and metal fraction (see the Analytical Methods section), yielded the following composition (in wt%): 0.09% P₂O₅, 0.57% TiO₂, 8.35% Al₂O₃, 0.29% Cr₂O₃, 22.73% FeO, 0.26% MnO, 18.28% MgO, 9.86% CaO, 0.01% Na₂O, 0.04% K₂O.

This composition was converted into molar proportions of elements, which, together with the average composition of the minerals obtained at the electron microprobe (Tables 1–5; see below), yielded an estimate of the molar proportions of these minerals in the rock, by applying the least-squares method (e.g., Fisher 1989). Then, weighting these molar proportions by the molar volumes of the minerals as given by the

<figure><img src="image_3.png" /><figcaption>Fig. 2. Complex corona of anorthite (An_c) + olivine (Ol_c) between primary spinel (Spl_g) and clinopyroxene (Cpx_g) (backscattered electron image). The olivine corona (Ol_c) is discontinuous and close to clinopyroxene Cpx_g, whereas the anorthite corona (An_c) is continuous and developed around spinel Spl_g. A–B, C–D: profiles in Fig. 6.</figcaption></figure>

data set of Holland and Powell (1998; upgraded in 2009), an estimate of the volumetric percentages of the minerals in the rock was obtained (“calculated mode” in Table 6).
<figure><img src="image_4.png" /></figure>

Fig. 3. Complex coronas of clinopyroxene (Cpxc) + spinel (SpIc) between primary olivine (Ol) and anorthite (Ang) (backscattered electron images). a) Spinel Splc shows a vermicular habit within a clinopyroxene corona (Cpxc), forming a symplectite. b) Spinel Splc constitutes an irregular corona, as well as a symplectite with clinopyroxene Cpxc; c) isolated grains of spinel Splc penetrate within primary anorthite Ang toward kamacite grains (white circle); E-F: profile in Fig. 6c. d) Spinel from the corona (SpIc) preferentially developed around large kamacite grains. Kamacite grains form a trail in Ang (1–2) and Ol, (3–4) interrupted by the corona (between 2 and 3).

The volumetric percentages of the various minerals in the studied section have been also evaluated by multiple thresholding of 28 BSE images, covering about 1 cm² of the polished section. The results are similar to those of the calculated mode (see “measured mode” in Table 6). Small differences, however, can be attributed to the problem of representativeness of the imaged area and analyzed volume, because of the relatively large grain size (mm-sized) and the small quantity of available material. In particular, the modal proportion of plagioclase is quite uncertain, as this mineral forms few, but large, crystals, inducing a possible “nugget effect.”

Both the calculated and measured modes indicate that olivine (46.9 and 46.2 vol%, respectively) and clinopyroxene (28.3% and 29.4%) are the main phases,

whereas plagioclase (12.6% and 8.5%) and spinel (3.6% and 8.5%) are less abundant. Iron oxide from the late cracks, together with minor oxidized sulfide and kamacite, accounts for 8.1–7.4 vol%. Following the nomenclature of peridotites, NWA 3164 can be called a spinel–plagioclase-bearing wehrlite.

Mineral Chemistry

Mineral compositions for NWA 3164 are reported in Tables 1–5 (see the Analytical Methods section, for the analytical conditions).

Olivine grains (Ol) are homogeneous, like the other phases in this rock. Its forsterite content is 57.8 ± 0.3 (1σ) mole% (i.e., $X_{Mg}$ = Mg/[Fe + Mg + Ni + Mn +
<figure><img src="image_5.png" /><figcaption>Fig. 4. Granular (An_g) and coronitic (An_c) anorthite (true color cathodoluminescence image). Large anorthite grains in the rock groundmass (An_g) and girdle-shaped anorthite corona (An_c) around spinel (Spl_g) clearly differ by their shape, size, and cathodoluminescence color. Calcite (Cal) that luminesces in orange can be observed in the veins.</figcaption></figure>

<table><caption>Table 6. Modal abundance of the minerals in NWA 3164.</caption><thead><tr><th></th><th>Ol</th><th>Cpx</th><th>An</th><th>Spl</th><th>others</th></tr></thead><tbody><tr><td>Measured mode</td><td>46.2</td><td>29.4</td><td>8.5</td><td>8.5</td><td>7.4</td></tr><tr><td>Calculated mode</td><td>46.9</td><td>28.3</td><td>12.6</td><td>3.6</td><td>8.1</td></tr></tbody></table>

The mineral abundances, expressed in vol%, have been measured by image analysis (measured mode) and estimated by matrix calculation from the chemical analyses (calculated mode; see text); others: kamacite grains and iron oxides (i.e., the alteration products that replaced kamacite or fill the veins).

Ca] = 0.578 ± 0.003) with a relatively high Ca content (1.80 ± 0.15 mole% of larnite Ca₂SiO₄; X_Ca = 0.018 ± 0.002) (Table 1). The FeO/MnO ratio is 84.15 ± 7.15. A slight zoning with a decrease in the Ca content is observed at a few micrometers from the clinopyroxene coronas that developed at the olivine–anorthite interface (X_Ca down to 0.014), whereas X_Fe increases and X_Mg remains almost unchanged (Fig. 6c). The secondary olivine from the coronas (Ol_c) that developed at the spinel–clinopyroxene interface is slightly richer in Mg (X_Mg = 0.586 ± 0.002: Table 1) and poorer in Ca (X_Ca = 0.012 ± 0.002).

Clinopyroxene grains (Cpx_g) are homogenous, with an average composition of En₃₃.₇ ± 0.₄ F₅₈.₅ ± 0.₈ WO₃₉.₈ ± 0.₈ Ca-Al-Ts₁₃.₉ ± 1.₂ Ca-Ti-Ts₃.₉ ± 0.₃ (mole% of endmembers; see Table 2), and an X_Mg ratio of

<figure><img src="image_6.png" /><figcaption>Fig. 5. Complex corona of anorthite (An_c) + olivine (Ol_c) around primary spinel (Spl_g). a) General view (backscattered electron image): the anorthite corona (An_c) developed preferentially at the expense of spinel Spl_g and extends far from the contact between spinel Spl_g and clinopyroxene Cpx_g, giving locally the appearance of a single anorthite corona around spinel; locally, secondary clinopyroxene (Cpx_c) and spinel developed between the anorthite corona An_c and primary olivine Ol_g. Traces of calcite (Cal) are observed within the late veins. b) Detailed view (false-color image generated by principal component analysis of several element maps from the scanning electron microscope); red: spinel; emerald green: olivine; dark green: clinopyroxene; violet: anorthite; pink: kamacite and iron oxide; blue: calcite. Olivine in the corona (Ol_c) is partly enveloped by clinopyroxene (white circles; see text). Small clinopyroxene grains (Cpx_c) and spinel films (Spl_c) formed between the secondary olivine (Ol_c) and anorthite (An_c) that developed around primary spinel Spl_g (see text).</figcaption></figure>
<figure><img src="image_7.png" /></figure>

Fig. 6. Chemical profiles showing zoning in reactants close to the coronas. $X_i$ refers to the molar fraction of element $i$ (see mineral chemistry). a) Clinopyroxene (Cpx$_g$): EPMA traverse from A to B (dotted line in Fig. 2) toward corona (Ol$_c$). At a few micrometers from the coronas, the composition of Cpx$_g$ slightly evolves with increases of $X_{Mg}$ and $X_{Ca}$, and slight decreases of $X_{Fe}$ and $X_{Ti}$. b) Spinel (Spl$_g$): traverse from D to C (dotted line in Fig. 2) toward corona (An$_c$). Close to the corona, Spl$_g$ becomes slightly enriched in Cr. c) Olivine (Ol$_g$): traverse from E to F (dotted line in Fig. 3c) toward corona (Cpx$_c$). A slight zoning with a decrease in $X_{Ca}$ and an increase in $X_{Fe}$ is observed at a few micrometers from the corona, whereas $X_{Mg}$ remains almost unchanged.

$0.364 \pm 0.016$ ($X_{Mg}$ = Mg/[Ca + Fe$^{2+}$ + Mg + Ni + Mn]). This pyroxene is mainly characterized by an anomalous high content in the Tschermak's endmembers (Ca-Al-Ts: CaAl$^{VI}$Al$^{IV}$SiO$_6$, and Ca-Ti-Ts: CaTi$^{VI}$Al$^{IV}_2$O$_6$) and so it is identical to the “fassaitic” clinopyroxene of the angrite meteorites. At a few micrometers from the olivine + anorthite coronas, toward spinel Spl$_g$, the composition of Cpx$_g$ slightly evolves with increases of $X_{Mg}$ and $X_{Ca}$, and a slight decrease of $X_{Fe}$ (Fig. 6a). The coronitic clinopyroxene Cpx$_c$ that developed with spinel Spl$_c$ at the olivine–anorthite interface is more ferrous and aluminous ($\text{En}_{32.7} \pm 2.2 \text{ Fs}_{10.3} \pm 0.9 \text{ Wo}_{38.2} \pm 4.9 \text{ Ca-Al-Ts}_{17.1} \pm 6.2 \text{ Ca-Ti-Ts}_{3.6} \pm 1.1$; $X_{Mg} = 0.357 \pm 0.030$: Table 2).

Spinel is a solid solution of spinel (Spl: MgAl$_2$O$_4$) and hercynite (Hc: Fe$^{2+}$Al$_2$O$_4$), with minor quantities of chromite (Chr: Fe$^{2+}$Cr$_2$O$_4$), magnetite (Mag: Fe$^{2+}$Fe$^{3+}_2$O$_4$), and ulvöspinel (Us: Fe$^{2+}_2$Ti$^{4+}$O$_4$). Its average composition is Spl$_{48.2} \pm 0.4 \text{ Hc}_{44.2} \pm 0.8 \text{ Mag}_{1.2} \pm 0.3 \text{ Chr}_{4.9} \pm 0.3 \text{ Us}_{1.1} \pm 0.1$ (Table 3). Again, this composition slightly evolves close to the coronas (Fig. 6b; Table 3). In particular, the spinel Spl$_g$ becomes slightly enriched in Cr ($X_{Cr}$ up to 0.58) at the contacts with the anorthite corona (An$_c$) that has grown at the spinel–clinopyroxene interface. Finally, the coronitic spinel Spl$_c$ that developed at the olivine–anorthite interface also shows a slightly different composition (Spl$_{44.1} \pm 0.5 \text{ Hc}_{48.7} \pm 0.9 \text{ Mag}_{1.1} \pm 0.5 \text{ Chr}_{5.8} \pm 0.3 \text{ Us}_{0.0} \pm 0.0$: Spl$_c$ in Table 3).

Plagioclase is nearly pure anorthite (An$_{99.5} \pm 0.2 \text{ Ab}_{0.5} \pm 0.2$: Table 4), with a negligible content in Na (Na$_2$O < 0.1 wt%), but a substantial quantity of Fe$^{3+}$ (Fe$_2$O$_3$ = $0.59 \pm 0.37$ wt%). The plagioclase from the coronas is also almost pure anorthite, but contains a little more ferric iron (Fe$_2$O$_3$ = $0.82 \pm 0.27$ wt%), which may explain the difference of color observed in cathodoluminescence (An$_c$ versus An$_g$ in Fig. 4).

Most of the kamacite patches are partially oxidized. Only a few small grains included in olivine and clinopyroxene are preserved. Their content in Ni is about 6.5 wt% (Table 5).

## DISCUSSION: PETROGENESIS

### Angrite Signature

The angrites are a small group of achondrites characterized by unusual mineral assemblages (clinopyroxene + olivine + anorthite + spinel + metal: e.g., Mittlefehldt et al. 2002; Jambon et al. 2005) and their richness in refractory elements, particularly Ca, Al, and Ti. In addition to their peculiar compositions, these meteorites have unusually old ages (approximately 4.56 Ga: e.g., Jagoutz et al. 2002; Baker et al. 2005), and specific oxygen isotopic compositions ($\delta^{18}$O = 4.02, $\Delta^{17}$O = -0.072: Greenwood et al. 2005).

Although all angrites share the above characteristics, they are quite variable in terms of mineralogy and
textures (1) Angra dos Reis is dominated by megacrysts of Al-Ti-rich diopside-hedenbergite (clinopyroxene formerly known as fassaite) and has generally been interpreted as an annealed magmatic cumulate (e.g., Prinz et al. 1977). (2) Other angrites, such as Sahara 99555, D'Orbigny, and NWA 1296, have igneous basaltic textures and contain abundant olivine, including kirschsteinite (CaFeSiO₄), and anorthite in addition to clinopyroxene (Mikouchi et al. 2000; Kurat et al. 2001a, 2001b; Mikouchi and McKay 2001). These angrites are characterized by their low Mg-content. Some of them contain a significant amount of olivine xenocrysts with Mg-rich cores (NWA 1670, LEW 87051, etc.), which shifts their bulk composition toward an ultramafic composition. (3) Finally, peridotitic angrites (NWA 2999, NWA 3164, and NWA 5167) show a granular texture, abundant olivine, and are characterized by a higher content of MgO relative to the other angrites.

Northwest Africa 3164 shows a number of petrological and mineralogical features that are typical of angrites, particularly peridotitic angrites:

1. The major phases are olivine, Al-rich clinopyroxene, and pure anorthite, as observed in angrites (e.g., Weisberg et al. 2006).

2. Spinel is present and orthopyroxene is absent, indicating silica undersaturation (e.g., Mittlefehldt et al. 2002).

3. The rock is poor in Na and K (Mittlefehldt and Lindstrom 1990; Prinz and Weisberg 1995; Mittlefehldt et al. 2002).

4. The $X_{Fe}$ ratio ($X_{Fe} = Fe/[Fe + Mg + Ni + Mn]$) of olivine, clinopyroxene, and spinel is abnormally high, with respect to the terrestrial peridotites and other ultramafic meteorites.

5. Olivine is relatively rich in CaO (approximately 1.2 wt%: Table 1), as in many other angrites (e.g., Mittlefehldt et al. 2002; Jambon et al. 2008).

6. The richness of clinopyroxene in Al and Ca, expressed by its high content in the Ca-Tschermak endmembers (13.8 ± 1.0 mole%), is also characteristic of angrites (Jambon et al. 2005, 2008).

All these features reflect a high CaO content (i.e., superchondritic Ca/Al) and a low SiO₂ content of the rock, which are typical of angrites (e.g., Mittlefehldt and Lindstrom 1990; Jambon et al. 2005). A few characters, however, are specific of NWA 3164. In contrast to other angrites (e.g., Kurat et al. 2004; Jambon et al. 2005, 2008), no kirschsteinite (CaFeSiO₄) has been found in NWA 3164. Spinel shows higher Al and lower Cr contents relative to spinel in other angrites (e.g., D'Orbigny: Mittlefehldt et al. 2002; NWA 1670: Jambon et al. 2008). Finally, NWA 3164 has a high magnetic susceptibility ($\log \chi = 4.25$: this study; $\log \chi = 4.54$: Rochette et al. 2009) relative to other

angrites ($\log \chi = 2.89 \pm 0.19$: Rochette et al. 2009), which is probably due to the relative abundance of metal in this peridotitic angrite (Rochette et al. 2009).

In contrast to most other magmatic-textured angrites, NWA 3164 has a granular texture that could indicate a slowly cooled plutonic origin, like the type angrite Angra dos Reis, which is interpreted as a clinopyroxene-rich cumulate (Ma et al. 1977). The homogeneity of the clinopyroxene composition of NWA 3164, compared with the wider compositional range in other angrites (Kuehner et al. 2006), also supports a plutonic origin. Alternatively, Jambon et al. (2012) infer that the granular texture could result from annealing of a former brecciated rock. In all cases, the observed coronas developed after the magmatic or annealing stage.

## Metamorphic Reactions

### Solid-State Reactions

The above coronitic microstructures can be interpreted as resulting from metamorphic reactions in the solid state, as commonly assumed for terrestrial metamorphic rocks since Sederholm (1916).

The olivine+anorthite coronas visible at the clinopyroxene-spinel interface (Figs. 2 and 5) could result from the following reaction:

$$ \begin{gathered} \text{clinopyroxene (Cpx}_g\text{)} + \text{spinel (Sp}_g\text{)} \rightarrow \\ \text{olivine (Ol}_c\text{)} + \text{anorthite (An}_c\text{)}. \end{gathered} \qquad (R1) $$

On the other hand, the clinopyroxene + spinel coronas at the olivine-plagioclase interface (Fig. 3) would form as follows:

$$ \begin{gathered} \text{olivine (Ol}_g\text{)} + \text{anorthite (An}_g\text{)} \rightarrow \\ \text{clinopyroxene (Cpx}_c\text{)} + \text{spinel (Sp}_c\text{)}. \end{gathered} \qquad (R2) $$

Several lines of evidence confirm that both types of coronas formed during a reaction in the solid state:

1. The coronas occur systematically between specific minerals, which would not be the case if they had crystallized from an interstitial melt. Minerals crystallizing from a melt form veins and/or pockets, neither coronas nor symplectites.

2. The coronas are asymmetric, particularly in the case of R1. The olivine (Ol_c) and anorthite (An_c) layers are close to the pre-existing clinopyroxene (Cpx_g) and spinel (Sp_g), respectively, indicating that Ol_c grew mainly at the expense of Cpx_g and An_c mainly from Sp_g, during a diffusion-controlled process in
<table><caption>Table 7. Stoichiometry of reaction R1 (Cpx + Spl → Ol + An).</caption><thead><tr><th rowspan="2"></th><th colspan="3">Reactants</th><th colspan="4">Products</th></tr><tr><th>Cpx</th><th>Spl</th><th>Σreactants</th><th>Ol</th><th>An</th><th>Σproducts</th><th>Residual</th></tr></thead><tbody><tr><td>Si</td><td>1.792</td><td>0.003</td><td>1.794</td><td>0.988</td><td>1.982</td><td>1.984</td><td>-0.190</td></tr><tr><td>Ti</td><td>0.038</td><td>0.011</td><td>0.044</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.043</td></tr><tr><td>Al</td><td>0.308</td><td>1.847</td><td>1.250</td><td>0.004</td><td>1.980</td><td>1.315</td><td>-0.065</td></tr><tr><td>Cr</td><td>0.006</td><td>0.102</td><td>0.058</td><td>0.001</td><td>0.003</td><td>0.003</td><td>0.055</td></tr><tr><td>Fe</td><td>0.197</td><td>0.552</td><td>0.479</td><td>0.828</td><td>0.038</td><td>0.587</td><td>-0.108</td></tr><tr><td>Mn</td><td>0.003</td><td>0.004</td><td>0.005</td><td>0.009</td><td>0.000</td><td>0.006</td><td>-0.001</td></tr><tr><td>Mg</td><td>0.679</td><td>0.479</td><td>0.923</td><td>1.150</td><td>0.000</td><td>0.780</td><td>0.143</td></tr><tr><td>Ni</td><td>0.001</td><td>0.000</td><td>0.001</td><td>0.002</td><td>0.001</td><td>0.002</td><td>-0.001</td></tr><tr><td>Ca</td><td>0.975</td><td>0.001</td><td>0.976</td><td>0.026</td><td>0.991</td><td>0.675</td><td>0.301</td></tr><tr><td>Na</td><td>0.002</td><td>0.000</td><td>0.002</td><td>0.000</td><td>0.009</td><td>0.006</td><td>-0.004</td></tr><tr><td>O</td><td>6.000</td><td>4.000</td><td>8.040</td><td>4.000</td><td>8.000</td><td>8.016</td><td>0.024</td></tr><tr><td>Coeff.</td><td>-1.000</td><td>-0.510</td><td>-1.000</td><td>0.678</td><td>0.663</td><td>1.000</td><td>1.000</td></tr></tbody></table>

Coeff. = stoichiometric coefficients $n_i$; all values are in mole; see text for the algorithm.

the solid state (e.g., Sederholm 1916; Bard 1986; Barker 1998). The hypothetical crystallization of Ol$_c$ and An$_c$ from a melt would have produced an equigranular aggregate of both phases, not asymmetric coronas.

3. The R2 coronas frequently consist of a symplectite made of Spl$_c$ + Cpx$_c$ intimate intergrowths (Fig. 3). Symplectites are also produced by solid-state reaction, the intergrown grains being elongated perpendicular to the reaction front (e.g., Sederholm 1916; Bard 1986; Barker 1998; Ashworth and Chambers 2000), as shown in Figs. 3a and 3b.

4. The crystals of Ol$_g$, An$_g$, and Cpx$_g$ display a zoning pattern close to the contact with the coronas (Fig. 6; see the Petrography section). This is typical of a solid-state multivariant sliding reaction, during which the composition of some of the reactants evolve in composition by intracrystalline diffusion (e.g., Hensen 1971).

“Corona formed in the solid state” does not exclude that a melt or a fluid was involved as a minor phase. Although not totally excluded, there is no evidence for such an intervention. Magmatic veins crystallizing from a melt and associated with the coronas have not been observed. Infiltration of an aqueous or carbonated fluid is not substantiated by the observations, as there is no mineral containing H$_2$O, CO$_2$, Cl, or F among the reaction products in the coronas (calcite only exists in the late veins that cross cut the coronas). This feature contrasts with many terrestrial rocks, in which metamorphic coronas formed at the olivine–anorthite interface commonly contain hydrous minerals, such as amphibole and, occasionally, biotite (e.g., Turner and Stüwe 1992; Claeson 1998).

## Balancing the Reactions

Ideally, a solid-state reaction is isochemical, as the composition of what is consumed during the reaction (i.e., the reactants) is equal to the composition of what appeared (i.e., the products). However, frequently the reaction zone is not entirely isochemical, as some elements involved in the reaction may have diffused between the reaction area and the outside, possibly through minor mobile phases like fluid or melt (e.g., Kretz et al. 1989; Barker 1998; Adjerid et al. 2013). To test their reliability and isochemical character, we balanced the inferred reactions R1 and R2 using the method of least squares (e.g., Fisher 1989; Godard and Martin 2000; Adjerid et al. 2013). This algorithm expresses the Lavoisier law of conservation of matter (each chemical component is in the same amount before and after the reaction) and provides the following vector equation:

$$ \sum n_i P_i + \text{Residual} = (0), \quad (1) $$

where the $i$ vectors $P_i$ are the known compositions of the $i$ minerals involved in the reaction (determined by electron microprobe: Tables 1–4), scalars $n_i$ are the corresponding stoichiometric coefficients (negative for reactants, positive for products), and Residual is a residual vector in the sense of the least-squares method. One $n_i$ is fixed arbitrarily and the others are obtained from the algorithm, so that the reactants and products are not determined a priori. Each coordinate of vectors $P_i$ and Residual corresponds to the content of one independent chemical component. Oxygen is considered to be independent of the other components, as iron is either Fe$^{2+}$ or Fe$^{3+}$; its content was calculated by stoichiometry (see Tables 1–4). The results of the algorithm are shown in Tables 7 and 8.
<table><caption>Table 8. Stoichiometry of reaction R2 (Ol + An → Cpx + Spl).</caption><thead><tr><th rowspan="2"></th><th colspan="3">Reactants</th><th colspan="4">Products</th></tr><tr><th>Ol</th><th>An</th><th>Σreactants</th><th>Cpx</th><th>Spl</th><th>Σproducts</th><th>Residual</th></tr></thead><tbody><tr><td>Si</td><td>1.001</td><td>1.988</td><td>1.995</td><td>1.788</td><td>0.001</td><td>1.789</td><td>0.206</td></tr><tr><td>Ti</td><td>0.001</td><td>0.001</td><td>0.001</td><td>0.028</td><td>0.002</td><td>0.029</td><td>-0.028</td></tr><tr><td>Al</td><td>0.001</td><td>1.993</td><td>1.246</td><td>0.313</td><td>1.857</td><td>1.249</td><td>-0.003</td></tr><tr><td>Cr</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.007</td><td>0.114</td><td>0.064</td><td>-0.065</td></tr><tr><td>Fe</td><td>0.808</td><td>0.008</td><td>0.613</td><td>0.210</td><td>0.580</td><td>0.502</td><td>0.110</td></tr><tr><td>Mg</td><td>1.157</td><td>0.000</td><td>0.870</td><td>0.673</td><td>0.440</td><td>0.895</td><td>-0.025</td></tr><tr><td>Ca</td><td>0.036</td><td>1.022</td><td>0.666</td><td>0.984</td><td>0.005</td><td>0.987</td><td>-0.321</td></tr><tr><td>Na</td><td>0.000</td><td>0.006</td><td>0.004</td><td>0.000</td><td>0.000</td><td>0.000</td><td>0.004</td></tr><tr><td>O</td><td>4.000</td><td>8.000</td><td>8.007</td><td>6.000</td><td>4.000</td><td>8.016</td><td>-0.007</td></tr><tr><td>Coeff.</td><td><strong>-0.752</strong></td><td><strong>-0.625</strong></td><td><strong>-1.000</strong></td><td><strong>1.000</strong></td><td><strong>0.504</strong></td><td><strong>1.000</strong></td><td><strong>1.000</strong></td></tr></tbody></table>

Coeff. = stoichiometric coefficients $n_i$; all values are in mole; see text for the algorithm.

The balance of a reaction is considered satisfactory when (1) the results account for the observed microstructures, with the disappearing and appearing minerals as reactants (i.e., $n_i < 0$) and products ($n_i > 0$), respectively, and (2) the residuals are low. As the number of independent equations (i.e., the number of chemical components) is greater than the number of independent unknowns (i.e., the number of coefficients to be estimated), the problem is overdetermined and the residuals (i.e., the coordinates of vector Residual) are not null. However, these residuals should still be low relative to the overall composition of the reactants or products (i.e., $\Sigma_{reactants}$ or $\Sigma_{products}$ in Tables 7 and 8). Ideally, the residual for a major chemical element is of the order of $\pm 2$ mole% of the overall content in the same element, as this is the reported accuracy for wavelength-dispersive microprobe analyses (Ancey et al. 1977). Residuals well outside this tolerance range imply that the mathematical solution is not totally satisfactory. This can occur for the following reasons: (1) the system was open to some mobile elements during the reaction; (2) the reaction was a sliding multivariant reaction and was accompanied by substantial changes in the composition of some minerals, typically through the $Fe_1Mg_{1+1}$ substitution (e.g., Hensen 1971), in which case the vector Residual is close to the substitution vector; (3) additional phases were involved in the reaction, in which case the vector Residual is similar to the composition of the missing phase(s).

The stoichiometric coefficients, expressed in mole, are converted to volume using the molar volumes of the involved minerals as given by the data set of Holland and Powell (1998; upgraded in 2009). For convenience, the coefficients are normalized to one of them.

**Reaction R1:** For the olivine + anorthite coronas that developed at the clinopyroxene–spinel interface, the algorithm yields the following result (in mole; see Table 7):

$$ 
\begin{aligned}
& 1 \text{ clinopyroxene (Cpx}_c\text{)} + 0.510 \text{ spinel (Spl}_c\text{)} \\
& = 0.678 \text{ olivine (Ol}_c\text{)} + 0.663 \text{ anorthite (An}_c\text{)} \\
& + 1 \text{ Residual,}
\end{aligned} 
\quad (2) $$

$$ 
\begin{aligned}
\text{with Residual} = & \text{Si}_{-0.190}\text{Ti}_{0.043}\text{Al}_{-0.065}\text{Cr}_{0.055}\text{Fe}_{-0.108} \\
& \text{Mn}_{-0.001}\text{Mg}_{0.143}\text{Ni}_{-0.001}\text{Ca}_{0.301} \\
& \text{Na}_{-0.004}\text{O}_{0.024}.
\end{aligned} 
\quad (3) $$

After conversion to volumes:

$$ 
\begin{aligned}
1 \text{ clinopyroxene} + 0.311 \text{ spinel} \rightarrow \\
0.460 \text{ olivine} + 1.011 \text{ anorthite} (\Delta V/V = +0.108).
\end{aligned} 
\quad (4) $$

The mass-balance study yields results compatible with the observed microstructures, with the disappearing (Cpxg, Splg) and appearing (Olc, Anc) minerals representing the reactants and products, respectively. Moreover, the calculated volume of anorthite Anc is about twice that of olivine Olc, as effectively observed in the coronas (Figs. 2 and 5). The residuals are significantly positive for Mg, Ca and negative for Fe, which explains why Cpxc is enriched and depleted, respectively, in those elements close to the corona (Fig. 6a). In the same way, the positive residual for Cr also accounts for the spinel enrichment in chromite during the reaction, as evidenced by the spinel zoning close to the corona (Fig. 6b). In other terms, these residuals indicate that the corresponding elements were either incorporated into (positive residuals) or extracted from (negative residuals) the reacting phase(s), whose composition evolved accordingly during the reaction (i.e., item (2) above). However, substantial residuals for Si and Ca also suggest that the reaction microdomain was not perfectly isochemical during the reaction (i.e., item (1) above), possibly due to the
intervention of a mobile phase (fluid or melt: i.e., item (3) above).

**Reaction R2:** As for the clinopyroxene + spinel corona that developed at the olivine–anorthite interface, it yields the following result (in mole; see Table 8):

$$ 
\begin{aligned}
0.752 \text{ olivine (Ol}_g\text{)} + 0.625 \text{ anorthite (An}_g\text{)} &= 1 \text{ clinopyroxene (Cpx}_c\text{)} + 0.504 \text{ spinel (Spl}_c\text{)} \\
&\quad + 1 \text{ Residual,}
\end{aligned}
\quad (5) $$

$$ \text{with Residual} = \begin{aligned}[t] & \text{Si}_{0.206}\text{Ti}_{-0.028}\text{Al}_{-0.003}\text{Cr}_{-0.065}\text{Fe}_{0.110} \\ & \text{Mg}_{-0.025}\text{Ca}_{-0.321}\text{Na}_{0.004}\text{O}_{-0.007}. \end{aligned} \quad (6) $$

After conversion to volumes:

$$ 
\begin{aligned}
0.510 \text{ olivine} + 0.953 \text{ anorthite} &\rightarrow \\ 
1 \text{ clinopyroxene} + 0.308 \text{ spinel}(\Delta V/V = -0.106).
\end{aligned}
\quad (7) $$

Reaction R2 is almost the reverse of reaction R1, the stoichiometric coefficients as well as the residuals and $\Delta V/V$ being more or less inverted relative to those of R1. Again, the residuals can be partly correlated with the zonation in the reacting Ol$_g$ close to the corona (enrichment in Fe; depletion in Ca and Mg: Fig. 6c), and the substantial residuals for Si and Ca also suggest that the reaction microdomain was not perfectly isochemical. Finally, the local modifications of the coronic microstructures close to kamacite (see the Petrography section) suggest that the latter very locally behaved as a reactant; in this case, some Spl$_c$ preferentially developed from kamacite, because both phases are Fe-rich and Si-free.

## Chronology of the Reactions

As noted above in the Petrography section, the olivine corona (Ol$_c$) that developed together with anorthite An$_c$ at the contacts between spinel Spl$_g$ and clinopyroxene Cpx$_g$ during reaction R1 is discontinuous; it consists of ovoid grains that are partly enveloped by clinopyroxene (white circles in Figs. 2 and 5b), suggesting that this olivine corona has been partly resorbed and replaced by clinopyroxene. Moreover, clinopyroxene (Cpx$_c$) and spinel (Spl$_c$) are also observed as very thin films between the olivine (Ol$_c$) and anorthite (An$_c$) coronas of the Spl$_g$–Cpx$_g$ interface (white arrows in Fig. 5b). These two observations indicate that the Ol$_c$ + An$_c$ coronas, already formed by reaction R1, were partly replaced by secondary clinopyroxene and spinel during reaction R2, which implies that R1 preceded R2. The complex zoning profile displayed by some Spl$_g$ grains close to the R1

corona (X$_{Fe}$ and X$_{Mg}$ in Fig. 6b) might be due to the successive and superimposed effects of R1 and R2 events. All these coronas are cross cut by the late veinlets. The coronas apparently interrupt the kamacite trails (1–4 interrupted between 2 and 3 in Fig. 3d), suggesting that these trails formed before the R1 and R2 reactions.

These observations lead to the following scenario, summarized in Fig. 7: (1) the rock first consisted of primary clinopyroxene (Cpx$_g$), spinel (Spl$_g$), olivine (Ol$_g$), anorthite (An$_g$), kamacite, and sulfide, with a granular texture (Fig. 7a); (2) the thin olivine (Ol$_c$) + anorthite (An$_c$) coronas developed at the contacts between Ol$_g$ and Spl$_g$ grains during reaction R1 (Fig. 7b), with An$_c$ extending far from this contact; (3) coronitic clinopyroxene (Cpx$_c$) and spinel (Spl$_c$) grew at the expense of olivine and anorthite during reaction R2, at the contacts between primary olivine (Ol$_g$) and anorthite (An$_g$) grains, as well as at the expense of the secondary R1 olivine (Ol$_c$) + anorthite (An$_c$) coronas (Fig. 7c); (4) a shock produced the late fracturing (Fig. 7d); (5) finally, sulfide, kamacite, and the late veinlets were almost completely oxidized on Earth.

## P–T Evolution

### Method

For 25 years, the P–T evolution of metamorphic rocks has been mainly modeled using P–T isochemical phase diagrams, otherwise called “P–T pseudosections” (e.g., Powell and Holland 2010). Such phase diagrams are graphical representations of the equilibrium relationships between minerals (or other phases such as melt), as a function of P and T. These relationships are governed by the laws of equilibrium thermodynamics. A P–T pseudosection is calculated considering one particular chemical composition, typically the bulk composition of the studied rock. The result predicts, as a function of P and T, the minerals that are stable, as well as their composition and modal abundance. Comparing the calculated P–T pseudosection with the mineralogy effectively observed in the rock, it is then possible to constrain the P–T conditions at which the rock formed.

The P–T pseudosection presented in Fig. 8a was calculated using the THERMOCALC tc333 computer program by Powell and Holland (1988), with an upgraded version of the internally consistent thermodynamic data set of Holland and Powell (1990, 1998; upgraded version 55s of 2009), and considering the modal abundances of the phases in NWA 3164 (Table 6), converted to mole%. The calculated and measured modes of Table 6 yield similar results, but the measured mode was preferred, being better
<figure><img src="image_8.png" /><figcaption>Fig. 7. Microstructural evolution of Northwest Africa 3164 (explanatory images representing the area of Fig. 5b). a) Primary assemblage. b) Coronas of olivine (Ol_c) + anorthite (An_c) developing during reaction R1 at the contacts between primary clinopyroxene (Cpx_g) and spinel (Sp1_g) grains. c) Secondary clinopyroxene (Cpx_c) and spinel (Sp1_c) developing during R2 at the expense of the pre-existing coronas of olivine (Ol_c) + anorthite (An_c). d) Late fracturing due to a shock.</figcaption></figure>

representative of the studied thick section. As pseudosection modeling can take into consideration only a few major components, the minor elements were either neglected (K, Na, P, Ti) or added to major elements known to behave identically (Cr to Al, Mn to Fe, Ni to Mg). Finally, metal and iron oxide from the veins (see Table 6) were not considered in this composition, because thermodynamic data on kamacite are not available in the database of Holland and Powell (1998), and the veins, injected into the rock, should not be considered while modeling its earlier evolution. Therefore, the considered bulk composition represents the average composition of olivine + clinopyroxene + spinel + anorthite in the rock, i.e., (in mole%): 37.50 SiO₂; 5.23 Al₂O₃; 28.01 MgO; 18.53 FeO; 10.72 CaO. The solution models used for the various phases are

Fo–Fa solid-solution model of Dachs and Geiger (2007) for olivine; Alm–Prp–Grs model of White et al. (2007) for garnet; En–Fs–Mg–Tschermak model of Powell and Holland (1999) for orthopyroxene; Di–Hd model after Holland and Powell (2003) for clinopyroxene; Spl–Hc model of Holland and Powell (1998) for spinel; liquid solution model of White et al. (2007) for the melt; pure anorthite for plagioclase. It is not possible to take into consideration the chromite, Ca–Tschermak, and larnite endmembers in spinel, clinopyroxene, and olivine solid solutions, respectively, as solid-solution models compatible with the SiO₂–Al₂O₃–MgO–FeO–CaO system and involving these endmembers are not available in the literature. This may contribute to the deterioration of the results, as these endmembers are not always negligible in NWA 3164 (5 mole% of
<figure><img src="image_9.png" /><figcaption>Fig. 8. *P*−*T* evolution of Northwest Africa (NWA) 3164 (thermodynamics-based model; see text). a) *P*−*T* pseudosection modeled for the bulk composition of NWA 3164 (see text); the curves labeled “av. *P*−*T*” (±*σ*) result from the multiequilibria method applied to the primary assemblage Ol~g~ + Cpx~g~ + Spl~g~ + An~g~ (see Average *P*−*T* in the text). b) Modal evolution (in vol%) at 0.1 and 0.5 GPa, showing a decrease of clinopyroxene and spinel together with an increase of olivine and anorthite, as *T* increases, which corresponds to reaction R1 (see text).</figcaption></figure>

chromite in spinel; 14 mole% of Ca-Tschermak in clinopyroxene; 1.7 mole% of larnite in olivine).

The modal abundances, in vol%, have been modeled for olivine, clinopyroxene, spinel, and plagioclase at various temperatures and pressures (Fig. 8b). To better constrain the *P*−*T* conditions at which the initial assemblage equilibrated, the multiequilibria approach (“Average *P*−*T*” method of Powell and Holland 1994) was used with the same thermodynamic data set, but considering the real compositions of the minerals (i.e., Ol~g~, Cpx~g~, Spl~g~, and An~g~, from Tables 1–4), instead of those from the model. Given a set of minerals that is interpreted to have once been in equilibrium (in this case, Ol~g~ + Cpx~g~ + Spl~g~ + An~g~), the endmembers of the minerals, for which there are thermodynamic data in the internally consistent data set, are used to write an independent set of equilibria. For each equilibrium, a relation (ΔG° + RT ln K = 0) can be written, in which ΔG° is a function of *P* and *T*, whereas *K* is a function of the composition of the minerals. The activities of the endmembers in the minerals, obtained from the compositions of the minerals using the AX software, are used to calculate *K*, so each equilibrium relationship is a curve in *P*−*T*. The information for the several independent

equilibria is then combined in a least-squares sense, using Thermocalc, to calculate the conditions of formation. The method also provides a confidence interval on *P*−*T* (0.68, corresponding to ±*σ*; see Fig. 8a) that reflects the uncertainties in the thermodynamic data and mineral compositions.

## Results

The *P*−*T* pseudosection for the considered composition (Fig. 8a) indicates that there is a wide range of *P*−*T* conditions (<1200 °C and <1.0 GPa) at which the observed mineral assemblage (Ol + Cpx + An + Spl) is stable. This assemblage partly melts at *T* above 1250 °C (Fig. 8a), which is comparable to the Ol + Cpx + An + Spl = Liq melting curve that has been well calibrated for the purely magnesian compositions (Green et al. 2012). As NWA 3164 does not show melting structure, such as melt veins, this limit was probably not overstepped during the metamorphic evolution, while the coronas formed in the solid state. The “Average *P*−*T*” method restrains the conditions of formation of the initial assemblage (Ol~g~ + Cpx~g~ + Spl~g~ + An~g~) to 914 ± 119 [1*σ*] °C at 0.0 GPa, and
<figure><img src="image_10.png" /></figure>

Fig. 9. *P*−*T* and *P*−*X*<sub>Mg</sub> pseudosections for terrestrial lherzolites (after Perkins and Anthony 2011). a) *P*−*T* pseudosection for a terrestrial lherzolite; solid lines show the stability fields of different assemblages for a given lherzolite composition; the dotted lines are the boundaries between the plagioclase, spinel, and garnet peridotite, as commonly published in the literature (see Perkins and Anthony 2011). b) *P*−*X*<sub>Mg</sub> pseudosection for a terrestrial lherzolite at 1100 °C, from Perkins and Anthony (2011); solid lines show the stability fields of different assemblages for a given lherzolite composition; the dark gray vertical band represents the common *X*<sub>Mg</sub> values for terrestrial lherzolites. For comparison, the dashed lines, cursive letters, and light gray vertical band represent, respectively, the stability fields, assemblages, and *X*<sub>Mg</sub> value for Northwest Africa 3164 (see text).

976 ± 121 °C at 0.9 GPa (see “av. *P*−*T*” in Fig. 8a). These thermobarometric calculations do not constrain accurately the pressure (*P* < 0.9 GPa).

As the “pseudosection” takes into account the
composition of the bulk rock less the kamacite and the
veins, it does not directly indicate at which *P*−*T*
conditions reactions R1 and R2 occurred, because these
are restricted to local microdomains. However, the
mineral abundances in NWA 3164 deduced from the
pseudosection modeling (Fig. 8b) can help to unravel
these conditions. It should be recalled here from the
above section (Metamorphic Reactions) that both
reactions produced a “sliding” evolution of their
reactants, whose composition slightly evolved during the
reaction. This implies that the reactions were
multivariant, not univariant (e.g., Hensen 1971); in other
terms, they developed progressively across a space of the
*P*−*T* diagram, not across a simple *P*−*T* univariant curve.
The model of Fig. 8b shows that the relative abundances
of the minerals slightly evolve within the
*P*−*T* field where the Ol + Cpx + An + Spl assemblage is
stable: At increasing *T*, clinopyroxene and spinel
decrease slightly, whereas anorthite and olivine increase
(Fig. 8b). These changes precisely correspond to the
effects of reactions R1, with Cpxg and Splg reacting

together to produce some An_c and Ol_c; R1 is thus related
to an increase of *T*. Indeed, for reasons of symmetry, the
inverse reaction R2 is due to a decrease of *T*. Because
these two reactions are accompanied by variations in
volume, positive for R1 and negative for R2 (see Δ*V*/*V*,
in Eq. 4 and 7), they are potentially favored by
decreasing and increasing pressures, respectively.
However, this effect is almost negligible in the model (see
dashed lines [0.5 GPa] versus full lines [0.1 GPa] in
Fig. 8b).

The *P*−*T* pseudosection obtained for NWA 3164 (Fig. 8a) is quite different from pseudosections for the most common terrestrial peridotites (i.e., lherzolite: Fig. 9a, 9b; e.g., Perkins and Anthony 2011). A typical pseudosection for terrestrial lherzolite displays the following successive assemblages while *P* increases: Ol + Opx + Cpx + An, Ol + Opx + Cpx + An + Spl, Ol + Opx + Cpx + Spl, Ol + Opx + Cpx + Spl + Grt, Ol + Opx + Cpx + Grt (Figs. 9a and 9b at *X*<sub>Mg</sub> ≈ 0.88; Perkins and Anthony 2011). In contrast, the sequence observed in the case of NWA 3164 composition would be: Ol + Cpx + An + Spl, Ol + Cpx + An + Spl + Grt, Ol + Cpx + Spl + Grt (Fig. 8a). Differences in composition between peridotitic angrites and terrestrial lherzolites can explain these divergences. In particular, the high content in CaO of
NWA 3164 (9.86 wt%) with respect to lherzolite (typically <4 wt%; e.g., Perkins and Anthony 2011) favors clinopyroxene instead of orthopyroxene; the relative richness of NWA 3164 in FeO (see the Angrite Signature section) also favors Fe-rich minerals, such as garnet and spinel (light gray vertical band in Fig. 9b); finally, the relatively low SiO₂ content of NWA 3164 (39.32 wt% versus approximately 45 wt% for terrestrial lherzolites; e.g., Perkins and Anthony 2011) explains the omnipresence of spinel in this rock (dashed lines and cursive letters in Fig. 9b).

DISCUSSION: ORIGIN OF THE METAMORPHIC CORONAS

Two types of reaction microstructures are found in NWA 3164 angrite:

1. Irregular coronas of anorthite and olivine have developed at the contacts between spinel and clinopyroxene, during a metamorphic reaction R1 (Spl + Cpx → An + Ol) due to a temperature increase, associated or not with a decompression.

2. Coronitic intergrowths of clinopyroxene and spinel have grown at the contacts between anorthite and olivine, during a second metamorphic reaction R2 (An + Ol → Spl + Cpx), which is almost the reverse of reaction R1. R2 is related to a temperature decrease and clearly postdates R1, as it also affects the anorthite + olivine coronas produced by reaction R1 (see above).

The above petrogenetic study implies some well-established consequences on the rock history:

1. As NWA 3164 is a peridotitic achondrite, and because the coronas developed at contacts between pre-existing crystals, it is clear that the parent body had already been differentiated when the metamorphic coronas formed.

2. As R2 clearly postdates R1, the general metamorphic evolution consisted first of heating (with or without decompression) followed by cooling, with a peak temperature in the range of 900–1200 °C.

3. Both reactions occurred in the solid state, like many metamorphic reactions in terrestrial rocks that produce similar coronitic microstructures. They produced a slight compositional zoning in some of the reacting crystals close to the produced coronas, which indicates incipient changes in composition of the reactants through diffusion during reaction. Such a process implies slow diffusion-controlled kinetics, requiring a long span of time similar to what occurs during terrestrial metamorphism.

4. The rock did not experience ductile deformation during or after the growth of the coronas; otherwise, these would have been destroyed.

5. Such reaction coronas are only observed among peridotitic angrites (see above; i.e., NWA 2999, NWA 3164, and NWA 5167), whereas the other angrites are devoid of them, although the potentially reacting minerals (Cpx, Spl, An, and Ol) are present in all cases. This suggests that the reason for this metamorphism is specifically related to peridotitic angrites.

6. After the formation of the coronas, a shock event produced some fracturing of the rock, generating small veins that cross cut the coronas.

Keeping in mind the six above prerequisites, several causes can be envisaged to explain this metamorphism, namely (1) heating during a close approach to the Sun, (2) heating resulting from a shock wave, (3) metamorphic evolution in the parent body.

Heating by Solar Radiation

One can imagine that moving toward and away from the Sun could result in heating and cooling, respectively. The models that give the surface temperature of a body versus its heliocentric distance (Marchi et al. 2009; Chaumard et al. 2012) yield a maximum distance from the Sun of 0.2 AU to attain the temperature reached at the metamorphic peak (>900 °C). In these conditions, the probability for the meteorite to be caught by the Sun is extremely high (in the order of 0.99: Marchi et al. 2009). Finally, this hypothesis does not explain why this metamorphism apparently only affects peridotitic angrites (i.e., prerequisite (5) above).

Heating Due to Shock Wave

Throughout the history of the solar system, asteroids have repeatedly collided with one another and with the planets, producing shock waves in the colliding bodies. The effects of shock in meteorites and asteroids have been discussed at length in many studies (e.g., Chao 1967, 1968; Heymann 1967; Kieffer 1971; Stöffler 1972, 1974; Ashworth 1985; Stöffler et al. 1991; Bischoff and Stöffler 1992; Scott et al. 1992; Leroux 2001). A shock event increases suddenly the temperature of the target that is expected to cool rapidly by energy dissipation within space, with cooling rates that range from subinstantaneous to 0.1 °C yr⁻¹ (Taylor and Heymann 1971; Smith and Goldstein 1977; Scott 1982; Laul 1986; Leroux et al. 2000; Goldstein et al. 2007).

In the case of NWA 3164, a shock could easily increase *T* up to 1000 °C (see prerequisite (2) above). However, the shock hypothesis is contradicted by several objections:

1. The observed fractures effectively resulted from a shock, but they cut across the reaction coronas
(Figs. 2–5 and 7d), which implies that this shock was posterior to the heating and cooling that induced reactions R1 and R2;

2. A shock would produce rapid heating and cooling (see above), which is hardly compatible with the growth of metamorphic structures that need diffusion, i.e., slow kinetics (see prerequisite (3) above). Evidently, shock metamorphism produces brecciation, melting, and annealing, but apparently neither coronas nor symplectites, on Earth as well as in meteorites (e.g., Stöffler et al. 1991). It is highly significant that the last event of fracturing, which was effectively produced by shock, did not produce any metamorphic coronas.

As NWA 3164 underwent first a slow metamorphic event that produced coronas without fractures and later a shock that generated fractures without coronas, ascribing the coronas to such a shock event implies a complicated scenario that would not obey “Occam’s razor” principle, but which cannot be totally ruled out.

## Metamorphism in the Parent Body

Coronas similar to those observed in NWA 3164 are well known in metamorphosed mafic and ultramafic terrestrial rocks. Olivine + plagioclase coronas between clinopyroxene and spinel, formed by R1-type reaction, have been reported in a few peridotites (e.g., Müntener and Piccardo 2003; Piccardo and Guarnieri 2010). Clinopyroxene + spinel coronas developing at the expense of olivine + anorthitic plagioclase by R2-type reaction are much more common, as they have been observed in many terrestrial peridotitic or gabbroic rocks (see Sederholm 1916, pp. 9–36; for the ancient bibliography; e.g., Griffin 1971; Griffin and Heier 1973; Donald 1976; Selverstone and Stern 1983; Rivers and Mengel 1988; Ballhaus and Berry 1991; Franz et al. 1996). In both cases, the metamorphic coronas are interpreted as resulting from changes in the *P*–*T* conditions undergone by the rocks in the solid state, during some geodynamic processes. For example, Müntener and Piccardo (2003) and Piccardo and Guarnieri (2010) argued that R1-type coronas in peridotites from Italy and Corsica, respectively, were linked to decompression related to the exhumation of these rocks. As for R2-type coronas, they are very commonly interpreted as resulting from retrograde metamorphic reactions linked to an isobaric cooling, typically from 1150 to 750 °C, at medium pressures (0.5–1.0 GPa) (e.g., Griffin 1971; Selverstone and Stern 1983; Ballhaus and Berry 1991).

By analogy to the Earth, the microstructures observed in NWA 3164 could be ascribed to similar

processes having occurred in the angrite parent body, which would match the above prerequisites (1), (3), and (6). To obey prerequisite (2), these geodynamic processes should first induce a temperature increase over approximately 1000 °C, associated or not with decompression (i.e., reaction R1; see *P*–*T* evolution above; Fig. 8), and a subsequent cooling (i.e., reaction R2; Fig. 8). Because the *P* evolution of NWA 3164 is not well constrained, it is difficult to argue in favor of or against decompression during R1, which would imply a partial exhumation of the rock in the parent body. On Earth, such coupled decompression and exhumation are related to tectonics that commonly induces ductile deformation, generating some strain structures in rocks, like schistosity, foliation, and/or lineation. Such deformation has not been observed in NWA 3164 and the hypothesis of tectonics in the parent body is highly speculative. Isobaric heating and subsequent isobaric cooling are a probable alternative. They could occur near the intrusion of a hot magma (*T* > 1200°C; typically basaltic or picritic) that produces a thermal effect without noticeable deformation (see prerequisite (4) above), a process that is well known on Earth as “contact metamorphism.” Some of the magmatic angrites (see the Angrite Signature section) could be representative of such magmatic intrusions, whereas the intruded rocks could be peridotitic angrites. This would explain why only the latter rocks contain metamorphic coronas (see prerequisite (5) above).

Contact metamorphism in the parent body is the preferred hypothesis, as it matches all the six above prerequisites. Above all, it obeys the principle of uniformitarianism, as both the inferred cause (i.e., contact metamorphism) and its effects (i.e., the coronas) are known to occur on Earth. Hypothetically, the late fracturing could be related to the extraction of the rock from the angrite parent body during a shock.

It has been suggested that angrites represent ejecta from Mercury (e.g., Irving et al. 2005; Kuehner et al. 2006), but this assumption is debated (Hutson et al. 2007). Nonetheless, this study confirms that the angrite parent body was not an inert asteroid, but probably a planetary-sized body, which, in addition to differentiation (e.g., Baker et al 2005; Shukolyukov and Lugmair 2007; Weiss et al., 2008; Kleine et al. 2009, 2012) and magmatism (Sugiura et al. 2005; Kleine et al. 2009), shows now evidence for metamorphism. Contact metamorphism requires a pluton, which in turn requires some confining pressure, otherwise the magma would erupt to the surface of the parent body. Confining lithostatic pressure involves gravity, which incidentally is attested by the cumulate texture of Angra dos Reis (e.g., Ma et al. 1977; Prinz et al. 1977), and thus a parent body of a significant size.
## CONCLUSION

The coarse-grained peridotitic angrite NWA 3164 shows metamorphic coronas, which is exceptional in meteorites. These have grown during two metamorphic reactions (1) discontinuous reaction coronas of olivine and anorthite developed around spinel grains in contact with clinopyroxene, indicating the reaction $C_{px} + Spl \rightarrow Ol + An$ (R1); (2) irregular coronas of clinopyroxene and spinel are observed around primary anorthite crystals in contact with olivine, and at the contacts between the olivine and anorthite that developed during reaction R1, involving the reaction $Ol + An \rightarrow C_{px} + Spl$ (R2). R2 is almost the reverse of reaction R1 and clearly postdates it. Both reactions occurred in the solid state, producing coronitic microstructures well known in metamorphic terrestrial rocks, with slight zoning in the reacting minerals. The corresponding *P-T* metamorphic evolution, modeled by calculating a *P-T* pseudosection using the upgraded thermodynamic data set of Holland and Powell (1998), consisted first of equilibrium at $940 \pm 120 \,^\circ\text{C}$ at $P < 0.9 \,\text{GPa}$, followed by heating during reaction R1, with or without decompression and a peak *T* of 1000–1200 °C, and subsequent cooling during reaction R2. This metamorphic evolution is very probably inherited from the angrite parent body, which had already been differentiated when the coronas developed. Contact metamorphism in the parent body, in the vicinity of a hot magmatic intrusion, is the preferred cause. The evolution of the rock ended with a shock that produced an intense fracturing. In addition to differentiation and magmatism, there is now evidence for metamorphism in the angrite parent body, which should have been of a sufficient size to generate a significant amount of gravity.

*Acknowledgments*—The authors thank D. Badia and O. Boudouma for their technical assistance with the electron microprobe and SEM. We are grateful to Luigi Folco and an anonymous reviewer for their helpful comments.

*Editorial Handling*—Dr. Edward Scott

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