# Northwest Africa 10414, a pigeonite cumulate shergottite

R.H. HEWINS<sup>1,2*</sup>, B. ZANDA<sup>1,3</sup>, S. PONT<sup>1</sup>, and P.-M. ZANETTA<sup>4</sup>

<sup>1</sup>Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Sorbonne Université, Muséum National d'Histoire Naturelle, UPMC Université Paris 06, UMR CNRS 7590, IRD UMR 206, 61 rue Buffon, 75005 Paris, France

<sup>2</sup>Earth and Planetary Sciences, Rutgers University, Piscataway, NJ 08854, USA

<sup>3</sup>IMCCE, Observatoire de Paris, CNRS UMR 8028, 77 Av. Denfert Rochereau, 75014 Paris, France

<sup>4</sup>Université Lille, CNRS, UMR 8207, UMET, 59000 Lille, France

*Corresponding author. E-mail: hewins@scarletmail.rutgers.edu

*(Received 07 February 2018; revision accepted 22 July 2019)

**Abstract**—Northwest Africa (NWA) 10414 is an unusual shergottite with a cumulate texture. It contains 73% coarse prismatic pigeonite, plus 18% interstitial maskelynite, 2% Si-rich mesostasis, 2% merrillite, and minor chromite-ulvöspinel. It contains no olivine, and only ~3% augite. Phase compositions are pigeonite (En<sub>68-43</sub>Fs<sub>27-48</sub>Wo<sub>5-15</sub>) and maskelynite An<sub>~54-36</sub>, more sodic than most maskelynite in shergottites. Chromite-ulvöspinel composition plots between the earliest and most fractionated spinel-group minerals in olivine-phyric shergottites. NWA 10414 mineralogically resembles the contact facies between Elephant Moraine 79001 lithologic units A and B, with abundant pigeonite phenocrysts, though it is coarser grained. Its most Mg-rich pigeonite also has a similar composition to the earliest crystallized pyroxenes in several other shergottites, including Shergotty. The Shergotty intercumulus liquid composition crystallizes pigeonite with a similar composition range to NWA 10414 pigeonite, using PETROLOG. Olivine-phyric shergottite NWA 6234, with a pure magma composition, produces an even better match to this pigeonite composition range, after olivine crystallization. These observations suggest that after the accumulation of olivine from an olivine-phyric shergottite magma, the daughter liquid could precipitate pigeonite locally to form this pigeonite cumulate, before the crystallization of overlying liquid as a normal basaltic shergottite.

## INTRODUCTION

The SNC meteorites (shergottites, nakhlites, and chassignites) have long been recognized as planetary igneous rocks (Papanastassiou and Wasserburg 1974; Reid and Bunch 1975) from Mars (Bogard and Johnson 1983). The shergottites, the most abundant Martian meteorite type, are a chemically and mineralogically diverse set of basaltic rocks. They reflect significant variation in mantle source regions, magma transport mechanisms, and conditions of crystallization. While shergottites are critical to understanding Mars' evolution, they are complex. They are divided into three main categories—poikilitic, olivine-phyric, and basaltic (Goodrich 2002; Bridges and Warren 2006; Shearer et al. 2008; Papike et al. 2009; Walton et al. 2012), with three superimposed chemical types—enriched, intermediate,

and depleted in incompatible trace elements. There are several mineralogical or textural variants, for example, Northwest Africa (NWA) 6963 (Filiberto et al. 2018), NWA 7635 (Lapen et al. 2017), and NWA 8159 (Herd et al. 2017). A significant development is the recognition of Al-rich and Al-poor shergottite suites, each modified by fractional crystallization, and each having a wide range of incompatible element concentrations (Treiman and Filiberto 2015), that clarify the roles of mixing and fractionation of these magmas.

Stolper and McSween (1979) and McSween et al. (1979a, 1979b) proposed that olivine-saturated magmas were parental to the basaltic shergottite suite. Experimental studies of the basaltic shergottites Shergotty and Zagami were expected to produce coexisting pigeonite and augite at liquidus temperatures (McKay et al. 2000), but only pigeonite was observed as
their liquidus phase (Stolper and McSween 1979; McCoy and Lofgren 1999; McKay et al. 2000; Dann et al. 2001). Elephant Moraine (EET)-A 79001 is a key sample with an igneous contact between olivine-phyric facies A and basaltic shergottite facies B (McSween and Jarosewich 1983; Goodrich 2003; Van Niekerk et al. 2007), with a pigeonite-rich contact zone. Goodrich (2003) argued that EET-B was the fractionated liquid lost after olivine phenocrysts were accumulated in EET-A, and other olivine-phyric parent and basaltic daughter pairs have been proposed (Filiberto et al. 2012; Treiman and Filiberto 2015). In EET-B, pigeonite crystallized before augite, as in many shergottites, providing the possibility of making a pigeonite cumulate after olivine has settled. The ~1 cm wide pigeonite-rich contact facies of EET-B with EET-A may thus have formed by settling of pigeonite (Van Niekerk et al. 2007).

Northwest Africa 10414 is an unusual shergottite relevant to the relationship between olivine-phyric and basaltic shergottites. It contains pigeonite but not olivine and very little augite. The study of NWA 10414 clarifies the conditions of pigeonite crystallization and its role in the transition from olivine-phyric to basaltic shergottites.

## SAMPLE AND METHODS

An 8 g sample of NWA 10414 was cut at the Institut de Minéralogie, de Physique des Matériaux, et de Cosmochimie (IMPMC), Muséum National d'Histoire Naturelle (MNHN), to prepare a polished section numbered AK for study using reflected light microscopy, scanning electron microscopy (SEM), Raman microspectroscopy, and electron probe micro analysis (EPMA). A backscattered electron (BSE) map and images of selected regions were made at MNHN using a Tescan VEGA II LSU SEM in conventional mode (mainly 15 keV, <20 nA, and 15.4 mm working distance). High resolution images were made at the IMPMC quarters at the Université Paris VI, using a Zeiss Ultra 55 field emission (FE) SEM using 15 keV and 7.5 mm working distance.

Modal analysis was achieved using a simplified version of the ACADEMY method (Zanetta et al. 2019). We acquired Si, Fe, Mg, Ca X-ray maps of the entire section on the VEGA SEM with 4 µm pixels and prepared a phase map. Using Matlab®, we selected the pixels of similar composition to reveal the localization of the different grains. Reference grains were later used to classify all pixels by K-means clustering, using XMapTools (Lanari et al. 2014). The large map allowed us to select a smaller representative area, for analysis at greater resolution and with a greater number of counts/pixel. We then acquired a hyperspectral map that was

processed using Hyperspy (de la Peña et al. 2017). This map was acquired on a representative 3.62 mm² region with a resolution of 2048 × 2048 pixels (1.7 µm/pixel). Compositional endmembers for eight phases were extracted. To construct the phase map and obtain a higher number of counts, the signal was rebinned to a 250 × 250 pixel hyperspectral map (10 µm/pixel) which was still sufficient to resolve most of the smaller grains. A convolution of these different reference spectra was fitted to the raw data. This allowed us to construct a more precise phase map and to estimate mixing proportions in pixels using the scaling coefficient of the model components (Zanetta et al. 2019).

Minerals were characterized on the MNHN SEM with an SD³ (Bruker) EDS detector. We used an inVia confocal Raman microscope with a 532 and 785 nm laser sources at MNHN for distinguishing feldspar phases and silica phases. All quantitative mineral analyses were made by wavelength-dispersive spectrometry on the Cameca SXFive electron microprobe at the Université Paris VI equipped with an LaB₆ source, mainly using 15 keV and 10 nA, but 10 keV and 20 nA for some phosphates, with a focused beam or small rasters. Dwell time was 10 s for peaks and backgrounds, increased to 20 or 60 s for some minor elements, for example, Mn. We used silicate, oxide, and phosphate standards, and checked the calibrations with San Carlos and Eagle Station internal standards.

## PETROGRAPHY

Northwest Africa 10414 consists mainly of an aggregate of euhedral, highly fractured, weakly zoned pigeonite prisms, up to 4 × 1 mm in size, with a cumulate texture (Figs. 1a and b). The pigeonite prisms are set in interstitial maskelnite-rich mesostasis patches. Augite is seen in X-ray map (Fig. 2) as interstitial grains, small grains in mesostasis, and as inclusions in pigeonite. Si-rich inclusions and maskelnite inclusions are also seen within pigeonite. Some apparent inclusions of augite are shown to be interstitial between two crystals of pigeonite by differences in orientation of exsolution lamellae (Fig. 3a). Maskelnite laths have euhedral Ca-rich cores visible in enhanced BSE (Fig. 1b), and zonation, though in normal contrast the feldspar appears to be smooth unfractured maskelnite. Raman spectra show weak broad peaks near 500 and 600 cm⁻¹ characteristic of maskelnite (Fritz et al. 2005). The preservation of zoning suggests solid-state formation though the maskelnite flowed elsewhere, in shock melt pockets.

A low-resolution phase map generated from X-ray images of the section is shown in Fig. S1 in supporting information. A higher resolution phase map (with binned 1.7 µm pixels) of a representative 3.62 mm region was prepared (Fig. S2 in supporting information),
<figure><img src="image_1.png" /><figcaption>Fig. 1. Backscattered electron (BSE) images of a polished section of Northwest Africa 10414 (BSE) showing cumulate texture. a) Pigeonite (gray with light gray rims), chromite (white), merrillite needles (off-white), and interstitial maskelynite and mesostasis (both dark gray). b) Cumulus pigeonite (p, white), with interstitial maskelynite laths (m, medium gray) and Si-rich residual melt (s, dark gray) containing merrillite needles (a, white). The arrow shows the position of the pyroxene traverse of Fig. 5c.</figcaption></figure>

using the ACADEMY approach of Zanetta et al. (2019), and phase abundances were calculated, as shown in Table 1.

Of the minor phases, chromite-ulvöspinel occurs mainly as inclusions in pigeonite, and merrillite (~1%) mainly as blades in mesostasis, though it is also found within pigeonite. Apatite occurs as thin rims on some merrillite blades. Other accessory minerals include a

silica phase, alkali feldspar, ilmenite, baddeleyite, and pyrrhotite. The pigeonite contains small blocky or irregular Si-rich inclusions typically < 100 µm in size (Fig. 3b), both sodic and potassic, as well as ~100 nm exsolution lamellae. Based on the good stoichiometry of the pigeonite analyses, these lamellae must be augite. The mesostasis consists primarily of an intergrowth of 1–10 µm sodic plagioclase closely intergrown with a
<figure><img src="image_2.png" /><figcaption>Fig. 2. Combined elemental map for Mg (blue), Al (red), and Ca (blue) K X-rays, showing pigeonite (dark blue), maskelynite (orange), augite (light blue), merrillite (brown), Si-rich mesostasis (brown).</figcaption></figure>

silica phase, similar to that in NWA6963 (Filiberto et al. 2014) but finer grained (Fig. 3c). Ilmenite as well as baddeleyite have been identified in mesostasis, though they are more prominent in association with small SiO₂-rich inclusions within pigeonite margins against mesostasis (Fig. 3d). The rock is unaltered except for miniscule barite granules peppering one or two pigeonite grains, and isolated Fe-rich phases associated with local high Mn and Ba concentrations in shocked pigeonite, probably related to desert weathering.

The meteorite is moderately shocked, containing a few small (~200 µm) patches of shock melt at some pigeonite-maskelynite contacts (Fig. 4a). The melt is Si-rich and both P- and S-bearing, due to melting of pyroxene and feldspar, and accessory phases. It contains smooth anhedral grains of pyroxene and silica up to ~30 µm in size, as well as merrillite. Its groundmass is heterogeneous on a submicron scale (Fig. 4b), containing Fe-rich phase(s), and its texture resembles that of shock veins in Zagami (El Goresy et al. 2013). The silica grains are surrounded by micron-thick rims of denser glass. The pyroxene and merrillite may be unmelted relics, but silica grains of this size are rare in the unshocked regions suggesting that they were produced by the shock. Both high pressure polymorphs of silica and silica glass (lechatelierite) occur in shergottites (El Goresy et al. 2013). The silica in NWA 140414 lacks fractures and has

Raman spectra without characteristic peaks suggesting that it is glass. Stoichiometric maskelynite occurs as veins in the impact melt, which in turn is interfingered into the melt (Fig. 4a). Any mixing between the two is at a submicron level. Pigeonite adjacent to the shock melt contains lamellar to fibrous deformation features, with lamella orientation changing on a scale of 10–20 µm (Fig. 4c). This pigeonite contains pyrrhotite inclusions, and streaks of Mg-free silicate and Fe oxide or hydroxide parallel to exsolution lamellae ~100 nm in width (Fig. 4d).

## MINERAL COMPOSITIONS

Northwest Africa 10414 pyroxene has an Fe/Mn atomic ratio of 34.2 SD 3.0 (n = 75) compared with an average value of 32 SD 6 for Martian basalts (Papike et al. 2003). The data plot on or slightly below the best fit line on the Fe-Mn plot of Papike et al. (2009) for pyroxene-phyric basalts, confirming the Martian origin of NWA 10414. The pigeonite crystals are normally zoned with Ca, Fe, Mn, and Ti increasing, and with Mg and Cr decreasing toward the rims. Pigeonite core and rim composition ranges from En₆₈Fs₂₇Wo₅ to En₄₃Fs₄₈Wo₁₅, and for interstitial augite, it is En₄₃Fs₂₈ Wo₃₇ to En₃₈Fs₂₈Wo₃₄ (Fig. 5a; Table 2). The composition range across a single ~1 mm wide pigeonite prism is (En₆₅Fs₂₉Wo₆ to En₄₈Fs₄₁Wo₁₁) (Figs. 5b and 5c; Table 2) matching the middle of the range for random grains given above traverse across this pigeonite prism revealed a ~15 µm inclusion of orthopyroxene in the interior and one of augite near the rim, neither of which could be distinguished in BSE. These compositions are indicated in the quadrilateral Fig. 5b and the traverse Fig. 5c.

The plagioclase (maskelynite) composition is An$_{46.7}$ ± 3.9Ab$_{50.8}$ ± 3.4Or$_{2.5}$ ± 0.6 (n = 111), with a range of An$_{~54–38}$ (Fig. 6a; Table 2). Individual maskelynite pseudomorphs after plagioclase laths are slightly zoned and differ from one another (Fig. 6b). Most shergottites contain more calcic early formed plagioclase than this (Basu Sarbadhikari et al. 2009). This is shown in Fig. 6a, for Shergotty and Zagami (Stolper and McSween 1979), EET 79001A, EET 79001B, and QUE 94201 (Mikouchi et al. 1998, 1999), NWA 1068 (Barrat et al. 2002b), NWA 2990 (Bunch et al. 2009), NWA 5718 (Wittke et al. 2010), NWA 5789 (Gross et al. 2011), NWA 6234 (Gross et al. 2013), and Los Angeles (Warren et al. 2004). The exceptions with less calcic early formed plagioclase are the enriched olivine-phyric shergottites NWA 1068 (Barrat et al. 2002b) and LAR 06319 (Basu Sarbadhikari et al. 2009). NWA 6963 is similar, though it contains rare calcic grains (Filiberto et al. 2014).
<figure><img src="image_3.png" /><figcaption>Fig. 3. Backscattered electron images of Northwest Africa 10414. a) Interstitial augite grains between two crystals of pigeonite. b) Na-KSi-rich inclusion containing three ilmenite needles and phosphate specks, in pigeonite with ~200 nm augite lamellae. c) Interstitial mesostasis with an intergrowth of sodic maskelynite and a silica phase, plus baddeleyite and ilmenite specks. d) Pigeonite (with fine SiO₂ inclusions, and ilmenite with baddeleyite), mesostasis (left), and merrillite (right). A is augite, B baddeleyite, I ilmenite, Mk maskelynite in mesostasis, P pigeonite, Ph merrillite, S silica.</figcaption></figure>

<table><caption>Table 1. Modal abundances.</caption><tr><td>Pigeonite</td><td>72.58%</td></tr><tr><td>Maskelynite</td><td>17.93%</td></tr><tr><td>Augite</td><td>3.41%</td></tr><tr><td>Chromite-ülvospinel</td><td>0.39%</td></tr><tr><td>Merrillite</td><td>2.03%</td></tr><tr><td>Pyrrhotite</td><td>0.21%</td></tr><tr><td>Si-rich mesostasis</td><td>1.92%</td></tr><tr><td>Shock melt</td><td>1.54%</td></tr><tr><td>Total</td><td>100.00%</td></tr></table>

Chromite-ülvöspinel compositions in NWA 10414 range from ~Spl₂₁Chr₆₅Usp₆Mag₈ to ~Spl₄Chr₁Usp₄₉ Mag₄₆ (Table 2). They are compared in Fig. 7 to those for Tissint and 10 other olivine-phyric shergottites (Allan Hills (ALH) 77005, Dar el Gani (Dag) 476, Dhofar (Dho) 019, Elephant Moraine (EET) 79001A, Larkman Nunatak (LAR) 06319, NWA 1068, NWA 1110, NWA 5789, NWA 6234, and Sayh al Uhaymir (SaU) 005). Olivine-phyric shergottite data shown are

from Mikouchi et al. (2001), Barrat et al. (2002b), Goodrich et al. (2003), Basu Sarbadhikari et al. (2009), and Gross et al. (2011, 2013) and we show separately our own data for Tissint (Chennaoui Aoudjehane et al. 2012). NWA 10414 oxide minerals are not quite as Cr-rich as chromites crystallized along with olivine in other shergottites (e.g., Goodrich et al. 2003) and are continuously fractionated to almost Cr-free compositions. Their Fe and Ti fractionation trend involves strong enrichment in Fe³⁺, as for most olivine shergottites but is unlike the trend more toward the ulvöspinel endmember for Tissint, Dar el Gani 476 (Mikouchi et al. 2001), and NWA 5789 (Gross et al. 2011). In basaltic shergottites, the spinel phase is mainly Cr-poor titanomagnetite (Stolper and McSween 1979; Goodrich et al. 2003) except in those with liquidus magnesian pyroxene like NWA 480 (Barrat et al. 2002a).

The ilmenite is near-endmember Ilm₉₉.₉₄Hem₀₁.₀₆. Since ilmenite coexisting with magnetite normally increases its Ti content with decreasing temperature
<figure><img src="image_4.png" /><figcaption>Fig. 4. Backscattered electron images. a) Patch of heterogeneous shock melt (melt) between pigeonite grains and maskelynite, with silica inclusions (black) and interfingering or veining of the P- and S-bearing silicate melt with mobilized maskelynite. b) Groundmass of shock melt pocket. c) Pigeonite adjacent to shock melt is lamellar to fibrous and deformed, with pyrrhotite inclusions, streaks of Mg-free silicate, and Fe oxide or hydroxide. d) Pigeonite contains exsolution lamellae ~100 nm in width. Symbols as before.</figcaption></figure>

(e.g., Buddington and Lindsley 1964), we assumed the most Ti-poor ilmenite coexisted in the groundmass with the most ferrian titanomagnetite ~Sp14Chr1Usp49Mag46. Such an assemblage would indicate equilibration at 825 °C and an oxygen fugacity (fO₂) of FMQ-0.3 log units, with standard deviations of 6 °C and 0.2 log units based on the composition range observed, calculated using the method of Sauerzapf et al. (2008). This temperature is consistent with oxide assemblages that equilibrated at ~800 °C in many shergottites (Herd et al. 2001). The calculated fO₂ is high for shergottites but similar to values for Shergotty (Herd et al. 2001), NWA 5298 (Hui et al. 2011), and a late stage assemblage in NWA 1110 (Herd 2003). Crystallization modeling (below) suggests that the pigeonite crystallized at a lower fO₂.

Among accessory phases, pyrrhotite in NWA 10414 occurs as small anhedral grains in mesostasis. Its composition is Fe₂_0.96S₁. Apatite rims on merrillite, calculated on a 3 P atomic basis, contain on average 0.40% F, 0.25% Cl, leaving an assumed 0.35% OH.

The mesostasis is seen in the SEM to be a micrographic intergrowth (Fig. 5b) like that in NWA 6963 (Filiberto et al. 2014) though the electron probe excitation

volume causes a bulk composition resembling Si-rich glass. In rare cases, the analysis resolved the silica phase, but generally the results are interpreted as mixtures of silica and 40-80% feldspar. The Si and Al contents for bulk mesostasis define mixing lines, which we extrapolate to pure silica plus extremes of plagioclase Ab~68 and alkali feldspar Or~66 but also to intermediate feldspar.

## DISCUSSION

### Crystallization of NWA 10414 Compared to That of Other Shergottites

Understanding relationships between shergottites, and particularly olivine-phyric and pyroxene-rich shergottites, requires a knowledge of liquid compositions. Shergotty and Zagami were recognized as phenocryst-enriched basaltic rocks rather than as crystallized parent liquids (McSween et al. 1979a; Stolper and McSween 1979; Hale et al. 1999), but NWA 480 (Barrat et al. 2002a) and QUE 94201 may have the compositions of the liquids from which they crystallized (McSween et al. 1996; Mikouchi et al. 1998; Barrat et al. 2002a; Filiberto and Dasgupta 2011; Gross
<figure><img src="image_5.png" /><figcaption>Fig. 5. Pigeonite compositions in Northwest Africa 10414. a) Pyroxene quadrilateral for pigeonite random cores and rims, and augite. b) Analyses of crystal 107; the outlier points are within the crystal. c) Traverse across pyroxene crystal 107 to show zoning; it includes a few points of augite and orthopyroxene. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

et al. 2013). In the case of olivine-phyric shergottites, Fe-Mg partitioning between olivine and melt has been used to tell whether olivine was in equilibrium with a liquid of bulk meteorite composition (e.g., Filiberto and Dasgupta 2011). Yamato 980459 (Greshake et al. 2004; Mikouchi et al. 2004; Musselwhite et al. 2006), NWA 5789 (Gross et al. 2011), NWA 2990 (Bunch et al. 2009; Filiberto and Dasgupta 2011), and NWA 6234 (Gross et al. 2013) have bulk compositions that could represent liquid compositions, and many other shergottites have accumulated excess olivine and/or pyroxene.

The silicate crystallization sequences in shergottites have olivine, if present, generally followed by sequential or simultaneous crystallization of two or three pyroxene

phases, and then plagioclase (Stolper and McSween 1979; Papike et al. 2009). There is evidence of a Ca-poor pyroxene phase preceding augite, and in part following it (Stolper and McSween 1979; Mikouchi et al. 1999; Van Niekerk et al. 2007; Papike et al. 2009; Herd et al. 2017).

Northwest Africa 10414 differs from other basaltic shergottites in that it has only one major pyroxene, pigeonite. It is a plagioclase-rich pyroxenite or pigeonite orthocumulate with only ~5% augite and no olivine. It has the classic orthocumulate texture as defined by Wager et al. (1960), with limited adcumulus growth on the subhedral pigeonite crystals. It consists mainly of an aggregate of euhedral, highly fractured, weakly zoned
<table><caption>Table 2. Electron microprobe analyses of pyroxene, spinel, and maskelynite in NWA 10414.</caption><thead><tr><th>Pyroxene</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>FeO</th><th>MnO</th><th>MgO</th><th>CaO</th><th>Na<sub>2</sub>O</th><th>Total</th><th>En</th><th>Fs</th><th>Wo</th></tr></thead><tbody><tr><td>Pigeonite 61</td><td>53.98</td><td>0.76</td><td>0.12</td><td>0.38</td><td>17.07</td><td>0.60</td><td>24.10</td><td>2.38</td><td>0.04</td><td>99.50</td><td>68.10</td><td>27.06</td><td>4.83</td></tr><tr><td>Pigeonite 81</td><td>53.34</td><td>0.84</td><td>0.14</td><td>0.26</td><td>18.55</td><td>0.61</td><td>21.65</td><td>3.70</td><td>0.10</td><td>99.24</td><td>62.37</td><td>29.98</td><td>7.65</td></tr><tr><td>Pigeonite 1</td><td>52.78</td><td>1.03</td><td>0.10</td><td>0.22</td><td>19.90</td><td>0.59</td><td>20.79</td><td>4.14</td><td>0.05</td><td>99.68</td><td>59.52</td><td>31.95</td><td>8.53</td></tr><tr><td>Pigeonite 39</td><td>52.84</td><td>0.91</td><td>0.33</td><td>0.19</td><td>22.29</td><td>0.68</td><td>17.82</td><td>5.41</td><td>0.11</td><td>100.61</td><td>52.09</td><td>36.54</td><td>11.37</td></tr><tr><td>Pigeonite 22</td><td>51.17</td><td>0.73</td><td>0.46</td><td>0.00</td><td>24.86</td><td>0.69</td><td>15.71</td><td>5.38</td><td>0.06</td><td>99.12</td><td>46.85</td><td>41.61</td><td>11.54</td></tr><tr><td>107 C 53<sup>a</sup></td><td>53.55</td><td>0.97</td><td>0.02</td><td>0.32</td><td>18.28</td><td>0.51</td><td>23.06</td><td>2.96</td><td>0.07</td><td>99.77</td><td>65.08</td><td>28.92</td><td>6.00</td></tr><tr><td>107 M 36</td><td>53.32</td><td>0.86</td><td>0.17</td><td>0.29</td><td>19.55</td><td>0.62</td><td>21.28</td><td>3.88</td><td>0.07</td><td>100.08</td><td>60.72</td><td>31.29</td><td>7.99</td></tr><tr><td>107 M 8</td><td>52.96</td><td>0.90</td><td>0.17</td><td>0.32</td><td>20.92</td><td>0.61</td><td>20.30</td><td>4.20</td><td>0.08</td><td>100.44</td><td>57.91</td><td>33.47</td><td>8.62</td></tr><tr><td>107 R 82</td><td>50.76</td><td>1.39</td><td>0.23</td><td>0.38</td><td>24.40</td><td>0.57</td><td>15.88</td><td>5.17</td><td>0.09</td><td>98.94</td><td>47.74</td><td>41.12</td><td>11.14</td></tr><tr><td>Augite 23</td><td>51.32</td><td>2.13</td><td>0.38</td><td>0.64</td><td>13.47</td><td>0.40</td><td>14.25</td><td>16.36</td><td>0.20</td><td>99.46</td><td>42.46</td><td>22.52</td><td>35.03</td></tr><tr><td>Augite 24</td><td>51.21</td><td>2.30</td><td>0.50</td><td>0.67</td><td>13.16</td><td>0.39</td><td>14.21</td><td>17.03</td><td>0.20</td><td>99.89</td><td>42.01</td><td>21.82</td><td>36.17</td></tr><tr><td>Augite 3</td><td>50.98</td><td>1.55</td><td>0.43</td><td>0.58</td><td>14.50</td><td>0.40</td><td>14.16</td><td>16.30</td><td>0.22</td><td>99.51</td><td>41.64</td><td>23.92</td><td>34.44</td></tr><tr><td>Augite 4</td><td>51.49</td><td>1.09</td><td>0.96</td><td>0.21</td><td>16.38</td><td>0.46</td><td>13.13</td><td>16.54</td><td>0.20</td><td>100.88</td><td>38.39</td><td>26.86</td><td>34.75</td></tr><tr><td>Augite 25</td><td>51.42</td><td>0.98</td><td>0.79</td><td>0.20</td><td>17.09</td><td>0.59</td><td>12.80</td><td>15.98</td><td>0.23</td><td>100.27</td><td>37.80</td><td>28.30</td><td>33.90</td></tr><tr><th>Spinel</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>FeO</th><th>MgO</th><th>Total</th><th>Spl</th><th>Chr</th><th>Usp</th><th>Mag</th></tr><tr><td>62/1</td><td>0.13</td><td>10.04</td><td>2.09</td><td>46.34</td><td>33.97</td><td>4.07</td><td>96.67</td><td>20.99</td><td>65.02</td><td>5.61</td><td>8.37</td></tr><tr><td>67/1</td><td>0.11</td><td>5.39</td><td>6.30</td><td>38.75</td><td>44.19</td><td>2.30</td><td>97.05</td><td>11.60</td><td>55.96</td><td>17.29</td><td>15.15</td></tr><tr><td>17</td><td></td><td>3.13</td><td>15.60</td><td>21.58</td><td>55.44</td><td>1.50</td><td>97.25</td><td>6.86</td><td>31.65</td><td>43.54</td><td>17.95</td></tr><tr><td>16</td><td></td><td>3.38</td><td>17.31</td><td>16.12</td><td>58.96</td><td>1.52</td><td>97.30</td><td>7.36</td><td>23.56</td><td>48.14</td><td>20.94</td></tr><tr><th>Maskelynite</th><th>SiO<sub>2</sub></th><th>Al<sub>2</sub>O<sub>3</sub></th><th>TiO<sub>2</sub></th><th>Cr<sub>2</sub>O<sub>3</sub></th><th>FeO</th><th>MgO</th><th>CaO</th><th>Na<sub>2</sub>O</th><th>K<sub>2</sub>O</th><th>Total</th><th>An</th><th>Ab</th><th>Or</th></tr><tr><td>20</td><td>54.29</td><td>28.04</td><td>0.04</td><td></td><td>0.75</td><td>0.08</td><td>11.10</td><td>5.12</td><td>0.30</td><td>99.85</td><td>53.56</td><td>44.75</td><td>1.70</td></tr><tr><td>33</td><td>55.50</td><td>27.73</td><td>0.07</td><td></td><td>0.50</td><td>0.06</td><td>10.09</td><td>5.64</td><td>0.33</td><td>99.91</td><td>48.78</td><td>49.34</td><td>1.88</td></tr><tr><td>14</td><td>56.01</td><td>27.04</td><td>0.08</td><td></td><td>0.58</td><td>0.03</td><td>9.70</td><td>5.80</td><td>0.48</td><td>99.76</td><td>46.71</td><td>50.55</td><td>2.74</td></tr><tr><td>10</td><td>56.99</td><td>26.34</td><td>0.14</td><td></td><td>0.73</td><td>0.03</td><td>8.91</td><td>6.19</td><td>0.53</td><td>100.04</td><td>42.96</td><td>53.99</td><td>3.04</td></tr><tr><td>74b-22C<sup>a</sup></td><td>55.96</td><td>28.17</td><td>0.11</td><td></td><td>0.53</td><td>0.07</td><td>10.69</td><td>5.26</td><td>0.27</td><td>101.21</td><td>52.05</td><td>46.37</td><td>1.58</td></tr><tr><td>74b-13R</td><td>58.35</td><td>25.87</td><td>0.04</td><td></td><td>0.51</td><td>0.04</td><td>8.72</td><td>6.23</td><td>0.55</td><td>100.39</td><td>42.24</td><td>54.58</td><td>3.17</td></tr><tr><td>2-2C</td><td>56.15</td><td>27.19</td><td>0.11</td><td></td><td>0.56</td><td>0.04</td><td>10.02</td><td>5.64</td><td>0.44</td><td>100.24</td><td>48.31</td><td>49.18</td><td>2.51</td></tr><tr><td>32-8R</td><td>58.12</td><td>26.23</td><td>0.27</td><td></td><td>0.60</td><td>0.04</td><td>8.23</td><td>6.57</td><td>0.63</td><td>100.80</td><td>39.42</td><td>56.99</td><td>3.59</td></tr><tr><td>74a-1C</td><td>57.38</td><td>26.52</td><td>0.11</td><td></td><td>0.64</td><td>0.05</td><td>9.28</td><td>6.08</td><td>0.52</td><td>100.72</td><td>44.42</td><td>52.63</td><td>2.95</td></tr><tr><td>74b-5R</td><td>59.33</td><td>25.56</td><td>0.07</td><td></td><td>0.53</td><td>0.07</td><td>7.84</td><td>6.66</td><td>0.60</td><td>100.75</td><td>38.03</td><td>58.52</td><td>3.45</td></tr></tbody></table>

pigeonite prisms, up to 4 × 1 mm in size (Fig. 4a). The texture is more like that of a coarse-grained shergottite such as NWA 6963 (Filiberto et al. 2014) than that of most other shergottites, but NWA 6963 contains much more abundant augite than NWA 10414. The key question is why the augite found in other basaltic shergottites is present only as a minor interstitial phase. To investigate the origin of this cumulate, we compare its oxide and pyroxene compositions to those of other shergottites, and its earliest pyroxenes to pigeonite crystallized in calculations or experiments with potential parent liquids.

The chromite-ulvöspinel of olivine-phyric shergottites shown in Fig. 7 have a wide range of composition, from Cr-rich (associated with early olivine) to Cr-poor close to or on the Usp-Mag join (in their groundmass or mesostasis). Basaltic shergottites contain Ti-magnetite rather than chromite, except those

with liquidus magnesian pyroxene like NWA 480 (Barrat et al. 2002a). NWA 10414 chromite-ulvöspinel is intermediate within this range with few analyses plotting near the Chr apex, and few near the Ti-Fe join (Fig. 7). The rock is thus intermediate between most olivine-phyric and basaltic shergottites. In addition, its Usp/Mag ratio resembles that of enriched and intermediate shergottites rather than that of the depleted shergottites Tissint and NWA 5789 (Fig. 7).

Pyroxene fractionation patterns reveal many differences between shergottites, as shown in the assembled pyroxene quadrilaterals of Mikouchi et al. (1999, fig. 3), Papike et al. (2009, fig. 10), and Meyer (2012). There are two styles of pyroxene crystallization in the basaltic shergottites (Mikouchi et al. 1999), (1) simple Fe enrichment in coexisting grains of pigeonite and augite and (2) sequential growth of magnesian pigeonite (or orthopyroxene) followed by augite mantles and ferroan

<footnote><sup>a</sup>C, M, and R refer to core, mantle, and rim. Blank entries indicate below detection limit.</footnote>
<figure><img src="image_6.png" /></figure>

Fig. 6. Plagioclase (maskelynite) compositions in mol % An versus Or. a) Total range for Northwest Africa (NWA) 10414 compared to early- and late-crystallized plagioclase (cores and rims, or extremes of range) in other shergottites (see text for details). b) Eight individual grains in NWA 10414 differ in composition. (Color figure can be viewed at wileyonlinelibrary.com.)

pigeonite rims on composite crystals. These trends are shown schematically in pyroxene quadrilaterals (Figs. 8a and 8b) based on data for Shergotty (Hale et al. 1999) and LAR 06319 (Basu Sarbadhikari et al. 2009; Peslier et al. 2010). Many of the basaltic shergottites like the classic Shergotty follow the first pattern (Papike et al. 2009; Meyer 2012). Others show a Ca enrichment in pigeonite followed by augite mantling (McSween et al. 1996; Mikouchi et al. 1998; Barrat et al. 2002a; Mikouchi and Barrat 2009); they have a complex pyroxene zoning trajectory, which is like that in olivine-phyric shergottites (Papike et al. 2009; Meyer 2012), and some Apollo 12 mare basalts (Kushiro et al. 1971), Apollo 15 quartz-normative basalts (Grove and Bence 1977), and terrestrial komatiites (Arndt and Fleet 1979). These differences in crystallization patterns were attributed by Mikouchi et al. (1999) to (1) near-equilibrium crystallization with

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

Fig. 7. The Cr-Ti spinel in Northwest Africa 10414 does not have the most Cr-rich compositions found in olivine-phyric shergottites, and it extends to the late Ti-magnetite typical of basaltic shergottites. See text for sources of shergottite data. (Color figure can be viewed at wileyonlinelibrary.com.)

no significant undercooling, and (2) rapid disequilibrium crystallization of undercooled magmas to give the complex zoning. A similar interpretation was used for the terrestrial and lunar occurrences.

The pyroxene in NWA 10414 is also broadly intermediate in composition between that of olivine-phyric and olivine-free shergottites, with the liquidus pigeonite Mg# higher than that in some basaltic shergottites and lower than that in many olivine-phyric shergottites. It shows modest enrichment toward the rims, in both Ca and Fe as in many olivine-phyric shergottites (Papike et al. 2009). However, the liquidus pigeonite composition in NWA 10414 (En68Fs27Wo5) is not unique; the earliest crystallized pigeonites in some shergottites, both basaltic and olivine-phyric, are very similar (Fig. 8c). The most magnesian pyroxenes for Shergotty (En68Fs24Wo8, Hale et al. 1999, fig. 3), EETA 79001B (En70Fs21Wo9, Van Niekerk et al. 2007, fig. 6), NWA 1068 (En72Fs23Wo5, Barrat et al. 2002b), NWA 6234 (En70Fs26Wo4, Gross et al. 2013, NWA 6234), and LAR 06319 (En71Fs25Wo4, Basu Sarbadhikari et al. 2009; Peslier et al. 2010) are all plotted as diamonds close to the most Mg-rich pigeonite in NWA 10414 (Fig. 8c). The above earliest pigeonites tend to be too poor in Ca and too magnesian to be in equilibrium with augite at liquidus temperatures for shergottite (Huebner 1980; Lindsley 1983). NWA 10414 pigeonite shows modest enrichment in both Fe and Ca, with rim pigeonite only slightly
<figure><img src="image_8.png" /><figcaption>Fig. 8. Shergottite pyroxenes and Northwest Africa (NWA) 10414 pigeonite (a) co-crystallization of pigeonite and augite; schematic, after Hale et al. (1999). b) Complex zoning sequence in LAR06319; schematic, after Basu Sarbadhikari et al. (2009) and Peslier et al. (2010). c) Comparison of NWA 10414 pigeonite with the earliest (most magnesian) pigeonite in Shergotty, NWA 1068, NWA 6234, Elephant Moraine 79001B, and LAR 06319, all plotted as diamonds. See text for sources of data. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

enriched in Ca in equilibrium with interstitial augite. Pigeonites in many other shergottites are either strongly enriched in Ca to augite mantles or in Fe to ferropigeonite. Among the shergottites with similar early pigeonite compositions, NWA 10414 most resembles the contact facies between EETA 79001 A and B in phase abundances: this zone has more pigeonite phenocrysts than the more evolved pyroxene assemblage in the main part of lithologic unit B (Goodrich 2003; Van Niekerk et al. 2007).

## Crystallization Experiments and Modeling

Reproducing the crystallization of shergottites by experiment or modeling depends on knowing the parent liquid composition, which is difficult because most meteorites consist of liquid plus accumulated crystals. We consider possible liquid compositions for basaltic shergottites with similarities to NWA 10414. Crystallization experiments were performed by McKay et al. (2000) and by Dann et al. (2001) on SILC (Shergotty intercumulus liquid composition) calculated by subtracting pyroxene core composition from bulk composition for Shergotty (Hale et al. 1999). The experiments used a combined range of 1 bar to 2 kb with an oxygen fugacity relative to the fayalite-magnetite-quartz buffer (FMQ) of -1 to +1 log units. Dann et al. (2001) estimated the crystallization pressure as 0.56 kb, as discussed below. For comparison purposes, we refer to experiments at 1 bar and FMQ, used in both studies. Pigeonite crystallizes from 1200 to 1110 °C (McKay et al. 2000, fig. 2) and 1175-1075 °C (Dann et al. 2001), where it is joined by augite. The composition of its liquidus pyroxene is En<sub>67</sub>Fs<sub>28</sub>Wo<sub>5</sub> (Dann et al. 2001) or En<sub>66</sub>Fs<sub>30</sub>Wo<sub>4</sub> (McKay et al. 2000), and it is very close to that of the most magnesian pigeonite compositions in NWA 10414 with a similar composition range (Fig. 8c).

Stolper and McSween (1979) and McCoy and Lofgren (1999) also showed experimentally that pigeonite crystallized before augite in melts with the compositions of bulk Shergotty and Zagami. Although pigeonite and augite are widely perceived as joint liquidus phases in such “basaltic” shergottites, they could have equilibrated mutually after the precipitation of augite joined pigeonite and after accumulation in the residual magma (Stolper and McSween 1979). The survival of magnesian pigeonite too poor in Ca to be in equilibrium with the augite in Shergotty (Hale et al. 1999) is consistent with this idea. McKay et al. (2000) considered their failure to produce the expected simultaneous crystallization of pigeonite and augite to be a paradox. However, as noted above, in some basaltic shergottites, pigeonite does crystallize before augite, particularly in those with the disequilibrium trend but even in Shergotty (Hale et al. 1999, fig. 4). Thus, we judge the experiments not to be paradoxical in the context of shergottites and other shergottite crystallization experiments, but their starting compositions need minor modification for Shergotty (more CaO, Dann et al. 2001) to accelerate the appearance of augite after pigeonite crystallization.

We modeled the crystallization of a number of shergottites at pressures between 1 bar and 2 kb, and oxygen fugacities between FMQ and FMQ-2 using
PETROLOG (Danyushevsky and Plechov 2011). This program calculates pseudo-liquidus temperatures using mineral-melt equilibria models for all possible minerals. We used the optional models of Sack et al. (1980) for melt oxidation state and Ariskin et al. (1993) for silicate mineral-melt equilibria. Calculated pseudo-liquidus temperatures are compared to determine which phase(s) should crystallize. Where PETROLOG indicated that orthopyroxene and pigeonite come onto the liquidus at the same temperature within error, we chose to crystallize pigeonite rather than orthopyroxene. We searched for liquids with pigeonite crystallizing alone, capable of forming cumulates, after the crystallization of any olivine and before the appearance of augite (or plagioclase). Although pressure changed crystallization temperature and Al content of pyroxene, it had a trivial effect on the calculated major element composition of pyroxene, so here we report only 1 bar calculations. The effect of a change of 2 log units in oxygen fugacity was ~1–4 mol% in Fs, with more ferroan pyroxene at the lower fO₂. We report below results for FMQ-2, except where otherwise stated, because they come closer to the composition of the pigeonite in NWA 10414 for the liquid compositions available.

We used PETROLOG first with basaltic shergottite compositions, starting with Zagami, and later with olivine-phyric shergottite potential parent melts, to search for extensive crystallization of pigeonite alone. The results are summarized in Figs. 9 and 10. We evaluated the use of PETROLOG for shergottites by comparing results for the crystallization of Zagami to those from experiments at 1 bar and FMQ (McCoy and Lofgren 1999). The liquidus temperatures are 1251 °C (PETROLOG) and 1255–1260 °C (McCoy and Lofgren); the first pigeonite compositions are FeO 14.1 versus 13.6 wt%, MgO 26.2 versus 26.8 wt%, and CaO 3.7 versus 2.5 wt%, respectively; the first augite appeared at 1169 °C versus 1175 °C, respectively. Although this confirms the utility of PETROLOG, both these pigeonite compositions are more magnesian than natural pigeonite compositions for Zagami (McCoy and Lofgren 1999) and NWA 10414 (Fig. 9a). The same is true for results shown in Figs. 9a and 9b for an oxygen fugacity of FMQ-2, probably because the bulk composition is not a true liquid composition (Stolper and McSween 1979).

Elephant Moraine A 79001B is a logical candidate for the parent liquid of NWA 10414, because of its pigeonite-rich border facies discussed above and their similar liquidus pigeonite compositions. Analyses of three relatively small samples (McSween and Jarosewich 1983; Warren et al. 1999; Arauza et al. 2010) gave slightly different compositions. We found using PETROLOG that they have different crystallization

sequences, pigeonite then augite, plagioclase then augite, and pigeonite then plagioclase, respectively. These differences are easily understood because the EETA 79001B bulk composition(s) plot very close to the pigeonite-augite-plagioclase invariant point (Stolper and McSween 1979; Arauza et al. 2010; Jones and Arauza 2010). Furthermore, Van Niekerk et al. (2007) observed different crystallization sequences as a function of the distance from the contact with lithology A, underlining the difficulty of defining the parent liquid composition. Two of these composition estimates (McSween and Jarosewich 1983; Warren et al. 1999) are capable of producing a pigeonite cumulate, though only over a temperature range of ~30 °C. The McSween and Jarosewich composition is shown in Fig. 9a, because its cumulate would have interstitial plagioclase formed after than augite, as in NWA 10414. However, the solitary pigeonite calculated for this composition crystallized over a temperature range of only 32 °C (Fig. 9a) and is too restricted in Fe/Mg and too calcic compared to that in NWA 10414 (Fig. 10a).

We modeled the crystallization of the Shergotty parent SILC using PETROLOG and compared the results to those of MELTS calculations (Hale et al. 1999, fig. 5a). For 1 bar and FMQ, we have pigeonite on the liquidus at ~1201 °C (MELTS) and 1206 °C (PETROLOG), while pigeonite formed between 1190 and 1075 °C in the experiments of Dann et al. (2001). Augite appears at about 1121 °C (MELTS) as opposed to 1143 °C (PETROLOG), too early to match the augite-free composition range of pigeonite in NWA 10414. Crystallization with FMQ-2 matches much better, with all except the most magnesian compositions reproduced (Fig. 10a).

Second, we considered the parent melts of olivine-phyric shergottites. Fractional crystallization of EETA79001A groundmass composition E<sub>g</sub> (McSween and Jarosewich 1983; Longhi and Pan 1989) yielded 15% olivine followed by 11% pigeonite, before the onset of augite at 1163 °C. The hypothetical pigeonite cumulate that could be formed has a composition that is too calcic and too restricted in Fe/Mg ratio to be a good match to NWA 10414 (Fig. 10b).

Northwest Africa 6234 is an olivine-phyric shergottite containing olivine microphenocrysts, pigeonite in the groundmass, and late interstitial augite, interpreted as a ferroan primary magma crystallized at upper crustal pressures (Gross et al. 2013). Modeling its crystallization using PETROLOG with FMQ + 2, a value between the extremes calculated for different mineral assemblages by Gross et al. (2013), yields a good match to the observed olivine and Ca-poor pyroxene compositions (Fig. S3 in supporting information). Applying the results to the crystallization
<figure><img src="image_9.png" /></figure>

Fig. 9. Bar chart giving temperature ranges for the crystallization of olivine, pigeonite, augite, and plagioclase calculated using PETROLOG for 1 bar and fayalite-magnetite-quartz buffer-2 (a) olivine-free shergottites and (b) olivine-phyric shergottites. The estimated temperature range for Northwest Africa (NWA) 10414 pigeonite cores (open circles) and rims (dots) based on the calculated temperature-composition correlation for NWA 6234 pyroxene is also shown in (b). The rectangles indicate temperature windows with neither olivine nor augite, where formation of a pigeonite cumulate is possible. Details of starting compositions given in text. (Color figure can be viewed at wileyonlinelibrary.com.)

of NWA10414, we find that the calculated pyroxene range also matches its pigeonite core compositions very well (Fig. 10b), better than with SILC.

Based on the similarity of the pigeonite compositions, we can estimate the temperature range of pigeonite in NWA 10414 using the regression line for En versus T °C from calculations on NWA 6234. This range (1261–1084 °C) is plotted in Fig. 10b for comparison with the temperature windows for solitary pigeonite crystallization calculated with PETROLOG for the other shergottites. We see that rim pigeonite (dots) continued to crystallize at similar temperatures to augite in many shergottites. The only obvious difference between calculated compositions for the first liquids saturated in pigeonite is that those in SILC and NWA 6234 had <10% CaO and those not reproducing our

pyroxene composition range had >10% CaO, which tended to produce more calcic and more magnesian pigeonite before the appearance of augite.

Dann et al. (2001) constructed a phase diagram for SILC as a function of P<sub>H2O</sub> (water pressure) and temperature (their fig. 1). This shows that in order to have pigeonite crystallize before augite, a low water pressure is required. Pigeonite crystallizes alone for ~60–95 °C at 0–0.5 kb, from this Shergotty parent liquid. Dann et al. (2001) used their high pressure phase compositions to estimate a crystallization pressure of Shergotty’s two-pyroxene assemblage as 0.56 kb. As NWA 10414 has much the same range of pigeonite compositions as in the 1 bar experiments of Dann et al. (2001) over an estimated temperature range of 170 °C, with no augite present, it probably crystallized at near-surface conditions.

The pyroxene minor element data for equilibrium crystallization experiments on the SILC composition (Dann et al. 2001) also show that formation at water pressures of 1 bar and 2 kb cannot be distinguished using Al and Ti concentrations in pigeonite (Fig. S4 in supporting information). Ti/Al ratios in pyroxenes are affected by growth rate and co-crystallizing phases. A lunar quartz-normative basalt composition crystallized in the laboratory yielded pigeonite with a very similar composition range to that for NWA 10414 (Grove and Bence 1977). The Ti/Al ratios for their pigeonite crystallized at cooling rates between 1.75 and 60 °C/h deviate from the equilibrium value, increasing toward unity as the quench temperature is decreased, because plagioclase forms, incorporating Al. Pigeonite cores for NWA 10414 pigeonite have Ti/Al ratios similar to those for equilibrium crystallization of the lunar basalt. However, pigeonite rims have much higher ratios up to 0.8 (Fig. S2), also probably due to fractionation during rapid crystallization, consistent with the localized incorporation of small plagioclase crystals in the rims in a weak subophitic texture.

## Formation of Pigeonite Cumulate Shergottite

Pigeonite is a Ca-poor pyroxene that has undergone the displacive transformation to the P₂₁/c space group and that on Earth occurs mainly in volcanic rocks, especially andesites (e.g., Deer et al. 1977) though also in komatiites (Arndt and Fleet 1979). On the Moon, it is found in mare basalts (Kushiro et al. 1971; Grove and Bence 1977). In plutonic and most hypabyssal rocks, pigeonite exsolves augite and inverts to orthopyroxene because of slow cooling (Hess 1960; Robinson 1980). The Palisades Sill of New Jersey, emplaced at a depth of ~3 km, shows inverted pigeonite with spectacular exsolved augite (Walker 1969). The
<figure><img src="image_10.png" /><figcaption>Fig. 10. Comparison of pigeonite composition for Northwest Africa (NWA) 10414 and pigeonite crystallized alone using PETROLOG at 1 bar and fayalite-magnetite-quartz buffer-2 for (a) basaltic shergottite parent liquids, and (b) olivine-phyric shergottite parent liquids. The best matches are obtained with (a) Shergotty intercumulus liquid composition (Hale et al. 1999) and (b) NWA 6234 (Gross et al. 2013). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

widespread occurrence of primary pigeonite in shergottites is thus an indication that most of them are volcanic or subvolcanic rocks. NWA 10414 has a classic orthocumulate texture, but it differs from terrestrial pyroxene cumulates, and many diabases, in that pigeonite is present rather than inverted pigeonite, suggesting near-surface cooling.

There are basaltic shergottites with complex pyroxenes representing the rapid crystallization of liquid, shergottites with coexisting pyroxene pairs where crystal cores indicate a limited amount of accumulation, and NWA 10414 where the abundance of only the higher temperature pyroxene indicates a true cumulate. Shergottites with co-crystallized pigeonite and augite (Shergotty and Zagami) contain exsolution lamellae a few tens of nm wide in both pigeonite and augite, but those with disequilibrium pyroxene crystallization

(EETA79001 and QUE94201) do not: their cooling must have been relatively rapid and extremely rapid, respectively, even in the subsolidus (Mikouchi et al. 1999). Brearley (1991) calculated that pyroxene exsolution in Zagami occurred during cooling at a rate of <0.02 °C/h and at a depth of >10 m. In NWA 10414, the augite lamellae in pigeonite are only ~100 nm wide, suggesting moderately rapid cooling in the subsolidus and a depth of some tens of meters.

Olivine-phyric EETA 79001 A may have been parental to the pigeonite-rich EETA 79001 B basaltic shergottite (Goodrich 2003). The crystallization of pigeonite earlier than augite in many shergottites, as documented above, provides the possibility of making a pigeonite cumulate after olivine has settled. This is the explanation specifically used by Van Niekerk et al. (2007) for the ~1 cm wide pigeonite-phyric contact facies of EET-B with EET-A. We illustrate the formation of a pigeonite cumulate in the context of the model of Van Niekerk et al. (2007) who envisaged eruption of an olivine-charged liquid in which olivine settled out before the solidification of an upper basaltic shergottite layer in the same flow (Fig. 11). We propose that NWA 10414 formed from a liquid like NWA6234 (Gross et al. 2013) but, for our purposes, it is not important if the olivine accumulated in a subvolcanic chamber, at the base of a thick single flow, or before the separation of a fractionated flow lobe. We illustrate the transport and accumulation of olivine in Figs. 11.1 and 2. The daughter liquid of NWA 6234 that precipitates pigeonite in Fig. 11.2 is quite similar to Shergotty SILC (Hale et al. 1999) after olivine fractionation, and both compositions are Ca-poor compared to other proposed parents. The pigeonite on the liquidus has a sufficient temperature interval to form a cumulate (Fig. 9), as shown in Fig. 11.3. The contact above the olivine cumulate in Fig. 11.3 would be like that in EET-A/B if crystallization occurred in a static flow or very shallow magma chamber, but with greater depth and slower cooling, more extensive pigeonite settling would have occurred. Augite and plagioclase crystallize along with pigeonite in the liquid overlying the pigeonite cumulate liquid (Fig. 11.4). If drained away, this liquid could have produced a normal basaltic shergottite flow and allowed the underlying cumulate to cool relatively rapidly (Richter et al. 2016). In summary, the fractionation of NWA 6234 liquid could have produced an olivine accumulation, not unlike EET-A, then a pigeonite cumulate (like NWA 10414), and subsequent daughter liquid could have formed fractionated basaltic shergottites. NWA 10414 captures the temperature interval in the solidification of a shergottite magma when olivine has ceased, but augite and plagioclase have not begun to crystallize.
<figure><img src="image_11.png" /><figcaption>Fig. 11. Formation of Northwest Africa (NWA) 10414 pigeonite from olivine-phyric liquid NWA86234, simplified after the model of Van Niekerk et al. (2007) for Elephant Moraine (EET) 79001. 1. A magma charged with olivine phenocrysts reaches a near-surface environment. 2. Olivine settles; pigeonite crystallizes in the daughter liquid. 3. Pigeonite accumulates. Point “a” is analogous to EET-A and the pigeonite-rich border of EET-B. 4. Pigeonite cumulate “b” is analogous to NWA 10414. Pigeonite, augite, and plagioclase crystallize in the daughter liquid in “c” to form a later basaltic shergottite. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

CONCLUSIONS

Northwest Africa 10414 is a so-far unique pigeonite cumulate shergottite, containing 73% pigeonite, no olivine, and only ~3% interstitial augite. However, there are similarities in mineral abundances to a 1 cm pigeonite-rich contact zone between EET 79001 olivine-phyric and basaltic facies, A and B (Van Niekerk et al. 2007), and in texture to NWA 6963 (Filiberto et al. 2014).

Its chromite-ulvöspinel compositions extend from near the most Cr-rich compositions found in olivine shergottites to the Ti-magnetite of basaltic shergottites. The composition of the most magnesian pigeonite cores is En<sub>68</sub>Fs<sub>27</sub>Wo<sub>5</sub>, similar to the earliest pyroxenes for basaltic shergottites Shergotty (En<sub>68</sub>Fs<sub>24</sub>Wo<sub>8</sub>, Hale et al. 1999, fig. 4) and EET 79001B (En<sub>65</sub>Fs<sub>27</sub>Wo<sub>8</sub>, Mikouchi et al. 1998; Van Niekerk et al. 2007), and for olivine-phyric shergottites NWA 1068 (En<sub>72</sub>Fs<sub>23</sub>Wo<sub>5</sub>, Barrat et al. 2002b), NWA 6234 (Gross et al. 2013, En<sub>70</sub>Fs<sub>26</sub>Wo<sub>4</sub>), and LAR 06319 (En<sub>71</sub>Fs<sub>25</sub>Wo<sub>4</sub>, Basu Sarbadhikari et al. 2009; Peslier et al. 2010). Its most calcic maskelnite composition is An<sub>53.6</sub>Ab<sub>44.7</sub>Or<sub>1.7</sub>, more sodic than the most calcic plagioclase in most other shergottites.

The NWA 10414 most magnesian pigeonite composition is similar to that of the liquidus pigeonite (En<sub>67</sub>Fs<sub>28</sub>Wo<sub>5</sub>) of a Shergotty parent liquid SILC at 1

bar and FMQ (Dann et al. 2001) that crystallizes pigeonite in cooling through 100 °C, before co-crystallizing with augite. The range of pigeonite compositions can be matched by crystallization with PETROLOG at 1 bar and FMQ-2, quite well for the SILC basaltic composition, and very well for the NWA 6234 olivine-phyric liquid composition, after the end of olivine crystallization.

The juxtaposition of pigeonite-phyric shergottite with olivine-phyric shergottite (Van Niekerk et al. 2007) and the phase relations for shergottites suggest the formation of NWA 10414 from an olivine-saturated parent liquid. After olivine crystallization from a liquid composition like NWA 6234, accumulation of pigeonite could have occurred from a resulting daughter liquid quite like SILC. NWA 10414 is a link between olivine-phyric and basaltic shergottites.

*Acknowledgments*—We are indebted to L. Labenne for the sample and his insights; to M. Fialin and N. Rividi for help with the electron probe analyses; to N. Castle and two anonymous reviewers for thorough, critical, and very helpful reviews which led to much improvement of the manuscript; to M. J. Carr for IGPET, as always; and to L. Danyushevsy for PETROLOG. We are grateful for funding through ANR grant MARS-PRIME ANR-16-CE31-0012 (P.I. N. Mangold).
Editorial Handling—Dr. Cyrena Goodrich

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## SUPPORTING INFORMATION

Additional supporting information may be found in the online version of this article:

**Fig. S1.** Phase cartography of NWA 10414 based on Si, Fe, Mg, and Ca $K_\alpha$ X-ray maps.

**Fig. S2.** Phase cartography of NWA 10414 with binned 1.77 µm pixels, based on nine-element X-ray maps.

**Fig. S3.** Olivine and pyroxene compositions in NWA6234 (Gross et al. 2013) compared with those

calculated for the crystallization of liquid of bulk composition, using PETROLOG at 1 bar and FMQ-2.

**Fig. S4.** Pigeonite crystallized from Shergotty SILC by Dann et al. (2001) shows no systematic variation of Al and Ti as a function of water pressure, and NWA 10414 pigeonite is similar in composition.

**Table S1.** Compositions of parent liquids used in crystallization calculations with PETROLOG.