# Petrology of a nonindigenous microgranitic clast in polymict ureilite EET 87720: Evidence for formation of evolved melt on an unknown parent body

A. Beard, H. Downes, M. Chaussidon

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A. Beard, H. Downes, M. Chaussidon. Petrology of a nonindigenous microgranitic clast in polymict ureilite EET 87720: Evidence for formation of evolved melt on an unknown parent body. Meteoritics and Planetary Science, 2015, 50 (9), pp.1613-1623. 10.1111/maps.12484. hal-01770587

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# Petrology of a nonindigenous microgranitic clast in polymict ureilite EET 87720: Evidence for formation of evolved melt on an unknown parent body

A. D. BEARD<fnref n="1" />*, H. DOWNES<fnref n="1" />,<fnref n="2" />, and M. CHAUSSIDON<fnref n="3" />

*(Received 03 September 2014; revision accepted 14 June 2015)

**Abstract**–EET 87720 is a polymict ureilite breccia known to contain numerous nonindigenous fragments. We have discovered a microgranitic clast in an interior chip of Elephant Moraine (EET) 87720. The clast consists of a granophyre-like intergrowth of a pure SiO₂ phase (tridymite) and albite, mantling a zoned oligoclase phenocryst. In the intergrowth, the tridymite occurs as thin elongate vermicular blebs within larger albite crystals. The granophyre-like intergrowth and the oligoclase phenocryst share a common margin, suggesting that the clast was originally part of a larger fragment. An estimate of its bulk composition is equivalent to that of granite (77 wt% SiO₂). Patches of high-Si K-bearing glass occur interstitially within the clast; they have high concentrations of SO₃ (11–12 wt%) and contain Cl (0.6 wt%), suggesting that the clast formed on a volatile-rich parent body perhaps resembling early Mars. The mean oxygen isotope composition of the feldspar and tridymite in the clast is very different from the oxygen isotope compositions of ureilites, and is similar to those of silicate inclusions in IIE and IVA irons. Thus, the clast is not indigenous to the ureilite parent body, but it provides evidence for the formation of evolved melts on an unknown parent body in the early solar system.

## INTRODUCTION

A fundamental question in studies of the early solar system is whether the process that formed granitic rocks on the early Earth (Guitreau et al. 2012) also occurred on other terrestrial planets or differentiated planetesimals. Bonin (2012) reviewed the occurrences of “extraterrestrial granites” and suggested that such granites are more common than had previously been thought. Granitic clasts have been found in the Adzhi-Bogdo LL3-6 chondrite meteorite (Bischoff et al. 1993) and the Buzzard Coulee chondrite (Ruzicka et al. 2012). Pb-Pb dating of the Adzhi-Bogdo clasts suggests that granitic melts formed on a primitive asteroid 4.53 Ga ago (Terada and Bischoff 2009), within a few Ma of the formation of CAIs. These “grandparent” primitive asteroids along with their granitic igneous bodies were subsequently fragmented by impact, mixed and reaccreted as the brecciated Adzhi-Bogdo meteorite

parent body ago (Terada and Bischoff 2009). Day et al. (2009) also reported the occurrence of quartz-normative andesitic meteorites (GRA 06128/9) with an age of 4.517 Ga. Felsic or siliceous inclusions are also known from IIE irons and the ungrouped iron Sombrerete (Ruzicka et al. 2006; Ruzicka and Hutson 2010; Ruzicka 2014). We report here mineral, chemical, and oxygen isotopic data for a microgranitic clast within a polymict ureilite meteorite, which adds to the evidence for the formation of evolved magmas on early solar system bodies.

Ureilites are the second largest group of achondritic meteorites (Mittlefehldt et al. 1998; Goodrich et al. 2004). Most ureilites are unrecciated ultramafic rocks consisting of olivine and pyroxene, representing the mantle of the ureilite parent body (UPB). However, several are polymict breccias, considered to represent the regolith formed on or near to the surface of the UPB (Warren and Kallemeyn 1989; Goodrich et al.

<footnote n="1">¹Department of Earth and Planetary Sciences, UCL/Birkbeck Centre for Planetary Sciences, Malet Street, Birkbeck, London WC1E 7HX, UK</footnote>

<footnote n="2">²Department of Earth Sciences, Natural History Museum, Cromwell Rd, London SW7 5BD, UK</footnote>

<footnote n="3">³Institute of Physics of the Globe, 1, Rue Jussieu, 75238 Paris, France</footnote>

<footnote>*Corresponding author. E-mail: a.beard@ucl.ac.uk</footnote>
<figure><img src="image_2.png" /><figcaption>Fig. 1. BSE image of EET 87720,41 showing location of the microgranitic clast. The clast can be clearly separated from the enclosing ureilitic material, but different phases within the clast cannot be distinguished.</figcaption></figure>

2004). Polymict ureilites contain a large variety of clasts, some of which are clearly ureilitic, but others are different from common ureilitic material and must have been derived from other solar system bodies (Goodrich et al. 2004; Kita et al. 2004; Bischoff et al. 2006; Downes et al. 2008). The fall of Almahata Sitta (Goodrich et al. 2014; Horstmann and Bischoff 2014) has yielded new insights into the nature of the UPB, as approximately 20–30% of the recovered mass of this meteorite was nonureilitic. Bischoff et al. (2014) have reported the discovery of a trachyandesitic meteorite with ureilitic affinities among the recovered Almahata Sitta stones. In this study, we extend the variety of clasts found in polymict ureilites with the discovery of an igneous-textured granitic clast in an Antarctic polymict ureilite Elephant Moraine (EET) 87720.

## SAMPLES AND ANALYTICAL TECHNIQUES

The sample investigated in this study is a polished thick-section prepared from an interior chip of EET 87720. The section (41) was previously studied by Downes et al. (2008). EET 87720 is a cataclastic aggregate (Fig. 1) dominated by clasts of olivine, up to 3 mm in diameter, plagioclase and minor amounts of

low-Ca pyroxene with traces of Ni-rich iron, troilite, suessite, and graphite, set in a clast-supported matrix of finer mineral clasts. It is petrologically similar to other polymict ureilites such as EET 83309, North Haig, and Nilpena (Prinz et al. 1987; Downes et al. 2008). Detailed electron microprobe and ion microprobe analyses on the clast population have been reported in several previous studies (Guan and Crozaz 2000, 2001; Cohen et al. 2004; Kita et al. 2004; Downes et al. 2008). EET 87720 contains solar-implanted gases indicating a regolith origin (Rai et al. 2003).

Major element mineral analyses were obtained using a Jeol JXA8100 Superprobe (WDS) at Birkbeck. Analysis was carried out using an accelerating voltage of 15 kV, current of 25 nA and a beam diameter of 1 µm. The counting times for all elements were 40 s on the peak and 20 s each on the high and low backgrounds. However, counting times for Na and K for the glass were reduced to 20 s on the peak and 10 s for the background to reduce possible volatile loss. The analyses were calibrated against standards of natural silicates, oxides and Specpure ☐ metals with the data corrected using a ZAF program. Electron microprobe data for mineral phases and glass in the microgranitic clast are given in Table 1, and
<table><caption>Table 1. Electron microprobe data for constituent minerals and glass in the microgranitic clast in EET 87720,41.</caption><thead><tr><th rowspan="2"></th><th colspan="2">Granophyre-like intergrowth</th><th colspan="3">Oligoclase</th><th rowspan="2">Glass phase</th></tr><tr><th>Silica phase</th><th>Albite</th><th>Core</th><th>Rim 1</th><th>Rim 2</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>98.19</td><td>69.60</td><td>63.81</td><td>68.16</td><td>69.81</td><td>SiO<sub>2</sub></td><td>72.51</td></tr><tr><td>TiO<sub>2</sub></td><td>0</td><td>0</td><td>0</td><td>0</td><td>0</td><td>TiO<sub>2</sub></td><td>0.06</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>1.16</td><td>17.93</td><td>21.71</td><td>17.75</td><td>18.37</td><td>Al<sub>2</sub>O<sub>3</sub></td><td>9.36</td></tr><tr><td>FeO</td><td>0.12</td><td>0.10</td><td>0.94</td><td>0.51</td><td>0.15</td><td>FeO</td><td>0.95</td></tr><tr><td>MnO</td><td>0.01</td><td>0</td><td>0</td><td>0</td><td>0</td><td>MnO</td><td>0.01</td></tr><tr><td>CaO</td><td>0</td><td>0.84</td><td>4.89</td><td>0.03</td><td>0.02</td><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.01</td></tr><tr><td>MgO</td><td>0.02</td><td>0</td><td>0</td><td>0</td><td>0</td><td>NiO</td><td>0.13</td></tr><tr><td>Na<sub>2</sub>O</td><td>0.12</td><td>10.09</td><td>8.48</td><td>10.33</td><td>11.02</td><td>CaO</td><td>0.42</td></tr><tr><td>K<sub>2</sub>O</td><td>0.08</td><td>1.10</td><td>0.25</td><td>0.97</td><td>1.02</td><td>MgO</td><td>2.84</td></tr><tr><td>Total</td><td>99.70</td><td>99.67</td><td>100.08</td><td>97.75</td><td>100.39</td><td>Na<sub>2</sub>O</td><td>0.39</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>K<sub>2</sub>O</td><td>1.18</td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>SO<sub>3</sub></td><td>11.36</td></tr><tr><td></td><td>An</td><td>1.93</td><td>12.29</td><td>0.07</td><td>0.05</td><td>Cl</td><td>0.60</td></tr><tr><td></td><td>Ab</td><td>92.12</td><td>86.22</td><td>97.12</td><td>99.26</td><td></td><td></td></tr><tr><td></td><td>Or</td><td>5.92</td><td>1.49</td><td>2.81</td><td>0.79</td><td>Total</td><td>99.81</td></tr></tbody></table>

<table><caption>Table 2. Estimated modal proportions for the microgranitic clast in EET 87720,41, compared with other known extraterrestrial granitic clasts.</caption><thead><tr><th></th><th>EET 87720,41<br>Polymict<br>Ureilite</th><th>Lunar<br>Apollo<br>14 Breccia<br>14321</th><th>Lunar<br>Apollo<br>14 breccia<br>14321</th><th>Adzhi-Bogdo<br>Chrondrite<br>(LL3-6)<br>Clast A</th><th>Adzhi-Bogdo<br>Chrondrite<br>(LL3-6)<br>Clast B</th><th>Adzhi-Bogdo<br>Chrondrite<br>(LL3-6)<br>Clast C</th><th>Adzhi-Bogdo<br>Chrondrite<br>(LL3-6)<br>Clast D</th></tr></thead><tbody><tr><td>%</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Quartz</td><td>30</td><td>40</td><td>23</td><td>22.9</td><td>26</td><td>43.7</td><td>30</td></tr><tr><td>K-feldspar</td><td></td><td>60 (Or<sub>89</sub>)</td><td>32(Or<sub>93</sub>)</td><td>58.9(Or<sub>96.1</sub>)</td><td>35(Or<sub>96.4</sub>)</td><td>55.9(Or<sub>96.2</sub>)</td><td>68.8(Or<sub>97.3</sub>)</td></tr><tr><td>Albite</td><td>55 (Ab<sub>99</sub>)</td><td></td><td>33 (An<sub>86-59</sub>)</td><td>15.8(Ab<sub>93.7</sub>)</td><td></td><td></td><td></td></tr><tr><td>Plagioclase</td><td>15 (An<sub>12</sub>)</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Clinopyroxene</td><td></td><td>Trace</td><td>11</td><td>1.3</td><td>1.6</td><td></td><td></td></tr><tr><td>Ilmenite</td><td></td><td>Trace</td><td>1</td><td>0.8</td><td>21.7</td><td>&lt;0.1</td><td>0.1</td></tr><tr><td>Comments<br>and<br>accessory<br>phases</td><td>Granophyre-like<br>intergrowth</td><td>Graphic texture<br>(qtz-K-feld<br>intergrowth)</td><td>Graphic<br>texture (qtz-<br>K-feld<br>intergrowth)</td><td>Apatite<br>Zircon</td><td>Apatite<br>Whitlockite<br>Zircon</td><td>Zircon</td><td>Apatite Zircon</td></tr></tbody></table>

modal mineral abundance (%) is given in Table 2, compared with other known meteoritic and lunar granitic clasts.

Because of its small size, the bulk elemental composition of the microgranitic clast could not be directly analyzed. However, it was possible to obtain its bulk element composition by multiplying the average compositions of the mineral phases (Table 1) by the modal fraction of the phases (Table 2). The mineral modal fraction was obtained by calculating the proportion of minerals within each 1 cm² grid, superimposed on enlarged elemental distribution maps of the clast. The resulting bulk elemental composition is shown in Table 3. The bulk data were then used to determine the normative mineralogy via a CIPW normalization program (Table 3). The normative

mineralogy was then used to classify the rock type using the QAP diagram of Streckeisen (1976).

Spatially resolved, nondestructive X-ray microdiffraction was performed on the silica phase in the microgranite clast in EET 87720,41 using a Rigaku D/MAX-RAPID II X-ray diffractometer at the Natural History Museum, London, UK. The diffractometer was configured with a semicylindrical image-plate detector around a 2-axis sample goniometer, upon which the thin section was mounted. The Cu K alpha beam was collimated and restricted to a diameter of 30 µm by an exit pin hole, and with the sample oriented at an angle of 20° to the incident X-ray beam the footprint of the beam on the sample was approximately 100 µm (e.g., Lambiv-Dzemua et al. 2013). The sample was continually rotating throughout the 19 h data
<table><caption>Table 3. Calculated bulk rock and CIPW compositions for the microgranitic clast in EET 87720, 41, using the mineral compositions in Table 1 and modal proportions in Table 2. Glass (Table 1) was not included in the calculations.</caption><thead><tr><th></th><th></th><th>CIPW Norm</th><th></th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>77.31</td><td>Quartz (Q)</td><td>32.91</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>13.47</td><td>Orthoclase (Or)</td><td>3.96</td></tr><tr><td>FeO</td><td>0.23</td><td>Albite (Ab)</td><td>58.05</td></tr><tr><td>CaO</td><td>1.20</td><td>Anorthite (An)</td><td>3.98</td></tr><tr><td>Na<sub>2</sub>O</td><td>6.86</td><td>Diopside (Di)</td><td>0.79</td></tr><tr><td>K<sub>2</sub>O</td><td>0.67</td><td>Wollastonite (Wo)</td><td>0.45</td></tr><tr><td>Total</td><td>99.73</td><td>Total</td><td>100.14</td></tr></tbody></table>

collection. Phase identification was performed with the aid of the ICDD PDF-2 powder diffraction file database.

Oxygen isotope compositions were obtained from albite and silica phases in the microgranite clast at CRPG-CNRS (Nancy, France) using a multicollector Cameca ims 1280HR2 ion microprobe. The analyses were made using procedures previously described (Chaussidon et al. 2008) using a primary Cs beam of ≈ 0.5 nA intensity and 30 µm diameter and electron gun for charge compensation. The ¹⁶O⁻, ¹⁷O⁻, and ¹⁸O⁻ ion beams were measured on three collectors (the off axis L'2 Faraday cup, the central electron multiplier EM and the off axis H2 electron multiplier, respectively) at a mass resolution M/ΔM of 2500 for ¹⁶O⁻ and ¹⁸O⁻ ion and of ≈6000 for ¹⁷O⁻. Dead times of 44 and 68 ns were determined and used for correction of data acquired on the two electron multipliers, EM and H2, respectively. A set of four “in house” standards (quartz, olivine, magnetite, calcite) were used to determine the instrumental fractionation law of the three oxygen isotopes, which was then used to correct data acquired on the minerals of the microgranitic clast. The measurements were corrected for matrix effect, taking into account that the spots analyzed are mainly on feldspar, using the instrumental fractionation determined on the quartz standard, and the law previously determined which shows that matrix effects are primarily dependent on the SiO₂ content of the silicate mineral analyzed (Chaussidon et al. 2008).

The oxygen isotopic compositions are expressed in the classical delta notation in permil relative to the SMOW international standard (δ¹⁷,¹⁸O<sub>sample</sub> = 1000 × [(¹⁷,¹⁸O/¹⁶O)<sub>sample</sub>/(¹⁷,¹⁸O/¹⁶O)<sub>standard</sub> − 1]). The two sigma errors given in Table 4 for δ¹⁷,¹⁸O values are the quadratic sum of the errors due to counting statistics (≈±0.35‰ and ±0.15‰ for δ¹⁷O and δ¹⁸O, respectively), the external reproducibility (two sigma error) on the quartz standard, and the two sigma error

<table><caption>Table 4. Oxygen isotope data for feldspars and tridymite in microgranitic clast within EET 87720,41, analyzed by SIMS (CNRS-CRPG, Nancy, France).</caption><thead><tr><th></th><th>δ¹⁸O‰</th><th>±2σ</th><th>δ¹⁷O‰</th><th>±2σ</th><th>Δ¹⁷O‰</th><th>±2σ</th></tr></thead><tbody><tr><td>Point 1</td><td>3.92</td><td>0.51</td><td>2.72</td><td>0.66</td><td>0.68</td><td>0.60</td></tr><tr><td>Point 2</td><td>2.59</td><td>0.56</td><td>1.78</td><td>0.61</td><td>0.43</td><td>0.56</td></tr><tr><td>Point 3</td><td>1.95</td><td>0.52</td><td>1.45</td><td>0.76</td><td>0.44</td><td>0.70</td></tr><tr><td>Point 4</td><td>3.93</td><td>0.53</td><td>2.44</td><td>0.73</td><td>0.40</td><td>0.67</td></tr><tr><td>Point 5</td><td>3.85</td><td>0.55</td><td>2.58</td><td>0.78</td><td>0.58</td><td>0.73</td></tr><tr><td>Point 7</td><td>3.66</td><td>0.51</td><td>2.49</td><td>0.70</td><td>0.59</td><td>0.64</td></tr><tr><td>Point 8</td><td>2.72</td><td>0.55</td><td>2.56</td><td>0.74</td><td>1.15</td><td>0.69</td></tr><tr><td>Point 9</td><td>3.69</td><td>0.53</td><td>2.87</td><td>0.70</td><td>0.95</td><td>0.64</td></tr><tr><td>Point 10</td><td>0.32</td><td>0.50</td><td>2.13</td><td>0.64</td><td>0.47</td><td>0.57</td></tr><tr><td>Point 11</td><td>3.14</td><td>0.54</td><td>2.55</td><td>0.77</td><td>0.92</td><td>0.72</td></tr><tr><td>Mean</td><td>3.26</td><td></td><td>2.36</td><td></td><td>0.66</td><td></td></tr><tr><td>±2σ</td><td>1.34</td><td></td><td>0.88</td><td></td><td>0.52</td><td></td></tr></tbody></table>

due to the correction for matrix effect. The ¹⁶O excesses are expressed as Δ¹⁷O values with Δ¹⁷O = δ¹⁷O − 0.52 × δ¹⁸O. The two sigma errors given for the Δ¹⁷O values in Table 4 are calculated by summing in quadratic mode the errors due to counting statistic on the δ¹⁷,¹⁸O values (before correction for instrumental fractionation) with the external reproducibility (two sigma errors) on the four terrestrial standards analyzed (19 measurements) giving Δ¹⁷O = 0 ± 0.11‰ (two sigma error).

## PETROLOGY AND MINERAL CHEMISTRY OF THE MICROGRANITIC CLAST IN EET 87720

The microgranitic clast in EET 87720,41 is approximately 550 × 850 µm in size (0.47 mm²) and consists largely of albite and oligoclase together with a pure silica phase which has been tentatively identified by micro-XRD as tridymite. Tridymite and other silica polymorphs have been previously reported in igneous-textured clasts in the Parnallee (LL3) chondrite (Bridges et al. 1995), and identified in Martian meteorite NWA 856 (Leroux and Cordier 2006) and the LaPaz Icefield lunar basaltic meteorites (Righter et al. 2005). Figure 2 shows element distribution maps of the entire clast, emphasizing the texture and mineral variation, together with an interpretative schematic of the clast mineralogy. Oligoclase occurs as the core of a single large (150 × 225 µm) euhedral phenocryst, which is strongly zoned to an albitic rim. The tridymite and albite occur as a granophyre-like intergrowth, covering an area of 150 × 200 µm and mantling the oligoclase crystal. In the intergrowth, the tridymite occurs as thin elongate vermicular blebs, with an average diameter of 5–10 µm and up to 100–120 µm in length within larger albite crystals. Outside the intergrowth, the tridymite and
<figure><img src="image_3.png" /></figure>

Fig. 2. Elemental distribution maps of Si, Al, Na, K, Ca within the microgranitic clast in EET 87720, 41, showing the granophyre-like intergrowth and the zoned oligoclase sharing a common margin. Colors indicate relative intensity of elemental concentrations (i.e., reds = high concentrations; pinks = medium concentrations; greens = low concentrations). Oligoclase phenocryst is the rectangular object on the right-hand side (clearly shown on Si, Na, and Al images); zonation from core to rim is shown in Ca image. Top right image BSE image of the clast shows that its constituent phases all have very similar low mean atomic numbers.

albite also occur as anhedral crystals up to 150 µm in diameter along with two anhedral areas of glass, up to 100 µm in diameter (Fig. 2). Although the normal granophyre mineralogy of K-feldspar and quartz does not occur within this clast, the texture is very similar to that of terrestrial granophyres. The granophyre-like intergrowth and the oligoclase crystal share a common margin, suggesting that the microgranitic clast was originally part of a larger fragment.

Albite within the granophyre-like intergrowth has a slightly variable composition (Ab$_{91}$$_{-94}$) with a low K$_2$O content (~1 wt%) and traces of FeO (0.1–0.3 wt%). The tridymite in the intergrowth is slightly impure, containing up to 1.3 wt% Al$_2$O$_3$, 0.11 wt% FeO, and 0.2 wt% Na$_2$O, similar to quartz in granitic clasts found in the Adzhi-Bogdo LL3-6 chondrite (Bischoff et al. 1993). The phenocryst is strongly zoned from oligoclase (An$_{12}$) in its core to an albitic rim (An$_{0.05}$Ab$_{99}$Or$_{0.7}$). On an Ab-An-Or ternary diagram (Fig. 3), the albite and oligoclase have compositions similar to those found in felsic clasts from polymict ureilite DaG 319 (Ikeda et al. 2000; Ikeda and Prinz 2001) and albite in the granitic clasts in Adzhi-Bogdo (Bischoff et al. 1993). K-feldspar and more anorthitic

plagioclase from small lunar granite samples from the Apollo 14 landing site (Ryder 1976; Warren et al. 1983; Jolliff et al. 1990) are also plotted for comparison.

Glass occurs as two small patches within the microgranite clast. Even under high magnification in backscattered images, it appears completely devoid of crystallites. As the glass and the tridymite/albite intergrowth share a common broken margin, the glass is clearly part of the clast and has not been introduced from outside. It may therefore be a melt residue that was quenched after crystallization of the minerals within the granophyre-like intergrowth. It contains up to 72 wt% SiO$_2$, 9.7 wt% Al$_2$O$_3$, 3 wt% MgO, and 1.2 wt % K$_2$O. It has an extremely unusual volatile composition with a very high SO$_3$ content (11–12 wt%) and 0.6 wt% Cl (Table 1).

The bulk composition of the microgranite clast, calculated from modal abundance and mineral compositions, plot in the granite field of the TAS diagram (Fig. 4), as used by Bonin (2012) for extraterrestrial granites. It plots in a similar region as the bulk compositions of several glassy inclusions from IIE irons (Takeda et al. 2003). Its Na$_2$O content
<figure><img src="image_4.png" /></figure>

Fig. 3. Ab-An-Or ternary diagram showing the plagioclase composition for the microgranitic clast in EET 87720,41. Feldspar compositions from the granitic clasts in the LL3-6 chondritic breccia Adzhi-Bogdo (Bischoff et al. 1993), lunar granite samples (Ryder 1976; Warren et al. 1983; Jolliff et al. 1990), feldspathic clasts from polymict ureilite DaG 319 (Ikeda et al. 2000; Ikeda and Prinz 2001), and mean plagioclase compositions in silicate inclusions in iron meteorites (Ruzicka 2014) have been plotted for comparison.

<figure><img src="image_5.png" /></figure>

Fig. 4. Total alkali versus silica diagram showing estimated bulk chemical composition of microgranitic clast in EET 87720,41, compared with compositions of glassy inclusions in IIE irons Colomera, Sombrerete, and Weekeroo Station (Takeda et al. 2003; Ruzicka et al. 2006) and in granitic inclusions in Martian meteorite NWA 6963 (Gross and Filiberto 2014a, 2014b).

greatly exceeds its K₂O content, as is common in silicate glass inclusions in iron meteorites (Bonin 2012) but differs from the Adzhi-Bogdo granitic clasts. The composition of granitic material found within a Martian gabbro (Gross and Filiberto 2014a, 2014b)

also shown plots near the mean composition of the microgranite clast from EET 87702,41.

The normative composition of the clast plots in the alkali granite field of the QAP diagram (Fig. 5). The calculated CIPW normative mineralogy for granitic clasts in Adzhi-Bogdo (Bischoff et al. 1993), silica-rich glass inclusions in polymict ureilite DaG 319 (Ikeda et al. 2000; Ikeda and Prinz 2001), silica-rich glass inclusions in the Colomera IIE iron (Takeda et al. 2003), lunar granites (Ryder 1976; Warren et al. 1983), felsic glass spherule in howardite NWA 1664 (Barrat et al. 2009), and selected silica-rich glass inclusions in olivine from Martian meteorites DaG 489 (Folco et al. 2000) and Chassigny (Varela et al. 2000) have been plotted on the QAP diagram for comparison (Fig. 5). The majority of granitic clasts from Adzhi-Bogdo and glass inclusions from DaG 319 and Colomera IIE iron plot in a confined range from alkali syenite to alkali granite. Lunar granite samples have a wider range, plotting within the alkali granite and syenogranite fields. A glass inclusion from the Martian meteorite Chassigny plots in the syenogranite field, while glass inclusions from shergottite DaG 489 plot in the monzogranite and quartz granite fields. The felsic glass spherule in howardite NWA 1664 plots in the monzogranite field.
<figure><img src="image_6.png" /></figure>

Fig. 5. QAP diagram (Streckeisen 1976) showing the microgranitic clast in EET 87720,41 plotting in the alkali granite field. Granitic clasts in the LL3-6 chondritic breccia Adzhi-Bogdo (Bischoff et al. 1993), silica-rich glass inclusions in polymict ureilite DaG 319 (Ikeda et al. 2000; Ikeda and Prinz 2001), silica-rich glass inclusions in the Colomera IIE iron (Takeda et al. 2003), lunar granites (Ryder 1976; Warren et al. 1983), felsic glass spherule in howardite NWA 1664 (Barrat et al. 2009), and selected silica-rich glass inclusions in olivine from Martian meteorites DaG 489 (Folco et al. 2000) and Chassigny (Varela et al. 2000) have been plotted for comparison.

## OXYGEN ISOTOPE ANALYSIS

The data obtained for 10 spots made on the feldspar and tridymite within the microgranitic clast in EET 87720,41 are shown in Table 4. Mean values with their two sigma standard deviation errors are: $δ^{18}O = 3.26 ± 1.34‰$, $δ^{17}O = 2.36 ± 0.88‰$, and $Δ^{17}O = 0.66 ± 0.52‰$. The relatively large errors on the $δ^{17,18}O$ values are likely due to the low secondary ion intensity (required for a small size of the beam) and to small variations of matrix effects which cannot be totally corrected for. On a three isotopes diagram (Fig. 6), the present data plot within error on the YR line of slope 1 (Young and Russell 1998), significantly above the terrestrial fractionation line, far from the region of ureilites, and close to the field of H chondrites and of silicate inclusions in IIE irons (Clayton and Mayeda 1996). The microgranitic clast has, within error, the same $Δ^{17}O$ as a similar object previously found in

the LL3-6 Adzhi-Bogdo ordinary chondrite ($Δ^{17}O = 0.95 ± 0.27‰$ but with a ≈ 3.5‰ higher $δ^{18}O$ value; Sokol et al. 2007). Two other similar clasts from Sokol et al. (2007) have much higher $Δ^{17}O$ values (+1.79 ± 0.16‰ and +1.93 ± 0.19‰, respectively) and cannot derive from the same differentiated parent body as the clast in EET 87720,41. The data plot marginally above the fractionation line for Martian (SNC) meteorites (Clayton and Mayeda 1996) and are on an extension of the fractionation line for Mars meteorites NWA 7034 (Agee et al. 2013) and NWA 7475 (Wittmann et al. 2015).

## DISCUSSION

It is well known that the regolith of the ureilite parent body contains nonindigenous clasts derived from a number of different parent bodies which were transported to the surface of the ureilite parent body as
<figure><img src="image_7.png" /><figcaption>Fig. 6. Three isotopes of oxygen plot showing mean values for microgranite clast in EET87720,41 (Table 4), mean values for three granitic clasts in Adzhi-Bogdo (Sokol et al. 2007), bulk ureilites (Clayton and Mayeda 1988), field for ordinary chondrites, silicate inclusions in IIE and IVAB iron meteorites, field for Martian meteorites (Clayton and Mayeda 1996), field for Mars breccia meteorites NWA 7034 (Agee et al. 2013) and NWA 7475 (Wittmann et al. 2015), GRA 06128/9 = andesitic meteorites (Day et al. 2009); TFL = terrestrial fractionation line; CCAM = carbonaceous chondrites anhydrous mineral line; Young and Russell slope 1 line (Young and Russell 1998).</figcaption></figure>

meteorites (Goodrich et al. 2004; Kita et al. 2004). A petrological study of clasts in polymict ureilite (DaG 319) by Ikeda et al. (2000) resulted in a classification scheme consisting of 7 major groups with 24 types of lithic fragments and 22 types of mineral fragments. It is not therefore surprising that new varieties of clast are still being identified (e.g. Downes et al. 2008; Goodrich and Gross 2015). However, few of the previously described clasts could be easily related to the microgranitic clast described here, although Ikeda et al. (2000) reported that the mesostasis in a felsic clast in DaG 319 was a mixture of feldspar and silica.

Igneous-textured granitic clasts are relatively rare among granitic material in meteorites (Sokol et al. 2007) and returned lunar samples (Warren et al. 1983; Kuehner et al. 2007), although granitic material showing graphic intergrowths has recently been reported from within a gabbroic Martian meteorite (Gross and Filiberto 2014a, 2014b). Granophyric intergrowths in terrestrial rocks consist of branching/radiating quartz in a larger single K-feldspar grain, often occurring as a mantle around a pre-existing subhedral-euhedral plagioclase grain. A graphic intergrowth consists of subhedral skeletal quartz crystals in a K-feldspar or sodic plagioclase crystal. The quartz crystals usually have a regular geometric orientation that resembles “cuneiform” writing. All of the different types of

granitic intergrowths are indicative of rapid cooling and the simultaneous crystallization of quartz, K-feldspar, and/or plagioclase. In EET 87720, the microgranitic clast consists of an intergrowth of branching tridymite crystals in a larger albite grain, and occurs as mantle around a much larger zoned subhedral oligoclase phenocryst. Although the host feldspar is albite rather than K-feldspar, the general texture is more typical of a granophyre intergrowth rather than myrmekitic or graphic intergrowths.

On Earth, approximately 86 vol% of the upper continental crust is granitic in composition or origin (Wedepohl 1991), with smaller volumes of granitic rocks in the lower continental crust (charnockites), oceanic crust (plagiogranites), and upper mantle (Bonin and Bébien 2005). No other body in the solar system is thought to have such an evolved crust, although andesitic and trachyandesitic meteorites have recently been reported (Day et al. 2009; Bischoff et al. 2014) that are expanding the range of magma compositions that erupted on asteroidal surfaces. Other reports of granitic rocks with an extraterrestrial origin include granitic rock fragments within breccias and soil samples returned from several Apollo landing sites (Rutherford et al. 1976; Ryder 1976; Warren et al. 1983; Martinez and Ryder 1989; Morris et al. 1990), granitic clasts in the Adzhi-Bogdo LL3-6 chondrite (Bischoff et al. 1993),
a granitic clast displaying a micrographic texture in gabbroic shergottite (NWA 6963) (Gross and Filiberto 2014a), and granophyric clasts in lunar meteorite NWA 4472 and NWA 4485 (Kuehner et al. 2007). Barrat et al. (2009) proposed that evolved magmas may also have occurred on the asteroid 4 Vesta because of the presence of glass impact spherules with granitic compositions from several howardite meteorites.

Formation of granitic magma on Earth can occur by anatexis of pre-existing silica-rich feldspar-bearing material, by extensive fractionation of basaltic magma, or by partial melting of pre-existing igneous rocks. The latter two mechanisms do not require any continental crustal or sedimentary material to be present. For lunar granite petrogenesis various models have been considered, including fractional crystallization producing acidic residual melts (Ryder 1976) and fractional crystallization of a basaltic melt followed by silicate-liquid immiscibility (Rutherford et al. 1976; Jolliff et al. 1990). Hagerty et al. (2006) proposed an additional model whereby basaltic underplating of the lunar crust resulted in its partial melting and the generation of small-scale granite intrusions. The recently discovered granitic compositions on Mars have been ascribed to extreme fractional crystallization of a basaltic magma (Gross and Filiberto 2014a, 2014b). Usui et al. (2015) studied low pressure experimental partial melting of a H chondrite analog which showed that siliceous (quartz-normative) melts can be formed by very small degrees of melting under reducing conditions. Experimental studies by Gardner-Vandy et al. (2014) confirm that evolved melts can be formed on an asteroid-sized body through low-degree melting of an oxidized, volatatile-rich chondritic precursor. Terada and Bischoff (2009) suggested that the granitic clasts in the Adzhi-Bogdo meteorite were formed by fractional crystallization or liquid immiscibility from large-scale melting produced the granitic melt. Our data are not sufficient to distinguish between these alternatives for the origin of the microgranitic clast in EET 87720.

The high-Si glass inside the microgranite clast has an extremely unusual composition with 11–12 wt% SO₃ and some Cl. None of the other granitic or high-Si meteoritic clasts reported in the literature contain these high amounts of volatiles. The high S and Cl contents indicate a volatile-rich parent body, which is unlikely to be the parent body of ureilite meteorites which are known to be depleted in volatile elements. The only extant planetary body known to contain abundant SO₃ and Cl is Mars (Leshin et al. 2013), which is thought to have experienced a major impact event early in its history (Marinova et al. 2008). The oxygen isotope composition of the microgranite clast is close to those of Martian meteorites (Fig. 6), and lies on an extension

of the $\Delta^{17}\text{O} = +0.6\text{‰}$ fractionation trend for Mars breccia meteorites (Agee et al. 2013; Wittmann et al. 2015). Thus, although the identity of the parent body of the microgranite clast remains unresolved, some of the evidence points toward Mars.

## CONCLUSIONS

This study has shown that the microgranitic clast in ureilite EET 87720 was derived from a S- and Cl-rich parent body, with a $\Delta^{17}\text{O}$ value of +0.66 ‰, similar to that of Mars breccias NWA 7034/7475 and some of the silicate inclusions in IIE and IVAB iron meteorites. The clast provides further evidence for formation of evolved melt by either extreme fractional crystallization, liquid immiscibility, or very low degrees of partial melting on terrestrial parent bodies in the early solar system. The parent body was subsequently impacted or fragmented and a small fragment of it accreted onto the ureilite parent body.

*Acknowledgments*—We thank the Meteorite Working Group for providing the sample, and Caroline Smith (Natural History Museum, London), Katie Joy (University of Manchester), and Nachiketa Rai (NHM/Birkbeck) for discussions about the origin of this clast. Reviews by B. Cohen, K. Gardner-Vandy, J. Gross, A. Ruzicka, and associate editor Cyrena Goodrich greatly improved the final version of this paper. We thank Paul Schofield and Jens Najorka (NHM) for identifying the tridymite by micro-XRD.

*Editorial Handling*—Dr. Cyrena Goodrich

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