# The first main group ureilite with primary plagioclase: A missing link in the differentiation of the ureilite parent body

Cyrena A. Goodrich, Max Collinet, Allan Treiman, Tabb C. Prissel, Markus Patzek, Samuel Ebert, Michael J. Jercinovic, Addi Bischoff, Andreas Pack, Jean-Alix Barrat, et al.

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Cyrena A. Goodrich, Max Collinet, Allan Treiman, Tabb C. Prissel, Markus Patzek, et al. The first main group ureilite with primary plagioclase: A missing link in the differentiation of the ureilite parent body. Meteoritics and Planetary Science, 2022, 57, pp.1589-1616. 10.1111/maps.13889. insu-03779803

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# The first main group ureilite with primary plagioclase: A missing link in the differentiation of the ureilite parent body

Cyrena A. GOODRICH<sup>ID 1*</sup>, Max COLLINET<sup>ID 2</sup>, Allan TREIMAN<sup>ID 1</sup>, Tabb C. PRISSEL<sup>1,†</sup>, Markus PATZEK<sup>ID 3</sup>, Samuel EBERT<sup>ID 3</sup>, Michael J. JERCINOVIC<sup>4</sup>, Addi BISCHOFF<sup>ID 3</sup>, Andreas PACK<sup>5</sup>, Jean-Alix BARRAT<sup>6</sup>, and Stephan DECKER<sup>7</sup>

**Abstract**–MS-MU-012, a 15.5 g clast from the Almahata Sitta polymict ureilite, is the first known plagioclase-bearing main group ureilite. It is a coarse-grained (up to 4 mm), equilibrated assemblage of 52% olivine (Fo 88), 13% orthopyroxene (Mg# 89.2, Wo 4.5), 11% augite (Mg# 90.2, Wo 37.3), and 14% plagioclase (An 68), plus minor metal and sulfide. The plagioclase grains have been secondarily remelted and internally recrystallized, but retain primary external morphologies. Melt inclusions occur in olivine. Rounded chadocrysts of olivine and orthopyroxene are enclosed in augite grains. In terms of texture, mineralogy, major and minor element mineral compositions, and oxygen isotopes, MS-MU-012 is virtually identical to the archetypal Hughes-type main group ureilites, with the significant addition of primary plagioclase. We conclude that MS-MU-012 formed as a cumulate in a common lithologic unit with the Hughes-type ureilites. Based on reconstructed compositions of melts trapped in olivine, orthopyroxene, and augite in the Hughes-type samples, we infer that the parent magma of the Hughes unit originated as a late melt in the incremental melting of the ureilite parent body (UPB), near the end of the melting sequence, but was not completely extracted from the mantle like earlier melts and was emplaced in an intrusive body. MELTS calculations indicate that olivine began to crystallize at ~1260 °C, followed shortly thereafter by co-crystallization of orthopyroxene and augite. Plagioclase began to crystallize at ~1170–1180 °C. Graphite was buoyant in the melt and became heterogeneously distributed in flotation cumulates. Residual silicate liquid was extracted from the cumulate pile and could have crystallized to form the “labradoritic melt lithology” (with plagioclase of An ~68-35), which is partially preserved as clasts in polymict ureilites. The final equilibration temperature recorded by the Hughes unit was ~1140–1170 °C, just before catastrophic disruption of the UPB. MS-MU-012 provides a critical missing link in the differentiation history of this asteroid.

## INTRODUCTION

Ureilites are a major group of achondritic meteorites that represent a partially differentiated, carbon-rich,

parent asteroid. Some of the most fundamental aspects of the formation and igneous evolution of this body remain poorly understood. In this work, we studied a new ureilitic lithology, discovered as a clast (MS-MU-012)

<footnote n="1">¹Lunar and Planetary Institute, Universities Space Research Association, 3600 Bay Area Blvd., Houston, Texas 77058, USA</footnote>

<footnote n="2">²Institute of Planetary Research, German Aerospace Center (DLR), Rutherfordstraße 2, 12489 Berlin, Germany</footnote>

<footnote n="3">³Institut fuer Planetologie, University of Münster, Wilhelm-Klemm Str. 10, D-48149 Münster, Germany</footnote>

<footnote n="4">⁴Department of Geosciences, University of Massachusetts, Amherst, Massachusetts 01003, USA</footnote>

<footnote n="5">⁵Universität Göttingen, Geowissenschaftliches Zentrum, Goldschmidtstraße 1, D-37077 Göttingen, Germany</footnote>

<footnote n="6">⁶Univ Brest, CNRS, IRD, Ifremer, LEMAR, Institut Universitaire Européen de la Mer (IUEM), Place Nicolas Copernic, F-29280 Plouzané, France</footnote>

<footnote n="7">⁷Oberstrasse 10a, D-55430, Oberwesel, Germany</footnote>

<footnote>†Current address: Now at Jacobs JETS, Astromaterials Research & Exploration Science Division, NASA Johnson Space Center, Houston, Texas 77058, USA</footnote>

<footnote>*Corresponding author. E-mail: goodrich@lpi.usra.edu</footnote>

<footnote>(Received 24 August 2021; revision accepted 25 May 2022)</footnote>
from the Almahata Sitta (AhS) polymict ureilite, which provides a critical piece that was missing from this puzzle. We begin by reviewing the petrology of ureilites and the current understanding of ureilite petrogenesis, in order to provide context for interpreting the significance of MS-MU-012.

## Petrology of Ureilites

The ureilite group consists of ~95% main group and 5% polymict samples. Main group ureilites are unrebranded, coarse-grained (average ~0.5–1 mm) ultramafic rocks consisting of olivine and pyroxene, with ≤10% interstitial carbon (graphite and minor diamond), metal, and sulfides (Mittlefehldt et al., 1998). To first order, they can be subdivided into the low-Ca pyroxene ureilites (olivine–pigeonite and olivine–orthopyroxene), and the less common (~4–5% of main group ureilites) augite-bearing ureilites (Goodrich et al., 2004). Olivine–pigeonite (Wo ~5–14) and olivine–orthopyroxene (Wo ~4–5) ureilites (Wo = 100 × molar CaO/[CaO + MgO + FeO]) typically have equilibrated textures, with gently rounded grain boundaries and common triple junctions, and in some cases show a fabric defined by both a lineation and a foliation (Berkley et al., 1980). Olivine compositions range continuously from ~Fo 75 to Fo 95 among samples (Fo = 100 × molar MgO/[MgO + FeO]), but within each sample are homogeneous except for secondary reduced zones along grain boundaries and around included carbon phases (known as “reduction rims”). In ureilites of Fo 75–85, the dominant (usually only) pyroxene is pigeonite; in ureilites of Fo ~86–95, orthopyroxene occurs in addition to or instead of pigeonite. Pyroxene abundance (vol% pyroxene/ [pyroxene + olivine]) is correlated with Fo among the low-Ca pyroxene ureilites and ranges from ~10% to ≥50% (Collinet & Grove, 2020a; Goodrich et al., 2007; Singletary & Grove, 2003).

The augite-bearing main group ureilites (supporting information S1) contain up to 40 vol% augite of Wo ~33–37, in addition to orthopyroxene and/or pigeonite. Although they occur over the same range of Fo (75–95) as the low-Ca pyroxene ureilites, they do not show the same correlations of pyroxene abundance and low-Ca pyroxene type (pigeonite versus orthopyroxene) with Fo. They also show more complex textures than the olivine + low-Ca pyroxene ureilites. Many augite-bearing ureilites have poikilitic textures in which large (up to cm-sized) oikocrysts of low-Ca pyroxene (orthopyroxene or pigeonite) enclose chadocrysts or polycrystalline areas of olivine and/or augite (Berkley, 1986; Berkley et al., 1985; Berkley & Goodrich, 2001; Chikami et al., 1997; Goodrich, 1999, 2001; Goodrich et al., 2006, 2014; Prinz

et al., 1994; Takeda, 1989; Takeda et al., 1979, 1988, 1989; Weber et al., 2003). Some of these, referred to as having bimodal texture, also have areas of equilibrated texture similar to the low-Ca pyroxene ureilites but with augite occurring instead of low-Ca pyroxene. In other poikilitic samples, oikocrysts of augite enclose chadocrysts of olivine and orthopyroxene. Another unique feature of many augite-bearing ureilites (i.e., not observed in low-Ca pyroxene ureilites) is the presence of melt inclusions in olivine and/or the pyroxenes (Goodrich et al., 2001, 2009, 2014). Almost half of the known augite-bearing ureilites have olivine of Fo ~86–88, as well as other common characteristics (Table 1), and have been designated (after Hughes 009) as the “Hughes cluster” (Downes et al., 2008).

Polymict ureilites are fragmental and regolith breccias dominated by lithic and mineral clasts of main group ureilite materials showing the same range and distribution of petrologic types and Mg# as main group ureilites themselves (Downes et al., 2008; Goodrich et al., 2004; Ikeda et al., 2000; Jaques & Fitzgerald, 1982; Van Kooten et al., 2017). They also contain a few volume percent of small (typically a few µm to mm in size) feldspathic (plagioclase-bearing) clasts (Cohen et al., 2004; Goodrich, Treiman, & Boyle, 2017; Ikeda et al., 2000) that appear to be related to ureilites based on oxygen and chromium isotopes (Kita et al., 2004, 2006; Van Kooten et al., 2017), as well as a variety of types of xenoliths (Bischoff et al., 2006; Boleaga & Goodrich, 2019; Downes et al., 2008; Goodrich et al., 2004, 2016, 2018, 2019; Goodrich & Gross, 2015; Goodrich, Hartmann, et al., 2015; Goodrich, Kita, et al., 2017; Goodrich, Mikouchi, & Treiman, 2015; Ikeda et al., 2000, 2003; Jaques & Fitzgerald, 1982; Patzek et al., 2018; Prinz et al., 1986, 1987; Ross et al., 2010). Among the feldspathic clasts, the most common type (~50% by number) appears to represent an “albitic” igneous lithology (Cohen et al., 2004; Goodrich, Hutcheon, et al., 2010; Goodrich, Treiman, & Boyle, 2017; Ikeda et al., 2000). This lithology consists of plagioclase of An ~0–30, FeO-rich pyroxenes, phosphates, Fe-Ti oxides, and Fe(Mn,K,P,Ti)-rich glass. The next most common type (~30% by number) has “labradoritic” (An ~30–70) plagioclase, but the lithology they represent is less well defined (Cohen et al., 2004; Goodrich, Treiman, & Boyle, 2017). Only a few labradoritic lithology clasts contain pyroxenes. Most are just plagioclase mineral fragments, although related pyroxene mineral fragments may have been overlooked because they are less easily recognized than plagioclase in a sea of main group ureilitic pyroxene fragments.

The AhS polymict ureilite fell in 2008 when asteroid 2008 TC₃ impacted in northern Sudan (Jenniskens et al., 2009; Shaddad et al., 2010). It is considered to be anomalous because it disintegrated in the atmosphere,
<table><caption>Table 1. Hughes-type ureilites.</caption><thead><tr><th>Sample name</th><th>Fo</th><th>Dominant texture</th><th>Pyroxenes</th><th>Chadocrysts</th><th>Melt inclusions</th><th>Modal abundances</th><th>Olivine Fe-Mn-Mg<sup>b</sup></th><th>Carbon phases</th><th>Cr<sub>ol</sub>-HPx T (°C)<sup>c</sup></th><th>Ref.</th></tr></thead><tbody><tr><td colspan="11"><em>Archetypal Hughes-type samples</em></td></tr><tr><td>Hughes 009</td><td>87.3</td><td>Equilibrated</td><td>Opx (Mg# 88.3, Wo 4.9)<br>aug (Mg# 89.2, Wo 37.0)</td><td>Oliv and aug<br>in opx</td><td>In oliv, opx,<br>and aug</td><td>23-47% oliv; 12-<br>56% opx; 7-52%<br>aug</td><td>Hughes<br>group</td><td>None observed</td><td>1167</td><td>1.2</td></tr><tr><td>FRO 90054/<br>93008</td><td>87.6</td><td>Equilibrated</td><td>Opx (Mg# 88.0, Wo 4.8)<br>aug (Mg# 88.8, Wo 38.0)</td><td>Oliv and aug<br>in opx</td><td>In oliv, opx,<br>and aug</td><td>23-47% oliv; 12-<br>56% opx; 7-52%<br>aug</td><td>Hughes<br>group</td><td>None observed</td><td>1150</td><td>1-4</td></tr><tr><td>FRO 90228</td><td>87.2</td><td>Equilibrated</td><td>opx (Mg# 88.2, Wo 4.6)<br>aug (Mg# 88.2, Wo 38.0)</td><td>nd</td><td>In oliv</td><td>nd</td><td>Hughes<br>group</td><td>None observed</td><td>1134</td><td>4.5</td></tr><tr><td>NWA 11754</td><td>87.5</td><td>Equilibrated</td><td>Opx (Mg# 88.4, Wo 4.9)<br>aug (Mg# 89.7, Wo 36.5)</td><td>Oliv and opx<br>in aug</td><td>In aug</td><td>39.5% oliv; 40.2%</td><td>Hughes<br>group</td><td>None observed</td><td>1156</td><td>7</td></tr><tr><td>NWA 3222</td><td>86-88</td><td>Equilibrated</td><td>Opx (Mg# 87.7, Wo 4.7)<br>aug (Mg# 89.4, Wo 36.8)</td><td>Oliv and opx<br>in aug; oliv in<br>opx</td><td>In oliv</td><td>27% oliv; 36%<br>opx; 37% aug</td><td>Hughes<br>group</td><td>None observed</td><td>1156</td><td>6.8</td></tr><tr><td>AhS #15<sup>a</sup></td><td>88.3</td><td>Equilibrated</td><td>Opx (Mg# 88.2, Wo 4.9)<br>aug (Mg# 88.8, Wo 37.0)</td><td>Oliv in aug</td><td>In oliv<br>and aug</td><td>19% oliv; 40%<br>opx; 40% aug</td><td>Hughes<br>group</td><td>None observed</td><td>1170</td><td>9</td></tr><tr><td>MS-MU-012<sup>a</sup></td><td>88.1</td><td>Equilibrated</td><td>Opx (Mg# 89.2, Wo 4.5)<br>aug (Mg# 90.2, Wo 37.3)</td><td>Oliv and opx<br>in aug</td><td>In oliv</td><td>52% oliv; 13%<br>opx; 11% aug;<br>14% plag</td><td>Hughes<br>group</td><td>None observed</td><td>1144</td><td>6</td></tr><tr><td colspan="11"><em>Other Hughes-type samples</em></td></tr><tr><td>EET 96314/<br>96293/96331</td><td>86.9</td><td>Equilibrated</td><td>Opx (Mg# 87.5, Wo 4.5)<br>aug (Mg# 89.5, Wo 36.7)</td><td>Aug in opx<br>and oliv</td><td>None<br>observed</td><td>43% oliv; 50-%<br>opx; 2-5% aug</td><td>Hughes<br>group</td><td>Graphite<br>present</td><td>1124</td><td>6.7</td></tr><tr><td>NWA 2895</td><td>87.8</td><td>Poikilitic;<br>oikocrysts =<br>pigeonite</td><td>Pig (Mg# 87.7, Wo 9.6)<br>opx (Mg# 88.4, Wo 4.8)<br>aug (Mg# 89.8, Wo 35.6)</td><td>Oliv and aug<br>in pig</td><td>nd</td><td>1-2% aug</td><td>Hughes<br>group</td><td>Graphite<br>present</td><td>1193</td><td>6.8</td></tr><tr><td>MIL 091004</td><td>88.2</td><td>Equilibrated</td><td>opx (Mg# 89.2, Wo 4.9)<br>aug (Mg# 89.9, Wo 36.7)</td><td>Oliv and aug<br>in opx; aug<br>in oliv</td><td>None<br>observed</td><td>~50:50 oliv:opx;<br>1% aug</td><td>Hughes<br>group</td><td>One graphite-<br>rich region</td><td>1161</td><td>6.10</td></tr><tr><td>Calama 001</td><td>88.4</td><td>Equilibrated</td><td>Opx (Mg# 89.7, Wo 4.9)<br>aug (Mg# 89.7, Wo 37.2)</td><td>nd</td><td>in oliv</td><td>45% oliv; 50%<br>opx; 5% aug</td><td>Hughes<br>group</td><td>Graphite flakes<br>to 1 mm</td><td>1145</td><td>6.8</td></tr></tbody></table>

References: 1) Goodrich et al. (2001); 2) Goodrich et al. (2009); 3) Fioretti and Molin (1996); 4) Smith et al. (2000); 5) Goodrich et al. (2006); 6) This work; 7) Collinet and Grove (2020a); 8) Inoue et al. (2016); 9) Goodrich, Goldstein, et al. (2010); 10) Day et al. (2017).

aug = augite; nd = not determined; oliv = olivine; opx = orthopyroxene; pig = pigeonite.

Fo = 100 * molar MgO/(MgO + FeO) in olivine.

<sup>a</sup>Clasts from polymict ureilite Almahata Sitta.

<sup>b</sup>See Fig. 4 and supporting information S5.

<sup>c</sup>Temperature from Collinet and Grove (2020a) or calculated using supporting information table S8 of that paper. See supporting information S4 and S5.
with its clasts landing on Earth as separated, centimeter-sized, stones (Goodrich, Hartmann, et al., 2015; Jenniskens et al., 2009). Two of these stones, MS-MU-011 (24.2 g) and MS-MU-035 (26.9 g), are ureilitic trachyandesites that appear to be larger and more representative clasts of the albitic lithology than those found in other polymict ureilites (Barnes et al., 2019; Bischoff et al., 2014, 2016; Delaney et al., 2015). The sample studied in this work, MS-MU-012, is a new type of feldspathic ureilitic lithology discovered as a clast from AhS.

## Accretion and Differentiation of the Ureilite Parent Body (UPB)

Several lines of evidence indicate that all ureilites originate from a single UPB (Goodrich et al., 2004; Goodrich, Hartmann, et al., 2015; Scott et al., 1993). Thus, a fundamental question is how the observed large range of Mg# (Fo of olivine or bulk sample Mg#) was established on this asteroid. The observed variation in Mg# does not conform to igneous fractionation trends. For example, igneous fractionation would produce a trend of nearly constant Fe/Mn over a large range of Fe/Mg in olivine (or pyroxenes). In contrast, ureilite olivine shows a trend of nearly constant Mn/Mg over a large range of Fe/Mg ratio (i.e., variation in FeO only), which appears to represent either reduction-oxidation or mixing processes (Goodrich & Delaney, 2000). Furthermore, the Mg# variation is correlated with a variation in $\Delta^{17}O$ that is much greater than that in any other group of achondrites or related igneous rocks (Clayton & Mayeda, 1988, 1996; Rumble et al., 2010), as well as with variation in $^{12}C/^{13}C$ ratio (Barrat et al., 2017). Although pressure (depth)-dependent, carbon-controlled redox control (known as equilibrium smelting) is an obvious candidate for producing the Mg# variation (Berkley & Jones, 1982; Goodrich et al., 2004, 2007; Singletary & Grove, 2003; Walker & Grove, 1993), smelting cannot produce the observed correlation of Mg# with $\Delta^{17}O$, which suggests that the Mg# and correlated isotopic variations were primordial, accretionary characteristics (Barrat et al., 2017; Clayton & Mayeda, 1988; Rai et al., 2020; Sanders et al., 2017; Warren, 2012; Warren & Huber, 2006; Warren & Kalllemeyn, 1992).

If the Mg# and correlated isotopic variations among ureilites were inherited from the nebular precursors of the UPB, it raises the question of how such heterogeneities were preserved during high-temperature igneous processing. At least part of the answer to this question seems to be that ureilites are predominantly partial melt residues from similar degrees of melting (Goodrich, 1999; Goodrich

et al., 2004, 2007; Scott et al., 1993; Warren & Kalllemeyn, 1992), rather than cumulates as they were once thought to be (Berkley et al., 1980; Goodrich et al., 1987). The principal arguments for this interpretation are that isotopic and chemical heterogeneities are more likely to be preserved in unmelted residues than in minerals that crystallized from melts (Goodrich et al., 2004, 2007; Scott et al., 1993; Warren & Kalllemeyn, 1992), and that main group ureilites are so abundant that they are more likely to represent the predominant rock type on their parent asteroid rather than the volumetrically minor rock type they would be in a fractionation/ accumulation scenario (Scott et al., 1993). The interpretation that ureilites are residues is currently the prevailing paradigm in which to understand the differentiation of the UPB.

Thermal and physical modeling, combined with evidence from short-lived radionuclides, indicates that the UPB accreted $\le 1$ Myr after calcium-aluminum-rich inclusion (CAI) formation and shortly thereafter was heated by decay of $^{26}Al$, experiencing partial melting and melt extraction (Baker et al., 2012; Goodrich, Hartmann et al., 2015; Kita et al., 2013; Van Kooten et al., 2017; Wilson et al., 2008). Peak temperatures up to 1280 °C were recorded by the olivine-pigeonite and olivine-orthopyroxene ureilites (Collinet & Grove, 2020a). Physical modeling indicates that extraction of melts from the mantle of the UPB would have been highly efficient, as a result of the rapid formation of networks of veins and dikes in response to high initial rates of melt production (Wilson et al., 2008; Wilson & Keil, 2012). This process would have been chemically equivalent to fractional (as opposed to batch) melting, which could explain the preservation of oxygen isotope and Mg# heterogeneity in the residual mantle (Goodrich et al., 2007). Furthermore, Collinet and Grove (2020a, 2020b) showed experimentally that incremental melting (with small increments of ~2–5 wt%, approaching fractional melting) of chondritic materials produces characteristic properties of main group ureilites that batch melting cannot, in particular the dominance of pigeonite as the main pyroxene at temperatures above 1200 °C (Goodrich, 1999). Based on this experimental work, combined with the mineralogy and equilibration temperatures of main group ureilites and their bulk rare earth element (REE) concentrations (Barrat et al., 2016), cumulative degree of melting ranged from ~15 to 24 wt% (Collinet & Grove, 2020a). Plagioclase was completely eliminated from the source regions at ~15 wt% melting (~1120–1160 °C, depending on Mg#), after which the rate of melt extraction (as a function of time) decreased significantly due to the removal of the main heat source, $^{26}Al$ (Collinet & Grove, 2020b; Goodrich, Hartmann, et al., 2015;
Wilson et al., 2008) and the decrease in melt productivity (as a function of temperature increase) following the disappearance of plagioclase (Collinet & Grove, 2020a).

The fate of the extracted melts is not directly known, as there are no “basaltic” (or other igneous type) ureilites. Some fraction of the melts may have erupted explosively and been lost to space due to generation of CO by pressure-release carbon reduction (disequilibrium or runaway smelting) during ascent (Warren & Kallemeyn, 1992), while at least 30% of the total melts produced could have been retained in shallow intrusions (Wilson et al., 2008). The indigenous feldspathic clasts in polymict ureilites may be remnants of the igneous rocks that crystallized in such intrusions. The mineralogy of the “albitic” lithology is consistent with that expected for very low-degree melts on the UPB, while the labradoritic lithology(ies) could represent higher degree melting increments (Collinet & Grove, 2020c; Goodrich, Treiman, & Boyle, 2017; Goodrich & Wilson, 2014). Alternatively, these lithologies have been interpreted as products of secondary, impact-induced melting rather than primary igneous rocks (Van Kooten et al., 2017).

The augite-bearing ureilites also appear to contain a melt component. Based on their distinctive properties, they have been argued to be cumulates or paracumulates (defined as “mushy, cumulate-like partial melt residues” by Warren & Kallemeyn, 1989) rather than residues (Berkley, 1986, 1990; Berkley & Goodrich, 2001; Collinet & Grove, 2020a; Goodrich, 1992, 1999; Goodrich et al., 2001, 2004, 2006, 2009, 2014; Prinz et al., 1994; Takeda et al., 1989; Warren & Kallemeyn, 1994). These properties include (1) the presence of poikilitic and solid-melt reaction textures, (2) the presence of melt inclusions, (3) Fe-Mn-Mg compositions of olivine suggesting igneous fractionation relative to the low-Ca pyroxene ureilites, (4) δ¹⁸O compositions suggesting igneous fractionation relative to the low-Ca pyroxene ureilites, (5) higher Al and Ca contents than the low-Ca pyroxene ureilites, (6) pyroxene/olivine ratios that do not conform to the low-Ca pyroxene ureilite trend as a function of Fo, and (7) lower temperatures of equilibration than the low-Ca pyroxene ureilites. Thus, although they are ultramafic rocks, the augite-bearing ureilites may provide a link between the low-Ca pyroxene ureilites (mantle residues) and the feldspathic clasts in polymict ureilites (crustal rocks).

## Catastrophic Disruption of the Ureilite Parent Body

The igneous evolution of the UPB was interrupted (i.e., terminated prematurely) before it had cooled substantially from magmatic temperatures. All main group ureilites show petrologic evidence of very rapid

cooling (1–20 °C h⁻¹), accompanied by a drop in pressure, through the temperature range from ~1050 ± 50 to ~600 °C (Herrin et al., 2010; Mittlefehldt et al., 1998). This distinctive thermal history is interpreted to result from catastrophic impact disruption of the parent body, followed by reassembly of subsets of the debris into daughter bodies (Downes et al., 2008; Goodrich et al., 2004, 2021; Goodrich, Hartmann, et al., 2015; Herrin et al., 2010; Michel et al., 2015). These daughter bodies, then, became the source for the ureilitic materials that we have sampled, with polymict ureilites representing regolith developed on their outer layers. Catastrophic disruption of the UPB may explain the paucity of ureilitic crustal rocks in the meteoritic record; that is, the crust may have been completely shattered and largely dispersed by the impact, leaving only small fragments to be found in polymict ureilites. Thus, polymict ureilites offer our best chance to find missing pieces from the history of differentiation of the UPB.

## This Work

Here, we describe MS-MU-012, a recently discovered stone from the AhS polymict ureilite (Bischoff et al., 2015) that provides one of these pieces. MS-MU-012 is the first known plagioclase-bearing main group ureilite. We present detailed petrologic, oxygen isotope, and trace element data for this sample, and discuss its implications for the differentiation of the UPB. We argue that MS-MU-012 provides a critical link between the olivine + low-Ca pyroxene ureilitic residues, the augite-bearing ureilitic cumulates, and the albitic and labradoritic lithology clasts in polymict ureilites that represent ureilitic melts, and thus helps us piece together a more complete picture of the differentiation history of the UPB.

## METHODS

MS-MU-012 is an ~15.5 g stone (Fig. 1) from the AhS meteorite. We studied one polished thin section (PL14065) with an area of ~81.2 mm² (Fig. 2), and one polished thick section of similar area, made from this sample. The sections were studied by transmitted and reflected light microscopy using a Leica DMRX microscope equipped with a SpotFlex 64MP camera at the Lunar and Planetary Institute (LPI, USRA-Houston) and a ZEISS polarizing microscope (Axiophot) at the Institut für Planetologie at the University of Münster. They were studied by scanning electron microscopy using the JEOL 7600-FE at the Astromaterials Research and Exploration Science (ARES) division at NASA, Johnson Space Center (JSC), the JEOL 6610-LV at the Institut für
<figure><img src="image_2.png" /><figcaption>Fig. 1. Almahata Sitta stone MS-MU-012, with original mass 15.5 g and approximate dimensions of 2.8 × 2.5 × 2.4 cm. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

Planetologie at the University of Münster, and the ZEISS EVO in the Department of Geosciences at the University of Massachusetts, Amherst. Quantitative analysis of phases by wavelength-dispersive spectrometry was conducted using the JEOL 8530-FE and CAMECA SX-100 electron microprobes at ARES-JSC, the Cameca SXFIVE-Tactis at the University of Massachusetts (Amherst), and the JEOL JXA 8530F electron microprobe at the Institut für Mineralogie at the University of Münster. Natural and synthetic minerals, metals, and glasses were used as standards. ZAF matrix corrections were applied to all analyses. Mafic silicates were analyzed using 15 kV, 15–30 nA beam current, and ~1 µm diameter spot size. Feldspathic materials and glassy areas were analyzed using 15 kV, 10–15 nA beam current, and 2–5 µm diameter spot sizes. Metallic phases were analyzed using 15 kV, 40 nA beam current, and ~1 µm diameter spot size. All analyses are given in supporting information S2. Average or representative compositions of phases are given in tables in the paper.

Elemental X-ray maps were obtained using the NSS
Thermo Fischer Scientific 3 software with the energy-
dispersive detector on the JEOL 7600-FE at ARES
operating at 15 kV and 30 nA beam current. These
maps were combined and processed in ImageJ. Modal
abundances of phases were obtained by point counting
a grid of 1122 points overlain on a combined Mg-Ca-Al
X-ray map of the entire section in which olivine, augite,
orthopyroxene, and feldspathic areas were distinguished
by color (Fig. 2a).

We separated three aliquots of the plagioclase-rich
areas (~1.4, 2.4, and 2.5 mg) by digging them out of
the butt from which the polished section was made
with a needle and spatula, and then examining

them with a binocular microscope. Two of these
aliquots appeared to be pure plagioclase, while one was
possibly contaminated with olivine and/or pyroxene.
The three-oxygen isotope compositions of the separates
were measured at the University of Göttingen by laser
fluorination in combination with gas chromatography
(Pack et al., 2016). The δ¹⁷O and δ¹⁸O values are
reported relative to the standard mean ocean water
standard and Δ¹⁷O was calculated as 1000 × ln (δ¹⁷O/
1000 + 1) – 0.528 × 1000 × (δ¹⁸O/1000 + 1) following
Miller and Pack (2021). San Carlos olivine was used as
the in-house standard with Δ¹⁷O = –0.052‰ (0.528
reference line, average of Pack et al., 2016; Sharp
et al., 2016; Wostbrock et al., 2020). The uncertainties
were 0.1‰ and 0.2‰ for δ¹⁷O and δ¹⁸O, respectively,
and <0.01‰ for Δ¹⁷O.

Concentrations of REE in two spots (50 and 85 µm diameter) in one augite crystal were measured by laser ablation inductively coupled mass spectrometry (LA-ICP-MS) at the Institute of Mineralogy, University of Münster using an Analyte G2 Excimer Laser Ablation System (193 nm wavelength, 4 ns pulse length, and a repetition rate of 10 Hz at 3 J cm⁻²) coupled to a Thermo Scientific ELEMENT 2™ ICP-MS. He was used as the carrier gas. Backgrounds were measured for 20 s and peaks were measured for approximately 40 s. To verify external reproducibility, the USGS reference glasses BIR-1G and BCR-2G were analyzed sequentially (supporting information S3) and compared with previously published data (Jochum et al., 2005; https://georem.mpch-mainz.gwdg.de). All obtained concentrations are in the range of these published values. NIST 612 was analyzed as an external reference material and Si was used as an internal reference. The raw data were reduced using the Glitter software by manually defining the measured background and the sample.

Concentrations of REE in two 90 µm diameter
spots in plagioclase-rich areas were measured by LA-
ICP-MS at Institut Universitaire Européen de la Mer,
Plouzané. The analyses were performed under a He
atmosphere using an Excimer (193 nm wavelength) laser
ablation system (Geolas Pro102), connected to a
Thermo Element 2 spectrometer. Analytical and data
reduction procedures followed those used by Barrat
et al. (2009). Results were normalized to CaO
abundances, measured by electron microprobe as
internal standards to account for a variable ablation
yield. The BCR-2G glass standard was used for external
calibration of relative element sensitivities, using values
given by Jochum et al. (2005). Replicate analyses of the
USGS basaltic-glass standard BIR-1G run (supporting
information S3) at intervals during the analytical session
(Barrat et al., 2014) yielded an external reproducibility
generally better than 5% (1 RSD).
<figure><img src="image_3.png" /></figure>

<figure><img src="image_4.png" /><figcaption>Fig. 2. Studied polished section of MS-MU-012. a) Combined elemental X-ray map. Red = Mg; Green = Ca; Blue = Al. b) Backscattered electron image (BEI). Oliv = olivine; opx = orthopyroxene; aug = augite; plag = plagioclase. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>
# RESULTS

## Petrology

The analyzed section of MS-MU-012 consists (by area) of ~52% olivine, 14% plagioclase-rich areas, 13% orthopyroxene, 11% augite, and minor metal and sulfide. Graphite was not observed. The silicate minerals show a coarse-grained, equilibrated texture, with gently rounded grain boundaries and common triple junctions (Fig. 2). Olivine and augite grains range up to 3.9 mm in maximum dimension. Orthopyroxene grains range up to 1.2 mm in maximum dimension. Small (~0.1–0.4 mm), sub-equant (subhedral to spherical) grains of olivine and orthopyroxene are poikilitically enclosed in augite grains (Figs. 2 and 3a,b). Melt inclusions (~5–13 µm in maximum dimension) consisting of glass ± subhedral crystals of Ca-rich pyroxene occur in several olivine crystals (Fig. 3c and 3d). The olivine + pyroxene mineralogy, textures, and occurrence of melt inclusions in MS-MU-012 are very similar to those of the Hughes-type augite-bearing main group ureilites (Table 1) (Goodrich et al., 2001, 2006, 2009).

The olivine, augite, and orthopyroxene have very homogeneous compositions (Table 2), except for the presence of very narrow reduction rims and veins in olivine, typical of all ureilites. Olivine is Fo 88.1 ± 0.1, with 0.25 ± 0.01 wt% CaO, 0.55 ± 0.01 wt% MnO, and 0.40 ± 0.01 wt% Cr₂O₃. Orthopyroxene is Mg# 89.2 ± 0.1, Wo 4.5; augite is Mg# 90.2 ± 0.1, Wo 37.3 ± 0.1. These compositions, including minor element Al, Ti, Cr, Mn, and Na contents in the pyroxenes, are very similar to compositions of olivine and primary pyroxenes in augite-bearing ureilites Hughes 009 and FRO 90054 (Goodrich et al., 2001, 2006, 2009). The molar Fe/Mg versus Fe/Mn composition of the olivine (Fig. 4) plots within the cluster of Hughes group augite-bearing ureilites, which are offset from the trend of the low-Ca pyroxene ureilites. Compositions of glasses and daughter pyroxene crystals in the melt inclusions in olivine (Table 3) fall within the range of compositions of those in melt inclusions in olivine in Hughes 009 and FRO 90054 (Goodrich et al., 2001, 2009).

The plagioclase-rich areas have overall shapes and sizes similar to those of the olivine and augite, ranging up to 3.9 mm in maximum dimension, and also poikilitically enclose a few olivine and augite grains (Fig. 2a). Internally, these areas are heterogeneous (Figs. 5 and 6). A few patches (up to ~150 µm in size) appear to be homogeneous plagioclase in low-magnification images, but even these show a “mottled” texture at higher magnifications (Fig. 6b–d). The “mottled” texture appears as fine laths of plagioclase

with abundant interstitial sulfides and metal in optical microscopy (Fig. 5c and 5d). In backscattered electron images (BEI), it appears as numerous inclusions of submicron-sized equant rounded or subhedral “grains,” occurring in clusters or chains. These “grains” are brighter than most of the plagioclase, are surrounded by halos that are darker (Fig. 6b–d). In addition, the plagioclase areas contain ~10–150 µm diameter clusters of radially oriented skeletal pyroxene crystals and feldspar laths (Figs. 5e,f and 6e,f), as well as small “islands” of Ca-pyroxene that have a ragged, reacted appearance due to idiomorphic re-entrants of plagioclase (Fig. 6e). Where larger augite grains are poikilitically enclosed in the plagioclase areas, there is evidence of reaction at the augite–plagioclase contact. The augite has a narrow rim of Mg-enriched composition (based on energy-dispersive X-ray spectroscopy [EDS] spectra) and shows a resorbed texture (Fig. 7). In places along the contact, there are narrow regions consisting of numerous fine, idiomorphic plagioclase laths (often protruding into the augite), with interstitial glass (+crystallites) and sulfide (Fig. 7).

The composition of plagioclase in the most homogeneous areas is An 68.1 ± 2 (average of 32 analyses, Table 4). High magnification images of these areas suggest that they may consist of multiple fine laths of plagioclase all of the same composition, rather than larger single crystals. Broad beam analyses of other areas of the plagioclase show a similar composition, with slightly greater variations (Table 4). Broad beam analyses aimed at the skeletal pyroxene + feldspar clusters (Figs. 5e,f and 6e,f), in which individual phases are too small for quantitative analyses, show various non-stoichiometric compositions consistent with a mix of Na-rich plagioclase and calcic pyroxene (Table 4). Based on EDS spectra, compositions of the submicron-sized brighter grains in the mottled areas (Fig. 6b and 6d) appear to be significantly enriched in Al, and Mg, and depleted in Si, compared to surrounding plagioclase. Features that are texturally and compositionally very similar to these occur within plagioclase in the NWA 7325 ungrouped achondrite (Goodrich, Kita, et al., 2017; Irving et al., 2013; Weber et al., 2016), and have been inferred to be Mg-Al spinel grains surrounded by Na-enriched plagioclase (Goodrich, Kita, et al., 2017). Idiomorphic plagioclase crystals in the reaction zones between plagioclase and augite (Fig. 7) also appear to be enriched in Na compared with surrounding plagioclase, based on EDS spectra.

Metal (Si- and P-bearing kamacite) occurs along silicate grain boundaries in elongated strips, along with Cr-bearing troilite (Fig. 8a; Table 5), as in all ureilites. Phosphide grains occur along the edges of the metal
<figure><img src="image_5.png" /><figcaption>Fig. 3. BEI of polished section of MS-MU-012. a) Rounded chadocrysts of olivine and orthopyroxene in augite. b) Rounded chadocryst of olivine in augite. c, d) Melt inclusions in olivine, consisting of daughter pyroxene and glass. Ol = olivine; opx = orthopyroxene; aug = augite; pyx = pyroxene; gl = glass. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

(Fig. 8b and 8c; Table 5), a feature that is common in the Hughes-type augite-bearing ureilites (Fioretti et al., 1996; Goodrich et al., 2001, 2013; Goodrich, Goldstein, et al., 2010; Ross et al., 2011). Metallic spherules in the plagioclase areas consist of metal-phosphide-sulfide assemblages (Fig. 8d).

## Oxygen Isotopes

Oxygen isotope analyses of the three separates from plagioclase-rich areas in MS-MU-012 yielded average $\delta^{17}\text{O} = 2.63 \pm 0.57\%_\odot$ (2$\sigma$), $\delta^{18}\text{O} = 6.98 \pm 0.99\%_\odot$ (2$\sigma$), and $\Delta^{17}\text{O} = -1.0 \pm 0.06\%_\odot$ (2$\sigma$) (Table 6). This composition is within the range of compositions of main group ureilites, main group ureilitic stones from AhS, and ureilitic mineral clasts in polymict ureilites (Fig. 9). On a plot of $\Delta^{17}\text{O}$ versus Fo of olivine (Fig. 9b), MS-MU-012 plots among the cluster of Hughes-type samples, which have $\Delta^{17}\text{O}$ of approximately -1.0 (-0.8 to -1.1). On a three-oxygen isotope diagram (Fig. 9a), a least squares linear regression calculated for MS-MU-012 and five other Hughes-type samples for which data are available (three main group ureilites and two other clasts from polymict

ureilites) has a slope of ~0.46 ± 0.11 (1$\sigma$), consistent with a mass-dependent fractionation trend among these samples.

## Rare Earth Elements

Measured abundances of REE in augite- and plagioclase-rich areas of MS-MU-012 are given in Table 7 and shown on CI-normalized (Barrat et al., 2012) plots in Fig. 10. Results for the two augite analyses are very similar to one another and show an average CI-normalized pattern with a light REE (LREE) depletion (La = 0.24 and Sm = 2.94 × CI), a small negative Eu anomaly with Eu/Eu* (McLennan, 1989) = 0.33, and a slight heavy REE [HREE] depletion (Gd = 3.77 and Lu = 2.25 × CI). These abundances are essentially identical to those in the primary augite in augite-bearing ureilites Hughes 009 and FRO 90054 (Goodrich et al., 2009) within the observed variation of the latter (Fig. 10a). CI-normalized abundances of REE in the two analyses of plagioclase-rich areas are similar to one another and show a strong LREE depletion (average La = 0.009 and Sm = 0.142 × CI), large positive Eu
<table><caption>Table 2. Compositions of mafic minerals in MS-MU-012 from EMPA (wt% oxides).<sup>a</sup></caption><thead><tr><th rowspan="2"></th><th colspan="2">Olivine</th><th colspan="2">Orthopyroxene</th><th colspan="2">Augite</th></tr><tr><th>Avg. (39)</th><th>stdev (1σ)</th><th>Avg. (11)</th><th>stdev (1σ)</th><th>Avg. (28)</th><th>stdev (1σ)</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>40.0</td><td>0.1</td><td>56.0</td><td>0.1</td><td>53.5</td><td>0.1</td></tr><tr><td>TiO<sub>2</sub></td><td>bdl</td><td></td><td>0.09</td><td>0.01</td><td>0.18</td><td>0.02</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>bdl</td><td></td><td>1.20</td><td>0.03</td><td>2.00</td><td>0.06</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.466</td><td>0.01</td><td>0.95</td><td>0.01</td><td>1.23</td><td>0.02</td></tr><tr><td>FeO</td><td>11.5</td><td>0.1</td><td>6.9</td><td>0.0</td><td>3.9</td><td>0.0</td></tr><tr><td>MgO</td><td>47.8</td><td>0.2</td><td>32.2</td><td>0.1</td><td>20.0</td><td>0.1</td></tr><tr><td>MnO</td><td>0.548</td><td>0.01</td><td>0.54</td><td>0.01</td><td>0.39</td><td>0.01</td></tr><tr><td>CaO</td><td>0.254</td><td>0.01</td><td>2.38</td><td>0.02</td><td>18.3</td><td>0.1</td></tr><tr><td>Na<sub>2</sub>O</td><td>bdl</td><td></td><td>0.04</td><td>0.01</td><td>0.25</td><td>0.01</td></tr><tr><td>Total</td><td>100.6</td><td>0.3</td><td>100.3</td><td>0.2</td><td>99.7</td><td>0.2</td></tr><tr><td>Fe/Mg</td><td>0.135</td><td>0.001</td><td>0.121</td><td>0.001</td><td>0.108</td><td>0.001</td></tr><tr><td>Mg#</td><td>88.1</td><td>0.1</td><td>89.2</td><td>0.1</td><td>90.2</td><td>0.1</td></tr><tr><td>Fe/Mn</td><td>20.7</td><td>0.4</td><td></td><td></td><td></td><td></td></tr><tr><td>Wo</td><td></td><td></td><td>4.5</td><td>0.0</td><td>37.3</td><td>0.1</td></tr></tbody></table>

bdl = below detection limit.

Fe/Mg = molar FeO/MgO; Mg# = 100 × molar MgO/(MgO + FeO); Fe/Mn = molar FeO/MnO; Wo = 100 × molar CaO/(CaO + MgO + FeO).

anomaly (Eu/Eu* = 269), and slight HREE depletion (Gd = 0.119 and Lu = 0.083 × CI).

DISCUSSION

The olivine–augite–orthopyroxene mineral assemblage of MS-MU-012 indicates that this ureilite belongs to the augite-bearing group of main group ureilites (supporting information S1). The major and minor element compositions of the minerals and melt inclusions, REE abundances in augite, oxygen isotope compositions, and apparent absence of carbon phases, all indicate further that it belongs to the Hughes-type subgroup of augite-bearing ureilites (Table 1). However, compared with other Hughes-type ureilites (and all other main group ureilites), MS-MU-012 is unique in containing plagioclase.

The overall sizes and external shapes of the plagioclase areas in MS-MU-012 are similar to those of the mafic minerals, which suggests that they represent primary plagioclase grains. However, the internal features of these areas indicate that they experienced secondary melting and recrystallization. These features, including the mottled textures (due to the presence of submicron-sized grains inferred to be Mg-Al spinel), the clusters of Ca-rich pyroxene + Na-rich plagioclase laths, the reacted-looking pyroxene “islands,” and the reaction zones surrounding augite inclusions in the plagioclase, are all very similar to those observed in plagioclase in the ungrouped NWA 7325 achondrite (Goodrich, Kita, et al., 2017; Irving et al., 2013; Weber et al., 2016) and

in some enstatite chondrite impact melts (Keil, 2007). Following the arguments of Irving et al. (2013), Weber et al. (2016), and Goodrich, Kita, et al., (2017), we suggest that these features result from a short duration heating event, most likely due to impact shock. Peak temperatures would have been sufficient to totally melt plagioclase, Fe,Ni sulfide, and some augite, but not olivine or orthopyroxene. The clusters of skeletal pyroxene + Na-feldspar laths and the pyroxene “islands” (Fig. 6e and 6f) could result from partial melting and reaction of small augite inclusions in the original plagioclase, followed by rapid recrystallization, as detailed in Goodrich, Kita, et al., (2017). Based on these arguments, we infer that plagioclase was a primary mineral in MS-MU-012, making it the first known plagioclase-bearing main group ureilite.

Petrogenesis of MS-MU-012 and Hughes-Type Augite-Bearing Ureilites

Petrologic properties of the nine Hughes-type augite-bearing main group ureilites that have been studied beyond initial reports in the Meteorological Bulletin Database are summarized in Table 1. Five of these (Hughes 009, FRO 90054/93008, FRO 90228, NWA 11754, and NWA 3222), which we will refer to as the archetypal Hughes-type samples, have virtually identical properties that are consistent with them having formed in a common lithologic unit. These features include mineral assemblage (olivine–augite–orthopyroxene), modal abundances, major and minor element mineral

<footnote><sup>a</sup>All analyses given in supporting information S2.</footnote>
<figure><img src="image_6.png" /></figure>

Fig. 4. Plot of molar Fe/Mg versus Fe/Mn in olivine in ureilites. The olivine + low-Ca pyroxene ureilites show a well-defined trend of nearly constant Mn/Mg consistent with redox processes (pure Fe variation). Most augite-bearing ureilites plot at higher Fe/Mg relative to this trend, consistent with igneous fractionation (which produces nearly horizontal trends on this diagram). Hughes-type augite-bearing ureilites cluster tightly and are distinguished from the low-Ca pyroxene ureilite trend. Olivine in MS-MU-012 and AhS #15, both Hughes-type clasts from polymict ureilite Almahata Sitta, plot within the Hughes group. Trend for olivine + low-Ca pyroxene ureilites (a power law fit forced through the origin, calculated as in Goodrich et al., 2006) is based on a self-consistent, high-precision set of data (Goodrich et al. 2011) and references therein). Data for MS-MU-012 from this study. Data for Hughes-type samples: Hughes 009 (Goodrich et al., 2001); FRO 90054/93008 (Goodrich et al., 2006); FRO 90228 (Goodrich et al., 2006); NWA 11754 (Collinet & Grove, 2020a); Calama 001 (Inoue et al., 2016); MIL 091004 (Day et al., 2017; this work); NWA 3222 (Inoue et al., 2016); NWA 2895 (Inoue et al., 2016); EET 96314/96293/96331 (Collinet & Grove, 2020a; Goodrich et al., 2006; Inoue et al., 2016; Singletary & Grove, 2003); and AhS #15 (Goodrich, Goldstein, et al., 2010). Complete compositions of olivine in Hughes-type ureilites are given in supporting information S5. Data for other augite-bearing ureilites (Chikami et al., 1995; Collinet & Grove, 2020a; Day et al., 2017; Goodrich et al., 2001, 2006, 2009, 2014; Inoue et al., 2016; Rosén et al., 2019; Singletary & Grove, 2003; Sinha et al., 1997; Takeda, 1987; Takeda et al., 1989; Weber et al., 2003). (Color figure can be viewed at wileyonlinelibrary.com.)

compositions (e.g., Fig. 4), equilibrated (typical ureilitic) textures, the presence of melt inclusions in at least olivine (in some cases also the pyroxenes), the rarity (or apparent absence) of carbon phases, oxygen isotope compositions (Fig. 9), minor poikilitic features (chadocrysts of one phase in another), and equilibration temperatures based on the $Cr_{oliv-lpx}$ thermometer (Collinet & Grove, 2020a)

(supporting information S4). The common Mg#s (Fo 87–88) of these five samples, in particular, strongly suggest that they are derived from the same source region on the UPB.

MS-MU-012 shares all of these characteristic features of the archetypal Hughes-type ureilites. We suggest that it formed in the same lithologic unit as Hughes 009, FRO 90054/93008, FRO 90228, NWA 11754, and NWA 3222, and provides further insight into the formation of this unit, which we will refer to as the Hughes unit. Finding a Hughes-type ureilite clast in polymict ureilite AhS is not in itself surprising, since Hughes-type clasts are abundant in other polymict ureilites (Downes et al., 2008). Another AhS clast (AhS #15, a 75.5 g stone) sharing all the properties listed above has already been reported (Goodrich, Goldstein, et al., 2010; Ross et al., 2011; Zolensky et al., 2010). However, the presence of primary plagioclase in MS-MU-012 makes this sample unique compared with any previously studied Hughes-type (or other main group) ureilite sample.

Although some augite-bearing ureilites appear to be paracumulates, that is, partial melt residues in which significant amounts of melt were retained or introduced and then crystallized in place (Berkley, 1986, 1990; Berkley & Goodrich, 2001; Collinet & Grove, 2020a; Goodrich, 1992, 1999; Goodrich et al., 2001, 2004, 2006, 2009, 2014; Prinz et al., 1994; Takeda et al., 1989; Warren & Kallemeyn, 1994), the presence of melt inclusions in all three silicate minerals in the Hughes-type ureilites (particularly in olivine, which would have been the highest temperature phase) strongly suggests that these ureilites must have completely crystallized from melts, that is, they must be cumulates. The mass-dependent oxygen isotope trend observed among Hughes-type samples (Fig. 9a), as well as olivine Fe/Mg-Fe/Mn compositions that are consistent with igneous fractionation relative to the low-Ca pyroxene ureilites (Fig. 4), also support crystallization from a melt.

The magmatic evolution of Hughes 009 and FRO 90054 inferred from reconstructed compositions of the primary trapped liquids (PTL) in olivine, augite, and orthopyroxene (Goodrich et al., 2009) can be taken as the starting point for understanding the crystallization of the Hughes unit, including MS-MU-012. Goodrich et al. (2009) assumed that the melt trapped in olivine (olPTL) in Hughes 009 and FRO 90054 represented the parent magma of these rocks, and presented evidence that at the time of trapping, this melt was in equilibrium with olivine of ~Fo 83, rather than the current Fo 87–88 composition of the olivine. In contrast, the melts trapped in augite and orthopyroxene (augPTL and opxPTL, respectively) were in equilibrium with Fo 87–88 at the time of entrapment. Therefore,
<table><caption>Table 3. Compositions of phases in melt inclusions in olivine in MS-MU-012 from EMPA (wt% oxides).<sup>a</sup></caption><thead><tr><th rowspan="2"></th><th colspan="4">Inclusion 1</th><th colspan="4">Inclusion 2</th><th colspan="4">Inclusion 3</th><th colspan="4">Inclusion 4</th><th>Inclusion 5</th></tr><tr><th>Glass</th><th>Glass</th><th>Pyx</th><th>Pyx</th><th>Glass</th><th>Glass</th><th>Pyx</th><th>Pyx</th><th>Glass</th><th>Pyx</th><th>Pyx</th><th>Pyx</th><th>Pyx</th><th>Pyx</th><th>Pyx</th><th>Glass</th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>59.0</td><td>58.6</td><td>50.7</td><td>51.0</td><td>57.7</td><td>57.8</td><td>52.9</td><td>52.7</td><td>58.8</td><td>50.0</td><td>51.7</td><td>50.3</td><td>59.1</td><td></td><td></td><td></td></tr><tr><td>TiO<sub>2</sub></td><td>0.25</td><td>0.28</td><td>0.52</td><td>0.53</td><td>0.29</td><td>0.26</td><td>0.29</td><td>0.28</td><td>0.26</td><td>0.71</td><td>0.64</td><td>0.71</td><td>0.22</td><td></td><td></td><td></td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>22.6</td><td>22.4</td><td>5.7</td><td>5.5</td><td>21.9</td><td>21.9</td><td>3.6</td><td>3.7</td><td>22.2</td><td>5.8</td><td>4.5</td><td>6.1</td><td>23.8</td><td></td><td></td><td></td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.03</td><td>0.04</td><td>1.67</td><td>2.46</td><td>bdl</td><td>bdl</td><td>1.30</td><td>1.37</td><td>bdl</td><td>2.09</td><td>2.32</td><td>2.26</td><td>bdl</td><td></td><td></td><td></td></tr><tr><td>FeO</td><td>1.48</td><td>1.93</td><td>2.82</td><td>2.60</td><td>3.89</td><td>3.77</td><td>5.48</td><td>5.66</td><td>1.88</td><td>2.87</td><td>2.30</td><td>2.47</td><td>1.30</td><td></td><td></td><td></td></tr><tr><td>MgO</td><td>1.72</td><td>1.83</td><td>15.4</td><td>15.2</td><td>1.95</td><td>1.88</td><td>20.3</td><td>20.3</td><td>1.6</td><td>15.4</td><td>16.8</td><td>15.4</td><td>1.26</td><td></td><td></td><td></td></tr><tr><td>MnO</td><td>0.18</td><td>0.24</td><td>0.29</td><td>0.28</td><td>0.25</td><td>0.25</td><td>0.59</td><td>0.58</td><td>0.15</td><td>0.21</td><td>0.34</td><td>0.35</td><td>0.14</td><td></td><td></td><td></td></tr><tr><td>CaO</td><td>5.0</td><td>5.6</td><td>23.2</td><td>23.2</td><td>5.4</td><td>5.2</td><td>16.3</td><td>16.2</td><td>5.7</td><td>23.0</td><td>21.9</td><td>22.7</td><td>4.9</td><td></td><td></td><td></td></tr><tr><td>K<sub>2</sub>O</td><td>0.08</td><td>0.06</td><td>bdl</td><td>0.02</td><td>0.71</td><td>0.65</td><td>bdl</td><td>0.04</td><td>0.11</td><td>bdl</td><td>0.03</td><td>bdl</td><td>0.05</td><td></td><td></td><td></td></tr><tr><td>Na<sub>2</sub>O</td><td>10.2</td><td>9.7</td><td>0.58</td><td>0.67</td><td>7.6</td><td>7.49</td><td>0.29</td><td>0.31</td><td>10.1</td><td>0.66</td><td>0.64</td><td>0.68</td><td>10.5</td><td></td><td></td><td></td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>1.27</td><td>0.89</td><td>0.35</td><td>0.28</td><td>1.11</td><td>1.26</td><td>0.02</td><td>0.01</td><td>0.91</td><td>0.91</td><td>0.10</td><td>0.40</td><td>0.39</td><td></td><td></td><td></td></tr><tr><td>Total</td><td>101.8</td><td>101.4</td><td>101.3</td><td>101.6</td><td>100.9</td><td>100.4</td><td>101.0</td><td>101.1</td><td>101.8</td><td>101.6</td><td>101.2</td><td>101.4</td><td>101.6</td><td></td><td></td><td></td></tr><tr><td>Fe/Mg</td><td>0.48</td><td>0.59</td><td>0.10</td><td>0.10</td><td>1.12</td><td>1.12</td><td>0.15</td><td>0.16</td><td>0.66</td><td>0.10</td><td>0.08</td><td>0.09</td><td>0.58</td><td></td><td></td><td></td></tr><tr><td>Mg#</td><td>67.4</td><td>62.8</td><td>90.7</td><td>91.2</td><td>47.2</td><td>47.1</td><td>86.8</td><td>86.5</td><td>60.3</td><td>90.5</td><td>92.9</td><td>91.7</td><td>63.3</td><td></td><td></td><td></td></tr></tbody></table>

bdl = below detection limit.

Fe/Mg = molar FeO/MgO; Mg# = 100 × molar MgO/(MgO + FeO).

<sup>a</sup>All analyses given in supporting information S2.

Goodrich et al. (2009) proposed that the Hughes parent magma originated and began to crystallize olivine at a depth slightly greater than that at which it was finally emplaced (where the pyroxenes crystallized and the Hughes unit cumulates were formed) and was reduced (losing FeO) during ascent by local carbon redox control. The main argument of Goodrich et al. (2009) for original trapping in olivine of Fo 83 was that the Fe/Mg ratio of the residual liquid in the melt inclusion (i.e., the melt inclusion minus the olivine that crystallized onto the walls of the original trapped melt volume), which should represent reequilibration with the original host olivine composition (Danyushevsky et al., 2000), corresponds to equilibration with Fo 83, assuming $K_{DFe/Mg}^{ol/liq} = 0.33$. However, $K_D$ values for liquids with low MgO + FeO contents, like those of the residual liquid in the inclusions, may be lower than 0.33 (Collinet & Grove, 2020c; Toplis, 2005), possibly as low as 0.25. If the latter value is appropriate, then the residual liquid could have been in equilibrium with Fo ~86–87, closer to the present olivine composition. Nevertheless, an independent argument for reduction of the magma before emplacement is that normal magmatic evolution of olPTL by crystallization of olivine followed by olivine + augite does not lead to the compositions of augPTL or opxPTL (see fig. 12 of Goodrich et al., 2009). However, if olPTL evolves to slightly short of augite cosaturation by crystallization of olivine (or originates at this point) and then is reduced, the resulting melt composition (olPTL-R) is very similar to augPTL. This strongly suggests that some reduction occurred.

As discussed in Goodrich et al. (2009), the amount of olivine in olPTL is unconstrained, other than the minimum required for olivine saturation. The olPTL composition given in table 5 of Goodrich et al. (2009) was an example with a much higher olivine content than the minimum. Comparison with the experiments of Collinet and Grove (2020a, 2020c) shows that such a high olivine content (implying 14.5 wt% MgO) is unrealistic. The MgO content of primary melts produced in chondritic parent bodies correlates positively with the temperature of melting. A melt with an MgO content of 14.5 wt% would indicate temperatures of 1340 ± 20 °C in the source (Fig. 11). This is at least 60 °C higher than temperatures experienced by even the hottest pigeonite–olivine ureilites, assumed to represent the residual mantle. Experiments also show that at temperatures higher than 1280 °C, pigeonite completely disappears from mantle residues. Any melt produced above that temperature, in equilibrium with orthopyroxene + olivine, would have much less CaO (4–7 wt%) than the olPTL shown in table 5 of Goodrich et al. (2009). We have therefore calculated a revised olPTL, shown in Table 8, which has only slightly more than the minimum required olivine component, and is approximately the composition at which reduction would have begun, assuming the model outlined above. The MgO content of this melt (~10 wt %) is consistent with a melt derived from a pigeonite–olivine mantle at a temperature of 1260 ± 20 °C (Fig. 11). At this temperature, the CaO content of partial melts is at a maximum of 10–12 wt%, which is identical to the concentration in olPTL (fig. 11 of
<figure><img src="image_7.png" /></figure>

Fig. 5. Images of MS-MU-012. a) Plane-polarized light image showing the major phases olivine (oliv), orthopyroxene (opx), augite (aug), and plagioclase-rich areas (plag). b) Cross-polarized light image of same area as in (a). c) Higher magnification, plane-polarized light image of a plagioclase-rich area, showing mottled texture consisting of fine laths of plagioclase with abundant interstitial sulfides and metal (opaque, black). d) Cross-polarized light image of same area as in (c). e) Backscattered electron image (BEI) of a plagioclase-rich area. f) Combined elemental X-ray map of same area as in (e). Red = Mg; Green = Ca; Blue = Al. Area outlined by yellow box in (e) and (f) is shown at higher magnification in Fig. 6e and 6f. (Color figure can be viewed at wileyonlinelibrary.com.)

Collinet & Grove, 2020a). Based on this revised olPTL, we also recalculated olPTL-R, reduced from olPTL to be in equilibrium with Fo ~87. This composition is also shown in Table 8, along with augPTL and opxPTL from Goodrich et al. (2009).

Based on equilibrium crystallization calculations with MAGPOX, Goodrich et al. (2009) found that olPTL-R

and augPTL were initially saturated only with augite, but after a few percent crystallization, they become saturated with orthopyroxene and consistent with opxPTL (which is very similar to augPTL). Based on these results, they concluded that augPTL and opxPTL were trapped in their hosts at approximately the same time. Here, we have used MELTS (alphaMELTS 1.9) to calculate the early
<figure><img src="image_8.png" /><figcaption>Fig. 6. a) BEI of a plagioclase area that appears to be relatively homogeneous. b–d) BEI showing plagioclase areas containing numerous submicron-sized grains (brighter than the plagioclase and surrounded by darker halos). These grains are inferred to be spinel surrounded by Na-enriched plagioclase. e, f) BEI of the area outlined in Fig. 5e and 5f. The plagioclase-rich areas also contain ~10–150 µm diameter clusters of radially oriented skeletal pyroxene + feldspar crystals. Area outlined by box in (e) is shown at higher magnification in (f). Area circled in (e) shows an “island” of Ca-plagioclase with idiomorphic plagioclase re-entrants. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

crystallization sequence of the revised olPTL/olPTL-R. The scenario in which olPTL becomes reduced by ascent is simulated by combining calculations at $fO_2 = IW - 1.6$ and $fO_2 = IW - 1.9$ (Fig. 12a and 12b). Results show the crystallization of olivine only from 1260 to 1205 °C, followed by orthopyroxene + olivine from 1205 to 1195 °C, orthopyroxene + augite from 1195 to 1175 °C, and then plagioclase (An 65). We also calculated the crystallization sequence of augPTL and opxPTL (Fig. 12c and 12d), which show a brief interval of crystallization of olivine only from 1220 to 1200 °C, with augite appearing

between 1200 and 1210 °C, feldspar (An 69–71) at 1185 °C, and orthopyroxene at 1170 °C. These results are similar to those of Goodrich et al. (2009), and consistent with observed poikilitic relationships in the Hughes-type samples in indicating that olivine crystallized before either of the pyroxenes. We note that small differences in bulk CaO content have a strong influence on the temperatures and the order of appearance of the pyroxenes (in addition to plagioclase compositions and saturation temperatures). Slightly modified compositions (CaO reduced by 0.6 wt%) for augPTL and opxPTL (augPTL2 and opxPTL2)
<figure><img src="image_9.png" /><figcaption>Fig. 7. BEI showing contact between grains of augite (aug) enclosed in plagioclase (plag). a) Augite shows a resorbed texture. b, c) Augite commonly shows a band of Mg-enriched composition along the contact. In places along the contact, there are narrow regions consisting of numerous fine, idiomorphic plagioclase crystals, with interstitial glass (+crystallites) and sulfide. (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

<table><caption>Table 4. Compositions of plagioclase-rich areas in MS-MU-012 from EMPA (wt% oxides).<sup>a</sup></caption><thead><tr><th rowspan="2"></th><th colspan="2">"Homogeneous" plagioclase</th><th colspan="2">Other plagioclase</th><th colspan="6">Broad beam analyses of radial pyroxene + Na-plag clusters in plagioclase</th></tr><tr><th>Avg. (32)</th><th>Stdev (1σ)</th><th>Avg. (53)</th><th>Stdev (1σ)</th><th></th><th></th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>51.4</td><td>0.3</td><td>51.0</td><td>0.6</td><td>53.7</td><td>52.7</td><td>52.4</td><td>53.5</td><td>52.7</td><td></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>0.22</td><td>0.40</td><td>0.30</td><td>0.17</td><td>0.41</td><td></td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>30.1</td><td>0.24</td><td>29.7</td><td>0.60</td><td>15.2</td><td>3.5</td><td>4.0</td><td>13.4</td><td>4.1</td><td></td></tr><tr><td>FeO</td><td>0.33</td><td>0.14</td><td>0.28</td><td>0.15</td><td>0.28</td><td>0.48</td><td>0.49</td><td>0.27</td><td>0.44</td><td></td></tr><tr><td>MgO</td><td>0.30</td><td>0.06</td><td>0.37</td><td>0.23</td><td>9.2</td><td>19.3</td><td>20.0</td><td>12.1</td><td>19.9</td><td></td></tr><tr><td>MnO</td><td>0.01</td><td>0.02</td><td>0.02</td><td>0.02</td><td>0.18</td><td>0.39</td><td>0.40</td><td>0.22</td><td>0.46</td><td></td></tr><tr><td>CaO</td><td>13.9</td><td>0.39</td><td>13.4</td><td>0.4</td><td>15.2</td><td>21.2</td><td>20.1</td><td>17.2</td><td>20.0</td><td></td></tr><tr><td>K<sub>2</sub>O</td><td>0.01</td><td>0.01</td><td>0.01</td><td>0.01</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td>bdl</td><td></td></tr><tr><td>Na<sub>2</sub>O</td><td>3.6</td><td>0.2</td><td>3.9</td><td>0.2</td><td>3.1</td><td>0.3</td><td>0.3</td><td>2.3</td><td>0.4</td><td></td></tr><tr><td>Total</td><td>99.6</td><td>0.3</td><td>98.7</td><td>0.6</td><td>97.1</td><td>98.3</td><td>98.0</td><td>99.2</td><td>98.3</td><td></td></tr><tr><td>An</td><td>68.1</td><td>1.5</td><td>65.6</td><td>1.7</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Or</td><td>0.04</td><td>0.05</td><td>0.05</td><td>0.1</td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ab</td><td>31.9</td><td>1.5</td><td>34.3</td><td>1.7</td><td></td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>

An = molar CaO/(CaO + Na₂O + K₂O); Or = molar K₂O/(CaO + Na₂O + K₂O); Ab = molar Na₂O/(CaO + Na₂O + K₂O); bdl = below detection limit.

<sup>a</sup>All analyses given in supporting information S2.

illustrate this effect (Fig. 12e and 12f). In addition, for those compositions, plagioclase appears after both pyroxenes (compare Fig. 12c to Fig. 12e and Fig. 12d to Fig. 12f). Overall, MELTS calculations suggest that augite and orthopyroxene co-crystallized between 1200 and 1180 °C in olPTL, augPTL, and opxPTL. Furthermore, textural relationships of the Hughes-type samples are ambiguous regarding the relative appearances of the pyroxenes. Some samples show augite enclosed in orthopyroxene while others show orthopyroxene enclosed in augite, and some show both (Table 1). These

observations suggest that orthopyroxene and augite essentially co-crystallized, with local variations in which pyroxene appeared first. Co-crystallization is also consistent with the dominantly equilibrated, rather than poikilitic, textures, of these samples.

The two-pyroxene (orthopyroxene-augite) equilibration temperature determined for Hughes 009 and FRO 90054 using QUILF (Andersen et al., 1993; Lindsley, 1983) based on Ca exchange is 1250° ± 38 °C (Goodrich et al., 2009). However, the experiments of Collinet and Grove (2020a) showed that the transition
<figure><img src="image_10.png" /><figcaption>Fig. 8. BEI showing metal in MS-MU-012. a) Metal occurs mainly along silicate grain boundaries, as in all ureilites. b) c) Metal consists mostly of Si-bearing kamacite. Phosphide (schreibersite) occurs commonly as grains or discontinuous borders along the margins of the metal where in contact with silicates, which is a feature typically seen in Hughes-type ureilites (Fioretti et al., 1996; Goodrich et al., 2013; Goodrich, Goldstein, et al., 2010; Ross et al., 2010). d) Metallic spherule in plagioclase-rich area consists of Si-rich kamacite with lamellae and blebs of phosphide, plus Cr-bearing troilite (FeS). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

<table><caption>Table 5. Compositions of metallic phases in MS-MU-012 from EMPA (wt%).<sup>a</sup></caption><thead><tr><th rowspan="2"></th><th colspan="2">Metal<sup>b</sup></th><th colspan="2">Sulfide</th><th colspan="2">Phosphide</th></tr><tr><th>Avg. (17)</th><th>Stdev (1σ)</th><th>Avg. (3)</th><th>Stdev (1σ)</th><th>Avg. (4)</th><th>Stdev (1σ)</th></tr></thead><tbody><tr><td>Fe</td><td>97.9</td><td>0.4</td><td>59.2</td><td>0.1</td><td>83.2</td><td>0.2</td></tr><tr><td>Mn</td><td>bdl</td><td></td><td>0.39</td><td>0.19</td><td>bdl</td><td></td></tr><tr><td>P</td><td>0.85</td><td>0.20</td><td>bdl</td><td></td><td>13.7</td><td>0.0</td></tr><tr><td>Si</td><td>0.46</td><td>0.13</td><td>0.03</td><td>0.03</td><td>0.06</td><td>0.02</td></tr><tr><td>Cr</td><td>0.02</td><td>0.01</td><td>2.81</td><td>0.37</td><td>0.04</td><td>0.00</td></tr><tr><td>S</td><td>bdl</td><td></td><td>38.0</td><td>0.1</td><td>0.23</td><td>0.01</td></tr><tr><td>Ni</td><td>1.58</td><td>0.06</td><td>0.02</td><td>0.01</td><td>2.17</td><td>0.10</td></tr><tr><td>Co</td><td>0.11</td><td>0.01</td><td>bdl</td><td></td><td>0.08</td><td>0.01</td></tr><tr><td>Total</td><td>100.9</td><td>0.4</td><td>100.5</td><td>0.1</td><td>99.5</td><td>0.3</td></tr></tbody></table>

bdl = below detection limit.

<sup>a</sup>All analyses given in supporting information S2.

<sup>b</sup>Some metal analyses may have overlapped thin lamellae or inclusions of phosphide (e.g., Fig. 8d).

from two pyroxenes to pigeonite-only stability occurred at lower temperatures than given by QUILF, by ~50 °C (consistent with the minimum temperature of pigeonite stability of Sack & Ghiorso, 1994), and would place an upper limit of ~1190–1200 °C on the pyroxene crystallization temperatures for Hughes/FRO, in agreement with our alphaMELTS calculations (Fig. 12).

These temperatures can be interpreted to correspond to the temperature of emplacement of the Hughes unit magma (i.e., at the depth corresponding to equilibrium with Fo 83), and crystallization of the main cumulate phases. The Cr<sub>oliv-lcp</sub> temperatures (based on the distribution of Cr between olivine and low-Ca pyroxene) of the Hughes-type ureilites are lower, ~1140–
<table><caption>Table 6. Oxygen isotope compositions (‰) of aliquots from the plagioclase-rich areas in MS-MU-012.</caption><thead><tr><th></th><th>mass (mg)</th><th>δ<sup>17</sup>O</th><th>δ<sup>18</sup>O</th><th>Δ<sup>'17</sup>O</th></tr></thead><tbody><tr><td>MS-MU-012 - sample 5</td><td>1.419</td><td>2.44</td><td>6.66</td><td>-1.023</td></tr><tr><td>MS-MU-012 - sample 4</td><td>2.536</td><td>2.50</td><td>6.73</td><td>-0.9996</td></tr><tr><td>MS-MU-012 - sample 3<sup>a</sup></td><td>2.39</td><td>2.96</td><td>7.55</td><td>-0.966</td></tr><tr><td>average</td><td></td><td>2.63 ± 0.57 (2σ)</td><td>6.98 ± 0.99 (2σ)</td><td>-0.996 ± 0.06 (2σ)</td></tr></tbody></table>

1170 °C (Collinet & Grove, 2020a) (supporting information S4 and S5), which may represent the main period of equilibration and the temperature of the cumulate pile just before impact disruption of the parent body.

The most significant difference between MS-MU-012 and the other Hughes-type ureilites is the presence of plagioclase in the former. Poikilitic relationships indicate that the plagioclase crystallized later than olivine and augite, that is, it was the last phase to crystallize. The temperature of equilibration of MS-MU-012 (1144 °C) is slightly lower than the other Hughes type ureilites and could explain why it is the only sample containing plagioclase. Based on our MELTS calculations for olPTL-R and augPTL2 (Fig. 12), plagioclase should have begun to crystallize at 1170–1180 °C, which is slightly higher than the plagioclase in-out temperature (1140–1170 °C) of Collinet and Grove (2020b, 2020c) at the appropriate f/O₂ (IW – 1.9). Regardless, the equilibration temperatures of all Hughes-type ureilites seem within error of the temperature of plagioclase appearance (fig. 8 of Collinet & Grove, 2020a) (supporting information S4 of this paper). The absence of plagioclase in the other Hughes-type ureilites could be due to delayed nucleation (e.g., Grove & Bence, 1979) or the efficient expulsion of the residual melt from the cumulate unit prior to further cooling from 1180 °C (MELTS plag-in) to 1140–1170 °C, the final equilibrium temperature recorded by Hughes ureilites.

The compositions of plagioclase crystallizing from augPTL and opxPTL, based on MELTS, are An 50–70 (An 65–71 at plagioclase saturation), which is similar to the compositions of the most anorthitic plagioclase clasts from the “labradoritic lithology” in polymict ureilites (Cohen et al., 2004; Goodrich, Treiman, & Boyle, 2017). The pyroxenes in these clasts also have Mg# of ~88–90, similar to those in the Hughes-type samples (Goodrich, Kita, et al., 2017). Thus, the labradoritic lithology may have crystallized from a residual melt that was expelled from the Hughes unit. Furthermore, the composition of the plagioclase in MS-MU-012 is An ~68, which is consistent with early crystallization from a melt similar to augPTL or opxPTL, supporting the formation of this sample in the Hughes unit.

If this interpretation is correct, abundances of REE in the plagioclase should reflect crystallization from essentially the same melt as that from which the pyroxenes crystallized. Figure 10c shows calculated REE abundances for the melt from which the average augite in MS-MU-012, as well as the average augite in Hughes 009 and FRO 90054 (Goodrich et al., 2009), crystallized, based on partition coefficient values ($D_{cpx/melt}$) from Barrat et al. (2016). They are essentially identical to those calculated to be in equilibrium with the augite and the orthopyroxene in Hughes 009 and FRO 90054 by Goodrich et al. (2009). Figure 10c also shows calculated REE abundances for melts predicted to be in equilibrium with the average composition of the two 90 {m-sized spots from the plagioclase-rich areas in MS-MU-012, based on six different sets of experimentally determined values for $D_{plag/melt}$ (Barrat et al., 2016; Dygert et al., 2020; Sun et al., 2017). None of these predicted melts provides a good match to the melt in equilibrium with augite. The $D_{plag/melt}$ values from Barrat et al. (2016), which were specifically selected to model the REE concentrations of ureilites, provide the best fit overall. The predictions for Sm, Eu, Gd, and Eu/Eu* are within a factor of ~1.8 × those of the melt in equilibrium with augite. The most significant difference is that the melt predicted from the plagioclase REE concentrations shows a strong LREE depletion (La/Sm = 0.03), while the melt in equilibrium with augite has an essentially flat LREE pattern (Fig. 10c).

It seems likely, given the obvious evidence for secondary disturbance of the plagioclase-rich areas, that the measured REE abundances do not represent the primary plagioclase, but rather have been affected by the remelting and recrystallization they experienced. Both the textures (Fig. 6) and compositions of the plagioclase-rich areas suggest that some amount of surrounding augite was remelted and incorporated into the plagioclase-rich melt, and this is supported by reaction textures at augite–plagioclase contacts (Fig. 7). However, if we assume a theoretical plagioclase composition calculated to be in equilibrium with the same melt as the augite (Fig. 10d), it is apparent that addition of augite to this composition cannot result in the low LREE abundances and extreme LREE depletion observed in the plagioclase-

<footnote><sup>a</sup>Sample 3 may be contaminated by surrounding olivine and/or pyroxene.</footnote>
<figure><img src="image_11.png" /><figcaption>Fig. 9. a) Three-oxygen isotope diagram for main group ureilites and clasts in polymict ureilites. Five Hughes-type samples including MS-MU-012 yield a linear regression with slope 0.46 ± 0.11, consistent with a mass-dependent fractionation trend among these samples. b) Δ¹⁷O (‰) versus Fo in olivine for main group ureilites and clasts in polymict ureilites (AhS and EET 87720). Hughes-type samples (including clasts from polymict ureilites) plot in a cluster with Δ¹⁷O approximately -1‰. MS-MU-012 (this work) plots within this cluster. Symbols same as in (a). Sources of data: main group ureilites including Hughes 009 and FRO 90054 (Clayton & Mayeda, 1996); ureilitic clasts from AhS including AhS #15 (Horstmann & Bischoff, 2014; Rumble et al., 2010); olivine and pyroxene in other polymict ureilites, including EET 87720 Gr23 (Downes et al., 2008); plagioclase in polymict ureilites (Kita et al., 2004, 2006); EET 87720 Gr 23 (Downes et al., 2008); MIL 091004 (Day et al., 2017); ALM-A (Bischoff et al., 2014). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

rich areas because the augite has higher LREE abundances than the plagioclase (Fig. 10d). A contaminant with lower LREE abundances, such as

olivine, is required. However, a simple calculation shows that contamination of this theoretical plagioclase by >50% olivine would be required to produce the measured composition. This is unlikely, considering that there is no evidence for remelting of olivine, even along plagioclase-olivine contacts.

The only process that is likely to have led to the extreme LREE depletion observed is loss of a small fraction of incompatible element-enriched melt during remelting. There is, in fact, evidence for segregation of a small amount of melt along the contacts between the plagioclase-rich areas and adjacent or enclosed augite grains (Fig. 7b and 7c), and such melts may not have been included in the analyzed plagioclase areas. Ureilites that have suffered a large degree of shock melting (e.g., Warren & Rubin, 2010) are believed to have lost a substantial fraction of their REE (Collinet & Grove, 2020a). In MS-MS-012, the shock melting of plagioclase could have mostly affected the incompatible LREE, leaving the middle REE and HREE concentrations largely unchanged. The concentration of the latter in the plagioclase-rich areas and the modeled plagioclase concentrations assuming augite-plagioclase-melt equilibrium (Fig. 10d) are in overall agreement and consistent with the co-crystallization of augite and plagioclase from the same melt.

Carbon in the Hughes Unit

The apparent absence of carbon phases is one of the characteristics of Hughes-type samples (Downes et al., 2008; Goodrich et al., 2001), and is common to MS-MU-012 and the archetypal Hughes-type samples (Table 1). Nevertheless, the presence of typical ureilite olivine reduction rims in all these samples suggests the former presence of graphite, thus implying that although the abundance of carbon in the Hughes unit magma was low (with most of it having been consumed by secondary reduction), it was not zero. The low abundance of carbon might be explained if these samples were cumulates rather than paracumulates, since graphite very likely had a lower density than the silicate melt and therefore could have formed flotation cumulates separated from the silicate cumulates that we have sampled. In addition, graphite is a refractory phase that would have been preferentially retained in the unmelted (residual) mantle, and despite its low density may not have been carried in large quantities as a solid phase in ascending melts due to its large grain size relative to the size of melt conduits. The latter explanation has been invoked to explain the absence of carbon in feldspathic clasts in polymict ureilites that represent the extracted ureilitic melts (Wilson et al., 2008).
<table><caption>Table 7. Concentrations (ppm) of rare earth elements in augite- and plagioclase-rich areas in MS-MU-012.</caption><thead><tr><th rowspan="2"></th><th colspan="4">Augite<sup>a</sup></th><th colspan="4">Plagioclase-rich areas</th></tr><tr><th>4-a</th><th>4-b</th><th>Average</th><th>SD<sup>b</sup></th><th>1</th><th>2</th><th>Average</th><th>SD<sup>b</sup></th></tr></thead><tbody><tr><td><sup>139</sup>La</td><td>0.072</td><td>0.086</td><td>0.079</td><td>±0.007</td><td>0.002</td><td>0.002</td><td>0.002</td><td>±0.0001</td></tr><tr><td><sup>140</sup>Ce</td><td>0.481</td><td>0.588</td><td>0.535</td><td>±0.054</td><td>0.020</td><td>0.024</td><td>0.022</td><td>±0.0021</td></tr><tr><td><sup>141</sup>Pr</td><td>0.152</td><td>0.158</td><td>0.155</td><td>±0.003</td><td>0.005</td><td>0.006</td><td>0.005</td><td>±0.0007</td></tr><tr><td><sup>146</sup>Nd</td><td>1.120</td><td>1.420</td><td>1.270</td><td>±0.150</td><td>0.041</td><td>0.048</td><td>0.044</td><td>±0.0033</td></tr><tr><td><sup>147</sup>Sm</td><td>0.599</td><td>0.660</td><td>0.630</td><td>±0.031</td><td>0.027</td><td>0.017</td><td>0.022</td><td>±0.0050</td></tr><tr><td><sup>153</sup>Eu</td><td>0.077</td><td>0.106</td><td>0.092</td><td>±0.015</td><td>2.001</td><td>2.107</td><td>2.054</td><td>±0.0529</td></tr><tr><td><sup>157</sup>Gd</td><td>0.940</td><td>1.230</td><td>1.085</td><td>±0.145</td><td>0.023</td><td>0.026</td><td>0.025</td><td>±0.0013</td></tr><tr><td><sup>159</sup>Tb</td><td>0.166</td><td>0.214</td><td>0.190</td><td>±0.024</td><td>0.004</td><td>0.006</td><td>0.005</td><td>±0.0015</td></tr><tr><td><sup>163</sup>Dy</td><td>0.970</td><td>1.320</td><td>1.145</td><td>±0.175</td><td>0.012</td><td>0.040</td><td>0.026</td><td>±0.0141</td></tr><tr><td><sup>165</sup>Ho</td><td>0.229</td><td>0.251</td><td>0.240</td><td>±0.011</td><td>0.004</td><td>0.010</td><td>0.007</td><td>±0.0033</td></tr><tr><td><sup>166</sup>Er</td><td>0.534</td><td>0.640</td><td>0.587</td><td>±0.053</td><td>0.010</td><td>0.021</td><td>0.016</td><td>±0.0058</td></tr><tr><td><sup>169</sup>Tm</td><td>0.074</td><td>0.094</td><td>0.084</td><td>±0.010</td><td></td><td></td><td></td><td></td></tr><tr><td><sup>172</sup>Yb</td><td>0.428</td><td>0.600</td><td>0.514</td><td>±0.086</td><td>0.015</td><td>0.039</td><td>0.027</td><td>±0.0121</td></tr><tr><td><sup>175</sup>Lu</td><td>0.069</td><td>0.083</td><td>0.076</td><td>±0.007</td><td>0.001</td><td>0.003</td><td>0.002</td><td>±0.0010</td></tr></tbody></table>

## Origin of the Hughes Unit Parent Magma

The inferred parent magma of the Hughes unit has the characteristics expected for a late, CaO-rich, alkali-poor melt in the context of a fractional (incremental batch) melt extraction model for the UPB (Collinet & Grove, 2020a). From a MELTS calculation for an OC-like starting composition similar to that inferred by Collinet and Grove (2020a) as a suitable starting composition for the UPB, we find that the most calcic plagioclase composition produced in a batch melting scenario, just at the point of exhaustion of plagioclase, would be An ~46. This composition is consistent with results of the experiments of Collinet and Grove (2020b, 2020c). However, using MELTS iteratively to simulate incremental batch (i.e., near fractional) melt extraction, we find that the final plagioclase composition would be An ~65–70, consistent with the compositions of the earliest plagioclase crystallizing from augPTL and opxPTL, and with the plagioclase in MS-MU-012. Furthermore, the compositions of augPTL and opxPTL are quite similar (Table 8) to the composition of such a “late” melt from the experiments of Collinet and Grove (2020a; their table 4). They estimated that such a melt would have 1–2 wt% Na₂O and would crystallize plagioclase of An 50–70, again consistent with augPTL, opxPTL, and MS-MU-012. Thus, the composition of the Hughes unit parent magma reinforces the conclusion that melt extraction on the UPB was a “fractional” (incremental) rather than batch process (Collinet & Grove, 2020a; Goodrich et al., 2007). We suggest that this late melt did not fully escape from the UPB mantle due to a substantial decrease in the rate of melt extraction late in the melting process, but was

emplaced after a short ascent into a shallow intrusion, possibly just below the inferred crustal intrusions formed by the bulk of the UPB melts (Wilson et al., 2008).

## Other Hughes-Type Samples

The discussion so far has been based on the archetypal Hughes-type ureilites (Table 1). Four other main group ureilites designated as Hughes-type in Table 1 (MIL 091004, Calama 001, EET 96314/96293/ 29262, and NWA 2895) share essential properties with the archetypal samples, for example, olivine-orthopyroxene-augite mineral assemblage, major and minor element mineral compositions (e.g., Fig. 4), and oxygen isotope compositions when available (Fig. 9), which suggests a similar petrogenesis. However, they also show some differences from the archetypal samples. In particular, they all have lower modal abundances of augite (a few vol% compared with 20–50 vol%), and all contain significant amounts of graphite. The lower abundance of augite might be explained if these samples were cumulates from a slightly later melt (i.e., later in the incremental melt extraction sequence) than the parent melt of the archetypal samples, produced after the depletion of augite from the source region. The presence of graphite might be explained if they were paracumulates rather than cumulates, but the similarity of their olivine Fe/Mg-Fe/Mn compositions to those of the archetypal samples argues against this. Alternatively, the degree of melting at this point might have been so high that residual graphite was disaggregated from its matrix and carried in the melt. MIL 091004 may be informative in this regard, because

<footnote><sup>a</sup>Analysis spot 4-a was 85 µm diameter, analysis spot 4-b was 50 µm diameter.</footnote>

<footnote><sup>b</sup>Standard deviation for the two analyses.</footnote>
<figure><img src="image_12.png" /><figcaption>Fig. 10. a) CI-normalized abundances of rare earth elements (REE) in augite and plagioclase (plag)-rich areas in MS-MU-012. Shown for comparison are REEs in augite in Hughes 009 and FRO 90054 (Goodrich et al., 2009). b) Six different sets of experimentally determined coefficients for the partitioning of REEs between plagioclase and silicate melt, *D*<sub>(plagioclase/melt)</sub>. c) CI-normalized REE abundances in melts calculated to be in equilibrium with the average augites in MS-MU-012 and Hughes 009/FRO 90054, and the average plagioclase-rich areas in MS-MU-012. Melts in equilibrium with augite calculated using *D*<sub>(cpx/melt)</sub> from Barrat et al. (2016). Melts in equilibrium with plagioclase-rich areas calculated for the six sets of *D*<sub>(plagioclase/melt)</sub> given in (c). d) CI-normalized REE abundances in a hypothetical plagioclase calculated to be in equilibrium with the same melt as the augite in MS-MU-012, using *D*<sub>(cpx/melt)</sub> and *D*<sub>(plagioclase/melt)</sub> from Barrat et al. (2016). CI rare earth element abundances from Barrat et al. (2012). (Color figure can be viewed at wileyonlinelibrary.com.)</figcaption></figure>

it appears to be mostly graphite-free like the archetypal
samples but contains one area that is extremely rich
(34 vol%) in graphite (Day et al., 2017) (see also
supporting information S6). This observation suggests
that the apparent absence of graphite in the archetypal
samples might be explained simply by large (>cm)-scale
heterogeneity in its distribution due to flotation in the
melt. NWA 2895 differs from all the other Hughes-type
samples in having a dominantly poikilitic texture and
containing pigeonite as the dominant low-Ca pyroxene

(the oikocrysts). It also has a higher Cr<sub>ol-pyx</sub> equilibration
temperature of 1193 °C (Table 1), consistent with the
orthopyroxene–pigeonite transition temperature (Collinet
& Grove, 2020a, Fig. 8; and supporting information S4),
which suggests that it originated from a slightly higher
degree melt than the archetypal Hughes-type samples. In
sum, the melts from which the “low-augite” Hughes-type
formed most likely originated from the same mantle
source region as the archetypal Hughes-type samples.
These melts may have been emplaced in a common
<figure><img src="image_13.png" /></figure>

Fig. 11. MgO content of melts as a function of temperature. Batch and incremental melting experiments of Collinet and Grove (2020a, 2020c) correlate positively with temperature. The trend can be fitted with linear and quadratic regressions, resulting in some uncertainty at the highest temperatures. Note that pMELTS and MELTS predict a similar melt MgO content trend during the crystallization of olPTL (parent magma composition inferred from melt inclusions in olivine in Hughes 009 and FRO 90054). Those trends can be used to estimate the temperatures at which estimated olPTL compositions would have been produced in the UPB. The extrapolated temperature (1325–1350 °C) of “olPTL old,” which had the maximum possible olivine component estimated by Goodrich et al. (2009), is not consistent with the melting of a pigeonite–olivine residue (needed to produce the high CaO content of olPTL) because pigeonite completely disappears above 1280 °C (Collinet & Grove, 2020a). The new olPTL calculated in this work, which has only slightly more than the minimum possible olivine component (see text), has a lower MgO content consistent with a melt derived from a pigeonite–olivine mantle at 1260 ± 20 °C. At this temperature, the CaO content of partial melts is at a maximum of 10–12 wt%, consistent with the concentration in the newly calculated olPTL (Table 8). (Color figure can be viewed at wileyonlinelibrary.com.)

intrusion with the archetypal samples, or in one or more
similar intrusions nearby.

Timing of Melt Generation, Extraction, and
Crystallization on the Ureilite Parent Body

Both thermal modeling and evidence from short-
lived radionuclides indicate that the earliest melts on
the UPB were generated and extracted from their
source regions in a short time period around ~1 Myr
after CAI (Baker et al., 2012; Goodrich, Hartmann,
et al., 2015; Kita et al., 2013; Van Kooten et al., 2017;
Wilson et al., 2008). However, later melts, such as the
one we propose to have been the parent melt of the
Hughes-type ureilites and the labradoritic feldspathic
clasts in polymict ureilites, may not have been produced

and extracted until several million years later due to the
significant decrease in melt productivity that followed
the exhaustion of plagioclase at ~15% melting (Collinet
& Grove, 2020a, 2020b; Goodrich, Hartmann, et al.,
2015; Wilson et al., 2008). The timing of crystallization
of melts on the UPB is less clear. Goodrich, Hutcheon,
et al. (2010) obtained concordant ²⁶Al-²⁶Mg and
⁵³Mn-⁵³Cr ages of 5.4 ± 0.4 Myr after CAI for clasts
from the albitic lithology. A Pb-isotopic isochron age of
5.4 ± 3.4 Myr after CAI for MS-MU-011 (Amelin
et al., 2015) is consistent with this timing. A younger
age of 6.5 ± 0.5 Myr after CAI was suggested by
²⁶Al-²⁶Mg data for seven equilibrated feldspar grains in
MS-MU-011 (Bischoff et al., 2014), but this remains to
be tested with an internal isochron. Goodrich,
Hutcheon, et al. (2010) argued that the 5.4 Myr after
CAI age was most plausibly interpreted as the time of
catastrophic disruption of the UPB (i.e., quenching of
the melt) as both mineral thermometry and thermal
modeling indicate that both the mantle and crust of the
UPB were still quite hot at this time. Thus, the parent
melt of the Hughes-type ureilites and labradoritic clasts
could have been generated, extracted, and emplaced in
an intrusive body at ~3–4 Myr after CAI, providing ~1–
2 Myr of slow cooling, and equilibration before impact
quenching at 5.4 Myr after CAI.

Alternatively, Van Kooten et al. (2017) proposed that feldspathic lithologies in polymict ureilites originated as impact melts, based on a study of Mg and Cr isotopes in clasts from polymict ureilite DaG 319. They obtained an ⁵³Mn-⁵³Cr age of 3.8 ± 1.5 Myr after CAI for nine feldspathic clasts (8 albitic, 1 labradoritic), marginally consistent with the 5.4 ± 0.4 Myr after CAI age of Goodrich, Hutcheon, et al. (2010). They also found that five olivine–orthopyroxene clasts of Fo 89–90, which they designated as HT clast (for Hughes-type) but characterized as different from Hughes-type main group ureilites, plotted at the intersection of the ⁵³Mn-⁵³Cr isochron for the feldspathic clasts and an older ⁵³Mn-⁵³Cr isochron defined by main group ureilitic clasts. They argued that the HT clasts were residues, which formed at ~3.8 Myr after CAI during impact remelting of older, Hughes-type cumulate ureilites, with extraction of melts to form the feldspathic clast lithologies.

We disagree with several aspects of this
interpretation (see also supporting information S10).
First, Van Kooten et al. (2017) characterized the HT
clasts as distinct from Hughes-type ureilites because of
the absence of augite. This might imply that the HT
clasts are similar to the low-augite Hughes-type
samples, rather than the archetypal ones (Table 1).
However, the absence of graphite in the HT clasts (Van
Kooten et al., 2017) suggests just the opposite.
<table><caption>Table 8. Melt compositions (wt% oxides).</caption><thead><tr><th></th><th>olPTL<sup>a</sup></th><th>olPTL-R<sup>b</sup></th><th>augPTL</th><th>opxPTL</th><th>"Late melt"<sup>c</sup></th></tr></thead><tbody><tr><td>SiO<sub>2</sub></td><td>51.4</td><td>53.2</td><td>54</td><td>54.8</td><td>55.7</td></tr><tr><td>TiO<sub>2</sub></td><td>0.74</td><td>0.77</td><td>0.4</td><td>0.41</td><td>0.3</td></tr><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>10.4</td><td>10.8</td><td>11.8</td><td>12.8</td><td>10</td></tr><tr><td>Cr<sub>2</sub>O<sub>3</sub></td><td>0.29</td><td>0.3</td><td>0.69</td><td>0.63</td><td>0.65</td></tr><tr><td>FeO</td><td>11.1</td><td>8.0</td><td>7.1</td><td>6.6</td><td>12.1</td></tr><tr><td>MgO</td><td>10.1</td><td>10.5</td><td>9.2</td><td>8.6</td><td>8</td></tr><tr><td>MnO</td><td>0.29</td><td>0.3</td><td>0.37</td><td>0.36</td><td>0.55</td></tr><tr><td>CaO</td><td>11.6</td><td>12.0</td><td>13.2</td><td>12.4</td><td>12.1</td></tr><tr><td>K<sub>2</sub>O</td><td>0.03</td><td>0.03</td><td>0.04</td><td>0.03</td><td>0.05</td></tr><tr><td>Na<sub>2</sub>O</td><td>2.27</td><td>2.4</td><td>2.05</td><td>2.52</td><td>1</td></tr><tr><td>P<sub>2</sub>O<sub>5</sub></td><td>1.72</td><td>1.8</td><td>1.11</td><td>0.86</td><td>0.1</td></tr><tr><td>Total</td><td>100</td><td>100</td><td>100</td><td>100</td><td>100.6</td></tr><tr><td>Fe/Mg</td><td>0.613</td><td>0.427</td><td>0.431</td><td>0.430</td><td></td></tr><tr><td>equil Fo</td><td>83.2</td><td>87.6</td><td>87.6</td><td>87.6</td><td></td></tr></tbody></table>

Fo = 100 × molar MgO/(MgO + FeO).

<sup>a</sup>Based on method of Goodrich et al. (2009), with 30 mol% olivine added to urLAG, and Fe/Mg adjusted for equilibrium (equil) with Fo 83 assuming KD<sub>FeMg</sub>(ol/liq) = 0.33.

<sup>b</sup>olPTL reduced by removal of FeO.

<sup>c</sup>Collinet and Grove (2020a), Table 4.

Therefore, it seems likely that all Hughes-type ureilites and HT clasts have a common petrogenesis, with differences in graphite abundance being due to flotation of graphite in the melts, as suggested above. Van Kooten et al. (2017) also state that the Mn/Mg ratios of orthopyroxene in the HT clasts differ from those of Hughes-type ureilites. However, a plot of Fe/Mg-Fe/Mn for pyroxenes in all main ureilites and ureilitic clasts in polymict ureilites (supporting information S7 and S8) shows that this is not the case. Orthopyroxenes in all Hughes-type ureilites plot in a cluster that is offset to higher Fe/Mg (for the same Fe/Mn), relative to the trend of low-Ca pyroxenes in the olivine + low-Ca pyroxene ureilites. Orthopyroxene in the HT clasts plot within this same cluster, as do Hughes-like clasts (with and without augite) in other polymict ureilites studied by Downes et al. (2008). Thus, there does not appear to be any petrologic distinction between the HT clasts of Van Kooten et al. (2017) and Hughes-type main group ureilites, which undermines the hypothesis that HT clasts are remelted residues while Hughes-type main group ureilites are cumulates. Second, the HT clasts show the same grain sizes and textures as all Hughes-type ureilites, which is inconsistent with them having been remelted, particularly by impact melting.

The hypothesis that we have presented, in which indigenous melting and melt extraction due to internal heat sources on the UPB were a continuous process lasting several million years, and all Hughes-type ureilites (and clasts in polymict ureilites derived from them) represent late melts in this sequence, are consistent with petrologic and isotopic data for ureilites,

as well as thermal modeling and experimental petrology. This hypothesis could be tested if it were possible to obtain precise age data that corresponded to the time of melt generation/extraction, rather than crystallization, for Hughes-type main group ureilites.

## CONCLUSIONS

MS-MU-012 is the first known plagioclase-bearing main group ureilite. It is an olivine–orthopyroxene–augite ureilite of Fo 88, similar in all respects to the archetypal Hughes-type ureilites except for the significant presence of ~14% primary plagioclase of An ~68. We conclude that it formed as a cumulate in a common lithologic unit (the Hughes unit) with the archetypal Hughes-type ureilites. Based on the compositions of melts trapped in olivine, orthopyroxene, and augite in the Hughes-type samples, we infer that the parent magma of the Hughes unit originated as a late melt in the incremental melting of the UPB, but was not completely extracted from the mantle-like earlier melts and was emplaced in an intrusive body. Olivine began to crystallize at ~1260 °C, followed shortly thereafter by co-crystallization of orthopyroxene and augite. Plagioclase began to crystallize at ~1170–1180 °C. Graphite was buoyant in the melt and became heterogeneously distributed in flotation cumulates. Residual silicate liquid was extracted from the cumulate pile and formed the labradoritic melt lithology now partially preserved as clasts in polymict ureilites. The final equilibration temperature recorded by the Hughes unit, probably just before impact disruption of the parent body was ~1140–1170 °C.
<figure><img src="image_14.png" /></figure>

Fig. 12. MELTS equilibrium crystallization sequences for modeled melts. a) olPTL/olPTL-R compositions crystallizing at constant $fO_2$: IW-1.6 (a) and IW-1.9 (b). The olPTL composition (Table 8) was used as starting composition but the superliquidus melt composition imposed by alphaMELTS at those redox conditions is more reduced due to formation of metal. Two possible scenarios (discussed in the text) are simulated by the two $fO_2$ conditions. If crystallization started at IW-1.6 (olivine $Fo_{84}$), reduction to IW-1.9 must have occurred before the appearance of pyroxene (Mg# 89) at 1200 ± 10 °C, as suggested by the absence of pigeonite in Hughes-type samples. c–f) Trapped liquid compositions reconstructed from augite- and pyroxene-hosted melt inclusions: augPTL and opxPTL from Table 8; augPTL2 and opxPTL2 slightly modified by lowering CaO contents by 0.6 wt%. Note that the order of appearance of orthopyroxene and augite is strongly sensitive to small variations in the bulk CaO content. Overall, alphaMELTS calculations are consistent with textural observations and suggest that orthopyroxene and augite co-saturated at 1200 ± 10 °C, before the appearance of plagioclase. (Color figure can be viewed at wileyonlinelibrary.com.)

We propose the following scenario for differentiation of the UPB. Silicate melting began ~1 Ma after CAI and proceeded rapidly (near-peak temperatures being reached

in <1 Myr), resulting in continuous, near-fractional (incremental) melt extraction (Collinet & Grove, 2020a; Wilson et al., 2008). The earliest melts were generated and
extracted from the mantle within a short time period around 1 Myr after CAI. Late melts, generated several million years later, are represented by melt inclusions trapped in the Hughes-type ureilites. These melts were not completely extracted from the residual mantle but were emplaced in an intrusive body or bodies. The archetypal Hughes-type samples are cumulates from these melts, while at least some of the labradoritic feldspathic clasts in polymict ureilites represent fractionated melt that was expelled from the cumulate pile. Main group olivine + low-Ca pyroxene ureilites represent the final mantle residues after ~15-24% incremental melt extraction. It remains to be determined whether the other augite-bearing ureilites (which have Fo ranging from ~75 to 95) formed by similar processes to the Hughes-types but are derived from different source regions. This issue is related to the unresolved questions of what determined the large range of Mg#s of source regions and how these source regions were arranged spatially in the UPB. MS-MU-012 provides a critical missing link in the differentiation of the ureilite parent asteroid.

*Acknowledgments*—We thank Lindsay Keller and Dave Draper for making the JSC electron beam facilities available for this work. We thank Eve Berger, Ulla Heitmann, Anne Peslier, Kent Ross, and Dustin Ward for technical and analytical assistance. We thank reviewers Hilary Downes, Richard Greenwood, an anonymous reviewer, and associate Editor Katie Joy for helpful comments, which led to improvement in this manuscript. This work was supported by the Lunar and Planetary Institute (USRA-Houston) and NASA grant NNX17AH09GS03. M.C. gratefully acknowledges financial support from the DLR Management Board Young Research Group Leader Program and the Executive Board Member for Space Research and Technology. T.C.P. gratefully acknowledges the Gordon McKay Fellowship. This work is also partly funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—Project-ID 263649064—TRR 170 (A.B. and M.C.). This is LPI publication #002707 and TRR170 Publication No. 170.

*Data Availability Statement*—All data will be made available in a suitable repository.

*Editorial Handling*—Dr. Katherine Joy

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