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Chemie der Erde 75 (2015) 155–183

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Chemie der Erde

j o ur na l ho mepage: www.elsev ier .de /chemer

nvited Review

steroid (4) Vesta: I. The howardite-eucrite-diogenite (HED)lan of meteorites

avid W. Mittlefehldt ∗

I3/Astromaterials Research Office, Astromaterials Research and Exploration Sciences Division, NASA/Johnson Space Center, 2101 NASA Parkway, Houston,X 77058, USA

r t i c l e i n f o

rticle history:eceived 23 November 2013ccepted 21 August 2014ditorial handling – K. Keil

eywords:owarditesucritesiogenitesestaasaltic achondritesifferentiated asteroids

a b s t r a c t

The howardite, eucrite and diogenite (HED) clan of meteorites are ultramafic and mafic igneous rocksand impact-engendered fragmental debris derived from a thoroughly differentiated asteroid. Earth-basedtelescopic observation and data returned from vestan orbit by the Dawn spacecraft make a compellingcase that the asteroid (4) Vesta is the parent asteroid of HEDs, although this is not universally accepted.Diogenites are petrologically diverse and include dunitic, harzburgitic and noritic lithologic types in addi-tion to the traditional orthopyroxenites. Diogenites form the lower crust of Vesta. Cumulate eucrites aregabbroic rocks formed by accumulation of pigeonite and plagioclase from a mafic magma at depth withinthe crust, while basaltic eucrites are melt compositions that likely represent shallow-level dikes and sills,and flows. Some basaltic eucrites are richer in incompatible trace elements compared to most eucrites,and these may represent mixed melts contaminated by partial melts of the mafic crust. Differentiationoccurred within a few Myr of formation of the earliest solids in the Solar System. Evidence from oxy-gen isotope compositions and siderophile element contents favor a model of extensive melting of Vestaforming a global magma ocean that rapidly (period of a few Myr) segregated and crystallized to yield ametallic core, olivine-rich mantle, orthopyroxene-rich lower crust and basaltic upper crust. The igneouslithologies were subjected to post-crystallization thermal processing, and most eucrites show texturaland mineral-compositional evidence for metamorphism. The cause of this common metamorphism isunclear, but may have resulted from rapid burial of early basalts by later flows caused by high effusionrates on Vesta. The observed surface of Vesta is covered by fragmental debris resulting from impacts,
and most HEDs are brecciated. Many eucrites and diogenites are monomict breccias indicating a lack ofmixing. However, many HEDs are polymict breccias. Howardites are the most thoroughly mixed polymictbreccias, yet only some of them contain evidence for residence in the true regolith. Based on the numbersof meteorites, compositions of howardites, and models of magma ocean solidification, cumulate eucritesand their residual ferroan mafic melts are minor components of the vestan crust.
© 2014 Published by Elsevier GmbH.

ontents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1562. Lithologies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1563. Mineralogy and petrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157

3.1. Diogenites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
3.2. Cumulate eucrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Basaltic eucrites. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3.4. Petrologically anomalous eucrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

3.5. HED polymict breccias . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Tel.: +1 281 483 5043; fax: +1 281 483 1573.E-mail address: [email protected]

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4. HED compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1674.1. Oxygen isotopic composition. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1674.2. Lithophile element compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1694.3. Siderophile element compositions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1714.4. Noble gas contents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172

5. HED ages. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1726. HED meteorite petrogenesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1747. Thermal metamorphism of the HED parent body crust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1758. Fluid-mediated metasomatism? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1769. Mixing of the vestan crust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17610. Ungrouped basaltic achondrites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17811. The future . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 178

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

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References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

Meteorites of the howardite, eucrite and diogenite (HED) clanake up the largest suite of crustal igneous rocks available for study

rom any solar system body, baring the Earth and Moon. As of July014, there were over 1450 named HED meteorites with 270 newED meteorites announced during the previous year. Because mostre from collection fields in Antarctica and northern Africa, pairingsreatly reduces the number of individual fall events represented.EDs include basalts, cumulate gabbros, norites, orthopyroxenites,arzburgites and rare dunites, plus brecciated mixtures of these

gneous lithologies. The HED clan provides an unmatched look atifferentiation processes that occurred on asteroidal-sized bodiesarly in the history of the solar system.

Aside from lunar and martian meteorites, the HED meteoritelan is the only group for which we have a strong candidate for thearent body, the asteroid (4) Vesta. McCord et al. (1970) showedhat the visible and infrared reflectance spectrum of Vesta is closely

atched by the laboratory spectrum of the basaltic eucrite Nuevoaredo. Subsequently, Consolmagno and Drake (1977) argued thatesta was indeed the parent body of the HED meteorites. How-ver, there appeared to be severe dynamical problems with movingaterial from Vesta to Earth-crossing orbits and this cast doubt on

vestan origin for HEDs (Wasson and Wetherill, 1979). Vesta isar from orbital resonances such that only energetic events couldropel material into regions where they would have a reasonablerobability of evolving into Earth-crossing orbits. Cruikshank et al.1991) showed that three near-Earth asteroids (NEAs) ∼1–3 km iniameter have spectral characteristics like those of Vesta and theED meteorites. They suggested that these asteroids are the imme-iate sources of many of the HED meteorites, but Cruikshank et al.1991) did not favor Vesta as the ultimate source for the Earth-pproaching asteroids. Binzel and Xu (1993) found 20 asteroidsith diameters in the range of 4–10 km in orbits similar to Vesta’s

hat have Vesta-like spectra. These vestoids form a “trail” in orbitallement space from near Vesta to near the 3:1 resonance fromhich material can be perturbed into NEA orbits. Binzel and Xu

1993) proposed that these vestoids are blocks spalled off Vestay impacts, and that some spalls reached the 3:1 resonance andvolved into NEAs, ultimately delivering HED meteorites to thearth. Thomas et al. (1997) determined the shape of Vesta to have aattened southern hemisphere, which they interpreted as indicat-

ng that a large impact basin was located approximately coincidentith the southern rotation axis. Asphaug (1997) modeled an impact

y a moderate-sized asteroid onto Vesta and showed that Vesta
ould have survived such an impact, forming the southern basinnd the family of vestoids identified by Binzel and Xu (1993).
The Dawn mission to Vesta, the subject of a forthcomingeview (McCoy et al., 2014), has provided a wealth of detailed

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 179

geological, mineralogical, compositional and geophysical informa-tion on Vesta. The returned data are consistent with the identityof Vesta as the HED parent asteroid (McSween et al., 2013). Never-theless, there continues to be a minority view that Vesta is not theHED parent. Wasson (2013) contends that the numerous V-typeasteroids in the inner main belt are debris from the disruption ofseveral differentiated asteroids, and that these are more probableas sources for HEDS than debris knocked off Vesta by the impactthat formed the Rheasilvia basin. He argues that the isotopic simi-larity of IIIAB irons and HEDs indicates that they were derived fromthe same asteroid, and that asteroid must have been completelydisrupted in order to liberate IIIAB irons from the core. AlthoughWasson (2013) argues against Vesta being the parent asteroid ofHEDs, none of his arguments requires that this be true. The evi-dence outlined by McSween et al. (2013) in favor of Vesta as the HEDparent asteroid is more compelling in my opinion, but again, doesnot require that this be true. We could only be certain by returningsamples from Vesta so that the rocks and soils could be studied inlaboratories for direct comparison with HEDs.

This article will cover the mineralogy, petrology, chemistry andpetrogenesis of HED meteorites, and is an updated and expandedversion of the material covered in Mittlefehldt et al. (1998). Sincethe publication of that paper, several basaltic achondrites that arecompositionally, mineralogically and petrographically very similarto HED meteorites, but demonstrably distinct, have been iden-tified (Bland et al., 2009; Mittlefehldt, 2005; Scott et al., 2009;Yamaguchi et al., 2002). These are classified here as ungroupedbasaltic achondrites. The silicates of mesosiderites are also petro-logically similar to HEDs, but are distinct (Mittlefehldt, 1990; Rubinand Mittlefehldt, 1992). All of these basaltic materials are thoughtto have been derived from different parent asteroids based onisotopic and/or petrologic differences. I will briefly discuss theungrouped basaltic achondrites as they provide important infor-mation that may help constrain the nature of vestan differentiation.Mesosiderite silicates are too complex a topic to be included in thisreview.

2. Lithologies

The lithologic diversity of the HED clan is greater than can beinferred from classification designations. This has largely resultedfrom the explosive growth in meteorite recoveries beginning withthe systematic harvesting of meteorites from Antarctica and laterby the retrieval of meteorites from desert regions of the temperatezones. With greater numbers of rocks available, the chance of rare
lithologic types being in collections increases. One side effect hasbeen that some unique lithologies have been shoehorned into anexisting classification, and that can result in obscuring importantpetrogenetic information.

ie der Erde 75 (2015) 155–183 157

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There are three general types of lithologic diversity in theED clan; one engendered by igneous fractionation processes, one

esulting from thermal metamorphism, and one caused by impactrocessing of the vestan crust. Of primary concern for decipher-

ng the geological evolution of Vesta is the diversity of igneousock types, the major focus of this paper. Metamorphic grade isn important indicator of the thermal evolution of the crust ands briefly considered here. Most HEDs are breccias, but the brec-iation process is not a major focus of this paper. The terminologyor breccias used here follows Bischoff et al. (2006), and the clas-ification of HED polymict breccias largely follows that of Delaneyt al. (1983) and Miyamoto et al. (1978). Fig. 1 graphically showshe igneous and breccia lithologic types; modal mineralogy used inig. 1a, b are given in Table S1. This paper is structured around theajor lithologic types given in the figure.

. Mineralogy and petrology

.1. Diogenites

The majority of diogenites are coarse-grained ultramafic rocksominated for the most part by magnesian orthopyroxene. Most arerthopyroxenites but several are harzburgites (Beck and McSween,010; Beck et al., 2013; Mason, 1963; Mittlefehldt et al., 1998).ittke et al. (2011) suggest that meteoriticists follow IUGS nomen-

lature for terrestrial ultramafic rocks and thus diogenites withetween 10 and 40 vol% olivine should be referred to as olivine-rthopyroxenitic diogenites rather than harzburgitic diogenites. Iill follow the Beck and McSween (2010) system here (Fig. 1a).

few dunites seem related to diogenites. Beck et al. (2011) pre-ented petrologic, compositional and oxygen isotopic data on Millerange (MIL) 03443 and concluded that it is related to HEDs (cf.rawczynski et al., 2008; Mittlefehldt, 2008). They argued that ithould be classified as a dunitic diogenite. Several other ultramaficeteorites have been suggested to be dunites from Vesta (North-est Africa (NWA) 2968, NWA 5784, NWA 5968), but they are onlyocumented in abstracts (Bunch et al., 2006, 2010); their pedi-ree remains to be verified. Some diogenites that contain moreerroan orthopyroxene, pigeonite and plagioclase are transitionalo cumulate eucrites. Meteorites paired with Yamato (Y-) 75032ere originally classified as Yamato Type B or Y-75032-type dio-

enites (e.g., Takeda and Mori, 1985; Takeda et al., 1979) or asigeonite cumulate eucrites (Delaney et al., 1984a,b). An averageode weighted by area of seven of these meteorites (Delaney et al.,

984a,b) has 85 vol% total pyroxene (opx:pig:aug ∼ 62:17:6) and2 vol% plagioclase. In terrestrial plutonic rock nomenclature, theseould be referred to as clinopyroxene norites or possibly gab-

ronorites (Streckeisen, 1976). Following Wittke et al. (2011), Iecommend classifying these and similar plagioclase-rich, ferroaniogenites (e.g., Queen Alexandra Range (QUE) 93009, Mittlefehldtt al., 2012a) as noritic diogenites (Fig. 1b). A caveat is that diogen-tes that are plagioclase-rich by virtue of admixed eucritic debrisre polymict diogenites, not noritic diogenites.

Most diogenites are breccias (Fig. 2a and b), and some areolymict breccias. An important type of diogenitic breccia is dimictreccia composed of orthopyroxenitic and harzburgitic lithologiesBeck and McSween, 2010; Mittlefehldt et al., 2012a) (Fig. 2c). Theriginal grain sizes of the diogenite protoliths are not well known.he typical brecciated orthopyroxenitic diogenite is composed ofoarse orthopyroxene clasts up to 5 cm in size set in a fine-grainedragmental matrix of orthopyroxene (Mason, 1963). Dunitic dio-
enite MIL 03443 has a similar texture but with olivine as theajor mineral (Fig. 2e); olivine clasts up to 2.5 mm in size are
resent (Beck et al., 2013). Dimict orthopyroxenitic-harzburgiticiogenites are composed of fragmental breccias of more magnesian

Fig. 1. Modal mineralogy and classification of diogenites (a) and diogenites andeucrites (b). Modal mineralogy data taken from Table S1. Classification of HED brec-cia types (c) after Delaney et al. (1983) and Miyamoto et al. (1978).

Zena Robert


Fig. 2. Diogenite images. (a) Photomicrograph in plane polarized light of orthopyroxenitic diogenite LaPaz Ice Field (LAP) 91900. (b) X-ray elemental map of brecciatedo Al. (ct xeniteb

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rthopyroxenitic diogenite MET 01084 colorized using red = Mg, green = Ca, blue =riangular magnesian harzburgitic clast contained within less magnesian orthopyrorecciated dunitic diogenite MIL 03443.

arzburgite and more ferroan orthopyroxenite that are moderatelyntermingled (Beck and McSween, 2010). The Yamato Type B noriticiogenites are brecciated rocks composed of subangular mineralnd lithic clasts in a black, glassy matrix containing fine-grained,lastic debris (Mittlefehldt and Lindstrom, 1993; Takeda and Mori,985; Takeda et al., 1979).

Some diogenites are unbrecciated but the abundance of thems not well documented; “unbrecciated” is not a descriptor usedor diogenites in the Meteoritical Bulletin Database. Three unbrec-iated orthopyroxenitic diogenites, Grosvenor Mountains (GRO)5555, Tatahouine and Y-74013 and pairs, have metamorphic orhock textures. GRO 95555 has a polygonal-granular texture ofnhedral orthopyroxene grains up to 2.4 mm in size (Antarcticeteorite Newsletter, 19(2), 1996; Papike et al., 2000). The Yamato

ype A, or Y-74013-type, diogenites have a granoblastic textureomposed of equant, rounded orthopyroxene grains a few tens of
m to mm in size, containing inclusions dominantly of chromitend troilite (Mittlefehldt and Lindstrom, 1993; Takeda et al., 1978,981). Tatahouine is composed of numerous individual orthopy-oxene fragments of cm size that are generally free of fusion crust
) Backscattered electron (BSE) image of dimict diogenite LEW 88679, showing a. (d) BSE image of unbrecciated harzburgitic diogenite GRA 98108. (e) BSE image of

indicating the meteorite broke up late during entry. Pyroxenesin Tatahouine show patchy extinction under crossed polars indi-cating shock damage. Two unbrecciated harzburgitic diogeniteshave quite different textures. Graves Nunataks (GRA) 98108 hasan allotriomorphic-granular texture composed of orthopyroxeneand olivine grains up to mm size, with smaller grains of plagioclaseand diopside (Fig. 2d). MIL 07001 has a poikilitic texture in whichnumerous olivine grains a few hundred �m in size are enclosedin mm-sized orthopyroxene, and olivine-free zones contain sub-hedral orthopyroxene grains with interstitial groundmass rich intridymite (Mittlefehldt and Peng, 2013). Unbrecciated orthopyrox-enitic diogenite NWA 4215 has an unusual medium-grained texturecomposed of zoned xenomorphic orthopyroxene grains ∼0.5 mmin size, and larger, irregularly shaped chromite grains (Barrat et al.,2006). Dhofar (Dho) 700 is a medium-grained unbrecciated rockcomposed of ∼99% slightly zoned orthopyroxene (mg# from 68 to
71) with interstitial plagioclase and silica phases (Yamaguchi et al.,2011).
Orthopyroxene is the major phase of most diogenites. Harzbur-gitic and orthopyroxenitic diogenites contain from ∼64 to

D.W. Mittlefehldt / Chemie der Erde 75 (2015) 155–183 159

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have higher Ti, Mn and Fe and lower Mg, Si and Cr in rims comparedto cores. They concluded that this is a record of original igneouszoning and rejected the hypotheses that the compositional varia-tions were caused by either interaction of pyroxene grains with a

ig. 3. Pyroxene quadrilateral diagrams for diogenites (a), cumulate eucrites (b), ban unmetamorphosed basaltic clasts from polymict eucrite Y-75001 is taken from Byroxene exsolutions are shown in panels a-c. Data plotted are from Table S2. Pasa

100 vol% orthopyroxene (Beck and McSween, 2010; Beck et al.,013; Bowman et al., 1997; Sack et al., 1991, 1994) (Fig. 1a). AsukaA-) 881548 contains ∼37 vol% orthopyroxene (Yamaguchi et al.,011) but these authors note that because of the small sampleize, coarse grain size and heterogeneous distribution of olivine,his mode might not be accurate. Dunitic diogenite MIL 03443 con-ains only ∼5 vol% orthopyroxene (Beck et al., 2011). The pyroxene

ode of Yamato Type B noritic diogenites is given above. Differentections of individual diogenites can have widely different orthopy-oxene contents (Beck et al., 2013; Bowman et al., 1997; Yamaguchit al., 2011), demonstrating that modal abundances determined on

single thin section can have low fidelity.The compositions of pyroxenes in many diogenites are well

haracterized; Table S2 gives representative pyroxene com-ositions. For diogenites that contain distinct sub-lithologies,uch as Elephant Moraine (EETA) 79002 (Mittlefehldt, 2000)nd the dimict orthopyroxenite-harzburgite diogenites (Becknd McSween, 2010), representative compositions for distinctithologies are presented. Fig. 3a shows pyroxene composi-ions for diogenites. Orthopyroxenitic and harzburgitic diogenitesenerally contain orthopyroxene of uniform major elementomposition of ∼Wo2±1En74±2Fs24±1 (Fig. 3a) and mg# (molar00 × MgO/(MgO + FeO)) of 74–77 (Fig. 4). Orthopyroxenes in MIL3443 are at the magnesian edge of the range typical of orthopy-oxenitic and harzburgitic diogenites: Wo3.0En75.3Fs31.7, mg# 78Fig. 3a, Fig. 4). Some diogenites are exceptionally magnesian orerroan. The diogenites with the most magnesian orthopyrox-ne are NWA 1461 with orthopyroxene mg# of 86 (Bunch et al.,007; Barrat et al., 2010) and Meteorite Hills (MET) 00425 with anrthopyroxene mg# of 84 (Mittlefehldt, 2012; Table S2); both arerthopyroxenitic diogenites. Noritic diogenites are more ferroanhan typical diogenites; QUE 93009 contains orthopyroxene with

g# of 70 (Mittlefehldt et al., 2012a; Table S2) and Yamato Type diogenites contain orthopyroxene with mg# of 66 (Fowler et al.,994; Mittlefehldt and Lindstrom, 1993; Table S2). Dimict diogen-

te Lewis Cliff (LEW) 88008 is composed of two orthopyroxeniticiogenite lithologies and the more ferroan contains orthopyroxeneith mg# 68 (Beck and McSween, 2010).

ucrites (c) and ungrouped basaltic achondrites (d). The field for primary pyroxeneset al. (2011). Representative tie lines between low-Ca pyroxene hosts and high-Ca

zoning trends (arrows) are from Schwartz and McCallum (2005).

Some diogenites contain orthopyroxene grains with varyingFe/Mg. Fowler et al. (1994) showed that individual grains in Garland

Fig. 4. Histograms of low-Ca pyroxene mg#s in diogenites, cumulate eucrites andbasaltic eucrites. Data plotted are from Table S2.

1 ie der Erde 75 (2015) 155–183

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rapped melt phase or metamorphic equilibration between moreagnesian and more ferroan lithologies in this polymict breccia.owever, details of the textural context were not provided thatould allow full evaluation of the hypothesis. Fowler et al. (1994)id note that some fractured grains show zoning only on a portionf the grain margin and thus that compositional zonation occurredrior to the final brecciation and assembly. Fine-grained NWA 4215ontains orthopyroxene grains that are zoned in Fe/Mg, Ca, Al andi that is thought to have resulted from interaction between cumu-us crystals and an interstitial melt that was ultimately expelledrom the rock (Barrat et al., 2006). Yamaguchi et al. (2011) stud-ed a suite of diogenites and showed that compositional variations

ithin orthopyroxene grains are present in three. Two (Dhofar 700nd the Yamato Type A diogenite Y-74097) have variations in Fe/Mghile A-881377 has variation in Ca content. These authors refer toiogenites having compositional variations within orthopyroxenerains as unequilibrated; those that do not are equilibrated in theirarlance.

Minor and trace element contents of diogenite orthopyroxeneshow considerable variation (Barrat et al., 2006; Fowler et al., 1994,995; Mittlefehldt, 1994; Shearer et al., 1997, 2010). The averagerthopyroxene Al and Cr within the diogenite group vary by factorsf 5 and Ti by a factor of 17 (Table S2). Fowler et al. (1994, 1995)howed that minor and trace incompatible element contents forifferent grains within individual diogenites can be quite variable.arrat et al. (2006) showed that within zoned fine-grained orthopy-oxenes there is a strong Ti–Al correlation. Mittlefehldt and Peng2013) demonstrated substantial Al and Ti variations on the scale of

few hundred microns in orthopyroxene in unbrecciated harzbur-itic diogenite MIL 07001. Minor and trace incompatible elementsre decoupled from orthopyroxene mg# (Fowler et al., 1994, 1995;ittlefehldt, 1994). Incompatible element contents of diogenite

rthopyroxenes are positively correlated in general (Fowler et al.,994, 1995; Mittlefehldt, 1994), although some diogenites arenomalous. For example, Al and Ti are positively correlated forhe most part, although some diogenites have anomalously high orow Ti/Al ratios (Fig. 5a). Pyroxenes in the noritic diogenites haveery high Ti/Al ratios, consistent with co-crystallization with pla-ioclase. Individual diogenites can show a weak positive correlationetween orthopyroxene Cr content and mg# (Berkley and Boynton,992), but this is not evident when considering the diogenite groups a whole (Fig. 5b).

Olivine is a minor to major mineral making up between 0 and3 vol% of orthopyroxenitic and harzburgitic diogenites (Beck andcSween, 2010; Beck et al., 2013; Bowman et al., 1997; Sack

t al., 1991, 1994), and ∼91 vol% of MIL 03443 (Beck et al., 2011).-881548 contains ∼63 vol% olivine but this mode might not beccurate (Yamaguchi et al., 2011). Again, different sections of indi-idual diogenites can have very different olivine contents (Beckt al., 2013; Bowman et al., 1997; Yamaguchi et al., 2011). Olivinerains are commonly a few hundred �m to ∼mm in size, but theyre often crystal fragments as clasts and matrix grains. Olivine haseen found with its original textural context with orthopyroxeneartially or completely preserved in only a few diogenites – Allanills (ALHA) 77256, Dhofar 700, GRA 98108, LEW 88679, MIL 03443,IL 07001, NWA 4215 and Roda (Barrat et al., 2006; Beck andcSween, 2010; Mittlefehldt, 1994, 2008; Mittlefehldt and Peng,

013; Sack et al., 1991; Yamaguchi et al., 2011) (Fig. 2c–e). Olivinen harzburgitic diogenites MIL 07001 and NWA 5480 and in duniticiogenite NWA 5784 show lattice-preferred orientations similar tohose observed in olivine from terrestrial mantle peridotites thatere affected by high-temperature solid-state plastic deformation

Tkalcec and Brenker, 2014; Tkalcec et al., 2013).Olivine compositions have been determined for many dio-

enites, although mostly for diogenites containing olivine-richithologies; few analyses exist for olivine in orthopyroxenitic

Fig. 5. (a) Ti (apfu) vs. Al (apfu) for diogenite orthopyroxenes. (b) Cr (apfu) vs. mg#for diogenite orthopyroxenes. Data plotted are from Table S2.

diogenites. Table S3 summarizes representative olivine composi-tions for diogenites, and Fig. 6 gives a histogram of olivine mg#.When multiple lithologies are present in a diogenite, olivine isusually associated with the more magnesian lithology and thisis independent of the mg# of the rock. Mittlefehldt (2000) con-cluded that olivine in EETA79002 was in equilibrium with the mostmagnesian orthopyroxenes in this genomict breccia, and suggestedthe parent lithology was a harzburgite more magnesian than theorthopyroxenite that constituted the bulk of the breccia. Beck andMcSween (2010) documented that for a set of orthopyroxenite-harzburgite dimict breccias, the harzburgite lithology is always themore magnesian. Further, some of the harzburgite lithologies aremore ferroan than some olivine-free orthopyroxenite lithologiesin different meteorites (Beck and McSween, 2010). This is one ofthe most significant new findings in diogenite petrology because itsuggests that the rocks were not formed as a single, continuously
evolving magmatic sequence. This had previously been inferredfrom trace element distributions in orthopyroxene separates of dio-genites (Mittlefehldt, 1994). Olivine is present in the anomalous


Fig. 6. Histogram of olivine mg#s for diogenites. Data represent either a single mete-of

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than a few microns in size included within orthopyroxene. The lat-

rite for unbrecciated and monomict diogenites, or each distinct lithology in the rockor dimict and polymict rocks. Data plotted are from Table S3.

iogenites Dhofar 700 and NWA 4215; in the latter the grains haveodest Fe/Mg zoning (Barrat et al., 2006; Yamaguchi et al., 2011)Chromite is a ubiquitous minor mineral making up from a trace

o 5 vol% (Beck and McSween, 2010; Bowman et al., 1997). It occurss mm-sized equant grains in igneous contact with orthopyrox-ne, as clasts in the breccia and as grains a few tens to hundreds ofm in size poikilitically enclosed in orthopyroxene. Table S4 givesverage chromite compositions for diogenites. Chromite grains inypical diogenites are variable in Cr/Al and mg# but have a limitedariation in Ti contents (Fig. 7). Sack et al. (1991, 1994) show a rangen mg# for EETA79002 chromites from ∼14 to 32. This is most ofhe range found for the averages of chromites for all diogenites. Theange in Al2O3 contents of average chromites is considerable, from.5 to 21.8 wt%, (Beck and McSween, 2010; Bowman et al., 1999;omanik et al., 2004; Mittlefehldt, 1994; Yamaguchi et al., 2011).ithin individual diogenites, chromite grains can also be variable

n Al2O3 composition. Chromite grains in Roda range in Al2O3 from.6 to 18.6 wt%, although most grains contain between 8.6 and0.2 wt% (Gooley, 1972). Small grains are often of different com-osition than large grains, but the differences are not systematicMittlefehldt, 1994; Mittlefehldt and Lindstrom, 1993). Chromiterains in NWA 4215 show substantial variations in compositionBarrat et al., 2006) but this is a very unusual diogenite and itsroperties were established by processes not common to diogeniteetrogenesis.

Other minor silicate minerals include plagioclase (≤5 vol%),igh-Ca pyroxene (≤12 vol%) and a silica phase (≤2 vol%) (Beckt al., 2010; Bowman et al., 1997; Domanik et al., 2004, 2005;ittlefehldt, 1994). The Yamato Type B noritic diogenites also con-

ain on average ∼17 vol% inverted pigeonite, which has exsolvedugite as lamellae and blebs (Delaney et al., 1984a,b; Mittlefehldtnd Lindstrom, 1993; Takeda and Mori, 1985; Takeda et al.,979). Plagioclase is often found as crystal fragments in theatrix (Fig. 2b), but does occur in primary textural context in

ome diogenites (Fig. 2d). Plagioclase is usually anorthitic in theange An82–96, but grains as sodic as An73–77 are found in someBeck and McSween, 2010; Domanik et al., 2004; Floran et al.,
981; Fredriksson, 1982; Gooley, 1972; Mittlefehldt, 1979, 1994;ittlefehldt et al., 2012a; Table S5, Fig. 8). Domanik et al. (2004)
eport plagioclases as sodic as An46 in Bilanga, but they do not

Fig. 7. Compositions of chromites and ulvöspinels in HEDs. Data plotted are fromTable S4.

give an analysis. High-Ca pyroxene is often present as thin lamel-lae or blebs in orthopyroxene, but discrete grains in the matrixor interstitial to orthopyroxene grains are also found (Beck andMcSween, 2010; Domanik et al., 2004; Mittlefehldt, 1994, 2000).The lamellae are the products of subsolidus exsolution from theorthopyroxene and can have sub-micron widths; these are iden-tified as augite (Mori and Takeda, 1981a). Lamellae and discretegrains large enough for electron microprobe analysis include augiteand diopside; representative compositions are given in Table S2.Note that low-Ca clinopyroxene is also present in some diogen-ites were it was formed by shock deformation of orthopyroxene(Mori and Takeda, 1981a). A silica phase can occur as equant grainsa few tens of �m in size, but the textural setting has not beenwell described. In MIL 07001 silica is an interstitial phase betweenorthopyroxene grains (Mittlefehldt and Peng, 2013).

Troilite (≤3 vol%) and metal (≤1 vol%) are common minor phases(Beck et al., 2010; Bowman et al., 1997; Domanik et al., 2004).Troilite occurs as equant grains or polycrystalline aggregates sev-eral hundred microns in size in the matrix, or as small grains less

ter often form inclusion curtains within orthopyroxene along withsome combination of metal, chromite and silica (Domanik et al.,2004; Gooley and Moore, 1976; Mori and Takeda, 1981a). The metal

162 D.W. Mittlefehldt / Chemie der

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ig. 8. Histograms of plagioclase An contents in diogenites, cumulate eucrites andasaltic eucrites. Data plotted are from Table S5.

hases are kamacite, taenite and tetrataenite that occur as grainsn the matrix and as inclusions in orthopyroxene (Domanik et al.,004; Gooley and Moore, 1976; Mittlefehldt, 2000). Kamacite andaenite grains have wide ranges in Co and Ni contents and Ni/Coatios, even within a given diogenite (Gooley and Moore, 1976;ittlefehldt, 2000).Trace phases include Ca-phosphates, K-feldspar, pentlandite

nd native copper (Domanik et al., 2004, 2005; Mittlefehldt, 1994;ittlefehldt and Peng, 2013). The phosphates in Bilanga and

oda are rich in light rare earth elements with wt% quantities ofa2O3 and Ce2O3 (Domanik et al., 2004, 2005; Mittlefehldt, 1994).lmenite is a trace phase in some nortitic diogenites (Delaneyt al., 1984a,b; Mittlefehldt and Lindstrom, 1993; Mittlefehldt et al.,012a).

.2. Cumulate eucrites

Cumulate eucrites are medium- to coarse-grained gabbrosomposed principally of low-Ca clinopyroxene and calcic pla-ioclase with minor chromite and accessory silica, phosphate,lmenite, metal, troilite and zircon (e.g. Delaney et al., 1984a,b;omes and Keil, 1980). Several cumulate eucrites are unbrec-iated but many are breccias. Binda is a polymict breccia (Delaneyt al., 1983; Garcia and Prinz, 1978; Yanai and Haramura, 1993)nd some polymict eucrites and howardites contain abundantumulate eucrite material (e.g., Gardner and Mittlefehldt, 2004;
ittlefehldt and Lindstrom, 1993; Saiki et al., 2001; Takeda, 1986,
991). The typical texture of unbrecciated cumulate eucrites isquigranular with subequal amounts of pyroxene and plagio-lase grains 0.5–5 mm across (Duke and Silver, 1967; Hess and

Erde 75 (2015) 155–183

Henderson, 1949; Lovering, 1975; Mayne et al., 2009; Mittlefehldtand Lindstrom, 1993), for example, Moore County and Moama(Fig. 9). The cumulate eucrites have been subdivided into thefeldspar- and orthopyroxene-cumulate eucrites based on modalabundances (Delaney et al., 1984a,b), but this terminology is notwidely used. Binda is the only orthopyroxene-cumulate eucriteidentified by Delaney et al. (1984a). However, Takeda et al. (1976)had previously demonstrated that the primary igneous pyroxene inBinda is a low-Ca pigeonite that had exsolved augite and invertedto hypersthene.

The original igneous pyroxene of most cumulate eucrites waspigeonite, which subsequently underwent subsolidus exsolutionof augite and for some the pigeonite inverted to orthopyroxeneaccompanied by additional augite exsolution (Hess and Henderson,1949; Harlow et al., 1979; Lovering, 1975; Mori and Takeda, 1981b;Takeda et al., 1976). The result is a complex pyroxene texture thatcan involve as many as seven distinct phases (Mori and Takeda,1981b). These exsolution textures can be used to model coolingrates for the cumulate eucrites (e.g., Miyamoto and Takeda, 1977).Table S2 gives representative low-Ca and high-Ca pyroxene analy-ses for several cumulate eucrites, and pyroxene compositions areshown in Fig. 3b. ALH 85001 and Binda are the most magnesianof the cumulate eucrites, with low-Ca pyroxene mg# of 66.1 and64.6 (this work; Pun and Papike, 1995). These cumulate eucritesmerge into the pyroxene compositional range of diogenites: Yam-ato Type B diogenites, 66.5 (Mittlefehldt and Lindstrom, 1993); MIL07613, 62.7 (Mittlefehldt et al., 2013a). The most ferroan cumu-late eucrite, Y-791195, contains low-Ca pyroxene with mg# of 42.6(Mittlefehldt and Lindstrom, 1993).

Cumulate eucrites contain plagioclase with compositions inthe range An91–95 and with very low K2O contents, mostly<0.05 wt% (Lovering, 1975; Mayne et al., 2009; Mittlefehldt, 1990;Mittlefehldt and Lindstrom, 1993; Treiman et al., 2004). These com-positions are on average more calcic than those of basaltic eucriteplagioclases. Cumulate eucrite plagioclase compositions are shownin Fig. 8 and representative compositions are given in Table S5.

Chromite/ulvöspinel is a minor mineral in all cumulate eucrites(Delaney et al., 1984a,b). It occurs in a variety of textures, includ-ing discrete gains interstitial to pyroxene and plagioclase, elongategrains intergrown with tridymite and as inclusions in pyrox-ene (Ghosh et al., 2000; Hostetler and Drake, 1978; Kanedaet al., 2000; Lovering, 1975; Mittlefehldt and Lindstrom, 1993).Chromite/ulvöspinel grains in cumulate eucrites typically havehigher TiO2 contents and lower mg# than those in diogenites(Fig. 7), but the Al2O3 contents overlap (e.g., Bunch and Keil,1971; Ghosh, 2000; Hostetler and Drake, 1978; Lovering, 1975;Mayne et al., 2009; Mittlefehldt and Lindstrom, 1993). In manycases, the compositional variability of chromite/ulvöspinel grainswithin cumulate eucrites is not well documented because averageanalyses are often presented. Ferroan cumulate eucrite Y-791195contains chromite/ulvöspinel grains that vary from 8.6 to 15.7 wt%in TiO2, which is negatively correlated with Al2O3 and Cr2O3contents (Mittlefehldt and Lindstrom, 1993). Average or represen-tative chromite/ulvöspinel compositions for cumulate eucrites aregiven in Table S4.

Ilmenite occurs in many cumulate eucrites but it has notbeen reported in ALHA81313, Moama or Vissannapeta (Delaneyet al., 1984a,b; Ghosh et al., 2000; Lovering, 1975; Mayne et al.,2009). Ilmenite occurs as individual grains, composite grains withchromite and as exsolution lamellae in chromite (Bunch and Keil,1971; Hostetler and Drake, 1978; Mittlefehldt and Lindstrom,1993). Minor constituents of ilmenite – MgO, MnO and Cr2O3 –
usually are at <3 wt%, and mg# varies from 9 to 5 (Bunch and Keil,1971; Mayne et al., 2009; Mittlefehldt and Lindstrom, 1993). Aver-age or representative ilmenite compositions for cumulate eucritesare given in Table S6.


F Coung

M

mMHed

ig. 9. Colorized mineralogy maps for select unbrecciated cumulate eucrites (Moorerain sizes.

odified from Mayne et al. (2009).

A silica polymorph is commonly present but the specific poly-orph is often not identified. Tridymite is present in Moama,
oore County and Serra de Magé (Duke and Silver, 1967; Hess andenderson, 1949; Lovering, 1975; Treiman et al., 2004). Treimant al. (2004) document the presence of quartz veinlets in Serrae Magé in a textural context similar to that of antitaxial veinlets
ty and Moama) and basaltic eucrites (all others), showing the range of textures and

in terrestrial rocks. These authors conclude the quartz veins wereprecipitated from aqueous fluids (see Section 8).

Metal in several unbrecciated cumulate eucrites has low Nicontents, ≤0.5 wt% (Duke, 1965; Lovering, 1964, 1975; Mayne et al.,2009). Polymict cumulate eucrite Binda contains some metal grainswith ∼2 wt% Ni (Lovering, 1964). The textural setting of this metal


Fig. 10. Basaltic eucrite images. (a) Photomicrograph in plane polarized light of brecciated eucrite QUE 97430. (b) Photomicrograph in plane polarized light of unbrecciated,porphyritic basaltic eucrite LEW 88009. (c) BSE image of anomalous unbrecciated, metamorphose basaltic eucrite EET 90020. (d) Initial processing image of unbrecciatedm tridymm dymitb ght-to

wc

3

matltuht8-aU

etamorphosed basaltic eucrite GRA 98098. The light-toned “veins” contain long,

orphosed basaltic eucrite GRA 98098. The medium-gray cracked grain is coarse triasaltic eucrite GRA 98098 showing a portion of one of the plagioclase-tridymite li

as not given, and possibly the high-Ni metal is not innate to theumulate gabbro lithology of this polymict breccia.

.3. Basaltic eucrites

Basaltic eucrites are pigeonite-plagioclase rocks with fine toedium grain size. Most are brecciated, composed of mineral

nd lithic fragments set in a fine-grained, generally fragmen-al matrix (Fig. 10a). Original igneous textures are preserved inithic clasts in brecciated eucrites and are generally subophitico ophitic (Duke and Silver, 1967). A few basaltic eucrites arenbrecciated (Figs. 9 and 10b), but some of these have beenighly metamorphosed, resulting in recrystallized, granoblasticextures (Fig. 10c–f) (Mayne et al., 2009). Two of these, A-
81388 and A-881467, are interpreted to be granulitic breccias
brecciated eucrites that were highly metamorphosed to yieldn overall granulitic textured rock (Yamaguchi et al., 1997a).nbrecciated, igneous-textured basaltic eucrites vary in texture

ite needles. (e) Photomicrograph in crossed polarized light of unbrecciated meta-e from one of the light-toned “veins.” (f) BSE image of unbrecciated metamorphosedned “veins.”.

from very fine-grained vitrophyric texture consisting of pyroxenemicrophenocrysts set in a groundmass of pyroxene microcrystsand glass of mixed plagioclase-silica composition (ALHA81001) tocoarse-grained subophitic texture with fine-grained, recrystallizedmesostasis (paired Pecora Escarpment (PCA) 91078, PCA 91245)(Howard et al., 2002; Mayne et al., 2009) (Fig. 9). NWA 5073 is arare, unbrecciated and almost unmetamorphosed basaltic eucritecomposed of coarse-grained zoned pyroxenes, skeletal plagioclasegrains, dendritic chromite grains and a fine-grained mesostasis(Roszjar et al., 2011).

The original igneous pyroxene in basaltic eucrites was fer-roan pigeonite, but some original pyroxene in Sioux Countywas orthopyroxene (Takeda et al., 1978a). Most eucrites havebeen metamorphosed and the original pigeonite underwent sub-
solidus exsolution of augite and homogenization of original Fe/Mgzonation. Inversion of pigeonite to orthopyroxene is uncommon.Metamorphism also engendered clouding of pyroxene and pla-gioclase through exsolution and/or redox of minor elements into

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ccessory phases (Harlow and Klimentidis, 1980; Schwartz andcCallum, 2005). Takeda and Graham (1991) described the min-

ralogic characteristics of basaltic eucrite pyroxenes, and relatedhem to a metamorphic sequence of types from 1 to 6. The least

etamorphosed, type 1, basaltic eucrites contain pigeonite withnly very narrow, <0.1 �m thick, augite exsolution lamellae, theyroxenes are not cloudy, and the original igneous Fe/Mg/Ca zon-

ng is preserved. The type 1 pyroxenes have cores of magnesianigeonite with low Ca contents and are zoned to ferroaugite orubcalcic ferroaugite compositions. Type 6 basaltic eucrites, theighest metamorphic grade, contain pigeonite with augite lamel-

ae several �m thick and exhibit partial inversion of pigeonite torthopyroxene. The grains are cloudy and homogeneous in Fe/Mgomposition. Preservation of original Ca zoning is evident in someasaltic eucrites where grain rims contain higher densities of augite

amellae than do cores. Note that Takeda and Graham (1991) usedreservation of chemical zoning of the pyroxene as one of the meta-orphic type criteria. Basaltic eucrites exhibit a range of original

rain sizes indicating differences in cooling rates; quite likely notll had the same original igneous zoning. Yamaguchi et al. (1996)dded a type 7 metamorphic grade containing pyroxenes with aixture of characteristics from types 4 and 6. The exsolution tex-

ures of basaltic eucrite pyroxenes have been used to quantify theooling rates for the rocks (e.g., Miyamoto and Takeda, 1977). Rep-esentative analyses for low-Ca and high-Ca pyroxenes are given inable S2, and are shown in Fig. 3c.

Plagioclase in basaltic eucrites is calcic, in the range of bytowniteo anorthite (Table S5). Unlike cumulate eucrites where plagioclaserains are typically homogeneous, plagioclase grains in individ-al basaltic eucrites can show considerable range in composition.lagioclase compositions in Chervony Kut span the range An75–94Mayne et al., 2009), while those in Nuevo Laredo span the rangen74–92 (Warren and Jerde, 1987). These represent most of theange observed for all HEDs (cf. Fig. 25 of Delaney et al., 1984a,b).he K2O contents are low, usually ≤0.2 wt% (e.g., Mayne et al.,009; and see Mittlefehldt et al., 1998), although some basalticucrites contain plagioclase with K2O contents of ∼0.5 wt% (Maynet al., 2009; Mittlefehldt and Lindstrom, 1993). The An contents ofasaltic eucrite primary plagioclases are shown in Fig. 8. Secondarylagioclase is present in late-stage veins transecting pyroxenes inome basaltic eucrites (Barrat et al., 2011); these are discussed inection 8.

Chromite/ulvöspinel is a common minor mineral in basalticucrites. Chromite/ulvöspinel grains show wide ranges in com-ositions, with TiO2 ranging between ∼1 and 23 wt%, Al2O3etween ∼3 and 18 wt% and Cr2O3 between ∼18–59 wt%; rep-esentative compositions are given in Table S4 (Bunch and Keil,971; Christophe-Michel-Levy et al., 1987; Mayne et al., 2009;arren et al., 1990; Yamaguchi et al., 1994, 2009; this work).hile in diogenites the major substitution is Al ↔ Cr, in basaltic

ucrite it is Ti ↔ (Al + Cr), with a fairly constant Cr/Al (Fig. 7a).hromite/ulvöspinel compositions in basaltic eucrites completelyverlap the range for cumulate eucrites in molar Al–Cr–Ti, but areore ferroan (Fig. 7).Ilmenite grains are mostly ilmenite-picroilmenite solid solu-

ions; representative compositions are given in Table S6. The minorlement contents of MgO, Al2O3, Cr2O3 and MnO are <1.4 wt%, andg# varies from 1.6 to 5.3.Other minor and accessory phases are the silica polymorphs

ridymite and quartz, ferroan olivine, metal, troilite, whitlock-te, apatite, zircon and baddeleyite (e.g., Delaney et al., 1984a,b;aba et al., 2014; Mayne et al., 2009; Saiki et al., 1991). These
hases typically occur interstitial to pyroxene and plagioclase, or
n mesostasis if present. Zircon is often intergrown with ilmenitee.g., Misawa et al., 2005). Cloudy pyroxene and plagioclase grainsontain inclusions of silica, metal, troilite and phosphate phases

Erde 75 (2015) 155–183 165

(Harlow and Klimentidis, 1980; Schwartz and McCallum, 2005).Olivine in basaltic eucrites is rare and very iron-rich. It occursin the most ferroan rocks and in the mesostasis of the least-metamorphosed basalts (e.g. Mittlefehldt and Lindstrom, 1993;Takeda et al., 1994). Ferroan olivine is also present in late-stageveins transecting pyroxenes in some basaltic eucrites (e.g., Barratet al., 2011); these are discussed in Section 8.

Calcium-phosphate phases are apatite and merrillite, and bothphases can occur in a given eucrite. Merrilite is rich in rare earth ele-ments (REE) while apatite contains lower concentrations of them(e.g., Hsu and Crozaz, 1996). Eucrites are generally considered tobe volatile-poor igneous rocks, but the apatites in them containvolatile elements in the structural X site. Sarafian et al. (2013) haveshown that apatite grains in basaltic and cumulate eucrites havemostly F in the X sites, but some grains in Juvinas contain up toabout 25% OH in the X sites. The apatite grains in eucrites are poorin Cl, with <3% of the X site filled by it. However, unbrecciated, meta-morphosed basaltic eucrite GRA 98098 contains apatite grains with∼11% of the X sites filled by Cl (Sarafian et al., 2013).

3.4. Petrologically anomalous eucrites

A few eucrites have anomalous textures and compositions thatcannot be easily fit into the cumulate gabbro or basalt/metabasaltcategories. Magnesian eucrite Pomozdino is an orthocumulate –a rock consisting of a mixture of 30 ± 10% cumulus crystals witha solidified melt (Warren et al., 1990). It is a monomict brecciabut contains two distinctly different types of mafic clasts; coarse-grained ophitic-poikilophitic and fine-grained, anhedral-granularclasts. Two distinct primary pyroxenes are present, one is composi-tionally similar to the primary pigeonite of cumulate eucrite MooreCounty, and the other shows Ca zoning similar to that documentedin pyroxenes of the basaltic eucrites Bouvante and Stannern (TableS2). Both of these pyroxene types are magnesian, with mg# of ∼47and ∼52, quite different from pyroxenes from basaltic eucrites suchas Bouvante, which have mg#s of ∼38 (Christophe-Michel-Levyet al., 1987). Chromite and ilmenite in Pomozdino are similar tothose in Moore County and more magnesian than those of basalticeucrites (Tables S4, S6). Plagioclase is more sodic than those incumulate eucrites, An81–87 vs. An91–95 (Table S5).

EET 90020 is an unusual unbrecciated, metamorphosed eucritecontaining two distinct lithologies, one coarse-grained (Fig. 10c),one fine-grained (Yamaguchi et al., 2001). The coarse-grainedlithology shows a REE pattern similar to those of cumulate eucrites,but other incompatible lithophile element (Hf and Ta) contentsare typical of basaltic eucrites (Mittlefehldt and Lindstrom, 2003;Warren et al., 2009). Pyroxene and spinel compositions are fer-roan, like those of basaltic eucrites (Mayne et al., 2009; Yamaguchiet al., 2001; this study). It seems clear that EET 90020 is a meta-morphosed basaltic eucrite, but the process that engendered itsanomalous REE pattern is obscure. Yamaguchi et al. (2001) con-cluded that the rock was heated to the point of partial melting andloss of some of the melt engendered the cumulate eucrite-like REEpattern. In contrast, Mittlefehldt and Lindstrom (2003) concludedthat the depletion of light REE but not of other highly incompati-ble elements (Hf, Ta) reflected subsolidus REE exchange betweenthe Ca-phosphates in the coarse- and fine-grained lithologies cou-pled with non-representative sampling of this heterogeneous rock.Rare-earth-element-rich merrillite is present only in the fine-grained lithology (Sarafian et al., 2013; Yamaguchi et al., 2001).Several other basaltic eucrites with granulitic texture (Agoult, A-87272, DaG 945, and NWA 2362) have REE patterns with some
similarities to that of EET 90020, and these have again been inter-preted to be restites (Yamaguchi et al., 2009). As was the case forEET 90020, these other granulitic-textured basaltic eucrites do nothave depletions in other highly incompatible elements (Yamaguchi

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t al., 2009). Y-86763 is described as having a texture like that ofET 90020, and the trace element compositions of pyroxenes areike those of pyroxenes from the coarse-grained lithology of EET0020 (Floss et al., 2000; Yamaguchi et al., 2001).

ALHA81001 is a fine-grained, quench-textured unbrecciatedasalt unlike any other eucrite (Mayne et al., 2009; Mittlefehldtnd Lindstrom, 2003; Warren and Jerde, 1987). The mg# of theow-Ca pyroxene is higher than typical for basaltic eucrites, andhe feldspar component occurs as a glass with the composition of alagioclase–silica mixture (Mayne et al., 2009). The bulk rock com-osition is also at the high end of the basaltic eucrite mg# range,he Na content is anomalously low, and refractory incompatible ele-

ent contents are higher than typical for eucrites (Mittlefehldt andindstrom, 2003; Warren and Jerde, 1987). Warren and Jerde (1987)oted compositional similarities between ALHA81001 and Ibitira,hile Mittlefehldt and Lindstrom (2003) considered it an anoma-

ous member of the Stannern-trend eucrites. Delaney et al. (1984b)onsidered an impact-melt origin as one possible explanation forhe petrologic characteristics of ALHA81001, but Warren and Jerde1987) cited low upper limits for Ni and Au contents as makingn impact-melt origin unlikely (see Warren, 1999; Warren et al.,996, for published siderophile element data on this meteorite).LHA81001 holds remanent magnetization that was acquired byooling in a field of crustal remanent magnetization formed by anarly vestan core dynamo (Fu et al., 2012). The remanent magneti-ation was acquired 3.69 Gyr ago based on Ar-Ar chronometry (Fut al., 2012).

.5. HED polymict breccias

Airless rocky bodies in the Solar System are covered with aayer composed of fragmental debris, lithified breccias and solidi-ed melt particles formed by hypervelocity meteoroid impacts ontohe surface (McKay et al., 1991). Howardites are polymict brec-ias, the lithified remnants of that debris layer from Vesta, and areostly composed of diogenitic and eucritic debris (Duke and Silver,

967). There are also polymict breccias consisting only of debrisrom different types of eucrites (Miyamoto et al., 1978; Olsen et al.,978; Takeda et al., 1978b), and a few diogenites contain basalticucritic clasts (e.g., Lomena et al., 1976). Thus, the suite of polymictreccias includes rocks with eucrite:diogenite-mixing ratios out-ide the range of “traditional” howardites (Mason et al., 1979). TheED classification system now recognizes a continuum of brec-ia types from monomict basaltic eucrite to monomict diogenitereccias (Fig. 1c). Polymict eucrites are eucrite-rich breccias con-aining <10% diogenitic material, and polymict diogenites contain10% eucritic material (Delaney et al., 1983). Very few polymictreccias are composed only of mixtures of cumulate and basalticucrite materials. Binda is one (Yanai and Haramura, 1993), and islassified as a polymict cumulate eucrite (Delaney et al., 1983). Sim-larly, polymict breccias composed only of diogenites and cumulateucrites seem very rare; Y-791073 is an example (Takeda, 1986).owardites fall into two subtypes: regolithic howardites, the lithi-ed remnants of the active regolith of Vesta; and fragmentalowardites, simpler polymict breccias (Mittlefehldt et al., 2013b;arren et al., 2009). Most of the material in the polymict brec-

ias is essentially the same as basaltic eucrites, cumulate eucritesr diogenites, and their descriptions need not be repeated. Here Iill focus on material formed by regolith gardening and on unusual

ithologic components.Many polymict breccias have fragmental matrixes that have

een little modified by metamorphism, for example the howardites
holghati, Kapoeta and Frankfort (Mason and Wiik, 1966a,b; Reidt al., 1990). Some polymict breccias have metamorphosed matri-es. Polymict eucrite Y-792769 has a fine-grained, sintered matrixTakeda et al., 1994), while howardite LEW 87002 shows petrologic
Erde 75 (2015) 155–183

evidence for Fe-Mg exchange among pyroxenes on a mm-scale(Mittlefehldt et al., 2013b). Some polymict breccias contain glassyor glass-rich matrices. Paired polymict eucrites LEW 85300, LEW85302, LEW 85303 and LEW 88005 have matrices rich in dense,dark brown glass (Kozul and Hewins, 1988). Polymict diogenite Y-791073 is heavily shocked with a vesicular glassy matrix (Takeda,1986). However, there has been no systematic study of matrix typesin the polymict breccias and an evaluation of the degree of meta-morphism and the fraction of melt-matrix types are unknown.

The polymict breccias, especially the howardites and polymicteucrites, contain lithic components formed during gardening onthe surface of Vesta. Bunch (1975) recognized three types of brec-cia clasts: crystalline matrix breccias with fine-grained fragmentalmatrix, glassy matrix breccias having glassy or devitrified matrix,and sulfide matrix breccias having troilite matrix. Labotka andPapike (1980) and Fuhrman and Papike (1981) recognized sul-fide matrix breccias as an important clast type, but they combinedcrystalline and glassy matrix breccias and melt rocks into theirdark-matrix breccia type. Labotka and Papike (1980) consideredthe dark-matrix breccia clasts to be fused soil and thus vestananalogs of lunar agglutinates. However, true agglutinates are rarein howardites (Noble et al., 2010). The matrices of melt rocks varyfrom glassy to cryptocrystalline to fine-grained quench-textured(Bunch, 1975; Delaney et al., 1984a,b; Hewins and Klein, 1978;Klein and Hewins, 1979; Mittlefehldt and Lindstrom, 1997, 1998).Mittlefehldt et al. (2013b) distinguished between melt-matrix anddark-matrix breccias, with the difference being the grain size of thematrix, but did not set rigorous limits for separating the two types.

Glassy spheres and irregularly shaped particles are commonlyfound in howardites and can be composed of glass sensu stricto ordevitrified glass (e.g., Barrat et al., 2009a; Bunch, 1975; Hewins andKlein, 1978; Labotka and Papike, 1980; Olsen et al., 1990). Warrenet al. (2009) used the abundance of glass particles, especially brown,turbid glass, as one indicator that a howardite might be regolithic.Glassy particles may contain microphenocrysts of olivine and/orlow-Ca pyroxene (Barrat et al., 2009a; Bunch, 1975; Hewins andKlein, 1978; Labotka and Papike, 1980; Mittlefehldt and Lindstrom,1998; Olsen et al., 1990; Singerling et al., 2013). Most glassy par-ticles in HED polymict breccias have petrologic characteristics andcompositions indicating that they are impact-melt particles of thevestan regolith (e.g., Olsen et al., 1990; Singerling et al., 2013).However, some have unusual compositions suggestive of formationfrom specific lithologic components, such as evolved mesostasis(Singerling et al., 2013), or distinctly different lithologic terrains(Barrat et al., 2009a,b).

Most igneous clasts in polymict breccias are very similar oridentical to the lithologies found as basaltic eucrites, cumulateeucrites and diogenites. However, some unusual lithologies arefound only as clasts. Fine-grained porphyritic or microporphyriticclasts composed of pyroxene phenocrysts or microphenocrysts inholocrystalline or variolitic groundmass of acicular plagioclase andpyroxene have been described in few howardites (Dymek et al.,1976; Mittlefehldt and Lindstrom, 1997), but these seem likely tobe impact-melt clasts of howarditic composition (Mittlefehldt andLindstrom, 1997). Texturally and mineralogically, they are similarto microporphyritic glassy spherules and fragments (Barrat et al.,2009a; Olsen et al., 1990; Singerling et al., 2013). Howardite Y-7308 contains olivine-orthopyroxene clasts with variable amountsof plagioclase, chromite and high-Ca clinopyroxene (Ikeda andTakeda, 1985). The olivine is euhedral to anhedral with mg# of65–73; the orthopyroxene is similar in composition to those in theYamato Type B noritic diogenites with mg# 69–74.

Numerous clasts of material as ferroan as or more so thanthe most evolved eucrites – Lakangaon and Nuevo Laredo, low-Ca pyroxene mg# of 28–32 (Mason et al., 1979; Warren andJerde, 1987) – have been described from howardites and polymict

ie der Erde 75 (2015) 155–183 167

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ucrites. Ikeda and Takeda (1985) described a clast in Y-7308omposed of fayalitic olivine (mg# 10–14), hedenbergitic pyrox-ne (mg# 27–32), tridymite, plagioclase (An78–85) and minorlmenite, chromite, troilite, Fe-metal and whitlockite. Mittlefehldtnd Killgore (2003) describe a cm-sized ferroan clast composedf euhedral to subhedral plagioclase grains (An83–86) poikiliticallynclosed in a framework consisting of a single ferroan augite grainmg# 21); fine-grained accessory silica, chromite, ilmenite, troilitend metal decorate the boundaries between plagioclase and augite.everal polymict breccias contain ferroan clasts consisting of pla-ioclase and iron-rich pyroxenes, some with an assemblage of ailica, fayalitic olivine and hedenbergite in symplectitic texture thatre likely breakdown products of pyroxferroite or metastable fer-oan pyroxene (Barrat et al., 2012; Buchanan et al., 2000, 2005;atzer and McSween, 2012). The olivine in these has mg# in theange 5–19, and high-Ca pyroxene has mg# as low as ∼3 (Barratt al., 2012; Buchanan et al., 2000; Patzer and McSween, 2012).ccessory phases in these clasts include ilmenite, merrillite, apatite,yalophane, troilite, zircon and baddeleyite (Barrat et al., 2012).

Chondritic clasts were inferred to occur in the dark portion ofapoeta based on higher C content of this material compared to the

ight portion (Müller and Zähringer, 1966), and were subsequentlydentified in thin section (Wilkening, 1973). Since then, chondriticlasts have been identified as a minor component of many HEDeteorites, especially the polymict breccias. Zolensky et al. (1996)

nd Gounelle et al. (2003) found that most chondritic clasts in HEDsre CM2 materials, with CR2 chondritic materials being less abun-ant. Some clasts appear to be thermally processed CI chondritesBuchanan et al., 1993), and some clasts are closest to CV3 chon-rites in mineralogy and petrology (Zolensky et al., 1992). Recenturveys of exogeneous materials in HEDs have shown that ordinaryhondrite, mesosiderite and possibly pallasite clasts are presentBeck et al., 2012; Lorenz et al., 2007; Prettyman et al., 2012).

. HED compositions

Basaltic eucrites show very limited ranges in major and minorlement composition, while cumulate eucrites and diogenites areore diverse. The polymict breccias, to first order, are simple mix-

ures of basaltic eucrites and orthopyroxenitic diogenites (e.g.,érome and Goles, 1971; Dreibus et al., 1977). Trace lithophile ele-

ent and siderophile element contents show much wider ranges. have maintained a database of achondrite compositions forecades. Whole rock and separated clast major, minor and tracelement compositions for HED meteorites and ungrouped basalticchondrites are given in Table S7, a version of my current databasencluding only published data. Note that this database always lagsehind the publication rate to some extent so some data, especiallyhe most recent, may not be included.

.1. Oxygen isotopic composition

With the advent of the laser-assisted fluorination techniqueor oxygen isotope analysis, HED meteorites have been showno be uniform in oxygen isotopic composition to high precisionDay et al., 2012; Greenwood et al., 2005, 2014; Scott et al.,009; Wiechert et al., 2004). Fig. 11a shows the oxygen isotopicompositions of HEDs, petrologically similar meteorites, and thesotopically similar angrites and main-group pallasites in Clayton-

ayeda oxygen-isotope space, and Fig. 11b focuses in on the regionccupied by HEDs. The narrow range shown in Fig. 11b is crowded
ith material derived from numerous differentiated asteroids, all ofhich may have been formed in the same general region of the solarebula. The IIIAB iron data are not shown in Fig. 11 because only
ower precision data are published (Clayton and Mayeda, 1996).

Fig. 11. Oxygen isotope compositions of HEDs, other basaltic achondrites,mesosiderites and pallasites. Data are from Bland et al. (2009), Floss et al. (2005),Greenwood et al. (2005, 2006), Scott et al. (2009), Wiechert et al. (2004).

High precision O isotopic analyses for IIIAB irons are reportedto have a mean �17O identical to that of main-group pallasites(Franchi et al., 2013).

There is a systematic bias in �17O between data from theCarnegie Institution of Washington (Day et al., 2012; Wiechert et al.,2004) and the Open University (Greenwood et al., 2005, 2014; Scottet al., 2009). I have adjusted data from the former institution so thatthe mean values from the two laboratories agree; see discussion inScott et al. (2009). For the remainder of this paper, only data fromGreenwood and colleagues at the Open University will be used.

Scott et al. (2009) calculated a 2× standard error of the mean(SEM) on �17O for eucrites and diogenites they measured of±0.004‰, although they preselected the data to exclude analy-ses that are more than 3 standard deviations from a penultimatemean. Using the same methodology, they calculated essentiallyidentical 2× SEM for the data from Greenwood et al. (2005) andWiechert et al. (2004). Using the same procedures, I calculate a 2×SEM for non-polymict HEDs of 0.0022‰ for Open University data(Greenwood et al., 2005, 2014; Scott et al., 2009) and of 0.0072‰ forCarnegie Institution of Washington data (Day et al., 2012; Wiechertet al., 2004). In spite of the greater SEM for the latter dataset,an important conclusion of these studies is that the HED igneous
lithologies point to an isotopically uniform system. This was takenas evidence for isotopic homogenization by the magmatic processesoperating on Vesta, and it was concluded that >50% melting of theasteroid occurred (Greenwood et al., 2005).


F mposr given

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ig. 12. Select element-element diagrams for HED meteorites showing distinct coanges for polymict breccias. Data shown are averages calculated from the analyses

Mittlefehldt (2005) noted that the unbrecciated eucrite Ibitira
as a �17O that is 16� outside the mean of other HEDs (Wiechertt al., 2004), and demonstrated that significant petrologic and tracelement differences are also observed. I concluded that Ibitira isn ungrouped basaltic achondrite from a distinct parent asteroid.
itional ranges for diogenites, cumulate eucrites and basaltic eucrites, and mixing in Table S7. The field “Sg” encloses the data for Stannern group of basaltic eucrites.

Scott et al. (2009) describe other “eucrites” that are discrepant by
>4� from the mean of other HED data, and ascribe them as hav-ing been derived from distinct parent asteroids. Note that thereis circularity in the logic here; Scott et al. (2009) first excludedoutliers from calculation of the mean, and then concluded that

ie der Erde 75 (2015) 155–183 169

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Fig. 13. Ti and Hf vs. mg# and Hf vs. Sc for basaltic eucrites showing the distinct


iscrepant meteorites are likely from distinct parents. Some sup-ort for this conclusion is derived from recent Cr isotopic studies.enedix et al. (2014) and Sanborn and Yin (2014) showed that sev-ral basaltic achondrites that have O isotopic compositions distinctrom eucrites also have �54Cr that is resolved from eucrites. Thesenclude Pasamonte and PCA 91007 that have �17O displaced only5� above the HED mean (Scott et al., 2009). However, Trinquiert al. (2007) determined that the Cr isotopic composition of Pasa-onte is identical to those of eucrites and diogenites, a discrepancy

hat requires resolution. The alternative scenario is that Vesta is iso-opically heterogeneous. The implications of this will be discussedn the section on vestan geologic evolution.

.2. Lithophile element compositions

The HED meteorites have several distinctive compositionalharacteristics. Among these is a general paucity of moderatelyolatile and volatile lithophile elements. The Na/Al ratios of basalticucrites is roughly a factor of 10 lower than that of solar abundancesnd of ordinary chondrite abundances, and a factor of ∼3 lower thanor CV and CK chondrites (McSween et al., 2011). The latter are the

ost volatile-depleted chondrite groups (see Lodders and Fegley,998). The more volatile alkali elements (Rb, Cs) are at much lowerbundances relative to refractory elements (Mittlefehldt, 1987).stimates of the bulk composition of the HED parent asteroid havebundances of moderately volatile and volatile elements similar tohose of the Moon (e.g., Anders, 1977; Dreibus and Wänke, 1980).ther distinctive characteristics of eucrites and diogenites are theirery low abundances of siderophile elements. For example, typicalI-normalized abundances for basaltic eucrites are Co ∼10−2, Ni10−3 to 10−6, and Ir ∼10−4 to 10−7 (Warren et al., 2009). Cumu-

ate eucrites and diogenites have on average higher siderophilelement abundances than basaltic eucrites, but their abundancesre low (Warren et al., 2009). In spite of their very low siderophilelement abundances, basaltic eucrites are rich in FeO, with the mostrimitive basalts having mg#s of ∼40–42.

The compositions for select major and minor elements on HEDsre shown in Fig. 12. Basaltic eucrites show very little variation inulk rock Mg, Al, Si, Ca and Fe; the most ferroan basaltic eucritesave only ∼15% more Fe than the group average, and Mg-richasaltic eucrite EET 87520 has ∼12% less Fe (Fig. 12a). Cumulateucrites show greater variations in all but Si, in part due to vari-tions in the mg# of their pyroxene (Fig. 3b) and in part due toariations in modal pyroxene/plagioclase ratio (Fig. 1b; cf. Tablec of Delaney et al., 1984a,b). Among igneous lithologies, diogen-

tes show the widest ranges in major element composition. This isostly a reflection of their lithologic diversity including dunites,

arzburgites, orthopyroxenites and norites (Beck and McSween,010; Beck et al., 2011; this work). Dunitic diogenite MIL 03443 hashe highest Mg and Fe contents and lowest Al, Si and Ca (Fig. 12).owever, some orthopyroxenitic diogenites contain exceptionallyagnesian pyroxenes; these diogenites have higher bulk Mg and Si

oupled with lower Fe. Diogenites include coarse-grained chromites a ubiquitous minor phase and bulk rock diogenites show a wideange in Cr contents. This reflects a combination of the difficultyn obtaining a representative sample of coarse-grained cumulatesnd real heterogeneity in the diogenite suite. Bulk rock analysesor Cr in some diogenites are always higher than typical for theroup indicating a more chromite-rich lithology rather than simplentra-sample heterogeneity (Mittlefehldt et al., 1998). Unbrecciatedrthopyroxenitic diogenite Dhofar 700 stands out as being mod-
stly anomalous in composition. Its Sc content (29.4 �g/g; Barratt al., 2008) is higher than for any other orthopyroxenitic diogenite5.6–21.6 �g/g; Table S7), and overlaps the range for main-groupucrites (∼27–36; Table S7). Orthopyroxenitic diogenites with the
groupings of main-group, Stannern-group and Nuevo-Laredo-group eucrites. Datashown are averages calculated from the analyses given in Table S7.

closest Sc contents to Dho 700 are all breccias, and their Sc contentsmight have been enhanced by basaltic components in the breccia.

The basaltic eucrites were subdivided based on mg# and Ticontents into the main-group, the Nuevo Laredo-trend and theStannern-trend (Stolper, 1977). With the availability of a muchlarger database, trends among basaltic eucrites have become morecomplex (McSween et al., 2011). Fig. 13a, is a Ti vs. mg# plot forbasaltic eucrites, a variant of the diagram used by Stolper (1977)to discuss basaltic eucrite petrogenesis. Most basaltic eucrites plotwithin a region of mg# ∼36–42, Ti ∼2.3–4.9 mg/g; I will refer tothese as the main group as is common practice. Some eucrites plotwithin this same mg# band, but at higher Ti contents; I will referto these as the Stannern group. These equate with the Stannern-
trend of Stolper (1977), but with the discovery of new membersand the redefinition of Ibitira as an ungrouped basaltic achondrite(Mittlefehldt, 2005), a “trend” from Ti-rich Stannern-group eucritesto high mg# main-group eucrites no longer exists. The Stannern

1 ie der Erde 75 (2015) 155–183

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Fig. 14. (a) Plagioclase An vs. low-Ca pyroxene En for basaltic eucrites and basalticclasts in polymict breccias, modified after Delaney et al. (1981) and Ikeda and Takeda(1985). Fields “p” and “e” are peritectic and evolved basalt groups of Delaney et al.(1981), and trends “A” and “B” are from Ikeda and Takeda (1985). (b) An equivalentdiagram based on bulk compositions shows that the main-group is equivalent to


roup shows less dispersion on a Hf vs. mg# plot and is well-esolved from the main group (Fig. 13b). A ferroan group of threeucrites – Igdi, Lakangaon and Nuevo Laredo – is separated fromhe main group with mg# ∼33, Ti ∼5.4 mg/g, Hf ∼1.7 �g/g. The cur-ent population of basaltic eucrites shows a significant gap in mg#etween the main group and the trio of ferroan eucrites. The dataorm separate groups, rather than trends. Unless the gap is closedhrough discovery of new basaltic eucrites, I recommend abandon-ng the term “trend” to describe these eucrites and instead refer tohem as the Nuevo Laredo group. The Nuevo Laredo group is tightlylustered in Fig. 13a and b, while the main group is not. There is

crystal fractionation sequence between main-group and Nuevo-aredo-group eucrites (cf. Stolper, 1977), but there might be two orore fractionation sequences represented by main-group eucrites

McSween et al., 2011). Pomozdino has a higher mg#, and highi and Hf contents. It is characterized as an orthocumulate fromn Stannern-group parent magma (Warren et al., 1990). McSweent al. (2011) used a plot of Hf vs. Sc to distinguish the main grouprom the Stannern group. On this plot (Fig. 13c), the Stannern groups cleanly separated from the main group, while the Nuevo Laredoroup is on the edge of the field of main-group eucrite data.

Yamaguchi et al. (2009) have proposed that eucrites like EET0020 are a distinct compositional group, which they refer to asresidual” eucrites. (Restitic eucrites would be a better term as theypothesis is that these eucrites are anatectic residues. This wouldvoid confusing them with residual melts like Nuevo Laredo.)amaguchi et al. (2013) have done melting experiments on a main-roup eucrite at temperatures just bracketing the solidus. Theynd that at small degrees of melting, the mesostasis-rich regionselt, leaving the pyroxene-plagioclase framework largely intact.

he minor/accessory phases apatite and ilmenite and by inferenceircon, enter the melt phase (Yamaguchi et al., 2013), which thenhould carry the bulk of the Zr, Nb, REE exclusive of Eu, Hf and Ta.irconium, Nb, Hf, Ta in the alleged restitic eucrites are sometimest basaltic eucrite abundances, and sometimes abundances equalo those of La and Ce (Mittlefehldt and Lindstrom, 2003; Warrent al., 2009; Yamaguchi et al., 2009). The formation scenario pro-osed by Yamaguchi et al. (2001; 2009) does not address these

nconsistencies, and because of this, the “restitic” eucrite group isot considered here.

Petrologic study of basaltic eucrites and clasts in polymictreccias suggested that there are two groups of basalts – called peri-ectic and evolved basalts – defined by differences in plagioclases. orthopyroxene compositional trends (Delaney et al., 1981). Theeritectic group has petrologic similarities to main-group eucrites,hile the evolved group shows similarities to Stannern (Delaney

t al., 1981). Ikeda and Takeda (1985) extended this observationo clasts in howardite Y-7308, gave the trends the neutral desig-ations A and B, equivalent to the peritectic, and evolved trendsf Delaney et al. (1981). Fig. 14a is a schematic plot of plagioclasenorthite content vs. low-Ca pyroxene enstatite content showinghe mineralogical differences between trends A and B. Fig. 14b isn equivalent bulk compositional plot of Na/Al vs. mg# for basalticucrites. In basaltic eucrites, most of the Na and Al are contained inlagioclase, and the Na/Al ratio thus tracks the plagioclase compo-ition. Because the mineralogical basalt groups are defined based onlagioclase anorthite content, Fig. 14b plots Na/Al in reverse order.he Stannern-group basalts generally have higher Na/Al, consis-ent with trend B basalts. Thus the bulk compositional data areonsistent with the Delaney et al. (1981) supposition that trend

(peritectic) basalts are equivalent to main-group eucrites, whilerend B (evolved) basalts included Stannern (cf. McSween et al.,
011).
Hewins and Newsom (1988) noted that the compositions of newembers of the Stannern group demonstrated that a single partialelting trend as originally defined (Stolper, 1977) was incorrect.

Trend A while the Stannern group is equivalent to Trend B. Data shown are averagescalculated from the analyses given in Table S7.

They noted that the two new eucrites with trace element charac-teristics similar to Stannern (Bouvante and Y-74450) plot on trend Bof Ikeda and Takeda (1985), presaging the equivalence of the min-eralogical groups and compositional groups argued for here andin McSween et al. (2011). Hewins and Newsom (1988) favoredfractional crystallization of distinct parent magmas as the originof the two mineralogical/compositional (trend A/main group andtrend B/Stannern group) sequences. However, they noted a poten-tial problem with modeling the trace element contents of trend Bbasalts, and suggested that trend B magmas may have resulted fromcontamination or magma mixing with incompatible-element-richresidual melts, presaging the model for Stannern-group formationpresented by Barrat et al. (2007).

The polymict breccias show extreme heterogeneity in bulkmajor and minor element composition, and span the ranges frombasaltic eucrites to diogenites (Fig. 12). The linear relationships inFig. 12 are consistent with the petrographic evidence that basaltclasts are similar to basaltic eucrites and that orthopyroxene clastsare similar to diogenites (e.g., Bunch, 1975; Duke and Silver, 1967;Mason et al., 1979). Note that on Al vs. Mg and Ti vs. Mg dia-grams (Fig. 12c, e) the polymict breccia data do not show evidencefor mixing with a substantial cumulate eucrite component (cf.,Jérome and Goles, 1971; Dreibus et al., 1977; McSween et al.,2011; Mittlefehldt et al., 2013b). Some polymict breccias con-tain Stannern-group basalts as their dominate mafic component(Fig. 12e), but the majority have compositions consistent with mix-tures of main-group eucrites and diogenites (Mittlefehldt et al.,2013b). Note that on an Hf vs. Mg diagram there is less evidence
for involvement of Stannern-group basalts in the polymict breccias(Fig. 12f). (Many of the Ti data are from wet chemistry analyses thatcan be inaccurate and/or imprecise for minor elements like Ti.)


Fig. 15. Incompatible lithophile trace element diagrams for HEDs showing good correlations and chondritic ratios for refractory elements (a), poorer correlations formoderately volatile-refractory data (b) and volatile-refractory data (c) with decreasing element/refractory ratio for increasingly volatile elements, and moderately wellcorrelated volatile/moderately volatile element ratios (d) that are lower than the CI ratio. Data shown are averages calculated from the analyses given in Table S7.

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almost exactly match the CM chondrite ratio, while some basalticeucrites, cumulate eucrites and diogenites have Ir/Ni ratios that are

The contents of incompatible trace lithophile elements gener-lly increase in the sequence diogenite, cumulate eucrite, basalticucrite, but there are overlaps of the ranges for cumulate eucritesnd diogenites for some elements (Fig. 15). For refractory highlyncompatible elements, element-element ratios are generallyquivalent to CI ratios (Fig. 15a). For moderately volatile incompati-le elements such as K, the correlation with refractory incompatiblelements breaks down for diogenites, and basaltic and cumulateucrites have moderately-volatile/refractory incompatible elementatios much lower than CI (Fig. 15b). For volatile incompatible ele-ents such as Cs or Tl, the correlation with refractory incompatible

lements completely breaks down and volatile/refractory incom-atible element ratios are again much lower than CI (Fig. 15c).olatile element Cs is generally correlated with moderatelyolatile Rb, but with a ratio lower than that of CI chondritesFig. 15d).

The contents of refractory incompatible lithophile elements iniogenites show wide ranges compared to those of basaltic orumulate eucrites (Fig. 15a). Part of this variation is due to varyingixtures of a trapped-melt component in the cumulate rocks (cf.,

arrat et al., 2010; Mittlefehldt, 1994). However, even after consid-ring the potential effects of included trapped melt on diogeniteompositions, wide ranges in refractory incompatible lithophilelement contents remain (Mittlefehldt, 1994). These ranges cannote explained by simple models for HED petrogenesis that invokerystallization of a global magma ocean (Barrat et al., 2008, 2010;
ittlefehldt, 1994; Shearer et al., 1997, 2010). This topic will be
xplored in more detail in Section 6.

4.3. Siderophile element compositions

Siderophile elements are at low to very low abundancesin HED igneous lithologies, and are generally highest in thepolymict breccias. Chondritic debris is found even in supposedlymonomict breccias (e.g., Mittlefehldt, 1994), and for this reason, thesiderophile element contents of HED igneous lithologies need to beevaluated cautiously. Cobalt is a moderately siderophile elementand HED igneous lithologies show ranges in Co contents from ∼3to 9 �g/g for basaltic eucrites and up to ∼11–30 �g/g for diogenites.Nickel contents show much wider ranges: 0.1–50 �g/g for basalticeucrites, 1–150 �g/g for diogenites (Fig. 16). Carbonaceous chon-drites have about 20–200 times as much Co, but 80 to 123,000 timesthe Ni. Contamination by chondritic debris in monomict brecciasis therefore much less a problem for Co. A mixing model betweenthe most Ni-poor basaltic eucrite and CM chondrites shows that theentire Ni range can be explained by as little as 0.5% chondritic debrisin the breccias (Fig. 16), but this engenders only a modest increasein the Co content. For the most part, the ranges in Co contentsof basaltic eucrites, cumulate eucrites and diogenites likely reflectthose of pristine igneous lithologies.

The highly siderophile element contents will be similarly verysusceptible to chondritic contamination. Warren et al. (2009)demonstrated that the polymict breccias have Ir/Ni ratios that

consistently ∼0.4 times lower. These igneous HEDs have Ni and Irconcentrations that span a range of ∼600 times (lowest to highest)

172 D.W. Mittlefehldt / Chemie der

Fig. 16. Ni vs. Co for HEDs showing asymptotic approach to CM chondrite ratio withincreasing siderophile element contents in polymict breccias. Small amounts of CMcsg

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hondrite contamination causes substantial increase in Ni with little change in Co ashown by the mixing model. Data shown are averages calculated from the analysesiven in Table S7.

nd includes monomict breccias and unbrecciated rocks, provid-ng good evidence that these are endogenous siderophile elementontents. Within this subset, diogenites have the highest Ni and Irontents and basaltic eucrites have the lowest. Dale et al. (2012) anday et al. (2012) determined highly siderophile element (Ru, Pd, Re,s, Ir, and Pt) contents for suites of eucrites and diogenites. Theseuthors found ranges in concentrations spanning nearly 4 ordersf magnitude, and most of the eucrites and diogenites have nearlyhondritic patterns for highly siderophile elements (Fig. 17). Thenbrecciated harzburgitic diogenite MIL 07001 has the highest con-entrations, with abundances at ∼10−2 times CI for these elementsDay et al., 2012). Unbrecciated harzburgitic diogenite NWA 5480as ∼10−3 times CI abundances of the elements, and unbrecciatedrthopyroxenitic diogenite Tatahouine has ∼4 × 10−5 to 10−4 timesI abundances. Some of the brecciated diogenites have lower abun-ances. Brecciated diogenite MET 00424 has lower abundances ofighly siderophile elements than any of the unbrecciated diogen-

tes (Fig. 17), while brecciated AHLA77256 and LAP 91900 havebundances lower than those of NWA 5480.

Basaltic eucrites have abundances of highly siderophile ele-ents typically between 10−4 and 10−5 (Dale et al., 2012) (Fig. 17).

amel Donga has among the highest siderophile element contents.

ig. 17. CI-normalized abundances of highly siderophile elements for select basalticucrites (diamonds) and diogenites (circles) plotted in order of decreasing compat-bility in silicate systems. Shown are averages of data from Dale et al. (2012), Dayt al. (2012) and Warren et al. (2009).

Erde 75 (2015) 155–183

This is a relatively metal-rich basaltic eucrite (Cleverly et al., 1986;Palme et al., 1988) that also has a high Ni content (Barrat et al.,2007). Béréba and Juvinas have generally increasing abundancesof highly siderophile elements from Os, the most compatible, toRe, the least compatible. Stannern generally shows this same trendexcept that Os is at a higher abundance than Ir and Ru (Fig. 17).

4.4. Noble gas contents

The noble gas contents of HED igneous lithologies are generallyquite low (e.g., Eugster and Michel, 1995), but some howardites arerich in noble gases compared to other HED meteorites. Howarditeswith higher noble gas contents contain two components, planetary-and solar-type gases (e.g., Mazor and Anders, 1967). These authorsthought that these gases were added to the regolith by a singlecarrier phase because the planetary- and solar-type gas contentswere correlated. Characterization of carbonaceous chondrite frag-ments in howardites (Wilkening, 1973) led to the identification ofthese clasts as the carriers of the planetary-type gases (Wilkening,1976). Experiments have shown that the solar-type gases are a sur-face correlated component in mineral and glass fragments of HEDparentage, not just of the chondritic materials (Black, 1972; Caffeeet al., 1983; Padia and Rao, 1989; Rao et al., 1991). These solar-typegases represent solar wind and fractionated solar wind implantedin the outer few �m of grains in the breccias. (The fractionatedsolar wind component was formerly referred to as a solar energeticparticle component and thought to be derived from solar flares,cf. Grimberg et al., 2006.) The solar wind component shows thatsome howardites were part of the true regolith of their parent body(see discussion in Cartwright et al., 2013, 2014; Mittlefehldt et al.,2013b).

5. HED ages

The crystallization ages of HED igneous lithologies as deter-mined by long-lived chronometer systems – Rb-Sr, Sm-Nd, Pb-Pb– demonstrate that magmatism on Vesta occurred very early inSolar System history (e.g., Allègre et al., 1975; Birck and Allègre,1978; Nyquist et al., 1986; Papanastassiou and Wasserburg, 1969;Smoliar, 1993; Tera et al., 1997). However, almost all eucrites anddiogenites have been brecciated and/or thermally metamorphosed,and the different geochronometers have responded differently tothese post-crystallization perturbations depending on the relativediffusivities of the parent and daughter elements. The unbrecciatedcumulate eucrites Moore County and Serra de Magé yield Pb-Pbmodel ages (Tera et al., 1997) that are resolvably lower than the bestestimate of the age of magmatism for the basaltic eucrites based onRb-Sr data (Smoliar, 1993). The cumulate eucrites have texturesand pyroxene exsolution characteristics that indicate they wereformed deep in the vestan crust and cooled slowly (e.g., Miyamotoand Takeda, 1977; Takeda, 1979). Similarly, a Rb-Sr isochron agefor diogenites Johnstown and Tatahouine (Takahashi and Masuda,1990) is resolvably lower than the estimate age of basaltic eucritemagmatism. Johnstown is a breccia and Tatahouine suffered shockdamage, potentially resetting the Rb-Sr chronometer. Thus, whilelong-lived chronometer systems provide evidence that magmatismoccurred very early on Vesta, the derived ages are neither robustenough nor precise enough to detail the fine-scale history of vestanglobal igneous evolution.

Fine-scale interrogation of the relative ages of events in the dif-ferentiation history of Vesta is provided by an array of short-lived
chronometers. These short-lived chronometers can be tied into anabsolute age scale if an accurate, precise, long-lived chronometersystem can be applied to a milestone meteorite. Precise long-livedchronometry is achievable with the Pb-Pb model age system that

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an yield ages with uncertainties on the order of 120–200 Kyr forgneous meteorites that are ∼4565 Myr old (Amelin, 2008; Wadhwat al., 2009). The short-lived chronometer systems 26Al-26Mg (t½.73 Ma), 53Mn-53Cr (t½ 3.7 Ma) and 182Hf-182W (t½ 8.9 Ma) are

mportant fine-scale chronometers for probing the timing of dif-erentiation of Vesta. The 60Fe-60Ni system (t½ 2.62 Ma) appearso have suffered late-stage disturbance, possibly related to the Fe-

g exchange documented in pyroxenes, and has not yet providedhronological constraints on HED formation (Quitté et al., 2011).

The Al–Mg offers fine-scale temporal resolution by virtue ofhe very short half-life of the parent nuclide. Because they aren incompatible-compatible lithophile element pair, the system isell fitted for dating mantle differentiation events. Bizzarro et al.

2005) determined that 26Mg* anomalies (radiogenic 26Mg) com-ared to the Earth, Moon, Mars and chondrites are present inumulate and basaltic eucrites. This demonstrated that 26Al wasive during magmatism on Vesta. The data were confirmed bychiller et al. (2010) who calculated that magmatism occurred onesta 2.6–3.2 Myr after formation of CAIs. Study of an extensiveuite of diogenites shows that they exhibit a range in 26Mg* thatorrelates with the contents of Ca, Ti and Yb in the orthopyroxene,nd that are lower than the 26Mg* of basaltic eucrites (Schiller et al.,011). These authors conclude that the data are consistent with the

ngrowth of radiogenic Mg while the HED suite was forming viaolidification of a global magma ocean. Schiller et al. (2011) calcu-ate that diogenites crystallized from 0.6 to 2.5 Myr after formationf CAIs, and that the cumulate and basaltic eucrite reservoirs wereormed between 2.1 and 2.8 Myr after CAI formation. As mentionedn Section 4.2, the variations in refractory incompatible lithophilelement contents of diogenites cannot be explained by simplelobal magma ocean models. Thus, the interpretation of the 26Mg*ariations in diogenites espoused by Schiller et al. (2011) cannote correct in detail. Nevertheless, the correlations of 26Mg* with

ncompatible lithophile elements in diogenites does support theeneral conclusion of ingrowth of radiogenic Mg with increasingractionation. This is explored in more detail in Section 6.

Lugmair and Shukolyukov (1998) determined that diogenites,umulate eucrites and basaltic eucrites define an isochron in the3Mn-53Cr system. They indexed a precise Pb-Pb absolute age and3Mn-53Cr isotopic systematics for the angrite LEW 86010 to theED Mn-Cr isochron to derive an estimate for the time of differ-ntiation of the HED parent body at 7.1 ± 0.8 Myr before formationf angrite LEW 86010, or an absolute age of 4564.8 ± 0.9 Ma. LEW6010 is a plutonic and metamorphosed angrite (see Keil, 2012),nd volcanic, unmetamorphosed angrite D’Orbigny is 5.9 Myr olderhan LEW 86010 (Amelin, 2008), indicating that HED parent bodyifferentiation occurred roughly 1 Myr before magmatism on thengrite parent asteroid. Trinquier et al. (2008) confirmed this abso-ute age for HED parent body differentiation, and pegged this evento 2.3 Myr after CAI formation.

The 182Hf–182W system is uniquely able to constrain the timingf core-mantle separation on differentiated asteroids because Ws a moderately siderophile element while Hf is lithophile. At theow oxygen fugacity conditions of vestan petrogenesis, a significantractionation of W from Hf occurred during metal-silicate separa-ion (Newsom and Drake, 1982). In the silicate phase, both elementsre incompatible and thus not very sensitive to mantle-crust frac-ionation. Early work in the W-Hf system suggested ages for vestanifferentiation of a few million years after formation of ordinaryhondrites (e.g., Quitté et al., 2000), but these analyses may haveeen compromised by contamination with terrestrial W (Kleinet al., 2009; Touboul et al., 2008). At present, the best estimate for
ore-mantle differentiation of Vesta is 2.5 ± 1.2 Myr after CAI for-ation (Touboul et al., 2008), which is consistent with estimates
f HED parent asteroid differentiation from Mn-Cr (Trinquier et al.,008). However, these estimates are inconsistent with formation

Erde 75 (2015) 155–183 173

of the earliest diogenite cumulates at 0.6 Myr after CAI formationderived from the Al-Mg system (Schiller et al., 2011). Schiller et al.(2011) concluded that the very short time interval between CAI anddiogenite formation is inconsistent with derivation of HEDs froman asteroid as large as Vesta because a large magma ocean could notreach the point where diogenites were crystallizing in only 600,000years.

Metamorphic and impact ages are best determined using the39Ar-40Ar method, and Bogard (2011) has summarized these stud-ies for eucrites and howardites. The data show that the HED suitecontains evidence for significant Ar degassing in the period of about4.1 to 3.3 Ga ago. The broad age probability spectrum in this timerange shows highs and lows that are interpreted to indicate group-ings of ages that may be related to different large impact eventson Vesta (Bogard, 2011). Relatively few HED samples have Ar-Arages older than 4.1 Ga. Bogard and Garrison (2003) showed that asuite of unbrecciated, metamorphosed basaltic eucrites and cumu-late eucrites have Ar-Ar ages consistent with a single thermal eventat 4.48 Gyr ago. They interpreted this result as indicating that therocks had resided at depth where temperatures were sufficient tokeep the K-Ar system open until a large impact excavated themand allowed rapid cooling and K-Ar closure to occur. Cohen (2013)determined Ar-Ar ages for impact-melt clasts from the howarditesEET 87513, QUE 94200 and QUE 97001, and compared her resultswith literature data from several howardites. She concluded thatthe impact-melt clasts may indicate that impacts occurred at atyp-ically high velocities during the time period 3.3–3.8 Ga, and thata greater number of impacts may not be the cause. In cases wheremultiple samples have been analyzed from the same meteorite, theAr-Ar ages are not always concordant. For example, the results com-piled by Bogard (1995) show that Ar-Ar ages of different samples ofKapoeta vary from ∼3.44 to 4.48 Ga, and Cohen (2013) found agesof 3.37 to 3.73 for impact-melt clasts from QUE 94200. This indi-cates that the Ar-Ar ages of howardite clasts predate the assemblyof the breccias.

Ages of thermal metamorphis can also be determined using theHf-W system applied to metal and pyroxene. Under the tempera-ture conditions of eucrite metamorphism, radiogenic 182W diffusesout of pyroxene (high Hf/W) and is sequestered in metal (Hf/W ∼0) allowing computation of the timing of thermal metamorphism ifthe 182Hf/180Hf ratio at the time of formation of the eucrite can bedetermined (Kleine et al., 2005, 2009). Using this technique, Kleineet al. (2005) determined that the thermal metamorphism recordedin a suite of three main-group eucrites occurred 13–18 Myr aftercrystallization.

Cosmic ray exposure ages for HED meteorites range from ∼3Ma for the howardite Kapoeta (Wieler et al., 2000) to 110 Ma forthe howardite Lohawat (Sisodia et al., 2001). Eugster and Michel(1995), Herzog (2007), Shukolyukov and Begemann (1996) andWelten et al. (1997) synthesized the cosmic ray exposure data thatwere available. Eugster and Michel (1995) found that 80% of the 67HED meteorites studied fell into five exposure age clusters of 6 ± 1,12 ± 2, 21 ± 4, 38 ± 8 and 73 ± 3 Ma. Shukolyukov and Begemann(1996) found five CRE age clusters for eucrites. Three correlate withthe Eugster and Michel (1995) clusters at 6, 21 and 38 Ma, butShukolyukov and Begemann (1996) concluded that there are twoclusters at 10 ± 1 and 14 ± 1 Ma instead of a single cluster at 12Ma. Welten et al. (1997) calculated exposure ages for the diogen-ites they analyzed, and combined their data with literature data toreevaluate the clustering of HED exposure ages. They parsed thedataset differently than did Eugster and Michel (1995), and use 63HEDs for their analysis. They found only two statistically significant
age clusters in the data, 22 ± 2 and 39 ± 5 Ma. The other age clus-ters identified by Eugster and Michel (1995) were not statisticallyrobust. Herzog (2007) also concluded that two impacts at ∼20 and∼40 Myr could have ejected most HEDs from their parent object.

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he 22 and the 39 Ma age clusters each contain eucrites, polymictucrites, diogenites and howardites demonstrating the individualmpact events can liberate the range of petrologic types. Many cos-

ic ray exposure ages have been published since Herzog (2007)see for example Cartwright et al., 2013, 2014), and a reevaluationf HED exposure age clusters would be timely given that the Dawnission has provided a detailed record of cratering on Vesta (e.g.,archi et al., 2012).

. HED meteorite petrogenesis

The earliest petrogenetic model for asteroidal differentiationas based on the petrology and mineralogy of HED meteorites,allasites and iron meteorites and envisioned total melting andrystallization to produce a suite of igneous rocks. Mason (1962)bserved that the lithologies of the HED clan could represent aractional crystallization sequence from early magnesian orthopy-oxenites, through a series of rocks with increasing plagioclaseontent and Fe-enrichment of pyroxene, ending with basalticucrites as residual melts. He inferred that an iron core and duniticantle would occur in the deep interior. (Note that at this time,ason thought howardites were a brecciated igneous rock type,

nd not an impact-engendered mixture of distinct lithologies.)The Mason model was the standard until Stolper (1977) pre-

ented the alternative anatexis model. He did a series of meltingxperiments on basaltic eucrites and found conditions of temper-ture and oxygen fugacity at which some eucrites are saturatedith pyroxene, plagioclase, olivine, metal and spinel, a mineral

ssemblage like that of ordinary chondrites. Thus he concluded thatome eucrites are primary partial melts of their parent body. Someucrites are more Fe-rich, and he noted that these follow majorlement trends of glasses produced in his experiments at differingegrees of crystallization. He interpreted the more ferroan eucriteso be residual melts from differing degrees of crystallization of pri-

ary partial melts. The partial melting/fractional crystallizationodel can successfully explain trace incompatible lithophile ele-ent contents of basaltic eucrites (e.g., Consolmagno and Drake,

977; Mittlefehldt and Lindstrom, 2003). Melting experiments onhondritic meteorites at the oxygen fugacity inferred for eucriteetrogenesis yield melts from some chondrite types with compo-itional characteristics similar to basaltic eucrites (Jurewicz et al.,993, 1995). In these experiments, moderately volatile elements

ike Na were allowed to escape from the system and the resultingelts had eucrite-like Na contents.However, details of eucrite and diogenite compositions have

evealed several problems with the Stolper (1977) petrogenesischeme. Warren (1985, 1997) used mass-balance constraints fromajor and trace elements to conclude that the mafic fraction of

owardites was mostly residual after crystallization of diogenites,nd by extension, most eucrites must also be. This contradicts theonclusion based on the eucrite melting experiments that basalticucrites are unlikely to be residual melts after extensive crystalliza-ion of more magnesian magmas (Stolper, 1977). A related problems that although experimental melts of a Na-poor CM chondritere similar to basaltic eucrites in composition, there is insufficientyroxene in the restite to produce abundant orthopyroxene cumu-

ates from higher temperature partial melts (Jurewicz et al., 1993).ontrarily, high temperature melts in experiments on Na-poor LLhondrites do contain sufficient normative pyroxene to explain dio-enites, but the low temperature melts produced are not a goodatch for eucrite compositions (Jurewicz et al., 1995).
The siderophile element contents of basaltic eucrites also sug-
est that a primary partial melt model for basaltic eucrites isncorrect. Calculations using metal/silicate partition coefficients forhe moderately siderophile elements Co, Mo and W indicate that

Erde 75 (2015) 155–183

basaltic eucrite melts did not equilibrate with metal (McSweenet al., 2011; Newsom, 1985; Palme and Rammensee, 1981), con-trary to the inference from melting experiments (Stolper, 1977).The abundances of P, Co, Ni, Mo and W in basaltic eucrites are bestfit by a model in which metal and silicate are totally molten (Righterand Drake, 1996); a Mason-type petrogenetic model.

High precision oxygen isotope measurements have shown thatHEDs are very uniform in �17O (Greenwood et al., 2005, 2014;Scott et al., 2009; Wiechert et al., 2004). Greenwood et al. (2005)cited models that match the bulk composition of the HED parentasteroid by mixing known chondrite types as indicating that priorto differentiation Vesta would have been heterogeneous in O iso-tope composition on a many-km scale. They then concluded thatthe extreme uniformity in O isotopic composition of HEDs (�17Owithin ±3% of the mean) indicated homogenization via a magmaocean stage, which they calculated must have been formed by >50%melting. Note that this conclusion stands on the assumption thatbecause the bulk composition of Vesta can be modeled as a mixtureof known chondrite types, the post-accretion interior was com-posed of large-scale disparate compositional regions. As mentionedabove, the uniformity in �17O inferred for HEDs is partially becauseoutliers were excluded from calculations of the means.

Partly because of these problems with the Stolper (1977) petro-genetic scheme, Mason-type magma ocean models have regainedfavor. Righter and Drake (1997) and Ruzicka et al. (1997) madedetailed calculations of major and trace element fractionationduring crystallization of a magma ocean, while Warren (1997) pre-sented a more conceptualized view of vestan petrologic evolutionby magma ocean crystallization. Ruzicka et al. (1997) modeledbasaltic eucrites, excluding Stannern-group basalts, as havingformed in a magma ocean that first fractionally crystallized dunitesand orthopyroxenites, and then crystallized under conditionsapproaching equilibrium to produce the suite of basaltic eucrites. Incontrast, Righter and Drake (1997) modeled the early stage of pet-rogenesis as equilibrium crystallization forming harzburgites, withthe basaltic residual liquid then undergoing fractional crystalliza-tion to form the main-group basaltic eucrite suite. Stannern-groupbasalts represent other residual melts. Ruzicka et al. (1997) andRighter and Drake (1997) come to opposite conclusions as to whichlithologies require equilibrium vs. fractional crystallization for theirformation. The Righter and Drake (1997) model is more firmlygrounded in magma physics; they computed that a vestan magmaocean would undergo vigorous convection until sufficient crystalswere present to cause convective lockup, and this informed theirmodeling of the early stage as an equilibrium process.

However, there are problems with these simple magma oceanmodels in that they do not obviously describe the ranges of incom-patible trace element variations observed for diogenites. Bulksamples and pyroxene separates of diogenites show wide ranges inincompatible element contents, which would imply a wide rangein fractional crystallization, but they have restricted ranges in mg#and mineralogy, suggesting a very narrow range in degree of crys-tallization (Mittlefehldt, 1994). In situ measurements of minor andtrace elements in diogenite pyroxenes support this general conclu-sion (Fowler et al., 1994, 1995; Shearer et al., 1997). Fowler et al.(1995) and Shearer et al. (1997) concluded that the diogenite suitecan be modeled as cumulates arising via small amounts (10–20%)of fractional crystallization of a suite of distinct basaltic magmasformed either by fractional melting of a homogeneous source, orequilibrium melting of heterogeneous sources. This scenario is atodds with magma ocean models.

The most recent magma ocean model incorporates polybaric
crystallization and periodic tapping of a deep magma layer, andmay have achieved resolution of this conundrum. The concep-tualized model is shown in Fig. 18. Mandler and Elkins-Tanton(2013) propose a model that includes an early stage of equilibrium

D.W. Mittlefehldt / Chemie der

Fig. 18. Conceptualized differentiation model for Vesta, after Mandler and Elkins-Tanton (2013). Vestan structure: 1. metallic core; 2. possible restitic dunite lowermll

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rystallization à la Righter and Drake (1997). With an increasingbundance of crystals, convective lockup occurs and the melt isssentially filter-pressed to the top of the mush where crystal-ization becomes non-equilibrium (fractional). An initial thermaloundary layer of primitive chondritic material is graduallyeplaced by a mafic crust through impact disruption, founderingnd magma extrusion/intrusion; the mafic crust thickens over time.elt from the residual magma ocean intrudes into the mafic crust

nd crystallizes a sequence of diogenitic and eucritic cumulates.eriodic recharge of these crustal plutons from the residual magmacean melt buffers the major element composition of the magmaut allows for increases in the incompatible lithophile trace ele-ent contents. This causes decoupling major elements from trace

lements in the cumulate diogenites and restricts the evolution ofasaltic eucrite compositions. Eruption of the mafic magmas formsasaltic eucrites.

The Mandler and Elkins-Tanton (2013) model is more detailednd dynamic than previous magma ocean models, but is still broad-rush and does not treat the differences between the main-groupnd Stannern-group basalts. Barrat et al. (2007) put forth a crustal-ontamination model to explain the genesis of Stannern-groupasalts. They showed that small degrees of partial melting of arust with the composition of main-group eucrites, and mixingf this crustal melt with main-group basaltic eucrite magmas,ould replicate the trace element compositions of Stannern-groupasalts. Stannern-group basalts have higher Na/Al than main-groupasalts (Fig. 14b, and see McSween et al., 2011), and higher averageodal tridymite/quartz (Delaney et al., 1984a,b), consistent with

he crustal contamination model of Barrat et al. (2007). Yamaguchit al. (2009) have concluded that eucrites with light REE-depletedatterns are restites from the process that formed Stannern-groupasalts. However, when other refractory incompatible elementsZr, Nb, Hf, Ta) are considered, these rocks do not match the mod-ling of Barrat et al. (2007). Thus, there is not a compelling reasono connect EET 90020 and similar eucrites with the formation oftannern-group eucrites.

Barrat and Yamaguchi (2014) contend that the Mandler andlkins-Tanton (2013) model is incapable of explaining some of thencompatible trace element characteristics of the diogenite suite,
or example the Dy/Yb ratios. But again, even though the Mandler-lkins-Tanton is a great improvement over older magma oceanodels (Righter and Drake, 1997; Ruzicka et al., 1997), the model
s still of necessity rather simplistic in the way it treats geological

Erde 75 (2015) 155–183 175

processes. Future extensions of the model will determine whetherthe shortcoming identified by Barrat and Yamaguchi (2104) repre-sents a fatal flaw.

The petrologic and compositional characteristics of diogenitesdo seem inconsistent with magma ocean models for vestan petro-genesis (e.g., Barrat, 2004; Fowler et al., 1995; Mittlefehldt, 1994;Shearer et al., 1997). Barrat et al. (2008, 2010) and Yamaguchiet al. (2011) have concluded that the trace element characteristicsof diogenites are explained by a complex petrogenetic scheme inwhich olivine- and pyroxene-rich magma ocean cumulates wereremelted, some were contaminated with melts derived from thebasaltic crust, and some were formed as shallow intrusions or evensurface flows. At present, the energy source for this process hasnot been identified. The heat engine powering vestan petrogene-sis is thought to be primarily the decay of short-lived 26Al with alesser contribution from 60Fe (for example, Formisano et al., 2013).Olivine-pyroxene-rich cumulates will have low contents of Al. Theirsolidus temperature will also be higher than the temperature atwhich they crystallized from the magma ocean because of theirseparation from the more felsic magma of their formation. Thus,a plausible scenario that allows this refractory cumulate to remeltwill be challenging to develop for Vesta.

The calculations of Wilson and Keil (2012, 2013) have chal-lenged the magma ocean concept altogether. They argue that thefast melt production and easy melt migration in differentiatedasteroids, including those of the size of 4 Vesta, imply formationof giant sills, not magma oceans. Thus, large-scale “magma oceans”in asteroid mantles should not form.

The cumulate eucrites are accumulations of plagioclase andpyroxene from a crystallizing mafic melt. Cumulate eucrites are tooiron-rich to be adcumulates from basaltic eucrites (Stolper, 1977).However, Barrat et al. (2000) and Treiman (1997) showed that themajor and trace element compositions of Binda, Moore County andSerra de Magé can be modeled as mixtures of cumulus mineralsand trapped melt from magmas similar to basaltic eucrites. Barrat(2004) and Mittlefehldt and Lindstrom (2003) showed that thetrace element contents of many cumulate eucrites are consistentwith mixtures of cumulus pyroxene and plagioclase mixed withtrapped melt, and that the parent magmas were similar to basalticeucrites in trace element composition.

Trace element data from mineral separates and in situ anal-yses on cumulate eucrites have been taken to indicate that theparent magmas of cumulate eucrites were richer in REE thanbasaltic eucrites and had unusual light REE-enriched patterns (Hsuand Crozaz, 1997; Ma and Schmitt, 1979; Ma et al., 1977). Punand Papike (1995) also calculated nominal equilibrium melt REEcontents from their in situ analyses of cumulate eucrites, but cau-tioned that unusual parent melt compositions provided only onepossible explanation of the result. They noted that subsolidusredistribution of the REE or inappropriate partition coefficientscould be the cause of the unusual calculated patterns. Treiman(1996, 1997) showed that subsolidus redistribution of the REEin cumulate eucrites and diogenites causes parent melts calcu-lated by inversion of ion microprobe data to have unusual REEpatterns that have no bearing on the original parent melt composi-tion. There is no compelling evidence that magmas with very highlight REE/heavy REE ratios existed during formation of the vestancrust.

7. Thermal metamorphism of the HED parent body crust

Most HED igneous lithologies show textural evidence for ther-mal annealing expressed as variable degrees of Fe-Mg equilibration,exsolution of augite from pigeonite sometimes accompaniedby inversion of pigeonite to orthopyroxene, and mineral grain

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9. Mixing of the vestan crust


ecrystallization and clouding (Harlow and Klimentidis, 1980;ayne et al., 2009; Metzler et al., 1995; Schwartz and McCallum,

005; Takeda and Graham, 1991; Yamaguchi et al., 1996). Pyroxeneextures are generally correlated with mg#; pyroxenes in cumu-ate eucrites (magnesian lithologies) have coarser exsolutions thanhose in basaltic eucrites (ferroan lithologies). These pyroxene tex-ures can be used to model quantitatively the cooling rates of theocks, which inform estimates of the depth of formation. Thesebservations led to development of a layered-crust model for Vestae.g., Miyamoto and Takeda, 1977; Takeda, 1979; Takeda et al.,976). The pyroxene textures were interpreted to be caused largelyy slow cooling from the magmatic stage and hence the extent ofyroxene exsolution and inversion is related to the depth of forma-ion of the rock (Takeda, 1979). Some eucrites do not fit the generalrend, suggesting that later metamorphism was an important pro-ess in specific cases (e.g., Takeda and Graham, 1991). Possibleechanisms for later metamorphism include impact heating and

eating by subsequent flows or intrusions (contact metamorphism)e.g., Nyquist et al., 1986; Takeda and Graham, 1991). Impacteating was invoked to explain the pyroxene textural differencesetween monomict eucrites and basalt clasts in polymict breccias.onomict basaltic eucrites were thought to represent material

rom crater floors where impact energy is deposited and heat isetained for longer times, while the basaltic clasts were ballisti-ally dispersed in the polymict ejecta that was cooler (e.g., Nyquistt al., 1986; Takeda and Graham, 1991). Impact heating, not globalrustal metamorphism, was considered to be the cause of thermaletamorphism recorded in the Hf-W systematics of basaltic eucriteetal (Kleine et al., 2005, 2009). However, impact heating and con-

act metamorphism by later intrusions/extrusions seem incapablef explaining the great preponderance of metamorphosed igneousithologies in the HED suite (Keil et al., 1997; Yamaguchi et al.,996).

Yamaguchi et al. (1996, 1997b) developed a model to explain therevalence of thermally metamorphosed basaltic eucrites, which

ikely were extrusive flows, shallow dikes and/or shallow sill-likentrusions on Vesta. These authors posit that eruption rates wereery high, resulting in rapid burial of early formed basalts. In theirodel, heat conducted from the mantle through the crust would

e sufficient to anneal eucrites. Thus, reestablishment of the par-nt body thermal gradient causes global thermal metamorphism inhe crust. The least metamorphosed basaltic eucrites would be theatest extrusions. The maximum eruption rates used by Yamaguchit al. (1996, 1997b) are consistent with the maximum magma vol-me fluxes calculated by Wilson and Keil (2012) based on magmahysics for asteroids of the size of Vesta. Haraiya is one of theetamorphic type 7 basaltic eucrites of Yamaguchi et al. (1996)

nd thus represents a deeply buried rock in their model. However,chwartz and McCallum (2005) found that exsolution textures ofaraiya are inconsistent with simple burial metamorphism. Rather,

hey concluded that a short-lived thermal pulse provided the peaketamorphic temperature.Bogard and Garrison (2003) determined that a number of

nbrecciated cumulate and metamorphosed basaltic eucrites haver-Ar ages of 4.48 Ga which they interpreted as indicating that theocks were at depth at temperatures above Ar diffusion closure until

large, basin scale impact excavated them, promoting rapid coolingo below the Ar closure temperature. One of these is the anoma-ous basaltic eucrite EET 90020. Yamaguchi et al. (2001) presentedvidence that they interpret as indicating that EET 90020 was at870 ◦C at the time of excavation, and was briefly heated to above

he solidus (∼1060 ◦C) by this process, resulting in partial meltingnd melt migration. The scenario developed by Yamaguchi et al.2001) implies that at depth but within the crust of Vesta, temper-
tures of ∼870 ◦C were maintained (or achieved) for ∼80 Myr afterlobal differentiation.
Erde 75 (2015) 155–183

8. Fluid-mediated metasomatism?

Historically, HEDs have been considered to have formed on avolatile-depleted asteroid and that fluid phases were not involvedin melting, magma crystallization or subsolidus processing.Mittlefehldt and Lindstrom (1997) presented evidence from animpact-melt clast in the EET 92014 howardite that fluid-mediatedmetasomatism occurred. Magnesian orthopyroxene phenocrysts inthe clast contained veins of Fe-rich pyroxene that have anoma-lously high Fe/Mn ratios, precluding igneous fractionation as thecause of Fe enrichment. Similar Fe-rich, high Fe/Mn veining isfound in numerous magnesian orthopyroxene clasts in severalElephant Morraine howardites (Mittlefehldt et al., 2011, 2013b)suggesting that a metasomatized orthopyroxenite protolith wasthe source. These studies did not attempt to characterize the natureof the fluid agent. Treiman et al. (2004) concluded that the quartzveinlets in Serra de Magé are the result of precipitation from aque-ous solutions. The water was exogenic in origin, and aqueousmobilization of SiO2 in Serra de Magé was engendered sometimeafter formation of eucritic basalts and gabbros (Treiman et al.,2004).

Some Fe-enrichment zoning patterns in Pasamonte pyroxenesare interpreted to have resulted from post-magmatic metasoma-tism by a dry Fe-rich vapor (Schwartz and McCallum, 2005). Incontrast, Barrat et al. (2011), documented a variety of composi-tional and mineralogical changes crosscutting pyroxenes in severaleucrites, which they concluded were cause by interaction with late-stage aqueous fluids. The veins included simple Fe-enrichment inPasamonte, Fe-enrichment coupled with formation of fine-grainedferroan olivine and sometimes very calcic plagioclase (An97–99)in several eucrites, and this same assemblage but on wider scaleand with development of Al-depletions (Barrat et al., 2011). Theseauthors conclude that aqueous solutions were plausible agent ofmetasomatism based on terrestrial hydrothermal analogs, but theydid not discuss at what point in the post-magmatic-crystallizationstage the metasomatism occurred.

Warren et al. (2014) described a set of late-stage alteration fea-tures in brecciated and metamorphosed Stannern-group eucriteNWA 5738. They document two types of microveins: one dom-inantly of calcic plagioclase (An∼95), ferroan olivine (mg# ∼14)and Cr-spinels containing Fe3+ (from stoichiometry); one domi-nantly composed of very pure Fe-metal. These authors concludethat the two types of veins were formed at distinct times, and thatthermal metamorphism did not postdate vein formation. Warrenet al. (2014) conclude that the veins were formed from aqueous-rich fluids derived from carbonaceous chondrite impactor debris onVesta.

Zhang et al. (2013) have documented ferroan pyroxenes asmineral fragments and in lithic clasts that are partially replacedby fine-grained troilite, magnesian augite or hedenbergite, and asilica phase in melt-breccia eucrite NWA 2339. They concludedthat the primary grains suffered sulfurization likely the result ofimpact heating mobilizing S-rich vapors. This is essentially thesame scenario as invoked by Palme et al. (1988) for the formationof abundant Fe metal in brecciated eucrite Camel Donga.

At present, evidence for metasomatism is not widespread,documents different types of processes, and appears to indi-cate late-stage, post-magmatic processes unrelated to formationof primary lithologies on Vesta. The evidence for aqueous-basedmetasomatism points to an exogenous source for the water.

The Dawn spacecraft has returned images, visible and infraredreflectance spectra, and gamma ray and neutron spectra of the

ie der Erde 75 (2015) 155–183 177

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Fig. 19. Variation in calculated percentage of eucrite material (POEM) in HEDpolymict breccias estimated from data averages calculated from the analyses given


egolith surface of Vesta. Howardites, polymict eucrites andolymict diogenites preserve records of regolith processes thatccur on Vesta but at smaller spatial scales. Most HED polymictreccias are fragmental breccias that do not contain material thatpent significant time in the actively gardened soil at the top ofhe debris layer, the true regolith (McKay et al., 1991). Bischofft al. (2006) suggest that ∼38% of howardites are gas-rich, andhus regolith breccias, but the details behind this census have noteen published. Warren et al. (2009) determined that ∼12% ofhe howardites they discussed are regolithic. However, subsequentoble gas analyses have shown that several howardites studiedy Warren et al. (2009) contain implanted solar wind gases andhus are regolithic (Cartwright et al., 2013, 2014). Adding theseo the list and correcting for pairings, ∼21% of the howarditesiscussed in Warren et al. (2009) are regolithic. Cartwright et al.2013, 2014) found that 33% of the howardites they studied con-ain solar wind implanted Ne. Based on these observations, roughlyne quarter to one third of howardites contain material exposed inhe true regolith. Considering that howardites make up roughlyalf of all HED polymict breccias listed in the Meteoritical Bulletinatabase, the abundance of regolithic howardites may be ∼15% ofED polymict breccias. This estimate is consistent with theoreticalodeling of asteroidal regoliths that shows that a packet of frag-ental debris on Vesta will spend less time exposed to the space

nvironment than the equivalent debris on the Moon (Housen andilkening, 1982).Warren et al. (2009) noted that the few regolithic howardites

dentified by them tended to have a narrower range in compositionhan shown by the suite as a whole. Their Al contents suggesteducrite:diogenite-mixing ratios of ∼2:1, which they posited washat of ancient, well-mixed vestan regolith. Mittlefehldt et al.2013b) found that the additional regolithic howardites identi-ed by Cartwright et al. (2013, 2014) have a range of mixingatios of ∼1:1 to ∼3:1. Fig. 19 is a histogram of the percentagef eucritic material (POEM) in HED polymict breccias highlight-ng those that are regolithic. The POEM index, first introduced byérome and Goles (1971), is calculated as described in Mittlefehldtt al. (2013b). Ignoring MacAlpine Hills (MAC) 02666 (POEM8) which is only possibly regolithic (Warren et al., 2009),ucrite:diogenite mixing for regolithic howardites covers ∼40%f the range observed for howardites generally. About a third ofhe howardites shown have not had noble gas analyses publishedn them, and thus cannot be subtyped with certainty. Comparingnly those that have been characterized, there are numerous frag-ental howardites with POEM <50 but no regolithic howardites

n this range; the average POEM for fragmental howardites is 55nd for regolithic howardites is 63. The data are suggestive thategolithic howardites generally have a more restricted mixing ratio,ut there are too few well-characterized howardites to allow firmonclusions.

The compositions of howardites also inform us of the naturef the mafic end member of the mixtures. An early study focusedn quantitative modeling using linear regression analysis by pair-ise mixing of specific basaltic eucrites and diogenites to yield

best fit for a suite of major and trace elements in individ-al howardites (Fukuoka et al., 1977). These authors found thatowardite compositions were generally well matched by combi-ations of main-group eucrites and orthopyroxenitic diogenites.ässing was the only howardite that was better fit using a Nuevo-

aredo-group eucrite (Fukuoka et al., 1977). They did not testumulate eucrite-diogenite mixtures, or ternary mixtures of cumu-ate and basaltic eucrites with diogenites. Usui and Iwamori (2013)
ave modeled the entire HED suite using independent componentnalysis to ferret-out mixing and fractionation trends. They con-rmed that mixing of the HED polymict breccias is dominantlyetween main-group eucrites and orthopyroxenitic diogenites.
in Table S7. Howardites with “?” in the symbol are only possibly regolithic. Excludingthose, regolithic howardites range in POEM from about 50 to 80.

This is also the conclusion reached through qualitative evaluationof mixing diagrams. McSween et al. (2011) used averages of litera-ture data and showed that most polymict eucrites and howarditesare consistent with simple main-group eucrite-diogenite mixtures;only a few contain a significant Stannern-group basalt component,and only a few contain a significant cumulate eucrite compo-nent. Mittlefehldt et al. (2013b) similarly found that of the 29HED polymict breccias they analyzed, only one was dominatedby a Stannern-group basaltic component, one contained substan-tial Nuevo-Laredo-group basalt debris, and none were dominatedby cumulate eucrite material. They ran linear regressions on dif-ferent incompatible refractory lithophile elements vs. Al, whichconsistently passed through the composition of main-group eucriteJuvinas. Mittlefehldt et al. (2013b) concluded that because diogen-ites are a major component of HED polymict breccias yet wereformed lower in the crust than cumulate eucrites (Takeda, 1979),the paucity of cumulate eucrites and their complementary fer-roan basalts is plausibly an indication that they are truly rarein the vestan crust, not just under sampled. The petrogenetic
model of Mandler and Elkins-Tanton (2013) predicts that cumu-late eucrites should be much less common than basaltic eucrites ordiogenites.

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casual glance comparing cosmic ray exposure ages with mete-orite petrology and compositions does not suggest any bias, but a


0. Ungrouped basaltic achondrites

Northwest Africa 011 was the first basaltic achondrite describedhat is petrologically very similar to eucrites but demonstrablyrom a different parent asteroid (Yamaguchi et al., 2002). It and itsairs remain the most distinct from HEDs in O isotope compositionFig. 11a). Subsequently, I argued that the unusual basaltic eucritebitira was also unrelated to HEDs. This rock is one of a very few

eteoritic basalts to have vesicles (Wilkening and Anders, 1975)nd has unusually low alkali element contents (Stolper, 1977). Itas an O isotope composition like angrites and very different fromEDs (Greenwood et al., 2005; Wiechert et al., 2004), an Fe/Mn

atio in pyroxenes significantly higher than for basaltic eucrites,nd ratios for some incompatible trace element that are differentrom those of basaltic eucrites (Mittlefehldt, 2005). Subsequently,igh precision O isotope analyses have shown that A-881394 andunburra Rockhole have similar O isotope compositions with �17Ohat lie ∼15� away from the HED mean composition (Benedix et al.,014; Bland et al., 2009; Scott et al., 2009). Preliminary data onnbrecciated cumulate gabbro EET 92023 and basaltic achondritemmaville, both classified as eucrites, show that they have O iso-ope compositions that are close to those of A-881394 and Bunburraockhole (Greenwood et al., 2012, 2013). Dhofar 007 is a polymictreccia composed mostly of cumulate gabbro debris and is rich inetal and siderophile elements (Dale et al., 2012; Yamaguchi et al.,

006). It contains materials with differing �17O within the rangeound for Bunburra Rockhole (Greenwood et al., 2012). NWA 2824as rare-earth-element contents like those of basaltic eucrites, but

s more Mg-rich than eucrites (Bunch et al., 2009). Its O-isotopicomposition is very similar to Ibitira. Pasamonte and PCA 91007 lie5� from the HED mean, and NWA 1240 lies ∼4� from the mean

Scott et al., 2009) (Fig. 11b).Sayh al Uhaymir (SaU) 493 is truly anomalous (Irving et al.,

011). It is composed of exsolved pigeonitic pyroxene and calciclagioclase with an annealed igneous texture, and has a rare-arth-element pattern like that of cumulate eucrites. Other highlyncompatible elements (Zr, Nb, Hf, Th) are at abundance levelsike those of basaltic eucrites. SaU 493 contains substantial Fe3+

n pyroxene and as hematite, indicating formation at higher oxy-en fugacity conditions than those of any other mafic achondriteIrving et al., 2011).

All of these mafic rocks have broadly eucrite-like mineralogy:alcic plagioclase and pyroxene compositions like those of cumu-ate (A-881394) or basaltic eucrites (Fig. 3d) (see discussion in Scottt al., 2009). NWA 1240 is unusual in that it has highly zoned pyrox-nes but a trace element signature of a cumulate gabbro (Barratt al., 2003). This rock is interpreted to be an impact melt. Thextreme view based on O isotope compositions is that six parentsteroids are represented by the basaltic achondrites: HEDs, NWA11 and pairs, Ibitira (and possibly NWA 2824), A-881394 and Bun-urra Rockhole (and possibly EET 92023, Emmaville and Dhofar07), Pasamonte and PCA 91007, and NWA 1240 (Scott et al., 2009).ittlefehldt (1990) and Rubin and Mittlefehldt (1992) have argued

hat mesosiderite silicates cannot be from the same parent asteroids HEDs, and this is supported by an evaluation of the preliminarye composition of the vestan surface determined by the Gammaay and Neutron Detector on the Dawn spacecraft (Mittlefehldtt al., 2012b). The implication is that generally similar asteroids inerms of low moderately volatile element contents (Na, K), oxy-en isotope composition (except for NWA 011 and pairs), and withn oxygen fugacity near that of the iron-wüstite buffer (exceptor SaU 493) differentiated to produce basaltic crusts. These par-nt asteroids are low in moderately volatile elements comparedo all chondrites (McSween et al., 2011). There is no obvious rea-
on why basaltic meteorites should be dominated by Na,K-poorompositions.
Erde 75 (2015) 155–183

11. The future

There are several areas where our current knowledge of HEDsand vestan geologic evolution is lacking, and research directed intothese areas could help us more fully understand differentiation ofasteroids.

• How many parent asteroids? The very large difference in oxygenisotope composition between HEDs and NWA 011 et al. make astrong case that a distinct parent asteroid is required (Yamaguchiet al., 2002). Ibitira is substantially different in O isotopes, andhas additional compositional differences that also favor a dis-tinct parent asteroid (Mittlefehldt, 2005). However, some basalticachondrites are less distinct, Pasamonte for example, which nev-ertheless are purported to be from different asteroids than HEDs(Scott et al., 2009). My sense is that some of these noted distinc-tions are not robust, and the number of differentiated asteroidsrepresented might be less than thought. If true, the oxygenisotopic composition of Vesta is not as uniform as thought. Addi-tional high precision O isotopic work, especially on HEDs thathave petrologic and/or compositional anomalies ought to be doneto evaluate this issue further. In particular, O isotope analyses ofdiogenitic and eucritic clasts separated from howardites couldprovide strong tests. Oxygen isotopic variations (or uniformity)within an individual polymict breccias would allow for moremuscular conclusions.

• The unusual Mg isotopic result on diogenites that suggests theywere formed only 0.6 Myr after CAIs (Schiller et al., 2011) shouldbe revisited. These authors conclude that the time scale is tooshort to fit thermal models for the differentiation of a Vesta-sized asteroid. If this result stands up upon further testing, thenwe would have to conclude that HEDs do not come from Vestaafter all. Additional isotopic work and more sophisticated aster-oid thermal models are required to investigate this issue.

• A related but separate issue is whether diogenite petrogenesisoccurred as part of magma ocean solidification (Mandler andElkins-Tanton, 2013), or via remelting of magma ocean cumulateswith or without contamination by basaltic-crust-melts (Barratet al., 2008; 2010). The trace element and mineralogic character-istics of diogenites remain difficult to factor into magma oceanmodels (e.g., Barrat and Yamaguchi, 2014; Mittlefehldt et al.,2014). Further petrologic/compositional study of diogenites andcontinuing efforts to increase the sophistication of petrogeneticmodels are needed to close this gap.

• The evidence that fluid phases might have metasomatized mate-rials on Vesta is poorly understood. The timing of metasomaticevents with respect to the history of the rocks has not been welldefined, but is certainly post-brecciation in some cases (Warrenet al., 2014; Zhang et al., 2013). The source of the fluid phase(endogenic vs. exogenic) is unknown, but has been inferred tobe exogenic in some cases (Treiman et al., 2004; Warren et al.,2014). Several types of fluids have been suggested, including hotdry vapors (Schwartz and McCallum, 2005), aqueous-based flu-ids (Barrat et al., 2011; Treiman et al., 2004; Warren et al., 2014)and S-rich fluids (Palme et al., 1988; Zhang et al., 2013). A moresystematic study of metasomatic effects in HEDs would allow forgreater understanding of the role of fluids on Vesta.

• The cosmic ray exposure ages show that substantial numbers ofHEDs were excavated from their immediate parent object, pre-sumably a vestoid, by only two impacts (e.g., Herzog, 2007). Thus,many HEDs sample the same limited region of Vesta. Has thislimited sampling biased our view of vestan differentiation? A

more rigorous petrologic and compositional comparison of HEDsthat are not members of the 22 or 39 Myr age groups with those

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that are should be done to ascertain the representativeness ofour sampling of the vestan crust. Further study of the lithologicdiversity of igneous clasts in HED polymict breccias would alsohelp in addressing this issue.

cknowledgments

My career-long infatuation with HEDs and mesosiderites begans a graduate student at UCLA working under John T. Wasson.egardless of the fact that we did not (and still do not) see eye-o-eye on several aspects of HEDs and mesosiderites, this reviewould not have been possible without his mentoring; special

hanks to him. Collaborations and discussions with several col-eagues over the years have helped me refine my thinking on HEDenesis: A.W. Beck, D.D. Bogard, P.C. Buchanan, J.S. Delaney, R.reenwood, R.H. Hewins, J.H. Jones, A.G.J. Jurewicz, K. Keil, M.M.indstrom, H.Y. McSween, Jr., L.E. Nyquist, H. Palme, J.J. Papike, R.G.ayne, C.H. Shearer, H. Takeda, P.H. Warren. Seminal papers by

.-A. Barrat and A. Yamaguchi have had considerable influence ony understanding of HED meteorites. I thank these individuals for

heir insights. I thank B. Mandler and R. Mayne for some of theraphics used here, and K. Ross for some of the SEM BSE imagessed. I thank Associate Editor K. Keil for inviting me to write thiseview and for his handling of the manuscript. Journal reviews by.-A. Barrat and A. Yamaguchi resulted in substantial improvementsn the manuscript. Support for this work came from the NASA Cos-

ochemistry Program.

ppendix A. Supplementary data

Supplementary data associated with this article can beound, in the online version, at http://dx.doi.org/10.1016/j.chemer.014.08.002.

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