Metallic Phases and Siderophile Elements in Main Group Ureilites: Implications for Ureilite Petrogenesis

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Geochimica et Cosmochimica Acta 112 (2013) 340–373 www.elsevier.com/locate/gca Response Metallic phases and siderophile elements in main group ureilites: Implications for ureilite petrogenesis

Cyrena A. Goodrich a,⇑, Richard D. Ash b, James A. Van Orman c, Kenneth Domanik d, William F. McDonough b

a Planetary Science Institute, 1700 E. Ft. Lowell Drive, Suite 106, Tucson, AZ 85719-2395, USA b Dept. of Geology, University of Maryland, College Park, MD 20742, USA c Dept. of Geological Sciences, Case Western Reserve University, Cleveland, OH 44120, USA d Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

Received 12 May 2011; accepted in revised form 22 June 2012; available online 3 August 2012

Abstract

Metallic phases and siderophile elements are critical to understanding the petrogenesis of the enigmatic ureilite meteorites. We obtained petrographic, major and minor element, and the first in situ trace element data for metallic phases (metal, sulfides, phosphide, carbide) in 24 main group ureilites of various petrographic types with Fo 75–95. The most abundant type of metal (1–3 vol.%) occurs as 10–40 lm-wide strips along silicate grain boundaries. Ni contents of this metal range from 0 to 7.3 wt.% and are correlated with Co among all samples (Ni/Co = 0.64 CI). A less abundant type of metal occurs as 5–150 lm diameter metallic spherules, consisting of cohenite (Fe3C), metal, phosphide and sulfide, enclosed in silicates (preferentially low-Ca pyroxene). Most samples contain 2 types of sulfide: (1) low-Cr (<0.1 wt.%) troilite, and (2) lamellar intergrowths of daubreelite (FeCr2S4) and troilite. Abundances of 17 (mostly siderophile) elements were measured by LA-ICP-MS in grain boundary metal, spherules, graphite, sulfides and silicates. Average compositions of grain boundary metal in 10 samples show decreasing CI-normalized abundance with increasing volatility, interrupted by depletions in W, Mo, Ni and Zn, and enrichments in Au, As, Ga and Ge. CI-normalized Os abundances range from 2 to 65, and are correlated with increasing Os/Pt, Os/Ni and Os/Pd ratios. CI-normalized Pt/Os ratios range from 0.3 to 1. Bulk cohenite-bearing spherules have siderophile element abundances indistinguishable from those of grain boundary metal in the same sample. CI-normalized patterns of most siderophile elements in the metal are, within error, identical to those of the bulk rock (at 25–40 higher abundances) in each sample. There are no correlations between siderophile element abundances and Fo. We infer that at T P 1200 °C ureilites contained immiscible Fe–C (3–4 wt.% C) and Fe–S melts, small samples of which were trapped as the spherules within silicates. The Fe–S melt was largely extracted from the rocks, and the bulk of the residual Fe–C melt is now represented by the grain boundary metal. Assuming that ureilite precursor materials had CI or CV abundances of siderophile elements, the large fractionations of HSE observed in metal in 7 of the 10 samples require extremely high degrees (>98%) of batch Fe–S melt extraction, which implies very high xFeS (= wt. FeS/[Fe + FeS]) in the precursors. Fur- thermore, at such high degrees of fractionation, the HSE are so strongly concentrated into the residual metal that to match their relatively low absolute abundances in the ureilite metal, very high initial metal contents are required. Together, these constraints would imply that ureilite precursors had abundances of Fe metal and FeS (20–35 wt.% each) far exceeding those of known CC or OC. These requirements could be relaxed, permitting lower (more plausible) degrees of melting and lower

⇑ Corresponding author. E-mail address: [email protected] (C.A. Goodrich).

0016-7037/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2012.06.022 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 341 initial metal and sulﬁde abundances, if ureilite precursors were volatile-depleted to a greater extent than bulk CV. We suggest that ureilite precursors contained, to various degrees, an overabundance (relative to chondrites) of refractory-enriched material such as CAIs. Excess CAIs could also account for observed depletions of W and Mo (otherwise diﬃcult to explain) in the ureilite metal, and lead to the observed range of siderophile element patterns and abundances among samples. Such a model can potentially explain the lack of correlation between siderophile element abundances and FeO (or olivine Fo), and reconcile the metal and siderophile element data with a redox model for ureilite petrogenesis. Ó 2012 Elsevier Ltd. All rights reserved.

1. INTRODUCTION 1.1. Previous work on metallic phases and siderophile elements in ureilites Ureilites are carbon-rich ultramafic (olivine + domi- nantly low-Ca pyroxene) meteorites whose petrogenesis is In early work on ureilites, when the number of known poorly understood (Goodrich, 1992; Mittlefehldt et al., samples was 66(Vdovykin, 1970; Berkley et al., 1976, 1998). Although they are clearly achondritic (both textur- 1980), they were described as having a dark carbonaceous ally and compositionally), and generally thought to be res- matrix (“C-matrix”) that intruded the silicates. The matrix idues of 15–30% partial melting (sufficient to completely was a “chaotic melange” of carbon polymorphs (fine-grained remove the basaltic component from a chondritic composi- graphite, diamond, and lonsdaleite), with veins of Fe,Ni me- tion), many of their characteristics are difficult to reconcile tal (identified as kamacite on the basis of low Ni contents) with typical igneous processes. and sulfides. This led to common use of the term “vein metal” One major question is the origin of extensive FeO- to describe the dominant metal in ureilites. This view of urei- variation among ureilites. Ureilite olivine compositions lites changed with the discovery of the diamond-free (appar- (which are homogeneous within grain cores in each sam- ently low-shock) sample ALHA78019 (Berkley and Jones, ple) span a large and essentially continuous range of Fo 1982), in which mm-sized euhedral graphite crystals were (= 100 molar MgO/[MgO + FeO]) values, from 74 to intergrown with metal and sulfides along silicate grain 95, and show an FeO/MgO vs. FeO/MnO trend of near- boundaries. Berkley and Jones (1982) inferred that these tex- constant MnO/MgO (Mittlefehldt, 1986; Goodrich et al., tures represented the primary state of ureilite carbon and me- 1987; Goodrich et al., 2004). This is not an igneous tal, which had been obscured in previously studied samples fractionation trend, which would have constant FeO/ by shock. With the subsequent increase in number of samples MnO. Rather, it is a trend of nearly pure FeO-variation, (currently >300), it has become even more clear that the which suggests that ureilites are related by a redox dominant occurrence of both graphite and metal in ureilites (oxidation or reduction) process (Goodrich and Delaney, is along silicate grain boundaries. In this paper we use the 2000). term “grain boundary metal” rather than “vein metal.” However, any redox process, whether nebular or plan- Available petrographic and compositional data for me- etary, would lead not just to variation in FeO content, tal and sulfides in ureilites are sparse (a few grains in a but also to anti-correlated variation in Fe metal content. few samples), due at least in part to pervasive effects of ter- No such variation has been observed among ureilites. restrial weathering. These data show variable Ni, Cr, Si and Metal abundances of all ureilites studied to date are low P contents in metal, and highly variable Cr contents in sul- (a few percent), and their bulk Fe contents (which are sub- fides (Mittlefehldt et al., 1998). Rare phosphides and car- chondritic) are principally determined by the FeO contents bides have also been described (Goodrich and Berkley, of their olivine and pyroxene (Jarosewich, 1990). In addi- 1986; Fioretti et al., 1996). tion, siderophile element abundances in bulk ureilites are All previous discussions and modeling of siderophile ele- near-chondritic (0.1–1 CI), and not correlated with ments in main group ureilites have been based on bulk rock FeO (or Fo), which appears to rule out significant metal data, plus a few analyses of physically separated C-rich loss. This observation has been used (Mittlefehldt et al., “vein” material from Havero¨ and Kenna. Although abun- 2005; Warren and Huber, 2006; Warren et al., 2006; dances of siderophiles in bulk samples are near-chondritic, Rankenburg et al., 2008) as an argument against the equi- their patterns are not unfractionated. A number of studies librium smelting model, in which ureilite FeO-variation is (Wa¨nke et al., 1972; Higuchi et al., 1976; Goodrich et al., attributed to reduction by carbon (FeO + C ! Fe0 + CO) 1987; Warren et al., 2006) have pointed out a general trend during high-temperature igneous processing on the ureilite of decreasing abundance with increasing volatility, but parent body (Goodrich et al., 1987, 2007; Walker and noted that the decrease is not monotonic and therefore vol- Grove, 1993; Singletary and Grove, 2003). However, it atility cannot be the only (or the dominant) control. Almost may also be an argument against models in which ureilite all studies, from Wa¨nke et al. (1972) based on 4 ureilites, to FeO-variation is considered to be inherited from chon- Warren et al. (2006) based on 29 ureilites, have noted signif- dritic precursors (e.g., Goodrich and Delaney, 2000). If icant correlations between the highly refractory (e.g., Ir, Re, all redox models for ureilite petrogenesis are ruled out, Os) and the less refractory (“normal” to volatile) sidero- then new types of models must be considered. Clearly, me- phile elements (e.g., Ni, Co, Au, Ge, Pd), and have attrib- tal and siderophile elements are critical to understanding uted this to the presence of two metal components – one ureilite petrogenesis and the nature of ureilite precursor with high Ir/Ni (or Ir/Au) and the other with low Ir/Ni materials. (or Ir/Au). Interpretations of these components have 342 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 varied. In early studies (Wasson et al., 1976; Higuchi et al., Gabriel and Pack, 2008) in ureilites. These new data provide 1976; Boynton et al., 1976) it was accepted that the carbo- a more complete picture of the diversity of types and compo- naceous material (and associated metal) in ureilites was sitions of metallic phases in ureilites, and allow more detailed non-indigenous (injected late in ureilite history by impact). modeling of their origin. The trace element data obtained for Based on this assumption, Wasson et al. (1976) identified different types of metal, sulfides and silicates allow us to the low Ir/Au component with the injected material and examine the distribution of siderophile elements between the high Ir/Au component with the mafic silicates. In con- phases, and to model not only elemental patterns, but abso- trast, Boynton et al. (1976) interpreted the high Ir/Ni com- lute abundances as well. A further goal of the work was to ponent to be the foreign material and the low Ir/Ni assess the role of carbon in ureilite petrogenesis, by modeling component to be an indigenous residue of partial melting. ureilite siderophile elements using previously unavailable Higuchi et al. (1976) also interpreted the high Ir/Ni compo- partition coefficients for the Fe–S–C system (Hayden et al., nent to be the foreign material, and considered the low-Ir 2011). Our overarching objective was to determine the origin component to be the “bulk rock.” With the discovery of of metal in ureilites, and assess its implications for the origin euhedral graphite + metal intergrowths in ALHA78019 of the extensive FeO-variation among ureilites. (Berkley and Jones, 1982), and of cohenite ([Fe,Ni]3C) in This work focuses on main group ureilites (single-lithol- metallic spherules within ureilite silicates (Goodrich and ogy samples previously referred to as monomict). Polymict Berkley, 1986), the carbon and associated metal in ureilites ureilites (regolith and fragmental breccias) contain a variety came to be regarded as indigenous. Based on this new view, of metallic phases that have not been observed in main group Janssens et al. (1987) and Goodrich et al. (1987) attempted ureilites, such as suessite ([Fe,Ni]3Si) and other silicides (Keil to attribute all siderophile element characteristics of urei- and Berkley, 1982; Herrin et al., 2007, 2008; Smith et al., lites to indigenous processes. Both of these studies pointed 2008, 2010; Ross et al., 2009, 2011; Goodrich et al., 2010). out that abundances of siderophile elements in ureilites However, these phases may be products of secondary mixing show a correlation with compatibility in S-rich metallic sys- and shock processes that have not affected main group sam- tems. Janssens et al. (1987) proposed that the high-Ir com- ples. By limiting this work to main group ureilites, we target ponent represented the grain boundary metal, and was a the primary petrogenesis of ureilites on their parent body. product of either 90% partial melting or 10% fractional crystallization. They also suggested that the low-Ir compo- 2. SAMPLES AND ANALYTICAL TECHNIQUES nent might be identified with the spherules. Goodrich et al. (1987) modeled a multi-stage cumulate scenario (now Twenty-four ureilites were studied in the form of pol- widely considered to be wrong), but reached a similar con- ished sections (Table 1). All samples were characterized clusion regarding the origin of the grain boundary metal. by scanning electron microscopy (SEM) using back-scat- Recent studies, such as Warren et al. (2006) and Ranken- tered electron imaging (BEI) and energy dispersive spectral burg et al. (2008), have also focused on removal of S-rich (EDS) analysis. Selected phases in all samples were ana- metallic liquid as the dominant control on ureilite sidero- lyzed by electron microprobe (EMPA) using wavelength phile element patterns. However, a very different view has dispersive spectral (WDS) analysis. Selected phases in four- been espoused by Gabriel and Pack (2008, 2009), who ar- teen samples were analyzed using laser-ablation inductively gued from Ni and Co data that ureilite metal is not in equi- coupled mass spectrometry (LA-ICP-MS). Analytical tech- librium with coexisting silicates, and cited Ni isotope niques are described in Electronic Annex EA-1. evidence (Quitte´ et al., 2010) that the metal and the silicates are derived from distinct nucleosynthetic reservoirs. From 3. PETROGRAPHY these arguments, Gabriel and Pack (2008, 2009) concluded that the grain boundary metal was introduced by an impac- 3.1. General tor, thus reviving the view that the dominant metal in ureilites is not indigenous. If this is correct, then ureilite metal Table 1 summarizes petrographic characteristics (ureilite would provide no constraints on the petrogenesis of the type, olivine Fo, pyroxene type and Wo, shock features, ureilite silicate assemblage. alteration level) and occurrences of metallic phases for the 24 samples studied. Following the classification of Good- 1.2. Objectives of this study rich et al. (2004), thirteen of the samples are olivine-pigeonite ureilites of Fo 74.9–89.0, six are olivine-orthopyroxene The first goal of this work was to augment available pet- ureilites of Fo 85.1–92.3, and five are augite-bearing urei- rographic and compositional data for metallic phases (metal, lites of Fo 75.9–95.1. Special properties, silicate composi- sulfides, phosphides and carbides) in ureilites. Thus, we tions (for samples that have not previously been described made detailed petrographic observations, and obtained main published papers), shock features, and weathering fea- jor and minor element compositions for metallic phases in 24 tures are described in more detail in EA-2. ureilites. For each sample, we examined the entire area of at least one section, and analyzed every metallic grain that was 3.2. Metallic phases sufficiently large. In addition, we obtained the first in situ data for trace siderophile elements in individual metallic 3.2.1. Grain boundary metal and silicate phases (with the exception of a few measure- The most abundant type of metal (or its inferred oxi- ments of Ni and Co in olivine reported in an abstract by dized equivalent) in all samples occurs along silicate grain Table 1 Petrographic features and occurrences of metallic phases in 24 ureilites. Sample Section Fo Pig Opx Degree of Shock Fe-C-S-P spherules Cr-rich lamellar Inter.-Cr Low-Cr troilite-metal globs g.b. Ref. Wo Wo alteration level (host) sulfides sulfide troilite in shock-melt phosphide Olivine-Pigeonite LAP 03587 ,9 74.6 10 mild low yes yes 343 1,2 340–373 (2013) 112 Acta Cosmochimica et Geochimica / al. et Goodrich C.A. NWA 3109 NAU 76.3 12.8 severe low yes yes yes 1 CMS 04048 ,5 76.4 6.9 mild low yes 1 ALHA78019 ,14 77.4 9.7 moderate v. low 1 (ol) yes yes 3,4 Kenna MPI 78.0 9.6 severe medium 5,6 ALHA81101 ,10 78.5 8.1 mild v. high yes yes 7 ALHA78262 ,14 78.7 8.4 mod/sev low 5 (pig), 6 (ol) 3,4 GRA 95205 ,11 79.1 7.6 moderate medium yes yes 6 PCA 82506 ,24 79.5 6.0 moderate low >35 (pig) 3,4 EET 96042 ,10 81.4 9.1 moderate low 4 (ol) yes yes yes 1 ALHA77257 ,104 85.6 6.8 moderate low >47 (pig) 3,4 EET 83225 ,12 88.7 11.2 mod/sev medium yes 6 EET 90019 ,13 89.0 9.0 severe low 1,5 Olivine-Orthopyroxene LAP 02382 ,5 78.3 4.5 mild ? 2 (ol) yes yes yes 1 EET 96328 ,5 85.1 4.7 severe low 1 (ol), 1 (opx) yes 1,5 LEW 85440 ,18 91.0 4.9 severe medium yes yes 5,8 Y-74659 ,81 90.9 7.2 4.5 mod/sev medium 1 (opx) yes yes yes 1,9 Y-791538 ,111 91.3 4.9 Severe Medium —————Yes*————— 5,8 EET 87517 ,26 92.3 4.7 severe low yes yes 6 Augite-Bearing META78008 ,49 ,48 75.9 4.4 mild medium 1 (ol) yes yes 5,10 EET 96293 ,8 86.9 5.0 severe medium >35 (opx) 5,11 Hughes 009 B1 87.3 4.9 severe medium yes (mi) yes 12 FRO 90054 ,10R 87.6 4.8 mod/sev medium yes (mi) yes yes 12,13 ALHA82130 ,9 95.1 4.5 severe medium 2 (ol), 8 (opx) 3,4,10 Fo = 100 molar Mg/(Mg + Fe); Wo = 100 molar Ca/(Ca + Mg + Fe); pig = pigeonite; opx = orthopyroxene; v = very; g.b. = grain boundary; Ref. = Reference. Sev. = Severe; mi. = melt inclusions. References: (1) This work. (2) Warren and Rubin (2010). (3) Goodrich et al. (1987). (4) Goodrich and Righter (2000). (5) Goodrich et al. (2006). (6) Singletary and Grove (2003). (7) Berkley (1985). (8) Takeda (1989).(9)Takeda (1987). (10) Takeda et al. (1989). (11) Antarctic Meteorite Newsletter 21, #2 (1998).(12)Goodrich et al. (2001).(13)Goodrich et al. (2009). Ureilite classification scheme of Goodrich et al. (2004). * A few tiny grains were observed but their type could not be identified. 344 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

(a) (b)

(e) (f)

Fig. 1. Back-scattered electron images (BEI) of grain boundary metal (m), Fe-oxides (ox), sulfides (sulf) and phosphides (phos) in ureilites. Surrounding silicates are olivine (ol) or pyroxenes (pig = pigeonite; opx = orthopyroxene). (a,b) PCA 82506. (c) LAP 02382. (d) ALHA78019. (e) FRO 90054. (f) ALHA81101. boundaries, forming narrow (10–40 lm wide), anhedral In nine samples (Table 1) we observed rare patches of strips or linings (Fig. 1; EA-3). Continuous strips of metal phosphide intergrown with grain boundary metal (or altered up to 350 lm in length were observed (Fig. 1a), but most metal) in a fine-grained symplectic-like texture (Fig. 2; EA- are shorter and terminated by Fe-oxides, “empty” cracks, 4). Hughes 009 and EET 83225 contain more extensive areas sulfides or carbonaceous material (Fig. 1b–e; EA-3). All of this intergrowth than other samples (Fig. 2b; EA-4-a and grain boundary metal appears homogeneous in reflected f). In FRO 90054 (Fig. 1e, EA-4-j), grains of phosphide are light or BEI. common in all grain boundary metal (oxide), occurring as Carbonaceous material (graphite and/or other poly- discontinuous borders along the edges of or thin needles morphs of C) occurs along silicate grain boundaries along within the metal/oxide (see also Fioretti et al., 1996). The with metal in all the samples, but varies greatly in abun- few grains of phosphide that were large enough to analyze dance. There is no clear textural relationship between the were identified from EMPA as schreibersite. grain boundary metal and the graphite. Intergrowths of euhedral graphite crystals and metal such as those described 3.2.2. Grain boundary sulfides by Berkley and Jones (1982) were not observed. Graphite is The most abundant type of sulfide is massive troilite, particularly abundant in META78008, and commonly con- occurring along grain boundaries as narrow, curvilinear tains patches of metal (EA-3-g). Grain boundary metal in strips similar to grain boundary metal (Fig. 1c, 3a and b; ALHA81101 occurs mainly as rounded masses in triple- EA-5-a and b). In some cases, straight boundaries between junction areas inferred to represent the boundaries of pri- troilite and grain boundary Fe-oxides suggest that the troi- mary (before shock and recrystallization) silicate grains lite has been preferentially preserved relative to metal (Fig. 1f). (Fig. 3b). In others, the sulfide appears to have been C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 345

(a) (b)

Fig. 2. Symplectic-like intergrowths of phosphide and metal (or weathered metal) along grain boundaries in ureilites. (a) EET 96328. (b) Hughes 009. Labels as in Fig. 1. BEI.

(a) (b)

Fig. 3. Sulfides in ureilites. (a and b) Massive low-Cr (0.2–1.0 wt.%) grain boundary troilite. (c and d) Lamellar intergrowths of daubreelite (ideally FeCr2S4) and low-Cr troilite. (a, c and d) EET 96042. (b) NWA 3109. tr = troilite; daubr = daubreelite; other labels as in Fig. 1. (a– c) = BEI. (d) = combined BEI and chromium Ka X-ray map (online version; BEI only in printed version). partially altered as well (Fig. 3a). In rare occurrences, the weathered, and sulfides have been previously reported in troilite contains tiny, rounded blebs of metal suggestive of several of them (Berkley et al., 1976, 1980). unmixing (EA-5-b). Cr-rich lamellar intergrowths of sulfides were observed, 3.2.3. Modal abundances of grain boundary metal and sulfide principally along grain boundaries, in 10 of the samples There are many uncertainties in estimating original (several of which also contain massive troilite). They com- modal abundances of grain boundary metal and sulfides. monly occur as small patches having rounded boundaries These uncertainties are discussed in EA-6. Our best esti- with metal (Fig. 3c; EA-5-d–f). In a few cases they occur mates indicate metal abundances of 1–3 vol.%, similar in significantly larger areas, particularly in EET 96042 to the 3–6 wt.% determined by wet chemistry in 3 ureilite (Fig. 3c and d; EA-5-g). Internally, they consist of thin falls (Vdovykin, 1970). Original metal/sulfide ratios were lamellae of very Cr-rich sulfide, intergrown in various pro- probably 1. portions with Cr-poor sulfide. The Cr-rich sulfide appears to be preferentially weathered (EA-5-g–j). In NWA 3109, 3.2.4. Cohenite–metal–phosphide–sulfide spherules in these intergrowths are abundant as rims around chromite silicates grains (EA-5-j). No contacts between lamellar sulfides and Highly rounded metallic spherules, 5–150 lm (mostly massive troilite were observed. 10–50 lm) in diameter, occur as inclusions within the cores In 7 of the samples no grain boundary sulfides were ob- of silicate minerals. They are not typically associated with served (Table 1). However, all but one of these is severely other inclusions or features in their hosts. These spherules 346 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 are assemblages of cohenite, metal, phosphide and sulfide (Fig. 4c and e; EA-8-c, e, n). Such areas were referred to (Fig. 4; EA-8). They were first identified by Goodrich and as “the mottled phase” by Goodrich and Berkley (1986) Berkley (1986) in five ureilites, and in this work were found and inferred to be fine-grained mixtures of cohenite and in an additional eight (Table 1). Their abundance in these metal. Phosphide (not recognized by Goodrich and samples varies from only 1–2 up to >50 spherules per Berkley, 1986) occurs in almost all spherules as small dis- section. In all cases they are volumetrically minor compared persed grains and/or discontinuous rims (Fig. 4; EA-8). to grain boundary metal. In samples that have high Some phosphide-rich areas appear to be fine-grained inter- abundances (>35) of spherules, they occur exclusively growths of phosphide and a second phase, probably Fe,Ni within low-Ca pyroxene (Table 1), commonly in very high metal (EA-8c and o). Sulfide typically occurs in 1–2 rela- concentration in only 1 or 2 crystals (EA-8-m). In Hughes tively large areas within each spherule, showing smooth, 009, FRO 90054 and EET 96328, they also occur within melt rounded boundaries with the areas of cohenite + metal. inclusions (Goodrich et al., 2001, 2009; Goodrich, 2001). Occasionally the sulfide contains tiny blebs of metal (EA- The spherules exhibit a variety of phase assemblages and 8-q). Some spherules consist only of the “mottled phase” textures (Fig. 4; EA-8). The majority are dominated by plus phosphide and sulfide. One unusual spherule (in oliv- intergrowths of cohenite and Fe,Ni metal in a range of ine in LAP 02382) contains a grain of chromite (Fig. 4e; grain sizes and apparent modal abundances. Cohenite crys- EA-8-e and f). Another unusual spherule (in olivine in tals are commonly subhedral to euhedral. Many of the lar- EET 96293) appears to be attached to a poikilitically en- ger areas of metal do not appear homogeneous in BEI closed grain of augite (EA-8-g,h).

(a) (b)

(e) (f)

Fig. 4. Cohenite–metal–phosphide–sulﬁde spherules enclosed in silicates in ureilites. (a and b) ALHA77257. (c,d) PCA 82506. (e) LAP 02382. (f) ALHA77257. Elemental X-ray maps in [b] and [d] correspond to areas shown in [a] and [b], respectively. Other images are BEI. chr = chromite; other labels as in Fig. 1. Spherule in [a and b] was analyzed by LA-ICP-MS for trace element composition. Spherule in [e] is the only one in which chromite was observed (see EA-8-f for combined elemental X-ray map of this spherule). In (f), sulﬁde (outlined) cannot be distinguished from surrounding silicates due to contrast settings. C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 347

3.2.5. Reduction rim metal (Figs. 5a, and 6a), with constant (within uncertainty) Ni/ Silicate darkening, particularly in proximity to grain Co ratio of 13.6 (0.64 CI). Individual samples show a boundaries, is a characteristic feature of ureilites (Wlotzka, large variety in both range and absolute abundance of Ni 1972; Goodrich, 1992; Mittlefehldt et al., 1998; Rubin, in grain boundary metal. LAP 03587 (98 analyses) spans 2006), and is present to various degrees in all samples stud- the entire range of observed values, from 0–7 wt.%, with ied (Figs. 1 and 3). It is caused by the presence of densely a large standard deviation (Table 2), while all other samples concentrated micron to sub-micron-sized (occasionally lar- show smaller ranges at various absolute levels (Figs. 5a and ger) grains of metal within areas of Mg-enriched (relative to 6a). ALHA78019 (48 analyses) shows the smallest range silicate core compositions) silicate composition. These areas and tight clustering around the average value are generally referred to as reduction rims (although they (5.6 ± 0.2 wt.%). There is no correlation of range or abso- commonly occur not only as rims but also as crosscutting lute abundance of Ni in grain boundary metal with ureilite veins or patches within silicates), and are interpreted as a type or composition (Fo) of core olivine (Fig. 7a). product of in situ reduction of silicates by carbon (in situ Si contents of grain boundary metal are near the detec- smelting), due to pressure release during a late impact or tion limit of 0.01–0.02 wt.% in the majority of the olivine- parent body disruption (Mittlefehldt et al., 1998). Reduc- pigeonite ureilites (Table 2, EA-9). However, in several tion rims are most common in olivine, with the Mg-rich sil- samples they are significantly higher (Fig. 5b). EET 96042 icate being forsteritic olivine and/or enstatite (Fig. 1d). has the highest values, with all 56 analyses (from 12 differ- Reduction rims have been reported on pyroxenes in a few ent metal areas) yielding 3.6 to 5.1 wt.% Si. The only two cases (Mittlefehldt et al., 1998). We observed them on analyses obtained from EET 90019 also fall in this group. pigeonite in ALHA78019, with the Mg-rich silicate being EET 83225 and LAP 02382 form distinct groups at 0.5– enstatite (Fig. 1d). 1 wt.% Si and 1.5 wt.% Si, respectively. NWA 3109 forms two distinct groups, one (3 metal areas) showing Si contents 3.2.6. Troilite–metal globules in shock-smelted pyroxenes of 2.5 wt.% Si and the other (2 metal areas) showing Si In five samples (Table 1) we observed rounded globules contents near detection limit; there are no obvious petro- of troilite + metal in areas of shock-(s)melted pyroxene graphic differences between metals in the two groups. Si (EA-7). These globules are 10–20 lm in diameter, and contents of grain boundary metal in all the olivine-orthopy- show a variety of internal textures, all featuring rounded roxene and augite-bearing ureilites also form distinct boundaries between troilite and metal that suggest unmix- groups, at values ranging up to 2.5 wt.% (Fig. 6b). In ing and/or liquid immiscibility (EA-7-c,d). Hughes 009, metal intergrown with phosphide (Fig. 2b) shows 0.75% Si while other metal has Si near detection 3.2.7. Shock-redistributed metal and sulfide limit. Similarly variable Si contents were reported previ- Olivines and pyroxenes in all samples also contain trails ously for 5 ureilites (Gabriel and Pack, 2008). In or patches of sub-micron-sized metal and sulfide grains that META78008, metal closely associated with graphite shows are not associated with Mg-enrichment in the silicate higher Si and Ni contents than other metal (Fig. 6b). Other- (Fig. 1a). Such occurrences have been described previously wise, there is no correlation between Si and Ni contents of (Singletary and Grove, 2003; Rubin, 2006), and were inter- grain boundary metal (Figs. 5b and 6b), or between Si con- preted by Rubin (2006) to be a result of shock melting and tent of metal and core Fo (Fig. 7b). mobilization of Fe–FeS. Cr contents of grain boundary metal range from <0.2 to 0.6 wt.% (Figs. 5c and 6c), and show no correlation with 4. COMPOSITIONS ureilite type, core Fo, or presence of Cr-rich sulfides. P contents are in the range 0.2–0.6 wt.% in the majority of the 4.1. Major and minor element compositions from EMPA samples (Figs. 5d, and 6d). They are lower (0.05 wt.%) in ALHA81101 and NWA 3109 (Fig. 5d), and significantly 4.1.1. Grain boundary metal higher (0.8–1.2 wt.%) in metal coexisting with coarse Table 2 gives average compositions and the range of phosphides (Fig. 1e) in FRO 90054 (Fig. 6d). Metal in measured Ni, Co, Si, Cr and P contents in grain boundary the fine-grained intergrowth with phosphide in Hughes metal in 20 samples. A table of complete analyses and other 009 (Fig. 2b) does not show these high values (Fig. 6d). statistics is given in EA-9. The total number of analyses ob- Based on point counting of metal and phosphide in this tained, and the number of distinct grains (or metal areas) intergrowth, its bulk composition has 9 wt.% P (Table 3). analyzed, varies greatly (EA-9), depending on degree of All Cr, P and Si contents obtained for grain boundary me- weathering. All analyses of grain boundary metal are tal in this study are within the ranges previously reported shown in Figs. 5 and 6 on plots of Ni vs. Co, Si, Cr and (Berkley et al., 1980; Berkley and Jones, 1982; Goodrich P (olivine-pigeonite ureilites in Fig. 5; olivine-orthopyrox- and Bird, 1985; Berkley, 1985; Fioretti et al., 1996; Good- ene and augite-bearing ureilites in Fig. 6). rich et al., 2001). Ni contents of grain boundary metal range from near Carbon contents of grain boundary metal in PCA 82506 detection limit to 7.3 wt.%, consistent with previous re- were determined to be 0.00 ± 0.15 wt.%, from the carbon ports (Neuvonen et al., 1972; Berkley et al., 1980; Berkley calibration curve method (Fig. EA-1-b). For other samples, and Jones, 1982; Berkley, 1985; Goodrich et al., 1987; C contents of grain boundary metal were only constrained Fioretti et al., 1996; Goodrich et al., 2001; Gabriel and to less than 0.8 wt.% (see EA-1), but are inferred to be Pack, 2008). Ni and Co are significantly correlated similar to those in PCA 82506. 348 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

Table 2 Average compositions of grain boundary metal in ureilites from EMPA (complete data in EA-9). Fe Ni Co Cr P Si Total LAP 03587 Avg (98) 93.9 4.07 0.27 0.10 0.37 0.02 98.7 Stdev 2.1 1.75 0.11 0.03 0.11 0.01 Max 99.3 6.91 0.45 0.18 0.99 0.03 Min 90.7 0.04 0.04 0.03 0.15 0.01 NWA 3109 Avg (8) 92.8 3.87 0.31 0.41 0.07 1.22 98.7 Stdev 1.5 2.16 0.17 0.24 0.02 1.27 Max 94.5 6.43 0.49 0.80 0.10 2.52 Min 90.2 1.00 0.09 0.09 0.04 0.02 CMS 04048 Avg (75) 94.8 3.37 0.29 0.13 0.52 0.05 99.2 Stdev 0.7 0.51 0.05 0.09 0.11 0.05 Max 96.5 3.97 0.37 0.29 0.91 0.19 Min 93.4 2.17 0.16 0.03 0.29 0.01 ALHA78019 Avg (48) 90.2 5.63 0.40 0.04 0.35 2.70 99.3 Stdev 0.6 0.16 0.05 0.02 0.04 0.08 Max 91.9 6.09 0.51 0.12 0.42 2.87 Min 89.0 5.41 0.32 0.03 0.25 2.55 Kenna (1) 92.6 4.54 0.35 0.18 0.42 0.41 98.5 ALHA81101 Avg (75) 97.5 1.13 0.10 0.08 0.11 0.04 98.9 Stdev 0.9 0.78 0.04 0.04 0.06 0.03 Max 99.3 2.97 0.20 0.21 0.36 0.21 Min 95.4 0.10 0.04 0.03 0.03 0.01 ALHA78262 Avg (16) 91.4 4.94 0.38 0.11 0.34 0.03 97.2 Stdev 0.8 0.54 0.05 0.04 0.12 0.02 Max 92.6 6.22 0.47 0.17 0.61 0.06 Min 90.0 3.76 0.27 0.06 0.22 0.01 GRA 95205 Avg (42) 94.6 3.60 0.28 0.09 0.40 0.07 99.0 Stdev 1.2 0.77 0.06 0.04 0.10 0.09 Max 97.4 5.23 0.44 0.21 0.59 0.48 Min 92.4 2.14 0.15 0.02 0.17 0.01 PCA 82506 Avg (36) 94.8 3.46 0.30 0.25 0.41 0.10 98.9 Stdev 0.9 0.66 0.05 0.08 0.06 0.10 Max 96.2 4.40 0.40 0.35 0.50 0.29 Min 93.2 2.40 0.22 0.16 0.34 0.02 EET 96042 Avg (56) 89.5 4.87 0.35 0.06 0.28 4.20 99.2 Stdev 0.6 0.32 0.03 0.02 0.08 0.44 Max 90.5 5.27 0.41 0.13 0.56 5.07 Min 88.1 3.72 0.25 0.03 0.19 3.61 ALHA77257 Avg (37) 94.4 4.14 0.33 0.37 0.31 0.05 99.6 Stdev 0.7 0.25 0.03 0.05 0.04 0.02 Max 95.7 4.76 0.40 0.49 0.37 0.11 Min 92.3 3.77 0.28 0.28 0.25 0.02 EET 83225 Avg (29) 94.6 1.64 0.11 0.26 0.28 0.50 97.4 Stdev 0.8 0.16 0.02 0.09 0.05 0.12 Max 95.6 1.89 0.14 0.58 0.37 1.08 Min 93.0 1.20 0.07 0.16 0.14 0.36 EET 90019* Avg (2) 89.6 4.57 0.34 0.04 0.35 1.57 100.0 Stdev 0.3 0.06 0.01 0.01 0.00 0.02 Max 89.8 4.61 0.35 0.16 0.38 4.90 Min 89.4 4.53 0.33 0.15 0.38 4.88 LAP 02382 Avg (90) 90.7 5.73 0.38 0.05 0.35 1.57 98.8 Stdev 0.7 0.65 0.03 0.02 0.08 0.05 Max 92.2 7.27 0.45 0.15 0.59 1.73 Min 88.8 4.09 0.28 0.03 0.20 1.47

(continued on next page) C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 349

Table 2 (continued) Fe Ni Co Cr P Si Total LEW 85440 Avg (5) 92.7 3.78 0.30 0.09 0.24 0.88 98.0 Stdev 0.4 0.09 0.02 0.02 0.01 0.03 Max 93.4 3.89 0.33 0.11 0.25 0.93 Min 92.1 3.67 0.27 0.06 0.23 0.85 Y-74659 Avg (12) 92.4 4.69 0.35 0.07 0.18 2.20 99.9 Stdev 0.5 0.29 0.03 0.02 0.02 0.25 Max 93.3 5.03 0.40 0.11 0.21 2.54 Min 91.7 3.97 0.30 0.05 0.16 1.76 META78008 Avg (40) 95.6 3.46 0.30 0.11 0.41 0.09 99.9 Stdev 1.2 0.90 0.08 0.07 0.09 0.08 Max 97.4 4.66 0.46 0.24 0.58 0.26 Min 93.4 2.09 0.18 0.03 0.24 0.01 META78008 (gph) Avg (48) 93.9 4.93 0.41 0.16 0.34 0.31 100.0 Stdev 0.8 0.57 0.07 0.09 0.05 0.05 Max 95.8 5.75 0.51 0.30 0.47 0.42 Min 92.1 3.83 0.22 0.03 0.23 0.17 Hughes 009 (1) 96.5 2.47 0.18 0.07 0.66 0.03 99.9 FRO 90054 Avg (11) 97.3 1.66 0.12 bdl 1.00 0.43 100.6 Stdev 0.3 0.08 0.02 0.11 0.05 Max 97.8 1.82 0.15 1.12 0.50 Min 96.8 1.53 0.09 0.80 0.37 ALHA82130 Avg (6) 93.7 4.30 0.32 0.16 0.27 1.06 99.8 Stdev 0.4 0.09 0.03 0.03 0.05 0.08 Max 94.4 4.43 0.36 0.20 0.37 1.13 Min 93.1 4.22 0.29 0.13 0.22 0.94 Notes: Does not include metal intergrown with phosphide (see Table 3). Complete analyses and additional statistics are given in EA-9. All values in wt.%. In all analyses, S contents were below detection limit (bdl); in some samples, Mn, V and Ti were analyzed and found to be below detection limit (see EA-9). * Metal grain in vein extending into olivine reduction rim from grain boundary. Metal enclosed in or closely associated with graphite.

Analyses of grain boundary Fe-oxides in 7 samples are (Table 4, EA-11). The majority of grain boundary troilite given in EA-10. Compositions are highly variable, both in LAP 02382 has Cr contents similar to FRO 90054, but within and among samples, with Fe = 43–68 wt.%, 2 out of 12 areas analyzed showed very low Cr content O=13–41 wt.%, and lesser amounts of Ni, Co, S, P, (Table 4, EA-11). In the metal-sulfide globules in shock- Cr, Si, Mg and Ca. All analyses showed low analytical to- melted pyroxenes (EA-7), the sulfide is low-Cr troilite. tals (85–95%). Most analyzed areas appeared to be of Coexisting metal shows variable Ni and Si contents mixed phases (Fig. 1d and e; EA-3), many were highly por- compared to grain boundary metal in the same sample ous, and they are likely to be hydrous. Cl was observed in (Table 2,4). All analyses of non-lamellar sulfides have stoi- EDS spectra in some areas but was not analyzed. These chiometry consistent with FeS–CrS solid solution (Fig. 8). analyses were obtained primarily to provide normalization values for LA-ICP-MS analyses, and no attempt was made 4.1.3. Lamellar, Cr-rich sulfides to improve them or identify the minerals present. Low S Almost all analyses of the lamellar sulfides are probably contents suggest that these oxides represent mainly altered mixtures, considering the narrow widths of the Cr-rich metal, rather than sulfides. lamellae. One exceptionally wide (3 lm) lamella in CMS 04048 (Fig. EA-5-e) yielded a composition with 32 wt.% 4.1.2. Non-lamellar sulfides Cr, 1.5 wt.% Mn, 0.4 wt.% V and 0.08 wt.% Ti (Table 4), Compositions of sulfides are summarized in Table 4; and stoichiometry consistent with that of daubreelite: complete analyses are given in EA-11 and EA-12. Grain Fe1.06Mn0.08V0.02Cr1.85S3.98 (Fig. 8). All other analyses boundary troilite in ALHA78019, EET 96042 and NWA showed 5–20 wt.% Cr (Table 4; EA-12), consistent with 3109 contains 0.2–1.2 wt.% Cr and <0.2 wt.% each Ni and being mixtures of daubreelite and troilite (Fig. 8). In addi- Mn. These compositions are consistent with those previ- tion, in one area of lamellar sulfides surrounding chromite ously reported for low-Cr troilite in a few ureilites (Berkley in NWA 3109 (EA-5-j), we found a sulfide with 51 wt.% et al., 1980; Berkley and Jones, 1982; Berkley, 1985; Good- Cr (Table 4) and stoichiometry close to that of brezinaite: rich et al., 1987). Grain boundary troilite in FRO 90054 has Fe0.16V0.03Cr2.82S3.98 (Fig. 8). Brezinaite has been reported higher Cr contents of 2.3–3.5 wt.%, as well as higher Mn along with other Cr-rich sulfides surrounding chromites in (0.5–0.9 wt.%), V (0.03 wt.%) and Ti (0.09 wt.%) contents the ureilite LEW 88774 (Prinz et al., 1994). 350 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

4.1.4. Cohenite–metal–phosphide–sulfide spherules bulk compositions of the spherules (neglecting sulfide), Compositions of phases in selected cohenite–metal– assuming that they all have similar bulk composition. The phosphide–sulfide spherules are given in Table 5. Complete modal abundances implied by these analyses (40–60% analyses are given in EA-13. Cohenite and coexisting metal cohenite, 60–40% metal, a few% phosphide) are within in 10 coarse-grained spherules (e.g., Fig. 4a,c and e) have Ni the range of those observed in the coarse-grained spherules, contents of 1.4–2.8 wt.% and 7–10 wt.%, respectively. and similar to those inferred by Goodrich and Berkley Metal has 0.2–0.3 wt.% C, as determined by the calibra- (1986). The largest uncertainty is in their C contents. The tion curve method (Fig. EA-1-b). No phosphides in any most reliable determinations, from the calibration curve of the spherules were large enough for analysis. Sulfides method, imply bulk C contents of 3–4 wt.%. analyzed in a few spherules are low-Ni troilite with 1.4– 1.7 wt.% Cr. These compositions are similar to those re- 4.2. Compositions from LA-ICP-MS ported by Goodrich and Berkley (1986), with the exception of C contents of the metal (see EA-1). We also obtained Abundances of 17 (mostly siderophile) elements mea- defocused beam analyses of a variety of finer-grained spher- sured by LA-ICP-MS in various phases in fourteen ureilites ules. Some of these (e.g., EA-8-g and k) consist of recogniz- are summarized in Table 6. Complete data are given in EA- able intergrowths of cohenite, metal and phosphide grains 14. Results are shown on CI-normalized abundance plots in too small to analyze individually. Others appear to consist Fig. 9 (metals, oxides and sulfides) and Fig. 10 (silicates). In only of the “mottled phase” (EA-8-i). These analyses all these tables and plots, the elements are arranged in order of showed 4–6 wt.% Ni and 0.5–1.6 wt.% P. These Ni con- decreasing condensation temperature (increasing volatility) tents would correspond to 50:50 to 60:40 mixes (by in a gas of solar composition (Lodders, 2003). weight) of cohenite:metal, based on the Ni contents of cohenite and metal in the coarse-grained spherules. 4.2.1. Grain boundary metal and Fe-oxides Apparent modal abundances in the coarse-grained For 10 samples that showed low to moderate degrees of spherules vary. However, the possibility of unrepresentative weathering, we obtained 6–14 analyses each of unaltered sectioning of such small objects is very high, and our statis- grain boundary metal (Fig. 9a–j). Abundances of Co (as tics (only 10 spherules) are poor. Therefore, the defocused well as Ni when it was not used for normalization) obtained beam analyses probably provide the best estimate of the from these analyses are consistent with those obtained by

LAP 03587 NWA 3109 CMS 04048 ALHA78019 Kenna ALHA81101 Grain Boundary Metal in Olivine-Pigeonite Ureilites LAP 02382 ALHA78262 GRA 95205 PCA 82506 EET 96042 ALHA77257 (a) 5 (b) 0.6 EET 83225 EET 90019 4

0.4 3 CI

2 Si (wt.%) Co (wt.%) 0.2 1

0.0 0

0.8 (c)1.0 (d)

0.6 0.8

0.6 0.4

P (wt.%) 0.4 Cr (wt.%) 0.2 0.2

0.0 0.0 012345678 012345678 Ni (wt.%) Ni (wt.%)

Fig. 5. Minor element contents (from EMPA) of grain boundary metal in 14 olivine-pigeonite ureilites. (a) Ni and Co are signiﬁcantly correlated among all samples, with Ni/Co = 0.64 CI. (b) Si contents in the majority of samples are near detection limit, but in some samples they are signiﬁcantly higher, with individual samples forming discrete clusters. (c and d) Cr and P contents are not correlated with Ni. Note that some samples show large ranges in Ni content, while others show little variation. C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 351

Grain Boundary Metal Olivine-Orthopyroxene in Olivine-Orthopyroxene and Augite-Bearing Ureilites LEW 85440 Y-74659 Augite-Bearing 5 META78008 0.6 (a) (b) META78008 (gph) 4 FRO 90054 Hughes 009 0.4 ALHA82130 3 CI 2 Si (wt.%) Co (wt.%) Co 0.2 1

0.0 0

0.8 (c) 1.0 (d)

0.6 0.8

0.6 0.4

P (wt.%) P 0.4 Cr (wt.%) Cr 0.2 0.2

0.0 0.0 012345678 012345678 Ni (wt.%) Ni (wt.%)

Fig. 6. Minor element contents (from EMPA) of grain boundary metal in 2 olivine-orthopyroxene and 4 augite-bearing ureilites. All elements show patterns indistinguishable from those of olivine-pigeonite ureilites (Fig. 5). Phosphorus contents of metal in FRO 90054, which also contains abundant phosphides, are notably high (up to 1 wt.%). Metal closely associated with graphite in META78008 shows higher Ni and Si contents than other grain boundary metal in this sample.

olivine-pigeonite olivine-orthopyroxene augite-bearing 5

6 4

4 3

2 2 1 (a) (b) 0 0 Ni (wt.%) in grain boundary metal 76 80 84 88 92 96 Si (wt.%) in grain boundary metal 76 80 84 88 92 96 Fo of olivine cores Fo of olivine cores

Fig. 7. Ni (a) and Si (b) contents (from EMPA) of grain boundary metal vs. Fo of olivine cores in 24 ureilites. There are no correlations of Ni or Si content with Fo or ureilite type.

EMPA of the same areas, within the analytical errors. In in Mo and Ni, and most show enrichments in Au, As, Ga each of the 10 samples, the majority of analyses showed and Ge. In addition, CI-normalized abundances of Zn in similar elemental abundance patterns, with absolute abun- all samples are 10–100 lower than those of other ele- dances typically varying by factors of 2–20. Average ments of similar volatility. Among the 10 samples, average compositions of grain boundary metal for all samples show CI-normalized abundances of Re and Os in grain boundary a general pattern of decreasing CI-normalized abundance metal range from 2 to 65, with ALHA81101 having the with increasing volatility (Figs. 9 and 11). However, relative lowest and GRA 95205 the highest values (Fig. 11). Abun- to a completely smooth volatility-correlated pattern, half dances of Re, Os, Ir and (to a lesser extent) Pt are signiﬁ- the samples show depletions in W, most show depletions cantly correlated. ALHA81101 shows the lowest 352 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

Table 3 Compositions of phosphide-metal intergrowths in ureilites from EMPA. Fe Ni Co Cr P Si S Total Hughes 009* Schreibersite Avg (2) 80.9 2.73 0.06 0.05 14.6 0.07 0.16 98.5 Coexisting Metal Avg (4) 96.5 2.13 0.10 0.04 0.33 0.74 bdl 99.9 Bulk 86.8 2.50 0.07 0.04 9.16 0.33 0.10 99.0 FRO 90054** Schreibersite Avg (4) 82.2 2.76 0.08 0.05 15.2 0.03 0.18 100.4 Coexisting metal Avg (11) 97.3 1.66 0.12 0.02 1.00 0.43 bdl 100.6 NWA 3109§ Schreibersite (1) 78.0 4.92 0.36 0.15 14.9 0.32 0.13 98.8 * Fine-grained intergrowth of phosphide and metal. See Fig. 2b and EA-4-b, d and f. Bulk estimated as 62% schreibersite, 38% metal. ** Coarse schreibersite along margins of grain boundary metal areas, e.g., Fig. 1e. § Spherule of phosphide in ﬁne-grained intergrowth with inferred metal (weathered), EA-4-h.

Table 4 Compositions of sulﬁdes in ureilites from EMPA. Fe Ni Co Cr P Si S Mn V Ti Total Non-lamellar grain boundary sulﬁdes ALHA78019 Avg (13) 62.3 <0.07 bdl 0.56 bdl bdl 36.6 0.07 bdl bdl 99.5 EET 96042 Avg (4) 61.6 0.25 bdl 0.72 bdl bdl 36.5 0.09 bdl bdl 99.2 NWA 3109 Avg (13) 62.0 0.14 bdl 0.49 bdl bdl 36.6 0.08 bdl bdl 99.3 LAP 02382 Avg (17) 58.6 0.13 bdl 2.88 bdl bdl 36.5 0.63 0.03 0.09 98.9 LAP 02382 (1) 61.7 0.14 bdl 0.74 bdl bdl 36.2 0.15 bdl bdl 98.9 FRO 90054 Avg (13) 59.6 <0.07 bdl 2.51 bdl bdl 36.8 0.57 0.03 0.07 99.6

Sulfide-metal blebs in shock-melted pyroxene areas EET 96042 sulfide Avg (5) 61.8 0.09 bdl 0.55 bdl bdl 36.5 0.09 bdl bdl 99.0 coexisting metal Avg (5) 94.8 2.90 0.20 0.07 0.29 0.02 bdl 98.3 ALHA81101 sulfide Avg (2) 62.2 0.10 bdl 0.31 bdl 0.11 36.4 0.02 bdl bdl 99.2 coexisting metal Avg (5) 92.9 4.74 0.22 0.08 0.29 0.04 bdl 98.2

Lamellar sulﬁdes – selected analyses CMS 04048 Daubreelite 19.9 0.09 bdl 32.0 bdl bdl 42.5 1.51 0.36 0.08 96.5 EET 96042 Mixed 54.2 0.05 bdl 6.5 bdl bdl 37.3 0.15 0.12 0.39 98.7 Mixed 39.1 0.04 bdl 19.6 bdl bdl 39.5 0.57 0.13 0.44 99.4 Mixed 50.8 0.04 bdl 8.6 bdl bdl 37.7 1.16 0.08 0.19 98.6 GRA 95205 Mixed 43.8 0.01 bdl 11.1 bdl bdl 38.0 0.44 0.08 0.02 93.5 LAP 03587 Mixed 45.0 0.08 bdl 12.9 bdl bdl 39.3 0.57 0.34 0.02 98.2 Mixed 55.6 0.06 bdl 5.0 bdl bdl 36.9 0.46 0.11 bdl 98.2 Mixed 51.0 0.11 bdl 9.1 bdl bdl 37.8 0.61 0.16 0.03 98.8 LEW 85440 Mixed 50.1 0.01 bdl 7.1 bdl 0.02 37.7 1.10 0.11 0.10 96.2 NWA 3109 Mixed 55.4 0.07 bdl 5.0 bdl 0.02 37.2 0.43 0.08 0.05 98.2 Mixed 46.9 0.03 bdl 10.3 bdl 0.07 38.4 0.11 0.14 0.08 96.1 Brezinaite 3.1 0.00 bdl 51.0 bdl 0.04 44.3 0.12 0.59 0.07 99.2 All values in wt.%. abundances of all analyzed elements (except Zn), at showed elemental abundance patterns indistinguishable 1–2 CI. Among the 10 samples, degree of fractionation, (within variation) from those of grain boundary metal as indicated by Os/Pt, Os/Ni or Os/Pd ratio, increases with not associated with graphite, and an average composition abundance of Os (Fig. 11). There are no correlations of any with abundances of all measured elements (except Zn) elemental abundances or ratios with olivine core Fo 1.3–2 higher (Fig. 9f). This is consistent with the (Fig. 11) or ureilite type. factor of 1.4 diﬀerence in Ni and Co contents between For META78008, we also obtained 15 analyses of these two populations of metal obtained from EMPA metal closely associated with graphite. These analyses (Table 2). C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 353

S low-Cr grain boundary troilite intermediate-Cr non-lamellar sulfide lamellar sulfides 600°C estimated bulk lamellar sulfide

FeCr2S4

(Fe,Cr)S (Fe,Cr) + FeCr S 2 ss 2 Cr2S3

Cr3S4

(Fe,Cr)ssS (Cr,Fe)ssS

(Fe,Cr)ss + (Fe,Cr)ssS (Fe,Cr)ss + FeCr2S4 + (Cr,Fe)ssS

(Fe,Cr)ss + (Fe,Cr)ssS + FeCr2S4

(Cr,Fe)ss + (Cr,Fe)ssS Fe Cr

Fig. 8. Compositions of sulﬁdes analyzed in this study shown on Fe–Cr–S phase diagram (in weight fractions) at 600 °C (simpliﬁed from El

Goresy and Kullerud, 1969). Star indicates composition close to breziniate (Cr3S4) found in lamellar sulﬁdes around chromite in NWA 3109.

Table 5 Compositions of phases in cohenite–metal–phosphide–sulfide spherules from EMPA. Fe Ni Co Si C* Cr P S Total PCA 82506,24 Spherule 1(coarse) Metal 87.4 10.1 0.63 0.02 0.24 na na na 98.5 Cohenite 90.3 2.1 0.31 0.01 6.19 na na na 98.9 Spherule 2 (coarse) Metal 87.5 9.7 0.60 0.03 0.07 na na na 97.9 Spherule 3 (coarse) Metal 90.7 8.3 0.56 0.04 0.33 na na na 99.9 Metal 90.0 8.5 0.57 0.02 0.28 na na na 99.4 Cohenite 90.7 1.6 0.27 0.02 6.23 na na na 98.9 ALHA77257,104 Spherule 8§ (coarse) Cohenite 91.2 1.6 0.27 0.02 6.3 0.11 0.28 bdl 99.8 Metal (avg. of 3) 88.3 9.3 0.58 0.04 <0.8 0.07 0.17 bdl 99.4 Sulfide (avg. of 2) 60.6 0.12 0.02 0.02 na 1.67 0.01 37.0 99.4 Spherule 11 (coarse) Cohenite 90.8 1.7 0.23 0.02 6.4 0.41 0.01 bdl 99.6 Metal 88.7 8.6 0.55 0.04 <0.8 0.08 0.41 bdl 99.3 Spherule 21 (coarse) Cohenite 90.1 2.8 0.38 0.03 6.30 0.09 0.50 bdl 100.2 Metal (avg. of 2) 88.0 9.0 0.59 0.03 <0.8 0.08 0.12 0.06 98.7 Spherule 23 (coarse) Cohenite 92.1 1.2 0.21 0.02 6.3 0.32 0.00 bdl 100.2 Metal 90.2 6.4 0.41 0.02 <0.8 0.09 0.23 bdl 98.1 Spherule 29** (coarse) Cohenite 91.5 1.8 0.27 0.02 6.4 0.06 0.02 bdl 100.1 Metal 88.7 9.0 0.64 0.04 <0.8 0.06 0.3 bdl 99.5 Sulfide 61.3 0.1 0.00 0.02 na 0.85 0.01 36.7 99.0 Spherule 1 (“mottled”) “Mottled phase” 89.4 5.4 0.42 0.08 2.4 0.09 0.29 bdl 98.1 “Mottled phase” 88.8 5.2 0.52 0.07 2.5 0.12 0.24 bdl 97.5 “Mottled phase” 89.8 5.4 0.47 0.06 2.0 0.09 0.28 bdl 98.1 Spherule 3 (fine-grained) Cohenite + phosphide 88.7 3.4 0.30 0.04 6.5 0.13 1.41 0.27 100.9 Mixed 85.7 7.1 0.47 0.03 1.2 0.13 2.04 0.20 96.9 Spherule 31 (fine-grained) Sulfide 61.0 0.2 0.00 0.03 na 1.39 0.00 37.2 99.8 Mixed 88.2 4.2 0.36 0.02 1.3 0.08 1.60 0.09 96.0 * In PCA 82506, C was analyzed by the carbon calibration curve method (see EA-1.2.2). In ALHA77257, C was analyzed by the standard u-q-z method; in these analyses, C contents reported for mixed analyses and “mottled” phase have high uncertainties. § Shown in Fig. 4a. ** Shown in EA-8-a. Table 6 340–373 (2013) 112 Acta Cosmochimica et Geochimica / al. et Goodrich C.A. 354 Summary of trace element abundances in various phases in ureilites from LA-ICP-MS (complete data in EA-14). ReOsW IrRuMoPtRhNiCoPdAuAsCuZnGaGe LAP 03587 g.b. Metal – avg. 1.2 18 1.0 15 7.9 7.6 16 0.94 41227 2671 1.7 0.85 5.1 92 11 20 78 Std. dev. (1r) 0.8 15 0.7 13 4.6 4.5 12 0.45 13099 952 0.6 0.33 1.4 29 4 3 16 # of values* 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 11 Sulf.-metal bleb 0.011 0.014 0.21 <0.07 <0.27 0.56 <0.24 <0.04 812 300 <0.09 0.30 4.1 88 8.0 18 69 CMS 04048 g.b. Metal – avg. 0.75 11 1.9 12 12 9 16 1.6 35409 2978 1.4 0.88 7.2 76 11 30 206 Std. dev. (1r) 0.18 5 0.4 83270.43221 340 0.2 0.11 1.3 8 8 5 23 # of values* 12 12 12 12 12 12 12 12 12 12 12 12 12 12 2 12 12 Kenna Metal in graphite <1.2 1.0 45 1.5 <7.0 235 3.9 <2.32 3317 <1255 <3.7 4.9 55 710 <62 17 <75 Graphite 0.14 1.5 0.22 1.3 1.4 1.4 2.3 0.15 1130 111 0.11 1.6 0.73 5.1 <0.29 1.3 38 g.b. Oxides – avg. 0.25 7.3 1.2 5.6 7.8 6.8 6.0 0.86 14833 1864 0.22 0.36 1.1 5.2 2.5 14 115 Std. dev. (1r) 0.60 15 2.1 11 16 9.0 11 1.8 12544 2052 0.25 0.66 0.6 3.1 2.1 21 155 # of values* 9989999999 8976899 Olivine core 0 0.019 <0.02 0.003 <0.06 <0.18 0.023 <0.01 138 46 <0.02 <0.01 <0.15 3.7 58 0.34 <0.71 Pigeonite core 0 0.005 0.004 0 <0.51 <1.27 <0.06 <0.04 <128 <55 <0.17 <0.08 <0.89 <8 23 3.8 <3.4 Reduction rim# <0.03 <0.08 0.028 0.010 <0.17 0.39 <0.07 <0.03 1800 214 <0.07 <0.04 <0.43 6.5 <1.3 2.4 28 ALHA81101 g.b. Metal – avg. 0.07 0.96 0.08 1.1 1.1 1.2 2.3 0.23 16532 1319 0.80 0.43 2.5 77 20 16 19 Std. dev. (1r) 0.02 0.72 0.07 0.7 0.6 0.5 1.5 0.18 10204 567 0.50 0.23 28 14 2 2 # of values* 4989999999 9919299 GRA 95205 g.b. Metal – avg. 2.1 33 2.3 26 18 10 22 1.7 34537 2692 2.1 0.61 <137 95 11 19 192 Std. dev. (1r) 1.0 26 1.6 26 10 6 11 1.2 7777 926 1.2 59 10 7 70 # of values* 14 14 13 14 14 14 14 14 14 14 14 1 14 14 13 14 14 PCA 82506 g.b. Metal – avg. 0.60 9.6 2.5 9.9 11 10 13 1.3 35511 3237 1.4 0.77 7.9 76 26 22 137 Std. dev. (1r) 0.19 2.8 1.5 3.8 3 3 3 0.4 10052 639 0.2 0.16 1.7 23 24 3 17 # of values* 5555555555 4535555 Spherules – avg. 0.93 12 2.0 12 11 9.5 19 1.5 55512 4467 2.0 1.5 14 170 41 46 479 Std. dev. (1r) 0.37 7 0.3 8 7 1.8 12 1.0 3818 357 0.9 0.7 6 56 37 15 203 # of values* 3333333333 3333333 EET 96042 g.b. Metal – avg. 0.66 14 1.4 13 9.8 6.7 16 1.4 45075 3466 1.9 1.4 10 131 <21 52 191 Std. dev. (1r) 0.31 6 0.6 5 3.1 2.9 6 0.4 9012 707 0.5 0.3 3 28 4 29 # of values* 6666636666 6626666 Lamellar sulfides 0.028 0.48 n.m. 0.52 <1.0 5.9 7.2 0.22 15406 n.m. 1.0 0.30 n.m. 352 6.2 n.m. n.m. Lamellar sulfides <0.40 0 n.m. <0.38 <0.98 <4.7 <1.3 <0.24 943 n.m. <0.41 <0.22 n.m. 735 <5.1 n.m. n.m. Low-Cr troilite 0.036 0.39 n.m. 0.20 0.41 <2.7 <0.64 <0.08 9395 n.m. <0.18 <0.07 n.m. 478 5.4 n.m. n.m. Sulf.-metal bleb <0.17 0.21 n.m. <0.23 <0.65 <3.0 1.0 <0.25 5272 n.m. <0.48 0.15 n.m. 400 6.1 n.m. n.m. (continued on next page) Table 6 (continued) ReOsW IrRuMoPtRhNiCoPdAuAsCuZnGaGe

ALHA77257 g.b. Metal – avg. 0.60 7.6 1.5 7.4 12 7.5 12 1.3 42317 3425 2.9 1.3 6.6 59 4.0 32 176 Std. dev. (1r) 0.14 2.5 0.7 2.5 5 0.7 4 0.3 3494 523 1.6 0.1 2.3 14 7 46 # of values* 2546525665 6422146 Spherules – avg. 0.6 7.4 1.2 7.2 9.0 7.2 13 1.3 50080 4065 1.9 1.6 11 107 10 56 397 Std. dev. (1r) 0.1 1.8 0.4 2.3 2.2 2.1 4 0.4 11214 983 0.5 0.6 3.1 44 2 16 133 # of values* 9 101010101010101010101010108 1010 Olivine core <0.03 0 <0.04 <0.02 <0.16 <0.39 0.004 <0.04 <87 <54 <0.04 <0.07 <0.88 <3.4 26 <1.2 <3.1 Pigeonite core <0.04 0.007 <0.06 0.039 <0.20 <0.67 <0.06 <0.03 <76 <70 <0.14 <0.12 <1.1 <4.4 355 28 2.2 <3.1 340–373 (2013) 112 Acta Cosmochimica et Geochimica / al. et Goodrich C.A. Reduction rim# <3.8 5.1 <2.61 6.8 <9.7 <31 11 <1.9 32300 2149 <4.5 <2.2 <30 <118 <72 <53 <103 Reduction rim# <0.05 0.2 0.066 0.14 <0.34 <0.39 0.30 0.040 1132 72 <0.06 <0.07 <1.3 4.2 8.6 1.9 12 EET 90019 g.b. Metal 0.86 8.8 <2.26 6.3 7.9 8.0 14 1.4 45300 4137 2.6 1.4 <17 77 <33 42 215 g.b. Oxides – avg. 0.42 0.61 0.20 0.32 0.88 1.5 1.0 0.03 16956 1369 0.22 0.17 <4.5 34 4.1 <5 18 Std. dev. (1r) 0.34 0.72 0.16 0.51 0.62 1.0 0.6 0 2859 587 0.16 0.11 63 1.9 18 # of values* 9999888199 5599899 Olivine core <0.02 0.004 <0.02 0.001 <0.10 <0.21 <0.06 <0.02 55 <27 <0.05 <0.03 <0.42 <1.4 63 <0.50 <0.97 Pigeonite core <0.02 0 <0.04 0.001 <0.13 <0.24 <0.04 <0.01 <40 <28 <0.04 <0.03 <0.42 <1.3 46 1.8 <1.4 LAP 02382 g.b. Metal – avg. 1.8 26 1.8 24 27 11 38 3 51982 3610 2.1 2.2 24 209 <68 48 245 Std. dev. (1r) 0.7 18 0.9 14 13 4 15 2 6466 480 0.4 0.5 4 55 9 59 # of values* 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 12 Spherule 1.6 23 2.4 27 18 16 37 2.2 46155 4171 1.9 2.2 17 88 57 43 575 Inter.-Cr troilite** <0.25 2.2 0.036 0.86 3.4 10 2.3 <0.68 7527 531 <1.5 <0.57 <9.9 462 <84 <1.9 63 inter.-Cr troilite** <0.08 0 <0.40 <0.33 <1.6 4.3 <1.8 <0.36 1247 79 <0.79 <0.34 <6.4 455 <63 <1.5 <14 Low-Cr troilite <0.12 0.35 <0.42 <0.34 <1.5 3.4 <1.0 0.23 11143 713 <0.55 0.43 5.1 258 <37 4.9 <14 Y-74659 g.b. Metal – avg. 0.51 6.4 2.2 9.0 13 8.4 12 2.0 38392 3394 2.5 <6 134 64 393 Std. dev. (1r) 0.29 3.6 1.1 3.8 6 4.8 4 0.5 10538 716 1.2 46 26 126 # of values* 6888776888 68 8 88 META78008 g.b. Metal – avg. 1.0 14 1.9 9.6 9.0 8.5 13 0.9 33688 3179 1.1 0.6 3.8 50 14.6 23 196 Std. dev. (1r) 0.8 11 1.0 7.6 7.3 2.6 10 0.6 7799 831 0.6 0.3 2.1 44 24.4 15 141 # of values* 8888888888 876788 Metal in gph- avg. 1.3 18 2.8 16 13 12 20 1.6 47840 4150 1.8 1.5 10 85 6.6 33 344 Std. dev. (1r)0.460.675270.59605 909 0.4 0.4 3 31 2.5 12 112 # of values* 15 15 15 15 15 15 15 15 15 15 15 15 14 15 13 15 15 Olivine core 0 0 <0.02 <0.01 <0.05 <0.20 <0.03 0.00 63 35 0.007 <0.04 <0.24 <1.2 67 0.56 <0.86 Reduction rim# 0.9 18 2.6 12 13 25 12 1.3 50000 3104 0.55 <0.51 4.3 21 75 23 423 Hughes 009 Metal§ 0.050 <0.29 <0.37 0.19 <1.6 <4.3 <1.1 <0.40 21300 889 0.90 0.46 4.9 120 <9.2 85 84 g.b. Oxides – avg. 0.01 0.07 0.04 0.02 0.16 0.44 0.10 <0.06 5450 366 0.12 2.27 <0.62 20 1.0 2.7 4.2 Std. dev. (1r) 0.01 0.06 0.03 0.02 0.05 0.07 0.09 3371 198 0.04 4.75 19 0.1 2.8 2.7 # of values* 2688648888 8888488 356 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

In four samples that were severely weathered (Kenna, Hughes 009, ALHA82130 and EET 90019) we obtained <4.1 <3.5 8–10 analyses each of grain boundary iron oxides. Ni abundances obtained by EMPA (EA-10) were used for normalization, thus ensuring accurate determination of elemental 17 138 76 250 <0.51 <2.9 <0.39 <3.0 <1.8 <4.7 <2.879 <9.1 350 abundances to within the 10% uncertainty introduced by low EMPA totals. For Kenna we also obtained 22 analyses of grain boundary iron oxides that had not previously 9.3 57 222 18 2.8 0.8 9 4.4 <4.4 27 9.1 15 6.7 been analyzed by EMPA and were therefore all normalized to an Fe abundance of 90 wt.% (EA-14). These analyses all grain boundary; gph = graphite; showed elemental abundances and patterns similar to the 142 7.6 42 223 others, but because their normalization is less certain, they are not shown in Fig. 9 or included in the averages given in Table 6. In each of the four severely weathered samples, ele- <15 <64 <18 <58 <7.55 <1.1 <7.8 mental abundances in grain boundary iron oxides varied by factors of 20–400, much more than in grain boundary metal within the less weathered samples (Fig. 9k–n). For 1.1 <2.4 <37 0.68 Kenna and ALHA82130, the average grain boundary oxide shows a pattern of decreasing CI-normalized abundance with increasing volatility, similar to and within the abun- for details).

<2.1 dance range of the 10 average grain boundary metal patterns (Fig. 11), though with some diﬀerences in details. In

EA-14 particular, all analyses of grain boundary oxides in ALHA82130 show large enrichments in W relative to other 3195 1382 0.68 1.2 2.0 21 1967 1.7 3106 2.0 1.2 10 126 <1363411 <0.28 2.3 <0.14 <1.3 1.3 <9.1 elements of similar volatility (Fig. 9l). Average grain boundary oxides in Hughes 009 and EET 90019 show lower abundances and more erratic patterns. 41700 21560 50700 42400 301 43300 In each of these four samples we also obtained 1–2 analyses of small (610 lm) grains of metal. Two grains in one area of Kenna showed abundances of most elements 0.58 <1.9 <0.06 <111 <55 <0.07 similar to those in the average iron oxide, but with extreme enrichments in W, Mo and Au (Fig. 9k). Two grains in ALHA82130 show abundances and patterns similar to <4.2 11 1.0 0.011 those of the iron oxides, with the same enrichment in W (the possibility of overlap with surrounding oxides cannot be ruled out). One grain in EET 90019 (Fig. 9m) shows higher elemental abundances than the iron oxides, very 8.0 <6.1 similar to the average grain boundary metal in ALHA77257 (Fig. 9d) except possibly for a modest W enrichment. Two analyses of metal intergrown with schre- <4.5 <10 <3.8 <1.1 <3.9 <0.33 <1.6 <0.09 <0.09 <157 <117 <0.12 <0.14 <1.6 <7.3 <0.48 <1.7 <0.40 <0.12 <0.38 <1.1 ibersite in Hughes 009 (Fig. 2b) show nearly flat CI-normalized abundance patterns (Fig. 9n) at 1–2 CI (similar to average grain boundary metal in ALHA81101), 4.2 7.7 7.0 7.7 0.003 except for large enrichments in Au. Au enrichments are also seen in some of the iron oxide analyses. This sample had previously been Au-coated, and we cannot rule out <0.15 <0.02 <0.45 <0.94 <0.08 <0.10 <96 <57 <0.17 <0.04 <0.95 <6.5 <0.04 <0.04 <0.10 <0.74 <0.08 <0.11 <104 <57 <0.11 <0.05 <0.90 <7.7 <1.2 <0.04 <0.02 <0.46 <0.92 <0.09 <0.08<0.33 <115 <47 <0.11 <2.82 <0.65 <7.4 the possibility that residual Au-coating was included in these analyses. 0.65 2.8 1.8 1.9 2.2 1.5 3.4 0 0.018 0.004 0 5.5 2.3 5.2 6.7 0.15 0 0 4.2.2. Cohenite–metal–phosphide–sulfide spherules We obtained analyses of cohenite–metal–phosphide–sulfide spherules in ALHA77257, PCA 82506, LAP 02382 and ALHA82130. In ALHA77257 (10 analyses) and PCA 82506 <2.7 <0.50 <0.36 <0.10 000000198137 3 9 10 8 103 <0.02 7 1010101010107109 <0.38 <0.07 (4 analyses), the average spherule composition is indistinguishable from the average grain boundary metal (Fig. 9d ) 0.14 4.8 6.7 4.4 7.1 9.7 5.2 0.09 6251 395 0.21 1.5 0.1 7 0.3 10 213 # #

r and g). This is also true of the one spherule analyzed in * LAP 02382 (Fig. 9i). The one spherule analyzed in ALHA82130 has a composition similar to that of the average grain boundary iron oxides and within the range of the Rims or internal patches of highly reduced olivine plus variable amounts of included metal. Intermediate-Cr sulﬁde. Std. dev. (1 # of values Number of values includedTroilite-metal in blebs average in (upper shock-melted limits pyroxene are areas. included if comparable to or lower than measured values; see Metal intergrown with schreibersite.

# few analyzed grains of grain boundary metal; it shows the Reduction rim Spherule Opx core 0.004 0 Misc. metal g.b. Oxides – avg. 0.22 2.8 7.3 2.5 4.8 7.1 4.3 0.32 Augite core 0.005 0 ALHA82130 g.b. Metal Pyroxene core Reduction rim All values in ppm. Niopx values = in orthopyroxene. boldface areRims from or EMPA, used internal for patches normalization. of Zero highly values reduced mean olivine that plus no counts variable were amounts obtained. of n.m. included = metal. not measured; g.b. = Olivine core 0.15 0 Olivine core * ** § C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 357 same enrichment in W (relative to a smooth volatility- 4.2.3. Sulﬁdes correlated elemental abundance pattern) as all oxides and We obtained a few analyses of grain boundary sulﬁdes metal analyzed in this sample (Fig. 9l). in EET 96042 and LAP 02382, including Cr-rich lamellar

Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge (a) – (g)

100 (a) grain boundary metal average grain boundary metal Fe-C-P-S spherules 10 average spherule metal in graphite 1 average metal in graphite

CI-Normalized Abundance 0.1 GRA 95205 Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge

100 (b) 100 (e)

10 10

1 1 CI-Normalized Abundance 0.1 CI-Normalized Abundance 0.1 CMS 04048 Y-74659

100 (c) 100 (f)

10 10

1 1 Normalized Abundance CI-Normalized Abundance 0.1 CI- 0.1 ALHA81101 META78008

100 (d) 100 (g)

10 10

1 1 Normalized Abundance -Normalized Abundance CI 0.1 CI- 0.1 ALHA77257 PCA 82506

Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge

Fig. 9. CI-normalized abundances of 17 (mostly) siderophile elements in metallic phases in 14 ureilites, from LA-ICP-MS (a few of the Ni data are normalization values from EMPA, see Table 6 and EA-14). For grain boundary metal (a–n), Fe–S–C–P spherules (d, g and i), metal in graphite (f), graphite (k) and grain boundary oxides (k–n) all individual analyses are shown (symbols for different analyses not distinguished), as well as (where applicable) average compositions. Open down-pointing triangles indicate upper limits. For sulfides (h and i), individual analyses are shown in different symbols of same color (upper limits are not plotted). CI abundances from Palme and Beer (1993),as updated in Palme and Jones (2005). Elements are arranged in order of increasing volatility in a gas of solar composition (Lodders, 2003). 358 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

(h) – (n) Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge grain boundary (and misc.) metal 100 (h) average grain boundary metal grain boundary Fe-oxides average grain boundary Fe-oxide 10 Fe-C-P-S spherules graphite 1 Cr-rich sulfide

-Normalized Abundance low-Cr troilite

CI 0.1 troilite-metal blebs in shock-melted pyx EET 96042 Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge

100 (i) 100 (l)

10 10

1 1 CI-Normalized Abundance 0.1 CI-Normalized Abundance 0.1 LAP 02382 ALHA82130

100 (j) 100 (m)

10 10

1 1 Normalized Abundance - CI-Normalized Abundance 0.1 CI 0.1 LAP 03587 EET 90019

Hughes 009 100 (k) 100 (n)

10 10

1 1 CI-Normalized Abundance 0.1 CI-Normalized Abundance 0.1 Kenna

Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge

Fig 9. (continued) sulfides in EET 96042, intermediate-Cr non-lamellar sul- show similar, but more scattered, elemental abundances fides in LAP 02382, and massive low-Cr troilite in both at levels 20–2000 lower than in average grain bound- samples. In EET 96042, both types of sulfide show CI-nor- ary metal in the sample. Again, Zn is strongly depleted malized elemental abundances (Fig. 9h) that are roughly and Mo appears to be enriched, at levels similar to those constant or slightly increasing with volatility, at refractory in grain boundary metal. In both samples, CI-normalized element levels of 1 CI (20–70 lower than in aver- abundances of Cu are slightly higher than in average grain age grain boundary metal in this sample). Significant boundary metal. exceptions include Zn, which is strongly depleted to levels We also analyzed troilite-metal globules in shock-melted similar to those grain boundary metal, and Mo, which is pyroxenes in EET 96042 and LAP 03587 (one in each sam- enriched to levels similar to those in grain boundary metal. ple). In both samples, these globules showed abundances of Both intermediate-Cr and low-Cr sulfides in LAP 02382 refractory elements to those of the grain boundary sulfides, C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 359

average grain boundary metal average grain boundary Fe-oxides olivine cores pyroxene cores “reduction rims” (reduced olivine + included metal)

Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge 100 ALHA82130 (a) 100 EET 90019 (d)

10 10

1 1

0.1 0.1 -Normalized Abundance CI-Normalized Abundance CI 0.01 0.01

1E-3 1E-3 100 META78008 (b) 100 Hughes 009 (e)

10 10

1 1

0.1 0.1 CI-Normalized Abundance CI-Normalized Abundance 0.01 0.01

1E-3 1E-3 ALHA77257 100 Kenna 100 (c) (f)

10 10

1 1

0.1 0.1 Normalized Abundance -Normalized Abundance - 0.01 CI CI 0.01

1E-3 1E-3 Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge Re Os W Ir RuMo Pt Rh Ni Co Pd Au AsCu ZnGa Ge

Fig. 10. CI-normalized abundances of 17 (mostly) siderophile elements in silicate cores and olivine reduction rims in 6 ureilites, compared to compositions of average grain boundary metal or iron oxides in the same samples. Upper limits are not shown. For Hughes 009, blue square = augite and blue asterisk = orthopyroxene. For the average metal in Hughes 009, aberrant Au values have been eliminated (this sample was previously Au-coated). Elements are arranged in order of increasing volatility in a gas of solar composition (Lodders, 2003). and abundances of more volatile elements (Au through Ge 145 ppm and <27 to 46 ppm, respectively (Table 6, EA- in Fig. 9) indistinguishable from those of grain boundary 14). These are consistent with the few previous data for metal (Fig. 9h and j). Ni and Co (Neuvonen et al., 1972; Berkley et al., 1976; Goodrich et al., 1987; Gabriel and Pack, 2008), but do 4.2.4. Silicates not show the correlation with FeO observed by Gabriel Analyses of olivine and pyroxene cores in 6 ureilites and Pack (2008). Zn abundances in the olivine are similar showed siderophile element abundances (except Zn) in the to or higher than those in the metal. range of 100–6000 lower than in grain boundary metal Analyses of olivine “reduction rims” (i.e., reduced oliv- in the same samples (Fig. 10). In ALHA82130 and EET ine plus variable amounts of included metal, whether in 90019, CI-normalized abundances of W are elevated rims or interior patches) in 5 samples showed highly vari- relative to Os and Ir by factors of 10 (Fig. 10a and d). able compositions. In some cases, siderophile element abun- Ni and Co abundances in olivine cores range from <44 to dances are similar to those of olivine cores (Fig. 10c,e and 360 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

GRA 95205 (Fo 79) It is apparent from Fig. 12 that within each sample, CI- LAP 02382 (Fo 79) normalized abundance patterns in the average grain bound- Average Grain Boundary Metal LAP 03587 (Fo 75) ary metal (or for Kenna, iron oxides) are very similar to META 78008 (Fo 76) those of the bulk rock. This comparison is quantified in in Ten Ureilites CMS 04048 (Fo 77) EET 96042 (Fo 82) EA-15-a, which shows that for the 8 samples, Re/Ir, Os/ PCA 82506 (Fo 79) Ir, Os/W, Os/Pt, Os/Pd, Os/Au and Os/Ni ratios in the me- ALHA77257 (Fo 85) tal vs. those in the bulk rock all plot on a 1:1 line, within the Y-74659 (Fo 91) observed internal variations in metal analyses and pub- ALHA 81101 (Fo 79) lished bulk rock data. The only exceptions are Zn and Ga. Relative to other trace elements elements of similar vol- 10 atility, both Zn and Ga are depleted in the metal but enriched in the bulk rocks (Fig. 12). CI-normalized Co/Zn ratios, for example, range from 70–210 in the metal compared with 0.2–0.5 in the bulk rocks. CI-normalized Co/Ga ratios are all (outside of uncertainties for all samples except Kenna where the comparison is to iron oxides) 1.5– CI-normalized Abundance 4 higher in the metal than in the bulk rocks EA-15-b). 1 These discrepancies arise because Zn is principally hosted in olivine and pyroxene, and Ga is principally hosted in Re Os W Ir Ru Mo Pt RhNiCo Pd AuAsCu Zn GaGe pyroxenes (Fig. 10). Absolute abundances of siderophile elements in the Fig. 11. Comparison of CI-normalized abundances of 17 (mostly) average grain boundary metal are 25–40 higher than siderophile elements in average grain boundary metal in 10 in corresponding bulk rocks for 7 of the samples shown ureilites. Degree of fractionation (e.g., Os/Pt) increases with Os abundance. Elements are arranged in order of increasing volatility in Fig. 12, and 60 higher for ALHA81101. For Kenna, in a gas of solar composition (Lodders, 2003). absolute abundances of these elements in the average grain boundary Fe-oxides are 10 higher than in the bulk rock. Assuming that grain boundary metal is the principle host of siderophile elements, these factors imply metal f). In others, they are virtually identical to those of grain abundances of 2.3–4 wt.% for most samples and boundary metal (Fig. 10a–c). This variation might be ex- 1.7 wt.% for ALHA81101, and an iron oxide abundance plained if the metal produced by in situ reduction of olivine of 10% for Kenna. These values are consistent with esti- partly equilibrated with grain boundary metal. However, mates from point counting. this seems unlikely because of the low temperatures (<600–700 °C; Mittlefehldt et al., 1998) at which this would 5. DISCUSSION have occurred. More likely, some of the larger grains of metal in these areas are primary metal of the same derivation 5.1. Types and occurrences of metallic phases as grain boundary metal (despite not being located on grain boundaries), rather than a product of in situ reduction of Our results reinforce previous observations that metal in olivine. ureilites occurs mainly along silicate grain boundaries, and that pre-weathering metal abundances were low (mostly 4.2.5. Comparison to bulk rock compositions 1–3 vol.%). In many cases, the metal probably formed Bulk rock siderophile element data are available for 10 nearly continuous “linings” around the silicate grains, of the samples we analyzed. Most published datasets in- which suggests that it crystallized from an interstitial metal- clude only a selection of the elements we analyzed. lic liquid. Fig. 12 shows average CI-normalized compositions of grain The cohenite–metal–phosphide–sulfide spherules in urei- boundary metal or (in the case of Kenna) iron oxides in 8 lite silicates remain rare and volumetrically minor, but the samples, compared to bulk rock data for 12 of the most observation that they are concentrated in low-Ca pyroxene commonly analyzed elements. The bulk rock datasets in preference to olivine provides a new constraint on their shown for each sample are from multiple sources, and thus origin. Goodrich and Berkley (1986) interpreted these may not be completely consistent with one another. Given spherules to represent metallic liquids that were immiscible typical sample sizes for bulk rock analyses (by INAA or in the silicate liquid from which the olivine and pyroxene ICP-MS), there is a significant probability that the different crystallized. However, this interpretation was based on analyzed aliquots of the sample contained different the assumption that ureilites are cumulates (Berkley et al., amounts of metal (or the carrier phase for the element of 1980; Goodrich et al., 1987), whereas they are now thought interest). In some cases, this problem can be overcome by by many workers to be residues (Warren and Kallemeyn, normalizing to a common measured element, but in many 1992; Scott et al., 1993; Goodrich, 1999; Singletary and cases there are no common elements (Fig. 12). Despite these Grove, 2003; Goodrich et al., 2007). It is difficult to under- uncertainties, some robust comparisons can be made be- stand how rounded inclusions of metallic liquid could between siderophile elements in the grain boundary metal come trapped inside solid silicate crystals. Alternatively, and the bulk rock for these 10 ureilites. models which consider the olivine to be residual but the C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 361

Re Os W Ir Ru PtPd Ni Co Au Zn Ga Re Os W Ir Ru PtPd Ni Co Au Zn Ga

average grain boundary metal

100 (a) 100 (e) bulk rock [1] bulk rock [4] bulk rock [2] bulk rock [8] bulk rock [3] bulk rock [9] 10 10

1 1 Normalized Abundance Normalized Abundance - 0.1 0.1 CI CI-

GRA 95205 ALHA81101 0.01 0.01 (b) 100 100 (f)

10 10

1 1 Normalized Abundance 0.1 0.1 CI- CI-Normalized Abundance PCA 82506 ALHA82130 0.01 0.01 (c) 100 100 (g)

10 10

1 1 Normalized Abundance - 0.1 0.1 CI-Normalized Abundance CI

EET 96042 META78008 0.01 0.01

average Fe-oxides bulk rock [5] 100 (d) 100 (h) bulk rock [6] bulk rock [7]

10 10

1 1 Normalized Abundance Normalized Abundance - - 0.1 0.1 CI CI ALHA77257 Kenna 0.01 0.01 Re Os W Ir Ru PtPd Ni Co Au Zn Ga Re Os W Ir Ru PtPd Ni Co Au Zn Ga

Fig. 12. CI-normalized abundances of selected siderophile elements in average grain boundary metal or iron oxides in 8 ureilites, compared to published bulk rock data. For META78008, the average metal includes both grain boundary metal and metal associated with graphite. Sources of bulk rock data: [1] Rankenburg et al. (2008); [2] Warren and Kallemeyn (1992); [3] Lee et al. (2009); [4] Warren et al. (2006); [5] Wasson et al. (1976); [6] Boynton et al. (1976); [7] Janssens et al. (1987); [8] Spitz (1992); [9] Wang and Lipschutz (1995). Elements are arranged in order of increasing volatility in a gas of solar composition (Lodders, 2003). low-Ca pyroxene to have crystallized from a melt, such as immiscible Fe–S and Fe–C liquids. However, in the Fe–S–C smelting models (Singletary and Grove, 2003; Goodrich system the two-liquid miscibility gap does not close at high et al., 2007) or paracumulate/mixing models (Warren and temperatures (Wang et al., 1991), so it is unlikely that there Kallemeyn, 1989; Goodrich, 2001), might explain the pref- ever was a single Fe–S–C liquid. It is plausible, however, erential trapping of metallic liquids in low-Ca pyroxene. that immiscible Fe–S and Fe–C melts were able to lower Goodrich and Berkley (1986) interpreted the rounded their energy by together adopting a spherical geometry boundaries between the sulfides and the cohenite–metal within silicate melt, since the interfacial energy between S- assemblages in these spherules (Fig. 4a,c) as evidence that rich and C-rich metallic liquids is likely to be much less than an originally homogeneous Fe–S–C liquid unmixed to form the interfacial energy between either of these and silicate 362 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 melt. If this is the case, ratios of Fe–S to Fe–C assemblages previously unrecognized variation (Figs. 5 and 6). Ni con- in the spherules may not have been determined by phase tents of metal span a large range both within and among relations. samples, and are strongly correlated with Co over this Our observations on sulfides in ureilites clarify the previ- whole range. Both Ni and Co are sensitive to fO2 in the ous picture of wildly varying Cr contents (e.g., Berkley range (IW-1.5 to IW-3) inferred for ureilites from oliv- et al., 1980) by showing that all “high”-Cr sulfides are prob- ine-silica-metal equilibria (Nitsan, 1974), and thus might ably lamellar intergrowths of daubreelite (FeCr2S4) and be expected to correlate with primary differences in fO2 (re- low-Cr troilite. However, they reveal a new mystery. It is flected in Fo of olivine, if metal and silicates are cogenetic). clear that there were two distinct types of sulfides: (1) Cr- Thus, the lack of correlation between Ni (or Co) and Fo ar- poor (<1 wt.%) troilite (the nearly pure Fe endmember of gues against redox control on variations in metal composi- the monosulfide solid solution [Fe,Cr]ssS); and (2) a more tion, and suggests that the Ni–Co correlation is instead a Cr-rich monosulfide solid solution (mss) that in most cases mixing relationship (see Section 5.5.4). exsolved daubreelite. Fig. 8 shows all sulfide data obtained Metal/silicate partition coefficients for Cr and P are even in this study on the 600 °C section of the Fe–Cr–S system more sensitive to fO2 than Ni or Co (Schmitt et al., 1989), (El Goresy and Kullerud, 1969). All analyses of the lamellar but subsolidus unmixing of phosphides and Cr-rich sulfides sulfides are consistent with being mixtures of daubreelite may have obscured any primary variations of Cr and P con- and a mss having 5 wt.% Cr, and indicate a bulk compo- tent in metal. Si is not appreciably siderophile in the range sition mss with 9–10 wt.% Cr. Comparison to the 600° C of primary fO2 of ureilites, but becomes so in the lower fO2 phase boundaries shows that the exsolution must have oc- range relevant to in situ reduction. Thus, its apparently er- curred below 600 °C. The intermediate-Cr (2.5–3 wt.%) ratic behavior (Figs. 5b and 6b) might be due to local mss that occurs in FRO 90054 and LAP 02382 apparently reduction effects. This seems to be supported by the occur- did not contain sufficient Cr to exsolve daubreelite at that rence of silicides in brecciated ureilites (Keil and Berkley, temperature. Phase relations in the Fe–Cr–S system do 1982; Herrin et al., 2007, 2008; Smith et al., 2008, 2010; not, however, explain the coexistence of Cr-poor and Cr- Ross et al., 2009), which may have experienced high degrees rich mss in the same sample. Either could have crystallized of secondary reduction in carbon-bearing regolith. How- from a S-rich, Cr-bearing Fe–S liquid (in the (Fe,Cr)ssS+ ever, among the main group ureilites there is no apparent (Fe,Cr)ss phase field as shown in Fig. 8), but crystallization correlation between Si content of metal and either shock le- of two distinct monosulfide phases would require two dis- vel or degree of secondary reduction (criteria of Wittke tinct liquids. There is a liquid miscibility gap that might et al., 2007). Both of these effects can be very heteroge- produce two liquids of the appropriate composition (Oika- neous, but in most samples we observe no correlations even wa et al., 2000), but only at much higher temperatures on a local scale. The one exception is META78008, in (>1500 °C) than maximum ureilite equilibration tempera- which metal in close proximity to graphite shows higher tures (1300 °C). Ni and Si contents than other metal (Fig. 6b). This, how- The occurrence of brezinaite (Cr3S4) in sulfide assem- ever, cannot be explained by local reduction of silicates, be- blages surrounding chromite in NWA 3109 is consistent cause metal produced by such reduction does not contain with the interpretation that similar Cr-rich sulfides in other appreciable Ni. The origin of high-Si metals in these urei- chromite-bearing ureilites form during late, in situ reduc- lites remains mysterious. tion of chromites (Prinz et al., 1994; Warren and Kalle- meyn, 1994). However, our observations suggest that 5.3. Trace element distribution and comparisons to bulk daubreelite was present before the late reduction, and its compositions presence in non-chromite-bearing ureilites supports this. Brezinaite likely forms from daubreelite at temperatures be- The trace element data obtained for different types of low 500 °C(Bunch and Fuchs, 1969). metal, sulfides and silicates allow us to examine the distri- Our observations indicate that schreibersite in associa- bution of these elements between phases, and assess their tion with grain boundary metal is more common than pre- contributions to the bulk rocks. As discussed above, the ra- viously recognized, but still volumetrically minor. Hughes tios of most of the analyzed elements in grain boundary me- 009 and FRO 90054 appear to have had significantly higher tal are very similar to those in the corresponding bulk rocks P contents than any of the other samples. All occurrences of (Fig. 12; EA-15). Here we discuss the most obvious (Zn, phosphide are consistent with exsolution from low-P Fe,Ni Ga) and some more subtle (Ni, Co, W, Mo) exceptions to metal at subsolidus temperatures. For FRO 90054, the dis- that statement. tribution of Ni and P between schreibersite and metal Previous workers have noted that Zn is enriched in urei- (Doan and Goldstein, 1970; Romig and Goldstein, 1980) lites relative to other elements of similar volatility (Good- indicates an equilibration temperature of 750 °C. For rich, 1992; Warren et al., 2006). Goodrich (1992) Hughes 009, the distribution of P indicates a temperature speculated that this is because it is sited in olivine in prefer- of 550 °C, but Ni contents are not on equilibrium tielines. ence to metal or sulfides. Our data confirm this, and indicate that pyroxene may be a significant host as well 5.2. Minor element compositions of grain boundary metal (Fig. 10). The geochemical behavior of divalent Zn is similar to that of divalent Fe and Mn (e.g., Norman et al., This study significantly augments the available data for 2005), and Zn abundances of olivine for 5 samples mea- compositions of ureilite grain boundary metal, and shows sured in this study correlate well with their FeO contents. C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 363

Ga abundances measured in silicates here (Fig. 10) decrease 182Hf–182W systematics (Lee et al., 2009) provide no chro- in the order augite > low-Ca pyroxene > olivine, which is nological information about the UPB. It would also remove consistent with Ga crystal/melt partitioning behavior in one of the major arguments against redox models, namely forsterite and diopside (Malvin and Drake, 1987). Mass the lack of correlation of metal or siderophile elements with balance indicates that 40–60% of bulk Ga is in the pyrox- Fo. ene in the samples studied. Thus, deductions about ureilite We have carried out an analysis of Fe–Ni and Fe–Co ex- metal based on bulk rock Ga data (e.g., Spitz and Boynton, change equilibria using the olivine and metal data obtained 1991, 1992) may need to be re-examined. in this study and the partitioning model of Seifert et al. Co/Ni ratios for grain boundary metal (0.07–0.09) are (1988). Similar to Gabriel and Pack (2008), we find that lower than those of corresponding bulk rocks (0.08–0.11). the olivine does not appear to be in equilibrium with the Boynton et al. (1976) made a similar observation in com- metal. Even at the maximum temperatures experienced by paring Havero¨ “vein metal” separates to bulk rock, and ureilites (1200–1300 °C), measured abundances of Ni, concluded that a significant fraction of the Co must be in and to a lesser extent Co, in the olivine imply corresponding the silicates. Our data show Co/Ni ratios of 1.8–5 in oliv- values in the metal that are 2–10 times higher than those ine and pyroxene, which support this conclusion, and is observed. However, we do not necessarily conclude from consistent with Co and Ni metal/silicate partitioning behav- this that the metal is exotic. ior (Jones and Drake, 1986; Schmitt et al., 1989). First, the datasets for Ni and Co in olivine are small Tungsten is important in interpreting several aspects of (both ours and Gabriel and Pack’s), and our results show ureilite petrogenesis. 182Hf–182W chronometry of 8 bulk large internal variations (beyond uncertainties). One con- samples provides evidence for differentiation of the ureilite cern is that tiny metal inclusions are pervasive in ureilite parent body within 1–2 Ma of CAI (Lee et al., 2009). Bulk olivine, and may have been included in analyzed areas (de- rock W data (showing chondrite-normalized abundances spite best efforts to avoid them). Although it is generally as- similar to those of other refractory siderophile elements) sumed that this metal is a product of in situ reduction of the were used by Lee et al. (2009) and Spitz and Boynton olivine, and therefore unlikely to contain significant Ni, our (1992) to argue against significant solid silicate/liquid sili- analyses of olivine reduction rims and patches (Fig. 10) cate fractionation in the absence of metal. Our data confirm showed this was not always the case. Thus, it is possible that W is primarily sited in the metal, and support this that Ni and Co contents of the olivine were overestimated. interpretation. W/Pt ratios are important for determining Second, the olivine-metal partitioning model does not whether C was present during solid metal/liquid metal frac- account for the possible presence of non-metallic elements tionation (Hayden et al., 2011). Available bulk rock data do such as carbon in the metal, at the conditions of equilibra- not permit robust assessment of W/Pt because there are no tion. Carbon has repulsive interactions with Ni and Co in datasets that include both elements. Thus, our data for the the metal phase (Lupis, 1983), and thus would increase grain boundary metal fill a serious gap. However, marked the olivine/metal partition coefficients. It is difficult to eval- W enrichments in several of the highly weathered samples uate the magnitude of this effect, however, because the (particularly ALHA82130 and Kenna) should be treated influence of carbon on Co and Ni activities in the metal with suspicion, as they may be of terrestrial origin. seems to have been studied only at concentrations well be- Mo/Ge ratios are important for distinguishing between low carbon saturation (Lupis, 1983). solid/liquid and liquid/liquid fractionation in the Fe–S–C Finally, we suggest that even if ureilite grain boundary system (Hayden et al., 2011). Again, there are no bulk rock metal and silicates are not (currently) in equilibrium, they datasets that include both of these elements. Our data show may still be cogenetic. In the modeling below (Section 5.5.4) that Mo abundances in the sulfides are approximately equal we suggest one such scenario, in which the metal was orig- to those in the metal, which implies that 10–50% of bulk inally in equilibrium with the silicates, but the siderophile Mo is in sulfides rather than metal. Thus any bulk rock Mo element signature in the metal was subsequently altered values would have to be carefully interpreted. by processes that are not recorded in the silicates. Gabriel and Pack (2009) also cited Ni isotope data for 4 5.4. Is the metal in ureilites indigenous? ureilites (Quitte´ et al., 2010; presented in abstract form by Gabriel et al., 2008) as evidence for impact injection of A first-order question is whether the grain boundary me- ureilite metal. These data show differences in e60 60 58 60 58 tal in ureilites is indigenous and therefore provides informa- ([( Ni/ Ni)sample/( Ni/ Ni)standard 1] 10000) between tion on primary petrogenesis. Gabriel and Pack (2008, bulk samples (presumably dominated by grain boundary 2009) examined Fe–Ni and Fe–Co distribution between metal) and silicates, which are argued to require distinct olivine and metal in 5 ureilites, and concluded that these nucleosynthetic reservoirs. However, the uncertainties on phases were not in thermodynamic equilibrium. Based on these measurements are fairly high, and the most precisely this result, they suggested that the metal had been intro- measured sample shows no significant difference between duced by impact, thus reviving a once popular (Higuchi bulk and silicates. et al., 1976; Boynton et al., 1976; Wasson et al., 1976) but In sum, we do not feel that there is strong evidence at subsequently abandoned (Berkley and Jones, 1982; this time that the grain boundary metal in ureilites is exotic Janssens et al., 1987) view. The conclusion that the grain (however, if it is, the modeling presented below will help boundary metal in ureilites is not indigenous would have constrain its provenance). In addition, there is at least far-reaching implications. For example, it would imply that one observation which strongly suggests that it is indige- 364 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 neous – namely, that grain boundary metal and cohenite- Fractional Melt Extraction GRA 95205/CI bearing spherules enclosed in silicates in the same sample x GRA 95205/CV have the same trace element composition. It is difficult to FeS F=0.99 0.86 ALHA81101/CI see how this could be the case if the grain boundary metal ALHA81101/CV was injected by impact, unless ureilites are total impact 100 Fe-S System melts. F=0.92 0.8 7% initial metal 5.5. Modeling of siderophile trace elements in grain boundary 0.7 0.2 metal 10 F=0.23

Previous discussions of siderophile elements in ureilites have focused on three processes to explain bulk rock patterns: volatility fractionation, solid metal/liquid metal frac- 1 tionation and mixing. Our results show that it is the grain boundary metal that is primarily responsible for the bulk rock patterns, thus our modeling of the grain boundary me- abundance in metal relative to CI or CV (a) tal focuses on these 3 processes as well. We agree with pre- Os Re Ir Pt Ru W Ge Co Au Mo Pd Ni Cu vious workers (Wa¨nke et al., 1972; Higuchi et al., 1976; x Goodrich et al., 1987; Warren et al., 2006) that volatility- FeS Fe-S-C System F=0.99 controlled fractionation cannot be the only process respon- 0.86 7% initial metal sible for the observed patterns, but there may be an imprint 100 of volatility fractionation inherited from precursors. Thus, F=0.92 in this section we model the data normalized not only to 0.8 CI, but also to CV chondrites. The latter have been sug- 0.7 gested as ureilite precursors on the basis of oxygen isotopes 0.2 10 F=0.23 (Clayton and Mayeda, 1988, 1996), and are of interest here because they are among the most volatile-depleted chondrites, relative to CI. As noted for bulk rock patterns (Janssens et al.,1987; 1 Goodrich et al., 1987; Warren et al., 2006; Rankenburg abundance in metal relative to CV et al., 2008), chondrite-normalized siderophile element abundances in the grain boundary metal (except (b) ALHA81101, which is unfractionated on average) show Re Os Ir W Pt Ru Ge Au Mo Co Pd Ni Cu a signiﬁcant correlation with compatibility in Fe–S metallic systems, leading to the inference that fractionation of Fig. 13. Results of fractional melting calculations in the Fe–S S-rich metallic liquid was the dominant control on urei- system (a) and Fe–S–C system (b), compared to CI- and CV- lite siderophile element patterns. In particular, we can normalized siderophile element abundances in average grain think of no other process capable of producing the high boundary metal in the most-fractionated (GRA 95205) and least- fractionated (ALHA81101) ureilites. Calculated curves give abun- degrees of HSE fractionation (e.g., subchondritic Pt/Os dances of siderophile elements in residual solid metal (relative to ratios) seen in some samples. A critical question is initial abundance in Fe + FeS) after fractional melting (initial whether carbon was a primary component of ureilites, conditions = 7 wt.% Fe metal and chondritic siderophile element and was therefore present during melting of Fe–FeS. abundances in bulk rock, with a range of FeS abundances).

All previous quantitative modeling of this process has Calculated patterns are labeled on left with initial value of xFeS been conducted in the pure Fe–S system, although sev- (= wt. FeS/[Fe + FeS]); some are also labeled with corresponding eral studies (Goodrich et al., 1987; Warren et al., 2006; value of F (degree of melting). CV abundances from Wasson and Rankenburg et al., 2008) have discussed possible effects Kallemeyn (1988). Elements are arranged in order of decreasing of C. To first order, this is reasonable even if C was compatibility in solid metal (decreasing solid/liquid partition coefficient) in (a) the Fe–S system (Chabot and Jones, 2003)or present (at least below the Fe–C eutectic at 1153 °C), be- (b) the Fe–S–C system (Hayden et al., 2011) at the eutectic. cause the C content of the Fe–S–C eutectic liquid is so low (Vogel and Ritzau, 1931). Recently, however, Hay- den et al. (2011) have determined the first solid metal/li- cussed this higher temperature stage, but only examined quid metal partition coefficients for a large selection of solid metal/liquid metal fractionation within the pure siderophile elements in the ternary Fe–S–C system. Sev- Fe–C system (see below). Hayden et al. (2011) also ob- eral elements show significantly different behavior com- tained the first liquid/liquid (immiscible Fe–C and Fe–S pared to the pure Fe–S system, which may allow us to liquid) partition coefficients for siderophile elements in determine whether C was present. Furthermore, at tem- the Fe–S–C system, which will allow us to more realisti- peratures >1150 °C in the Fe–S–C system, solid metal cally model the high-temperature stage. Again, several is replaced by liquid metal and a large two-liquid (Fe– elements behave differently during liquid/liquid compared C and Fe–S) immiscibility field forms (Vogel and Ritzau, to solid/liquid fractionation, and thus could be useful in 1931; Wang et al., 1991). Rankenburg et al. (2008) dis- determining the temperature at which extraction of S-rich C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 365

batch melt extraction Fe-S system ureilite grain boundary metal/CV ureilite grain boundary metal/CV ureilite grain boundary metal/CI 100 ureilite grain boundary metal/CI 1.6 ° ° xFeS = 0.54, T = 1150-1275 C xFeS=0.54, T = 1150-1275 C 10% initial 1.4 80 metal 5% initial 1275 metal 1.2 F=0.70 F=0.28 F=0.84 60 1.0 F=0.98 1050 20% initial 1250 1250 metal 1200 40 0.8 1300 F=0.66 (1150C C) 1250 F=0.83 (1250 C) 1100 0.6 F=0.91 (1265 C) 20 F=0.93 1100

Pt/Os in metal relative to CI or CV or CI to relative metal in Pt/Os 0.4 F=0.96 1250 F=0.997 (1275 C) (a) (b) 1100 0

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 abundance of in Os metal relative to CI or CV 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 Pd/Os in metal relative to CI or CV Pt/Os in metal relative to CI or CV

GRA 95205 grain boundary metal/CV GRA 95205 grain boundary metal/CV PCA 82506 grain boundary metal/CV 1000 GRA 95205 grain boundary metal/CI 1000 ALHA81101 grain boundary metal/CV ALHA81101 grain boundary metal/CV 1275 ALHA81101 grain boundary metal/CI

xFeS=0.54 xFeS=0.54 100 1265 100 5 wt% initial metal 1275 30 wt% initial metal 5.9 wt.% initial FeS 35.2 wt.% initial FeS 1050 1265 10 10

1050

1 1 (c) (d) abundance in metal relative to CI or CV or CI to relative metal in abundance CV or CI to relative metal in abundance Os Re Ir Ru Pt W Ge Co Mo Ni Pd Au As Cu Os Re Ir Ru Pt W Ge Co Mo Ni Pd Au As Cu --

Fig. 14. Results of batch melting calculations in the Fe–S system. (a) Pd/Os and Pt/Os ratios in residual metal. Curves labeled in °C (1050– 1300) each show loci of results for a range of intial xFeS values at constant temperature. Each curve begins (on the right) at xFeS = 0.2 (corresponding to F = 0.24, 0.28, 0.31, and 0.45 at 1050 °C, 1200 °C, 1250 °C and 1300 °C, respectively) and ends at the maximum xFeS possible at that temperature (xFeS = 0.83, 0.71, 0.65 and 0.43, at 1050 °C, 1200 °C, 1250 °C and 1300 °C, with F = 0.98 to 0.99 in all cases). Dotted curve (green in online version) shows locus of results for fixed xFeS (= 0.54) at temperatures ranging from 1150 °C to 1275 °C. Plotted for comparison are average grain boundary metal in ureilites, normalized to CV and CI. (b) Pt/Os ratios and Os abundances in residual metal, corresponding to dotted (green) curve (xFeS = 0.54 at a range of temperatures) in (a). Note that ALHA81101 is not plotted. Results are shown for 5, 10 and 20 wt.% initial metal (corresponding to, respectively, 5.9, 11.7 and 23.5 wt.% initial FeS). (c) Calculated abundances of siderophile elements, corresponding to 5 wt.% initial metal curve in (b) for various temperatures (labeled in degrees C, 1050–1275), compared to the average grain boundary metal in the most-fractionated (GRA 95205) and least-fractionated (ALHA81101) ureilites, normalized to CI and CV. (d) Same as in (c) but for 30 wt.% initial metal, and also showing PCA 82506. In (c) and (d), elements are arranged in order of decreasing compatibility in solid metal (decreasing solid/liquid partition coefficient) in the Fe–S system (Chabot and Jones, 2003)at T P 1100 °C. liquid occurred if carbon was present. In the following melt extraction. We do not account for partitioning of sid- modeling, we test all 3 sets of partition coefficients. Par- erophile elements into either solid or liquid silicates at high tition coefficients for the Fe–S system are based on the temperatures. Based on metal/silicate partition coefficients parameterizations of Chabot and Jones (2003) and Cha- from Schmitt et al. (1991) and Jones and Drake (1986), this bot et al. (2009). omission has no significant effect on any of our results. We consider a starting material with bulk chondritic abundances of siderophile elements (either CI or CV). All 5.5.1. Fractional melting siderophiles are initially contained entirely in the Fe me- Pure fractional melting in the Fe–S system is a highly tal + Fe-sulfide (Fe + FeS) component. Both initial abun- constrained process in which a single parameter, the initial dance of Fe metal and initial abundance of FeS in the composition of the Fe + FeS system (xFeS), completely bulk material are treated as variables. We specify the latter determines the degree of melting, F (within the Fe + FeS by defining xFeS = wt. FeS/(Fe + FeS). Melting begins at system), and the degree of siderophile element fraction- the Fe–S eutectic (988 °C), slightly below the temperature ation. When melts are continuously removed, all melting at which silicate melting probably began (1050 °C) on the happens at the eutectic (86.6 wt.% FeS), and proceeds until UPB (Goodrich et al., 2007). Because the percolation either FeS or Fe metal is exhausted (i.e., up to the maxi- threshold for Fe–S liquids in solid silicate matrices is uncer- mum degree of melting, F = xFeS/0.866). For bulk compo- tain (Walker and Agee, 1988; Yoshino et al., 2004; Rush- sitions on the Fe side of the eutectic, only solid metal mer et al., 2005), we consider both fractional and batch remains. At this point, melting ceases and would not re- 366 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

batch melt extraction (solid/liquid) CI, CV,CM,CO CI Fe-S system 1.0 CR Fe-S-C system Allende CM GRA 95205 metal/CV CO 0.8 1100°C CV LL average 100 x =0.8 (F=0.98) OC FeS 5% initial metal 0.6 E L average

0.4 E H average 10 = wt. FeS/(FeS+Fe) FeS

x CR 0.2

to CH,CB 0.0 0 2 4 6 8 1012141618202224 (a)

abundance in metal relative to CI or CV 1 wt.% Fe (m) Os Re Ir Ru Pt W Ge Co Mo Au Pd Ni Cu

Fig. 15. Abundances of Fe metal (wt.%) and xFeS (= wt. FeS/ [Fe + FeS]) in major groups of chondrites. Data from Jarosewich Fe-C liquid/Fe-S liquid fractionation GRA 95205 metal/CV (1990) and Mason and Wiik (1962). in Fe-S-C System GRA 95205 metal/CI xFeS=0.70 (F=0.92) xFeS=0.745 (F=0.988) sume until the Fe–C eutectic (1153 °C) was reached (if C 1200°C was present). The degree of melting at the eutectic thus de- 100 7% initial metal pends only on the initial bulk composition of the Fe + FeS system. The partition coeﬃcients are ﬁxed, because they depend only on the S content of the liquid (Chabot and Jones, 2003) and all melting happens at the eutectic (31.6 wt.% S). Fig. 13 shows abundances of siderophile elements in 10 residual metal, calculated with the standard equations for non-modal fractional melting (Shaw, 1970; Albare`de, 1995), for an initial metal abundance of 7 wt.% and a range of initial xFeS (0.1–0.86). CI-normalized and CV-normalized abundances in average grain boundary metal in the (b)

abundance in metal relative to CI or CV 1 most-fractionated (GRA 95205) and the least-fractionated (ALHA81101) ureilites are shown for comparison. The Re Os W Ir Ru Pt Mo Ge Au Co Pd Ni Cu highest degrees of fractionation are achieved at the highest Fig. 16. Results of batch melting in the Fe–S–C system. (a) Solid degrees of melting, where the fraction of remaining metal is metal/liquid metal fractionation at 1100 °C, comparing Fe–S–C extremely small (for example, at F = 0.99, <0.5 wt.% metal and Fe–S systems. Calculated curves show siderophile element remains in the rock, compared to the initial 7%). Neverthe- abundances in residual metal, for xFeS = 0.8 and 5 wt.% initial less, even at such high degrees of melting, the highly com- metal (20 wt.% initial FeS), compared to average grain boundary patible elements (Os, Re, Ir, Pt, Ru, W and Ge), metal in GRA 95205 normalized to CV. (b) Liquid/liquid (Fe–C/ including the HSE, remain virtually unfractionated Fe–S) fractionation at 1200 °C. Calculated curves show siderophile element abundances in the Fe–C liquid, for 7 wt.% initial metal and (Fig. 13a). This is true in the Fe–S–C system as well, despite two different xFeS (0.7, F = 0.92, 16.3 wt.% initial FeS; 0.745, other differences between the two systems (Fig. 13b). Thus, F = 0.99, 20.5 wt.% initial FeS), compared to average grain fractional melting in either system cannot match the sider- boundary metal in GRA 95205 normalized to CI and CV. In (a) ophile element patterns of the most-fractionated ureilites, elements are arranged in order of decreasing solid/liquid partition or even many of the less-fractionated samples (cf. Fig. 11). coefficient in the Fe–S–C system at 1100 °C(Hayden et al., 2011). Rankenburg et al. (2008) also modeled fractional melt- In (b) elements are arranged in order of decreasing liquid/liquid ing in the Fe–S system in ureilites. However, they treated partition coefficient in the Fe–S–C system at 1200 °C(Hayden the S content of the liquid (which determines the partition et al., 2011). coefficients; Chabot and Jones, 2003) as a free parameter, which it is not, thus effectively applying the fractional melt- conditions of fractional melting, are very similar to ours ing equations over a range of temperatures above the eutec- in showing essentially no fractionation of Pt from Os even tic where they do not apply. Furthermore, they treated the at very high degrees of melt extraction. amount of melt extraction and the S content of the liquid as independent parameters, which they are not even in the less 5.5.2. Batch melt extraction – Solid/liquid fractionation constrained case of batch melting (see Section 5.5.2). Thus, Batch melting is not as constrained as fractional melting the majority of the parameter space they explored is non- because the extraction temperature must be specified. For a physical. We note, however, that their results for a S con- given temperature, degree of melting (F) is determined by tent of 30% in the liquid, which is close to the S content xFeS. The liquid composition used to calculate the partition of the eutectic (31.6 wt.%) and therefore approximates the coefficients for the Fe–S system is the liquidus at the extrac- C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 367 tion temperature. Fig. 14 shows results of batch melting cal- er initial FeS), implying combinations of xFe and xFeS that culations in the Fe–S system, for a range of conditions. To are unknown among chondrites (Fig. 15). approach the full spectrum of possibilities systematically, Finally, we note that even if we were to accept that very we first varied xFeS while holding temperature constant at high values of F and very high initial metal + sulfide con- each of 1050 °C, 1100 °C, 1200 °C, 1250 °C and 1300 °C. tents are plausible, then it is still not possible to generate Based on those results, we then varied temperature (from the entire range of ureilite metal compositions with a single 1100 °C to 1275 °C) while holding xFeS constant at 0.54. process. From Fig. 14a and b it would be tempting to argue Fig. 14a shows loci of Pd/Os and Pt/Os ratios in residual that all ureilites could be explained by varying a single metal from these calculations (note that the results of parameter – for example, varying xFeS from 0.2 to 0.7 at Rankenburg et al., 2008 shown on this same diagram in 1250 °C (very heterogeneous precursors) or varying temper- their Fig. 5 appear to be similar to ours, but in fact are ature from 1050 °C to 1275 °CatxFeS = 0.54 (homogeneous not comparable because their models were incorrectly cal- precursors with extraction at different temperatures, per- culated using fractional melting equations). Batch melting haps more plausible). However, Fig. 14d shows that even at high temperatures (1200–1300 °C) leads to significantly the HSE cannot be explained this way, because none of greater fractionation among the HSE compared to frac- the calculated results is capable of producing siderophile ele- tional melting, which makes it possible to match the HSE ment abundances in metal as low as those in ALHA81101. patterns (e.g., Pt/Os) of the highly fractionated ureilites. Fig. 16a shows results of batch melting in the Fe–S–C Nevertheless, very high degrees of melting are required to system compared to the Fe–S system, at 1100 °C and do so. For GRA 95205, F must be >0.98, and for 6 of F = 0.98. Although there are some differences, they are the remaining 9 samples, F must be >0.90 (Fig. 14a). It is small compared to the mismatches between either set of re- not clear whether such high values of F are plausible. The sults and the ureilite metal, and provide no indication of degree of HSE fractionation in the residue is extremely sen- which system is more appropriate. sitive to small differences in these high values (Fig. 14a), which translate into small differences in temperature for 5.5.3. Batch melt extraction – Liquid/liquid fractionation the case of constant xFeS (homogeneous starting composi- At temperatures of 1150 °C and higher in the Fe–S–C tion). It seems unlikely that at temperatures above system, solid metal (in equilibrium with S-rich liquid) is re- 1200 °C, where a significant amount of silicate melting placed by C-rich liquid metal, which is immiscible with re- should have occurred, extraction of metallic melt would spect to the S-rich liquid (Vogel and Ritzau, 1931; Wang be retarded. et al., 1991). This means that if extraction of S-rich metallic Another problem is that, except at 1300 °C (which is too liquid occurred at temperatures >1150 °C (as suggested by high for most ureilites, including GRA 95205), all solutions the results in the previous section), and graphite was pres- that match the Pt/Os ratios significantly underestimate the ent, the residual phase would have been C-rich metallic li- Pd/Os ratios of the ureilites (Fig. 14a). In fact, these solu- quid (which subsequently crystallized to become the grain tions underestimate all of the less compatible elements boundary metal), rather than solid metal. Thus, we can test (Co, Mo, Ni, Au, As and Cu) relative to the compatible whether C was present by modeling liquid/liquid fraction- ones, and do not match their pattern (Fig. 14d). ation using partition coefficients from Hayden et al. We can also now model absolute abundances of sidero- (2011). Rankenburg et al. (2008) examined the possible phile elements in the metal, and thus place constraints on importance of C in this high-temperature stage of ureilite the metal and sulfide contents of the starting materials. melting by modeling solid metal/liquid metal fractionation Absolute abundances of siderophile elements in the residual in the Fe–C system (Chabot et al., 2006). However, this metal depend on the abundance of metal in the starting modeling would only be relevant if all the S had been re- materials (the higher the initial abundance of metal, the moved from the system at lower temperatures. The results lower the initial abundances of siderophile elements in the in the previous section show that this could not have been metal) and the degree of melting, F (which determines the the case. Furthermore, if all the S had been removed, the degree to which the abundance of metal decreases, or the bulk composition in the Fe–C system would have been on degree to which siderophile elements are concentrated). the C side of the eutectic (since ureilites contain excess At the very high values of F required to match the Pt/Os ra- graphite), and the relevant fractionation would have been tios of the most-fractionated ureilites, very small fractions between graphite (or cohenite) and liquid, rather than Fe of the initial Fe metal remain (typically <10%, relative) so metal and liquid. the siderophile element concentration factor is very high. Fig. 16b shows calculated siderophile element abun- This means that very high initial metal contents are re- dances in the Fe–C liquid, for liquid/liquid fractionation quired to yield HSE abundances as low as those observed at 1200 °C. These results are similar to those for the pure in the ureilite (presumably residual) metal. For example, Fe–S system (solid/liquid fractionation) in terms of the to match GRA 95205 would require 20–30 wt.% initial main points discussed above. However, there are two key metal (Fig. 14b and d). Since xFeS must be high to yield elemental ratios that differ between the two systems. (1) the high F values, very high initial FeS contents are also re- In liquid/liquid fractionation, the ratios of partition coeffi- quired. For xFeS = 0.54, these values would be 23– cients for W relative to HSE such as Os, Re and Ir are on 32 wt.%. Among known chondrites, such combinations of the order of 1, whereas in solid/liquid fractionation they Fem and FeS content are similar only to EH chondrites are much less than 1 (Hayden et al., 2011); and (2) in li- (Fig. 15). All other solutions require higher xFeS (thus high- quid/liquid fractionation, the ratio of the partition coeffi- 368 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

GRA 95205 metal/CV 5.5.4. Mixing mix of 20:80 model (1275°C) metal:liquid Most studies of siderophile elements in bulk ureilites mix of 20:80 model metal:model ALHA81101 have inferred that some form of mixing is required to explain their patterns (Boynton et al., 1976; Higuchi et al., residual metal 100 batch melting 1976; Wasson et al., 1976; Janssens et al., 1987; Warren 1275°C x = 0.54 FeS et al., 2006; Rankenburg et al., 2008), and our data show 7 wt.% initial metal that elemental correlations suggestive of mixing are present in the metal itself (Figs. 5a, 6a and 11). If we accept that high degrees of batch melt extraction are required to yield

10 residual metal with the high degrees of HSE fractionation 1275 seen in some ureilites, then various degrees of admixture of a second component might explain the apparent over- complementary S-rich liquids abundance of the less compatible elements (i.e., Pd, Au) 1250 1050 in these samples, and provide the full range of observed sid- 1265 erophile element abundances and ratios among all samples 1 model ALHA81101 (Fig. 11). Mixing might also explain the apparent disequi- abundance in metal relative to CI or CV (a) librium in Ni and Co distribution between olivine and metal Os Re Ir Ru Pt W Ge Co Mo Ni Pd Au As (via admixture of a component that lowers Ni and Co contents of the metal) without invoking an exotic origin for the metal. Ureilite Metal We consider a mixing model in which one endmember is a residue from very high degrees of batch melting of Fe + FeS. Possible candidates for the other endmember in- 10 clude pure Fe metal produced by smelting, retained S-rich liquid (inefficient melt extraction), a total Fe + FeS melt, and non-indigenous (impact-injected) chondritic metal. For the present, we consider only the Fe–S system. We note that since we are trying to match abundances of siderophile elements in the metal itself (rather than just the bulk rocks), a mixing model would require that the admixed component totally equilibrated with the solid residual metal. CV-normalized abundance If we assume that the starting materials had Fe metal (b) and FeS abundances in the range of most carbonaceous 1 chondrites (67 wt.% Fe metal and xFeS ffi0.5–1; Fig. 15), Co Mo Ni Pd Au As then abundances of the highly compatible elements in the Order of decreasing D (solid/liquid) in Fe-S at 1275 C residue would need to be reduced by a factor of >5 to Fig. 17. (a) Results of mixing calculations compared to abun- match HSE abundances (e.g., Fig. 17a). Dilution with pure dances of siderophile elements in metal in GRA 95205. One Fe metal produced in smelting could easily accomplish this, endmember is assumed to be residual metal from batch melt since the amount of metal produced in equilibrium smelting extraction at 1275 °C, for starting material with 7 wt.% metal and models (e.g., Goodrich et al., 2007) is very high compared xFeS = 0.54. Retention of the complementary (1275 °C) melt in to the amount of residual metal predicted by our calcula- proportions sufficient to reduce HSE abundances to those in GRA tions (up to 11 wt.% vs. 62 wt.% relative to bulk rock). 95205, results in overabundances of the less compatible elements. However, simple dilution would have no effect on elemental Lower-degree (lower temperature) melts of the same material are ratios, and so could not solve the problem of decreasing the more fractionated (curves with temperatures labeled in °C, 1050– overall slope of the pattern and matching the abundances of 1275), but admixture of such melts cannot reproduce the charac- the less compatible elements (Figs. 14d and 17b). teristic pattern of the less compatible elements in ureilites (illustrated in [b]). Admixture of an unfractionated melt with In contrast, admixture of retained S-rich liquid has the siderophile element abundances at 1 chondritic (a model potential to lower abundances of the HSE and increase ALHA81101 endmember) yields a similar result. Order of elements abundances of the less compatible elements simultaneously. as in Fig. 14. In detail, however, no simple scenarios appear to work. One problem is that the degree of melting required to fractionate HSE in the residue is so high that the complementary liquid cient for Mo relative to Ge is greater than 1, whereas in so- approaches a total melt (i.e., nearly unfractionated), and if lid/liquid fractionation it is less than 1 (Hayden et al., retained in proportions sufficient to dilute the HSE to the 2011). Relative to the prediction for liquid/liquid fraction- required levels, significantly overestimates abundances of ation, ureilites show strong depletions of both W and Mo the less compatible elements (Fig. 17a). Lower-degree (Fig. 16b). This seems to argue against liquid/liquid frac- (i.e., lower temperature) melts of the same starting material tionation in the Fe–S–C system and therefore against the (migrated from other source regions?) would be more frac- presence of carbon during the melting process. We discuss tionated, but mixes with such melts fail to match the ureilite this issue further in Sections 5.7 and 5.8. pattern for the less compatible elements (Fig. 17a). In fact, C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 369 this is not surprising, as most of the ureilites show a pattern cohenite-bearing spherules reinforce this interpretation. of CV-normalized Au,As > Co,Mo > Ni,Pd (Fig. 17b), In terms of mineral assemblages, textures and major/min- which does not correspond to compatibility in either the or element compositions, these spherules appear to be Fe–S system (at any temperature) or the Fe–S–C system. small, trapped samples of coexisting immiscible Fe–C (3– Admixture of any unfractionated melt (whether an 4 wt.% C) and Fe–S liquids at temperatures P1150 °C indigenous total melt or an introduced chondritic compo- (Wang et al., 1991). A major question is why these spher- nent) results in similar problems. Even though an exotic ules have siderophile element abundances identical to metal component could be invoked at any absolute abun- those of coexisting grain boundary metal (we note that dance level, no unfractionated component can lead to the this observation rules out a suggestion by Janssens et al. observed ureilite pattern for the less compatible elements. (1987) that the spherules represent the low-Ir component This rules out a mixing model in which ALHA81101, which of a mixing trend). It seems likely that the siderophile ele- might seem a natural choice because it has the least-frac- ments measured in the bulk spherules belong mainly to the tionated pattern and lowest absolute abundances, is taken Fe–C liquid (i.e., siderophile abundances in the Fe–S li- as the second endmember (Fig. 17a). In fact, ALHA81101 quid were much lower). If this is the case, then it is reason- is particularly problematic. Rankenburg et al. (2008) sug- able to assume (as in Section 5.5.3) that the grain gested that the very low HSE abundances of bulk boundary metal represents the solidified equivalent of this ALHA81101 could be explained by partial loss of metallic same liquid, with the Fe–S liquid having been largely ex- total melts. While this was reasonable when only bulk rock tracted from the rocks. The presence of the metastable abundances were known, it clearly cannot explain the very phase cohenite in the spherules could be explained by crys- low siderophile elements abundances of the metal itself. In tallization within small closed systems, whereas the stable sum, although our results support the inferences of previous phase graphite crystallized from the grain boundary metal. workers that the range of ureilite siderophile element abun- The scarcity of graphite–metal co-crystallization textures dances and patterns is a product of mixing, the tests we might be explained if excess primary graphite was already have done reveal no simple scenarios that can explain all present, so that the two types of graphite became indistin- observations. guishable. This raises the question of why the S-rich melt would be preferentially lost and the C-rich liquid retained. 5.6. Sulfide-metal equilibrium and origin of sulfides The answer may simply be that the S-rich melts were more mobile because they remained wholly liquid until much The modeling in Section 5.5 invokes very high degrees of lower temperatures (Rankenburg et al., 2008). A more extraction of the Fe–S component from ureilite precursor serious problem is why W and Mo are depleted in the urei- materials, consistent with the low (subchondritic) bulk S lite metal, relative to predictions for a C-bearing system contents of ureilites (Warren et al., 2006; Rankenburg (Section 5.5.3). We suggest a solution to this apparent con- et al., 2008). Nevertheless, all ureilites contain some sul- tradiction in Section 5.8. fides. Do these represent small amounts of retained S-rich liquid, as suggested in the previous section? In such a 5.8. Summary and conclusions from modeling of siderophile model, the retained Fe–S liquid must mix with the residual elements in ureilite metal metal (Fe–C liquid), but upon cooling sulfides would precipitate. Assuming that ureilite precursor materials had CI or CV Based on troilite-metal partition coefficients for Ge, Mo, abundances of siderophile elements, two main processes are Au, Co, As, Pd and Cu at temperatures slightly below the evidenced in the patterns and absolute abundances of sider- Fe–S eutectic (Jones et al., 1986; Hayden et al., 2011), the ophile elements in ureilite grain boundary metal – fraction- sulfides analyzed in two samples appear to be in equilibrium ation of S-rich metallic melt, and admixture of one or more with coexisting grain boundary metal. The bulk Cr-rich siderophile element-bearing components. Very high degrees sulfides do not have significantly different trace element (>98%) of batch S-rich melt extraction (implying abundances from the troilite, probably due to low-T T > 1200 °C), are required to explain the high degree of equilibration. This equilibration is consistent with the inter- HSE fractionation observed in many ureilites. However, pretation that the sulfides are derived from retained Fe–S various degrees of admixture of a second component are re- liquid, but does not preclude other possibilities (e.g., migra- quired to match abundances of the less compatible elements tion from other parts of the parent body, or an exotic (e.g., Pd, Au), and to generate the full range of siderophile origin). element patterns and abundances among samples. The very high degrees of melt extraction required may be physically 5.7. The role of carbon implausible, and moreover would require that ureilite precursors had initial abundances of Fe metal and FeS (20– The presence of euhedral graphite crystals (Berkley and 35% each) far exceeding those of plausible (CC- or OC-like) Jones, 1982; Treiman and Berkley, 1994) and cohenite- chondritic precursors. These difficulties lead us to suggest bearing metallic spherules enclosed in silicate grains that ureilite precursors may not have had chondritic relative (Goodrich and Berkley, 1986) in ureilites provide convinc- abundances of siderophiles. In particular, starting material ing evidence that carbon was an abundant primary com- that was volatile-depleted to an even greater extent than ponent and was present during high-temperature igneous bulk CV, would permit lower degrees of melt extraction, processing. The new observations made here about the and therefore lower initial Fe + FeS contents. 370 C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373

Non-chondritic patterns of siderophile elements in the correlation of ﬁnal siderophile element abundance with precursors may also explain the apparent depletions of W FeO. A model of heterogeneous excess CAI component in and Mo in the metal. Depletions of W and Mo relative to ureilite precursors should be further explored, in both redox elements of similar volatility are a characteristic feature of and non-redox models. many CAIs (Fegley and Palme, 1985; Campbell et al., 2003; Humayun et al., 2007). Thus, an excess of CAIs (rel- ACKNOWLEDGMENTS ative to bulk chondrites) in ureilite precursor material could lead to inherited depletions of W and Mo in the metal. At We thank Michael Jercinovic for assistance with SEM and the same time, an excess CAI component would contribute EMPA at the University of Massachusetts. We thank Joseph to overall volatile-element depletion and, if present in vari- Goldstein, Andreas Pack and Leslie Hayden for valuable discus- ous amounts, might explain the range of siderophile ele- sions. We also thank Joseph Goldstein for assistance with calculat- ment patterns and abundances among samples (i.e., fulﬁll ing phosphide-metal equilibration temperatures. We are grateful to the reviewers, Munir Humuyun, Andreas Pack and Anonymous for the requirements for mixing). We suggest that a model of their insightful comments and suggestions, and to the AE Hiroko varying excess CAI in ureilite precursors could kill several Nagahara for her constructive and professional handling of this birds with one stone. submission. This work was supported by NASA Grants NNX09 AI27G and NNX11AG55G to C.A. Goodrich, NNX08AH76G to 5.9. Implications for ureilite petrogenesis W.F. McDonough, and NNX09AB93G to J.A. Van Orman.

If the metal in ureilites is not indigenous, then the results of our work may help to determine its provenance, but pro- APPENDIX A. SUPPLEMENTARY DATA vide no constraints on the primary petrogenesis of ureilites. In that case, the only clues to the origin of the FeO-varia- Supplementary data associated with this article can be tion among ureilites must come from the silicates. For found, in the online version, at http://dx.doi.org/10.1016/ example, a correlation between FeO and Ni content of oliv- j.gca.2012.06.022. ine (Gabriel and Pack, 2008), if confirmed by further data, would support a redox model, consistent either with inherited nebular redox variation or planetary smelting. Other characteristics, such as valence states of Cr in silicates (Goodrich et al., 2012) may provide evidence of the redox REFERENCES conditions under which they formed independent of their FeO contents or assumptions about metal. Albare`de F. (1995) Introduction to Geochemical Modeling. If the metal in ureilites is indigenous, then the lack of Cambridge University Press, U.K. correlation between FeO and either metal content or abun- Antarctic Meteorite Newsletter 21, #2 (1998). Berkley J. L. (1985) Four Antarctic ureilites: Petrology and dances of siderophile elements is difficult to reconcile with a observations on ureilite petrogenesis. Meteoritics 21, 169–189. redox model. This argument has already been made in the Berkley J. L. and Jones J. H. (1982) Primary igneous carbon in context of planetary redox (equilibrium smelting) models ureilites: Petrological implications. Proc. 13th Lunar and (Mittlefehldt et al., 2005; Warren and Huber, 2006; Warren Planetary Sci. Conf., Part 1. J. Geophys. Res. 87(Suppl.), et al., 2006; Rankenburg et al., 2008). We suggest that it is A353–A364. also applies to nebular redox models. For example, if urei- Berkley J. L., Brown, IV, H. G., Keil K., Carter N. L., Mercier J.- lite precursors had an OC-like trend of increasing metal C. C. and Huss G. (1976) The Kenna ureilite: An ultramafic with decreasing FeO content, then the lowest-FeO ureilites rock with evidence for igneous, metamorphic, and shock origin. must have lost the most metal, which is no different from a Geochim. Cosmochim. Acta 40, 1429–1437. smelting model. Berkley J. L., Taylor G. J., Keil K., Harlow G. E. and Prinz M. (1980) The nature and origin of ureilites. Geochim. Cosmochim. On the other hand, these arguments assume that all urei- Acta 44, 1579–1597. lite precursors had chondritic patterns and absolute abun- Boynton W. V., Starzyk P. M. and Schmitt R. A. (1976) Chemical dances of siderophile elements. This would not have been evidence for the genesis of the ureilites, the achondrite the case if, as suggested above, ureilite precursors contained Chassigny and the nakhlites. Geochim. Cosmochim. Acta 40, various excesses of CAIs, and a correlation of FeO with sid- 1439–1447. erophile element abundance would not necessarily be ex- Bunch T. E. and Fuchs L. H. (1969) A new mineral: Brezinaite, pected. For example, Singletary and Grove (2006) have Cr2S4, and the Tucson meteorite. Am. Min. 54, 1509–1518. also proposed a model in which ureilite precursors had a Campbell A. J., Simon S. B., Humayun M. and Grossman L. heterogeneous CAI component. Their model was invoked (2003) Chemical evolution of metal in refractory inclusions in to explain the oxygen isotope characteristics of ureilites, CV3 chondrites. Geochim. Cosmochim. Acta 67, 3119–3134. Chabot N. L. and Jones J. H. (2003) The parameterization of solid and requires that the lowest-FeO ureilites had the largest metal–liquid metal partitioning of siderophile elements. Mete- CAI component. Adding siderophile element consider- orit. Planet. Sci. 38, 1425–1436. ations to this picture, we find that those ureilites which Chabot N. L., Campbell A. J., Jones J. H., Humayun M. and (in a redox model) would have lost the most metal, are Lauer H. V. J. (2006) The influence of carbon on trace element those that had the highest siderophile element abundances partitioning behavior. Geochem. Cosmochim. Acta 70(5), 1322– to begin with, which could effectively cancel out any 1335. C.A. Goodrich et al. / Geochimica et Cosmochimica Acta 112 (2013) 340–373 371

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