Siderophile elements in brecciated HED meteorites and the nature of projectile materials in HED meteorites

Earth and Planetary Science Letters 437 (2016) 57–65

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Earth and Planetary Science Letters www.elsevier.com/locate/epsl

Siderophile elements in brecciated HED meteorites and the nature of projectile materials in HED meteorites N. Shirai a,∗ , C. Okamoto a,1 , A. Yamaguchi b,c , M. Ebihara a a b c

Department of Chemistry, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan National Institute of Polar Research, Tachikawa, Tokyo 190-8518, Japan Department of Polar Science, School of Multidisciplinary Science, SOKENDAI (The Graduate University for Advanced Studies), Tachikawa, Tokyo 190-8518, Japan

a r t i c l e

i n f o

Article history: Received 8 July 2015 Received in revised form 2 December 2015 Accepted 20 December 2015 Available online 18 January 2016 Editor: B. Marty Keywords: platinum group elements Vesta impact processes heavy bombardment

a b s t r a c t Petrological, mineralogical and geochemical studies were performed on five brecciated HED meteorites (ALH 76005, EET 92003, LEW 85300, LEW 87026 and GRO 95633) in order to elucidate the nature of impactors on the HED parent body. Some brecciated HED meteorites contain exotic materials such as FeNi-metal grains with low Co/Ni ratios (ALH 76005, EET 92003 and GRO 95633) and carbonaceous chondrite clasts (LEW 85300) in a clastic and/or impact melt matrix. Such exotic materials were incorporated during brecciation. Platinum group element (PGE) abundances vary significantly (CI × 0.002–0.05), but are higher than those of pristine rocks from the HED parent body. The PGE ratios for the five HED meteorites are inconsistent with each other, implying that the impactor components of each HED meteorites are different from each other. The various PGE ratios are consistent with those for metals from chondrites and iron meteorites, and carbonaceous chondrites. This study provides the evidence that IAB and IVA iron meteorites, and carbonaceous chondrites (CM, CO, CV, CK, CB and CR), ordinary chondrites (L and H) and enstatite chondrite (EL) are candidates of the impactor materials on the HED parent body. It is highly probable that significant amounts of siderophile elements were incorporated into the inner solar system objects like the HED parent body from both chondritic materials and differentiated materials like iron meteorites during heavy bombardment. The HED meteorites in this study and metals from mesosiderite have different Pd/Ir ratios, probably implying that HED meteorites and mesosiderites formed either at distinct settings on one common parent body or on similar parent bodies. © 2016 Elsevier B.V. All rights reserved.

1. Introduction

Apollo 15, 16 and 17 highland rocks and lunar meteorites showed that resetting of isotopic ages by thermal heating (e.g., Cohen et al., 2000) clustered between about 3.8 to 4.1 Ga. It is widely accepted that heavy bombardment occurred during this period. The time period over which heavy bombardment occurred on Moon is explained by the tail end of accretion or a spike in the impact rate (e.g., Hartmann et al., 2000; Ryder et al., 2000). Although the traces of these early impact events on Earth are erased by igneous processes, evidence for heavy bombardment was found from Mercury, Venus and Mars (Strom and Neukum, 1988). There-

*

Corresponding author. Tel.: +81 42 677 2548; fax: +81 42 677 2552. E-mail address: [email protected] (N. Shirai). 1 Current address: Organization of Advanced Science and Technology, Kobe University, 1-1, Rokkoudai-cho, Nada-ku, Kobe, Hyogo 657-8501, Japan. http://dx.doi.org/10.1016/j.epsl.2015.12.024 0012-821X/© 2016 Elsevier B.V. All rights reserved.

fore, it is certain that the heavy bombardment created the cratered terrains on inner solar system bodies. Howardite, eucrite and diogenite (HED) meteorites are the largest group of achondrite which are thought to have been derived from a asteroid (4) Vesta (e.g., Binzel and Xu, 1993). Recently, the Dawn mission confirmed that Vesta’s surface is mineralogically and chemically similar to HED meteorites (Prettyman et al., 2012). Eucrites are basalts or gabbro, diogenites are orthopyroxenenites or harzburgites, and howardites are mixtures of eucrites and diogenites with minor xenolithic components. These rocks are products of differentiation and extensive melting which took place ∼4.55 Ga (e.g., Misawa et al., 2005). The textures, mineralogy and bulk chemical compositions suggest that almost all HED meteorites experienced impact events. Bogard (2011) suggested that the impact events took place at ∼3.4–4.1 Ga based on Ar–Arz ages for eucrites. Dawn observations reveal that Vesta’s surface contains abundant impact craters similar to those on Moon (Jaumann et al., 2012). Therefore, Vesta also experienced heavy bombardment. Carbonaceous chondrites clasts have been re-

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ported in HED meteorites, and are thought to be the most common impactors on Vesta (e.g., Zolensky et al., 1996; Gounelle et al., 2003). This was proved by the Dawn mission (Reddy et al., 2012; Nathues et al., 2014). Reddy et al. (2012) and Nathues et al. (2014) reported that low albedo dark materials were found in and around impact craters and concluded that possible sources of such dark materials are carbonaceous chondrites. Recent analysis data from the Dawn spacecraft suggested that the olivine-rich sites on Vesta are remnant ordinary chondrite impactors (Le Corre et al., 2015; Nathues et al., 2015). A wide variety of exogenic materials (carbonaceous, ordinary and enstatite chondrites, and ureilite) from eucrites and howardites were reported by Lorenz et al. (2007). Siderophile elements can be an excellent tracer for identifying projectile materials (e.g., Norman et al., 2002). Siderophile elements are highly depleted in pristine silicate portions of Vesta because these elements are strongly partitioned into metallic phases during core–mantle formation (e.g., Dale et al., 2012). Siderophile element abundances for chondrites and iron meteorites are several orders of magnitude higher than those for (pristine) crustal materials. Thus, the elevated siderophile element abundances in brecciated HED meteorites are due to the addition of meteoritic components to pristine crustal materials. Thus, the relative abundances of siderophile elements are a function of the type of impactors. In the previous studies, refractory and volatile siderophile elements were used for identification of impactors (e.g., Anders et al., 1973). Recent studies using improved analytical methods such as inductively coupled plasma mass spectrometry (ICP-MS) indicated that platinum group elements (PGE; Ru, Pd, Os, Ir, Pt and Rh) are fractionated among chondrites (e.g., Fischer-Gödde et al., 2010). As PGE are less likely to be fractionated by impact, they provide a high-resolution identification tool for identifying the type of impactors. In order to improve our understanding of the nature of the heavy bombardment of Vesta, we performed mineralogical, petrological and geochemical characterization of five brecciated HED meteorites. Characterization of impactors that bombarded the surface on Vesta was done by determining PGE abundances in brecciated HED meteorites. 2. Sample and experimental Three polymict eucrites (ALH 76005, LEW 85300 and LEW 87026), one brecciated eucrite (EET 92003) and one howardite (GRO 95633) were part of this study, and provided as chips by the Meteorite Working Group, NASA/JSC. Polished thin sections (PTSs) were made for three eucrites (ALH 76005, EET 92003 and LEW 85300) and the GRO 95633 howardite. Chips close to those used for chemical analyses were selected to make PTSs. The PTS could not be made for LEW87026 because the specimen were not of sufficient mass for mineralogical and petrological analyses. PTS samples for three eucrites and one howardite were examined by an optical microscope, a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (JEOL JSM5900LV) and an electron microprobe analyzer (EPMA) (JEOL JXA8800M) at National Institute of Polar Research, Tokyo (NIPR). A portion of the chips of each HED meteorite sample (weighing 200 to 500 mg for each meteorite) were firstly analyzed by prompt gamma-ray analysis (PGA) for the major and minor element compositions. The chips were sealed in fluorinated ethylene polyethylene film bags and were irradiated with thermal neutrons at Japan Atomic Energy Agency (JAEA). Most major and minor elements were non-destructively determined by PGA. The same specimens used for PGA were powdered in an agate mortar at Tokyo Metropolitan University (TMU). An aliquant of each powdered meteorite was used for instrumental neutron activation analysis (INAA) and instrumental photon activation analysis (IPAA).

Two separate samples weighing 20–50 mg and 45–65 mg were prepared for each meteorite for INAA and IPAA, respectively. In our INAA and IPAA, samples were irradiated several times with different irradiation periods which are adjusted for the half lives of the nuclides usable for determining elemental abundances. INAA was performed by using the JRR-3 and JRR-4 research reactors of JAEA, while IPAA was conducted at the Laboratory of Nuclear Science, Tohoku University. The detailed analytical procedures of PGA, INAA and IPAA were described by Shirai and Ebihara (2009). PGE were determined by ICP-MS at TMU. For each analysis, a new batch of chips (about 500 mg) were powdered in an agate mortar. PGE abundances were determined by using NiS fire assay coupled with the isotope dilution method for Ru, Pd, Os, Ir and Pt. 99 Ru/101 Ru, 105 Pd/106 Pd, 189 Os/190 Os, 191 Ir/193 Ir and 196 Pt/195 Pt ratios were used for isotope dilution analysis. As Rh is a monoisotopic element, its abundance was determined by using a calibration method with internal standard elements (In and Tl). The ICPMS procedure was described in Shirai et al. (2003) in detail. 3. Results 3.1. Textures and mineralogy PTSs of four HED meteorites (ALH 76005, EET 92003, GRO 95633 and LEW 85300) were examined petrologically, focusing on the occurrences of exotic materials such as metal grains and chondritic clasts. Backscattered electron images for ALH 76005, GRO 95633, LEW 85300 and EET 92003 are shown in Fig. 1. Chemical compositions for pyroxene and metal obtained by using EPMA are indicated in the Supplementary Fig. S1 and Fig. S2, respectively. 3.1.1. ALH 76005 The PTS displays a clastic matrix composed of a variety of mineral fragments and dark impact melt clasts. There is a large impact melt clast (1.4 × 0.8 mm) which contains tiny droplets of FeS–FeNi (Fig. 1(a)). The compositions of pyroxene vary widely (Wo5.4 En64.5 –Wo12.1–33.5 En21.5–38.4 ) (Supplementary Fig. S1). Large variation was also observed for the chemical composition of plagioclase (An93.2 Or0.1 –An67.0 Or2.5 ). ALH 76005 is observed to be a polymict eucrite mainly composed of basaltic eucrites with minor cumulate eucrites, consistent with previous studies (Grossman et al., 1981; Fuhrman and Papike, 1981). FeNi-metal grains (<10 μm in size) occur as fragments in the clastic matrix and impact melt clasts, or as inclusions in pyroxene fragments where FeNi–FeS coexisted with silica minerals, indicating that such phases formed by reduction of pyroxene. Three FeNi-metal grains were analyzed by EPMA. Two FeNi-metal grains from the impact melt clast (IM) are plotted close to the area of pristine metals (Supplementary Fig. S2). One metal grain in the clastic matrix has the Ni/Co ratio (20.1) similar to those for ‘meteoritic’ metals (Hewins, 1979) (Supplementary Fig. S2). 3.1.2. GRO 95633 GRO 95633 is a howardite composed of a clastic matrix with mineral fragments (<1 mm) of pyroxene, plagioclase, silica minerals and oxide phases. This rock is composed of eucrites with minor diogenitic components. The compositions of low-Ca pyroxene show a range of Wo∼1–2 En40.8–75.8 (Supplementary Fig. S1). Plagioclase displays a slight chemical variation (An90.1–95.5 ). The pyroxene fragments and matrix are darkened due to the presence of very fine opaque minerals (FeS) along cracks and fractures probably formed by shock (i.e., shock darkening). FeNi-metals occur in the clastic matrix. There are relatively large subrounded FeNi-metal grains (<120 μm) with FeS along the rims in the clastic matrix. The grains of FeNi-metal are plotted in the range of meteoritic iron on Ni–Co plot (Ni = 8.2–10.0 wt%), indicating that the metals are xenoliths (Supplementary Fig. S2).

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Fig. 1. Backscattered electron images of xenolithic clasts in clastic or impact melt matrices of the brecciated HED meteorites studied in this investigation, (a) ALH 76005, (b) GRO 95633, (c) LEW 85300, and (d) EET 92003. In ALH 76005, there is a large impact melt clast (“IM”) which contains tiny grains of Fe-metal. In LEW 85300, there are fragments of vesiculated carbonaceous chondrites in an impact melt vein, which are shown within the dotted outline in (c). The black portion within the outlined area is probably an amoeboid olivine aggregate (“AOA”). White areas in (a), (b) and (d) are FeNi metal (FeNi), or mixtures of FeNi metal and sulfide (FeNi–S in (d)). Medium-gray = pyroxene; dark-gray = plagioclase or silica minerals; black = cracks or epoxy.

3.1.3. LEW 85300 Polymict eucrite LEW 85300 is composed mostly of impact melt matrix. About one third of the PTS is occupied by a basaltic clast which shows a subophitic texture composed of pyroxene and plagioclase. Most of the pyroxene fragments in the matrix are chemically similar to those of the basaltic clast. Impact melt matrix contains a small amount of diogenitic orthopyroxene. There are a few large (∼1 mm) fragments of pyroxene and plagioclase. LEW 85300 is reported to have chondritic clasts petrologically similar to CM chondrites (e.g., Zolensky et al., 1992). We found clasts of carbonaceous chondrite which are partially vesiculated in the melt matrix (Fig. 1(c)). There are a few Ni-poor metal grains surrounded by weathering products. 3.1.4. EET 92003 EET 92003 is a monomict eucrite composed of fine-grained, recrystallized basaltic clasts set in the clastic matrix with gradational boundaries. Pyroxenes in the clasts and matrix of EET 92003 have fairly uniform chemical compositions (Wo∼13 En∼38 ) (Supplementary Fig. S1). Fine FeNi-metal grains (∼10 μm) occur in the clastic matrix (Fig. 1(d)). Cobalt and Ni abundances for these FeNi-metal grains in EET 92003 are consistent with those for ‘meteoritic’ metal (Supplementary Fig. S2). 3.2. PGA, INAA, IPAA and ICP-MS results PGA, INAA and IPAA results for bulk chemical compositions of the five brecciated HED meteorites studied in this investigation are given in Supplementary Table S1, where literature values are also shown for comparison. Our six PGE values are indicated in Table 1. Our data for these five HED meteorites are in good agreement with literature values except for Co, Ni and Sr. In the following paragraph, these discrepancies will be discussed. Our Sr values for EET 92003 and LEW 85300 are lower than literature values (Mittlefehldt and Lindstrom, 2003). It is well known that chemical compositions for meteorites found in hot deserts tend to be variable for some elements due to terrestrial weathering (e.g., Barrat et al., 2001). Because the five HED

meteorites analyzed in this study were collected from Antarctica, not from hot deserts, it is less likely that these discrepancies are due to terrestrial weathering (e.g., Crozaz et al., 2003; Saunier et al., 2010). Our Sr abundances for the five HED meteorites were determined by IPAA, while literature values were obtained by using INAA. Strontium abundances determined by INAA are not generally as precise as those by IPAA and ICP-MS due to poor sensitivity in INAA, indicating that our Sr values are more reliable. As shown in Supplementary Table S1, consistencies of Co and Ni abundances between our data and literature values are also poor. The difference in Ni between our data and the corresponding literature value is more than a factor of about 20 in LEW 85300. This will be discussed later. All PGE abundances for the five HED meteorites were determined by ICP-MS in this study. Compared with Os and Ir data, which were mostly obtained by using INAA and RNAA (e.g., Mittlefehldt and Lindstrom, 2003; Warren et al., 2009), Ru, Rh, Pd and Pt abundance data were less available in the literature. In this study, Ir was determined by both INAA and ICP-MS for GRO 95633. Iridium abundances for other HED meteorites are below the detection limit in INAA. Iridium abundances show poor consistency for abundances of GRO 95633 between INAA and ICPMS (Table 1). Used sample weight in ICP-MS (about 500 mg) was about 10 times higher than that in INAA (about 50 mg). As FeNimetals are present in the studied HED meteorites as mentioned in the previous section, the host phase of PGE is considered to be FeNi-metal. The difference in Ir values between INAA and IDICP-MS must be due to the heterogeneous distribution of FeNimetals. Although our Ir value for EET 92003 is in good agreement with literature values, those for other HED meteorites are inconsistent with corresponding literature values. These inconsistencies between our data and literature values must be also due to the heterogeneous distribution of FeNi-metals. Considering that higher amounts of well-homogenized powder were used in ICP-MS compared to INAA, the ICP-MS data must be more representative for HED meteorites analyzed in this study than the INAA data. Therefore, we use PGE abundances obtained by ICP-MS in the following discussions.

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Table 1 PGE abundances (in ng/g) for five HED meteorites analyzed in this study.

Ru Rh Pd Os Ir Pt a b c d

ALH 76005

EET 92003

This worka ICP-MS

This worka ICP-MS

3.25 ± 0.16 0.58 ± 0.03 3.8 ± 0.9 2.0 ± 0.2 2.09 ± 0.09 3.6 ± 0.3

9.2 ± 0.4 1.40 ± 0.06 4.9 ± 0.3 4.9 ± 0.8 4.31 ± 0.21 6.5 ± 0.3

LEW 85300 Literature

5c

This worka ICP-MS

<0.2 0.36 ± 0.04 1.1 ± 0.3 <0.1 1.08 ± 0.26 1.5 ± 0.4

LEW 87026 Literature

3.5c

This worka ICP-MS

<0.2 0.66 ± 0.01 2.0 ± 0.1 1.7 ± 0.1 1.57 ± 0.07 2.6 ± 0.1

GRO 95633 Literature

This worka ICP-MS

2.46d 38c , 2d

35.4 ± 1.2 6.28 ± 0.13 19.4 ± 0.7 25.7 ± 1.4 23.9 ± 1.2 43.5 ± 1.7

Literature INAAb

6.3 ± 1.4

6.9, 8.0c

Error are due to standard deviations (1σ : n = 10) in ICP-MS. INAA data are taken from Supplementary Table S1. Mittlefehldt and Lindstrom (2003). Warren et al. (2009).

4. Discussion 4.1. Siderophile element abundances in the five HED meteorites 4.1.1. Ni and Co HED meteorites are characterized by the most severely depleted abundances of siderophile elements among achondrites (e.g., Dale et al., 2012). When the parent body(ies) having chondritic elemental compositions were thermally differentiated, siderophile elements were concentrated into Fe–Ni metallic alloys and were easily separable from the silicate material. HED meteorites are well acknowledged to have been delivered from such a silicate layer of largely differentiated parent body(ies). According to the reflection spectrum obtained on the Earth and the surface geomorphology observed by the Dawn spacecraft, asteroid 4 Vesta is proposed as the parent body of HED meteorites (e.g., Binzel and Xu, 1993; Gaffey, 1997; Reddy et al., 2010, 2011, 2013; Prettyman et al., 2012). Eucrites are supposed to have originated from the outermost part of such a body, considering their petrographical and geochemical features. Nickel and Co are grouped together as siderophile elements but their degrees of siderophile nature are different from each other. Fig. 2 shows CI-normalized (Ni/Co) ratios (designated as (Ni/Co)CI ) and Ni abundances in various meteorites. As the condensation temperatures of Ni and Co are almost identical (1352 K and 1353 K for Ni and Co, respectively, at 10−4 atm of the solar system composition (Lodders, 2003)), Ni and Co don’t widely fractionate from each other during condensation, staying on the CI line (Fig. 2). Further, Ni/Co ratios in iron meteorites and metals from chondrites don’t significantly deviate from those of chondritic meteorites because of the high affinity of Ni and Co for the Fe–Ni metal alloy. Mesosiderites which are mixtures of silicate and metal have large variations of (Ni/Co)CI ratios as shown in Fig. 2. (Ni/Co)CI ratios for metals from mesosiderites are higher than those for their silicates and similar to those for iron meteorites (Kong et al., 2008). Thus, it is likely that such large variations result from the different proportions of FeNi-metal and silicate in mesosiderite samples. As shown in Fig. 2, almost all eucrites and howardites have lower (Ni/Co)CI ratios than chondritic ratios. Nickel abundances and (Ni/Co)CI ratios systematically decrease from polymict eucrites and howardites to monomict and unbrecciated eucrites (Fig. 2). Eucrites and howardites have similarly larger variations of Ni abundances and (Ni/Co)CI ratios to those of mesosiderites. Polymict eucrites and howardites have similar Ni and (Ni/Co)CI ratios to each other and also to those of mesosiderites. LEW 85300 has the lowest Ni abundances and (Ni/Co)CI ratios among the five HED meteorites, and the others overlap with the area of polymict eucrites and howardites. There are two possible explanations for the large deviation of eucrites from (Ni/Co)CI ; addition of chondrites and/or iron meteorites to pristine HED meteorites and igneous processes on the HED parent body.

Fig. 2. (Ni/Co)CI ratios vs. Ni abundances for several types of extra-terrestrial samples. Solid curves represent mixing between EET 87520 and chondrites, and EET 87520 and iron meteorites. The proportional contributions of the chondritic and iron meteorite end members are shown as percentages along the mixing curves. Cobalt and Ni abundances for three end members (EET 87520, chondrites and iron meteorites) are 4.9 ppm and 0.1 ppm, 502 ppm and 11 000 ppm, and 4940 ppm and 84 000 ppm, respectively. The sources of literature values are provided in the Supplementary information.

Previous studies (e.g., Zolensky et al., 1996) and this study revealed that some HED meteorites have extraneous materials (chondritic clast and/or FeNi-metal). These extraneous materials have higher Co and Ni abundances than those for differentiated meteorites such as HED meteorites. Considering that large variations of Ni abundances and (Ni/Co)CI ratios in mesosiderites are due to the different proportions of FeNi-metal and silicate in the samples, systematic increase of Ni abundances and (Ni/Co)CI ratios are mostly caused by the addition of FeNi-metals and/or chondrite clasts. We consider a possibility that Ni and Co abundances for eucrites and howardites are controlled by the addition of chondrites or iron meteorites to pristine eucritic materials. Here, assuming that EET 87520 analyzed by Warren et al. (2009) represents the pristine eucritic material with low Co and Ni abundances, we calculated Ni abundances and (Ni/Co)CI ratios of the mixture of two end components (LEW 87520 and chondrites or iron meteorites) by changing their mixing ratios (Fig. 2). Apparently, eucrites and howardites fall on mixing lines obtained for chondrites or iron meteorites (Fig. 2) as Co and Ni-rich end members. Estimated mass fractions of chondrites and iron meteorites are 0.35 and 0.035% for ALH 76005, 3.6 and 0.36% for EET 92003, 1.8 and 0.18% for GRO 95633, 0.02 and 0.002% for LEW 85300, and 0.33 and 0.033% for LEW 87026, respectively.

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Differentiated planetary bodies like Earth have undergone their own differentiation histories and thereby their silicate layers have different element compositions from those of chondrites. For example, as Ni and Mg behave similarly during igneous processes, each planetary body has its own characteristic Ni/Mg ratio in the silicate layer. Cobalt and Ni behave as compatible elements during igneous processes in which metal and sulfide are not involved, with compatibility of Ni and Co being different from each other. Righter and Drake (1997) estimated Ni and Co abundances in silicate portion of HED parent body by using a correlation of Ni and Co vs. FeO + MgO. Terrestrial samples show a good correlation between these two variables, while HED meteorites don’t show such a correlation. Righter and Drake (1997) suggested that poor correlations were caused by the addition of chondritic metals. Later, Warren et al. (2009) observed that polymict, monomict and unbrecciated eucrites have similar Co abundances, while polymict eucrites have slightly higher Ni abundances than monomict and unbrecciated eucrites. There are no positive correlations among MgO and, Co and Ni abundances in eucrites (Supplementary Figs. S3). Therefore, Ni and Co abundances for eucrites including our samples were not influenced by igneous processes but addition of chondritic materials and/or meteoritic irons. Foreign meteoritic components have been reported to be present in HED meteorites (e.g., Zolensky et al., 1996). Petrological and mineralogical studies revealed that most of the foreign meteoritic components confirmed in eucrites and howardites so far are carbonaceous chondrites. Chou et al. (1976) determined siderophile and chalcophile element abundances for howardites, and observed that howardites have higher abundances of these elements relative to the indigenous level. They considered that this excess was due to chondritic contamination, and estimated 2.4–3.3% of chondritic component in mass. This component was chemically similar to CM chondrite. In addition to howardites, some eucrites were reported to have chondritic clasts. Kozul and Hewins (1988) observed a dark CM chondritic clast in polymict eucrite LEW 85300 and Mittlefehldt and Lindstrom (1988) found that this clast was essentially chondritic with a unusual M-shaped CI-normalized REE abundances pattern. Gounelle et al. (2003) observed a large number of carbonaceous chondrite clasts in three howardites. Lorenz et al. (2007) also found a wide variety of foreign meteoritic components from howardites and polymict eucrites. Those components are mineralogically similar to ordinary chondrites, carbonaceous chondrites, enstatite chondrites and ureilites. Lorenz et al. (2007) also found that howardites and eucrites contain metal particles of both meteoritic and endogenic origins. Based on mineralogy, chemistry and oxygen isotopic composition, it was concluded that howardite JaH 556 contains H chondrite materials (Janots et al., 2012). We also commonly observed such metal grains in our PTS samples of ALH 76005, EET 92003 and GRO 95633 by SEM (Fig. 1). Amoeboid olivine aggregates (AOA) were also found in LEW 85300. Chondritic clasts, however, were not found in any studied HED meteorites except LEW 85300. Therefore, we conclude that a part of Ni and Co of the HED meteorites were introduced by impact event of not only carbonaceous chondrites suggested by previous studies and this study, but also iron meteorites and/or metals from chondrites. 4.1.2. Platinum group elements Dale et al. (2012) reported PGE abundances for cumulate and monomict eucrites, and found that these eucrites have 10−4 to 10−5 times the CI levels and that Os, Ir and Ru are slightly depleted relative to Pd and Pt. As shown in Fig. 3, our PGE abundances are at least 10 times higher than literature values for cumulate eucrites (Dale et al., 2012). PGE abundances in LEW 85300 are the lowest among the five HED meteorites we analyzed. PGE abundances in GRO 95633 are 10–50 times higher than those in

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Fig. 3. CI-normalized PGE (platinum group element), Ni and Co abundances for eucrites. For comparison, those for cumulate eucrites and monomict eucrites (Dale et al., 2012) also are shown.

LEW 85300. ALH 76005 and LEW 87026 have a few times higher PGE abundances than LEW 85300. The CI-normalized PGE abundance patterns for these five HED meteorites are nearly flat. PGE abundances are positively correlated with Ni and Co, implying that PGE are partitioned into the same phase as that for Ni and Co. Therefore, PGE in the five HED meteorites analyzed in this study are mostly contributed by meteoritic components. 4.2. Identification of impactors PGE signatures of studied HED meteorites were used for the identification of types of meteoritic impactors that bombarded Vesta. This technique assumes that PGEs were not fractionated during the incorporation of the meteoritic impactor into breccias. There are at least two possible causes for the PGE fractionation after brecciation of impactors (projectiles): (1) vapor phase fractionation during impact events, (2) terrestrial weathering processes. We reject the first possibility because PGE are so refractory that they were not fractionated in vapor phase (Norman et al., 2002). We also reject the second possibility for the samples we studied because the terrestrial weathering of metallic phases was not significant. Therefore, we conclude that the PGE signatures in the HED meteorites analyzed in this study indicate primary meteoritic compositions. FeNi-metal for ALH 76005, EET 92003 and GRO 95633 have different Ni and Co abundances from each other (Supplementary Fig. S2) which could not be explained by mixing of kamacite with taenite. Therefore, we considered that individual HED meteorites contain remnants of the different impactors. Our PTSs for ALH 76005, GRO 95633 and EET 92003 contain FeNi-metals, while carbonaceous chondritic clasts are found in our PTS of LEW 85300. Therefore, PGE signatures for ALH 76005, GRO 95633 and EET 92003, and LEW 85300 are compared with those for mesosiderites, iron meteorites and metal from chondrites, and carbonaceous chondrites. For LEW 87026, comparison is made among mesosiderites, iron meteorites, metals from chondrites and carbonaceous chondrites. Fig. 4(a) compares Pt/Ir and Pd/Ir ratios for the four HED meteorites with those for metals from mesosiderites and iron meteorites. Platinum/Ir ratios for the four HED samples are almost constant, while Pd/Ir ratios vary among them. Palladium/Ir ratios for the four HED meteorites do not likely result from volatile redistribution of PGE and terrestrial weathering as discussed above. Therefore, different Pt/Ir ratios also provide evidence that each HED meteorite contains remnants of the different impactors. Platinum/Ir and Pd/Ir ratios for GRO 95633 are the most consistent

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(a)

(b)

(c)

(d)

(e) Fig. 4. Comparison of Pt/Ir vs. Pd/Ir ratios for eucrites to those for iron meteorites and mesosiderites (a), metals from chondrites (b–d) and bulk carbonaceous chondrites (e). Pt/Ir and Pd/Ir ratios for IAB iron meteorite, enstatite, ordinary and carbonaceous chondrites are presented by the field. For other iron meteorites (IC, IIAB, IIC, IID, IIE, IIIAB, IIIE, IIIF, IVA and IVB), these two PGE elemental ratios are shown by the correlation lines of fractional crystallization (see the Supplementary information). The sources of literature values are provided in the Supplementary information.

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with those for IAB and IVA among meteorites compared. Other HED meteorites (ALH 76005, EET 92003 and LEW 87026) have the same Pt/Ir and Pd/Ir ratios as those of IAB. Our data are also compared to values for metals from chondrites in Figs. 4(b)–(d). Compared to the case for iron meteorites, fractionations of Ir from Pt and Pd are not large among metals from chondrites. Four HED meteorites (ALH 76005, EET 92003, GRO 95633 and LEW 87026) have similar Pt/Ir and Pd/Ir ratios to those for metals from H and CB chondrites. Platinum/Ir and Pd/Ir ratios for ALH 76005 and GRO 95633 also fall within the ranges of L and EL, and CR chondrites, respectively. In Fig. 4(e), Pt/Ir and Pd/Ir ratios for LEW 85300 and LEW 87026 are compared to those for carbonaceous chondrites. Platinum/Ir and Pd/Ir ratios for LEW 85300 overlap with CO, CM, CV and CK chondrites, while those ratios of LEW 87026 are different from those for carbonaceous chondrites. It is concluded that the five HED meteorites analyzed in this study contain different types of impactors (IAB, L, H, EL CB for ALH 76005, IAB, H and CB for EET 92003, IAB, IVA, H, CB and CR for GRO 95633, IAB, H and CB for LEW 87026, and CM, CO, CV and CK for LEW 85300). 4.3. Bombardment history of the HED parent body Because the Ar–Ar system has a relatively low closure temperature compared to other isotopic chronometers, Ar–Ar dating provides useful information about impact chronology. Bogard (2011) summarized Ar–Ar data for HED meteorites and observed that Ar– Ar ages of HED meteorites are strongly peaked at 3.4–4.1 Ga and ∼4.48 Ga. The ages for cumulate and unbrecciated eucrites cluster at ∼4.48 Ga, while brecciated eucrites cluster at 3.4–4.1 Ga. Many lunar highland rocks also give impact reset ages in the range of 3.8–4.1 Ga. Therefore, it is likely that heavy bombardment on the HED parent body occurred during the same time period as that on Moon. Kunz et al. (1995) analyzed two clasts from ALH 76005 and observed that these clasts had different thermal histories, implying that they were embedded into the breccia. Ar–Ar ages for these clasts are in the range of 3.5–3.7 Ga. The Ar–Ar ages for clasts from LEW 85300 also range between 3.5–3.8 Ga (Nyquist et al., 1991). These meteorites experienced Ar degassing during the heavy bombardment, which likely caused the impact brecciation and lithification. Clasts commonly found in HED meteorites are CM-like clasts with lesser amount of CR-like clasts (Zolensky et al., 1996; Gounelle et al., 2003). Based on siderophile and chalcophile elements abundances, Chou et al. (1976) once estimated that 2.4–3.3% of the chondritic material was present in HED meteorites. The Dawn Framing Camera observed that the dark materials are common in and near the crater on Vesta (Reddy et al., 2012). Band depth and albedo of these dark materials were compared with those for mixture of carbonaceous chondrite (CM) and eucrites, and both values were found to be consistent with each other (Reddy et al., 2012). Thus, it is highly probable that carbonaceous chondrites once impacted on Vesta. Petrological observation of this study showed that only LEW 85300 has the AOA structure commonly observed in carbonaceous chondrites but that PTSs of the three brecciated HED meteorites examined in this study contain FeNi metals. Based on PGE abundance patterns for the five brecciated HED meteorites, we assigned potential impactors on Vesta to be iron meteorites (IAB and IVA), carbonaceous chondrite (CO, CM, CV, CK, CB and CR), ordinary chondrite (H and L), enstatite chondrite (EL). This study and previous studies (Zolensky et al., 1996; Lorenz et al., 2007) clearly show a large diversity of impactor materials during bombardment of Vesta. This conclusion is similar to the case for Moon. Fischer-Gödde and Becker (2012) determined PGE abundances for lunar breccias and impact melt rocks, and found that the impactor components in these samples

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have close affinities to chondritic meteorites. Fischer-Gödde and Becker (2012) also observed that PGE signatures in some impact melts are similar to those in iron meteorites. Apparently, similar heavy bombardments occurred on Moon and Vesta. It seems likely that variable impactor materials were incorporated during the heavy bombardment (3.4–4.1 Ga) (Bogard, 2011) in the inner solar system. Pyroxene fragments from ALH 76005, GRO 95633 and LEW 85300 have large chemical variations (Supplementary Fig. S1). These pyroxenes are mineralogically similar to those from Juvinas (Takeda and Graham, 1991). Miyamoto and Takeda (1977) calculated cooling rates for some eucrites based on the width of exsolution lamellae of augite in pigeonite and suggested that pyroxene fragments of Juvinas were derived from relatively deep interior (∼ several km deep). Therefore, pyroxene fragments of the brecciated eucrites studied in this work must have sampled material from a range of depths of the parent body. If so, impacts would have penetrated the eucritic crust, which is estimated ∼10–40 km in thickness (Ruzicka et al., 1997). Empirical consideration indicates that the size of craters required to excavate ∼10–40 km thick eucritic crust would be several tens to several hundreds km in diameter (Melosh, 1989). This estimate seems to be consistent with the heavily cratered surface of Vesta as observed by the Dawn mission (Jaumann et al., 2012). 4.4. Relationship between HED meteorites and mesosiderites Mesosiderites are breccias composed of approximately equal amounts of silicate and metal phases. With respect to petrology, mineralogy and chemical compositions, some basaltic and gabbroic clasts in mesosiderites are indistinguishable from eucrites (e.g., Ikeda et al., 1990). High-precision oxygen isotope measurements indicated that HED meteorites and mesosiderites have the same oxygen isotopic compositions (Greenwood et al., 2006). Rosing and Haack (2004) observed a mesosiderite-like clast in DaG 779 (howardite). These evidences indicate that mesosiderites and HED meteorites originated from the same parent body. Mesosiderites have two kinds of metals – nodule metals and matrix metals. Kong et al. (2008) analyzed nodule metals and matrix metals from the Dong Ujimqin Qi mesosiderites. The chemical compositions for nodule and matrix metals are slightly different from each other. Palladium and Au abundances for nodule and matrix metals are consistent with each other, while nodule metals have higher Re, Os, Ir, Ru and Pt abundances than those for matrix metals. The nodule metals have a clear boundary with the silicate matrix, indicating that they were incorporated as solids during metal-silicate mixing. In contrast, the matrix metals were liquid when metals and silicates were mixed. Tamaki et al. (2006) performed petrological, mineralogical and bulk chemical analyses for the silicate portion from the Mount Padbury mesosiderite, and observed two types of metals in the silicate portion; one is present as droplets with troilite while the other is present as angular grains. The CI-normalized PGE abundance pattern for the silicate portion from Mount Padbury was not flat. As a result, Tamaki et al. (2006) concluded that such fractionation of PGE observed in the silicate fraction resulted from partial melting of metal and sulfide. Therefore, some metals from mesosiderites were metamorphosed after the impact of solid metal bodies on the surface of the mesosiderite parent body. The four brecciated HED meteorites examined in this study contain FeNi-metals incorporated during brecciation and/or lithification on the HED parent body, and were observed to have higher siderophile element abundances compared to the pristine eucritic materials. These four HED meteorites have similar Co and Ni abundances to those for mesosiderites (Fig. 2). However, PGE ratios of these four HED meteorites are different from those of

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mesosiderites. As shown in Fig. 4(a), although Pt/Ir ratios for the four brecciated HED meteorites are similar to those for metals from mesosiderites, these four brecciated HED meteorites have higher Pd/Ir ratios. Therefore, metals from HED meteorites and mesosiderites are not related to each other. Modal abundances of metals in brecciated HED meteorites are much lower than those for mesosiderites. Mesosiderites show high modal abundances for silica minerals and phosphate, while these minerals could not be found in the four HED meteorites except LEW 87026. Probably, HED meteorites and mesosiderites formed either at distinct settings on one common parent body or on similar parent bodies. 5. Conclusions and implications Petrological, mineralogical and bulk chemical studies were performed for five brecciated HED meteorites (ALH 76005, EET 92003, LEW 85300, LEW 87026 and GRO 95633) in order to elucidate the nature of impactors on the HED parent body. Our PTSs showed that FeNi-metals (ALH 76005, EET 92003 and GRO 95633) and carbonaceous chondrite clasts (LEW 85300) were present in clastic matrix and/or impact melt. Brecciated HED meteorites have higher siderophile element abundances compared to those for the pristine eucritic materials. Incorporations of meteoritic components during heavy bombardment on HED meteorites are responsible for these elevated abundances of siderophile elements in HED meteorites. The PGE abundance signatures obtained for HED meteorites in this study indicate that candidates for impactors are carbonaceous chondrites, iron meteorites and/or chondrites with metals. Bogard (1995) suggested that inner solar system objects experienced intense meteorite bombardments which caused a widespread resetting of various isotopic clocks ∼3.0–4.5 Ga ago. Kring and Cohen (2002) suggested that there was an impact cataclysm which affected the entire inner solar system, resurfacing the terrestrial planets. They suggested that most of the impact craters on Mercury, the Moon, the ancient cratered highlands of Mars in the inner solar system were produced by the impact cataclysm. Earth and Venus were also affected by the cataclysm, but geologic activities have erased the impact-cratering surfaces. This study showed that Vesta also experienced meteorite bombardments. Significant amounts of siderophile elements were incorporated into the mantle and crust by these events. A previous study suggested that carbonaceous chondrites were related to the regolith processes on the HED surface (Zolensky et al., 1996). Our results indicate that the impactors on the HED parent body were not only chondritic but also differentiated materials (iron). Interestingly, the impactors that formed lunar basins are also both chondritic and differentiated meteoritic metal (Fischer-Gödde and Becker, 2012). These facts imply that the impactors causing cataclysmic bombardments could be fragments of chondritic meteorites and differentiated meteorites perturbed in the early history of solar system. Acknowledgements We thank MWG for providing us with the meteorite samples, Masahito Arakawa, Hideyasu Kojima and Kevin Righter for discussion. We are grateful to Kevin Righter for improving English. PGA and INAA analyses were made possible by an inter-university cooperative program for the use of JAEA facilities, supported by the University of Tokyo. We thank Tsutomu Ohtsuki and Hideyuki Yuki of the Laboratory for Nuclear Science, Tohoku University for their assistance with IPAA analyses. Yasuji Oura is acknowledged for his help throughout this work. We would like to thank for V. Reddy and L.E. Nyquist for providing useful and constructive comments on the manuscript. Financial support was provided in part via JSPS

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