Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids

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Geochimica et Cosmochimica Acta 75 (2011) 3757–3780 www.elsevier.com/locate/gca

Shock effects in “EH6” enstatite chondrites and implications for collisional heating of the EH and EL parent asteroids Alan E. Rubin a,*, John T. Wasson a,b,c a

Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA b Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095, USA c Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA Received 10 January 2011; accepted in revised form 5 April 2011; available online 15 April 2011

Abstract Of the six chondrites that were listed as EH6 or EH6-an during the course of this study, we confirm the EH classification of Y-8404, Y-980211 and Y-980223 and the EH-an classification of Y-793225; two chondrites (A-882039 and Y-980524) are reclassified as EL (the former contains ferroan alabandite and both contain kamacite with 1 wt% Si). All of the meteorites contain euhedral enstatite grains surrounded by metal ± sulfide (although this texture is rare in Y-793225), consistent with enstatite crystallizing from a mixed melt. All contain enstatite with <0.04 wt% MnO; the three EH chondrites average 0.25 wt% Mn in troilite. (Literature data show that typical EH3–EH5 chondrites contain enstatite with 0.13–0.20 wt% MnO and troilite with 0.05–0.11 wt% Mn.) The three EH chondrites contain keilite [(Fe>0.5,Mg<0.5)S], which has been interpreted in the literature as a product of impact melting. Y-8404 and Y-980223 contain abundant silica (13 and 10 wt%, respectively), a rare phase in most enstatite chondrites. We suggest that all six meteorites have experienced impact melting; Mn was preferentially partitioned into sulfide during subsequent crystallization. The silica-rich samples may have become enriched in the aftermath of the impact by a redox reaction involving FeO and reduced Si. A-882039, Y-8404, Y-980211, Y-980223 and Y-980524 were incompletely melted; they contain rare relict chondrules and are classified as impact-melt breccias; Y-793225 is a chondrule-free impact-melt rock. If these EH and EH-an chondrites (which were previously listed as petrologic type 6) have, in fact, been impact melted, it seems plausible that collisional heating is generally responsible for EH-chondrite metamorphism. This is consistent with literature data showing that a large fraction (P0.7) of those chondrites classified EH5–7 and a significant fraction (P0.3) of those chondrites classified EH4 and EH4/5 possess textural and mineralogical properties suggestive of impact melting. In addition, 60% of classified EL6–7 chondrites (now including A-882039 and Y-980524) appear to have formed by impact melting. It thus seems likely that collisional heating is mainly responsible for EL- and EH-chondrite metamorphism. Ó 2011 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Many EH chondrites have been described as impactmelt breccias or impact-melt rocks (e.g., Rubin, 1997a; Rubin and Scott, 1997; Lin and Kimura, 1998; Kimura and Lin, 1999; Burbine et al., 2000; Fagan et al., 2000; Keil, 2007). The heat generated by the associated impacts could ⇑ Corresponding author. Tel.: +1 310 825 3202.

E-mail address: [email protected] (A.E. Rubin). 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2011.04.002

potentially cause thermal metamorphism in these materials, a process involving recrystallization, mineral equilibration, and partial loss of highly volatile elements. A correlation between petrologic type and recognizable impact effects would be consistent with collisional heating being mainly responsible for chondrite metamorphism. In contrast, if the decay of short-lived radionuclides were the main heating mechanism for EH chondrites, no correlation between petrologic type and the number and intensity of impact effects would be expected.

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The current classification scheme for petrologic types is based on criteria developed by Van Schmus and Wood (1967) and modified by later workers. It can be applied to ordinary, carbonaceous, enstatite and R chondrites. In all of these groups, type-3 chondrites contain very sharply defined chondrules (some with primary igneous glass), finegrained silicate-rich matrix material, polysynthetically twinned low-Ca clinopyroxene, and compositionally unequilibrated olivine and low-Ca pyroxene. Type-4 chondrites contain well-defined chondrules (with little or no glass), matrix material that is either absent or recrystallized, relatively few grains of polysynthetically twinned low-Ca clinopyroxene, equilibrated olivine and largely equilibrated low-Ca pyroxene. Type-5 chondrites have readily delineated chondrules with no primary glass, no fine-grained matrix material, no polysynthetically twinned low-Ca clinopyroxene, and compositionally uniform olivine and low-Ca pyroxene. Type-6 chondrites contain poorly defined chondrules with no glass, no fine-grained matrix material, no polysynthetically twinned low-Ca clinopyroxene, uniform olivine and low-Ca pyroxene compositions, and many plagioclase grains that exceed 50 lm. (Olivine is absent in many type-4 and all type-5 and -6 enstatite chondrites.) Although some chondrites have been designated type 7 because they are highly recrystallized or contain melt (or contain orthopyroxene with >1 wt% CaO; Dodd, 1974), many of these rocks (e.g., L7 PAT 91501; EL7 Ilafegh 009) have been impact melted (e.g., McCoy et al., 1995; Mittlefehldt and Lindstrom, 2001) and should be classified as impact-melt rocks or impact-melt breccias. There are also many ordinary chondrites (OC) listed in the on-line Meteoritical Bulletin Database (MBDB) as type 6 that have been shown to be impact-melt breccias: e.g., H6 Yanzhuang (Xie et al., 1991), L6 Chico (Bogard et al., 1990) and LL6 Bison (Dominik and Bussy, 1994). Even a few OC listed as type 5 are impact-melt breccias: e.g., H5 Rose City (Mason and Wiik, 1966; Fruland, 1975; Rubin, 1995a) and L5 Cat Mountain (Kring et al., 1996). If the principal mechanism responsible for chondrite metamorphism is collisional heating (e.g., Rubin, 1995b, 2004, 2005) and not the decay of short-lived radionuclides (e.g., Lee et al., 1976; Miyamoto et al., 1981; Grimm and McSween, 1993; McSween et al., 2002), then the distinction between impact-melt breccias and type-5, -6 and 7 chondrites becomes blurred. In the present study, we examined all six enstatite chondrites that had been classified by others (and listed in the MBDB during the course of this study) as EH6 or EH6-an to determine whether or not they have petrologic features consistent with having been heated by impacts. These rocks include five EH6 chondrites (A-882039, 384 g; Y-8404, 10.7 g; Y-980211, 36.9 g; Y-980223, 116.2 g; Y-980524, 24.3 g) and one EH6-an chondrite (Y-793225, 75.6 g). Other than the study by Lin and Kimura (1998) that described Y-8404 as an impact-melt rock and Y-793225 as a new kind of enstatite chondrite (intermediate between EH and EL), the meteorites classified as EH6 chondrites have received scant attention. As shown below, our new data have led us to accept the EH classifications of Y-8404, Y-980211 and Y-980223,

to accept the EH-an classification of Y-793225, and to reclassify A-882039 and Y-980524 as EL chondrites. We also show that all six chondrites in this study exhibit textural and mineralogical characteristics indicative of impact melting (Appendix A) and are considered impact-melt rocks or impact-melt breccias. 2. ANALYTICAL PROCEDURES The following thin sections were borrowed from the National Institute of Polar Research, Japan: A-882039, 72-1; Y-793225,51-2; Y-8404,51-2; Y-980211,51-1; Y-980223, 61-2 and Y-980524,51-1. These sections were examined in transmitted and reflected light with an Olympus BX60 petrographic microscope. Mineral compositions were determined with the JEOL electron microprobe at UCLA using natural and synthetic standards, a sample current of 15 nA, an accelerating voltage of 15 keV, 20-s counting times per element, ZAF corrections, and a focused beam. Cobalt values were corrected for the interference of the Fe-Kb peak with the Co-Ka peak. It is possible that the enstatite FeO values in Table 1 are too high because of electron-beam overlap on tiny metal blebs included within the enstatite grains or because of excitation of Fe atoms in neighboring kamacite grains by bremsstrahlung or doubly backscattered electrons (Wasson et al., 1994). The modal abundance of silica was estimated in some samples by point counting on the electron microprobe (n = 50), moving the stage a fixed distance, and identifying the phase under the crosshairs by EDS. We used instrumental neutron activation analysis (INAA) to determine the bulk compositions of adjacent chips of Y-793225 (290.2 mg), A-882039 (273.8 mg), Y980223 (299.9 mg) and EH4 Indarch (237.9 mg). Meteorites belonging to other chondrite groups (CM, CK, CR and R) were included in the runs, affording more control of the quality of the data set. Samples were generally analyzed as 3-mm-thick rectangular prisms. The sawn surfaces were cleaned with SiC paper; rusty patches on other surfaces were flaked off with stainless-steel dental tools. The samples have experienced only mild terrestrial weathering that should have minor to negligible effects on their bulk compositions: A-882039 is listed as weathering category A (Meteorite Newsletter 15), Y-980223 is listed as category A/B (Meteorite Newsletter 16), and although no weathering category is given for Y-793225, our examination of a thin section shows it to be weathering group W2 on the scale developed by Wlotzka (1993). Samples were irradiated for 3 h at the TRIGA Mark I reactor of the University of California, Irvine with a rotating-tray (lazy-susan) flux of 1.8  1012 neutrons cm2 s1 for the determination of isotopes with half-lives of several hours and more. For counting, the samples were mounted on cardboard slides. For standards we used the Allende meteorite (Jarosewich et al., 1987), the USGS international reference materials GSP-1 and BHVO-1 (Govindaraju, 1994), and the North Chile (Filomena) IIAB iron meteorite. Analyses were carried out following a protocol similar to that described in Kallemeyn et al. (1989) and Choe et al. (2010).

Table 1 Mean compositions (wt%) of silicate phases. Enstatite

SiO2 TiO2 Al2O3 Cr2O3 FeO MnO MgO CaO Na2O K2O Total Fs Wo Ab Or

Y-793225 1 7

Y-793225 2 14

Y-8404 1 1

Y-8404 2 11

Y-980211 1 3

Y-980223 1 2

A-882039 1 22

Y-980524 1 30

Y-793225 1 4

Y-793225 2 17

Y-8404 2 10

A-882039 1 4

Y-980524 1 8

Y-8404 1 2

59.2 <0.04 0.23 <0.04 0.06 <0.04 39.2 0.72 <0.04 <0.04 99.4 0.08 1.3

60.0 nd 0.09 nd <0.04 <0.04 39.2 0.49 0.04 <0.04 99.8 <0.04 0.89

59.0 0.08 0.30 <0.04 0.82 <0.04 38.8 0.27 0.04 <0.04 99.3 1.2 0.49

60.5 nd 0.08 nd 0.16 <0.04 39.7 0.19 0.05 <0.04 100.7 0.22 0.34

59.2 <0.04 0.10 <0.04 0.57 <0.04 40.0 0.27 <0.04 <0.04 100.1 0.79 0.48

59.7 <0.04 0.09 <0.04 0.55 <0.04 40.0 0.28 <0.04 <0.04 100.6 0.76 0.50

60.0 <0.04 0.22 <0.04 0.22 <0.04 39.7 0.79 <0.04 <0.04 100.9 0.31 1.4

59.3 <0.04 0.20 <0.04 0.32 <0.04 39.2 0.82 <0.04 <0.04 99.8 0.32 1.5

65.3 <0.04 21.0 <0.04 0.26 <0.04 0.05 2.8 9.3 0.90 99.6

66.0 nd 20.9 nd <0.04 <0.04 0.04 2.3 9.5 0.79 99.5

70.2 nd 18.6 nd <0.04 <0.04 <0.04 <0.04 11.1 1.1 101.0

66.5 <0.04 21.7 <0.04 0.11 <0.04 <0.04 3.2 8.9 0.80 101.2

66.8 <0.04 21.4 <0.04 0.21 <0.04 <0.04 3.0 9.3 0.77 100.5

98.6 <0.04 0.77 <0.04 0.16 <0.04 <0.04 <0.04 0.36 0.11 100.0

81.3 5.2

84.1 4.6

93.9 6.1

79.5 4.7

80.9 4.4

Collisional heating of enstatite chondrites

Source No. of grains

Silicaa

Plagioclase

Source: 1 = this study; 2 = Lin and Kimura (1998). nd = not determined. a Analysis normalized to 100 wt%.

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3. RESULTS 3.1. Petrography Detailed petrographic descriptions of Y-8404, Y-980211, Y-980223, Y-980524, Y-793225 and A-882039 are in Appendix B; corresponding images are in Figs. 1–8. A summary of the main petrographic features of these meteorites appears below. All of the rocks have major orthoenstatite; in all samples except Y-793225, euhedral enstatite grains are abundant (Figs. 1, 3, 4 and 6). Plagioclase is present in all of the samples and abundant silica occurs in Y-8404 and Y-980223. All of the rocks contain kamacite and troilite; other opaque phases present in several samples include schreibersite, graphite and daubre´elite (e.g., Fig. 7). Keilite occurs in Y8404, Y-980211 and Y-980223; ferroan alabandite is present in Y-793225 and A-880239. Enstatite in each of these meteorites has sharp to moderately undulose extinction, corresponding to shock stages S1–S2 (Sto¨ffler et al., 1991; Rubin et al., 1997).

Fig. 1. Euhedral enstatite grains displaying prominent cleavage in Y-8404. (a) Black areas are opaque grains. Plane-polarized transmitted light. (b) The same region as in (a). White areas are kamacite; medium-gray areas are troilite. Reflected light.

In all of the samples, enstatite is intergrown with other silicates. In Y-980211 and Y-980223, an opaque groundmass surrounds the silicate assemblages (Fig. 3). Y-793225 has a hypidiomorphic-granular texture (Fig. 5). Relict chondrules (e.g., Figs. 2, 4 and 8) are present in all of the rocks except Y-793225. In some cases, enstatite grains appear to have nucleated on the relict chondrules. 3.2. Mineral chemistry In order to determine the proper classification of the meteorites in this study, it is necessary to compare their mineral chemistry to the established ranges for enstatite chondrites. EH3, EH4 and EH5 chondrites typically contain enstatite with 0.13–0.20 wt% MnO (e.g., Keil, 1968; Grossman et al., 1985); the MnO content of enstatite in each of the chondrites studied here is below the detection limit, i.e., <0.04 wt% (Table 1). Enstatite with very low

Fig. 2. Relict radial pyroxene chondrule in Y-8404. The chondrule does not form a sharp boundary with surrounding host. (a) The chondrule contains radiating bars of enstatite (light gray) and intercalated troilite (black). Plane-polarized transmitted light. (b) The abundant troilite (light gray) is visible between the enstatite bars (medium gray). Reflected light.

Collisional heating of enstatite chondrites

Fig. 3. Kamacite–troilite nodule with euhedral enstatite grains in Y-980211. The troilite-rich side of the nodule at right grades into the groundmass. kam = kamacite; troi = troilite. Reflected light.

MnO also occurs in EL6 chondrites (Keil, 1968) and in the Abee EH impact-melt breccia (Rubin and Keil, 1983; Rubin and Scott, 1997; Rubin, 2008). Plagioclase in Y-793225 (Ab81 Or5; Table 1) is less sodic and much more potassic than that in EH4 Indarch

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(Ab97.6 Or0.8; Keil, 1968). The published plagioclase composition of Y-8404 (Ab94 Or6) (Lin and Kimura, 1998; Table 1) appears to be non-stoichiometric; it is moderately enriched in SiO2 and Na2O and moderately depleted in Al2O3. Plagioclase in A-882039 (Ab79.5 Or4.7) and Y-980524 (Ab80.9 Or4.4) (Table 1) are within the range of typical EL6 chondrites (Ab79.4–82.9 Or2.9–4.6; Keil, 1968) (although A-880239 plagioclase is slightly more potassic than the extreme value reported by Keil, 1968). Silica in Y-8404 contains 1.5 wt% minor oxides (Table 1), consistent with the phase having an open crystal structure (i.e., cristobalite and/or tridymite) rather than quartz. (Quartz occurs in several EH3, EH4, EH5, EL3 and EL6 chondrites as well as in the Y-82189 EH impact-melt rock (Mason, 1966; Kimura et al., 2005).) Whereas most EH3–5 chondrites have troilite with 0.05–0.11 wt% Mn (e.g., Keil, 1968), Y-8404, Y-980211 and Y-980223 contain troilite with much higher Mn (i.e., 0.19–0.30 wt%; Table 2). In contrast, the low Mn contents of troilite in Y-793225 (<0.04–0.09 wt%; Table 2) and in A-882039 and Y-980524 (0.09 and 0.07 wt%, respectively; Table 2) are the same as that of troilite in average EL6 chondrites (Keil, 1968). Niningerite [(Mg>0.5,Fe<0.5)S] in most EH3–5 chondrites contains 6.5–11.8 wt% Mn (e.g., Keil, 1968), but keilite

Fig. 4. Y-980223. (a) Euhedral enstatite grains (medium gray) surrounded by abundant troilite (light gray) and small amounts of kamacite (white). Reflected light. (b) Relict radial pyroxene chondrule. Plane-polarized transmitted light. (c) Same chondrule (chd) as in (b). The chondrule is surrounded by abundant troilite (light gray). There is no sharp boundary between the chondrule and the host. Kamacite (white) occurs at upper left and lower right. Reflected light. en = enstatite; troi = troilite; kam = kamacite.

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Because the Fe–Ni phase diagram (Yang et al., 1996) indicates that the maximum Ni content of kamacite is 7 wt%, it is plausible that the metal phase in some of the chondrites in this study that contains 7.7–7.9 wt% Ni is martensite. This phase occurs in the Adhi Kot EH impact-melt breccia (Rubin, 1983a), the Blithfield brecciated EL impact-melt rock (Rubin, 1984) and the Hvittis EL6 breccia (Rubin, 1983b). However, the effect of 5 at.% Si on the phase diagram is not known. The mean Si and Ni contents of kamacite in A-882039 (1.0 and 6.1 wt%, respectively; Table 3) and Y-980524 (1.1 and 6.2 wt%, respectively; Table 3) are similar to those in average EL6 chondrites (1.3 ± 0.2 and 6.4 ± 0.3 wt%, respectively; Keil, 1968). 3.3. Bulk chemical composition

Fig. 5. Texture of EH6-an Y-793225. (a) Abundant enstatite grains form a hypidiomorphic-granular texture. Grains display prominent cleavage. Crossed nicols. (b) Different region than in (a). The rock has a high silicate/opaque modal abundance ratio. Opaques include kamacite (white) at bottom left and top left, troilite (medium gray) and terrestrial weathering products (light gray). Reflected light. kam = kamacite; sil = silicate; troi = troilite.

[(Fe>0.5,Mg<0.5)S] in Y-8404, Y-980211 and Y-980223 has significantly lower Mn (3.8–4.1 wt%; Table 2). A-882039 contains ferroan alabandite; no niningerite or keilite were observed in this rock. Troilite and daubre´elite were the only sulfides found in Y-980524. In contrast to the composition of daubre´elite in most EH3–EH5 chondrites, daubre´elite in Y-793225 contains more Mn (1.9 vs. 0.73–1.0 wt%) and much less Zn (<0.04 vs. 4.3–5.2 wt%) (Table 2; Keil, 1968). It is more similar in composition to that in EL6 chondrites (2.5 wt% Mn and 0.12 wt% Zn; Keil, 1968). Daubre´elite in A-882039 contains less Mn and less Zn (1.7 and 0.05 wt%, respectively; Table 2) than average EL6 chondrites. Daubre´elite in Y-980524 contains 2.8 wt% Mn (Table 2), similar to that in EL6 chondrites. Kamacite in Y-793225 is richer in Si (3.3 ± 0.1 wt%; 3.1–3.4 wt%; n = 18) and poorer in Ni (5.5 ± 0.4 wt%; 4.8–6.7 wt%) than kamacite in Y-8404, Y-980211 and Y-980223 (2.6 wt% Si and 7.7–7.9 wt% Ni; data from this study; Table 3). Kamacite in the latter three rocks is much richer in Si than kamacite in average EL6 chondrites (1.3 ± 0.2 wt%; 1.1–1.7 wt%; n = 8; Keil, 1968).

Duplicate instrumental neutron activation analyses (INAA) were completed on A-882039, Y-793225 and Y980223 (Table 4). In one irradiation run we also included a sample of EH4 Indarch as a control. The data are normalized in Fig. 9a and b to Cr and to mean EH chondrites (Sears et al., 1982; Kallemeyn and Wasson, 1986) and plotted as abundance ratios. The mean composition of EL chondrites is shown as a curve with the points connected. The elements are grouped in the traditional way for more-oxidized chondrites; “lithophiles” are plotted in Fig. 9a, siderophiles and chalcophiles are plotted in Fig. 9b. However, the traditional meaning of chalcophiles as elements that largely partition into FeS is less useful here than in the more-oxidized chondrite groups because of the presence of additional sulfide minerals. For example, the “lithophile” rare earth elements partition strongly into oldhamite (CaS) rather than into oxide minerals (Larimer and Ganapathy, 1987; Lundberg and Crozaz, 1988; Floss and Crozaz, 1993); in addition, “lithophile” Mn is a major constituent of niningerite, keilite and ferroan alabandite, and Cr a major constituent of daubre´elite. The principal indicator elements that are useful for distinguishing between enstatite chondrite groups are the siderophiles and volatiles; both sets of elements are appreciably higher in EH than EL chondrites. For example, the volatile element Zn is 3–30 times higher in EH than EL (Fig. 9b). Based on mean data given by Kallemeyn and Wasson (1986), elements that are particularly good discriminators between EH and EL chondrites include Zn (mean values of 250 and 17 lg/g in EH and EL, respectively), Ga (16 and 11 lg/g), As (3.45 and 2.20 lg/g), Se (25.5 and 13.5 lg/g), Sb (196 and 90 ng/g) and Au (330 and 225 ng/g). Analyses of the three chondrites from the first run are plotted with filled symbols; those from the second run are plotted with open symbols. To simplify their resolution on the diagrams, the second analyses are plotted slightly to the right of the first analyses. The duplicate analyses of the siderophiles and chalcophiles (Fig. 9b) are in good agreement; most agree to within 10%. The largest differences between duplicates are found for Sb in A-882039: the analyses differ by a factor of 1.8.

Collisional heating of enstatite chondrites

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Fig. 6. Euhedral enstatite grains in A-882039. (a) Several euhedral enstatite grains (medium gray) occur adjacent to troilite (light gray) at the side of a large kamacite grain (white). (b) Euhedral enstatite grain and a cluster of more-equant enstatite grains (dark gray) in a sulfide assemblage consisting of troilite (light gray), ferroan alabandite (medium gray) and tabular daubre´elite (smooth texture, very light gray; top of assemblage). (c) Euhedral enstatite grains (medium gray) within kamacite (white) and adjacent to massive schreibersite (light gray; distinctly darker than kamacite). Also present within the metal are several intergrowths of graphite (dark gray). (d) Massive silicate assemblage (medium gray) protruding into kamacite grain (white) that contains graphite clusters and tabs (dark gray). en = enstatite; daub = daubre´elite; troi = troilite; sil = silicate; alab = ferroan alabandite; kam = kamacite; sch = schreibersite; gr = graphite. All images in reflected light.

In Fig. 9b the patterns for EH4 Indarch and EH6 Y980223 are quite similar, implying that the EH group is largely isochemical. All data plot within 10% of the EH line with the exception of Zn, for which abundance ratios are 1.5–2 times higher than mean EH. Kallemeyn and Wasson (1986) also reported that Indarch had the highest Zn value among the EH falls that they studied, and was 1.4 times higher than their reported EH mean. The Indarch fall is one of the best-preserved EH4 chondrites, one that has largely avoided impact alteration. At the other extreme is the pattern for Y-793225. All of the elements plot low; nevertheless, the replicates agree to within a factor of 1.2 with the exception of Zn (where the values vary by a factor of 1.5). It is possible that the systematically low abundance ratios result from Cr values that are unrepresentatively high. (The data are normalized to Cr.) In fact, the Cr concentrations of 4.4 mg/g in Y793225 are 33% higher than in any of the Kallemeyn and Wasson (1986) data for EH or EL chondrites. On the other hand, there is no petrographic evidence that either of the two values (which are essentially identical) is unrepresenta-

tive for Y-793225. One test is provided by the lithophile plot (Fig. 9a) in which some well-determined elements (Sc, Sm) plot near the EH line. As discussed in this study, the bulk composition of Y-793225 seems to have been affected by impact processes, and it is possible that our INAA samples were enriched in a high-Cr component, perhaps daubre´elite. (Lin and Kimura (1998) reported 1 vol% daubre´elite in Y-793225.) In Fig. 9b the pattern of A-882039 is more similar to the mean EL pattern than to the mean EH line. The agreement with mean EL chondrites for Sb and the eight more-refractory elements would be excellent if the Cr values were 1.2 times higher; in fact, the Cr concentrations are lower in A-882039 than those in all of the EH chondrites and all except two of the EL chondrites studied by Kallemeyn and Wasson (1986). The two more-volatile elements, Se and Zn, are slightly lower than EL, but are within the range observed for EL chondrites by Kallemeyn and Wasson (1986), even if one were to increase the Cr value by a factor of 1.2. The patterns appear less regular on the “lithophile” plot (Fig. 9a), but this is partly because the plotted range in

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In Fig. 9a the abundance ratios for A-882039 agree to within a factor of 1.2 for all elements. The pattern is reasonably close to the EL curve, and would be even closer if Cr were increased by a factor of 1.1. 4. DISCUSSION 4.1. Group classification of impact-melted enstatite chondrites

Fig. 7. Graphite books in A-882039. (a) Elongated graphite book (medium gray) occurring mainly inside silicate (dark gray) but with opposite termini touching troilite (light gray) and keilite (lightmedium gray). (b) Isolated graphite book (medium gray) completely enclosed within silicate (dark gray). gr = graphite; keil = keilite; sil = silicate; troi = troilite; kam = kamacite. Both images in reflected light.

abundance ratios is only a factor of 5, much smaller than the factor of 120 plotted in Fig. 9b. In fact, all the replicate ratios agree to within a factor of 1.25 with the exception of Eu in Y-793225 and Ca, La, Sm and Eu in Y-980223; in all the latter cases, the abundance ratio in the second replicate is much lower than in the first. Even though Eu is a moderately well-determined element by our INAA technique, the overall range in Eu for the analyzed enstatite chondrites is a factor of 4. The differences between replicates for individual meteorites are much smaller; the factors range from 1.3 to 1.7. The bulk composition of siderophiles and chalcophiles in Y-980223 is very similar to that of EH4 Indarch in Fig. 9b, but the two meteorites show very different patterns for “lithophiles” (Fig. 9a). The two replicates of Y-980223 show large differences that are almost certainly the result of sampling variations. Because of the scatter in the Y-980223 “lithophile” data (Fig. 9a), it is difficult to use these data to determine whether this meteorite is more closely related to EH or EL chondrites. In contrast, the siderophile and Zn data (Fig. 9b) indicate that Y-980223 is an EH chondrite.

The principal petrographic, mineralogical and bulk compositional distinctions between EH and EL chondrites include mean chondrule size (ca. 220 lm in EH vs. 550 lm in EL; Rubin, 2000), the Si content of kamacite (2.3–3.8 vs. 1.0–2.1 wt%, respectively; Keil, 1968), the identity of the cubic (Mg,Fe,Mn)S sulfide (niningerite or keilite in EH, ferroan alabandite in EL; Keil, 1968, 2007) and the bulk abundances of siderophile and chalcophile elements (high in EH, low in EL; Se and Zn are particularly good discriminators; e.g., Kallemeyn and Wasson, 1986). Because the chondrites studied here are recrystallized and evidently melted (Section 4.2), average chondrule size cannot be used as a distinguishing characteristic; most chondrules (particularly the small ones) have been recrystallized and rendered unrecognizable in these rocks. Y-8404, Y-980211 and Y-980223 are confirmed to be EH chondrites. Consistent with being members of the EH group, they contain P2.6 wt% Si in kamacite (Table 3); they also contain keilite and lack ferroan alabandite (Table 2). Y-980223 has relatively high abundances of Se and Zn (Table 4). (Bulk compositions were not determined for Y8404 or Y-980211.) These meteorites have all experienced high temperatures: they contain orthoenstatite to the exclusion of clinoenstatite; they have recrystallized textures and relict chondrules that are texturally partially integrated with the matrix. A-882039 and Y-980524 are reclassified as EL chondrites. They average 1.0 and 1.1 wt% Si in kamacite, respectively (Table 3). They both lack niningerite and keilite. A-882039 contains ferroan alabandite, but only troilite and daubre´elite were identified in Y-980524 (Table 2). The low bulk abundances of siderophiles, Se and Zn in A882039 (Table 4) are consistent with an EL classification. (A bulk composition was not determined for Y-980524.) A-882039 and Y-980524 exhibit high degrees of recrystallization and share a paucity of recognizable chondrules, consistent with annealing at high temperatures. We confirm the classification of Y-793225 as EH-an, although Lin and Kimura (1998) classified it as being intermediate between the EH and EL groups, mainly on the basis of its distinctive mineral chemistry. The property that is most consistent with an EH classification is the Si content of kamacite (3 wt%; Table 3) which is near the middle of the EH range (2.7–3.8 wt%) and is appreciably higher than that of EL chondrites (1.0–2.1 wt%) (Table 5 of Keil, 1968). This is an important classificatory parameter and one we find persuasive. Nevertheless, the low Zn and low siderophile contents of bulk Y-793225 are more characteristic of EL than EH chondrites, supporting the classification of the rock as anomalous.

Collisional heating of enstatite chondrites

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Fig. 8. Relict chondrules in A-882039. (a) Relict radial pyroxene chondrule fragment displaying radiating bars of enstatite. The border between the chondrule and the surrounding matrix is difficult to discern, but lies approximately at the ends of the elongated pyroxene laths. Transmitted light. (b) Same image as in (a) showing the radiating bars of enstatite (medium gray) intergrown with kamacite (white) and troilite (light gray). A thin fusion crust layer is visible at lower left at the interface with the glass slide. Reflected light. (c) Relict porphyritic pyroxene chondrule with quasi-equant grains of enstatite and plagioclase. The chondrule is bordered by opaque phases on all sides except bottom left; there is no sharp boundary between the chondrule and the surrounding host. Transmitted light. (d) Same image as in (c) showing the intergrowth of enstatite (medium gray) and plagioclase (dark gray). Adjacent to the chondrule are grains of kamacite (white) and troilite (light gray). Reflected light. en = enstatite; plag = plagioclase; troi = troilite; sil = silicate; kam = kamacite.

4.2. Impact-melting in the EH6 chondrites Impacts can produce extreme heterogeneities in the degree of shock damage of target rocks. Shock waves can become chaotic as they interact with inhomogeneities in the rock (e.g., voids, cracks and solid components of different densities). The initial (nanosecond-scale) peak pressure in the shock front and the resulting shock temperature can have grain-to-grain variations of an order of magnitude (Sharp and DeCarli, 2006). Because many EH and EL chondrites have been altered by impacts (e.g., Rubin and Scott, 1997; Rubin et al., 1997; Kimura and Lin, 1999; Burbine et al., 2000; Fagan et al., 2000; Patzer et al., 2004; Keil, 2007) and because impact processes can produce changes in bulk composition (e.g., Rubin et al., 2009) and mineralogy (e.g., Rubin, 1983b; Rubin and Scott, 1997) on a scale of 10 mm (e.g., Rubin, 1984; Rubin et al., 2009), the standard taxonomic parameters may yield different results for different samples. Caution must therefore be taken in weighting these parameters.

The three confirmed EH chondrites in this study (Y-8404, Y-980211 and Y-980223) are discussed here; EH-an Y-793225, EL A-882039 and EL Y-980524 are discussed in the following sections. Y-8404, Y-980211 and Y-980223 each exhibits some unusual mineralogical and textural characteristics that appear to have resulted from impact melting as discussed in Appendix A. Y-8404, Y-980211 and Y-980223 contain euhedral enstatite grains surrounded by metal ± sulfide, indicating growth of the enstatite from a mixed melt. Y-980211 contains a millimeter-size kamacite globule (Fig. 3), similar to some in Abee (Dawson et al., 1960; Rubin and Keil, 1983), that most likely formed during melting. All three contain enstatite with <0.04 wt% MnO and all average P0.19 wt% Mn in troilite, implying transfer of Mn from enstatite to troilite. All contain keilite with low Mn (i.e., with 3.8–4.1 wt% Mn) (Keil, 2007; this study). Y-8404 (and its paired specimens) average 13 wt% silica (Lin and Kimura, 1998); Y-980223 also contains abundant silica (10 wt%). In contrast, Y-980211 contains <1 wt%

4.3. Impact-melting origin of EH-an Y-793225

14.4 3.1 44.9 0.23 0.07 <0.04 <0.04 0.08 37.6 100.4 59.3 nd 0.19 nd nd nd 0.32 1.9 36.4 98.1

61.2 0.10 0.30 <0.04 0.08 <0.04 0.33 1.9 36.2 100.1

60.6 0.08 0.27 <0.04 <0.04 <0.04 0.36 1.9 36.2 99.4

62.3 <0.04 0.09 <0.04 0.08 <0.04 0.57 0.79 36.2 100.0

62.6 <0.04 0.07 <0.04 nd <0.04 0.41 0.85 35.9 99.8

36.9 11.5 3.8 1.6 0.22 0.94 0.08 1.8 41.7 98.5

39.4 11.4 3.8 1.9 <0.04 0.81 0.09 1.7 41.0 100.1

37.3 12.4 4.1 1.9 <0.04 0.89 0.07 1.8 41.9 100.4

silica, similar to that in most EH3–5 chondrites (Rubin, 2008). These characteristics imply that Y-8404, Y-980211 and Y-980223 have been impact melted. The high proportions of euhedral enstatite grains in these meteorites imply that melting was pervasive. Because they all contain rare relict chondrules, it is apparent that melting in these rocks was incomplete. We designate them as impact-melt breccias.

Source: 1 = this study; 2 = Lin and Kimura (1998); 3 = Keil (2007). nd = not determined.

61.0 <0.04 0.09 <0.04 <0.04 <0.04 1.1 0.92 36.4 99.5 Fe Mg Mn Ca Zn Na Ti Cr S Total

17.4 <0.04 1.9 <0.04 <0.04 <0.04 0.08 35.4 42.9 97.7

17.1 <0.04 1.9 nd <0.04 <0.04 0.09 35.3 43.3 97.7

18.3 <0.04 1.7 <0.04 0.05 <0.04 0.11 35.4 43.6 99.2

16.8 <0.04 2.8 <0.04 nd <0.04 0.05 35.5 42.2 97.4

59.9 nd <0.04 nd nd nd 1.1 0.94 36.5 98.4

Y-8404 Y-980211 3 1 27 1 Y-793225 1 3 Y-980524 1 7 A-882039 1 11 Y-793225 2 29 Y-793225 Source 1 No. of grains 3

Daubre´elite

Table 2 Mean compositions (wt%) of sulfide minerals.

Troilite

Y-793225 2 23

Y-8404 2 18

Y-980211 1 2

Y-980223 1 6

A-882039 1 25

Y-980524 1 15

Keilite

Y-980223 1 2

A.E. Rubin, J.T. Wasson / Geochimica et Cosmochimica Acta 75 (2011) 3757–3780

Ferroan alabandite A-882039 1 11

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Y-793225, which is considered anomalous mainly because of its low siderophile abundances, differs in texture and mineral chemistry from the other EH chondrites described in this study. Those samples are dominated by euhedral enstatite grains surrounded by metal ± sulfide, but this texture is very rare in Y-793225. Whereas the three EH chondrites contain Mn-rich troilite (averaging 0.19–0.30 wt%; Table 2), troilite in Y-793225 is poor in Mn (60.09 wt%). Y-793225 contains daubre´elite and ferroan alabandite (this study; Lin and Kimura, 1998), but does not contain keilite. The whole-rock silicate/opaque-phase ratio is higher in Y793225 than in Y-8404, Y-980211 and Y-980223. Y-793225 appears to have experienced high temperatures as reflected by the absence of chondrules, the (very rare) occurrence of euhedral enstatite grains surrounded by kamacite, and the low MnO content (<0.04 wt%) of enstatite. Although heating via the decay of short-lived radionuclides cannot be ruled out, heating of this rock can plausibly be attributed to impact. For the present discussion, we assume that Y-793225 was melted during an impact event. Y-793225 may have been more extensively melted and more slowly cooled than Y-8404, Y-980211 and Y980223. More extensive melting would have allowed some separation of an immiscible metal–sulfide melt from the silicate melt, accounting for the high silicate/opaque-phase modal abundance ratio and the low abundance of siderophiles (Fig. 9b). Daubre´elite exsolved from Cr-bearing troilite as temperatures dropped below 600–700 °C (El Goresy and Kullerud, 1969). Because chondrules are absent, we conclude that Y-793225 is an impact-melt rock. 4.4. Impact-melting origin of EL6 A-882039 and Y-980524 A-882039 and Y-980524 contain euhedral enstatite grains surrounded by metal and sulfide, enstatite with very low MnO (<0.04 wt%; Table 1), and euhedral blades of graphite. By analogy to petrographic and mineralogical properties of Abee (Appendix A) and the EH chondrites Y-8404, Y-980211 and Y-980223, it seems likely that A-882039 and Y-980524 underwent impact melting and crystallization. The high proportions of euhedral enstatite imply widespread melting. The presence of relict chondrules indicates that these meteorites are EL impact-melt breccias. 4.5. Origin of compositional fractionations Elements that differ significantly between the replicates of EH Y-980223 include Ca, La, Sm, Eu and Lu

Collisional heating of enstatite chondrites

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Table 3 Mean compositions (wt%) of kamacite and schreibersite. Kamacite

Schreibersite

Source No. of grains

Y-793225 1 18

Y-793225 2 75

Y-8404 1 1

Y-8404 2 30

Y-980211 1 13

Y-980223 1 15

A-882039 1 23

Y-980524 1 26

A-882039 1 4

P Si Fe Ni Cr Co Total

<0.04 3.3 90.1 5.5 <0.04 0.33 99.2

<0.04 3.0 91.1 5.5 nd 0.38 100.0

0.07 2.6 88.8 7.9 <0.04 0.35 99.7

0.18 2.8 89.2 7.6 nd 0.34 100.1

0.06 2.6 89.0 7.8 <0.04 0.33 99.8

0.06 2.6 88.2 7.7 <0.04 0.31 98.9

<0.04 1.0 92.2 6.1 <0.04 0.40 99.7

0.07 1.1 92.8 6.2 <0.04 0.37 100.5

14.5 0.05 56.9 27.6 <0.04 0.09 99.1

Source: 1 = this study; 2 = Lin and Kimura (1998). nd = not determined.

(Fig. 9a). All of these elements partition into oldhamite (CaS) (e.g., Lundberg and Crozaz, 1988; Floss and Crozaz, 1993). The fact that the most volatile “lithophile” elements (Na, K) have not been lost from Y-980223 indicates that Ca, Sm, Eu and Lu (which are more refractory than Na and K) were not lost through simple volatilization processes. We assume here that Y-980223 is indeed an impact-melt breccia. Given that assumption, we suggest that impact-induced physical fractionations of molten phases affected oldhamite to a greater degree than other phases such as troilite that have lower melting temperatures (e.g., Rubin et al., 2009); in other words, CaS-rich liquids were preferentially lost relative to FeS-rich liquids. Such CaS-rich liquids could have been concentrated elsewhere in veins and clasts. These might resemble the oldhamite-rich vein in EL6 Jajh deh Kot Lalu (6.7 wt% oldhamite; Rubin et al., 1997) and the oldhamite-rich clast in the Bustee aubrite (30 vol% oldhamite; Keil, 2010). Because oldhamite is water soluble (e.g., Merrill, 1915), it is important to consider whether loss of oldhamite due to terrestrial weathering or sample-preparation procedures involving water could be responsible for the compositional fractionations observed in Y-980223. As discussed in Rubin et al. (2009), EL6 falls exhibit significant REE fractionations, rendering terrestrial weathering as an unlikely process responsible for the fractionations in Y-980223. Furthermore, many Antarctic enstatite chondrites that were cut using water as a lubricant have less fractionated compositions than typical EL6 falls; this suggests that routine laboratory preparation procedures probably did not cause appreciable loss of REE and Ca from Y-980223. In Fig. 9b, Y-793225 exhibits a strongly fractionated pattern, but, with minor exceptions at Au and Se, the abundances decrease monotonically with increasing volatility. In contrast, the meteorite exhibits a much more complex fractionation pattern among “lithophiles” in Fig. 9a. We speculate that the fractionations result mainly from mechanical separations produced during impact melting. The low Zn, K and Na abundance ratios presumably reflect volatile loss; the undepleted abundances of Mn and Se (which are only slightly less volatile) suggest that these elements may have been in different phases that were not lost during impact melting and vaporization.

We conclude that the moderate fractionations of several elements exhibited by the enstatite chondrites in this study mainly reflect local differences associated with the impact production of melts and the mechanical fractionations of melts from solids. Such differences are expected to result from impact processing. 4.6. Absence of high-pressure minerals A possible objection to the impact-melting model for these enstatite chondrites is that impact-generated highpressure minerals have not been observed. Ringwoodite, wadsleyite, majorite, akimotoite, plagioclase with the hollandite structure, and other phases have been identified in some shocked OC (e.g., Binns et al., 1969; Binns, 1970; Smith and Mason, 1970; Price et al., 1983; Rubin and Read, 1984; Mori, 1994; Chen et al., 1996, 2003; Sharp et al., 1997; Tomioka and Fujino, 1997; Gillet et al., 2000) and majorite, wadsleyite and coesite were reported in the Gujba bencubbinite (Weisberg and Kimura, 2004; Weisberg et al., 2006). Such high-pressure phases are largely confined to shock melt veins or melt pockets and adjacent unmelted material (e.g., Sharp and DeCarli, 2006). However, in impact-melt rocks, any high-pressure minerals that may have formed probably experienced high-temperature annealing after pressure release and were transformed to their low-pressure polymorphs. We suggest that this is why high-pressure phases have not been reported in OC or enstatite–chondrite impact-melt rocks such as PAT 91501, Ilafegh 009 and Happy Canyon (McCoy et al., 1995; Mittlefehldt and Lindstrom, 2001). 4.7. Heat source for metamorphosing EH chondrites Recognition that the four chondrites listed in the MBDB as EH6 and EH6-an have been impact melted allows two alternative inferences about the dominant heat source of the EH parent asteroid: (1) Y-793225, Y-8404, Y-980211 and Y-980223 have been properly classified as EH6 and EH6-an chondrites (as they are currently listed in MBDB, 2011).

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Table 4 Duplicate neutron activation analyses of 23 elements in some enstatite chondrites. Sc (mg/g)

Cr (mg/g)

7.0 7.9 7.4

7.57 7.82 7.70

4.43 4.42 4.42

615 563 589

5.5 6.9 6.2

6.92 7.15 7.04

Mean

6.77 8.31 7.54

737 820 779

5.3 8.2 6.8

EH

6.76

662

7.50

As (lg/g)

Se (lg/g)

EH-an Mean

A-882039

EL

Y-980223

EH

Mean

Indarch

Y-793225

EH-an

A-882039

EL

644 597 620

5.48 5.21 5.35

Br (lg/g)

Mn (mg/g)

Fe (mg/g)

Co (lg/g)

Ni (mg/g)

Zn (lg/g)

Ga (lg/g)

3.67 1.93 2.80

234 229 232

646 644 645

12.6 12.4 12.5

7.10 5.00 6.05

6.9 7.5 7.2

2.65 2.73 2.69

1.64 0.83 1.23

256 284 270

774 919 846

16.0 17.0 16.5

8.40 10.6 9.50

11.4 12.7 12.0

6.32 7.68 7.00

2.89 3.24 3.07

2.90 1.82 2.36

318 301 310

890 842 866

19.8 18.6 19.2

381 513 447

14.5 14.2 14.4

5.80

3.06

2.16

311

874

17.7

342

14.7

Ru (ng/g)

Sb (ng/g)

La (ng/g)

Sm (ng/g)

Eu (ng/g)

Lu (ng/g)

Os (ng/g)

Ir (ng/g)

Au (ng/g)

15.6 14.7 15.2

4.79 3.97 4.38

840 537 689

70.0 58.5 63.8

284 313 298

196 235 216

51 69 60

39.8 38.0 38.9

747 536 642

516 481 499

211 216 214

2.69 3.43 3.06

13.0 12.4 12.7

5.81 8.51 7.16

840 860 850

72.0 136 104

212 201 206

133 153 143

68 88 78

19.3 22.5 20.9

738 859 799

615 716 666

269 317 293

Mean

3.94 3.63 3.78

28.9 29.4 29.2

5.56 5.89 5.72

1050 995 1023

174 201 188

136 90 113

42 28 35

11.5 12.7 12.1

625 551 588

510 502 506

354 328 341

EH

3.58

23.5

890

172

180

66

23

685

557

357

Mean

Indarch

5.54 6.23 5.89

Ca (mg/g)

2.06 2.48 2.27

Mean

Y-980223

K (mg/g)

EH

21.1

54.0 17.0 35.5 124

A.E. Rubin, J.T. Wasson / Geochimica et Cosmochimica Acta 75 (2011) 3757–3780

Y-793225

Na (mg/g)

Collisional heating of enstatite chondrites

Because these samples have been impact melted, we can infer that the mechanism mainly responsible for EH-chondrite metamorphism is collisional heating. (2) These meteorites have been misclassified; they are impact-melted rocks and are not type-6 chondrites. In this case, “real” EH6 chondrites are unknown and the source of heating of “normal” EH-chondrites is unidentified. One way to choose between these alternatives is to examine other metamorphosed EH chondrites, i.e., those listed in the MBDB as EH4, EH4/5, EH5 and EH7. If many

a

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of them appear to have been impact melted, then impact heating is probably the dominant mechanism responsible for EH-chondrite metamorphism. If few show evidence of shock heating, then other heat sources (e.g., the decay of short-lived radionuclides) may be more likely. At the time of this writing, the MBDB lists 24 EH4 and EH4/5 chondrites including two probable pairing groups: {EET 96135, 96202, 96217, 96223, 96299, 96309, 96341} and {MET 00636 and 00783}. In addition, Kimura and Lin (1999) thought it likely that Y-791810 and Y-791811 are paired due to their similarities in texture, mineral modal abundance and noble-gas composition (Patzer and Schultz,

2.0

abundance ratio (EH, Cr norm)

1.5

1.2 1.0

0.8 0.7 0.6 0.5

Y 793225 A 882039 Y 980223 Indarch EL

0.4 Sc

Ca

La

Sm

Eu

Yb

Lu

Cr

Ni

Co

Fe

Au

As

Ga

Mn

Na

K

Sb

Se

Zn

b 2.0 1.5

abundance ratio (EH, Cr norm)

1.0 0.8 0.5 0.3 0.2

0.1

0.05 0.03 0.02

Y 793225 A 882039 Y 980223 Indarch EL S i 10

Os

Ir

Fig. 9. Elemental abundance ratios of the enstatite chondrites in this study; duplicate analyses are plotted. Data are normalized to mean EH chondrites and to Cr and are arranged from left to right in order of increasing volatility (except for REE which are listed in order of increasing atomic number). For most elements in each meteorite, the duplicate analyses are in good agreement. EH4 Indarch, which was included in one analytical run, plots near mean EH chondrites. Mean EL chondrites are shown for comparison. (a) “Lithophile” elements. There is a fair amount of spread among replicates, particularly for Ca, Sm, Eu and Lu (elements that partition into oldhamite). (b) Siderophile and chalcophile elements. Both A-88039 and Y-793225 have very low Zn abundances.

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1998). Little published information is available for Y791812, but its EH4 classification and sequential catalog number render it plausible that it is paired with Y-791810 and Y-791811. For the purposes of the present manuscript we assume that Y-791812 is indeed paired with Y-791810 and Y-791811, but our arguments are not dependent on this assumption. We conclude that, at present, there are 15 independent EH4 and EH4/5 chondrites in our collections. At least four of these rocks (Abee, Adhi Kot, ALH 82132, Y-791811) have been described as impact-melt breccias (Rubin, 1983a, 1997a; Rubin and Keil, 1983; Rubin and Scott, 1997; Keil, 2007). Keil (2007) reported keilite in Abee, Adhi Kot and Y-791811. According to Keil (2007), “keilite occurs only in enstatite chondrite impact-melt rocks and impactmelt breccias;” it mainly formed from niningerite and troilite. In addition, euhedral graphite blades occur in Abee, Adhi Kot and Y-791810 (Rubin, 1983a, 1997a; Rubin and Keil, 1983). ALH 82132 is also an impact-melt breccia; it resembles Abee in containing euhedral enstatite grains enclosed within kamacite globules (Rubin and Scott, 1997). The wide-spread occurrence of euhedral enstatite and/or graphite in these rocks indicates that melting was extensive. The MBDB lists six EH5 chondrites: A-881475, LEW 88180, QUE 93372, RKP A80259, St. Mark’s and SaintSauveur. At least three of these are probably impact-melt breccias: LEW 88180 contains keilite (Keil, 2007), RKP A80259 displays an igneous texture and contains impactmelted feldspar and keilite (Zhang et al., 1995; Fagan et al., 2000; Keil, 2007), and Saint-Sauveur contains keilite, low-MnO enstatite, euhedral enstatite grains surrounded by metallic Fe–Ni, cristobalite, and subhedral grains of fluorrichterite (Keil, 1968, 2007; Rubin, 1983a, 2008; Kimura et al., 2005). One meteorite, QUE 94204 (2.43 kg), is listed in the MDB as EH7. This is a chondrule-free rock that contains abundant euhedral enstatite grains with <0.03 wt% MnO and exhibits an igneous texture (Weisberg et al., 1997). Its paired specimen, QUE 99059 (22.1 g), contains abundant silica, although QUE 94204 and the other paired specimens (QUE 97348 (50.7 g) and QUE 97289 (51.9 g)) apparently do not (McCoy and Welzenbach, 2001). QUE 94204 was classified as an EH impact-melt rock by Rubin et al. (1997) and Burbine et al. (2000); apparent differences in the abundance of silica among paired specimens may reflect local heterogeneities commonly seen among impact products (e.g., Rubin, 1983b, 1997b; Rubin and Scott, 1997). Although QUE 94204 contains niningerite instead of keilite, Keil (2007) suggested that the rock was impact melted, buried and annealed; any keilite that crystallized from the melt could have transformed back into niningerite during cooling. In summary, at least four of 15 EH4 and EH4/5 chondrites, at least three of six EH5 chondrites, all four EH6 and EH6-an chondrites, and the sole EH7 chondrite currently listed in the MDB appear to have been impact melted. Thus, the fraction of impact-melt rocks and impact-melt breccias among officially classified EH5–7 chondrites (to one significant figure) is P0.7; it is P0.3 among EH4 and EH4/5 chondrites. These results suggest that impact heating is primarily responsible for EH-chondrite metamorphism.

The less-metamorphosed EH materials tend to show less evidence of impact processes. The occurrence of excess 53Cr (formed by the decay of 53Mn; t½ = 3.7 Ma) in enstatite chondrites is restricted to EL3, EH3 and EH4 rocks (Birck and Alle`gre, 1988; El Goresy et al., 1992; Guan et al., 2007) that have not experienced impact melting. Pre-existing isotopic anomalies would have been obliterated in the enstatite–chondrite impact-melt breccias and impact-melt rocks. There is an alternative to the acceptance of collisional heating as the dominant agent of EH-chondrite metamorphism. Although we have concluded that most of the chondrites listed in the MBDB as EH5–7 were probably heated mainly by collisions, it is possible that a few of these rocks (e.g., those that show few shock-related features) were never appreciably shocked and were instead metamorphosed by a different heat source, e.g., by the decay of short-lived radionuclides. The chief problem with this alternative is Occam’s razor; it seems unnecessary to invoke more than one principal heat source for these rocks. Shocked chondrites and unshocked chondrites can potentially be derived from adjacent regions of the parent body in the proximity of an impact crater but still record similar amounts of post-impact annealing. 4.8. Heat source for metamorphosing EL chondrites Our conclusion that the chondrites listed in the MBDB as EH6 were heated mainly by collisions requires us to consider whether EL6 chondrites experienced a similar history. One way to approach this problem is to inventory equilibrated EL chondrites and determine if many of them are impact-melt breccias or impact-melt rocks. At the time of this writing, the MBDB lists nine EL4 and EL4/5 chondrites (DaG 734, DaG 1031, FRO 03005, Grein 002, HaH 317, MAC 02747, PCA 01006, QUE 94368, SaU 188). Not all of these rocks have been studied thoroughly, but at least three appear to be impact-melt breccias. QUE 94368 was described as an impact-melt breccia that contains abundant euhedral enstatite grains protruding into kamacite, euhedral graphite grains surrounded by metal and silicate, and euhedral grains of sinoite (Si2N2O) that probably crystallized from the impact melt (Rubin, 1997b; Bischoff et al., 2005a,b). Grein 002, which was partly impact melted, contains euhedral grains of enstatite, graphite and sinoite (Patzer et al., 2004). Many of the euhedral enstatite grains are surrounded by metal clasts or nodules (Patzer et al., 2004; Rubin, 2006). MAC 02747 contains euhedral grains of enstatite and graphite and is also probably an impact-melt breccia (Rubin, 2006); the meteorite also contains several 2–3-mm-long metal shock veins (Rubin et al., 2009). PCA 01006 is very small (0.75 g) and has not been adequately characterized. The MBDB lists five EL5 chondrites (Adrar Bous, NWA 1222, NWA 1810, Tnz 031, TIL 91714). Detailed descriptions are generally not available, but Bischoff et al. (2005a) reported sinoite in TIL 91714, suggesting that this rock might possibly have experienced impact melting (Rubin, 1997b). After taking probable pairings into account, we find that there are 48 distinct EL6 chondrites currently listed in the

Collisional heating of enstatite chondrites

MBDB, some of which have not been described in detail. If we omit the seven poorly described unpaired samples with masses <25 g, then 41 EL6 chondrites remain. Nevertheless, several larger EL6 chondrites have also been inadequately characterized. Rubin (2006) summarized the impact-related features of several EL6 chondrites. Blithfield is a chondrule-free brecciated impact-melt rock with large sulfide-rich, kamacitepoor clasts, a metal-rich matrix, and centimeter-size metal veins (Easton, 1983; Rubin, 1984). Hvittis is a fragmental breccia containing 5 vol% impact-melt-rock clasts (Rubin, 1983b). Atlanta contains a sulfide-rich, kamacite-poor clast and centimeter-long metal veins (Rubin, 1983c). Eagle is a breccia containing centimeter-size black inclusions (probably formed by the shock melting and dispersion of metal and sulfide) (Olsen et al., 1988; Rubin et al., 1997). NWA 2213 has euhedral grains of enstatite and graphite, kamacite grains with small troilite blebs, and troilite grains with small kamacite blebs. Forrest 033, GRO 95626 and QUE 97462 contain euhedral grains of enstatite and graphite surrounded by kamacite and sulfide. Other EL6 chondrites also contain abundant euhedral enstatite grains. It is plausible that sinoite in the EL6 chondrites crystallized from impact melts as it is inferred to have done in EL4 QUE 94368 and Grein 002 (Rubin, 1997b; Patzer et al., 2004; Bischoff et al., 2005a,b). Euhedral grains of sinoite occur in ALH A81021, EET 90102, Forrest 033, GRO 95626, Hvittis, Jajh deh Kot Lalu, LEW 88714, LON 94100, Neuschwanstein, Pillistfer, Ufana and Yilmia (Bischoff et al., 2005a,b; Rubin, 2006). The EL6 sinoite grains are similar in morphology to those in the impact-melted regions of the EL4 chondrites QUE 94368 and Grein 002 (Rubin, 1997b; Patzer et al., 2004), suggesting that EL6 sinoite is not granoblastic (i.e., produced metamorphically), but instead crystallized from a melt. The melt associated with the EL6 chondrites was probably recrystallized during the same annealing event that healed the silicate lattices and thereby lowered the apparent shock stages of these rocks (e.g., Rubin et al., 1997; Rubin, 2006). There are additional potential impact-melt breccias among EL6 chondrites. MET 00951 is described in the MBDB as having experienced shock heating; it contains numerous quenched metal–sulfide intergrowths. The MBDB lists NWA 002 as being “partly melted,” but no details are given. It may be paired with NWA 2965, NWA 2736 and NWA 2828 (see below). NWA 4650 is listed in the MBDB as containing millimeter-size, metal-rich recrystallized veins; it seems likely that these are shock veins. Rubin et al. (1997) reported opaque-rich veins and clasts in other EL6 chondrites (i.e., Atlanta, Blithfield, Eagle, Hvittis, Jajh deh Kot Lalu and Khairpur) that are of probable impact origin. Finally, the present study has identified A-882039 and Y-980524 as probably having experienced impact melting. In summary, at least 23 of the 41 individual EL6 chondrites (i.e., 60%, to one significant figure) show some evidence of being impact-melt breccias or impact-melt rocks. The MBDB lists two EL6/7 chondrites (Happy Canyon, NWA 2965) and one EL7 chondrite (Ilafegh 009). McCoy et al. (1995) described Happy Canyon as an impact-melt rock; the kamacite Si content of 0.80 wt% Si indicates EL

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parentage. The rock contains a coarse-grained igneous lithology, a metamorphosed diopside-bearing fine-grained lithology (in which one 12-lm-size grain of osbornite (TiN) was identified), and enstatite megacrysts up to 5 mm in size (Olsen et al., 1977; McCoy et al., 1995). The origin of NWA 2965 is unclear and additional studies are needed. The paucity of opaque phases may reflect severe terrestrial weathering (Kuehner et al., 2006; Irving et al., 2010). Ilafegh 009 is a chondrule-free EL impact-melt rock with a hypidiomorphic texture (Bischoff et al., 1992; McCoy et al., 1995). The occurrence of ferroan alabandite and kamacite with 1.0 wt% Si indicates an EL-group affinity. The rock also contains K-rich inclusions, silica, sinoite and rare 10–20-lm-size grains of osbornite (Bischoff et al., 1992, 2005a; McCoy et al., 1995). Another impact-melt rock is Zakłodzie (e.g., Burbine et al., 2000), which contains sinoite inclusions within keilite (Karwowski et al., 2007), polysynthetically twinned clinoenstatite grains, and kamacite with a mean Si content of 1.6 wt% (Pryzlibski et al., 2005). The kamacite composition is within the overall EL range (1.0–2.1 wt%) and is similar to that of mean kamacite in EL Blithfield (1.6 wt% Si) and EL6 Ufana (1.7 wt% Si) (Keil, 1968). Because of the large fraction of type 4–7 EL chondrites that appear to have been heated by impacts (and the existence of several EL impact-melt rocks), it seems reasonable to conclude that collisions constituted a major mechanism for heating EL chondrites. The EL4–6 chondrites may have formed from EL3 chondrites by variable degrees of impact heating, slow cooling and annealing beneath insulating regolith material. If this model is correct, then the apparent absence of shock features in some EL4–6 chondrites could be due to annealing having erased shock effects in pyroxene, producing grains with sharp optical extinction. Subsequent impacts caused undulose extinction to develop in pyroxene, changing the shock-classifications of many of these rocks from stages S1 to S2 (e.g., Rubin, 2006). The EL materials that were heated to the greatest degree by the impact events were largely to completely melted and formed impact-melt rocks. The conclusion that EL chondrites were heated mainly by impacts is consistent with the rapid cooling rates down to blocking temperatures of 1000–1200 °C determined for the EL6 chondrites Pillistfer and Yilmia based on their sphalerite compositions: 0.8 °C/day and 3  104 °C/day, respectively (Kissin, 1989). About 60% of EL6 chondrites have similar 21Ne CRE ages (27 ± 6 Ma) (Crabb and Anders, 1981; Patzer and Schultz, 2001), suggesting that these rocks were in the same location on their parent asteroid and experienced similar shock and thermal histories. 5. CONCLUSIONS We examined the four officially classified EH6 and EH6an chondrites (and two EL chondrites that were initially misclassified as EH6) and found that each meteorite exhibits evidence of impact melting. The principal petrographic features associated with the formation of impact melts in

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EH and EL chondrites include euhedral enstatite grains, nucleation of enstatite on relict chondrules, low-MnO enstatite, high-Mn troilite, high-Mn oldhamite, keilite, relatively abundant silica, euhedral graphite, euhedral sinoite, fluor-richterite and fluorphlogopite. Y-8404 (EH), Y-980211 (EH), Y-980223 (EH), Y-980524 (EL) and A-882039 (EL) contain rare relict chondrules and are impact-melt breccias; Y-793225 (EH-an) is a chondrule-free impact-melt rock. The conclusion that these chondrites have an impact-melt origin is based on their unusual textural and mineralogical characteristics including the occurrence of euhedral enstatite grains with low MnO that crystallized from the melt. The high abundance of these grains indicates that melting was extensive. These EH chondrites also contain high-Mn troilite (due to partitioning of Mn into sulfide during crystallization) and keilite [(Fe>0.5,Mg<0.5)S], a sulfide previously interpreted to be a product of impact melting. Although silica is rare (<1 wt%) in most enstatite chondrites, it is abundant in Y-8404 (13 wt%) and Y-980223 (10 wt%). Silica constitutes up to 16 wt% of some centimeter-size clasts in the Abee EH impact-melt breccia. The origin of these wide variations is not clear; we suggest that impact-mobilization of Si-bearing phases and the heterogeneous production of melt are the probable causes. In the Adhi Kot EH impact-melt breccia there are 3–5-mm-size chondrule-free clasts that contain up to twice as much silica (18–28 wt%) as enstatite (12–14 wt%). Silica may have crystallized from an impact melt after oxidation of reduced Si. Literature data show that many metamorphosed EH chondrites have been impact melted: at least 70% of EH5–7 chondrites and at least 30% of EH4 and EH4/5 chondrites possess textural and mineralogical properties attributable to impact melting. It thus seems likely that collisional heating is the principal mechanism responsible for EH-chondrite metamorphism. The proportion of impact-melt breccias and impact-melt rocks among EL6–7 chondrites is 60%. This suggests that collisional heating is also mainly responsible for EL-chondrite metamorphism. Shock and post-

shock annealing are ubiquitous natural processes that occurred on all asteroids and affected all chondrites to a greater or lesser degree. ACKNOWLEDGMENTS We thank the curators at the National Institute of Polar Research for the provision of samples for INAA and the loan of thin sections. We are grateful to E.R.D. Scott, P.S. De Carli, M. Kimura, A. Ruzicka, A. El Goresy and an anonymous referee for reviews on earlier versions of this manuscript. We also thank S.S. Russell for her handling of the paper. This work was supported by NASA Cosmochemistry Grants, mainly NNG06GF95G (A.E. Rubin) with additional support by NNG06GG35G (J.T. Wasson).

APPENDIX A. SHOCK FEATURES OF ENSTATITE CHONDRITES A.1. Petrologic and geochemical indicators of impact and melting A.1.1. Petrographic shock features Several petrologic and mineralogic features in enstatite chondrites have been interpreted previously as having resulted from impact processing. They are described below and summarized in Table 5. Many of these features are present in the six chondrites studied here and, on that basis, we conclude that these rocks experienced impact melting. However, no single chondrite contains all of these features. (1) Brecciation. Several enstatite chondrites contain clasts that form sharp boundaries with the host. EH breccias include Abee (Dawson et al., 1960; Rubin and Keil, 1983; Kempton, 1996), wherein clasts have similar textures to that of the host (Rubin and Keil, 1983; Rubin and Scott, 1997), Adhi Kot, which contains silica-rich and chondrulerich clasts (Rubin, 1983a), and Parsa, in which there occurs a porphyritic impact-melt-rock clast (Bhandari et al., 1980; Nehru et al., 1984; Rubin, 1985). EL breccias include Hvittis, which possesses impact-melt-rock clasts (Rubin,

Table 5 Shock-produced petrologic and mineralogical characteristics of enstatite chondrites. Feature

Implication

Examples

References

1

Brecciation Diamonds Euhedral enstatite grains Nucleation of enstatite on relict chondrules Low-MnO enstatite; high-Mn troilite; high-Mn oldhamite Keilite

Abee; Adhi Kot; Blithfield; Hvittis; Parsa Abee Abee; ALH 82132; RKPA 80259 Abee Abee; Adhi Kot; St. Sauveur

1–5

2 3 4 5

Mechanical disruption High pressure Melting Melting Melting Impact melting

Abee; Adhi Kot; St. Sauveur; RKPA 80259 Abee; Adhi Kot; Y-8404; Y-980223 Abee; A-882039; Adhi Kot QUE 94368; Hvittis; Neuschwanstein Abee; St. Sauveur; Y-82189

6 7 8 9 10

Relatively abundant silica Euhedral graphite Euhedral sinoite Fluor-richterite; fluorphlogopite

Melting Melting Impact melting Impact melting

6, 7 7, 8 7 9, 2, 10 11 9, 2, 12, 13 9, 2, 13, 14 15–18 19, 20, 2, 12

References: 1 = Dawson et al. (1960); 2 = Rubin (1983a); 3 = Rubin (1984); 4 = Rubin (1983b); 5 = Bhandari et al. (1980); 6 = Russell et al. (1992); 7 = Rubin and Scott (1997); 8 = Fagan et al. (2000); 9 = Rubin and Keil (1983); 10 = Keil (1968); 11 = Keil (2007); 12 = Lin and Kimura (1998); 13 = this study; 14 = Rubin (1997a); 15 = Rubin (1997b); 16 = Rubin (2006); 17 = Bischoff et al. (2005a); 18 = Bischoff et al. (2005b); 19 = Douglas and Plant (1969); 20 = Olsen et al. (1973).

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1983b), Atlanta, in which a sulfide-rich, metal-poor clast is present (Rubin, 1983c) and Blithfield, an impact-melt rock that has centimeter-size sulfide-rich, metal-poor clasts (Rubin, 1984). (2) Diamonds. Abee contains 100 lg/g diamonds (Russell et al., 1992) twinned on {111}; they were interpreted as a shock feature by Rubin and Scott (1997). This twinning feature is shared with shocked synthetic diamonds (Daulton et al., 1994). Abee diamonds contain low N (<50 lg/g; Russell et al., 1992) as do impact-produced diamonds (<60 lg/ g N; Shelkov et al., 1996). Abee diamonds also have d13C values (2&; Russell et al., 1992) that are fairly similar to Abee bulk C that combusts at temperatures >700 °C (8&; Grady et al., 1986); this C component is mainly graphite, the likely precursor of the diamonds. Shock-produced diamonds are not restricted to enstatite chondrites; they also occur in ureilites (Lipschutz, 1964; Vdovykin, 1972; Berkley et al., 1976), IAB irons (Ksanda and Henderson, 1939; Frondel and Marvin, 1967; Clarke et al., 1981; Russell et al., 2003) and bencubbinites (Mostefaoui et al., 2002). (3) Euhedral enstatite grains. The presence of euhedral enstatite grains shows that a melt was present. Partial melting experiments conducted at 1450 °C on EH4 Indarch produced near-millimeter-size lath-like enstatite grains (McCoy et al., 1999). Euhedral enstatite grains also occur in some igneously produced microporphyritic clasts in the Norton County aubrite (Okada et al., 1988) and in the Ilafegh 009 EL impact-melt rock (McCoy et al., 1995). Euhedral enstatite grains in enstatite chondrites (e.g., Abee, Adhi Kot, RKPA 80259, ALH 82132, Y-791790, Y-791810) are inferred to have crystallized from local pyroxene-normative melts (e.g., Rubin and Scott, 1997; Lin and Kimura, 1998; Fagan et al., 2000; Rubin, 2008). Because these rocks exhibit other petrographic evidence of shock (e.g., brecciated textures, and, in the case of Abee, shock-produced diamonds), we infer that euhedral enstatite in enstatite chondrites is evidence of impact melting. A high abundance of euhedral enstatite grains indicates that melting was extensive. (4) Nucleation of enstatite on relict chondrules. Enstatite grains crystallizing from a melt would tend to nucleate on unmelted (relict) enstatite grains. The surfaces of relict pyroxene-rich chondrules in Abee served as nucleation sites for euhedral enstatite grains crystallizing from the surrounding melt (e.g., Fig. 2d of Rubin and Scott, 1997). Many of the relict chondrules themselves appear to have been partly resorbed. The low abundance of chondrules in Abee (2–3 vol%; Rubin and Keil, 1983) relative to those in unmelted EH3 chondrites (30 vol%) implies that most of the chondrules that were initially present in Abee were melted. (5) Low-MnO enstatite, high-Mn troilite, and high-Mn oldhamite. Most EH3, EH4 and EH5 chondrites contain enstatite with 0.13–0.20 wt% MnO (Keil, 1968; Grossman et al., 1985). In contrast, enstatite in those EH chondrites that contain euhedral enstatite grains (and, hence, are inferred to have been melted) (e.g., Abee, Adhi Kot, St. Sauveur) have very low MnO (i.e., below the normal detection limits of the electron microprobe, e.g., <0.04 wt%; Keil, 1968; Rubin and Keil, 1983; Rubin, 1983a; Lin and Kimura, 1998). Abee, Adhi Kot and St. Sauveur also differ from unmelted EH4 chondrites such as Indarch in containing troi-

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lite and oldhamite with relatively high Mn contents. Whereas Indarch troilite averages 0.11 wt% Mn (Keil, 1968), troilite grains in Abee, Adhi Kot and St. Sauveur average 0.24 wt% Mn, 0.34 wt% Mn and 0.17 wt% Mn, respectively (Keil, 1968; Rubin, 1983a). Whereas Indarch oldhamite averages 0.22 wt% Mn (Keil, 1968), oldhamite grains in Abee, Adhi Kot and St. Sauveur average 0.36 wt% Mn, 0.39 wt% Mn and 0.39 wt% Mn, respectively (Keil, 1968). It seems plausible that after MnO-bearing enstatite melted along with sulfide, Mn partitioned preferentially into crystallizing sulfide phases. The relatively high Mn contents of troilite and oldhamite may also be due in part to enrichment of Mn in the melt by the melting of pre-existing (Mn-rich) niningerite. (Indarch niningerite averages 6.5 wt% Mn (Keil, 1968).) (6) Keilite [(Fe>0.5,Mg<0.5)S]. As proposed by Keil (2007), keilite could have formed from niningerite via reaction with troilite at high temperatures: 2ðMg0:6 Fe0:4 ÞS þ FeS ¼ 3ðMg0:4 Fe0:6 ÞS Keil (2007) reported that keilite occurs only in enstatite chondrites that appear to have been impact melted, e.g., Abee, Adhi Kot, St. Sauveur and RKPA 80259. The relatively low Mn content of keilite (4 wt%; Keil, 2007) compared to niningerite in EH4 Indarch and EH3 Kota-Kota (6.5 and 11.6 wt% Mn, respectively; Keil, 1968) is consistent with keilite formation from a melt after the Mn (and Mg) concentrations of the original niningerite grains were diluted by the concomitant melting of troilite. Sulfide-rich impact melts thus have higher Fe/Mg and Fe/Mn ratios than niningerite. (7) Relatively abundant silica. In many enstatite chondrites, the modal abundance of silica is low. The two analyzed chondrule-rich clasts in Adhi Kot contain 61 wt% silica (Rubin, 1983a). No silica was reported in EL6 Atlanta (Rubin, 1983b) or EL6 Hvittis (Rubin, 1983c). Binns (1967) reported accessory cristobalite in EL Blithfield, but Rubin (1984) found no silica grains. In contrast, the mean modal silica content of Abee is 7 wt% and some centimeter-size clasts contain up to 16 wt% silica (Table 1 of Rubin and Keil, 1983). In Adhi Kot there are 3–5-mm-size chondrule-free clasts (plausibly impact products) that contain up to twice as much silica (18–28 wt%) as enstatite (12– 14 wt%). As a result of melting, it is possible that some of the reduced Si derived mainly from Si-bearing kamacite and perryite [(Ni,Fe)5(Si,P)2] reacted with FeO from melted enstatite to produce silica and metallic Fe during crystallization: Si þ 2FeO ¼ 2Fe þ SiO2 (Some phosphide should have been produced by the breakdown of perryite; consistent with this conclusion is the occurrence of schreibersite in Y-8404, Y-980211, Y980524, Y-793225 and A-882039.) Silica could also have been produced if the bulk composition of the melt was in the enstatite + liquid field; after enstatite crystallized, silica could have crystallized along the enstatite–silica cotectic. Although the mechanism for the production of abundant

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silica is uncertain, it is apparent that it crystallized from SiO2-rich melts. Mobilization of Si-bearing phases and the heterogeneous production of melt are probably responsible for variations in the abundance of silica among impact products. (8) Euhedral graphite. Whereas graphite in unmelted enstatite chondrites typically occurs as irregular or compacted aggregates within kamacite (e.g., Fig. A37 of Ramdohr, 1973), some or all of the graphite in euhedralenstatite-bearing enstatite chondrites (e.g., Abee, Adhi Kot) is present as euhedral grains (“books” in the terminology of El Goresy et al., 2001) with pyramidal terminations (Rubin, 1983a, 1997a; Rubin and Keil, 1983). These grains texturally resemble those that crystallized from melts in the ALHA 78019 and Nova 001 ureilites (Berkley and Jones, 1982; Treiman and Berkley, 1994) and in terrestrial ultramafic xenoliths in alkali basalts (Figs. 5–7 of Kornprobst et al., 1987). Euhedral graphite grains in enstatite chondrites probably crystallized from melts. Their precursors probably occurred as aggregates within kamacite. (9) Euhedral sinoite (Si2N2O). Euhedral grains of sinoite occur in many EL6 chondrites (e.g., Forrest 033, Hvittis, Jajh deh Kot Lalu, Neuschwanstein, Pillistfer, Ufana, Yilmia, ALHA81021, EET90102 and LEW88714; Rubin, 2006), leading to the suggestion (Petaev and Khodakovsky, 1986; Fogel et al., 1989; Meunow et al., 1992) that sinoite formed at EL6 metamorphic temperatures (i.e., 950 °C; Meunow et al., 1992; Wasson et al., 1994) over geologic timescales. However, sinoite has been reported in portions of the EL4 chondrites QUE 94368 (Rubin, 1997b) and Grein 002 (Patzer et al., 2004) that also contain euhedral grains of enstatite and graphite and are thus inferred to have been formed by impact melting. Kimura et al. (2005) reported sinoite in EH6-an Y-793225, a meteorite described in this study as an impact-melt rock. Sinoite may have formed at temperatures of 1400– 1500 °C (Brosset and Idrestedt, 1964) by a reaction such as that proposed by Ryall and Muan (1969): SiO2 þ 3Si þ 2N2 ðgÞ ¼ 2Si2 N2 O (with the N2 possibly being derived from nitride condensates; e.g., Rubin and Choi, 2009). Such high temperatures are far beyond those envisioned for thermal metamorphism in EL4 or even EL6 chondrites (700–950 °C) and would have caused wide-spread melting of metal and sulfide. However, high temperatures are briefly present in portions of impact melts and it seems likely that sinoite in EL4 chondrites crystallized from such melts. Euhedral sinoite in EL6 chondrites may have crystallized directly from impact melts (Rubin, 1997a; Bischoff et al., 2005a) or grown coarser via post-impact annealing. A single grain of sinoite was also reported in a perchloric-acid-resistant residue of Abee (Lee et al., 1995). (10) Fluor-richterite [Na2Ca(Mg,Fe)5Si8O22F2] and fluorphlogopite [KMg3(Si3Al)O10F2]. Fluor-richterite occurs in Abee as rare 3.5-mm-long radiating acicular grains bundled in clusters associated with enstatite, troilite and keilite (Douglas and Plant, 1969; Olsen et al., 1973). In St. Sauveur, fluor-richterite occurs as 40  100-lm-size subhedral

grains (Rubin, 1983a). The fluor-richterite grains probably crystallized from the melt. Fluor-richterite has also been reported in the Mayo Belwa aubrite (Bevan et al., 1977; Graham et al., 1977) where it occurs as bundles of acicular grains in vugs; some bundles are up to 3 mm long and individual fluor-richterite blades are up to 1 mm long. Rubin (2010) interpreted Mayo Belwa as an impact-melt breccia on the basis of its possession of an intergranular melt matrix containing euhedral silicate grains, small opaque grains and numerous vugs. He proposed that volatiles in Mayo Belwa were vaporized during the impact event, forming cavities in the melt; fluor-richterite may have condensed in some of the cavities from the cooling vapor. Fluorphlogopite occurs in the EH impact-melt rock Y82189 as rare subhedral, 10–30-lm-size grains in association with enstatite, silica and albite (Lin and Kimura, 1998). This phase most likely crystallized from the melt. It is important to note that the only enstatite meteorites in which fluor-richterite and fluorphlogopite have been reported were interpreted as having been impact melted on the basis of other features. This correspondence is suggestive that these F-rich phases are themselves petrographic indicators of impact processing. A.1.2. Geochemical shock indicators Bogard et al. (2010) reviewed literature data and determined Ar–Ar ages for a suite of enstatite meteorites. They concluded that several EH and EL chondrites had experienced collisional heating early in solar system history that was sufficiently intense to have disturbed both the Ar–Ar and Rb–Sr chronometers. Argon–argon data (corrected for errors in the 40K decay constant) show that the Abee and RKP 80259 EH impactmelt breccias have Ar–Ar ages of 4.52 and 4.24 Ga, respectively (Bogard et al., 2010 and references therein). The Blithfield brecciated EL impact-melt rock has a corrected Ar–Ar plateau age of 4.534 Ga (Bogard et al., 2010). These dates range from 31 to 135 Ma after accretion occurred 4.565 Ga ago (Carlson and Lugmair, 2000; Wadhwa et al., 2008; Kleine et al., 2009) and are long after any 26Al (t½ = 730,000 years) that was initially present had decayed away. Impacts are the only plausible heat source at these late dates. A.2. Formation of Abee and other enstatite chondrites by impact melting Abee, a large, extensively studied observed fall, is the classic enstatite–chondrite impact-melt breccia. It is useful to review the numerous indicators for impact melting (and contra internal heating) in Abee and to compare these features to the properties of the enstatite chondrites in the present study. The brecciated texture of Abee and the occurrence of diamonds that were probably produced by shock (see above) suggest that other melt features in Abee (e.g., euhedral enstatite and euhedral graphite) could have resulted from impact melting.

Collisional heating of enstatite chondrites

Several petrologic characteristics of Abee are inconsistent with long-term melting and slow cooling as expected from heating via the decay of short-lived radionuclides: (1) Many type-3 enstatite chondrites contain two varieties of enstatite grains: low-MnO grains that exhibit blue luminescence under electron bombardment and MnO-bearing grains that exhibit red luminescence (e.g., Keil, 1968; Leitch and Smith, 1982; Weisberg et al., 1994). Although dominated by low-MnO enstatite (Keil, 1968), Abee contains both varieties (Fig. 7 of Leitch and Smith, 1982), indicating that it was incompletely melted and incompletely homogenized. (2) The presence of readily recognizable chondrules also indicates incomplete melting. (3) SiC is abundant in EH3 chondrites. The presence of SiC in Abee is inferred from the identification of trace amounts of Ne–E (H) in an acid-etched residue (Huss and Lewis, 1995); SiC is the host phase of Ne– E (H) (e.g., Amari et al., 1994). (4) Igneous textures in Abee occur both within the clasts and in matrix regions between clasts; this indicates that Abee experienced two separate melting episodes (Rubin and Scott, 1997; Rubin, 2008). (5) Different clast and matrix regions in Abee contain appreciably different modal abundances of major phases (Rubin and Keil, 1983): enstatite (22– 52 wt%), silica (0.9–16 wt%), plagioclase (4– 14 wt%), keilite (3–13 wt%), troilite (5–13 wt%) and kamacite (20–65 wt%). Igneous rocks that formed by slow melting processes would likely be much more homogeneous unless post-crystallization brecciation mixed diverse lithologies. (6) The occurrence of keilite in Abee indicates that the melt was quenched; if Abee had cooled slowly (or had been subsequently annealed), keilite would have exsolved into troilite and niningerite (Keil, 2007). Quenching of Abee with little or no subsequent annealing is consistent with the occurrence of tridymite and cristobalite and the absence of quartz (Kimura et al., 2005). The presence of cohenite exsolution lamellae in Abee kamacite also indicates rapid cooling (Herndon and Rudee, 1978; Rudee and Herndon, 1980). (7) The acicular morphologies of fluor-richterite grains in Abee and St. Sauveur and their occurrence in these two rocks which have been interpreted as impactmelt breccias suggest that fluor-richterite crystallized from the impact melt (Rubin, 2008). Insofar as the other enstatite chondrites in this study resemble Abee in containing euhedral enstatite and graphite and having relict chondrules on which enstatite nucleated from the melt, it is likely that these rocks formed by similar, impact-related processes. The occurrence of euhedral sinoite grains in regions of QUE 94368 that also contain euhedral enstatite and euhedral graphite (Rubin, 1997b) suggests that the sinoite formed from an impact melt. Regions of QUE 94368 that

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do not contain sinoite have normal chondritic textures and do not contain euhedral enstatite or euhedral graphite. By extrapolation, other enstatite chondrites that contain euhedral sinoite are likely to have experienced impact melting. APPENDIX B. PETROGRAPHIC CHARACTERISTICS OF EH6 CHONDRITES B.1. Y-8404 The dominant phase is orthoenstatite, occurring mainly as euhedral grains, typically 10  50 lm in size, but ranging up to 25  120 lm (Fig. 1a and b). Many grains show welldeveloped {210} cleavage; most grains exhibit sharp optical extinction consistent with a whole-rock shock stage of S1. The majority of the euhedral enstatite grains are intergrown with other randomly oriented grains of euhedral enstatite and smaller (15–60-lm size) more-equant grains of enstatite, plagioclase and silica. Some of the euhedral enstatite grains are surrounded by 20–150-lm-diameter opaque patches consisting of troilite ± kamacite (Fig. 1b). Some opaque patches range up to 1500 lm in size and are nearly free of kamacite; they consist almost entirely of troilite surrounding silicate assemblages containing euhedral enstatite grains. Silica is abundant; Lin and Kimura (1998) reported a modal abundance of 17.7 vol%. Other opaque phases include minor keilite and accessory schreibersite. According to Lin and Kimura (1998), the silicate/opaque modal abundance ratio is 3.7. Although Lin and Kimura (1998) did not find chondrules or chondrule fragments in Y-8404, we found recognizable remnants of two radial pyroxene (RP) chondrules in the available thin section. The larger is 2 mm in diameter (Fig. 2a and b). It consists of radiating enstatite laths separated by 20-lm-thick bands of troilite; troilite constitutes 50 vol% of this chondrule (Fig. 2b). The smaller RP chondrule is 400 lm in diameter and contains 5–10 vol% troilite occurring between radiating bars of enstatite. B.2. Y-980211 The dominant phase is orthoenstatite, occurring mainly as euhedral grains ranging in size from 10  40 to 60  300 lm. Many grains show well-developed {210} cleavage. Although most euhedral enstatite grains exhibit sharp optical extinction, about 25% exhibit undulose extinction, consistent with a whole-rock shock stage of S2. The majority of the euhedral enstatite grains are intergrown with more-equant, 10–40-lm-size grains of enstatite and plagioclase. Plagioclase grains range from 30 to 80 lm in maximum dimension. Silica was not identified; point counting (n = 50) on the electron microprobe gives an upper limit of <2 vol% (i.e., <1 wt%). Most silicate assemblages are surrounded by an opaque groundmass that, in different regions, consists of (a) kamacite and troilite, (b) kamacite, massive schreibersite and troilite, and (c) troilite with accessory amounts of kamacite. Minor keilite is associated with the troilite. There is one millimeter-size kamacite globule with a rounded edge that

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grades into the uniformly dispersed opaque groundmass (Fig. 3). Similarly sized kamacite globules occur in Abee (Dawson et al., 1960; Rubin and Keil, 1983). One relict RP chondrule fragment was identified in Y980211. It is 80  110 lm in size and is composed of a sheaf of radiating enstatite laths with no interleaved troilite. B.3. Y-980223 The meteorite is dominated by an opaque groundmass consisting mainly of kamacite and troilite. Although the groundmass is fairly uniformly distributed, different regions consist mainly of either kamacite or troilite. Keilite is a minor phase. Also present are 150–700-lm-diameter rounded kamacite globules. Throughout the groundmass and inside the globules are euhedral grains of orthoenstatite ranging in size from 4  18 to 30  320 lm (Fig. 4a). Many grains show well-developed {210} cleavage. Most enstatite grains exhibit significant undulose extinction, consistent with a whole-rock shock stage of S2. Plagioclase grains range from 20 to 70 lm in size. Silica is abundant, constituting 15 vol% of the whole rock. Relict chondrules include RP and porphyritic pyroxene (PP) types. One relict RP chondrule fragment is 250 lm across and contains 10-lm-size patches of troilite located between bars of enstatite (Fig. 4b and c). It has been partly resorbed but still contains much of its original radial texture. A few surrounding euhedral enstatite grains appear to have nucleated at the chondrule surface (Fig. 4c). Also present in the meteorite groundmass are a few quasirounded, 200–250-lm-diameter, silicate clumps consisting of 40-lm-size grains of enstatite and plagioclase; these objects are probably relict PP chondrules. B.4. Y-980524 The meteorite consists of intergrown euhedral and anhedral orthoenstatite (12  30 to 100  270 lm in size), plagioclase grains with very fine (0.5 lm) polysynthetic twin lamellae (and one grain with a Carlsbad twin), troilite (some grains with patches of daubre´elite or daubre´elite exsolution lamellae), kamacite, graphite (occurring as quasi-equant and irregular 10–100-lm-size patches within kamacite, silicate and schreibersite, and as elongated (5  55 lm) grains within kamacite), and patches of schreibersite (8–1000 lm) adjacent to kamacite. The thin section also contains a boomerang-shaped metal vein that is 8 mm in length. A few small (2–8 lm) kamacite and troilite grains are enclosed within silicate grains. Most silicate grains exhibit sharp optical extinction indicating that the rock is of shock-stage S1. Two relict chondrules were identified in the thin section. An RP chondrule is 1000  1060 lm in size; a PP chondrule (500  550 lm) contains 30–70-lm-size enstatite phenocrysts. B.5. Y-793225 This rock is composed mainly of quasi-equant enstatite grains 40–70 lm wide and 150–420 lm long (Fig. 5a and

b). Most grains show well-developed {210} cleavage and exhibit sharp optical extinction consistent with a wholerock shock stage of S1. Although most enstatite grains are anhedral or subhedral, some rare euhedral grains also occur. The enstatite grains are randomly oriented and are intergrown with plagioclase and minor silica, forming a hypidiomorphic-granular texture. We observed one 10  60 lm euhedral enstatite grain attached to a more rounded silicate assemblage protruding into surrounding kamacite. Plagioclase grains range in size from 40 to 160 lm. The rock is coarser grained than Y-8404, Y-980211 and Y-980223 and has a higher silicate/opaque-phase ratio (Fig. 5b). According to Lin and Kimura (1998), the silicate/opaque modal abundance ratio in Y-793225 is 7.6. There are scattered opaque patches 30–350 lm in size. They consist mainly of kamacite and troilite. Some of the troilite grains are associated with coarse grains of daubre´elite, ranging in size from 20  30 to 50  100 lm; in addition, a few troilite grains contain relatively thin (8-lm wide) lamellae of daubre´elite. Other opaque phases include accessory schreibersite and trace amounts of graphite, perryite and a Mn-rich, (Mg,Mn,Fe)S solid solution (probably ferroan alabandite) (Lin and Kimura, 1998). Kimura et al. (2005) also reported two 10-lm-size grains of sinoite. No keilite was observed. No chondrules or chondrule fragments were found. B.6. A-882039 This rock is composed of major orthoenstatite intergrown with plagioclase. Both equant and euhedral enstatite grains occur; the euhedral grains range from 10  50 to 120  450 lm (Fig. 6a–c). Very few of the grains show pronounced cleavage. Most enstatite grains exhibit undulose extinction but lack polysynthetic twins, indicating shock stage S2. Plagioclase grains are generally irregular in shape and range up to 70 lm in size. Many silicate intergrowths are surrounded by 30–1100lm-size opaque assemblages. Opaque phases include kamacite, schreibersite, troilite, daubre´elite, ferroan alabandite [(Mn,Fe)S], and graphite. Daubre´elite typically occurs within troilite, either as exsolution lamellae (typically 4– 6 lm wide) or as tabular grains up to 70 lm thick (Fig. 6b). Ferroan alabandite occurs as massive grains adjacent to kamacite and/or troilite (Fig. 6b) and as rare 8–20lm-thick exsolution lamellae within troilite. Schreibersite occurs within kamacite near the border with surrounding silicate; in some opaque assemblages, the schreibersite is massive, ranging up to 250 lm in thickness (Fig. 6c). Graphite occurs within kamacite grains as 2–20-lm-size clusters and as large tabular grains up to 50  210 lm in size (Fig. 6c and d). Graphite is also present as isolated books (using the terminology of El Goresy et al., 2001) ranging from 6  100 to 30  250 lm. Some graphite books are adjacent to kamacite and/or troilite (Fig. 7a); others are completely enclosed within silicate (Fig. 7b). Rare relict chondrules and chondrule fragments are present. One 650  700-lm-size relict radial pyroxene (RP) chondrule fragment consists of a radiating sheaf of

Collisional heating of enstatite chondrites

enstatite bars and minor sulfide (Fig. 8a and b). One 750  820 lm relict PP chondrule is composed mainly of 100–200-lm-size quasi-equant enstatite and plagioclase phenocrysts (Fig. 8c and d). REFERENCES Amari S., Lewis R. S. and Anders E. (1994) Interstellar grains in meteorites: I. Isolation of SiC, graphite, and diamond; size distribution of SiC and graphite. Geochim. Cosmochim. Acta 58, 459–470. Berkley J. L. and Jones J. H. (1982) Primary igneous carbon in ureilites: petrological implications. Proc. Lunar Planet. Sci. Conf. 13, A353–A364. Berkley J. L., Brown H. G., Keil K., Carter N. L., Mercie J.-C. C. and Huss G. (1976) The Kenna ureilite: an ultramafic rock with evidence for igneous, metamorphic, and shock origin. Geochim. Cosmochim. Acta 40, 1429–1437. Bevan A. W. R., Bevan J. C. and Francis J. G. (1977) Amphibole in the Mayo Belwa meteorite: first occurrence in an enstatite achondrite. Mineral. Mag. 41, 531–534. Bhandari N., Shah V. G. and Wasson J. T. (1980) The Parsa enstatite chondrite. Meteoritics 15, 225–234. Binns R. A. (1967) Olivine in enstatite chondrites. Am. Mineral. 52, 1549–1554. Binns R. A. (1970) (Mg,Fe)2SiO4 spinel in a meteorite. Phys. Earth Planet. Inter. 3, 156–160. Binns R. A., Davis R. J. and Reed S. B. J. (1969) Ringwoodite, natural (Mg,Fe)2SiO4 in the Tenham meteorite. Nature 221, 943–944. Birck J.-L. and Alle`gre C. J. (1988) Manganese–chromium isotope systematics and the development of the early Solar System. Nature 331, 579–584. Bischoff A., Palme H., Geiger T. and Spettel B. (1992) Mineralogy and chemistry of the EL-chondritic melt rock Ilafegh 009 (abstract). Lunar Planet. Sci. 23, 105–106. Bischoff A., Grund T., Jording T., Heying B., Hoffmann R.-D., Rodewald U. C. and Po¨ttgen R. (2005a) Occurrence, structure, and formation of sinoite in enstatite chondrites (abstract). Meteorit. Planet. Sci. 40, A20. Bischoff A., Grund T., Jording T., Heying B., Hoffmann R.-D., Rodewald U. C. and Po¨ttgen R. (2005b) First refinement of the sinoite structure of a natural crystal from the Neuschwanstein (EL6) meteorite. Z. Naturforsch. 60b, 1231–1234. Bogard D. D., Garrison D. H., Scott E. R. D., Keil K., Taylor G. J., Vogt S., Herzog G. F. and Klein J. (1990) The Chico, NM, L-6 chondrite: a large, 500 My-old impact melt with a long cosmic ray exposure (abstract). Lunar Planet. Sci. 21, 103–104. Bogard D. D., Dixon E. T. and Garrison D. H. (2010) Ar–Ar ages and thermal histories of enstatite meteorites. Meteorit. Planet. Sci. 45, 723–742. Brosset C. and Idrestedt I. (1964) Crystal structure of silicon oxynitride, Si2N2O. Nature 201, 1211. Burbine T. H., McCoy T. J. and Dickinson T. L. (2000) Origin of plagioclase-“enriched”, igneous, enstatite meteorites (abstract). Meteorit. Planet. Sci. 35, A36. Carlson R. W. and Lugmair G. W. (2000) Timescales of planetesimal formation and differentiation based on extinct and extant radioisotopes. In Origin of the Earth and Moon (eds. R. Canup and K. Righter). University of Arizona Press, Tucson, pp. 25– 44. Chen M., Sharp T. G., El Goresy A., Wopenka B. and Xie X. (1996) The majorite-pyrope + magnesiowu¨stite assemblage: constraints on the history of shock veins in chondrites. Science 271, 1570–1573.

3777

Chen M., Shu J. F., Xie X. D. and Mao H. K. (2003) Natural CaTi2O4-structured FeCr2O4 polymorph in the Suizhou meteorite and its significance in mantle mineralogy. Geochim. Cosmochim. Acta 67, 3937–3942. Choe W. H., Huber H., Rubin A. E., Kallemeyn G. W. and Wasson J. T. (2010) Compositions and taxonomy of 15 unusual carbonaceous chondrites. Meteorit. Planet. Sci. 45, 531–554. Clarke R. S., Appleman D. E. and Ross D. R. (1981) An Antarctic iron meteorite contains preterrestrial impact-produced diamond and lonsdaleite. Nature 291, 396–398. Crabb J. and Anders E. (1981) Noble gases in E-chondrites. Geochim. Cosmochim. Acta 45, 2443–2464. Daulton T. L., Eisenhour D. D., Buseck P. R., Lewis R. S. and Bernatowicz T. J. (1994) High-resolution transmission electron microscopy of meteoritic and terrestrial nano-diamonds (abstract). Lunar Planet. Sci. 25, 313–314. Dawson K. R., Maxwell J. A. and Parsons D. E. (1960) A description of the meteorite which fell near Abee, Alberta, Canada. Geochim. Cosmochim. Acta 21, 127–144. Dodd R. T. (1974) Petrology of the St. Mesmin chondrite. Contrib. Mineral. Petrol. 46, 129–145. Dominik B. and Bussy F. (1994) The Bison LL6 breccia. Meteoritics 29, 235–237. Douglas J. A. V. and Plant A. G. (1969) Amphibole: first occurrence in an enstatite chondrite (abstract). Meteoritics 4, 166. Easton A. J. (1983) Grain-size distribution and morphology of metal in E-chondrites. Meteoritics 18, 19–27. El Goresy A. and Kullerud G. (1969) Phase relations in the system Cr–Fe–S. In Meteorite Research (ed. P. M. Millman). D. Reidel, Dordrecht, pp. 638–656. El Goresy A., Wadwha M., Nagel H.-J., Zinner E. K., Janicke J. and Crozaz G. (1992) 53Cr–53Mn systematics of Mn-bearing sulfides in four enstatite chondrites (abstract). Lunar Planet. Sci. 23, 331–332. El Goresy A., Gillet P., Chen M., Ku¨nstler F., Graup G. and Sta¨hle V. (2001) In situ discovery of shock-induced graphite– diamond phase transition in gneisses from the Ries Crater, Germany. Am. Mineral. 86, 611–621. Fagan T. J., Scott E. R. D., Keil K., Cooney T. F. and Sharma S. K. (2000) Formation of feldspathic and metallic melts by shock in enstatite chondrite Reckling Peak A80259. Meteorit. Planet. Sci. 35, 319–329. Floss C. and Crozaz G. (1993) Heterogeneous REE patterns in oldhamite from aubrites: their nature and origin. Geochim. Cosmochim. Acta 57, 4039–4057. Fogel R. A., Hess P. C. and Rutherford M. J. (1989) Intensive parameters of enstatite chondrite metamorphism. Geochim. Cosmochim. Acta 53, 2735–2746. Frondel C. and Marvin U. (1967) Lonsdaleite, a hexagonal polymorph of diamond. Nature 214, 587–589. Fruland R. M. (1975) Volatile movement in the Rose City meteorite, and implications concerning the impact and late thermal history of ordinary chondrites (abstract). Meteoritics 10, 403–404. Gillet P., Chen M., Dubrovinsky L. and El Goresy A. (2000) Natural NaAlSi3O8-hollandite in the shocked Sixiangkou meteorite. Science 287, 1633–1636. Govindaraju K. (1994) Compilation of working values and descriptions for 383 geostandards. Geostand. Newslett. 118, 1– 158. Grady M. M., Wright I. P., Carr L. P. and Pillinger C. T. (1986) Compositional differences in enstatite chondrites based on carbon and nitrogen stable isotope measurements. Geochim. Cosmochim. Acta 50, 2799–2813.

3778

A.E. Rubin, J.T. Wasson / Geochimica et Cosmochimica Acta 75 (2011) 3757–3780

Graham A. L., Easton A. J. and Hutchison R. (1977) The Mayo Belwa meteorite: a new enstatite achondrite fall. Mineral. Mag. 41, 487–492. Grimm R. E. and McSween H. (1993) Heliocentric zoning of the asteroid belt by 26-Al heating. Science 259, 653–655. Grossman J. N., Rubin A. E., Rambaldi E. R., Rajan R. S. and Wasson J. T. (1985) Chondrules in the Qingzhen type-3 enstatite chondrite: possible precursor components and comparison to ordinary chondrite chondrules. Geochim. Cosmochim. Acta 49, 1781–1795. Guan Y., Huss G. R. and Leshin L. A. (2007) 60Fe–60Ni and 53 Mn–53Cr isotope systems in sulfides from unequilibrated enstatite chondrites. Geochim. Cosmochim. Acta 71, 4082–4091. Herndon J. M. and Rudee M. L. (1978) Thermal history of the Abee enstatite chondrite. Earth Planet. Sci. Lett. 41, 101–106. Huss G. R. and Lewis R. S. (1995) Presolar diamond, SiC, and graphite in primitive chondrites: abundances as a function of meteorite class and petrologic type. Geochim. Cosmochim. Acta 59, 115–160. Irving A. J., Bunch T. E., Rubin A. E. and Wasson J. T. (2010) Northwest Africa 2828/Al Haggounia 001 is a weathered unequilibrated EL chondrite: trace element and petrologic evidence. Meteorit. Planet. Sci. 45, A90. Jarosewich E., Clarke R. S. and Barrows J. N. (1987) The Allende meteorite reference sample. Smithson. Contrib. Earth Sci. 27, 1– 49. Kallemeyn G. W. and Wasson J. T. (1986) Compositions of enstatite (EH3, EH4, 5 and EL6) chondrites: implications regarding their formation. Geochim. Cosmochim. Acta 50, 2153– 2164. Kallemeyn G. W., Rubin A. E., Wang D. and Wasson J. T. (1989) Ordinary chondrites: bulk compositions, classification, lithophile-element fractionations, and composition–petrographic type relationships. Geochim. Cosmochim. Acta 53, 2747–2767. Karwowski L., Kryza R. and Przylibski T. (2007) New chemical and physical data on keilite from the Zakłodzie enstatite achondrite. Am. Mineral. 92, 204–209. Keil K. (1968) Mineralogical and chemical relationships among enstatite chondrites. J. Geophys. Res. 73, 6945–6976. Keil K. (2007) Occurrence and origin of keilite, (Fe>0.5, Mg<0.5)S, in enstatite chondrite impact-melt rocks and impact-melt breccias. Chem. Erde 67, 37–54. Keil K. (2010) Enstatite achondrite meteorites (aubrites) and the histories of their asteroidal parent bodies. Chem. Erde 70, 295– 317. Kempton R. (1996) Abee—more questions than answers. Meteorite 2, 18–19. Kimura M. and Lin Y. (1999) Petrological and mineralogical study of enstatite chondrites with reference to their thermal histories. Antarct. Met. Res. 12, 1–18. Kimura M., Weisberg M. K., Lin Y., Suzuki A., Ohtani E. and Okazaki R. (2005) Thermal history of the enstatite chondrites from silica polymorphs. Meteorit. Planet. Sci. 40, 855–868. Kissin S. A. (1989) Application of the sphalerite cosmobarometer to the enstatite chondrites. Geochim. Cosmochim. Acta 53, 1649–1655. Kleine T., Touboul M., Bourdon B., Nimmo F., Mezger K., Palme H., Jacobsen S. B., Yin Q. Z. and Halliday A. N. (2009) Hf–W chronology of the accretion and early evolution of asteroids and terrestrial planets. Geochim. Cosmochim. Acta 73, 5150– 5188. Kornprobst J., Pineau F., Degiovanni R. and Dautria J. M. (1987) Primary igneous graphite in ultramafic xenoliths: I. Petrology of the cumulate suite in alkali basalt near Tissemt (Egge´re´, Algerian Sahara). J. Petrol. 28, 293–311.

Kring D. A., Swindle T. D., Britt D. T. and Grier J. A. (1996) Cat Mountain: a meteoritic sample of an impact-melted asteroid regolith. J. Geophys. Res. 101, 29353–29371. Ksanda C. J. and Henderson E. P. (1939) Identification of diamond in the Canyon Diablo iron. Am. Mineral. 24, 677–680. Kuehner S. M., Irving A. J., Bunch T. E. and Wittke J. H. (2006) EL3 chondrite (not aubrite) Northwest Africa 2828: an unusual paleometeorite occurring as cobbles in a terrestrial conglomerate. AGU Fall Meeting 2006. #P51E-1247 (abstr.). Larimer J. W. and Ganapathy R. (1987) The trace element chemistry of CaS in enstatite chondrites and some implications regarding its origin. Earth Planet. Sci. Lett. 84, 123– 134. Lee M. R., Russell S. S., Arden J. W. and Pillinger C. T. (1995) Nierite (Si3N4), a new mineral from ordinary and enstatite chondrites. Meteoritics 30, 387–398. Lee T., Papanastassiou D. and Wasserburg G. J. (1976) Demonstration of Mg-26 excess in Allende and evidence for Al-26. Geophys. Res. Lett. 3, 41–44. Leitch C. A. and Smith J. V. (1982) Petrography, mineral chemistry and origin of Type I enstatite chondrites. Geochim. Cosmochim. Acta 46, 2083–2097. Lin Y. and Kimura M. (1998) Petrographic and mineralogical study of new EH melt rocks and a new enstatite chondrite grouplet. Meteorit. Planet. Sci. 33, 501–511. Lipschutz M. E. (1964) Origin of diamonds in the ureilites. Science 143, 1431–1434. Lundberg L. L. and Crozaz G. (1988) Enstatite chondrites: a preliminary ion microprobe study (abstract). Meteoritics 23, 285–286. Mason B. (1966) The enstatite chondrites. Geochim. Cosmochim. Acta 30, 23–39. Mason B. and Wiik H. B. (1966) The composition of the Bath, Frankfort, Kakangari, Rose City, and Tadjera meteorites. Am. Mus. Novitates 2272, 1–24. McCoy T. and Welzenbach L. (2001) Thin section (,4) description of QUE 99059. Antarct. Met. Newslett. 24(1). McCoy T. J., Keil K., Bogard D. D., Garrison D. H., Casanova I., Lindstrom M. M., Brearley A. J., Kehm K., Nichols R. H. and Hohenberg C. M. (1995) Origin and history of impact-melt rocks of enstatite chondrite parentage. Geochim. Cosmochim. Acta 59, 161–175. McCoy T. J., Dickinson T. L. and Lofgren G. E. (1999) Partial melting of the Indarch (EH4) meteorite: a textural, chemical, and phase relations view of melting and melt migration. Meteorit. Planet. Sci. 34, 735–746. McSween H. Y., Ghosh A., Grimm R. E., Wilson L. and Young E. D. (2002) Thermal evolution models of asteroids. In Asteroids III (eds. W. F. Bottke, A. Cellino, P. Paolicchi and R. P. Binzel). University of Arizona Press, Tucson, pp. 559–571. Merrill G. P. (1915) On the monticellite-like mineral in meteorites, and on oldhamite as a meteoric constituent. Proc. Natl. Acad. Sci. USA 1, 302–308. Meteoritical Bulletin Database [MBDB] (2011) (updated 04.01.11). Meunow D. W., Keil K. and Wilson L. (1992) High-temperature mass spectrometric degassing of enstatite chondrites: implications for pyroclastic volcanism on the aubrite parent body. Geochim. Cosmochim. Acta 56, 4267–4280. Mittlefehldt D. W. and Lindstrom M. M. (2001) Petrology and geochemistry of Patuxent Range 91501, a clast-poor impact melt from the L-chondrite parent body and Lewis Cliff 88663, an L7 chondrite. Meteorit. Planet. Sci. 36, 439–457. Miyamoto M., Fujii N. and Takeda H. (1981) Ordinary chondrite parent body: an internal heating model. Proc. Lunar Planet. Sci. 12B, 1145–1152.

Collisional heating of enstatite chondrites Mori H. (1994) Shock-induced phase transformations on the Earth and planetary materials. J. Mineral. Soc. Jpn. 23, 171–178. Mostefaoui S., El Goresy A., Hoppe P., Gillet Ph. and Ott U. (2002) Mode of occurrence, textural settings and nitrogen-isotopic compositions of in situ diamonds and other carbon phases in the Bencubbin meteorite. Earth Planet. Sci. Lett. 204, 89–100. Nehru C. E., Prinz M., Weisberg M. K. and Delaney J. S. (1984) Parsa: an unequilibrated enstatite chondrite (UEC) with an aubrite-like impact melt clast (abstract). Lunar Planet. Sci. 15, 597–598. Okada A., Keil K., Taylor G. J. and Newsom H. (1988) Igneous history of the aubrite parent asteroid: evidence from the Norton County enstatite achondrite. Meteoritics 23, 59–74. Olsen E., Huebner J. S., Douglas J. A. V. and Plant A. G. (1973) Meteoritic amphiboles. Am. Mineral. 58, 869–872. Olsen E. J., Bunch T. E., Jarosewich E., Noonan A. F. and Huss G. I. (1977) Happy Canyon: a new type of enstatite achondrite. Meteoritics 12, 109–123. Olsen E. J., Huss G. I. and Jarosewich E. (1988) The Eagle, Nebraska, enstatite chondrite. Meteoritics 23, 379–380. Petaev M. I. and Khodakovsky I. L. (1986) Thermodynamic properties and conditions of formation of minerals in enstatite meteorites. In Chemistry and Physics of Terrestrial Planets (ed. S. K. Saxena). Springer-Verlag, New York, pp. 106–135. Patzer A. and Schultz L. (1998) The exposure age distribution of enstatite chondrites (abstract). Meteorit. Planet. Sci. 33, A120– A121. Patzer A. and Schultz L. (2001) Noble gases in enstatite chondrites: I. Exposure ages, pairing, and weathering effects. Meteorit. Planet. Sci. 36, 947–961. Patzer A., Schlu¨ter J., Schultz L., Tarkian M., Hill D. H. and Boynton W. V. (2004) New findings for the equilibrated enstatite chondrite Grein 002. Meteorit. Planet. Sci. 39, 1555–1575. Price G. D., Putnis A., Agrell S. O. and Smith D. G. W. (1983) Wadsleyite, natural beta-(Mg,Fe)2SiO4 from the Peace River meteorite. Can. Mineral. 21, 29–35. Pryzlibski T. A., Zago_zd_zon P. P., Kryza R. and Pilski A. S. (2005) The Zakłodzie enstatite meteorite: mineralogy, petrology, origin and classification. Meteorit. Planet. Sci. 40, A185–A200. Ramdohr P. (1973) The Opaque Minerals in Stony Meteorites. Elsevier, Amsterdam, 245p. Rubin A. E. (1983a) The Adhi Kot breccia and implications for the origin of chondrules and silica-rich clasts in enstatite chondrites. Earth Planet. Sci. Lett. 64, 201–212. Rubin A. E. (1983b) Impact melt-rock clasts in the Hvittis enstatite chondrite breccia: implications for a genetic relationship between EL chondrites and aubrites. Proc. Lunar Planet. Sci. Conf. 14, B293–B300. Rubin A. E. (1983c) The Atlanta enstatite chondrite breccia. Meteoritics 18, 113–121. Rubin A. E. (1984) The Blithfield meteorite and the origin of sulfide-rich, metal-poor clasts and inclusions in brecciated enstatite chondrites. Earth Planet. Sci. Lett. 67, 273–283. Rubin A. E. (1985) Impact melt products of chondritic material. Rev. Geophys. 23, 277–300. Rubin A. E. (1997a) Igneous graphite in enstatite chondrites. Mineral. Mag. 61, 699–703. Rubin A. E. (1997b) Sinoite (Si2N2O): crystallization from EL chondrite impact melts. Am. Mineral. 82, 1001–1006. Rubin A. E. (2000) Petrologic, geochemical and experimental constraints on models of chondrule formation. Earth-Sci. Rev. 50, 3–27. Rubin A. E. (2004) Post-shock annealing and post-annealing shock in equilibrated ordinary chondrites: implications for the thermal and shock histories of chondritic asteroids. Geochim. Cosmochim. Acta 68, 673–689.

3779

Rubin A. E. (2006) Shock and annealing in EL chondrites. Meteorit. Planet. Sci. 43, A154. Rubin A. E. (2008) Explicating the behavior of Mn-bearing phases during shock melting and crystallization of the Abee EHchondrite impact-melt breccia. Meteorit. Planet. Sci. 43, 1481– 1486. Rubin A. E. (2010) Impact melting in the Cumberland Falls and Mayo Belwa aubrites. Meteorit. Planet. Sci. 45, 265–275. Rubin A. E. (1995a) Fractionation of refractory siderophile elements in metal from the Rose City meteorite. Meteoritics 30, 412–417. Rubin A. E. (1995b) Petrologic evidence for collisional heating of chondritic asteroids. Icarus 113, 156–167. Rubin A. E. (2005) What heated the asteroids? Sci. Am. 292, 80–87. Rubin A. E. and Choi B.-G. (2009) Origin of halogens and nitrogen in enstatite chondrites. Earth Moon Planets 105, 41–53. Rubin A. E. and Keil K. (1983) Mineralogy and petrology of the Abee enstatite chondrite breccia and its dark inclusions. Earth Planet. Sci. Lett. 62, 118–131. Rubin A. E. and Read W. F. (1984) The Brownell and Ness County (1894) L6 chondrites: further sorting-out of Ness County meteorites. Meteoritics 19, 153–160. Rubin A. E. and Scott E. R. D. (1997) Abee and related EH chondrite impact-melt breccias. Geochim. Cosmochim. Acta 61, 425–435. Rubin A. E., Scott E. R. D. and Keil K. (1997) Shock metamorphism of enstatite chondrites. Geochim. Cosmochim. Acta 61, 847–858. Rubin A. E., Huber H. and Wasson J. T. (2009) Possible impactinduced refractory-lithophile fractionations in EL chondrites. Geochim. Cosmochim. Acta 73, 1523–1537. Rudee M. L. and Herndon J. M. (1980) The thermal history of Abee (abstract). Meteoritics 15, 361. Russell S. S., Pillinger C. T., Arden J. W., Lee M. R. and Ott U. (1992) A new type of meteoritic diamond in the enstatite chondrite Abee. Science 256, 206–209. Russell S. S., Zipfel J., Folco L., Jones R., Grady M. M., McCoy T. and Grossman J. N. (2003) The Meteoritical Bulletin, No. 87, 2003 July. Meteorit. Planet. Sci. 38, A189– A248. Ryall W. R. and Muan A. (1969) Silicon oxynitride stability. Science 165, 1363–1364. Sears D. W., Kallemeyn G. W. and Wasson J. T. (1982) The compositional classification of chondrites: II. The enstatite chondrite groups. Geochim. Cosmochim. Acta 46, 597–608. Sharp T. G. and DeCarli P. S. (2006) Shock effects in meteorites. In Meteorites and the Early Solar System II (eds. D. S. Lauretta and H. Y. McSween). University of Arizona Press, Tucson, pp. 653–677. Sharp T. G., Lingemann C. M., Dupas C. and Sto¨ffler D. (1997) Natural occurrence of MgSiO3-ilmenite and evidence for MgSiO3-perovskite in a shocked L chondrite. Science 277, 352–355. Shelkov D., Milledge H. J., Verchovsky A. B., Kaminsky F. and Pillinger C. T. (1996) Preliminary C, N, He and Ar study of shock produced diamonds from the Popigai Crater and Ebeliakh Placer, Siberia (abstract). Lunar Planet. Sci. 27, 1187–1188. Smith J. V. and Mason B. (1970) Pyroxene–garnet transformation in Coorara meteorite. Science 168, 832. Sto¨ffler D., Keil K. and Scott E. R. D. (1991) Shock metamorphism of ordinary chondrites. Geochim. Cosmochim. Acta 55, 3845– 3867.

3780

A.E. Rubin, J.T. Wasson / Geochimica et Cosmochimica Acta 75 (2011) 3757–3780

Tomioka N. and Fujino K. (1997) Natural (Mg,Fe)SiO3-ilmenite and -perovskite in the Tenham meteorite. Science 277, 1084– 1086. Treiman A. H. and Berkley J. L. (1994) Igneous petrology of the new ureilites Nova 001 and Nullarbor 010. Meteoritics 29, 843– 848. Van Schmus W. R. and Wood J. A. (1967) A chemical–petrologic classification for the chondritic meteorites. Geochim. Cosmochim. Acta 31, 747–765. Vdovykin G. P. (1972) Forms of carbon in the new Havero¨ ureilite of Finland. Meteoritics 7, 547–552. Wadhwa M., Srinivasan G. and Carlson R. W. (2008) Timescales of planetesimal differentiation in the early solar system. In Meteorites and the Early Solar System II (eds. D. S. Lauretta and H. Y. McSween). University of Arizona Press, Tucson, pp. 715–732. Wasson J. T., Kallemeyn G. W. and Rubin A. E. (1994) Equilibration temperatures of EL chondrules: a major downward revision in the ferrosilite contents of enstatite. Meteoritics 29, 658–662. Weisberg M. K. and Kimura M. (2004) Petrology and Raman spectroscopy of shock phases in the Gujba CB chondrite and the shock history of the CB parent body. Lunar Planet. Sci. 35. Lunar Planet. Inst., Houston. #1599 (abstr.).

Weisberg M. K., Kimura M., Suzuki A., Ohtani E. and Sugiura N. (2006) Discovery of coesite and significance of high pressure phases in the Gujba CB chondrite. Lunar Planet. Sci. 37. Lunar Planet. Inst., Houston. #1788 (abstr.). Weisberg M. K., Prinz M. and Fogel R. A. (1994) The evolution of enstatite and chondrules in unequilibrated enstatite chondrites: evidence from iron-rich pyroxene. Meteoritics 29, 362–373. Weisberg M. K., Prinz M. and Nehru C. E. (1997) QUE 94204: an EH-chondritic melt rock. Lunar Planet. Sci. 28, #1358 (abstr.). Wlotzka F. (1993) A weathering scale for the ordinary chondrites (abstract). Meteoritics 28, 460. Xie X., Li Z., Wang D., Liu J., Hu R. and Chen M. (1991) The new meteorite fall of Yanzhuang, a severely shocked H6 chondrite with black molten materials (abstract). Meteoritics 26, 411. Yang C. W., Williams D. B. and Goldstein J. I. (1996) A revision of the Fe–Ni phase diagram at low temperatures (<400 °C). J. Phase Equilib. 17, 522–531. Zhang Y., Benoit P. H. and Sears D. W. G. (1995) The classification and complex thermal history of the enstatite chondrites. J. Geophys. Res. 100, 9417–9438. Associate editor: Sara S. Russell