Fe-Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites

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1 Meteoritics & Planetary Science 43, Nr 7, (2008) Abstract available online at Fe-Ni metal in primitive chondrites: Indicators of classification and metamorphic conditions for ordinary and CO chondrites M. KIMURA 1, J. N. GROSSMAN 2, and M. K. WEISBERG 3, 4 1 Faculty of Science, Ibaraki University, Mito , Japan 2 U.S. Geological Survey, 954 National Center, Reston, Virginia 20192, USA 3 Department of Physical Sciences, Kingsborough College of the City University of New York, Brooklyn, New York 11235, USA 4 Department of Earth and Planetary Sciences, American Museum of Natural History, New York, New York 10024, USA Corresponding author. makotoki@mx.ibaraki.ac.jp (Received 2 May 2007; revision accepted 20 December 2007) Abstract We report the results of our petrological and mineralogical study of Fe-Ni metal in type 3 ordinary and CO chondrites, and the ungrouped carbonaceous chondrite Acfer 094. Fe-Ni metal in ordinary and CO chondrites occurs in chondrule interiors, on chondrule surfaces, and as isolated grains in the matrix. Isolated Ni-rich metal in chondrites of petrologic type lower than type 3.10 is enriched in Co relative to the kamacite in chondrules. However, Ni-rich metal in type chondrites always contains less Co than does kamacite. Fe-Ni metal grains in chondrules in Semarkona typically show plessitic intergrowths consisting of submicrometer kamacite and Ni-rich regions. Metal in other type 3 chondrites is composed of fine- to coarse-grained aggregates of kamacite and Ni-rich metal, resulting from metamorphism in the parent body. We found that the number density of Ni-rich grains in metal (number of Ni-rich grains per unit area of metal) in chondrules systematically decreases with increasing petrologic type. Thus, Fe-Ni metal is a highly sensitive recorder of metamorphism in ordinary and carbonaceous chondrites, and can be used to distinguish petrologic type and identify the least thermally metamorphosed chondrites. Among the known ordinary and CO chondrites, Semarkona is the most primitive. The range of metamorphic temperatures were similar for type 3 ordinary and CO chondrites, despite them having different parent bodies. Most Fe-Ni metal in Acfer 094 is martensite, and it preserves primary features. The degree of metamorphism is lower in Acfer 094, a true type 3.00 chondrite, than in Semarkona, which should be reclassified as type INTRODUCTION Unequilibrated ordinary (O) chondrites and some carbonaceous (C) chondrites have been classified into petrologic types , based on their thermoluminescence (TL) sensitivity, chemical, and petrologic characteristics (e.g., Sears et al. 1980; Keck and Sears 1987). Scott and Jones (1990) determined the petrologic types of unequilibrated CO chondrites by using the distribution of Fa content in olivine and the minor-element composition of Fe-Ni metal. The petrologic features of amoeboid olivine inclusions can also be used to determine petrologic type in CO chondrites (Chizmadia et al. 2002). Bonal et al. (2006, 2007) showed that the structural grade (maturation) of the organic matter in the matrix is related to the extent of metamorphism in CV and CO chondrites and thus is an indicator of petrologic type. The least metamorphosed chondrites (i.e., those close to petrologic type 3.0) are important materials because they are the most likely to preserve primordial petrologic features and isotopic compositions (e.g., Brearley and Jones 1998; Kita et al. 2000), and thus are the best records of primary processes in the early solar system. However, mineralogical criteria for distinguishing differences among chondrites of very low petrologic type (<3.1) are not well established. The major element chemistry of many silicates is generally not a sensitive indicator of metamorphic temperature. For example, the compositional heterogeneity of olivine and pyroxene seems to be generally similar among types (Sears et al. 1980). However, the least metamorphosed chondrites can be very accurately classified into petrologic types by using the minor-element compositions of olivine. Grossman and Brearley (2005) showed that the distribution of Cr in FeO-rich olivine changes systematically as the metamorphic degree 1161 The Meteoritical Society, Printed in USA.

2 1162 M. Kimura et al. increases between type 3.0 and 3.2. Additionally, they suggested that alkali elements were mobilized from the matrix and entered chondrules during the early stages of metamorphism; at the same time, sulfides were removed from the fine-grained matrix and entered other components of the chondrite. From the distribution of Cr in olivine, Grossman and Brearley (2005) and Grossman and Rubin (2006) divided types 3.0 and 3.1 into 3.00 through 3.15 in O chondrites, and 3.00 to 3.1 in CO chondrites, respectively. These metamorphic trends can be used to identify the most primitive chondrites. Fe-Ni metal is a common constituent of meteorites, and is a useful mineral for evaluation of the cooling history of meteorites (e.g., Wood 1967; Hopfe and Goldstein 2001). In chondrites, Fe-Ni metal shows a wide range of textural and compositional variation, and also can be used to distinguish the chemical group of the host chondrite (e.g., Afiattalab and Wasson 1980; Rubin 1990). In unequilibrated O chondrites, Fe-Ni metal is present in chondrules and matrix. This metal may contain minor elements such as Cr and Si (Rambaldi et al. 1980), and tiny inclusions of phosphate, chromite and other phases (Zanda et al. 1994). Some Fe metal formed by reduction of silicates, and is marked by a strong depletion in Ni (Rambaldi and Wasson 1984). Plessitic intergrowths have been observed in chondrule metal (Reisener and Goldstein 1999). These features do not exist in metal in equilibrated O chondrites, implying that they are metamorphic indicators. However, the relationship between these features and petrologic type has not been established. In contrast, metal grains in primitive CH and CR chondrites show a positive correlation between Ni and Co, consistent with condensation models for their origin (e.g., Grossman et al. 1988; Weisberg et al and 1995; Meibom et al. 1999). This characteristic has not been reported for metal in O and CO chondrites, except for bulk metal compositions in Semarkona chondrules (Kimura and Weisberg 2004), and the bulk chemical composition of Semarkona chondrules (Grossman and Wasson, 1985). Because CH and CR chondrites are thought to be hardly metamorphosed, it is possible that the disappearance of the Ni-Co correlation is also a metamorphic effect. In a preliminary report, Kimura et al. (2006a) showed that the characteristic features of Fe-Ni metal, such as texture and composition, are highly sensitive to thermal metamorphism in O and CO chondrites, and seem to be consistent with the classification system proposed by Grossman and Brearley (2005). Thus, the features of Fe-Ni metal in the most primitive chondrites may shed light on their parent body thermal history. Toward this end, we are conducting a systematic study of Fe-Ni metal in O and C chondrites. The purposes of this study are 1) to explore the primitive nature of metal, 2) to investigate the metamorphic effects on metal in type 3 chondrites, 3) to evaluate the metamorphic conditions in the chondrite parent bodies, and 4) to reinvestigate the classification of type 3 chondrites. Here we report our results on ordinary and CO chondrites, plus the ungrouped type C3 chondrite, Acfer 094. SAMPLES AND EXPERIMENTAL METHODS A list of the samples analyzed in this study is given in Table 1. Included among these samples are Yamato (Y-) and Y , which we have reclassified as LL3.15 using the method of Grossman and Brearley (2005) (the original classification of LL3.0 by TL was done by Ninagawa et al and 2002); the average Cr 2 O 3 content and standard deviation of ferroan olivine in these two samples are (in wt. %) 0.24 and 0.14, and 0.21, and 0.14, respectively. Including these, we studied 14 O chondrites: two thin sections of Semarkona, two petrologic type 3.05 chondrites, one 3.10, three 3.15, four 3.2, one 3.5, one 3.7, and one 3.9 (Table 1). They are all classified as LL chondrites, except for the two 3.05 chondrites which are classified as L, and one of the 3.2 chondrites which is H, and is one of the most primitive H3 chondrites known (Kimura et al. 2002). Although Semarkona was classified as type 3.00 (Grossman and Brearley 2005), we will show below that it should be reclassified as We also studied three primitive CO chondrites; Allan Hills (ALH) A77307 and Y-81020, classified as CO3.03 and CO3.05, respectively (Bonal et al. 2007; Grossman and Rubin 2006) and Asuka (A-) , classified as CO3.1 (Chizmadia and Bendersky 2006). In addition, we analyzed Fe-Ni metal in Y (CO3.6). Finally, we studied the Fe-Ni metal in Acfer 094, a unique and ungrouped carbonaceous chondrite, with properties similar to CO and CM chondrites (Newton et al. 1995). The primitive matrix mineralogy (Greshake 1997), abundant presolar grains (Newton et al. 1995), and unaltered nature of its refractory inclusions (Krot et al. 2004) indicate that Acfer 094 preserves primordial features and was not significantly modified on the parent body. This is consistent with the classification of Acfer 094 as a type 3.00 chondrite by the Cr distribution in its olivine (Grossman and Brearley 2005). Backscattered electron (BSE) imaging and mineral analyses were obtained using the JEOL 733 electron-probe microanalyzer (EPMA) at Ibaraki University. We conducted quantitative single point analyses at 15 kv and a sample current of 70 na for Fe-Ni metal. The X-ray overlaps of K β on K α lines of some successive elements were corrected with a deconvolution program. The ZAF matrix method was used for correction. The analytical points were randomly selected to cover a range of cores and rims in individual metal grains, and

3 Fe-Ni metal in primitive chondrites 1163 Table 1. List of the samples. Meteorite Key Group Type Section Shock 2 Semarkona Sem LL 3.01 ( ) AMNH 4128 and S2 EET EET L 3.05 JSC-,5 S2 QUE QUE L 3.05 JSC-,13 S2 NWA 1756 NWA LL 3.10 NAU ADRC-158 S1 Y Y60 LL 3.15 NIPR 81-2 S2 Y Y24 LL 3.15 NIPR 51-1 S3 Y Y84 LL 3.15 NIPR 51-1 S2 Krymka Kry LL 3.2 AMNH S3 Y Y48 LL 3.2 NIPR 91-6 S2 Y Y96 LL 3.2 NIPR 51-1 S2 Y Y38 H 3.2 NIPR 61-3 S1 ALHA77260 A60 LL 3.5 NIPR 71-3 S3 ALHA77304 A04 LL 3.7 NIPR 83-3 S3 Bo Xian BoX LL 3.9 GIG S3 ALHA77307 A07 CO 3.03 NIPR 85-4 S1 Y Y20 CO 3.05 NIPR B1 S2 A A32 CO 3.1 NIPR 51-4 S1 Y Y17 CO 3.6 NIPR 81-2 S1 Acfer 094 Acf CC ung 3.00 Mün PL S1 1 Type 3.00 after Grossman and Brearley (2005). 2 After Koblitz (2003), Grossman and Brearley (2005), Kimura et al. (2002), and this work. to include tiny Ni-rich grains in the plessite. We avoided visible inclusions (phosphates, oxides, etc.). Tables 2 and 3 show the detection limits for analyses. Some BSE images for Semarkona metal were taken with the Hitachi S4700 field emission scanning electron microscope (FE-SEM), equipped with a GW Centaurus backscattered electron (BSE) detector and a PGT IMIX quantitative EDS, at the American Museum of Natural History, New York. RESULTS Characteristic Features of Fe-Ni Metal The textures of Fe-Ni metal and troilite are sensitive to shock events; distinguishing features, such as coarsegrained plessite, melted metal-troilite assemblages, martensite, polycrystalline kamacite and troilite, have been reported in highly shocked chondrites (Taylor and Heymann 1971; Smith and Goldstein 1977; Bennett and McSween 1996). However, the opaque minerals in the samples studied here do not show these shock-induced features. The shock stages of all the studied samples are below S3 (Table 1). Fe-Ni metal in O and CO chondrites occurs in and around chondrules, and in the matrix (Fig. 1). As shown below, the composition and texture of metal are related to the petrographic location of the metal in primitive chondrites. Therefore, we classified the metal as chondrule interior metal, chondrule surface metal, or isolated matrix metal, similar to the classification used by Nagahara (1982). Fe-Ni metal grains in chondrule interiors are typically 5 60 µm in size and spherical to ellipsoidal in shape (Fig. 1a), whereas grains in the other occurrences are generally µm in size and irregularly shaped in O and CO chondrites. Most Fe-Ni metal grains in Acfer 094 are spherical in all petrographic locations. The texture and chemical composition of Fe-Ni metal in type 3 chondrites show wide variation. Table 2 gives selected analytical data and Table 3 shows average compositions of Fe-Ni metal in the samples studied here. In this paper, we call all Fe-Ni metal with <7.5 wt.% Ni kamacite (Brearley and Jones 1998), and all other metal is referred to as Ni-rich metal. Semarkona Chondrules in Semarkona commonly contain Fe-Ni metal grains in their interiors (Fig. 1a). The metal spherules in the chondrule interiors commonly contain submicrometer size Ni-rich grains (Fig. 1b). Reisener and Goldstein (1999) interpreted the interior metal to be a fine-grained plessitic intergrowth of kamacite and tetrataenite. We observed more than 100 chondrules, and found that the interior metal in most chondrules has a similar plessitic texture. An intergrowth of coarser-grained kamacite and Ni-rich metal, several micrometers in size, also occurs in a few Semarkona chondrule interiors. The two textural types of metal, plessite and coarser-grained intergrowths, never occur together in the same chondrule. Additionally, there is no relationship between the chondrule texture or composition and the type of interior metal it contains. Fe-Ni metal spherules in several Semarkona chondrule interiors contain schreibersite grains of submicrometer size. Tiny inclusions of chromite and Fe- or Ca-phosphate, smaller than a few micrometers in size, also occur in metal spherules

4 1164 M. Kimura et al. Table 2. Selected analytical data of Fe-Ni metal in type chondrites (wt.%). Class Meteorite Occurrence 1 Note Si P Cr Fe Co Ni Total LL3.01 Semarkona Chond. int. Ni-Co-poor Chond. int Chond. int. Plessite Chond. int. b.d. b.d b.d IM b.d. b.d. b.d Chond. int. Schreibersite metal L3.05 EET90161 Chond. int b.d IM b.d. b.d. b.d IM Co-rich 0.09 b.d. b.d L3.05 QUE97008 Chond. int IM 0.11 b.d. b.d LL3.10 NWA 1756 Chond. int. Schreibersite b.d LL3.15 Y Chond. int. Ni-Co-poor 0.06 b.d Chond. sur. b.d. b.d. b.d Chond. int b.d Chond. int b.d. b.d Chond. int. Co-rich 0.18 b.d CO3.03 ALHA77307 Chond. int IM b.d. b.d. b.d CO3.05 Y Chond. int b.d Chond. sur. b.d CO3.1 A Chond. int b.d IM 0.16 b.d Cug Acfer 094 Chond. int IM Martensite Chond. int. = Chondrule interior, Chond. sur. = Chondrule surface, IM = Isolated mineral. b.d.:below detection limits (2σ, wt%), 0.02 for Si and Cr, 0.03 for P and Co. of many chondrules (Fig. 1b), as previously reported (e.g., Zanda et al. 1994). Fe-Ni metal grains on chondrule surfaces and occurring as isolated minerals in the matrix of Semarkona do not show plessitic intergrowth textures. In rare cases, they show intergrowths of coarse-grained kamacite and Ni-rich metal, more than several micrometers in size. Metal on chondrule surfaces and isolated grains is generally associated with troilite (Fig. 1a) and in some cases magnetite, which was identified by optical properties and composition (giving 93% totals with Fe expressed as FeO). In some cases, the metal is surrounded by troilite and magnetite (Fig. 1c). The metal composition in Semarkona also depends on the occurrence (Fig. 2a). Most of the Fe-Ni metal grains on chondrule surfaces and occurring as isolated minerals are highly enriched both in Ni ( wt%) and Co ( %), with only a few kamacite grains ( % Ni and % Co) found in these textural settings. Similar Ni-Corich metal has been reported as rare occurrences by Afiattalab and Wasson (1980) and Reisener and Goldstein (1999) from Semarkona, and a few type 3 chondrites (Smith et al. 1993; Scott and Jones 1990). However, the textural settings of the Ni-Co-rich metal were not well documented. We find that Ni- Co-rich metal is common in Semarkona and other highly primitive chondrites, as shown later, and it occurs exclusively as isolated minerals in the matrix and on chondrule surfaces. Rubin (1990) predicted the occurrence of Ni-Co-rich metal to explain the Co-poor kamacite in some type 3 chondrites. Fe-Ni metal in chondrule interiors in Semarkona shows a wide range of compositions, % Ni and % Co, at least partly because it has plessitic structure and we report single-point analyses. However, most grains are Nipoor (3 14% Ni) with % Co. Some coarse-grained Nirich metal ( % Ni), coexisting with kamacite in chondrule interiors, is depleted in Co ( %). This is clearly different from the Ni-Co-rich metal on chondrule surfaces and occurring as isolated grains in the matrix. Fe-Ni metal in chondrule interiors and in the other occurrences contains and % Si, , and % P, and and % Cr, respectively (Table 3 and Fig. 3). The contents of these minor elements are higher in chondrule interior metal than in metal in the other textural settings. The minor-element contents of the interior metal overlap with compositional ranges for metal that have been previously reported (e.g., Zanda et al. 1994). Metal analyses for some Semarkona chondrules have high amounts of P (up to 7.9%), which correlates with Ni contents (up to 30.6%) (Table 2). These metal grains contain submicrometersize schreibersite grains and the high Ni and P are likely due to electron beam overlap onto schreibersite during analysis. Regardless of their occurrence, none of the metal grains studied shows compositional zoning for both major and minor

5 Fe-Ni metal in primitive chondrites 1165 Table 3. Average compositions of kamacite and Ni-rich metal (wt%). No. of Class Meteorite Phase Occurrence 1 Analyses Si P Cr Fe Co Ni Total LL3.01 Semarkona Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM ± ± ± ± ± ± Ni-rich metal Chond. int ± ± ± ± ± ± Ni-rich metal Chond. sur. and IM 59 b.d. b.d ± ± ± ± L3.05 EET90161 Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM 30 b.d. b.d. b.d ± ± ± Ni-rich metal Chond. int ± ± ± ± ± ± Ni-rich metal Chond. sur. and IM 15 b.d. b.d ± ± ± ± L3.05 QUE97008 Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM 7 b.d. b.d. b.d ± ± ± Ni-rich metal Chond. int ± ± ± ± ± ± Ni-rich metal Chond. sur. and IM ± 0.07 b.d. b.d ± ± ± LL3.10 NWA 1756 Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM ± 0.05 b.d ± ± ± ± Ni-rich metal Chond. int ± 0.06 b.d ± ± ± ± Ni-rich metal Chond. sur. and IM 41 b.d. b.d. b.d ± ± ± LL3.15 Y Kamacite Chond. int ± 0.07 b.d ± ± ± ± Kamacite Chond. sur. and IM ± 0.07 b.d. b.d ± ± ± Ni-rich metal Chond. int ± 0.10 b.d ± ± ± ± Ni-rich metal Chond. sur. and IM ± 0.06 b.d. b.d ± ± ± LL3.15 Y Kamacite All ± 0.03 b.d ± ± ± ± Ni-rich metal All ± 0.04 b.d ± ± ± ± LL3.15 Y Kamacite All 26 b.d. b.d ± ± ± ± Ni-rich metal All 22 b.d. b.d. b.d ± ± ± LL3.2 Krymka Kamacite All 30 b.d. b.d ± ± ± ± Ni-rich metal All 24 b.d. b.d ± ± ± ± LL3.2 Y Kamacite All 31 b.d. b.d. b.d ± ± ± Ni-rich metal All 7 b.d ± ± ± ± ± LL3.2 Y Kamacite All 44 b.d. b.d ± ± ± ± Ni-rich metal All 10 b.d. b.d. b.d ± ± ± H3.2 Y Kamacite All 21 b.d. b.d ± ± ± ± Ni-rich metal All 30 b.d. b.d ± ± ± ± LL3.5 ALHA77260 Kamacite All ± 0.01 b.d. b.d ± ± ± Ni-rich metal All 8 b.d. b.d. b.d ± ± ± LL3.7 ALHA77304 Kamacite All 21 b.d. b.d. b.d ± ± ± Ni-rich metal All 26 b.d. b.d. b.d ± ± ± LL3.9 Bo Xian Kamacite All 7 b.d. b.d. b.d ± ± ± Ni-rich metal All 12 b.d. b.d. b.d ± ± ± CO3.03 ALHA77307 Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM ± ± ± ± ± ± Ni-rich metal Chond. int ± ± ± ± ± ± Ni-rich metal Chond. sur. and IM 39 b.d. b.d ± ± ± ± CO3.05 Y Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM 17 b.d ± ± ± ± ± Ni-rich metal Chond. int ± ± ± ± ± ± Ni-rich metal Chond. sur. and IM 55 b.d ± ± ± ± ±

6 1166 M. Kimura et al. Table 3. (Continued). Average compositions of kamacite and Ni-rich metal (wt%). No. of Class Meteorite Phase Occurrence 1 Analyses Si P Cr Fe Co Ni Total CO3.1 A Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM ± ± ± ± ± ± Ni-rich metal Chond. int ± 0.04 b.d ± ± ± ± Ni-rich metal Chond. sur. and IM ± 0.04 b.d ± ± ± ± CO3.6 Y Kamacite All ± 0.04 b.d ± ± ± ± Ni-rich metal All ± 0.03 b.d ± ± ± ± CC ung Acfer094 Kamacite Chond. int ± ± ± ± ± ± Kamacite Chond. sur. and IM ± ± ± ± ± ± Ni-rich metal Chond. sur. and IM 33 b.d. b.d. b.d ± ± ± Chond. int.: Chondrule interior, Chond. sur.: Chondrule surface, IM: Isolated mineral. b.d.: below detection limits (2σ, wt%), 0.02 for Si and Cr, 0.03 for P.

7 Fe-Ni metal in primitive chondrites 1167 Fig. 1. Backscattered electron (BSE) images of a) a chondrule in Semarkona, consisting of olivine phenocrysts (Oli) and groundmass glass (Gla). Spherical Fe-Ni metal (Met) is abundant in this chondrule. On the chondrule surface, a metal and troilite association (Met + Tro) is observed. Width of field is 1mm. b) A plessitic intergrowth in an Fe-Ni metal spherule in a Semarkona chondrule. Image taken by FE-SEM. Width of field is 30 µm. c) An isolated Fe-Ni metal grain in the matrix of Semarkona, which is surrounded by troilite (Tro) and magnetite (Mag). Width of field is 30 µm. d) A low-ca pyroxene phenocryst (Lpx) in a type II chondrule of Semarkona with abundant metal inclusions. Note that pyroxene compositions increase in MgO content toward the metal inclusions (darker area around metal), e.g., from En69 to En83. Width of field is 55 µm. elements. This is in contrast to the zoned metal grains that are common in CH and CB chondrites (e.g., Meibom et al. 1999; Weisberg et al. 2001). In the Semarkona thin sections studied here, three chondrules, two type I and one type II (Fig. 1d), contain very Ni-Co-poor metal grains 5 to 40 micrometers in size. These metal grains are homogeneous in texture (not plessitic), and contain % Ni and % Co, with %

8 1168 M. Kimura et al. Fig. 1. Continued. BSE images of g) a chondrule metal grain in Krymka (LL3.2), consisting of kamacite and coarse-grained Ni-rich metal. Width of field is 30 µm. h) A chondrule metal grain in ALHA77307 (CO3.03). Fine-grained Ni-rich metal is distributed within the kamacite. Width of field is 30 mm. i) A metal spherule in an A (CO3.1) chondrule. Width of field is 30 mm. j) An isolated Fe-Ni metal grain (martensite) in the matrix of ungrouped chondrite Acfer 094, which contains 8.2% Ni. Note that the metal is homogeneous in texture. Width of field is 230 mm. Si, % P and % Cr (Table 2). These chondrules also contain Ni-free metal grains that are submicrometer in size and are interpreted to be in situ reduction products of FeO in silicates (e.g., Rambaldi and Wasson 1982). It is noted that silicate compositions increase in MgO content toward the Ni-poor metal, such as En 69 to En 83 in the type II chondrule (Fig. 1d), consistent with a reduction origin for the Ni-poor metal. L/LL , Y (LL3.15) and CO Fe-Ni metal in chondrule interiors of L/LL and CO chondrites never show the plessitic intergrowths that are typical of Semarkona (Figs. 1e, 1f, 1h, and 1i). Nirich metal grains in these chondrules are generally larger than in the fine intergrowths of Semarkona metal. In type chondrites, many of the metal spherules in chondrule interiors contain small (usually less than several micrometers in size) schreibersite grains (Fig. 1e) and tiny inclusions of chromite and phosphate (Figs. 1e, 1h and 1i). As in Semarkona, Fe-Ni metal on chondrule surfaces and occurring as isolated grains in the matrix of chondrites at or above petrologic type 3.03 also do not show plessitic intergrowths. Although Fe-Ni metal grains in L/LL , one LL3.15 (Y-74660) and CO chondrites do not show plessitic texture in chondrules, they do show wide chemical variation and compositional dependence on their occurrence. Metal in chondrule interiors usually contains % Ni and % Co. Metal on chondrule surfaces and occurring as isolated matrix grains are generally enriched both in Ni ( %) and Co ( %), similar to corresponding occurrences in Semarkona (Table 3 and Fig. 2). In Y-74660, one type I chondrule that we studied contains Ni-Co-poor metal ( % Ni and % Co). Dusty Ni-free metal is also found in association with the Ni- Co-poor metal. Fe-Ni metal in L/LL and CO chondrites contains and % Si, and % P, and and % Cr in chondrules and other occurrences, respectively (Table 3 and Fig. 3). The minor-element contents of this metal overlap the range shown by metal in Semarkona.

9 Fe-Ni metal in primitive chondrites 1169 Fig. 2. c) Ni versus Co plot of Fe-Ni metal in CO chondrites and Acfer 094. Ni-rich metal occurring as isolated mineral grains and on chondrule surfaces in CO chondrites contains more Co than kamacite in the chondrules from these chondrites. Fe-Ni metal in Acfer 094 has a similar Co/Ni ratio to CI chondrites. Fig. 2. a) Ni versus Co (wt%) plot for random point analytical data of Fe-Ni metal in Semarkona. C = metal occurring in chondrule interiors, I/S = isolated mineral grains in the matrix and occurring on chondrule surfaces. Only Ni-rich metal occurring as isolated mineral grains and on chondrule surfaces contain higher Co contents than the kamacite in chondrules. The dotted line shows the CI chondrite (solar) Co/Ni ratio, after Anders and Grevesse (1989). b) Ni versus Co (wt%) plot of Fe-Ni metal in H/L/LL3.05 to 3.9 chondrites. Note that Ni-rich metal occurring as isolated mineral grains and on chondrule surfaces in type chondrites and an LL3.15 (Y-74660) contain higher Co contents than kamacite in the chondrules from these chondrites. In the other chondrites, kamacite has higher Co contents than the Ni-rich metal. Fig. 3. a) Plot of Ni and Si contents of Fe-Ni metal in Semarkona, L/ LL , CO , and Acfer 094. Metal in these chondrites contains minor elements. Co-rich metal (up to 13.5%) occurs in EET 90161, Y74660, and Y-81020, as it did in Semarkona (Table 2). The grains are usually several micrometers in size, and typically occur as isolated minerals. Co-rich metal was also reported from some LL chondrites (Afiattalab and Wasson 1980; Rubin 1990) and carbonaceous chondrites (Kimura and Ikeda 1992; Hua et al. 1995). Chondrule metal grains in H/L/LL and Y (CO3.6) show much coarser-grained intergrowths of kamacite and Ni-rich metal, up to ~10 µm (Fig. 1g), than do those in type chondrites. Thus, the texture of metal in chondrule interiors in type chondrites is intermediate between those in Semarkona and H/LL chondrites and metal texture seems to correlate with petrologic type. Schreibersite in QUE (L3.05) and NWA 1756 (LL3.10) contains % Ni (Table 2). H/L/LL and CO3.6

10 1170 M. Kimura et al. from the other chondrites. Intergrowths of coarse-grained kamacite and Ni-rich metal are extremely rare in Acfer 094, even in the isolated metal grains of this meteorite. Although most of the Fe-Ni metal in Acfer 094 seems to be homogeneous in texture, the composition depends on the occurrence: chondrule metal has % Ni and % Co, whereas isolated metal has % Ni and % Co (Table 3). The Ni/Co ratio is close to the solar ratio (Fig. 2b). We did not find compositional zoning in Acfer 094 metal in any of its occurrences. The compositions of the abundant, isolated metal grains, which contain % Ni (Fig. 2c), fall within the kamacite + taenite field of the Fe-Ni phase diagram (e.g., Meibom et al. 2005). However, these grains do not show decomposed textures, as does the plessite in Semarkona. In all cases, the texture appears to be homogeneous (Fig. 1j). Therefore, it is probable that the metal in Acfer 094 is martensite. Such martensitic metal was also reported from CH and CB chondrites (e.g., Meibom et al and 2005; Krot et al. 2000; Campbell et al. 2005a). Fe-Ni metal in Acfer 094 contains and % Si, and % P, and and % Cr in chondrules and other occurrences, respectively. The minor element content of Acfer 094 metal generally overlaps with the composition of metal in Semarkona and type chondrites. One isolated metal grain, 80 µm in size, is highly enriched in Si (up to 3.0%), although it seems to contain no silicide or silicate inclusions. Metal with high Si content was also observed in ALH 85085, a CH chondrite (Weisberg et al. 1988). Number Density of Ni-Rich Metal in Chondrule Fig. 3. b) Plot of Ni and P contents of Fe-Ni metal in Semarkona, L/ LL , CO , and Acfer 094. c) Plot of Ni and Cr contents of Fe-Ni metal in Semarkona, L/LL , CO , and Acfer 094. In type chondrites, except Y (LL3.15), kamacite ( % Ni) has a higher Co content ( %) than does Ni-rich metal ( Ni and % Co), independent of its occurrence (Table 3 and Fig. 2). This is consistent with previous studies of type 3 6 chondrites (e.g., Afiattalab and Wasson 1980; Nagahara 1982). Fe-Ni metal in these chondrites rarely contains P, Si and Cr ( % Si, % P, and % Cr) (Table 3). Acfer 094 Fe-Ni metal is fairly abundant in the chondrules and matrix of Acfer 094. Most of the metal in Acfer 094 is nearly homogeneous (Fig. 1j), in contrast to the plessitic intergrowths in the metal in Semarkona chondrules and the intergrowths of kamacite and Ni-rich metal in chondrules Metal textures in chondrules vary continuously from fine-grained plessite in Semarkona to coarse-grained aggregates of kamacite and Ni-rich metal in type chondrites. In order to evaluate these textural variations quantitatively, we measured the number density and average area of Ni-rich metal grains in chondrules from the Semarkona, L/LL and CO chondrites. The number density is here defined as the number (N) of Ni-rich metal grains per unit area of an individual metal spherule (N/µm 2 ). The average area is calculated from areas of individual Ni-rich metal grains in each metal spherule in chondrules. In each chondrite, we calculated the average by measuring several metal spherules in chondrules (Table 4). Figure 4a shows that there is a clear negative correlation between the number density and area of Ni-rich metal. With increasing petrologic type, the number density of Ni-rich metal grains drastically decreases and the area increases, as fine-grained plessite is replaced by coarse-grained Ni-rich metal coexisting with kamacite. As mentioned above, Co is enriched in isolated Ni-rich metal in the matrix, relative to kamacite in chondrules in

11 Fe-Ni metal in primitive chondrites 1171 Table 4. Area and number density of Ni-rich metal. Class Meteorite Chondrule 1 Area 2 Density 3 LL3.01 Semarkona ± ± L3.05 EET ± ± L3.05 QUE ± ± LL3.10 NWA ± ± LL3.15 Y ± ± LL3.15 Y ± ± LL3.2 Krymka ± ± LL3.2 Y ± ± LL3.2 Y ± ± CO3.03 ALHA ± ± CO3.05 Y ± ± CO3.1 A ± ± CO3.6 Y ± ± Number of measured chondrules in each meteorite. 2 Average area of individual Ni-rich metal grain (µm 2 ). 3 Average number density of Ni-rich metal grain per metal spherule (N/µm 2 ). Semarkona and type chondrites. Figure 4b shows the clear positive correlation between the number density and the ratio of Co in isolated Ni-rich metal grains to Co in chondrule kamacite. DISCUSSION Primitive Martensitic Metal in Acfer 094 Acfer 094 has clearly experienced little metamorphism, based on the primitive nature of its matrix, abundant presolar grains and other features (e.g., Newton et al. 1995). Our observations are consistent with this. Plessitic textures are absent in Acfer 094 metal. Instead, most Fe-Ni metal grains in Acfer 094 are martensite, like the metal in CH and CB chondrites. Martensite forms by quenching from high temperature conditions, and it decomposes into plessite under low temperature conditions, as suggested by Reisener et al. (2000). Therefore, the presence of martensite strongly indicates that Acfer 094 was subjected to almost no metamorphism on the parent body, as suggested for CH chondrites by Meibom et al. (1999). Therefore, Acfer 094 is more primitive than Semarkona, which contains plessite. This idea is supported by the Co/Ni ratio of the metal in Acfer 094: for kamacite in the chondrule interiors, for kamacite on chondrule surfaces and isolated minerals in the matrix, and for Ni-rich metal (Table 3). These values generally agree with the CI (i.e., solar) ratio (0.046 after Anders and Grevesse, 1989) (Fig. 2c), suggesting that Ni and Co were neither fractionated in the solar nebula, nor redistributed between kamacite and Ni-rich metal in the parent body. We conclude that Fe-Ni metal in Acfer 094 preserves its primordial composition. We also conclude that the primitive metal in Acfer 094 was quenched from high temperatures within chondrules before accretion to parent bodies, in the solar nebula. Thermal History of Fe-Ni Metal in Semarkona Among the chondrites we studied, fine-grained plessitic intergrowths are only found in metal from the interiors of Semarkona chondrules. Plessite has also been reported in highly shocked meteorites, mainly S5-S6 (Taylor and Heymann 1971; Smith and Goldstein 1977; Bennett and McSween 1996). However, Semarkona and the other samples studied here do not show evidence of being heavily shocked; all are S3. Our observations suggest that the plessitic intergrowths of metal in Semarkona chondrule interiors resulted from mild metamorphic heating of martensite in the parent body (Reisener and Goldstein 1999). It seems likely that the plessite in Semarkona probably resembled that in Acfer 094, both in terms of its texture and composition, prior to secondary thermal processing. Semarkona chondrule metal shows pervasive plessitic intergrowths in the sections we studied, consistent with a parent body process. Reisener et al. (2000) suggested that if the parent taenite cools >>5 K/day, it transforms to martensite, and martensite decomposes into a taenite + kamacite intergrowth during 1 year at temperatures as low as 573 K. Thus, Fe-Ni metal in Semarkona chondrules records the earliest stages of metal decomposition to form plessite in the parent body. Physical conditions for the heating process that resulted in metal decomposition are discussed below. In contrast, Ni-Co-rich metal on chondrule surfaces and occurring as isolated minerals in the matrix of Semarkona has a different texture from metal in chondrule interiors. This suggests that the former metal was not directly derived from broken chondrules. Additionally, the composition of the metal in chondrule interiors is different from that on chondrule surfaces and as isolated grains. This observation suggests disequilibrium and indicates low degrees of metamorphism in the parent body. The metal on chondrule surfaces and as isolated grains is generally surrounded by or associated with

12 1172 M. Kimura et al. solar Co/Ni ratio. In contrast, Fe-Ni metal on chondrule surfaces and as isolated grains has a distinctly lower Co/Ni ratio than the solar value (Fig. 2a), which might be balanced by the occurrence of isolated Co-rich metal. Thermal History of Fe-Ni Metal in H/L/LL and CO Chondrites Fig. 4. a) Average area versus number density of Ni-rich metal grains in metal spherules in chondrules from Semarkona, L/LL and CO chondrites. With increasing petrologic type, the number density drastically decreases and the area increases. Petrologic type is evidently related to these characteristic features of Fe-Ni metal. b) Plot of the number density and the ratio of Co in isolated Ni-rich metal grains to Co in chondrule kamacite. troilite and magnetite. Therefore, it probably formed through sulfidation and oxidation of Ni-Co-poor metal. In the oxidized CV chondrites, Ni-rich metal and magnetite are common (e.g., McSween 1977; Hua and Buseck 1998). In Allende, Ni-rich metal contains % Ni and % Co (Kimura, unpublished data), which overlaps in composition with the Ni-Co-rich metal in Semarkona. Our focused-beam electron probe analyses are nonrepresentative samples of kamacite and Ni-rich metal occurring in individual plessite particles in chondrules, and the metal in Semarkona chondrule interiors does not have the solar Ni/Co ratio, as does that in Acfer 094 (Fig. 2). However, the bulk composition of chondrules (Grossman and Wasson 1985), and the bulk metal compositions in chondrules of Semarkona (Kimura and Weisberg 2004) do plot along the Fe-Ni metal in the other (higher petrologic type) chondrites studied here has different features from that in Semarkona and Acfer 094. Fe-Ni metal in H/L/LL , except Y (LL3.15), has the following features: 1) higher Co contents in kamacite than in Ni-rich metal, regardless of occurrence, 2) intergrowths of coarse-grained kamacite and Ni-rich metal in chondrule interiors and elsewhere, and 3) nearly undetectable (<0.03 wt.%) minor element contents. Metal in equilibrated chondrites has textural and chemical features similar to those outlined for metal in type chondrites (e.g., Afiattalab and Wasson 1980). The metal compositions in type chondrites are interpreted to be the result of parent body metamorphism. Cobalt is preferentially redistributed into kamacite during thermal metamorphism, as expected from partitioning under equilibrium conditions (Rasmussen et al. 1988). Through metamorphism, plessite is transformed into intergrowths of coarser-grained kamacite and Ni-rich metal. Although Fe-Ni metal in L/LL and Y (LL3.15) does not show plessitic intergrowths, isolated metal in the matrix is usually enriched in Ni and Co like that in Semarkona. The metal in L/LL and Y (LL3.15) still contains detectable minor elements, Si, P and Cr (although we cannot rule out the presence of tiny inclusions below the resolution of the SEM). Thus, L/LL chondrites have intermediate features between Semarkona and H/LL Therefore, the degree of thermal metamorphism inferred from the silicate mineralogy of L/ LL chondrites is also reflected in the composition and texture of their metal. Fe-Ni metal in chondrule interiors in L/LL chondrites did not equilibrate with the isolated metal grains. Fe-Ni metal in ALHA77307, Y-81020, and A (CO ) has similar textural and chemical features to the metal in L/LL chondrites (Fig. 4). Therefore, metal in these CO chondrites was subjected to metamorphic effects similar to metal in L/LL chondrites. Similarly, metal in Y (CO3.6) has features close to metal in LL3.2 or higher. Classification of Primitive Chondrites from Fe-Ni Metal The diagrams of the number density of Ni-rich metal grains and Co ratios (Figs. 4a and 4b) can be used to distinguish petrologic types for O and CO chondrites. Thus, the metal features are consistent with the petrologic type of most O and

13 Fe-Ni metal in primitive chondrites 1173 CO chondrites proposed by Grossman and Brearley (2005). We conclude that Fe-Ni metal is a sensitive indicator for identifying the most primitive chondrites. The composition and texture of metal are more sensitive to the earliest onset of thermal metamorphism than TL sensitivity or chemical heterogeneity of silicate phases. Fe-Ni metal features as well as the Cr distribution in ferroan olivine can be used to accurately determine the metamorphic grade in O and CO chondrites. Both Semarkona and ALHA77307 have been classified as type 3.00 (in the LL and CO groups, respectively: Grossman and Brearley 2005; Grossman and Rubin 2006). Scott and Jones (1990) also suggested that ALHA77307 is the most primitive CO chondrite. However, in Figs. 4a and 4b, ALHA77307 plots in the region of L/LL and CO , and does not plot near Semarkona. Plessitic intergrowths that are typical in Semarkona are not observed in ALHA Instead, it has intergrowths of coarser-grained kamacite and Ni-rich metal. Therefore, we can use the properties of metal to conclude that ALHA77307 was subjected to a somewhat greater degree of secondary thermal modification than was Semarkona. Based on the maturation of organic matter, Bonal et al. (2007) concluded that ALHA77307 experienced higher peak metamorphic temperatures than Semarkona, but they also clearly recognized that it had properties indicating lower temperatures than the meteorites considered to be type 3.05 by Grossman and Brearley (2005). Specifically, type 3.05 O chondrites show evidence for incipient exsolution of Cr-rich phases from ferroan olivine, loss of S from fine-grained matrix, and much greater entry of alkalis into type I chondrules than is seen in either Semarkona or ALHA Consequently, Bonal et al. (2007) classified ALHA77307 as type Although we cannot resolve ALHA77307 from the type chondrites by using the metal data (Fig. 4), our data do not conflict with such a classification. The similarity in texture and chemistry of Fe-Ni metal between L/LL and CO chondrites suggests similar metamorphic conditions for these chondrites. Metamorphic Conditions of Primitive Chondrites Semarkona All the textural and chemical data for Fe-Ni metal obtained here indicate that Semarkona is one of the most primitive chondrites, consistent with previous studies, mainly based on silicate phases (e.g., Brearley and Jones 1998; Grossman and Brearley 2005). Despite this, it does show evidence for secondary (low temperature) aqueous alteration, such as the occurrence of smectite (Hutchison et al. 1987) and altered mesostasis in chondrules (Grossman et al and 2002). Krot et al. (1997) suggested that carbide-magnetite assemblages in type 3 O chondrites, including Semarkona, formed as the result of hydrothermal alteration of metal. Alexander et al. (1989) suggested that the metamorphic Fig. 5. Distribution of temperatures based on Co partitioning between kamacite and Ni-rich metal for Fe-Ni metal in types chondrites, using the method of Afiattalab and Wasson (1980). temperatures for Semarkona did not exceed 260 C, based on the hydrous phases. Pentlandite chemistry also supports such temperature conditions (Zanda et al. 1995). Keller (1998) suggested that the maximum temperature for Semarkona on the parent body did not exceed 500 C from the occurrence of carbides. Rubin et al. (1999) reported the occurrence of tetrataenite in metal-troilite spherules, indicative of mild metamorphism at temperature below 320 C. It is under these low-temperature conditions that the plessitic textures formed from martensite in the LL chondrite parent body. Other Chondrites Fe-Ni metal in the other O and CO chondrites that we studied partly or completely lost its primordial character through thermal metamorphism; all metal in these chondrites lacks martensitic or plessitic intergrowth textures, and Co is preferentially partitioned into kamacite in the type chondrites. In order to estimate the metamorphic conditions, we calculated the metal temperature based on Co partitioning between kamacite and Ni-rich metal determined by Afiattalab and Wasson (1980). Fe-Ni metal in the chondrule interiors of type O and CO chondrites give temperatures in the range of C (Fig. 5). The temperatures within and between chondrules in a single chondrite, and for the various chondrites studied here overlap, regardless of petrologic type or chemical group. By using the same method, Shibata and Matsueda (1994) calculated metal

14 1174 M. Kimura et al. temperatures of C for Y (CO3.05), and Nagahara (1982) obtained temperatures lower than 500 C. All these metal temperatures for type 3 chondrites are within the same range. Schreibersite often occurs within Fe-Ni metal in the chondrules (Fig. 1e and Table 2). The schreibersite-metal geothermometer (Zhang and Sears 1996) gives 465 and 471 C in QUE (L3.05) and NWA 1756 (LL3.10), respectively. These temperatures are consistent with those determined from metal composition. Keck and Sears (1987) estimated metamorphic temperatures of C for CO chondrites in a TL study. Jones and Rubie (1991) suggested that the maximum temperatures for CO chondrites were about 500 C from Fe- Mg diffusion in olivine. All these temperatures are generally consistent with the results obtained here. Therefore, it is probable that these O and CO chondrites experienced metamorphism under relatively low-temperature conditions, but higher than that for Semarkona. Grain size of Ni-rich metal increases and number density decreases through thermal processing. Yet, the Ni-Co distributions in Fe-Ni metal in type chondrites are still similar to that in Semarkona metal, and are different from those in chondrites of higher types. Only local diffusion took place, on the scale of a metal spherule in a chondrule. Chondrule interior metal did not equilibrate with isolated metal in the matrix under such low-temperature conditions in Semarkona and type chondrites. Spinel Group Minerals and Fe-Ni Metal Kimura et al. (2004 and 2006b) suggested that ilmenite and spinel group minerals are metamorphic indicators in LL3 chondrites. The compositions of these minerals can be used to distinguish chondrites of type from higher types. However, the spinel composition cannot be used to distinguish the subgroups between types 3.0 and 3.3. The olivine-spinel geothermometer gives igneous crystallization temperatures >1000 C for chondrules in types , whereas for chondrules in types it gives metamorphic temperatures of C (Johnson and Prinz 1991; Kimura et al. 2006b). Therefore, diffusion barely took place in the spinel of type chondrites. In contrast, Fe-Ni is a sensitive metamorphic indicator for chondrites with petrologic subtypes between 3.00 and 3.2. This difference between the behavior of metal and oxide phases is due to the large contrast in their diffusion rates; diffusion in metal is much faster than in oxides (Dean and Goldstein 1986; Freer and O Reilly 1980). Under the lowtemperature metamorphic conditions for the lowest type chondrites (type <3.3), diffusion in Fe-Ni metal is significant. Comparison of O and CO Chondrites The characteristic features of Fe-Ni metal suggest that CO3 chondrites were metamorphosed under similar conditions to those of type 3 O chondrites. This is supported by similar geothermometries and Cr distribution trends in olivine between O and CO chondrites (Grossman and Brearley 2005; Grossman and Rubin 2006). A wide range of petrologic types is observed in CO3 chondrites, as in type 3 O chondrites (Keck and Sears 1987). Therefore, the thermal evolution of these chondritic parent bodies might have been very similar to each other. However, O and CO chondrites originated on different parent bodies, based on oxygen isotopic compositions (e.g., Clayton 1993) and the lack of breccias composed of CO and O chondrites. All CO chondrites are type 3 (Brearley and Jones 1998), whereas most O chondrites are types 4 6. This observation suggests that the internal structures and/or sizes were different between the O and CO parent bodies. However, it is probable that the thermal conditions for the regions where the type 3 chondrites resided in the O and CO parent bodies were similar. Revision of the Petrologic Subtypes Grossman and Brearley (2005) showed that there is a wide range of metamorphic features among O and C chondrites that traditionally have been classified as types 3.0 or 3.1. They recommended adding a second decimal place to the classification scheme in order to distinguish primitive meteorites that had clearly experienced different degrees of thermal processing. Instead of the original two categories, Grossman and Brearley (2005) separated types 3.0 and 3.1 into four divisions: 3.00, 3.05, 3.10, and They suggested that Semarkona is the only LL3.00 chondrite. We can use our data to continue the refinement of the metamorphic scale, as we have presented evidence that Semarkona is more metamorphosed than Acfer 094. Rubin et al. (1999), and Grossman and Brearley (2005) also showed evidence that Semarkona had experienced small degrees of metamorphism. We suggest that the ubiquitous presence of martensitic metal with the solar Ni/Co ratio is the hallmark of chondrites that have escaped significant thermal processing on the parent body, and should therefore be classified as type Thus, we assign type 3.00 to Acfer 094, and revise Semarkona to LL3.01. By this definition, the characteristic feature of type 3.01 chondrites should be the presence of plessitic metal in chondrules. Other chondrites also contain primitive martensitic metal similar in composition to that in Acfer 094, and should be assigned to type These include all of the CR, CH, and CB chondrites, and probably most CM chondrites (Weisberg et al and 2001; Grossman and Olsen 1974). Campbell et al. (2005b) suggested that the metal in CH and CB chondrites provide information on condensation processes in the solar nebula. Most CR and CM chondrites are currently classified as type 2 chondrites, a designation that indicates an abundance of phyllosilicates produced during aqueous alteration. There is no reason to assume that all type 2 chondrites have entirely