Nepheline and sodalite in the matrix of the Ningqiang carbonaceous chondrite: Implications for formation through parent-body processes

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Nepheline and sodalite in the matrix of the Ningqiang carbonaceous chondrite: Implications for formation through parent-body processes Megumi Matsumoto a,⇑, Kazushige Tomeoka a, Yusuke Seto a, Akira Miyake b, Mitsuhiro Sugita a a

Department of Earth and Planetary Sciences, Faculty of Science, Kobe University, Kobe 657-8501, Japan b Department of Geology and Mineralogy, Faculty of Science, Kyoto University, Kyoto 606-8502, Japan

Received 1 July 2013; accepted in revised form 11 November 2013; available online 22 November 2013

Abstract Ningqiang is an ungrouped carbonaceous chondrite that chemically and petrologically resembles CV3 chondrites. The matrix of Ningqiang shows much higher abundances of Na, K, and Al by factors of 4.4, 2.7, and 1.6, respectively, than in CV3 chondrites. Our scanning and transmission electron microscope observations and synchrotron radiation X-ray diffraction measurements reveal that the major proportions of these elements can be attributed to the presence of nepheline and sodalite. Rietveld refinement of X-ray diffraction data shows that the feldspathoids constitute 7.7 vol.% of all crystalline phases in the matrix. Nepheline and sodalite occur mostly as discrete, equidimensional grains 2–5 lm in diameter that are dispersed homogeneously in the matrix. Most of the grains contain inclusions of Fe-rich olivine and minor Ca pyroxene, magnetite, troilite, and pentlandite. Despite the high abundances of Na, K, and Al in the matrix of Ningqiang, the bulk meteorite abundances of these elements are comparable to those of the CV group (e.g., Rubin et al., 1988). This means that the chondrules, which constitute a major proportion of the volume other than the matrix in Ningqiang, are depleted in Na, K, and Al. In fact, our analyses and observations show that the chondrules in Ningqiang overall contain very small amounts of these elements. Our interpretation of these findings suggests that nepheline and sodalite in the Ningqiang matrix were originally formed by Na-metasomatism of the chondrules and Ca–Al-rich inclusions in the meteorite parent body. Afterward, they were likely disaggregated and scattered into the matrix. However, it is difficult to envisage that the disaggregation and scattering occurred in situ in the present setting of the meteorite. Hence, we suggest that the Ningqiang meteorite underwent these processes before final lithification. Ó 2013 Elsevier Ltd. All rights reserved.

1. INTRODUCTION Sodium is a relatively minor constituent of carbonaceous chondrites (0.3–0.5 wt.%; Wasson and Kallemeyn, 1988). However, because of its volatility and high mobility in aqueous fluids, Na serves as an important indicator of thermal metamorphism and aqueous alteration in carbonaceous chondrites. ⇑ Corresponding author. Tel.: +81 78 803 5746.

E-mail address: [email protected] (M. Matsumoto). 0016-7037/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.gca.2013.11.016

Sodium-rich nepheline (NaAlSiO4) and occasionally sodalite (Na4Al3Si3O12Cl) are present in chondrules, Ca–Al-rich inclusions (CAIs), amoeboid olivine inclusions (AOIs), and matrices in CV3 (e.g., Grossman and Steele, 1976; MacPherson and Grossman, 1984; Wark, 1986; Hashimoto and Grossman, 1987; Ikeda and Kimura, 1995; Kimura and Ikeda, 1995, 1997, 1998; Krot et al., 1997, 1998a, b; Wasserburg et al., 2011) and CO3 chondrites (e.g., Kurat and Kracher, 1980; Tomeoka et al., 1992; Kojima et al., 1995; Jones, 1997; Russell et al., 1998; Tomeoka and Itoh, 2004). Previous studies showed

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that the feldspathoids were produced by replacing mesostasis in chondrules (e.g., Kimura and Ikeda, 1995) and Ca–Al-rich silicates in CAIs (e.g., Tomeoka et al., 1992) and AOIs (e.g., Hashimoto and Grossman, 1987). Whether the Na-metasomatism occurred by a reaction with gas in the solar nebula (e.g., Grossman and Steele, 1976; Ikeda and Kimura, 1995) or by a reaction that took place in the meteorite parent body (e.g., Krot et al., 1997) has been controversial. However, more recent studies have shown abundant evidence suggesting that the Na-metasomatism occurred in the parent bodies (e.g., Kojima et al., 1995; Jones, 1997; Krot et al., 1997, 1998a, b, 2004; Itoh and Tomeoka, 1998; Russell et al., 1998; Tomeoka and Itoh, 2004). The Ningqiang meteorite was described by Rubin et al. (1988) as an anomalous CV3 chondrite containing low abundances of CAIs and refractory lithophiles. Thus, they suggested that Ningqiang formed late in the solar nebula, after most refractory inclusions had condensed and been incorporated into other objects. Ningqiang was later described as being chemically related to the CK group (Kallemeyn et al., 1991). However, more recent studies (Kallemeyn, 1996; Wang and Hsu, 2009; Wasson et al., 2013) concluded that Ningqiang does not fit into any of the established chondrite groups, and that it should be considered as an ungrouped C3 chondrite. Among the anomalous characteristics of Ningqiang, we were especially interested in the high abundance of Na in its matrix. Previous studies have reported that the Ningqiang matrix contains 1.2 wt.% Na2O (Rubin et al., 1988), 1.2 wt.% Na2O (Kimura et al., 1997; average of the values presented for the dark matrix), and 1.9 wt.% Na2O (Wang and Hsu, 2009; after normalizing the analytical total to the same value as Rubin et al., 1988). All of these values are much higher than the averages reported for CV3 and CO3 matrices (0.37 and 0.31 wt.%, respectively; McSween and Richardson, 1977). However, Rubin et al. (1988) and Kimura et al. (1997) did not report any minerals that could be responsible for the high Na content in the matrix. Wang and Hsu (2009) described that nepheline and sodalite occur in the Ningqiang matrix, but in very minor amounts. In this paper, we present the results of mineralogical investigation of the matrix of the Ningqiang carbonaceous chondrite. The investigation was conducted using scanning and transmission electron microscopes (SEM and TEM, respectively), an electron probe microanalyzer (EPMA), and synchrotron radiation X-ray diffraction (SR-XRD). TEM enables us to observe and analyze materials on scales down to the nanometer range. SRXRD is capable of yielding X-ray diffraction data from areas 100 lm in diameter on petrographic thin sections, and this technique enables us to identify extremely small minerals in the meteorite matrix and also to determine their relative abundances. Our goal in this study is to find the actual phase that accounts for the anomalously high Na content in the Ningqiang matrix and to investigate the origin of the Na-rich phase. This study focuses on results for the matrix of Ningqiang. Details of results for

other components of Ningqiang will be presented elsewhere. 2. MATERIALS AND METHODS The sample analyzed in the present study is from a limited area of 320 mm2 on a polished thin section (total area of 560 mm2) of the Ningqiang carbonaceous chondrite. It was studied using a JEOL JSM-6480LAII scanning electron microscope (SEM) equipped with an energy dispersive Xray spectrometer (EDS) and a JEOL JXA-8900 electron probe microanalyzer (EPMA) equipped with wavelength dispersive X-ray spectrometers (WDS). For most SEM observations, we used back-scattered electron imaging. EDS analyses were obtained at 15 kV and 0.6 nA, and WDS analyses at 15 kV and 12 nA. Data collections were made by the ZAF method for the EDS analysis and by the Bence-Albee method for the WDS analysis. Standards used for the analyses (and elements analyzed) were: jadeite (Na), periclase (Mg), corundum (Al), quartz (Si), KTiPO5 (P), pyrite (S), NaCl (Cl), KTiPO5 (K), wollastonite (Ca), rutile (Ti), eskolaite (Cr), manganosite (Mn), fayalite (Fe), and NiO (Ni). For the analysis of each mineral, we used a focused electron beam of 2 lm in diameter. For the bulk analysis of matrix, we used a defocused electron beam of 50 lm in diameter; areas including grains and clasts larger than 20 lm were avoided. In the WDS analyses of matrix, relative uncertainties (2r), based on counting statistics, for major elements (Mg, Si, and Fe) are estimated to be <1%. In the EDS analyses of nepheline and sodalite, relative uncertainties for Na, Al, and Si are estimated to be 2%, 1%, and 1%, respectively. In order to check the reliability of measurements, silicate standards were measured as a monitor before every series of measurements. These measurements showed that standard deviations for major elements in the WDS and EDS analyses are <2% and <3%, respectively. The X-ray elemental maps were acquired on the SEM using an electron beam of 2 lm diameter, and on the EPMA using an electron beam of 10 lm diameter. Cathodoluminescence images were acquired on the SEM using a Gatan Mini-CL detector. Specific portions were extracted from the thin section for transmission electron microscope (TEM) observations using a focused ion beam (FIB) system (FEI, Quanta 200 3DS) at Kyoto University. After a specific area (15  1 lm) was cut out to a depth of 10 lm using a Ga+ ion gun, it was lifted from the sample and mounted on a TEM grid. The extracted samples were thinned to a thickness of 50–100 nm by a Ga+ ion beam at 30 kV and 0.1–30 nA for initial rough processing and later at 5 kV and 16 pA for removing damaged surface layers. The processed samples were studied using a JEOL JEM-2010 TEM equipped with an EDS, operated at 200 kV. Crystal structural identification was based on selected-area electron diffraction (SAED). TEM images were recorded with imaging plates and films and analyzed using the software ImageJ (http://rsbweb.nih.gov/ij/). SAED patterns were analyzed using the software ReciPro (http://pmsl.planet.sci.kobeu.ac.jp/~seto/).

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Synchrotron radiation X-ray diffraction (SR-XRD) experiments were performed using the BL10XU beam line at the synchrotron radiation facility SPring-8 in Harima Science Park City, Japan. Two samples of 500  500  30 lm in size were cut out from the meteorite thin section and mounted on copper grids. The incident X-ray ˚ beam was monochromatized to a wavelength of 0.4161 A and collimated to 100 lm in diameter. The exposure time was 300 s for each measurement. Two-dimensional Debyering patterns recorded on flat imaging plates were converted to 2h-intensity profiles using the software IPAnalyzer (Seto et al., 2010). The 2h-intensity profiles were then analyzed using the software PDIndexer (Seto et al., 2010). Rietveld refinement was carried out using the software GSAS-II (Toby and Von Dreele, 2013). The sources for crystal structural data used for the Rietveld refinement are: Redfern et al. (2000) for olivine, Raudsepp et al. (1990) for clino-pyroxene, Haavik et al. (2000) for magnetite, Buerger et al. (1947) for nepheline, Andreozzi and Lucchesi (2002) for hercynite, King and Prewitt (1982) for troilite, Tsukimura et al. (1992) for pentlandite, and Hassan et al. (2004) for sodalite. Compositions of the minerals were adjusted to those in the meteorite by changing site occupancies of atoms, if that was appropriate. All analyses were conducted based on an assumption that there is no elastic strain and lattice preferred orientation in the samples. The samples used for the SR-XRD measurements were reused for TEM observations after reprocessing with a Gatan DuoMill Model 600 ion mill, operated at 3 kV. The samples were thinned to a thickness of 50–100 nm using an Ar+ ion beam. For the measurements of modal abundances of chondrules, AOIs, CAIs, and matrix in the meteorite, back-scattered electron images were used. These objects were colored on the back-scattered electron images, and the colored areas were calculated as a percentage of the whole area using the Adobe Photoshop software package. 3. RESULTS 3.1. General petrography The Ningqiang sample contains 386 chondrules (55 vol.%), 81 AOIs (4.0 vol.%), and 21 CAIs (1.0 vol.%) embedded in a fine-grained matrix (40 vol.%). The modal abundances of these components are similar to those of CV group meteorites except for that of CAIs, which is much lower; the modal abundances of the CV group are: 35–51 vol.% (matrix), 32–51 vol.% (chondrules), 3–9 vol.% (CAIs), and 1–9 vol.% (AOIs) (McSween, 1977). Previous studies have also reported relatively low abundances for CAIs in Ningqiang, although the values vary between studies (1.0 vol.% (Rubin et al., 1988), 2.5 vol.% (Lin and Kimura, 2003), 0.94 vol.% (Hezel et al., 2008), 2.0 vol.% (Wang and Hsu, 2009)). Most of the chondrules, AOIs, and CAIs in Ningqiang are surrounded by fine-grained rims ranging in thickness from 20 to 400 lm. Because the rims resemble the host matrix in mineralogy and texture and the boundaries between them are not easily discernible, we include the rims in the matrix in this study.

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3.2. Chondrules The chondrules range in diameter from 50 to 3400 lm, with a mean of 430 lm. Spherical chondrules are rare, and most of the chondrules are irregular in shape commonly with many topographic depressions on their surfaces (Fig. 1a). The chondrules are mostly of the porphyritic olivine-pyroxene type; other minor textural types include porphyritic olivine, porphyritic pyroxene, and barred olivine. In CV3 and CO3 chondrites, chondrules normally include mesostasis, which consists primarily of plagioclase and glass, and occasionally phenocrysts of plagioclase. The mesostasis and plagioclase contain major proportions of Na, K and Al in the bulk meteorites. However, our SEM observations and X-ray elemental mapping analyses of Ningqiang show that mesostasis and plagioclase are very low in abundance. We studied all 68 chondrules that are >100 lm in diameter in a limited area of 60 mm2 of the thin section in detail using SEM and EPMA, and found that only six (9%) relatively large chondrules (>1000 lm in diameter) contain plagioclase-bearing mesostasis in their cores; a back-scattered electron image and an X-ray map for Na in a representative chondrule are shown in Fig. 2a and b, respectively. In the mesostasis of the cores, plagioclase was partially replaced by nepheline (Fig. 2c) and occasionally sodalite. In the remaining 62 chondrules (91%; most of which are <1000 lm in diameter) and mantles of the six large chondrules, mesostasis was almost completely replaced by fine-grained nepheline and various other minerals (Fig. 2d) – including hedenbergite, sodalite, Fe-rich olivine, troilite, and magnetite – and became porous. No plagioclase phenocrysts are present in any of the 68 chondrules. 3.3. Matrix 3.3.1. Bulk chemical composition and elemental distributions The bulk chemical composition of the matrix obtained by defocused beam (50-lm diameter) analyses is shown in Table 1. The composition shows good agreement with those reported by Rubin et al. (1988) and Wang and Hsu (2009). Previous studies indicated that Ningqiang, whose petrologic type is 3, is compositionally most similar to the CV and CK groups (Rubin et al., 1988; Kallemeyn et al., 1991; Wang and Hsu, 2009). Most of the CV chondrites studied previously are of petrologic type 3, whereas those of the CK chondrites are of types 4–6. Thus, in this paper we compare the bulk matrix composition of Ningqiang to that of the CV group (Table 1). Among 13 major elements (Na, Mg, Al, Si, P, S, K, Ca, Ti, Cr, Mn, Fe, Ni), the Na content of the Ningqiang matrix is much higher than the average of CV matrices (4.4  CV; McSween and Richardson, 1977). The K and Al contents are also very high (2.7  CV and 1.6  CV, respectively). To examine the distributions of the 13 major elements along with Cl in the Ningqiang meteorite, we performed X-ray elemental mapping analysis using the EPMA-WDS. The results indicate that Al, Na, and K are almost homogeneously concentrated within the matrix, whereas the chondrules generally contain very small amounts of Al, Na, and

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a

c

Na

b

Al

d

K

Fig. 1. (a) Back-scattered electron image of a portion of the Ningqiang thin section. (b–d) X-ray maps for Al, Na and K, respectively, of the area in (a). All three elements are concentrated in the matrix and depleted in the chondrules. Chon = Chondrule.

K (Fig. 1b–d). These results are consistent with the low abundances of unaltered mesostasis and plagioclase in the chondrules as described above. The X-ray elemental mapping analyses also indicate that Al, Na, and K are poor in AOIs (also see Fig. 1b–d), and that Cl is concentrated in the matrix. 3.3.2. SEM observations The matrix is a porous aggregate consisting mainly of fine grains (<1–20 lm in diameter) of Fe-rich olivine and minor amounts of Ca–Mg–Fe pyroxene (hereafter referred to as Ca pyroxene), magnetite, nepheline, hercynite, troilite, pentlandite, sodalite, and awaruite. Most of the olivine grains are very small (<1 lm in diameter) and thus difficult to analyze with a focused electron beam. However, there are also minor amounts of relatively coarser-grained olivine (5–20 lm in diameter), and our EDS analyses indicate that these grains have an average composition of Fa54.0 ± 3.3. In the matrix, narrow discontinuous bands (5–50 lm in thickness) that are rich in fine-grained opaque minerals (<1–20 lm in diameter), including magnetite, troilite, pentlandite, and awaruite, occur along intermediate regions between the chondrules and other coarse-grained

components (Fig. 3a and b), and the bands form a network throughout the meteorite. These bands may correspond to the bright matrix reported by Kimura et al. (1997). The defocused beam analyses of the matrix described above (Table 1) include these opaque mineral-rich bands. In order to visualize the distributions and abundances of minerals in the matrix, we performed high-magnification Xray elemental mapping on seven randomly selected areas of 40  30 lm. A back-scattered electron image and representative elemental maps from one of these areas are shown in Fig. 4a–c. The results show that nepheline grains are almost homogeneously distributed in high density in all seven areas (Fig. 4b; in this regard, also see Figs. 3b and 6c). Sodalite grains are also present, although in much lesser amounts (Fig. 4c). Both nepheline and sodalite occur as discrete, equidimensional grains, typically 2–5 lm in diameter (Fig. 4a and b; Table 2). Most of them contain various amounts of small inclusions (<1 lm in diameter), which are mostly Fe-rich olivine (Fig. 5a and b) and minor amounts of Ca pyroxene and opaque minerals; these inclusions will be described in detail later. It is difficult to distinguish nepheline and sodalite by back-scattered electron images. However, we found that sodalite exhibits distinc-

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a

b

c

d

445

Na

Fig. 2. (a) Back-scattered electron image of a large chondrule with a core (surrounded by a light blue dotted line) containing plagioclasebearing mesostasis. (b) X-ray map for Na of the area in (a) showing that Na is concentrated in the core, whereas it is comparatively poor in the mantle. (c) Image of boxed area c in (a) showing the mesostasis (plagioclase (Pl) and diopside (Di)) in the core was partially replaced by nepheline (Nph), hedenbergite (Hd), and troilite (Tro). (d) Image of boxed area d in (a) showing that the mesostasis in the mantle was almost completely replaced by fine-grained nepheline, troilite, and Fe-rich olivine and became porous. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

tively strong cathodoluminescence, and thus we could easily distinguish them. While most grains of nepheline and sodalite (hereafter referred to as nepheline/sodalite) contain abundant inclusions, there are also minor amounts of nepheline/sodalite grains that are relatively poor in inclusions. The inclusion-rich grains tend to exhibit jagged outlines (Fig. 5a). In comparison, the inclusion-poor grains have a tendency to exhibit angular morphology with straight edges (Fig. 5b). In the matrix, we found five regions (ranging from 20  20 lm to 800  400 lm in size) in which only inclusion-poor nepheline/sodalite grains are present (Fig. 6a–c). We think that these regions are actually clasts of matrix produced during brecciation. Inclusion-rich nepheline/sodalite grains are much less noticeable in backscattered electron images than inclusion-poor nepheline/ sodalite grains (Fig. 6b and c). Another notable feature of the Ningqiang matrix is the presence of Fe-rich spinel ((Fe, Mg) Al2O4; referred to as hercynite), which occurs as very small grains (<1 lm in

diameter) in all seven X-ray mapped areas (Fig. 4a and b). Another abundant constituent is Ca pyroxene, which has a range of compositions close to diopside–hedenbergite. It also occurs as very small grains (<2 lm in diameter) homogeneously distributed throughout the matrix; almost all Ca-rich regions in the X-ray elemental map (Fig. 4c) correspond to Ca pyroxene. 3.3.3. Synchrotron radiation X-ray diffraction measurements Synchrotron radiation X-ray diffraction measurements were performed on seven randomly selected areas of 100 lm in diameter in the matrix. The X-ray diffraction pattern obtained by averaging four of the seven patterns, which do not contain diffraction peaks for copper from the sample holders, is shown in Fig. 7. Dominant diffraction peaks for olivine and weaker but distinct peaks for Ca pyroxene (clino-pyroxene), magnetite, and nepheline were observed in all seven X-ray diffraction patterns. Weaker but significant peaks for hercynite and sodalite were observed in five patterns, and peaks for troilite are also

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Table 1 WDS defocused beam analyses of matrix in Ningqiang and an average composition of CV chondrite matrices (wt.%). No. of analyses

CV*

Ningqiang 24 (S.D.)

Na2O MgO A12O3 SiO2 P2O5 S Cl K2O CaO TiO2 Cr2O3 MnO FeO NiO

1.64 15.5 4.05 28.8 0.26 0.38 0.11 0.12 2.24 0.06 0.38 0.21 34.3 0.57

Total

88.6

0.41 0.55 0.43 1.28 0.13 0.25 0.03 0.12 0.30 0.02 0.04 0.02 1.81 0.34

(S.D.) 0.37 19.2 2.54 29.9 – 0.76 – 0.04 1.99 0.08 0.39 0.20 33.4 1.57

0.19 1.92 0.90 2.62 – 0.58 – 0.04 0.62 0.01 0.08 0.07 4.35 0.41

90.4

*

Average of eight CV chondrite matrix analyses from McSween and Richardson (1977).

observed in three patterns and for pentlandite in one pattern. Minerals identified from the seven areas are summarized in Table 3. The lattice volume of olivine, which was calculated from the lattice constants that were refined based on the X-ray diffraction profiles of olivine in the matrix, is ˚ 3. This value corresponds to Fa54 based on 299.6 ± 0.2 A the measure established by Akimoto and Fujisawa (1968), and the composition is in good agreement with the results of focused beam analyses of relatively coarse-grained olivine (Fa54.0 ± 3.3) (5–20 lm in diameter) described in Section 3.3.2. These results indicate that the composition

a

of fine-grained olivine (<1 lm in diameter), which constitutes a dominant proportion of the matrix olivine, is identical to that of coarse-grained olivine. Rietveld refinement of the X-ray diffraction pattern shown in Fig. 7 was performed to estimate the relative abundances of the eight identified minerals (olivine, Ca pyroxene, magnetite, nepheline, hercynite, troilite, pentlandite, and sodalite) in the matrix. In the calculations, we refined volume fractions, crystal sizes, and lattice constants of the eight minerals as variable parameters using the software GSAS-II (Toby and Von Dreele, 2013). As a result, the weighted profile R-factor (Rwp) converged to 8.8%, and the entire diffraction profile was successfully reproduced (Fig. 7). The abundances of the eight minerals derived from the above calculations are shown in Table 4. The second-most abundant mineral after the dominant olivine (68.6 vol.%) is Ca pyroxene (9.7 vol.%), followed by magnetite (8.1 vol.%), nepheline (7.1 vol.%), and hercynite (3.4 vol.%). Assuming that Na-bearing minerals in the matrix are only nepheline and sodalite and that these minerals have stoichiometric compositions, the calculated total Na2O-content for these minerals in the matrix is 0.98 wt.% (after normalizing the total of all constituent elements to the total of the bulk matrix composition in Table 1), which is 60% of the Na2O-content of the bulk matrix composition (1.64 wt.%). This means that the remainder of Na2O presumably occurs in a non-crystalline phase (or phases) and/or as components of minerals other than nepheline/sodalite. 3.3.4. TEM observations Fine Fe-rich olivine grains that constitute the dominant proportion of the matrix range in size typically from 200 to 700 nm. They show a range of morphologies, including a lath-like one (Fig. 8a). EDS analyses of 23 olivine grains show that they are Fa54 in average, which is again in good agreement with the results for coarse-grained olivine.

b

Na Al Fe

Fig. 3. (a) Back-scattered electron image of a portion of the matrix. Indicated by arrows are bands consisting of fine-grained opaque minerals. (b) Combined X-ray map for Na (red), Al (blue), and Fe (green) of the area in (a). Grains that appear green are opaque minerals. Note that grains of nepheline and minor sodalite (both appear pink) are distributed homogeneously in the matrix. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Table 2 Selected EDS analyses of nepheline and sodalite in the Ningqiang matrix (wt.%). Nepheline 1

a

Na2O A12O3 SiO2 Cl K20 CaO FeO

18.1 33.4 44.6 n.d. 1.62 1.02 1.14

Total

99.9

Sodalite 2 18.0 33.7 45.2 n.d. 1.73 0.87 1.30 100.8

3 17.5 31.9 44.6 n.d. 1.68 0.90 2.06 98.6

26.0 32.4 36.7 5.84 n.d. n.d. n.d. 100.9

n.d. = not detected. Detection limits: 0.32 for Cl, 0.19 for K2O, 0.30 for CaO, 0.40 for FeO.

b

Na Al

c

Ca Cl

Fig. 4. (a) Back-scattered electron image of a portion of the matrix consisting mainly of fine grains of Fe-rich olivine (Fe-Ol) and minor Ca pyroxene (Cpx), nepheline (Nph), hercynite (Hc), sodalite (Sdl), and pentlandite (Pn). (b) Combined X-ray map for Na (red) and Al (blue) of the area in (a) showing that nepheline grains (pink) are distributed at high density. Small amounts of sodalite grains (pink) also occur. Smaller grains that appear blue are hercynite (Hc). (c) Combined X-ray map for Ca (green) and Cl (red) of the area in (a) showing that small grains of Ca pyroxene (green) are widely dispersed. Grains that appear red are sodalite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The Ca pyroxene grains range in size from 200 to 1000 nm and exhibit irregular shapes (Fig. 8b). They have an average

composition of En34 ± 8Fs24 ± 6Wo42 ± 6. Most of the magnetite, troilite, pentlandite, and awaruite grains range in size from 50 to 700 nm and display irregular shapes. The hercynite grains range in size from 100 to 200 nm and have irregular shapes and an average composition of Hc68 ± 5. Many of the hercynite grains occur along the margins of olivine (Fig. 8c), magnetite, troilite, and pentlandite grains. Based on selected-area electron diffraction, we found that most of the nepheline/sodalite grains are single crystals (Fig. 9a and b). We also found a poorly crystalline to amorphous Si–Al–Na–K-rich material similar in size, morphology, and composition to the nepheline/sodalite grains. This material may be indigenous to Ningqiang. However, considering the sensitive nature of nepheline/sodalite to electron irradiation, we cannot exclude the possibility that the material was derived from nepheline/sodalite amorphization during observations in the TEM. Because Na in nepheline/sodalite and Cl in sodalite evaporate rapidly by electron irradiation in the TEM, we did not acquire quantitative analyses from these minerals. The TEM observations confirm that almost all nepheline/sodalite grains contain various amounts of inclusions that range in diameter from 100 to 1000 nm (Fig. 9a and b). These inclusions are predominantly Fe-rich olivine, with minor amounts of Ca pyroxene, magnetite, troilite, and pentlandite. These inclusions are indistinguishable in grain size, morphology, and composition from the counterpart minerals that occur outside of nepheline/sodalite grains. 4. DISCUSSION 4.1. Comparison to the composition of CV3 chondrite matrices The Na, K, and Al contents in the matrix of Ningqiang are distinctly higher by factors of 4.4, 2.7, and 1.6, respectively, than those in the CV group (Table 1). The results of our study reveal that the major proportions of these elements can be explained by the presence of nepheline/sodalite. Although nepheline/sodalite have been known to occur in the matrices of several CV3 chondrites (e.g., Krot et al., 1998b), their abundances are very low and there have been

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a

b

Fig. 5. (a) Back-scattered electron image of a typical nepheline (Nph) grain in the matrix, which contains abundant inclusions of Fe-rich olivine (Fe-Ol) and exhibits a jagged outline (light blue dotted line). (b) Image of an inclusion-poor nepheline grain in the matrix, which exhibits angular morphology with straight edges. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

no reports of nepheline/sodalite at such high abundances as found by us in the Ningqiang matrix. The primary reason why such high abundances of nepheline/sodalite have not been noticed in the previous studies of Ningqiang is probably due to their very small grain sizes and the presence of their abundant inclusions that are mineralogically similar to the grains outside of nepheline/sodalite grains (see Fig. 5a for example). Despite the high abundances of Na, K, and Al in the matrix of Ningqiang, the bulk meteorite abundances of these elements are comparable to those in the CV group (1.1  CV (Na), 1.0  CV (K), 0.88  CV (Al); Rubin et al., 1988; Wasson and Kallemeyn, 1988). These results mean that the chondrules, which constitute a major proportion (55 vol.%) of the Ningqiang meteorite other than the matrix (40 vol.%), are depleted in Na, K, and Al. In fact, our SEM observations and X-ray elemental mapping analyses indicate that most of the chondrules in Ningqiang contain very small amounts of Na, K, and Al. Previous studies also reported similar characteristics for Ningqiang chondrules. Kallemeyn et al. (1991) described that “plagioclase in Ningqiang is absent outside of the rare refractory inclusions” (p. 885). Wang and Hsu (2009) reported that “the abundance of Al-rich chondrules (which contain major amounts of plagioclase) is very low. Only three such chondrules were found in 15 sections” (p. 778). Kimura et al. (1997) performed bulk analyses of chondrules in Ningqiang and found that Na, K, and Al contents are significantly lower than those in CV3 chondrites. To explain the anomalous distributions of Na, K, and Al in Ningqiang, we consider that there are two possibilities (to simplify the following models, we assume that the meteorite only consists of chondrules and matrix): (1) such fractionations had already been established in the solar nebula before accretion to the parent body; that is, Ningqiang was formed by accretion of chondrules depleted in Na, K, and Al and matrix grains enriched in these elements, or (2) when chondrules and matrix grains in Ningqiang ac-

creted from the nebula, their elemental abundances were equivalent to those in the CV group. However, afterwards and due to some parent-body process, Na, K, and Al in the chondrules were preferentially transported to the matrix – this process essentially took place in a closed system. Before we discuss which of these models is more applicable, we review past studies on nepheline/sodalite in CO3 and CV3 chondrites and consider the occurrence and the origin of these minerals. 4.2. Occurrence and formation mechanism of nepheline/ sodalite in CO3 and CV3 chondrites Nepheline/sodalite have been reported mainly from chondrules (e.g., Ikeda and Kimura, 1995; Jones, 1997; Kimura and Ikeda, 1998; Tomeoka and Itoh, 2004), CAIs (e.g., MacPherson and Grossman, 1984; Wark, 1986; Tomeoka et al., 1992; Kojima et al., 1995; Russell et al., 1998), and AOIs (e.g., Grossman and Steele, 1976; Hashimoto and Grossman, 1987; Kojima et al., 1995) in CO3 and CV3 chondrites. Nepheline/sodalite were produced primarily by replacing mesostasis and plagioclase in chondrules (e.g., Kimura and Ikeda, 1995; Tomeoka and Itoh, 2004), melilite and plagioclase in CAIs (e.g., Tomeoka et al., 1992; Russell et al., 1998), and plagioclase in AOIs (e.g., Hashimoto and Grossman, 1987). 4.2.1. Chondrules and CAIs in CO3 chondrites CO3 chondrites have gone through minor degrees of thermal metamorphism in their parent body and can be divided into petrologic subtypes 3.0–3.8 based on the degree of metamorphism (e.g., Chizmadia et al., 2002). Previous studies showed that CAIs (Kojima et al., 1995; Itoh and Tomeoka, 1998; Russell et al., 1998) and AOIs (Kojima et al., 1995) in CO3 chondrites of higher subtypes tend to show higher degrees of Na-metasomatism. Tomeoka and Itoh (2004) studied mesostasis of chondrules in seven CO3 chondrites ranging in subtype from 3.0 to 3.7 and

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found that the amounts of nepheline in chondrule mesostasis systematically increase with increasing petrologic subtype of the host meteorites. These results suggest that nepheline in chondrules, CAIs, and AOIs formed synchronously as a result of effects related to thermal metamorphism. Among the precursors of nepheline, melilite (Ca2(Mg, Al)(Si, Al)2O7) in CAIs contains virtually no Na, and there is evidence that the alteration of melilite in CAIs occurs preferentially from the CAI edges where the melilite is in direct contact with matrix (Tomeoka et al., 1992). If we assume that Na-metasomatism occurred in a closed system and no Na was introduced from the outside, these facts suggest that nepheline in CAIs was formed by relocation of Na originally contained mainly in mesostasis and plagioclase in chondrules through some transportation media. Nomura and Miyamoto (1998) performed hydrothermal experiments on melilite and found that nepheline hydrate (NaAlSiO4
1/2H2O) forms from melilite by reactions with NaOH solutions at 200 °C, and that it is dehydrated to nepheline at 700 °C. Based on these observations and experimental results, Tomeoka and Itoh (2004) suggested that nepheline in chondrules and CAIs in CO3 chondrites formed through hydrothermal alteration in the presence of small amounts of aqueous fluid and subsequent thermal metamorphism that took place after the exhaustion of aqueous fluid.

a

b

c

449

Na Al

Fig. 6. (a) Back-scattered electron (BSE) image of a region (surrounded by a light blue dotted line) in the matrix containing only inclusion-poor nepheline and sodalite grains. (b) Image of boxed area b in (a) containing the boundary between the matrix (upper side) and the region containing only inclusion-poor nepheline and sodalite grains (lower side). Note that inclusion-poor nepheline (Nph) grains (lower side) are more noticeable than inclusion-rich nepheline grains (upper side) in the BSE image. (c) Combined X-ray map for Na (red) and Al (blue) of the area in (b) showing that inclusion-poor nepheline grains (pink; lower side) are indistinguishable in size and density from inclusion-rich nepheline grains (also pink; upper side). Fe-Ol = Fe-rich olivine, Hc = hercynite, Mag = magnetite, Pn = pentlandite, Sdl = sodalite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

4.2.2. Oxidized Allende-like CV3 chondrites and dark inclusions It has been known that the oxidized group of CV3 chondrites can be divided into a Bali-like subgroup and an Allende-like subgroup (Weisberg et al., 1997; Krot et al., 1998b; Weisberg and Prinz, 1998). The CV3 chondrites that contain nepheline/sodalite in chondrules, CAIs, and AOIs almost exclusively belong to the Allende-like subgroup. Minor amounts of fine-grained nepheline/sodalite were also reported from matrices of the meteorites in this subgroup (Krot et al., 1998b) and an igneous chondrule rim in Allende (Rubin, 1984). The CV3 chondrites in the Allendelike subgroup were interpreted to have experienced aqueous alteration with relatively small amounts of aqueous fluid and subsequent dehydration by thermal metamorphism (e.g., Krot et al., 1998a, b). Dark inclusions are lithic clasts that are common in CV3 (e.g., Fruland et al., 1978; Johnson et al., 1990) and CO3 chondrites (Itoh and Tomeoka, 2003). Nepheline/sodalite are also contained in chondrules, CAIs, and matrix in dark inclusions (e.g., Kurat et al., 1989; Kojima and Tomeoka, 1996; Buchanan et al., 1997; Krot et al., 1998a; Itoh and Tomeoka, 2003). Although some early authors (e.g., Kurat et al., 1989; Weisberg and Prinz, 1998) favored the idea that dark inclusions are unaltered aggregates of nebular condensates, more recent studies showed much evidence indicating that they have undergone aqueous alteration and subsequent thermal metamorphism in the parent body (e.g., Kojima et al., 1993; Kojima and Tomeoka, 1996; Buchanan et al., 1997; Krot et al., 1997, 1998a, b). Remarkable among the various occurrences of nepheline in dark inclusions is where it formed in fracture-filling veins (see Fig. 13a and b in Kojima and Tomeoka, 1996), which

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Intensity

Olivine Clino-pyroxene Magnetite Nepheline Hercynite Troilite Pentlandite Sodalite

5

10

15

2θ (degree) Fig. 7. X-ray diffraction pattern of the Ningqiang matrix obtained by synchrotron radiation (average of X-ray diffraction patterns from four different areas) (indicated by blue crosses). The result of Rietveld refinement is indicated by a green solid line. The lower trace is the difference between observed and calculated patterns. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 3 Minerals identified from the seven areas in the Ningqiang matrix analyzed by SR-XRD. s = present,  = absent.

Area Area Area Area Area Area Area

1 2 3 4 5 6 7

Olivine

Clino-pyroxene

Magnetite

Nepheline

Hercynite

Troilite

Pentlandite

Sodalite

s s s s s s s

s s s s s s s

s s s s s s s

s s s s s s s

 s  s s s s

   s  s s

     s 

s s  s s  s

Table 4 Modal abundances of the minerals in the Ningqiang matrix obtained by the Rietveld refinement of the X-ray diffraction data (vol.%). Olivine

Clino-pyroxene

Magnetite

Nepheline

Hercynite

Troilite

Pentlandite

Sodalite

68.6 ± 0.3

9.7 ± 0.3

8.1 ± 0.1

7.1 ± 0.3

3.4 ± 0.1

1.6 ± 0.1

0.8 ± 0.1

0.6 ± 0.2

provides strong evidence for the formation of nepheline by the activity of aqueous fluid. 4.3. Origin of nepheline and sodalite in the Ningqiang matrix As reviewed above, there is now abundant evidence suggesting that nepheline/sodalite in CO3 and CV3 chondrites and their dark inclusions were formed by hydrothermal alteration in the presence of small amounts of aqueous fluid and subsequent thermal metamorphism in the meteorite parent bodies. Previous studies of Ningqiang also showed that nepheline/sodalite in CAIs were produced by replacing

melilite (Lin and Kimura, 2003; Sugita et al., 2009) and nepheline/sodalite in chondrules by replacing mesostasis and plagioclase (Wang and Hsu, 2009). Our study also demonstrates that mesostasis in the chondrules in Ningqiang was replaced by nepheline/sodalite and other minerals. Therefore, it is reasonable to assume that nepheline/sodalite in Ningqiang were also produced by this process in the parent body. Thus, we favor the explanation provided by model (2) in Section 4.1. Then, the next question to be addressed is how the present distributions of these minerals were established. We think that there are two possibilities: (2-a) Na, K, and Al, which were originally contained pri-

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b* a*

a

b*

b

b* a*

b* a*

c Fig. 8. (a) TEM image of Fe-rich olivine (Fe-Ol) grains in the matrix. Some of the grains exhibit a lath-like morphology. (b) Image of Ca pyroxene (Cpx) and Fe-rich olivine grains. (c) Image of an aggregate of Fe-rich olivine, hercynite (Hc), and magnetite (Mag) grains. The hercynite grain occurs along the edge of the olivine grain. Selected-area electron diffraction patterns of Fe-rich olivine, Ca pyroxene, and hercynite are also shown in the insets of (b) and (c).

marily in chondrules, were transported outward by aqueous fluid and deposited as nepheline/sodalite in matrix, or (2-b) nepheline/sodalite formed originally by reactions with aqueous fluid in chondrules and afterwards were disaggregated and distributed into matrix.

451

Model (2-a) assumes that the aqueous activity occurred in situ in the present setting of the meteorite. In that case, we expect that nepheline/sodalite were formed continuously in extended areas where aqueous fluid infiltrated. A typical example of such evidence are the veins containing nepheline in a dark inclusion as described above (Kojima and Tomeoka, 1996). However, such evidence is absent in the Ningqiang matrix. Instead, nepheline/sodalite occur as discrete, equidimensional grains with sharp edges (see Fig. 5a and b) homogeneously distributed throughout the matrix. These occurrence and morphology suggest that the minerals went through a fragmentation process such as brecciation, and thus appear to be inconsistent with deposition from aqueous fluid. Previous SEM observations showed that small CAI fragments are widely scattered in the Ningqiang matrix (Sugita et al., 2009), supporting the idea that such a fragmentation process occurred. In addition, it is difficult to explain that the situation in which Na, K, and Al were enriched in the chondrules and depleted in the matrix was completely reversed simply by the activity of aqueous fluid. In comparison, model (2-b) appears to better explain the occurrence and morphology of nepheline/sodalite grains, because this model potentially involves mechanical processes such as the disaggregation of nepheline/sodalite in chondrules. However, there are also serious drawbacks to this model. That is because this model should comprise three stages: (i) formation of nepheline/sodalite by replacing mesostasis in the chondrules, (ii) preferential disaggregation and removal of nepheline/sodalite from the chondrules, and (iii) scattering of separated nepheline/sodalite grains into the matrix and their homogeneous mixing with matrix grains. Among these stages, stage (i) is essentially a chemical process and could occur in situ in the present setting by the activity of aqueous fluid. However, stages (ii) and (iii) represent physically dynamic processes, and these processes likely could not be realized in situ; such processes should have taken place before the formation of the present lithology. Accordingly, if we assume that all the processes in both models (2-a) and (2-b) occurred in situ in the present setting of the meteorite, we still cannot explain consistently the anomalous distributions of nepheline/sodalite in Ningqiang. From these observations and considerations, we hypothesize that the Ningqiang meteorite underwent some sort of physical process before final lithification in the meteorite parent body. Such a process has not been previously envisioned from studies of other meteorites. 5. CONCLUSIONS The matrix of Ningqiang has anomalously high abundances of Na, K, and Al, whereas the chondrules have relatively low abundances of these elements. The results of our SEM and TEM observations and SR-XRD measurements reveal that the major proportions of Na, K, and Al in the matrix can be explained by the presence of nepheline and sodalite, which occur as discrete, equidimensional grains (2–5 lm in diameter) homogeneously distributed in the matrix. In the chondrules, mesostasis, which normally contains major amounts of Na, K, and Al, is low

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a*

a*

b*

a

b

Fig. 9. (a) TEM image of a nepheline (Nph) grain (outlined by a blue dotted line) containing Fe-rich olivine (Fe-Ol) inclusions. (b) Image of a sodalite (Sdl) grain (outlined by a blue dotted line) containing inclusions of magnetite (Mag), pentlandite (Pn), and Fe-rich olivine. Both nepheline and sodalite occur as single crystals. Selected-area electron diffraction patterns of nepheline, sodalite, and pentlandite are also shown in the insets. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

in abundance and was extensively replaced by fine-grained nepheline, hedenbergite, troilite, sodalite, Fe-rich olivine, and magnetite. We infer that Ningqiang originally contained higher amounts of mesostasis in chondrules and higher amounts of CAIs than the present levels, and that previous levels may have been comparable to those found in CV3 and CO3 chondrites. After accretion to the parent body, Ningqiang likely underwent Na-metasomatism in the presence of aqueous fluid similarly to the CV3 and CO3 chondrites, and large amounts of nepheline and sodalite were produced in chondrules and CAIs. The nepheline and sodalite were subsequently disaggregated from chondrules and CAIs, scattered into matrix, and mixed homogeneously with matrix grains. However, it is obvious that the disaggregation, scattering, and mixing did not occur in situ in the present setting of the meteorite. We suggest that these processes probably occurred before final lithification on the parent body. ACKNOWLEDGMENTS We thank D. Nishio-Hamane, I. Ohnishi, and all members of the Planetary Material Science Laboratory at Kobe University and of the Mineralogical Laboratory at Kyoto University for technical supports and discussion. We also thank M. Kimura, W.U. Reimold, and A.E. Rubin for helpful and constructive reviews. Electron microprobe analysis and transmission electron microscopy were performed at the Instrumentation Analysis Division in the Center for Supports to Research and Education Activities, Kobe University. Synchrotron radiation X-ray diffraction measurements at SPring-8 BL 10 XU were performed under the proposals 2009B1238, 2011B1339, and 2012B1344. This research was supported by the Grant-in-Aid for Scientific Research (No. 20340150 to K.T. and Nos. 25287143 and 23740392 to Y.S.).

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