Heterogeneous distribution of solar and cosmogenic noble gases in CM chondrites and implications for the formation of CM parent bodies

Geochimica et Cosmochimica Acta, Vol. 63, No. 2, pp. 257–273, 1999 Copyright © 1999 Elsevier Science Ltd Printed in the USA. All rights reserved 0016-7037/99 $20.00 1 .00

Pergamon

PII S0016-7037(98) 00278-8

Heterogeneous distribution of solar and cosmogenic noble gases in CM chondrites and implications for the formation of CM parent bodies TOMOKI NAKAMURA,1,* KEISUKE NAGAO,2,† KNUT METZLER,3 and NOBUO TAKAOKA1 1

Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University 33, Hakozaki, Fukuoka 812-8581, Japan 2 Institute for Study of the Earth’s Interior, Okayama University, Misasa, Tottori 682-0193, Japan 3 Institut fu¨r Planetologie, Universita¨t Mu¨nster, Wilhelm-Klemm-Strasse, 10, 48149 Mu¨nster, Germany (Received March 26, 1998; accepted in revised form September 2, 1998)

Abstract—Distribution of solar, cosmogenic, and primordial noble gases in thin slices of Murchison, Murray, and Nogoya CM carbonaceous chondrites was determined by the laser microprobe analysis so as to put some constraints on the parent-body processes in the CM chondrite formation. The main lithological units of the three meteorite slices were located by electron microscope observations and classified into clastic matrix and clasts of primary accretionary rocks (PARs) based on the classification scheme of texture of CM chondrites. All sample slices contain both clastic matrix and PARs. Clastic matrix shows a comminuted texture formed by fragmentation and mechanical mixing of rocks due to impacts, whereas PARs preserve the original textures prior to the mechanical disruption. Solar-type noble gases are detected in all sample slices. They are located preferentially in clastic matrix. The distribution of solar gases is similar to that in ordinary chondrites where these gases reside in clastic dark portions of these meteorites. The heterogeneous distribution of solar gases in CM chondrites suggests that these gases were acquired not in a nebular accretion process but in parent body processes. Solar energetic particles (SEP) are predominant in CM chondrites. The low abundance of low energy solar wind (SW) component relative to SEP suggests preferential loss of SW from minerals comprising the clastic matrix, due to aqueous alteration in the parent bodies. Cosmogenic noble gases are also enriched in some portions in clastic matrix, indicating that some parts of clastic matrix were exposed to solar and galactic cosmic rays prior to the final consolidation of the CM parent bodies. Primordial noble gases are rich in fine-grained rims around chondrules in all three meteorites. However, average concentrations of heavy primordial gases in the rims differ among meteorites and correlate inversely to the degree of aqueous alteration that the meteorites have experienced. This appears to have been caused by aqueous alteration reactions between fluids and carbonaceous carrier phases of noble gases. Copyright © 1999 Elsevier Science Ltd the energetic particle environments and the regolith processes in the very early stage of the solar system evolution can be ellucidated based on the solar-gas distribution in CM chondrites. Brecciation processes have caused redistribution of noble gases due to mechanical mixing of rocks from different sites in a parent body. When solar-type noble gases were implanted into the surface of parent bodies and the surface regolith experienced subsequent brecciation, then these gases were heterogeneously distributed in the meteorite parent bodies as it is observed in the light (solar gas-poor portion) -dark (solar gas-rich portion) structures of ordinary chondrites (e.g., Gerling and Levski, 1956). CM chondrite parent bodies must have located in the asteroid belt at heliocentric distances similar to those of ordinary chondrite parent bodes when they were subject to solar-wind irradiation, according to the correlation between concentrations of solar gases and durations of garactic cosmic ray (GCR) irradiation on regolith (Anders, 1975; Pedroni, 1989; Wieler et al., 1989). Therefore it is expected that both types of parent body have developed similar regolith features. In fact, Black (1972b) found that the Nogoya CM chondrite has solar gas-rich and -poor portions. But the relationship between petrologic characteristics and solar-gas abundance in CM chondrites is still poorly known. On the other hand, aqueous alteration processes are known to have changed mineralogical and petrological characteristics of CM chon-

1. INTRODUCTION

CM carbonaceous chondrites are primitive material formed in the early stage of solar system evolution, but petrological and mineralogical investigations revealed that they are affected strongly by secondary processes on the meteorite parent bodies. The most prominently observed secondary processes in CM chondrites are aqueous alteration (e.g., McSween, 1979; Bunch and Chang, 1980; Tomeoka and Buseck, 1985; McSween, 1987; Zolensky and McSween, 1988) and brecciation (e.g., Bunch and Rajan, 1988). These processes have been well characterized by recent studies especially in the field of mineralogy and petrology (e.g., Metzler et al., 1992; Zolensky et al., 1993; Browning et al., 1996), but it remains to be known how the processes affected on the noble gas distribution in CM chondrites. Solar-type noble gases in CM chondrites are solar wind and flares emitted from an ancient sun, because they were implanted prior to the final compaction of meteorites that occured very early: compaction ages of Murchison, Murray, and Nogoya are 4.3 Gyr or earlier determined by the 244Pu fission track dating technique (Macdougall and Kothari, 1976). Therefore

*Author to whom correspondence should be addressed. † Present address: Laboratory for Earthquake Chemistry, Graduate School of Science, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan. 257

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drites, but it is unclear how the alteration affected the abundances of solar, cosmogenic, and primordial noble gases. Recently, Browning et al. (1996) found, based on literature data, that bulk primordial 36Ar concentration decreases when CM chondrites exhibit higher degree of aqueous alteration. But a systematic correlation between the degree of alteration and the various kinds of noble gases remains to be described. In this paper we report the results of laser-microprobe analysis of noble gases from thin slices of the Murchison, Murray, and Nogoya CM chondrites that consist of two different lithologies, PARs and clastic matrix (Metzler et al., 1992). Our goal is to understand the distribution and composition of solar and cosmogenic gases in CM chondrites and to evaluate the planetary processes that took place on the CM parent bodies. Some nebular processes for the formation of CM chondrites were revealed by mapping of primordial noble gases in PARs in Nakamura et al. (1999) (hereafter, Paper I), in which laser microprobe analysis was performed on the slices consisting entirely of PARs, the representative samples of internal parts of large PARs. In this paper we show that CM chondrites are samples of regolith breccia where solar and cosmogenic noble gases are heterogeneously distributed. 2. SAMPLES AND EXPERIMENTAL METHODS Three slices with thickness from 200 to 300 mm were prepared, one each from Murchison, Murray, and Nogoya. The slice of Murchison used in the present study is not the slice used in Paper I: a new slice was prepared from a different chip of Murchison. The new Murchison slice showed petrographycal features quite different from that used in Paper I, as discussed in the result section. All three slices were polished carefully and observed by electron microscopy to identify the lithologies using the methods described in Paper I. Major element compositions of the selected portions for noble-gas analysis were determined by an electron beam 50 –100 mm diameter and were later used for calculation of production rates of cosmogenic Ne. After petrographic characterization the three meteorite slices were analyzed for noble gases by the laser microprobe method. A non Q-switched neodymium-YAG laser with beam diameter from 50 to 150 mm was used to extract noble gases. A single laser shot made a through cylindrical hole in the sample slice. One measurement of noble gases was made on the gases from several laser pits. The detailed experimental configurations of the mass spectrometer were described in Paper I and elsewhere (Nagao et al., 1993; Nagao, 1994; Nagao and Abe, 1994). The noble gas concentrations in laser pits were determined from absolute amounts of noble gases through two stages of calculations; (1) tentative concentrations were calculated from the absolute amounts devided by the pit weights which were obtained from pit volume and densities of fused material, and (2) the true concentrations were calculated by applying the extraction efficiencies of the laser system to the tentative concentrations (see paper I for full description). The extraction efficiencies of noble gases by the laser ablation for fine-grained CM chondrite material in slices with thickness about 300 mm were determined in Paper I and applied to the present study; 2.486, 1.28, 1.64, 1.09, and 1.22 for He, Ne, Ar, Kr, and Xe, respectively. The densities of the pit material are assumed as follows; chondrule is 3.2 g/cm3 from forsterite (53.2 g/cm3), rims around chondrules is 2.4 g/cm3 from serpentine (Mg3Fe3)Si4O10(OH)8 (5 2.98 g/cm3) with 20% porosity, PCPs (Poorly Characterized Phases; defined by Fuchs et al., 1973), which are typically 50 mm in size and made of intergrowth of two hydrous phases, tochilinite and serpentine (e.g., Nakamura and Nakamuta, 1996), are 2.8 g/cm3 from 1:1 mixture of cronstedtite Fe2Fe2SiO5(OH)4 (53.34 g/cm3) and tochilinite 6FeS z 5[(Mg, Fe)0.9(OH)2] (53.67 g/cm3) with 20% porosity, and clastic matrix is 2.7 g/cm3 from the average density of bulk CM chondrites (Wasson, 1974). The terminology and abbreviations of noble gases are the same as those used in Paper I. The following indices will be used to denote the

noble-gas components: pri (primordial), cos (cosmogenic), sol (solar), SEP (solar energetic particle), SF (solar flare), SW (solar wind) and mes (measured). Solar gases consist mainly of SW and SEP. For isotopic ratios of Ne-SEP, -SW, -A, and -spallogenic components we use values from selected references; (20Ne/ 22Ne, 21Ne/22Ne)SEP 5 (11.2, 0.0295), (20Ne/22Ne, 21Ne/22Ne)SW 5 (13.8, 0.0328) from Benkert et al. (1993), (20Ne/22Ne, 21Ne/22Ne)pri 5 (8.2, 0.024) from Ne-A (Pepin, 1967; Black and Pepin, 1969), and (20Ne/22Ne, 21Ne/22Ne)cos 5 (0.8, 0.9) from average shielding for chondrites (e.g., Eugster, 1988). For comic-ray exposure agses 21Ne production rates were calculated from the major element compositions of the portions analyzed for noble gases, using the formula given by Schultz and Freundel (1985), P21 5 1.63 [Mg] 1 0.6 [Al] 1 0.32 [Si] 1 0.22 [S] 1 0.07 [Ca] 1 0.021 [Fe1Ni] ([X] is concentration of element X as weight fraction and P21 is (21Ne)cos production rate in units of 1028 cc STP/g per Ma), which is valid for an average shielding corresponding to (21Ne/ 22 Ne)cos 5 0.9. 3. RESULTS

3.1. Petrologic Characterization of Sample Slices The lithology of slices of Murchison, Murray, and Nogoya were identified based on the classification scheme proposed by Metzler et al. (1992) in which main lithologies of CM chondrites were divided into two different types; PARs and clastic matrix. PARs appear to preserve original textures of the material that accreted to form CM chondrites, whereas clastic matrix exhibits comminuted texture produced by mechanical disruption such as impacts on parent body (Metzler et al., 1992). All three meteorite slices are mixtures of PARs and clastic matrix with various proportions, indicating that they are samples of brecciated parts of meteorites. Proportions between the two lithological units are approximately 3 : 7, 6 : 4, and 5 : 5 for PARs : clastic matrix in Murchison, those in Murray, and those in Nogoya, respectively. The Murchison slice that was used in Paper I consists of 100% PARs. This indicates that the proportion of PARs to clastic matrix varies within a single meteorites. PARs are an aggregate of various components such as chondrules and PCPs. Most components in PARs are coated by fine-grained rims, therefore PARs are mainly composed of chondrules, rims around chondrules, and PCP-rich portions (see Figs. 1a, b, and c in Paper I). On the other hand, clastic matrix is a mixture of fragments of chondrules, PCPs, and various kinds and sizes of minerals such as cronstedtite, olivine with various compositions, enstatite, calcite, troilite, pentlandite, and FeNi-metal (Fig. 1a). Diameter of grains in clastic matrix varies greatly from submicron to 1 milimeter, but most grains are with diameters less than 30 mm. Clastic matrix fills the interstices between PARs. Figures 1b and c show an overview of a Nogoya slice where some PARs are recognized with distinct boundaries to the clastic matrix. 3.2. Noble-Gas Distribution in PARs Laser microprobe analyses were carried out on the portions selected by petrologic observations and all results are shown in Appendices 1 and 2. In this section noble gas signatures of the internal parts of PARs in Murchison, Murray, and Nogoya are summarized. The results from outermost parts of PARs, areas less than 300 mm to clastic matrix, are discussed separatedly in a later section. Primordial noble gases are prominent in PARs. Among the components of PAR, primordial gases are the richest in rims

Distribution of solar gases in CM chondrites

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Fig. 1. (a) A back-scattered electron (BSE) image of a portion of clastic matrix in the slice of Murray. Angular to subrounded shaped PCPs (white grains) and Mg-rich silicate of mainly forsterite (black grains) are set in a fine-grained phyllosilicate matrix. (b) A whole view of the Nogoya slice. (c) A schematic illustration of (b) showing some PARs and clastic matrix. A sequential analysis was performed on the laser pits from 53 to 60 and the results are shown in Figure 6.

around chondrules; this enrichment is observed in all three meteorite slices analyzed in this study. Elemental abundances of the PARs in Murchison (Fig. 2a), Murray (Fig. 2b), and Nogoya (Fig. 2c) indicate that noble gases in PARs are dominated by the planetary-type noble gases. Ne isotopic ratios of the PARs in Murchison (Fig. 3a), Murray (Fig. 3b), and Nogoya (Fig. 3c) suggest that Ne is a mixture of primordial and cosmogenic noble gases. Cosmic-ray exposure ages of the portions analysed were determined and average ages are 1.8 6 0.9, 3.4 6 1.1, and 0.1 6 0.1 Ma for Murchison, Murray, and Nogoya, respectively (Table 1). In Table 1 the exposure age for Murchison PARs (1.3 6 0.9 Ma) is given by averaging whole data obtained in Paper I and this paper. One location in a PCP-rich portion in the Nogoya sample showed 22Ne enriched compositions of 4.55 6 0.93 and 0.031 6 0.012 for 20 Ne/22Ne and 21Ne/22Ne, respectively (Fig. 3c), indicating that Ne-E carrier phases such as interstellar SiC grains are located in this portion. By taking together with the results of Paper I in which analyses were made on the meteorite slices of Yamato (Y-) 791198 and Murchison that consist entirely of PARs, general features of noble gases in the interior of PARs can be evaluated: (1) primordial noble gases are predominant, (2) a significant fraction of the primordial gases is contained in finegrained rims around chondrules, which shows a clear contast to enclosing chondrules with low gas concentration, and (3) average concentrations of heavy primordial noble gases in the rims are slightly variable among meteorites and decrease in the order of Y-791198, Murray, Murchison, and Nogoya (Fig. 4). 132 Xe concentration in the rims in Nogoya is comparable to that of bulk CI chondrites (Fig. 4).

3.3. Noble-Gas Distribution in Clastic Matrix Clastic matrix shows essentially no textural diversity unlike PARs (Fig. 1a). Most grains in clastic matrix are smaller than beam diameter (;150 mm) and thickness of sample slices (;300 mm), thus we cannot obtain noble-gas compositions of individual grains by laser microprobe analyses. Therefore the results are reported merely as “clastic matrix” in Appendices 1 and 2. Thirty sites of clastic matrices in the Murchison, Murray, and Nogoya were analyzed. Large amounts of light noble gases were extracted from many laser-pits in clastic matrix, resulting in a remarkable difference in noble-gas elemental abundances and isotopic compositions between clastic matrix and PARs. 3.3.1. Elemental abundance Noble-gas elemental abundances of the clastic matrices in Murchison (Fig. 2a), Murray (Fig. 2b) and Nogoya (Fig. 2c) differ from those of coexisting PARs. 4He and 20Ne abundances of the clastic matrices are up to 2 orders of magnitude higher than those of the PARs. These excesses are ascribed to the addition of the solar-type noble gases, which is also verified by Ne isotopic ratios as discussed later. However, 132Xe, 84Kr, and 36Ar do not show significant excesses. This is because the planetary-type gases are severely fractionated depleting the light gases relative to cosmic abundances while the solar gases are much less fractionated. Thus a factor of 100 excess of 20Ne, which is the largest excess observed in Nogoya, will be accompanied by only ;40, ;4, and ;0.6% excess of 36Ar, 84Kr, and 132 Xe, respectively. In this calculation noble gases in pit 51 in clastic matrix of Nogoya (Appendix 1) are separated into solar (Eberhardt et al., 1972) and planetary (Mazor et al., 1970) components.

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Fig. 1. Continued.

Distribution of solar gases in CM chondrites

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Fig. 2. Elemental abundance patterns of noble gases normalized to the planetary pattern (Mazor et al., 1970). 20Ne and Ar were corrected for cosmogenic gases. 4He was not corrected for cosmogenic and radiogenic gases, but their contribution to the total abundance is small. (a) Murchison. Clastic matrix shows an excess of He and Ne which is not seen in PAR. ‘Boundary’ is the results of an analysis of a portion in between PAR and clastic matrix. ‘Bulk’ data is from Bogard et al. (1971) (b) Murray. ‘Bulk’ data is from Mazor et al. (1970). (c) Nogoya. He and Ne in clastic matrix show an excess larger than those of bulk (Heymann and Mazor, 1967). 36

3.3.2. Primordial noble gases Concentrations of heavy primordial noble gases are relatively uniform in clastic matrix on a ;40 mg scale. This does not necessarily mean that gas concentrations of individual constituents in clastic matrix are constant, since a single laser shot fuses many constituents at once. Figure 5 shows 132Xe concentrations of the clastic matrices of the Murchison, Murray, and Nogoya samples. Distribution of 132Xe can be regarded as that of heavy primordial noble gases, since solar gas contribution on 132Xe is very small according to the reasoning in the previous paragraph. The average 132Xe concentrations tend to decrease from Murray (1.3 3 1028 cc STP/g, only one analysis), to Murchison (1.0 3 1028 cc STP/g), and to Nogoya (0.7 3 1028 cc STP/g). This order is the same as that of the average 132Xe concentrations in the rims in PARs (Fig. 4). The average 132Xe concentration in the clastic matrix in Murchison (1.0 3 1028 cc STP/g) is lower than that in the rims (1.5 3 1028 cc STP/g), close to that in the PCP-rich portions (0.9 3 1028 cc STP/g), and higher than that of the chondrules (0.3 3 1028 cc STP/g). The same tendency is also recognized

in the Nogoya sample. These facts indicate that clastic matrix could have originated from PAR-like precursor material by comminution and mixing of chondrules, rims and PCP-rich portions. 3.3.3. Solar and cosmogenic noble gases He and Ne isotopic ratios of the clastic matrices of Murchison (Fig. 3a), Murray (Fig. 3b), and Nogoya (Fig. 3c) are significantly different from those of coexisting PARs. 20Ne/ 22 Ne and 21Ne/22Ne ratios in the clastic matrices of Murchison and Nogoya mostly range from 10.4 to 11.6 and from 0.03 to 0.09, respectively, which are close to those of SEP (11.2, 0.0295; Benkert et al., 1993) and SF (11.6, 0.030; Rao et al., 1991). 3He/4He isotopic ratios are around 3.5 3 1024 in the clastic matrices of Murchison and Nogoya; those ratios are higher than those of the coexisting PARs (;2 3 1024) and close to those of SW (3.9 3 1024; Black 1972a) and SF (2.6 3 1024; Rao et al., 1991). These results clearly indicate that the large excesses of light noble gases in elemental abun-

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Fig. 3. He and Ne isotopic ratios determined by laser-microprobe analysis. (a) Murchison. ‘Bulk’ values are from Bogard et al. (1971). Ne data from clastic matrix are plotted close to SEP. (b) Murray. ‘Bulk’ values in the figure are from Mazor et al. (1970). (c) Nogoya. Both He and Ne isotopic data show a bimodal distribution at primordial (A) and solar (SEP) components. ‘Bulk’ values are derived from Heymann and Mazor (1967).

dance patterns of clastic matrix (Figs. 2a-c) are due to the presence of solar-type noble gases. The presence of solar gases leads to a conclusion that clastic matrix is regolith material that had been exposed to solar wind. Previously reported He and Ne isotopic ratios and He-Xe elemental abundances of bulk CM-chondrite samples (Murchison from Bogard et al., 1971; Murray from Mazor et al., 1970; Nogoya from Heymann and Mazor, 1967 and Black, 1972a) showed signatures of solar gases that are similar to those of clastic matrix determined in this study (Figs. 2a– c and 3a– c). Thus, it is inferred that the meteorite samples having been

analyzed in the previous studies must have contained clastic matrix. Heymann and Mazor (1967) and Black (1972a) have analyzed solar gas-rich and -poor samples of Nogoya and the results are quite similar to those of our analyses of clastic matrix and PARs, respectively, in Nogoya (Figs. 2c and 3c). Eight measurements on a line with approximately 500 mm intervals in the Nogoya sample were made to see solar gas distribution in clastic matrix. The results show that 4He and 20 Ne abundances vary up to two orders of magnitude within the clastic matrix (Figs. 6a and b; see also Figs. 1b and c). This indicates that in clastic matrix solar gases are heterogeneously

Distribution of solar gases in CM chondrites

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Fig. 3. Continued.

distributed, showing a clear contrast to the homogeneous distribution of primordial gases. The 4He and 20Ne distributions show a maximum peak at pit 57 with margins about 1 mm on either sides (Figs. 6a and b), suggesting that the solar gases are heterogeneous on a millimeter scale in the clastic matrix. The concentrations of pits 53 and 54 in the clastic matrix are similar to those of the PARs, pits 59 and 60, indicating that there are places in clastic matrix lacking solar gases. All these observations imply that some of clastic matrix has had a long residence time on the surface exposed to the solar wind, while other constituents have been heavily shielded. Clastic matrix in CM chondrites, therefore, is a sample of an immature regolith. Distribution of cosmogenic noble gases indicates that (21Ne)cos concentrations in many portions in the clastic matrices of Murchison and Nogoya are comparable to those in PARs. However, some portions of clastic matrix showed a distinct excess of (21Ne)cos up to 1 3 1028 cc STP/g relative to

the average PAR, which raises up an average (21Ne)cos concentration of clastic matrix in Nogoya relative to that of PARs (Table 1). The (21Ne)cos excesses in clastic matrix could not be caused by a difference in target chemistry, since bulk elemental abundances in clastic matrix and PARs are almost identical (Metzler et al., 1992). (21Ne)cos concentrations do not correlate with the spatial position in samples but with the lithology of analyzed areas, thus shielding depth is not responsible for the difference of (21Ne)cos. Therefore the (21Ne)cos excesses in some places in clastic matrix are explained that they were exposed to cosmic rays in a shallow depth of the parent bodies. 3.4. Noble-Gas Signatures at Boundaries Between PARs and Clastic Matrix In this section noble gas signatures of outermost parts of PARs and adjacent clastic matrix are summarized. Analyses of

Table 1. (21Ne)cos concentrations and exposure ages of CM chondrites determined by laser-microprobe analyses. Meteorites

Lithology

Number of data

Average (21Ne)cos (1029 cc/g)

Production rate (1029 cc/g)

Average exposure age (Ma)

Murchison

PAR area clastic matrix PAR area clastic matrix PAR area clastic matrix

29* 20 4 1 7 7

3.4 6 2.1 5.9 6 4.1 8.5 6 3.1 5.8 6 2.8 0.3 6 0.3 1.6 6 1.1

2.023.6 2.122.6 2.022.6 2.1 2.022.6 2.022.1

1.3 6 0.9 2.7 6 1.9 3.4 6 1.1 2.7 6 1.3 0.1 6 0.1 0.8 6 0.5

Murray Nogoya

*The number including the data of Murchison in Paper I.

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Fig. 4. Average 132Xe concentrations in rims around chondrules in CM chondrites. The concentration of Y-791198) 791198 is an average of 5 rims (data from Paper I) and that of Murchison is an average of 15 rims (10 out of 15 data from Paper I). 132Xe concentrations of bulk CI chondrites are shown for comparison (data from Mazor et al., 1970). Rims in CM chondrites show higher concentration than bulk CI material.

two locations in PARs in Nogoya (pits 50 and 59 in Fig. 1c), which are adjacent to the boundary between the PARs and clastic matrix, revealed that solar gases are present in the pit 50 and not in the pit 59. 4He and 20Ne concentrations of the pit 50 are 2 3 1024 and 9 3 1027 cc STP/g, respectively, that are approximately 5 times higher than those of average PARs. The pit 59 showed normal noble-gas characteristics of PARs. Dis-

Fig. 5. 132Xe concentrations in the clastic matrices of Murchison, Murray (M), and Nogoya.

tances from the pits 50 and 59 to the boundary are measured to be less than 300 mm. A similar phenomenon was also observed in the Murchison sample. A chondrule having a rim up to 300 mm in thickness that contacts to clastic matrix appeared to contain solar gases in an outer portion of the rim (pit 39 in Fig.

Fig. 6. Line profiles of 4He (a) and 20Ne (b) abundance in the Nogoya sample, showing heterogeneous distribution of solar-type noble gases. The pit numbers in the figures correspond to those in Fig. 1c.

Distribution of solar gases in CM chondrites

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Fig. 7. A BSE image of a portion of Murchison where a PAR clast and clastic matrix are in contact. Four laser pits, numbered from 38 to 41, are visible. The boundary between the PAR clast and the clastic matrix is indicated by a dashed line. Inset: A BSE image of a central chondrule prior to laser extraction showing a well developed rim around the chondrule.

7), although an inner portion of the rim was free of solar gases (pit 38 in Fig. 7). 4He and 20Ne concentrations of the pit 39 are approximately 4 ; 5 times higher than the average concentrations of PARs in Murchison. But in the pit 38, 200 mm away from the pit 39, He and Ne are in the range of concentrations and compositions of typical PARs. In summary, analyses of the boundary regions revealed that some amounts of solar gases are included in the edges of PARs, but the concentrations decrease steeply toward interiors of the PARs. 4. DISCUSSION

4.1. Distribution of Solar-Type Noble Gases and Brecciation Processes experienced by CM Chondrites Our present study showed that solar gases are contained in the meteorite slices of Murchison, Murray, and Nogoya which exhibit a complex texture consisting of some clasts of PAR and clastic matrix. Solar gases are distributed very heterogeneously on a centimeter scale, which is resulted from the absence of solar gases in PARs, typically 1 cm in size, and the presence of the gases in clastic matrix. This confirms that CM chondrites

are regolith breccias from the surface regions of their parent bodies, being consistent with the formation model for CM chondrites proposed by Metzler et al. (1992). Clastic matrix exhibits a comminuted texture and contains many small angular anhydrous mineral fragments in fine-grained phyllosilicates (Fig. 1a). This indicates that the reduction of grain size due to fragmentation and disaggregation took place upon impacts on the parent body, and constituents of clastic matrix were able to trap solar gases efficiently owing to high surface area to be exposed. According to the observation of olivine grains by Scott et al. (1992), shock effects of the CM chondrites including Murchison, Murray, and Nogoya are not significant. Thus low-velocity impacts might have caused formation of clastic matrix, which is consistent with the immature nature of clastic matrix inferred from heterogeneous distribution of solar gases: 4 He and 20Ne concentrations vary more than two orders of magnitude on a millimeter scale (Figs. 6a and b). The solar gas-rich portions in clastic matrix had been located on the topmost surface for some duration in the formation history, because the solar wind penetrates meteoritic materials to very shallow depths. When (20Ne)sol flux in the asteroid belt

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at 2.5 AU is 3.2 3 1029 cc STP/cm2 yr (Geiss et al., 1972) and SW-Ne/SEP-Ne flux ratio is 33 6 17 (Wieler et al., 1986), SEP exposure time of clastic matrix can be estimated to be 1000 and 3300 yr for Murchison and Nogoya, respectively. Following assumptions were made for this estimation: (1) (20Ne)SEP with a current flux accumulates in a 100 mm surface layer of the parent body, (2) during exposure the (20Ne)SEP concentrations of the layer increase with a rate of 3 3 1029 cc STP/g yr to the highest concentrations of (20Ne)sol in the clastic matrix, i.e., 3 3 1026 and 1 3 1025 cc STP/g for Murchison and Nogoya, respectively, and (3) (20Ne)sol in clastic matrix is dominated by SEP, which is verified in the later discussion. Our noble-gas measurements demonstrated that solar-type noble gases are totally absent in the interior of PARs. This is consistent with the results of Y-791198 and Murchison in Paper I. These observations strongly suggest that rim formation and subsequent PAR formation took place in a thick nebular cloud where solar wind was highly shielded. Marginal areas of some PARs directly adjacent to clastic matrix, however, showed a contribution of solar gases and following reasoning can be made to explain the presence of solar gases at the edges of PARs; (1) solar wind implantation to the surfaces of PARs in a period after formation of PAR, and (2) transportation of solargas implanted fine mineral particles from clastic matrices to the marginal portions of PARs through pores by the fluid activity during aqueous alteration. As a further element of consideration, it must be pointed out that a part of clastic matrix, close to the solar-gas bearing portion of PARs, contains large amounts of solar gases in either case for Murchison and Nogoya. We can not conclude here which reasoning is valid in accounting for the presence of solar gases at the edges of PARs, but in any case solar gases were added to PARs from the exterior after formation of PARs. Presence of the solar gases was also reported in a rim around a chondrule in Murchison by Woolum et al. (1995), although in this case texture and lithology of portions adjacent to the chondrule were not described.

Fig. 8. The calculation sequence to obtain 20Ne/22Ne ratios and 20Ne concentrations of solar gases in clastic matrix of CM chondrites, from the measured Ne isotopic ratios and concentrations. In this figure, we assume that all clastic matrix contain primordial Ne with Ne-A isotopic ratios (case I in text). Following is a whole calculation sequence. (1) (20Ne)pri concentrations of indivisual analysed portions were obtained using 132Xe concentrations of the portions and a average (20Ne/ 132 Xe)pri ratio (19.4). (2) The primordial Ne, having Ne-A isotopic ratios and concentrations determined in step (1), was corrected from the measured Ne isotopic ratios (point M) and concentrations, then we have Ne (point N) consisting of solar and cosmogenic Ne. (3) The (20Ne/ 22 Ne)sol ratios (point S) were determined by taking an intersection of a SW-SEP line and an extension line of Cos. (cosmogenic)-N. The (20Ne)sol concentrations were obtained by correction for (20Ne)cos from (20Ne)cos1sol concentrations.

11.42 yields a retained SEPNe/SWNe ratio of 10.7. In case II, as an extreme case of a large Ne-E contribution, isotopic ratios of the primordial Ne in Orgueil CI chondrite, (20Ne/22Ne, 21Ne/ 22 Ne)pri 5 (6.8, 0.0268) (Huss et al., 1996), was used. The weighted mean of the (20Ne/22Ne)sol ratios was 11.56 6 0.02 and the retained SEPNe/SWNe ratio was 6.2.

4.2. High SEPNe/SWNe Ratio in CM Chondrites and Possible Relevant Mechanisms Solar-wind-derived 20Ne concentrations ((20Ne)sol) and isotopic ratios ((20Ne/22Ne)sol) in the clastic matrices in Murchison, Murray, and Nogoya were determined through corrections of (Ne)pri and (Ne)cos from (Ne)mes. The whole calculation sequence is shown in Fig. 8. Following assumptions were made for the calculation: (1) isotopic ratio of primordial Ne is constant for all clastic matrix, and (2) (20Ne)pri concentrations are proportional to those of 132Xe with a ratio of (20Ne/132Xe)pri 5 19.4 for all clastic matrix, which is an average ratio determined by analyses of PARs in Murchison in Paper I and this paper. Again, it was confirmed that more than 99% of 132Xe in clastic matrix is planetary-type noble gas. Calculations were made for two cases. In case I, primordial Ne in clastic matrix is Ne-A with isotopic ratios of (20Ne/22Ne, 21Ne/22Ne)pri 5 (8.2, 0.024) (Pepin, 1967; Black and Pepin, 1969). The result is shown in Fig. 9 which indicates that with a few exceptions the (20Ne/ 22 Ne)sol ratios of the three meteorites are very close to those of SEP and SF. A weighted mean of all (20Ne/22Ne)sol ratios except for those lower than 11.2 was obtained to be 11.42 6 0.02. When partitioned between SW and SEP components, the

Fig. 9. (20Ne/22Ne)sol isotopic ratios (a) and (20Ne)sol concentrations (b) in clastic matrix of CM chondrites. The (20Ne/22Ne)sol ratios are far below the SW (Benkert et al., 1993) and scattered in a narrow range around the SEP (Benkert et al., 1993) and SF (Rao et al., 1991).

Distribution of solar gases in CM chondrites

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Fig. 10. Correlation between (21Ne)cos and (20Ne)sol concentrations in the clastic matrices of Murchison (a) and Nogoya (b). (21Ne)cos and (20Ne)sol concentrations of PARs are shown as a field of gray bars on the ordinate. (21Ne)cos excesses in clastic matrix relative to the gray bars are observed in some data points in both (a) and (b). There seems two group of data points in the diagram of Murchison (a); (21Ne)cos-rich group whose (21Ne)cos concentrations are larger than 1 3 1028 cc STP/g, and (21Ne)cos-poor group whose (21Ne)cos concentrations range from 0.1 to 0.8 3 1028 cc STP/g.

Results of calculation indicate that the mean (20Ne/22Ne)sol ratio is in a range from 11.4 to 11.6, demonstrating that solar Ne in clastic matrix in the three CM chondrites is dominated by SEP- or SF-component with energy much higher than that of SW. The retained SEPNe/SWNe ratio ranges from 6.2 to 10.7, which is clearly higher than ;1 deduced from lunar soil and regolith that record contemporary and 1 Ga-ago solar-particle environment (Nichols et al., 1994). Even when compared with other gas-rich meteorites that contain ancient solar-wind-derived Ne, the range for CM chondrites is still higher. For instance, the Fayetteville H4 chondrite and the Kapoeta howardite give SEPNe/SWNe ratios from 3 to 5.5 and from 1 to 1.3, respectively, as deduced from data reported by Wieler et al. (1989) and Black (1972a). Why do CM chondrites show such a high SEPNe/SWNe ratio? We here list three possible mechanisms to explain the high SEPNe/SWNe ratio. The ancient sun would have emitted solar wind with a SEPNe/SWNe ratio similar to the present sun. This case consists of two parts. (1) Preferential shielding of low energy particles such as SW occurred upon implantation, and (2) after solar wind implantation aqueous alteration on the parent body has released preferentially low energy particles from constituent minerals. The final mechanism is (3) The active young sun had emitted energetic particles with a SEP/SW flux ratio higher than that of the present sun. In mechanism (1) presence of water vapor on the surface of parent body is assumed. According to an experimental investigation for stopping power of water vapor (Matteson et al., 1977), at 200 K a layer of approximately 5 m thickness and at 175 K 400 m thickness of water vapor are able to stop SW He21. In mechanism (2) aqueous alteration replaced minerals from the surfaces where low energy solar gases were implanted, which

resulted in the preferential loss of SW component from CM chondrites. A detailed explanation for this mechanism is given in the later section. For mechanism (3) there is no compelling evidence to postulate the high SEP/SW flux ratio of the ancient sun, but there are some indications that the energetic-particle environment in the early solar system was quite different from that of the present day (e.g., Eberhardt et al., 1972; Geiss, 1973; Clayton and Thiemens, 1980). Combined effects of all three mechanisms might be responsible, but in our view aqueous alteration is the most effective to establish the high SEPNe/ SWNe ratio. There is evidence that aqueous alteration, the mechanism (2), has worked even after the final consolidation of CM chondrites, thus the alteration might have modified past records of solar gases established by other mechanisms, as discussed in the later section. 4.3. Distribution of Cosmogenic Noble Gases and Pre-Compaction Irradiation of CM Chondrites Unlike solar-gas distribution that shows a large variation in concentrations between places up to factors of 100, the variation of spallogenic noble gases in CM chondrites is much less significant. GCR exposure ages of PARs in Murchison and Nogoya were determined to be 1.3 6 0.9 and 0.1 6 0.1 Ma, respectively (Table 1). On the other hand, from radioactive nuclides such as 26Al and 10Be, GCR exposure duration of Murchison in a transit from the meteorite parent body to the earth was determined to be 1.6 6 0.3 Ma (Herzog et al., 1997), which shows an agreement with the average PARs’ exposure age determined in this study. Similarly GCR exposure duration of Nogoya, 0.15 Ma from 26Al and 0.21 Ma from 53Mn (e.g., Goswami et al., 1984), is consistent with that of Nogoya PARs.

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These facts suggest that PARs in the two meteorites have not experienced “pre-compaction irradiation” (e.g., Wieler et al., 1989; Rao et al., 1997). The variation of spallogenic-gas concentrations in clastic matrix along with that of solar gases can be seen in a diagram showing a relationship between (21Ne)cos and (20Ne)sol in the clastic matrices of Murchison (Fig. 10a) and Nogoya (Fig. 10b) samples. The mean concentrations of (21Ne)cos and (20Ne)sol in PARs with a range of 61s are also shown as gray bars on the ordinate, since solar gases are almost absent from PARs (Figs. 10a and b). (21Ne)cos concentrations in some places in the clastic matrices show distinct excesses up to factors of four relative to those in PARs in the two meteorites (Figs. 10a and b). Major element concentrations such as Mg and Si, which are main sources for production of (21Ne)cos in spallation reactions, are not largely different between places in clastic matrix, which excludes a possibility that the (21Ne)cos excesses are caused by differences in target chemistry. The (21Ne)cos excesses are, therefore, ascribed to pre-compaction irradiation. The data points of Murchison (Fig. 10a) can be divided into two groups based on (21Ne)cos concentrations: one is within a range of PARs ((21Ne)cos -poor group) and the other is over the range of PARs ((21Ne)cos -rich group). Most data points are included in the former, suggesting that a major fraction of the clastic matrix in Murchison have experienced very short precompaction irradiation, i.e., less than 0.1 Ma. On the contrary, some portions in the clastic matrix whose Ne compositions are plotted in the latter were subject to pre-compaction irradiation for 10 Ma at most, when 2p exposure to contemporary GCR is assumed. Caffee et al. (1983, 1987) and Hohenberg et al. (1990) found pre-irradiated olivine grains in Murchison with very high (21Ne)cos concentrations up to 36 3 1028 cc STP/g, corresponding to 144 Ma exposure age. There is a possibility that such pre-irradiated olivine grains are contained in the spallogenic Ne-enriched portions found in this study. In summary, taking together with solar gas distribution, PARs are almost free of pre-compaction irradiation. Most parts of clastic matrix have experienced very short pre-compaction irradiation, which is verified by the presence of solar gases and the absence of excesses of spallogenic gases. Only some locations of clastic matrix exhibit obvious effects of pre-compaction irradiation where high concentrations of solar gases and excesses of spallogenic gases were observed. 4.4. Aqueous Alteration of CM Chondrites and Its Effects on the Abundance of Noble Gases There are much evidence that aqueous alteration of CM chondrites occurred in the meteorite parent bodies (e.g., Tomeoka and Buseck, 1985). Recently, Hanowski and Brearley (1997) found iron-rich aureoles spreading from a metal inclusion in Murchison and they concluded that it resulted from in-situ aqueous alteration. Similar texture was observed in clastic matrix in Murchison where a large type-I (Fe-rich) PCP, an alteration product of metallic grains (Tomeoka and Buseck, 1985), generates the iron-rich aureoles that encompass fragmented silicate minerals (Fig. 1b in Nakamura and Nakamuta, 1996). All these observations suggest that clastic matrix has experienced aqueous alteration after it became present structure. On the other hand, solar-gas implantation took place

Fig. 11. Stepwise release patterns of cosmogenic 21Ne from Y-791198 and Nogoya. Two patterns from solar-gas-free samples of Nogoya (Black, 1972a) show that much 21Ne is released at low temperature steps, which is distinctly different from that of Y-791198 (data from Paper I).

before clastic matrix became present structure. Therefore it is concluded that solar-gas implanted clastic matrix has undergone aqueous alteration in the parent body and has never been exposed to the solar wind again. In the next three paragraphs effects of aqueous alteration on the abundance of solar gases are discussed. The aqueous alteration reactions have changed mineralogy of CM chondrites greatly and most minerals have suffered from replacement. There is evidence that the alteration process has lowered gas-retentivity of light noble gases in CM chondrites. Figure 11 shows that a cumulative release pattern of spallogenic 21Ne, being distributed in bulk mineral grains, in the stepwise heating analysis of Y-791198 is quite different from that of Nogoya (data from Paper I and Black, 1972a). Nogoya releases much more 21Ne in the low temperature steps compared with Y-791198, indicating lower Ne retentivity of Nogoya than that of Y-791198. Y-791198 is the least altered CM chondrite (Metzler et al., 1992), whereas Nogoya is one of the most altered CM chondrites (McSween, 1979; Browning et al., 1996). Assuming that Nogoya had a mineralogy similar to Y-791198 prior to the intense aqueous alteration, the low Ne retentivity of Nogoya in the present mineralogy would have been caused by the aqueous alteration. Therefore, it is suggested that the mineral replacement during the alteration has made CM chondrites “leaky” material by lowering retentivity of light noble gases. It is known that SEP penetrates meteoritic materials to 1 mm depth and SW to 1021 mm depth (e.g., Goswami et al., 1984). CM chondrites are mainly composed of fine-grained serpentine-type phyllosilicates with diameters less than 1 mm (Zolensky et al., 1993). The 1 mm size of grains is much smaller compared with penetration depth of SEP, thus like spallogenic noble gases SEP would be distributed homogeneously in finegrained minerals, while SW resided in surface layers of minerals. On the other hand, aqueous alteration reaction replaces minerals from the surfaces, thus the outermost parts of minerals become subject to alteration. Therefore it is likely that partially replaced minerals have lost solar-type noble gases, especially

Distribution of solar gases in CM chondrites

surface-located SW, due to low gas-retentivity of surface layers of minerals. Even when minerals were completely replaced by the alteration, SW would have been lost greater than SEP because of much shorter distance to the surface of minerals. In this case some parts of SEP and even cosmogenic gases might have lost from minerals. Based on the reasoning described above we believe that during the aqueous alteration constituent minerals in clastic matrix have lost most of SW, parts of SEP, and probably parts of spallogenic gases that were produced by pre-compaction irradiation. In our view, the alteration is mainly responsible for the high SEPNe/SWNe ratio in CM chondrites. As the alteration proceeds, a larger fraction of SW was lost from CM chondrites and thus the SEPNe/SWNe ratio should be increasing. Our calculation shows that the retained SEPNe/SWNe ratio of Nogoya (13.4) is higher than that of Murchison (5.6), assuming that Ne-A is a primordial Ne component in clastic matrix. The higher ratio of Nogoya is consistent with our view, since Nogoya is much more altered than Murchison (e.g., Browning et al., 1996). Further, there is another confirmation from terrestial analogue: the alteration of meteorites in Antarctic ice layers raised the SEPNe/SWNe ratio (Padia and Rao, 1986). The aqueous alteration seems to have affected also on the abundances of primordial noble gases. Our present examination and Paper I showed that rims around chondrules contain a large amounts of heavy primordial gases, suggesting a dense distribution of phase Q (e.g., Lewis et al., 1975). Y-791198 is least affected by aqueous alteration reactions (Metzler et al., 1992) and shows the highest abundance of heavy primordial noble gases in the rims (Fig. 4). Murchison and Murray, which have experienced mild aqueous alteration (McSween, 1979; Browning et al., 1996), show the primordial-gas concentrations in the rims lower than those of Y-791198, whereas Nogoya, the most aqueously altered sample among the four CM chondrites (McSween, 1979; Browning et al., 1996), shows the lowest gas concentrations in the rims (Fig. 4). Assuming that the rims in CM chondrites have sampled a common reservoir of nebular dust in the formation process, the inverse correlation between the concentrations of heavy primordial gases in the rims and the degrees of aqueous alteration can be attributable to noble-gas loss from phase Q during the alteration. This is consistent with the interpretation by Browning et al. (1996) who reported a similar correlation of CM chondrites between alteration intensities and whole rock (36Ar)pri contents given in the literature (Schultz and Kruse, 1989). The physico-chemical conditions of solution responsible for the aqueous alteration of CM chondrites were estimated by various geochemical indicators; ,20°C based on oxygen isotopes of minerals coexisting with the solution (Clayton and Mayeda, 1984) and ,50°C with pH 10 to 12 based on the stability fields of some hydrous minerals (Zolensky, 1984; Zolensky et al., 1993). But nature of the interactions between high pH solution and phase Q is poorly known, thus experimental studies are clearly needed in this field. 4.5. Nebular and Planetary Processes of CM Asteroids Based on Noble Gas Microdistribution In this section we summarize the constraints set on the formation processes of CM chondrites based on the results of

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this paper and Paper I as well as those from literature. The analyses of PARs revealed some nebular processes that individual CM components have experienced. At first chondrules and PCPs, or PCP precursor, have been formed in the nebula. The low concentrations of primordial noble gases in chondrules suggest efficient outgas during chondrule formation. Carrier phases of primordial noble gases such as phase Q and interstellar diamonds were accreted together with various kinds of solar system fine mineral particles onto chondrules and other material to form fine-grained rims around them. The rimmed material was agglomerated to form PARs in the nebula where energetic particles were highly shielded by nebular gas, which is verified by the absence of solar gases in the interior of PARs. Then the nebular gas was dispersed. There is no direct evidence for the timing of nebular dissipation, but studies on CI chondrites (e.g., Macdougall et al., 1984) provide some indications for this. According to Endress et al. (1996) carbonate vein fragments in the Orgueil and Ivuna CI chondrites were formed 20 Myr or less after the time of formation of the oldest known solar nebular condensate Allende CAIs. Assuming that solar gases were already present in CI parent bodies when the carbonate veins were formed through some brecciation and alteration processes, the nebular dissipation should have occurred before the vein formation. When the solar wind reached CI parent bodies, CM parent bodies must be exposed to solar wind, too. At that time CM parent bodies might have been still growing by collecting PARs. Surfaces of the parent bodies were exposed to the solar wind and GCR. Solar-wind fluxes might have been much higher than those of the present (Caffee et al., 1983, 1987; Hohenberg et al., 1990), which would be responsible for the extraordinarily large amounts of (21Ne)cos in olivine grains (e.g., Hohenberg et al., 1990) and the excesses of (21Ne)cos in some portions in clastic matrix. The exposed material was implanted with solar gases and was periodically mixed by another impacts so that new material was exposed to the wind, which resulted in clastic matrix consisting of irradiated and unirradiated material. Some material, like the inside of centimeter-sized PAR, was not exposed to the solar wind, although if it were within several meters of the surface, it would have been exposed to GCR. The difference in the depth to which solar wind and GCR penetrate explains why solar gases can vary by orders of magnitudes while cosmogenic gases are relatively constant between places in clastic matrix. Most parts of CM chondrites do not show long GCR irradiation in the regolith, suggesting that addition of newly accreted material buried CM chondrite material below at least several meters to the surfaces of the parent bodies. After that the CM chondrite material has never been exposed to solar wind and GCR for more than 4 Gyr. At some depth in the parent bodies the CM chondrite material underwent aqueous alteration. It is not clear when the alteration has started, but some petrological evidence showed that CM chondrites have experienced in situ aqueous alteration after the final consolidation (e.g., Hanowski and Brearley, 1997). In the course of the alteration constituent minerals of CM chondrites were replaced by secondary phases to varying degrees, from partially to completely. During the alteration most of SW, some parts of SEP and spallogenic gases were lost from minerals, resulted in high SEPNe/SWNe ratios in meteorites. The aqueous alteration reactions appear to have caused the

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partial loss of primordial noble gases. Concentrations of 132Xe in rims around chondrules in CM chondrites are correlated inversely with the degree of aqueous alteration. Acknowledgments—We are indebted to Drs. M. Sekiya, R. Wieler, M. E. Zolensky, and D. Woolum for helpful discussions. Thoughtful and perceptive review by Dr. G. Huss and comments by an anonymous reviewer are much appreciated. Thanks are also due to Messrs. R. Okazaki, K. Shimada, and Dr. V. Yang for assistance and support during the course of this study. This work has been supported by the Grant-in-aid of the Japan Ministry of Education, Science and Culture to TN (No. 09740398), to NT and TN (No. 09304055), as well as the grants from the Sumitomo Science Foundation to KN and TN and from the Yamada Science Foundation to NT. This work was carried out by joint research at the Institute for Study of the Earth’s Interior, Okayama University. REFERENCES Anders E. (1975) Do stony meteorites come from comets? Icarus 24, 363–371. Benkert J.-P., Baur H., Signer P., and Wieler R. (1993) He, Ne, and Ar from the solar wind and solar energitic particles in lunar ilmenites and pyroxenes. J. Geophys. Res. 98, 13147–13162. Black D. C. (1972a) On the origins of trapped helium, neon and argon isotopic variations in meteorites-I. Gas-rich meteorties, lunar soil and breccias. Geochim. Cosmochim. Acta 36, 347–375. Black D. C. (1972b) On the origin of trapped helium, neon and argon isotopic variations in meteorites. II. Carbonaceous chondrites. Geochim. Cosmochim. Acta 36, 377–394. Black D. C. and Pepin R. O. (1969) Trapped neon in meteorite. II. Earth Planet. Sci. Lett. 6, 395– 405. Bogard D. D., Clark R. S., Keith J. E., and Reynolds M. A. (1971) Noble gases and radionuclides in Lost City and other recently fallen meteorites. J. Geophys. Res. 76, 4076 – 4083. Browning L. B., McSween H. Y., and Zolensky M. E. (1996) Correlated alteration effects in CM carbonaceous chondrites. Geochim. Cosmochim. Acta 60, 2621–2633. Bunch T. E. and Chang S. (1980) Carbonaceous chondrites. II. Carbonaceous chondrite phyllosilicates and light element geochemistry as indicators of parent body processes and surface conditions. Geochim. Cosmochim. Acta 44, 1543–1577. Bunch T. E. and Rajan R. S. (1988) Meteorite regolith breccias. In Meteoritics and the Early Solar System (eds. J. F. Kerridge and M. S. Matthews), pp. 144 –164. Univ. Arizona Press. Caffee M. W., Goswami J. N., Hohenberg C. M., and Swindle T. D. (1983) Cosmogenic neon from precompaction irradiations of Kapoeta and Murchison. Proc. of 14th Lunar Planet. Sci. Conf. B267– B273. Caffee M. W., Hohenberg C. M., Swindle T. D., and Goswami J. N. (1987) Evidence in meteorites for an active early sun. Astrophys. J. 313, L31–L35. Clayton R. N. and Thiemens M. H. (1980) Lunar nitrogen: Evidence for secular change in the solar wind. In The Ancient Sun, (eds. R. O. Pepin, J. A. Eddy, and R. B. Merrill) Pergamon Press, New York, pp. 463– 473. Clayton R. N. and Mayeda T. K. (1984) The oxygen isotopic record in Murchison and other carbonaceous chondrites. Earth Planet. Sci. Lett. 67, 151–161. Eberhardt P., Geiss J., Graf H., Mendia M. D., Morgeli M., Schwaller H., Stettler A., Kra¨henbu¨hl U., and von Gunten H. R. (1972) Trapped solar wind noble gases in Apollo 12 lunar fines 12001 and Apollo 11 breccia 10046. Proc. 3rd Lunar Planet. Sci. Conf., 1821–1856. Endress M., Zinner E., and Bischoff A. (1996) Early aqueous activity on primitive meteorite parent bodies. Nature 379, 701–703. Eugster O. (1988) Cosmic-ray production rates for 3He, 21Ne, 38Ar, 83 Kr, and 126Xe in chondrites based on 81Kr-Kr exposure ages. Geochim. Cosmochim. Acta 52, 1649 –1662. Fuchs L. H., Olsen E., and Jensen K. J. (1973) Mineralogy, mineralchemistry and composition of the Murchison (C2) meteorites. Smithson. Contrib. Earth Sci. 10, 1–39. Geiss J., Buehler F., Cerutti H., Eberhardt P., and Filleaux, Ch. (1972)

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Appendix 1. He-Ar results by laser microprobe analyses on the Murchison, Murray, and Nogoya samples. Lithology

Component

Fused mass§

Murchison Murchison Murchison Murchison

PAR Boundary Clastic Clastic

Rim Rim/Matrix Matrix Matrix

42

Murray

PAR

43 44 45 46

Murray Murray Murray Murray

PAR PAR PAR Clastic

Rim PCP-rich portion Rim Rim Matrix

47 48 49 50 51 52 53 54 55 56 57 58 59

line line line line line line line

Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya Nogoya

PAR PAR PAR Boundary Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic PAR

60 line

Nogoya

PAR

38 39 40 41

line line line line

§

PCP-rich portion Rim Rim PCP/Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Rim PCP-rich portion

4

He#

3/4

1.3E-05 8.4E-06 8.4E-06 8.4E-06

5.44E-05 3.26E-04 2.68E-04 2.26E-04

0.00030 6 0.00001 0.00039 6 0.00003 0.00039 6 0.00002 0.00037 6 0.00004

2.2E-05

6.12E-05

2.3E-05 1.1E-05 8.3E-06 3.1E-05

20

Ne

20/22

21/22

4.05E-07 1.44E-06 1.17E-06 9.25E-07

7.16 6 0.72 9.88 6 1.14 10.41 6 0.71 9.76 6 0.31

0.091 6 0.010 0.064 6 0.009 0.061 6 0.008 0.063 6 0.007

0.00044 6 0.00003

6.58E-07

7.53 6 0.12

4.42E-05 5.48E-05 5.46E-05 1.62E-04

0.00051 6 0.00001 0.00034 6 0.00003 0.00055 6 0.00001 0.00035 6 0.00002

2.41E-07 2.88E-07 2.19E-07 8.52E-07

3.1E-05 3.6E-05 4.3E-05 1.0E-04 5.9E-05 3.4E-05 1.6E-05 1.6E-05 1.6E-05 1.6E-05 3.4E-05 1.6E-05 1.6E-05

2.58E-05 3.54E-05 2.94E-05 1.96E-04 1.28E-03 5.39E-04 1.44E-05 1.51E-05 9.40E-05 5.56E-04 1.02E-03 4.84E-04 2.42E-05

0.00019 6 0.00001 0.00019 6 0.00001 0.00019 6 0.00001 0.00030 6 0.00000 0.00033 6 0.00000 0.00032 6 0.00001 0.00027 6 0.00002 0.00022 6 0.00005 0.00031 6 0.00001 0.00036 6 0.00001 0.00032 6 0.00000 0.00033 6 0.00002 0.00021 6 0.00001

1.6E-05

1.52E-05

0.00028 6 0.00001

36

Ar

Kr

132

40/36

1.35E-06 9.71E-07 8.28E-07 9.24E-07

0.2306 6 0.004 0.1869 6 0.002 0.1848 6 0.006 0.1892 6 0.002

n.d. 15.64 6 3.0 14.74 6 3.5 40.68 6 2.7

1.44E-08 1.06E-08 8.90E-09 1.24E-08

1.60E-08 9.42E-09 1.16E-08 1.13E-08

0.173 6 0.007

9.53E-07

0.1868 6 0.001

16.54 6 1.3

1.76E-08

1.84E-08

7.92 6 0.65 7.84 6 0.52 7.25 6 1.29 9.79 6 0.34

0.207 6 0.015 0.230 6 0.012 0.210 6 0.032 0.093 6 0.003

7.64E-07 1.28E-06 8.54E-07 7.99E-07

0.1859 6 0.001 0.1889 6 0.001 0.1883 6 0.001 0.1879 6 0.001

20.53 6 1.5 43.65 6 0.8 11.29 6 0.7 8.37 6 1.1

1.26E-08 2.32E-08 1.32E-08 1.33E-08

1.04E-08 1.74E-08 1.22E-08 1.29E-08

9.35E-08 1.48E-07 1.44E-07 8.87E-07 9.85E-06 4.22E-06 6.24E-08 1.08E-07 6.67E-07 3.89E-06 7.89E-06 2.48E-06 7.40E-08

4.55 6 0.93 7.90 6 0.59 7.86 6 0.50 10.95 6 0.08 10.87 6 0.02 11.35 6 0.06 7.32 6 0.96 6.32 6 0.96 9.99 6 0.27 11.36 6 0.04 11.24 6 0.01 11.52 6 0.14 7.88 6 0.89

0.031 6 0.012 0.045 6 0.032 0.061 6 0.013 0.036 6 0.002 0.033 6 0.001 0.034 6 0.001 0.077 6 0.022 0.021 6 0.038 0.039 6 0.006 0.031 6 0.004 0.032 6 0.001 0.040 6 0.004 0.016 6 0.022

3.62E-07 5.28E-07 3.83E-07 2.64E-07 7.15E-07 2.62E-07 2.43E-07 2.69E-07 3.54E-07 4.39E-07 4.73E-07 4.01E-07 3.90E-07

0.1872 6 0.000 0.1887 6 0.001 0.1875 6 0.001 0.1894 6 0.000 0.1889 6 0.000 0.191 6 0.001 0.1832 6 0.004 0.1836 6 0.003 0.1892 6 0.002 0.1883 6 0.002 0.1877 6 0.001 0.1855 6 0.002 0.1857 6 0.003

32.13 6 2.1 25.91 6 1.5 36.06 6 1.6 21.12 6 0.8 19.50 6 0.8 25.94 6 2.7 56.74 6 4.6 30.65 6 4.6 31.34 6 3.5 29.69 6 2.9 29.37 6 1.8 5.87 6 3.4 15.90 6 3.5

7.06E-09 1.15E-08 7.79E-09 5.15E-09 1.08E-08 5.61E-09 5.02E-09 4.44E-09 5.80E-09 6.61E-09 7.97E-09 5.86E-09 6.75E-09

6.91E-09 1.16E-08 7.73E-09 5.10E-09 1.12E-08 5.18E-09 5.01E-09 5.41E-09 6.91E-09 7.18E-09 7.69E-09 7.53E-09 8.06E-09

6.83E-08

7.54 6 1.38

0.022 6 0.046

2.00E-07

0.185 6 0.003

26.70 6 6.2

3.61E-09

3.99E-09

Fused masses are given in grams. Noble gas concentrations are given in cc STP/g and uncertainty of concentrations are approximately 25% after correction by degassing efficiencies. & n.d. 5 not detected. * n.m. 5 not measured. #

84

38/36

Xe

T. Nakamura et al.

Sample

Appendix 2. He-Ar results by laser microprobe analyses on the Murchison sample. Sample

Component Rim Matrix Rim Rim Rim Rim Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Matrix Chondrule Matrix PCP-rich portion

61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84

Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison Murchison

PAR Clastic PAR PAR PAR PAR Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic Clastic PAR Clastic

85

Murchison

PAR

§ #

Fused mass§

4

He#

3/4

5.4E-06 5.3E-05 4.2E-05 9.6E-06 5.7E-05 5.7E-06 2.3E-05 1.2E-05 6.4E-06 1.3E-05 1.4E-05 1.4E-05 1.4E-05 4.6E-05 1.4E-05 3.4E-05 8.1E-06 1.2E-05 2.1E-05 1.4E-05 2.5E-05 2.1E-05 1.9E-06 1.1E-05

6.34E-05 5.79E-04 6.80E-05 6.58E-05 5.89E-05 7.23E-05 2.57E-04 3.48E-04 5.05E-05 3.25E-04 1.85E-04 4.40E-04 4.33E-04 5.21E-04 2.92E-04 4.89E-04 5.59E-04 1.04E-04 2.26E-04 6.14E-04 4.21E-04 2.26E-04 8.35E-05 4.21E-04

0.00014 6 0.00001 0.00035 6 0.00001 0.00030 6 0.00000 0.00023 6 0.00002 0.00026 6 0.00001 0.00044 6 0.00004 0.00036 6 0.00000 0.00025 6 0.00000 0.00106 6 0.00004 0.00037 6 0.00001 0.00045 6 0.00001 0.00039 6 0.00001 0.00038 6 0.00000 0.00037 6 0.00001 0.00039 6 0.00001 0.00037 6 0.00000 0.00036 6 0.00001 0.00044 6 0.00001 0.00039 6 0.00000 0.00041 6 0.00001 0.00044 6 0.00000 0.00038 6 0.00001 0.00141 6 0.00003 0.00042 6 0.00001

1.6E-05

4.36E-05

0.00021 6 0.00000

20

Ne

20/22

21/22

2.15E-07 3.07E-06 3.96E-07 4.42E-07 4.11E-07 2.99E-07 1.25E-06 1.27E-06 1.98E-07 1.47E-06 9.40E-07 2.23E-06 1.58E-06 2.55E-06 1.55E-06 2.21E-06 1.83E-06 5.95E-07 1.23E-06 2.92E-06 2.08E-06 1.20E-06 4.85E-08 1.95E-06

7.45 6 1.71 10.96 6 0.05 7.73 6 0.04 7.20 6 0.21 7.69 6 0.03 6.36 6 0.42 10.17 6 0.09 10.98 6 0.08 6.74 6 0.30 10.73 6 0.16 9.56 6 0.19 10.73 6 0.11 11.12 6 0.07 11.37 6 0.12 10.45 6 0.13 10.60 6 0.07 10.48 6 0.21 10.28 6 0.09 9.80 6 0.13 10.71 6 0.05 9.76 6 0.09 10.77 6 0.02 4.44 6 1.39 10.43 6 0.07

0.193 6 0.027 0.050 6 0.001 0.107 6 0.005 0.143 6 0.016 0.061 6 0.002 0.193 6 0.020 0.060 6 0.004 0.048 6 0.007 0.187 6 0.024 0.062 6 0.007 0.127 6 0.005 0.040 6 0.004 0.047 6 0.003 0.052 6 0.001 0.062 6 0.006 0.064 6 0.002 0.052 6 0.003 0.059 6 0.013 0.071 6 0.003 0.081 6 0.003 0.099 6 0.004 0.040 6 0.005 0.447 6 0.045 0.092 6 0.002

2.11E-07

7.96 6 0.30

0.075 6 0.030

36

Ar

84

Kr

132

38/36

40/36

Xe

7.30E-07 1.19E-06 1.05E-06 9.58E-07 9.36E-07 8.74E-07 1.36E-06 4.19E-07 9.65E-07 7.87E-07 5.28E-07 7.75E-07 5.41E-07 4.52E-07 5.98E-07 8.40E-07 4.03E-07 3.64E-07 7.89E-07 5.73E-07 7.08E-07 6.11E-07 1.40E-07 5.08E-07

0.187 6 0.002 0.188 6 0.000 0.187 6 0.001 0.190 6 0.001 0.188 6 0.001 0.189 6 0.002 0.189 6 0.001 0.190 6 0.002 0.186 6 0.001 0.188 6 0.001 0.186 6 0.001 0.185 6 0.001 0.188 6 0.001 0.190 6 0.000 0.186 6 0.001 0.187 6 0.001 0.188 6 0.002 0.187 6 0.001 0.186 6 0.001 0.189 6 0.001 0.187 6 0.001 0.189 6 0.001 0.250 6 0.028 0.186 6 0.001

1.65 6 7.3 5.1 6 0.5 4.9 6 0.3 10.2 6 1.4 4.7 6 0.2 18.0 6 2.6 3.4 6 0.5 11.3 6 2.7 12.2 6 2.3 10.9 6 1.4 13.6 6 1.8 11.0 6 1.3 11.8 6 1.7 5.2 6 1.0 11.7 6 1.3 5.9 6 0.5 18.9 6 3.6 16.0 6 3.5 4.5 6 0.9 30.6 6 1.8 22.9 6 0.8 10.0 6 1.2 55.9 6 17.1 17.4 6 3.2

1.32E-08 1.59E-08 1.31E-08 1.25E-08 1.21E-08 1.16E-08 1.36E-08 5.84E-09 7.39E-09 8.13E-09 8.12E-09 1.01E-08 7.65E-09 8.98E-09 1.06E-08 1.04E-08 6.58E-09 4.92E-09 9.49E-09 9.61E-09 1.08E-08 8.31E-09 3.75E-09 7.33E-09

1.44E-08 1.71E-08 1.41E-08 1.43E-08 1.35E-08 1.30E-08 1.29E-08 6.38E-09 6.12E-09 7.91E-09 8.50E-09 1.00E-08 8.67E-09 9.80E-09 1.10E-08 1.18E-08 6.29E-09 5.30E-09 1.12E-08 9.48E-09 1.04E-08 9.32E-09 2.75E-09 7.36E-09

9.22E-07

0.188 6 0.001

2.3 6 1.1

9.71E-09

9.46E-09

Distribution of solar gases in CM chondrites

Lithology

Fused masses are given in grams. Noble gas concentrations are given in cc STP/g and uncertainty of concentrations are approximately 25% after correction by degassing efficiencies.

273