Presolar SiC in chondrites: How variable and how many sources?

Vol.

0016-7037/93/[email protected]

Gemhimica o Cosmochimica Ado 51. pp. 2869-2888 Copyright 0 1993 Pergamon Press Ltd. Printed in U.S.A.

+ .OO

Presolar Sic in chondrites: How variable and how many sources? C. M.

O’D ALEXANDER

McDonnell Center for theSpace Sciences, Washington University, St Louis, MO 63 130, USA

(Received JuIy 13, 1992; accepted in revised form January 11,1993)

Abstract-The carbon and silicon isotopic compositions of 246 isotopically anomalous Sic grains found in relatively low concentration acid residues prepared from nine chondrites are reported. However, two of these residues, from Leoville (CV3) and Qingzhen (EH3), only produced one anomalous SIC grain each. The other meteorites studied included Murchison and six UOCs. The results for Murchison are broadly similar to those found by previous Murchison Sic studies. However, no two Murchison studies are identical, implying that despite the number of grains analyzed to date, we still do not have a representative sample of Murchison Sic. Statistical comparisons between the Sic in Murchison and other meteorites are also hampered by the small sample sizes relative to the range of isotopic compositions observed. Nevertheless, of the six UOCs studied only the results obtained from Inman are clearly different from Murchison. Several prebious studies have concluded that most of the Sic formed in AGB stars, although the silicon isotopes remain problematic. However, few, if any, C-rich AGB stars have ‘%/“C ratios less than 20. Those Sic grains with the isotopically heaviest carbon ( “C/ “C c 20) may have come from s-process poor J- and R-type carbon stars. Four small groups of grains, which appear to require a minimum of four distinct Sic sources, have been identified in a compilation of 308 individual Sic analyses but the overall lack of groupings in the compilation suggest that either there were a large number of stellar sources or that there were a few sources whose isotopic compositions varied considerably. Simple calculations suggest that one could expect 10 to 100 AGB stars to have contributed Sic to a protosolar system embedded in a mature molecular cloud. Even a few 10s of sources are enough to explain the apparent differences seen in the various studies of Murchison Sic and, with the exception of Inman, the differences observed from meteorite to meteorite. This number of sources would also appear to favor galactic chemical evolution as the explanation of the slope 1.3 array formed by SIC in silicon three isotope plots rather than explanations which appeal to a single star or rare high mass AGB stars.

release profiles suggest there may be significant differences in both the abundance and carbon isotopic composition of SIC from meteorite to meteorite (ASH et al., 1990; ALEXANDER et al., 1990a). The three anomalous noble gas components (Ne-E( H), s-Xe, and s-Kr) carried by Sic also vary in their absolute and relative abundance from meteorite to meteorite ( HUSS, 1990; OTT et al., 199 1). There is some evidence that these variations are, at least in part, a function of grain size (LEWIS et al., 1990; OTT et al., 199 1). The initial ion probe investigations of carbon and silicon isotopes in Sic (ZINNER et al., 1989; VIRAG et al., 1992) suggested that the compositions ofthese elements are also grain size dependent. For instance, the carbon in the finer grained Sic seemed, on average, to be isotopically heavier than in the coarser grains. However, the recent work of AMARI et al. ( 199 1) suggests the differences observed between grain size fractions may simply be a statistical consequence of the relatively small number of coarser grains that have been analyzed to date. Another notable result is that in Allende (CV3), despite the high abundance of interstellar diamond, there is scant evidence for the noble gas components associated with SIC (FRICK et al., 1983; HUSS, 1990), nor is there evidence for much Sic in the carbon isotope systematics measured in stepped combustion studies (ASH et al., 1990). Neither Sic nor interstellar diamond appears to be present in any ordinary chondrites with metamorphic grades much above type 3.6 ( ALAERTS et al., 1979; MONIOT, 1980; SCHELHAAS et al.,

INTRODUCTION THE

ISOTOPICCOMPOSITIONof a number of elements, including carbon, silicon, and the noble gases, measured in individual Sic grains and bulk Sic samples have demonstrated that most of the Sic isolated from primitive meteorites is presolar in origin ( SWART et al., 1983; TANG and ANDERS, 1988; ZINNER et.al., 1989; LEWIS et al., 1990; OTT and BECEMANN, 1990;AMARI et al., 199 1; STONE et al., 199 1; NICHOLS,JR ., et al., 1992; VIRAG et al., 1992). To date, studies of single Sic grains have concentrated on material separated from the Murchison (CM2) meteorite, although Murray (CM2) and Orgueil (CIl ) have also been used (ZINNER et al., 1989; STONE et al., 199 1). However, noble gas and carbon isotope stepped combustion experiments indicate, albeit indirectly, that Sic is present not only in other carbonaceous chondrites ( Huss, 1990; VERCHOVSKY et al., 199 1; RUSSELL et al., 199 la), but also in several unequilibrated ordinary chondrites (S~HELHAAS et al., 1990; ALEXANDER et al., ( 1990a); Huss, 1990; RUSSELL et al., 1991a) and enstatite chondrites (Huss, 1990; RUSSELLet al., 199 la). The presence of piesolar Sic in the latter two meteorite groups is surprising given that the vast majority of their constituents were so clearly formed by high-temperature nebular processes which would have destroyed any pre-existing material. The release of isotopically heavy carbon at high temperature during stepped combustion experiments indicates the presence of Sic in many chondrites, and variations in these

1990; ALEXANDER et al., 1990a; Huss 2869

1990).

On the other

2870

C. M. O’D Alexander

hand, Sic and diamond persist in metamorphosed enstatite chondrites such as Indarch (EH3/4) (STONE et al., 1991; HUSS, 1990; OTT et al., 1991). The absence of Sic in the more metamorphosed members of the CV carbonaceous chondrites and ordinary chondrites has led to the suggestion that it was destroyed by oxidation during metamorphism (ALEXANDER et al., 1990a; Huss, 1990; RUSSELL et al., 1992). A similar process may have destroyed the organic matter and interstellar diamond in ordinary chondrites with petrologic types greater than 3.6 (ALEXANDERet al., 1990a; Huss, 1990; RUSSELLet al., 1992). ALEXANDERet al. ( 1990a) suggested that differing reaction rates during oxidation could fractionate the Sic from the interstellar diamond. Rather surprisingly, the data of HUSS ( 1990) suggest that the Sic oxidizes more quickly than the diamond and this may be the reason for the absence of Sic in Allende. However, other mechanisms are capable of producing the observed variations in the absolute and relative abundances of Sic and other interste!lar matter, the most obvious being grain size sorting in the nebula. Here the results of a survey of seven chondrites, Murchison and six unequilibrated ordinary chondrites (U@Cs), are used in an attempt to determine whether there are systematic differences from meteorite to meteorite in the isotopic composition of meteoritic Sic. In conducting this survey relatively low concentration acid residues have been used to both reduce the initial sample sizes required and to speed sample preparation. The concentration factors (one over the fraction of material remaining) of the acid residues used here range from 100 to 10780. This compares with the concentration factor of the order of 200000 necessary to produce a pure Sic residue from Murchison. During the course of this work 246 isotopically anomalous SiC grains were analyzed and the results are compared to the distribution of SIC isotopic compositions expected from asymptotic giant branch ( AGB) stars, the currently favored sources of most meteoritic Sic.

TECHNIQUES To reduce the likelihood of contamination, the meteorite samples used to prepare the acid residues were, where possible, interior chips and free of sawn surfaces. Inman and Leoville were the only excep-

tions. In the case of Inman, sawn surfaces were removed by two half hour treatments in HF-HCI. Each acid treatment was followed by ultrasonic cleaning to remove loose material adhering to the surfaces. The sawn surfaces of the Leoville sample were removed by polishing with coarse corundum abrasives. Again the polishing was interspersed with several episodes of ultrasonic cleaning to remove loose material from the surfaces. The concentration of SIC and other presolar material by chemical etching was largely developed by E. Anders and co-workers at the University of Chicago (e.g., AMARIet al., 1993) and the procedures used for this study follow or were influenced by their work. The results of the acid treatments used to prepare samples for this work are summarized in Table I. The acid treatments used to prepare the

ordinary chondrite residues analyzed in this study have already been described by ALEXANDER et al. ( 199Oa)and NICHOU, JR., et al. ( I99 I ). The Krymka residue was prepared in a similar way to the other ordinary chondrites ( LEVSKY et al., 1989) and kindly provided by K. L. Levsky and U. Ott. The Murchison, Leoville, and Qingzhen residues were prepared by a series of acid treatments with various combinations of concentrated HF, HN09, and HCI in sealed containers. After every step the acids were either evaporated off or decanted after centrifugation. A last step in the preparation of the Qingzhen residue was a treatment with HBr. The final Qingzhen was very small and no attempt was made to weigh the sample. In preparation for ion probe analysis, small dry aliquots of each residue were, with the exception of Qingzhen, pressed into high-purity gold foil, mounted on an SEM stub. alone with standard svnthetic sic as described by ZINNERet al. ( 1689). The Qingzhen sakple was deposited on the gold as a suspension in water. To reduce the carbon background the samples were both combusted in air overnight at approximately 500°C and ashed in an oxygen plasma for up to one hour. In most cases the samples were not examined in the SEM prior to ion probe analysis, as in other Sic studies ( ZINNERet al., 1989; STONEet al., 1991; VIRAGet al., 1992), because cracking of the diffusion pump oil in the electron beam increases the carbon background and because it proved more straightforward to locate the Sic grains in the ion probe directly. Thus, no morphological data arc available for any of the grains analyzed. The silicon and carbon isotopes of each SIC grain were measured in the ion probe using a Cs+ primary beam and negative secondary ions. The grains were located in the ion probe as hot spots in the ‘*C and *‘Si ion images. As already mentioned, the purpose of this study was to survey a large number of meteorites, hence the use of relatively low concentration residues. Despite the acid treatments, the combustion and ashing, and the predominance of oxide grains in these residues, all had significant carbon and silicon backgrounds. In general, the SIC grains were smaller than the ion beam and, as a result, these backgrounds will have contributed to the SiC analyses. The background is presumably from fine-grained Sic, material within the oxide grains and material left over from the acid treatment that is adhering to grain surfaces.

Table 1. The initial bulk sample weights, concentration factors (Cf) of the acid residues used and concenaated acid treatments used to prepart the xsidues. The concentration factor is one over the fraction of material remaining. Sample Murchison (CM2) Leoville (CV3)

Weight (g)

Cf

0.4969 0.7132

1400 1679

Semarkona (LL3.0)1 0.7001 Bishunpur (uLL3.1)’ 1.1911 Krymka (LL3.l$2 Chainpur f&3.4)1 2.0838 Inman (L/LL3.4)‘.3 40.7047 Tieschitz (L/H 3.6$3,* 4.4500 Qingzhen (EH3)

0.9584

101 193 3850 257 10780 8380 5486

Acid Treatment HF-HN03, HCI HF-HN03, HCI

HP-HCl HF-HCI HP-HCI, HC104, H3P04 HP-HCl HP-HCl, Cr2O7-, HC104 HP-HCI, Cr2O7-.HC104 HP-HN03, HCI, HBr

1 Previously analyzed by ALEXANDER et al. (1990); 2 Pwio~~ly analyzed by LEVSKYet al. (1989); 3 Previously analyzed by NICHOLS Jr. et al. (1991). * The Tieschitz sample was a fine grained separate.

Presolar SiC in chondrites Whatever the source, the backgrounds for both aubon and silicon were quite variable both within and between residues. The mean and standard deviation of the background carbon and silicon count rates and isotopic compositions were determined for each residue. For given count rates of carbon and silicon per unit area of beam occupied by either SIC or background, an estimate of the fractional contribution of the background to any analysis can be made using a simple mixing relationship: GiorC

=

X*CsicC(l

-X)-Cl&,

where Csi arc is the observed count rate for either silicon or carbon, C,c is the count rate for an infinite Sic grain, C,, is the count rate for an infinite area of background, and X is the fractional area of the beam occupied by the Sic grain being analyzed. The total range of X for the grains used in this study was from 2.5 X 10e3 to 0.71 with an average of about 0.07. The carbon and silicon count rates from the background were determined as previously described and the count rates for an infinite Sic were obtained from the Sic standards. With the conditions used in this work typical values of C& and ChL are250,000 cts/s and 500-2000 cts/s, respectively.Thus, on average the background contribution was of the order of 5-1096. The uncertainty in the background count rates and isotopic compositions results in a significant, and often dominant, contribution from the background subtraction to the overall error for each grain. Although the size of the grains was not measured directly, their relative sizes could be estimated from the values of X for each grain. Normalizing all meteorites to Murchison, the mean radii of analyzed grains were: Murchison, 1.0; Bishunpur, 0.58; Chainpur, 0.46; Tieschitz, 0.85; Inman, 0.82; Krymka, 0.82 and; Semarkona 0.84. Too few:grains were found in Qingzhen and Leoville for a meaningful mean. The mean and standard deviation of the instrumental mass fractionation for carbon and silicon were determined on Sic standards. In silicon three-isotope diagrams, the major axis of the instrumental mass fractionation error ellipse is directed along the terrestrial fmctionation line which has a slope of 0.5. On the other hand, the major axes of the error elipses due to the counting statistics and background subtraction are parallel to the plot axes. Combining these three sources of error produces an error ellipse for individual analyses that will be inclined at a range of angles to the plot axes depending on which sources of error dominate (VIRAG et al., 1992). These errors are shown in all silicon isotope figures illustrating data from this work. However, in the tables the quoted errors are projections of the error ellipses onto the axes. To ensure that only the best analyses are used only those grains with B/C ratios, normalized to that of the standard SiCs, in the range 0.75-1.5 were accepted. Some variation in the Si/C ratio is inevitable as a result of counting statistics and the influence of geometry on ion production. However, contamination and innate variations in the SIC may also be a factor. The lower limit of the allowed Si/C range was chosen on the basis of the lowest observed ratio (0.72) from single grains by VIRAGet al. ( 1992). The upper Si/C limit of 1.5 is higher than observed by VIRAC et al. (1992) but was chosen because the smaller grains may have been oxidized at their surfaces by the combustion and ashing of the residues (ZINNERet al., 1989). A second constraint applied to the analyses was to only use those analyses with at least one isotope more than three standard deviations from the standard Sic composition. Synthetic Sic has a very limited range of isotopic compositions ( ‘v/‘3C = 92, 629Si = 0460,G”Si = 0%) and this condition should remove any terrestrial contamination. It is likely that it would also remove SiC that formed in the solar nebula. The number of isotopically normal grains rejected in this way is given in Table 3, but is not included in the calculation of the mean SIC compositions. It should be noted that this procedure may introduce some bias in that it will tend to exclude small presolar grains, which will inevitably have relatively large errors, that do not have particularly anomalous compositions. RESULTS

The Origin of the Isotopically Normal Sic This paper is principally concerned with those Sic grains that show significant isotopic anomalies. However, in almost

2871

every meteorite studied here and elsewhere some Sic grains have been found with isotopically normal silicon and carbon (e.g., VIRAG et al., 1992). In fact, the relative abundance of normal and anomalous grains is one of the most striking differences in the samples examined here (Table 3). A few Murchison Sic grains which have isotopically normal silicon and carbon are isotopically anomalous in other elements, such as nitrogen ( VIRAGet al., 1992), and, therefore, probably presolar; but for those other grains which are isotopically normal in all measured elements, a solar system origin seems likely. An obvious source of normal SIC is synthetic Sic since it is an abrasive that is commonly used for cutting and polishing. In the case of the Leoville sample, which was prepared from a sawn slab, this is a likely explanation for the normal grains but, with the exception of Inman, all other residues were prepared from interior chips free of sawn surfaces. In the latter cases a nonterrestrial but, nevertheless, solar system origin must be a possibility. For instance, Sic can precipitate from iron metal under reducing conditions but in situ searches of Semarkona and Leoville, two meteorites with a considerable fraction of normal Sic, have so far failed to reveal any Sic in their metal (ALEXANDERet al., 1992). One might expect the enstatite chondrites with their highly reduced assemblages to be the most likely samples in which to find indigenous solar system Sic. Indeed LARIMER and BARTHOLOMAY( 1979) have predicted that Sic would condense from the nebula in the enstatite chondrite forming region and STONE et al. ( 1991) reported finding isotopic-ally normal SIC in Indarch (EH3/4). Sic has also been found in an Abee (EH4) residue (RUSSELLet al., 199 lb) but has an unknown isotopic composition. However, an in situ search of Qingzhen ( EH3 ) found silicon nitride and not Sic (AL EXANDER et al., 199 1). As will be discussed in a following section, one Sic grain was found in the Qingzhen residue but this was isotopically anomalous. Thus, in the absence of any evidence which demonstrates that the normal Sic formed either in the nebula or in the meteorites themselves its origin is, at present, a mystery but is most likely to be terrestrial contamination. The Isotopically Anomalous Grains For carbon, at least, the isotopic variations are large enough to warrant the use of ratios rather than delta values and it is for this reason that ‘zC/‘3C ratios are listed in all tables and figures. The total range of ‘2C/‘3C ratios is approximately 2-320. As will become apparent, most grains have 12C/13C ratios between 20 and 100 and are more or less normally distributed. The silicon isotopes of most grains form a rather broad distribution but do not obviously correlate with the carbon isotopes. Consequently, carbon and silicon isotopes will generally be discussed and plotted separately in the following sections. In summarizing the results of each meteorite, the grains have been divided into three according to their “C/ 13Cratio (O-20,20- 100 and > 100). Table 2 lists the number of grains in each range and the mean carbon and silicon isotopic compositions for the 20- I00 range. Those grains that fall outside the 12C/ 13C= 20- 100 range have not been used to calculate the means in Table 2 because despite their relative scarcity

2872

C. M. O’D Alexander Table 2. Summary of the distribution of isotopic compositions of single Sic grains found in different meteorites. The grains from each have been divided into three according to their 1%/13C ratio. The first three columns are the number of grains that fall into the three *W/W ranges. The remaining columns are the mean isotopic compositions (with standard emu) and standard deviations of the grains in the r2c/t3C=26lOO range. The Mumhison data are the results from the relatively pun Murchison SIC residues studied by ZINNBRet al. (1989) and VIRAGet al. (1992) and bv STONEet al. (1991). These three studies have been combined to serve as a reference for compkson with the ‘resultsof this study. Lastly, all the single grain data reported here and by

ZINNERet al. (1989). STONE

et al. (1991) and

020

20-100

ml-

3 2 5

38 6 44

0

Murehison

0

Bishunuur Semadcona

6 3 2 0

ikz?

23

VIRAG et al. (1992) have been combined to

12fJ13c

(J

ggSi

o

g3Qsi

o

Murchison

zinnm+virag Stone et al.

Combined

1

56.8kI2.3 58.2rts.a 57.M2.1

14.0 14.1 13.9

49.5f5.0 39.3f9.0 47.9zt4.5

3 1.0 23.9 30.0

50.6f3.7 52.1f5.7 50.ak3.2

22.8 15.2 21.6

30 30 30 22 6

0 3 1 2 4

56.0k7.9 57.4f2.8 51.8f2.6 58.6k3.5 55.*10

10.0 15.2 14.1 16.5 24.7

62.2t7.0 46.7f7.7 63.28.8 47.9f9.1 49.M 1.4

38.2 42.9 47.9 42.1 28.0

62:6f5.7 47.5f5.6 72.lm.a 4a.ifa.o 35.af7.4

31.0 30.9 37.1 37.5 la.1

71 27

1

67.5f3.8 53.akl.7

14.0 19.6

24.af4.4 25.7fa.5

44.0 36.9

2l.lf7.1 36.41t4.4

36.9 36.7

13.4

44.2f2.6

41.9

4a.af2.2

34.8

1

This study

Chain& Ties&z

All combined’ 23 2fQ 14 55.0ko.a * Does not include nine grains from Krymka. See text.

their extreme compositions tend to inordinately influence the means, especially for small sample sizes. Because of the large range of isotopic compositions, most figures do not display all grains; but those grains which fall outside the limits are relatively rare. As has been noted in several previous studies (ALEXANDER et al., 1990; STONE et al., 1990; AMARI et al., 1991; STONE et al., 1991; VIRAG et al., 1992), the silicon isotopes of most Murchison grains form an array that is significantly steeper than the terrestrial fractionation line, the best fit to which gives a slope of about 1.3 or 1.4. The terrestrial fractionation line and a slope 1.3 line are displayed in all silicon isotope plots. The shaded regions in silicon isotope plots (Figs. 1 and 3) were drawn around two clusters in the Murchison data obtained in the course of this study and are simply intended to serve as reference points in the forthcoming graphical comparisons.

Comparisons

between Previous Murchison Studies

The Sic in Murchison is by far the most comprehensively studied Sic from any meteorite and is the obvious standard to use both for checking that the analysis of Sic grains in impure residues is possible and as a reference to which all other meteorites can be compared. However, for Murchison to be a useful standard it is important that the single grain analyses reported to date are similar to one another and, therefore, form a representative sample. The largest study of single Murchison Sic grains by AMARI et al. ( 199 1) (residue KJG, 1S-3.0 pm across) does not list individual analyses but reports the mean carbon and silicon isotopic compositions for 75 grains (without the very anomalous grain X) as being ‘2C/‘3C = 35.3 + 4.0, az9Si = 50.7 -t 5.7%0 and G”Si = 49.6 + 4.0%0 (the errors are the standard errors of the means). These results compare favorably with the combined means of forty-one isotopically anomalous

separates HO ( I- 12 pm; ZINNER et al., 1989) and LS and LU ( 1.5-25 pm; VIRAG et al., 1992) which are ‘2C/‘3C = 31.4 +- 8.8, 629Si = 48.8 f 5.1%0 and S3’Si = 48.7 ? 3.7%0. S~o~~etal. (1991) measured 9 Sic grains (2-6 pm) from a Murchison residue which have an average composition (minus their smallest grain) of ‘2C/‘3C = 39.2 * 10.7, d29Si = 43.8 + 8.56 and S%i = 54.6 f 5.2%0. As well as having similar mean compositions the data of ZINNER et al. ( 1989) STONE et al. ( 199 1 ), and VIRAG et al. ( 1992) exhibit comparable ranges ofcarbon and silicon isotopes (Figs. 1 and 2). However, the similarity in the means of these three sets of data may be somewhat illusory. The VIRAG et al. ( 1992) data are dominated by two groups of anomalous grains which appear unique to that study. The largest of the two groups (their group II) creates the high frequency of grains with ‘*Cl 13C ratios between 45 and 50 (613C = 980 - 780) in Fig. 2 and the cluster of grains with silicon isotopic compositions at about 6 29Si = 6 3osi x 40%0 in Fig. 1. The second anomalous A. Virag et al. group (their group III) has a composition of about b29Si = 90% 630Si = 80%0, and 12C/13C = 52 ’ (613C = 700%0)). Also, because the means were calculated using delta values, there is a tendency for the means and their errors to be disproportionately influenced by a few very anomalous grains. This is particularly true for carbon where a few grains have 6 13Cvalues of 1OOOOb to 30000%0 ( ‘zC/‘3C x 8-3). However, the means of only those grains with “C/ 13C = 20- 100 are still fairly similar (Table 2). Even so, the fact that no grains belonging to the groups identified by VIRAG et al. ( 1992) are present in the other Murchison studies suggests we do not yet have a completely representative sample of Murchison Sic, even for the most common grains ( 12C/ 13C = 20-100). Nevertheless, in the absence of any alternative the ZINNER et al. ( 1989), STONE et al. ( 199 1) , and VIRAG et al. (1992) data will be used as a reference, and as such

grains from three other Murchison

Presolar SIC in chondrites

2873

6 3oSi (%o) FIG. 1. The silicon isotopic compositions of 5 I single, isotopically anomalous Murchison Sic grains reported in three previous studies. The grain compositions in these three studies cover a similar range with most grains falling about a slope 1.3 line (SIC fit). However, the WRAG et al. ( 1992) grains exhibit two marked clusters that are not seen in the other studies. Also included are the terrestrial fractionation line and two shaded areas that will serve as reference markers in subsequent silicon three-isotope plots.

have been combined in Table 2. They will, in the following sections, be referred to collectively as the pure residue grains. Comparison Between Present lbrchison Previous Studies

Results and

The mean carbon isotopic composition of Murchison Sic grains measured in this study (Table 2) is quite similar to that of the pure residue data (Table 2), as is the range of compositions (Fig. 2). The silicon isotope means are slightly different but still within two standard errors of one another. The differences in the means probably reflect the very broad distribution of silicon isotopes in presolar Sic. Certainly, the grains appear to be distributed along the slope 1.3 line (Fig. 5a) in a similar fashion to the pure residue grains. However, there is no evidence of grains belonging to the VIRAG et al. ( 1992) groups. The only striking difference between the pure residue grains and the grains in this work is the absence of grains lying outside the ‘2C/13C = 20-100 range (Table 2). Of the 50 anomalous SIC grains for which carbon isotopes are reported by ZINNER et al. ( 1989))STONE et al. ( 199 I ), and VIRAG et al. ( 1992) only five have a 12C/13C < 20 (Table 2 and Fig. 2). An almost identical frequency has been found in the KJG residue (S. Amari, pers. commun.). As will be seen in the next section, these isotopically very heavy grains have been found, with varying frequency, in most of the other meteorite residues studied here. Therefore, it seems unlikely that the differences between this study and the pure residues are a consequence of the analytical technique. Given what are still relatively small samples it seems likely, at this stage, that the absence of grains with ‘2C/‘3C < 20 in the results of this

study is simply a consequence of statistical fluctuations and not heterogeneity within Murchison. From the previous comparison of the Murchison Sic studies, it is clear that there are differences, but given the

-I 0

10

20

30

40

50

60

70

80

90

100

l2 CPJC FIG. 2. A comparison of the range of Murchison Sic carbon isotope ratios measured in three previous studies and in this work. The bulk of the grains in both sets of data falls in the range of 40 to 70, but the grains with ratios less than 20 are absent from this study’s data. The large number of grains between 45 and 50 in the VIRAGet al. ( 1992) data is due a group of grains that appears to be unique to that study.

C. M. O’D Alexander

2874

50

6 3oSi (%o)

6 Y3i (%+)

-so _I

-50

, .‘.

.

I

.

0

6 3oSi (%+)

1

.

.

.

,

.

.

SO 6 3oSi (%o)

.

.

I

.

loo

.

.

I

I 150

1Fig. 3e Semarkona ]

100

-I-

-50 6

30

Si (%o)

0

50 6 30Si

100

150

(%o)

FtG. 3. The silicon isotopic compositions of Sic grains measured in this work from: (a) Murchison (CM2); (b) Bishunpur (L/LL3.1); (c) Chainpur (L3.4); (d) Tieschitz (H/L3.6); (e) Semarkona (LL3.0): (f) Krymka (LL3.0); and (g) Inman (L3.4). The stippled regions in each plot are the compositions of the backgrounds measured in each residue. Also included in all figures are the terrestrial fractionation line, a slope I .3 line and two shaded areas that serve as reference markers. These figures do not show the entire range of silicon isotopic compositions observed.

very wide range of isotopic compositions exhibited by the Sic and the relatively small number of grains that has been analyzed to date, statistical variations seems the most likely

explanation for the differences. The impure nature of the residue used here does not appear to be a factor, except for the larger analytical errors.

2815

Presolar Sic in chondrites

6 3oSi (%0)

FIG. 3. (Continued)

(Fig. 3g). In Inman, the silicon isotopes cluster almost exclusively about the larger and isotopically lighter of the two shaded regions (Fig. 3g). This difference is reflected in the mean silicon isotopic compositions (Table 2) which are significantly lighter, isotopically, than the Murchison results obtained in this work and the previous studies. It is possible that the strong clustering demonstrated by the silicon isotopes of Inman SIC is an artifact resulting from an underestimate of the background contribution. Yet, the larger grains, which will have a proportionately lower background contribution, have similar compositional ranges to the smaller ones. In most of the other meteorites studied grains with compositions up to 6 29Si = 120% are fairly common (Table 2, Fig. 3g) but in Inman 629Si = 80% is about the isotopically heaviest value (Table 3g and Fig. 3g). Since the background in Inman has a composition of about 629Si = O%O,to produce a shift of grain compositions from 629Si = 120% to 629Si = 80% would require that there remains

Comparison of Murchison with the UOCs With the exception of Krymka and Inman, which will be discussed separately, the mean silicon and carbon isotopic compositions for the SIC grains in the unequilibrated ordinary chondrites (UOCs) are similar to the Murchison data of this work and the pure residue grains (Table 2). The same is true for the distribution of their silicon and carbon isotopes (Figs. 3 and 4). For silicon all UOCs exhibit rather more scatter than the pure residue grains, primarily due to the errors associated with the blank correction, but still fall around the slope 1.3 line. The scatter is particularly pronounced in Chainpur because the Sic grams are significantly smaller in this meteorite than in the others. The bulk of the carbon isotopes in Sic from the UOCs is distributed over a similar range to the Murchison data of this work (Fig. 4). There is, however, one striking difference and that is the much higher frequency of grains with extreme carbon isotopic compositions, particularly in Bishunpur and Semarkona. As in the comparison of the various Murchison studies, statistics are probably the root cause of these differences and not intrinsic differences between the meteorites. The Krymka mean isotopic compositions are strongly influenced by nine grains (Kl-3, K15, K22-25 and K28; see Table 3) that only just passed the three-sigma cut used to remove isotopically normal grains. The recalculated means without these grains ( ‘zC/‘3C = 56.0 + 3.1, 629Si = 40.1 + 11.O, 6%i = 3 1.9 + 9.0) are quite consistent with the other meteorites. Why the removal of what are probably normal grains was less successful in Krymka than in Semarkona, where 54 out of 68 grains were removed, is not clear, but it seems that, apart from these nine grains, the Sic in Krymka is very similar to that in the other meteorites. In Inman Sic the mean (Table 2) and distribution (Fig. 4) of carbon isotopes in the 12C/13C = 20-100 range are comparable to those in other meteorites, but despite the relatively large number of grains analyzed there appears to be a deficit of the isotopically extreme grains (Table 2). However, it is the silicon isotopes of the Inman grains that contrast most obviously with the results from the other meteorites

f 0

10

20

30

40

50

60

70

80

90

l2 CPC FIG. 4. The carbon isotope ratios of Sic from seven meteorites studied in this work. There are a few grains with ratios greater than 100 that are not shown. In general all the SIC in each meteorite exhibit similar distributions, with most grains having “fZ/“C ratios between 40 and 80. There is some variation in the abundances of rare grains with ‘*C/ 13Cratios less than 20, but this is probably due to the number of grains analyzed from each meteorite being statistically small.

2876

NOS

Q/*3c

@?Si %)

doSi(960)

SE

1~0s

SK

‘2Cll3c

@Si

53.7f 1.4 62.7f 1.6 48.lf 2.7 56.Szt1.8 54.3* 2.0 5o.w 1.3 74.W 0.6 56.4f 2.1 64.3f 2.4 6lAf 2.1 46.lf 0.3 59&t 6.3 46.3i 1.3 62.7f 1.8 65.2f 2.7

993f 6.8 6l.Sf 5.6 23.6f 10.4 8O.kt 8.0 114.M 10.5 70&t 6.8 18.6f 2.9 64.e 11.9 31&t 7.9 49.4f 11.9 39.Szt24 92.3* 21.0 -29hf 6.3 33Xt 5.2 SO.4+ 13.8

853f 6.1 50&k 5.1 S2.4f 10.0 7lM 7.3 118.lf 9.5 71.7f 6.4 26.2f 3.9 63.2+ 11.2 37&t 7.5 64.lf 11.3 56.8i 3.5 93&t 15.2 2.4f 6.5 30.9f 5.6 40.3f 13.1

1.01 090 1.14 1.07 0.95 0.96 125 0.85 1.03 0.92 1.13 0.78 0.91 094 0.85

60.9f 1.7 73.61 1.3 64.5f 1.3 46.2f 0.8 S4.lf 0.7 5.OLt0.1 12.4i 1.6 32.2 0.4 6S.lf 0.6 63X 1.1 15.2f 0.3 49.3f 1.4 45.k 0.7 45.M 0.7 48.8f 0.9 4.3f 0.1 56.lf 2.0 47.W 0.6 126.7f 7.0

37.M 17.9 s9.6f 9.8 70.9f 9.7 -51.9& 10.5 53.a 6.6 128.OLt 15.1 60.9f 10.3 37.4* 7.0 43&t 4.4 -28.W 14.6 -26.H 12.3 1015f 15.7 66.6f 7.6 51.5f 9.2 97.4f 10.6 93.Sf 6.0 -2&t 18.9 -17.3f 7.3 -59.5f 22.6

82.7f 18.4 44.7f 10.2 63.2f 9.5 -0.4f 10.9 SSLJtt7.2 1Ol.W 14.6 46.3f 10.5 193f 7.6 35% 5.1 -7.7* 15.0 -2.5f 12.7 67.e 155 61.9f 8.0 54&t 9.6 99% 10.4 62.lf 6.4 19.M 19.7 6.8f 7.8 4.oi 22.9

0.86

46.7f 0.5 77.e 1.7 57.3f 0.7 48.e 0.5 47.3* 0.3 42.H 1.8 S8.W 1.6

144.4* 7.3 64.4f 14.0 77.w 9.3 119.of 8.0 48&t 3.0 5.a 30.2 104.7f 16.4

142.8f 7.0 73.w 13.5 825f 9.0 los.cn 7.7 6l.lf 2.9 4l.lf 29.8 131&k 16.3

1.04 1.00 0.99 0.97 1.Ol 1.09 0.99

(%o)

dosi@W)

MlUehlson

M3 M4 MS M6 M7 Z!

2.5.!% 0.1 56.lf 1.1 49.M 1.3 48.X 4.2 53.e 1.6 5l.lf 2.5 7o.e 0.7 52.9f 0.6 60&z 2.5

Ml0 Ml1 Ml2 Ml3 Ml4 Ml5

42.&t 4.9 SO.* 0.8 68.9k 1.5 57.w 2.1 52.01t1.2 75&k 1.3

-1lM 2.1 69.9k 7.1 27.W 12.6 124.2t32.0 1lO.M 8.9 106.8f 14.1 48.sH 4.6 117&t 11.6 %.2A 3.9 1Omt 29.0 64.a 2.9 53.01:5.4 36.2f 12.6 79.8f 15.2 37.e 4.4

11.2t 3.4 58.&t 6.7 43.e 12.2 142.lf 29.4 52X 8.0 99.u 12.3 64.3* 5.1 102.9f 10.0 98.&t 4.5 95.&t 21.9 56.lf 3.8 49.2f 5.7 45.85.12.2 61&t 14.6 33,s 4.9

1.13 1.02 1.16 0.77 1.16 0.93 0.90 1.00 1.01 0.76 1.09 1.29 1.15 0.94 0.91

Ml6 Ml7 Ml8 Ml9 M20 M21 M22 M23 M2.4 M2.5 M26 M27 M2E Mz9 M30

Bishunpur Bl B2 B3 B4 B5 B6 B7 E BlO Bll B12 B13 B14 B15 B16 B17 Bl8 Bl9 BU)

8.5f 0.7 87.2t 1.0 91&z 4.4 67.of 0.6 64.M 4.5 59&t 2.9 9.9.k1.1 167.e 65.e 15.3 1.4 llS.Ozt7.9 57.of 2.8 44.4* 1.2 7.7* 0.2 26&t 3.2 27&t 0.5 S4.3f 4.0 77.e 2.2 55.* 3.4 fA.ti 0.7 52.6zt1.3

37.3f 9.2 8.2 3.8 47.4f 13.1 37.W 2.7 84.4zt21.5 60.5zt12.7 5.lf 11.7 -2533f 16.9 -6.a 6.3 13.3f 14.6 124&t 16.2 82.M 4.9 211.8f 4.2 78.M 13.0 63.e 5.0 7.w 15.7 13&k 11.9 113.6f 16.0 4l.W 5.3 64.W 12.9

2S.3* 8.7 36.9i 3.6 23.Oi 12.3 16.Si 3.0 100.3f 21.3 47.7f 12.1 24.e 11.3 -398.lf 19.8 14&t 6.1 12.w 13.8 100.4f 15.3 84.lk 3.1 155.6ct 3.0 48.w 11.9 72.2f 3.6 38.21t14.9 19.Qt 11.5 97.w 15.0 34&t 6.0 42.W 13.1

0.91 0.87 1.11 0.90 0.84 0.94 1.06 0.78 0.90 0.99 0.87 0.94 1.03 0.82 0.94 0.86 0.81 0.79 0.94 0.96

B21 B22 B23 B2A B25 B26 B27 B28 B29 B30 B31 B32 B33 B34 B3S B36 B37 B38 B39

::: 0.86 0.91 1.00 0.91 0.88 0.93 1.07 1.13 1.00 1.01 0.96 0.90 1.16 1.08 0.94 0.81

Chainpur Cl c2 c3 c4 c5 C6 c7

76.4zt1.7 46.w 1.5 65&t 0.5 51.* 0.3 74.4* 0.7 51.e 1.3 47.&k0.9

-92 12.9 139.4*29.2 6.7f 4.0 3X!& 3.6 44.Sf 4.1 96&t 21.6 63.lf 14.6

14.2f 12.6 123.O.k 28.3 18.4f 4.3 51.9k 4.0 s3.Qt 4.4 125&z 21.4 105.u 14.4

1.13 0.85 1.00 1.01 1.08 0.89 1.02

about a 35% contribution to the silicon from the background. This would significantly increase the mean Si/C ratios of the grains, which is I .08, unless the carbon background has been underestimated to the same degree. As already mentioned, the carbon isotopes of Inman grains have a similar range to the other meteorites in Fig. 4. However, the composition of the background is 12C/“C = 59, which is fairly close to the mode of grains in Fig. 4, and a 35% contribution of background carbon would only slightly alter the compositional range of Inman grains (a ‘*C/“C range of 40-80 would alter to 45-71). Even if the carbon background in Inman has been underestimated to this degree the very heavy carbon grains should still be evident, albeit with less extreme isotopic compositions.

C8 C9 Cl0 Cl2 Cl3 Cl4 Cl5

After all, a grain with a 12C/“C = 8 would still have an apparent composition of ‘2C/‘3C < 20 even if 60% of the carbon analyzed had a composition of ‘*ClL3C = 89, the terrestrial ratio. Thus, the deficit in grains, relative to the pure Murchison residues, with compositions greater than ‘2c/ “C < 20 is real whether or not the background in Inman has been underestimated. An underestimation of the background to the degree necessary to create the rather different distribution of Silicon isotopes in lnman seems unlikely but cannot be ruled out. In summary, comparisons of the distribution of isotopic compositions of single SIC grains both from several different studies of the same meteorite, Murchison, and between this study’s results for several different meteorites suggest they all

PresolarSic in chondrites

2811

Table3 an&wed. Nor

12c/‘3C

@ji@)

Cl6 Cl7 Cl8 Cl9 C2O

50.&f 57.9 46.e 65.B 45s 38.& 35.2f 3M 22&k

%.lf 9.7f 99.9& 53& 45.7* -3l.lf 61&t 7.4* 27.W

z E

1.1 3.3 2.7 2.1 l-4 1.1 0.8 0.0 09

#hi+) 14.0 32.7 33.9 18.6 20.0 14.6 10.8 10.6 323

94.2t 13.6 56.e 32.2 83.W 32.6 39.3f 18.2 58.W 19.7 26.9Lt 14.7 loo.‘lf 10.7 22.3* 10.4 2l.W 31.9

12c/!3c

i329S@I)

Six:

No0

1.12 1.23 1.44 1.11 1.02 1.11 1.14 1.23 0.82

c25 C26 C27 C29 C30 C31 C32 c33 C34

26&t 70.lf 35f 106Af 43.5k 35.4* 59.4* 435f 73&t

0.95 l.O8 1.41 0.87 1.21 1.32 1.09 1.19 1.11 1.14 1.M

T14 Tl5 T16 T17 Tl8 Tl9 T2o T21 T22 T23 T24

0.91 1.35

GE

63.7* 2.7 63.5k 35 53.M 2.4 74s 3.8 28&t 2.8 56.B 5.4 44.w 1.4 91.3f 5.5 163&t 21.9 61.9zt 4.9 74&t 3.4 120.&t 8.2 7.lf 0.7

0.7 2.1 0.1 1.5 0.7 0.3 0.8 0.7 1.9

53.w -7&f 104Jf 85f lo4.B 1072 64&t 104.2k 19&t

11.2 15.9 16.0 5.1 12.0 6.4 8.5 12.0 14.8

@Si

(%n)

78.7f 13Ai 90.4* 32.B 71.4* 9O.lf 48.7* 7lAf 48&t

si

11.3 15.8 15.5 5.3 115 6.2 8.3 11.5 15.1

1.19 090 1.03 0.95 105 0.92 0.94 1.05 l.oo

57&t 6.1 70.lf 6.6 127.4f 8.9 49.lf 5A 44.7f 12.7 MS& 11.8 45.w 4.5 21&t 5.6 149.3* 16.6 58.5.k 7.9 3.ti 5.3 22&t 6.8 27.4* 10.6

100 1.26 1.27 1.31 1.33 135 1.20 1.28 1.22 1.~ 1.2a 093 1.38

Tiesdlib

z T3 T4 T5 T6 n TS T9 TlO Tll T12 T13

71.4* 3.4 5O.Qt 2.4 34.&t 4.8 19.E 2.3 65.2 7.8 62.Qt 13.2 64.e 4.0 44.&t 7.5 92.& 19 49.W 0.6 52.skt 1.9 55.4* 4.0 34.7* 2.2

15.7f 10&t -3.lf 5.4* 143f 413f 66.lf 110&t -8.zt 81.M 88.lf 66.4f 128si

5.9 5.2 12.1 9.2 5.6 13.4 7.6 31.3 2.9 2.6 7.2 11.7 13.4

24.M 6.2 -3.w 5.5 -14.2t 12.4 16kt 11.4 n.2f 7.0 w.3f 10.0 68.&L 6.6 84.B 18.9 -18.9 3.5 73.2t 2.4 73.8 5.9 76.W 10.3 1o9.s 10.1

7l.lf 7.4 43&t 5.9 127.B 10.2 38s 4.7 43.7* 13.5 552 13.1 5o.lf 3.7 -8% 4.6 184.9zt 23.6 45.w 8.3 -24.2 4.3 -27.e 7.4 -2.23: 10.3

Semarkona Sl s2 ii S5

67&t 1oo.4* 53.w 47.s 211.Qt

3.6 4.3 3.3 1.8 16.3

39.8* -6&k 95.w 563f 305f

4.6 4.6 19.7 9.2 12.4

29.w -17m 26.e 62&t 99.9

4.8 5.1 18.2 8.9 12.3

0.98 0.94 0.88 0.95 0.90

93.2 87.kt 9o.ok 62.M 8&t 37.M 58.7f 63.2t 53.7* 40.&t 2.s 61.3f 66.W W.& 91.7* 2.w

0.9 0.8 0.4 SA 1.6 3.4 6.1 6.0 0.4 4.5 0.1 3.6 2.7 3.5 1.2 0.1

-1lM 0.3f 4.w 56.e 113.7* 27.e 28xt o.Bf -8.ti 34.4* 49&t 75s 17zkt 151.2f -6.&i 74&k

4.0 2.7 1.9 10.4 27.3 113 19.9 2o.2 2.8 11.6 2.9 8.2 5.3 27.9 6.5 3.6

-18.2t 4.B 4.4f 8.W 1122f 2O.a 23&t 31.e 22.s 26.8f 7o.4* 56.lf 2.x 93.h -28.&t 64.M

4.4 5.5 25 26.0 13.8 14.2 27.0 23.5 3.8 15.9 4.0 14.7 8.6 10.1 8.3 2.8

0.99 0.98 1.18 0.87 1.01 1.27 0.88 0.93 1.03 1.23 1.50 1.09 1.34 1.07 0.89 1.29

S6 s7 S8 s9 SlO

187.7* 23.lf 103xt 47.lf 97.M

5.0 0.6 15 0.8 5.1

-0.4* 54.M -2% 37&t 11.2

3.8 7.2 3.3 2.0 2.9

K17 Kl8 Kl9 K20 K21 K22 K23 Ku K.25 K26 K27 K28 K29 K30 K31

44.lf 5.9 43.w 5.3 69&t 2.9 319&t 90 57.7* 6.6 85.4* 19 932 15 88.m 0.9 94.lf 2.6 73.4* 2.4 78.lf 3.1 9o.w 1.9 562 0.4 6l.?f 0.6 52.5zt 0.4

25.3i 1143f -16.W -243.7* 21.B -9&t -34.7* o&t 3lM n.ti 3.7i -2.3* -10.7* 83% 92.3*

14.4 22.7 11.9 90.5 14.8 7.8 8.9 4.4 12.0 8.8 15.4 4.6 2.9 2.9 2.5

53.H 44.lf 44&t 32.3f 77.3*

16.9 16.1 14.4 7.9 11.2

59.lf 47a -2.W 373f 10&k

3.7 6.9 3.2 39 4.6

1.02 0.87 0.84 0.98 1.01

2.0.~ 131.e -ll.Qt 459.M 4% -12.&t -16.3* 3.6f 47.W -9.lf 4.e 21% 8.4f 72.3* 66xt

23.1 25.0 14.2 166 25.0 9.8 9.4 4.6 125 11.3 16.8 5.0 3.4 3.1 29

1.03 1.05 0.89 0.93 1.03 l.OO 0.80 1.11 0.78 1.17 0.99 1.03 0.93 1.03 1.09

30.lf 68.B 80.M 56.&t 78.3*

15.4 16.7 16.2 8.3 10.3

lA4 1.14 1.06 1.35 0.99

Krymka Kl K2 K3. K4 ii K7 K8 Ic9 KlO Kll K12 K13 K14 Kl5 K16

Inman I1 12 ii I5

55.8f 45.4* 42.7* 48.M 59.4i

2.5 16.4 5.0 4.9 12.5

46.7f 3.9 52&t 15.4 45.u 5.0 24.3* 6.2 O.&t 14.7

.57.4* 34.3* 52.7f 66.e 54.9+

3.8 14.0 5.3 6.9 14.4

0.86 1.42 1.50 1.34 0.81

sampled the same (or very similar ) population of Sic grains (Table 2). Differences in the results summarized in Table 2 probably reflect the small sample sizes, relative to the large range of Sic isotopic compositions. Therefore, combining all the single Sic grain data reported here and in the literature should produce a reasonable estimate of the overall Sic population (Table 2 and Figs. 5 and 6). Figure 5 indicates that most grains have carbon isotopic compositions that appear to form a fairly well-defined normal distribution. There may also be a second population of grains with ‘*C/ 13Cc 20 but these grains and those with ‘2C/‘3C > 100 are still relatively ram. On the other hand, the silicon isotopes form a single, broad distribution (Fig. 6) and do not correlate with the carbon isotopes. The broad distribution of silicon isotopes in

I6 17 18 19 110

44.5* 18.4 53.4* 17.2 41.7* 11.5 47.5Lt 8.2 48.M 5.5

Fig. 6 is probably not due to the relatively large errors in this work since the pure residue results exhibit similarly broad distributions. Qingzhen ( EH3) and Leoville (0’3) The search of Qingzhen and Leoville residues did not produce enough anomalous grains to make meaningful comparisons. In Qingzhen, only one Sic grain (629Si = 49.6 f 3.4 and 630Si = 58.0 + 3.8, the carbon isotopes were not measured and no background correction made) was found, the dominant silicon-bearing phase in the residue being silicon nitride rather than silicon carbide (ALEXANDERet al., 199 1). Although silicon nitride is a possible stellar condensate, most

2878

C. M. O’D Alexander Table 3 cmtiawd. NOS Ill 112 113 114 115 116 117 118 119 120 121 I22 I23 I24 125 126 127 128 I29 130 131 132 133 134 135 136 137 138 139 140 141 142

grains measured and carbon, precipitated

S%i (960)

52.lf 6.5 63% 2.3 53.4* 4.3 51.3* 14.1 54.W 7.1 51&t 9.4 20% 4.1 48.U 3.2 S4.& 5.3 57% 35 55.2t 6.0 69.W 4.2 56.7i 3.8 4l.lf 5.2 43.at 4.7 43% 5.8 35.2 6.4 59.4* 1.9 SO.lf 3.9 61.W 6.3 39&t 10.2 58&t 4.5 47.3* 4.0 37.e 3.1 214.&t 7.7 76.Qt 8.9 SO.6f 6.2 46.X 1.4 56.&t 5.0 53&t 3.2 91.7* 4.9 18.Qf. 1.6

18.lf 12.1 31.7* 7.0 4l.W 4.7 32&t 11.7 -52.W 13.0 86sit 23.3 22.2 7.1 -56.e 4.6 o&t a.2 -71.5* 8.1 27.2 6.2 33.M 5.8 3O.W 4.8 So.& 9.1 325f 5.9 27.W 12.6 21&t 5.9 56.5f 3.0 -9.2 6.6 21% 7.5 62.Sf 18.4 32.3* 4.8 2.0.t 4.6 67.lf 6.6 -265.e 11.7 2Q.W 11.0 6.2 5.9 47.7* 2.5 57% 7.0 20.M 5.1 O&t 6.2 4.7* 3.5

have isotopically

and where

found

from iron metal,

phide (ALEXANDER isolated

‘W3c

et al.,

normal

Sx

~

I.& 12.5 22.lf 6.7 49&t 5.2 45% 12.3 -84% 17.6 64.9Lt19.0 23.9f 7.9 -31&t 5.0 10.3f 85 17.4* 5.8 12.3* 6.8 41.7* 6.2 46.5f 5.5 47& 8.6 33% 6.2 37% 12.3 7.If 7.0 55&t 3.8 -9.lf 7.5 30.2t 7.9 60.8f 16.5 2ECXt4.5 36&t 5.0 31.&t 5.4 -456.& 19.7 47.lf 11.2 14.7* 6.3 48.3* 3.1 53.4i 6.3 24% 5.4 38.lf 6.5 7.3* 4.1

1.03 0.76 1.13 1.01 1.05 0.86 0.99 1.01 1.15 0.91 1.23 1.19 0.94 1.14 1.05 1.15 1.49 1.01 0.93 0.95 1.20 0.88 1.19 1.04 1.07 1.15 1.50 0.98 1.12 0.89 0.99 1.42

143 144 145 146 147 148 149 150 151 152 153 IS4 155 156 IS7 I58 IS9 ::

silicon,

nitrogen,

in situ they appear nickel silicide,

199 1). Two

silicon

from the residue had isotopically

but this may

Psi(%)

to have

and iron phosnitride grains

anomalous

carbon

have simply

been due to small Sic grains adhering to them. Thus, it is likely that most if not all the silicon nitride is solar system in origin. The Leoville residue contained a large number of isotopically normal Sic grains. Fifteen normal grains were accurately measured, and approximately forty more were examined briefly, in a search that produced only one anomalous grain with a composition of ‘2C/‘3C = 62.5 + 0.6, 629Si = 62.6 ~fr 15.4, 630Si = 48.4 rt 9.5. This grain has not been blank corrected since the normal grains scatter evenly about the normal composition, suggesting the background contribution is minimal. Comparison Results

between Ion Probe and Stepped Combustion

Stepped combustion experiments, which should not be plagued by the statistical problems inherent in the singlegrain work, have been carried out on the same residues of Inman, Tieschitz, and Krymka (ALEXANDER et al., 1990a) as analyzed peak isotopic

here and revealed compositions

significant

differences

in the

of the carbon released (Table

4).

CM chondrites typically give peak carbon isotopic compositions of ‘2C/‘3C = 37-39 (Table 4). Stepped combustion experiments performed on Inman produced peak carbon isotopic compositions and release profiles that are very similar to the CM chondrites. Tieschitz and Krymka, however, gave much lower 13C enrichments in their peak carbon isotopic compositions ( ‘*Cl 13C= 58.2 and 5 1.5, respectively, see Ta-

162 163 164 165 166 167 168 169 170 171 172 173 174

%‘3C 65&t 46&k 81.4* 53.3* 60% 6l.M 582 36% 53.3* 62.W 52% 65.4* 72.W 46.3* 124&t 49.4i 4s.ti 46.&t 52.7*

3.0 2.8 2.7 7.0 6.4 1.9 8.2 2.6 3.1 4.9 6.9 2.0 3.7 4.2 2.9 2.6 4.7 3.5 2.7

50.8f 5.4 55.2f 5.8 42.7* 2.2 57.U 12.4 53.4* 10.0 71.3* 3.2 56.W 4.6 31.3* 3.1 44.4* 5.5 44&t 0.8 50.&t 3.7 57% 4.0 4&t 0.6

629Si (%I)

-cxp,

SK!

22% 3.7 55.9f 6.0 22.7* 3.7 IO..% 8.5 25.4* 7.5 14.H 2.8 713f 18.8 33.7* 4.6 22.M 4.9 24.M 5.6 29.W 12.0 -56%~ 3.1 38.3* 4.3 -59&t 8.6 30.7* 3.9 18.8f 4.4 -1.6f 9.8 -62.M 11.1 19.2f 4.8 ll.Cn 8.3 21.2t 5.9 1532 7.9 42-B 12.9 42.M 11.7 22& 3.8 49.3* 6.4 38&t 7.5 8% 8.6 70.5f 2.6 56.lf 6.4 2.8it 4.8 -17&t 10.8

27.e 4.2 38% 55 22.2& 4.1 22.W 8.3 45.4* 7.8 36.1* 3.4 14.lf 15.2 49.M 4.9 16.4* 5.2 29.5i 5.9 43.M 11.8 72.7* 3.3 39% 4.1 -115.7* 12.6 SO.4* 4.4 32.U 4.8 25.W 9.6 70.7* 55 15.a 5.1 1.W 8.9 16.2f 6.1 112.W 6.1 37.7* 11.9 53.B 11.5 37.7* 43 179.e 10.6 47.U 7.2 IS& 8.9 68.4* 3.2 68.3* 6.2 19% 5.1 -24&t 11.8

1.05 0.98 0.96 1.29 1.11 1.07 0.78 0.95 0.98 0.98 0.89 1.00 1.01 1.18 0.92 0.91 0.87 1.02 1.01 0.97 1.31 1.06 1.26 1.43 1.18 1.49 094 0.92 1.04 0.94 1.03 093

ble 4) and their release profiles are narrower than those of Inman or the CMs. Barring the presence of one or more unidentified carbonaceous components in these residues, the significant differences seen from sample to sample (Table 4) imply a surprisingly heterogeneous carbon isotopic composition for Sic in the ordinary chondrites. Direct comparison of stepped combustion and single-grain work is questionable since the stepped combustion will be strongly influenced by the submicron grains not analyzed in the single-grain work. However, the isotopic composition, as measured in the ion probe, of Murchison fine-grained Sic aggregates is very similar to the single-grain mean (AMARI et al.,

1991)

and

to the stepped

combustion

results

(Ta-

ble 4). The ion probe and stepped combustion both give mean/ bulk CM Sic carbon compositions of ‘2C/‘3C = 37. However, the single-grain work shows that both the mode and median of the carbon distribution are at about ‘*Cl 13C = 50 (Fig. 5), demonstrating the importance of the rare grains with isotopically very heavy carbon to the bulk Sic composition. From the single-grain work, Tieschitz and Krymka do not appear depleted enough in the isotopically very heavy carbon grains to explain their stepped combustion results. In the case of Krymka the single-grain work suggests that the rather subdued peak 13C excesses observed in the stepped combustion experiments, compared to CMs, is probably due to the presence of isotopically normal Sic. The Tieschitz residue also contains some normal Sic grains but it is not clear that it is enough to explain the stepped combustion results. The fact that Inman appears very similar to a CM in stepped combustion experiments adds credence to the suggestion that statistics lies behind the apparent lack of isotopically extreme Sic grains in the single-grain work, but

2819

Presolar Sic in chondrites 18 16

Carbon stars

14 12 10 8 6 4 2

18 16

Sic

14 12 10 8 6 4 2

0

20

40

60

80 100 120 140 160 180 200 220 240 260 280 300 320 12c/1w

FIG. 5. The carbon isotopic compositions of 305 individual SIC grains and in 30 carbon stars. The Sic compositions are from ZINNERet al. ( 1989), STONE et al. ( 1991), VIRAGet al. ( 1992) and this work. The carbon star compositions are from ( LAMBERT et al., 1986). Most grains have ‘v/ “C ratios between 20 and 100,and their distribution resembles that of the carbon stars. The carbon stars with ‘%/13C ratios less than 20 are from J-type stars and Sic grains with similar compositions may come from these stars. The grains with ‘zC/‘3C ratios greater than 100 do not have carbon star counterparts and some, the grains X, may come from supernovae (AMARIet al., 1992a).

makes the difference in the silicon isotopes, compared Murchison, even more perplexing.

to

How Many Stellar Sources Were There?

ZINNER et al. ( 1989) have suggested that at least four and probably more than six different sources of Sic are needed to explain the range of isotopic compositions they measured. In the simplest of all possible worlds, each source would have

produced Sic with a very limited range of isotopic compositions, such as the groupings reported by VIRAG et al. ( 1992). However, the behavior of the VIRAG et al. ( 1992) grains ap pears to be the exception rather than the rule. The majority of grains studied have silicon isotopes that are distributed more or less uniformly along the slope. 1.3line in three isotope plots (Fig. 7) and in plots of “C/W vs. 629Si form a wedgeshaped field (Fig. 8). Since most of the VIRAG et al. ( 1992) grains are typically larger than the grains isolated in other

2880

C. M. O’D Alexander

14

1

-100

-50

0

50

loo

150

200

6 29Si (%0) FIG. 6. The distribution of 629Si compositions of 304 of the 307 SIC grains reported by ZINNERet al. ( 1989), STONEet al. ( 1991), VIRAGet al. ( 1992) and this work. A few grains have compositions that fall outside the range of this plot. The grains appear to form a single broad distribution. This broad distribution largely explains the variations in the mean silicon isotopic compositions listed in table 2. Typically, the silicon isotopes do not correlate with the carbon isotopes.

SIC studies, their apparently unique properties may be indicative of a strong compositional variation with grain size or possibly a heterogeneous distribution of SIC grains in Murchison at the scales sampled. The absence of simple groupings in the Sic isotopic compositions is not, perhaps, too surprising since most stars are both chemically and isotopically stratified and their compositions evolve with time. As a result, grains that formed at different stages in a star’s evolution may form a trend, rather than a group, as newly synthesized material is mixed into the envelope. Mixing has been proposed as an explanation for the slope 1.3 silicon three-isotope line in a morphological subset of SIC grains by STONE et al. ( 199 1)) but the general lack of any correlation between carbon and silicon isotopes argues against mixing, at least if it only involved two components. Although most grains form an ungrouped trend in Fig. 7 and field in Fig. 8, some grains are quite distinct from the rest. For instance, there are the three grains (B8, 135, and K20) that are isotopically very light in both carbon and silicon. Two of the grains have been previously reported, without blank corrections, by ALEXANDER et al. ( 1989b and 199Ob), and all three are isotopically similar to the Murchison grains X of AMARI et al. ( I99 I and I992a,b). In addition to the three ordinary chondrite grains X, one other small grain with very light silicon (S29Si = -470 + 61960,S?Gi = -285 +- 40%) was found in Tieschitz. This grain is included in Fig. 7 although it contained little or no carbon (Si/C > IO). The grain is clearly not Sic, but its mineralogy is unknown. Silicon nitride is one possibility since silicon nitride has been observed in a Tieschitz residue and

may contain isotopically heavy nitrogen (LEE et al., 1992). A number of grains (B24, 115, I1 8,120,154,156,160, and 168) lie below the main Si isotope array (shaded region in Fig. 7 ) . These grains all have fairly similar 629Sicompositions of between -30 to -6OL, but their 6?!ji compositions range from - 120%0 to + I80%0. Also included in Fig. 7 is a Murchison grain found in situ (ALEXANDERet al., 199Oc) which is even more 29Sidepleted than the other grains. The carbon isotopic compositions of all these grains are mostly between ‘*C/ 13C = 66 and 45 and are, apparently, independent of the silicon isotopes. AMARI et al. ( 1992b) have identified a group of grains, which they have named grains Y, that also plot at the base and to the right of the main silicon array. However, the grains Y all have light carbon isotopic compositions ( 12C/ 13C> 100) and do not seem to be related to those previously discussed. This new group will be referred to as grains Z in the rest of this paper. There are several grains that have 12C/13C > 100 in Table 3, and four of these (B39, T25, S5, and S6) do appear to be related to the grains Y in that their silicon isotopes fall on the same line (slope of about 0.5) as the grains Y and there is a positive correlation between their ‘*C/13C ratio and h3’Si. A second group ofgrains with 12C/13C > 100 (BIO, C29, S2, S8, and 157) falls close to the terrestrial fractionation line and also appears to have a positive correlation between their ‘*C/ 13C ratio and 6?ji. Indeed, the two groups (grains Ya and Yb, perhaps) have similar gradients, in plots of their ‘*C/ 13C ratio vs h3’Si, of about 0.9. Thus, these two groups of grains may have formed by a similar process. One other grain, T22, also has a ‘2C/‘3C > 100 but its silicon isotopes are significantly heavier than the other grains. Of the 246 anomalous grains measured here, three grains X and four grains Y have been found. AMARI et al. ( 1992b) found in 786 grains five grains X and five grains Y. They do not report grains belonging to the second Y-like group or the grains Z. Thus, although these two large SIC samples have similar abundances of these unusual grains they are still not identical, suggesting still larger numbers are needed to obtain a truly representative sample of the presolar Sic. Conse-

Table 4. Comparison of the peak carbon isotopic compositions observed by stepped combustion from three CM chondrite residues and the three highest concentration ordinary chondrite residues studied here. Sample

12cPC

Murchison (CM2)1 Murray (CM2)2 Cold Bokkeveld (CM2)3

36.8 37.6 38.3

Krymka (LL3.0)3 Inman (IJLL3.4)3 Tieschitz (H/L3.6)3

51.5 37.5 57.0

1 ASH et al. (1991); 2 WRIGHT et al. (1988); 3 ALEXANDER et al. (199Oa).

Presolar Sic in chondrites

-600

-500

-400

-300

-200

2881

-100

0

100

200

6 3oSi (%o) FIG. 7. The silicon isotopes of 313 isotopically anomalous Sic grains reported previously (ZINNER et al., 1989; ALEXANDERet al., 199Oq STONE et al., 1991; AMARI et al., 1992a; VIRAG et al., 1992) and in this work. The errors of only the most unusual grains are shown to avoid confusing the plot. The vast majority of grains plot about the slope 1.3 line. The most notable exceptions are the grain Xs of AMARI et al. (1992a) and this study. The stippled region delineates what may be a new group of grains, here called grains 2. Also included (open symbol) is a Tieschitz grain of unknown mineralogy with very unusual silicon isotopes but which containedhttle or no carbon.

120

loo

80 U r? 260 cl 40

20

(I -100

-d&z

*

*d+

I”..,‘...,....,....,....,....

-50

0

50

6 29Si

loo

150

200

250

(%o)

FIG. 8. A plot of the lrC/13C and 6?Si for 301 of the 307 isotopically anomalous BC grains reported by ZINNER et al. ( 1989), STONE et al. ( 1991), VIRAG et al. ( 1992) and this work. A few grains have compositions that fall outside the range of this plot. There are remarkably few groups or trends in the data despite the number of grains in the plot. The range of isotopic compositions exhibited in this plot can be produced by the various nucleosynthetic processes that occur in AGB stars but probably not in a single AGB star. The stippled region encloses the grains Z.

2882

C. M. O’D Alexander

quently, it is not surmising that there are differences in the distribution of Sic isotopic compositions obtained in previous Murchison studies and for the various meteorites examined in this work, since these only involved samples of 9 to 74 grains. Despite the fact that Figs. 7 and 8 include more than 300 individual Sic grains reported here and in the literature, and although a few grains that may be associated have been identified, in general these grains appear to bear surprisingly little relationship to one another. Simple cluster analysis performed on this .and unpublished Murchison data (S. Amari, pers. commun.) shows that few grains can be incorporated into groups of four or more. Therefore, it would seem that either most grains come from separate sources or there were relatively few sources whose isotopic compositions varied considerably during their lifetime. These two possible interpretations of the carbon and silicon isotope data are discussed in the following sections but if one assumes that the Sic sources did not vary isotopically with time, the scarcity of clusters of four or more grains implies that there were of the order of seventy-five or more sources. What Were the Stellar Sources? The astronomical evidence and astrophysical models

Of the possible stellar sources of Sic, the currently most favored are the asymptotic giant branch ( AGB) stars by virtue of their relative abundance (all single stars of 1-9 Mo will probably become AGB stars) and their apparent ability to produce the s-process nuclides and Ne-E(H) found in Sic. A prerequisite for Sic formation is a C/O P- 1 and, indeed, most carbon stars appear to form during the AGB phase of evolved stars, shortly before the formation of a planetary nebula. The characteristic infrared spectrum of Sic has been observed in the circumstellar envelopes of numerous carbon stars ( TREFFERS and COHEN, 1974; MERRILL and STEIN, 1976a,b; COHEN, 1984; LITTLE-MARENIN, 1986) and in planetary nebulae ( ROCHE, 1989; LENZUNIet al., 1989). Carbon stars and planetary nebulae are thought to be the two major suppliers of refractory carbonaceous material to the interstellar medium (BODE, 1988). Wolf-Rayet stars, as well as other carbon-rich astrophysical settings such as novae, are possible Sic sources but are far less common than typical carbon stars ( VIRAG et al., 1992 ). A brief description of the evolution of stars in this mass range on leaving the main sequence is given next (for detailed reviews see IBEN, 198 1, 1985 and 1991; IBEN and RENZINI, 1983). After spending most of their life on the main sequence burning H via the pp process and the CNO cycle the H in the core is exhausted. At this point the He-rich core contracts and the stellar envelope expands rapidly, driven by the energy from a thin H-burning shell around the core. This is the first red giant phase. At about this time there occurs the so-called first dredge up when material from the H-burning shell is brought to the surface via convection. The C, N, and 0 abundances, as well as the “C/‘? ratio, of red giant atmospheres in this phase show clear evidence for the addition of CNO cycle material, which has a C:N:O ratio of approximately

0.01:0.04 at low temperatures and 12C/13C = 3.4, but their C/O ratios remain less than one at this stage. The inert core continues to contract and gain mass from the H-burning shell until eventually He-burning begins. Core He-burning is accompanied by contraction of the envelope and the star leaves the red giant field. The two major products of He-burning are 12Cand, to a lesser extent, 160. When He is finally exhausted in the core, the core contracts and Heburning continues in a surrounding shell. At the onset of shell He-burning the envelope expands again, extinguishing the H-burning shell, and the star enters its second red giant or AGB phase. In the more massive stars, a second dredge up of CNO processed material occurs at this stage and, as a result, nitrogen abundances in the stellar envelope can rival that of oxygen but the C/O ratio remains less than one. In the early AGB phase, there is no H-burning shell but as the He-burning shell expands it approaches the base of the H-rich envelope and H-burning is re-ignited, at which point He-burning falls off dramatically. From this stage on the star begins to thermally pulse. Each cycle begins with the Hburning shell moving outwards and contributing material to the quiescent He-shell below. Eventually the He-shell reaches a critical mass and a thermonuclear runaway occurs. During this pulse the He-burning shell expands rapidly, quenching the H-burning shell and itself cools until burning ceases. This is followed by a phase of contraction and re-ignition of the H-burning shell, beginning the whole cycle again. Several important processes occur during the thermally pulsing AGB phase. At the end of each He-burning pulse convection carries material rich in “C from the He-burning shell into the H-burning shell region and the outer envelope. As a result, after only a few pulses the envelope can develop a C/O ratio that is greater than one. However, between Heshell pulses some or all of the ‘*C in the H-burning shell and, possibly, at the base of the convective envelope is converted, via the CNO cycle, to 14N. The process of nucleosynthesis at the base of the convective envelope is often referred to as hot bottom burning. The efficiency with which the “C is converted to 14N during hot bottom burning is dependent on both the mass of the star and the nature of the convection and, will determine both the carbon isotopic ratio of the envelope and whether or not the star becomes a carbon star. Metallicity also plays a role in determining whether a star becomes a carbon star. The lower the metallicity the fewer dredge ups are required to produce a carbon star and the higher the ‘2C/‘3C ratio for a given number of dredge ups. At the temperatures experienced by stars less than about 7 Mo, hot bottom burning may affect a shift of about -40% in &29Siand IS~‘S~by converting “Al to 28Si through proton capture (BROWNand CLAYTON, 1992). Higher temperature hot bottom burning in more massive stars leads to preferential destruction of 29Si, compared to the other silicon isotopes, resulting in a decrease in 629Si and an increase in ?i3’Si ( BKOWNand CLAYTON, 1992). In the next He-burning pulse, some of the CNO processed material is overrun by the expanding He-burning shell and 14N is converted to 22Ne. Not only could this be the source of Ne-E( H) but *‘Ne is also a potent source of neutrons via the *‘Ne( cY,n)“Mg reaction. The 22Ne(a,n)25Mg and/or

Presolar SIC in chondrites

13C(a n)“O reactions (the “C also having come from the CNO processed material) in the He-burning shell are probably responsible for s-process nucleosynthesis in these stars. As a result of neutron capture during the s-process, the silicon isotopes in the He-shell become much heavier and because of relatively large 29Si and 33S(n,a) cross sections the silicon isotopes evolve along a line with a slope of about 0.5 on a silicon three-isotope plot ( GALLINO et al., 1990;OBRADOVIC et al., 1991). Throughout the red giant phases of stellar evolution stars lose a significant fraction of their mass via powerful stellar winds. It is in these winds, as they stream away from the central star and cool, that most refractory grain growth is thought to occur (e.g., BOWEN, 1988). Towards the end of the AGB phase these winds may become even more powerful superwinds that are possibly driven by hot bottom burning. These winds can lead to the complete loss of the H-rich stellar envelope and, if sufficiently strong, prevent the more massive stars (~5 Ma) from becoming supernovae. The final remnant of the central portion of those stars that do not become supernovae is a C- and O-rich white dwarf surrounded by an extended circumstellar halo. As the white dwarf contracts and heats up, its surface temperature can reach more than 30,000“C at which point the radiation from this hot, compact remnant is suficient to ionize the inner portion of the halo producing a planetary nebula. Up to this point, the discussion of AGB evolution has been qualitative. Quantitative modeling has demonstrated just how sensitive the chemical evolution of an AGB star’s envelope is to input parameters such as metallicity, initial stellar mass, mass loss rate, and convection ( RENZINIand VOLI, 198 1). According to these authors stars in the 2-5 Mo range will become carbon stars for all assumed conditions, while stars with initial masses as low as 1 Mo will become carbon stars if they have sufficiently low metallicity, and those with masses greater than 5 Mo will become carbon stars only if a poorly convecting envelope or high mass loss rates prevent hot bottom burning from becoming important. It has long been thought that the M, MS, and S Q-rich giant stars form an evolutionary sequence from first ascent giants (M-stars) to AGB star (MS and S stars). Carbon, nitrogen, oxygen, and s-process element abundances are broadly consistent with this picture (SMITH and LAMBERT, 1990). The so-called normal cool, or N-type, carbon stars ( ‘*C/ 13C > 20) appear to be more evolved AGB stars than the MSand S-stars ( LAMBERTet al., 1986). The elemental and isotopic compositions of the MS-SC sequence appear to result from the addition of He-shell material to stellar envelopes previously enriched in CNO material by the first and second dredge up processes ( LAMBERTet al., 1986; SMITH and LAMBERT, 1990). There is little evidence for hot bottom burning having played a significant role in these stars. Six of the 30 carbon stars studied by LAMBERTet al. ( 1986) have ‘*C/ “C < 20 and four of these have ‘*C/ r3C ratios between 3 and 4.5. Their compositions are inconsistent with the simple addition of He-shell material and appear to require a H-burning component. In one case hot bottom burning appears a reasonable explanation despite the fact that it is only for a very restricted set of conditions that hot bottom

2883

burning can produce a “C-rich carbon star (HARRIS et al., 1987). The remaining five stars, which are J-type carbon stars, are not enriched in s-process elements and, therefore, may not be AGB stars. Another group of carbon stars that are “C-rich and depleted in s-process elements is the warm, or R-type, carbon stars ( HOMINY, 1985 ) . At present there is no completely satisfactory explanation for either of these types of star. Comparison of astrophysical models and observations with the isotopic compositions of Sic grains The carbon stars measured by LAMBERTet al. ( 1986), and whose compositions are explainable in terms of the addition of He-shell material to the envelope, have a very similar range of ‘*C/13C ratios to that of the Sic (Fig. 6). In addition, despite the uncertainties in the correct values for the model parameters GALLINO et al. ( 1990) concluded that, given a reasonable set of assumptions, AGB model predictions produced an acceptable fit to bulk Sic noble gas data. Since both observational evidence and theoretical models point to an AGB origin for most of the Sic the question is, then, whether a single AGB star can produce the range of isotopic compositions observed in meteoritic Sic. Certainly, by a judicious mixing of equilibrium CNO cycle material (‘2C/‘3C = 3.4, and @%i = OL), He-burning material (pure ‘*C and 629Si 5 O%O), and s-process products ( b29Si ET ; 000% ) one can, with the exception of the heaviest grains ( ‘*C/13C < 3.4) and the grains X, reproduce the observed range of carbon and silicon isotopic compositions in Fig. 8. Hot bottom burning can briefly produce 12C/‘3C x 2.5 ( SCALO et al., 1975), which could explain the very heavy carbon grains if it is possible to maintain the C/O < 1 at the same time. However, it is not clear that these endmembers can be mixed together in a physically reasonable setting and still produce the range of Sic isotopic compositions observed. For instance, on a simplistic level one might expect to see a positive correlation between the ‘*C/ r3C ratio and 629Si as the proportion of s-process rich He-burning shell material rises in the envelope. Yet, except for the grains Y, there is no evidence for this correlation (Fig. 8 ), perhaps because the endmember is not pure He-burning shell material but one in which some of the ‘*C has been reprocessed through the CNO cycle by hot bottom burning. Indeed, those grains with 12C/‘3C < 20 require efficient hot bottom burning but, as already mentioned, it is difficult to produce a carbon star under these conditions. Perhaps most problematic to a single AGB source for the Sic are the silicon isotopes. Both GALLINO et al. ( 1990) and OBRADOVIC et al. ( 199 1) predict that the silicon isotopes from an AGB star should evolve along a slope of about 0.5, as opposed to the slope 1.3 line found for most of the Sic. The only recognized group of grains that conforms fairly closely to the predicted slope are the grains Y. BROWN and CLAYTON ( 1992) have proposed a way of producing a slope of about 1.3 in an AGB star by invoking what they term as Mg-burning, conversion of *‘Mg and *‘Mg at about 4.5 X 10’ K to 29Si and “Si by ((w, n) reactions, in the very hot

2$fu.

C. i\/r.O’D Alexander

He-burning shell of a massive (~7 Mo) AGB star. However, the temperatums required for this process are lm higher than steUar models predict and if such a massive star is to become a carbon star, hot bottom burning would have to have been more or less suppressed by processes such as high mass loss rates. While it cannot yet be ruled out, it is apparent that there are a number of objections to a single stellar source for the meteoritic Sic. Indeed, from the silicon and carbon isotopes ZINNERet al. ( 1989) argue that there must be four or more sources. With the grains X, the two grains Y groups, the grains 2, and the remainder of the grains, four sources seem to be the absolute minimum. Based on the krypton isotope systematics, GALLINOet al. ( 1990) also prefer, but cannot prove, a multistar origin for the Sic. Ba, like Kr, varies ~topi~lly with grain size probably reflecting differing neutron exposures during the s-process ( ZINNERet al., 199 la). As already mentioned, He and **Ne concentrations vary enormously from grain to grain (NICHOLS et al., 1992), as does the 26Al/27Al ratio ( ZINNERet al., 199 1b). The Ti isotopic compositions of SiC grains exhibit broad trends that deviate su~~nti~ly from AGB star model calculations (IRELAND et al., 1991) but could be made to be more or less compatible with the models if the grains came from several stars with different initial 46Ti enrichments, relative to solar. Perhaps the most persuasive evidence for more than one source of Sic is the fact that different Mu~hison SiC size fractions have cosmic ray exposure ages that range from 133 m.y. to 13 m.y., but there is a possibility that the smaller, younger grains have been partially degassed (LEWIS et al., 1993). All in all, the interpretation of the isotopic data remains far from unambiguous but points to more than one AGB source of Sic. Multiple AGB sources with different ages could overcome the problem of producing the slope 1.3 silicon array because the mean silicon isotopic composition of the galaxy should evolve along a slope of approximately one (CLAYTON, 1988 ) . In apparent support of the CLAYTON (1988) suggestion is the observation that the isotopic composition of both the present-day galactic cosmic rays ( MEWALDT, 1989) and the interstellar medium (WOLF, 1980) are greatly enriched in 29Si and %i relative to “Si compared to solar. In fact, the galactic cosmic ray composition lies close to an extension of the main Sic array. The test of a galactic evolution explanation will be to find other isotopes that are not affected signi~~n~y by nucleosynthesis in AGB stars which should grow like the silicon isotopes as the galaxy evolves. IRELAND et al. ( 199 I ) suggest the &Ti contents may have varied from star to star. CLAYTON ( 1988) did not calculate how the titanium isotopes should evolve in the galaxy with time but there does appear to be good correlation between the silicon isotopes and the qi/&Ti ratio (IRELAND~~al, 1991; HOPPE et at., 1993). The attraction of multiple, and probably evolving, sources for the Sic grains is that the range of Sic compositions can be explained quite easily by invoking, albeit in an ad hoc fashion, stars of different ages, masses, and metallicities. It would also explain the similarity of the carbon isotope dist~butions for Sic and carbon stars. The apparent rarity of hot bottom burning in AGB stars makes those grains with

the isotopically heaviest carbon hard to explain in terms of AGB evolution. They may instead come from s-process poor J- and R-type stars. Trace element con~n~tions in these grains may allow one to differentiate between AGB and sprocess poor stellar origins. Isotopically, the only grains that clearly do not fit into models of AGB stars are the grains X. AMARI et al. ( 1992a) suggest a supernova origin for the grain X s based on the large 44Ca and 4% enrichments they observed. It should be pointed out that the precursor of the supernova could have been a high mass AGB star. The major drawback of a supernova origin is that it would require mixing of material from several zones, and it is not obvious that this would result in a C/O ratio greater than one. Is a multi-AGB origin for SIC consistent with the astronomical observations? It has just been argued that the range of Sic isotopic compositions can best be explained in terms of several sources. The estimated number of sources depends on how variable stars are during their evolution but is almost certainly more than four and may be as high as seventy-five. Here simple arguments are developed to examine whether it is reasonable to expect this number of AGB stars to contribute Sic to one solar mass of material embedded in a mature molecular cloud. Typically, molecular clouds and stars have relative velocities of about 10 km/s. As a result, numerous stars, some of which will be in their carbon-rich AGB (C-AGB) phase, will pass through a cloud during its lifetime. Sic grains emitted from those C-AGB stars that pass through the cloud will, as a result of turbulent motions in the cloud (LARSON, 198 I), mix throu~out some volume of the cloud. The distance, and therefore the volume, the grains can mix is given by x L-;(v.L.t)0.5,

(1)

where V is the r.m.s. velocity of the gas, L the mixing length (in this case the size of the cloud), and t is time. Setting X equal to L. one obtains the so-called turnover time, T = LI V, which is roughly the time it takes to mix the whole system. In his study of turbulence in molecular clouds, LARSON ( 198 1) showed that both the velocity and the gas density of a cloud are a function of its mass. Using the LARSON( 198 1) relationships, the turnover time of a cloud is approximately given by T(m .y.) = 0 . 184MbF6 I

(2)

with MMc being the mass of a cloud, in solar masses. From the present mass distribution of molecular clouds, the solar system is most likely to have formed in clouds with masses between 10’ Mo and lo6 Ma (e.g., ELMEGREEN, 1985). Clouds in this mass interval have a range of turnover times from about 8 to 17 m.y., respectively. Molecular clouds may have lifetimes of 10’ yrs (e.g., ELMEGREEN,1985), a lifetime which is supported by the 133 m.y. mean cosmic ray exposure age for the coarser Sic grains (LEWIS et al., 1993), and the gas in these clouds should, therefore, be well mixed. It follows that, provided the Sic grains are coupled to the gas, the num-

Presolar Sic in chondrites ber of C-AGB stars that can contribute Sic to any part of a cloud is roughly the number of C-AGB stars that pass through the cloud. The turbulent velocities of the gas in molecular clouds are typically supersonic (LARSON, 198 I ). Since the maximum relative velocity between a grain and the gas is likely to be of the order of the gas velocity ( 4-6 km/s) , supersonic drag forces will determine the coupling between the gas and dust. A grain that can be slowed from these maximum relative velocities to a subsonic velocity, about 0.3 km/s in cool molecular clouds, in a time that is shorter than the turnover time must be reasonably well coupled to the gas. Using the MCCREA and WILLIAMS ( 1965) formulation for the supersonic drag force and the LARSON ( 198 1) relationships for the variation of gas density and velocity with cloud mass, the time it takes for a grain of radius R (pm) to slow to a sonic velocity, C (km/s), is given by t (m.y.) = R1.3 X 10e3 M~~9(0.42flti~C-’

- 1).

(3)

For grains of radius one micron in clouds between 10’ Mo and lo6 Mo, the braking times are between about 1 and 5 m.y., respectively. Thus, given the turnover times calculated earlier, the grains in the size range studied here are indeed fairly well coupled to the gas and will, therefore, be well mixed in the clouds. The number of C-AGB stars that will on average pass through a cloud during its lifetime is approximately given by the product of the mean number of AGB stars that form throughout the galaxy during the lifetime of a cloud and the volume fraction of the galaxy occupied by the cloud, which is NAGS = &d~cV~d%cIM~~c,

(4)

Where: RPNis the rate of formation of white dwarfs/planetary nebulae in the galaxy ( l-6 yr-‘, MILLER and !SCALO,1979) which, if star formation has reached a steady state, is equal to the rate of AGB star formation and here is taken to be 3 yr-‘; VG,C is the fractional volume occupied by molecular clouds in the galaxy (2.5 X 10w4, CAMERON, 1993); tMCis the lifetime of a molecular cloud, which is of the order of IO8 yrs; MMc is the mass of the cloud and; MoMC the total mass of material in molecular clouds in the galaxy ( IO9 Mo, CAMERON, 1993). Estimates made using Eqn. 4 should be regarded as a minimum estimate as no allowance is made for the grains a cloud may have inherited when it formed or the grains it will accumulate as it passes through the intercloud medium. Equation 4 does not take into account the relative motions of the clouds and stars. These relative motions will increase the volume of the galaxy which a cloud can sample. If one pictures the stars as being uniformly distributed and at rest, then for a moving cloud the effective increase in its volume is the cylinder of space the cloud sweeps through during the average lifetime of a C-AGB star. A C-AGB star has a lifetime of the order of 10’ years ( RENZINI and VOLI, 198 1). Using Larson’s relationships the diameter of a cloud is L (PC) = o.079f@J6.

(5)

2885

A cloud with a velocity of about 10 km/s relative to the can travel about 10 pc during the lifetime of a C-AGB The effective fractional increase in the cloud’s volume, therefore the number of AGB stars that can contribute is F, = 1 + 190M$e526.

stars star. and to it, (6)

Combining Eqns. 4 and 6, and using the values given above for the variables in Eqn. 4, NAoB becomes, for lo5 Mo and lo6 Mo clouds, 11 and 85, respectively. These estimates Of N,,oB would be entirely consistent with the conclusions from the isotope data but for the fact that they may be in error by a factor of three simply due to the uncertainty in RpN. The possible range in the number of CAGB stars that could contribute to the solar system is, therefore, 4 to 250. If errors in the estimates of VGMCand MGMC combine to give another factor of two or so to the overall error, the lower limit for NAG*may be as low as 1 or 2. Nevertheless, the most probable value for NAoa suggests many Sic sources are likely. An additional complication is the fact that some recent estimates of cloud lifetimes suggest they may be of the order of only 10’ yrs (see ELMEGREEN,1985). In this case, the 133 m.y. cosmic ray exposure age of the coarse Sic could be interpreted as the average age of SIC that accumulated in the material from which the cloud formed. Since molecular clouds probably form from lower density atomic clouds, with their correspondingly larger volumes, NAoa may, in fact, be considerably larger than previously estimated. The previous estimates are far from unequivocal. They suggest a number of SIC sources are likely, but the errors are such that one cannot completely rule out a single source. However, the concentration of Sic in meteorites can be used to obtain an independent estimate of NAGS. If it is assumed that the material returned to the interstellar medium from a C-AGB star, as well as the interstellar medium itself, is ap proximately solar in composition then the concentration of Sic (ppm) in the condensable fraction (not including H, He, and ices) of a well-mixed molecular cloud is approximately SiCMc = 106N~~,M~~~,FsiclM~c,

(7)

where McAGBis the average mass of material, in solar masses, returned by a C-AGB star and Fsic is the weight fraction of Sic in the condensable material. A reasonable estimate for McAG~, given the dominance of low mass stars in the stellar mass function ( SCALO, 1978), is about 0.5 Mo per star. If all the silicon in the C-AGB ejecta is in Sic but all the other condensable elements are oxides, then Fsic is about 0.2. The concentration of Sic observed in Murchison, when it is corrected for the 50 ~01% of high-temperature nebular products ( chondrules, CAIs etc ) , is about 12 ppm. From Eqn. 7 a Sic concentration of 12 ppm in the condensable fraction of a lo5 Mo cloud requires NAoB to be about 10 and for a lo6 Mo cloud to be about 100. This, of course, assumes that no SiC is destroyed in the molecular cloud or in the solar nebula (except for in the material from which chondrules, etc., formed). Since McAGBis unlikely to be in error by more than a factor of two and the value used for Fsic is probably an upper limit, the conclusion from this estimate that of the

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order of 10 or more AGB stars contributed to the solar system seems a fairly robust one. The estimates for NAGBobtained from the concentrations of Sic in meteorites are consistent both with those estimated from a simple model of how a cloud may acquire its Sic and the number of sources indicated by the isotope data. Although the value of NAos depends on the cloud mass and lifetime it seems likely, from these calculations, that one could expect between IO and 100 C-AGB stars to have contributed Sic to the solar system.

SUMMARY AND CONCLUSIONS The carbon and silicon isotopic compositions of 246 isotopically anomalous SIC grains have been measured in low concentration residues prepared from nine chondrites (6 UOCs, Qingzhen (EH3), Leoville (CV3), and Murchison (CM2)). The Murchison results are similar to previously reported studies which used essentially pure Sic separates. However, no two Murchison Sic studies are identical, probably due, in large part, to the statistics of small samples and possibly heterogeneity within Murchison. In the absence of a better alternative, Murchison is used as a standard to which all the other meteorites studied are compared. The range of isotopic compositions exhibited by UOC Sic appear similar to Murchison, except in Inman. Inman Sic seems to have a distinctly different distribution of its silicon isotopic composition compared to the other meteorites. Residues from Qingzhen and Leoville produced only one anomalous SIC grain each. Isotopically normal SIC is present in almost every residue despite the care taken to avoid contamination. It is possible to form SIC by condensation in a reduced nebula or by exsolution from metal. However, in situ searches for Sic in the enstatite chondrite, Qingzhen, and in chondritic metal in other meteorites have been unsuccessful. The one Sic grain found in the Qingzhen residue is isotopically anomalous and the dominant silicon-bearing phase is isotopically normal silicon nitride. At present the origin of the isotopically normal SIC remains uncertain. The observed range of Sic carbon isotopic compositions ( ‘2C/‘3C = 2-320) can be produced by the nucleosynthetic processes that occur in AGB stars. On the other hand, the silicon isotopes, which generally lie on a slope 1.3 line in a three-isotope diagram, are problematic. Although the grains with the ‘2C/‘3C < 20 could, in principle, be produced by hot bottom burning, it is probably a rare process in carbonrich AGB stars. S-process poor J- and R-type carbon stars are likely alternative sources for these grains. The range of isotopic compositions exhibited by Sic is enormous. There appear to be at least four groups of grains present in a compilation of more than 300 grains, but most grains are ungrouped. The range of isotopic compositions and the few groups that have been identified indicate there were at least four SiC sources and possibly as many as seventyfive. Simple calculations suggest it is likely that 10 to 100 AGB stars could have contributed Sic to the protosolar system if it were imbedded in a mature and well-mixed molecular

cloud. These simple calculations imply that explanations for the silicon isotope data that require a single star (STONE et al., 1991) or rare stars (BROWN and CLAYTON, 1992) are unlikely, and appear to favor a galactic chemical evolution interpretation. Acknowledgments--The

author is indebted to R. Walker and E. Zinner for their invaluable help and advice throughout the course of this project, to J. Arden, J. Pier and C. Prombo who prepared the acid residues and to P. Swan for his guidance on the SEM. The author would also like to thank G. Huss, R. Lewis, U. Ott and an anonymous reviewer for their helpful comments, and Colin PiUinger who provided the initial impetus for this project. This research was supported by NASA grant NAG 9-55. Editorial handling: C. Koeberl

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