On the siting of noble gases in E-chondrites

0016-7037/82/l

Geochimica d Cosmochimica Ado Vol. 46, pp. 23.5 I-2361 0 Pergamon Pras Ltd. 1982. Printed in U.S.A.

12351-I I$O3.00/0

On the siting of noble gases in E-chondrites JANE CRABB* and EDWARD ANDERS Enrico Fermi Institute and Department of Chemistry, University of Chicago, Chicago, IL 60637

(Received March 24, 1982; accepted in revisedform August 18, 1982)

Abstract-We have investigated the siting of noble gases in 6 Echondrites, by analyzing fractions seg arated by density, grain size, and chemical resistance from Qingzhen (E3), Indarch (E4), Abee and Saint Sauveur (E4-5), and Yilmia and North West Forrest (E6). The new “subsolar” (i.e. Ar-rich) component in E6’s is concentrated in the main, enstatite-rich fraction of the meteorites, with density 3.06-3.3 g/cm’. It is unatfected by HCl and HNOs treatments of such fractions and remains in unchanged concentration when the samples are partially dissolved by HF. These properties suggest that the subsolar component is located in enstatite, or less likely, in a phase closely associated with it. E4-5’s have at least half of their subsolar gases in HCl- and HNO-resistant sites (enstatite?), but fail to show the increasinggasconcentration with decreasing grain size that is characteristic of most other primordial gas carriers. This may mean that the subsolar gases originally were in some other phase, but were then transferred to enstatite by metamorphism. Most of the ‘29Xe,of E6’s is concentrated in the same fractions as the subsolar gases, again suggesting enstatite or an associated phase as the host. Only a few percent of the ‘29Xe, is contained in fractions enriched in other major and minor minerals. In E4’s, on the other hand, ‘29Xe, is enhanced in finegrained, low density fractions and is also partly associated with chondrules. Perhaps ‘29Iwas originally contained in fine-grained matrix, but was transferred to enstatite during metamorphism. A carbon-rich fraction of Indarch (E4) is enhanced in Ne-A, CCF-Xe, and LXe. Interestingly, both the isotopic composition of Xe and the Ne/CCF-Xe ratios resemble those of C-chondrites, yet these two meteorite classes probably formed rather far apart. Thus, if these components were mixed at a late stage, it must have been in fairly constant ratio over a large scale. Alternatively, they may have been mixed at an earlier stage, into a common carrier that was spread through a significant portion of the solar nebula. The primordial gases of Qingzhen (E3) resemble those of Indarch: they are present in moderate amounts (20Ne, = 1.2 X lo-’ cc/g, 132Xe= 10 X lo-” cc/g), with little or no contribution from the subsolar component. Thus Qingzhen reinforces our earlier finding that E-chondrites show no regular increase in noble gas content with decreasing petrologic type. One notable feature of Qingzhen is its very low ‘He/“Ne ratio of 1.07, which indicates that ‘He has been lost by solar heating. Solar heating may also account for its low, discordant gas retention ages (U,Th-He age = 1.1AE, K-Ar age = 3.2AE).

1. INTRODUCTION E-CHONDRITEShave long been recognized for their special properties: they are highly reduced (almost all of their iron is in metal or sulfide) and they contain a number of unusual minerals (Prior, 19 16; Mason, 1966; Keil, 1968). These properties suggest that Echondrites formed from a gas of high C/O ratio (k0.83 vs. the solar value of -0.6, Larimer, 1968; Larimer and Bartholomay, 1979). Not surprisingly, noble gases in these meteorites also show some peculiarities: their radiogenic ‘29Xe contents are higher than those of most other chondrites (Reynolds, 1960; Zahringer, 1968) primordial’ Ne is present even in Present address: Max-Planck-lnstitut fur Chemie, Saarstr. 23, D-6500 Maim, Federal Republic of Germany. ’ We use the following terminology for noble gas components. Primordial: the broadest term, covering all gases trapped l

at an early stage, but excluding radiogenic, cosmogenic, and solar wind components. Planetary: the most common type of primordial gas, which dominates in carbonaceous and ordinary chondrites. Subsolar: primordial gas with Ar/Xe and Kr/Xe ratios higher than the usual planetary values. It is not yet known whether its elemental composition is unique, or, like the planetary component, covers a range in ratios. For definition of other components, see Matsuda er al. (1980). 2351

types 4 and 5 (Zahringer, 1962, 1968; Crabb and Anders, 198 1, hereafter CA), and “Ar/“‘Xe ratios are high and variable (up to 2700 VS.40- 120 for normal planetary gases; Zahringer, 1968; CA). Mineral separations and stepped heating have shown that the third feature reflects the presence of a new noble gas component in E-chondrites (called “sub-solar” to indicate that its Ar/Xe and Kr/Xe ratios are intermediate between solar and planetary values, Crabb, 1980; CA; Wacker and Marti, 1982). A step toward learning about the origin of these unusual noble gases is to locate their host phases. This is easier said than done, however, as purely physical methods do not give clean separations of such intergrown or fine-grained minerals (cf: density separates of Indarch, Schaeffer ef al., 1965), and an HF/ HCl treatment, which is successful in enriching gas carriers in other meteorite classes, removes two of the most interesting components-the subsolar gases and 129Xer(Srinivasan et al., 1977; CA). We have followed up on our earlier attempts to locate the host phases (CA) by combining separations based on density, size, and chemical resistance. A major goal of the separations was to find out more about the siting of the subsolar component. We had previously shown that the host in E6’s is resistant to HCl and HN03 but soluble in HF, which strongly

2352

1. C’rabh and E. Anderc

(private commun., 1982) rcclassttied to E3. thereby greatly enhancing its

A. El Goresy

Qingzhen interest.

2. EXPERIMENTAL 2. I Mass spectrometrj

suggests a Si-bearing mineral (CA). Likely candidates are silica polymorphs, sinoite (Si2N20), enstatite, or a phase enclosed by one of these minerals. In the present work, candidate minerals were separated by density, and in the case of enstatite, further purified by chemical treatment. Two other approaches were used to check for an encapsulated phase: acid treatment of different size fractions, and partial dissolution by HF. Another question of interest is whether the host for the subsolar component is enriched in fine-grained material, like most other primordial noble gas components (Table 10 of Alaerts et al., 1980). For this purpose we turned to E4-5 chondrites, which we disaggregated into natural grain-size fractions by freezing and thawing in HCl. These separations also were expected to provide further information about the siting of lz9Xer. Having previously shown that most of the ‘29Xer in E6’s follows the subsolar gases in acid-resistant phases, we now wanted to separate a meteorite non-destructively, to learn something about i29Xer in acid-soluble phases. Another separation was designed to investigate the primordial Ne in E-chondrites. Previous work had shown that Ne in E-chondrites is concentrated in HF/ HCl-resistant residues, and resembles Ne-A in its isotopic composition (Srinivasan et al., 1977; CA). If it is indeed Ne-A, with the same kind of carrier, then it should persist upon etching with HNO,, just as in other chondrites (Lewis et al., 1975). This experiment should also provide information about the properties of CCF-Xe in E-chondrites, since this anomalous component is generally enhanced in such etched residues from primitive chondrites. The slightly elevated ‘36Xe/‘32Xe ratios of some E-chondrites suggest that they contain CCF-Xe (Krummenacher et af., 1962; Srinivasan et al., 1978), but this component has never been resolved from trapped Xe in these meteorites. At a late stage of our work, we included Qingzhen, a recent fall originally classified as type EA (Wang and Xie, 1977). It is unusually friable, and hence provided an opportunity to study separated chondrules and matrix. After this work was completed,

Ne. Ar, Kr, and Xe from separates of Abee and lndarch were measured on a glass mass spectrometer, following our usual procedure (Alaerts et al., 1979a). Samples of other meteorites were measured on a metal spectrometer, with the following modifications. While admitting Ar to the spectrometer and pumping excess Ar from the sample system, the activated charcoal was held at - 78°C. Kr and Xe were then released by heating the charcoal, and admitted to the spectrometer together. A few sweeps were made over *“Kr, then a complete Xe analysis was performed. Neon data were obtained only for those E4’s and 4-5’s where primordial Ne might be visible above the spallogenic Ne contribution. Blanks have been subtracted from all samples, with an error equal to the correction compounded into the errors for the 3 heavier gases. For He and Ne the blanks varied by ~20% and <30%, respectively, so % of the effect of blank subtraction for He and % for Ne was included in the stated errors. Nominal errors associated with the absolute sensitivities of the spectrometers are: glass spectrometer rlO% (Ne, Xe), ?20% (Ar, Kr); metal spectrometer ?lO% (He. Ne, Ar, Xe), *IS% (Kr); but the actual errors were larger in some cases, as discussed in the next paragraph. Analysis of an extended set of samples run on the metal spectrometer showed that there is a problem with some of the Xe data. Of 19 cases where there was an independent way to estimate the 13’Xe or rZ9Xe,content (expected Ar/ Xe ratio, redundant samples), 7 showed Xe contents too low by 1.5X to 3X. In just a few cases Ar was low as well.

Sedmebtotux

<4P

Ylf 5.8%

4-15p

Ylm

30.4%

3M HCI

FIG. 1. Separation schemes for E6 chondrites. Percentage yields refer to the bulk sample =lOO%. For Yilmia various chemical treatments and size separations were used to prepare enstatite-rich samples (7 1 fractions). Grain size is indicated by f = fine, m = medium, and c = coarse. Chemical treatment is indicated by L = HCI, H = HNO,, and F = HF and HCI.

2353

Noble gases in E-chondrites

3M HCI

3M HCI Freeze,/Thau

6M HCI FrtezqlThaw

FIG. 2. Separation schemes and yields for E3 to E4-5 chondrites. Qingxhen, Saint Sauveur, and Abee samples were initially disaggmga ted with HCI to produce natural grain size separates. Samples analyzed for noble gases are. outlined by heavy boxes.

We have excluded the samples where Ar was too low, and have listed the Xe data as lower limits in the cases where there was clearly a discrepancy. Data for which there is no check on the Xe content must be regarded as suspect, and have been listed in italics. Analyses of the Xe standard for the metal spectrometer agreed to within +5% over a period of several months, and background peaks were relatively constant from day to day, so we believe that the problem is associated with retrapping or incomplete extraction of Xe during sample heating. 2.2 Sample preparation Sample preparation and yields are shown for E6’s in Fig. 1 and for E3’s to E4-5’s in Fig. 2. Samples were examined by SEM/EDX to confirm solubilities of minerals (Table 1) and concentration of minerals according to density, and to provide size ranges for the sedimentation separates. Further details are given below. Density separations. These separations were performed on ~100 p powders, using combinations of bromoform (p = 2.87 g/cm’), methylene iodide (3.32 g/cm3), and acctone. E-chondrite minerals fall into the following density ranges: ~2.4 g/cm3-graphite, tridymite, cristobalite; 2.42.87 g/cm’-feldspar, oldhamite, quartz, roedderite, sinoite; 2.87-3.06 g/cm3-insures removal of sinoite (p = 2.87 9/ cm’) from enstatite-rich fractions; 3.06-3.32 g/cm’-enstatite; >3.32 g/cm’-FeNi metal, troilite, niningerite, alabandite, sphalerite, djerfisherite, daub&elite, schreibersite, cohenite, perry&, osbomite. Enstatite-rich fractions are designated by y. HF/HCl-insoluble residues. 10 g of Abee and I .5 g of Indarch were treated at room temperature with a 1: 1 mixture of 5 M HF and 1 M HCl. (The Abee sample was worked up separately, by H. Takahashi). Subsequent treatments with 3-6 M HCI (room temperature for Abee, up to 50°C for March) and CS2 were used to remove precipitated fluorides and elemental sulfur. The Abee sample came from the same 56 g piece as those ofSrinivasan et al. (1977, 1978). HNO,etched samples. All of these samples (denoted by H) were treated with concentrated HNO, for 4-5.5 hr at 80°C. We used this treatment not only for demineralized HF, HCl-resistant residues, but also for silicate-rich fractions

(e.g. Abee bulk, Yilmia ylcL), in an attempt to remove planetary gases without affecting the subsolar gases. Nomenclature. Sample Designation

Treatment Chemical:

HF/HCl HCI HN03

F L H

Sedimentation:

colloid fine medium coarse

co1 f m C

Disaggregated samples. Chips of Qingzhen up to 5 mm in size completely disintegrated upon ultrasonication in 3 M HCl, overnight at 45°C. This material was separated by density and size. Chondrules and their fragments were concentrated in the fraction > 100 ~c; however, the low yield (3.4%) compared to a visual estimate of chondrule abundance (lo- 15%, Wang and Xie, 1977) suggests that the combination of HCI and ultrasonication probably broke up some chondrules. Harsher techniques were required to disaggregate Abee and Saint Sauveur, and even those were only partially successful. Chips several millimeters in size were treated with 3-6 M HCI for several days to dissolve exposed metal and sulfide, during which a small amount of fine material was generated. Next, the samples (still in HCl) were cycled between - 196’C and an ultrasonic bath at 60°C which produced additional fine material. As the process slowed down, the larger clumps were gently broken up with a stirring rod to expose interior metal and sulfide. After several hundred cycles further progress was extremely slow, and so cycling was stopped, even though small clumps remained. The density and size separations were performed at this point. The finer-grained fractions (Abee f, m; Saint Sauveur pcol, rf, ym) consisted mainly of individual crystals, with some enstatite crystals showing lath-like shapes. For Saint Sauveur, only these fine-grained fractions were analyzed for noble gases, but unfortunately, such tractions account for only l/4 of the HCl-insoluble material. The Abee samples

YIlMlC Bulk' ”

$2

vf UlC

VlCL TlCH vlcF ylcF' YlCLf *lmL ylmH NORTH WEST Nt

2'14 4.x 25 3.6 34 4.9 45 4i 43 34 44 1.4 44 53

,100 1,n 10.3 1.0 20.9 1.0 ~ 52.7 48.1 ~ 47.4 42.7 : 24.5 0.71 29.9 1 29.5

FORREST 79

39.3 4.72 f.07 74.0 ~4.8731.036 5.6t.c IZ.39 t.23 38.3 6.0 34.5 53.3 58.2 i 60.3 56.6 62.6 / 116 1 33.1 ; 34.0

6510 ~21000 ' 1450o?z‘mo 2000 4802 zzo 2700' 900 3070

11.4 55 2.7~1.0 lB.if3.7 3.B -7 la.5 16.R

4.X

I

I 5100 / 12.B1.5 6.0 NLcs 3.9 / 282.7 , 96 26 f 4 4.37 1.17 ~ ,‘::: NLcyH 16 5.222r.037 /~3100016000 273Oi 390 1 ;:.4’6 *Italicizedvaluesmaybeiowdueto exoer,menta* difficulties.

+Crabb

and Amiers

68

44.5

'5.17

f.08

3. RESULTS

Ar, Kr, and Xe data for Yilmia and North West Forrest (E6) fractions are given in Table 2. The noble gas data for separates of E4 and E4-5 chondrites (Qingzhen, Indarch, Ake, and Saint Sauveur) are given in Tables 3, 4, and 5. The Xe isotopic composition for Yilmia fraction y 1c is also included in Table 5. It is typical of other enstatite-rich fractions of Yilmia (e.g. -rlcH, ylcLF’). Xe isotopic data for all of the Yilmia fractions are available from the authors upon request. Abee residue. The Abee HF/HCI-resistant residue (sample F, Tables 3, 4, and 5) contains 43% of the primordial Xe. This value is comparable to yields for other low-petrologic-type chondrites of all classes (Alaerts et al., 1979a,b; Moniot, 1980; Matsuda et al., 1980; CA), but is substantially higher than yields for Abee reported in two other studies. Srinivasan et al. (1977) found that an HF/HCl residue preTABLE

QINGZHEN &ilk BCOl

;

(mg)

bulk

4? 88 36

100 10.3 a.6 30.7 3.8

":.a, 0.96

'","., 0.3

(E4-5*)

4.2 4.5

3.13 z1.2 :1.4 1.12r.42 :1.2

21 ia

NOBLE

GASES

4. DISCUSSION

4.1 The subsolar component We have investigated the siting of subsolar gases in both E6 and E4 chondrites. E6’s have the advan-

IN SEPARATES

OF E3

to

0.878 0.724 0.768 0.922 0.934

t.016 t.0u f.019 f.OlB *.0z1

0.859 *.021 0.0533?.0027 0.0422'.0024

0.86 3.7 21.9 55.2

SAINT SAlJYEUE4(E4-5 100 Bulk 9.7 17 EC01 5.9 1.0 uf 15 2.6 Ym

3.

pared under harsher conditions (more concentrated acids, higher T) contained only about 3% of the xenon, whereas Wacker and Marti (1982)found only 15% of the xenon in a fraction treated with CuCl* followed by concentrated HF. The differences among these three samples may reflect procedural differences--in the case of Srinivasan et al. ( 1977). other meteorites processed together with Abee are known to have lost gas due to overly harsh treatment and loss of tine material (cf: Omans 1C 1 vs. IC2, Alaerts et al., 1979b). Apparently the treatment of Wacker and Marti also causes some loss of the gas carrier. In view of the relatively high recovery of xenon for three E-chondrites demineralized under comparatively mild conditions (Abee, this work; Indarch and Yilmia, CA), we think that the usual “Phase Q” gas carrier (Lewis et al., 1975) accounts for a large part of the planetary noble gases in E-chondrites, rather than a silicate carrier as proposed by Wacker and Marti ( 1982). This view is further supported by the strong enhancement of noble gases in the fine fractions of Qingzhen (Table 3), just as expected for a Q-type carrier.

E4-5

CHONDRITES 36Ar+

(E4)

Fy FH ABEE F ml

% of

(E3) 198

uf YC chond 'N~PK~

wt.

36.9

2.06 t.04 1.92at.olh 2.072+.039

(19kl).

m and c covered a wider size range (2- 150 p vs. i 10 p) and a larger weight fraction (VI of the recovered HCI-insoluble fraction), but, judging from optical microscopy, only about half of the grains of Abee c were single crystals.

Sample 1'

3.25 '.ii 4.22 4.1.1 2.088d.oso 2.076*.525 6.16 +.m 6.91 3.99 6.98 i.10 h.87 + 1J i1.10 1.35 1.36 +.07 6.02 ..i: 0.13 '.j('

0.8 0.80 0.91 0.92

t.3 2.18 ?.0B f.08

0.869 0.866

2.017 *.0x

a.79 43.3

20.8 2.99 1.49?.11 6.28 540. '110 83. + 17

10.5 11.0 f1.4 14.9 fl.O 13.8 ~1.6

T

4.79ot.044 5.230~.044 5.1m.042 4.792*.015 5.89 2.13

3140 2250 2040 970 a40+

140

23.6 52.6 24.3 3.21 O.Ot0.6

2355

Noble gases in E-chondrites TABLE 4. Kr IN W/W

RESIOUES

.62?f.O11 4.030f.038 20.39-i.1420.32t.1z 31.07*.1s

~

.615f.o1o 4.235r.033 20.19t.12 20.15+.10 30.93t.14

that they are relatively rich in subsolar gases and their minerals can be separated more cleanly due to their coarse grain size. However, the extensive thermal metamorphism of E6’s makes it doubtful that the subsolar gases are still in their original host. E4 chondrites, on the other hand, are more likely to reflect the original, pre-metamorphic distribution of gases, but unfortunately, they have generally low concentrations of subsolar gases. Host phase in E6 ‘s. Having previously shown (CA) that the most likely carriers of subsolar gases are enstatite, silica polymorphs, and sinoite, we separated these minerals by density. In Yilmia fractions (Fig. 3, Table 2) the subsolar component-typified by 36Ar-strongly coheres with enstatite. 36Ar is enhanced 1.4~ in the main fraction with density between 3.04 and 3.3 g/cm3 (ylc), where enstatite is the only phase detectably enriched. In contrast, the density fractions (72, (Y,/I) that are enriched in SiOz and presumably sinoite (indistinguishable from SiOZ by SEM/EDX), carry only a small part of the subsolar gases (l-2%). Two fine-grained fractions (a and y 1cLf) are high in 36Ar, but this is due to enrichment of planetary gases, judging from their low 36Ar/‘32Xe ratios of 210 and 430 (Table 2). Indeed, enstatite can quantitatively account for the subsolar Ar in Yilmia: 56% MgSi03 (Buseck and Holdsworth, 1972) with an 36Ar concentration equal to fraction y 1cH (>90% MgSi03) would yield a bulk 36Ar concentration of 34 X 10e8 cc/g, which is close to the measured value of 38 x 10e8 cc/g. The separates of North West Forrest duplicate the trend for Yilmia: enhancement of 36Arin an enstatite-rich fraction (NLcyH) and depletion in a low density fraction (NLcP; Table 2).

tages

TABLE 5. Sample

1=X,

l=Xe

lz6Xe

There is a loophole in this argument. Rather than enstatite itself, the gas-carrier could conceivably be a minor phase tightly associated with enstatite, and so not separated from it by density. To test this possibility we performed several etching experiments. In the first, we used HF to etch two portions of an enstatite-rich fraction (Yilmia y 1CL) until - 10% and -50% had dissolved. This should show a change in gas concentrationif the gases are unevenly distributed (e.g., mainly at the surface or in small grains) or if they are in a mineral that dissolves at a different rate than does enstatite. (An example of such preferential solubility is feldspar, cf low 40Ar in y 1cLF compared to y 1CL). But we find no significant difference in 36Ar concentration upon HF-treatment (y 1cLF and y 1cLF’ have 56 and 62 X lo-’ cc/g compared to 58 X 10m8 cc/g for their parent sample ylcL). Thus, the hypothetical minor phase must be fairly evenly distrib uted in enstatite and as readily soluble in HF. As the hypothetical minor phase showed no differential solubility in HF, we next tried two acids that do not attack enstatite: HCl and HNO3. In fact, etching a coarse fraction (y lc, 20- 100 CL)showed no decrease in gas content (7 1cL and y 1cH contain 72% and 74% of the total 36Ar vs. 73% for y lc). Thus, any inclusions would have to be either so small that few are exposed (~5 p), or insoluble in these acids. Etching of an enstatite-rich fraction of smaller grain size (y 1m, 4- 15 CL)gave ambiguous results. The gas contents of y 1mL and y 1mH are virtually identical, and 0.5X those of the corresponding coarse fractions (e.g. y 1CL). However, as we have no usable Ar data for the untreated parent sample ylm (its Ar content was too low due to experimental difficulties; Sec. 2.1), we cannot tell whether the low gas content is intrinsic (e.g. due to crushing) or caused by acid treatment). Also, the medium fraction is not strictly equivalent to the coarse fraction, although both are predominantly enstatite-about half of fraction y 1m was fines removed from the coarse fraction (Fig. 1). A better test for inclusions of an acid-soluble gas-

Xe IN E-CHONORITE SEPARATES 128X.

1Z9X,

lo-1°cm3/g QINGZHEN

;;J:

::.'

Yf Yt chand

25 I%

0.406? 0.460* 0.463+ 0.448?

.oos ,009 .010 .OIS

INDARCH

bulk* F FH

8.7 740 86*12

0.443+ .010 0.478t ,009 0.589t .027

ABEE

bulk' F m c H

10.0 500 14s 10.9 7.4

0.473t 0.4705 0.480t 0.469? 0.490+

SAINT SAUVEUR

YILMIA

lCrsbb

and +srin1vasen

bulk BCOI Yf Yrn

4.7 ::i 4.3

YlC

4.8

hders (1981) .Pt 111. (1978)

130X,

l=Xe

134Xe

136Xe

132x, I 100 0.370+ 0.410' 0.406t 0.410t

.606 .a08 .011 .o14

0.405f ,012 0.421 f ,007 0.418+ ,030

7.71 f 8.15t B.ZO? 0.49? 10.09?

.06 155.0t 0.5 .07 135.3~ 0.6 .07 157.0* 0.6 .o9 375.2* 3.6 .z' 1520 uo.

80.2 81.5 81.8 81.6 81.3

f t t f f

.4 ., .4 .6 .9

38.13t 38.39t 38.422 3e.29t 38.B4t

.1? 32.74+ .13 .17 32.37? .1s .1e 32.33* .PO .29 32.21 f .25 .17 32.851 .35

38.46~ .30 38.532 .I4 45.0 f .B

32.33t .21 32.62i .13 42.7 +LZ

16.49% 16.24* 16.3Bt 16.11* 16.04'

.13 .1o .16 .14 .17

82.965 81.8 f 81.9 f 82.2 t 82.5 f

37.77* 37.82* 38.07* 38.07t 37.82t

31,2J+ 31.7Bi 31.91? 31.64t 31.64t

0.4712 .018 0.436? .oz9

8.65t .Jl 1171 t1, 8.68~ .16 1572 t3o 8.29t.11 784 tee a.gzt .22 1280 *7o

15.92t 15.96? 15.91? 16.10?

.16 .20 .31 .21

80.7 80.2 80.2 80.8

0.463* ,012

9.20+ .09

16.27r .11 82.2 2 .6

.oo6 .*o7

,013

0.421t 0.416+ 0.424i ,014 0.429+ .OIS 0.429*

.008

.o14 .011

0.431+ .011 0.416t .012 0.446t .015 0.42Oi .016

0.423t .o12

8.71+ B.32* 8.555 B.81i B.79?

351.4? 2.8 123.55 0.8 134.4t 3.8

.08 .12 .10 .~s .2B

16.27t .13 81.5 f .6 16.20? .I2 81.6 2 .4 16.2Bt .18 82.0 f .6

.o13 .OII

B.24? .13 8.302.05 8.56t .09

15.66* 16.152 16.19t 16.03* 16.06*

.13

586.05 5.0 128.4% 1.1 680 960. .07 783 f60. .07 687 f46.

.05 .Jl

616

t B

.25 .# .5 .4 .s

t .5 f .7 i12 f .,

38.11' 38.2Of 38.62i 38.1Ot

.22 .16 .36 .J7

.22 .28 .38 .5J

.42

37.73* .2o

32.082 32.29i 32.27* 32.25*

.1e .15 .54 .73 .2o .a .32

.a .38

31.28* .1e

2356

.I. Crabb and E. Anders

rf, and ym, respectively (Fig. 3), while in Abee j6Ar is 30 X 1Om8cc/g in m vs. 40 X 1Om8cc/g in c (Table 3). Thus, if anything, 36Ar is slightly depleted in the fine-grained fractions. If there ever was a dependence on grain size (e.g. as expected from adsorption, next section), it apparently has been obscured by metamorphism even in E4-5’s.

Bulk Q

0 Y2 s 1.0

Yf

3.06-3.3

/YlC

; ylcL

--i--

I ylcH

47 1

TY

I YlCF

53 I 46 I

HCI

43 I

,I,-

Bulk

loo

PC01

IO

Yf

I.0

Ym

2.6 0

5

IO

15

0

From their more detailed study of Abee clasts and separates, Wacker and Marti (1982)suggest enstatite as a likely carrier of subsolar gases (their “Ar-rich” component) in Ahee, in general agreement with our results. However, enstatite cannot he the only carrier, since their HCl- and HN03-treated separates contain only about half of the total 36Ar.

50

100

FIG. 3. In Yilmia over 70% of the 36Ar (indicative of the subsolar component) is concentrated in the main enstatiterich fraction (ylc) of density 3.06-3.3 g/cm’. The Ar content of enstatite-rich fractions (enclosed by dashed lines) is unaffected by etching with HCl and HNO,, which dissolves - 10% of non-silicate material (samples y 1CL, y IcH). Nor does the Ar concentration change as up to half of the enstatite is dissolved by HF (y 1cF, y 1cF’). Evidently enstatite, or less likely, a closely associated phase, is the host for the subsolar component in E6’s. The high 36Ar contents of two fine-grained fractions (a and y1cLf) reflect enhancement of planetary rather than subsolar gases. HCI-disaggmgated fractions of Saint Sauveur (E4-5) show little dependence of Ar concentration on grain size. This

may mean that the subsolar gases have been redistributed by metamorphism, even in the lower petrologic types.

carrier would require repeating the etching experiment on a fine-grained fraction, with an analysis of the untreated fraction. If the acid treatment caused the Ar loss of the medium fraction, by dissolving Arrich inclusions, then the inclusions can at most make up 1% of the meteorite. But such inclusions would still have to meet the requirements set by our etching experiments on the coarse fraction: they must be fairly evenly distributed in enstatite and at least as soluble in HF, and either small (t5 p) or resistant to HCl and HN03. Siting in E4-5 ‘s. The purpose of the Abee and Saint Sauveur separates was to look for a grain size dependence of the subsolar gases (cf enrichment of planetary gases with decreasing grain size in Qingzhen, Tables 3 and 5). In fact, we find little variation in “Ar concentration among these HCl-treated size separates. In Saint Sauveur 36Ar is 1 I X 10e8 cc/g, 15 X lOma cc/g and 14 X 1OM8cc/g in fractions &ol,

Origin of subsolar component. We must try to explain the unusual properties of the subsolar component: an elemental pattern of Ar, Kr, and Xe intermediate between those of solar and planetary components, and a variable and often high abundance (subsolar 36Ar ranges from ~1 up to 750 X lo-* cc/g, with no obvious correlation with other properties such as petrologic type; CA). If this component is ultimately derived from solar gas, then two types of process must be considered: non-selective trapping, followed by a separate fractionation step that depletes the lighter gases, and selective trapping, which fractionates gases as they are trapped. All non-selective processes run into two problems, one general, one specific. First, the large loss of lighter gases must inevitably lead to large isotopic fractionation of Ar: a IO-fold decrease in Ar/Xe ratio, as for South Oman, should lower ‘6Ar/38Ar to about 5.0, yet samples rich in subsolar Ar fall near the solar wind ratio of 5.33 (Eberhardt et al., 1972), e.g., 5.46 -+ .04 in South Oman bulk and 5.22 rt .04 in North West Forrest NLcyH (CA; Table 2). Second, every specific mechanism for non-selective trapping has additional difficulties of its own. For example, if the solar gas was somehow trapped by occlusion in voids, then very high pressures would be required for the observed gas concentrations (> 100 atm for a solar Ar/H* ratio). Or, if it was trapped by solar wind irradiation in a regolith, as observed in other meteorite classes, then it is necessary to explain many differences: low (He, Ne)/Ar ratios (e.g. 20Ne/36Ar 5 0.003 in South Oman vs. 30-40 in solar wind; Marti et al., 1972) absence of regolith features (tracks, agglutinates, microcraters), lack of surface concentration, and the unbrecciated textures of most E-chondrites. Solar wind irradiation before accretion (Wacker and Marti, 1982) is still less satisfactory, as the solar wind cannot penetrate the nebula while gas is still present (the range of solar wind ions is
2351

Noble gases in Echondrites

trapping mechanisms are available: solubility and adsorption, corresponding to equilibrium distribution of the gas in the interior or on the surface of the solid. Solubility apparently cannot explain the data. The volubility of Ar in an enstatite melt at 1500°C (Kirsten, 1968) corresponds to a distribution coefficient, Kh,, of 2 x 10s5 cc/g-atm, so to explain the most Arrich E-chondrite (South Oman, 36Ar = 760 X lo-* cc/g; CA) one needs PA, = 0.4 atm-some 10% higher than the value in the solar nebula, at 10m3atm total pressure (Cameron, 1982). Higher pressures may be available in gaseous protoplanets (DcCampli and Cameron, 1979), but not by a factor of 10’. Moreover, enstatite melts give the wrong fractionation pattern, raising rather than lowering Ar/Xe relative to the reservoir. There is no obvious way to resolve these differences. Solid enstatite should be even worse at 1500°C as noble gases, like other incompatible elements, should preferentially enter the melt rather than the solid. Lower temperatures would help, but by only -lo’-10*X, judging from typical heats of solution (-5 to - 10 k&/mole, Gmelin, 1970). Thus one must appeal to the unknown-either a trace mineral or a special form of enstatite of very high capacity for noble gases and the right fractionation pattern. The latter is not so far-fetched: Kirsten’s enstatite contained significant FeG, and it is conceivable that meteoritic enstatite, having formed under much more reducing conditions, has a much greater capacity for noble gases (e.g. due to a greater abundance of anion vacancies). Adsorption looks somewhat more promising. No laboratory data are available for enstatite, but this may not matter, as recent studies show that a variety of solids-carbon, chromite, magnetite, Fe&Jr-sulfides, etc.-give essentially similar fractionation patterns and distribution coefficients, with temperature and surface area as the main variables (Yang et al., 1982; Yang and Anders, 1982a,b). Let us use carbon as a stand-in, since it has been studied in greatest detail (Chackett and Tuck, 1957; Cole et al., 1974). The Ar/Kr/Xe fractionation of South Oman is roughly matched by carbon at 400-500 K (Fig. 6 of Yang and Anders, 1982b). KAr for C at 400 K is 2.2 X 10-3cc m-* atm-‘, so if the gas carrier in South Oman had a similar K and a surface area of lo3 m*/ g, P*,, would have to be 3.4 X lob6 atm to account for the observed 36Ar content. In a solar gas, this corresponds to a total pressure of - 1 atm, which, though favored by some authors (Blander, 1971; Sears, 1980; but see also Laul et al., 1973; Larimer and Bartholomay, 1979), is unattainable in standard models of the solar nebula. Thus one can either invoke protoplanets or appeal to other factors that can boost gas content: surface area or a more adsorptive mineral, with a greater number of active sites and perhaps lower selectivity, allowing it to produce a subsolar pattern at lower T, where K is higher. One

‘*gxe, ( Io-‘“cc/g) 0

IO

20

30

Fmctlon of ‘%Y,

0

50

100

(%)

150 WI% 100 1.0 IO 1.0 21 1.0 48 47 0.7 100 IO 8.6 31 30

FIG. 4. In Yilmia (E6), the bulk of the rz9Xe, is in acidresistant, enstatite-rich fractions (y 1cL and -r lcH). Apparently ‘29Xenlike the subsolar gases, is either in enstatite or a closely associated phase. There is little ‘29Xe, in other phases, such as sulfides, FeNi metal, and phosphide, which are enriched in the other density fractions (u, j3, 17).The yields in excess of 100% for rlcL and rlcH may indicate loss of 129Xe,from the bulk sample during extraction, or less likely, inhomogeneity in ‘29Xe, distribution on a centimeter scale. For Qingzhen (E3) and Saint Sauveur (E4-5, not shown), a fine-grained, low density fraction (ficol) is enriched in ‘*‘Xe, relative to bulk. The chondrule-rich fraction of Qingzhen has the same concentration of ‘*‘Xe, as bulk, in contrast to its 6-fold depletion in trapped Xe. These HCl-disaggregated fractions of Qingxhen account for only half of the total 129Xe,, a proportion similar to that previously found for the E4 chondrite Indarch (CA).

or more of these variables must have varied among E-chondrites, to explain the 103-fold spread in abundance of the subsolar component within this class. 4.2 Siting of radiogenic “‘Xe Type 6. The density separates of Yilmia yield new information on the siting of ‘*‘Xe, in E6’s, despite reservations about some of the Xe data (‘32Xe and ‘*‘Xer are sometimes too low, Sec. 2.1). Figure 4 shows that the fractions with density 3.06-3.3 g/cm3 (rlcL and y 1cH) contain the bulk of the ‘*‘Xer. These samples had been etched with HCl and HN03, hence the host of ‘*‘Xe, must also be resistant to these acids. Both characteristics match those of the host of the subsolar component and so ‘*‘Xe, must also be located in enstatite or a minor phase associated with it. The other density fractions (a, 6, 72, a) each account for only a few percent of the total ‘29Xe+,suggesting that the sulfides, carbon, feldspar, metal, and phosphides concentrated in these fractions are at most minor hosts for ‘*‘Xq. Type 3 to 4-5. In contrast to E6’s, the lower pet-

2358

J. Crabb and k. Anders

rologic types retain only half of their lz9Xe, upon etching with HN03 or HCI. Abee H contains 49% of the bulk ‘29Xer (Table 5) and Qingzhen HCl-disaggregated fractions, 5 1% (Fig. 4). Indarch shows similar behavior (CA). Apparently, there exist additional, acid-soluble sites for ‘29Xer in these types. A new feature is the distribution of ‘29Xer among the acidresistant fractions: for both Qingzhen and Saint Sauveur the low density/colloidal fractions (&ol) are enriched 2-fold in ‘29Xercompared to coarser, enstatite-rich fractions, and account for up to half of the HCl-insoluble ‘29Xe At the other end of the size range, Qingzhen ch&drules also have appreciable ‘29Xer concentrations (Fig. 4, Table 5). despite their 6-fold depletion in trapped Xe. In this respect they resemble chondrules from other primitive meteorites (Merrihue, 1966; Rowe, 1968). (As mentioned in Sec. 2.2, many chondrules broke up during the disaggregation, and so the actual fraction of ‘29Xer in chondrules is substantially larger than the 4% for Qingzhen chond.). Wacker and Marti (1982) found through stepwise heating that at least two carriers of ‘29Xer remain in Abee after treatment with HCl or HN03. They suggest enstatite as the carrier of the major, high temperature component, and plagioclase as the carrier of the low-T component. In support of the latter suggestion they cite a rough correlation between ‘@Ar and ‘29Xe,. We also see rough correlations between 40Ar and ‘29Xer for Abee, Saint Sauveur, and Qingzhen, but they are only qualitative, not quantitative. In particular, Abee fractions m, c, and H have 28,000, 12,000, and 11,400 X 1Om8cc/g of ‘k’Ar and 8 1, 74, and 43 X lo-” cc/g of ‘29Xer,respectively. A feldsparrich Indarch fraction of Schaeffer et al. (1965) actually had only about one-tenth the ‘29XeJ40Ar ratio of other density fractions of that meteorite. Thus, feldspar does not appear likely to contain much ‘29Xer.We do agree that enstatite is probably the high temperature carrier, as in E6’s. A potentially useful clue is that fractions with high ‘29Xe/‘32Xe ratios also have high “Arf3’Ar ratios (5.94 in Saint Sauveur /3col, 5.80 in Qingzhen chond.), presumably due to neutron capture by Cl during the cosmic-ray era. This observation suggests that it may be possible to use the more abundant chlorine in future work to locate at least some of the hosts for ‘29Xer in E4’s. 4.3. Primordial Ne and CCF-Xe Primordial Ne and CCF-Xe are discussed together, as they often occur together in primitive chondrites (Lewis et al., 1975, 1977; Alaerts and Lewis, 1978; Ott et af., 1981). Primordial Ne has been found in many E4 and E5 chondrites, in contrast to other chondrite groups, where it is detectable in bulk samples only in petrologic type 3 or lower (Bhringer, 1968; Heymann and Mazor, 1968; CA). Recent work has shown that,

like Ne-A of other chondrite classes (Alaerts 6’1ui.. 1980) the primordial Ne in E4’s is concentrated in the small fraction of the meteorite resistant to F-IF and HCl, and has a similar isotopic composition (20Ne/22Ne -8 t I; Srinivasan ct al., 1977: CA). We have taken the comparison a step farther; if the Ne in E-chondrites is in fact typical Ne-A in a similar host, then it should remain upon etching with HNO,. This is indeed observed for an Indarch (E4) residue (sample FH VS.F. Table 5). The 20Ne/22Ne ratio of sample FH (7.7 1 f 0.33) is marginally lower than the value for Ne-A2 of carbonaceous chondrites (8.37 +- 0.03, Ott et al., 1981); it would be interesting to find out whether the difference is real. The etched residue consists predominantly of carbon, a major carrier of planetary Ne in other chondrites (Lewis ti/ al., 1975; Frick and Moniot, 1977; Ott ef u/., 198 1). The separates of Qingzhen show a similar enhancement of primordial Ne in fine-grained fractions, with parallel enrichment of 4He as well. Apparently any metamorphism and the solar heating that caused loss of radiogenic 4He for Qingzhen (Sec. 4.4) have not completely driven off primordial He, although the 4He/20Ne, ratios (- 120 in @co1 and rf, Table 3) are lower than those of Cl and c‘?. chondrites (-350 and -220, Mazor et al., 1970). The Indarch etched residue FH (Table 5) also provides new information about CCF-Xe in E-chondrites. Bulk analyses of several E-chondrites show ‘36Xe/‘32Xe ratios slightly above estimates for planetary xenon (0.3 10 extrapolated by Lewis et al.. 1975; 0.3123 f 0.0007 measured in the Kenna ureilite, Wilkening and Marti, 1976) suggesting that CCF-Xe is present in E-chondrites (Krummenacher et ul., 1962; Marti et al., 1966; Srinivasan et al., 1978: CA). But Indarch FH gives stronger and more direct evi-

A lndarch FH l

Allende

3C2

1.4-

b-

x” 5 %i v, \

-1 u 1.2n.

E c=:

_

+ 6

1.0

:

------+-A i

124

126

128

130 I31 132

134

136

Mass Number FIG. 5. Xe isotope pattern for an Indarch (E4) etched residue, normalized to solar Xe (Pepin and Phinney, 1982). The pattern resembles that of carbonaceous chondrites, typified here by Allende 3C2 (Lewis et al., 1975).

2359

Noble gases in E-chondrites dence for CCF-Xe in E-chondrites,

and shows that this component is resolvable from planetary Xe by the same HNQ etch treatment that proved successful for other primitive chondrites (Lewis et al., 1975). Sample FH shows the striking enrichment of heavy Xe isotopes characteristic of CCF-Xe (e.g., i3’jXe/ “2Xe _ 1.4X the trapped value, Fig. 5), accompanied by other isotopic effects usually associated with it (Lewis et al., 1975): enrichment of 124Xe and other light Xe isotopes (=L-Xe), small enhancements of the heavy isotopes of Kr and Ar, and Ne-A (Tables 3, 4, and 5). Thus, whatever the origin of CCF-Xe and related anomalies, they are widespread, so far extending to E, C, H, and LL-chondrites (Fig. 5; Lewis et al., 1975; Alaerts et al., 1979a,b; Moniot, 1980). It is interesting to compare anomalous Xe from Indarch with that from other classes. Figure 5 shows that the Xe isotopic pattern of Indarch FH generally parallels that of carbonaceous chondrites, with only marginal discrepancies at isotopes 128 and 130. The agreement can be seen in detail on a plot of ‘24Xe/ ‘32Xe vs. 136Xe/132Xe(Fig. 6), where I&arch FH falls within error of the trend line for carbonaceous chondrites. This agreement is interesting, as the enrichments of ‘24Xe and 136Xere5ect L-Xe and CCF-Xe, respectively, which are thought to have different origins (Pepin and Phinney, 1982; Matsuda et al., 1980). Ordinary chondrites, in fact, do not always

I

0.35

I

I

I

0.40 136Xe/132Xe

I

1

I

,

I

0.45

FIG. 7. The Indarch etched residue (open star) fits into the trend between planetary Ne and CCF-Xe defined by carbonaceous chondrites (circles). Taken at face value, this observation suggests both that Indarch has lost little Ne during metamorphism, and that Ne apparently is a steady companion of CCF-Xe-GXe, in E as well as C-chondrites.

have the same ratio of L-Xe to CCF-Xe as carbonaceous chondrites (Fig. 6; Alaerts et al., 1979a;

Moniot, 1980). The similarity between Indarch FH and carbonaceous chondrite etched residues extends even farther; the Ne/CCF-Xe ratio of Indarch FH fits right into the correlation between these two components previously noted for carbonaceous chondrites (Fig. 7; Lewis et al., 1975, 1977; Alaetts and Lewis, 1978; Ott et al., 1981). The correlations in Figs. 6 and 7 simply represent mixing lines between planetary gas in the lower left and a mixture of anomalous gases (with a constant Ne-A/L-Xe/CCF-Xe ratio) in the upper right. The similarity of the Ne-A/L-Xe/CCFXe ratios in two such different classes as E and Cchondrites, which probably formed rather far apart and under quite different conditions, poses a problem for any model where the anomalous gases are mixed in situ (e.g., local trapping of mass fractionated gas to produce L-Xe and Ne-A, combined with in situ Cl C2 C3C C3V LL3 H3 E4 fission of a superheavy element to produce CCF-Xe; Unelcted 0 . A v W + * Anders et al., 1975; Lewis et al., 1977). On the other Etched 0 0 n v 0 0 b hand, if the gases were mixed at a pre-solar stage into 0.35 Xe136,Xe13z 0.40 0.45 a carrier that found its way into a significant portion FIG. 6. Correlation between heavy (CCF-Xe) and light of the solar system (Black, 1975; Dziczkaniec and Heymann, 1980; Ott et al., 1981), it is not obvious (LXe) Xe isotopic anomalies, represented by ‘36Xe and lz4Xe. An Indarch etched residue (open star) falls within 1TV why the L-Xe/CCF-Xe ratio sometimes varies (e.g. of the trend line for these components in carbonaceous H or LL-chondrites vs. C-chondrites, Fig. 6). Thus, chondrites (line labelled Allende; line and data points from neither a local nor a presolar origin for the anomalous Matsuda et al., 1980 and references therein). This obsernoble gases is without problems. vation implies that the ratio of CCF-Xe to L-Xe is similar for C and E-chondrites, in contrast to the more variable The agreement of the Indarch Ne-A/CCF-Xe ratio with ratios for ordinary chondrites. that for carbonaceous chondrites raises a question regarding

2360

J. Crabb and E, Anders

the noble gases in South Oman. A bulk sample of this me(CA), which for the Ne-A/CCF-Xe ratio of Indarch and a ‘36Xe/‘32Xe ratio of 0.310 for trapped Xe would predict a ‘36Xe/‘32Xe ratio of 0.315. Yet the measured 136Xe/‘32Xe ratio is 0.3095 + 0.0020, in agreement with the estimates for planetary Xe. Perhaps the Ne is from another source, P.R. the subsolar component or atmospheric contamination (cf: Wacker and Marti. 1982), though at least the latter seems unlikely since Indarch, Saint Sauveur, and Bethune all have 10.3 x 10-s cc/g of *‘Ne attributable to air. Alternatively, the Ne-A/ CCF-Xe ratio may vary among E-chondrites, or the trapped Xe in South Oman has a lower ‘36Xe/‘32Xe ratio (closer to solar, in keeping with its subsolar Ar/Xe ratio). The above alternatives could be distinguished by looking for Ne-A and CCF-Xe in an etched residue of South Oman.

teorite has (2.2 f 0.2) X IO-* cc/g of %e,

4.4. Qingzhen (E3)

As the only known E3 chondrite, this meteorite is of particular interest. Unfortunately, it became available only near the end of this study, and its reclassification from “Type A” to “E3” became known only afterwards. Data for a bulk sample and HCldisaggregated size separates of Qingzhen are given in Tables 3 and 5. Ages. The gas retention ages for Qingzhen are low: the U,Th-He age is 1.1 AE (on the assumption of 9.9 ppb U, 33 ppb Th; average for E4’s and E5’s from CA) and the K-Ar age is 3.2 AE (for 910 ppm K, Wang and Xie, 1977). However, judging from the very low 3He/2’Ne ratio (1.07) 70-80s of the 3He was lost during the cosmic-ray exposure era, and by analogy to other chondrites with such low ratios, radiogenic gases should have been lost as well. Thus the low gas retention ages do not necessarily imply late reheating in the meteorite parent body. Also, 4He appears to contain a contribution from primordial He (Sec. 4.3) so the 4He age is only an upper limit. The nominal *‘Ne exposure age for Qingzhen is 11.4 Myr, for a production rate of 0.252 X lo-* cc/ g-Myr (chemical analysis of Wang and Xie, 1977, production rate equation of Cressy and Bogard, 1976, renormalized to the revised production rate of Nishiizumi et al., 1980). Though the very low 3He/2’Ne ratio of 1.07 implies that at least 3He was lost by solar heating during the exposure era, such loss for *‘Ne is expected to be slight, as Huneke et al. ( 1969) found that pyroxene, the main carrier of spallogenic Ne in Qingzhen, is fairly retentive of *‘Ne (the diffusion parameter for *‘Ne is lo*-103X smaller than that for 3He). Qingzhen shows the largest effect of solar heating of any E-chondrite-its 3He/2’Ne ratio is approached only by that of Bethune (E4-5, ‘He/*‘Ne - 1.8, CA), but in that case part of the ‘He loss may be due to extensive weathering. The exposure age for Qingzhen falls between those of Abee (- 10 Myr) and Indarch (16 Myr). Thus Qingzhen reinforces the earlier finding that E-chondrites of petrologic type 5 or lower have lower exposure ages than E6’s: 7 out of 9 E~‘s-E~‘s fall below 20 Myr, whereas only 1 out of 9 E6’s does (CA). Such a difference

may indicate

different

sources

for the

various petrologic types, or a single parent body that is still stratified on the scale of individual impacts (production of E6’s may be dominated by a single event -35 Myr ago; CA). Primordial gases. Despite its classification as E3, Qingzhen resembles Indarch (E4, Tables 3, 5). The primordial gases are present in similar amounts (including *‘Ne, at 1.2 +- 0.6 X 1O-’ cc/g), and there is little or no contribution from the subsolar component (e.g. 3hAr/“2Xe = 83 in sample pcol). The bulk sample is heavily contaminated with atmospheric Xe (cf: low 36Ar/‘32Xe and ‘3”Xe/‘32Xe ratios), which may reflect the unusually friable nature of this meteorite. The 13*Xecontent corrected for such contamination is 10 X 10-l’ cc/g. Thus one of the peculiar features of E-chondrites carries over to the first E3-in contrast to other chon-

drite classes, there is no strong correlation of noble gases with petrologic type (CA). Though an E3, Qingzhen does not have an exceptionally high Ne content or Ne/Ar ratio; on the contrary, several E4’s and E45’s surpass it in these respects. Acknowledgments-We thank H. Takahashi for preparing the Abee residue, and K. Marti for a thorough review. P. Pellas and Ouyang Ziyuan kindly donated meteorite samples. This work was supported in part by NASA Grant NGL 14-001-010.

REFERENCES Alaerts L. and Lewis R. S. (1978) Noble gases in meteoritic gas-rich minerals: Some implications for the formation of the solar system. In Protostars and Plunets (xl. T. Gehrels), pp. 439-449. Univ. Arizona Press. Alaerts L., Lewis R. S., and Anders E. (1979a) Isotopic anomalies of noble gases in meteorites and their originsIII. LL-chondrites. Geochim. Cnsmochim. Acta 43,13991415. Alaerts L., Lewis R. S., and Anders E. (1979b) Isotopic anomalies of noble gases in meteorites and their originsIV. C3 (Omans) carbonaceous chondrites. Geochim. Cosmochim. Acta 43, 1421-1432. Alaerts L., Lewis R. S., Mats&a J-I., and Anders E. (1980) Isotopic anomalies of noble gases in meteorites and their origins-VI. Presolar components in the Murchison C2 chondrite. Geochim. Cosmochim. Acta 44, 189-209. Anders E., Higuchi H., Gros J., Takahashi H., and Morgan J. W. (1975) Extinct superheavy elements in the Allende meteorite. Science 190, I262- 127 1. Black D. C. (1975) Alternative hypothesis for the origin of CCF-Xe. Nature 253, 4 17-4 19. Blander M. (197 1) The constrained equilibrium theory: sulphide phases in meteorites. Geochim. Cosmochim. Acta 35,61-76. Buseck P. R. and Holdsworth E. F. ( 1972) Mineralogy and petrology of the Yilmia enstatite chondrite. Mefeoritics 7,429-447. Cameron A. G. W. (1982) Elementary and nuclidic abundances in the solar system. In Nuclear Astrophysics (eds. C. Barnes, D. D. Clayton, and D. N. Schramm), in press. Cambridge Univ. Press. Chackett K. F. and Tuck D. G. (1957) The heats of adsorption of the inert gases on charcoal at low pressure. T&s. Faraday Sot. 53, 1652-1658. Cole J. H., Everett D. H., Marshall C. T., Paniego A. R., Pow1 J. C., and Rodriguez-Reinoso F. (1974) Thermodynamics of the high temperature adsorption of some

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