Trapped noble gases in unequilibrated ordinary chondrites

Geochrmrca et Cosmochimrco Copyright 0 1990 Pergamon

0016.7037/90/$3.00 + .OO

Acta Vol. 54, pp. 2869-2882 Press pk. Pnnted in U.S.A.

Trapped noble gases in unequilibrated ordinary chondrites N. SCHELHAAS,U. OTT, and F. BEGEMANN Max-Planck-Institut ftir Chemie (Otto-Hahn-Institut), SaarstraBe23, D-6500 Maim, Germany (Received December 2 1,

1989; accepted in revised form August 3, 1990)

Abstract-The abundances and isotopic compositions of noble gases were determined in bulk samples and acid-resistant residues of eight unequilibrated (type 3.0-3.8) ordinary chondrites (two LLs; four Ls, with two from a paired fall; two Hs) including the most primitive one, Semarkona. HL-Xe was found in all cases except Dhajala (H 3.8). The H-part of HL-Xe is isotopically indistinguishable from that in Allende; the same we find true for the ratio H-Xc/L-Xe. Thus, our data do not confirm previous reports of variations of this ratio among ordinary chondrites nor do they confirm that in ordinary chondrites this ratio is any different from that in Allende. Constraints on the origin of HL-Xe as well as the trapping mechanism are discussed. Also present in all meteorites except Dhajala is an HF/HCl soluble component with a high Ar/Xe ratio. The abundances of both isotopically ordinary Xe and HL-Xe correlate with petrological subtype. Combustion of a Dhajala acid-resistant residue yielded Ne with abundance ratios 20Ne/22Ne = 10.1 + 0.2 and 2’Ne/22Ne I 0.04 for that part of the Ne (“Q-Ne”) which is associated with the bulk of the (ordinary) heavy noble gases. The noble gases released in this experiment were associated with the combustion of only a minor portion of C; the highest 13’Xe/C atom ratio of 1.3 X lo-’ was found for the first combustion step and is only 10 times lower than the solar ratio. In Semarkona (LL 3.0) there is evidence for the presence of Ne-E; in ALH A 77278 about half the Ne is of solar origin (SEP-Ne). I. INTRODUCIlON

orites analyzed so far are not unexpected simply for statistical reasons, however. Hence, because of the obvious implications for the origin and the history of the anomaly, we felt it worthwhile to analyze some more ordinary chondrites and check for the constancy of the L-Xc/H-Xe ratio. In contrast to the previous studies by ALAERTS et al. (1979a) and MONIOT (1980) we concentrated on unequilibrated (type 3) ordinary chondrites, where sufficient amounts of HL-Xe for analysis were expected to occur. We also included in our study L-chondrites, for which no previous data existed. Of special interest were the results for LL 3.0 Semarkona which in terms of its thermal history is the most primitive ordinary chondrite (GROSSMAN,1985; ANDERSand ZADNIK, 1985; but see also HUTCHISON et al., 1987, and ALEXANDERet al., 1989). As it turned out, we were also able to obtain significant data for the Ne component associated with the isotopically normal (major) part of the heavy noble gases.

BECAUSEOF THEIR LOW abundance in solid matter, the isotopic composition of noble gases is especially sensitive to the addition of foreign components. Noble gases, therefore, have always figured prominently on the list of elements with isotope abundance anomalies. This is especially true for Xe where, by virtue of its many stable isotopes, several anomalous components could be identified. Most contentious of these has undoubtedly been what used to be called CCF-Xe but which is now mostly referred to as HL-Xe (cf. ANDERS, 1988). This Xe is characterized by an overabundance of the heavy (HXe) and the light (L-Xe) isotopes as compared to, say, the relative abundance of the same isotopes in atmospheric Xe or in solar wind Xe. There never was much disagreement that a simultaneous enrichment of the light and the heavy isotopes could not have come about in one single process, nucleosynthetic or other; rather, what was contentious about this Xe was whether the mixing together of its sub-components had occurred prior to the incorporation into the meteorites (MANUEL et al., 1972; FRICK, 1977) or only within the meteorites (ANDERS et al., 1975; ANDERS, 1981) i.e., whether or not in the meteorites this Xe is a single welldefined component. Since a crucial result would be if it were possible to separate the individual sub-components again, considerable effort and ingenuity has been spent to achieve such a separation. For carbonaceous chondrites all such endeavors have failed. H-Xe was invariably found to be accompanied by L-Xe, and there is no evidence for any difference in the mixing ratio of the two (ALAERTSet al., 1979b; ANDERS, 198 1). For ordinary chondrites, on the other hand, the situation is less clear. Among 12 meteorites analyzed, one LL-chondrite (Krymka; ALAERTSet al., 1979a) yielded a low ratio, and one H-chondrite (Dimmitt; MONIOT, 1980) yielded a high ratio of L-Xe/ H-Xe. Two deviations on the order of 2a among the mete-

II. EXPERIMENTAL A. Samples A list of the meteorites used in this study and the sources of our samples is given in Table 1. HF/HCl-resistant residues (designated R 10 in the tables) were prepared according to the methods of LEWIS et al. (1975) and ALAERTSet al. (1979a); their compositions were checked on a SEM/EDX. As in previous work, chromite and spine1 were found to be the major non-carbonaceous phases. A small sample, typically around 1 mg, was used for noble gas analysis. The rest (ca. lo-20 mg) was treated with cont. HN03 at 80°C for 17 h in order to enrich HL-Xe relative to normal primordial Xe. After washing with Hz0 and acetone, this yielded the “oxidized” samples designated Rl 1. Mass yields after the HF/HCl treatment as well as after the HN03 treatment are also given in Table I. B. Noble Gas Analysis Noble gases were analyzed as described for the Mainz series of measurements in OTT et al. (1984) except that the electron multiplier 2869

2870

N. Schelhaas, U. Ott, and F. Begemann Table

Meteorite samples used in this study. Petrographic sub-classification based on TL from SEARS et al. (1982) and SEARS and WEEKS (1983). ALH A 81030 is probably paired with ALH A 77214 (L3.4).- Mass yields are given [in wt.% of the bulk meteorite] for the HF/HCI residues of the bulk samples as well as for the residues after HN03 treatment.

source*

HF/HCl-yield

(%)

HN03-yield

Meteorite

Tvoe

Semarkona

LL3.0

NMNH; USNM 1805

1.04

0.38

ALH A 77278

LL3.6

JSC;

0.86

0.46

lnman

L3.3

BM; 1982,

1.60

0.70

ALH A 81030 ALH A 77214

L3.4

splits 3,51,56

M.ll

JSC; splits 3,13,17

1.64

1.12

JSC; split 54

0.98

0.85

0.85

0.70

0.91

0.57

0.38

0.29

Khohar

L3.6

BM;1915,

ALH A 77299

H3.7

JSC;

87

Ohajala

H3.8

MPIM; T 67

split 6

(%)

‘NMNH: National Museum of Natural History, New York;= Antarctic meteorite collection, Johnson Space Center, Houston;BM: British Museum, London;MPIM: Max-Planck-Institut, Mainz.

(model: Balzers 2 17) was used in the counting mode rather than in the analog mode. In addition, there has been a change of the gas extraction system. It now consists of a W crucible contained within a Ta tube which is heated by radiation from a resistance-heated W mesh filament contained in an auxiliary vacuum. The calibration of temperature vs. furnace power for the system has been studied by MTZ (1984) using both optical pyrometry and a W/W-Re thermocouple. In routine work, input power is set in order to reach the desired temperature. Temperatures listed in the tables are thought to be accurate to better than k5O”C. Since our main interest was in the isotopic composition of trapped Xe, we attempted to remove atmospheric contamination as much as possible by preheating the samples at higher temperatures (- 150°C) and for longer times than usually employed in the study of meteoritic noble gases. This holds especially for a few instances where new samples were loaded before all samples from the previous batch had been analyzed. In these cases effective preheating times were several days. Bulk samples and HF/HCl residues were usually degassed in two steps, a 600°C step for the release of lightly bound/adsorbed gases

and the major extraction step at 1800°C. In order to achieve better separation of HL-Xe from ordinary trapped (“planetary”) Xe, the “oxidized” Rl 1 samples were degassed in three steps at 600, 1000, and 1800°C for 30 min each. System blanks for extraction at 1800°C corresponded to the following gas amounts (in cm3 STP): *‘%e, 2 X lo-“; 22Ne, 2 X lo-‘*; 36Ar, 1 X lo-“; %r, 2 X 10-13; 13’Xe, 1 X 10mL3.Re-extraction at 2000°C was performed in several cases to make sure of complete gas release at 1800°C. This temperature step always contained less than 2% of any of the noble gases. In addition to gas extraction by pyrolysis, gases from a sample of the Dhajala HF/HCl-residue were released in a stepwise combustion experiment. For the combustion, CuO contained in an alumina crucible within a stainless steel finger was heated to approximately 900°C to yield ca. 20 mbar of 02, while the sample was heated to a given temperature in a separate stainless steel finger for a total time of 60 min, with -45 min at the final temperature listed in Table 4. After recombination of unused O2 with the cooling Cu, the abundance of gaseous combustion products (essentially CO and COZ) was read off a Pirani gauge which had been calibrated by combusting known amounts ofgraphite in the system. Cleanup of the combustion prod-

Table 2a. Ne, Ar and Kr in type 3 ordinary chondrites (bulk samples). Gas concentrations are in units of 10-8cm3 STP/g except for Kr (l~-‘~crn~ STP/g).

Sample

Temp.

22Ne

20Ne 22Ne

21Ne

Semarkona-8 (56.51 mg)

600-C

0.134 ? ,004 3.83 .Ol 3.96 .Ol

6.712 f ,097 1.802 ,005 1.968 ,008

0.705 ,002 7.47 19 8:17 .lV

1.359 ,025 3.576 .OlO 3.385

0.030 ,001 18.6

0.734 ,150 1.133 ,009

0.877 ,017 0.886 ,005

0.02 .Ol 26.7 .6

0.366 ,063 0.236

0.042 ,003 17.3 .3

18:; .3

1.132 ,009

0.886 ,005

26.7 .6

0.237 ,002

17.3 .3

18OO'C TOTAL A77278-8 (77.83 mg)

600°C 1800°C TOTAL

Inman-B (87.61 mg)

600-C 18OO’C TOTAL

,011

36Ar

38Ar 36Ar

84Kr

0.338 f ,008 0.751 ,006 0.737 ,006

0.80 f .Ol 59.7 .5 60.5 .5

0.2014 + .0061 0.1938 .0030 0.1939 .0029

4.43 ? .18 41.6 1.7 46.1 1.7

0.787 ,005 0.619 ,003 0.633

0.04 .Ol 24.8 2.2 24.9

0.2338 .0644 0.2056 .0059 0.2056

0.15 .Ol 16.7 .7 16.9

"Ne

,003

2.2

.0059

,002

.7

A81030-B (60.72 mg)

1800'C

1.12 .15

2.378 .037

0.730 ,004

87.3 1.2

0.1836 .0051

45.7 1.7

A77214-B (63.47 mg)

1800'C

1.00 .03

2.611 ,020

0.711 ,004

94.6 .4

0.1903 .ooov

44.9 1.0

Khohar-B

1800'C

7.27

1.169 ,010

0.865 ,005

22.9 .5

0.2096 .0017

17.7 .3

0.925 ,006

0.866 .005

22.0 1.2

0.2326 .0073

12.5 .v

(89.80

mg)

A77299-B (85.13 mg) Dhajala-B

(72.80

.ll 18OO'C VOO'C

mg) 1800-C TOTAL

11.2 .6 2.19

0.940

0.938

10.6

0.1978

.05 0.447 .OlO

,011 0.912 ,019

,081 0.973

,:;1

.0020 0.2001 .OlOl

11.0 .4 1.05 .06

2.63 .05

0.935 .OlO

0.944 ,067

11.6 .3

0.1980 .0020

12.0 .4

.007

.03

2871

Noble gases in unequilibrated chondrites Table 2b.

chondrites. Gas Ne, Ar and Krin HF/HCI-resistant residuesfrom type 3 ordina % 3 concentrations are in units of 10m8cm3 STP/g except for Kr (lo- cm STY/g).

I Sample

Temp.

22Ne

Semarkona-RlO

6OO'C

0.19

(1.04 mg)

18OO'C TOTAL

A77278-RlO (2.71 mg) Inman-RlO (2.67 mg)

1800°C 6OO'C 1800°C TOTAL

5

5

36Ar

38nr 36A,

84Kr

22Na

22Nt?

10.3 t4.6 7.837 ,045 7.859 ,061

0.055 F ,039 0.042 ,001 0.042 .Oll

8.78 .22

7.656 .054

0.236 ,002

244 34

0.190 ,020

410 17

0.280 ,010 7.63 .13 7.91 .I3

1.0 .6 5.409 .061 5.254 ,061

0.797 ,027 0.396 ,003 0.410 ,003

2.0 1 400'

3.5 .l 361

40; 9

0.287 ,015 0.1906 .0015 0.1911 .0015

36: 6

/

I

+ .08 21.2 .5 21.4 .5

47.3 t .7 1052 9 1099 9

0.1871 ? .0066 0.1868 .0029 0.1868 .0028

64.9, f 2.7 1152

I

12:; 46

1 I 1

A81030-RlO (1.79 mg) A77214-RlO (2.17 mg) Khahar-RIO (2.14 mg) A77299-RIO (1.44 mg)

1800°C

5.35 .I7

4.41 .I2

0.515 ,005

2522

0.1925 .0032

298 3

Dhajala-RlO

18OO'C

3.69 .lO

6.39 .12

0.358 .007

741 18

0.1889 .0047

912 33

8.596

0.0456

2158 _.

0.1879

2185 _.

(1.10 mg) Allende-RIO

1800-C

(1.01 mg)

91.4

2.0

performed in our standard purification line using Ti sponge and Al-Zr getters under the usual conditions. Combustion blanks were similar to the blanks for pyrolysis extraction in the W crucible system (see above). ucts was

III. RESULTS AND DISCUSSION Results for the analyses performed in the standard mode (pyrolysis) are listed in Tables 2a-c and 3a-c, those from the combustion of the Dhajala HF/HCl residue R 10 in Table 4. A complete list of the data including He, 40Ar, and the Kr isotopic compositions is contained in SCHELHAAS (1987). A. Trapped Gases: Mass Balance and Elemental Abundance Ratios The noble gas balance for the meteorites investigated in this work is similar to that found in the previous study of LL chondrites by ALAERTS et al. (1979a) and in the study of H chondrite Dimmitt by MONIOT (1980). HF/HCl residues (0.38-1.64% by weight; see Table 1) contain between 8 and 24% of the trapped 36Ar present in the bulk meteorites, between 21 and 34% of the 84Kr, and between 30 and 72% of 13*Xe, with A 77278 and Inman being the extreme cases at the low and high side, respectively. Etching with HN03 removes 13 to 63 wt% of the residues (Table 1), while the gas losses range between 85% (Kr in A 8 1030; but see also below) and 98% (Ar, Kr, and Xe in Dhajala). Dhajala is unique in that there is much less difference in the behavior of Ar, Kr, and Xe than in all other meteorites. Its residue R10 contains 24, 29, and 30% of the total 36Ar, 84Kr, and ‘32Xe, respectively, and gas losses upon treatment of the residue with HN03 are uniformly high for all three gases (Tables l-3). The different behavior upon etching of the various gases is obvious from a plot of the elemental abundance ratios

,044

0006

52

.0015

19 1

36Ar/‘32Xe vs. s4Kr/‘32Xe (Fig. 1). Except for Dhajala the data points for bulk samples and the respective residues do not plot together; rather, they follow a slightly curved trend that is similar to the more pronounced trend in abundance ratios previously seen in ureilites (GOBEL et al., 1978), CO3 chondrites (ALAERTS et al., 1979b), and E-chondrites (CRABB and ANDERS, 198 1). We refer to these papers for a discussion of possible causes for this trend. Note, however, that utilizing the 20Ne/36Ar ratio as a cosmothermometer (ALAERTS et al., 1977) is no longer justified, since the laboratory trapping data of LANCET and ANDERS ( 1973), on which such an interpretation was based, have been shown by YANG et al. (1982) to be faulty. Exceptions to the general behavior are the Rl 1 residues from A 8 1030 and A 77214. We attribute their deviation from the normal trend to the presence in our samples of terrestrial atmospheric gases. Strong evidence in favor of this explanation comes from the Xe isotopic data (see discussion below), which suggest that ca. 60% (A 81030) and 75% (A 772 14) of 13*Xe in these samples are of terrestrial origin. Additional support is provided by the 40Ar/36Ar ratios of 48 and 73, respectively, which are significantly higher than in all other residues (< 15). Assuming all 40Ar to be of atmospheric origin and correcting 36Ar accordingly, 36Ar,,,/‘32Xecorr is in the range 80-100 (the ratio in the contaminating component is - 15). Similarly, admixture of an equivalent amount of elementally fractionated noble gases from the atmosphere will increase the Kr/Xe ratio (YANG and ANDERS, 1982). Both corrections together bring the data points for A 8 1030R 11 and A 772 14-R 11 into the “normal” range of Fig. 1. The basic behavior of the heavy noble gases upon chemical treatment of LL- and L-chondrites is that after HF/HCl treatment the 36Ar/‘32Xe and 84Kr/‘32Xe ratios are lower than in the bulk samples (cf. also the data in ALAERTS et al., 1979a) but that they are higher again in the HN03-treated samples.

N. Schelhaas, U. Ott, and F.Begemann

2872

Table 2c. Ne, Ar and Kr in HNOyetched residues from type 3 ordinary chondrites. Gas concentrations are in units of 10‘%m3 STP/g except for Kr (10~*“cm3 STP/&.

____._.._._~-iample

Temp.

22Ne

iemarkona-RI1 2.65 mg)

600-C

0.32 t .03 15.9 .4 la.6

0.2048

54:: 1.8

0.2010 0.2032

1.2: .O* 12.0

62:; 1.5 81.0 1.5

0.1988 .0019 0.1997 .0017

42:: .6

14.8 .l 23.2 .2 21.1

a.210 .014

0.426 .033 0.052 .OOl 0.053 ,001 0.065 ,001

0.1879 .0029 0.1926 .0029 0.1893 .0029 0.1903 .0017

31.9 1.3 64.3 2.6 63.8 2.6 160 4

3.28 .I6 2.64 .I3 5.92 .21

a.23 .I0 a.17 .12 a.207 ,077

0.060 .OOl 0.056 .OOl 0.058 ,001

21.4 1.1 15.0 .8 36.4 1.4

0.1915 .0062 0.1937 .0065 0.1924

71.7 5.0 62.3 4.2 134 7

600°C

0.071

2.36

0.704

2.67

1OOO’C

.002 2.86 .05 1.72 .03 4.66 .05

.61 3.116 ,036 4.303

,026 0.644

.06 21.8

0.495

3.544 ,031

0.590

13:: .3 37.5 .6

1.05 .03 5.03 .12

0.829 ,011 0.537

6.55 .09 lo.8

1.73

1.320 ,031 3.800 ,014 3.423

0.591

.04 7.82 .I3

3.383 .015

0.588

15:: .2 32.4 .2

0.419 ,035 1.39 .05 0.440 ,151 2.24 .16

1.98 .68 1.91 .21 3.8 2.5 2.30 .54

0.764 ,059 0.728 ,018 0.73 .25 0.734 .051

a.427

0.0407 .0004 0.0449

0.437 .033 0.201 0.293

23::

7.735 .017

0.235

59:: .4

0.108 .003 6.37 .lO 7.33 .12 13.8 .2

2.14 .47 4.239

0.705 .017 0.502

1.35 .03 16.9

5.364 .047 4.824 ,031

0.284 .002 0.388

6OO’C

0.189

1OOO’C

4.07 .09 1.18 .03 5.44 .10

5.89 .I3 8.292 .014 8.299

600’C

TOTAL

18OO’C TOTAL 1OOO’C Iaoo*c TOTAL

1800x TOTAL A77299-Rll (11.46 mg)

6OO’C 1OOO’C 1800°C TOTAL

Dhajala-Rll (3.05 mg)

600’C 1OOO’C laoo~c TOTAL

Allende-RI1 (2.48 ng)

0.1920 ? .0043 0.1885 .0029 0.1889

*4Ki+

14.8 1.0 20.2 1.2 19.3

5,942 .076 7.953 ,017 7.593

iaoo’c

mg)

36Ar

0.1996 .0042 0.2016 .0043 0.2102

1

1.03 .03 10.0 .2 2.40 .06 13.5 .2

1OOO’C

(10.68

304 2

.0005

600’c

TOTAL

Khohar-Rll

0.0452 .0004

.0005

34:: .6

laoo’c

R77214-RI1 (5.48 mg)

0.0460

26.0 f .3 142 1 136

TOTAL

1OOO’C

481030~RI1 (25.65 ing)

9.02 f .99 a.047 ,027 7.965 .025 a.012 .020

36Ar

22N.Z -0.078 + .Oll 0.0436

38nr

0.1890 .oozo

1OOO’C

fnnan-Rll (a.86 w)

22Ne

.-.I_ &

29.7 t 1.2 109 4 93.1 3.8 232 6

Laoo’c

\7727a-Rll (8.56 mg)

*ON@

looo’c

,044

*006

30.8 52::

laoo*c TOTAL

8:‘: 1:4

,036

,034

.052

.026

,054

8.309 ,045 a.353 ,035

9.35 .ll 26.5

,002 .003 .003

,003 .002

59:: .3

.004

.004 .003

,005 ,006 .004

4.94 .35 13.4 .5 5.81 .36 24.1 .7

.0004

0.0433

.0003

39.8 1.0 96.4 2.4 136 3

.0033

.0036 .0025 ._~ .0054

.0043

.0045

0.1968

.002a 0.1980 .oola 0.2116 .0024

0.2027 .0014 0.2146

29::

2.8 24:: 14:: .3 41.7 .5

0.2272 .0041 0.2392 .0039 0.2302 .0025

13.2 1.5 12.9 .5 16.3 .7 42.4 .s

0.1920 .0050 0.1941

5.7 z 14:;

0.218 ,012 0.1997

5.1

.0046

.oozo .0034

.____. 0.2040 .OOlQ 0.1999 0020 0.2010 .0015

.E

24:: 1.2 35.1 1.5 79.5 3.0 115 3

.-

For Semarkona (LL 3.0) the ratios in the final residue Ri I are actually almost as high again as they are in the bulk sample. For H-chondrites, on the other hand, there does not seem to be an increase of the ratios associated with the HNOj treatment. In the case of Dhajala, the elemental ratios remain basically unaffected by both steps of the acid treatment, in spite of the heavy gas losses that the treatment causes. The explanation could be simple. Since there is no evidence for the presence of HL-Xe in this meteorite (see below), it may

well be that the ca. 2% of the gases Ieft after HNC$ treatment are only remnants of the ‘“planetary component” of ordinary trapped Xe which was incompletely lost in the treatment, and trivially the elemental composition has not changed. But this explanation does not hold for A 77299, where HL-Xe is observed. It is also noteworthy that the trend of increasing 36Ar/‘32Xe and 84Kr/‘32Xe with HN03 treatment is opposite to what is observed for Allende residues (Tables 2 and 3, and, e.g., LEWIS

2873

Noble gases in unequilibrated chondrites Table

3~

Xe in type

Sample

3 ordinary

Temp.

Semarkona

6OOOC 1800°C TOTAL

A77278

I man

1800°C

chondrites

132X,

TOTAL

samples).

Concentrations

arc in units

of 10-lo

cm3 STP/g.

130x,

131x,

134x,

132X,

132X,

132X,

l32x,

0.08222 .00076

1.0233 f .0068 1.1632 .0065 1.1477 .0059

0.1522 t .0013 0.1618 .OOll 0.1607 .0009

0.7930 i .0042 0.8206 .0035 0.8175 .0032

0.3863 t .0023 0.3801 .0021 0.3808 .0019

0.3270 i .0015 0.3202 .0016 0.3210 .0014

124x,

126x,

%129X,

132x,

‘32X,

132X,

132~~

0.00371 ? .00011 0.004665 .000041 0.004559

0.004064 f .000089 0.004233 .000051 0.004214

11.7 .4

0.00486 .00019

0.00434 .00015

0.0827 .OOlO

1.673 ,011

0.1628 .0014

0.8197

0.3808 .0029

0.3168 .0025

0.00486 .00093 0.004782 .000058 0.004782 .000058

0.0061 .0017 0.004275 .000074 0.004279 .000074

0.0858 .0093 0.08297 .00052 0.08298 .00052

1.051 ,045 1.0935 .0035 1.0934 .0035

0.157 ,012 0.1624 .0009 0.1624 .0009

0.795 .045 0.8203

0.8203 .0034

0.388 ,023 0.3800 .0017 0.3800 .0017

0.320 ,018 0.3191 .0021 0.3191 .0021

.Ol 13.9 14:: .I

.000038

0.0778 + .0013 0.08277

136x,

4.10 ? .13 32.8 1.2 36.9 1.2

6OOOC 0.03 18OO’C

(bulk

.00084

.000046

.0034

.0034

A81030

18OO’C

36.4 .7

0.004556

0.00404 .00016

0.08113

1.0710

0.1597 .0012

0.8177 .0032

0.3839 .0028

0.3218 .0016

A77214

18OO’C

38.2 .3

0.004476

0.004534

0.08486 .00063

1.0551 .0066

0.1608 .0006

0.8171 .0035

0.3809 .0017

0.3196 .0017

A77299

18OO’C

11.8 .8

0.004737 .000042

0.004219 .000072

0.08290

1.2672

0.1628 .0009

0.8200

0.3375

0.3160 .0019

9oooc

15.9

0.004630 .000059 0.00482

0.00413 .00012 0.0036 .OOlO 0.00407 .00015

0.08453 .00087 0.0801 .0013 0.08412 .00080

1.652 ,010 1.241 ,015 1.6144 .0098

0.1623 .0012 0.1636 .0019 0.1624 .OOll

0.8226 .0063 0.8270 .0081 0.8229

0.3810 .0035 0.381

0.3149

0.3810

0.3156 .0026

Dhajala

18OO’C TOTAL

1::1 .09 17.5 .9

.000055 .000065

.00020

0.004648

.000056

.00060

.000061

.00062

3b.

Xe in HF/HCI-resistant

Sample

Temp.

132X,

Semarkona -RlO

6OO’C

117 ff 4 1509

18OO’C TOTAL A77218 -RlO

18OO’C

lnman -RlO

6OO’C 18OO’C TOTAL

A81030 -RlO

18OO’C

A77214 -RlO

18OO’C

Khohar -RlO

6OO’C 18OO’C

TOTAL 77299 -RlO

18OO’C

residues

from

type

3 ordinary

.0060

.0051

.0022

,023

.0058

.0038

.0028

0.3230

.0042

is similar for the HF/HCl soluble component, for our samples as well as for those of ALAERTS et al. (1979a). In contrast, little variation is apparent in the “planetary” (HF/HCl insoluble, HN03 soluble) component, a feature noted also by CRABB and ANDERS ( 198 1) for enstatite chondrites.

et al., 1975, and OTT et al., 1981). It thus appears that independent of possible isotopic variations to be addressed in the following section, each meteorite group has its own (range of) elemental composition in the gas component accompanying HL-Xe. It is not clear whether this could possibly reflect the properties of the trapping process or might rather be due to the degree of metamorphic losses. A dedicated investigation of more meteorites of different types to look for a correlation between the content of HL-Xe and the elemental abundance ratios might be helpful in addressing this question. The case

Table

.0057

B. Xenon The occurrence primitive ordinary

chondrites.

Concentrations

and composition of HL-Xe in a suite of chondrites has been the central question

are in units

of 1O-‘o

cm3 STP/g.

124x,

126x,

128x,

129x,

130x,

131x,

134x,

132X,

‘32X,

132X,

132X,

132X,

132X,

132X,

132X,

0.0794 f .0017 0.08233 .00073 0.08212 .00069

1.0243 f .0062 1.0377

0.1591 f .0013 0.1623 .OOll 0.1621 .OOlO

0.8048 f .0043 0.8207 .0035 0.8196 .0033

0.3809 t .0026 0.3820

0.3236 ? .0025 0.3202

0.3819 .0019

0.3204 .0017

136x,

0.004139 ? .000079 0.004189

16:: 53

0.00415 f .00015 0.004651 .000040 0.004615 .000039

578 21

0.004731 .000059

0.00409 .OOOll

0.08320 .00076

1.0642 .0074

0.1628 .0012

0.8250

0.3807 .0030

0.3171

4.0 .I 624

0.00445

0.00438

6286 6

0.004626

0.1619 .0009 0.1619 .0009

0.805 ,014 0.8195 .0036 0.8194 .0035

0.3821

0.004143 .000069 0.004145 .000069

1.069 ,015 1.0508 .0037 1.0509

0.1630

0.004627

0.0789 .0034 0.08198

0.3799 .0018 0.3799 .0018

0.3108 0067 0.3169 .0021 0.3168 .0021

918 11

0.004451 .000084

0.00400 .00014

0.08089 .00095

1.0365

0.1603 .0012

0.8169 .0032

0.3804 .0029

0.3179 .0017

1392 90

0.004512 .000064

0.004068 .000065

0.08252 .00075

1.0493 .0077

0.1609 .OOlO

0.8185

0.3821

0.3230 .0030

0.00495 .00035 0.004565

0.00526 .00035 0.004096

0.0905 .0033 0.08267

1.240 ,036 1.0679

0.1695 .0038 0.1624 .0009 0.1625 .0009

0.3810 .0066 0.3786 .0017 0.3786 .0017

0.3358 .0084 0.3158 .0019 0.3159 .0019

6.0 957 .l

.00034 .000040

.000040

.000046

0.004185 .000043

.00053

.00063

0.08196

.00063

.0060

1.0367

.0056

.0037 .0056

9 963 9

.000046 0.004568 .000046

.000070 0.004103 .000069

.00053 0.08272 .00053

.0034 1.0690 .0034

482 4

0.004663

0.00383 .00015

0.0783 .0015

1.0530

.000082

.0074

.0040

.0042

.0053

-0.8192 .0036 --

.0020

.0074

.0025

.0018

.0020

0.1616 .0006

0.8237

0.3820 .0021

0.3173 .0019

.0036

Dhajala -RlO

18OO’C

1372 72

0.004580 .000089

0.00419 .00012

0.08239 .00065

1.0464 .0062

0.1624 .0012

0.8210 .0065

0.3786 .0029

0.3171 .0023

Al lende -RlO

18OO’C

2341 124

0.004788 .000066

0.004195 .000090

0.08302 .00069

1.1155 .0068

0.1624 .0012

0.8209 .0060

0.3925 .0028

0.3396 .0025

2814

N. Schelhaas, U. Ott, and F. Begemann Table 3c.

Xe in HN03-etched

Sample Semarkona -Rll

Temp. 6oooc 1000oc 1aoooc TOTAL

A77278 -RI1

6OOOC 1ooooc 1aoo~c TOTAL

Inman -Rll

600% 1ooooc 18OO~C TOTAL

A81030 -Rll

6OO’C 1ooooc 1aoooc TOTAL

A77214 -Rll

600°C 1ooooc 1aoooc TOTAL

Khohar -Rll

600°C 1OOO~C 1aoooc TOTAL

A77299 -Rll

6OO’C 1ooooc 18OO~C TOTAL

Dhajala -Rll

6OO’C 1ooooc 1aoooc TOTAL

Al lende -Rll

1oooec 1800% TOTAL

HF/HCI-residues

from type 3 ordinary chondrites.

124x,

126x,

128x,129x,

13*x,

132X,

13*Xe

132X, 36.6 1.4 77.7 2.7 105

Concentrations

130x,

131x,

134x,

132X,

132X,

132X,

132X,

1.0627 t .0077 1.0866

0.1632 t .0019 0.1611 .0012 0.1601 .0015 0.1610 .0009

0.8214 f .0055 0.8242 .0047 0.8297

0.3816 f .0026 0.4085

0.8263 .oo*a

0.4560 .0027 0.4267 .0017

0.3194 f .0023 0.3624 .0019 0.4346

0.1585 .0013 0.1611 .0015 0.1615 .0015 0.1605 .0009

0.8074 .0041 0.8280

0.3837 .0024 0.4476

0.4215

0.8270

0.3929

0.3357

0.8213 .0033

0.4084 .0017

0.3603 .0017

0.1599

0.828 ,015 0.8241

0.3796

0.3133

0.4073

0.3581 .0023 0.3694

0.00397 f .00011 0.004287 .000084 0.004620 0.004393 .000051

0.0812 f .0018 0.0839 .0013 0.0854 .0015 0.08415 .00091

20.5 1.1 22.3 .a 23.5 .a 66.3 1.6

0.004307 .000045 0.005437

0.00411 .00012 0.00459 .OOOll 0.00428 .OOOll 0.004328 .000066

0.0785 .OOll 0.0849 .0013 0.0839 .OOll 0.08253 .00069

1.0806 .OOlO 1.1210

1.23 .03 12.7 .l 31.7 .4 45.6 .4

0.00465

0.00405

0.005089 .ooooa1 0.005132 .000084 0.005107

0.00419 .OOOll 0.004352 .000093 0.004298

0.0795 .0030 0.08580 .00090 0.08441

1.098 ,013 1.1427

0.08466

1.0777 .0034

0.004303 .000053 0.004744

0.003999

0.004064

0.004084 .000065 0.003671

0.07844 .00079 0.0796 .OOlO 0.07581

1.0309 .0059 1.0337 .0058 1.0085

0.004276 .000028

0.003839 .000034

0.07727 .00050

1.0193

21: 5

30.3 1.0 34.2 1.2 79.0 2.8 143 3 4.93 .32 28.8 1.9 79.0 5.2 113 6 3.05 .04 23.1 .2 22.5 .2 48.7 .3

.000087

0.00475 .00012 0.004844

.000054 .00034

.000064

.000065

.000035

0.00510 .00041 0.00464 .00016 0.004042

.000077

.000074

.00048

.000074 .000043

.000053

.00060

.00050

.00074

-0.003869 .000059 0.004199 .000096

0.0767 .OOlO 0.0804 .0012

0.004241 .000070 0.004637

.000089

0.004599 .000081 0.004731

.000058

0.004662

.000047

0.00424 .00019 0.004219

.000085

0.004239 .000077 0.004229 .000055

0.0842 .0013 0.08464 .00069 0.08349 .00075 0.08408

.00048

20.9 1.5 13.6 .5 27.4 1.0 62.0 1.8

0.00443 .00096 0.00505 .OOOll 0.004690 .000089 0.00468 .00033

0.003930 .000054 0.00439 .00012 0.004134 .000057 0.004121 .000041

0.07839 .00096 0.0827 .0017 0.08214 .00091 0.08099

10.6 .a 21.9 1.3 8.91 .71 41.4 1.6

0.00481

0.00447 .00021 0.00461 .00063 0.00459 .00020

0.0058 .oo*o 0.0045 .OOlO 0.0085 .0024 0.00567

0.0807 .0035 0.0863

57.7 3.1 144 8 202 a

.00040

0.00693 .00026 0.00674 .OOOll 0.006790 .000107

.00064

.0062

1.0501

.0081

1.0652

.0046

.0073

1.0935 .0069 1.0987 .0035

.0057

1.0509

.0042

.0056 .0036

1.106 ,013 1.0303 .0069 1.0038 .0057 1.0150 .0044 1.0878

.0074

1.1116 .0036 1.0749 .0039 1.0931 .0026 1.0396 .0059 1.1002

.0086

1.1087

.0065

1.0835 .0041 1.144

,036

0.0858

1.090 ,018 1.175

0.0848 .0020

1.122 .016

0.00507 .00048 0.00513

0.0876 .OOll 0.08747

1.127 ,011 1.0567

0.00511 .00023

0.08751 .00067

1.0767 .0061

.00091

.00026

we have tried to answer in this study. Dhajala (H 3.8) is the only case in which HL-Xe was not observed, neither in the pyrolysis of the HN03-treated Rl 1 sample (Table 3) nor in the combustion ofthe HF/HCl residue RlO (Table 4). Upper limits to the ratio H-‘36 Xe/“‘Xe (~10-~) and to the abundance of H-‘36Xe (cl.2 X lO-‘3 cm3 STP/g) are more than ten times lower than what is observed in the other H-chondrite of similar petrological type (A 77299, H3.7). For all other

.0023

tOO60

.00083

cm3 STP/g.

l3*xe

0.00459 f .00015 0.00500 .00010 0.00565 .00010 0.005242 .000066

i

we in units of ill-lo

,044

.0072

.0038

0.1620 .0015 0.1625 .OOlO 0.1623

.0008

.0044

.0057

.0063

.0043

0.8217 .0035 0.8225

.oo*a

.0025

.0032

.0024

.0082

.0021

0.4131 .0018 0.4106 .0014

0.1577 .OOll 0.1565 .0013 0.1547 .OOlO 0.1557

0.8067 .0036 0.8110 .0035 0.8028 .0037 0.8056 .0023

0.1602

0.8064

0.1552 .OOlO 0.1527 .OOlO 0.1537

0.8098 .0047 0.7983 .0052 0.8016

0.1624 .0019 0.1619 .0009 0.1619 .0009 0.1619

0.8109 .0071 0.8197 .0034 0.8253 .0036 0.8217

0.1563 .OOll 0.1607 .0021 0.1595 .OOll 0.1586

0.8043 .0039 0.8250 .0041 0.8186 .0035 0.8152

0.1595 .0041 0.1626 .0029 0.1636 .0038 0.1620

0.817 .015 0.819 .Oll 0.847 .014 0.8244

0.387 ,061 0.388 .030 0.401 .073 0.391 ,027

0.1576 .0017 0.1560 .0017 0.1565 .0013

0.8349 .0069 0.8338 .0062 0.8341 .0049

0.537 ,014 0.5228 .0061 0.5269 .0058

.0007 .0023

.oooa

.0006

.0007

.oozo

.0078

.0040

.0024

.0022

.0075

0.3803

.0022

0.4228

,0024

0.3967

.0022

0.3995 .0014

136x,

.oo*o

0.3897 .0016 0.3222

.oo*o .0026

.oo*o

.0052 .0024

0.3647 .0018 0.3212 .0014 0.3847

.0021

0.3415 .0018 0.3475 .0012

0.3785 .0047 0.4160 .0022 0.4010

0.3716 .0026 0.3516

0.4038 .oo*o

0.3553 .0016

0.3799

0.3127

0.3834 .oozo 0.3884

0.3223 .oo*o 0.3319 .0022 0.3261 .0015

.0026

.0042

.0023

0.3855 .0015 0.3875 .0021 0.4251 .0030 0.3970

.0024

0.3999 .0015

/

0.3197

.0040 .oo*o

.0034

0.3277 .0014 0.3860 .0024 0.3431 .0016 0.3473 .0012 0.3137

.0055

0.3199

.0053

0.3178

.0077

0.3179

.0036

0.5629

,0057

0.5381 .0049 0.5452 .0039

meteorites the presence of H-Xe is obvious; it is most pronounced in the 1000 and 1800°C steps of the RI 1 samples (Table 3). In a ‘34Xe/132Xe vs. ‘36Xe/‘32Xe plot (Fig. 2) these data points fall along the line defined by the Allende data. Hence, the H-Xe in ordinary LL-, L-, and H-chondrites has a composition indistinguishable within errors from that in the carbonaceous chondrite Allende (cf. Table 5). Whether the ratio L-Xc/H-Xe is also identical is best vi-

2875

Noble gases in unequilibrated chondrites Table

and composition of noble Abundance (10.'cm3STP/9) Dhajala RlO. Also included is the pressure due to in the reaction vcsscl. Note lack oi evidence for

4.

r

P

T

[mbar]

[‘Cl

I-

22No

630

370

0.063

415

0.032

460

0.140

510

0.190

540

0.201

610

0.072

680

0.077

095

0.080

945

0.030

0.212 fO.017

Total

21Ne

22Ne

"Ne

2.93 to.50

0.717 0.023 0.320 0.019 0.686 0.023 0.410 0.019 0.123 0.016 0.030 0.016 0.056 0.016 0.061 0.026 0.123 0.016

9.83 0.14 10.12 0.39 10.06 0.16 10.17 0.20 10.27 0.51 8.4.4 1.84 2.13 1.06 0.95 0.15 1.01 0.99

3.54

6.94

0.06

L

"Ne

0.12

0.722 to.051

0.045 0.003 0.031 0.005 0.038 0.004 0.041 0.005 0.044 0.014 0.143 0.059 0.667 0.159 0.093 0.017 0.009 0.092 0.326

t

6+

’+ u 00

%&?+ u_

q

-9 0

0.5

1.0

1.5

30Ar

04Kr

132Xe

24.6

15; 4 215 5 97.9 1:::

k0:s:

---

136Xe 132Xe

36Ar

fl.1 351

of CO/CO,)

0.1954

0.465

0.614

f0.0022 0.1811 0.0051 0.1927 0.0031 0.2055 0.0076 0.1805 0.0015 0.1869 0.0018 0.1867 0.0092 ___

f0.020 4.32 0.16 1.80 0.07 2.58 0.09 0.981 0.036 0.162 0.008 0.011 0.004 ___

to.044 6.750 0.294 2.809 0.123 3.994 0.174 1.437 0.063 0.214 0.009 0.017 0.003 0.007 0.002 0.029 0.003

---

___

---

___

0.016 0.004 ___

---

___

_.-

0.3078

iO.0027 0.3140 0.0024 0.3137 0.0026 0.3137 0.0025 0.3160 0.0032 0.3175 0.0035 0.319 0.021 0.300 0.048 0.310 0.013 -_-

__.

0.010

sualized in a plot of ‘24Xe/‘32Xevs. ‘36Xe/‘32Xe.As is obvious from Fig. 3 the situation is less clear cut in this case. Although again most of the data points fall along the Allende reference line, there are quite a few which definitely lie below (or to the right ol) the line. Several possibilities come to mind to explain deviations from such a correlation line. Spallation Xe, for all plausible

+

3GAr

gases released in combustion reaction products (basically the prcscnco of IIL-Xc.

2.0

*’ Kr/'32Xe PIG. 1. MAr/‘32Xe vs. %r/‘32Xe in bulk samples(B, filled symbols), HF/HCl-resistant residues (RlO, half-filled), and HN03-etched residues (RI 1, open). Note that the ratios are highest in bulk samples and (in general) lowestin HF/HCl residues.For ALH A 8 1030/77214 see discussion ‘Intext.

target elements, has a very much higher ‘24Xe/‘32Xe ratio than any of the values measured here. Thus, its presence would cause the data points to fall above the correlation line. The only instance where it may play a role is for Inman whose cosmic-ray exposure age of 45 Ma (based on the Ne data in Table 2 and the production rates of EUGSTER, 1988) is the longest among our suite of samples and whose concentration of Xe in the Rl 1 residue is lowest. In all other instances the deviations of the data points from the correlation line are opposite to what spallation Xe would cause. Fission Xe would produce deviations in the right direction. In order to explain the magnitude of the observed shifts, however, the concentration in or around the material of the residues of Pu and/or U would have to have been unreasonably high (e.g., 80 ppb Pu in the case of A 81030). Furthermore, since there are no corresponding effects seen in the ‘34Xe/‘32Xevs. ‘36Xe/‘32Xeplot of Fig. 2 we can safely exclude fission Xe as responsible for the outliers in the 124Xe/132Xe vs. ‘36Xe/‘32Xe diagram. The position in Fig. 3 of the deviating points, in particular those for the two Antarctic L-chondrites A 77214 and A 8 1030, rather suggests an admixture of terrestrial atmospheric Xe. This is born out by plots such as 130Xe/132Xevs. ‘36Xe/ “*Xe and ‘3’Xe/‘32Xe vs. ‘29Xe/‘32Xe (Fig. 4a, b), which are especially sensitive to admixture of air Xe. Based on Fig. 4 we estimate that approximately 75% of “‘Xe in A 77214Rl 1 and 60% of that in A 8 1030-Rl l are of atmospheric origin. Since this interpretation is also in full agreement with the effects observed in the elemental abundance ratios (see above) we feel that there can be no reasonable doubt that it is correct. The source of the “contamination” is unknown, however, and its occurrence is puzzling. It is not clear how significant it is that the amount of residue after HN03 treatment is higher for A 77214 and A 81030 than for all other meteorites investigated, independent of their exact petrographic classification. If real, there appear to be two sources of “excess res-

2876

N. Schelhaas, U. Ott, and F. Begemann

031

0.L

'0.3

0.5

‘36Xe/‘32

0.6

Xe

FIG. 2. ‘34Xe/‘32Xevs. ‘36Xe/‘32Xein HN03-etched residues (lOOO”C/opensymbols and 18OO”C/filled).Data for ordinary chondrites and Allende fall on a single mixing line.

idue” that might contain terrestrial noble gases. First, precipitates formed from dissolved matter during acid dissolution of the meteorites could have trapped gases originally dissolved in the acids. But then it is puzzling why it should have happened only for the two fragments from one meteoroid (note that A 77214 and A 8 1030 are paired). More plausible might be the explanation that secondary minerals formed during weathering in Antarctica and, in this process, trapped noble gases. Such an effect should be more pronounced in A 772 14 and A 81030 which are of weathering class C (“severe rustiness”) and B/C, respectively, than in the other two Antarctic meteorites which are both of weathering class A (“minor rustiness”) (MARVIN and MACPHERSON,1989) and which show much less, if any, evidence for a terrestrial Xe component. But we find it surprising that such weathering products should survive the acid treatments and should require extraction temperatures in excess of lOOO”C, to say nothing about the extremely high concentrations of Xe in these putative minerals. Another process that may account for high abundances of trapped atmospheric gases in meteorites of long terrestrial s

Composition of HL-Xe

in ordinary

chondrites and C3V Allende.

124X, This work1 Dinmitt (Moniot,

t

1980)

Allende (Frick and Moniot,

1977)

Allende (Lewis et al., 1975) ' excluding

ALH A 81030

age has been suggested by ANDERS (pers. comm., 1990). It is the diffusion into a “labyrinth of pores” in amorphous C, the adsorption on interior surfaces and the effective trapping of the gases due to the effect of “choke points” that restrict their movement (cf. WACKER, 1989). Perhaps a systematic survey of Antarctic meteorites with a variety of terrestrial ages and, in particular, a more detailed characterization of the constituents of the residues of A 77214 and A 81030 might help to resolve the problem. The preceding discussion should not distract from the fact that only A 77214/A 81030 show a significant deviation from the Allende reference line in Fig. 3 but that all other data points plot along the line, implying that the L-Xe/HXe ratio is constant. Since the deviations from the reference line in the case of A 772 14/A 8 1030 have a perfectly natural explanation we have found no compelling evidence that the relative proportion of L-Xe and H-Xe is any different in LL-, L-, and H-chondrites than it is in the carbonaceous chondrite Allende. For a comparison with the carbonaceous chondrite data, it is useful to compare the ‘36Xe-normalized compositions of “pure HL-Xe” (i.e., extrapolated to j3’Xe = 0 and calculated in the manner of FRICK, 1977, and MONIOT,1980), although it appears that this “pure component” does not exist in nature and HL-Xe is always accompanied by some amount of isotopically normal (“planetary”) Xe (cf. TANG and ANDERS, 1988a). The results for our samples are listed in Table 5, where they are compared with the HLcompositions reported for Allende (LEWISet al., 197.5;PRICK, 1977) and for Dimmitt (MONIOT, 1980). As in the studies by ALAERTSet al. (1979a) and MONIOT (1980) we find also that in bulk ordinary chondrites the ‘36Xe/ 13*Xeratio is lower than in AVCC-Xe. Since this Xe, which is usually found for bulk carbonaceous meteorites, contains contributions from HL-Xe (PEPIN and PHINNEY, 1978) the mixing ratio HL-Xc/planetary Xe is lower in ordinary chondrites than in carbonaceous chondrites. The best representation of the HL-Xe-free trapped component in our study is given by the data for Dhajala RlO, since Dhajala shows no evidence for the presence of HL-Xe in either pyrolysis of the HNO,-treated R 11 sample or the combustion of R 10. Its composition (Table 3)-124/126/128/129/130/131/132/ 1341136 = 0.00458/0.00419/0.0824/1.046/0.1624/0.821/ = l/0.379/0.317-is very similar to that derived for Forest Vale trapped Xe by combusting that H4 chondrite (0.00458/ 0.00411/0.0823/1.038/0.1619/0.819/= 1/0.380/0.318;L~v1ELLEand MARTI, 1988). This composition is likely to represent “Q-Xe” (i.e., the major trapped [planetary] component of Xe, which is lost during HNO, treatment) as it is found in ordinary chondrites. The ‘36Xe/‘32Xeratio of 0.3 17 k 0.002

126X,

128X,

0.0087 .0008

0.0043 * .oooa

0.013 f .011

0.0204 .0026

0.0093 .0032

0.054 ,008

0.0098 .0003 __

0.0044 .OOOl

0.034 ,001

and ALH A 77214

__ (see text).

‘30X,

131X,

132X,

134X,

- 0

0.117 f ,055

0.057 ?. .048

0.658 f ,031

I

1 ,,

IO

0.047 .039

0.049 .040

0.648 ,040

=

1

- 0

0.173 ,009

0.146 .009

0.677 ,013

=

1

0

0.156 .Oll

0.092 ,015

0.671 ,017

f

1

136X,

2811

Noble gases in unequilibrated chondrites

AVCC M

0.oo.G

All21L Pu

+ Atmosph.

fission

t

0.003

’

0.3

I

I

I

I

0.L ‘36Xe/‘32

0.5

I

0.6

Xe

FIG. 3. ‘24Xe/“2Xe vs. ‘36Xe/‘32Xeat 1000°C (open symbols) and 1800°C (filled) for HNOg-etched residues. Both Allende and all ordinary chondrite data points from this study fall within 20 of the mixing line, indicating a constant L-Xc/H-Xe ratio. Deviations for ALH A 81030/77214 are attributed to admixture of terrestrial Xe (see text).

**Ne. see, e.g., EBERHARDT et al., 198 1). Nemonoisotopic E may also be the ;eason for the one deviating point from Inman (the 1800°C step of R 11) which falls distinctly below the regression line through the points of this or all other meteorites analyzed here. This interpretation is supported by a recent report of ALEXANDER et al. (1989b) who found Ne-E in Inman. A 77278 is the most puzzling case. The data points define fairly well a mixing line with (*‘Ne/**Ne), = 9.84 f 0.11 which is identical within error with the ratio in the terrestrial atmosphere. Since there is no evidence for the presence of atmospheric contamination in any of the other noble gases, and since some Ne-A2 with *%Je/**Ne - 8.4 has to be present in this meteorite as well (given the presence of HL-Xe), the agreement with the terrestrial ratio appears fortuitous and is very likely the result of mixing Ne-A2 with another component. The high *‘Ne/**Ne ratio required for this putative component, the high 4He/20Ne ratio in A 77278-Rl l of 880 as compared to 200-300 for the other Rl 1 samples, and the high *‘NeJ3’jAr ratio in the bulk sample (0.92 vs. co.23 for the rest of our samples) all point to a solar component. Solar flare Ne (SEP-Ne, WIELER et al., 1986) appears to be more likely than the presence of solar wind Ne/Ne-B since solar wind gases reside in the outermost layer of grains and therefore would presumably not have survived the acid treatments

is distinctly different from the ratio of 0.310 that has been derived for Q-Xe in Allende from differences between etched and unetched samples (ANDERS, 1977). Etching experiments of the kind performed by WIELER et al. ( 1989), in which the released gases are measured directly, may be able to clarify whether there is a difference between ordinary and carbonaceous chondrites in this respect.

(al

C. Trapped Neon

0.3

0.L

0.5

0.6

I36Xe/‘32 Xe The trapped Ne component in acid-resistant residues from carbonaceous chondrites is Ne-A with its subcomponents NeAl and Ne-A2 (ALAERTS et al., 1980; see also SWINDLE, 1988). The abundance of Ne-A2 (“chromite-Ne”; *‘Ne/**Ne = 8.37 + 0.03; *‘Ne/“Ne = 0.035 f 0.001; OTT et al., 1981) correlates with that of HL-Xe (LEWIS et al., 1977; MANUEL, 1980; ANDERS, 1988), and it is the dominant component in C3V Allende as well as in the primitive ordinary chondrites of ALAERTS et al. (1979a). Accordingly, Ne-A2 is the dominant trapped Ne component in the meteorites investigated here, and most of the data points fall, in a *‘Ne/**Ne vs. *‘Ne/ 22Ne diagram, on a mixing line connecting Ne-A2 and spallogenic Ne (Fig. 5). There are a few notable exceptions, however. For Semarkona a fit through the data points yields a trapped (2%e/22Ne), ratio of 8.11 + 0.09 (for 2’Ne/22Ne = 0.03 + 0.0 1) which is on the low side. A possible explanation is the presence in this thermally least affected of all ordinary chondrites (SEARS et al., 1980; GROSSMAN, 1985) of Ne-E (basically,

o.86 3

o.aL E 2 -\

z

0.82 -

x” 0.80 f

0.78 0.95

’

Almosph.

’ 1.00

’

’ 1.05

’

’ ’ ’ ’ 1.15 1.20 1.10

‘2qXe/‘32Xe FIG. 4. ‘30Xe/“2Xe vs. ‘36Xe/‘32Xe(a) and ‘3’Xe/‘32Xe vs. lz9Xe/ 13’Xe (b) in HN03-etched samples. Note shift of ALH A 81030/ 772 14 data points away from the general trend and towards the terrestrial atmospheric composition.

2878

N. Schelhaas, U. Ott, and F. Begemann

_ I

I

I

0.L

0.6

0.8

I 1.0

2’Ne /‘*Ne FIG. 5. 2oNe/2”Nevs. 2’Ne/ZZNe.Not included are some data points (two from Semarkona, one from Dhajala) with large errors. For reference the composition of air Ne, Ne-A 1, Ne-A2, and Ne-B (SWINDLE, 1988;Ne-A 1 from TANG and ANDERS, 1988b) are also shown, as well as mixing lines between a “typical” cosmogenic composition and Ne-B and Ne-A2, respectively. Semarkona plots slightly below Ne-A2, possibly due to the presence of Ne-E, which can account also for the position of Inman-R11/18OO”C. The trapped component in Dhajala is thought to be Q-Ne rather than Ne-A2, while ALH A 77278 contains an apparently solar (SEP-Ne?) component in addition to Ne-A2. Data points for these two meteorites plot clearly above the mixing line connecting spallogenic Ne and Ne-A2.

involved in the preparation of A 77278-Rl l (cf. the experiments of WIELER et al., 1986). It is possible to deduce the 20Ne/22Ne ratio of the second component from a mass balance calculation based on the correlation that exists between the ratios 2oNeJ’32Xe and ‘36Xe/‘32Xe (Fig. 6). The data points for the A 77278 samples plot significantly above the mixing line between “planetary” gas (lower left) and the (HL-Xe+Ne-A2) component, which dominates the oxidized samples (upper right off scale), and this deviation is a measure of the fraction of “extra” (solar) 20Ne. Assuming that a mixing line passes through our Allende Rl 1 data point on the right and through the average composition of our R 10 HF/HCl resistant residues ( ‘36Xe/‘32Xe = 0.3 18; 20Ne/‘32Xe = 6) on the left, gives 55% as the fraction of “extra” 2oNe in A 77278-R 11. This in turn requires a corresponding 2”Ne/22Ne ratio of 11.5 in order to yield, by mixing with 45% of Ne-A2 with 20Ne/22Ne = 8.37, the observed 20Ne/ “Ne ratio in the mixture. The important result is, of course, that a value of 11.5 for the 20Ne/22Ne ratio is in excellent agreement with the value of 11.3 + 0.3 for solar energetic particles (SEP-Ne) obtained by WIELEP et al. (1986). A fit through the data points for bulk, pyrolyzed R 10, and Rl 1 from Dhajala (Fig. 5), gives a nominal (20Ne/22Ne), of 9.4 t 0.3 which is distinctly higher than Ne-A2. The spallogenic end is, however, not well defined because of the large error in 2’Ne/22Ne in the bulk analysis. A fit through the more precise data points (R 10 and R 1 1- 1000°C only) leads to an even higher value of - 10.4. A high 20Ne/22Ne ratio for trapped Ne in Dhajala is not surprising. As mentioned before, there is no evidence for the presence of HL-Xe in Dhajala, and, therefore, because of the 2oNe/‘32Xe vs. ‘36Xe/‘32Xe correlation (Fig. 6) the absence

of Ne-A2 can be expected. The Ne present should be that associated with the normal “planetary” or “Q-type” gases. ALAERTS et al. (1979b), summarizing their results and a number of previous analyses, have concluded that (20Ne/ “Ne)o = 10.3 + 0.4. This value is only based on isotopic differences between HN03-etched and unetched samples, however, and the error given is the result of pooling a large number of values that have large individual uncertainties each. Dhajala offers the possibility to determine the Q-Ne composition in this meteorite by a direct measurement and hence with greater confidence in the result. In our approach, Dhajala RIO was first heated to 600°C to release gases that are given off at this temperature on pyrolysis. Subsequently, the carbonaceous host phases of trapped Ne were combusted at low temperatures in order to keep cosmogenic contributions to a minimum. The results, listed in Table 4 and plotted in Fig. 7, show that the five combustion steps from 370 to 54O’C are indeed almost free from cosmogenic contributions and hence constitute a direct measure of the composition of Q-Ne. It follows from these steps that (2’Ne/22Ne)o < 0.04 and, from the correlation through all data points, that (20Ne/ 22Ne)q = 10.11 f 0.16 (for (2’Ne/22Ne)a = 0.03 & 0.01). WIELER et al. ( 1989) have utilized a closed system to perform on the Allende meteorite the HN03 etching that releases the Q-gases (LEWIS et al., 1975). Their analysis ofthe released noble gases yielded (20Ne/22Ne)o = 10.65 + 0.15, which is significantly higher than our value. A reason for the difference is not obvious except that some real difference may exist between the Q-Ne composition in H-chondrites and in carbonaceous chondrites. In any case, both our value and the one of WIELER et al. (1989) are similar to the composition of Ne-U (20Ne/22Ne = 10.4 +- 0.3 and 2’Ne/22Ne = 0.027

2879

Noble gases in unequilibrated chondrites k 0.003) observed in ureilites (OTT et al., 1985) which strengthens the case for a link between noble gases in ureilites and the Q-type noble gases in the various classes of chondritic meteorites (OTT et al., 1984; WIELER et al., 1989). D. Carbon and Xenon in Dhajala

From the combustion experiment on the Dhajala residue RlO, the abundances of released noble gases and the total pressure corresponding to the sum of all gaseous combustion products were determined simultaneously. The latter was read off a Pirani gauge after back-reaction of excess O2 with Cu (see above) but before exposure of the gases to the Ti and Al-Zr getters. It is essentially a measure of the combustion products CO and/or CO2 formed from the sample and will be used as such in the discussion below. Nitrogen is expected to be present also but only at a level of - 1% of the C (see, e.g., OTT et al., 198 1); SO2 is apparently gettered by the hot metal walls of our combustion system, as was ascertained by cornbusting known amounts of ZnS, and Hz0 is most likely reduced at the hot metal surfaces with subsequent dissolution of hydrogen in the metal. The release of Xe upon combustion of Dhajala RlO is compared to the production of CO plus CO* in Fig. 8. Major release for both species is below 600°C in agreement with observations of FRICK and PEPIN(198 1) and OTT et al. (198 1) on equivalent residues from the Allende CV3 meteorite. Note that the two curves are not parallel but that the Xc/C ratio is particularly high at the lowest two combustion temperatures w3r-l 4””

400

300

I

0.5

1

0.6

136Xe/‘32 Xe FIG. 6. Trapped 20Ne/‘32Xe vs. ‘36Xe/‘3zXe. Symbols are the same as in Fig. 1. Most bulk samples and HF/HCl residues plot within the small circle at the lower left. For HN03-etched samples both ratios are enhanced. The mixing line shown connects the mean of our RlO residues for ordinary chondrites with our Allende data point. Note evidence for extra Ne component in ALH A 77278.

0

02

01

J 06

08

10

“Ne /“Ne FIG. 7. Composition of Ne released in stepwise combustion of Dhajala-RlO. Combustion temperatures are indicated (630 = preheating pyrolysis step at 630°C). The region of trapped Ne composition is shown on a larger scale in the inset. A composition for QNe in Dhajala of “Ne/*‘Ne < 0.04 and Z”Ne/2ZNe= 10.11 + 0.16 is inferred from these data.

of 370 and 415°C. Provided the release of noble gases is caused solely by total combustion of their carrier phase (as opposed to being released due to structural changes initiated by partial combustion), it follows that only a small portion of the total carbon present can be host to the “planetary” or “Q’‘-gas, again in agreement with previous work (OTT et al., 198 1, 1984). Based on the calibration of our system by the combustion of known amounts of graphite the 13*Xe/Catom ratio in the first temperature step is 1.3 X lo-* and still 1.1 X IO-* at 415°C. With the same assumption as above, this, then, is a lower limit to the ratio in the actual host phase. It is 40 times higher than the mean total “*Xc/C ratio for type 3 chondrites of -3.2 X 10-l’ (ANDERSand ZADNIK, 1985) and 30 times higher than the ratio of 4.2 X IO-” obtained by WACKER( 1986) for a gas-rich phase in the ureilite ALHA 780 19. It is still an order of magnitude higher than the lower limit of 1.8 X 10m9found by OTT et al. (1986) in combustion of the same ureilite; from the solar value of 1.2 X lo-’ (ANDERSand GREVESSE,1989) it falls short by a factor of 10 (cf. also the discussion of distribution coefficients in YANG and ANDERS, 1982, and ZADNIK et al., 1985). With a total C content of 4.0% for Dhajala residue RlO and a mass yield of the residue of 0.38 wt% of bulk Dhajala (Table l), the abundance of acid-insoluble C in Dhajala bulk is 0.015%. We are not aware of a C measurement for Dhajala, but compared to other type-3 ordinary chondrites an abundance of 0.0 15% seems very low. According to SEARSet al. (1980) the C abundance expected for a type 3.8 chondrite is in the range 0.21-0.24% which is more than an order of magnitude higher (cf. also GRADY et al., 1982). Handling

2880

N. Schelhaas, U. Ott, and F. Begemann

losses of such a magnitude can be ruled out simply from the mass balance for the noble gases (see above), so that we are forced to conclude that Dhajala either has an unusually low C abundance, compared to its other properties (including the abundance of noble gases), or that the major portion of its C occurs in a form that is susceptible to attack by HF/HCl. IV. SUMMARY

AND CONCLUSIONS

In the apparent absence of any process capable of causing the simultaneous enrichment or overproduction of the light and the heavy Xe isotopes, the occurrence of HL-Xe as a single, monolithic component requires that Xe from two sources had to form a well-mixed reservoir from which it was trapped by its host phase which, according to LEWIS et al. (1987), is diamond. Ion implantation rather than processes such as sorption or co-condensation appears to be the favored process for trapping HL-Xe (cf. ANDERS, 1988). The argument usually cited in support of implantation is the lack of a significant overabundance of rz9Xe in HL-Xe. LEWISand ANDERS(198 1) argue that, if a chemically selective process had been at work, ‘=‘Xe should be strongly enhanced, because its precursor from the r-process, ‘291with a half-life of 16 Ma, should have been favored over Xe. We tend to believe that the conclusion is correct, but with our current knowledge the argument is not fully convincing. The reason is that it is questionable whether the process that made HL-Xe would have also made “‘1 in any significant amounts. First, a contribution to lz91(Xe)from the process that made L-Xe is certainly low judging from the low absolute ratio of the L-Xe to H-Xe isotopes in HL-Xe (cf. Table 5). Second, as far as the H-Xe contribution to the abundance of ‘29Xe is concerned, it is useful to bear in mind that the composition of H-Xe is quite different from “normal solar-system r-process Xe.” For the latter, ‘31Xe/‘32Xe/‘34Xe/ ‘36Xeis -2.4/2.4/l .2/ = 1 (where the r-process contributions

LOO

550

Temperature

700

850

1000

(OC)

FIG. 8. Percentage release of gaseous combustion products and ‘32Xeas a function of temperature in combustion of Dhajala-Rl~. Xe release is essentially complete at 6OO’C.The first combustion steps have high Xc/C ratios.

to 13’Xe and 13’Xe have been taken as 96 and 76%, respectively; see CLAYTONand WARD, 1978). In contrast, the approximate composition of H-xenon is 0.1.5/O.l/0.7/ = 1 (Table 5). By inference, a very low yield at mass 129 would also be expected (HEYMANN, 198 1). In effect, we cannot rule out the possibility that in spite of preferential trapping of lz91 there is no significant overabundance in HL-Xe of iz9Xe simply because of a low production ratio “91(Xe)/‘36Xe. An important clue to the mechanism of inco~rating HLXe into its host minerals would be to know the elemental fractionation that occurred during trapping of the HL component. But, although we know the relative abundances of the “anomalous” Kr and Ar that accompany HL-Xe (e.g., LEWIS et al., 1975; FRICK, 1977), we do not know the elemental composition at the source. Calculations such as the ones by HEYMANNand DZICZKANIEC( 1979, 1980a,b) and CLAYTON(1989) address the isotopic, but not the elemental pattern. What may be helpful, though, is the fact that even in the most HL-Xe enriched samples the characteristic ratio ‘36Xe/132Xenever exceeds -0.68 (TANG and ANDERS,1988a; the highest measured value appears to be the 0.685 of LEWIS and ANDERS, 1988). Just like s-Xe (the Xe produced in the slow neutron capture process of nucleosynthesis and found in primitive meteorites), HL-Xe, therefore, may not occur in pure form but may always be accompanied by some planetary Xe (TANG and ANDERS, 1988a; OTT et al., 1988). If so, and with the planetary component possibly related to a solar-like composition, the relevant fmctionation pattern may be one in which the fractionation factor is an exponential function of mass (see Fig. 13 in OTT et al., 198 I) or nuclear charge. It remains to be demonstrated what kind of trapping process can account for such a pattern. HEYMANNand DZICZKANIEC(1979,198Oa.b) have shown that HL-Xe and its associated Kr com~nent can be produced in different zones of a supernova in explosive nucleosynthesis. An interesting alternative scheme for the production of HXe, involving, in a first stage, the production of free neutrons via a neutrino burst by (v, v’n)-reactions and the usual (cu,n) reaction contributing in a second stage, has been proposed by CLAYTON(1989). Unfo~unately, none of the calculations gives the yield at mass 129. In any case, in spite ofthe criticism voiced above of the LEWIS and ANDERS (198 1) argument, we agree with the conclusion that for trapping the observed quantities of HL-Xe in diamond, ion implantation is a promising scheme. CLAYTON (198 1) has shown a way in which this could have happened in a single star, although the need to produce both C condensates and the p- and r-process products usually associated with a supernova poses problems. An attractive alternative approach is that of JORGENSEN (1988), where the carbon carrier and the HL-nuclides to be implanted are produced in the two stars of a binary system. Acknowledgments-Samples used in this study were kindly provided by Dr. R. S. Clarke, Dr. R. Hutchison, and the Meteorite Working Group. Essential for the noble gas analyses was the assistance of H. P. Liihr and his maintenance of the mass spectrometer. Besides the constructive reviews of Drs. R. K. Moniot and C. M. O’D Alexander vaiuable comments were provided by Dr. E. Anders. Editorial handling: K. Marti

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