Krypton and xenon isotopic composition in three carbonaceous chondrites

EARTH AND PLANETARY SCIENCE LETTERS 3 (1967) 249-257. NORTH-HOLLAND PUBLISHING COMP., AMSTERDAM

KRYPTON AND XENON ISOTOPIC COMPOSITION IN THREE CARBONACEOUS CHON!JRITES O. EUGSTER *, P. EBERHARDT and J. GEISS

Physikalisches Institut, University of Berne, Berne, Switzerland Received 27 November 1967

The concentrations and isotopic composition of Kr and Xe have been measured in the three carbonaceous chondrites Cold Bokkeveld, Lancé and Orgueil. Spallation corrections and, in the case of Cold Bokkeveld also neutron effects, are negligible and for the average Kr and Xe isotopic composition in carbonaceous chondrites we obtain :

686 =- (3 .4 ±0.8 )% 586 =- (2 .4 ±0 .8 )% 6 82 =-(1 .7 ±0 .4 )% 86 8 6 6 =- 0 .35 10.35)% 8 6g6 =- (1 .35 ±0 .35)%

and

5 130 -+(21

.0±3 .0)%

26=+(16 6 1 30

.0±1 .5)%

5128=+(

8.3 ±1 .0)%

5130

( 2.3 ± 0-8)%

5 130 -

( 5.8±0.7)%

7.3 ± 0.9)% 5 130 - ( 136__ 8.3 ± 0:9)% 5130 The significance and the possible origin of these anomalies are briefly discussed.

1 . INTRODUCTION Carbonaceous chondrites contain relatively large amounts of trapped Kr and Xe [1, 21 . Concentrations as high as 10 -8 cm 3 STP/g have been measured. They are one to two orders of magnitude higher than the terrestrial Kr and Xe abundances (assuming a completely outgassed earth). This comparison has led to 1e assumption that depletion of Kr and Xe relative ~o the nonvolatiles has been smaller in carbonaceous chondrites than in the earth, and therefore these meteontes should contain a less altered sample of the primordial noble gases. Present address: California Institute of Technology, Pasa,.lena, Calif.

In 1960 Reynolds [ 1 J discovered that the isotopic composition of Xe in carbonaceous chondrites deviates significantly from the composition of atmospheric Xe. Some ratios differ by as much as 40%. These anomalies cannot have resulted from the addition of cosmic ray produced spallation Xe [ 1 ] or other particle irradiation [3, 41 . Several different models have been proposed to explain the Xe general anomalies j3, 5, 61 . So far, none of these succeeded in fully explaining the observed differences quantitatively without resorting to yet unproven hypotheses . Until recently the isotopic composition of trapped Kr was only incompletely known. Krummenacher, Merrihue, Pepin and Reynolds [3] reported a small enrichment by 1-2% of the heavy Kr isotopes in carbonaceous chondrites . Background on mass 78 made

O. EUGSTER, P. EBERHARDT and J. GEISS

250

Table 1 Mason [111 ; classification acSome re1zvant data on the investigated carbonaceous chondrites. General data from Hey [ 101 and b) Stauffer [ 131, c) Wyttenbach 11 .11, other data from : a) Zähringer 121, Schmus and Wood (121 ; cording to Wiik 191 and Van were calculated from 21 Ne sp, allowing for the component. The radiation ages sp denotes spallation 1151 . Index d) Lieberrtiann composition. differences in the chemical Meteorite

Sample number

Classificatien

Cold Bokkeveld Lancé Orgueil

BE-366 BE-367 BE-261

C'2 C3 C1

Date of fail

Recovered weight (k g)

3Hesp

10 13 .1838 7 .23.187 ;. 5 .14.1864

3 52 10

<0 . 5a)

1

21 Ne sh

10- 8 cm 3 STP/g <7 .7a)

0.04a) 1 .3 b) 0.8 a)

Radiation age (M .Y .)

Br (1~pm)

Source of sample

0.12 3.2 2.9

2.30; 2.24(-) 3.6 *d) 5.72 d)

A B C

* Br in Lancé h y not been determined, and the average for carbonaceous chondrites type III is given. A: Professor .l . cel, Musée National d'Histoire Naturelle, Paris B : Professor W. choller, Naturhistorisches Museum, Wien C : Professor E. nders, University of Chicago, Chicago, Ul .

a measurement of the light Kr isotope impossible . A few moilthe ago Eugster, Eberhardt and Geiss [71 have published the isotopic composition of Kr in gas-rich and uhegiuilibrated chondrites. It was found that the non-spallation Kr showed small but distinct anom,ilies, the light isotopes being depleted in meteoritic Kr. The anomaiies in the heavy isotopes were identical with those observed by Krummenacher et al. [31 in carbonaceous chondrites. Several possible mechanisms were discussed which could explain the observed difference between trapped meteoritic Kr and atmospheric Kr. Because the investigated meteorites had fairly high radiation ages we could not com0 7 `el3r(n,yß)80Kr pleteiy exclude the possibility that the reaction had changed the observed Kr abundance. Marti [81 has recently measured the isotopic composition of trapped Kr -,md Xe in several meteorites . In some of these, spallation corrections are entirely negligible. Marti's trapped Kr spectrum agrees well with that given by Eugster et al. [71 . In this paper we report concentrations and isotopic compositions of Kr and Xe in three carbonaceous chondrites selected from the three subclasses I, Il and III [9] . Our aim was threefold. : 1) to measure the abundance of all Kr isotopes in carbonaceous chondrites and to compare the results with those obtained in gas-rich and urrequdibrated chondrites ; 2) to determigne the '80Kr abundance in a meteorite with small radiation age excluding any possible contribution from the '79Br(n,yß) 8OKr reaction ; 3) to redetermine the Xe isotopic composition in carbonaceous chondrites, especially the abundances of the light isotopes .

In table 1 data concerning the investigated three meteorites are compiled. The Br concentrations given for Cold Bokkeveld were determined with neutron activation by Wyt-tenbach [ 141 on aliquots of the crushed meteorite sample used for the rare gas determination (for techniques cf. Wyttenbach, Von Gunten and Scherle [ 161). Two independent determinations were made . 69 17 of the Br was leachable with water (900C for 20 main). After the water leach less than 1% of the Br was leachable with toluene. The meteorite Cold Bokkeveld was especially selected because of its low radiation age . As will be shown later, eff.-cts of cosmic ray produced secondary neutrons can be excluded with a high degree of certainty in this meteorite.

2. EXPERIMENTAL TECHNIQUE ANI) RESULTS The sample preparations, extraction and measuring techniques used were essentially the same as previously described by Eberhardt, Eugster, Geiss and Marti [ 171 ; Marti, Eberhardt and Geiss [4] ; Eugster et al. [7, 181 . As usual the samples were preheated at approximately 70')C for 2-5 days in vacuum prior to the rare gas extraction. Additional Ti-sponge getters on t' . extraction system and in the mars spectrometer inlet manifold had to be used to handle the very large amounts of gases released by the carbonaceous chondrite samples during the extraction. With six stages of clean-up it was then possible to reduce hydrocarbon

KRYPTON AND XENON ISOTOPIC COMPOSITION

25 1

Table 2 Results of Kr measurements on 4 aliquots of the Cold Bokkeveld carbonaceous chondrite . The errors given for the average isotopic composition are two times standard deviation of the average plus 0 .1 %.

Meteorite

Sample size ~ (9

Cold Bokkeveld

0.15

Kr-fraction

0.4%

Cold Bokkeveld

0.~.

Kr-fraction

Cold Bokkeveld

0.3

0.3

Cold Bokkeveld

Gas

Hydrocarbon correction on mass 78

totKr

86Kr

78Kr

80Kr

82Kr

83 Kr

84 Kr

86 Kr

2150 *

1 .947 x.025

12.67 x.06

65 .24 x .30

65 .41 ±0.30

322 .4 ±1 .0

100

0.7%

1950 *

1 .922 x.020

12.59 x.07

64 .86 x .40

64 .90 x.30

323 .4 ±1 .0

100

Kr-fraction

0 .3%

2120 *

1 .929 ±0.010

65 .49 ±0.30

--

65 .09 ±0.40

323 .5 ±1 .1 323 .9 ±1 .7

100

Kr in Xe fraction Kr-fraction

12.50 65 .15 ±0.06 ±0.20 12 .78 . 65 .16 x .17 x.40

-

64 .81 ±0.50 65 .10 ±0 .35

64 .91 x .40 65 .15 ±0 .30

322 .8 ±1 .5 322 .5 ±2 .0

100

Kr in Xe fraction

12 .69 ±0.07 12 .65 x.10 ;12 .65 ,+0 .09

55 .05 x .20

65 .16 x.25

323 .1 ±0.8

100

fraction

Average for Cold Bokkeveld

10-12 cm3 STP/g

-

1 .8%

2010 *

1 .931 ±0.020

11700 ±2300

-

-

2060 ±400

1 .932 x .012

100

100

* Total concentration of 86Kr, i .e . 86Kr in Ar, Kr and Xe fractions are included .

Table 3 Results of Xe measurements on 3 aliquots of the Cold Bokkeveld carbonaceous chondrite . The errors given for the average isotopic composition are two times standard deviation of the average plus 0 .1%, but not smaller than 0 .5%.

Meteorite

132Xe Sample totXe 124Xe size (g) 10- 1 2 cm3 STP/g

Cold Bokkeveld

0.15

6460

0 .478 ±0.030

Cold Bokkeveld

0 .2

6550

Cold Bokkeveld

0 .3

5550

0.454 ±0.020 0.454 x .020

Average for Cold Bokkeveld

24000 ±5000

6200 ±1300

0 .462 ±0 .016

126Xe

128Xe

129X e

130X e

131Xe

132Xe

134X e

136X e

0 .417 x.020 0.409 ±0.010

105 .5 ±1 .0

16 .27 x .20

100

38 .1 ±0 .3

105 .7 ±0 .7

16 .07 ±0.20

81 .9 ±1 .0 82 .0 ±1 .0

100

38 .2 ±0 .2

32 .1 ±0 .4 31 .9 ±0 .2

0.405 10 .020

8 .21 x .10 8 .22 x .10 8 .17 x.10

105 .5 11 .0

16.02 x.10

81 .6 ±1 .0

100

38 .3 ±0 .3

32 .2 ±0 .2

0 .410 ±0 .007

8.20 ±0.04

105 .6 ±O:5

16 .12 x .15

81 .8 ±0 .4

100

38 .2 ±0 .2

32 .1 ±0 .2

and other background to virtually negligible levels . In most cases hydrocarbon corrections were less than 1% on mass 78 and less than 0.1% on all other Kr and the Xe isotopes. Extraction blanks were a few times 10-1 2 cm3 STP for Kr and Xe and can be completely neglected .

The mass spectrometer discrimination was determined and corrected for by running Kr and Xe standards immediately prior and after each meteorite analysis. These standards were prepared from spectral pure Kr and Xe. The amounts of standard Kr and Xe were adjusted to be identical within ±30% of the meteorite

O . EUGSTER, P . EBERHARDT and J . GEISS

25 2

Table 4 Results of Kr measurement: on the Lancé and Orgueil carbonaiceous chondrites . Meteorite Lancé

Sample size ig)

Gas fraction

Hydrocarbon correction on mass 78

0.4

Kr-fraction

0 .3%

totKr J: 86 K r 10-12 cm 3 STP/g

Kr in Xe fraction

0 .2

80Kr

82 Kr

83 Kr

84 Kr

86 Kr

1 .938 ±0 .025

12 .66 Jû.14

65 .27 ±0.40

323 .2 ±1 .2 322.4 ±1 .2

100

1 .938 x.025

64 .77 ±0 .40 65 .02 ±0.40

321 .5 ±1 .2

100

12 .85 j0 .17 12 .76 îO.14

65 .00 ±0.440 64 .517 f0.4Û 64 .99 ±0.40

1 .903 ±0.025

12 .75 10.12

65 .17 ±0 .40

65 .1.8 ±0 .40

321 .9 ±1 .6

100

12.61 JA. 13 12.68 10.12

64 .87 ±0 .40

65 .05 ±0 .40 65 .12 ±0 .40

322 .7 ±2 .0

100

--

Average Orgueil

7 8 Kr

Kr-fraction

1 .90z')

Kr in Xe fraction Average

14000 ±3000

2500 ±500

19000 ±4000

3300 ±700

1 .903 ±0.025

65 .02 x .40

322 .3 ±1 .6

100

100

Table 5 Results of Xe measurements on the Lancé and Orgueil carbonaceous chondrites. Meteorite Lancé

_

Orgueil

Sample size (g)

totK r

132Xe

10- 1 2 cm 3 STP/g

124Xe

126X e

128X e

129Xe

130;Ke

131Xe

132X e

134X e

136X e

0 .4

19000 ±4000

4800 ±900

0 .452 x .015

0 .409 ±0.010

8.23 ±0.08

113 .6 ±0.9

16 .00 ±0 .10

81 .3 ±1 .0

100

38.0 ±0.3

31 .9 ±0 .2

0.2

42000 ±8500

10900 ±2200

0 .456 x .005

0.410 x .006

8.16 x.05

105 .5 ±0. 5

16 .04 ±0.fî8

81 .8 ±0.2

100

38.4 ±0.2

32 .3 ±0 .2

swnples. For atmospheric Xe we used the isotopic composition given by Nier 1191, for Kr that given by Eugster et al. [ 71 . The absolute amounts were estimated from ion beam intensities alone, they have thus an uncertainty of ± 20%. Xe, K.r and Ar were always separated by adsorption on charcoal at different temperatures in the extraction system . However, the Xe fractions still contained between 9% and 28% of the total Kr. In most cases the Xe fraction was therefore subjected to an additional Kr-Xe separation in -the inlet system of the mass spectrometer and thus we were able to measure one Xe sample and two Kr samples from the same extraction . Four independent extractions were made on aliquots of the Cold Bokkeveld meteorite. One of the

Xe fractions was lost. For Lancé and Orgued only one extraction each was made . The results for Cold Bokkeveld are given in tables 2 and 3 and for the other two meteorites in tables 4 and 5 . Within the experimental uncertainty the isotopic composition of lr:i and Xe agrees in all measured samples, except for l. 29Xe and possibly 'OKr. The variations in the 129?:e abundance result from the decay of the extinct 1291, the possible variations in 80Kr from the 79Br(n,,yß)8OKr reaction . The contribution of spallation Kr ,end Xe during the exposure of the meteorite to the cosmic radiation can easily be estimated from the known radiation ages (cf. table 1) of the three meteorites,. With equations (5) and (6) given by Marti, Eberhardt and Geiss [41 we obtain for Cold Bokkeveld 78 Ki~s~.) ~/ 78 Kr = 10-4 and 126Xespall/ 1. 26Xe = 3 X 10-5. TI1e observed Kr

KRYPTON AND XENON ISOTOPIC COMPOSITION

and Xe abundances are thus not altered by the spallation component produced during the cosmic ray age. In Orgueil the spallation contribution is 78Krs all/ 78Kr = 1 .5 X 10'3 and 126Xesoall 126Xe = 4 1'0 4 { and can be neglected relative tô the experimental uncertainties. In Lancé 78Krs all/78Kr = 3 X 10-3 and 126Xespai1/126Xe = 1 .4 X 10-3 and a very small spallation correction for 78 Kr is necessary. An upper limit for 80Kr, 82Kr and 128Xe produced from the capture of slowed-down secondary cosmic ray neutrons can be obtained from the known radiation ages and the Br contents (cf. table 1). Eberhardt, Geiss and Lutz [20] have calculated neutron slowing down densities q(r,r) in chondritec meteorites of different size. Experimentally determined neutron slowing down densities, mainly from excess 80Kr resulting from the reaction 79 Br(n,yß)80Kr have so far been in reasonable agreement with the theoretical calculations [4, 21 ] . The carbonaceous chondrites contain large amounts of hydrogen, and the calculations of Eberhardt, Geiss and Lutz [20] cannot be directly applied in this case . An upper limit for the slowing down density is, however, obtained if we assume : a) no neutron loss by diffusion (infinitely large moderator); b) the highest possible neutron production which corresponds to a spherical meteorite of 20 to 30 cm radius . The slowing down density then does not depend on the Fermi age r and would be q = 0.55 neutrons cm'3 sec- l [201 . With t 1 tot ^- 0 .5 cm-1 , corresponding to the H2 content of Cold Bokkeveld, and neglecting neutron losses due to capture by other nuclides, we obtain as an upper limit for 80 Krn produced by epithermal neutrons on 79Br in Cold Bokkeveld : 80Kr,, < 0.15 X 10-12 cm3 STP/g. Under the moderator conditions assumed above also the 80 Kr production by slow neutrons must be considered and an upper limit of0.35 X 10-12 cm3 STP 80Krn / derived. Thus not more than a total of 0.5 X 10cm3 STP 80Kr/g could have been made in Cold Bokkeveld by neutron capture during the exposure to cosmic radiation. Possible corrections due to this neutron produced krypton component on mass 80 are thus smaller than 0.2% and on mass 82 smaller than 0..02%. The ontribution to 128Xe by the reaction I(n,yß) Xe is also completely negligible . The radiation ages of Lancé and Orgueil are more than a factor 10 higher than the radiation age of Cold

s

c

253

Bokkeveld and neutron effects on masses 80, 82 and 128 cannot be ruled out a priori . In tables 6 and 7 the average isotopic composition for non-spallation Kr and Xe in carbonaceous chondrites is given as derived from our measurements . For 80Kr only the Cold Bokkeveld results were used. Our "CC (average for carbonaceous chondrites) Kr isotopic abundances agree well with the trapped Kr isotopic composition observed in gas-rich and unequilibrated chondrites [7] except perhaps for a small difference in 80Kr. The 80Kr abundance in our earlier paper [7] had been based on the results obtained from Tieschitz. Because of its relatively high radiation age we had not been able to exclude completely the presence of (n,y)80Kr. In fact the slightly lower 80Kr abundance observed in Cold Bokkeveld might indicate the presence of a very small 80 Krn component in Tieschitz . For the Kr isotopes 80 to 86 our AVCC values agree well with the Berkeley AVCC krypton [3] . Recently Marti [8] has analyzed Kr and Xe in several carbonaceous chondrites, unequilibrated chondrites, a chondrite with very short radiation age and a ureilite. His isotopic composition for trapped Kr agrees extremely well with our AVCC values, the average deviation being less than 0.2%. The isotopic composition of non-spallation Kr is thus the same in all the investigated meteorite classes and'well represented either by our or by Marti's abundances as given in table 6. The composition of AVCC xenon derived from our measurements agrees for the heavy isotopes with the Berkeley AVCC [3] . For the light isotopes we obtain systematically slightly lower abut :ances; however, the values almost agree. Our AVCC c(- :position is in good agreement with the isotopic composition of trapped Xe--given by Marti [81, the average deviation being; 0 .7%. Our AVCC xenon agrees also with the nonspallation Xe in Mez6-Madaras [4] . It is thus not any longer necessary to assume an air-like Xe component in this meteorite. The 500-1300 0C Xe fraction of Renazzo [22] is in good agreement with our AVCC xenon. The significance of the air-like Xe component released at lower temperatures from this meteorite is not clear at present.

0. EUGSTER, P. EBERHARDT and 3 . GEISS Table 6 c'.h-asp. s~ ~3 of -air Kr results with those of other investigators. Our AVCC data are the averages from all measurements reported in ths papet Ver i OX r only the Col=' Bokkcveld data are used . The errors given for our results are two times standard deviation of the average plus 0 .19%

per

AV CC . ,his

C sri?ch and ancquilibrated thondrites 171 A 'f

i

1 .927 +0 .014

12 .65 +-0 .09

65 .04 ±0.20

65 .11 ±1.20

321 .8 ±0 .8

100

1 .914 ±0.02

12.80 ±0.1

64 .81 ±0.3

64 .97 ±0 .3

322 .1 ±1 .2

100

12 .75 ±0,1

64.63 ±0A5

64 .8S ±0 .45

324 .0 ±1 .6

100

, Berkeley (31

-

k> z rag,: tripped Kr, Marti (A[

1 .920 ±0 .020

12 .63 ±0 .12

64 .90 ±0 .45

65 .07 ±0 .35

323 .0 ±1 .5

100

Atmosr here 17'

1 .995 +0 .008

12.96 ±0.04

66 .17 ±0.16

66 .00 ±0 .14

327 .3 ±0 .7

100

average for carbonaceous chondrites.

Table 7 Comparison of our Xe results with those of other investigators . Our AVCC data are the averages from all measurements reported in this paper . The errors given for our results are two times standard deviation plus 0 .1% . !

124X,.

126 X,

th n~ paper

1

0 .459 ±0 .010

0 .410 ±0 .004

Berk-_!ey 1,31

!

0 .487 +0 .01`)

Verage trapped Xe, arti [ sj enazzo . 500-1 221 iczo Mactaras on spallation Xe [41 .ttnosph0re 1191

130X e

131 Xe

132X e

8 .20 +-0 .04

16 .08 ±0 .11

81 .7 . .3

0 .436 ±O .021

8 .29 ±0 .04

16 . 3 ±0 .(8

0 .452 ±0.010

0 .406 =b.008

8.09 ±0.07

0 .464 ±0 .010

0 .413 ±0 .008

0 .455 j ±0 .02

1

0 .3575 ±0 .0016

128X e

134X e

136Xe

100

38 .2 fO .2

32 .1 -0 .2

81 .11 ±0 .4

100

38 .2 10.2

32 .2 ±0.2

16 .13 -+0.G 8

81 .5 :W . :;

100

38 .1 ±0.2

32 .0 .2

8 .20 ±0 .06

16 .^, 7 ±0.10

81 M ±O . :R

100

38 .4 * ±0 .2

32 .5 ±0 .2

0 .411 ±0 .02

8 .39 ** -+0 . :12

15 .~

±o .:

81 .E +0 . ;+

100

38 .0 ±0 .3

32 .0 ±0 .3

0 .3331 ±() .0015

7 .137 ±0 .021

15 .15 ±0 .05

78 .76 ±o-!5

100

38 .82 ±0 .11

32 .98 ±0.08

A!'CC : average for cz bonaceous chordrites . Sti stematic variatic,n : in the temperature release curves indicate that E,. separate fission Xe component is present in Renazzo [ 22) ~~` tilezo Madaras contains a 128X, excess produced by epithermal neutrons [41 .

KRYPTON AND XENON ISOTOPIC COMPOSITION

3 . DISCUSSION

86

NKr/ 86Kr )meteorite - 1 X 100% (MKr/86 Kr)atmosphere

and 30Xe)me teorite 6M _ (MXe /' 1 X 100% . 130 (MXe/'30Xe)atmosphere We use 130Xe as reference isotope in preference to 132Xe because it has a negligible fission yield. 8-values for our AVCC Kr and Xe are given in table 8, and in figures i and 2 they are plotted against the mass number. The possible mechanisms which could lead to the observed Kr anomalies have been discussed in our previous paper [7] . We had concluded that the Kr anomalies observed in the non-spallation Kr of unequilibrated and gas-rich chondrites could be explained by a mass dependent fractionation process. However, a much better agreement was obtained if, in addition to fractionation, a fission Kr component (either thermal neutron 235U or spontaneous 2238U fission) is assumed. Our new AVCC 6-values agree with those of Eugster et al . [71, but they have smaller errors and it is thus appropriate to discuss again the origin of the Kr anomalies. The systematic trend of the 8-values with mass number suggests that a mass fractionation process is responsible for the Kr anomalies. The 6-values for 78Kr to 83 Kr alone would lie exactly on a straight line (cf. fig. 1). 6 84 however, falls, below this line . It seems unlikely that our 84Kr abundances are systematically low by 0.5%, a figure which would, be required to bring the 8 86 value on co the `fractionation line" defined by the isotopes 78Kr to 83 Kr. Our 84 Kr/ 86Kr ratio agrees to within 0.06% with Marti's result, which wmq obtained with_ a different extraction system and mass spectrometer . Using a slightly higher mass fractionation it is possible to draw a straight line through all 6-values within the limits of the experimental error. Our errors correspond to a 95% confidence level and it is unlikely that

Tabic. 8

86 and s 130 values for our A% CC Kr, and

S

For the further discussion it is convenient to use the 6-values, defined as

6M

25 5

78K r

_ _8OKr

.-

$2Kr

83Kr

_84

Xe .

Kr

- 3 .4

- 2 .4

- 1 .7

- 1 .35 t0.35

-1 .35 1 ±035

124X e

126Xe

128X e

131Xe

132Xe

134X e

21 .0 ±3 .0

16 .0 ±1 .5

8.3 ±1 .0

-2 .3 ±0 .8

-5 .8 ±0.7

-7 .3 ±0~9

±0.8

±0.8

±0.4

136X, . s.3 __~

9-J

for all isotopes the deviation between the measured and the true 6-values is as large as the given error. The present evidence suggests that because of the relatively low 84Kr/86 Kr ratio observed in all meteorites with large amounts of trapped Kr the Kr anomalies cannot be due to mass fractionation alone. A fission component in meteoritic Kr alone also cannot explain the Kr anomalies, since the meteoritic 78Kr/82Kr is lower than the atmospheric . Mass fractionation, combined with a small fission component in meteoritic Kr would give the observed anomalies (cf. fig. 1). From the data alone it cannot be decided whether atmospheric or meteoritic Kr is fractionated relative to primordial Kr . The Kr concentration in carbonaceous chondrites is larger than the amount of Kr present in the terrestrial atmosphere relative to the total mass of the earth, and it. could be argued itat the earth is only partially outgassed and that atmospheric Kr is a fraction enriched in the light isotopes . The difference between our AVCC Xe spectrum and the previously obtained one [31 is not significant enough to remove the difficulties encountered by the models proposed for the origin of the Xe anomalies . The Cameron-Kuroda model [5, 61 would still require a very high 132Xe yield for the proposed fission component in atmospheric Xe. Even the spontaneous fission products in Ca-rich achondrites do not seem to have a suitable spectrum [4, 23, 241 . Furthermore the differences observed between atmospheric Kr and our AVCC Kr are contradictory to the predictions of the Cameron-Kuroda model [3, 61 . Krummenacher, Merrihue, Pepin and Reynolds [31 have proposed mass fractionation and the addition of fission Xe to meteoritic Xe as an alternate model . As can be seen in fig. 2, our 6-values for the light isotopes would be well represented by a mass fractionation of

Q. FUGSTER, P. EBERHARDT and J. GEISS

78

MASS

PO

f'â

M'

81

MASS

e6

84

M

AWC - F RvPTON

MA SS FRACTIONATION DE~INED BY We - KrC3 MxTuRE OF FRACTIONATED

ATMOSPHERIC AND THERMAL NEUTRON FiSSbN KRYPTON

et

1-ag_ 1 . A%-erke krypton r, -values fnr carbonaceous chondrites Jvnvcd â :~ m out measurements. The dashed line represents the resulting from mass fractionation of atmospheric Kr t(ra tonatiion factor - 0.42% per mass unit). The solid line rpresents the s -values insulting for a mixture of fracnonated . 3txnosTheric Kr (fractionation factor -0 .3 ;T, per mass unit) and 2,SL thermal neutron fission Kr ( 86 Krfssion/ 86 Kr = 0.7~' 1 k, good fit to the experimental data could also be ob235U tained ht u%ing 238 U spontaneous fission Kr instead of thermal neutron fission Kr .

3.8 per mass unit. The fission component required to bring the heavy isotopes of our AVCC Xe on the mass fractionation line in fig . 2 would The ,5

-t

I5 . 1 ' 2Xe=38± 2I, 134Xe =64 ± 10, 136Xe = 100 .

this again is different from all known fission spectra . Furthermore there is the difficulty in finding a suitable source for the large quantities of fission Xc necessary in this model. Our best model for the Kr anomalies also requires mass fractionation and the addition of :fission gas to meteoritic Kr. The Kr fractionation is much smaller than the Xe fractionation and is furthermore of opposite sign so Chat at least two separate fractionation processes ,vou ld have to be invoked for Kr and Xe. In carbonaceous chondrites the average g6 Kr/ 136 ),e tRA*, ~,-, , ratio is 1 .3) and With R6,r ""'fission/ "-itr = 0.00 ~~ and 1 ?bXefïssion,"136Xe = 0 .16 we would obtain for the !I)Ass fractionation-fission model (g6Krj'136Xe)fission = 0 .06. For 7351* thermal neutron fission this ratio %--_)old be 0-3, 1, for 23SU spontaneous fission 0.12 and for ,4 1 .1 spontaneous fission ^- 0.0025 [241 .

Fig. 2. Average aenon 6-values for carbonaceous chondrites derived from our measurements . The straight line represents the S -values resulting from a linear mass fractionation of at mospheric Xe (fractionation factor 3 .8% per mass unit).

However, these comparisons may not be very conclusive, because (here is no evidence that these Kr and Xe fission components -- if at all present -- belong; to a closed system . ACKNOWLEDGEMENTS We would like to thank Professors E. Anders, J . Orcel and W. Scholler for providing meteorite samples . We are grateful to Dr. K. Marti for discussions and for allowing us to include his recent results in our tables and to Mr. H. Graf for his help with the measurements . We would like to thank Dr. J. Labeyrie for his support during an important phase of this work. This work was supported by the Swiss National Science Foundation (grant NF 453'0. K Ei - ERENC E:>

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