Chemically fractionated fission-xenon in meteorites and on the earth

Geochimica et Cosmochimica Acta. Vol. 5X, No. 14. pp. 3075-3092. 1994 Copyright 0 1994 Elsevier Science Ltd Printedin the USA. All rightsreserved 00 16-7037194 $6.00 + .OO

Pergamon

0016-7037(94)00103-O

Chemically YURI A. SHUKOLYUKOV,’

‘The Vernadsky

fractionated fission-xenon

ELMAR K. JESSBERGER,*.* ALEXANDER

in meteorites and on the Earth P. MESHIK,’

DANC

Vu MINH,~

and

Institute of Geochemistry and Analytical Chemistry, 1 I7975 Moscow, Kosygin Stranitsa. *Max-Planck-lnstitut fiir Kernphysik, Postfach 10 39 80. 69029 Heidelberg. Germany 3Academy of Sciences, Hanoi, Vietnam (Received

March

9, 1992; acwptc~d in rwrsed

/i,rnl .4pril

JIMMY L. JORDAN’.+

19, Russia

18. 1994)

Abstract-This is a report on the nature of isotopically anomalous xenon, which has been detected in two Ca-Al-rich inclusions of the Allende carbonaceous chondrite. It is extremely enriched in 13’Xe, lz9Xe, and to a lesser extent in 13’Xe. Similar large excesses of 13’Xe as well as of “‘Xe, ‘34Xe, and lz9Xe have previously been found in material processed in a natural nuclear reactor (Oklo phenomenon). Excess of these isotopes had also been encountered in MORB-glasses, in an ancient Greenland anorthosite. Thus, this Xe-type, which had previously been termed “alien” (JORDAN et al., 1980a) does not seem to be unique. To determine the origin of “alien” Xe, we analysed Xe (a) in neutron irradiated pitchblende and in the irradiation capsule, (b) in non-irradiated extremely fine-grained pitchblende (so-called Colorado-type deposit), and (c) in sandstone taken from the epicentre of an atomic explosion. In addition, the isotopic composition of xenon released by stepwise degassing and after selective dissolving of rocks from the Oklo natural reactor was determined. The results of these dedicated experiments demonstrate that the formation of alien Xe is due to the migration of the radioactive precursors of the stable isotopes ‘34Xe, 13*Xe, 13’Xe, and ‘29Xe. Due to this reason we now call it CFF-Xe-Chemically Fractionated Fission Xenon. Prerequisites for its formation are the simultaneous prevalence of two conditions: ( 1) fission (of 238U, 235U, and/ or 244Pu) and (2) a physicochemical environment (temperature, pressure, fluidity) at which the precursors of xenon (mainly Te and I) are mobile. Taking into account the occurrence of xenon in meteorites and connected terrestrial rocks, not all excesses of ‘29Xe in mantle rocks and natural gases are necessarily with the decay of primordial 12’? INTRODUCTION

Russian Academy of Science) as well as the Max-Planck-Institut ftir Kernphysik. Xenon from the various samples-individual inclusions in meteorites, uranium ore bulk rocks and separated mineral phases, insoluble residues after chemical treatment of uranium mineralswas analysed using various types of equipment. Therefore. we will first describe the techniques applied for xenon extraction and analysis.

IN THIS ARTICLE WE report on a new isotopic anomaly in inclusions of the carbonaceous chondrite Allende: large excesses of 13’Xe, ‘19Xe. and smaller excesses of 13’Xe. ‘34Xe. Some of us had previously termed this strange xenon “alien Xenon” (JORDAN et al., 1980a). As we will demonstrate, it is difficult if not impossible to explain the anomaly on the basis of probable nuclear reactions alone. But we noticed that there is a similarity between alien xenon and anomalous xenon contained in some other meteorites (SHUKOLYUKOV and DANG Vu MINH, 1984), some terrestrial rocks (JEFFERY, 197 I; MARTY, 1989), especially in samples from the Oklo uranium deposit (SHUKOLYUKOV, 1982). A genetic link between these anomalous xenon types has first been proposed by KIRSTEN et al. (1984). It is the aim of this study to determine the mechanism for the natural formation of alien xenon. A short account has been published as an abstract for the 23rd Lunar and Planetary Science Conference (JESSBERGER et al.,

Xenon Extraction

(a) For the xenon analyses in samples from Oklo uranium ores at the Institute of Geochemistry and Analytical Chemistry we used a 5kW vacuum resistance furnace with a tantalum heater for gas extraction (SHUKOLYUKOV et al., 1982a,b). The extraction temperature was determined both with a W-Re thermocouple and an optical pyrometer. Before beginning with the measurements the furnace was degassed for 8-10 h at 1900°C and better than IO Pa. The samples, wrapped in aluminium foil, were predegassed for 3 h at 2OO’C. A lock device allowed to load the samples into the crucible without breaking the vacuum. In this way a constant blank of the gas extraction system was maintained. For removing chemically active gases from xenon and for separating xenon from other inert gases, we used compound getters (It, Zr, Al, etc.. operated at 300-700°C). glass traps at -82°C (for sorption of water) and charcoal at +3OO”C (for activation), at - 196°C (for sorp tion of Xe, Kr, Ar), at ~82°C (for desorption of Kr and Ar), and at +25O”C (for desorption of xenon). The entire system was made of stainless steel as a single unit, so that the number of necessary vacuum seals was minimised, the length of the pipes reduced, and overall a rather compact apparatus emerged. As a result. the blank for xenon at 1500°C did not exceed 8 X IO-l4 cm3 STP (and not 10~” cm3 STP at 20°C). This is thousands of times less xenon than released from the samples under consideration. At the Institute of Precambrian Geology and Geochronology of the Russian Academy of Sciences in then Leningrad, xenon analyses in samples from Oklo uranium ores were done with a system with higher blank (SHUKOLYUKOV et al.. 1976, 1977, 1979; SHUKOLYU-

1992).

EXPERIMENTAL

and Blank

TECHNIQUES

Our study was performed over quite a time span in three laboratories, i.e., the Institute of Precambrian Geology and Geochronology, the Institute of Geochemistry and Analytical Chemistry (both of the

* Author to whom correspondence should be addressed, + Present uddress: Lamar University, Geology Department. Office of Lunar Base Research. P.O. Box 10031, Beaumont, TX 77710, USA. 3075

3016

Y. A. Shukolyukov

KOV and DANG Vu MINH, 1977a,b), but still the xenon blank never exceeded 1% of the sample’s xenon. (b) Xenon from the Allende inclusions was analysed at the MaxPlanck-Institut fiir Kemphysik. The samples were washed in ethyl alcohol, dried at room temperature, weighed, wrapped in aluminium foil, and pre-degassed in the glass-made extraction system at 200°C for about 24 h. Gas extraction was performed in a pre-degassed MO crucible incorporated into a glass vessel with water cooled walls by induction heating. Only two to three temperature steps (IOOO”C. 1500°C. and in some cases 1650°C) were used because of the small xenon amounts expected from the small samples (Table I). Following the routines described in JORDAN et al. (1980b) and HAAG (1975) for xenon purification and separation, we used Ti-getters, Al-Zr getters, and activated charcoal. Since the total amounts of xenon released from the samples in these series of experiments was usually rather low, particular attention was given to the xenon blank. After outgassing the furnace and baking the ion source of the massspectrometer at lOO”C, xenon of the extraction system did not exceed the background of the spectrometer and was practically close to zero after subtraction of the latter. At 1500°C the blank was still rather small (about 1 X 10ei4 cm3 STP ‘32Xe), and the typical blank correction did not exceed 5-10%. At 1650°C the blank was usually not higher than 1500°C because of the shorter heating duration (10 vs. 30 mins). Thus, we are confident that alien xenon is unaffected by any blank uncertainty. In the first measurement of the important sample #29 from Allende we did a second extraction at 165O”C, 1650-R, which still yielded a small amount of anomalous xenon (see Table I). In addition, to prove our ability to correctly analyse such small amounts of xenon. we analysed a small sample of Bjurbiile for which the concentration and isotopic composition of xenon is well established (JORDAN et al., 1980b; HOHENBERG and KENNEDY, 1980). The size of the sample, 0.8 mg, was chosen such that the amount of extracted xenon would approximately equal that from Allende inclusion #29 with alien xenon. The 100 and 1500°C extraction yielded

et al.

xenon that is composed ofatmospheric, AVCC, and plutogenic xenon, with trapped (AVCC) xenon approaching 95% (Table I). Despite the small sample weight and the possible heterogeneity of the xenon compositional distribution, the measured xenon represents correctly xenon of the BjurbGle meteorite. No indication of any “‘Xe excess was found, and the amount of xenon released in the 1650°C step was comparable to the blank. This is important, since BjurbGle was analysed together with the second two samples ofinclusion #29 (light and dark), in which the ‘32Xe excess was reproduced (Table 1). Mass

Spectrometry

The first measurement of samples with anomalous xenon from the natural nuclear reactor at Oklo were performed in the Institute of Precambrian Geology and Geochronology using a Russian-made mass spectrometer of the early MI 1305 series with a mass resolution ofabout 500. When we discovered unexpected excess of “*Xe, “‘Xe, ‘29Xe, it was decided to use a mass spectrometer with higher resolution in order to completely exclude any possible interference from other ions. Therefore, in the Institute of Precambrian Geology and Geochronology and later in the Institute of Geochemistry and Analytical Chemistry, we used three types of mass spectrometers. MB-2302 is an all-metal 1go-degree spectrometer with heterogeneous magnetic field. an accelerating voltage of 5,000 V, a radius of about 30 cm, and a mass resolution of about 5,000. It is equipped with a Faraday cup and a secondary electron multiplier. A low background of chemically active gases was maintained by freshly evaporated Ti-films. Isotopic mass discrimination did not exceed 0.6%/ amu as measured by pipetted amounts of atmospheric xenon. This spectrometer was also used to analyse xenon in rocks from the natural nuclear reactor-the Oklo uranium deposit. MI-l 201 B is an improved model of the spectrometer MI-1201 with increased mass resolution (up to 2,000) and sensitivity-up to 5 X 10m5 A/Pa. Its advanced computer allows us to determine the isotopic composition of xenon and krypton in the same gas sample

Table 1. Xenon released from Allende inclusions #18 and #29 and from a small whole rock sample of Bjurbiile. comparison baseline compositions are also given. t32Xe= 100.

Allende inclusions, this work #18-I (4.64)

loo0 1500 Total

17.8 32.4 so.2

30*9 26-I!? 27+6

30*7 34+4 33*5

77~12 65+7 69+8

16*5 10+1 12*2

105*12 312+15 238+17

-

_

#18-3 (27.8)

loo0 1500

15.7 26.6

35+_8 52+12

32k6 51+10

91+10 46+5

20f2 7.5-F 1

267+16 959+44

6+5

_

Total

42.3

45*10

44?8

63k7

l2fl

702+35

-

35.3 88.3 82.7 206.3 14.4

29+3 33&4 7+2 22+4 4*3

33*5 2a+5 8C2 21+4 16+3

83+9 70*6 24+2 54+5 42k6

13*2 13f3 2.9+0.7 9*2 6*1

292k20 173*13 75+2 154&S ao+s

24+5 _ -

27.1 90.7 17.2

31+1 37*1 33+4

38+1 40+2 41+3

75*3 77+2 76.~5

16+1 14fl 11+2

101*5 923k16 323+15

_

#29-1 (4.2)

loo0 1500 1650 TOtal 1650 nqz.,

#29-D (7.2)

1000 1500 1650 Total

#29-L (9.76)

3.112 0.4+2 0.4+2 0.8?4

-0.1 _

-0.2 _

135.0

35+1

40*2

76+2

14*1

681k19

-

loo0 1500

14.5 36.8

32+2 44+2

42*2 51*2

85k5 71*2

17+1 14+1

33a+ 15 940+19

s+1 -8

-0.7 _

-0.7 _

1650 Total

9.8 61.0

47+3 42+5

44+3 47*2

61f7

8+2

73+5

14+1

191+12 677+22

-10 -8

_

_

_

Bjurb&, (0.8)

=8 -7

2*2 _ o.s+2 55 _

this work

loo0 1500

199 615

36*3 35fl

36*5 41+1

sly5 88+3

14*3 15+1

110*11 153+5

a*3 a+2

-

TOlal

814

35+1

40+2

86*3

15*2

142*7

s*4

-

30.1 32. 32.9

37.1 38.2 38.8

82.3 81.7 78.9

16.5 16.1 15.1

105 variable 98.3

8.39 8.20 7.14

0.434 0.410 0.330

Polar Xo SLJC!OR(U AVCC-Xcnor@ Earth’s atmosphere Kbima and Podosek.

I

1983; Wlcynolds

et al., 1978

0.416 0.459 0.351

For

Anomalous

isotope composition

and to extrapolate not only by a straight line but by polynomials of different degrees. The latter is especially important in the measurement of large isotopic shifts when “memory” effects are substantial. Usually 8 to i0 mass-spectra of xenon and krypton were registered, and the analvtical error ofthe isotonic composition (with IO-” cm3 STP Xe) does’not exceed approximately 0.7%. This spectrometer was used for the analyses of xenon and krypton in the Colorado-type deposit, in the samples from New Mexico and also from the Oklo natural reactor. Xenon analyses of the Allende samples at the Max-Planck-Institut fiir Kernphysik were performed with a glass-and-metal mass spectrometer with a secondary electron multiplier (JORDANet al., 1980b: HAAG, 1975).

EXAMINED

SAMPLES, OBTAINED AND DISCUSSION

RESULTS,

Before discussing the Xenon data obtained from the two Allende inclusions and other samples, a note on the representation of isotopic xenon data seems appropriate. Most widely used are three-isotope diagrams with a neutral isotope as the common denominator, normally 13’Xe. Their disadvantage is, however, that of the nine xenon isotopes only three are displayed at a time. Therefore, to visualise the fine structure of the entire xenon spectrum we plot AA = (AXe/‘30Xe)sa,,,Qle - (AXe/‘30Xe),r,rence. Useful reference spectra, which by definition have no excess of 13’Xe, are solar, average carbonaceous chondrite (AVCC) or atmospheric xenon, the selection depending on the sample under discussion. However, in the case of strong radiogenic xenon such a representation may be misleading (if at all possible) because 13’Xe is very low. That is why to compare anomalous xenon from Allende inclusions and strong radiogenie xenon, a twofold normalisation is used:

AA/A

(AXe/‘30Xe),,,l,- (AXe/‘30Xe),ference ‘36= (136Xe/'30Xe),,,l, - ('36Xe/'30Xe)reference ’

Alien Xenon in Inclusions of the Allende C3V Chondrite The high apparent 40Ar-39Ar ages, 4.9 Ga, obtained for a few Ca-Al-rich inclusions from Allende (JESSBERGER and DOMINIK, 1979; JESSBERGER et al., 1980; JESSBERGER, 1984) motivated the search for peculiarities in other isotope systems. Therefore, two chips, samples #IS. 1 and #18.3, from the inner part of one ofthese inclusions, #18 (JESSBERGER, l984), were selected for xenon analysis. The inclusion has been described by JESSBERGER and DOMINIK (1979) as a typical coarsegrained Type B (GROSSMAN, 1980) inclusion with a diameter of about 1 I mm consisting mainly of clinopyroxene with melilite and anorthite. Analyses of both samples gave the first indication of the presence of an anomalous type of xenon (Table I). Rather pure alien xenon, however, is present in inclusion NMNH-29, a typical melilite-rich Type A inclusion (GROSSMAN, 1975). Based on its relatively weakly fractionated lanthanides (B. Mason, pers. commun.), it belongs to Group 1 in the classification scheme of the Smithsonian Institution (MASON and MARTIN, 1977). It contains fremdlinge with coexisting MO- and Ru-bearing Ca-phosphates-the stimulus for the xenon analysis-and Fe- and V-rich oxides (A. El Goresy, pers. commun.).

of Xe in Allende inclusions

3077

The first sample from this inclusion, #29-l, shows in its 1650°C fraction the strangest xenon composition (see below). We tried to confirm this finding by analysing two samples from the same inclusion, which were prepared by handpicking a light and dark phase, samples #29-L and #29-D, respectively. X-ray analysis showed that #29-L was dominated by melilite and #29-D by clinopyroxene. Because of the small sample size the expected amounts of xenon were very low and the samples were degassed in not more than three extraction steps (Table 1): 1000°C (30 min); 1500°C (30 min)-where the largest amount of xenon is released containing the usual meteoritic xenon components including fission xenon; 1650°C (IO min). Results after blank correction are listed in Table I. Xenon in the two inclusions studied contains a component with an unusual isotopic composition (JORDAN et al., 1980a). It is mixed with other known components in variable proportions. The new type of xenon is most prominent in the 1650°C extraction of sample #29- 1. 1) The first feature of this xenon is that is undoubtedly enriched in “‘Xe (I 500 and 1650°C fractions, Fig. 1). Although the error bars are rather large because of the low gas amounts, the deviations from established xenon compositions are sufficiently large. This is especially true for spontaneous component fission xenon (244Pu. 23xU) or the nucleogenetic Xe-H (PEPIN and PHINNEY, 1978). With the exception ofthe 1500°C fraction and bulk sample #29L, the data points plot outside of the area occupied by mixtures of normal (AVCC, solar or atmospheric) xenon with products of spontaneous fission 244Pu and 238U or Xe-H. In Fig. la. ‘3’Xe/‘32Xe vs. ‘36Xe/‘3’Xe, it seems that the anomalous xenon falls near a mixing line with s-process xenon (SRINIVASAN and ANDERS, 1978; LEWIS et al., 1990). However, Fig. I b,c indicate the virtual absence of the even s-only isotope 13’Xe. Therefore, s-Xe cannot be the source of the “‘Xe excess. Likewise, no combination of s-Xe with the products of the other basic nucleosynthetic process-the rprocess-leads to the observed isotopic composition. We have examined the possibility of “‘Xe production by cosmic protons. Irradiation with protons (energy range 25 < E > 30000 Mev) of possible target elements (Ba, Ce, and also La, Cs, Pr, Dy, Sm, Eu, Gd, Tb, Lu. Ho, Er, Tm, Yb, Ta, W, Re), however, establish that the yield of 13’Xe at all proton energies exceeds the yield of 13’Xe (SHUKOLYUKOV, 1989)-the opposite of the alien xenon pattern (Table 2). Thus, spallation cannot be its source. 2) The second feature of alien xenon in the Allende inclusions is the additional presence of considerable 13’Xe excesses (up to 60%) somehow correlated with the ‘32Xe excesses. Essentially, the above considerations on ‘32Xe formation apply to “‘Xe as well. In addition, 13’Xe may be formed by neutron capture on “‘Ba and 13’Te. However, the slight correlation between the 13’Xe and 13*Xe excesses shows that a neutron capture origin of the excesses is hardly possible because only “‘Xe (and not 13’Xe) can be produced by neutron-induced reactions. 13’Xe is rather abundant in spallogenic xenon formed by high-energy proton irradiation. But if the observed 13’Xe excess were really spallogenic, it would be accompanied by corresponding excesses of ‘24Xe and ‘26Xe, which are not ob-

Y. A. Shukolyukov et al.

3078

s

O-1 o-2 e-3 e-4 o-5 A-6 A-7 a-a

0.8 -

R i x" ;r

.---.-.-.-

&.H

0.6 ./'

0.4 - -I-

16.5A

./ f

+,

.’

.’

.&S

165

0.2

XeSj

I 0.6

I 0.5

I 0.4 ‘=Xe/‘32Xe

I 0.3

I 0.2

0.1

(b)

10 '\

a20 -

'\ '\

0.15 2 “D i x” E 0.10 -

ao5~ ;; , , , , 0.2

0.1

0.3

0.4

0.5

olb .-

(c)

16.5R --I--

0.05

bI

16.5

I

I

0.1

0.2

I

I

0.3 0.6 ‘=Xe/‘=Xe

I

I

0.5

0.6

FIG. I. isotopic correlation of Xenon from incfusions of the Allende C3v meteorite, alien Xenon. Numbers next to the data points are extraction temperatures in hundreds of “C. (I) solar Xenon (SUCOR) fOZlh4A and PWOSEK. 1983); (2) AvCC Xenon (REYNOLDS et al., 1978); (3) atmospheric xenon (OZIMAand PODOSEK,1983). The Allende samples are (4) #I 8.I: (5) # 18.3; (6) #29- 1: (7) #29D: (8) #29L. The index R denotes repeated extraction.

Anomalous isotope composition of Xe in Allende inclusions

Table 2. Isotopic composition of Xe produced by irradiation of Ba and Ce with protons of various energies (Shukolyukov, 1989). 130Xe= 1.

3079

rather of variable composition as is obvious from Fig. 1 and 2. Thus, despite the fact that the excesses of ‘36Xe, ‘34Xe, r2*Xe, and ‘29Xe in the Allende inclusions can be explained one way or another, the main enigma remaining is the large excess of r3’Xe, which is coupled to weaker excesses of 13’Xe and, possibly, ‘34Xe and ‘29Xe.

proton energy

IwXe

I32Xe

I3lXe

Alien Xenon in the Earth’s Lithosphere

MeV

2.5 50 I?. 100

730(l) 1000 30000

target: Ba 52 35 37

68 69 87

6400 4400 790

27500 16500 7770

1010 760 770 940

11500 1830 I.590 2650

1800

46.50 196 6130 475 614

Surprisingly alien xenon was also found in several terrestrial rocks. More than 20 years ago JEFFERY(197 1) observed xenon in an ancient Greenland anorthosite (JEFFERY, 1971, Fig. 3) with an anomalous pattern similar to that of alien xenon. However, recently AZUMA et al. ( 1993) tried to confirm the data of JEFFERY(197 1) with anorthosites from the same sampling site as that studied by Jeffery, but failed. Anomalous xenon appears to be present also in midoceanic ridge basalt glasses analysed by MARTY (1989). The

target: Cr 50

73 100 730n) 1000

LO.1

0.3 60 0.6 6.7

105 695 87 43

(I)Funk et al., 1967; @Hohenberg and Rowe, 1970

served (Table 1). An excess of 13’Xe can also be due to the addition of L-Xe (PEPIN and PHINNEY, 1978) to common trapped components. Unfortunately, the small amounts of ‘24Xe and ‘26Xe and the accompanying large uncertainties did not allow to estimate the possible presence of L-Xe in 13’Xe using ‘24Xe and ‘26Xe abundances. In any case, the slight correlation of 13’Xe and ‘32Xe in alien xenon precludes L-Xe, since 13*Xeis practically absent in L-Xe. 3) The third feature of alien xenon is most easily demonstrated by using 13’Xe as the normalising isotope: besides the dominant 13’Xe excess and the weaker ‘(‘Xe excess, also the heavy isotopes 136Xeand ‘34Xeare enriched in some fractions. Generally, the ‘36Xe overabundance exceeds that of li4Xe as is the case for spontaneous fission of 244Puand 238U. Thus,’ the CAIs contain some fission xenon together with alien xenon as is also obvious from Fig. 1. Unfortunately, using only the ratio ‘34Xe/“6Xe, it is very difficult to decide exactly from which heavy element (244Pu,238Uor some other) alien xenon was formed, 4) The fourth feature of the inclusions is the presence of excess ‘29Xe relative to solar xenon (Table 1). Excess ‘29Xe or a significant portion of it evidently stems from the p-decay of now extinct “‘I. Capture of thermal neutrons by iodine may also be a cause of the excess “‘Xe. Such an excess undoubtedly is present only in the 1650°C fraction of sample #29- 1, while in contrast the “‘Xe excess is present in all samples. Anyway, whether there is a relation between lzsXe and alien xenon is questionable, because of the lack of correlation between both. We would like to note that alien xenon is not unique to one sample or extraction fraction, and that it is not a single component of well-determined isotopic composition, but

128 130 132 134 136 A FIG. 2. The structure of the xenon isotopic composition extracted at certain temperatures from Allende inclusions normalized to solar xenon. AAis the excess of atoms with mass A relative to solar xenon normalized to “‘Xe: AA = (AXe/‘30Xe)sample - (AXe/‘30Xe)W,a,.

3080

Y. A. Shukoiyukov et al

0.701,

,

1

I

,

0.35

,I,

I,

1

0.40

t34Xe/t32Xe

,,.L

I

I

I

I

I

,I,

I

,

,

,

0.35

0.30

136Xe/132Xe

FIG. 3. Comparison of xenon isotopic composition of (1) MORB-glasses (shown are the data from MARTY(1989) where alien xenon is most pronounced-(a) CH31-DRl I-NG, 1500°C; (b) CH31-DRI I-NC, 750°C; (c) CH98-DRI 1, 750eC), (2) Greenland anorthosite (JEFFERY,197I), (3) solar xenon (OZIMAand PODOSEK,1983), (4) atmospheric xenon, mixed with xenon from 244Puand 238Uspontaneous fission.

data points are outside the range occupied by mixtures of atmospheric xenon, solar xenon, and xenon of spontaneous fission of 238Uand *&Pu. Therefore, the concept of the presence of plutonogenic xenon in MORB-glasses (MARTY, 1989) does not seem to be supported by the data (Fig. 3). In fact, both some MORB-glasses and Greenland anorthosite (JEFFERY, 1971) seem to contain anomalous xenon with the isotopic signature of alien xenon (Fig. 4): excess of 13’Xe,

2-

13’Xe, ‘34Xe, ‘36Xe, and lz9Xe relative to 13’Xe. The fact that the excesses are correlated (Fig. 5) suggests their formation in a common process. It is important to note that ‘29Xeusually interpreted as the decay product of extinct ‘291-is correlated with the excesses of the heavy isotopes. This association must cast doubt on the postulate of a specific (iodogenic) origin of excess ‘29Xe in the Earth mantle. However, the most surprising fact was that alien xenon similar to xenon from Allende inclusions was found in samples of uranium ore from the natural nuclear reactor-the

(a)

7 1 ~~ (b) Xet 6’o II 1.8

Xeat,

6.0 t

128 130

132

134 136 A

FE. 4. StNCtUfe of xenon spectra in MORE&glassesand Greenland anorthosite normalized to atmospheric xenon. AA = (AXe/‘ufXe)l.mD,le - (AXe/‘30Xe)atmospher,~.

11

’

6.0

2x)

’

1

’

”

2.4 2.6 2.8 13sXe/‘30Xe

2.2

/

6.5

f

/

I

7.5 7.0 8.0 132Xe/‘30Xe

1’

3.0

1

3.2

!

1

8.5

FIG. 5. Isotopic correlation of xenon in (I) MORB-glasses (MARTY, 1989) and (2) Greenland anorthosite (JEFETRY, I97 1). The data points are far from the area of mixing (3) solar, (4) atmospheric xenon, and 244Puand 238Uspontaneous fission Xe,

3081

Anomalous isotope composition of Xe in Allende inclusions Table 3. Xe in bulk ore samples from the Oklo nahlral nuclear reactor, well SC20 (Shukolyukov et al., 1976, 1977); l%e= 100.

samples directly from the chain reactionzone 5.95

1348

samples from

7.30

1364

7YO

I368

8.40

1371

0.05 0.00082

O.W26

0.00048

-

239pu+"*lx

2.9

11

126.4k1.4

83.3kl.O

47.5kO.9

O.IJ~O.02

13.5*0.2

14.8

10

124.Ci+o.7 77.7kl.O

47.8kO.7

0.16+0.02

13.5kO.2

22.1

20

128.5kl.O

48.3kO.7

0.17+0.02

13.2+0.1

20.0

42

124.9+1.2 73.9+0.8

45.0*0.5

0.11~0.02 11.9?0.3

0.609

715

0.13

124 3+1.4

77.2+0.9 _

45.3kO.6

0.3*0.1

13.lkO.3

I05

0.051

122.111.4

71.2+1 0

44.9e1.0

0.12~0.01

-

12.i*o.1

163

0.098

125.7+2 2

73.6+0.4 _

46.OeO.3

0.16kO.02

13.6~0.2

748

0.073

122.I;1.7

73.8+1.0

44.6kO.7

0.14?10.02

12.6+0.2

0.740

973

0.039

124.7kO.8

70.3cO.7

43.6kO.6

0.11~0.02

12.4*0.2

149

0.026

123.2*1.5

70.4*1.1

IJ.J+O.Y

0.09~0.01

1l.trL.z

906

0.053

125.2+0.9

71.6kO.5

45.5io.5

0.17+0.01

13.0*0.2

971

0.014

123.4k1.3

70.6kl.O

43.4kl.l

0.23+0.05

-

12.6&O .I

207

0.019

122.6+0.8

69.8*0.9

44.9kO.2

0.35+0.02

12.6?0.2

978

0.010

122.8kO.8

70.4kO.7

44.6kO.5

0.5+0.02

12.6+0.2

124.1+1.3

67.2kO.2

41.1*0.1

-

10.5+0.3

_

82.5kO.6

57.9+0.9

8.2+0.3

-

0.2kO.l

_

112.5+0.6

79.1tO.3

57.7t2.1

-

25.7t1.3

_

nw+n,cl,u8u,w

0.577

80.4kO.5

envimnmenlsl rocks

6.80

1361

26.85

-

,lKlwn an*l"=:enCYa :n II .:...A:~trd ."_*.,,:r rr n.,. ..,-.._nL,.:".A ,,"Ar_r.lra*,."A:,:r\"r *I #-_,.".nb,,. l....",ol Mhvn

anal&is ofXe in h&h-REE

samples (samarskite, monazite,betafile)

WFields et al,1966

uranium deposit at Oklo (Republic of Gabon). Xenon extracted from Oklo uranium ore has also been studied by other groups (NAUDET, 1974; NAUDET and RONSON, 1978; IAEA, 1978; DROZD et al., 1974; BASSIEREet al., 1975; RUFFENACH, 1978). But none of them has noticed one feature of this xenon that we accomplished to explain. In the mid-seventies we analysed samples from the Oklo deposit (SHUKOLYUKOV et al., 1976; SHUKOLYUKOV and DANG VU MINH, 1977a). Xenon in bulk uranium ore taken from zone 2 (well SC20; depth of sampling 5.95 m) contains an isotopic peculiarity that was unknown before: excess of ‘32Xe, ‘3’Xe.‘34Xe, and ‘29Xe relative to ‘36Xe (after subtraction of a very small admixture of atmospheric xenon). (Table 3). The isotopic composition of this xenon falls outside the

triangle spanned by thermal neutron fission of 235U and 239Pu and fast neutron fission of 238U (Fig. 6). The same pattern was later also observed in another sample taken from well C-52 in ore zone 3 (SHUKOLYUKOV et al., 1985). The strange xenon (and krypton that is enriched in 84Kr and s3Kr) comprises l- 10% of total xenon (and krypton). Stepwise heating bulk samples of Oklo uranium ore (Table 4) shows that the anomalous xenon is extracted mainly at low temperatures whereas xenon of neutron-induced 235U fission is degassed at higher temperatures (Fig. 7). In the range 400-800°C the ‘32Xe/‘3hXe, ‘3’Xe/‘36Xe, and ‘29Xe/‘36Xe ratios are often two to three times, ‘34Xe/‘36Xe 7-12%, higher than the corresponding ratios for xenon from thermal neutron fission of *YJ.

0.80

FIG. 6. Distribution of xenon isotopic ratios from pitchblende (Well SC-20, Zone 2 from the Oklo deposit). Also shown is the range of isotopic compositions given by mixtures of (1) 238U spontaneous fission Xe, (2) “% and (3) *39Pu thermal neutron induced fission xenon, and (4) 23*Ufast neutron induced fission xenon (Tables 3, 12).

3082

Y. A. Shukolyukov

et al.

Table 4. Xe released by stepwise heating from ore samples #is48 selected from the OMo uranium deposit (well X20); 13%e= 100

22.1

20.0

20.0

20.0

2.9

14.9

250 400 600 800

0.04 0.09 0.94 1.10

123.Okl.S 128.6rtO.5 137.211 128.1-(_1.4

loo0 1105 1350

9.10 3.60 s.10

total

19.97

131.3+1.5 125.8kO.7 123.910.5 128.s+l.l

300 500

0.39 0.33

123.Okl.5 135.5+0.5

700 900 1100 1250

I.10 I .40 30.0 8.80

rota1 300

72.910.3 118.0&2.0 192.0f 1.O 103.7k2.2 74.lkl.O 71.820.2 70.6+o.s 80.2*0.8

44.6k0.5 SfZ.OkO.8 78.8k 1.5 56.1&1.2 47.3kl.O 44.2kO.l 4s.a+o.s 48.5%0.7

1.OQ*o.10 0.38kO.03 0.08*0.02 0.11~0.01 0.10~0.01 0.05*0.01 0.3810.07 0.17~0.03

13.3kO.S 17.4kO.3 22.8+0.1 15.3kO.6 12.3kO.l 12.210.1 13.510.1 13.3+0.1

42.02

134.!J*o.3 125.ot0.5 123.840.6 127.3+3.4 125.Of 1.2

74.9kO.8 162.011.0 166.0+0.4 73.1+0.6 70.3f0.3 72.0+2.3 74.0f0.7

48.8kO.6 72.0~0.6 78.3t0.3 45.2kO.4 43.8CO.9 43.8+1.4 45.0&i .o

O.IJ~O.05 0.24*0.02 0.09~0.01
13.3kO.3 18.0~0.1 19.7+0.1 12.0*0.1 11.5+0.2 12.0+0.4 11.9-to.2

500 600 totalt’)

0.09 0.20 0.28 0.57

124.S_eO.5 139.4* I .o 13s.1+1.0 136.4kO.9

77.2fO.2 18S.Ojrl.O 197.0+2.0 173.9kl.4

44.3kO.8 76.7+0.1 91.2+1.0 78.7+0.7

0.38kO.l 0.24*0.01 0.12+0.01 0.20+0.02

13.5io.s 21.4*0.2 27.4+0.5 23.1kO.4

300 500 600 totIm

0.013 0.14 0.08 0.233

123.Srtr3.0 134.9f0.8 141.0+1.0 136.4kl.O

so.3*1.0 184.0f0.2 230.0t5.0 194.lfl.9

46.1k4.0 80.3kl.O 94.5+1.0 83.3+1.2

3.63tO.M 0.13+0.02 O.S?+O.OS 0.35t0.03

17.2&0.5 22.81tO.S 24.6+0.4 23.11tO.S

144.1&1.0 136.2k3.0

221.0+2.0 169.Ort3.0

104.5~5.0 76.Ok3.0

1.4to.s <0.14

26.0&0.5 21.9_+o.s

72.5+1.0 68.5kO.7

44.8kl.O 41.850.5

0.14+0.05 0.920.5

13.OiO.2 11.5+0.1

52.8kO.2 70.2k3.2 59.X+1.0 46.2-iO.5 4n+o.5 47.SkO.7

0.22+0.05 0.37+0.05 0.47+0.05 0.1 I *o.os 0.09+0.03 0.15~0.04 ----

15.7kO.2 22.8kO.O I8.6+_0.5 12.7kO.3 12.2+0.1 13.7+0.1

41.liO.l

-

10.5+0,3

0.33 750 1.00 loo0 (fraction lost) 1150 3.00 1350 6.80

12F%.4*2.0 124.5*1.0

300 500 800 1065 1160 total

127.4k3.0 126.9+2.7 131.3+3.0 123.5+0.3 124.6+0.4 125.0~0.8

92.24 1.O 137.5k5.0 105.6+3.0 70.1 iO.5 7o.s+o.5 77.9kl.O

124.1&1.3

67.2kO.2

0.24 0.47 1.00 3.90 4.10 9.71

ZS” + nih(?> (I)Only of the given fractions.

t

(2)Own data, cf. Table 3

1

500

F

1000 1500

500

Extractton

1

I

1000 1500

Temperature

500

[‘Cl

I

1000 1500

FIG. 7. Variation of the isotopic composition of xenon during stepwise degassing pitchbiende from Zone 2, well SC20, Oklo deposit (22.1 mg sample from Table 4). The dashed lines correspond to xenon from 235U thermal neutron fission.

Anomalous

6

isotope composition

ANORTHOSITE, GREENLAND

4

3083

for the occurrence of the strange xenon in the Oklo natural reactor will, in all likelihood, also provide an explanation for the alien xenon found in other places.

Is Anomalous Xenon in the Natural Nuclear Reactor at Oklo a Product of Nuclear Reactions?

OKLO, PITCHBLENDE, 20 mg, 600°C

:[

)

The effective thermal neutron fluence experienced by the Oklo deposit material was 1O*‘- 1O*’ neutron/cm* which may have caused formation of xenon isotopes by reactions on tellurium and barium (and of krypton isotopes on selenium and brominal). As far as tellurium as a target is concerned, previous experimental studies of Te-containing minerals irradiated by neutrons of different energies (0.5-7000 eV) revealed only 13’Xe and ‘29Xe as the dominant products and, in addition, the absence of any 13*Xe excess (BROWNEand BERMAN, 1973). In addition, in tellurium minerals enriched in uranium we have observed only a very small 13*Xe excess compared to 13’Xe and ‘29Xe (GERLING et al., 1967). Therefore, we examined experimentally xenon produced by irradiation of BaO with thermal and fast neutrons via the reactions: n,p(13’Xe, 13”Xe); n,2n(13’Xe, ‘29Xe); n,a(‘3’Xe); n,c’(‘34Xe, “*Xe, 13’Xe, ‘29Xe), etc. The resulting xenon isotopic compositions, given in Table 5, differ drastically from that of anomalous xenon from the Oklo samples. We conclude that direct formation of anomalous xenon by n-irradiation of barium is excluded. An indirect connection, however, with the n-reactions on barium is possible: 13*Xe formation by epithermal neutron capture of 13’Xe (cross-section 90 barn, resonance integral 870 barn). With a total fluence of 4 X 102’ neutrons/cm2 and 8 X lo’“-2 X 10zo neutrons/cm2 of epithermal neutrons

hMoRB

40 :A

ALLENDE, 29-1.

1650” C

20 0

of Xe in Allende inclusions

--128

-----

--

130 132 134

136

A structure in Greenland (the second 20 mg sample in Table 4), MORB glass CH3 I-DRI I-NC, 750°C (MARTY, 1989) and the Allende sample 29-1, 1650°C.The latter is normalized to solar xenon, the others to terrestrial atmospheric xenon. FIG. 8. Similarity

of the xenon

isotopic

anorthosite (JEFFERY, 197 I), Oklo pitchblende

Most strikingly, however, is that the fine structure of the anomalous xenon component in Oklo resembles that of alien xenon from Allende inclusions, in MORB-glasses, and in an ancient Greenland anorthosite (Fig. 8). Thus, the explanation

(BASSIEREet al., 1975; RUFFENACH, 1978) one would estimate the maximum value of ‘32Xe/‘3’Xe (a) to appear in pitchblende and (b) not to exceed 2. In fact. xenon in pitchblende is the standard 235U fission product without any indication of 13’Xe conversion to “*Xe, and the anomalous xenon is characterised by ‘32Xe/‘3’Xe = 4.4. Thus, the 13*Xe isotope anomaly in the Oklo materials is neither connected to direct nuclear reactions with barium or tellurium nor to 13’Xe burning. Next we consider fission of other heavy nuclides (Fig. 9). But the “fine structure” of alien xenon differs quite significantly from known yield curves for spontaneous or neutroninduced fission of heavy nuclides including transuranium elements (Pu, Am, Cm, Cf). We note, however, a qualitative

Table 5. Isotopic composition of Xe produced by neutron irradiation of BaO. For comparison the respective values are given for the Oklo samples, which contain anomalous Xe. 131Xe= 100.

Y. A. Shukolyukov et al.

3084

6

0 129

131 132

134

136

129

131 ?32 A

A

134

136 129

131132

134

136

A

FIG.9. Comparison of (if anomalous xenon from the Oklo deposit (#6G. 15mg after treated with acids (Table 9)) with (2) xenon of 238LJ.244Pu,“*Cf, ‘42Cm, z40Pu,254Fmspontaneous fission, (3) “‘Th, “‘Th, ‘?J, ““Pu, *42Am.248Cf, 235Uthermal neutron fission, and (4) Z32Th,24’Am, ?I fast neutron induced fission (see text).

Alien Xenon and the Separation of Precursor Elements in the Oklo Deposit

similarity nuclides:

to curves of fission fragments from the following “‘Frn, 257Fm, 259Md, 260Md, “*No, and 260(104) (GORBACHEV et al., 1976; SCWMITT and MOSEL, 1972: RAGAINI et al.,1974). In both cases a maximum at ls2Xe (via its double magic precursor ‘32Sn50)can be observed. However, it is absolutely unclear how these heavy nuclides could have formed in the natural nuclear reactor with a moderate neutron flux (10’ n/cm2/s) since they have never been discovered in high flux technical reactors ( 10’1-10’4 n/cm’/s). Thus, neither known nuclear reactions nor fission of heavy nuclides could directly produce alien xenon in a natural nuclear reactor. Nevertheless, this mysterious xenon is connected with fission, but not directly. In the next chapter we will show that anomalous xenon has been formed by chemical separation of radioactive precursors, which are fission fragments.

Xenon and krypton isotopes have the rather long-lived @active precursors Sb, Te, I, Sn, and Se, Br, respectively. Their half-lives are sufficiently long that the precursors may diffuse from the fuel (pitchblende) into other mineral phases. To test this hypothesis we have analysed xenon in various fractions of sample 1348, which differed in density, magnetic property, colour and grain size. The results show (Table 6) that anomalous xenon is indeed contained in pitchblende but is enriched in one of the light fractions, notably in sample #9. The available amount of this sample, however, was too small for further separation. Therefore, in order to gain further insight, we have studied using stepheating which ofthe mineral and den-

Table 6. Xe in rninerai and density fractions prepared of sample #I348 from the Oklo deposit; 13aXe= 100. grain

sampla

densit g/cm 1

No.

#14: lorggc crystals

0:

500-200

c&/g

‘We

‘We

I3’Xe

ixie

‘nXe

6

22

123.2~1.1

70.8kO.S

44.1 io.3

0.086+0.003

11.8+0.2

SW-200

1s

69

124.2kO.3

71.7+0.2

44.6kO.l

0.083iO.001

11.7~0.1

66

of uraninite

26.0

200-50

10

Bulk

24.3

5-00-20

15

#2: Bulk

14.3

550

49

X9: Bulk

64.3

ZOO-50

46

#7: Bulk

3.4-4.3

500-200

30

#6: Bulk

$3.3

500-200

43

6E

3.0

6~

17.0

4.4 12 1.9 30 2.1

123.1~0.4

70.8kO.4

43.6f0.3

0.081+0.003

126.4f0.3

81.3+1.0

48.5?10.4

0.14*0.02

124.5f0.7

s4.3*0.5

45.0+0.3

0.086~0.001

132.7k1.7

128.6f1.2

65.1+0.6

0.12+0.03

126.6kO.6

85.0*0.8

49.0+0.2

0.085+0.005

126.7kO.8

84.1+0.6

48.1t0.5

0.95f0.03

2.8

125.3*1.1

0.21

1x3+0.8

11.7fO.i 13.3*0.1 13.140.1 16.8~02 13.2kO.5 13.5*0.3

82.6+1.3

47.9fO.6

co.01

13.1_+0.2

102.1*1.3

5s.s+o.3

SO.4

14..5*0.3

6D

15.0

0.81

127.3kl.O

119.0t0.8

60.3iO.S

SO.2

16.1 eO.2

68

15.0

0.59

128.7kO.3

133.OkO.9

65.6tO.J

0.34+0.12

i-/.9+0.1

6G

17.0

2.9

129.7+0.9

128.6+1.0

62.4kO.4

0.12?~0.02

17.6+0.1

61

16.0

4.0

130.1+0.6

131.l?rl.l

63.3iO.6

0.12+0.02

17.5+0.1

6E: brown chlorite; akr

ST:

with admixtures

26.0 #16: small crystals

weight mg

of umninite

~6.0 #12: large crystals

SalllpfC 1rrXe

size v

minerals,

possibly

nonmagnetic;

removal

of samples

hematite;

nonmagentic;

6B: yellowmineral,

possibly

6D and 6E, nonmagnetic.

6A: brown chlorite;

mineral,

magnetic;

possibly

hematite;

6G: remains

after

sslccred removal

manually;

of samples

6D: ye&w 6~ and 68,

minerals, magnetic;

pssibty 6.1: mmins

3085

Anomalous isotope composition of Xe in Allende inclusions

sity fractions ##14, 16, 5, 12, 2, 7, 6 would be the closest analogue to sample #9 (Table 7). The degassing behaviour of the anomalous xenon (Fig. 10)

shows that sample #6, which is the lightest fraction of the bulk sample, may be considered as such an analogue. Therefore, fraction #6 was used for further manual and electro-

Table 7: Xe releasedby stepwise heating of mineral and density fractions prepared from aarnple #I348 from the Oido deposit, well C20, ore zone No.2. For sample description cf. Table 6. IsaXe= 100. sample

NO.

0.3*01 0.22kO.07 0.08j10.01 0.08+0.01

12.4kO.3 19.7310.3 12.9f0.2 ll.S+O.t

123.2iO.4 71.4kO.4

43.8kO.4

0.08~0.01

11.7k0.1 14.1kO.3 20.5*0.1 l&2+0.1 12.6+0.1 13.3-1-0.1

ii::: 3.9 4.42

12S.Okl.8 132.0&1.4 131.0+0.6 125.8+0.3 126.420.4

107.5i2.0 170.0*0.5 126.2k2.0 72.9+0.9 81.1+1.0

45.1jzl.O 75.5zkO.Z 6S.OkO.6 45.9+0.4 48.4k0.4

K 4.0 65 .O _

126.Ok1.8 136.5fl.l 126.3+0.7 124.0+0.3 124.2&0.3

95.2k3.1 182.Oi3.0 92.4k1.5 70.2+2.0 71.7t2.0

51.6k1.3 80.1k1.2 52.2t0.4 44.1+0.1 44,QO.l

0.18~0.06 0.31kO.02 0.08kO.05 0.08iO.01 0.08~0.01

14.2kO.4 22.3i0.4 14.2tO.2 11.9+0.1 12.OfO.f

126.6k1.4 134.OkO.S 132.6kO.7 122.8+0.7 125.7kO.7

100.0~3.0 198.0*2.0 152.0&-2.0 69.4+0.5 94.6+1.0

53.3k1.6 81.2kO.6 69.6kl.O 44.2+0.2 51.9kO.4

0.23+0.01 0.14+0.01 0.08+0.02 0.08+0.01 O.OSi-0.01

14.4t0.S 22.3kO.2 18.6t0.2 I I .9+0.1 13.9kO.i

0.1 0.16

2

44.3il.O 73.7k0.9 48.1kl.l 42.9f0.3

59.1zk2.3 fX'9:$;

0.41+0.15 0.18+0.06 0.69+0.10 0.10+0.02 0.14zk0.03

0.05

12

125.5k2.4 75.8ztl.0 135.SjrO.6 155.Ok2.0 125.4t0.8 83551.3 122.8+0.3 68.9+0.3

ii:! 9.5 56.0

5.97 5

14.9t1.0 18.4kO.3 14.3*0.1 t1.7*0.2 11.8f0.2 ;;:";":I 8 02

2:: 21.47

"0::: 15.0

16

43.8*0:3 43.7*0.4 42.7+0.2 44.110.3

<2 <0.6 0.09~0.02 0.09+0.02 0.10*0.&5 0.07+0.05 O.lOkO.04

129.9f3.S 101.0+4.0 130.6k1.3 117.Ort4.0 127.250.3 91.3i0.4 122.9kl.l 69.3i0.5 124.1~1.471.6~0.9 122.2+0.9 68.4+0.6 123.2kl.l 70.7kO.6

0.05

14

::: 1.16 7 8::: 5.8 23.0 ”

126.Oi1.4 112.0*0.7 145.6*0.6 268.Ort3.0 129.420.4 125.OiO.6 1?5.7+0.6 74.6+0.8 126.8iO.6 88.6+0.8

0.25+_0.08 56.0+0.7 101 .OC I .O 0.06~0.03 0.09+0.02 6U$$.; 0.08+0.05 50.0&0:$ 0.08+0.04

130.5&3.0 146.4k0.9 147.1~1.8 126.1 f2.0 128.1+1.8

63.7k0.8 118.0&2.0 120.0&1.0

16.3+0.1 26.1 i-O.4 17.0*0.2 12.3+0.6 13.5*0?i

is:: 65.0+0.8

3.3io.4 0.92iO.02 0.19iO.01 0.06*0.01 0.10+0.04 0.13&0.03

29.4kO.4 32.9kO.3 29.4kO.2 11.2*0.5 12.7.tO.2 16.8rtO.~

129.4* I .5 81.0+0.6 159.024.0 346.Ok2.0 140.8~1.0231.0~1.0 124.8kO.8 73.4f0.5 125.8i0.9 70.11tl.O 124.0+0.8 72.6+0.4 126.1i0.8 84.7kO.6

48.6f2.1 119.Op4.0 95.Ok2.0 45.4kO.4 43.5+0.5 42.8+0.5 48.3+0.6

1.2+0.1 2.5+0.4 0.14-tO.02 0.09+0.02 0.08tO.05 0.07+0.05 0.09+0.03

16.9f0.8 35.6*1.0 26.6kO.6 12.5~0.1 II.! 20.1 11.7+0.2 13.3*0.’

EC! ._‘,,

125.8+1.0 99.8i2.0 125.9+0.3 151.2kl.O 125.541.5 86.1+2.3 133.3+1.5 85.0+0.5 ,783 ,Z,O?G_.+. 14 _.t.

44.lk1.2 71Sk0.5 48.4kO.2 54.2fO.l 558.f. 03

co.7 0.41+0.10 CO.1 CO.6 5. 037

20..5+0.5 20.5+0.2 12.3kO.4 11.3+0.2 14.3kO.5

E: 0.15 2.45 _.I

139.120.5 130.2kl.O 141.11-1.0 185.3+1.0 119.6tO.9 lOG.O~l.1 124.5+1.1 74.4+1.3 154 11827 .*. ,3 __+-.

69.4ztO.S 76.Okl.O 53.6kO.5 45.4+o.c5 480.*. 06

so.7 so.3 so.3 ~0.08 s.-G,?

16.7&O.?. 22.OkO.3 15.4*0.1 13.4+0.2 13.1t0.Z

68

0.006 0.230 0.200 0.150 586

125.4+_0.5 138.7kO.l 124.3kO.4 122.8+0.5 129.6kO.3

165.9+1.0 216.9*1.0 100.4+0.8 77.2+0.3 140.51tO.S

56.4+0.5 93.1+_0.2 %4$;.; 68.2+0:4

50 Y 0.36+0.10 0.28+0.08 so.4 so.35

15.8*0.6 ?5.4iO.i 14.8+0.1 13.4eo.1 18.6kO.r

6J

0.014 0.043 1.800 2.OaO 0.165 02

124.5+0.5 131.7kO.7 136.7iO.4 124.9fO.S 122.4+0.3 130.2kO.6

132.0-10.5 278.4kl.O 200.2+1..5 71.0i0.9 72.0+0,2 131.3i1.1

60.8+0.2 93.OkO.5 85.9+0.6 44.OkO.6 44.4+0.2 63.4+0.6

SO.6 SO.2 0.13~0.10 SO.09 0.28+0.02 ~0.12

0.01 0.21 0.20 0.39 1

153.6k2.0 139.1~1.5 124.0f0.6 123.4+ I.0 128.Okl.O

282.6i2.0 161.6k1.5 81.1+0.6 73.9+0.6 lW.9*0.9

123.5kl.5 107.1*0.8 47.5fO.S 45.4co.3 62.9kO.5

cl so.2 so.09 SO.05 co.11

0.03 0.06 1.20

118.4iO.7 153.2k1.5 133.9rt1.3 125.920.3

75.1kO.5 317.2il.O 195.9k2.2 72.3kO.3

41.3cO.6 114.01tO.5 84.2kO.6 44.6i0.t 44.0fO.T

10.9 so.5 0.14kO.10 0.13~0.10 0.11+0.10

0.003 0.03

9

I.?“: 1x7 19t+1.8 6 EC? 0.15 I.1 0.44 0.42 _. 0.009 0.051

6A

6E

60

6G

1350 lOLlI

Il:E _.

105.0f0.3 327.OkS.O 284.Ok2.0 78.0+ 1.6 73.3+1.5 128.3i1.7

15.5*0.2

26.8+0.5

23.8f0.3 12.2+0.4 12.8i.O.l 17.6tO.F

12.2co.2 16.8*O.f 14.1+0.2 29.1*0.4 26.4+0.4 12.4fO.I 12.1+0.1 18.6+0.5

Y. A. Shukolyukov et al

3086

N9

determined the isotopic composition of U, Nd, Sm, Ce, and K in sample 6G. A suspension of sample 6G was treated with 30%~ HNOs during 12 h at room temperature, whereby the major portion of pitchblende was dissolved. The isotopic analyses of Nd, Sm, U, and Ce in the solution showed unequivocal signs of the 235U fission (Table 8). Isotopic abundance ratios ‘43Nd/‘44Nd. ‘4sNd/144Nd, iJhNd/‘44Nd, 14XNd/ 144Nd in acid extract turned out to be close to those produced from neutron-induced fission of 23sU. Excesses of “(Srn and ““Nd are due to high neutron fluence accompanying nuclear chain reaction. The higher (compared to normal) ‘42Ce/‘4*Ce ratio results from the production of ‘42Ce due to the chain reaction. As evidenced by the uranium isotopic composition, a significant portion of 235U was already burned out. Thus, pitchbiellde had existed, and was in the zone of neutron reaction, during the operation of the natural reaction in the deposit. The K-bearing phases (mica, feldspar), which remained undissolved by HN03, were probably formed after the termination of the fission chain reactions, i.e., they are secondary. This is evident not only from the normal or nearly normal isotopic composition of Ba. Nd, Sm. and Ce in the residual after treatment with HNO,, but from the approximately normal potassium isotopic composition (Table 8). From the amount of burned out 235U in the bulk sample 1348, one can estimate the effective thermal neutron fluence to be about 4.2 X 10’” neutrons/cm2. If the K-bearing mineral phases

N6

1.6 t

500

1000 1500 500

Extraction

Temperature

1000 1500

[“C]

FIG. IO. Variation of xenon isotopic composition in stepwise extraction of mineral fractions 9 and 6 of the sample 1348 (well SC20. Zone 2, Oklo deposit). The dashed lines correspond to 23sU-i- nth.

magnetic su~ivision. However, stepwise degassing of the subfractions obtained showed that such techniques did not succeed in separating the mineral-carrier(s) ofthe anomalous xenon. That is why from all subfractions obtained, we have selected sample 6G, of which the 500°C fraction was by far the richest in anomalous xenon. All further experiments were carried out with this subfraction. X-ray and microprobe analyses of this sample indicated the presence of two main phases, muscovite type dioctahedronic mica and dioctahedronic chlorite. Minor mineral phases are typical pitchblende, carbonates (mainly calcite) and sulphides (pyrites, galena). Mica contains l-2% barium both evenly distributed and as aggregates with sizes up to 20 pm. In some cases these are probably barytes, as evidenced by the characteristic lines of barium and sulfur, while in other cases we identified hyalophan ( 13% KzO, 58% SiOz, 2 1% A1203, 0.1% CaO, 0.12% FeZ03, 8% BaO). To find out which mineral phases of sample 6G were formed before or after the fission chain reaction ceased, we

Table 8. Isotopic composition of Nd, Sm, Ce, U, K, and Ba in acid extract and residual after treatment of sample 6G (cf. Table 6) with 30% HNOs. The normal isotopic compositions am from De Bievre and Barnes (1985). The stated uncertainties correspond to the last given digit of the isotope ratios. n.d. denotes IWI determind. U&f denotes isotopic composition of element produced by thermal-neutron induced fission (Gorbachev et al., 1976). ratio

extract

residual

normal

“‘u$j$

%PK

7659+_320

W%

SO+ lb

8578+ 109 592+ 4

7991 577

I .07

‘30Bn/‘38Ba IJ?Ba,i”8Ba

n.d.

o.O014s+7

0.00148

n.d.

0.00142*7

o.cO141

zo

134&p3Ba

ad. n.d.

0.03368+ 12 0.0913Okb

0.0337

ad. n.d.

0.10856_+ 13 0.15546+14

0.0919 0.1095 0.1566

1.12 -0

0.511~0.005

EO.1

0.1252

‘35Ba/‘38Ba ‘3”Ba/‘38Ba ‘37Ba/‘3*Ba “%3ol’*ce

-0 ~0 0.933

“‘Nd/‘“Nd

0.092 & 4

1.068*19

1.142

-0

‘43Nd/‘uNd “‘Ndt’J4Nd

0.848*9 0.608*8

0..568+ 10 0.394*9

0.513 0.349

1.071 0.708

‘~dJ’~Nd ‘“Ndi’“Nd

OS27fS 0.293 44

0.71657 0.259&S

0.721 0.242

0.547 0.304

lmNd/‘“Nd

0.166+4

0.237*5

0.237

0.119

‘%m/‘%Sm

I .43 kO.01

SO.3

0.277

0.003

TJPU

171.5*0.1

n.d.

137.88

-

3087

Anomalous isotope composition of Xe in Allende inclusions Table 9. Xenon released by stepwiae degassing from aliquots of sample 6G (cf. Table 6) after various acid treatments. 136Xe= 100. weight [mg] concrnwation of acid [%]bcfore atIer HF acid trea~~~ent HNO, HCC 3

10

30

67

-

-

_

-

-

-

22

22

22

20

18

17

300 500 800 1100 1350

0.011 0.028 1.723 7.630 1.866

123.9+1.1 142.8kO.9 134.1iO.9 122.6*0.8 126.7+0.6

1os.s*2.9 337.9k3.2 178.5*2.0 73.9+0.3 74.2+0.3

%5.7*1.0 106.8+0.7 79.5kl.2 44.8kO.2 45.8+0.1 50.4+0.3

51.4 51.5 50.06 ZGO.09 so.08 so.09

15.8kO.4 40.7kO.7 22.0*0.4 12.4+0.1 12.5+0.1 14.0+0.2

Iota1

11.258

125.1 f0.8

90.6kO.6

300 500 800

0.007 0.051 0.871

132.8k2.0 151.0*2.1 147.8k2.3

295.7+5.0 371.0_+3.8 312.6+3.7

90.1+5.0 116.8+1.2 122.4kl.8

12.0 SO.7 10.07

40.9* 1.4 36.2iO.4 34.6kO.4

1100 1350

0.058 0.016

129.2kO.9 136.4+2.0

159.6k2.0 145.9+3.0

81.0+3.0 82.9+3.2

50.2 Cl.4

1.003

146.6k2.2

303.9t3.6

118.9+1.9

SO.14

20.2+0.2 22.2+0.5 33.7kO.4

iolal 500 800 1100 1350

0.072 0.556 0.043 0.024

160.0*1.5 153.5+1.0 131.4*0.9 136.7+2.2

463.3k4.5 337.6kl.8 159.7k2.3 180.0+2.0

133.1+2.2 132.5+1.3 74.9kl.l 87.6+2.1

SO.5 SO.1 SO.2 s1.1

lotal

0.695

152.2+1.1 _

334.5k2.1

127.4k1.4

50.1X

11.2~0.6 3x.3*0.3 20.6+0.6 2X.8+0.6 37.2kO.4

-

22

18

300 500 800 1100 1350 tora1

0.006 0.160 0.475 0.093 0.030 0.764

139.1~1.0 155.8+1.6 154.8iO.4 135.6kO.6 121.0+1.0 151.2+0.7

334.821.5 433.8e2.2 333.Ok3.3 162.2+1.1 a7.4+3.0 323.7k2.8

202.6k3.0 128.Ok3.0 129.5kO.7 76.6i0.5 55.6+1.5 120.4kl.2

~3.0 SO.3 so.15 so.7 50.6 50.29

39.9+2-o 38.6tO.5 37.2+0.4 21.5+0.1 15.3+0.2 34.7kO.4

300 500 800 1100 1350

0.004 0.090 0.356 0.080 0.029

303.0~10.0 452.8k4.7 325.2k3.2 169.0*3.0 124.8+2.5 312.8k3.4

91.6k2.4 130.4kl.l 128.1~0.9 82.7kO.8 71.6+1.7 118.8+1.0

L3 SO.4 zZO.1 so.5 51 SO.27

32.Ok2.2 41.0*0.4 37.lkO.4 23.6kO.2 23.4+1.3 35.OkO.4

30

6

-

32

31

lOUI

0.559

129.0+3.5 154.6+1.2 154.OiO.6 130.6*1.6 126.7+2.2 149.2k0.9

30

12

-

38

36.5

300 500 800 1100 1350 roral

0.003 0.076 0.644 0.033 0.025 0.781

131.2+1.1 153.8+1.0 153.4kl.O 130.1kl.0 123.5+1.5 151.4+1.0

288.Oi5.0 427.7k5.0 32O.Okl.2 168.1f3.5 136.0+1.0 318.0+1.7

92.8k2.7 126.6+1.7 127.0*1.1 80.0+1.1 74.9+1.3 123.2kl.2

52 ~0.3 so.5 so.3 52 50.5

32.7+0.3 38.1*0.3 36.4kO.3 26.4kO.3 22.0+0.5 35.7kO.3

30

18

-

35

34

300 500 800 1100 1350 lOlaI

0.005 0.093 1.260 0.072 0.026 I.456

125.3+4.2 153.1+1.6 152.0*0.3 131.6*0.6 12.5.0+1.0 150.5*0.4

206.4k2.0 408.2k6.0 312.5?10.7 166.2~1.0 134.4+2.0 307.s*1.1

86.7k2.7 123.0+1.6 122.5kO.6 82.2+0.3 66.5+1.5 119.4*0.7

53 co.2 ~0.07 SO.5 51 so.13

32.OCll 37.9co.5 34.6+0.5 24.1 kO.6 22.1+3.0

300 500 800 1100 1350 1OM

0.003 O.OY5 0.887 0.079 0.021 I.085

148.0+3.0 152.3+‘.6 152.3;O.E 127.8k1.4 124.8+1.6 ISO.O+l.O

288.0+5.0 470.4?5.0 327.0kl.l 165.6kl.7 117.8+2.0 323.6kl.5

93.lk5.0 126.0&?.4 126.3+0.9 82.1&0.5 65.9+1.X 121.8+1.0

52 50.3 50.08 co.3 c 1.5 so.15

33.1*0 8 37.X+0.6 36.3t0.1 23.1 to.5 IX.OCO.2 35.1 yo.2

500 800

0.049 0.227

173.9k2.5 165.6+2.0

600.0~10.0 453.lk2.0

161.Ok3.0 158.Ok3.2

51.5 ~3.0

1100 total

0.047 0.323

127.4+3.0 161.3k2.2

157.8+2.5 432.4+3.3

79.9+1.3 147.1+2.9

s4.0 52.9

45.OeO.6 42.Ok3.0 30.7+0.5 40.x*2.3

30

24

30

-

30

30

-

-

unprocessed

-

48 3osec

48 60.~

48 120sec

sample 6G

35

32

30

15

30

30

9

5

34.1~0.6

500 800

0.033 0.111

158.2+1.0 143.7k4.3

539.5*8.0 348.4k4.8

153.9t3.0 131.5k3.0

52 52.5

1100 toLa1

0.030 0.174

140.1+2.0 145.853.3

175.2+3.0 354.855.1

83.6+2.0 127.5+2.X

c3.0 52.5

53.0+1.0 34.3kl.O 27.2+0.6 36.6kO.9

500 800 1100 lOlaI

0.038 0.260 0.008 0.306

161.2kO.5 149.8&-2.0 115.6+1.0 150.3klr8

495.3k6.0 331.1?2.4 96.9+1.0 345.4k2.8

168.1&3.0 154.4+1.6 59.8+2.3 153.6+ 1.8

13 52 I? cl

46.7~1.8 33.2kO.8 23.6+0.5 34.6kO.9

total

2.86

128.8kO.9

128.7*1.1

62.5kO.4

so.14

18.6+0.2

had been formed prior to the end of the fission chain reactor, a sixfold enrichment of 40K would have resulted from this neutron fluence via 39K(n,y)40K(u = 1.96 barn). Thus, there are a number of arguments that the mineral phases remaining after the acid treatment are of secondary geochemical origin.

To enrich the phase in sample #6G that contains the anomalous xenon, the following sequence of experiments was performed: (1) Aliquots of sample 6-G were subjected at room temperature for 12 h to 3, IO, 30, and 67% HN03, and xenon was analysed in the residues. (2) Aliquots of sample #6G

Y. A. Shukolyukov et al

3088

were treated with 30% HN03 for 12 h and portions of the residue were further processed at room temperature: (2a) for 12 h with 6-, 12-, 18-, and 24%-HCl; (2b) with 48%-HF during 30, 60 and 180 s, respectively. Xenon was analysed in all undissolved residues (Table 9). The xenon content in the acid-processed samples decreases with increasing removal of pitchblende which contains the major amount of xenon. At the same time, the isotopic composition of xenon becomes more and more anomalous: e.g., the 500°C extraction fraction of the residue after treatment with 30%-HNOJ and 48%-HF for 30 s differed from primary *j5U n-induced xenon by 40% in 13“Xe/‘36Xe, 800% in ‘32Xe/ ‘I’Xe, 290% in ‘3’Xe/‘36Xe, and by 330% in ‘29Xe/‘36Xe. It is worthwhile to note that with the removing of common nfission component, anomalous xenon in residual becomes more and more similar to alien xenon from the Allende inclusions. The anomalous xenon was not only found in well SC-20, ore zone 2, but also in wells C-52 and SC-36 of zone 3 (SHUKOLYUKOV et al.. 1985) which demonstrates that it is not a unique, but widespread phenomenon in Oklo. To check our hypothesis concerning the origin of anomalous alien xenon in the Oklo deposit due to precursor migration we have carried out additional experiments. In order to mimic (to some extent) the conditions prevailing in a natural nuclear reactor, pitchblende #l-82 from a Siberian uranium-deposit was irradiated in an evacuated super-pure quartz ampoule with a thermal neutron fluence of 4 X 10”

Table before amount during

10. Xe released by stepwise degassing pitchblende No.l-82 and after irradiation with 4 X 1015n/cm2. Also given are the and isotopic composition of Xe accumulated in the ampoule n-irradiation. 136Xe= 100. The data are corrected for minor contributions of atmospheric Xe based on 130Xe.

‘=xbxe T

STPcm'

"C

X10-"

'"XC

"%e

"'Xe

'?xe

2.5*2

unirradiatcd sample, 18.3 rng 600

0.11

91.3*4.9

54.3*11.0

18.0*8.6

820

0.97

88.7kl.2

58.2kl.7

16.6kl.2

1.8kl.3

930

1.62

91.4+1.0

59.7kO.9

16.3kO.4

1.9*0.3

1090

10.2

91.2+0.8

59.5f0.6

16.liO.3

1.8kO.2

1310

5.4

90.6cl.O

59.3f0.7

17.5kO.2

I.SrtO.2

1750

0.92

90.6+1.1

59.8+1.7

15.9+1.1

1.9+1.0

kxal

19.22

90.9*1.0

59.3kO.9

16.6*l.6

1.8*0.8

1.3

58.6*0.8

33.0*0.3

18.6~1.0

6.2kl.l

ampoulc

irradiakdsample, 26.9 mg 460

0.47

187.0+1.0

97.2t2.0

66.7k5.4

2.Ok1.8

590

1.3

134.0+2.0

77.7kl.3

52.4kO.8

3.5*0.4

720

1.9

129.Okl.O

73.6kl.l

48.6+1.0

3.5*0.5

900

4.2

12O.Okl.O

69.7kO.8

4O.OkO.4

0.4kO.2

1070

37.0

104.SkO.7

64.3f0.5

27.9*0.3

1.3*0.1

1400

16.0

102.OkO.8

62.9kO.5

25.6kO.3

1.4*0.1

1700 1OM

2.3 63.17

1OS.O+I.O

64.4+0.8

24.3+0.3

1.3+0.1

106.9il.O

65.lkO.8

29.4*0.5

1.4fO.l

sumofampoulegasand

z~"+"*(I) "'Own data;cf.Table 3

the 460-9CQ'C

fractionsofirradirled sample:

l18.6~1.0

67.8+0.8

41.9*0.5

2.3kO.4

124.1

67.2

41.1

IO.5

L.”

0.8

0.6 I

0.2 1 0

I

I

I

I

0.2

0.4

0.6

0.8

FIG. 11. Comparison of xenon isotopic compositions: (1) xenon released by stepwise heating from n-irradiated pitchblende; (2) xenon in the irradiation ampoule; (3) the sum of the 460-900°C release fractions and the ampoule gas, which is closed to (4), n-induced xenon from 235U fission. The totals of xenon, in (5) irradiated and (6) unirradiated pitchblende lie on the mixing line of n-induced xenon derived from “‘U and (7) xenon produced by 238U spontaneous fission (Table 10).

n/cm*. Before irradiation, the xenon isotopic composition of sample #l-82, determined by stepwise degassing, was as expected for 130 million years spontaneous fission of 238U and fission of 235U induced by the background neutrons (Table 10). But xenon in the sample and the ampoule after n,,,-irradiation differs markedly from what should dominate now, 235U(n) fission xenon (Table 10): xenon in the ampoule, which was released from the sample probably by some inevitable heating during irradiation in the reactor, qualitatively is a “mirror image” of xenon extracted from the irradiated pitchblende at low temperature (Fig. 11) in that deficits of some isotopes in the ampoule correspond to excesses of the same isotopes in the pitchblende itself. Consequently, as shown in Table 10, the sum of xenon in the ampoule and of xenon released between 460 and 900°C from the sample yields no isotopic anomalies, but turned out to be close to the total xenon in the sample. This confirms our concept that anomalous (“alien”) xenon (and krypton) is connected to the migration (or chemical fractionation) of the radioactive precursors of xenon in the b-decay chains (Fig. 12). That is why (following the suggestion of Frank A. Podosek) we call this xenon “CFF-Xe” (for Chemically Fractionated Fission). Not only are the diffusion rates of radioactive precursors of fission xenon different from those of xenon, but also the various precursors migrate in minerals with different rates since they are isotopes of elements with different geochemical

Anomalous

isotope composition

3089

of Xe in Allende inclusions

1361 86s -136x, 134Sb 50s ,134~~ 4srn_ 134152.5"'_134~, 132s" 2e2m_132Sb 2.'m,132~e

77

131Sn~131Sb~131TC

24m_

1321

h_

2.3 h c

1311

132x,

8d_131x,

868, 84s, 3.3m_

848,

83se*B_

838,

FIG. 12. Simplified scheme of p-active precursors of xenon and krypton formed by thermal of 23sU. Data from MOSES (1978) and SEELMANN-EGGEBERT et al. (I 98 1).

properties (Fig. 12). That is why the isotopic composition of xenon can become anomalous in minerals where the precursors are formed as well as in newly formed minerals into which they diffuse. Since this effect should be most pronounced in very fine-grained minerals, we analysed xenon in Colorado-type deposits, i.e., in extremely fine-grained pitchblende. Xenon in two samples is isotopically even more anomalous (Table I 1) than in pitchblende #l-82 irradiated in the technical reactor because of the extremely favourable conditions (small grain size) for the diffusional loss of the precursors. Finally we analysed xenon in samples for which two conditions prevailed simultaneously: rapid accumulation of 235U fission xenon and high temperatures allowing rapid migration of the p-active precursor isotopes of xenon: partially melted or sintered sandstone from the epicentre of the first US nuclear bomb explosion executed in 1945 at Alamogordo, New Mexico, USA. Our sample contains 6 ppm U, 18 ppm Th, 3 ppm Sm, 0.6 ppm Gd, and 0.5 ppm Eu. The isotopic compositions of xenon and krypton determined by total extraction are unprecedentedly anomalous (Table 12): compared to normal (n) 235U fission xenon and krypton, they range from +480 (for s4Kr/86Kr) to 23000% (for ‘3’Xe/‘36Xe). Since the precursors of ‘36Xe and 86Kr are very short-lived (Fig. I2), li6Xe and *‘Kr were formed within the rock. Since the temperature was very high during the first 5-8 minutes after the explosion, ‘36Xe and *‘Kr could rapidly diffuse out, thereby increasing the ratios with these isotopes in the denominator. The diffusive loss of 136Xe and 86Kr alone,

neutron

induced

fission

however, is insufficient to produce the observed isotopic compositions. If also diffusional loss of the xenon precursors had played a role, one expects that the shorter their lifetimes are, the smaller are the excesses of the corresponding xenon and krypton isotopes relative to ‘36Xe and *‘Kr. The experimental data clearly prove this relationship (Fig. 13). Evidently

Table 11. Xenon released by stepwise type deposit. rSe~e= 100. Corrected atmospheric Xe based on 130Xe. For compositions are also

heating from Coloradofor minor amounts of comparison fission Xe given.

sample 22/S T, 1.7 g 410

0.39

266*3

138k5

151k6

520

3.45

278k3

45*2

67+2

47*2

750

1.89

126+3

59*2

29il

2*2

1200

0.72

135+3

46+2

21+1

5+2

kxal

6.45

217k3

55*3

56+2

26+2

sample

f

29/84,

5*2

I .6 g

500

0.45

117*3

87*2

41+1

16+1

750

0.10

12Ok3

90*2

71+2

3+2

1200

0.12

109+3

59+2

73+2

3+2

total

0.61

116+3

82+2

51+2

12+2

‘qJ,W

-

82.5

57.9

8.21

so.2

23J~+“h(l)

-

124.1

67.2

41.1

10.5

%ur data,

cf. Table 3.

Y. A. Shukolyukov et al.

3090

Table 12. Xe and Kr released by total fusion (sample weight 46.6 mg)and stepped heating (sample weight 34.8 mg) from sandstone taken from the epicentre of the first atomic bomb test in Alamogordo, New Mexico, USA. IsaXe= 100; 86Kr= 100. For comparison fission Xe and Kr compositions are also given. Iralics &lore dora before nrmospheric corrections which are based on 130Xe and 82Kr. respectively.

fusion corr.

4.2

1450+10

763Ok80

8590+80

5.3*01

34.5*0.5

4.0

313+30

321+30

32.Jf0.4

3.7

1620?12

8590+100

9680~150

ro

SO.5

2.1

300*10

56Ok20

~0

297*30

59&6

56+6

0.3

109flO

144*15

106?8

15*2

103ClO

0.38

corr.

0.2

lOSkI

66k38

41*25

EO

-a+34

0.06

0.2

101*12

133flS

102~10

IS*?

100~10

con-.

0.13

93*19

so+39

41+28

10

3534

1.9

13025

281+10

1.6

132+6

277*

1.J

604+8

0.9

69X + 17

1.7

387+4

1688+20

1877k20

540 700 1080 corr. 1250 corr. I360

0.22 absanr

undsfmd 329*30 _

67k6

95+5

61~3

245*10

7.5+0.5

47*5

0.94

247kl2

r0

-2+6

0.08

3104&30

3405*30

7.4kO.S

49+3

0.78

307+ JO

J6J k3

3440+80

4010+80

=O

If7

0.30

273+31

316+28

3.3k2.0

26&J

0.41

309*10

lJY+3

12

305klO

66&6 _

undcfmd

I.5

408+6

1800~20

2OOOk30

EO

Sk2

0.04

0.5

247*5

783&10

849+10

9.7kO.3

75*4

0.29

car.

0.4

282+7

910+18

1010~20

10

15+7

0.05

2.9

97_+2

82k3

44*1

3.3+2

18+2

0.71

co*.

2.7

95+2

65+4

29+2

SO

-4+4

0.17

8.6

236*5

874&-12

930&11

5.8k1.4

38*3

3.73

7.4

2SOt7

915k20

lCQ9k22

=o

0+6

0.70

"U+n,h"'

124.1

67.2

41.1

-

10.5

52.1

23.4

~%+llp'

112.4

73.0

45.4

13.7

95.0

51.0

29.0

63.8 _

30.9

"%h+"p

_

82.5

57.9

8.21

-

LO.2

14.0

3.34

corr. 1480 1750 tOtal corr.

=%,p

4Jk2 d 6OiJ

undefined 280+30

65&S

ss+s

undstincd 295*10

46??

SO*3

undefined 303+15

94+4

54+3

undefined

_

Irn.".A.,..rC TlhL ?

_-,,

““.“(

“..

,

“1.1

_

b)Shukolyukov, 1982

intensive migration of xenon, krypton and of their radioactive precursors I, Te. Sb, Sn, Br, and Se proceeded during cooling of the rock directly after the explosion. Excess lZ9Xe is lacking in the Alamogordo sample because its precursors ‘I91 (T,,z = 16 Ma) did not decay detectably during the 48 years since the explosion. Its absence proves the absence of unaccounted nuclear reactions, e.g., on tellurium, occurring in parallel to fission. The isotopic variations of xenon released by stepwise degassing a sample from Alamogordo supports our contention that diffusion of fission xenon and its precursors at high temperature some minutes or dozens of minutes after the explosion was instrumental for the formation of the xenon (and krypton) anomaly. Xenon from the low (540 and 700°C) as well as from the dominant highest (1750°C) temperature extractions is a mixture of normal 238U-spontaneous and 23.FU neutron-induced fission xenon (Fig. 14), while over 95% of the anomalous xenon is extracted in the intermediate relatively high temperature range 1250- 1480°C (Table 12). Since the most stable phases from which xenon is released at 1750°C were also the earliest to be stable after the explosion, no xenon or xenon precursor loss occurred, and, consequently, the xenon isotopic composition is normal fissiogenic. From intermediately stable phases (degassed between 1250 and 148O’C) some ‘36Xe diffused out whereas simultaneously xenon urecursor atoms where trauped while the rock was still hot: The data points lie on a &ing line (Fig. l4), which reflects contributions from both processes, accumulation and migration. The difference in the isotopic composition of the two Alamogordo samples is not surprising because of the

uranium concentration and thermal conditions may vary in a large extent even at distances of a few centimeters. The krypton data do not display such a regular relationship. Probably the migration of the %r and 83Kr precursors (Se and Br isotopes are geochemically more different than I and

100000 LI

10000 6. %

FIG. 13. Correlation of the value 6 = (r,,,/r,;‘) X 100 [W](r,,, = measured isotopic ratios, r. = isotopic ratios of % neutron induced fission xenon and krypton) with the sum of life times of the respective P-radioactive precursors. The data are from the Sandstone sample 46.6 mg (Table 12).

309 I

Anomalous isotope composition of Xe in Allende inclusions

100T

g

x \ x” B

1 E b”Ik, 31.,8lW~ a “7 , /’

bulk. 34.8rW, E , _ 8’

,* 1250’ ,~‘lW

f1250’ /’ ,, * 1360” ,

10 /’

-

/’

I A ,480’

i’

~‘1ulO”

7 108W

IT - 2Z”,, _

100

540’

I 01 HI1 I I tlliiu’ 1 0.1

’ ’ lillll’ I ’ “” 100 10 ‘3’Xe/‘36Xe

10

1 ‘34Xe/‘35Xe

FIG. 14. Isotopic com~sition of xenon released by stepwise heating (extraction temperatures given in “C) from sandstone from the epicentre of the nuclear bomb test in Alamogordo. Atmospheric xenon has been corrected for. Some error bars are smaller then the symbols.

Te) during cooling after the explosion had not been controlled by temperature and time alone and, therefore, the points on three-isotope diagrams are scattered. The possible reason of this is more effective migration of krypton compared to xenon. It should be noted that there is no full analogy between the anomalous xenon from the Oklo deposit and from the epicentre of the nuclear explosion. While the main featurethe excess of ‘j’Xe relative to ls6Xe and ‘“*Xe-is obvious in both, 13’Xe is significantly higher in the epicentre of the explosion. This can be understood by the different peak temperatures and heating durations of the rocks in the two environments. In both cases the intensive 235U fission reactions were accompanied by a temperature increase. While at the Oklo deposit the temperature did not exceed 250-450°C but lasted for 500 000 years, it could have reached 1000°C and more in the AIamogordo explosion during some 10 min. Elevated temperatures are a prerequisite for the diffusion of xenon and its precursors, but the relative diffusion rates of Te, I, Se. and Br most probably vary differently with the temperature. which then results in different isotopic compositions of retained “fission” xenon and krypton. Irrespective of the interpretation of the details, the main conclusion of this chapter is that we provided experimental proof that diffusion and geochemical fractionation of xenon and its precursor isotopes after fission reactions are the clue for understanding the isotope anomalies of xenon in the Oklo uranium deposit. From that starting point it is also possible to explain alien xenon in other objects of the solar system. CONCLUSIONS

occurred. The main prerequisite is the operation of a fission process under conditions such that the freshly created radioactive precursors of the stable xenon isotopes have an opportunity for rapid diffusion. This is likely to be the case in very fine-grained minerals and at elevated temperatures. The fission may migrate products of *‘*II and 244Pu spontaneous if the temperature is rather high during the whole period of xenon accumulation. Redistribution of xenon between minerals can then lead to “*Xe , 13’Xe. ‘34Xe, and lx9Xe excesses (relative to fission produced 13”Xe) in certain minerals and, vice versa, to their deficiency in others. In particular, at high temperatures, the inclusions of the Allende chondrite could have retained the radioactive isotopes of Te, I, and other elements, and have lost some xenon, both products of spontaneous 244Pu fission, during some short high-temperature stage in the protoplaneta~ gas dust cloud. Later, decay of the radioactive Te and I isotopes then caused the formation of CFF-Xe with its unusual isotopic composition. If indeed the fractionation process of the xenon precursors and of xenon from fission is not so unique as supposed by CAFFEE and HUDWN ( 1987) but rather common in nature, the long-standing difficulty to understand the “‘Xe excess in MORB-glasses (MARTY, 1989) and some natural gases (PHINNEY et al., 1978; STAUDACHER, 19X7) may be overcome. Is the “‘Xe observed excess really connected to primordial nucleogenetic ‘DI, or is it related to ‘191as a product of *‘*U and 244Pu fission, which then migrated from the place of origin into other minerals? This question should be answered before using radiogenic “‘Xe and ‘“‘Xe in models for the early evolution of the Earth and its atmosphere.

ON THE ORIGIN

METEORITES

AND

OF CFF-Ke IN ON EARTH

The qualitative resemblance of the fine structure of CFFXe in samples from the natural nuclear reactor, in inclusions of the Allende carbonaceous chondrite, in anorthosite from Greenland and in rocks from the epicentre of a nuclear bomb explosion (Fig. 8) suggests a common mechanism ofthe origin of the isotope anomaly. The mechanism explored in the previous chapter is the diffusion of xenon and the diffusional or geochemical fractionation of the radioactive xenon precursors in the o-decay chains that originate in fission reactions. It is not required that the chain reaction of ‘j5U fission indeed

iltCnnw/~~~~~~c~}?f.s-We are very grateful to M. Ozima. an anonymous reviewer and especially to U. Ott for their critical and most constructive reviews and to F. A. Podosek for his involved handling of the manuscript.

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