On the origin of excess heavy xenon in primitive chondrites

aCochImicaet Coamochtmh Acta, 1068,Vol. 22, pp. 1317to 1328. Pergamon Pwss.hintedinNorthern Imland

On the

originof excess

heavy xenon

in primitive

chondrites

M. W. Rowx* Miller Institutefor Basic Research in Science and Department University of California, Berkeley 94720 (Received 15 May 1968; accepted in revised form

of Physics

1 Aecguet 1968)

Al&r&--In heating experiments, xenon was studied in separated matrix and chondrule samples of the Mokoia carbonaceous chondrite and the Chainpur unequilibrated ordinary ohondrite. Larger isotopic anomalies in the heavy xenon were found than heve previously been seen in carbonaceous chondrites. Pum spontaneous fission decay in situ is an unlikely source for the excess heavy xenon. A ‘&wrier” hypothesis is suggested a.s an alternative source.

THE PASAMONTE eucrite, a Ca-rich achondritic meteorite, was found to contain

excess amounts of heavy xenon isotopes which appear to have originated from extinct PuW decay, in situ (Row-s and KURODA, 1965; ROWE and BOCURD, 1966; HOHENBERO et al., 1967). Investigations of other Ca-rich achondritic material (eucrites and silicates from mesosiderites) have indicated that the same origin is generally applicable for fiasiogenic xenon in eucritic matter (KUXLODAet al., 1966; MARTI et al., 1966; PEPIN, 1966; ROWE, 1967; SABU and K~RODA, 1967,REYNOLDS, 1968). Only FISHER (1967) has questioned this interpretation for the excess heavy xenon in Ca-rich achondrites. Recent events have led me to conclude that the same origin for the excess heavy xenon in carbonaceous chondrites is unlikely, although all previous workers have assumed an origin from in situ decay of extinct PuaU (REYNOLDS, 1963; PEPIN, 1964; REYNOLDS and TURNER, 1964; TURNER, 1965; FUNK et al., 1967; HOHENBERG et al., 1967; PEPIN, 1967). (Xenon from some temperature fractions is enriched with Xe131-136when compared with a more primitive trapped component. This enrichment is called “excess heavy xenon” in this paper.) It is the purpose of this paper to present data on the composition of xenon extracted by stepwise heating from the Mokoia carbonaceous chrondite and the Chainpur unequilibrated chondrite and to present an alternative to ila situ Puze4 decay. The xenon in carbonaceous chondrites is more difficult to resolve than in the Ca-rich achondrites where large fractional increases of fissiogenic xenon are observed. This problem has been discussed by previous workers [e.g. see Table 2, ROWE and BOGURD (1966) and discussion thereof]. It should suffice here to point out that the major difficulty in resolution of the various components of xenon in the carbonaceous chondrites is a lack of knowledge of the exact composition of primordial xenon, coupled with the small fractional increases caused by the addition of the heavy xenon isotopes in question. FUNK et al. (1967) most recently reviewed this problem in detail and found that any reasonable assumption of the composition of the primitive trapped xenon led to the same conclusion reached by earlier investigations (REYNOLDS, 1963; PEPIN, 1964; REYNOLDS and TURNER, 1964; TURNER, 1965), i.e. that the amounts of excess heavy xenon in the carbonaceous chondrites are too * Presentaddress: Dept. of Chemistry, University of Washington, Seattle, Wash. 98105. 1317

great to have been produced from the in situ decay of extinct 1’11244if the Yu244 is assumed to have been made from the steady state mode of element production as described by BURBIDGE et al. (1957). For their calculations, FRANKet ml. (1967) adopted a compromise value for the composition for the primordial xenon which probably slightly underestimates the amount of excess xenon present, in a particular sample. The estimation of the composition of the excess xenon is unaffected by this choice. I point out that FUNK et al. did not intend to suggest that the composition of primitive trapped xenon that they adopted was the best estimate available. Rather it was chosen so that it would not bias the result toward an overestimation of the amount of excess xenon. Further, the composition of the heavy xenon in excess to the primordial trapped xenon in carbonaceous chondrites (REYNOLDS and TURNER, 1964 ; FUNK et al., 1967 ; PEPIN, 1967) differs from that observed in the Pasamonte achondrite (ROWE and KURODA, 1965; EBERHARDT and GEISS, 1966; ROWE and BOGARD, 1966; HOHENBERCJet al., 1967; ROWE, 1967). For example, REYNOLDS and TURNER (1964) found the composition of the excess heavy xenon in the Renazzo carbonaceous chondrite to be Xe131 : Xe132 : Xe13* : Xe13s = 13 & 10 : 16 f 10 : 72 -& 4 : 100, whereas the composition of fissiogenic xenon from the Pasamonte achondrite is Xe131 : Xe132: Xe134 : Xe136 = 33 + 3 : 93 + 10 : 91 f 3 : 100 (EBERHARDT and GEISS, 1966, using the Pasamonte data of ROWE and BOGARD, 1966). The composition of the heavy xenon excess in carbonaceous chondrites is generally simiIar to known spectra of fission xenon (as compiled by HYDE, 1964) although different in detail. Whereas CAMERON (1962) argues that the Xe132 yield should be enhanced in Pu244 spontaneous fission decay relative to U238 spontaneous fission (WETHERILL, 1953) the observed yield of Xe132 in carbonaceous chondrites is deficient. The present investigation was undertaken to accumulate more data concerning the problem of unexpectedly large amounts of excess heavy xenon in “primitive” meteorites. (The term “primitive” is used here to indicate those meteorites which have apparently suffered relatively little metamorphism since their formation.) The isotopic effects seen in the xenon extracted from Murray and Renazzo (REYNOLDS, 1963; REYNOLDS and TURNER, 1964) could be explained in another way if fissiogenic xenon had been incorporated on primitive grains which later aggregated to form the stone meteorites without completely mixing the fission xenon with the other xenon trapped in the material. Since there would probably have been less mixing in the low temperature matrix material, I analyzed xenon from separated samples of matrix and chondrules from two “primitive” meteorites: the Mokoia carbonaceous chondrite and the Chainpur unequilibrated ordinary chondrite [classified as C-2 and LL-3, respectively, by VAN SCHMUS and WOOD (1967)]. Ch ain p ur was analyzed to determine if the great excess of the heavy xenon isotopes seen in Murray and Renazzo extended to primitive ordinary chondrites as well. The xenon was released by stepwise heating and analyzed in a Reynolds’ type mass spectrometer (REYNOLDS, 1956). The mass spectrometer used for this work was described in HOHENBERG et al. (1967) as system 2. Mass discrimination errors for this instrument are discussed there.

1319

On the origin of excess heavy xenon in primitive chondrites

Table 1. Isotopic composition and abundance of xenon released from the Mokoia and Chainpur meteorites by stepwise heating Sample Temp. ("C)

x&34

_--X&s*

XC+

XEW

XI+

X&B

Xe"*

X&8"

XfPO

-X+8*

XeN

x,$*4

xe1*4

-&1**

Xelsr

m

xe19**

(lo-'Occ/g)

Mokoia matrix 350 600 850 1050 1250 1450 1600 Average

0.00388 It9 0.00498 12 0.00513 10 0.00487 6 0.00472 9 0.00456 70 0.0053 6 0.00475 3

0.00351 9 0.00429 14 0.00430 13 0.00433 7 0.00427 10 0.0055 4 0.0059 7 0.00419 4

0.0743 5 0.0832 5 0.0844 7 0.0838 6 0.0836 8 0.0826 28 0.0775 11 0.0824 3

1.021 0.1543 15 4 1.242 0.1579 7 I9 1.507 0.1624 8 13 1.287 0.1642 11 5 1.397 0.1627 3 12 0.1634 2.53 40 3 1.53 0.1600 3 50 1.321 0.1617 6 3

0.801 2 0.821 4 0.828 3 0.827 3 0.824 2 0.823 11 0.815 5 0.822 1

0.387 0.327 2 2 0.436 0.393 2 3 0.412 0.370 2 2 0.382 0.319 2 1 0.386 0.321 2 2 0.388 0.338 5 5 0.388 0.331 19 12 0.3925 0.3349 10 10

0*0040 4 0.0047 3 0.0048 5 0.0048 3 0.0056 4 -

0.0035 3 0.0042 3 0.0046 4 0.0043 1 0.0054 3 -

0.0760 15 0.0782 23 0.0832 17 0.0836 24 0.0837 17 0.0828 77 0.0817 10

I.114 8 1.366 20 3.05 2 2.70 2 3.02 2 11.65 13 2.373 11

0.1577 22 0.1554 23 0.1619 31 0.1640 15 0.1630 30 0.156 9 0.1612 11

0.796 9 0.792 15 0.810 7 0.819 3 0.815 8 0.781 30 0.808 3

0.387 0.327 5 6 0.395 0.351 7 5 0.402 0.355 3 5 0.383 0.323 2 5 0.389 0.332 3 5 0.463 0.420 19 16 0.3935 0.3387 23 19

0.00544 0.0761 16 9 0.00484 0.0832 31 15 0.00417 0.0834 5 4 0~00401 0.0813 11 5 0.00414 0.0820 5 8 0.0822 0.0060 5 30 0.09 2 0.00447 0.00417 0.0817 2 3 5

1.015 4 1.120 6 1.119 23 1.124 3 1.077 3 1.224 12 1.1 1 1.110 2

0.1572 11 0.1592 18 0.1640 7 0.1616 I 0.1629 9 0.1610 30 0.156 8 0.1622 4

0.798 2 0.822 5 0.821 3 0.823 2 0.820 2 0.813 8 0.817 13 0.820 1

0.385 0.327 5 2 0.408 0.362 4 3 0.383 0.324 2 2 0.382 0.323 2 2 0.380 0.320 3 3 0.388 0.328 4 4 0.393 0.331 4 12 0.3895 0.3239 7 8

0.0045 0.0045 8 6 O-00470 om44 2 10 0.00469 0.0044 10 2

1.066 19 1.211 6 I.206 6

0.1549 20 0.1624 9 0*1621 9

0.792 0.391 0.333 5 5 4 0.820 0.384 0.326 4 2 2 0.819 0.3841 0.3265 15 9 3

2.64 1.00 3.72 6.90 4.18 0.074 0.013 18.5

Mokoiachondmles 350 600 850 1050 1250 1450

0~00502 0.00462 16 28

Average

0.46 0.27 0.47 0.91 0.53 0.15 2.79

Chainpurmatrix 350 600 850 1050 1250 1450 1600 AVWbg0

0.00414 14 0.00479 29 0~00451 5 0.00442 4 0.00460 9 0.0054 6 -

1.8 1.1 9.8 18.2 6.1 0.13 0.02 37.1

Cheinpw magnetic chondrules 350

0.0756 16 0.0833 6 0.0830 5

0.48 13.3 13.8

1320

M. IV. ROWE Table 1 (Continued)

Chainpur non-magnetic chondrules 350 600 850 1050 1250 1450 Average

Atmosphere

(NIEB, 1960)

AVCCt

Average UOQ

0.00484 17

0*00530 22

0*0761 14 0.111 5 0.0834 20 0.0791 16 0.0885 23 0.09 I 0*0811 10

0.00358 0.00466 -

0.00333 0*00417 -

0.0714 0.0819 0.0843

0.0048

4 cmm 14 0*0046 4 0.00453 23 0.0061 6

-

0.0047 7

0023 3 0.0032 7 0.00516 22 0.0068 6

O-987 8 l-062 3 1.158 3 1.073 4 1.266 12 I.1 1 1.096 3

I.024 -

-

0.155 3 Ol64 7 0.1626 16 0.163 3 0.165 3 O-163 3 @1620 18 0*1515

0.1611 0.1630

0.333 G 0.354 10 0.333 r 0.32; 3 0.322 ” 0.3PJ L 0.3273 21

0*797

0.3%

IQ 0.819 21 0.822 8 0.825 6 0.822 13 0.81 4 0.820 4

8 0410 12 0.392 6 0.380 3 0.388 11 0.46 6 0.3858 24

0.788 0.817 0.819

0388 0.330 O-382 0.321 0.383 O-322

* Theuncertainty in theabundance of XslSeis thoughtto be N* 10%. t Referencea: AVCC-Averageof valuesby KRUMMENACHEB et ol. (lS62), EUGSTERet al. (1967) for ten carbonaceous ohonciritm.AverageUOC-HEYMANN and MAZOR (1968).

O-66

O-11 0.8H 2-61 o-47 O-03 4.76

(1967) and MART

RESULTS

The results of the me~s~ernen~ on Mokoia and Chimps are shown in Table 1. The errors tubules for the isotope ratios are the mean deviation in the measured ratio from a value which drifts linearly with time, due to memory effects. The errors for Murray [see Fig. 3 of FnNKet al. (1967)] clnd forRenazzo (REYNOLDS and TURNER, 1964) were similarly oalcnlated so that direct comparison with those results is possible. The isotopic ratios for atmospheric xenon (NINE, 1950), xenon with the com~sition fonnd on the average in ~&rbon~~o~ ~hon~ites (~~~ACEER et al., 1962; EUQSTER et al., 1967; IY~~LZ~TJ., 1967) and xenon found on the average in unequilibrated ordinary chondrites (HEYMANN and MAZOR, 1968) are included for comparison. The average carbonaceous chondrite xenon, abbreviated AVCC, is the average of data by KRUMMENACHER et al, (1962), EU~STER et al. (1967) and B~~RTI (1967) FUNK

et al. (1967) have shown that a plot of Xe130/Xe132versus Xe136~Xe13~ (or Xe131/Xe13*or Xel”/Xeia2) is helpful in resolving mixtures of trapped, a;tmospheric, and G&on xenon. Such a plot is shown in Fig. 1. The shaded area indicates the region previously populated by data from stepwise temperature release experiments (REYNOLDS, 1963; REYNOLDS and TUBNER, 1964; PIPIN, 1967). The 600°C fraction of Renazzo (point .I$)is shown be-use it has the lowest Xd*@/Xel*s value measured in caarbon~~~~~chondrites to date (REYNOLDS and TUBNER, 1964). Roth the atmospheric {point ~4) and AVCC (point C) values are also plotted. The point P is an estimate of the primitive trapped component adopted by FUNKet aE.(1967) in a similar analysis of the Murray and Renazzo data. In order to simplify the diagram

On the origin of excess heavy xenon in primitive chondrites

70 RENAZZO

FISSION

1321

COMPOSITION

o”550.150

-TO PASAMONTE FISSION COMPOSI7ION [Eberhordt and Geirr, 1966; Row and Bogard, 19661 I

0.310

0.310

I 0.330

1 0.340

I 0.350

I 0.360

v Chainpur magnatic chondrules A Chainpur nonmognatic chondrvler 0 Block symbols. ar.rage of twnpwatura Y&Region of Renorro and Murray data I I

0.370

0.380

0.390

0.400

0.410

0.420

runs

0.430

Xe”6/Xe”2

Fig. 1. A correlation plot of the temperature run of the Mokoia and Chainpur chondrule and matrix samples. The shades ares representswhere previous data have plotted (REYNOLDS, 1963); REYNOLDS and TURNIGR,1964; PRPIN, 1967). Point R is the 600°C fraction from Renezzo (REYNOLDSand TTTRNIZR, 1964). Point P is the value for the primitive trapped component adopted by Favg et al. (1967). Point C representsAVCC xenon (KRUMHLZ NACHERet al., 1962; EUUSTER et al., 1967; MARTI, 1967), snd Point A, atmospheric xenon (NIRR, 1950).

(Fig. l), no error bars are shown on the new data presented here which were found to In general, they are of the same magnitude as those which are shown.

lie in the region previously populated.

DISCUSSION Using Fig. 1, the amounts of the excess XelS6were estimated by the same method employed by FUNKet aE.(1967) and are illustrated in Fig. 2 where the excess Xe1S6is plotted for each temperature. Essentially the data plotted in Fig. 1 are used to partition the xenon between the fission and trapped components. The upper limit on the estimate of excess XefS6 reported here was set by assuming that the primitive trapped component was R, the value itself by assuming that the primitive trapped component was P, and the lower limit by assuming that the primitive trapped component was C if this choice were dominant in producing the error. In other cases, the error in the isotope ratio was of greater significance than the choice of trapped component. The larger of the two was used to indicate the error. Previous estimates of the excess xenon in carbonaceous chondrites were somewhat uncertain due to the combination of small observed anomalies and the uncertainty in the knowledge of the composition of the primitive trapped xenon. The large anomalies observed, both in the 600°C and 85O’C fraction of the Mokoia matrix and chondrules and in the 600°C fraction of the Chainpur matrix, make the exact composition of the primitive trapped component of less importance for these samples. It is clear from Fig. 2 that large excesses of Xelss occur in Mokoia and Chainpur, irrespective of the trapped component adopted, especially in the Mokoia matrix.

1322

o 0

Mokoio Mokoia

III Chainpur Choinpur 0. Chainpur

v

0

200

400

matrix chondruler matrix magnetic chondrutes nonmagnetic chondrufes

600

850

1000

Temperature,

1200

1400

1600

*C

Fig. 2. Amount of excws XcP6 observed at each temperature in the Mokoia and Chainpur samples. f confirmed the previously observed difference between the corny&ion of the excess heavy xenon in the primitive chondrites compared with that from the Pasamonte achondrite. In Fig. 1 the upper line points to the composition of the excess heavy xenon found in the Renazzo earbonaceo~s chondrite (REYNOLDSand TUBBE=, 1964) i.e. Xels2/Xe 136= 0.72 ; the lower line points to the composition of Pasamonte fission xenon (EBEBHARDT and GEISS, 1966 ; ROWEand BOGARD,1966 ; HOHJGNBERG et aE., 1967) i.e. Xe132/Xe~6 = O-91. Most points with larger Xe136/Xe1s*ratios are xenon of the Renazzo type. The 600°C fraction of the Mokoia chondrules seemingly contains Pasamonte-type fission xenon. Another equally plausible explanation is possible. In terms of the model used, mixing of Renazzo-type xenon of composition shown in Fig. I as * with an approxima~ly equal amount of atmospheric xenon yields a point coinciding with the 600°C fraction of Mokoia chondrules. Only a small amount of xenon was released from the Mokoia chondrules at BOO”C, SO that a relatively small amount of atmospheric xenon would have a relatively large effect on the result. Several authors ~~U~~~ACHER et d., 1962; EUCSTER et d., 1967; MART& 1967) have called attention to the uniformity of xenon from carbonaceous chondrites. For example, the Xelso/Xelga ratios in the ten carbonaceous meteorites measured have a total spread of only 2 %, while the Xe1@/XelS2 ratios vary by only l-2 %. This uniformity occurs, despite the existence of excess Xelsl, Xef32, Xexs and Xel”

On the origin of excess heavy

xenon in primitivechondrites

1323

in some temperature fractions, because of two factors. First, the xenon appears uniform because the fractional amount of excess heavy xenon is small. The largest fractional increase in the Xelm/Xelaa ratio is reported here and is 29 % ; more typically it is 6% or less. Secondly, the amounts of the excess heavy xenon are roughly proportional to the total xenon concentration in chondrites. This relationship is shown in Fig. 3 where excess Xelw is plotted against the Xe132content of the

lo-'0

IO-p Xe13',cc STP/g

1o-8

FICL3. A plot illustrating that the excess Xelss is proportional to the trapped primitive xenon content. The values for Renazzo md Murray are those calculated by FUNK et al. (1967); that for Bruderheimwas calculated here using the data of MERRIEDJE (1900) from a 3.3-g chunk of Bruderheim.

primitive meteorites for which data are available. The excess Xe’w was calculated as in Fig. 2. The line of proportionality is drawn at 45’. Only the Mokoia samples lie significantly above the proportionality line. Furthermore, the average composition of Mokoia differs from AVCC xenon (Table 1). Reviewing the evidence, four observations lead me to conclude that decay of extinct Pus” &nsitu does not provide a suitable source for the excess heavy xenon found in primitive chondrites. (1) Far too much excess xenon is present (50 f 17 x 10-l” cc/g Xe136 in the Mokoia matrix, for example) to be consistent with the steady-state theory of element production. This theory* predicts only ~3 x lo-l2 cc XelS6/g from extinct Pu244 decay in situ, if the meteorite began to retain xenon immediately after cessation of steady state nucleosynthesis ((BURBIDGE et al., 1957). * FLJZISCEER et al. (1908) found evidence against this theory in their examination of fossil fission tracks in the Toluca iron meteorite.

1324

M.

Ii;.

ROWE

(2) The composition of the excess heavy xenon in the primitive chondrites differs from that in the Pasamonte achondrite. The amount of fissiogenic xenon in Pas:&monte is consistent with an origin from spontaneous fission of extinct Pu24* in &p/l. (3) The amount of excess Xe136 in primitive chondrites was found to be pro portional to the amount of trapped xenon, and not proportional to the uranium content of the samples which is approximately constant varying by a factor of ~3 on gram sized samples (REED et al., 1969 ; GOLES and ANDERS, 1962 ; MORGAN and LOVERINO, 1964). It is perhaps possible, but seems unlikely, that a Xe136-uranium correlation is dominant in Fig. 3. Although FLEISCHER (1968) found extremefy large variations in uranium concentration even within a single sample. his tecllni~~~~~~ samples ~10~ times less material than those discussed here. (4) The yield of excess Xe132 relative to excess Xe136 in primitive chondrites is lower than in U23* spontaneous fission, not enhanced as expected from Pu244 decay. This argument admittedly is weak since it is based on estimations of a mass yield curve not yet measured. None of these arguments is proof against in situ decay of extinct Pu2*4. Taken collectively, however, these arguments constitute evidence against that mode of production of the xenon. Although REYNOLDS (I 967) reviews “the evidence for extraordinarily thorough isotopic mixing in the solar system . . .” which “. . . is the most important single thing to be said about isotopes”, perhaps the single most important observation to be made about the xenon isotopes in primitive chondrites is that they are not ~~rn~l~~~~~ mixed. Otherwise, no variation in isotopic composition of xenon would occur at different temperatures. PUNK et ak. (1967) have shown that neither atmospheric contamination, nor diffusion, nor xenon produced from cosmic ray bombardment of the samples, are capable of producing the variations in xenon isotopic composition observed in Murray and Renazzo. Their arguments are also valid for the new data presented here. An alternative to in situ decay of Pu244 is sought, therefore, to explain the xenon anomalies in the primitive chondrites. I propose that the excess heavy xenon in the primitive meteorites was adsorbed onto small particles, probably carbonaceous material, very soon after cessation of nucleosynthesis [see ANDERS (1964) ; MAZOR and ANDERS (1967) and LARIMER and AWDERS (1967), for detailed discussion of this process]. The subsequent conditions affecting this material were such that the initially unmixed excess heavy xenon never became completely mixed with the other trapped xenon which accompanied it. The xenon content of a given primitive meteorite appears to indicate the amount of carbonaceous “carrier” contained in the meteorite. This relationship was demonstrated by OTTING and Z_&ESINBER(1967), who pointed out that the total xenon content of many chondrites is proportional to the total carbon content of the meteorite. We expect from this hypothesis and observe from the data that the xenon composition is largely independent of the “dilutant” meteorite material with which the carrier becomes mixed. Initial adsorption and laboratory desorption of xenon on carbonaceous grains should be more prominent at lower temperatures. We note that no pronounced “fission” component was observed in the ~rbonaceous chondrites or ~hainp~ above lOOOY! in the temperature runs. (It is important to understand that u~o~n~a of “fission”

On the origin of excess heavy xenon in primitive chondrites

1326

are being emphasized. The “fission” xenon in the 146O’C Mokoia chondrule component is prominent due to the extremely small amount of trapped xenon present, but the amount of “f&ion” xenon is very small.) The larger anomalies were found at 600°C and 860°C in the measurements of Mokoia and Chainpur. The excess heavy xenon is more prominent in the matrix, which has probably been subjected to lower temperatures than the chondrules. The trans-bismuth elements produced near the end of nucleosynthesis in the r-process (BURBIDBE et al., 1957), many of which decay entirely or in part by spontaneous fission, provide a possible source for the large amounts of excess heavy xenon in the primitive chondrites. Ackraowledgmtmti-1 am grateful to Professor J. H. REYNOLDS for his encouragement, suggestions, and discussion throughout all aspects of this work. Discussion with Professor E. ANDERS, Dr. R. GANAPATHY,Dr. J. LBRIMER,Dr. 0. K. MANUEL,Dr. C. HOHENBERU,and Mr. F. PODOSEKwas also helpful. My thanks go to the referees, Drs. R. L. FLEISOIZER and R. 0. PEPIN, who provided constructive criticism. Mr. D. MCLAINassisted in reading charts. The meteorite samples used in this study were generouslysuppliedby ProfessorC. B. MOORE Center for Meteorite Studies, Arizona State University, Tempe. This work was supported in part by the U.S. Atomic Energy Commission and bears AEC Code Number UCB-34P32-64. REFERENCES A.NDERSE. (1964) Origin, age and composition of meteorites. space Sci. Rev. 3, 683-714. BTJRBIDQE E. M., BUR~ID~E G. R., FOWLERW. A. and HOYLE F. (1957) Synthesis of the elements in stars. Rev. Mod. Phya. 29, 647-650. C-RON A. G. W. (1962) The formation of the sun and planets. Icam 1, 13-69. EBERHARDT P. and GEISSJ. (1966) On the mass yield spectrum of fissionxenon in the Pasamonte meteorite. Earth Plan. i&i. Lett. 1, 99-101. EU~STERO., EBI~HARDTP. and GEISSJ. (1967) Krypton and xenon isotopic composition in three carbonaceouschondrites. Earth Plan. ~5%. L&t. 8,249-257. FISHERD. E. (1967) On the origin of 6ssiogenic xenon in meteorites. J. Geophya. Re.s. 72, 765-769. FLEISCEER R. L. (1968)Uranium distributionin stone meteoritesby the fissiontrack technique. Geochim. Coanzoohim. Acta 32, 989-998. FLEISCHER R. L., PRICEP. B. and WALKERR. M. (1968) Identification of Puaa4fission tracks and the cooling of the parent body of the Toluca meteorite. Qeochim.Cosmoch$m. Acta 83, 21-31. FUNK H., PODOSEKF. and Rowx M. W. (1967) Fissiogenic xenon in the Renazzo and Murray meteorites. aeochim. Cosmochim.Acta 31, 1721-1732. Corns G. G. and ANDERSE. (1962)Abundances of iodine, tellurium, and uranium in meteorites. Geochim. Cosmochdm. Acta 26, 723-727. HEYMANN D. and &OR E. (1968) Noble gases in unequilibratedordinary chondrites. Geochim. Coemochim. Acta 32, 1-19. HOHENBER~C. M., MUNE M. N. and REYNOLDSJ. H. (1967) Xenon and krypton from the Paaamonte aohondrite; Extinct plutonium-244 in meteorites; Relative ages of chondrites and achondrites. J. aeophys. Rec. 72, 3139-3177. HYDE E. K. (1964) The Nucleur Properties of the Heavy Elemente, VoZ. 3 F&&m Phenomena. Prentice-Hall. KRUMMENACEER D., MERRIETJE C. M., PEPIN R. 0. and REYNOLDSJ. H. (1962) Meteoritic krypton and barium versus the general anomalies in meteoritic xenon. Geochim.Coamochim. Acta 26,231-251. K~RODAP. K., ROWE:M. W., C-LA~KR. S. and CANAPATHY R. (1966) Galactic and Solar nucleosynthesis. Nature 212, 241-243.

1326

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ROWE

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