ICARUS 8, 481--485 (1968)
Plutonium-Xenon and Iodine-Xenon Chronology of Three Light-Dark-Structured Chondrites M. N. RAO, J. L. MEASON, AND P. K. K U R O D A Chemistry Department, University of Arkansas, Fayetteville, Arkansas
Communicated by ZdenSk Kopal Received December 8, 1967 ~"Pu/~36Xe and 1291/129Xedec~y intervals of the dark phases of three chondrites-Leighton, Fayetteville, and Pantar--are calculated and the results are discussed with reference to their origin from a material similar in composition to that of carbonaceous chondrites. I. INTRODUCTION
II. EXPERIMENTAL
The origin of light and dark structure in some stony meteorites has been a subject of much attention since KOnig et al. (1961) discovered this effect in Pantar. Several explanations like the shock hypothesis of Frederikson and Keil (1963) and the solar wind hypothesis of Signer and Suess (1963). Suess et al. (1964), and Eberhardt et al. (1965) were proposed, but none of them could offer a satisfactory solution to different problems associated with it. Recently, Miiller and Ziihringer (1966) proposed that the origin of dark phase in the gas-rich meteorites and in the carbonaceous chondrites seem to be closely associated. This hypothesis is closely examined here b y measuring the abundance and isotopic composition of K r and Xe, which are predominantly planetary-type gases and are little effected b y solar wind, in three bronzite chondrites, Leighton, Pantar, and Fayetteville. In the classification of Van Schmus and Wood (1967), these meteorites belong to group H~ and they consist of typical dark and light phases. At the time the carbonaceous and other types of chondrites were formed, two now-extinct nuclides, u4Pu and r-'9I, were present and a study of their decay products in different phases of these gasrich meteorites should throw light on the nature of their origin.
The experimental details about the measurement of abundances and isotope composition of xenon and krypton in the dark phase of Leighton are reported in a short communication elsewhere b y Meason and Rao (1967). The xenon data in the dark phase of the Fayetteville meteorite were measured b y Manuel (1968) and the data given in Table I are obtained by summing up various anmunts of xenon released at different temperatures in a stepwise-heating experiment. Further his data are renormalized to 13°Xe for the reasons previously given b y Kuroda (1960). The data for the dark phase of P a n t a r are taken from Merrihue et al. (1962) and are renormalized to 13°Xe, as mentioned before.
481
III. RESULTS AND DISCUSSION In the gas-rich meteorites, it is well known that the gas content in the dark phase is considerably greater than that of the light phase. Manuel and Kuroda (1964) showed that the dark phase of Fayetteville contained about 13.4 X 10-11 cc ~3°Xe S T P / g m whereas the light phase gave only 5.4 X 10-11 cc 13°XeS T P / g m . Also Merrihue et al. (1962) found that the dark phase of P a n t a r contained 10.8 X 10- n cc u°Xe S T P / g m while the light phase gave only 0.9 X 10-H cc 13°Xe S T P / g m . ~3°Xe (dark)/
482
M. N, R A O :ET A L .
TABLE I ISOTOPIC COMPOSITION AND ABUNDANCES OF X E N O N IN THE D A R K PHASE OF LEIGHTON~ I)ANTAR~ AND ]~AYETTEVILLE CHONDRITES Xenon isotope
124 126 128 129 130 131 132 134 136 Gas content 13°Xe (cc STP/gm)
Leighton (Dark)
0.028 0.025 0.504 15.34 ------1.000 5.060 6.241 2.383 1.991 5.2 ×
4- 0~002 -+ 0.002 + 0.006 +0.055 ±0.018 4-0.025 _+0.010 ±0.009 10 11
Fayetteville a (Dark)
0.031 0.030 0.510 8.241 ~1. 000 5.136 6.317 2.420 2.039 8.3 ×
_ 0.001 + 0.001 _+0.010 +0.050 ±0.008 ±0.010 4- 0.009 +_0.008 10 11
Pantarb (Dark)
0.029 ± 0.016 0.028 ± 0.016 0.529 ± 0.021 14.311±0.041 --~1.000 5.096 +_0.117 6.254_+0.016 2.378 ±0.096 2.045 ±0.067 10.8 X 10-11
Murray~
0.030 0.027 0.508 6.47 ~1. 000 5.03 6.12 2.35 1.97
+ 0.001 _+0.001 4- 0.004 _+0.03 ±0.02 4-0.03 4- 0.01 4-0.01
Renormalized data from Manuel (1968). Renormalized data from Merrihue et al. (1962). Reynolds (1960). ~3°Xe (light) in Fayetteville is about 2.5 but the same ratio for P a n t a r is about 12. However, there is an error of about 10% in the measurement of the absolute gas contents. The same ratio for Leighton could not be given because the rare gases in the light phase of Leighton are not measured. The 84Kr/mXe ratio for Leighton (dark) is about 8, whereas the same ratio of air is about 28, according to Nier (1950) and about 1.3 for the carbonaceous ehondrites of T y p e 1, according to Mazor and Anders (1967). I t needs to be pointed out that there could be an error of about 35% in the measurements of the absolute 84Kr content because xenon was measured first and about an hour later Kr was measured in the mass spectrometer (Meason and Rao, 1967). The S4Kr/mXe ratio for Fayetteville (dark) is about 3, as obtained from Manuel and Kuroda (1964). However, the same ratio in P a n t a r is not given b y Merrihue et al. (1962) because of the large errors in krypton measurements.
A. P u / X e and I / X e Decay Intervals It is interesting to calculate the 244Pu/ 136Xe and ~-gI/129Xe decay intervals for the dark phases of these three ehondrites and to see how these intervals eompare with one another. The 13°Xe content of Leighton chondrite was determined in this work (Meason and
Rao, 1967) to be 5.17 X 10-11 cc S T P / g m . We can calculate the excess l~6Xe and i29Xe, using the data given in Table I, as well as those values given for M u r r a y b y Reynolds (1960) also given in Table I. The light xenon isotope ratios luXe/l~°Xe, 126Xe/iS°Xe and i28Xe/13°Xe in the dark phase of Leighton do not seem to show a n y contribution from cosmic ray spallation reactions (Rowe et al., 1966), and therefore the cosmic-ray-produced 13°Xe is considered to be negligible. I n such a case, the relation (1) follows, (I~°Xe)ob~ = (13°Xe)p
(11
where p refers to primordial. Then, the amount of excess la6Xe can be calculated from ~136-
(l~6Xe/13°Xe)L~i~hton --
(136Xe/'~°Xe)M . . . .
y
(2)
and
(136Xei) = ~l:~6('~°Xe),
(3)
where ( 1 3 6 X e / 1 3 ° X e ) M . . . . y = 1.97 4- 0.01 (Reynolds, 1960) and the superscript f refers to the excess fissiogenic component. So, we have ~136 = 0.021 4- 0.010 and (~36Xei) = (1.086 4- 0.40) X 10-I~ cc S T P / g m .
XENON CHRONOLOGY OF LIGHT-DARK CHONDRITES
483
Z:~4 = (260 ± 1 2 0 ) X 106 years
The uranium content of Leighton (dark) was measured b y Clark et al. (1967) and the average value of 34 ppb is used as the uranium content of this meteorite. Thus,
and
(I~6Xel/U) = (3.19 ± 1.6) X 10-~ cc 136Xe] S T P / g m U.
I n a similar manner, the 244pu-136Xe and 1:91-129Xe decay intervals are calculated for the dark phases of Fayetteville and P a n t a r
I n a similar manner, the a m o u n t of excess 129Xe~ in Leighton (dark) is calculated (r refers to the radiogenic component) to be 45.9 X 10-11 CC 129Xer S T P / g m from the data given in Table I. The iodine content of the dark phase of this meteorite was measured b y Clark et al. (1967) and the average value of 750 ppb is t a k e n to be its iodine content. Thus, l:9Xe*/I = 0.611 X 10-3 cc 129Xe* S T P / g m I. According to Kuroda (1960) the :44Pu/ 238U ratio (a) and the 1291/127Iratio (fl) at the end of nucleosynthesis, are given b y = 2.35 X 10-5 and 2 = 3.2 X 10-~ for the continuous nucleosynthesis model. These values correspond to the (136Xef)0/U and (129Xe~)0/I ratios of (136Xef)0//U = 3.2
X 10 -4 cc ~36Xe/STP/gm U and (l:9Xe~)0/I = 5.3 X 10-' cc l:9Xe" S T P / g m I for the meteorite which started to retain xenon immediately after the cessation of galactic nucleosynthesis. T h e 244pu-l~6Xe decay interval (~-~244)and the 129I-1:9Xe decay interval (ZL~9) can be calculated b y following Eqs. (4) and (5) (136Xe]/U) = 3.2 X 10-~ X exp(-h:4~:44)
(4)
and (l:~Xe~/I) = 5.3 X 10-1 X exp(--X129Z129).
(5)
The following values are obtained for E:~4 and ~1:9 for the dark phase of Leighton,
~1:9 = (160 ± 10) X 106 years.
TABLE II 244pu/la6Xe AND 12~I/129Xe DECAY INTERVALS OF THE DARK ])HASES OF LEIGHTON~ PANTAR, AND ~7AYETTEVILLE CHONDR1TES Meteorite
~Pu/13~Xe decay interval, ~244 (years)
1~I/129Xedecay interval, ~2~ (years)
Leighton (260 ± 120) X 106 (160 ± 10) X l0 s (dark) Fayetteville (80 ± 30) × 108 (110 ± 10) X 106 (dark) Pantar __a (90 ± 20) X106 (dark) The fission xenon excess for Pantar (dark) is way too high and yields a negative ~244 value if we use U = 13.5 ppb (Reed, 1963). This problem is discussed in Section IV. and the values are given in Table I I . I t needs to be mentioned t h a t a correction for the cosmic-ray-produced 13°Xe in F a y e t t e ville has been applied to the total 13°Xe measured in the same meteorite. I n the case of Pantar, no correction is applied.
B. U and I Abundances in both Dark and Light Phases of these Three Chondrites The uranium and iodine contents for the dark and light phases of Fayetteville, Leighton, and P a n t a r are given in Table I I I along with the corresponding references. F o r the calculations, shown in Section I I I , A , the average values for the uranium and iodine contents are used. The average values are also given in Table I I I . This method is used because the distribution of uranium and iodine is generally known to be heterogeneous in stony meteorites. The average uranium concentration Of Leighton (dark phase) is about 34 ppb. The uranium content of the dark phase of P a n t a r was determined b y Reed (1963) to be 13.5 ± 1.5 ppb. Brief remarks are m a d e about the shnilarities and dissimilarities in the trace~
484
M.N.
RAO ET AL.
TABLE III V AND I CONTENTS IN THE LIGHT AND D A R K ~)HASES OF LEIGHTON~ PANTAR~ AND FAYETTEVILLE CHONDRITES Meteorite
Leightond~rk Leightonlight ~ l~ayettevilledark ~
Faye~te¥illelight ~ Pantar-dark Pantar-light
U (ppb) ~
I (ppb)
47.0 _ 3.7 21.9 +_ 3.3 Average = 34 50.6 ± 5.6 18.1 ± 2.7 Average = 34 28 S ± 4.6 69.0 ± 4.6 23 ± 3~ 22 ± 3~ 18 ± 3~ Average = 33 29 4- 4~ 33 ± 3b 32 ± 3~ Average = 31 13.5 ± 1.5 ~ 13.3 ± 0.2 ~
507 + 33 994 +_ 169 Average = 750 292 ± 19 316 ± 54 Average = 304 162 ± 19 43.4 ± 7.4
108 77
4- 9 ± 12
60 ± 15~ 4.4 +_ 0.9 ~
Clark et al. (1967). b Nix and Kuroda (1967). Reed (1963). Turner (1965). *ppb = parts per billion. element concentrations of light and dark phases of Leighton only as the other meteorites were discussed before b y other workers (Reed, 1963; Mueller and Z~hringer, 1966). The average U and I contents of Leighton (light) are 34 p p b and 304 ppb, respectively. As shown in T a b l e I I I , in comparison, the average U and I values for Leighton (dark) are 34 ppb and 750 ppb, respectively. The light and dark phases of this chondrite show similar U contents while I is more a b u n d a n t in the dark phase relative to the light. IV. CONCLUmNG R E ~ K S T h e l~SI/12SXe and 244pu/136Xe decay intervals of the dark phases of three bronzite chondrites are given in Table I I . The calculation of these decay intervals for the d a r k phase of Leighton is described in detail in Section I I I , A . T h e error limits of the 129I/1~9Xe decay interval are relatively small compared to the ea*pu/~ssXe decay
interval because relatively large errors are associated with the fission xenon content in chondrites. In view of this, within the range of experimental error, these two intervals in Leighton (dark) could be considered as concordant. I n case of F a y e t t e ville (dark), the 2a4pu/issXe decay interval was calculated to be (80 i 30) X 106 years which is in good agreement with the value of (110 :t: 10) X 106 years, obtained for the 1"~I/r-'~Xe decay interval. In case of P a n t a r (dark), the 129I/1~9Xe decay interval was calculated to be (90 =t= 20) × 10 s years. However, in the calculation of Zu~ for the dark phase of this meteorite, there is a problem. If we t a k e the h e a v y xenon isotope ratios of Merrihue et al. (1962) and calculate the excess 136Xef, it turns out to be about 8 X 10 -12 cc 13sXeS/gm. [The isotope ratios of 124Xe/13°Xe, 126Xe/13°Xe and ~2sXe/laXe in P a n t a r (dark) indicate t h a t the spallation component in this meteorite is negligible.] The excess fission xenon contents in the dark phases of Leighton and Fayetteville are about 1.1 X 10-~2 cc la~XeS S T P / g m and 5 X 10 -~" cc S T P '~6XeY/gm, respectively. This value is fairly high in Fayetteville (dark) and the dark sample analyzed b y Manuel (1968) contained some admixture of light (O. K. Manuel, personal communication). B u t the fission xenon excess in P a n t a r (dark) is way too high. This could arise from possible atmospheric contamination of the meteoritic xenon while being measured on the mass spectrometer. I t is significant to add t h a t the fission xenon excess calculated in case of Leighton (dark) agrees closely with the value of (1.1 -4- 0.4) X 10 -1~ cc 13sXel S T P / g m calculated for Bruderheim chondrite b y M e a s o a el al. (1967). T h e agreement between ~29 values for the dark phases of Fayetteville and P a n t a r needs to be pointed out, even though the ~29 value for Leighton dark phase seems to indicate t h a t the material comprising this dark phase had formed comparatively later relative to t h a t of Fayetteville and Pantar. At this stage, it is interesting to look into the hypothesis, proposed b y Mueller and Z~hringer (1966) and supported b y Mazor and Anders (1967). T h e y showed t h a t a
XENON CHRONOLOGY OF LIGHT-DARK CHONDRITES
carrier of carbonaceous chondrite composition was involved in the for~nation of the gas-rich meteorites. Further, Mazor and Anders (1967) suggested that the same type of carrier seems to be required for all meteorites studied. The composition of this carrier seems to be similar to those of carbonaceous chondrites Type I and II, but not Type III, because of its low carbon content. We look into the case of Meghei, a Type II carbonaceous chondrite. For this meteorite, the xenon isotope data are available from Krummanacher et al. (1962) and the iodine contents are given by Goles and Anders (1962). Two Type II carbonaceous chondrites, Murray and Meghei, show an iodine content of 230 -4- 80 ppb and 310 -440 ppb, respectively, and the dark phases of Leighton, Pantar, and Fayetteville are enriched in iodine relative to their light phases. Also, the ~3°Xe content of the dark phases of Leighton, Pantar, and Fayetteville vary from 50-100 X 10-12 cc STP/gm whereas Murray and Meghei have a ~a°Xe content of 1600-1800 X 10-~2 cc STP/gm. The 129I/~29Xe decay interval for Meghei is calculated and compared with those for the dark phases of Fayetteville, Pantar, and Leighton. The E~29 value for Meghei, carbonaceous chondrite Type II is calculated to be 150-4-20 million years. This value seems to compare well with the ~129 values for Leighton dark phase whereas it differs considerably from the ~29 values of Pantar and Fayetteville dark phases. But it is interesting to note that the ~ 9 values for Pantar and Fayetteville dark phases show reasonable agreement within experimental errors. From these results, it appears more experimental data are needed to clearly show whether the material comprising the dark phases of these chondrites has an origin from a source similar in composition to that of carbonaceous chondrites. ACKNOWLEDGMENT This work is supported by National Science Foundation contract Ga 695. It is a pleasure to thank Dr. O. K. Manuel of the University of Missouri at Rolla for various discussions on this problem.
485
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