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Isotopic search for live 135Cs in the early solar system and possibility of 135Cs–135Ba chronometer
Earth and Planetary Science Letters 193 (2001) 459^466 www.elsevier.com/locate/epsl
Isotopic search for live possibility of
135 135
Cs in the early solar system and Cs^135Ba chronometer
Hiroshi Hidaka a; *, Yohei Ohta a , Shigekazu Yoneda b , John R. DeLaeter c a
Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan b Department of Science and Engineering, National Science Museum, Tokyo 169-0073, Japan c Department of Applied Physics, Curtin University of Technology GPO U1987, Perth, WA 6001, Australia Received 4 December 2000; received in revised form 3 September 2001; accepted 13 September 2001
Abstract Here we report the isotopic excess of 135 Ba studied from Ba isotopic measurements of acid leachates of Allende calcium^aluminum rich inclusions (CAIs) and two primitive chondrites, Beardsley (H5) and Zag (H6 clasts). From the excess 135 Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for 135 Cs/133 Cs = (4.8 þ 0.8)U1034 is proposed as the initial ratio in the early solar system. A correlation between excess 135 Ba abundances and Cs/Ba ratios observed in the Beardsley and the Zag meteorites reveals the possible existence of primordial 135 Cs in the two meteorite parent bodies. On the assumption of two cases for 135 Cs/133 Cs initial in the early solar system, the 135 Cs^135 Ba isochron suggests that primitive (aqueous) alteration in the Beardsley and Zag meteorite bodies occurred at 8.2V11.9 Ma and 13.9V17.6 Ma after CAI formation, respectively. Our results are apparently consistent with previously reported data from Pb^Pb model age determinations, and 53 Mn^53 Cr chronometry for chondrites. This is the first report of isotopic excess of 135 Ba in meteorites, which may relate to live 135 Cs in the early solar system and the 135 Cs^135 Ba chronological application. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: radioactive isotopes; cesium; barium; absolute age; calcium^aluminum inclusions; chondrites
1. Introduction Primitive meteorites contain a repository of cosmochemical information which has the potential to develop models of chemical evolution in the early solar system. There are several isotopic systems which enable the time scale for the formation and accretion of planetesimals to be de-
* Corresponding author. Tel.: +81-824-24-7464; Fax: +81-824-24-0735. E-mail address: [email protected] (H. Hidaka).
termined, and to decipher the occurrence of metamorphic events on protoplanetary bodies. Shortlived radioisotopes with half-lives less than 100 Ma were present in the early solar system, although they are now extinct nuclides. Among such nuclides, 26 A1 (T1=2 = 0.72 Ma), 41 Ca (0.1 Ma), 53 Mn (3.7 Ma), 107 Pd (6.5 Ma), 129 I (15.7 Ma) and 146 Sm (103 Ma) have been used to impose temporal constraints on evolution models of the early solar system from the detection of their decay products, 26 Mg, 41 K, 53 Cr, 107 Ag, 129 Xe and 142 Nd, respectively [1]. 135 Cs (2.3 Ma), which decays to 135 Ba, is one of these presently extinct
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 1 3 - 1
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nuclides. Precise determinations of the isotopic composition of Ba in early solar materials may allow us to detect excess 135 Ba. However, previous isotopic studies of Ba in meteorites have not found evidence of 135 Ba excess as a decay product of 135 Cs. The large di¡erence in volatility between Ba and Cs makes it di¤cult to get isotopic information of 135 Cs decay to 135 Ba even in the earliest solar system materials such as calcium^aluminum rich inclusions (CAIs) found in carbonaceous chondrites [2]. Furthermore, it is known that the isotopic composition of Ba in primitive chondrites may be disturbed by some nucleosynthetic processes [2^6]. The FUN inclusion C1 of Allende indicates de¢cits of 135 Ba due to condensation of Ba before decay of 135 Cs in the early solar system [3]. On the other hand, some other isotopic investigations reported contribution of r-process isotopes to 135 Ba and 137 Ba for Allende and Vigarano CAIs and bulk Orgueil [2,4], and enrichment of s-process Ba in the Murchison meteorite [5,6]. We performed Ba isotopic analyses on acidleached fractions of Allende CAIs, and the Beardsley (H5) and the Zag (H3^6) chondrites to search for excess 135 Ba.
sample was leached by aqua regia, and the residue was decomposed by HF^HClO4 .
2. Experiments
In this study, chemical leaching was carried out for the Beardsley and the Zag chondrites to ¢nd high Cs/Ba phases in each bulk sample. The process for chemical leaching is partly based on the method of Shima and Honda [10]. After removal of the metallic fraction from the powdered sample, 0.5 g of the non-metallic fraction was leached by 10 ml of 0.1 M CH3 COOH^CH3 COONH4 , 0.1 M HCl, 2 M HCl, and aqua regia, successively. In each leaching process, the sample was ultrasoni¢ed for 1 h, and left for 23 h at room temperature. Then, the residue was decomposed by HF^ HClO4 . The leaching experiments were carried out on three di¡erent fragments of the Beardsley meteorite and on one fragment of the Zag meteorite.
2.1. Sample 2.1.1. Allende CAIs In order to ¢nd radiogenic 135 Ba and develop the 135 Cs^135 Ba chronometer, it is essential to determine precisely the Ba isotopic composition of a high Cs/Ba phase in primitive meteorites. Most CAIs are depleted in Cs and enriched in Ba because of a large di¡erence in volatility between the two elements. From our preliminary inductively coupled plasma^mass spectrometry (ICP^MS) analysis, however, the acid-leached fractions of CAIs show 10^20 times higher Cs/Ba ratios than the residues. In this study, six CAI samples weighing between 5.66 and 65.6 mg were used. One of the six CAIs is type A and mainly consists of melilite, whereas the other ¢ve samples are fragments from type B CAIs whose major minerals are fassaite, melilite, spinel and anorthite. Each
2.1.2. Beardsley (H5) and Zag (H3^6) chondrites Beardsley is a unique meteorite which shows a high 87 Sr/86 Sr ratio even for the whole rock. It contains a higher amount of Rb (14 ppm) than other H chondrites (1.8^3.9 ppm) [7]. We expected a high content of Cs and a large Cs/Ba abundance ratio in the Beardsley meteorite because of the chemical similarity of Cs with Rb. The presence of halite crystals in the Zag H3^6 chondrite and the Monahans H5 chondrite suggests that hydrothermal activities occurred in their parent bodies [8,9]. From the high Rb/Sr elemental ratio of the Monahans halite [8], and the large excess of 129 Xe in the Zag halite [9], it is expected that primitive halites in chondrites may contain detectable isotopic excess of 135 Ba from decay of primordial 135 Cs. It is reported that Zag consists of light H6 clasts and gray H3/4 matrix, and that halite grains are found in the gray matrix [9]. In this study, we used a fragment of H6 clasts of the Zag meteorite. 2.2. Sequential acid leaching treatment
2.3. Analytical procedures An aliquot of each fraction obtained by the above methods was divided into two portions
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for the isotopic measurements of Ba and for the determination of Cs/Ba elemental ratios. The major aliquot of the sample solution was used for the isotopic analysis. A Ba fraction was chemically separated using a conventional cation exchange method [11]. A VG54-30 thermal ionization mass spectrometer equipped with seven Faraday cup collectors was used to determine Ba isotopic compositions. Data collection with static multimode was performed. Con¢gurations of collectors were ¢xed to monitor ¢ve Ba isotopes, 134 Ba, 135 Ba, 136 Ba, 137 Ba and 138 Ba, and to monitor 139 La and 140 Ce for checking of isobaric interferences, 138 La, 136 Ce and 138 Ce, during data collection. Two minor Ba isotopes, 130 Ba and 132 Ba, were not monitored. For the correction of instrumental mass fractionation, the raw data of Ba isotopic ratios are normalized to 134 Ba/ 138 Ba = 0.033715 [4]. The careful calibration for the gain factors of each collector was carried out before the start of isotopic analysis, and then the analysis of standard material for Ba (Ba standard solution produced by SPEX Certi Prep, Inc.) was performed before and after analysis of each sample. The Ba sample was loaded onto a Re side ¢lament of triple Re ¢lament assembly. More than 2U10311 A of 138 Ba ion was obtained for more than 2 h from 100 ng of Ba. The minor aliquot of the sample solution was used for ICP^MS analysis. The solution was evaporated to dryness, and re-dissolved with 0.5 M HNO3 . Then, In solution was added to the sample solution as an internal standard element for Cs and Ba analysis. Analytical conditions of the instrument (VG Plasma-Quad III) were optimized to obtain maximum count rates for In. The detailed analytical procedures with ICP^MS were the same as those previously described [12]. Isotopic compositions of Ba and Cs/Ba elemental ratios in all samples are summarized in Table 1. 3. Results and discussions 3.1. Ba isotopic compositions of the Allende CAIs The isotopic compositions of Ba in the CAIs
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are variable, and most of the CAIs show negative and positive isotopic anomalies on 135 Ba and 137 Ba, possibly in association with a late input of an r-process nucleosynthetic component. The data set of 135 Ba and 137 Ba isotopic deviations observed in the CAIs is shown in Fig. 1. The data for Vigarano CAI [4] and Allende FUN inclusions C1 and EK1-4-1 [3] are also plotted in the same ¢gure. It is di¤cult to separate radiogenic e¡ects of 135 Cs decay from other nucleogenic e¡ects in the Ba isotopic compositions. The following discussion on the Ba isotopic data of the Allende CAIs includes some speculative factors, but it is interesting to estimate an initial 135 Cs abundance from our own experimental data. Assuming that the production ratio of a late input r-process isotopic component is constant, the production ratio is estimated as O135 Ba/O137 Ba = 0.81 þ 0.05 from the major data in Fig. 1. This ratio, O135 Ba/O137 Ba = 0.81, seems to be a reasonable value as a ratio of r-process products in supernovae as compared to theoretical estimates [13]. The data points for Vigarano CAI and FUN C1 fall below the O135 Ba/ O137 Ba = 0.81 line. This may be interpreted as early incorporation of Ba into the inclusions before decay of 135 Cs and before addition of a late input r-process component. On the other hand, it should be noted that four data points collected from acid-leached type B CAIs in this study, and from EK1-4-1 as previously reported, show O135 Ba/O137 Ba s 0.81. Processes other than the rprocess which may produce 135 Ba isotope should be considered for the four acid-leached type B CAI data points. As far as type B CAIs in this study are concerned, the results of Cs/Ba elemental analysis lend support to the possibility of ¢nding radiogenic 135 Ba from 135 Cs decay. Two of the three fractions with O135 Ba/O137 Ba s 0.81 show higher Cs/Ba elemental ratios than the other samples with O135 Ba/O137 Ba = 0.81. Determination of the Cs/Ba ratio for one of the three fractions was not successful because of the small aliquot of sample solution available for ICP^MS analysis. A correlation between extra-excess 135 Ba isotopic abundances ((O135 Ba)total ^(O135 Ba)rÿprocess ) and Cs/ Ba elemental ratios in CAIs is demonstrated in Fig. 2. In this study, assuming that the 135 Ba/
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Table 1 Ba isotopic data and Cs/Ba ratios of leaching fractions Allende Type A-L (aqua regia) R (residue) Type B#1L #1R Type B#2L #2R Type B#3L #3R Type B#4L #4R Type B#5L #5R Beardsley#1 whole rock 0.1 M HCl aqua regia residue Beardsley#2 whole rock 0.1 M CH3 COOH^ CH3 COONH4 0.1 M HCl 2 M HCl aqua regia Beardsley#3 whole rock 0.1 M CH3 COOH^ CH3 COONH4 0.1 M HCl Zag 0.1 M CH3 COOH^ CH3 COONH4 0.1 M HCl 134
O135 Ba
O136 Ba
O137 Ba
Cs/Ba
(O135 Ba)aextra
+0.27 þ 0.54 0 þ 0.67 +1.26 þ 0.72 30.05 þ 0.75 30.46 þ 0.43 30.69 þ 0.53 30.78 þ 0.32 30.40 þ 0.35 +1.30 þ 0.13 +1.89 þ 0.21 +1.53 þ 0.22 +1.39 þ 0.25
+0.19 þ 0.46 30.05 þ 0.47 +0.14 þ 0.39 30.57 þ 0.70 +0.08 þ 0.30 30.05 þ 0.21 +0.07 þ 0.23 30.23 þ 0.25 +0.05 þ 0.11 +0.27 þ 0.17 30.16 þ 0.17 +0.18 þ 0.19
+0.02 þ 0.33 +0.08 þ 0.31 +0.01 þ 0.41 30.19 þ 0.53 30.92 þ 0.22 31.60 þ 0.32 30.84 þ 0.18 30.65 þ 0.18 +1.20 þ 0.08 +2.26 þ 0.11 +0.91 þ 0.10 +1.48 þ 0.12
6 0.000642 6 0.000330 not determined 6 0.000512 not determined not determined 6 0.00125 6 0.000640 0.00214 0.000452 0.00888 0.0000503
+0.25 þ 0.60 30.06 þ 0.72 +1.25 þ 0.79 +0.10 þ 0.86 +0.29 þ 0.47 +0.61 þ 0.60 30.10 þ 0.35 +0.13 þ 0.38 +0.33 þ 0.16 +0.06 þ 0.25 +0.79 þ 0.24 +0.19 þ 0.28
30.01 þ 0.50 +5.30 þ 0.37 30.15 þ 0.64 30.25 þ 0.35
30.09 þ 0.10 30.06 þ 0.27 30.45 þ 1.07 30.38 þ 0.81
30.08 þ 0.46 30.03 þ 0.21 30.10 þ 0.84 30.19 þ 0.36
0.0868 3.43 0.327 0.00878
+43.1 þ 17.8
+0.45 þ 8.11
+0.08 þ 8.36
0.317 19.8
+17.3 þ 6.1 +0.46 þ 4.62 30.43 þ 0.60
31.00 þ 2.28 +0.77 þ 1.85 +0.04 þ 0.53
+0.01 þ 0.89 +0.29 þ 0.81 +0.17 þ 0.80
1.04 0.823 0.0113
not determined
not determined
not determined
0.288 9.19
+2.52 þ 1.83
+0.12 þ 1.25
30.10 þ 0.84
1.31
+2.44 þ 0.74
+0.57 þ 1.08
30.01 þ 0.67
7.29
30.62 þ 1.20
+0.74 þ 0.94
30.23 þ 0.50
1.49
The data are normalized to Ba/ Ba = 0.033715. Terrestrial standard values of Ba isotopic ratios are 135 Ba/138 Ba = 0.091952 þ 2, 136 Ba/138 Ba = 0.109544 þ 3, 137 Ba/138 Ba = 0.156557 þ 3. Uncertainties given for the last digit indicated are 2c of the mean. a (O135 Ba)extra = {(O135 Ba)total 3(O135 Ba)rÿprocess } = {(O135 Ba)total 3(O137 Ba)U(0.81 þ 0.05)}. 137
138
Ba isotopic ratio of a late input r-process component is equal to 0.81 þ 0.05, the r-process contribution of 135 Ba, (O135 Ba)rÿprocess , is estimated from (O137 Ba)U(0.81 þ 0.05). These systematic results suggest that the extra-excess 135 Ba isotope is derived from the decay of presently extinct 135 Cs. From the slope of the correlation line in Fig. 2, the initial 135 Cs/133 Cs ratio at the time of CAI formation was (4.82 þ 0.79)U1034 . This value is three times larger than that calculated from the 135 Ba isotopic de¢ciency of the Allende FUN inclusion C1 [14]. In addition, if the extra-excess
135
Ba observed in EK1-4-1 was only due to the decay of 135 Cs, EK1-4-1 should have a high Cs/Ba ratio (about 0.066). There are several ambiguous factors involved in the interpretation of the Cs^Ba system in FUN inclusions because of the lack of information of Cs/Ba abundances. On the other hand, it should be noted that our estimation is a model-dependent way on the assumption that the Ba isotopic compositions in the Allende CAIs were disturbed only by a late input of r-process nucleosynthetic component with a constant 135 Ba/ 137 Ba ratio. Therefore, it makes no sense if the Ba
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isotopic anomalies in the CAIs were signi¢cantly caused by nucleosynthetic heterogeneity. 3.2. Ba isotopic compositions of Beardsley and Zag Our ¢rst ICP^MS analysis of the bulk Beardsley meteorite showed that its Cs/Ba abundance ratio, 0.0868, is not signi¢cantly di¡erent from those in other chondrites (e.g. 0.0799 for CI) [15], and therefore it is unlikely to have an advantage in ¢nding an isotopic excess in 135 Ba in the bulk Beardsley meteorite. The results of two more analyses from other fragments of Beardsley, however, show higher Cs/Ba elemental ratios, 0.288 and 0.317. This shows that the contents of alkaline and alkaline earth elements of the Beardsley meteorite are not homogeneous in the whole rock. The ¢rst fractions of the sequential acid leaching of Beardsley#1 and #2, 0.1 M HCl and 0.1 M CH3 COOH^CH3 COONH4 leachates, have high Cs/Ba ratios, 3.43 and 19.8, show signi¢cant 135 Ba isotopic excesses with +5.3 þ 0.5 and +43 þ 18 in O units (parts per 104 ), respectively. In addition, 0.1 M HCl leachates for the second leaching fractions of Beardsley#2 and #3 also
Fig. 2. Correlation of Cs/Ba elemental ratio and extra-135 Ba excess of the Allende CAIs. Assuming that O135 Ba/O137 Ba from the contribution of late input r-process isotopes is 0.81 þ 0.05, extra-135 Ba excess without r-process contribution is calculated by the subtraction of r-process component ((O135 Ba)rÿprocess = (O137 Ba)U(0.81 þ 0.05)) from the total 135 Ba excess ((O135 Ba)total ). The slope of this isochron leads to 135 Cs/133 Cs = (4.82 þ 0.79)U1034 as an initial value of solar system material at the time of CAI formation. Therefore, the relationship between Ba and Cs isotopic compositions in the sample can be expressed as follows: 135 135 135 Ba Ba Cs 138 138 Ba 138 Ba Ba sample i 135
135 133 Ba Cs Cs 133 138 Ba Cs 138 Ba i
where (135 Ba/138 Ba)i is an initial value of solar system, which is considered to be equal to a terrestrial standard value (0.091952 þ 2). Error bars indicate individual analytical uncertainties. Determination of Cs/Ba elemental ratios was successfully performed on only four samples, #4L, #4R, #5L and #5R. In this ¢gure, ¢ve other samples whose Cs concentrations were not determined are also plotted with the upper limit of estimated Cs values, but they are omitted for the estimation of initial 135 Cs.
Fig. 1. Isotopic variations of 135 Ba and 137 Ba in the Allende CAIs. The data are expressed as a relative deviation of individual samples to the terrestrial standard in O units (parts per 104 ). Oi Ba = {(i Ba/138 Ba)CAI /(i Ba/138 Ba)standard 31}U104 . Three reference data from the Allende FUN inclusions EK14-1 and C1 [3] and the Vigarano CAI [4] are also plotted for comparison. 134 Ba/138 Ba = 0.033715 is used as a normalizing factor for a correction of instrumental mass fractionation. Error bars indicate individual analytical uncertainties. The labels used in the ¢gure are the same as those in Table 1.
show 135 Ba isotopic excesses with +17.3 þ 6.1 and 2.52 þ 1.83, respectively. The analytical errors are expressed as 2c of means. Isotopic result from the 0.1 M CH3 COOH^CH3 COONH4 leachate of Beardsley#3 is not shown here, because the data provided very poor statistics due to rapid decay of the Ba ion beam for TIMS analysis. Lack of 137 Ba isotopic excess in the fractions suggests that the 135 Ba excesses are derived not from r-process nucleosynthetic component but from decay of 135 Cs. Our leaching experiments suggest that Cs is enriched in a water-soluble phase of the sample. As a carrier of Cs in the sample, we suspected the
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existence of halite (NaCl) and/or sylvite (KCl) crystals observed recently in the Monahans and Zag meteorites [8,9]. However, there was no evidence of such crystals, or unusual alkaline minerals from our optical and scanning electron microscope observations of the polished sections for the Beardsley meteorite. The isotopic excesses of 135 Ba in the ¢rst and second leachates for individual experiments correlate with the Cs/Ba ratios. Most of the other leaching fractions show low Cs/Ba ratios ranging from 0.011 to 0.82, and no 135 Ba excesses within analytical errors. A correlation line between 135 Ba/138 Ba and 133 Cs/138 Ba, as shown in Fig. 3, provides the initial 135 Cs/133 Cs ratio of the sample. The data points from three leaching experiments almost lie on the same line, but a slight disturbance of the 135 Cs^135 Ba system can be seen in a data point from 0.1 M HCl leachate of Beardsley#2. It might also be that the assumption of 135 Cs homogeneity in the Beardsley protoplanetary body is unwarranted. Considering that Cs is a reactive element, the 135 Cs^135 Ba chronometer may be sensitive to post-accretion events, in particular to aqueous alteration. The deviations of some data points from the correlation line may be due to recent chemical fractionation between Cs and Ba in association with terrestrial alteration, because Cs is more reactive and soluble than Ba. All the data points except one from 0.1 M HCl leachate of Beardsley#2 yield 135 Cs/133 Cs = (1.34 þ 0.20)U1035 as an
Fig. 3. Correlation of Ba isotopic composition with Cs/Ba in two primitive chondrites, Beardsley and Zag. The solid lines represent 135 Cs^135 Ba isochrons obtained from Beardsley (¢lled symbols) and Zag (open symbols). Error bars indicate individual analytical uncertainties.
Fig. 4. Time variation of 135 Cs/133 Cs after CAI formation. Based on the assumption of (135 Cs/ 133 Cs)i = (4.82 þ 0.79)U1034 at CAI formation (see text), the data points for Beardsley and Zag provide 11.9 Ma and 17.6 Ma, respectively, as their formation intervals (vT). On the other hand, the use of (135 Cs/133 Cs)i = 1.6U1034 as an initial value in the early solar system [14] leads to 8.2 Ma and 13.9 Ma for Beardsley and Zag, respectively.
initial ratio of the Beardsley meteorite body. Fig. 4 shows a time-dependence of live 135 Cs abundance in the early solar system. The curves in the ¢gure were drawn for initial 135 Cs/133 Cs values of 4.8U1034 and 1.6U1034 at CAI formation, which are estimated from this study and from [3], respectively. Assuming that an initial solar system abundance of 135 Cs/133 Cs is (4.82 þ 0.79)U1034 on the basis of the systematic Ba isotopic data of the Allende CAIs, the initial 135 Cs/133 Cs of the Beardsley meteorite corresponds to a time di¡erence of 11.890:53 30:47 Ma after CAI formation. On the other hand, the use of 135 Cs/133 Cs = 1.6U1034 for an initial solar system leads to an 8.20:54 30:56 Ma interval between Beardsley and CAI formation. The values 11.9 Ma and 8.2 Ma indicate the formation interval of the Beardsley chondritic body after thermal metamorphism. The result from the Cs^Ba chronometer is consistent with the data from Pb^Pb model ages of meteoritic samples. Systematic Pb isotopic analyses of chondrites give 10^16 Ma for the formation interval between CAI (4566 Ma) [16] and H5 chondrites (4550^4556 Ma) [17]. A high Cs/Ba ratio ( = 7.29) and a small 135 Ba excess (O135 Ba = +2.4 þ 0.7) were found in the 0.1 M CH3 COOH^CH3 COONH4 fraction (open symbol in Fig. 3) of Zag. This result indicates that evaporite minerals such as halite and sylvite may
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be one of major carriers for Cs, although such minerals could not be found in the Beardsley meteorite, or our Zag sample. The evaporite minerals in Zag may be formed during aqueous activity, and detectable amounts of 135 Cs would have largely decayed away at the time of deposition of the evaporites. However, the presence of extinct 129 I in Zag halite [9], and the 4.70 þ 0.20 Ga of Rb^Sr model age in Monahans halite [8] suggest that aqueous activity in the asteroidal bodies occurred early in the solar system. On the basis of 135 Cs^135 Ba isochron of meteorites and 135 Cs abundance of the Allende CAIs, chronological constraints can be placed on the meteorites. 135 Cs abundance of Zag (135 Cs/ 133 Cs = 2.43U1036 ) corresponds to 13.91:8 31:1 and 17.61:8 31:2 Ma time intervals after formation of CAI with 1.6U1034 and 4.8U1034 initial value, respectively. The Cs^Ba chronological result from Zag shows a slightly later event in the range of H5 chondrite formation, which is signi¢cantly di¡erent from that of Beardsley. This di¡erence may be related to perturbation events on H chondrite parent bodies. The data from Beardsley suggest the primitive alkali-enriched fraction remains in the meteorite parent body without disturbance, and therefore allows the formation interval of the H5 chondritic body to be calculated. On the other hand, Zag is petrologically known as a breccia belonging to H3^6 [9,18]. According to precise determinations of Pb^Pb model ages for H chondrites [16,17], H6 chondrites give ages 30^50 Ma older than H4^5 chondrites. The Cs^Ba system from Zag may be disturbed by a metamorphic event in the H6 chondrite component, although it could provide the time scale for the termination of aqueous alteration on the H3^5 chondritic bodies. I^Xe chronometer from direct measurement of the Zag halite reveals that the halite formed 1.7 Ma later than formation of the Murchison magnetite which is known as the earliest I^Xe age [9]. Direct comparison of I^Xe ages with an absolute chronometer such as Pb^Pb ages is a little di¤cult [19]. However, judging from a calibration of the I^Xe ages with Pb^Pb ages by the measurement of the Bjurbo«le meteorite [20], formation age of the Zag halite by I^Xe chronometer seems older than the age by the Cs^Ba meth-
465
od. This also supports that our Cs^Ba age for Zag reveals the timing of a metamorphic event in the parent body of Zag. Further chronological work is required to determine the ages of aqueous activity and metamorphic events on the meteorite parent body. 4. Summary Systematic Ba isotopic measurements of the Allende CAIs suggest that the isotopic excess of 135 Ba in the CAIs mainly consists of r-process nucleosynthetic origin and decay products from extinct 135 Cs. From the excess 135 Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for 135 Cs/133 Cs = (4.82 þ 0.79)U1034 is experimentally determined as an initial ratio in the early solar system. Weak acid leaching fractions of Beardsley and Zag show very large Cs/Ba elemental ratios and isotopic excesses of 135 Ba. The correlation between Cs/Ba and 135 Ba excess suggests that the isotopic excess of 135 Ba of Beardsley and Zag is the result of 135 Cs decay. The 135 Cs^135 Ba isochron of two meteorites, Beardsley and Zag, and initial abundance of 135 Cs from the Allende CAIs show the possibility of chronological application on early activities in the meteorite parent bodies. To the best of our knowledge, the 53 Mn^53 Cr studies in carbonates from the Orgueil chondrite (CI) [21] and in fayalites from the Mokoia chondrite (CV3) [22] are clear cases which have enabled time constraints to be placed on the occurrence of aqueous alteration in primitive meteorite parent bodies. The 53 Mn^53 Cr chronometry shows that aqueous alteration on small protoplanetary bodies must have begun less than 20 Ma after CAI formation. Our data from 135 Cs^ 135 Ba chronometer have apparently no discrepancy with time constraints from the 53 Mn^53 Cr system, and may provide a more critical record for alteration processes on the early planetary body than the 53 Mn^53 Cr system does. But the Cs^Ba age for Zag is younger than the I^Xe age for the Zag halite [9]. One major problem is an estimation of the initial 135 Cs/133 Cs value in the
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early solar system. Our estimation depends on the model that the Ba isotopic compositions in the Allende CAIs were largely disturbed by a late input of r-process nucleosynthetic component with a constant 135 Ba/137 Ba ratio, because it is impossible to directly ¢nd isotopic evidence of 135 Cs decay from other nucleogenic e¡ects without assumption. Further work is necessary to make clear the primitive activities on meteorite parent body and to calculate a consistent and reliable age from Cs^Ba chronometry. Acknowledgements
[7] [8]
[9]
[10] [11]
We are grateful to Y. Takahashi, Y. Shibata and G. Burton for their technical support with ICP^MS, EPMA and TIMS analyses. Critical reading and comments by J. Bridges, L. Nyquist, and an anonymous journal reviewer were very helpful to improve the ¢rst draft of this paper. The suggestions from the Associate Editor, A.N. Halliday, were useful in revising the manuscript. A part of this study was ¢nancially supported by a Grant-in-Aid for Ministry of Education, Culture, Sports, Science and Technology (to H.H., no. 13440167).[AH]
[12]
[13] [14]
[15] [16]
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[1] F.A. Podosek, T.D. Swindle, Extinct radionuclides, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, University of Arizona Press, Tucson, AZ, 1988, pp. 1093^1113. [2] C.L. Harper, H. Wiesmann, L.E. Nyquist, The search for 135 Cs in the early solar system: Very high precision measurements of barium isotopes in bulk Allende and refractory inclusions, Meteoritics 27 (1992) 230^231. [3] M.T. McCulloch, G.J. Wasserburg, Barium and neodymium isotopic anomalies in the Allende meteorite, Astrophys. J. 220 (1978) L15^L19. [4] R.D. Loss, G.W. Lugmair, A.M. Davis, G.J. MacPherson, Isotopically distinct reservoirs in the solar nebula: Isotope anomalies in Vigarano meteorite inclusions, Astrophys. J. 436 (1994) L193^L196. [5] U. Ott, F. Begemann, Discovery of s-process barium in the Murchison meteorite, Astrophys. J. 353 (1990) L57^ L60. [6] C.A. Prombo, F.A. Podosek, S. Amari, R.S. Lewis, s-pro-
[18] [19]
[20]
[21] [22]
cess Ba isotopic compositions in presolar SiC from the Murchison meteorite, Astrophys. J. 410 (1993) 393^399. S.K. Kaushal, G.W. Wetherill, 87 Rb^87 Sr age of bronzite (H Group) chondrites, J. Geophys. Res. 74 (1969) 2717^ 2726. M.E. Zolensky, R.J. Bondar, E.K. Gibson, L.E. Nyquist, Y. Reese, C.-Y. Shih, H. Wiesmann, Asteroidal water within £uid inclusion-bearing halite in an H5 chondrite, Monahans, Science 285 (1999) 1377^1379. J. Whitby, R. Burgess, G. Turner, J. Gilmour, J. Bridges, Extinct 129 I in halite from a primitive meteorite: Evidence for evaporite formation in the early solar system, Science 288 (2000) 1819^1821. M. Shima, M. Honda, Distribution of rubidium^strontium age of chondrites using their separated components, Earth Planet. Sci. Lett. 2 (1967) 337^343. H. Hidaka, P. Holliger, A. Masuda, Evidence of ¢ssiogenic Cs estimated from Ba isotopic deviations in an Oklo natural reactor zone, Earth Planet. Sci. Lett. 114 (1993) 391^396. K. Shinotsuka, H. Hidaka, M. Ebihara, Detailed abundances of rare earth elements, thorium and uranium in chondritic meteorites: An ICP^MS study, Meteoritics 30 (1995) 694^699. D. Heymann, Barium from a mini r-process in supernovae, Astrophys. J. 267 (1983) 747^756. G.J. Wasserburg, M. Busso, R. Gallino, Abundances of actinides and short-lived nonactinides in the interstellar medium: Diverse supernova sources for the r-processes, Astrophys. J. 466 (1996) L109^L113. E. Anders, M. Grevesse, Abundances of the elements: meteoritic and solar, Geochim. Cosmochim. Acta 53 (1989) 197^214. C.J. Alle©gre, G. Manhe©s, C. Go«pel, The age of the earth, Geochim. Cosmochim. Acta 59 (1995) 1445^1456. C. Go«pel, G. Manhe©s, C.J. Alle©gre, U^Pb systematics of phosphates from equilibrated ordinary chondrites, Earth Planet. Sci. Lett. 121 (1994) 153^171. J.N. Grossman, The meteoritical bulletin, No.83, Meteorit. Planet. Sci. 34 (1999) A169^A186. T.D. Swindle, F.A. Podosek, Iodine^Xenon dating, in: J.F. Kerridge, M.S. Matthews (Eds.), Meteorites and the Early Solar System, University of Arizona Press, Tucson, AZ, 1988, pp. 1127^1146. R.H. Nichols Jr., C.M. Hohenberg, K. Kehm, Y. Kim, K. Marti, I^Xe studies of the Acapulco meteorite: Absolute I^Xe ages of individual phosphate grains and the BjurbÖle standard, Geochim. Cosmochim. Acta 58 (1994) 2553^ 2561. M. Endress, E. Zinner, A. Bischo¡, Early aqueous activity on primitive meteorite parent bodies, Nature 379 (1996) 701^703. I.D. Hutcheon, A.N. Krot, K. Keil, D.L. Phinney, E.R.D. Scott, 53 Mn^53 Cr dating of fayalite formation in the CV3 Chondrite Mokoia: Evidence for asteroidal alteration, Science 282 (1998) 1865^1867.
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Isotopic search for live possibility of
135 135
Cs in the early solar system and Cs^135Ba chronometer
Hiroshi Hidaka a; *, Yohei Ohta a , Shigekazu Yoneda b , John R. DeLaeter c a
Department of Earth and Planetary Systems Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan b Department of Science and Engineering, National Science Museum, Tokyo 169-0073, Japan c Department of Applied Physics, Curtin University of Technology GPO U1987, Perth, WA 6001, Australia Received 4 December 2000; received in revised form 3 September 2001; accepted 13 September 2001
Abstract Here we report the isotopic excess of 135 Ba studied from Ba isotopic measurements of acid leachates of Allende calcium^aluminum rich inclusions (CAIs) and two primitive chondrites, Beardsley (H5) and Zag (H6 clasts). From the excess 135 Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for 135 Cs/133 Cs = (4.8 þ 0.8)U1034 is proposed as the initial ratio in the early solar system. A correlation between excess 135 Ba abundances and Cs/Ba ratios observed in the Beardsley and the Zag meteorites reveals the possible existence of primordial 135 Cs in the two meteorite parent bodies. On the assumption of two cases for 135 Cs/133 Cs initial in the early solar system, the 135 Cs^135 Ba isochron suggests that primitive (aqueous) alteration in the Beardsley and Zag meteorite bodies occurred at 8.2V11.9 Ma and 13.9V17.6 Ma after CAI formation, respectively. Our results are apparently consistent with previously reported data from Pb^Pb model age determinations, and 53 Mn^53 Cr chronometry for chondrites. This is the first report of isotopic excess of 135 Ba in meteorites, which may relate to live 135 Cs in the early solar system and the 135 Cs^135 Ba chronological application. ß 2001 Elsevier Science B.V. All rights reserved. Keywords: radioactive isotopes; cesium; barium; absolute age; calcium^aluminum inclusions; chondrites
1. Introduction Primitive meteorites contain a repository of cosmochemical information which has the potential to develop models of chemical evolution in the early solar system. There are several isotopic systems which enable the time scale for the formation and accretion of planetesimals to be de-
* Corresponding author. Tel.: +81-824-24-7464; Fax: +81-824-24-0735. E-mail address: [email protected] (H. Hidaka).
termined, and to decipher the occurrence of metamorphic events on protoplanetary bodies. Shortlived radioisotopes with half-lives less than 100 Ma were present in the early solar system, although they are now extinct nuclides. Among such nuclides, 26 A1 (T1=2 = 0.72 Ma), 41 Ca (0.1 Ma), 53 Mn (3.7 Ma), 107 Pd (6.5 Ma), 129 I (15.7 Ma) and 146 Sm (103 Ma) have been used to impose temporal constraints on evolution models of the early solar system from the detection of their decay products, 26 Mg, 41 K, 53 Cr, 107 Ag, 129 Xe and 142 Nd, respectively [1]. 135 Cs (2.3 Ma), which decays to 135 Ba, is one of these presently extinct
0012-821X / 01 / $ ^ see front matter ß 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 0 1 ) 0 0 5 1 3 - 1
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nuclides. Precise determinations of the isotopic composition of Ba in early solar materials may allow us to detect excess 135 Ba. However, previous isotopic studies of Ba in meteorites have not found evidence of 135 Ba excess as a decay product of 135 Cs. The large di¡erence in volatility between Ba and Cs makes it di¤cult to get isotopic information of 135 Cs decay to 135 Ba even in the earliest solar system materials such as calcium^aluminum rich inclusions (CAIs) found in carbonaceous chondrites [2]. Furthermore, it is known that the isotopic composition of Ba in primitive chondrites may be disturbed by some nucleosynthetic processes [2^6]. The FUN inclusion C1 of Allende indicates de¢cits of 135 Ba due to condensation of Ba before decay of 135 Cs in the early solar system [3]. On the other hand, some other isotopic investigations reported contribution of r-process isotopes to 135 Ba and 137 Ba for Allende and Vigarano CAIs and bulk Orgueil [2,4], and enrichment of s-process Ba in the Murchison meteorite [5,6]. We performed Ba isotopic analyses on acidleached fractions of Allende CAIs, and the Beardsley (H5) and the Zag (H3^6) chondrites to search for excess 135 Ba.
sample was leached by aqua regia, and the residue was decomposed by HF^HClO4 .
2. Experiments
In this study, chemical leaching was carried out for the Beardsley and the Zag chondrites to ¢nd high Cs/Ba phases in each bulk sample. The process for chemical leaching is partly based on the method of Shima and Honda [10]. After removal of the metallic fraction from the powdered sample, 0.5 g of the non-metallic fraction was leached by 10 ml of 0.1 M CH3 COOH^CH3 COONH4 , 0.1 M HCl, 2 M HCl, and aqua regia, successively. In each leaching process, the sample was ultrasoni¢ed for 1 h, and left for 23 h at room temperature. Then, the residue was decomposed by HF^ HClO4 . The leaching experiments were carried out on three di¡erent fragments of the Beardsley meteorite and on one fragment of the Zag meteorite.
2.1. Sample 2.1.1. Allende CAIs In order to ¢nd radiogenic 135 Ba and develop the 135 Cs^135 Ba chronometer, it is essential to determine precisely the Ba isotopic composition of a high Cs/Ba phase in primitive meteorites. Most CAIs are depleted in Cs and enriched in Ba because of a large di¡erence in volatility between the two elements. From our preliminary inductively coupled plasma^mass spectrometry (ICP^MS) analysis, however, the acid-leached fractions of CAIs show 10^20 times higher Cs/Ba ratios than the residues. In this study, six CAI samples weighing between 5.66 and 65.6 mg were used. One of the six CAIs is type A and mainly consists of melilite, whereas the other ¢ve samples are fragments from type B CAIs whose major minerals are fassaite, melilite, spinel and anorthite. Each
2.1.2. Beardsley (H5) and Zag (H3^6) chondrites Beardsley is a unique meteorite which shows a high 87 Sr/86 Sr ratio even for the whole rock. It contains a higher amount of Rb (14 ppm) than other H chondrites (1.8^3.9 ppm) [7]. We expected a high content of Cs and a large Cs/Ba abundance ratio in the Beardsley meteorite because of the chemical similarity of Cs with Rb. The presence of halite crystals in the Zag H3^6 chondrite and the Monahans H5 chondrite suggests that hydrothermal activities occurred in their parent bodies [8,9]. From the high Rb/Sr elemental ratio of the Monahans halite [8], and the large excess of 129 Xe in the Zag halite [9], it is expected that primitive halites in chondrites may contain detectable isotopic excess of 135 Ba from decay of primordial 135 Cs. It is reported that Zag consists of light H6 clasts and gray H3/4 matrix, and that halite grains are found in the gray matrix [9]. In this study, we used a fragment of H6 clasts of the Zag meteorite. 2.2. Sequential acid leaching treatment
2.3. Analytical procedures An aliquot of each fraction obtained by the above methods was divided into two portions
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for the isotopic measurements of Ba and for the determination of Cs/Ba elemental ratios. The major aliquot of the sample solution was used for the isotopic analysis. A Ba fraction was chemically separated using a conventional cation exchange method [11]. A VG54-30 thermal ionization mass spectrometer equipped with seven Faraday cup collectors was used to determine Ba isotopic compositions. Data collection with static multimode was performed. Con¢gurations of collectors were ¢xed to monitor ¢ve Ba isotopes, 134 Ba, 135 Ba, 136 Ba, 137 Ba and 138 Ba, and to monitor 139 La and 140 Ce for checking of isobaric interferences, 138 La, 136 Ce and 138 Ce, during data collection. Two minor Ba isotopes, 130 Ba and 132 Ba, were not monitored. For the correction of instrumental mass fractionation, the raw data of Ba isotopic ratios are normalized to 134 Ba/ 138 Ba = 0.033715 [4]. The careful calibration for the gain factors of each collector was carried out before the start of isotopic analysis, and then the analysis of standard material for Ba (Ba standard solution produced by SPEX Certi Prep, Inc.) was performed before and after analysis of each sample. The Ba sample was loaded onto a Re side ¢lament of triple Re ¢lament assembly. More than 2U10311 A of 138 Ba ion was obtained for more than 2 h from 100 ng of Ba. The minor aliquot of the sample solution was used for ICP^MS analysis. The solution was evaporated to dryness, and re-dissolved with 0.5 M HNO3 . Then, In solution was added to the sample solution as an internal standard element for Cs and Ba analysis. Analytical conditions of the instrument (VG Plasma-Quad III) were optimized to obtain maximum count rates for In. The detailed analytical procedures with ICP^MS were the same as those previously described [12]. Isotopic compositions of Ba and Cs/Ba elemental ratios in all samples are summarized in Table 1. 3. Results and discussions 3.1. Ba isotopic compositions of the Allende CAIs The isotopic compositions of Ba in the CAIs
461
are variable, and most of the CAIs show negative and positive isotopic anomalies on 135 Ba and 137 Ba, possibly in association with a late input of an r-process nucleosynthetic component. The data set of 135 Ba and 137 Ba isotopic deviations observed in the CAIs is shown in Fig. 1. The data for Vigarano CAI [4] and Allende FUN inclusions C1 and EK1-4-1 [3] are also plotted in the same ¢gure. It is di¤cult to separate radiogenic e¡ects of 135 Cs decay from other nucleogenic e¡ects in the Ba isotopic compositions. The following discussion on the Ba isotopic data of the Allende CAIs includes some speculative factors, but it is interesting to estimate an initial 135 Cs abundance from our own experimental data. Assuming that the production ratio of a late input r-process isotopic component is constant, the production ratio is estimated as O135 Ba/O137 Ba = 0.81 þ 0.05 from the major data in Fig. 1. This ratio, O135 Ba/O137 Ba = 0.81, seems to be a reasonable value as a ratio of r-process products in supernovae as compared to theoretical estimates [13]. The data points for Vigarano CAI and FUN C1 fall below the O135 Ba/ O137 Ba = 0.81 line. This may be interpreted as early incorporation of Ba into the inclusions before decay of 135 Cs and before addition of a late input r-process component. On the other hand, it should be noted that four data points collected from acid-leached type B CAIs in this study, and from EK1-4-1 as previously reported, show O135 Ba/O137 Ba s 0.81. Processes other than the rprocess which may produce 135 Ba isotope should be considered for the four acid-leached type B CAI data points. As far as type B CAIs in this study are concerned, the results of Cs/Ba elemental analysis lend support to the possibility of ¢nding radiogenic 135 Ba from 135 Cs decay. Two of the three fractions with O135 Ba/O137 Ba s 0.81 show higher Cs/Ba elemental ratios than the other samples with O135 Ba/O137 Ba = 0.81. Determination of the Cs/Ba ratio for one of the three fractions was not successful because of the small aliquot of sample solution available for ICP^MS analysis. A correlation between extra-excess 135 Ba isotopic abundances ((O135 Ba)total ^(O135 Ba)rÿprocess ) and Cs/ Ba elemental ratios in CAIs is demonstrated in Fig. 2. In this study, assuming that the 135 Ba/
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Table 1 Ba isotopic data and Cs/Ba ratios of leaching fractions Allende Type A-L (aqua regia) R (residue) Type B#1L #1R Type B#2L #2R Type B#3L #3R Type B#4L #4R Type B#5L #5R Beardsley#1 whole rock 0.1 M HCl aqua regia residue Beardsley#2 whole rock 0.1 M CH3 COOH^ CH3 COONH4 0.1 M HCl 2 M HCl aqua regia Beardsley#3 whole rock 0.1 M CH3 COOH^ CH3 COONH4 0.1 M HCl Zag 0.1 M CH3 COOH^ CH3 COONH4 0.1 M HCl 134
O135 Ba
O136 Ba
O137 Ba
Cs/Ba
(O135 Ba)aextra
+0.27 þ 0.54 0 þ 0.67 +1.26 þ 0.72 30.05 þ 0.75 30.46 þ 0.43 30.69 þ 0.53 30.78 þ 0.32 30.40 þ 0.35 +1.30 þ 0.13 +1.89 þ 0.21 +1.53 þ 0.22 +1.39 þ 0.25
+0.19 þ 0.46 30.05 þ 0.47 +0.14 þ 0.39 30.57 þ 0.70 +0.08 þ 0.30 30.05 þ 0.21 +0.07 þ 0.23 30.23 þ 0.25 +0.05 þ 0.11 +0.27 þ 0.17 30.16 þ 0.17 +0.18 þ 0.19
+0.02 þ 0.33 +0.08 þ 0.31 +0.01 þ 0.41 30.19 þ 0.53 30.92 þ 0.22 31.60 þ 0.32 30.84 þ 0.18 30.65 þ 0.18 +1.20 þ 0.08 +2.26 þ 0.11 +0.91 þ 0.10 +1.48 þ 0.12
6 0.000642 6 0.000330 not determined 6 0.000512 not determined not determined 6 0.00125 6 0.000640 0.00214 0.000452 0.00888 0.0000503
+0.25 þ 0.60 30.06 þ 0.72 +1.25 þ 0.79 +0.10 þ 0.86 +0.29 þ 0.47 +0.61 þ 0.60 30.10 þ 0.35 +0.13 þ 0.38 +0.33 þ 0.16 +0.06 þ 0.25 +0.79 þ 0.24 +0.19 þ 0.28
30.01 þ 0.50 +5.30 þ 0.37 30.15 þ 0.64 30.25 þ 0.35
30.09 þ 0.10 30.06 þ 0.27 30.45 þ 1.07 30.38 þ 0.81
30.08 þ 0.46 30.03 þ 0.21 30.10 þ 0.84 30.19 þ 0.36
0.0868 3.43 0.327 0.00878
+43.1 þ 17.8
+0.45 þ 8.11
+0.08 þ 8.36
0.317 19.8
+17.3 þ 6.1 +0.46 þ 4.62 30.43 þ 0.60
31.00 þ 2.28 +0.77 þ 1.85 +0.04 þ 0.53
+0.01 þ 0.89 +0.29 þ 0.81 +0.17 þ 0.80
1.04 0.823 0.0113
not determined
not determined
not determined
0.288 9.19
+2.52 þ 1.83
+0.12 þ 1.25
30.10 þ 0.84
1.31
+2.44 þ 0.74
+0.57 þ 1.08
30.01 þ 0.67
7.29
30.62 þ 1.20
+0.74 þ 0.94
30.23 þ 0.50
1.49
The data are normalized to Ba/ Ba = 0.033715. Terrestrial standard values of Ba isotopic ratios are 135 Ba/138 Ba = 0.091952 þ 2, 136 Ba/138 Ba = 0.109544 þ 3, 137 Ba/138 Ba = 0.156557 þ 3. Uncertainties given for the last digit indicated are 2c of the mean. a (O135 Ba)extra = {(O135 Ba)total 3(O135 Ba)rÿprocess } = {(O135 Ba)total 3(O137 Ba)U(0.81 þ 0.05)}. 137
138
Ba isotopic ratio of a late input r-process component is equal to 0.81 þ 0.05, the r-process contribution of 135 Ba, (O135 Ba)rÿprocess , is estimated from (O137 Ba)U(0.81 þ 0.05). These systematic results suggest that the extra-excess 135 Ba isotope is derived from the decay of presently extinct 135 Cs. From the slope of the correlation line in Fig. 2, the initial 135 Cs/133 Cs ratio at the time of CAI formation was (4.82 þ 0.79)U1034 . This value is three times larger than that calculated from the 135 Ba isotopic de¢ciency of the Allende FUN inclusion C1 [14]. In addition, if the extra-excess
135
Ba observed in EK1-4-1 was only due to the decay of 135 Cs, EK1-4-1 should have a high Cs/Ba ratio (about 0.066). There are several ambiguous factors involved in the interpretation of the Cs^Ba system in FUN inclusions because of the lack of information of Cs/Ba abundances. On the other hand, it should be noted that our estimation is a model-dependent way on the assumption that the Ba isotopic compositions in the Allende CAIs were disturbed only by a late input of r-process nucleosynthetic component with a constant 135 Ba/ 137 Ba ratio. Therefore, it makes no sense if the Ba
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isotopic anomalies in the CAIs were signi¢cantly caused by nucleosynthetic heterogeneity. 3.2. Ba isotopic compositions of Beardsley and Zag Our ¢rst ICP^MS analysis of the bulk Beardsley meteorite showed that its Cs/Ba abundance ratio, 0.0868, is not signi¢cantly di¡erent from those in other chondrites (e.g. 0.0799 for CI) [15], and therefore it is unlikely to have an advantage in ¢nding an isotopic excess in 135 Ba in the bulk Beardsley meteorite. The results of two more analyses from other fragments of Beardsley, however, show higher Cs/Ba elemental ratios, 0.288 and 0.317. This shows that the contents of alkaline and alkaline earth elements of the Beardsley meteorite are not homogeneous in the whole rock. The ¢rst fractions of the sequential acid leaching of Beardsley#1 and #2, 0.1 M HCl and 0.1 M CH3 COOH^CH3 COONH4 leachates, have high Cs/Ba ratios, 3.43 and 19.8, show signi¢cant 135 Ba isotopic excesses with +5.3 þ 0.5 and +43 þ 18 in O units (parts per 104 ), respectively. In addition, 0.1 M HCl leachates for the second leaching fractions of Beardsley#2 and #3 also
Fig. 2. Correlation of Cs/Ba elemental ratio and extra-135 Ba excess of the Allende CAIs. Assuming that O135 Ba/O137 Ba from the contribution of late input r-process isotopes is 0.81 þ 0.05, extra-135 Ba excess without r-process contribution is calculated by the subtraction of r-process component ((O135 Ba)rÿprocess = (O137 Ba)U(0.81 þ 0.05)) from the total 135 Ba excess ((O135 Ba)total ). The slope of this isochron leads to 135 Cs/133 Cs = (4.82 þ 0.79)U1034 as an initial value of solar system material at the time of CAI formation. Therefore, the relationship between Ba and Cs isotopic compositions in the sample can be expressed as follows: 135 135 135 Ba Ba Cs 138 138 Ba 138 Ba Ba sample i 135
135 133 Ba Cs Cs 133 138 Ba Cs 138 Ba i
where (135 Ba/138 Ba)i is an initial value of solar system, which is considered to be equal to a terrestrial standard value (0.091952 þ 2). Error bars indicate individual analytical uncertainties. Determination of Cs/Ba elemental ratios was successfully performed on only four samples, #4L, #4R, #5L and #5R. In this ¢gure, ¢ve other samples whose Cs concentrations were not determined are also plotted with the upper limit of estimated Cs values, but they are omitted for the estimation of initial 135 Cs.
Fig. 1. Isotopic variations of 135 Ba and 137 Ba in the Allende CAIs. The data are expressed as a relative deviation of individual samples to the terrestrial standard in O units (parts per 104 ). Oi Ba = {(i Ba/138 Ba)CAI /(i Ba/138 Ba)standard 31}U104 . Three reference data from the Allende FUN inclusions EK14-1 and C1 [3] and the Vigarano CAI [4] are also plotted for comparison. 134 Ba/138 Ba = 0.033715 is used as a normalizing factor for a correction of instrumental mass fractionation. Error bars indicate individual analytical uncertainties. The labels used in the ¢gure are the same as those in Table 1.
show 135 Ba isotopic excesses with +17.3 þ 6.1 and 2.52 þ 1.83, respectively. The analytical errors are expressed as 2c of means. Isotopic result from the 0.1 M CH3 COOH^CH3 COONH4 leachate of Beardsley#3 is not shown here, because the data provided very poor statistics due to rapid decay of the Ba ion beam for TIMS analysis. Lack of 137 Ba isotopic excess in the fractions suggests that the 135 Ba excesses are derived not from r-process nucleosynthetic component but from decay of 135 Cs. Our leaching experiments suggest that Cs is enriched in a water-soluble phase of the sample. As a carrier of Cs in the sample, we suspected the
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existence of halite (NaCl) and/or sylvite (KCl) crystals observed recently in the Monahans and Zag meteorites [8,9]. However, there was no evidence of such crystals, or unusual alkaline minerals from our optical and scanning electron microscope observations of the polished sections for the Beardsley meteorite. The isotopic excesses of 135 Ba in the ¢rst and second leachates for individual experiments correlate with the Cs/Ba ratios. Most of the other leaching fractions show low Cs/Ba ratios ranging from 0.011 to 0.82, and no 135 Ba excesses within analytical errors. A correlation line between 135 Ba/138 Ba and 133 Cs/138 Ba, as shown in Fig. 3, provides the initial 135 Cs/133 Cs ratio of the sample. The data points from three leaching experiments almost lie on the same line, but a slight disturbance of the 135 Cs^135 Ba system can be seen in a data point from 0.1 M HCl leachate of Beardsley#2. It might also be that the assumption of 135 Cs homogeneity in the Beardsley protoplanetary body is unwarranted. Considering that Cs is a reactive element, the 135 Cs^135 Ba chronometer may be sensitive to post-accretion events, in particular to aqueous alteration. The deviations of some data points from the correlation line may be due to recent chemical fractionation between Cs and Ba in association with terrestrial alteration, because Cs is more reactive and soluble than Ba. All the data points except one from 0.1 M HCl leachate of Beardsley#2 yield 135 Cs/133 Cs = (1.34 þ 0.20)U1035 as an
Fig. 3. Correlation of Ba isotopic composition with Cs/Ba in two primitive chondrites, Beardsley and Zag. The solid lines represent 135 Cs^135 Ba isochrons obtained from Beardsley (¢lled symbols) and Zag (open symbols). Error bars indicate individual analytical uncertainties.
Fig. 4. Time variation of 135 Cs/133 Cs after CAI formation. Based on the assumption of (135 Cs/ 133 Cs)i = (4.82 þ 0.79)U1034 at CAI formation (see text), the data points for Beardsley and Zag provide 11.9 Ma and 17.6 Ma, respectively, as their formation intervals (vT). On the other hand, the use of (135 Cs/133 Cs)i = 1.6U1034 as an initial value in the early solar system [14] leads to 8.2 Ma and 13.9 Ma for Beardsley and Zag, respectively.
initial ratio of the Beardsley meteorite body. Fig. 4 shows a time-dependence of live 135 Cs abundance in the early solar system. The curves in the ¢gure were drawn for initial 135 Cs/133 Cs values of 4.8U1034 and 1.6U1034 at CAI formation, which are estimated from this study and from [3], respectively. Assuming that an initial solar system abundance of 135 Cs/133 Cs is (4.82 þ 0.79)U1034 on the basis of the systematic Ba isotopic data of the Allende CAIs, the initial 135 Cs/133 Cs of the Beardsley meteorite corresponds to a time di¡erence of 11.890:53 30:47 Ma after CAI formation. On the other hand, the use of 135 Cs/133 Cs = 1.6U1034 for an initial solar system leads to an 8.20:54 30:56 Ma interval between Beardsley and CAI formation. The values 11.9 Ma and 8.2 Ma indicate the formation interval of the Beardsley chondritic body after thermal metamorphism. The result from the Cs^Ba chronometer is consistent with the data from Pb^Pb model ages of meteoritic samples. Systematic Pb isotopic analyses of chondrites give 10^16 Ma for the formation interval between CAI (4566 Ma) [16] and H5 chondrites (4550^4556 Ma) [17]. A high Cs/Ba ratio ( = 7.29) and a small 135 Ba excess (O135 Ba = +2.4 þ 0.7) were found in the 0.1 M CH3 COOH^CH3 COONH4 fraction (open symbol in Fig. 3) of Zag. This result indicates that evaporite minerals such as halite and sylvite may
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be one of major carriers for Cs, although such minerals could not be found in the Beardsley meteorite, or our Zag sample. The evaporite minerals in Zag may be formed during aqueous activity, and detectable amounts of 135 Cs would have largely decayed away at the time of deposition of the evaporites. However, the presence of extinct 129 I in Zag halite [9], and the 4.70 þ 0.20 Ga of Rb^Sr model age in Monahans halite [8] suggest that aqueous activity in the asteroidal bodies occurred early in the solar system. On the basis of 135 Cs^135 Ba isochron of meteorites and 135 Cs abundance of the Allende CAIs, chronological constraints can be placed on the meteorites. 135 Cs abundance of Zag (135 Cs/ 133 Cs = 2.43U1036 ) corresponds to 13.91:8 31:1 and 17.61:8 31:2 Ma time intervals after formation of CAI with 1.6U1034 and 4.8U1034 initial value, respectively. The Cs^Ba chronological result from Zag shows a slightly later event in the range of H5 chondrite formation, which is signi¢cantly di¡erent from that of Beardsley. This di¡erence may be related to perturbation events on H chondrite parent bodies. The data from Beardsley suggest the primitive alkali-enriched fraction remains in the meteorite parent body without disturbance, and therefore allows the formation interval of the H5 chondritic body to be calculated. On the other hand, Zag is petrologically known as a breccia belonging to H3^6 [9,18]. According to precise determinations of Pb^Pb model ages for H chondrites [16,17], H6 chondrites give ages 30^50 Ma older than H4^5 chondrites. The Cs^Ba system from Zag may be disturbed by a metamorphic event in the H6 chondrite component, although it could provide the time scale for the termination of aqueous alteration on the H3^5 chondritic bodies. I^Xe chronometer from direct measurement of the Zag halite reveals that the halite formed 1.7 Ma later than formation of the Murchison magnetite which is known as the earliest I^Xe age [9]. Direct comparison of I^Xe ages with an absolute chronometer such as Pb^Pb ages is a little di¤cult [19]. However, judging from a calibration of the I^Xe ages with Pb^Pb ages by the measurement of the Bjurbo«le meteorite [20], formation age of the Zag halite by I^Xe chronometer seems older than the age by the Cs^Ba meth-
465
od. This also supports that our Cs^Ba age for Zag reveals the timing of a metamorphic event in the parent body of Zag. Further chronological work is required to determine the ages of aqueous activity and metamorphic events on the meteorite parent body. 4. Summary Systematic Ba isotopic measurements of the Allende CAIs suggest that the isotopic excess of 135 Ba in the CAIs mainly consists of r-process nucleosynthetic origin and decay products from extinct 135 Cs. From the excess 135 Ba abundances of CAIs, after subtraction of the r-process nucleosynthetic component in a model-dependent manner, a value for 135 Cs/133 Cs = (4.82 þ 0.79)U1034 is experimentally determined as an initial ratio in the early solar system. Weak acid leaching fractions of Beardsley and Zag show very large Cs/Ba elemental ratios and isotopic excesses of 135 Ba. The correlation between Cs/Ba and 135 Ba excess suggests that the isotopic excess of 135 Ba of Beardsley and Zag is the result of 135 Cs decay. The 135 Cs^135 Ba isochron of two meteorites, Beardsley and Zag, and initial abundance of 135 Cs from the Allende CAIs show the possibility of chronological application on early activities in the meteorite parent bodies. To the best of our knowledge, the 53 Mn^53 Cr studies in carbonates from the Orgueil chondrite (CI) [21] and in fayalites from the Mokoia chondrite (CV3) [22] are clear cases which have enabled time constraints to be placed on the occurrence of aqueous alteration in primitive meteorite parent bodies. The 53 Mn^53 Cr chronometry shows that aqueous alteration on small protoplanetary bodies must have begun less than 20 Ma after CAI formation. Our data from 135 Cs^ 135 Ba chronometer have apparently no discrepancy with time constraints from the 53 Mn^53 Cr system, and may provide a more critical record for alteration processes on the early planetary body than the 53 Mn^53 Cr system does. But the Cs^Ba age for Zag is younger than the I^Xe age for the Zag halite [9]. One major problem is an estimation of the initial 135 Cs/133 Cs value in the
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early solar system. Our estimation depends on the model that the Ba isotopic compositions in the Allende CAIs were largely disturbed by a late input of r-process nucleosynthetic component with a constant 135 Ba/137 Ba ratio, because it is impossible to directly ¢nd isotopic evidence of 135 Cs decay from other nucleogenic e¡ects without assumption. Further work is necessary to make clear the primitive activities on meteorite parent body and to calculate a consistent and reliable age from Cs^Ba chronometry. Acknowledgements
[7] [8]
[9]
[10] [11]
We are grateful to Y. Takahashi, Y. Shibata and G. Burton for their technical support with ICP^MS, EPMA and TIMS analyses. Critical reading and comments by J. Bridges, L. Nyquist, and an anonymous journal reviewer were very helpful to improve the ¢rst draft of this paper. The suggestions from the Associate Editor, A.N. Halliday, were useful in revising the manuscript. A part of this study was ¢nancially supported by a Grant-in-Aid for Ministry of Education, Culture, Sports, Science and Technology (to H.H., no. 13440167).[AH]
[12]
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[15] [16]
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