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Isotopic lead investigations on the Allende carbonaceous chondrite
Geochxmza
et Cosmochimica
Acta, 1976. Vol. 40. pp. 635 to 643. Pergamon
Press. PrInted I* Great Britain
Isotopic lead investigations on the Allende carbonaceous chondrite J. H.
CHEN
and G. R. TILTON
Department of Geological Sciences, University of California, Santa Barbara, California 93106, U.S.A. (Received 10 July 1975; accepted in revised form 17 November 1975) Abstract-Uranium and lead concentrations and the isotopic compositions of lead were determined on samples of total rock, matrix, white inclusion, pink inclusion, white aggregate and four chondrules from the Allende carbonaceous chondrite. Observed 206Pb/204Pb ratios varied from 10.004 to 107.29; *0’Pb/Z04Pb ratios from 10.695 to 69.07; 20*Pb/2”4Pb ratios from 30.062 to 207.96. In a ‘a’Pb/ zo4Pb-Z06Pb/204Pb diagram a regression line fitted to all of data has a slope of 0.6240 f 0.0015, corresponding to a single stage model age of 4.565 rt 0.004 AE. The regression line also includes the ratios for primordial lead as determined in previous investigations from Canyon Diablo troilite and the Mezo-Madaras chondrite. Although the lead in the matrix is not very radiogenic, the “‘Pb/ 206Pb ages of four samples average 4.505 AE, a value 0.06 AE younger than that of the chondrules and inclusions. The matrix age agrees closely with a total rock PbjPb model age previously reported for Allende by Tatsumoto, Knight and Allegre. The matrix Pb/Pb model age is also 0.06 AE younger than the PbjPb isochron ages determined by previous investigators on total samples of H and L chondrites. The H and L chondrite and Allende chondrule and inclusion Pb/Pb ages are indistinguishable. The lead isotope systematics require either that the matrix is cn. 0.06 younger than the silicate inclusions and chondrules (or that radiogenic lead was inherited from a younger external source) or that the initial lead in the matrix differed from primordial lead. The lead data cannot be reconciled to a model in which the bulk material of Allende first crystallized 4.57 AE ago, followed by transfer of radiogenic lead between phases since that time. In a concordia diagram four chondrules and three inclusions plot along a chord intersecting concordia at 4.57 and 0.28 k 0.07AE. This indicates disturbance of the U-Pb systems relatively recently, perhaps around 0.3AE ago. The time of disturbance is not readily understood and needs further confirmation. It correlates most closely with a possible cut-off in K-Ar and U, Th-He ages of chondrites. Although the Th/U ratios of the bulk samples and matrix are around the normal value of 3.8, much higher values are observed in some of the inclusions, the highest being 9.0.
INTRODUCTION
of meteorites are very fundamental to Earth and planetary sciences because these objects record events that are older than any that have been established directly on the Earth and Moon. In fact, isotopic data from meteorites form the basis for estimating the age of the Earth, Moon and solar system. Meteorites have generally been dated mainly by the U-Pb, Th-Pb, RbSr and K-Ar methods. Of these, the U-Pb system has two distinct advantages in principle: (1) an age can be calculated from the ratio of the two radiogenic daughter nuclides, “‘Pb and “‘Pb, for a given sample. This is simpler and yields more precise age values than methods involving determination of concentrations of parent and daughter nuclides. These ages must be considered as model ages, however, as the method may not indicate possible disturbances of the system which could affect the age value. (2) Disturbances, when present, can be studied by means of the concordia diagram (WEZTHERILL, 1956). In many instances the correct age and nature of the disturbance can be discerned when suites of samples are plotted in such a diagram. Only recently have techniques evolved to where meteorite STUDIES
data can be analyzed by means of concordia diagrams. PATTERSON(1956) first showed that several stone meteorites yield a 207Pb/206Pb model age of 4.55 AE, or 4.49 AE using the newer decay constants for 23*U and 235U (JAFFEY et al., 1971), the decay constants we will use in this paper. Patterson did not report U-Pb ages for his samples, but the U-Pb ages calculated from his data showed large excesses of radiogenie lead compared to the amount expected from the decay of uranium in the meteorites over 4.5AE. He realized that contamination by terrestrial lead was a possible explanation of the excess lead, especially since the lead concentrations were so low in the meteorites that contamination either by handling or in chemical processing was difficult to avoid or properly assess. More recently, TATSUMOTOet al. (1973a) and TILTON (1973) redetermined U-Pb ages on bulk samples of a number of stone meteorites, taking advantage of greatly improved analytical techniques that were not available to Patterson and other early workers. The new investigations indicate that much, if not all, of the excess radiogenic lead noted in the first studies is indeed due to terrestrial lead contami-
635
636
J. H. CHENand G. R. TILTON
should represent some of the oldest crystalline material from the solar system. GRAY et al. (1973) presented additional evidence for the antiquity of the high temperature materials from Rb-Sr studies. They found that the lowest measured isotopic composition of Sr in Allende (s7Sr/86Sr = 0.69877 2 0.00002) is less radiogenic than that given by PAPANASTASSIOU and WATERBURY (1969) for basaltic achondrites (0.69898 f O.~), the usual index for primordial Sr. They showed that if Allende and the basaltic achondrites were formed by differentiation from parent objects that contained Rb and Sr in average solar proportions, and Allende high temperature silicate material must have separated about 10m.y. earlier than the achondrites. Another important finding by Gray et al,, was the conclusive evidence that the Rb-Sr systems in some phases of Allende have been disturbed since the meteorite was formed. They found that, although the bulk sampte and several chondrules and inclusions define a model age of 4.6AE using BABI for initial strontium isotopic com~sition, other chondrules and Ca-Al-rich inclusions give much lower model ages. The data indicate that the Rb-Sr ratios in some of the phases have been disturbed more recently than THE ALLENDE CHONDRITE 3.6 AE ago, possibly very recently, but the Rb-Sr data can only place a limit on the time of disturbance. The Allende meteorite, classified as type-3 carbonaceous chondrite (WIIK, 1956) or type C3 (VAN SCHMUS WETHERILLet al. (1973) and NYQUISTet al. (1973) independently noted that the initial 87Sr/86Sr ratio and WOOD, 1967), is an unusually heterogeneous meteorite. It consists of a dark gray matrix (- 57%) for Allende was lower than the basaltic achondrite value (BABI). In spite of possible disturbances their in which are embedded many spherical chondrules initial ratio, when corrected for “Sr produced by ( - 3404) and less numerous irregular inclusions (-9%). In the present work we will refer to these decay of *‘Rb over 4.6AE, agreed closely with the spherical and subspherical objects as chondrules, fol- initial ratio of Gray et al., which was based on a lowing the classification of CLARKEet ai. (1970). but measured value. there is a serious question as to whether they are There is evidence that the U-Pb systems have also chondrules in the normal sense. Isotopic heterogeneibeen disturbed. TATSUMOTOet al. (1973a) and TILT~N (1973) noted that the U-Pb data in OS-l.Og samples ties in oxygen (CLAYTONet al., 1973) and magnesium (LEE and PAPANASTASSIOU, 1974. GRAY and COMP- were not consistent with closed system behavior over STON, 1974) indicate that these objects were never in the past 4.5 AE, starting with initial lead having the isotopic equilibrium with each other gr with a silicate same isotopic composition as that in Canyon Diablo melt. triolite (TATSUMOTOet al., 1973a) or the Me& Detailed studies of the mineralogy, petrography, Madaras chondrite (TILTON,1973). When initial lead and chemical composition of the components of the ratios are calculated from the observed U and Pb Allende were made by CLARKE et at. (1970). GREEN concentrations of the Allende samples, one of Tilton’s et al. (1971), and more recently by GROZSMAN{ 1975). samples yields a value close to primordial lead, but A brief description of the components of the Allende the sample of Tatsumoto et al., gives a much more meteorite for which we report data is given in the radiogenic lead. The second Tilton sample almost cerAppendix. tainly gives a more radiogenic lead, although the U Both chondruIes and inclusions, characterized by concentration was not directly determined. Contamihigh Ca and Al contents and refractory phases, are nation by terrestrial lead is unlikely in this case considered to have a composition and mineralogy because Allende was collected shortly after infall and that are comparable to those estimated for the initial should be one of the least contaminated of all meteorhigh temperature condensate from the solar nebula ites. The amount of terrestrial lead contamination as first suggested by LORD(1965), and later developed required to account for the data far exceeds the levels by LARIMER(1967) and by GROSSMAN(1972). More of the blanks for chemical processing of the samples. recently, GROKYMAN et al. (1975) have made detailed Since it is difficult to ascribe this excess lead to terresobservations on a pink inclusion by scanning electron trial contamination, the lack of balance between U microscopy that strongly suggest the minerals con- and Pb indicates either that the meteorite has not densed directly from a vapor phase. As such they been a closed system for these elements over the past
nation. Tatsumoto et al. found some remarkably concoidant ages for three achondrites and two chondrites (Beardsley and Modoc), but substantial excess radiogenie lead was reported in Plainview and Richardton. Plainview may be contaminated with some terrestrial lead since it is a find rather than a fall. Although Richardton is a fall, some fragments may likewise be contaminated because of the lapse of time between infall and recovery of samples, as noted by T~LTON (1973). The possibility of disturbances in the U-Pb system in at least some chondrite falls is indicated by the work of GALE et al. (1972), who reported large excesses of radiogenic lead in Appley Bridge that are difficult to ascribe to terrestrial contamination. The present investigations grew out of evidence in the earlier work of TATSUMOTO et ul. (1973a) and TILTON (1973) for disturbances in the U-Pb system of Allende. Both groups of investigators found that the uranium in bulk samples is not sufficient to account for the amount of radiogenic lead, asuming an age of 4.57 AE and initial lead identical to that in Canyon Diablo troilite.
637
The Allende carbonaceous chondrite
4.%AE, or that the initial lead assumption is erroneous, or both. Other age measurements on Allende include K-Ar determinations by FIREMAN et al. (1970) which gave values of 4.44 and 4.55 AE for a chondrule and the bulk sample. The U + Th/He ages of these samples were 4.2 and 3.4 AE, respectively. The objectives of the present study are: (a) to measure precise ages on the bulk sample and separated com~nents by the U-Pb as well as by the 207Pb/206Pb method; (b) to determine which components are responsible for the lack of balance between uranium and radiogenie lead noted in earlier investigations; (c) to try to determine a time of disturbance of the U-Pb system by evaluating the systematics of discordant ages in a concordia diagram. ANALYTICAL PROCEDURES Sample preparation
Samples from two fragments of the AIIende meteorite were used for analyses. One fragment, calfed Allende T,*
was a i-kg specimen; the other, called Allende W,* was a 100-g specimen. Both specimens contained black fusion crusts. Samples for the analyses of matrix, chondrules and inclusions were prepared as follows: fragments were broken into small pieces (c 1 g) between aluminum foil with a steel hammer on a steel plate. The analyzed samples were taken from the interior of the fragments, well away from any original surfaces. Six bulk analyses were made on these samples. The chunks were crushed carefully in a boron carbide mortar to avoid breaking the inclusions. The matrix in the form of fine dust was collected through a 325-mesh (44pm) screen (Spex nylon screen-h&e sieve set). The chondrules were ~on~ntrated on a IOO-mesh (149 pm) screen. More samples of matrix and chondrules were separated individually using stainless steel driils and chisels. Two analyses were performed on the individually separated matrix (M-3 and M-4) and on the matrix fine dust (M-l and M-2). The isotopic composition of lead in the matrix fragments is less radiogenic than that of the matrix fine dust (Table 3). This result indicates either the presence of fragments of the phases containing more radiogenic lead, i.e. chondrules and inclusions that passed through the 32%mesh screen, or some contamination from sieving. The chondrules obtained by sieving and digging were coated with fine dust of matrix, making identification and separation difTicu1t. The adhering dust was removed by washing the chondrules in distilled acetone. Analysis of the acetone wash for lead con~ntration and Iead isotopic composition gave about 2 ng of terrestrial lead; no evidence of leached meteoritic lead was found. The chondrules were then clearly visible and separates of calciumaluminum-rich chondrules (ChL-1 and ChL-2) and magnesium-rich chondrules (ChD and Ch3) were obtained by hand picking, using a suction device made of teflon. Chunks of white inclusion, pink inclusion and white aggregate essentially free of adhering matrix were extracted by drilling. The spherical white aggregate was intruded by the matrix material, indicating an event later than the formation of the aggregate. The mineral assemblages of the inclusions of Ca-Al-rich minerals (melilite, spine], anorthite, woliastonite and Al-rich pyroxene) were identified by using X-ray diffraction and the electron microprobe. * Purchased from Martin L. Ehrmann. 676 N. La Peer Dr.. Los Angeles. CA 90069. U.S.A.
The analyzed material of all of the bulk and two of the matrix samples (M-3 and M-4) were washed for 3-5 min in cold 2 N HCI. The acid rinse was used to dissolve any lead contamination from the sample handling. It also destroyed small amounts of sulfide from surfaces, but the effect on final concentrations was negligible, as shown by analysis of the acid wash of one bulk sample (W-2). Less than 1% of meteoritic lead was observed (Tables 1 and 2). The sieved matrix samples were washed twice in distilled acetone instead of 2 N HCI, because the surface to volume ratio of the fine material might be more significant. The acetone wash of the matrix sample (M-2) was also analyzed and gave no evidence of leached meteoritic lead. All teflonware was cleaned in hot aqua regia acid, 50% HNO,, and triple distilled water for at least 12 hr in each solution. Polyethyleneware was soaked in cold 50% HNO, for more than 48 hr and rinsed with triple distilled water. Chemical processing
Extensive precautions were taken to avoid contamination during analyses. All sample handling and chemical treatment was carried out in laminar flow hoods. Reagents were prepared by sub-boiling distillation in either teflon or silica glass containers (MA~IN~~N, 1972). The chemical procedures closely followed those given by CHURCH and TILTON(1974) for lunar work. The detailed procedures are given by CHEN(1974). Hanks
Reagent blanks for lead for l-g samples were in the range of 0.2-0.4 ng for the entire chemical processing, values which were small compared to the total blanks, which were 3-5 ng. For 0.1 g samples, lead blanks were O.s-2 ng. The individual measured blanks are shown in Table 2. Uranium and thorium blanks were about onetenth of the lead blanks for early analyses, and were later decreased to hundredths of nanograms when improved column cleaning procedures were instituted. Mass spectrometr~ The instrument and methods used for lead analyses were the same as described in CHURCHand TILTON(1974). The U-Th analyses were made initially by a triple filament method, but we later adopted the single filament method using phosphoric acid and carbon as described by ARDEN and GALE (1974). Superior intensities and stabilities were attained with the latter method. With an electron multiplier it was possible to make isotope analyses on l-ng samples of U, Th and Pb, e.g. in analyses of blanks. For all samples where 10-1OOng of element were present, analyses were made by Faraday cup collection as well as with the multiplier. The performance of the mass s~ctrometer was monitored by repeated measurements of the NBS 981 Pb standard during the course of the work. The observed ratios and standard deviations for 10 analyses were: *04Pb/Z06Pb 0.059065 rt 0.000040 20’Pb/206Pb 0.9 I426 + 0.00027 2osPb/206Pb 2.16306 _+0.00120 These values were sufficiently close to the absolute values (CATANZAROet al., 1968) so that no corrections were applied to the observed ratios from the meteorite samples. The Pb con~ntrations are believed accurate to ~D.>~.IY<,. while uncer~inties in U and Th are about
638
J. H.
and G. R. TILT~N
CHEN
Table I. Element concentrations Sample'
Weight
concentration
(mg) Pb
T,,232,,,238
"238,Pb204
(ppm)
u
Th
(v)
(k)
T-l
570.7
1.045
0.0148
0.642
T-2
1272.0
1.178
0.0143
0.569
T-3
661.0
1.088
0.0121
0.520
T-4
636.2
0.935
0.0120
0.573
W-l
805.8
1.069
0.0129
0,542
w-2
672.2
0.966
0.0119
0.546
M-1
693.8
1.139
0.014l
M-2
546,O
1.215
0.0138
0.0424
0.513
3.17
M-3
344.8
1.079
0.0130
0.0261
0.536
(2.09)
0.555
M-4
396.2
1.006
0.0175
0.0619
0.780
3.65
ChL-I
287.9
0.289
0.0230
0.1543
4.38
6.94
ChL-2
151.8
0.2784
0.0536
0.3164
18.47
6.10
Ch-3
225.5
0.3028
0.0263
0.0921
4.36
3.62
Ch-D
127.5
0.1675
0.0189
0.0944
6.62
5.15
WI
54.6
0.2667
0.0651
0.3636
23.30
5.77
WA
156.6
0.3122
0.0940
0.7093
PI
112.1
0.1834
0.0361
0.3146
leach solution (W-2)
7.80
129.6
9.00
23.23
0.0072
LSample identities: T, W: total samples; M: matrix from T; Ch: chondrules; WI: white inclusions; WA: white aggregate; PI: pink inclusions. See Appendix for more complete descriptions. 2. Concentration in pg of element removed/g of sample; sample leached 3-5 mm in cold 2 N HCI. Table 2. Isotopic composition of lead Observed
Corrected
Ratios' 208
Ratios2
Pbzo6
Pb207
Pb208
p
p
m
Blank, calculated (ng)
Blank, measured (n9)
Pb206 pp
Pb207
T-l
10.645
11.098
30.694
10.583
11.064
30.634
3+2
T-2
10.536
11.008
30.573
10.513
10.995
30.551
322
T-3
10.867
11.224
31.062
10.788
11.180
30.987
3+2
T-4
10.004
10.695
31.092
9.843
10.633
29.904
3;2
W-l
10.101
10.788
30.263
9.989
10.727
30.157
322
w-2
10.226
10.944
30.816
9.859
10.619
29.907
3+2
Atlende-l3
10.71
11.13
30.79
10.64
11.10
30.31
Allende-23
10.26
10.88
30.31
30.23
Sample6
Pb
Pb204
Allende'l
10.12
10.78
11.250
11.451
31.011
M-l
10.215
10.836
30.296
to.139
10.784
30.224
M-2
10.272
10.859
30.320
10.176
10.806
30.227
5
3+2
M-3
10.010
10.711
30.120
9.910
10.656
30.025
5
3+2 3+2 _
10.185
10.825
30.200
10.025
10.737
30.046
5
ChL-1
(IC)
14.696
13.671
35.009
14.654
13.652
34.972
0.6
ChL-1
(IO)
15.122
13.670
-
14.50
13.50
ChL-2
(IC)
26.511
20.782
53.412
29.214
22.623
ChL-2
(ID)
29.718
22.799
-
28.985
22.894
Ch3
(IC)
12.300
12.103
32.213
12.204
12.052
Ch3
(IO)
13.490
12.761
-
12.332
12.244
ChD
(IC)
16.367
14.596
37.005
15.816
14.378
ChD
{ID)
16.949
14.670
-
15.814
14.333
WI
(IC)
28.490
22.148
55.325
29.466
22.814
WI
(ID)
30.211
22.338
-
29.506
22.802
M-4
WA (IC)
101.60
69.67
WA
(ID)
107.29'
69.06
-
PI
(IC)
28.011
21.79
61.513
PI
(ID)
34.014
207.95
24.014
-
Leach
sol'"(W-1)'10.86
11.13
30.706
Leach
sol'n(W-2)511.90
11.73
-
134.06
88.12
134.22
88.07
33.673 33.881
25.642 25.403
1.0 58.673
5 0.8
32.221
0.8 5
36.659
2.4 1.8
57.030
0.6 0.6
274.426
1.8 1.0
75.756
1.0 1.5 1.0
176.9 111.0 76.4 75.4 84.4 141.1 60.4
1.0
67.1 32.9
0.5
21.7 98.7
1.2
4.4 1.2
Wt. (w)
57.9 70.7
1.5
41.4
1. Recalculated from the measured ratios, Pbm/Pb 206. The standard deviations are 0.04% for Pb207/Pb206 and PbZ08/Pb206 and O.l’A for Pbzo4/PbZo6 ratios, hence are cu. 0.1% for the ratios reported. 2. Isotopic composition of blank is PbZ”4:Pb2”“:Pb *07:pb20s = 1.00:18.90: 15.6:38.6--(ID) samples corrected for spikes and blanks. 3. From TILTON(1973). 4. From TATSUMOTO ef at. (1973a. b). 5. Washed in cold 2 N HCl for 3-5 min; W-2 was spiked with 1ng PbZo8; W-l unspiked. 6. Sample identities are the same as in Table 1. 7. Ratio from electron multiplier data only. All other ratios in table are Faraday cup data.
The Allende car~naceous chondritc
639
a white aggregate separate, a ratio close to that found in some achondrites (TATSUMOTO et al., 1973a). Simithe accuracy of the Pb determinations, great care was lar results were reported by TATSUMOTO et al. (1973b), taken in doing the blank correction. For the chondrules who found that the Pb in the matrix is less radiogenic and inclusions a reiterative calculation method was used than the whole meteorite, that Pb of Mg-rich chonwhereby the blanks in the isotopic composition and concentration determinations for lead were allowed to vary drules is moderately radiogenic (zo6Pb/Z04Pb ratios independently in 0.1 ng increments, Data were finally ranged from 18 to 26), and that Pb in Ca-Al-rich selected that gave close agreement between 206Pb/204Pb chondrules and aggregates is extremely radiogenic and 207Pb/Z04Pb ratios for the two samples. The method (zo6Pb/204Pb ratios reach 55.9). was not used for the matrix and bulk samples as the blank A plot of 207Pb/“04Pb ratios vs 206Pb/204Pb ratios corrections for these samples were less critical. is given in Fig. 1. A linear relationship exists among the bulk, matrix, chondrule and inclusion samples. RESULTS AND DISCUSSION The sIope of the regression line through all of the The concentrations of U and Th and Pb deter- data is 0.6240 sfr0.0015, which yields an isochron age mined by isotope dilution are given in Table 1, along of 4.565 + 0.004 AE. This line also passes through the with the values previously determined by TILTON primordial lead point defined by Canyon Diablo troilite (TATSUMOTOet al., 1973a) and MezG-Madaras (1973) and TATSUMOTO et al. (1973a). The Pb concentrations in the bulk samples range from 0.934 to chondrite (TILTON,1973). A calculated line including 1.178 ppm, about 3-6x lower than those of the the primordial lead data and our data is indistmatrix, 1.0061.212 ppm. The Pb concentrations of inguishable from the one shown here. If the matrix chondrules and inclusions range from 0.167 to and bulk sample data are eliminated, the slope of 0.312ppm, only about one-quarter of that in the the line through the remaining chondrule and inclusion data is again 0.6240. The leads clearly belong matrix. The U concentrations in the bulk sample and the matrix are similar, ranging from 0.012 to to an array that includes primordial lead as deterO.O148ppm, with the exception of M-4, which is mined by Canyon Diablo troilite and Mez~-Madaras. The meteorite lead data of TILTON(1973) and TATSU0.0175 ppm. The U concentrations in the chondrules MOTO efal. (1973a) also closely fit this isochron. are about 2-3 times higher than in the matrix, TATSUMOTO et al. (1975) also determined an internal whereas the U concentrations of the inclusions are 207Pb/20”Pb isochron for Allende. The two labora3-7 times higher than in the matrix. Chondrules and inclusions have Th concentrations about 2-10 times tories’ values are similar, but do not agree within stated error limits. TATSUMOTOet al. (1975) give higher than that of the matrix. The concentrations determined by TILTON (1973) and TATSUMOTOet al. 0.6188 + 0.0016 for the slope of the isochron; we find (1973a) for the bulk sample fall within the range of 0.6240 + 0.0015. The corresponding model Pb/Pb these data. The concentrations are also consistent ages are 4.553 and 4.565 AE. The cause of the differwith those of TAWJMOTOet al. (1973b), who reported ence is unknown, but can hardly involve mass specTh and U con~ntrations in the Ca--Al-pyroxene-rich trometer discrimination. For example, we obtain a chondrules and white and pinkish-white aggregate separates that were 5-10 times higher than that of the matrix. Their Pb concentrations, 87-485 ppb, in 6@tthe chondrules are less than half of that in the whole 61: rock and matrix, 1.5 ppm, and about 35% higher than 40 our matrix concentrations. The ratio 2’8U/204Pb (p) of the bulk and matrix samples range from 0.52 to 0.57 with the exception of samples T-l and M-4. The lead isotopic compositions for various fractions of the meteorite are shown in Table 2. The corrected ratios are obtained by subtracting the calcutated blanks. The Pb isotopic ratios for the whole meteorite are consistently lower than those obtained by TATSUMOTOet at. (1973a. b), but are similar to those reported by TILTON(1973). This is undoubtedly Fig. 1. 207Pb/204Pb-206Pb/ *O’Pb diagram for meteorite due to the fact that the Allende meteorite is extremely heterogeneous chemically and mineralogically. The samples, Slope of regression line = 0.6240 + 0.0015, corresponding to a model age of 4.565 & 0.004AE. Unlabeled isotopic composition of lead in the matrix is slightly solid circles = Allende total rock samples; solid triless radiogenic than that of the whole meteorite; the angles = Allende matrix. Coded Allende samples are desiglead in the specially separated matrix samples, M-3 nated by the same symbols used in the tables. Other and M-4, being less radiogenic than that in samples points: CD, B, PV, My, R, MO, A = Canyon Diablo troiM-l and M-2, which were prepared by sieving. The lite, Beardsley, Plainview, Murray, Richardton, Modoc and Allende total sample from TATSUMOTO et al. (1973a); MM, lead in the other phases is extremely radiogenic. The P. and A-l, A-2 = Me.&Madaras, Puftusk. and Allende ‘ ratio zohPh!“‘4Pb reaches values as high as 134 in total sample from TILTON(1973). *OS%. The blanks are approximately 0.3-7.2% of the total
Pb in the samples. Since the level of the blanks controlled
I
640
J. H. CHEN and G. R. TILTON
model Pb/Pb age of 4.560AE for sample WA, which has so little ‘04Pb that the age depends mainly on the zo1Pb/Z06Pb ratio. The discrimination correction on the 207Pb/206Pb ratio indicated by our NBS 981 analyses is 0.04%, while the difference in slope between the isochrons of Tatsumoto et al. and us is 0.8%. Moreover the two laboratories agree closely on the model PbjPb age of the matrix, where discrimination effects would be magnified since the corrections for primordial lead are large. Our average Pb/Pb model age for matrix samples is 4.505 AE (below) while Tatsumoto et al. find 4.496AE. A plot of 208Pb/204Pb ratios against 206Pb/204Pb ratios (not shown) yields a line for closed systems that correspond to 4.565AE old with a present day 232Th/238U ratio (k) of 3.8 (atomic ratio). This line adequately fits all the Pb data except that of the chondrules and the inclusions which have high Th/U ratios in Table 1, and which have higher 2osPb/204Pb ratios for given 206Pb/204Pb ratios. The cause of the high Th/U ratios is unknown. The variation in Th/U ratios in various phases of Allende probably signifies variation in Pu/U ratios as well. POWSEK and LEWIS (1972) have already noted the unusually high Pu/U ratios in white inclusions from Allende inferred from fissionogenic Xe data. Isotopic composition of initial lead Table 3 shows the isotopic composition of lead for the matrix and bulk samples corrected for radiogenic lead produced by U over the past 4.565 AE, assuming closed system behavior. The ratios vary from sample to sample, indicating that most or all of them have not been closed systems. We conclude that it is not possible to determine the isotopic composition of initial lead from direct measurements on the bulk meteorite or separated phases. In the discussion of U Ph ages that follows, we will assume that initial
lead ratios were identical to those in Canyon Diablo troilite and MezG-Madaras. As noted above, the regression line through the ratios in Fig. 1 is consistent with this hypothesis. Model ages for chondrules and inclusions are given in Table 4. All ages are calculated with the newer decay constants and the primordial lead values of TATSUMOTOet al. (1973a). The pink inclusion (PI) gives ages that are nearly concordant within estimated analytical errors; the remaining samples do not. It is significant that the most accurately determined data-those for the white aggregate (WAt yield distinctly discordant ages. The ages are best known for WA because the lead is the most radiogenie of that in all of the samples and because it contains relatively high concentrations of U and Pb compared to the remaining samples. We have not given model ages for the bulk and matrix samples because their lead is not sufficiently radiogenic to yield accurate UpPb ages. The 207Pb/206Pb ages for the four matrix samples are 4.481,4.482,4.509 and 4.549 AE. These ages are systematically younger than the 207Pb/206Pb ages for the chondrules and inclusions. The average value is 4.505AE, while the chondrule and inclusion Pb/Pb ages average 4.560AE. TATSUMOTOet al. (1975) also find a younger Pb/Pb model age for matrix samples (4.499 AE) than for inclusions and aggregates (4.548 AE). Both Tatsumoto et al. and we find a difference of ca. 0.05 AE between the Pb/Pb model ages of matrix and inclusions; however, our absolute age values differ systematically for unknown reasons. HUEY and KOHMAN (1973) have also determined the isotopic composition of lead in Allende, using a volatization process to separate the lead. The 207Pb/20hPb age calculated from their total sample data is 4.528 AE. This is again a lower age than the chondrule and inclusion ages we have measured for
Table 3. Calculated initial Pb isotopic compositions’ pb206,pb204
Sample
T-l
9.922
10.651
T-2
9.927
10.629
T-3
10.252
10.846
T-4
9.253
10.265
W-l
9.431
10.379
w-2
9.297
10.268
9.567
10.437
M-2
9.647
10.476
29.815
M-3
9.350
10.311
29.741
9.221
10.236
29.325
9.307
10.294
29.476
9.310
10.296
29.444
Olablo
Me%Madams3
pb208,pb204
M-l
M-4 Canyon
pb207,pb204
troilite'
l.Age = 4.565 x 109yr; Iz Uz3’ = 0.155125 x 10e9 yr-I, 10m9yr-‘, U238/U23’-137.88. 2. From TATSUMOKJet al. (1973a). 3. From TILT~N (1973). 4. Incorrectly given as 29.57 in TILTDN(1973).
I Uz3’ = 0.98485 x 10-9yr-‘,
j. Thz3’ = 0.049475 x
641
The Ailende carbonaceous chondrite Table 4. Model ages Atomic
..“,,,$,‘.
Model
Ratios
Age,
Million
Years'
_
I
ChL-1
1.207
104.42
0.6273
0.179
5104
4730
4573
3328
ChL-2
1.069
93.14
0.6317
0.257
4688
4615
4583
4622
Ch 3
5.672
56.49
0.6102
0.176
3312
4114
4533
3271
ChR
0.982
84.68
0.6255
0.211
4410
4619
4568
3862
WI
0.866
74.05
0.6199
0.205
4023
4385
4555
3773
WA
0.963
62.72
0.6227
0.243
4349
4496
4562
4391
PI
1.055
90.23
0.6202
0.223
4644
4583
4556
4063
1. See footnote of Table 3 for decay constants.
Allende, although the average Pb/Pb age for all of Huey and Kohman’s samples, including H and L chondrites is 4505 m.y. It is therefore uncertain to what extent their results agree with ours in indicting a younger age for the matrix. It is in any case likely that the Pb/Pb model age of the matrix is cu. 0.05 AE lower than that of the high temperature silicates. This indicates either that the matrix is younger than the silicates, or that it formed with a different initial lead, or both. The data for chondrules and inclusions have been plotted on a concordia diagram in Fig. 2. We have not plotted matrix and bulk samples because they contain relatively small proportions of radiogenic lead. All of the points except PI are clearly discordant. It seems signi~~ant that both samples which plot above the concordia curve are Ca-Al-rich chondrules. A linear regression line through the data is shown as a solid line in the figure. The chord intersects the concordia curve at an upper age of 4.57 & O.O2AE, a value consistent with the z”7Pb/206Pb age of 4.565 AE derived earlier. This indicates that the true age of the silicates is around 4.57AE. The age is significant since the sample suite includes high-temperature silicate material (see Appendix) that may, at least in part, have condensed directly from a vapor phase (GROSSMAN, 1975). As such it would likely represent the oldest solid matter to be found in meteorites. The age is not markedly older than the rather weIl-establish chemical differentiation age of chondrites in general. The uncertainties in the U-Pb ages for the Allende silicates and chondrite differentiation event easily allow for the 10 m.y. interval suggested by the initial Sr data of GRAY et al. (1973). Turning to the lower intersection on concordia, a linear regression line through all of the data gives an intersection at 0.28 + 0.07 AE (uncertainty calculated by distributing errors equally along ordinate and abscissa). A dashed line passing through the origin, corresponding to disturbance of the U-Pb system at 0 AE, is also given in Fig. 2. The data points differ significantIy from the dashed line. TA~SVMOTOet af.
(1975) report 0.107 + 0.070 AE for the lower intersection of their data on the concordia curve. The estimated errors nearly allow for overlap in the two intercept ages, but it seems probable that a small difference in results between the two laboratories remains to be resolved. Neither laboratory finds a 0 AE intercept, however. If the data do in fact lie along a chord intersecting concordia at 4.57 and 0.28 AE, they are consistent with disturbance 0.28 AE ago in systems that are now 4.57 AE old. This is an unusual result for meteorites. but it is comparable to the ‘cut-off’ in K-Ar and the U + Th/He ages of chondrites at ca. 0.4 AE first cited by ANDERS(1964). Such a limiting value is still plausible on the basis of a much larger amount of data cited by Z;~HRINGER(1968). Finally, one of the first 39Arj40Ar age measurements on chondrites gave an age of 0.5 AE for the most loosely held fraction in
---7 ChL.1
::i_ I / IO
I
09!-
0311
v
O2 ,(J 01
/$’
,/’
1 i
,’
Fig. 2. Concordia diagram showing U-Pb relationships for chondrules and inclusions from Ahende chondrite. Solid line is a regression line through all of the data, which intersects the curve at 4.57 and 0.28AE. The dashed line is drawn between 4.57 and OAE. Sample identi~cations are the same ones used in the tables.
642
J. H. CHEN and G. R. TILT~N
the Bruderheim chondrite (MERRIHUE and TURNER, 1966). It is not apparent how these events-shock or moderate heating-might have affected the U-Pb systems within individual phases of a chondrite such as Allende, but the gas retention data are in any case the only suggestion of significant events in chondrite histories around 0.3 AE age. GOPALAN and WETHERILL(1971) were able to demonstrate mobility of radiogenie Sr in highly shocked H chondrites, but these meteorites also have low K-Ar ages, while Allende does not. It is therefore not clear that the H chondrite observations have any bearing on the Allende case. The lead data do show that the disturbance of the U-Pb system occurred sometime between approximately 0.1 and 0.3 AE ago. This is undoubtedly the time of disturbance of the RbSr system as well, which confirms the supposition of GRAY et al. (1973) that it possibly took place in relatively recent time. In a strict sense the RbSr data show only that the disturbance occurred since 3.6 AE ago, as the authors stated. Finally we consider the bearing of these data on the apparent lack of U-Pb balance in bulk samples first evident in the work of TATSUMOTO et al. (1973a) and T~LTON(1973). The discordances found in all of the chondrules and in inclusions except ChL-1 and ChL-2 are in the opposite direction from that required to explain the bulk sample data-i.e. the bulk sample contains an excess rather than a deficiency of radiogenic lead. The effect is almost certainly related to the matrix. This seems to be confirmed by the matrix data in Table 3, showing that three out of four matrix samples give calculated initial lead ratios (relative to ‘04Pb) that are higher than the ratios for primordial lead. It should also be noted that for material balance of radiogenic lead in Allende our data require either that the matrix is ca. 0.05 AE younger than the high temperature silicates (or that younger radiogenic lead was inherited from an external source) or that the initial lead was not identical in isotopic composition in the two kinds of material. This is so because the Pb/Pb model age of the matrix is less than 457AE (the age of the high temperature silicates) even though the matrix appears to contain excess radiogenic lead, as stated above. These data cannot be reconciled to a model in which the bulk material of Allende first crystallized 4.57 AE ago with primordial lead identical to that observed in the Canyon Diablo and Me& Madaras meteorites. If this were true, transfer of radiogenic lead from any phases into the matrix would cause the Pb/Pb model age of the matrix to be equal to, or greater than 4.57 AE, depending on the time at which the transfer occurred. The same considerations probably apply to the data of TATSUMOTO et al. (1975) because their matrix appears to have a model Pb/Pb age that is younger than the model Pb/Pb age of the aggregates and inclusions. Their matrix samples, like ours, contain excess radiogenie lead.
Acknowledgetnents-These investigations were supported by National Science Foundation grant GA-40266 to G. R. TILTON.We are indebted to Mr. MARK STEINfor invaluable assistance with instrumental aspects of the work. We also thank Mr. DAVID CROUCH and Mrs. EVELYNGORDON and MERYLWIEDERfor assistance with preparation of the manuscript. C. J. ALLEGREand M. TATSUMOTO reviewed the manuscript and suggested many improvements. This paper is based in part on work for which J. H. CHENreceived half the Nininger Meteorite Award for 1974 from Arizona State University.
REFERENCES ANDERSE. (1964) Origin, age and composition of meteorites, Space Sci. Rev. 3, 5833714. ARDENJ. W. and GALE N. H. (1974) Separation of trace amounts of uranium and thorium and their determination by mass spectrometric isotope dilution. Anal. Chem. 46. 687-69 1. CATANZAROE. J., MURPHY T. J., SHIELDSW. R. and GARNERE. L. (1968) Absolute isotopic ratios of common, equal-atom and radiogenic lead isotopic standards. J. Res. Nati Bur. Stand. 12A, 261-267. CHEN J.
H. (1974) UTh-Pb investigations of the Allende carbonaceous chondrite. M.A. thesis, Univ. Calif. Santa Barbara, 49 pp. CHURCHS. E. and TIL~~N G. R. (1974) Lead isotope systematics of some Apollo 17 soils and some separated components from 75601. Proc. 5th Lunar Sci. Conf, Geochim. Cosmochim. Acta Suppl. 5, pp. 1389-1400. Pergamon Press. CLARKER. S.. JR., JAROSEWICH E., MACONB., NELENJ., G~MEZ M. and HYDE J. R. (1970) The Allende, Mexico meteorite shower. Smithsonian Contrib. Earth Sci. 5, 65 PP. CLAYTONR. E., GROSSMANL. and MAYEDAT. K. (1973) A component of primitive nuclear composition in carbonaceous chondrites. Science 182, 485-487. FIREMANE. L., DEFELICEJ. and NORTONE. (1970) Ages of the Allende meteorite. Geochim. Cosmochim. Acta 34, 873-881. GALE N.
H., ARDEN J. and HUTCHINSONR. (1972) Uranium-lead chronology of chondritic meteorites.
Nature 240. 5&58.
GOPALANK. and WETHERILLG. W. (1971) Rubidiumstrontium studies on black hypersthene chondrites: effects of shock and reheating. J. Geophys. Res. 76, 84848492. GRAY C. M., PAPANASTASSIOU D. A. and WASSERBURG G. J. (1973) The identification of early condensates from the solar nebula. Icarus 20. 2 13-239. GRAY C. M. and COMPST~N W. (1974) Excess 26Mg in the Allende meteorite. Nature 251, 495497. GREEN H. W., II, RADCLIFFE S. V. and HEVERA. H. (1971) Allende meteorite: a high-voltage electron petrographic study. Science 172, 936939. GROSSMANL. (1972) Condensation in the primitive solar nebula. Geochim. Cosmochim. Acta 36, 597-619. GROSSMANL. (1975) Petrography and mineral chemistry of Ca-rich inclusions in the Allende meteorite. Geochim. Cosmochim. Acta 39. 433-454.
GROSSMANL., FRULANDR. M. and MCKAY D. S. (1975) Scanning electron microscopy of a pink inclusion from the Allende meteorite. Geophys. Res. Left. 2, 3740. HUEY J. M. and KOHMANT. P. (1973) 207Pb-*0bPb isochron and age of chondrites. J. Geophys. Rex 78, 322773244. JAFFEYA. H., FLYNN K. F., GLENDENINL. E., BENTLEY W. C. and ESSLINGA. M. (1971) Precision measurement of the half-lives and specific activities of U235 and U238. P/II.\. Rr,r C4.
I X89
1906.
643
The Allende carbonaceous chondrite LARIMERJ. W. (19671 Chemical frnctionation in meteorites-1. Condensation of the elements. Geochim. Cosmochim. Actu 31, 1215-1238. LEE T. and PAPANASTASSIOU D. A. (1974) Mg isotopic
anomalies 0 and Sr LURD H. C.. tion in a
in the Allende meteorite and correlation with effects. Geophys. Res. Lett. 1, 225-229. III. (1965) Molecular equilibria and condensasolar nebula and cool stellar atmospheres. Icarus 4, 279-288. MATTINSONJ. M. (1972) Preparation of hydrofluoric, hydrochloric and nitric acids at ultralow lead levels Anal. Chem. 44, 1715-1716. MERRIHUEC. and TURNERG. (1966) Potassium-argon dating with fast neutrons. J. Geophys. Res. 71, 2852-2857. NYQWST L. E., HUBBARDN. J., GAST P. W., BANSALB. M.. WEISMANNH. and JAHNB.-M. (1973) RbSr systematics for chemically defined Apollo 15 and 16 materials. Proc. 4th Lunar Sci. Con& Geochim. Cosmachim. Acta Suppl. 4, pp. 1823-1846. Pergamon Press. PAPANASTASSIOU D. A. and WASSERBURG G. J. (1969) Initial strontium abundances and the resolution of small time differences in the formation of planetary objects. Earfh Planet. Sci. Lett. 5, 361-376.
PATTERSON C. C. (1956) Age of meteorites and the earth. Geochim. Cosmochim. Acta 10, 230-237.
PODOSEKF. A. and LEWIS R. S. (1972) iI91 and 244Pu abundances in white inclusions of the Allende meteorite. Earth Planet. Sci. Left. 15, 101-109. TATSUMOTO M., KNIGHTR. J. and ALLEGREC. J. (1973a) Time differences in the formation of meteorites as determined from the ratio of lead-207 to lead-206. Science 180, 1278-1283. TATSUMOTO M., UNRUH D. M. and DESBOROUGH G. A. (1973b) The U-Th-Pb systematics of the Allende carbonaceous chondrite. Meteoritics 8, 446 (Abstract). TATSUMOTQ M., UNRUH D. M. and DESB~ROUGHG. A. (1975) U-Th-Pb and RbSr systematics of Allende and U-Th-Pb systematics of Orgueil. Geochim. Cosmochim. Acta in press. TILTON G. R. (1973) Isotopic lead ages of chondritic meteorites. Earth Planet. Sci. Lett. 19. 321-329. VAN SCHMUSW. R., and Wool J. A. (1967) A chemical-petrologic classification for the chondritic meteorites Geochim. Cosmach~m. Acta 31, 747.--765.
WETHERILLG. W. (1956) Discordant uranium-lead Trtr~ts. Amer. Geophys. Union 37. 32%3X.
ages.
WETHERILL G. W., MARK R. and LEE-HU C. (1973) Chondrites: initial strontium-87/strontium-86 ratios and the early history of the solar system. Science 182, 281-283. WIIK H. B. (1956) The chemical composition of some stony meteorites. Geochim. Cosmochim. Acta 9, 279-289. Z~HRINGERJ. (1968) Rare gases in stony meteorites. Geochim. Cosmac~~im.Acta 32, 209238.
APPENDIX Sample description
Mineral constituents were studied by X-ray powder diffraction and electron microprobe. The following identifications were made: Pink inclusion (PI). A fine-grained, irregularly-shaped pink aggregate. Its dimensions are ca. 5 x 6 x 8 mm. Minerals identified are spinel, melilite, pyroxene, wollastonite, plagioclase and an Al-rich phase (corundum? Microprobe analysis = 92.1% AlzO,). li%ite inclusion ( W). A fine-grained, elongated inclusion which is ca. 3 x 4 x 5 mm in size, containing meiilite, plagioclase, and wollastonite. White aggregate (WA). Cu. I x 1 x 1.5cm in size. The inclusion is coarse-grained, subspherical-shaped, containing plagioclase, pyroxene, spine1 and melilite. The subspherical body was intruded at one end by the matrix material. Ca-rich c~ondrules (ChL-I and 2). Spherical material, ea. 0.7cm in diameter, referred to as Ca-rich chondrules by CLARKE et al. (1970) and GRAY et al. (1973). Coarsegrained, white to grey in color, has mineral constituents similar to those of the white aggregate. Mg-rich chondrules (ChD and Ch3). Containing coarsegrained, barred or granular forsteritic olivine, some clinoenstatite, interstitial glass and sulfides. Range in size from 0.2 to 0.5cm. Ch3 is a mixture of chondrules while ChD is a single chondrule. In the inclusions and chondrules we list only those minerals which were identified; others reported in the literature, and not cited here, may also be present, Matrix (M-l, 2. 3, 4). Consists mainly of euhedral ironrich olivine crystals up to 5 nm long and 1 pm in diameter. Carbon is located primarily at grain boundaries, and occurs as highly disordered graphite.
et Cosmochimica
Acta, 1976. Vol. 40. pp. 635 to 643. Pergamon
Press. PrInted I* Great Britain
Isotopic lead investigations on the Allende carbonaceous chondrite J. H.
CHEN
and G. R. TILTON
Department of Geological Sciences, University of California, Santa Barbara, California 93106, U.S.A. (Received 10 July 1975; accepted in revised form 17 November 1975) Abstract-Uranium and lead concentrations and the isotopic compositions of lead were determined on samples of total rock, matrix, white inclusion, pink inclusion, white aggregate and four chondrules from the Allende carbonaceous chondrite. Observed 206Pb/204Pb ratios varied from 10.004 to 107.29; *0’Pb/Z04Pb ratios from 10.695 to 69.07; 20*Pb/2”4Pb ratios from 30.062 to 207.96. In a ‘a’Pb/ zo4Pb-Z06Pb/204Pb diagram a regression line fitted to all of data has a slope of 0.6240 f 0.0015, corresponding to a single stage model age of 4.565 rt 0.004 AE. The regression line also includes the ratios for primordial lead as determined in previous investigations from Canyon Diablo troilite and the Mezo-Madaras chondrite. Although the lead in the matrix is not very radiogenic, the “‘Pb/ 206Pb ages of four samples average 4.505 AE, a value 0.06 AE younger than that of the chondrules and inclusions. The matrix age agrees closely with a total rock PbjPb model age previously reported for Allende by Tatsumoto, Knight and Allegre. The matrix Pb/Pb model age is also 0.06 AE younger than the PbjPb isochron ages determined by previous investigators on total samples of H and L chondrites. The H and L chondrite and Allende chondrule and inclusion Pb/Pb ages are indistinguishable. The lead isotope systematics require either that the matrix is cn. 0.06 younger than the silicate inclusions and chondrules (or that radiogenic lead was inherited from a younger external source) or that the initial lead in the matrix differed from primordial lead. The lead data cannot be reconciled to a model in which the bulk material of Allende first crystallized 4.57 AE ago, followed by transfer of radiogenic lead between phases since that time. In a concordia diagram four chondrules and three inclusions plot along a chord intersecting concordia at 4.57 and 0.28 k 0.07AE. This indicates disturbance of the U-Pb systems relatively recently, perhaps around 0.3AE ago. The time of disturbance is not readily understood and needs further confirmation. It correlates most closely with a possible cut-off in K-Ar and U, Th-He ages of chondrites. Although the Th/U ratios of the bulk samples and matrix are around the normal value of 3.8, much higher values are observed in some of the inclusions, the highest being 9.0.
INTRODUCTION
of meteorites are very fundamental to Earth and planetary sciences because these objects record events that are older than any that have been established directly on the Earth and Moon. In fact, isotopic data from meteorites form the basis for estimating the age of the Earth, Moon and solar system. Meteorites have generally been dated mainly by the U-Pb, Th-Pb, RbSr and K-Ar methods. Of these, the U-Pb system has two distinct advantages in principle: (1) an age can be calculated from the ratio of the two radiogenic daughter nuclides, “‘Pb and “‘Pb, for a given sample. This is simpler and yields more precise age values than methods involving determination of concentrations of parent and daughter nuclides. These ages must be considered as model ages, however, as the method may not indicate possible disturbances of the system which could affect the age value. (2) Disturbances, when present, can be studied by means of the concordia diagram (WEZTHERILL, 1956). In many instances the correct age and nature of the disturbance can be discerned when suites of samples are plotted in such a diagram. Only recently have techniques evolved to where meteorite STUDIES
data can be analyzed by means of concordia diagrams. PATTERSON(1956) first showed that several stone meteorites yield a 207Pb/206Pb model age of 4.55 AE, or 4.49 AE using the newer decay constants for 23*U and 235U (JAFFEY et al., 1971), the decay constants we will use in this paper. Patterson did not report U-Pb ages for his samples, but the U-Pb ages calculated from his data showed large excesses of radiogenie lead compared to the amount expected from the decay of uranium in the meteorites over 4.5AE. He realized that contamination by terrestrial lead was a possible explanation of the excess lead, especially since the lead concentrations were so low in the meteorites that contamination either by handling or in chemical processing was difficult to avoid or properly assess. More recently, TATSUMOTOet al. (1973a) and TILTON (1973) redetermined U-Pb ages on bulk samples of a number of stone meteorites, taking advantage of greatly improved analytical techniques that were not available to Patterson and other early workers. The new investigations indicate that much, if not all, of the excess radiogenic lead noted in the first studies is indeed due to terrestrial lead contami-
635
636
J. H. CHENand G. R. TILTON
should represent some of the oldest crystalline material from the solar system. GRAY et al. (1973) presented additional evidence for the antiquity of the high temperature materials from Rb-Sr studies. They found that the lowest measured isotopic composition of Sr in Allende (s7Sr/86Sr = 0.69877 2 0.00002) is less radiogenic than that given by PAPANASTASSIOU and WATERBURY (1969) for basaltic achondrites (0.69898 f O.~), the usual index for primordial Sr. They showed that if Allende and the basaltic achondrites were formed by differentiation from parent objects that contained Rb and Sr in average solar proportions, and Allende high temperature silicate material must have separated about 10m.y. earlier than the achondrites. Another important finding by Gray et al,, was the conclusive evidence that the Rb-Sr systems in some phases of Allende have been disturbed since the meteorite was formed. They found that, although the bulk sampte and several chondrules and inclusions define a model age of 4.6AE using BABI for initial strontium isotopic com~sition, other chondrules and Ca-Al-rich inclusions give much lower model ages. The data indicate that the Rb-Sr ratios in some of the phases have been disturbed more recently than THE ALLENDE CHONDRITE 3.6 AE ago, possibly very recently, but the Rb-Sr data can only place a limit on the time of disturbance. The Allende meteorite, classified as type-3 carbonaceous chondrite (WIIK, 1956) or type C3 (VAN SCHMUS WETHERILLet al. (1973) and NYQUISTet al. (1973) independently noted that the initial 87Sr/86Sr ratio and WOOD, 1967), is an unusually heterogeneous meteorite. It consists of a dark gray matrix (- 57%) for Allende was lower than the basaltic achondrite value (BABI). In spite of possible disturbances their in which are embedded many spherical chondrules initial ratio, when corrected for “Sr produced by ( - 3404) and less numerous irregular inclusions (-9%). In the present work we will refer to these decay of *‘Rb over 4.6AE, agreed closely with the spherical and subspherical objects as chondrules, fol- initial ratio of Gray et al., which was based on a lowing the classification of CLARKEet ai. (1970). but measured value. there is a serious question as to whether they are There is evidence that the U-Pb systems have also chondrules in the normal sense. Isotopic heterogeneibeen disturbed. TATSUMOTOet al. (1973a) and TILT~N (1973) noted that the U-Pb data in OS-l.Og samples ties in oxygen (CLAYTONet al., 1973) and magnesium (LEE and PAPANASTASSIOU, 1974. GRAY and COMP- were not consistent with closed system behavior over STON, 1974) indicate that these objects were never in the past 4.5 AE, starting with initial lead having the isotopic equilibrium with each other gr with a silicate same isotopic composition as that in Canyon Diablo melt. triolite (TATSUMOTOet al., 1973a) or the Me& Detailed studies of the mineralogy, petrography, Madaras chondrite (TILTON,1973). When initial lead and chemical composition of the components of the ratios are calculated from the observed U and Pb Allende were made by CLARKE et at. (1970). GREEN concentrations of the Allende samples, one of Tilton’s et al. (1971), and more recently by GROZSMAN{ 1975). samples yields a value close to primordial lead, but A brief description of the components of the Allende the sample of Tatsumoto et al., gives a much more meteorite for which we report data is given in the radiogenic lead. The second Tilton sample almost cerAppendix. tainly gives a more radiogenic lead, although the U Both chondruIes and inclusions, characterized by concentration was not directly determined. Contamihigh Ca and Al contents and refractory phases, are nation by terrestrial lead is unlikely in this case considered to have a composition and mineralogy because Allende was collected shortly after infall and that are comparable to those estimated for the initial should be one of the least contaminated of all meteorhigh temperature condensate from the solar nebula ites. The amount of terrestrial lead contamination as first suggested by LORD(1965), and later developed required to account for the data far exceeds the levels by LARIMER(1967) and by GROSSMAN(1972). More of the blanks for chemical processing of the samples. recently, GROKYMAN et al. (1975) have made detailed Since it is difficult to ascribe this excess lead to terresobservations on a pink inclusion by scanning electron trial contamination, the lack of balance between U microscopy that strongly suggest the minerals con- and Pb indicates either that the meteorite has not densed directly from a vapor phase. As such they been a closed system for these elements over the past
nation. Tatsumoto et al. found some remarkably concoidant ages for three achondrites and two chondrites (Beardsley and Modoc), but substantial excess radiogenie lead was reported in Plainview and Richardton. Plainview may be contaminated with some terrestrial lead since it is a find rather than a fall. Although Richardton is a fall, some fragments may likewise be contaminated because of the lapse of time between infall and recovery of samples, as noted by T~LTON (1973). The possibility of disturbances in the U-Pb system in at least some chondrite falls is indicated by the work of GALE et al. (1972), who reported large excesses of radiogenic lead in Appley Bridge that are difficult to ascribe to terrestrial contamination. The present investigations grew out of evidence in the earlier work of TATSUMOTO et ul. (1973a) and TILTON (1973) for disturbances in the U-Pb system of Allende. Both groups of investigators found that the uranium in bulk samples is not sufficient to account for the amount of radiogenic lead, asuming an age of 4.57 AE and initial lead identical to that in Canyon Diablo troilite.
637
The Allende carbonaceous chondrite
4.%AE, or that the initial lead assumption is erroneous, or both. Other age measurements on Allende include K-Ar determinations by FIREMAN et al. (1970) which gave values of 4.44 and 4.55 AE for a chondrule and the bulk sample. The U + Th/He ages of these samples were 4.2 and 3.4 AE, respectively. The objectives of the present study are: (a) to measure precise ages on the bulk sample and separated com~nents by the U-Pb as well as by the 207Pb/206Pb method; (b) to determine which components are responsible for the lack of balance between uranium and radiogenie lead noted in earlier investigations; (c) to try to determine a time of disturbance of the U-Pb system by evaluating the systematics of discordant ages in a concordia diagram. ANALYTICAL PROCEDURES Sample preparation
Samples from two fragments of the AIIende meteorite were used for analyses. One fragment, calfed Allende T,*
was a i-kg specimen; the other, called Allende W,* was a 100-g specimen. Both specimens contained black fusion crusts. Samples for the analyses of matrix, chondrules and inclusions were prepared as follows: fragments were broken into small pieces (c 1 g) between aluminum foil with a steel hammer on a steel plate. The analyzed samples were taken from the interior of the fragments, well away from any original surfaces. Six bulk analyses were made on these samples. The chunks were crushed carefully in a boron carbide mortar to avoid breaking the inclusions. The matrix in the form of fine dust was collected through a 325-mesh (44pm) screen (Spex nylon screen-h&e sieve set). The chondrules were ~on~ntrated on a IOO-mesh (149 pm) screen. More samples of matrix and chondrules were separated individually using stainless steel driils and chisels. Two analyses were performed on the individually separated matrix (M-3 and M-4) and on the matrix fine dust (M-l and M-2). The isotopic composition of lead in the matrix fragments is less radiogenic than that of the matrix fine dust (Table 3). This result indicates either the presence of fragments of the phases containing more radiogenic lead, i.e. chondrules and inclusions that passed through the 32%mesh screen, or some contamination from sieving. The chondrules obtained by sieving and digging were coated with fine dust of matrix, making identification and separation difTicu1t. The adhering dust was removed by washing the chondrules in distilled acetone. Analysis of the acetone wash for lead con~ntration and Iead isotopic composition gave about 2 ng of terrestrial lead; no evidence of leached meteoritic lead was found. The chondrules were then clearly visible and separates of calciumaluminum-rich chondrules (ChL-1 and ChL-2) and magnesium-rich chondrules (ChD and Ch3) were obtained by hand picking, using a suction device made of teflon. Chunks of white inclusion, pink inclusion and white aggregate essentially free of adhering matrix were extracted by drilling. The spherical white aggregate was intruded by the matrix material, indicating an event later than the formation of the aggregate. The mineral assemblages of the inclusions of Ca-Al-rich minerals (melilite, spine], anorthite, woliastonite and Al-rich pyroxene) were identified by using X-ray diffraction and the electron microprobe. * Purchased from Martin L. Ehrmann. 676 N. La Peer Dr.. Los Angeles. CA 90069. U.S.A.
The analyzed material of all of the bulk and two of the matrix samples (M-3 and M-4) were washed for 3-5 min in cold 2 N HCI. The acid rinse was used to dissolve any lead contamination from the sample handling. It also destroyed small amounts of sulfide from surfaces, but the effect on final concentrations was negligible, as shown by analysis of the acid wash of one bulk sample (W-2). Less than 1% of meteoritic lead was observed (Tables 1 and 2). The sieved matrix samples were washed twice in distilled acetone instead of 2 N HCI, because the surface to volume ratio of the fine material might be more significant. The acetone wash of the matrix sample (M-2) was also analyzed and gave no evidence of leached meteoritic lead. All teflonware was cleaned in hot aqua regia acid, 50% HNO,, and triple distilled water for at least 12 hr in each solution. Polyethyleneware was soaked in cold 50% HNO, for more than 48 hr and rinsed with triple distilled water. Chemical processing
Extensive precautions were taken to avoid contamination during analyses. All sample handling and chemical treatment was carried out in laminar flow hoods. Reagents were prepared by sub-boiling distillation in either teflon or silica glass containers (MA~IN~~N, 1972). The chemical procedures closely followed those given by CHURCH and TILTON(1974) for lunar work. The detailed procedures are given by CHEN(1974). Hanks
Reagent blanks for lead for l-g samples were in the range of 0.2-0.4 ng for the entire chemical processing, values which were small compared to the total blanks, which were 3-5 ng. For 0.1 g samples, lead blanks were O.s-2 ng. The individual measured blanks are shown in Table 2. Uranium and thorium blanks were about onetenth of the lead blanks for early analyses, and were later decreased to hundredths of nanograms when improved column cleaning procedures were instituted. Mass spectrometr~ The instrument and methods used for lead analyses were the same as described in CHURCHand TILTON(1974). The U-Th analyses were made initially by a triple filament method, but we later adopted the single filament method using phosphoric acid and carbon as described by ARDEN and GALE (1974). Superior intensities and stabilities were attained with the latter method. With an electron multiplier it was possible to make isotope analyses on l-ng samples of U, Th and Pb, e.g. in analyses of blanks. For all samples where 10-1OOng of element were present, analyses were made by Faraday cup collection as well as with the multiplier. The performance of the mass s~ctrometer was monitored by repeated measurements of the NBS 981 Pb standard during the course of the work. The observed ratios and standard deviations for 10 analyses were: *04Pb/Z06Pb 0.059065 rt 0.000040 20’Pb/206Pb 0.9 I426 + 0.00027 2osPb/206Pb 2.16306 _+0.00120 These values were sufficiently close to the absolute values (CATANZAROet al., 1968) so that no corrections were applied to the observed ratios from the meteorite samples. The Pb con~ntrations are believed accurate to ~D.>~.IY<,. while uncer~inties in U and Th are about
638
J. H.
and G. R. TILT~N
CHEN
Table I. Element concentrations Sample'
Weight
concentration
(mg) Pb
T,,232,,,238
"238,Pb204
(ppm)
u
Th
(v)
(k)
T-l
570.7
1.045
0.0148
0.642
T-2
1272.0
1.178
0.0143
0.569
T-3
661.0
1.088
0.0121
0.520
T-4
636.2
0.935
0.0120
0.573
W-l
805.8
1.069
0.0129
0,542
w-2
672.2
0.966
0.0119
0.546
M-1
693.8
1.139
0.014l
M-2
546,O
1.215
0.0138
0.0424
0.513
3.17
M-3
344.8
1.079
0.0130
0.0261
0.536
(2.09)
0.555
M-4
396.2
1.006
0.0175
0.0619
0.780
3.65
ChL-I
287.9
0.289
0.0230
0.1543
4.38
6.94
ChL-2
151.8
0.2784
0.0536
0.3164
18.47
6.10
Ch-3
225.5
0.3028
0.0263
0.0921
4.36
3.62
Ch-D
127.5
0.1675
0.0189
0.0944
6.62
5.15
WI
54.6
0.2667
0.0651
0.3636
23.30
5.77
WA
156.6
0.3122
0.0940
0.7093
PI
112.1
0.1834
0.0361
0.3146
leach solution (W-2)
7.80
129.6
9.00
23.23
0.0072
LSample identities: T, W: total samples; M: matrix from T; Ch: chondrules; WI: white inclusions; WA: white aggregate; PI: pink inclusions. See Appendix for more complete descriptions. 2. Concentration in pg of element removed/g of sample; sample leached 3-5 mm in cold 2 N HCI. Table 2. Isotopic composition of lead Observed
Corrected
Ratios' 208
Ratios2
Pbzo6
Pb207
Pb208
p
p
m
Blank, calculated (ng)
Blank, measured (n9)
Pb206 pp
Pb207
T-l
10.645
11.098
30.694
10.583
11.064
30.634
3+2
T-2
10.536
11.008
30.573
10.513
10.995
30.551
322
T-3
10.867
11.224
31.062
10.788
11.180
30.987
3+2
T-4
10.004
10.695
31.092
9.843
10.633
29.904
3;2
W-l
10.101
10.788
30.263
9.989
10.727
30.157
322
w-2
10.226
10.944
30.816
9.859
10.619
29.907
3+2
Atlende-l3
10.71
11.13
30.79
10.64
11.10
30.31
Allende-23
10.26
10.88
30.31
30.23
Sample6
Pb
Pb204
Allende'l
10.12
10.78
11.250
11.451
31.011
M-l
10.215
10.836
30.296
to.139
10.784
30.224
M-2
10.272
10.859
30.320
10.176
10.806
30.227
5
3+2
M-3
10.010
10.711
30.120
9.910
10.656
30.025
5
3+2 3+2 _
10.185
10.825
30.200
10.025
10.737
30.046
5
ChL-1
(IC)
14.696
13.671
35.009
14.654
13.652
34.972
0.6
ChL-1
(IO)
15.122
13.670
-
14.50
13.50
ChL-2
(IC)
26.511
20.782
53.412
29.214
22.623
ChL-2
(ID)
29.718
22.799
-
28.985
22.894
Ch3
(IC)
12.300
12.103
32.213
12.204
12.052
Ch3
(IO)
13.490
12.761
-
12.332
12.244
ChD
(IC)
16.367
14.596
37.005
15.816
14.378
ChD
{ID)
16.949
14.670
-
15.814
14.333
WI
(IC)
28.490
22.148
55.325
29.466
22.814
WI
(ID)
30.211
22.338
-
29.506
22.802
M-4
WA (IC)
101.60
69.67
WA
(ID)
107.29'
69.06
-
PI
(IC)
28.011
21.79
61.513
PI
(ID)
34.014
207.95
24.014
-
Leach
sol'"(W-1)'10.86
11.13
30.706
Leach
sol'n(W-2)511.90
11.73
-
134.06
88.12
134.22
88.07
33.673 33.881
25.642 25.403
1.0 58.673
5 0.8
32.221
0.8 5
36.659
2.4 1.8
57.030
0.6 0.6
274.426
1.8 1.0
75.756
1.0 1.5 1.0
176.9 111.0 76.4 75.4 84.4 141.1 60.4
1.0
67.1 32.9
0.5
21.7 98.7
1.2
4.4 1.2
Wt. (w)
57.9 70.7
1.5
41.4
1. Recalculated from the measured ratios, Pbm/Pb 206. The standard deviations are 0.04% for Pb207/Pb206 and PbZ08/Pb206 and O.l’A for Pbzo4/PbZo6 ratios, hence are cu. 0.1% for the ratios reported. 2. Isotopic composition of blank is PbZ”4:Pb2”“:Pb *07:pb20s = 1.00:18.90: 15.6:38.6--(ID) samples corrected for spikes and blanks. 3. From TILTON(1973). 4. From TATSUMOTO ef at. (1973a. b). 5. Washed in cold 2 N HCl for 3-5 min; W-2 was spiked with 1ng PbZo8; W-l unspiked. 6. Sample identities are the same as in Table 1. 7. Ratio from electron multiplier data only. All other ratios in table are Faraday cup data.
The Allende car~naceous chondritc
639
a white aggregate separate, a ratio close to that found in some achondrites (TATSUMOTO et al., 1973a). Simithe accuracy of the Pb determinations, great care was lar results were reported by TATSUMOTO et al. (1973b), taken in doing the blank correction. For the chondrules who found that the Pb in the matrix is less radiogenic and inclusions a reiterative calculation method was used than the whole meteorite, that Pb of Mg-rich chonwhereby the blanks in the isotopic composition and concentration determinations for lead were allowed to vary drules is moderately radiogenic (zo6Pb/Z04Pb ratios independently in 0.1 ng increments, Data were finally ranged from 18 to 26), and that Pb in Ca-Al-rich selected that gave close agreement between 206Pb/204Pb chondrules and aggregates is extremely radiogenic and 207Pb/Z04Pb ratios for the two samples. The method (zo6Pb/204Pb ratios reach 55.9). was not used for the matrix and bulk samples as the blank A plot of 207Pb/“04Pb ratios vs 206Pb/204Pb ratios corrections for these samples were less critical. is given in Fig. 1. A linear relationship exists among the bulk, matrix, chondrule and inclusion samples. RESULTS AND DISCUSSION The sIope of the regression line through all of the The concentrations of U and Th and Pb deter- data is 0.6240 sfr0.0015, which yields an isochron age mined by isotope dilution are given in Table 1, along of 4.565 + 0.004 AE. This line also passes through the with the values previously determined by TILTON primordial lead point defined by Canyon Diablo troilite (TATSUMOTOet al., 1973a) and MezG-Madaras (1973) and TATSUMOTO et al. (1973a). The Pb concentrations in the bulk samples range from 0.934 to chondrite (TILTON,1973). A calculated line including 1.178 ppm, about 3-6x lower than those of the the primordial lead data and our data is indistmatrix, 1.0061.212 ppm. The Pb concentrations of inguishable from the one shown here. If the matrix chondrules and inclusions range from 0.167 to and bulk sample data are eliminated, the slope of 0.312ppm, only about one-quarter of that in the the line through the remaining chondrule and inclusion data is again 0.6240. The leads clearly belong matrix. The U concentrations in the bulk sample and the matrix are similar, ranging from 0.012 to to an array that includes primordial lead as deterO.O148ppm, with the exception of M-4, which is mined by Canyon Diablo troilite and Mez~-Madaras. The meteorite lead data of TILTON(1973) and TATSU0.0175 ppm. The U concentrations in the chondrules MOTO efal. (1973a) also closely fit this isochron. are about 2-3 times higher than in the matrix, TATSUMOTO et al. (1975) also determined an internal whereas the U concentrations of the inclusions are 207Pb/20”Pb isochron for Allende. The two labora3-7 times higher than in the matrix. Chondrules and inclusions have Th concentrations about 2-10 times tories’ values are similar, but do not agree within stated error limits. TATSUMOTOet al. (1975) give higher than that of the matrix. The concentrations determined by TILTON (1973) and TATSUMOTOet al. 0.6188 + 0.0016 for the slope of the isochron; we find (1973a) for the bulk sample fall within the range of 0.6240 + 0.0015. The corresponding model Pb/Pb these data. The concentrations are also consistent ages are 4.553 and 4.565 AE. The cause of the differwith those of TAWJMOTOet al. (1973b), who reported ence is unknown, but can hardly involve mass specTh and U con~ntrations in the Ca--Al-pyroxene-rich trometer discrimination. For example, we obtain a chondrules and white and pinkish-white aggregate separates that were 5-10 times higher than that of the matrix. Their Pb concentrations, 87-485 ppb, in 6@tthe chondrules are less than half of that in the whole 61: rock and matrix, 1.5 ppm, and about 35% higher than 40 our matrix concentrations. The ratio 2’8U/204Pb (p) of the bulk and matrix samples range from 0.52 to 0.57 with the exception of samples T-l and M-4. The lead isotopic compositions for various fractions of the meteorite are shown in Table 2. The corrected ratios are obtained by subtracting the calcutated blanks. The Pb isotopic ratios for the whole meteorite are consistently lower than those obtained by TATSUMOTOet at. (1973a. b), but are similar to those reported by TILTON(1973). This is undoubtedly Fig. 1. 207Pb/204Pb-206Pb/ *O’Pb diagram for meteorite due to the fact that the Allende meteorite is extremely heterogeneous chemically and mineralogically. The samples, Slope of regression line = 0.6240 + 0.0015, corresponding to a model age of 4.565 & 0.004AE. Unlabeled isotopic composition of lead in the matrix is slightly solid circles = Allende total rock samples; solid triless radiogenic than that of the whole meteorite; the angles = Allende matrix. Coded Allende samples are desiglead in the specially separated matrix samples, M-3 nated by the same symbols used in the tables. Other and M-4, being less radiogenic than that in samples points: CD, B, PV, My, R, MO, A = Canyon Diablo troiM-l and M-2, which were prepared by sieving. The lite, Beardsley, Plainview, Murray, Richardton, Modoc and Allende total sample from TATSUMOTO et al. (1973a); MM, lead in the other phases is extremely radiogenic. The P. and A-l, A-2 = Me.&Madaras, Puftusk. and Allende ‘ ratio zohPh!“‘4Pb reaches values as high as 134 in total sample from TILTON(1973). *OS%. The blanks are approximately 0.3-7.2% of the total
Pb in the samples. Since the level of the blanks controlled
I
640
J. H. CHEN and G. R. TILTON
model Pb/Pb age of 4.560AE for sample WA, which has so little ‘04Pb that the age depends mainly on the zo1Pb/Z06Pb ratio. The discrimination correction on the 207Pb/206Pb ratio indicated by our NBS 981 analyses is 0.04%, while the difference in slope between the isochrons of Tatsumoto et al. and us is 0.8%. Moreover the two laboratories agree closely on the model PbjPb age of the matrix, where discrimination effects would be magnified since the corrections for primordial lead are large. Our average Pb/Pb model age for matrix samples is 4.505 AE (below) while Tatsumoto et al. find 4.496AE. A plot of 208Pb/204Pb ratios against 206Pb/204Pb ratios (not shown) yields a line for closed systems that correspond to 4.565AE old with a present day 232Th/238U ratio (k) of 3.8 (atomic ratio). This line adequately fits all the Pb data except that of the chondrules and the inclusions which have high Th/U ratios in Table 1, and which have higher 2osPb/204Pb ratios for given 206Pb/204Pb ratios. The cause of the high Th/U ratios is unknown. The variation in Th/U ratios in various phases of Allende probably signifies variation in Pu/U ratios as well. POWSEK and LEWIS (1972) have already noted the unusually high Pu/U ratios in white inclusions from Allende inferred from fissionogenic Xe data. Isotopic composition of initial lead Table 3 shows the isotopic composition of lead for the matrix and bulk samples corrected for radiogenic lead produced by U over the past 4.565 AE, assuming closed system behavior. The ratios vary from sample to sample, indicating that most or all of them have not been closed systems. We conclude that it is not possible to determine the isotopic composition of initial lead from direct measurements on the bulk meteorite or separated phases. In the discussion of U Ph ages that follows, we will assume that initial
lead ratios were identical to those in Canyon Diablo troilite and MezG-Madaras. As noted above, the regression line through the ratios in Fig. 1 is consistent with this hypothesis. Model ages for chondrules and inclusions are given in Table 4. All ages are calculated with the newer decay constants and the primordial lead values of TATSUMOTOet al. (1973a). The pink inclusion (PI) gives ages that are nearly concordant within estimated analytical errors; the remaining samples do not. It is significant that the most accurately determined data-those for the white aggregate (WAt yield distinctly discordant ages. The ages are best known for WA because the lead is the most radiogenie of that in all of the samples and because it contains relatively high concentrations of U and Pb compared to the remaining samples. We have not given model ages for the bulk and matrix samples because their lead is not sufficiently radiogenic to yield accurate UpPb ages. The 207Pb/206Pb ages for the four matrix samples are 4.481,4.482,4.509 and 4.549 AE. These ages are systematically younger than the 207Pb/206Pb ages for the chondrules and inclusions. The average value is 4.505AE, while the chondrule and inclusion Pb/Pb ages average 4.560AE. TATSUMOTOet al. (1975) also find a younger Pb/Pb model age for matrix samples (4.499 AE) than for inclusions and aggregates (4.548 AE). Both Tatsumoto et al. and we find a difference of ca. 0.05 AE between the Pb/Pb model ages of matrix and inclusions; however, our absolute age values differ systematically for unknown reasons. HUEY and KOHMAN (1973) have also determined the isotopic composition of lead in Allende, using a volatization process to separate the lead. The 207Pb/20hPb age calculated from their total sample data is 4.528 AE. This is again a lower age than the chondrule and inclusion ages we have measured for
Table 3. Calculated initial Pb isotopic compositions’ pb206,pb204
Sample
T-l
9.922
10.651
T-2
9.927
10.629
T-3
10.252
10.846
T-4
9.253
10.265
W-l
9.431
10.379
w-2
9.297
10.268
9.567
10.437
M-2
9.647
10.476
29.815
M-3
9.350
10.311
29.741
9.221
10.236
29.325
9.307
10.294
29.476
9.310
10.296
29.444
Olablo
Me%Madams3
pb208,pb204
M-l
M-4 Canyon
pb207,pb204
troilite'
l.Age = 4.565 x 109yr; Iz Uz3’ = 0.155125 x 10e9 yr-I, 10m9yr-‘, U238/U23’-137.88. 2. From TATSUMOKJet al. (1973a). 3. From TILT~N (1973). 4. Incorrectly given as 29.57 in TILTDN(1973).
I Uz3’ = 0.98485 x 10-9yr-‘,
j. Thz3’ = 0.049475 x
641
The Ailende carbonaceous chondrite Table 4. Model ages Atomic
..“,,,$,‘.
Model
Ratios
Age,
Million
Years'
_
I
ChL-1
1.207
104.42
0.6273
0.179
5104
4730
4573
3328
ChL-2
1.069
93.14
0.6317
0.257
4688
4615
4583
4622
Ch 3
5.672
56.49
0.6102
0.176
3312
4114
4533
3271
ChR
0.982
84.68
0.6255
0.211
4410
4619
4568
3862
WI
0.866
74.05
0.6199
0.205
4023
4385
4555
3773
WA
0.963
62.72
0.6227
0.243
4349
4496
4562
4391
PI
1.055
90.23
0.6202
0.223
4644
4583
4556
4063
1. See footnote of Table 3 for decay constants.
Allende, although the average Pb/Pb age for all of Huey and Kohman’s samples, including H and L chondrites is 4505 m.y. It is therefore uncertain to what extent their results agree with ours in indicting a younger age for the matrix. It is in any case likely that the Pb/Pb model age of the matrix is cu. 0.05 AE lower than that of the high temperature silicates. This indicates either that the matrix is younger than the silicates, or that it formed with a different initial lead, or both. The data for chondrules and inclusions have been plotted on a concordia diagram in Fig. 2. We have not plotted matrix and bulk samples because they contain relatively small proportions of radiogenic lead. All of the points except PI are clearly discordant. It seems signi~~ant that both samples which plot above the concordia curve are Ca-Al-rich chondrules. A linear regression line through the data is shown as a solid line in the figure. The chord intersects the concordia curve at an upper age of 4.57 & O.O2AE, a value consistent with the z”7Pb/206Pb age of 4.565 AE derived earlier. This indicates that the true age of the silicates is around 4.57AE. The age is significant since the sample suite includes high-temperature silicate material (see Appendix) that may, at least in part, have condensed directly from a vapor phase (GROSSMAN, 1975). As such it would likely represent the oldest solid matter to be found in meteorites. The age is not markedly older than the rather weIl-establish chemical differentiation age of chondrites in general. The uncertainties in the U-Pb ages for the Allende silicates and chondrite differentiation event easily allow for the 10 m.y. interval suggested by the initial Sr data of GRAY et al. (1973). Turning to the lower intersection on concordia, a linear regression line through all of the data gives an intersection at 0.28 + 0.07 AE (uncertainty calculated by distributing errors equally along ordinate and abscissa). A dashed line passing through the origin, corresponding to disturbance of the U-Pb system at 0 AE, is also given in Fig. 2. The data points differ significantIy from the dashed line. TA~SVMOTOet af.
(1975) report 0.107 + 0.070 AE for the lower intersection of their data on the concordia curve. The estimated errors nearly allow for overlap in the two intercept ages, but it seems probable that a small difference in results between the two laboratories remains to be resolved. Neither laboratory finds a 0 AE intercept, however. If the data do in fact lie along a chord intersecting concordia at 4.57 and 0.28 AE, they are consistent with disturbance 0.28 AE ago in systems that are now 4.57 AE old. This is an unusual result for meteorites. but it is comparable to the ‘cut-off’ in K-Ar and the U + Th/He ages of chondrites at ca. 0.4 AE first cited by ANDERS(1964). Such a limiting value is still plausible on the basis of a much larger amount of data cited by Z;~HRINGER(1968). Finally, one of the first 39Arj40Ar age measurements on chondrites gave an age of 0.5 AE for the most loosely held fraction in
---7 ChL.1
::i_ I / IO
I
09!-
0311
v
O2 ,(J 01
/$’
,/’
1 i
,’
Fig. 2. Concordia diagram showing U-Pb relationships for chondrules and inclusions from Ahende chondrite. Solid line is a regression line through all of the data, which intersects the curve at 4.57 and 0.28AE. The dashed line is drawn between 4.57 and OAE. Sample identi~cations are the same ones used in the tables.
642
J. H. CHEN and G. R. TILT~N
the Bruderheim chondrite (MERRIHUE and TURNER, 1966). It is not apparent how these events-shock or moderate heating-might have affected the U-Pb systems within individual phases of a chondrite such as Allende, but the gas retention data are in any case the only suggestion of significant events in chondrite histories around 0.3 AE age. GOPALAN and WETHERILL(1971) were able to demonstrate mobility of radiogenie Sr in highly shocked H chondrites, but these meteorites also have low K-Ar ages, while Allende does not. It is therefore not clear that the H chondrite observations have any bearing on the Allende case. The lead data do show that the disturbance of the U-Pb system occurred sometime between approximately 0.1 and 0.3 AE ago. This is undoubtedly the time of disturbance of the RbSr system as well, which confirms the supposition of GRAY et al. (1973) that it possibly took place in relatively recent time. In a strict sense the RbSr data show only that the disturbance occurred since 3.6 AE ago, as the authors stated. Finally we consider the bearing of these data on the apparent lack of U-Pb balance in bulk samples first evident in the work of TATSUMOTO et al. (1973a) and T~LTON(1973). The discordances found in all of the chondrules and in inclusions except ChL-1 and ChL-2 are in the opposite direction from that required to explain the bulk sample data-i.e. the bulk sample contains an excess rather than a deficiency of radiogenic lead. The effect is almost certainly related to the matrix. This seems to be confirmed by the matrix data in Table 3, showing that three out of four matrix samples give calculated initial lead ratios (relative to ‘04Pb) that are higher than the ratios for primordial lead. It should also be noted that for material balance of radiogenic lead in Allende our data require either that the matrix is ca. 0.05 AE younger than the high temperature silicates (or that younger radiogenic lead was inherited from an external source) or that the initial lead was not identical in isotopic composition in the two kinds of material. This is so because the Pb/Pb model age of the matrix is less than 457AE (the age of the high temperature silicates) even though the matrix appears to contain excess radiogenic lead, as stated above. These data cannot be reconciled to a model in which the bulk material of Allende first crystallized 4.57 AE ago with primordial lead identical to that observed in the Canyon Diablo and Me& Madaras meteorites. If this were true, transfer of radiogenic lead from any phases into the matrix would cause the Pb/Pb model age of the matrix to be equal to, or greater than 4.57 AE, depending on the time at which the transfer occurred. The same considerations probably apply to the data of TATSUMOTO et al. (1975) because their matrix appears to have a model Pb/Pb age that is younger than the model Pb/Pb age of the aggregates and inclusions. Their matrix samples, like ours, contain excess radiogenie lead.
Acknowledgetnents-These investigations were supported by National Science Foundation grant GA-40266 to G. R. TILTON.We are indebted to Mr. MARK STEINfor invaluable assistance with instrumental aspects of the work. We also thank Mr. DAVID CROUCH and Mrs. EVELYNGORDON and MERYLWIEDERfor assistance with preparation of the manuscript. C. J. ALLEGREand M. TATSUMOTO reviewed the manuscript and suggested many improvements. This paper is based in part on work for which J. H. CHENreceived half the Nininger Meteorite Award for 1974 from Arizona State University.
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H. (1974) UTh-Pb investigations of the Allende carbonaceous chondrite. M.A. thesis, Univ. Calif. Santa Barbara, 49 pp. CHURCHS. E. and TIL~~N G. R. (1974) Lead isotope systematics of some Apollo 17 soils and some separated components from 75601. Proc. 5th Lunar Sci. Conf, Geochim. Cosmochim. Acta Suppl. 5, pp. 1389-1400. Pergamon Press. CLARKER. S.. JR., JAROSEWICH E., MACONB., NELENJ., G~MEZ M. and HYDE J. R. (1970) The Allende, Mexico meteorite shower. Smithsonian Contrib. Earth Sci. 5, 65 PP. CLAYTONR. E., GROSSMANL. and MAYEDAT. K. (1973) A component of primitive nuclear composition in carbonaceous chondrites. Science 182, 485-487. FIREMANE. L., DEFELICEJ. and NORTONE. (1970) Ages of the Allende meteorite. Geochim. Cosmochim. Acta 34, 873-881. GALE N.
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APPENDIX Sample description
Mineral constituents were studied by X-ray powder diffraction and electron microprobe. The following identifications were made: Pink inclusion (PI). A fine-grained, irregularly-shaped pink aggregate. Its dimensions are ca. 5 x 6 x 8 mm. Minerals identified are spinel, melilite, pyroxene, wollastonite, plagioclase and an Al-rich phase (corundum? Microprobe analysis = 92.1% AlzO,). li%ite inclusion ( W). A fine-grained, elongated inclusion which is ca. 3 x 4 x 5 mm in size, containing meiilite, plagioclase, and wollastonite. White aggregate (WA). Cu. I x 1 x 1.5cm in size. The inclusion is coarse-grained, subspherical-shaped, containing plagioclase, pyroxene, spine1 and melilite. The subspherical body was intruded at one end by the matrix material. Ca-rich c~ondrules (ChL-I and 2). Spherical material, ea. 0.7cm in diameter, referred to as Ca-rich chondrules by CLARKE et al. (1970) and GRAY et al. (1973). Coarsegrained, white to grey in color, has mineral constituents similar to those of the white aggregate. Mg-rich chondrules (ChD and Ch3). Containing coarsegrained, barred or granular forsteritic olivine, some clinoenstatite, interstitial glass and sulfides. Range in size from 0.2 to 0.5cm. Ch3 is a mixture of chondrules while ChD is a single chondrule. In the inclusions and chondrules we list only those minerals which were identified; others reported in the literature, and not cited here, may also be present, Matrix (M-l, 2. 3, 4). Consists mainly of euhedral ironrich olivine crystals up to 5 nm long and 1 pm in diameter. Carbon is located primarily at grain boundaries, and occurs as highly disordered graphite.