U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

Geochimxa

et Cosmochimxz

~cta. 1976.

Vol.40% pp.617 to

634. Pergamon Press. Prmtcd ER Great B~itam

U-Th-Pb and RbSr systematics of Allende and U-Tb-Pb systematics of Orgueil MITSUNOBUTATSUMOTO. DANIELM. UNRUH and GEORGEA. DESBOROUGH U.S. Geological Survey, Denver, Colorado 80225, U.S.A. (Receioed 14 Maq’ 197.5; accepted in revised form 17 ~ove~zh~~ 1975)

Abstract-U-Th-Pb systematics study of Aliende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The zo6Pb/204Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the 206Pb/204Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates, and matrix on the zC7Pb/204Pb vs 206Pb/204Pb plot. The slope of the best fit line is 0.6188 + 0.0016, yielding an isochron age of 4553 + 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-AI-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 + 25 m.y. and 107 k 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting tha the RbSr system was also disturbed by a later event, as suggested by the U-Pb concordia data. Tk, e lowest observed 87Sris6Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (PAPANASTASSIOU,

Thesis, 1970).

The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.

INTRODUCTION

4.57 b.y., agrees within limits of uncertainty

with the

THE PURPOSEof this paper is to present (1) U-Th-Pb

Pb isotopic composition of Canyon Diablo troilite. systematics of the type I carbonaceous chondrite, In this paper we discuss the initial lead compositions Orgueil, and (2) U-Th-Pb and Rb-Sr systematics of of Orgueil and Allende. Thermodynamic calculations lead several authors inclusions in the type III carbonaceous chondrite, Pueblito de Allende. (e.g. LORD, 1965; LARIMER, 1967; LARIMER and 1972; GROSSMAN and LARRecently, TATSUMOTOet al. (1973) and TILTON ANDERS,1967; GROSSMAN, (1973) reported more precise lead isotopic composiIMER,1974) to suggest that the Ca-Al-Ti-rich phases tions and whole rock Pb-Pb ages of some meteorites in C2 and C3 carbonaceous chondrites represent the and verified PATTERSON’S (1955) meteorite age. Fur- early condensates of the cooling solar nebula during thermore, TATSC’MOTO et at. (1973) suggested age dif- high-t~~rature condensation. The mineralogy of ferences in the formation of meteorites from their the Pueblito de Allende carbonaceous chondrite Pb-Pb model ages. However, these Pb-Pb ages of reported by KING (1969), FUCHS (1969), CLARKEet ui. meteorites were determined by assuming that lead in (1970), MARVINet al. (1970), KEIL and FUCHS (1971), these bodies has evolved from the primordial lead and major and trace element studies of the Allende observed in Canyon Diablo troilite. The approach has inclusions (e.g. GROSSMAN,1972, 1973; TANAKAand been criticized because different classes or groups of MASUDA, 1973; W~;NKE et al., 1974) revealed that meteorites might have had different initial lead isotopit compositions (e.g. CAMERON,1973a). However,

TILTON(1973) reported that the L-3 chondrite MezijMadaras, which also has a high lead concentration, yields an initial Pb isotopic composition which, when corrected for Pb derived from in situ U decay for

refractory

lithophile

and

siderophile

elements

are

enriched in Ca-Al-rich inclusions and have supported the suggestions of thermodynamic calculations. Results of Xe studies (PODOSEKand LEWIS, 1972; LEWIS et al., 1975f. Sr isotope studies (GRAY et al., 1973; WETHERILLet at., 1973; NYQUISTer al., 617

M. TATSUMOTO, D. M. UNRUHand G. A. DESBOROUGH

618

1973) and 26Mg isotope anomaly studies (LEE and PAPANASTASSIOU,1974; GRAY and CC~MPSTON,1974) were in accordance with the~odynamic calculations in showing more primitive isotopic characteristics in Ca-Al-rich inclusions than in Mg-rich inclusions. Furthermore, CLAYTON et al. (1973) observed a remarkable pattern of oxygen isotope abundances which indicates that two kinds of oxygen originating from different nucleosynthetic processes were mixed in Allende inclusions. Even though GRAY et al. (1973) showed the extreme primitive 87Sr/s6Sr ratio of some Allende inclusions, they could not define an isochron for the inclusions

owing to possible disturbances of the Rb-Sr systematics which occurred less than 3.7 b.y. ago. We studied both the U-Th-Pb systematics and the Rb-Sr systematics of various inclusions and matrix in the Allende. meteorite in order to (1) obtain a concordant age relationship between the two methods, (2) obtain an ‘internal Pb-Pb isochron’ of the Allende inclusions, (3) to determine if the internal isochron is older than the Pb-Pb age of the matrix, and (4) to determine whether or not the ‘primordial’ Canyon Diablo troilite Pb isotopic composition is compatible with the Pb isotopic compositions of the Allende inclusions. Orgueil is a Cl carbonaceous chondrite (VAN SCHMUS and WOOD, 1967) which are considered to be the most primitive meteorites that escaped reheating by metamorphism or impacts {e.g. ANDERS, 1972), and elemental abundances in Cl carbonaceous chondrites are close to the solar abundances. The 12gI/1271 ratio of 1.45 x 10m4 (HERZOG et al., 1973; LEWIS et al.,

separates from 1975), obtained on magnetite this meteorite, is the second highest among the

meteorites so far analyzed and indicates that Orgueil is an early condensate from the solar nebula. Thus Orgueil was also studied for U-Th-Pb systematics in order to compare its initial lead with that of Canyon Diablo troilite.

PROCEDURE Aflende is very heterogeneous in gross inclusion distribution, and furthermore, each inclusion is mineralogicaliy heterogeneous on a microscopic scale (e.g. CLARKEet al., 1970). Chondrules, magnetic inclusions, a white aggregate separate, a pinkish-white aggregate separate, and the matrix from two specimens of Allende supplied by E. King, University of Houston, were analyzed. The whole-rock sample was the standard Allende powder {split 3, position 41) supplied by the US. National Museum of Natural History. The sample of the Cl carbonaceous chondrite, Orgueil. was supplied by E. Anders, University of Chicago. This Orgueil sample is considered to be least contaminated based on trace element studies by KR.&IENB~HLet at. (1973). The two Allende specimens were broken with a stainless steel chisel on a stainless steel plate and discrete chondrules were easily picked out. The chondrules are spheroidal, ranging from 2 to 12mm in diameter in our two specimens. The magnetic chondrules were separated with a hand magnet. The aggregates that were less than 5 mm in diameter in our two specimens were carefully separated from the matrix; however, the aggregates, owing to their irregular shape, could still contain small amounts of matrix. Similarly, the matrix analyzed could have contained some small aggregates and chondrules. White to gray aggregates were separated from pinkish-white aggregates. U-Th-Pb data for these two types of aggregates differ tremendously. All inclusion separation procedures were carried out with stainless steel tools on a laminar flow bench.

Table 1. Mineralogy of inclusions in the Allende meteorite, determined by electron microprobe analysest (G. A. Desborough; analyst) --_ FYlGNETIC C~DRULE

N

olivine orthopyroxene troilite

!!2 olivine Na-AI glass troilite (border)

olivine Mg-Al-Ca-Na glass magnetite troilite

fi grossularite ii-fassaite N+Al spine1 Ca-Al-MS glass me1i1ite tmflite

PINKISH-WHITE AGGREGATE

olivine Mg-Ca-Al-Na glass

!k?

augite Mg-Al-Fe spine1 Na-Al glass Ca-Al-@ pyroxene

olivine orthopyroxene clinopyroxene Ca-A? glass Na-Ca qlass troflite

ollvine Na-Al-Ca glass Fe-Al spine1 troilite

olivine orthopymxene troiltte

!4&

E Cd-Al-& silicate @Al spfnel Ti-fasraite augite Ca-Al-MS glass

!!?_ olivfne anorthite troilite pentlandite magnetite

E

N5

olivfne orthopyroxe"e* Na-Al glass clinopymxe"e wollartonite troilfte (border)

m olivine orthopyroxene "epheline i4g;i7;;-Na glass

WHITE AGGREGATE

olivfne ca-pymxene orthopy~xene &Al glass magnetite troilitepentlandite

Nr olivine orthopymxene Ea-pyroxene Ca-Al glass magnetite troi1ite

u orthopymxene olivine tmilite

E

m orthopyroxene olivine troilitepentlandite !&5

E

olivine orthapyroxene troi1it.e Na-Al glass

olivine orthopyroxene Na-Al glass

alivfne orthopymxene ca-augite Na-Al glass troilite

~;;;i;~ss

* Orthopyroxene = High Mg, low Ca and Al pyroxene. t Minerals are listed in order of abundance, i.e. most abundant listed first.

Nz olivine pyronene Na-Al glass

Ns olivine orthopynixene Na-Al glass troiiite

q7_ grossularite Mg-Al spine1 troilite Ca-Al-Mg glass g olivine orthopyroxene Na-glass troilite

619

Systematics of Ailende and Orgueil For small Allende inclusions, one-fourth of each chondrule was used for electron microprobe analysis and threefourths were used for either U-Th-Pb or Rb-Sr analyses. For large inclusions, one-half was used for U-Th-Pb analyses, one-fourth for the microprobe, and one-fourth for Rb-Sr analyses. Minerals contained in some inclusions and the matrix are hsted in Table 1. in this table minerals are listed in order of decreasing abundance. Chondrules N9, N28, N34, N40, and N41 were examined for mineralogy only with a microscope, Chondruies Nl-N14, N20-N27 and N3&N4L are Mg (olivine)-Rich chondrules and N17-N19 and N28 are Ca, Al-rich chondrules. The magnetic chondrules, white aggregates, and pinkish-white aggregates were composed of several grains. The chondrules were washed by double distilled acetone with the aid of an ultrasonic vibrator prior to analyses to liberate any matrix material but aggregates and the matrix were not washed with acetone. The Orgueil sample was also washed with acetone and the sample was reduced to powder by ultrasonic vibration. Analytical

procedures

A. U, Th and Pb anufyses. Samples Nl-N3 were split from solution after decomposition into concentration and composition portions prior to 235U-230Th-208Pb enriched spiking. Ail others were determined from solid sample, splits, and the concentration split was totally spiked before digestion. Most samples were digested in teflon bombs using an HF-HN03 solution, but chondrules N34, N40, and N41 were decomposed with an HF-HCIO., solution. As previously discussed (NUNFS et al., 1974), the totally spiked samples yielded more reliable Th concentration data but were not always homogeneous. However, as observed in Fie. 4. the zo7Pb/Z06Pb ratios of concentration and composition runs from powder splits agreed reasonably well. Differences in isotopic compositions in replicate runs are probably due to sample heterogeneity. The chondrules N34. N40 and N41 were analyzed only for concentration and their Pb concentrations were calculated by interpolating the z”*Pb/204Pb ratios from a zo8Pb,!206Pb vs zo4Pb/206Pb plot of other Allende inclusions. Lead was separated by a double resin (Dowel l-X8) column technique using HBr and HCI media after sample decomposition (NUNES et al., 1973). Lead blanks were 3-41~ in the early stage of this study but later were reduced to about 0.7-1.1 ng. Uranium and thorium were separated with a Dowex l-X8 NO; resin column and blanks were 0.03 and 0.05 ng, respectively. Pb mass spectrometry was done with the conventional silica gel-phosphate method on a single rhenium filament. U and Th were loaded as nitrates and run by triple filament (rhenium) ionization. B. K, Rb arid Sr analyses. Potassium, rubidium and strontium were determined by isotope dilution using spikes of “K s’Rb. and *%. Separation of the elements was done in the conventiona way using a Dowex 50-X12, 2OG-400mesh resin column in HCI medium. Mass spectrometry was done with triple filament (rhenium) ionization for Rb, K and single oxidized filament (tantalum) ionization for Sr. Rb and K were loaded as chloride and Sr was loaded as phosphate. K, Rb and Sr blanks determined ten times during this study were less than 1 pg, 0.015 ng and 0.5 ng, respectively. Muss spectrometry

Pb and Sr isotopic ratios were measured with an NBS (National Bureau of Standards) 12 in. radius 60 degree mass spectrometer equipped with programmed magnetic field and on-line digital data acquisition and processing. All data were obtained by Faraday cup (no multiplier) with a IO” R feedback resistor on a Cary 31 vibrating reed electrometer. The correction factor (0.1% per mass unit)

for Pb mass fractionation was obtained from replicate measurements on NBS equal atom standard, SRM-982 (TATSUMOTO et al., 1973). The uncertainties of the raw data are better than *0.25% for 206Pb/204Pb ratios and +O.l”,d in 207Pb/206Pb and r’sPb/ *06Pb ratios. However, uncertainties in the zflsPb/Z04Pb and *0*Pb/Z06Pb ratios corrected for analytical blank probably are up to IO:, for smaller samples (< 50 mg) due to a large amount of analytical blank Pb relative to sample Pb. The uncertainties of most of the corrected 20’PbiZ06Pb ratios remain almost the same (+0.2x) because the Zo7Pb!*0bPb ratios of the meteorite inclusions are nearly the same as those of terrestrial common Pb. Uncertainties in U, Th, and Pb concentrations are believed to be better than 2”; except for the Th values of chondrules Nl to N3. Sr isotopic compositions were measured on s4Sr spiked samples by the same mass spectrometer on which Pb isotopic ratios were measured. Corrections for mass spectrometric fractionation were made by normalization to s’Sr/ssSr = 0.11940. The errors given in Table 2 which lists s7Sr/*6Sr ratios are twice the standard deviation of the mean. Interlaboratory comparison and reproducibility of our Sr data were checked with replicate runs of NBS standard SRM-987 which was spiked with ‘%r in different degrees. The runs gave an average s’Sr!‘%r ratio of 0.710184 i 0.~69 (2~; 20, = 0.~25) ranging from 0.710153 to 0.710260 for 10 separate Sr isotope analyses during this investigation (TA~SUMOTO et a!.. 1975). The standard deviation indicates that we can resolve differences of less than 1 x lo-’ in 87Sr/s6Sr. Data from each laboratory will reflect a small bias in the *‘Srl”‘Sr ratios which must be considered if meaningful comparisons of small differences in 87Sr/86Sr ratios are to be made. We therefore standardize all data in this paper to the NBS certified value of 0.71014 for SRM-987. The K and Rb concentrations were determined with an NBS 6 in. mass spectrometer.

RESULTS AND DISCUSSION Thermodynamic

calculations

(LARIMER, 1967; LAR-

IMER and ANDERS, 1967; GROSSMAN,1972, 1973; GROSSMANand LARIMER,1974) have suggested that the meteorites and planets formed from a hot solar nebula by a condensation process, largely under equilibrium conditions. From a cooling gas of cosmic composition, some highly refractory compounds of Ca, Al, and Ti condense from below 2000°K to about 1450°K. Magnesium silicates and nickel-iron follow at 135tS12OO”K, then alkali silicates condense at about 1100°K. At _ 700°K sulfur begins to condense and forms troilite. The volatile elements Pb, Bi, Tl and In condense in the temperature interval of 600-800°K. Finally, water is bound as hydrated silicates below 400°K (GROSSMANand LARIMER,1974). The Cl chondrites consist only of matrix material and are believed to be the most primitive of all meteorites (e.g. ANDERS, 1972) because the relative abundances of the nonvolatile elements in Cl chondrites are similar to the solar abundances (CAMERON, 1968). The C2 and C3 chondrites contain ‘chondrule’ components in increasing abundances. Thus. it is important to compare trace element abundances in Orgueil (a Cl chondrite) to Allende (a C3 chondrite).

Og

I_3

Cl

C2

C2

c3

Canyon Diablo

M&-Madam

Orguei1

Murray

Hurchison

Allende

-

1097 1023

62.2 __ __

li.3

14.3

__

whole rock

whole rock

whole rock

1553

1m.l

__

43.0

1508

2434

39.6

28.6

--

4.07

3.71

--

and

This study

4.30 4.50

(1973)

11.233

11.466

31.458

x 10-9/yr,

0.6085

4.53

HUEV and KOHKAN (1973)

TILTON (1973)

(0.6047) 0.5978

30.31 30.23

10.64 10.12

(1973)

TATSUMOTO et

4.50

0.5953

31.352

11.250

(4.52) 4.51

This study

4.49 4.52

0.5931 0.6053

10.629 10.553

30.273 __

;:;:;u

29.699 29.69

10.499 10.56

HUEV and KOHMAN (1973) TILTON (1973)

9.77

29.939

10.594

4.45 4.52

TATSUHOTO et al. (1973)

TtLTON 11973)

4.45

0.5949 0.5953

TATSUFOTO et 0.3745

Age' Y 109 yrs

with

4.51

30.062 __

(207Pb/='f'Pb)Rt

compared

0.5710 0.6067

9.806 9.666

3.67

29.577

10.614 10.557

29.476

~"aPb/2"%

10.364

10.294

207pb,20'W

chondrites

0.6012

;:3

9.449

9.307

*asPb/20'+Pb

Isotopic Composition*

Pb concentrations of carbonaceous and Canyon Diablo troilite

3.49

--

5270

__

11.6

11.0

10.8

a.2

11.3

--

6868 4370

co.1

matrix

whole rock

whole rock

whole rock

whole rock

co.1

troilite

ThfU

(ppb) Pb

Th

Concentration

compositions and W, Th and those of Me.+Madaras

U

isotopic

Phase

2. Lead

* Corrected for Pb blank which is less than 1% of total Pb in sample studied in this paper. t Radiogenic component after subtraction of the primordial Pb. #Single-stage model age. Decay constants used are: 1238” = 0.155125 x 10-‘/yr, j/z=” = 0.98485 &jz7h = 0.049475 x LO-‘/yr. 23*U/z35U = 137.88. ’ Obtained from concentration runs after correction of the “‘Pb spike. ’ Corrected values (Tilton, personal communication, 1975).

Class

Meteorite

Table

621

Systematics of Allende and Orgueil A. Trace element abundances in Orgueil and Allende We have determined the concentrations of refractory elements Sr, Th, and U which are enriched in Ca, Al-rich minerals; K and Rb which are enriched in alkali silicates: and volatile Pb. Uranium, thorium, and lead concentrations of Orgueil are listed in Table 2 with data from other volatile-rich meteorites. MezG-Madaras, an L3 chondrite which TILTON (1973) analyzed, is exceptionally high in Pb. The Pb concentration of Orgueil is about half that of Mezb-Madaras but it is higher than that of other C2 and C3 carbonaceous chondrites. Uranium and thorium concentrations increase respectively from 8.2 and 29 ppb in the Cl chondrite Orgueil, to 15 and 62 ppb in the C3 chondrite Allende, whereas lead concentrations decrease from 2.4 to 1 ppm. The U concentration (8.2ppb) of Orgueil (Table 2) obtained by isotope dilution is similar to values obtained by neutron-activation (REED et a/., 1960; MORGAN and LOVERING, 1968; MORGAN, 1971; KRLHENBBHLet al., 1973), however, the Th concentration (28.6 ppb, Table 2), and the corresponding Th/U ratio (3.5, Table 2), are significantly lower than Morgan and Lovering’s values of 33.8 ppb for Th and 4.3 for the ThjU ratio. Morgan and Lovering’s lowest Th and Th/U values for Orgueil, however, seem to be still too high compared to those values of other Cl and C2 chondrites (see Table 1 of MORGAN and LOVERING, 1968; MORGAN, 1971). It seems that Morgan and Lovering’s samples were contaminated with respect to Th. The U, Th, and Pb concentrations in the Allende matrix are quite similar to those in C2 carbonaceous chondrites, suggesting that some chondrule material may have existed in our matrix sample and that the concentrations of the matrix may still be too high. Our ThjU ratios increase from type Cl

to type C3 carbonaceous chondrites. This interesting trend is probably due to an increasing abundance of chondrules formed at high temperature in C3 chondrites, as discussed later. U, Th, and Pb data for Allende inclusions are shown in Table 5 and compared to those for Orgueil. As previously mentioned, chondrules Nl-N3 were spiked after dissolution and their Th concentrations are very uncertain. U, Th, and Pb concentrations of Mg-rich (olivine-rich) chondrules Nl-N3 were determined using only 25- to 50-mg-sized samples which contained only 2-9 ng of Pb. Therefore an uncertainty of up to *25% in the Pb concentrations for these three chondrules is evident. Uranium concentrations of Ca-Al-rich chondrules (N17-N19) and of pink and white aggregates range from 67 to 112 ppb and are almost 10 times higher than those of the matrix and of Orgueil, whereas U concentrations of olivine-rich chondrules (Nl-N3 and N3@N41) range from only 13 to 20ppb. Both the pinkish and the white aggregates have Th concentrations about 19 times higher than Orgueil (Table 3 and 5), although the uranium concentrations are 13.7 times greater in the white aggregates and only about 6.2 times greater in the pinkish aggregates. Thus the Th/I-J ratio of the pinkish-white aggregates concentrations remarkably high. Thorium ;;7&370ppb) of Ca-Al-rich chondrules are about half as high as those of the aggregates (540ppb), whereas U concentrations are quite comparable. The ThjU ratios in olivine-rich chondrules range from 3.5 to 4.2, except for chondrule N34, which has a higher ratio than the whole rock (4.2). These results are extremely interesting since the refractory element thorium is concentrated in the pinkish and also white Ca-Al-rich aggregates in which an oxygen isotope

Table 3. Trace element concentrations

in Ca-Al-rich inclusions and the matrix of Allende and enrichment factors (E.F.) relative to Orgueil Allende

Pinkish-white aggregate

Orgueil

White aggregate

Ca- Al-rich chondrule Nla

(wm)*

E.F.t

Th

0.542

19.0

0.546

19.1

0.374

u

0.051 __

6.2

0.112

13.7

0.106 __

Sr

__

48.7 __

5.7

13.5

(m)

(0.12)1' 165 __

E.F.

-19.2 -_

(pm)

E.F.

Hatrix

Whole

(wm)

E.F.

(wm)

13.1

0.034

1.2

0.0286

12.9

0.0095

1.2

0.0082

--

(0.0091)

(13.2)

--

ia0

20.9

16.87

(130)

(15.1)

--

2.0 __

S.6& (8.60)

6.3

0.72

0.3

1.52

0.7

0.55

0.2

2.16JJ

K (5)

0.26 __

7.6 __

0.021 -_

0.6

0.017

0.5

0.009

0.3

0.03‘&

__

(0.021)

(0.44)

--

(0.048)

204Pb

0.00395

0.08

0.0025

0.05

0.0042

0.09

0.56

0.0473

Rb

All values are in ppm except K. t E.F. = enrichment factor = concentration divided by that of Orgueil. *

’ Data in parentheses are from WXNKE et al. (1974). ’ GOPALANand WETHERILL(1971). ’ GOLE (1971). 4 MURPHY and COMPSTON(1965).

-0.0267

622

M. TATSUMOTO, D. M. UNRUHand G. A. DESBOROUGH

anomaly (CLAYTON et al., 1973) and a 26Mg isotope anomaly (LEE and PAPANASTASSIOU,1974; GRAY and COMPSTON, 1974) were observed. GROSSMANand LARIMER (1974) suggested that Th may begin to condense from a nebula at 1647°K in perovskite (CaO.TiOJ. U02 may also condense in the perovskite, or condense at 1450°K as simple U02. Sr may also condense in solid solution with Ca in high-temperature Ca minerals. Thus, a large enrichment of U, Th, and Sr in the Ca-Al-rich aggregates relative to Cl chondrites provides further evidence that the Ca-Al-rich aggregates are high temperature condensates from the solar nebula. GROSSMAN(1973) showed that the refractory trace elements Ir, SC, La, Sm, Eu and Yb are enriched in the Ca-Al-rich inclusions of Allende by factors of from 18.3 to 25.7 relative to Cl chondrites. Th being enriched by a factor of about 19 (Table 3) relative to Cl chondrites in both pinkish and white aggregates behaves as a typical refractory trace element. Since uranium is enriched by factors of only about 6.2 in the pinkish aggregate and 13 in the white aggregate it does not behave as a typical refractory trace element. When using the U abundance in Allende as a basis for condensation model calculations for the Moon it is therefore necessary to use a correction factor (GANAPATHY and ANDERS, 1974). It might be better to use Th (or Sr) instead of U as a typical refractory element in condensation model calculations. Strontium is also enriched in the white aggregate by a factor of about 19 but is only enriched by a factor of 5.7 in the pink aggregates. The elemental abundances (except Th) relative to Cl chondrites, the refractory element ratios Th/Sr and Th/U as well as the Rb/Sr and K/U ratios of the Ca-Al-rich pinkish aggregates are distinctly different from those of either the Ca-Al-rich white aggregates or the Ca-Al-rich chondrules. Ca-Al-rich chondrules N17, N18, and N19 have K/U and Rb/Sr ratios similar to those of the white aggregate, while those of the pink aggregate are almost 2 orders of magnitude higher. The Th/U ratio of chondrule N17 is also similar to that of the white aggregate whereas the Th/U ratios of chondrules N18 and N19 and the Th/Sr ratios of the three chondrules are closer to that of the matrix; but these ratios are still within a factor of about 1.5 of those of the white aggregate. It appears that these chondrules may be a remelt of white aggregate material or a mixture of white aggregate material and a little matrix material. GRO~Z.MANet al. (1975) reported that feldspathoids occur as separate grains and euhedral crystals in combination with Ca-Al-silicates in pinkish white aggregates. Our high Rb and K abundances and consequently the high K/U and Rb/Sr ratios seem consistent with this observation. The apparent lesser enrichment of U and Sr relative to Cl chondrites may also be explained by dilution of the refractory-rich Ca-Alsilicates by these alkali-rich (Sr-U-poor) feldspath-

oids. If this were the case, however, we would expect Th to be similarly diluted. If we assume that the dilution hypothesis is correct we can determine how much feldspathoidal material is required to dilute the U enrichment relative to Orgueil from 13.7 to 6.2 and the Sr enrichment from 19 to 5.7 (Table 3). This would require the pink aggregate to consist of about S&700/, feldspathoidal material. Furthermore, if we assume that Th is diluted in the same proportions as U and Sr and calculate the ‘corrected’ enrichment of Th in the Ca-Al-rich phase of the pinkish aggregate relative to Orgueil we arrive at an enrichment factor of about 40-63. Both the required amount of feldspathoidal material and the calculated ‘corrected’ Th enrichment factors are unrealistic (Table 1; GROSSMAN,1973). We therefore cannot explain the unusual Th/U and Th/Sr ratios and consequently the low U and Sr enrichment of the pinkish aggregate relative to Orgueil, unless a thermal disequilibrium process is considered (ARRHENIUSand ALFVBN, 1971). The U concentrations of magnetic separates which contain magnetite and troilite are similar to those of olivine-rich chondrules but Pb is enriched in the magnetic separates. The Pb concentration of the matrix is exceedingly high and reflects more volatile element enrichment in later condensates. Observed UjPb ratios in chondrules do not directly indicate the extent of refractory element depletion, because total Pb concentrations are partially composed of radiogenie Pb produced from in situ U decay. However, ‘04Pb and observed 238U/204Pb ratios (p’s) may well reflect refractory depletion since ‘04Pb is not radioin white and pinkish genie. The ‘04Pb concentrations white aggregates and Ca-Al-rich chondrules are about one-tenth that of the matrix (Table 3) and the p’s of the chondrules N17, N18, and N19 and the $s of the white aggregates are extremely high whereas those of the matrix are very small. B. U-Pb

systematics

of Orgueil

The lead isotopic compositions of four carbonaceous chondrites are shown in Table 2. TILTON (1973) substantiated our value of the Canyon Diablo troilite lead isotopic composition by showing that the initial lead in Mezii-Madaras corrected for in situ uraniumderived lead agrees well with that of the primordial lead determined from Canyon Diablo. The Pb concentration of Orgueil is less than half that of MezoMadaras, whereas U concentrations in these two chondrites are nearly the same. Thus, the relative amount of Pb in Orgueil derived from in situ U decay is large compared to that of Mezo-Madaras. The early history of meteorites may be expressed in the following schematic way (e.g. ANDERS, 1972): Initial condensation t1

Chondrule formation t2

12vXee1291 chronology

accretion t3 showed

that

metamorphism t4 the

total

age

623

Systematics of Allende and Orgueil range, from t, to t4, for chondrites is about 15 m.y. (PODOSEK, 1970). HERZOG et al. (1973) also in this manner determined that the magnetite separates from unmetamorphosed Orgueil and Murchinson are 2.1 m.y. older than the metamorphosed C4 chondrite, Karoonda (PODOSEK, 1970) and they suggested that the entire interval between condensation and accretion must have been less than 2.1 m.y., perhaps much less, because cooling times even for small asteroids are on the order of 106-lo7 yr. The Cl chondrites are considered to be the most primitive condensates that escaped reheating by metamorphism or later impacts, thus it is interesting to compare the initial Pb isotopic composition of Orgueil with that of Canyon Diablo troilite. If we assume a closed system in which the Pb in Orgueil evolved in the observed 238U/204Pb (p) environment for the last 4.55 b.y., the calculated initial Pb isotopic composition for Orgueil is 206Pb/204Pb = 9.595; 207Pb/204Pb = 10.453 (from concentration run data). These values are significantly larger than the Canyon Diablo ‘primordial lead’ of 206Pb/204Pb = 9.307 and 207Pb/204Pb = 10.294. A terrestrial Pb contamination (about 7% of Orgueil Pb) prior to our analysis is a plausible explanation for the high initial Pb isotopic composition of Orgueil. However, because (1) the PbPb model ages of Orgueil and the matrix of Allende are both 4.50 b.y., (2) their U-Pb data both plot in a similar position above concordia in a U-Pb evolution diagram, (3) the Orgueil sample does not show any sign of terrestrial contamination for other trace elements (KRLHENBYJHLet al., 1973) and (4) because Allende is supposed to be the least contaminated among carbonaceous chondrites due to quick recovery of this meteorite, interpretations other than terrestrial contamination must be considered to explain the high initial Pb isotopic composition of Orgueil. If Orgueil consists totally of unmetamorphosed original condensate from the solar nebula, and if the time of all meteorite formation (tl-t3) is isochroneous [or even considering the time of metamorphism (t4 = - 2 m.y.) for other meteorites], the more radiogenic initial Pb isotopic composition of Orgueil than that of Canyon Diablo can not be explained by a single-stage model. This is true whether Canyon Diablo formed non-igneously by inhomogeneous accretion of the solar nebula (e.g. WASSON, 1970; SCOTT and BOLD, 1974) or it is an initial differentiation product of a large asteroidal object. If we assume that the proper initial Pb isotopic composition is that of Canyon Diablo troilite and consider a two-stage model for Orgueil then we still cannot account for the apparent excess radiogenic Pb in Orgueil with a young apparent 207Pb/206Pb age. If the parent body of Orgueil (asteroid?) suffered minor impact events after its accretion (t3), then a regolith may have formed on its surface, and Orgueil may represent the agglomeration of this regolith. The magnetite separates dated by Ander’s group would

RECENT

OISTURBAMCE

Fig. l(a). A two-stage model-a carbonaceous chondrite which accreted at t3 was disturbed in its U-Pb system by a meteorite impact at tS, Pb was enriched in the regolith formed at ts. The zo7Pb/Z06Pb model age (dashed line) appears to be older then t3. (b) A three-stage model-the U-Pb system of a carbonaceous chondrite which accreted at t3 was disturbed by a meteorite impact at ts. Both the regolith (R) and the chondrules (C) lost Pb into space (since Orgueil does not contain chondrules, we may assume that C represents another portion of the parent asteroid). At a recent disturbance in the U-Pb system, Pb in C moved into the regolith (matrix). The C becomes C, by losing Pb and R becomes RI by mixing with Pb from C. The recent disturbance is shown by dashed lines. therefore have to represent material in the regolith which was unaltered with respect to Xe-I. Pb, being more mobile, was lost from the parent material and was collected in the regolith causing Pb enrichment in the regolith, as is seen in lunar regolith (SILVER, 1970; TATSUMOTO, 1970; TATSUMOTO et al., 1971). 207Pb/206Pb model age of However, the apparent Orgueil should then be greater than the accretion age (Fig. la) as is also seen on the Moon. Unless Pb or U isotopic compositions in the solar nebula were not homogeneous (Canyon Diablo is not the proper initial Pb) and unless this Pb with a different 207Pb/206Pb ratio was added to the regolith from the impacting meteorites during regolith formation, the two-stage Pb migration model cannot explain the apparent young 207Pb/206Pb age. A three-stage model in which the parent of Orgueil suffered Pb depletion during regolith formation followed by Pb enrichment at a relatively recent time offers a possible explanation for the apparent young age of and the excess radiogenic Pb in Orgueil (Fig. lb). If the asteroidal parent of Orgueil was too small to retain in its gravitational field the Pb volatilized by minor impact events, one might expect the regolith to have been depleted in Pb. At a very recent time [4-5 m.y., cosmic ray exposure ages (FIREMAN and GOEBEL, 1970; JEFFERYand ANDERS, 1970)] the parent body of Orgueil may have broken up by collision with another asteroid. The Pb that was volatilized from the colliding bodies may have been collected on Orgueil and other remaining fragments. These conditions would result in Pb enrichment in Orgueil and, if the time of regolith formation were sufficiently young (for example, 4.45 b.y.) they could also account for the apparent young 207Pb/206Pb age (Fig. lb).

624

M. TATSUMOTO, D. M.

UNRUH

and G. A. DESBOROUGH

Table 4. K, Rb and Sr concentrations and “Sr/“Sr ratios and model ages for Allende inclusions and matrix

Sample

Weight dissolved (mg)

(PL

G.)

(P%

87Rb/86Sr

8'SrJ8%*

Model age + (initial 87Srls%)

Total meteorite (Smithsonian Split 3, Position 41) WRl

51.2

277

1.17

14.34

0.236

0.71415~10

4.48

WR2

39.9

252

1.03

12.49

0.240

0.71444516

4.49

MXl

38.3

76

0.51

16.87

0.088

0.70510*12

4.84

MX2

36.7

103

0.59

17.48

0.092

0.70504~10

4.60

a

mite

aggregate

WUC

68.4

195

0.99

156.5

0.018

0.69976*22

W6

11.8

73

1152

154.3

0.028

0.70030*4

Wll

29.8

206

0.72

165.2

0.013

0.69976'10

2597

13.50

0.806

0.74993*2

'(0.69857) 3.33 (0.69890)

Pinkish-white aggregate P

18.1

48.65

4.41

Ca-Al-rich chondrules N17

47.8

91

0.59

151.4

0.011

0.69934*4

N18

59.7

168

1.52

180.9

0.024

0.70037+10

N19

72.8

587

2.70

118.4

0.066

0.70291*4

N28

28.3

80

0.08

182.9

0.001

0.69884?8

(0.69860) 4.08 4.17 (0.69877)

Mg-rich chondrules N4

13.1

475

2.35

4.77

1.44

0.78851?4

4.34

N5

20.2

452

2.31

5.96

1.13

0.76963+8

4.36

N6

18.2

197

2.21

41.96

0.152

0.70846t8

4.36

N7

89.7

230

0.86

11.83

0.211

0.70686'10

2.64

NE

36.6

716

4.06

4.79

2.49

0.85941tlO

4.49

N9

32.5

744

3.65

6.52

1.63

0.79290?8

4.03

NlO

14.5

769

2.97

17.54

0.491

0.72977?10

4.38

Nil

27.9

466

2.61

12.70

0.596

0.73451?8

4.17

N12

36.1

641

4.20

11.14

1.096

0.76318+10

4.10

N13

23.7

556

2.65

5.57

1.386

0.78156e8

4.17

N14

31.2

305

1.05

9.16

0.333

0.71973*4

4.35

N25

11.8

1448

7.12

23.22

0.891

0.75238~16

4.19

N26

8.8

815

3.83

29.47

0.376

0.72171?8

4.22

N27

24.6

1001

5.37

14.33

1.08

0.76196e16

4.08

* Corrections for the ratios were made by normalization to *%/**Sr = 0.11940 for mass spectrometric fractionation and by interlaboratory standardization to *‘Sr/‘%r = 0.71014 for the National Bureau of Standards’ SRM 987 comparing to our measured s7Sr/s6Sr ratio of 0.710184 k 0.000025 (2a,,,). Errors correspond to the last figures given and are twice the standard deviation of the mean. t The model age is given for BABI initial = 0.69897 (standardized) and lqb = 0.0139 x 10m9 yr-‘. The initial *‘Sr/“Sr is given only for low measured s7Sr/86Sr assuming an age df 4.6 x lo9 yr. We think that this explanation is unlikely, however, because one would not expect Pb loss from minor impacts and Pb enrichment from major impacts on the same planetary body. We are, therefore, forced to conclude that the proper initial Pb isotopic composition of Orgueil is different from that of Canyon Diablo troilite.

standardized). The model ages of the Allende matrix are older than the whole-rock ages of 4.48 and 4.49 b.y. whereas most chondrules plot near the 4.2-b.y. model isochron. Our whole-rock model age is distinctly younger than those of GRAY et al’s (1973) (4.56 4 4.60 b.y.) but agrees with WETHERILLet al.‘s (1973) reported value. Since Wetherill and we analyzed the Smithsonian standard, the difference C. Rb-Sr systematics of Allende between ours and GRAY et al.‘s (1973) whole-rock The results for Allende inclusion, matrix, and model ages is probably due to sample heterogeneity. whole-rock samples are shown in Table 4 and Fig. 2. The 87Sr/86Sr ratios in Ca-Al-rich chondrules and Like GRAY et al. (1973), we also failed to obtain white aggregates are extremely low as are those of an internal isochron age for Allende. Our data are achondrites (PAPANASTASSIOU and WASSERBURG,1969) extremely scattered, more so than those of GRAY et and lunar anorthosites (TATSUMOTOet al., 1975; al. (1973) with model ages ranging from 2.6 to 4.8 PAPANASTASSIOU and WASSERBLJRG, 1973; NYQUISTet b.y. Model ages were calculated assuming an initial al., 1973). The lowest s7Sr/86Sr ratio we observed was Sr isotopic composition equal to BABI (PAPANASTAS- 0.69884 for one Ca-Al-rich chondrule (N28 in Table SIOU and WASSERBURG,1969; 87Sr/86Sr = 0.69897; 4) and is the same as Angra dos Reis’ 87Sr/86Sr ratio

625

Systematics of Allende and Orgueil I -_

I

I

/

ALLENDE

N6: /

0.64 i

0.620

/

I

///

-4

4

to

d c-

a 0 760 t

1 0.

1 0.S

I

I 1.5

1.0

, 20

5

Rb”/Sr”

ments are all 5 mm or less in diameter except chondrule N17 which was 12mm in diameter. Because the inclusion data scatter widely and do not form an isochron, the initial 87Sr/86Sr calculation assuming that the chondrules evolved in a closed single-stage system for 4.6 b.y. would not be valid. However, it is interesting to note that the lowest calculated initial 87Sr/86Sr values (0.69857 and 0.69860), by a single-stage system for 4.6 b.y. using observed Rb/Sr ratios, are similar to that obtained by GRAY et al. (1973). As discussed in the section on U-Th-Pb systematics, at least two events affected the Rb-Sr system of the Allende inclusions. The chemical system of aggregates and chondrules which accreted and metamorphosed To years ago (the short duration t,-t, is now expressed by To) were presumably disturbed when the carbonaceous meteorites were agglomerated with the matrix to form a parent body t5 years ago at an early stage of solar system formation. The system further underwent chemical fractionation at relatively recent time (t,twhich is also reflected in the U-Pb system as discussed later. The initial “Sr/‘%r (I,) then should be discussed, at simplest, by a threestage model: I, = (87Sr/86Sr), - (87Rb/86Sr), (e&3%- e&,t$)

Fig. 2. RbSr evolution diagram for Allende inclusions and matrix. The isochrons are shown for 4.56, 4.0 and 3.6 b.y. through N28 data with I = 1.39 x 10-l’ yr-‘. The low Rb/Sr data are expanded in the insert. The data for inclusions, except for N6 and N7, are scattered about the 4.2 b.y. isochron. The mode1 ages for the matrices exceed 4.56 b.y. suggesting that the matrices lost Rb (or gained Sr) during a later event. The sample numbers indicated on data points correspond to those in Table 3. WR, whole rock; MX, matrix; all others are inclusions.

of 0.69883 &- 0.00003 (PAPANASTASSIOU, 1970; standardized). It is still larger than the lowest ratio (IALL= 0.69876; standardized) in one Ca-Al-rich Allende chondrule observed by GRAY et al. (1973). If Ca-Al-rich inclusions which contain high concentrations of Sr but almost no Rb condensed at an early time and were isolated from the solar nebula, then the Rb/Sr ratio must have increased in the solar nebula, and Rb-rich phases which condensed later would have higher initial *‘Sr/*%r ratios than the earlier condensates. The differences among the initial 87Sr/86Sr ratios, then, could be used to determine the relative formation sequence of different components of planetary bodies but they could not be used to define the duration of condensation of an individual planetary body. We failed to find an Allende chondrule with a Rb/Sr ratio sufficiently low to allow accurate definition of an initial 87Sr/86Sr ratio. We presume that in the larger Ca-Al-rich chondrules and aggregates the Rb/Sr ratio is lower. Refractory elements apparently condensed at an earlier stage, formed Ca-Al-rich aggregates and chondrules, and grew larger than later condensates which contain more volatile elements. Chondrules in our two frag-

- (87Rb/86Sr), (e&t5 - e&f,) - (87Rb/86Sr)M(e&+ - l), To > T, 9 T6.

Ca-Al-rich inclusions which were among the earliest condensates in the solar system probably increased their Rb/Sr ratios at the time of agglomeration from (“Rb/*%r), to (87Rb/86Sr)z owing to associating with volatile-rich components (e.g. matrix), and, furthermore, the (87Rb/86Sr)z may have increased to the measured value, (87Rb/86Sr)M. This model would explain why all inclusion model ages are younger than the whole-rock model age, whereas the matrix model age is older. Thus, the l,, values calculated by a single-stage model from observed Rb/Sr ratios for Ca-Al-rich inclusions with primitive “Sr/“%r ratios may be over-corrected (i.e. too low). On the other hand, Rb-rich chondrules and aggregates may have lost Rb at the f6 stage thus yielding very old model ages as GRAY et al. (1973) observed for Rb-rich chondrules. In any case, the primitive 87Sr/86Sr ratios of Allende inclusions measured by GRAY et al. (1973) are lower than the initial “Sr/*%r of Angra dos Reis and clearly indicate that some Allende inclusions condensed earlier than the achondrite formed-that is, if the 207Pb/206Pb model age of Angra dos Reis (TATSUMOTO et al., 1973) is accepted, then some of Allende inclusions are older than 4.555 b.y. provided these meteorites condensed from a isotopically homogeneous solar nebula. D. I--Th-Pb

of AI/e&e

D. a. Pb isotopic composition. The Pb whole-rock isotopic composition of Allende reported in this study

626

M. TATWMOTO, D. M. UNRUHand G. A. DESBOROUGH Table 5. U-Th-Pb Sample type

Sample wt mg

"g Pb Contamination

Orgueil Orgueil

whole rock whole rock

404.5 131.2

4.0 2.5

!

Allende

whole rock

4.7

C,P

Sample

1027

Type of= Analysis

Th

Pb isotopic

Rawoata

wb U

concentrations,

Pb

232Th,238U

206Pb/204Pb

207Pb/206Pb

2osPb/206Pb

8.2

28.6

2434

3.62

9.855 9.777

1.0756 1.0802

3.040 __

15.3

62.2

1097

4.20

11.250

1.0159

2.778

34.0

1395

3.70

11.2

43.0

1520

3.85

9.878 10.033 9.986 10.094 9.773

1.0770 1.0665 1.0697 1.0645 1.0814

3.056 2.991

9.5

18.5

62.4

485

3.49

21.3

74.2

1040

3.60

11.166 11.923 11.902

1.0171 0.9914 0.9929

2.789 2.680 __

54.68

0.7022

1.582

50.95

0.7111

__ 2.625

Allende Inclusion Separates Ml I42 MZ M3 R4

matrix matrix matrix matrix matrix

381.9 173.6 98.3 676.3 128.4

1.8 4.0 4.0 4.0 2.4

mg 1 Mag 2 Mag 2

magnetic magnetic magnetic

412.9 167.2 62.7

4.0 3.0 3.0

White

white aggregate white aggregate

362.0

1.8

184.5

1.8

pinkish aggregate pinkish aggregate

122.7

1.7

White Pink Pink

P,C D

112

80.9

1.7

50.9

2.985 _-

546

445

5.04

26.229

0.7884

542

434

11.02

23.568

0.8096

185

(1.691

19.846 19.703

0.8453 0.8536

2.023 __

(1.43)

23.416 22.104

0.8133 0.8295

1.843 __

__

II:

chondrule chondrule

75.4 51.0

1.1 2.2

19.1

(31.3)'/

N2 N2

chondrule chondrule

85.7 51.9

1.1 2.2

17.3

(24.0)

N3 N3

chondrule chondrule

51.6 24.8

1.1 2.2

20.3

(37.6)

169

(1.90)

18.398 18.438

0.8629 0.8735

2.072 _-

N17 N17

chondrule chondrule

135.0 92.5

:::-

67.3

338

484

5.19

36.010 35.870

0.7281 0.7334

1.810 __ 1.743 __

86.1

N18 N18

chandrule chondrule

152.1 99.4

0.7 0.7

106

374

396

3.64

32.243 28.029

0.7561 0.7753

N19 N19

chondrule chondrule

207.6 186.2

0.8 2.0

105

274

399

2.68

51.797 47.920

0.7058 0.7140

1.335 __

N20 N20

chondrule chondrule

54.8 48.2

2.8 1.6

13.9

252

3.60

14.166 13.864

0.9303 0.9510

2.394 __

N3&'

chondrule

25.6

4.0

16.4

224

6.46

16.750

0.8811

__

N40

chondrule

33.5

4.0

14.2

58.8

155

4.27

19.758

0.8399

__

N41

chondrule

51.1

4.0

16.0

63.2

156

4.07

20.543

0.8326

__

48.5 103

* A ‘c’ indicates data from a concentration analysis, a ‘P’ from a composition analysis. t Pb concentration data are also corrected for “*Pb spike ‘contamination’. et al., 1971).l,,,,b $ Decay constants used are; &s, = 0.155125 x lo-’ yr-‘, 12235L, = 0.98485 x 10m9yr -I (JAFFEY 0.049475 x lo-’ yr-’ (LEROUXand GLENDENIN, 1963),and ‘3aU/ 23sU = 137.88 (SHIELDS, 1974). (Table 2) is similar to that determined by HUEY and KOHMAN(1973), who used a volatilization technique, but is more radiogenic than that given by TILTON (1973). Our data for the whole-rock sample of Allende

were obtained from the Allende standard sample pulverized by the U.S. National Museum of Natural History. The leads in C2 chondrites and in the matrix of Allende are very similar, and the difference in lead isotopic composition between TILTON’S(1973) wholerock data and ours may well be due to inhomogeneous distribution of inclusions, provided the Allende standard powder was not contaminated during preparation. Although our lead data (Table 2) are more radiogenic than Tilton’s, the uranium concentration is also higher.. Pb isotopic composition data of the Allende inclusions are shown in Table 5. Leads of Ca-Al-rich white aggregates and chondrule N19 are very radiogenic with 206Pb/204Pb ratios that exceed 50, whereas leads of the matrix and the magnetic separates are less radiogenic and approach the primordial value of Canyon Diablo. Mg-rich chondrules have a lead of intermediate ‘radiogenicity’ with zo6Pb/204Pb ratios ranging from 13 to 26. 206Pb/204Pb and 207Pb/204Pb

=

ratios of the pinkish-white aggregates are about half those of the white aggregates and are similar to those of the most radiogenic Mg-rich chondrules as expected from its U and Pb abundances. However, the *08Pb/ *04Pb of the pinkish-white aggregate separate is quite high-higher than that of N19 and close to that of the white aggregate separate as expected from its high thorium content. Lead in Ca-rich chondrules N17 and N18 is more radiogenic than Mg-rich chondrule leads but not quite as radiogenic as is lead from the white aggregate separate and chondrule N19. The data are plotted on a 23aU/204Pb vs 206Pb/204Pb diagram in Fig. 3. Most data scatter above the 4.55 b.y.-isochron suggesting that the U-Pb system of Allende was disturbed by a later event (or events), as was seen in the RbSr data. The Rb-Sr data of the inclusions scatter below the 4.55 b.y.isochron but most of the U-Pb data are above it apparently indicating that the more mobile elements Rb (parent of Sr) and Pb (daughter of U) moved into the inclusions from the matrix. However, unless U also migrates out of the inclusions, a simple transfer of the matrix Pb into the inclusions cannot cause such

627

Systematics of Allende and Orgueil compositions

and model ages of Orgueil and Allende Ages in yrx103 '

Corrected for Blank and Fractionation' 204Pb/*orPb

2a'Pb/204Pb

2oePb/2P'Pb

zo7Pb/2Q6Pb

2aePb/206Pb

ZSB",'W,ib ZOSpb,'38"

20'?Pb,235"

207Pb/2Q6Pb

LosPb/23*Th

9.845 9.750

10.614 10.557

30.02 __

1.0780 1.0827

3.049 __

0.149 0.148

9.86 8.95

5.78 5.60

4.495 4.497

14.4 __

11.250

11.451

31.35

1.0178

2.787

0.663

8.82

5.57

4.496

10.40

9.873 9.614 9.825 10.090 9.734

10.630 10.592 10.601 10.763 10.553

30.27 29.90 __ 30.23 __

1.0767 1.0793 1.0790 1.0667 1.0841

3.066 3.047

0.329 0.303 0.304 0.333 0.321

6.44 6.33 6.42 7.80 5.45

5.03 4.93 5.02 5.36 4.79

4.494 4.481 4.489 4.506 4.524

9.86 16.49

11.076 11.629 11.673

11.325 11.671 11.689

31.16 31.80 __

1.0225 1.0036 1.0014

2.813 2.734

1.80 0.996 1.00

4.41 7.75 7.82

4.45 5.34 5.34

4.467 4.492 4.483

4.81 10.1 -_

55.88

39.20

88.32

0.7014

1.580

40.16

4.96

4.68

4.557

5.15

52.93

37.45

0.7076

__

38.04

4.92

4.67

4.562

__

26.721

21.026

24.035

lg.287

20,046 20.709

16.939 17.431

24.435 26.415

__

2.996

9% __

0.7869

2.652

12.16

5.73

4.88

4.547

5.44

0.8025

__

10.94

5.50

4.81

4.533

__

40.566 __

0.8449 0.8417

2.024 __

8.33 8.60

5.34 5.44

4.78 4.02

4.552 4.569

(11.7) __

19.750 20.821

44.378 __

0.8083 0.7882

1.816 __

15.46 16.71

4.40 4.54

4.51 4.54

4.567 4.544

(10.40) _-

18.445 18.628

15.951 16.014

38.334 __

0.8648 0.8597

2.078 -_

7.71 7.78

::iZ

4.70

4.540 4.553

(9.551 __

36.440 36.660

26.515 26.484

66.010 __

0.7276 0.7224

i.811 __

15.60 15.70

6.50 6.50

5.05 5.04

4.503 4.488

7.52 _-

32.559 29.633

24.612 22.663

56.769 __

0.7559 0.7647

1.743 __

24.79 24.33

4.26 3.91

4.46 4.33

4.546 4.628

5.35 __

62.561 51.770

37.085 36.591

70.052 __

0.7056 0.7068

1.333 __

36.68 36.13

5.02 5.01

4.69 4.69

4.554 4.554

6.93 -_

13.450 13.389

12.797 12.847

33.360 _-

0.9514 0.9595

2.480 __

2.89 2.88

5.73 5.69

4.86 4.89

4.518 4.568

6.40 __

4.16

6.05

4.96

4.533

"-

6.52

6.66

5.10

4.521

__

7.53

6.35

5.04

4.538

_-

70.86 _”

15.773

14.240

__

0.9029

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17.436

__

0.8263

21.932

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__

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___ __

’ Pb, U, and Th concentrations in samples Nl, N2, and N3 were determined from an aliquot of dissolved sample. It appears that Th did not equilibrate between sample and spike. ’ Pb concentrations and Pb isotopic compositions of samples N34. N40 a&d N41 were determined by interpolating the 2osPb/206Pb ratios from a 2osPb/206 Pb vs 204Pb/*06Pb plot for other Allende separates. of the data from the 4.55 b.y.isochron observed in the 238U/204Pb-206Pb/Zo4Pb plot. We do not know whether the lower U enrichment factor of 13 in the white Ca-Al-rich inclusions (Table 3) was caused by U migration or it simply reflects the lesser refractory nature of U relative to Th and Sr. Preferential transfer of radiogenic Pb isotopes can, however, cause this scatter. Because the matrix (or the magnetic inclusion) is Pb-rich but contains little radiogenic Pb, the matrix may not be the radiogenic Pb donor. If only Pb is mobile and U is not, inclusions such as N18 which contain large amounts of radiogenic Pb may have acted as radiogenie Pb donors. When the Allende data are plotted on a “‘Pb/ ‘04Pb vs zo6Pb/204Pb diagram (not included), the data points scatter widely. This scatter may partly be caused by a later disturbance event but is mostly due to large variations in z3zTh/238U ratios in the discrete chondrules and aggregates, since the “‘Pb/ ‘04Pb vs “‘Pb/ ‘04Pb data form a liner array (Fig. 4). D. b. Internal isochron. The isotopic data are further plotted on a 206Pb/204Pb vs 207Pb/204Pb dialarge misplacements

gram (c+fi plot) in Fig. 4. Each sample has two determinations--+ne from the Pb isotopic composition run and one from the concentration run and are indicated by P and C, respectively, in Fig. 4. When P and C lie close together, which is the case for less radiogenic samples, the P and C notations are eliminated. Chondrules N20, N34, N40, and N41 have relatively large errors (up to 1% in the 207Pb/z06Pb ratio) because of large blank Pb to sample Pb ratios. All other data have errors of <0.2% for the 207Pb/206Pb ratio, which is smaller than the error limits drawn in Fig. 4. A striking linear relationship exists among the chondrules, aggregates, and matrix, except for chondrule N17. Chondrule N17 was half of a chondrule exposed on the broken surface of an Allende specimen and was located a few millimeters from the fused surface. We believe that the U-Pb system of chondrule N17 was severely disturbed by recent impact on the Earth. The slope of the best-fit line for Allende inclusions, excluding N17, was calculated using YORK'S (1969) method and is 0.6188 Ifr 0.0016 which corresponds to an age of 4553 f 4 m.y. This is the first precise Pb-Pb internal isochron obtained by meteorite lead isotope studies. GALE et cd.(1973)

M. TATSUMOTO. D. M. UNRUH and G. A. DESBOROUGH

628

/

.idG IO .knr 0

I 10

I 30

1 20

I 40

23s’J/204pb

Fig. 3. zo6Pb/204Pb vs 238U/ao4Pb plot of Allende inclusions and matrix. Only the Pb data from the concentration analyses are plotted since most samples were aliquoted from powder splits and sample heterogeneity is expected for the composition analyses. The solid line is a 4553 m.y. isochron using Canyon Diablo troilite Pb as the initial 20aPb/204Pb. The broken line is a best fit line through all of the data except the matrix and chondrules N17 and N18. The best fit line yields a slope of 1.143 k 0.027 (a) corresponding to an age of 4914 + 80 m.y., and an initial “‘Pb/ ‘04Pb of 10.58 -+ 0.36 (CT). Uncertainties in the ratios were calculated by quadratic addition of partial differentials of the equations used to calculate the data. The uncertainties are based primarily on; +50x for the Pb and U blanks, *0.03% per mass unit for Pb isotopic fractionation, f 2a, errors obtained from the mass spectrometry, and *0.2x for the zosPb/206Pb and 23sU/235U values in the spike solution. reported preliminary internal PbPb isochrons on Appley Bridge and Parnallee meteorites. These isochron ages of 4790 and 4870 m.y., however, appear suspiciously high. In our data, the single-stage model PbPb ages of individual inclusions (zo7Pb/206Pb)a fall within the interval 4535 m.y. to 4575 m.y., whereas the model ages for the whole meteorite and matrix appear to be somewhat younger-about 4500 m.y. Although the matrix contains only about 5% radiogenie “‘Pb and 3% radiogenic “‘Pb relative to Canyon Diablo troilite the younger 207Pb/206Pb model ages of the matrix are not attributable to errors in blank correction. The whole-rock and magnetic inclusions which also yield younger 207Pb/206Pb model ages contain up to 20”/, radiogenic Pb relative to Canyon Diablo troilite. Texturally speaking, one might expect the chondrules to be older since they appear to have formed and then to have been cemented together with the matrix. Xe-I methods (HERZOG et al., 1973; LEWIS and ANDERS, 1974; LEWLS et al., 1975) however, suggest that both the magnetic component of Orgueil and the chromite in the

Allende matrix contain older components and they are considered to consist of primary condensates from the solar nebula. We must therefore reconcile the apparent young PbPb age of the matrix and magnetic inclusions. Whether we include the matrix or not the internal isochron age does not substantially change. The Allende age of 4553 t_ 4 m.y. is in good agreement with the PbPb age of 4555 + 5 m.y. for Angra dos Reis although the position of one chondrule (N17) below this isochron may reflect a later disturbance. In view of the irregularities displayed by the U-Pb and RbSr data of Allende, it is perhaps surprising that the 207Pb/Z06Pb data are so linear. Angra dos Reis is a well-differentiated augite-rich achondrite but Allende is an agglomerate of chemically unequilibrated materials. Lead is a very mobile element compared to Sr. Possibly lead equilibrated but Sr did not at the time of agglomeration. If this is the case, the PbPb isochron age corresponds to the agglomeration age. Because a PbPb age is not sensitive to very recent U-Pb fractionation and because the Allende inclusions do not yield concordant UPb ages (Fig. 5) it seems likely that the hypothesized time of the U-Pb and Rb-Sr system disturbance is very recent. Another important point of these results is that the extrapolation of the Allende PbPb isochron passes, within uncertainty limits, through the point for Canyon Diablo troilite Pb (Fig. 4). This result appears to support the assumption commonly made when discussing PbPb model ages that iron meteorites and stone meteorites had the same primordial lead isotopic composition. TILTON (1973) also showed that when the correction for in situ U decay lead is made, the initial Pb for the Mezo-Madaras (L3) chondrite agrees with that of Canyon Diablo. This means either ALLiNDE

40

’

-

206Pb/=J*Pb 0

10

20

30

40

60

60

70

Fig. 4. 207Pb/ 204Pb vs 206Pb/204Pb diagram for Allende inclusions. The data, except N17, form an isochron whose slope is 0.6188 + 0.0016 which corresponds to an age of 4.553 + 0.004 b.y. Each sample, except N34, N40, and N41, has two determinations from Pb isotopic composition and concentration runs and are indicated by P and C, respectively. When P and C lie close together, the P and C notation is eliminated. C.D., Canyon Diablo troilite lead; MAT, matrix; MAG, magnetic inclusions. Other sample numbers correspond to those in Table 5.

Systematics of Allende and Orgueil

_

629

ALLENDE

Fig. 5. U-Pb concordia diagram for Allende inclusions. All data are corrected for Canyon Diablo troilite Pb. Aggregates and chondrules (Nl-N20) define a chord which intersects concordia at 4.548 +_0.025 b.y. and 0.105 rt 0.070 b.y. Points for the matrix (M) and the magnetic (MAG) chondrules do not lie on the chord suggesting that the initial lead (if it was equilibrated) was slightly different from that of Canyon Diablo troilite in isotopic composition. WRSTD is the NMNH whole rock standard. Two data points of each sample, one from Pb isotopic composition and U-Pb concentrations and the other from U-Pb concentration run only, are tied.

all meteorites formed and differentiated (or metamorphosed) within a short time interval from the same initial lead as seen in Canyon Diablo, or that our lead isotope abundance measurements are not precise enough to detect the differences. D. c. U-Pb concordia plot. The Allende data corrected for primordial Pb of Canyon Diablo troilite isotopic composition are plotted on a U-Pb concordia diagram (Fig. 5). As is the case for Allende wholerock samples (TATSUMOTO et al., 1973; TILTON, 1973) and Orgueil (this paper) the Allende matrix also contains a large excess of radiogenic Pb. Most inclusions also show excess radiogenic Pb in their U-P\, systems. Pink and white aggregates and chondrules Nl-N20 (except N17) define a chord which intersects concordia at 4.548 Ifr 0.025 b.y. and 0.105 k 0.07 b.y. (YORK,1969). The upper concordia intercept is, within error, the same as the age of the internal isochron of the cc-p plot and may correspond to the age of meteorite agglomeration, assuming Canyon Diablo troilite Pb is the proper initial Pb for the Allende system. Chondrules N34, N40, and N41 deviate from the chord and appear to have slightly younger model Pb-Pb ages. As previously stated these chondrules were small and consequently these data are not of the best quality. Thus, the deviations of these chondrules from the chord may reflect analytical errors. that these and possibly

Chondrule N17 was eliminated from the chord due to the apparent recent disturbance of this chondrule, as previously discussed. The matrix and the magnetic fraction also have slightly younger model PbPb ages and are not included in the chord. This means that either (1) the matrix actually is about 50 m.y. younger than the inclusions, and the Pb isotopic compositions of the matrix and inclusions were not homogenized at the time of agglomeration, (2) the initial Pb isotopic composition of Allende is slightly different from that of Canyon Diablo troilite Pb (assuming Pb isotopic equilibration at the tune of agglomeration), or (3) a complicated (at least 3-stage) post-accretional model is necessary to explain the apparent Pb enrichment and young model age of the Allende matrix. From Fig. 5 it appears that the matrix is enriched in radiogenic Pb. As is the case for Orgueil this enrichment together with a younger model Pb-Pb age cannot be explained by single or simple two-stage models. Any transfer of Pb (either radiogenic of total Pb) from the chondrules to the matrix would result in a model age for the matrix of older than 4.55 b.y. Similarly any addition of Pb from an external source of equivalent age to the Allende parent body would result in an older matrix model Pb-Pb age, as was the case for Orgueil (Fig. la). A three-stage model where (1) Pb was removed

630

M. TATSUMOM, D. M. UNRUHand G. A. DESBOROUGH

from the Allende parent body fairly early in the meteorite’s history (for example, 4.5 b.y. ago; t5 in Fig. lb) and (2) a recent disturbance caused Pb to migrate from the chondrules to the matrix may explain the young apparent age and the radiogenic Pb enrichment in the matrix. This explanation is not entirely satisfactory, however, as is discussed later in this section. GROSSMAN (1973) suggested that refractory U and Th were concentrated in early Ca-Al-rich condensates, whereas volatile Pb was concentrated in later condensates. If this is true, the 23*U/204Pb (p) ratios of the Ca-Al-rich condensates (aggregates and their remelts-chondrules) should approach infinity, and the ,u’s of the last condensates should approach 0. However, GROSSMAN et al. (1975) later observed that feldspathoids exist as discrete crystals in the pinkish CaAl-rich aggregates of Allende. Therefore it may be possible that volatile-rich chondensates also aggregated with earlier refractory-rich condensates. Alternatively, secondary crystal growth on surface coatings of volatile-rich components on refractoryrich condensates during condensation or post-accretional metamorphism could lower the p of the refractory-rich condensates. In either case one would still expect to see higher p’s in the earlier condensates than in the later ones. We observed this trend exactly in p’s of Allende inclusions. The Ca-Al-rich chondrules and aggregates have the highest p’s, about 50, and their lead is very radiogenic compared to those of Mg-rich chondrules. The p of the matrix is only 0.330.4 and lead is even less radiogenic. If the initial p of the solar nebula was small (< 1; CLAYTON, 1963, 1964; CAMERON, 1973b) and the meteorite parent bodies including Canyon Diablo accreted and differentiated in a short time, then the Pb in the solar nebula did not evolve significantly during the short time interval of chondrule accretion and the upper intercept age of 4.548 &-0.025 b.y. is close to the accretion time. However, if the p in the Canyon Diablo parent body was relatively large, the variation of the initial Pb isotopic compositions becomes significant, and the upper intercept has no age significance. As previously discussed for Orgueil, a three-stage model may explain the chord as follows: (1) aggregates, chondrules and a volatile-rich component accreted -4.57 b.y. ago and formed an asteroidal (?) body. (2) This asteroid may have been metamorphosed (or differentiated) by accretion energy or energy from other sources such as 26A1 radioactive decay within a very short time after or during accretion. (3) Asteroidal regolith could have been produced by minor impacts during the first, say, 100 m.y. after accretion. The I-Xe system of some regolith minerals may not have disturbed by minor impacts whereas the U-Pb system was. The volatile-rich component of the regolith may have been more depleted in Pb than the refractory-rich components (R and C, respectively, in Fig. lb) during minor impact events.

R and C agglomerated to form the matrix and inclusions (aggregates and chondrules) of the parent body of Allende. (4) the U-Pb system of this agglomerate was recently disturbed, perhaps during the break-up of the parent body. Radiogenic Pb migrated from the inclusions into the matrix producing excess radiogenie Pb in the matrix (Fig. 1). One would then expect the chondrules to be depleted in radiogenic Pb. However, in most cases the inclusions also show excess Pb (Fig. 5). This may be a result of impure physical separations in the laboratory. If most of the chondrules and aggregates contain significant matrix material then the chord in Fig. 5 (as well as the x-p plot in Fig. 4) may represent a mixing line, and neither intercept would have any direct age significance. The upper intercept would, however, reflect an age somewhere in between the accretion age and the age of regolith formation, and the lower intercept would be younger than the actual date of the recent event. If we draw a best fit line in Fig. 5 that includes all of the Allende data, the upper intercept is still about 4.55 b.y. but the lower intercept becomes negative. The cosmic ray exposure age (FIREMAN and GOEBEL,1970; JEFFERYand ANDERS,1970) of Allende, which is thought to be the age of the break-up of the parent body, is about 45 m.y., thus, a mixing line intersecting concordia slightly below this time or even below zero would fit the model. The 107 m.y. lower intercept (Fig. 5) would merely reflect biased selection of data for the chord which were least disturbed during regolith formation, and the lower intercept would again have no significance. Although the three-stage model appears to fit the U-Pb concordia data, it cannot satisfactorily explain the RbSr data (Fig. 2) or the 206Pb/204Pb vs 238U/Z04Pb data (Fig. 3). If a very recent disturbance caused volatile Pb to migrate from the inclusions into the matrix, one would also expect Rb to migrate from the inclusions to the matrix, if it moved at all. The inclusions would thus appear to be depleted in Rb and the matrix would appear enriched in Rb. One would also expect the Pb enrichment in the matrix relative to the chondrules to be reflected on a “‘Pb/ ‘04Pb vs 238U/204Pb plot (Fig. 3). Just the opposite is true. From Figs. 2 and 3 it is apparent that the matrix is depleted in Rb and Pb relative to the chondrules (assuming homogenization of RbSr and U-Pb isotopes at the time of accretion). We must, therefore, seek an alternative explanation for the U-Pb Allende data. The most logical alternative to explain the U/Pb data is to assume that the Pb isotopic composition of Canyon Diablo troilite is not the proper initial Pb for carbonaceous chondrites. As a first approximation, let us assume that the single-stage initial Pb isotopic composition of Orgueil (206Pb/204Pb = 9.595; zo7Pb/204Pb = 10.453; Section B, this paper) is the proper initial Pb in Allende. The Allende data corrected for this initial Pb are plotted on a U-Pb concordia diagram in Fig. 6. A best fit line (YORK, 1969)

Systematics of Allende and Orgueil

Fig. 6. U-Pb concordia diagram for Allende inclusions and matrix. Data are corrected for the single-stage initial Pb isotopic composition of Orgueil. The data define a chord (solid line) that intersects concordia at 4.553 +_0.07 b.y. and -0.13 + 0.15 b.y. The negative lower intercept and the old model age for M4 suggest that the data may be over-corrected. all of the Allende points now yields intercepts of 4.553 +_0.070 b.y. and -0.13 + 0.15 b.y. (2a, solid line, Fig. 6). Because the lower intercept is below 0, and because matrix point M4 has an unreasonably old model Pb-Pb age (-4.75 b.y.), we feel that the data may be slightly over corrected. The importance of this diagram, however, is in the relative positions of the matrix and inclusions, and not in the intercepts. The matrix data (except M3) now plot below concordia, indicating Pb depletion in the matrix during a very recent event. Most inclusions still show Pb enrichment as they did when corrected for Canyon Diablo type Pb. Chondrules N34, N40, and N41 now appear to fit the chord, where they did not in Fig. 5. The magnetic separates still remain anomalously young. With the different initial Pb, a two-(or three-)stage model now fits both the Rb-Sr and U-Pb data of Allende. At a very recent time, possibly the time of break up of the parent body, both Pb and Rb migrated episodically from the matrix into the inclusions, thus producing enrichment of Rb (Fig. 4) and Pb (Figs. 3 and 5) in most of the inclusions relative to the matrix. The Allende whole-rock appears still to be enriched in Pb and slightly enriched in Rb. This may simply be due to a heterogeneous distribution of inclusions in the whole-meteorite. The wholerock split analyzed in our laboratory may have had a greater abundance of inclusions than in the meteorthrough

631

ite as a whole. As previously discussed for Fig. 5, if a three-stage model is correct, the chord in Fig. 6 would have no real age significance. The upper intercept, however, would represent an intermediate age between accretion and regolith formation or agglomeration. D. d. Th-Pb of Allende. Most calculated 208Pb/232Th model ages for Orgueil and Allende whole-rocks and Allende inclusions (except pinkish white aggregates) are larger than 206Pb/238U model ages (Table 5). Although some of the Th concentration values are not of good quality, as stated previously, this general trend indicates that’ the initial 208Pb/204Pb ratio in the carbonaceous chondrites may be higher than that of Canyon Diablo trolite Pb. As stated previously, the Allende chondrules and aggregates which have high ThjU ratios also have significantly high 208Pb/204Pb ratios. The Allende data plotted on a 208Pb/204Pb vs 206Pb/204Pb diagram do not yield any regression line. This is probably due, as discussed above, to Th appearing more refractory than U in early condensates. Consequently Th/U ratios are quite large in the early condensates, while, Th and U both behave equally refractory in the later condensates. CONCLUSIONS (1) The carbonaceous chondrite, Orgueil, contains less U and Th than C2 and C3 carbonaceous chondrites, but more Pb. The Pb isotopic composition of Orgueil is much less radiogenic than that of C2 and C3 chondrites. The initial Pb corrected for in situ U-decay is more radiogenic (206Pb/204Pb = 9.595, 207Pb/204Pb = 10.453) than that of Canyon Diablo troilite. (2) RbSr data of Allende inclusions did not define an isochron, and they indicate that the Rb-Sr system was disturbed by a recent event. The lowest measured 87Sr/*6Sr ratio in our Allende inclusions is similar to that of Angra dos Reis. (3) The data of Allende inclusions plotted on an cc-p plot define an isochron which yields a 4.553 + 0.004 b.y. age. If lead in Allende inclusions reached isotopic equilibrium at the time of agglomeration, then the internal isochron age corresponds to the time of agglomeration. (4) U-Pb data of Allende inclusions corrected for initial Pb of Canyon Diablo troilite and plotted on a U-Pb concordia diagram define a chord which intersects concordia at 4.548 + 0.025 b.y. and 0.107 f 0.070 b.y. It appears that the upper intercept age corresponds to the agglomeration time and the lower intercept corresponds to a recent event which may also be responsible for the disturbance of the RbSr system. (5) The model presented above does not account for the apparent excess radiogenic Pb in combination with young model ages for Orgueil and the Allende

632

M. TATSUMOTO, D. M. UNRUHand G. A. DESBOROUGH

matrix, and cosmic ray exposure ages are not compatible with the lower concordia intercept. We are therefore forced to conclude that the initial Pb isotopic composition of these carbonaceous chondrites is slightly different from that of Canyon Diablo troilite, provided the samples are not contaminated with terrestrial Pb. We previously reported (TATSUMOTOet al., 1973) that the *“Pb/ ‘06Pb model age of Angra dos Reis referred to Canyon Diablo troilite is 4.555 b.y. The Pb in Angra dos Reis is very radiogenic (e.g. 206Pb/ ‘04Pb = 214) so that the small difference in the initial Pb isotopic values discussed in this paper does not significantly affect the model age of Angra dos Reis. It is remarkable that the PbPb internal isochron age of Allende is identical to the Pb model age of Angra dos Reis within experimental error. A PbPb internal isochron age is the age of the last Pb isotopic equilibration and the age obtained for Allende probably corresponds to the time of agglomeration of the Allende parent body. Angra dos Reis is a augite achondrite and is considered to be a welldifferentiated product of an astroidal body. Thus, the similarity between the Allende Pb-Pb internal isochron age and the Angra dos Reis model Pb age may indicate that formation of these meteorite parent bodies was almost contemporaneous, and subsequent global differentiation of the Angra dos Reis meteorite parent body occurred in a short time in the very early history of the solar system. PODOSEK (1970) reported that the total spread in the Xe retention ages for several meteorites of recrystallized H and L groups, carbonaceous chondrites and an iron meteorite is 14 m.y. Lead evolution over this short duration is small enough such that the ages obtained for meteorites that contain radiogenic lead, such as achondrites (TATSUMOTOet al., 1973), are still valid. On the other hand, Wasserburg’s group (e.g. GRAY et al., 1973) suggested a larger spread, on the order of 100 m.y., based on the initial s7Sr/86Sr values of meteorites, under the assumption that the Rb/Sr ratio in the solar nebula did not significantly change from the solar value. However, if the Rb/Sr ratio fractionated significantly (2 30 times) in a meteorite parent body from that of the solar nebula during early condensation, then the estimated spread in meteorite ages (probably initial metamorphic ages) estimated from the initial “Sr/%r will decrease to the order of 5 10 m.y. Our conclusion, reached in the previous discussion, that the initial Pb isotopic composition in the carbonaceous chondrites probably is slightly different from Canyon Diablo troilite Pb is consistent with those results obtained for oxygen (CLAYTON et al., 1973) and magnesium (GRAY and COMPSTON, 1974; LEE and PAPANASTASSIOU,1974) isotope anomalies. In our discussion we assumed the 238U/235U ratio of Allende is the same with the terrestrial value, however, it is possible that a U isotope anomaly also exists in Allende inclusions.

Note added in proof: CHEN and TILTON have recently presented their U-Th-Pb investigation of Allende at the 37th Annual Meeting of the Meteoritical Society (Abstract; Meteoritics 9, p. 325, 1974). Their results are very similar to ours presented at the 36th Annual Meeting of the same society (Abstract; Meteoritics 8, p. 446, 1973) and in this paper. However, the slope of their 207Pb/206Pb isochron was 0.6241 * 0.0015 and yielded an age of 4.565 I~I0.004 x lo9 yr which is slightly older than our isochron age of 4.553 f 0.004 x lo9 yr. Furthertheir ages are concordia intercept more, 4.56 _t 0.02 x lo9 and 0.37 f 0.07 x lo9 yr instead of our 4.548 k 0.025 x lo9 and 0.105 f 0.070 x lo9 yr. Possible sources for these discrepancies, in part, are blank correction and laboratory biases such as correction in isotope ratio mass fractionation, measurements, since Chen and Tilton used a Faraday cup and an electron multiplier but we used only a Faraday cup. These discrepancies, although small, also appeared in two previous papers (TILTON, 1973; TATSUMOTOet al.. 1973).

Acknowledgements-We benefited from the manuscript reviews of Professors EDWARDANDERS,University of Chicago; CLAUDEALLEGRE,University of Paris; GEORGETILTON,University of California; and Drs. P. D. NUNES and W. P. LEEMAN,U.S. Geological Survey. We also benefited from discussions of iron meteorite classification with Professors JOHN WASSONand GEORGEWETHERILL,University of California, Los Angeles. We are grateful for generous samples supplied by Professor ELBERTKING, University of Houston, for Allende; and by Professor EDWARD ANDERSfor Orgueil. We thank R. J. KNIGHTfor invaluable laboratory assistance in Sr isotope measurements and P. A. REEDfor Allende inclusion separations. REFERENCES ANDERSE. (1972) Conditions in the early solar system, as inferred from meteorites. In Nobel Symposium no. 21, From Plasma to Planet, (editor A. Elvius), pp. 133-156. Almqvist & Widsell. ARRHENIUSG. and ALFV~~N H. (1971) Fractionation and condensation in space. Earth .Planet. Sci. Left. 10, 253-267. CAMERONA. G. W. (1968) A new table of abundances of the elements in the solar system. In Origin and Distribution of the Elements, (editor L. H. Ahrens), pp. 125-143. Pergamon Press. CAMERONA. G. W. (1973a) Are large time differences in meteorite formation real? Nature 246, 3&32. CAMERONA. G. W. (1973b) Abundances of the elements in the solar system. Space Sci. Ret). 15, 121. CLARKER. S., JR., JAROSEWICHE., MASONB., NELEN J., GOMEZM. and HYDE J. R. (1970) The Allende, Mexico meteorite shower. Smithson. Contrib. Earth Sri. 5. CLAYTOND. D. (1963) A calculation of the abundances of uranium and thorium from the primordial PbZo6/PbZo7 ratio. J. Geophys. Res. 68, 3715. CLAYTOND. D. (1964) Cosmoradiogenic chronologies of nucleosynthesis. Astrophys. J. 139, 637. CLAYTONR. N., GROSSMANL. and MAYEDAT. K. (1973) A component of primitive nuclear composition in carbonaceous meteorites. Science 182, 485-488. FIREMANE. L. and GOEBELR. (1970) Argon 37 and argon 39 in recently fallen meteorites and cosmic ray variations. J. Geophys. Res. 75, 2115-2124.

Systematics of Allende and Orgueil FUCHS L. H. (1969) Occurrence

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