40Ar39Ar ages of Allende

ICARUS 42, 380-405 (19801

4°Ar-~gAr Ages of Allende E L M A R K. J E S S B E R G E R , B O G N A D O M I N I K , T H O M A S S T A U D A C H E R , AND G R E G O R Y F. H E R Z O G I Max-Planck-lnstitut ftir Kernphysik, P. O. Box I0 39 80, Heidelberg,. Germany Received February 20, 1979: revised February I. 1980 K - A r a n d / o r 4°Ar-3"qAr plateau ages of Allende s a m p l e s - - w h o l e rock, matrix, chondrules, white inclusions--range from 3.8 AE for matrix to ~5 AE for some white inclusions, but cluster strongly near 4.53 AE. This age marks the dominant K - A r resetting of Allende materials. Age spectra show disturbances due to ~ A r recoil or some other argon redistribution processes. Possible explanations for the apparent presolar ages (>4.6 AE) include: ~>20% loss of~gAr: ;~40% loss of~°K ~3.8 AE ago with no loss of ~"Ar; trapped argon of unique 4°Ar/a6Ar isotopic composition; admixture of "'very old" presolar grains.

One attractive feature of stepwise "~Ar:~gAr dating is that one can often separate the argon c o m p o n e n t s present in different phases on the basis of their activation energies. Lunar basalts, for example, show 4°Ar losses at low t e m p e r a t u r e s and from the magnitude of these losses and the diffusion p a r a m e t e r s o f 4"Ar, it has been possible to estimate the t e m p e r a t u r e history of such basalts (Turner, 19711. The average maximum t e m p e r a t u r e of ejecta and fallback blankets of a terrestrial crater has been inferred in a similar way (Jessberger et al., 1978: Jessberger and Staudacher, 1979). A second virtue is that with sufficient t e m p e r a t u r e resolution, stepwise heating can yield ages for minerals or phases included within a sample, but which are not easily separated for direct study. The first successful study of this kind was the dating of minute, relict, 4.47-AE-old plagioclase minerals in lunar highland breccia 65015 in which most of the plagioclase was - 4 . 0 AE old (Jessberger et al., 1974b). Similar results were obtained for Apollo breccias 67435 (Dominik and Jessberger, 19781 and 73215 (Jessberger et al., 19761 and were later confirmed by spotwise laser extraction (Eichhorn et al., 1978). In view of the evidence for late disturbances of Allende (Gray et al., 19731 and widespread speculation concerning possible extrasolar grains

INTRODUCTION

The Ailende C3V meteorite is a chondrite of unusual significance for it contains a variety of inclusions with extraordinary mineralogy and chemical and isotopic composition. The determination of ages for individual c o m p o n e n t s of this meteorite helps to establish a chronological framework for its formation and to constrain discussions of its origin. Studies to date have concluded that Allende formed about the same time as many other meteorites (Kirsten, 1978), i.e., - 4 . 5 5 AE ago, and that it has been disturbed at least once since then (Fireman et al., 1970; Podosek and Lewis, 1972; G r a y et al., 1973; Wetherill et al.. 1973; Chen and Tilton, 1976: Tatsumoto et al., 19761. As no full report of the ~°Ar-:~'~Ar ages of Allende has yet appeared and as different dating techniques may yield c o m p l e m e n t a r y information, we set out to measure the 4"Ar-:~"Ar ages of a representative suite of samples separated from AIlende. Results for one mineralogically unusual inclusion (Dominik et al., 1978) and for two inclusions with ages >4.5 AE (Jessberger and Dominik, 19791 have appeared elsewhere. k Present address: l~:partment of Chemistry, Rutgers University, New Brunswick, N.J. 08903. 380 0019- 1035/80/060380-26502.00/0 Copyright ~ 1980by Academic Ih-ess.Inc, All nghts of reproduction m any form reserved

4°Ar-39Ar AGES OF ALLENDE in Allende (Clayton et al., 1973; Clayton, 1975; Manuel and Sabu, 1975) it seemed probable that one or both of these capabilities of stepwise 4°Ar-39Ar dating might be turned to good account.

- - T h e black, zoned object is a metal-bearing chondrule significant amounts o f opaque seminated in forsterite and in ene.

381 (sample 8) containing phases disclinopyrox-

Fine-Grained Aggregates

SAMPLE DESCRIPTION The mineral composition and chemistry of the samples were determined using standard techniques: X-ray diffractometry, optical microscopy, SEM, and electron microprobe. The mineralogy of the fine-grained white inclusions was estimated from X-ray and SEM analysis only. All samples are numbered sequentially (cf. Table II). Our " w h o l e - r o c k " sample (sample 2) consisted of a 500-mg fragment chiseled from an 80-g piece of Allende, IBI. The sample was ground to - 5 0 / x m in an agate mortar and a 100-mg aliquot was irradiated without further treatment. In view of Allende's heterogeneity on a centimeter scale, this sample may not be truly representative of the whole rock. Three 3-mm slabs were sawed from IBI using a dry, diamond-impregnated wire. From these three slabs we collected the various samples. The " m a t r i x " sample (sample 1) comprised the residue from the powdered slabs after all visible inclusions and chondrules had been removed. A small but unknown amount of contamination with fragments of chondrules and inclusions undoubtedly remained. Chondrules

- - C h o n d r u l e s with isolated monosomatic olivine crystals (sample 3). These have tiny anorthitic glass inclusions which are similar in composition to those in porphyritic chondrules (McSween, 1977). - - B a r r e d chondrules (samples 4 and 5) are composed of olivine and Ca-Ai-rich glass. Pyroxene is usually absent. - - G r a n u l a r chondrules (samples 6 and 7) consist of forsterite, clinoenstatite, and C a Al-rich glass.

The three large (up to 5 ram) aphanitic white inclusions selected for study show a mineral composition typical of fine-grained aggregates (Wark and Lovering, 1977) previously described by Clarke et al. (1970) as type C. Two of them, samples 9 and 10, are oval; the third sample, II, is fluffy and irregular. X-Ray diffraction patterns of these inclusions show the presence of clinopyroxenes (hedenbergite and probably Tpx), sodalite, and nepheline; weak lines indicate a subordinate amount of forsterite and possibly melilite and spinel. SEM observation shows that the inclusions consist of individual angular or subrounded amoeboid aggregates from 1 to 12/xm. The larger ones, 7-12 p.m, are embedded in a very fine-grained matrix (1-3 Izm). The absence of contact rims between these aggregates is characteristic. They are rather loosely packed and isolated. The voids between the individual grains are up to 2 /xm in size. Some aggregates seem to contain only one mineral (Sodalite, nepheline, or hedenbergite), while others have nep.heline with acicular intergrowth of sodalite. Some amoeboid aggregates of sodalite crystals are found surrounded by nepheline. Wark and Lovering (1977) suggest that such compositions and sequences are identical to those observed in the rims o f both type A and type B coarse-grained inclusions. Coarse-Grained White Inclusions

The largest coarse-grained inclusion ( - 5 mm) was extracted from a slab using a dental drill; smaller inclusions ( - 1 mm) were handpicked under a binocular microscope after gentle grinding o f each slab in an agate mortar. Dust and matrix material adhering to the inclusions were removed as carefully as possible by means of tweezers,

382

JESSBERGER ET AL.

a dental drill, and ultrasonic washing in reagent-grade ethanol. In this manner a collection, sample 12, of five coarsegrained inclusions with a total weight of 108 mg was prepared. A small chip from the largest inclusion was selected for mineralogical and chemical analysis. The presence of clinopyroxene, spinel, melilite, anorthite, and traces of perovskite was established by microscopic examination and by X-ray diffraction m e a s u r e m e n t s . This mineral assemblage and the coarse crystalline structure are typical of type B Ca-rich inclusions (Grossman, 1975). Pyroxene forms highly fractured, prismatic or irregular crystals up to 250 p.m and smaller grains ( - 3 0 /xm) e m b e d d e d in melilite and anorthite: equal-sized melilite and anorthite occur as irregular or lath-shaped crystals. Spinel is present as small ( 5 4 0 / z m ) sub- or euhedral grains poikilitically enclosed in pyroxene or rarely in melilite and anorthite. Pentlandite and (Fe. Ni) were o b s e r v e d as minute isolated grains dispersed throughout the inclusion. In this sample, El Goresy et a/. (1978) detected refractory siderophile metals enclosed in silicates and spinel. The chemical composition (Table 1) of the main phases of our inclusion is consistent with that previously reported by others and summarized by G r o s s m a n (1975). The TABLE I AVERAGED ELECTRON MI(ROPROBI~ ANALYSIS o! MINERALS IN ONI: OF I H E COARSE-GRAINI..D C a - A I - R I ( H INCI.USIONS OF SAMPI.E 12

Aluminous titanopyroxene

Anorthitc

Melilitc

MnO FeO TiO., K~O CaO Si()e

0.01 {).1)3 1l.g8 0.05 23.96 33.16

0.01 ().06 0.03 0.07 2(). 19 43.26

0.03 0. I I 0.02 0.08 40.72 25.20

Na~O AI~O:~

0.03 36.10

0.03 31.09

MgO

0.(X) 22.62 6.67

0.06

1.66

Total

98.67

99.81

98.94

p y r o x e n e is of the AI-Ti-rich and Fe-poor variety typical of type B inclusions. The average TiO., (!1.8%) and AI.,O:~ (22.6%) contents are s o m e w h a t higher than those considered normal for type B, i.e., 6 - 8 and 15.1-21.4%, respectively. Accordingly, the MgO (6.7%) and SiO., (32.2%) contents are relatively lower. In all analyzed points, the FeO content is less than 0.05%. The plagioclase, which has nearly constant chemical composition, is almost pure anorthite. The Na.,O content is always less than 0.5%, and in some grains was not detectable (<0.01%). Melilite is rich in gehlenite and poor in akermanite. An examination of five grains revealed variations in the contents of MgO (0.99-2.52%), SiO,, (23.80-27.29%), and AI.,O.~ (29.24-33.88%). Average melilite is G%7.TAk12.:I. The mole percentage gehlenite in melilite varies from 82.2 to 92.8%. Sample 13 is a 5-mg sample from a small individual white, irregularly shaped inclusion. X-Ray diffraction patterns indicate a mineral composition typical of type B inclusions (Grossman, 1975). Samples 14-16 which were taken from a mineralogically unusual inclusion have been described by Dominik et al. (1978), samples 17 and 18 by Jessberger and Dominik (1979). Samples 19-23 were obtained from the National Museum of Natural History, Washington, D.C., through the courtesy of B. Mason and are described by Mason and Martin (1977). Their identification numbers are given in Table II. FXPERIMENTAL PROCEDURES AND DATA REDUCTION The samples were stacked in quartz ampoules which were then evacuated and irradiated in Mol/Belgium. Each ampoule contained three hornblende monitors (Schaeffer and Schaeffer, 1977) and one CaF2 monitor. After a 30-day cooling period the samples were loaded in the extraction system of the mass spectrometer. Stepwise degassing was peformed in a resistor-heated metal furnace (Staudacher et

4~Ar-:~'~Ar AGES OF ALLENDE al., 1978). This furnace has two separate e v a c u a t e d sections, an inner c h a m b e r for the crucible and sample gas and an outer one for the heating element and t h e r m o c o u ple. This construction facilitates repairs and lowers blanks. The extraction time was ~45 m i n / s t e p ; the furnace was not cooled between t e m p e r a t u r e steps. During extraction the gas underwent continuous, preliminary purification o v e r a hot Ti getter and two S A E S getters. During the last 10 min of each extraction step the Ar was transferred to the final cleanup line by adsorption on activated charcoal kept at -192°C. The s p e c t r o m e t e r background was monitored for 10 min prior to gas inlet. The background signal was always < 1 0 ~0 cm:~ S T P for 4~'Ar and was negligible for the other isotopes. After equilibration o f the gas in the spectrometer, 20 or more spectra were obtained in the m a s s / c h a r g e range 36 to 41 by computer-controlled magnetic field switching. The data were stored on magnetic tape for later detailed analysis. Before and after sample m e a s u r e m e n t s were carried out, a metered amount of atmospheric argon was admitted to the s p e c t r o m e t e r for to the s p e c t r o m e t e r for calibration. In reducing the data, we first eliminated doubtful, e x t r e m e values and then carried out a linear least-squares extrapolation of the measured isotopic ratios and a logarithmic least-squares extrapolation of the 4"Ar signals to gas inlet time. Additional corrections were made by following the list below: (1) mass discrimination (0.4 _+ 0.2)% /amu: (2) mass s p e c t r o m e t e r background: (3) d e c a y of :~gAr (negligible) and :~rAr during and alter the irradiation; (4) interfering nuclear side reactions on Ca, K, and CI (most of the needed factors were determined from the Ar composition of the CaF., and hornblende samples which were included in the ampoules): (5) flux gradient along the ampoules ( < - l ' ~ / c m along the 3-cm length); (6) blank (temperature in °C, 4"Ar in

383

10 -~° cm 3 STP) 1200, 6; 1300, 9; 1450, 18; 1550, 60; below 1450°C, the argon was atmospheric; an uncertainty o f 50% has been assigned to the blank. The data at this stage of reduction are compiled in the Appendix. The presence of chlorine in the samples prevented the correction of 4°Ar for trapped and spallogenic contributions. These corrections are thought to be small and normally would affect the K-derived 4°Ar/:~'~Ar ratios by less than 0.2%. The 4°Ar/'~:JAr ratios are converted to apparent ages using the J-values given for each sample in the Table AI and the 4°K d e c a y p a r a m e t e r s and K isotopic composition as r e c o m m e n d e d in Steiger and JS.ger (1977).

:~"Ar, :~SAr, a n d CI c o n c e n t r a t i o n s

:"~Ar and :~Ar of irradiated Allende comprise four c o m p o n e n t s in addition to possible atmospheric contamination: trapped :~Ar.t and :~Ar.r: spallogenic :~"Ar~ and 3SArc: reactor-produced ~8Ar~.~: and neutron-induced :~"Arc.~ (Smith et al., 1977). The spallogenic contribution can be estimated from (a) the known cosmic-ray exposure age of Allende, 5 my (Bogard et al.. 1971): (b) the :~"Arc production rate from Cressy and Bogard (1976) using K and Ca concentrations measured via :~'Ar and :~TAr, respectively (Table I1), and the Fe concentrations for c o m p a r a b l e samples from the literature (Clarke et al., 1970; G r o s s m a n , 1975: Mason and Martin, 1977): and (c) the conventional ratio :~Arc/:~Arc = 0.65. The Cl-derived c o m p o n e n t s are proportionally largest in the sample (15) with the lowest measured :";Ar/:~Ar ratio (= 0.019) and hence the lowest trapped c o m p o n e n t . After correction for spallogenic contributions we obtain (:3';Ar/a~Ar)c~ = 0.0148: this value is taken as the appropriate ratio for Cl-derived c o m p o n e n t s for the irradiation conditions of that sample. For other samples, adjustments must be made according to the value of J for the irradiations. The remaining c o m p o n e n t s can be resolved with the

384

JESSBERGER

ET AL.

T A B L E 11 Sample number

Weight (mg)

K (ppm)

Ca (%)

CI (ppm)

~OAr_agAr

K - A r age (AE) Age (AE)

Range (e/( a.,Ar ) -2-58

55.7 109.4

200 280

1.3 1.6

220 240

3.80 ± 0.09 4.43 ± 0.09

-4.57 ± 0.03

Chondrules 3 Monosomatic 4 Barred 5 Fine barred 6 Granular 7 Granular 8 Black

20.8 28.2 13.6 97.4 19.2 104.2

305 570 250 825 515 415

0.8 1.2 1.8 1.6 2.0 2.0

210 595 290 610 970 430

4.62 4.63 4.26 4.53 4.51 4.56

± ± + ± ± +

0.07 0.07 0.16 0.06 0.07 0.03

--4.56 ± 0.05 ----

White 9 10 11 12 13 14 15 16 17

88.3 85.9 81.2 108.4 5.0 26.0 5.6 10.7 15.1

840 2450 600 2(X) 670 240 450 480 155

1.4 6.9 6.4 16.0 17.5 2.2 6.2 7.8 11.9

560 1790 1250 620 151) 2810 4770 3430 170

4.52 4.33 4.44 4.82 4.49 4.20 4.40 4.23 5.12

-+ + -+ ± ± ± ± -~ =

0.07 0.08 0.07 0.06 0.03 0.13 0.11 0.10 0.03

-4.47 +- 0.01 4.55 ± 0.03 -4.50 ± 0.(12 4.52 +_ 0.02 4.53 ± 0.02 4.48 z 0.02 4.98 -- 0.06 5.43 -+ 0.04 4.92 _+ 0.03 5.37 ± 0.10 4.68 '- 0.12 5.27 _, 0.18 4.62 _+ 0.10 --

I 2

Matrix Whole rock

inclusions Fine Fine Iquffy Comp. of coarse Co arse Gray core" White core" White rim" Coarse n

18

Coarse n

40.9

70

10.6

50(1

5.08 ~ 0.08

19 20 21 23

Coarse" Co arse" Coarse" Coarse"

9.8 5.0 9.5 6.2

60 60 55 110

21.1 18.7 19.8 18.3

50 180 80 910

4.92 5.54 5.26 4.57

* 0. I 1 _+ 0.09 + 0.10 ± 0.41

-37-97 ---

31-91 0-80 -5-100 7-55 12-67 14-58 3-46 47-96 5-64 65-100 58-99" 64-100" 54-99" --

N o t e . S u m m a r y of the results of U'Ar-a~'Ar studies of whole rock, matrix, chondrules, and white inclusions samples of the Allende meteorite. Errors of K- and Ca c o n c e n t r a t i o n s are - 10'~. Those of the CI Concentrations are e s t i m a t e d to be < 3 0 % except for samp l e s I, 2, 13, 17, 18. 19-21 which may have no CI since (:~SAr/a'Arj,,,,,,,,,r~,,~ -. 0.65. The errors of the total K - A r ages include the weighted m e a n - s q u a r e variation around the average, the n-flux variation along the a m p o u l e s (*-1.4C~1. and the unc e rt a i nt y in ( ' " A r / K ) of the mo nitor (Schaeffer and Schaeffer, 19771. The errors of the plateau ages include the meansquare variation around the plateau value, the n-flux uncertainty (-0.5C~), and the error of the me a s ure d {4°Ar/:'"Ar) ratios of the monitors. " Dominik et al. ( 19781. r, Jessberg er and Dominik ( 19791. ' From high-temperature ('-900°C) correlation in the three-isotope diagram (U'Ar/:~'Arl vs (arAr/:"~Ar) (Fig.

5). a Samples taken from inclusions 4691 (No. 19). 3529,33 (No. 20). 3529,29 (No. 21). and 3529,26 (No. 231 of Mason and Martin ( 19771.

The

equations

conversion

(cm a STP/g) aSArc~

(a6Ar/aSAr)T+c I -

aSAr.,,+c~

(aGAr/aSAr)ct -

(a6Ar/aSAr)T (a~Ar/aSArh,

and

argon

analysis

of

been

C38

=

obtained

n-irradiated

glass (Pyrex,

Code 7740; 750 ppm

Su,

communication)).

dates (aSAr/aSAr)T = 5.35.

factor

has

dix.

private

of the glass The

resulting

are given C!

3SAr(.i/CI from

the

CI-bearing CI (Y.-S. The

in the

argon Appen-

concentrations

of

4°Ar-:~"Ar AGES OF ALLENDE

385

0

0

~-2

-2~ NO.I

-3

No.2

5

...,.- .,,L •

_

_

.

,

No. 4

--

MAIRIX

_,! ,.._----_._J

_ . .

WHOLE R O C K

i

BARRED OLIVINE

MONOSOM. CHONDRULE

.5 1 .5 FRACTION 39Ar N RELEASED

"

-3

4v

.5

0

No.3 _

2

.5

!

0

~-2 NO.5

-3 <

No 7

NO.8

-3

Lt

S

5~ < 4~

4 GRANULAR OLIVINE

GRANULAR OLIVINE

BLACK CHONDRULE

.'5

.5

.5

2 .5

1

2

FRACTION 39Ar ~ RELEASED

FIG. 1. Apparent ages and K/Ca ratios of matrix, whole rock, and chondrule samples from Allende versus the fraction',d :mAr release. Sample numbers refer to Table II.

Allende materials are given in Table !I. Lower limits of CI concentrations may be estimated as in Jessberger et al. (1974a).

almost perfect plateau, sample 5 with a high-temperature plateau, sample 10 with a well-defined intermediate-temperature plateau, and samples 2 and I I with low- to intermediate-temperature plateaus. The other spectra are disturbed and we are unable to interpret them in detail within the framework of the present understanding of the 4°Ar-:mAr technique. For all but three of these samples we have simply calculated K - A t ages from the integrated amounts of 4°Ar and 39Ar. Samples 19, 20, and 21 are

4°Ar-3"°Ar Results

Figures 1-4 show apparent age and K / C a spectra of the present study. Table II summarizes the results and also includes those published previously. From inspection of Figs. i - 4 it is evident that only a few Allende samples reported here show simple age patterns. These are sample 13 with an

0

I

g -2

t

'r No. 9

NO.10

~ No. 11

-3

v

"- .

4

FINE

3 2

GRAINED INCLUSION .5

-

WHITE INCLUSION ' i FINE-GRAINED ~ .5

o

1

"

N:~.12 . .

,

-2

.

3

I

WHITE FLUFFY INCLUSION .5

.

4 v

I COARSE-GRAINED i' INCLUSIONSCG1 1

3 ~ 2 <

.5

FRACTION 39Ar~ RELEASED

F r o . 2. Apparent age and K/Ca patterns of fine-grained inclusion samples (samples 9 - I I) and of a composite sample of several coarse-grained inclusions (sample 12) from A l l e n d e .

386

JESSBERGER ET AL. I ~0 -I

~_~

t

~

~ 1

;

I

I

I

I

I

T

I

I

1

N o . 13

10 .2

300

~: .0_3 I i

z..5

-

?-1_ !.~

AUendeo,/21::w20h',ncluslons te#

~.\ \

43 <

1

"#19

i

200 <

.

coarse - 9ramec

P~ldsion

<

100 0 FRACTION

39Ar*

the exceptions. They have the highest C a / K ratios (Table II) and hence the largest relative correction ( ~ 5 0 % ) for the undesired side reaction r ' C a ( n , a P " A r . These corrections introduce large errors. To obtain ages for these samples we resort to a three-isotope plot of n~Ar/S"Ar or :~rAr/39Ar before the correction for the side reaction (Fig. 5). Above an ~900°C release temperature. the data are well correlated (correlation factors >0.98). The x-intercepts correspond to the :3TAr/:~:~Ar ratio as determined from K-free CaF.,, the y-intercepts to the 4°Ar~"Ar ages as listed in Table II. In comparing the age patterns it is rei

........

i

z~q = ""

lo-' t -

"~ 6.0 <

<

-

~-F"~"

I

i

|

_# ?o

#21L k--

i: .....

#23

~ [: .... •

Ct 19

t_

J

- ~ ~.A- _ ~.; ~ . _ ~

Allende white inclusmns= ~#23

i # 2 O

-..

CaF2

4.0 2b Frachonal

39Ar

Or[ ~ 0

I L 500

i

L

i

I

I

1000

i'r,'i~

k-.l

37Ar/39Ar

F](;. 5. ( 4 ° A r / ' A r ) versus (~:Ar/S"Ar) before correction lbr Ca-derived :'Ar lbr three white Allende inclusions (of. Fig. 4). Filled symbols indicate data points of the -.900°C fractions used lot calculating the correlation lines, for which the correlation coefficients are 0.98 (sample 20) and .0.99 (samples 19 and 21).

markable that there appears to be no relation between the type of pattern and the nature of the samples. For example, olivine chondrules samples 4 and 6 have patterns similar to that of inclusion sample 9, a finegrained white inclusion: high ages in the first 50% of :~!'Ar release and younger ages in the second half. The integrated ages are about 4.5 AE (Table II) and a p p e a r to be meaningful. This observation suggests that closed-system redistribution of 4"Ar or :"'Ar produced the o b s e r v e d "'reversed-to-normal'" age patterns (Jessberger et al., 1977). DISCUSSION

l

5.0

o

•

RELEASED

Ft(;. 3. Age and K / C a spectra of coarse-grained Allende inclusion No. 13.

10"2

~

C.5

Release

FiG. 4. Age and K / C a spectra of four white inclusion samples from Allende. Large error bars result from low K / C a ratios.

By and large the K, Ca, and CI contents of the samples (Table lI) agree with literature values for c o m p a r a b l e samples (Marvin et al., 1970: Wakita and Schmitt, 1970: Clarke et al., 1970: Fireman et al., 1970: Martin and Mason, 1974: Wanke et al., 1974: G r o s s m a n , 1975: N a g a s a w a et al., 1977). The ages cluster strongly as evident from Fig. 6 which includes all ages from the "'Ar-:~"Ar studies of Allende. Ten out of

4°Ar-'~'~Ar AGES OF ALLENDE ,

4.53

100

i

8O

8O

40

3.80 0

3.0

, ~ 7 ,

,

4.0 5.0 /'0At- 3OAr Age [AE]

,

6.0

FiG. 6. 4°Ar-'~gAr age distribution in the Allende meteorite. The distribution curve should be read like a histogram and was obtained by s u m m i n g the error distribution c u r v e s - - a s s u m e d to be given by o"-t • (21r) -''2 e x p ( - x Z / ( 2 o a ) ) - - - o f t h e individual ages to take into account the widely varying uncertainties. The data are taken from Table II: plateau ages are used wher e possible. For samples 17 and 18, both the intermediate- and the high-temperature ages were used.

fourteen samples of white inclusions belong to that cluster. Thus 4.53 AE m a r k s a major resetting event in the K - A r history of Allende's constituents. This age is somewhat younger than the oldest model and isochron ages calculated from R b - S r and Pb data (Gray et al., 1973; T a t s u m o t o et al., 1976; Chen and Tilton, 1976), but is the same as the age of the solar system inferred by Kirsten (1980) from a literature survey to be 4.53 _+ 0.02 AE. The matrix has a K - A r age 0.7 AE less than that o f the main cluster. G r a y et al. (1973) previously inferred a late disturbance from R b - S r systematics. Perhaps some single disturbance 3.8 AE ago managed to reset the K - A r clock in Aliende's matrix without altering those in the rest o f the meteorite. As the relative labilities of K and 4°Ar under the relevant conditions in space are unknown, the 3.8-AE figure might not be definite even if the singleevent model is correct. The young age of the matrix can also be interpreted as reflecting long-term, low-temperature leakage of 4°Ar from fine-grained material (Green et al., 1971) as has also been ob-

387

served for the Murchison C2 chondrite (Dominik and Jessberger, 1979). Our most interesting results are the ages well in excess of 4.5 AE. The ages of samples 17 and 18 have been discussed elsewhere (Jessberger and Dominik, 1979). Here we will discuss five possible explanations for the exceedingly old ages using sample 12 as an example. It is pointed out that ages - 5 . 0 AE imply substantial ( > 2 0 % ) amounts of extra 4°Ar as c o m p a r e d to 4.5-AE ages. Extraneous Argon

The addition of 350 × 10- ~ cm '~ S T P / g of ";°Ar would have raised the K - A r age of sample 12 to 4.8 AE from the expected 4.5 AE. No more than 2 × 10 -" cm :~ S T P / g of this c o m p o n e n t is " l u n a r , " " p l a n e t a r y prim o r d i a l , " or cosmogenic for the ~°Ar/:"~Ar ratios of these three reservoirs of Ar are 0.4 ( H e y m a n n et al., 1975), 1.4 × 10 -3 (Begemann et al., 1976), and 0.3 ( H u n e k e et al., 1972), respectively, and the total o b s e r v e d 3';Ar content of sample 12 is only 4.6 × 10 ~ cm :~ STP/g. A trapped c o m p o n e n t with "~Ar/:~Ar - 7 5 would raise the K - A r age as needed. This putative c o m p o n e n t has n e v e r been observed. Its production in the nebula, p r e s u m a b l y from K decay, would require (1) long times or high-K material, (2) large-scale separation of 4°Ar from K, and (3) most importantly, preferential incorporation of 4°Ar relative to K from the gas phase into the inclusions. Contamination with terrestrial 4°Ar could explain the high age of sample 12. Figure 7 shows an isochron plot of the data which, were atmospheric contamination present, should intercept the y axis at (4°Ar/:~Ar)o = 295.5. Despite the scatter o f the data around the calculated K - A r age line, a fit yields an intercept of 20 _+ 20 which suggests that atmospheric argon is an unlikely source for the extra "'Ar. For samples 17 and 18, a t m o s p h e r i c argon has been dismissed as the source for the very old ages (Jessberger and Dominik, 1979). The present data, however, do not rule

388

JESSBERGER ET AL.

14oo

long irradiation of K and Ca if the grains are much larger than, say, 0.1 /zm.

/4OAr\ k~)

1200

R e c o i l loss o f "~'~Ar

K-Ar age : 4.82 AE ',

,ooo

.... -

/ 600

/

* 770

815o ~60

/.00 200

t'~°°~/e:sg{~° ,~1210 /ell20

17,0,~IIvI070

{39Ar,~ \36Ar)

~1460 O0

I

I

2

I

I

4

1

1

6

1

I

8

I

FI(;. 7. (~"Ar/:l'iAr) v e r s u s (:'"Ar/:l"Ar) for s a m p l e 12, a c o m p o s i t e o f w h i t e i n c l u s i o n s . N u m b e r s give the r e l e a s e t e m p e r a t u r e . A l s o s h o w n is the line t h r o u g h the o r i g i n with a s l o p e c o r r e s p o n d i n g to the K - A r age a r o u n d w h i c h the d a t a p o i n t s s c a t t e r .

out unequivocally atmospheric contaminat i o n - a s is always the case in extraterrestrial samples. Additional work will be needed to determine the extent to which coarse-grained inclusions may adsorb atmospheric argon. Stardust

Clayton (1975, 1977a) has predicted the existence of detectable amounts of "~Ar produced by 4"K d e c a y in " s t a r d u s t , " i.e., in interstellar grains. If 6% of the K, present in sample 12 4.5 AE ago, originated in micrometer-sized stardust an average of 8 AE ago and retained all its daughter 4°Ar, the 4.8-AE age would result (Clayton, 1977b). One objection to this hypothesis is that Ar loss evidently did not o c c u r but might have during accretionary heating (Chou et al., 1976). This objection, however, applies to all noble gas anomalies supposedly brought into the solar system via grains. A second objection is the absence of large amounts of cosmic-ray-produced argon that are expected from the

Neutron irradiation produces :~gAr, the experimental measure of K, via the reaction 3'JK(n, p)'~gAr. 3"~Arhas a recoil range of - 0 . 1 p,m (Turner and Cadogan, 1974) so that some '~'~Ar loss is expected w h e n e v e r the K-bearing phases are fine grained and concentrated close to grain surfaces. Herzog et al. (private communication) have shown that K is indeed c o n c e n t r a t e d in fine-grained veins and rim material in two type B inclusions that were not included in the present work. Some :~gAr recoil loss from the sample therefore seems probable but whether the amount would suffice to raise its age from 4.5 to 4.8 AE is uncertain. About 3 × 10 -~ cm :~S T P / g or - 2 0 % of the :~:~Arwould have had to recoil out of sample 12 to make this change. The largest recoil losses reported to date are only about 6% (Alexander et al., 1977). P o t a s s i u m Loss

R b - S r data (Gray et al., 1973) and the matrix K - A r age indicate an " e v e n t " late in the history of Allende. If Rb was mobilized, then K probably was too. To increase the total K - A r age of sample 12 from 4.5 to 4.8 AE, about 40% of the K would have had to have been lost 3.8 AE ago (or 17% at 1 my ago) assuming no loss o f ~°Ar occurred simultaneously. Other loss sequences can be imagined but all require the preferential removal of K o v e r Ar. Although a combined study of K migration and Ar diffusion has not yet been performed on extraterrestrial material, a larger mobility for K than for ""Ar seems physically improbable. A n o m a l o u s ~"K / K

Our age calculations assume that "~K = 1.167 × 10-' K, i.e., normal, terrestrial isotopic composition (Steiger and Jiiger, 1977). The 4.8-AE age could be explained if the "~K/K ratio were 20% higher than the terrestrial value. A ~°K e n h a n c e m e n t is

4°Ar-'~aAr AGES OF ALLENDE believed to be produced in s u p e r n o v a s ( C a m e r o n and Truran, 1977). H o w e v e r Stegmann and Begemann (1979) have shown that the isotopic composition in one sample of these apparently ancient inclusions (sample 18) is normal; other inclusions not dated by 4°Ar-~gAr technique also have normal K isotopic ratios ( B e g e m a n n and Stegmann, 1976; Birck et al., 1977). We have been informed by G. Lugmair (private communication) that one o f the anomalously old inclusions, sample 20, has an a n o m a l o u s 149Sm/lS°Sm ratio due to a neutron fluence calculated to be 8 × 1016 n / c m 2 and that the 6°C0 activity appears to indicate a recent (-<10 years) irradiation although there exists no record of an artificial irradiation. Pending experimental disproof, h o w e v e r , it can be speculated that the irradiation occurred early in the inclusion's history. Than a fluence o f this size would produce a aaArc~ concentration of 1.7 x l0 -r cm '~ S T P / g assuming a CI content of 175 p p m (Table II), a neutron absorption cross section o f 30 b, and the c o m p l e t e decay o f asCI. An alternative calculation, based on the 36Arc~ production rate of Smith et al. (1977), gives 3.4 × l0 -'q cm a STP/g. The value estimated from experimental data, 5.4 × l0 -~ cm "~ S T P / g agrees with the calculations a b o u t as well as can be e x p e c t e d considering the uncertainties involved. According to G. Lugmair (private communication) sample 20 also has a small but significant ~44Sm deficiency

389

which, h o w e v e r , is independent from a possible recent neutron irradiation.

CONCLUSIONS (1) The K - A t clocks in Allende materials underwent a major resetting on the average 4.53 _+ 0.03 AE ago. (2) The K - A t clock in Allende matrix was probably reset 3.8 AE ago. G r a y et al. (1973) inferred the identical u p p e r bound for a disturbance from their R b - S r results which suggests that the inequality may actually be an equality. Alternatively, the young age reflects the leakiness o f the fine-grained matrix material at low temperatures. (3) The anomalously high apparent ages o f some white inclusions m a y be caused by any o f a n u m b e r o f factors. The two explanations we find hardest to discredit are a t m o s p h e r i c contamination and the presence of ancient interstellar grains.

APPENDIX Argon data o f n-irradiated Allende materials are listed for each extraction fraction as the amount per gram for each isotope in units of 10 -n ( - - E - n) cm "~ S T P / g (see Table AI). The samples are ordered according to their sample n u m b e r (cL Table lI). The stage of reduction is described in the text. Errors are lcr. d = [exp(Mm) !] 39Arm/4°Arm is given for each sample.

390

JESSBERGER

ET AL.

TABLE

AI

SAMPLE rl

II* .0915 36AR E-11

37Ali E-l@

39AR c-11

39AR e-11

40AR E -9

+*

540 10

68 4

176 19

55 1

535 7

3.955 0.034

t-

264 9

69 5

115 12

45 2

lb0 6

2.452 Ln.087

+-

229 10

123 2

105 9

49 1

382 6

3.255 0.052

+-

171 7

149 3

7fi 11

93 2

4@9 6

3.590 0.068

l-

216 15

461 6

233 9

92 2

936 6

3,034 0.041

+-

152 14

4a7 5

260 15

73 3

751 6

4.1636 0.063

t-

199 8

342 5

209 11

64 3

525 6

3.679 0.064

+-

190 11

415 4

179 15

47 2

446 6

3.906 0.055

t-

293 8

664 b

201 8

49 3

432 6

3.929 0.092

t-

485 14

987 5

224 12

57 1

531 7

3.e93 0,040

t-

640 16

1326 9

294 15

61 2

706 9

4,244 0,062

+-

649 19

1267 8

242 17

60 2

554 e

3,969 0.053

t-

913 13

lb69 B

293 19

58 2

442 10

3.571 8.069

+-

1 445 13

2956 13

516 9

59 2

325 14

3.069 0.094

+-

1 35e 13

4797 24

762 14

44 2

234 12

3.034 0.100

+-

1 419 29

9331 55

799 19

51 2

359 12

3.426 0.069

l-

1397 11

7155 27

79b 18

bl 1

595 12

3.950 0.041

t-

601 B

2181 12

277 15

41 2

464 8

4.196 0.092

t-

301 20

Y

171 6

27 2

300 8

4.192 0.129

t-

226 11

701 6

171 10

20 3

263 9

4.455 0.239

+-

282 14

549 6

246 13

64 1

706 19

4.164 0.047

TOTAL +-

11956 63

36i02 12

6324 62

1127 9

9977 42

3.794 0.016

TEMP. tc1 350 391 430 470 510 560 61@ 650 700 760 920 970 9210 990 1038 1096 1140 1190 1254’ 1350 1500

APP.ACE I AE 1

40Ar-3gAr AGES

391

OF ALLENDE

TABLE AIXontinued SALPI&

JR .0Ql5

02

TMP. ICI

36AP E-11

37AR E-10

18AR e-11

350

t-

521 1

83 4

21;

*r

366 4

126 1

*-

283 5

+I

230 9

400 450 496

39AR c-11

125 5

“;‘;;“I” 5,067 0.046

ci7e s

4,396 0,032

731 20

101s 4

4,822 0,045

13;

1162 11

1701 3

4.619 0,015

%

Y

2111 4

4.s90 e.011

32:

1650 !I

2406 I

4,604 0.008

2s;

QSS 8

1104 3

4,409 0.011

36:

‘::

1166 3

4.671 0.028

‘::

45:

100 10 311 3

2U9 18

4e:

229 7

39E

157 5

211:

239 5

40AR e -9

226 5

323

236 13

518 Q

17% 3

4.659 0,020

t-

351 5

614 4

273 4

541 4

796 4

4,601 0.014

*-

412 1

824 3

la:

602 13

840 4

4,540 0,037

+-

419 6

1072 5

223 16

726 Q

925 5

709 1

1695 5

356 10

92s 6

1110 6

+r

llQ1 6

2125 8

399 10

1228 5

1376 11

1900 +=

961 7

37Q7 11

347 6

124 8

bT9 9

105F +-

133fi 10

7562 29

YT:

664 9

$2

3,621 0,040

1125 t-

1865 15

15951 40

12;;

916 11

Yi

3,739 1.836

1295 +r

2411 14

6956 25

981 11

1291 6

1435 +-

214 5

918 3

128 6

245 !I

TOTAL +-

12431 3’)

46516 65

760 810 855 900 9s0

t=

“E

11931 43

%

2’: 20616 3s

4.371 9.022 4.255 0.051 t*::: .

JESSBERGER

392

ET AL.

TABLE AI-Continued SAPYLB

x3

J* 36AR E-IQ

TEW. [ c I

3lAk E -9

39AR E-10

.8815

3?AF E-1L‘

4@AR E -8

APP.AGE I AE I

+-

53 3

3t: 2

ZIP 3

260 5

311 2

4.282 0.030

+-

14 2

29 2

38 2

160 6

227 2

4.561 h.061

+-

19 2

3c 2

68 2

lb3 3

217 2

4.468 L1.033

+-

17 2

46 1

66 3

152 2

266 2

4.919 ul.e23

+-

21 3

75 2

82 3

2Cl 6

34c 2

4.861 L3.@4E

+-

16 2

17.3 2

51 1

231 5

357 2

4.667 R.03.a

+-

12 4

552 3

19 3

256 5

356 2

4.533 u.030

+-

13 3

471 3

23 3

140 5

196 2

4.540 0.864

+-

2ti 3

760 3

26 3

141 6

213 2

O.@lYI

+-

11 3

242 3

18 2

38 4

66 5

4.092 0.213

TOTAL t-

195 9

7

412 8

1747 15

2549

36AF. t-1r

37AK E-lU

3BAR E-l@

39AIX E-12

4QAR E -e

AFP.AGE t AE I

tm

32 6

78 1

96 1

4@7 3

674 1

4.824 0.014

+-

34 2

121 1

179 2

310 3

527 1

4.865 0.014

+-

45 3

191 2

176 3

369 4

654 1

4.939 0.021

+-

24 4

176 1

164 3

265 3

414 1

4.726 0.021

+-

35 1

379 2

205 2

501 4

685 1

4.504 0.014

+-

23 2

864 3

99 1

437 4

549 1

4.363 B.017

+-

18 2

991 3

51 3

281 3

379 1

4.419 0.017

t-

23 2

4485 16

72 5

476 4

621 2

4.426 fd.015

+-

i

1259

5

40 3

146 3

104 3

4.372 0.03e

l-

:

138 2

:

29 4

44 4

4.665 0.243

8

3223 11

490

580 67fl 790 68@ 965 1Obl 114r 13U 1471’

sAI.PLC

TEW. I c

60~ 690 77b 66r 95w 104r 113e 123’Z 133L 146C

1

TUTAL +-

2357

$4

8663

18

lUE9

4.612 0.015

7 .C815

J’

247 9

4,667

4130 6

4,622 0.006

““Ar-3gAr AGES

TABLE

OF ALLENDE

AI-Continued

sAb!PtE. 15

TEMP. L c I

34b 41u 460 545 6la b9D

14W’

3BAR E-11

87

227 5

+-

i:

130 12

t-

7 6

455 21

14

189 28

4r)E 7

t-

-

39AR E-10

4EAR E -e

APP.AGE C AE I

148 3

45 2

2.246 0.068

69 4

35 2

2.945 0.126

90 8

135 2

4.521 0.141

13 7

lD4 2

4.560 0.111

t-

11 2

534 11

27 25

/SW 9

12u 2

3.635 0.096

t-

12 4

535 32

263 47

71 6

104 2

4.635 0.146

t-

7 4

635 13

289 25

7e 3

119 2

4.691 0.071

-

539 17

201 43

i4 9

93 3

4.312 0.201

-

lL55 19

269 39

95 12

144 3

4.686 0.218

11 4

2417 14

399 29

168 9

238 3

4.375 0.085

12 4

4759 42

442 40

ai, 13

130 3

4.668 0.254

t-

129e

E-l@

37Ali E-lcr

16 4

B3U

lE0P

JE .OB15

36AH

+-

76b

91@

393

tt-

t-

7 5

25ClU 11111’

1467 41

175 10

254 3

4.668 0,093

-

14C7E 78

If.03 71

98 9

130 4

4.451 0.152

3007 zc!

339 41

50 7

54 6

4.128 n.289

5365C 150

5360 14P

1452 31

1705 11

4.254 0.043

t-

To’fAL t-

84 12

JESSBERGER

394

ET AL.

TABLE AIXontinued sAPPLE 16 TEkt?. LCI

J= .oe1s 36AR E-11

37AR El1C

t-

123 2

186 1

t-

95 4

t*

APP.ACE t AE 1 -

39AR E-11

40AR E -9

169 2

1398 3

1563 5

4.183 0.007

299 1

191 4

20W4 4

3105 6

4.712 a.004

91 3

414 1

344 5

2704 5

4165 7

4.703 0.004

to

96 2

398 2

592 5

2549 5

3895 7

4,690 0.004

to

152 5

434 2

1255 5

2639 8

3846 8

4.611 0.006

t-

138 4

437 1

1132 4

1958 7

3125 8

4.7b2 0,007

t-

131 4

451 1

197 4

1762 6

3133 9

4.943 W.008

t-

141 5

JWU 2

en2 4

1778 6

309P l@

4.913 0.006

+-

120 5

569 2

922 4

1911 6

3064 11

4,770 ti,BQB

t-

129 5

9w 3

lC71 5

2682 5

3972 13

4,630 0.0’36

t-

153 s

1665 6

1181 8

4326 8

5935 13

4.510 0,005

134 21

2754 1u

865 9

5204 9

6895 16

4,453 Id.005

10513 t-

87 6

4941 16

384 9

4435 8

5390 18

4.3116 Ld.BQ6

lll(”

+-

92 B

8907 29

311 11

3523 9

3882 21

4.151 0.010

t-

101 8

6615 22

283 8

2@17 8

2531 23

4.363 0.016

t-

91 11

g4::

281 12

3778 11

4714 28

4,354 0.011

131rr

t-

22 12

2477 6

139 4

700 6

751 36

4.100 0.079

139F

l-

JE 15

3892 13

193 6

1159 4

1239 44

4.102 0.057

1501J

t-

76 18

1749 6

149 5

704 5

830 54

4.261 0.107

TOTAL t-

2818 41

47~92 54

40w 460 52W 570 630 690 730 785 83E 890 950

10loV +a

117v 1230

3EAf?

E-11

Y

47214 30

s51c

4.519 0.003

““Ar-3gAr AGES OF ALLENDE

TABLE sAN’LL

Ja 36AE

rci

E.ll’

3BAR

E.lC

39Ai-T

4i?AR

E-l@

L -8

APP.AGE

t AE I

+”

83

2

76 1

415 6

691 2

4.83) 0.025

t-

28 2

62 2

151 2

354 4

394 2

4.166 0.018

+-

22 4

17c 1

286 4

275 9

400 2

4.609 a.055

.

32 2

221 1

299 4

3kt6 5

373 2

4.318 0,027

t-

27 3

248 2

338 s

322 9

49P 2

4.648 0.047

t-

32 2

541 2

313 5

403 8

647 2

4.471 Ur.027

t-

25 2

1174 6

Bl 4

409 7

551 2

4.480 0.1627

t-

a7 2

1839 7

124 6

247 0

326 2

4.447 0.157

1404 6

77 3

147 12

197 s

4.381 0.137

5822 12

1747 12

2958 23

4Y51 a

r.sa1 0.013

6Ok’ 710 em l

901 1CW

1451‘

37AI: -9

E

./‘BlS

117 4

501:

l24f

AI-Continued

87

TEW.

111r

395

+.

TOTAL +-

309

e

396

JESSBERGER

ET AL.

TABLE AIXontinued 5AhPbL .a 39AR E-11

39AR E-11

40AR E -9

APP.AGE 1 AC I

+-

192 10

54 4

125 7

373 10

600

4.776 0.044

+-

46 6

73 3

66 5

292 13

333

+-

39 5

149 2

69 12

474 10

655

+-

b3 5

381 5

1124 20

1690

l-

37 5

494

1154 12

1664

+r

57 7

Ii44

1655

3b:

l-

63 9

312 2

54:

l-

b9 5

35:

42 7

409

+-

62 6

648

+-

l-

91 6

604

*-

b9 5

696

+-

75 7

972

350

390

470

510

550

590

635

670

720

765

905

660

905

ll2P

1002 14

1372

555 11

788 17

1193

445

602

3

4.922 0.037

3

4.6b3 0.030

3

4.592 0.017

3

4,599 0.010

3

4,507 0.023

3

4.673 0.035

3

4.671 0,023

3

4.716 0.034

3

4.735 0.027

3

4.653 0.030

910

3

9 545 11

753 15

1169

7

443 10

711 11

1117

5

467 11

826 15

1235

3

516 10

1147 12

1656

4

699

9

2061

130 7

1933 14

414 17

1729 21

2369

143 9

3537 13

443 9

1966 12

2462

1*9

5570 190

250 lb

1454 16

1914

25;

1185 19

1565

2166

4

4.207 0.072

4

4.595 0.017

3

4.541 0.021

4

4.509 0,020

4

4.446 0.011

4

4,442 0.019

4

4.449 0.027

4

4.54m 0,009

4

4.540 0.015

5

4,492 0.032 4.513 0.031

111 10

6545

106 7

6225 30

275 14

1551

l.

+-

84 6

11919 41

313 19

1791 16

2501

48 IF

6700 26

221 10

1072 20

1454

l-

l-

37 9

39e9 14

151 10

625 11

959 5

l.

20 12

2410 lfi

13;

456 19

659

l.

11 7

457

129P

1445

1525

7

3

1). ll7r:

1355

7

I474 19

l-

l-

435

9

l-

107~

e

3

6

11:

1020

263 6

1287

l-

9611

1225

.0915

37Afi E-10

Lc I

430

P 36AR E-11

TEW.

TOTAL

l-

lEl6 35

23

7

3

50 5

67 14

57890 2m0

7966 53

23661 72

333::

4.593

7

l.179

94 10

4,355 0.397 4.555 0.005

4oAr-3sArAGES OF ALLENDE

TABLE SAMPLC

TEW,

AI-Continued

t9

Jr 36AR E-11

[Cl

397

3lAR L-10

38~R E-11

WAR E-11

.08lS

UAR E -9

APP.AGE I AE I

5

604 B

577 4

3,922 0.022

54 3

67 3

520 9

614 3

4.264 0.029

53 6

129 2

111 9

12S0 7

ilS7 4

4.539 0.010

t’

50 3

204 2

188 1

1907 10

3111 4

4.799 0.009

*-

63 4

253 4

321 8

2421 12

3895 5

4.771 0.908

la

76 5

302 3

476 5

2673 18

4392 5

4,811 0.011

+-

81 5

323 3

859 9

2850 18

4422 4

4.715

+-

86 5

206 3

742 5

1576 12

24S3 4

4,721 0.013

t-

83 8

235 3

966 11

1736 14

2777 4

4,766 0.914

+-

137 6

407 3

g8i

3090 a2

5399 5

4.914 0.012

+-

145 4

317 3

613 11

1955 11

3b@2 4

5,004

t-

142 6

393 3

b73 6

1659 12

2847 4

4,084 0.012

l.

94 9

471 5

645 6

1766 16

2637 4

4.651 0.015

+-

105 7

775 4

2453 10

3219 4

4.437 0.007

*-

106 17

1362 7

679 9

3543 14

4316 4

4.314 0.006

+-

163 7

4289 1s

772 10

5698 21

6588 4

4,229 0.006

108f. +-

185 7

9723 32

464 13

5597 20

6169 4

4,151 0.006

+-

89 10

6931 24

211 12

2107 15

2395 5

4.202 0.012

l-

122 6

7043 25

163 13

22j6

11

26QP 4

4.294 0.009

129e t-

47 7

2836 12

1,:

730 19

691 5

3.904 0.643

13SP +-

31 5

3337 13

146 12

850 16

719 6

3.729 0.032

1496 +-

3i 8

2681 10

153 9

891 i4

Y:

3.602 0.037

TOTAL *-

2029 33

42328 56

659S9 a2

4.511 e.002

350

+-

113 5

57 2

l-

27 5

t-

398 430 475

515 550 590 630 680 130

770 815 860 910 950 102c

11% 12lP

l@l

1Y

48050 60

0.011

0,010

398

JESSBERGER

TABLE

ET AL.

AIXontinued

SArJPLk r1ll TENP, ICJ

J= .@815

3bAR E-11

37AR

E -9

38AR E-11

39AR E-le

40AR E

-8

APP.AGL [

AC

I

t-

lb4 5

296 1

663 11

t-

59 3

302 1

490 15

1253 3

1456 1

4,239 0.004

l-

90 3

473 2

6b7 23

2131 4

2736 2

4.400 0.003

t’

133 3

530 2

1416 29

2334 4

3092 3

4,447 0,003

t-

156 3

499 1

2619 21

1698 3

2530 3

4.462 0,002

t-

262 6

bU3 3

3331 29

1651 3

2202 3

4.463 0,003

392 5

114 3

5c91 23

1202 2

1615 2

4.476

+a

t-

329 4

535 2

4653 19

5ee 1

799 1

4.495 0,004

t-

232 6

352 2

3270 14

299 1

408 1

4,505 0,005

t-

213 5

365 2

2777 12

25s 1

344 1

4,486 Y.607

t-

113 5

230 1

1229 5

153 1

267 1

4.494 0.011

+-

126 5

3w 2

1256 6

173 1

219 1

4,376 w.013

+-

124 6

653 3

1115 6

169 1

219 1

4,233 0,012

l.

176 9

3102 14

922 32

253 1

222 2

3.766 0,015

t-

175 10

3416 lb

663 3s

229 1

134 2

3.162 0,024

t-

226 13

4562 22

1@50 47

260 2

112 2

2,574 0.030

t-

77 10

1379 7

555 15

63 1

21 3

2.391 0.165

t-

131 15

1909 9

651 21

106 1

23 4

l.E71 0.205

TOTAL +r

3160 3P

20321 34

32137 97

14029 9

410 459; 490 538 666 60V 640 670

liti 760 610 070 930 960 10x l09C It60 lZb@

968 2

044 1

3.776 0,003

1717;

0.003

4,322 0.001

399

40Ar-3gAr AGES OF ALLENDE

TABLE

AI-Continued

SAMWE ill

Jm .0BlS 36AR

L-11

37AR E -9

38AR Y-11

39AR Eill

40AR L 99

l-

764 4

139 1

690 5

3882 9

3e44 8

4.353 0.805

t-

208 4

87 1

447 5

2482 a

3s71 8

4.569 0.006

t-

193 5

104 1

b22 6

3119 6

4463 8

4.580 0.004

t-

216 4

137 1

1135 7

3777 7

5370 9

4.569 0.004

t-

271 6

158 A

1821 6

3198 10

4534 1R

0.006

t-

293 7

155 1

2172 10

2107 9

2906 10

4.519 0.009

t-

333 6

lb7 1

a474 8

1531 7

2231 11

4.610 0.011

t-

264 5

137 1

1645 6

1048 8

1619 11

4,707 0.017

t-

242 5

138 1

1210 6

936 10

1442 12

4.703 0,023

t-

352 6

213 1

125s 6

1168 6

1774 12

4,679 0.014

t-

388 6

302 1

12m0 5

1313 4

Y

4.389 0.013

t-

559 8

606 2

13Y

7

2282 14

4.460 0.0l2

+-

658 7

806 3

1861 8

2398 16

4.407 0.013

t-

655 8

1445 5

1050 16

1455 10

1671 19

4.219 0.e21

t-

1243 14

4789 16

1668 49

1905 19

1600 22

3.717 0.02b

t-

1087 14

4030 13

1377 42

1464 17

1165 25

3.631 0.038

t=

1221 13

29B6 10

723 13

678 28

3,898 0,072

t-

348 1%

1316 7

633 15

567 a

LB4 49

4.096 0.134

t-

146 20

10b6 3

526 13

572 6

587 58

4.035 0.162

TOTAb t=

9451 42

la794 2s

23938 80

34033 42

TEMP.

I c I

390 430 475 520 565 590 645 680 720 7i0 830 880 930 980

1040 1105 120P 1305 1400

l7i4

446z:

APP.ACE I AE 1

4.565

4.436 0,004

400

JESSBERGER

ET AL.

TABLE AI-Continued sA!‘FLC ~12 TEMF. [Cl

J= .ca15

36AR E-11

37AR E -9

38Ali c-11

39AR E-11

40AR E. -9

APP.ACE t Ae: I

*-

238 9

32 1

lU5 11

264 9

7@5 4

5.641 0,060

t-

187 6

66 1

122 7

373 11

787 3

5.235 0.051

t-

112 7

74 1

112 5

455 6

923 3

5.168 0.021

t-

99 4

70 1

87 1C

452 8

854 3

5.049 E.029

t-

82 5

108 1

166 8

662 e

1147 3

4.901 B.021

t-

80 3

159 1

291 6

692 7

1lW

3

4,756 0.017

t-

Y@ 5

218 1

372 6

592 7

855 3

4,596 0,020

t-

67 5

283 1

424 6

420 11

785 3

4.846 0.043

t-

234 6

546 1

636 8

782 9

1536 3

5.113 a.021

tr

64 2

143 1

171 2

203 3

404 3

5.135 0.024

t-

186 5

435 1

378 I

139 7

887 3

5.W85 0,027

t-

129 7

531 1

371 8

403 4

628 3

4.724 0.819

t-

157 6

818 2

458 11

SW 5

719 3

4.589 a.(n19

t-

251 8

1584 3

664 17

7ta9 6

955 3

4.481 0.(115

t-

352 13

3838 9

lOb8 41

745 15

917 3

4.330 ti.034

187C’ t-

402 15

4598 13

974 5F

606 18

807 3

4.460 0.051

1120

t-

375 In

3619 6

8LY 37

767 15

1051 5

4.518 0.033

t-

6~8 21

9895 21

1670 1OD

1202 38

1897 6

4,744 0.053

1321’ +-

595 30

13979 29

16b0 140

867 53

147s 6

4.869 cl.104

146P

245 15

6400 51

2413 69

224 25

352 16

4.734 rd.197

4554 _.

47388 __

1295P 21r

11376 81

18712 22

37t, 410 455 4916 535 585 625 670 72@ 77w 815 86ti 91li Y6ti l0lV

1216

t-

TOTAL t-

51

b5

4.e13 0.012

401

40Ar-39Ar AGES OF ALLENDE

TABLE sAKPLE

113 37AR E =a

+m

28 2

267 3

+-

52 3

*-

938 102P 114e 14w

19~~ e-10

Xf.

5:x

2i

573 25

650 16

4.441 0.064

a9 3

772 1P

Y

50) 9

Y

0,029 4.541

+-

123 4

1367 2c

66 5

64s 11

781 9

4,549 0.025

+.

‘“!

3840 140

::

790 30

931 as

4.502 0,048

+r

101 3

6690 110

:3

470 23

144 7

4.44s 0,079

+a

1°:

9470 22P

x:

456 12

843 I1

4.52V 0.040

+-

‘i’:

29V4C 280

2:;:

792 17

%

0.041 4.511

769 15

52740 400

590 93

s2::

0.017 4.499

4444 53

SARPLC; (19

670 790 910 103c 1150 127f 144P 155@

J.

.Q692

36AFf E-IQ

37AR E-10

3EAR E-11

39AR E-11

40AR E -8

*a

16 1

58 3

61 4

229 7

70 3

5,190 0.082

+-

27 1

196 5

144 5

754 14

145 3

4,004 0,042

t-

2V 1

290 1

161 4

393 6

71 3

4,871 0.072

t-

42 1

656 9

179 7

323 e

68 4

4.963 0,097

+.

74 2

3331 23

3::

320 16

58 4

4.692 0.146

+-

57 4

9554 53

576 29

244 39

42 S

4.610 0,335

*-

43 3

7156 42

434 24

156 34

PI 6

4,6Sl 0,584

+a

37 5

7513 39

527 29

322 35

58 13

4,692 0,409

+-

57 V

12744 01

623 44

136 68

:t

5.233 1,256

*-

26 12

“Y

257 26

_

_

400

47000 120

TEMP. ICI

550

APP .ACC I A,k 1

222 s

TarAL +.

400

,094s

40AR P =V

291 3

670 600

MAR E-10 B 1

630 550

Jr

3$AR e-10

TERP. [Cl

AI-Continued

TOTAL +a

16

33:x

2837 09

APP.AGE f AC I

4.086 0.096

402

JESSBERGER

TABLE

ET AL.

AI-Continued

sAlrPLli r2o

TEHP ,

38AR E-10

39AR E-10

40AR E -8

*r

69 2

176 6

42 1

64 1

274 5

6.706 0.944

I)-

105 2

239 7

96 2

63 1

152 5

5.192 0,065

+r

72 2

484 9

69 1

35 1

120 6

5.612 0,094

+r

70 3

1428 13

82 1

38 1

93 7

8,216 0.140

+.

72 3

4513 38

96 2

39 3

90 a

4.966 0,201

*r

46 4

4520 27

59 2

20 2

41 10

4.011 0,456

65 6

8565 83

“:

26 4

38 15

4,367 0.702

17 e

-

500 640

w 100p 1lZP

1279:

l.

150~

.0692

37AR $ -9

ICI

760

Jr

36AR e-10

14510 120

+r

TOTAL +=

499 9

34800 150

51:

321 10

5.325 6.067

795 23 J= .n692

sAFPLE 421 TEMF. t c 1

APP.AGE t AE I

36AR El1B

37bR E *9

38AR 6-l 1

39AR E-11

40AR E -a

APP.AGE t AB; 1

16 1

51

+=

4

178 4

61 3

5.184 0.083

t-

43 1

221 5

5b8 8

140 3

5.223 0.039

t-

125 2

214 5

563 9

385 10

66 3

5,950 0.074

t-

s2 1

1029 12

393 8

369 8

97 4

5.331 0,075

*c

49 2

17lk3 23

33@ 1n

309 11

51 4

4.535 0,149

102fw +a

59 3

57s3 28

507 20

423 29

65 5

4,437 0.174

114v+ tw

28 3

“ir:

326 14

168 20

a9 8

4,426 0.482

126fht t*

39 5

6735 34

560 23

317 34

47 13

4.376 0.490

137PI *r

68 9

12900 22c

634 53

394 62

5:

4.156 0,679

154n

38 12

4972 33

273 22

:x

*-

64 34

TOTAL +I

516 16

37610 230

400 650 670 790 910r

Y

3167 66

695 44

403

““AI--~~A~AGES OF ALLENDE

TABLE SAMPLE #23

AI-Continued J=

ALLEtiDE 3BAR

39AR

.6326.

MAR

APP.ACE

36AR E-1r

37AR E -9

*-

45 1

235 2

9ti7 8

74 2

191 3

3,967 11.843

+-

74 2

414 2

2431 23

64 1

201 3

4.371 9.037

+*

51 2

432 2

1211 22

29 1

123 3

4.970 0.069

+-

58 2

lC13 4

11164 25

33 1

93 3

4.206 0.871

+-

58 2

2464 11

630 51

24 2

71 3

4.204 (6.130

+--

51 2

2668 14

666 33

I7 2

67 3

4.762 O.lW

+-

169 3

9813 34

1540 280

6 3

111 4

1.46t3 1.338

+-

16 8

2817 12

235 45

TOTAL l-

461 10

19137 41

8760 220

246 6

e3k? 8

4.S21 0.244

TEHP, 1 C 1 55s 610 78@ 908 1llZP 114f 128f l44f

E-11

E-11

PYhEX-GLASS W

E -8

J=

COHRLCTIOI’ FLIP ATHOSPIIERIC ARGOL 36AR E-11

37AR E-11

G-11

30AR

39AR

t-

279 3

190 6

1329 6

a24 5

822 1

+-

173 3

4i# 2

337 5

436 4

S@8 1

TOTAL l-

432 4

1260 7

1329 2

TEMP. CC1 14kx 1556

220

6

1666

9

E-11

40AR

E-9

t

.OlO

AE

I

404

JESSBERGER ET AL. ACKNOWLEDGMENTS

CRESSY, P. J., AND BOGARD, D. D. (1976). On the calculation of cosmic-ray exposure ages of stone meteorites. Geochim. C o s m o c h i m . Acta 40, 749. DOMINIK, B., AND JESSBERGER, E. K. (1978). Early lunar differentiation: 4.42 AE old plagioclase clasts in Apollo 16 breccia 67435. Earth Planet. Sci. Lett. 38, 407-415. I~MINIK, B. AND JESSBERGER, E. K. (1979). 4°ArREFERENCES a"Ar dating of Murchison, Allende and Leoville ALEXANDER. E. C., JR., SAITO, K., DRAGON, J. C., whole rock samples. I n L u n a r P l a n e t . Sci. X, p. 306. COSCIO, M. R., JR., AND PEPlN,. R. O. (1977). '°ArThe Lunar Sci. Inst., Houston. ~aAr and rare gas studies of lunar soils. In L u n a r Sci. DOMINIK, B., JESSBERGER, E. K., STAUDACHER,TH.. VIII. pp. 10-12. The Lunar Sci. Inst., Houston. NAGEL, K., AND El GORESY, A. (1978). A new type BEGEMANN, F., AND STEGMANN, W. (1976). Implicaof white inclusion in Allende: Petrography, mineral tions from the absence of a 4'K anomaly in an chemistry, 4°Ar-.~Ar ages, and genetic implications. Allende inclusion. N a t u r e 259, 549-550. In Proc. Lunar Planet. Sci. Conf. 9th. p. 1249. BEGEMANN, F., WEBER, H. W., AND HINTENBERBER, Pergamon, Oxford. H. (1976). On the primordial abundance of argon-40. EICHItORN, G., JAMES, O. B., SCHAEFEFR, O. A.. Astrophys. J. 203, LI55-L157. AND M(JI.I ER, H. W. (1978). Laser a"Ar-4°Ar datBIRCK, J. L., LORIN, J. C., AND AI.I.EGRE, C. (1977). ing of two clasts from Consortium breccia 73215. Potassium isotopic determination in some meteoritic In Proc. Lunar Planet. Sci. ('on.f. 9th. p. 855. and lunar samples: Evidence for irradiation effects. Pergamon, Oxford. Meteoritics 12, 179-180. EL GORESY, A., NAGEL, K., AND RAMDOHR, P. BOGARD, D. D.. CLARK, R. S., KEITII, J. E., AND (1978). Fremdlinge and their noble relatives. In REYNOLDS, M. A. (1971). Noble gases and radionu,°roe. Lunar Planet. Sci. Con/: 9th. pp. 1279-1303. clides in Lost City and other recently fallen meteorPergamon, Oxford. ites. J. Geophys. Res. 76, No. 17, 4076--4083. FIREMAN, E. L., DEI:ELICE, J., AND NORTON, E. BOGARD, D. D., HUSAtN, L., AND WRIGHT, R. J. (1970). Ages of the Allende meteorite. Geochim. (1976). 4°Ar-aSAr dating of collisional events in C o s m o e h i m Acta 34, 873. chondrite parent bodies. J. Geophys. Res. 81, 5664GRAY, C. H., PAPANASrASSIOU, D. A., AND WAS5678. SERBURG, G. J. (1973). The identification of early CAMERON, A. G. W., AND TRURAN, J. W. (1977). The condensates from the solar nebula. Icarus 20, 213supernova trigger for formation of the solar system. 239. Icarus 30, 447-461. GREEN, H. W., RADCLIFFE, S. V., AND HEUER, A. H. CHEN, C. L., AND TILTON, G. R. (1976). Isotopic lead (1971). Allende meteorite: A high voltage electron investigations on the Allende carbonaceous chonpetrographic study. Science 172, 936-938. drite. Geochim. Cosmochim. Acta 40, 635-643. GROSSMAN, L. (1975). Petrography and mineral chemCHOU, C. L., BAEDL-CKER,P. A., AND WASSON, J. T. istry in Ca-AI-rich inclusions in the Allende meteor(1976). Allende inclusions: Volatile element distriite. Geochirn. Cosmochim. Acta 39, 433. bution and evidence for incomplete volatilization of HEYMANN, D., WALTON, J. R., JORDAN, J. L., LApresolar solids. Geochirn. C o s m o c h i m . Acta 40, 85KATOS, S., AND YANIV, A. (1975). Light and dark 94. soils at the Apollo 16 landing site. Moon 13, 81-110. CLARKE, R. S., Jr., JAROSEVICH, E., MASON, B., HUNEKE, J. C., AND SM, TH, S. P. (1976). The realities NELEN, J., GOMEZ, M., AND HYDE, J. R. (1970). of recoil: "~SArrecoil out of small grains and anomaThe Allende, Mexico, meteorite shower. Smithson. lous age patterns in aSArJ°Ar dating. In Proc. Lunar Contrib. Earth Sci.. No. 5. Sci. Conf. 7th. pp. 1987-2008. Pergamon, Oxford. CLAYTON, D. D. (1975). Z=Na, Ne-E, extinct radioacHUNEKE, J. C., PODOSEK, F. A., AND WASSERBURG, tive anomalies and unsupported 4°At. N a t u r e 257, G. J. (1972). Gas retention and cosmic ray exposure 36. ages of a basalt fragment from mare Fecunditatis. CLAYTON, D. D. (1977a). Precondensed matter: Key Earth Planet. Sci. Lett. 13, 375-383. to the early solar system. Moon & Planets 19, 109. JESSBERGER, E. K., AND DOMINIK, B. (1979). GeronCLAYTON, D. D. (1977b). Interstellar potassium and tology of the Allende meteorite. Nature 277, 554argon. Earth Planet. Sci. Lett. 36, 381-390. 555. CLAYTON, R. N. (1978). Isotopic anomalies in the JESSBERGER, E. K., AND STAUDACHER, Th. (1979). early solar system. Anna. Rev. Nucl. Sci. 28, 501. On the maximum initial temperature of the CLAYTON, R. N., GROSSMAN, L., AND MAYEDA, T. N6rdlinger Ries ejecta. Meteoritics 14, 432-434. K. (1973). A component of primitive nuclear compo- JESSBERGER, E. K., DOMINIK, B., KIRSTEN, T., AND sition in carbonaceous meteorites. Science 182, STAUDACHER, Th. (1977). New 4°Ar-.~gAr ages of 485-487. Apollo 16 breccias and 4.42 AE old anorthosites. In The authors would like to thank B. Mason for generously providing us with valuable Allende inclusions. We thank T. Kirsten, A. EIGoresy, J. L. Jordan. and D. D. Clayton for many stimulating discussions and two reviewers for helpful criticisms.

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