14C-39ArMe correlations in chondrites and their pre-atmospheric size

Earth and Planetary Science Letters, 25 (1975) 159-169 O North-Holland Publishing Company, Amsterdam - Printed in The Netherlands

14C-39ArMeCORRELATIONS

L_LJ

IN CHONDRITES AND THEIR PRE-ATMOSPHERIC SIZE

W. BORN and F. BEGEMANN Max-Planck-Institut f'tir Chemie (Otto-Hahn-lnstitut), Mainz (Germany)

Revised version received December 2, 1974 Cosmogenic 14C has been measured in 12 chondrites and the stone phase of the mesosiderite Bondoc. For the chondrites analysed the activities vary between 44 and 72 dpm]kg; the low value of (4.5 + 0.9) dpm/kg for Bondoc is essentially due to its large pre-atmospheric size and not to a terrestrial age of several half-lives of 14C. In eight cases agAr in the metal phase from the same meteorite specimens had been measured previously. The results are combined to derive the pre-atmospheric radii R o of the meteoroids and depth of burial of the samples investigated. Values ofR o between 35 and 82 cm are obtained; of 14 samples ten came from a depth of 10 cm or less. The preponderance of samples from shallow depths is ascribed to asymmetrical ablation losses of the meteoroids during their passage through the atmosphere. A compilation of all published C concentrations m chondrites shows that the variations between different specimens from the same meteorites are almost as large as those for samples from different meteorites. Thus, there is no need to invoke different orbits of the meteoroids and a strong spatial gradient in the primary cosmic-ray intensity to explain variations of low-energy-produced cosmogenic nuclides in different meteorites. 14

.

.

1. Introduction

Cosmogenic14C was first determined in stone and iron meteorites more than a decade ago by Goel and Kohman [1] and Suess and W~inke [2]. Aside from one single result o f Tamers [3] the only more recent and comprehensive study is that by Boeckl [4], although Tamers implies that more measurements on stone meteorites have been performed b y him. With the exception o f the latter the investigations were undertaken for two reasons. First, a number o f meteorites with known date o f fall ("observed falls") were analysed to investigate the general systematics o f the interaction o f cosmic rays with interplanetary matter. Secondly, after having established the average concentration o f 14C in "observed falls" this average value was then used to determine the terrestrial age of "finds", i.e. o f such meteorites for which the data o f fall is unknown. The latter procedure suffers from the fact, however, that "observed falls" exhibit widely disparate 14C activities. In stone meteorites, e.g., the total spread is more than a factor of two ( 3 4 . 4 - 7 7 . 8 dpm/kg) in spite o f only minor differences in target composition. Apparently, these variations are due to different fluxes

of those nuclear active particles which produce 14C, either because o f different orbits of the meteoroids or because o f different pre-atmospheric sizes. If different orbits were the reason there must at the same time be a gradient o f the cosmic ray intensity. Only then will different time-averaged distances o f the meteoroids from the sun - or different inclinations of their orbits with respect to the plane o f the ecliptic - result in different mean fluxes o f primary particles. At present it is impossible to make even an educated .guess as to what differences in the orbital parameters would be required to cause a difference of, say, a factor o f two in the mean primary flux. In spite o f considerable theoretical interest and quite some efforts by cosmic-ray physicists and meteoriticists there is still no consensus as to the gradient. Discrepancies exist between the values reported by various groups employing cosmic ray instruments on artificial satellites (cf. [5,6]); they exist as well between those derived from the 37Ar/39Ar ratios in the metal o f meteorites [7-10]. On the other hand, there is no doubt that the preatmospheric sizes o f meteoroids have not all been the same. Hence, different degrees o f shielding or build-up o f the secondary nucleon cascade will result in different

160 levels of activity even if the primary flux is constant. The evidence pointing to differences in the size of meteoroids comes not so much from the grossly different recovered meteorite masses although in extreme cases this certainly is a useful indicator. Rather, the more reliable and most widely employed methods are either the determination of (n, 3,)-produced nuclei like 36C1, 60Co, and 8°Kr or the comparison of the concentrations of such two cosmogenic nuclides which are produced by nuclear active particles in different energy regimes. Signer and Nier [ 11 ] used various stable rare-gas isotope and nuclide ratios measured in the iron meteorite Grant to deduce its size during the exposure to the cosmic radiation; more recently the same method has been applied to chondrites [12] by analysing extremely clean metal phases. In principle, numerous pairs of stable and/or radioactive nuclides are suitable although in bulk stone meteorites due to their chemistry it is hard to find a true high-energy product, i.e. a nuclide that is predominantly produced by projectiles with energies above a few hundred MeV. We therefore decided to compare the 14C activity in bulk samples with the 39Ar of the metal phase from the same meteorites. Using a pair of radionuclides instead of stable ones results perhaps in a somewhat larger error in the activity ratio, but for this particular nuclide ratio there are advantages as well. First, the half lives of both nuclides are short compared to all known exposure ages of meteorites. Thus, barring intensity changes of the cosmic radiation during the past 104 years or so, they are in radioactive equilibrium. They, furthermore, integrate the cosmic-ray flux over such a short period of time prior to the fall of the meteorites that their gradual decrease in size due to space erosion can be safely neglected. For the same reason the chance to be fooled by anomalous results which are due to a break-up of the meteoroid during its exposure history is greatly reduced. Finally, both nuclides can be expected to be completely retained in the meteorites, i.e. diffusion losses will not confuse the picture. The 39Ar data used in this analysis have been published previously [13]. Here we present the 14C resuits as well as an interpretation of the 14C/39Ar ratios based on model calculations by Born [14].

2. Experimental procedure and results Samples weighing about 20 g were pulverized in an agate mortar to < 100/l, occasional larger pieces of metal were broken up and added again. When necessary the fusion crust was removed prior to crushing the samples. The procedure to extract 14C was essentially that described by Goel and Kohman [ 1 ]. After addition of lamp black as carrier (-~ 1 g, produced by burning of inactive benzene), the sample was mixed with 100 g of PbCrO 4 + K2CrO 4 (10 : 1, pre-degassed at 850°C, pulverized and stored till use under dry nitrogen), transferred to an alumina crucible and heated at 1100°C in a closed system under 5 0 - 7 0 cm Hg of oxygen. The gases were circulated by means of an automatic Toepler pump. They passed through CuO at 700°C (CO ~ CO2), Pt-asbestos at 800°C (SO 2 ~ SO 3) and Ag-wool at 400°C (removal of SO 3 and halogenes). After 5 hours the CO 2 was removed from the system by means of a cold trap at the temperature of boiling nitrogen. The yield was always i> 96%, using for the native C content of the meteorites the median value of 0.1% given by Vdovykin and Morre [15]. Exception: Allende with 0.29% C [16]. The CO 2 was created with metallic lithium at 490°C, the ensuing lithium carbide converted with water to C2H2, and the acetylene separated from moisture and hydrogen by two cold traps at -80°C and -196°C, respectively [ 17]. At this stage the total yield was always t> 94%. The acetylene was used as counting gas in a thinwall anticoincidence proportional counter of the Oeschger-type [18,19] with a total volume of 2.5 liters and a central volume of 1.5 liters. It was housed within an iron shield (30 cm wall thickness), the cavity of which was lined with 2 cm of OFHC-copper. In order to avoid any fluctuations of background counting rate and efficiency due to different gas pressures the counter was always filled to a pressure of 200 torr. The efficiency was determined for various settings of the discriminators by calibration with C2H 2 prepared from an Amersham-Na2CO3 standard. The optimum figure of merit was obtained for an efficiency of 49%, corresponding to a background of 1.02 cpm. The latter was determined with acetylene produced from

161 l a m p black b y t h e same p r o c e d u r e used f o r t h e treat-

d p m / k g [ 2 0 ] , w e shall include it in t h e p r e s e n t dis-

m e n t o f t h e samples, using t h e same a m o u n t s o f

cussion.

reagents. In several instances t h e m e t e o r i t e + flux cake was pulverized u n d e r d r y n i t r o g e n , m i x e d w i t h a n o t h e r 100 g o f t h e c h r o m a t e s and r e h e a t e d . In n o case was

Fig. 1 s h o w s a h i s t o g r a m o f t h e d i s t r i b u t i o n o f all data r e p o r t e d in the literature. In case o f m u l t i p l e d e t e r m i n a t i o n s o n the same s p e c i m e n t h e average

any excess activity d e t e c t e d in the r e h e a t s , i.e. the possible c o n t r i b u t i o n to t h e t o t a l activity was < 5%. The results o b t a i n e d are listed in Table 1. With the e x c e p t i o n o f Plainview and B o n d o c all m e t e o r i t e s analysed are " o b s e r v e d falls". As Plainview, w h i c h was f o u n d in 1917, still has a 39Ar activity o f a b o u t 6

t h e same m e t e o r i t e s have b e e n a n a l y s e d t h e individual

value has b e e n p l o t t e d ; w h e n d i f f e r e n t s p e c i m e n s o f values are used. One result o b t a i n e d b y Boeckl for AUende (108 d p m / k g ) has b e e n o m i t t e d b e c a u s e " T h e r e is a r e m o t e possibility t h a t the value m i g h t b e a t t r i b u t e d t o c o n t a m i n a t i o n " [4]. As is evident f r o m t h e h i s t o g r a m the activities re-

TABLE 1 14C content of 12 chondrites and the stone phase of one mesosiderite. Where available the results obtained by other authors on different specimens from the same meteorites are included. All meteorites except Bondoc and Plainview are "observed falls".

Meteorite

Source

14C(dpm/kg)

Ref.

Remarks

AUende

C IIl

NASA, MSC -

60.4 -+ 3.1 64.3 -+ 4.3 108.0 +- 6.4

[4] [4]

crust 5 mm below crust

Beardsley

H

AML, 134.61

71.2-+ 2.0 51.2 -+ 2.7

[2]

stone phase

Bovedy

L

QUB

67.7 -+ 2.8

-

-

Forest City

H

NMC, 49 F

69.0 -+ 2.2

-

-

-

49

[11

-

H

NMNH, 4848-12

55.3 -+ 4.8

-

-

Nadiabondi

H

MNHN, 2370

55.0 -+ 3.2

-

-

Pantar

H

-

46.9 -+ 4.8 48.6 -+ 2.8

[2]

-

Plainview

H

AML, 92.363 -

71.8 -+ 2.3 61 -+4 43.2 -+ 2.9

[1] ~ [2]

found 1917

NMC, 100h

46.6 78 36.5 44.2

-+ 2.2 -+ 6 -+ 3.1 -+ 2.8

-

-

Lost

City

Richardton

H

-

-

+

5

)

[1]

-

[4] [4]

crust 5 mm below crust

St. Severin

LL

MNHN, D II 3'

58.1 -+ 2.1

-

-

Ucera

H

IVIC -

44.1 +- 2.1 34.4 +- 2.2

[3]

-

Zhovtnevyi

H

CMAN, 1199

70.2 -+ 2.3

-

-

Bondoc

Mes.

AML, (2) 684.87

4.5 +- 0.9

-

stone phase

CMAN: Committee on Meteorites, Academy Nauk U.S.S.R., Moscow; IVIC: Instituto Venezolano de Investigaciones Cientificas, Caracas; MNHN: Museum National d'Histoire Naturelle, Paris; NASA, MSC: Manned Spacecraft Center, Houston, Texas; NMC: Nininger Meteorite Collection, Tempe, Arizona; NMNH: National Museum of Natural History, Washington; QUB: The Queen's University of Belfast.

162

5&••j&v 0 x -I-

2

a*

a a

30

40

~AII chondrites 13/.)

•

~'l

~a

a

O + W &~[

50

60

70

80

3b' g o g o ~ ' ~ ' ~

30

40

50

60

70

1¢C [dpm/kg~"

"c Iapm/kgf

80

1~C [dpm/kg ]P

Fig. 1. Distribution of 14C activities measured in chondrites. The hatched boxes axe from this study, the others from the literature listed in Table 1. When more than one specimen from the same meteorite has been analysed this is marked to indicate the spreadobserved within one and the same meteorite. • = Allende; * = Beardsley; o = Bruderheim; + = Forest City; v = Harleton; o = Pantar; o = Plainview; A = Richardton; × = Ucera. ported here do not quite cover the same range as was found previously although the substantial spread is confirmed. When plotting H- and L-chondrites separately the distributions appear to be different. Due to the limited number of samples, however, it is not clear whether or not the difference is significant. At any rate it must be born in mind that L-chondrites contain about 10% (relative) more oxygen - the main target element; a correspondingly higher 14C is to be expected. It is noteworthy that about the same range of activities is observed for different meteorites as it is for different specimens from one and the same meteorite. The variations are especially pronounced in case of Richardton, Plainview, Beardsley, and Forest City, all meteorite showers where many different pieces adding up to large recovered masses have been collected over an area of up to 200 km 2 (cf. [21]). The only exception is Harleton where only a single stone of 8.36 kg has been recovered but where the 14C concentration reported by Goel and Kohman [1] is nevertheless 50% higher than that of Suess and W~inke [2]. Unfortunately, no rigorous inter-laboratory check has ever been undertaken to test the reproducibility of data obtained in different laboratories. However,

a comparison of the results reported by Boeckl [34] and Begemann et al. [33] for two comparable lunar samples (12002, depth 2 - 4 cm, 41.7 wt.% oxygen; 12053, depth 2 - 6 . 5 cm, 42.1 wt.% oxygen) shows good agreement within the experimental limits of error (26.7 -+ 3 and 29.7 -+ 3 dpm/kg). In the other cases we can only state that there appear to be no systematic differences between different laboratories.

3. Calculation of depth prof'des The fact that different specimens from the same meteorite have in extreme cases 14C contents which vary by more than a factor of two must obviously have its explanation in a strong dependence o f the production rate on the position o f the samples within the meteoroid. Hence, we shall first try to explain this observation and only then discuss whether or not there is any evidence in these data for different fluxes of the primary cosmic-ray intensity. For the calculation o f production rates in samples from different positions within meteoroids o f different size a number of models has been proposed in the past. Two approaches have been tried. One is to bombard thick targets with protons of various energies, measure the lateral and depth distribution of the product nuclides of interest, and then calculate the depth dependence for the case of an isotropic irradiation by a continuous spectrum of primaries [ 2 2 - 2 5 ] . The second one starts with calculating flux and spectral shape of the effective nuclear active particles as a function of position within and size of the meteoroids and then folding this with the excitation function for the production o f the nuclide in question ( [ 2 6 - 2 9 , 3 0 ] and references therein to numerous previous publications by this group). Unfortunately, none o f these calculations are directly applicable to the problem at hand because they either are not comprehensive enough or they do not contain the information needed in an accessible form. We, therefore, present depth profiles of our own but whenever possible shall compare the results with those obtained previously. Our approach is the second one mentioned above. In stone meteorites 14C is essentially produced on oxygen. The contributions from all other target elements can be neglected because of their smaller atomic

163 abundance, the larger mass difference AA = A (target) A (14C) and because 14C lies on the neutron-rich side of the/3-stability line which is not easily accessible to spaUation products proper. Experimental data for the production cross section on oxygen are available only for the bombardment with protons [31 ]. They are plotted in Fig. 2 (solid line). For neutrons as projectiles we shall adopt the excitation function derived by Reedy and Arnold [28]. For the spectral shape of the flux of nuclear active particles with E > 20 MeV we use the expressions given by Arnold et al. [27] for a depth of 100 g/cm 2 in an iron meteorite with a radius equivalent to 200 g/cm 2, i.e. for energies between 20 and 100 MeV.f(E)dE o: (E-1 + 0.01 E -2 + 1.1 X 10 -5 E -3) and for higher energies f ( E ) d E = (o~ + E) -2.5 with a = 200 MeV and 1 GeV for 100 MeV ~ 3 GeV, respectively. Although more recently Reedy and Arnold [28] have modified these expressions (for the case of the moon) by assuming the shape parameter a to depend on depth as well, the authors point out that this is of importance only for the production of nuclides which "have very low thresholds and high cross sections at very low energies". Both criteria are not fulfilled for the production of 14C from oxygen. Using these spectra the mean production cross sections o are calculated for the three energy ranges 20 200 MeV, 200 - 1000 MeV, and ~> 1 GeV. (Table 2). The total production rate is then: OO

P= f o

f(E)a(E)dE 200

=Clam

f 20

lOOO

f(E) dE +¢20"2 f

f(E)dE +

200

+e3~ 3 ? f(E)dE 1000 It varies within meteoroids because the ci are variable, i.e. absolute and relative contributions of the three energy regimes to the total production are depth and size dependent. Details of the rather involved calculations are given by Born [14]. Here, we restrict ourselves to briefly sketch the approach and to present the results which are all derived for spherical bodies. (1) E ~ 1 GeV. The flux is set to decrease exponentiaUy with an absorption mean free path of 190 g/cm 2

0 Irnbl

101

f

\

\

10-'

10-1

100

101 E [GeV]

Fig. 2. Cross section for the production of 14C from oxygen by protons (solid line [31]) and neutrons (dashed curve [28]).

and an interaction length of 94 g/cm 2. The latter is calculated from the average geometrical cross section for chondritic composition and a transparency of 15% [32] ; the ratio Xabs/Xint = 2 follows from the relation given in [27] with an inelasticity for light elements of 0.37. (2) 200 MeV ~
~n (rob)

~p (mb)

20 MeV < E ~ 200 MeV 200 MeV < E ~ 1 GeV E> 1 GeV

5.9 2.5 2.0

1.5 1.8 2.0

164 (3) 20 MeV ~ 200 MeV we use = 0.4"0 cm -2 sec -1 ster -1 raising by 10% the value derived by Kohman and Bender [23] for E > 300 MeV under the assumption that for cosmic-ray particles heavier than hydrogen the contribution of each nucleon is the same as that of a proton of the same energy. One third of this flux falls into the 200-MeV to 1-GeV region which is most seriously affected by solar modulation of the intensity; due to the preponderance in this energy region of secondary particles at shallow depth already no serious error is anticipated. The multiplicities are determined by fitting the calculated depth prof'des to the experimental data. As there are no meteorites with known preatmospheric size, however, this cannot be done unambiguously but only in the sense that the calculated curves cover the range of the measured activities. To determine m 1, the multiplicity for the production of secondaries with 200 MeV ~
as well as the metal phase from chondrites and stonyiron meteorites (cf. [9,13] and references therein). The highest 39Ar activity reported so far appears to be (31.4 + 2.5) dpm/kg for metal from the mesosiderite Veramin; all other values are below 28 dpm/kg (Fig. 3). From the cross-section data for the production of 39At upon bombardment of Fe and Ni [7,35] it is obvious that only projectiles with E > 200 MeV contribute significantly (see also fig. 2 in [7]). Hence, of the three energy regimes in our calculation just described only the first two have to be considered in this case. It turns out that for m 1 = 6 maximum activities of 28 dpm/kg are to be expected. Using then the value of m I = 6 the multiplicity m 2 is chosen so that maximum 14Cactivities of 80 dpm/kg are obtained - which is the case for m 2 = 5. It must be emphasized that at present not much meaning can be attributed to the value of m 2. What enters into the calculations is the product of multiplicity times cross section and for neutrons, which are the most important nuclear active particles, the latter is not known. Actually there is some indication that the cross sections for the production of 14Cby neutrons are considerably higher than estimated by Reedy and Arnold [28] (see below). The depth profiles for the production rate of 39Ar depicted in Fig. 4 can be compared with those calculated by Kohman and Bender [23] for 36C1 in iron meteorites. This comparison is legitimate because the measured 36C1/39Ar activity ratios in meteoritic metal, corrected for the decay of 39Ar since the time of fall

n 15-

10

5-

Iolo 20

o l-zl 3'o 3gAr [ dprn/kg J

_-

Fig. 3. Distribution of 39Ar activities in kon meteorites (~) and the metal from chondrites (rn) and stony-izon meteorites

(~).

165

_•

39At

30

~~~-~00

20I0

Q

~'C

h90

-60 tO-50-

-30

Fig. 4. Depth profiles for the production by galactic cosmic radiation of 39At in the metal from chondrites (solid lines) and 14C in bulk chondrites (dashed lines) for various preatmospheric radii Ro of spherical meteoroids. Assumed density: 3.5 g/cm3; oxygen content 33.8 wt.%.

of the meteorite, is always close to unity, i.e. the excitation functions for the production of the two nuclides appear to be very much the same. While the general shapes of the two sets of curves are very similar there are two main differences. Firstly, in our case the maximum activity is about 20% higher which simply reflects the fact that our choice for m 1 was such that decay rates of 29 dpm/kg occur somewhere within the meteorites while Kohman and Bender [23] arrived at maximum values of only about 23 dpm/kg. Secondly, for larger radii our curves are flatter resulting in activities in the center which are more than 20% higher. This comes about essentially because in stone meteorites the absorption length for nuclear active particles is twice the interaction length, while in iron the ratio is only 1.5 (see above). For 14C a comparison is possible with the depth profile given by Reedy and Arnold [28] for the moon (R = oo). Except for the top 15 cm the shapes of the two curves are in perfect agreement. The absolute activities of Reedy and Arnold, however, are about 40% lower than the ones calculated here which is almost certainly due to an underestimate of the cross section. According to Armstrong and Alsmiller [36] the flux of neutrons with E > 10 MeV at a depth of 10 g/cm 2 is around 2 neutrons/cm 2 sec. In order to produce the 14C activity of about 28 dpm/kg found at this depth

[33,34] the average cross section must be approximately 15 mb which for no reasonable shape of the neutron energy spectrum can be obtained from the excitation function given by Reedy and Arnold (Fig. 2).

4. Discussion In Table 3 the 14C contents of the bulk samples and the 39Armeasured in the metal phases are compiled. Only such observed falls are listed where the samples analysed came from the same meteorite specimen, the largest one having been smaller than fist size. In case of St. Severin and Bondoc the measured 14C activit~ (Table 1) has been normalized to the oxygen content of H-chondrites (33.8%) by assuming 38.8 and 41.8 wt.% oxygen, respectively [37,38]. To discuss the data we use the presentation of Fig. 5 which shows the theoretical results in a 14C39Ar diagram. Points of constant absolute depth within meteoroids of different radii are connected by thin lines. Note, that although the multiplicities were adjusted to obtain activities which cover the whole range observed, it is not trivial that all experimental points should fall inside the allowed region. From this diagram it appears that the pre-atmospheric radii of the meteorites analysed were between 35 and 82 cm (Table 3). Whenever there are 14C measurements available on more than one piece from the same meteorite (Beardsley, Forest City, Pantar, Richardton) the radii derived are compatible with the activities observed, i.e. in no case do the activities fall outside the range predicted for the individual R o determined from the 14C-39Ar correlation of our samples. For Richardton the high value of (78 -+ 6) by Goel and Kohman [1] can even be used to narrow the range in R o to 3 0 - 7 0 cm. This is, of course, due to the fact that this is the highest 14C activity observed so far in any sample and that our choice o f m 2 = 5 was made so that the maximum 14C content is 80 dpm/kg, i.e. barely exceeds the highest value observed. As this choice is somewhat arbitrary - although m 2 cannot be lower - it is perhaps worth mentioning that for any m 2 > 5 the R o arrived at would be even larger than given here if a sample falls into a region of the diagram where the R o = const, curves turn to the left (Beardsley, Forest City). The radii would be slightly smaller for Nadiabondi, St. Severin and Zhovtnevyi while in the

166 TABLE 3 14C in chondrites (normalized to 33.8 wt.% oxygen) and 39At in the metal from the same meteorite specimens. For the mesosiderite Bondoc the 36CI content is given instead of 39A1"(see tex0. In columns 4 and 6, respectively, are listed the pre-atmospheric radii R 0 of the meteoroids and the depth of burial d of the samples investigated as deduced from Fig. 6. The last column gives the equivalent radius calculated from the recovered mass

Beardsley Forest City Lost City Nadiabondi Pantar Richardton St. Severin Zhovtnevyi

14C(dpm/kg)

39Ar (dpm/kg)

71.2 69.0 55.3 55.0 46.9 46.6 50.6 70.2

19.5 18.5 20.0 25.1 20.0 21.0 23.2 25.7

Bondoc Beardsley Forest City Pantar Richardton

± 2.0 ± 2.2 ± 4.8 ± 3.2 ± 4.8 ± 2.2 ± 2.3 ± 2.3

3.6 ± 1 51.2 49 48.6 78 36.5 44.2

± 2.7 ±5 ± 2.8 -+ 6 ± 3.1 ± 2.8

Ro (em)

Range of Ro

+- 1 ±2 ± 1.5 -+ 2.5 ± 1.5 ± 2.5 -+ 1.5 ±2

70 75 82 35 82 70 50 35

1.5 ± 0.5

>200

-

-

30-70 -

-

r e m a i n i n g cases a larger m 2 will n o t a f f e c t R o at all as in this p a r t o f t h e diagram t h e R o = c o n s t , curves are a l m o s t vertical. A direct comparison with estimates of R o by other

[Cpmk/g] 80~-4d 70-

!

60- ~ 504020.

39At [dpm/kg metal]

Fig. 5. Correlation diagram between 14C content of bulk chondrites (33.8 wt.% oxygen) and 39~r in the metal for sphericM meteorites with various R o. Points of constant absolute depth d are connected by thin lines. The "surface" line does not pertain to the surface proper as the production of both nuclides by solar cosmic rays has not been taken into account. The positions of the meteorites listed in Table 3 are indicated. In case of Bondoc for the metal phase the value of the 36C1 activity is plotted rather than that of 39At (see text).

65-75 70-82 68-103 20-55 65-110 45-110 35-65 30-50

d (cm) 35 35 12 9 7 6 7 20

Range o f d 25-50 22-80 7-20 7-22 <12 <10 6-8 15-32

11 21 11 7 19 27 20

-

35

>200 8 7 8 60 ~2 5

r (cm)

< 10 <12 <10 >25 <5 <7

-

m e t h o d s is o n l y possible for St. Severin a n d L o s t City. A c c o r d i n g t o N y q u i s t et al. [12] t h e m i n i m u m pre-atm o s p h e r i c radius o f St. Severin was 35 c m ( 6 0 0 kg), B i b r o n et al. [39] give 3 0 c m (for a very p e c u l i a r prea t m o s p h e r i c shape), a n d Marti et al. [40] f i n d b e s t a g r e e m e n t o f t h e i r 54Mn d a t a w i t h t h e c a l c u l a t i o n s o f K o h r n a n a n d B e n d e r [23] f o r R o ~ 9 0 cm, a value o n w h i c h t h e y place little reliance, h o w e v e r . F o r Lost City, o n t h e o t h e r h a n d , t h e R o o f b e t w e e n 65 a n d 105 c m derived here is grossly at variance w i t h t h a t o f less t h a n 25 c m d e d u c e d b y Cressy [41] f r o m (n, 7)-prod u c e d 60Co. Most p r o b a b l y t h e e x p l a n a t i o n f o r this d i s a g r e e m e n t is t h a t t h e p r e - a t m o s p h e r i c s h a p e w a s n o t spherical as a c c o r d i n g t o M c C r o s k y et al. [42] " f o r L o s t City t h e evidence for a n initial flat s h a p e is r a t h e r g o o d " . This m a y affect q u i t e seriously t h e c o n c l u s i o n s b a s e d o n 60Co a n d t h e 14C-39Ar c o r r e l a t i o n s w h i c h b o t h h o l d for spherical b o d i e s only. W h a t c a n be c o m p a r e d in a d d i t i o n are t h e n u m b e r f r e q u e n c y d i s t r i b u t i o n s o b t a i n e d h e r e a n d in p r e v i o u s studies (Fig. 6). O b v i o u s l y all o u r samples lie o n t h e h i g h side in t h e h i s t o g r a m , b u t it m u s t b e k e p t in m i n d t h a t t h e K r - B r as well as t h e s p a l l a t i o n m e t h o d yield minimum values for R o w h i c h w o u l d p e r t a i n i f all samples a n a l y s e d c a m e f r o m t h e p r e - a t m o s p h e r i c c e n t e r

167

t,,

R0lcm] Fig. 6. Distribution of pre-atmospheric radii of stone meteorites as determined by various methods: m = this paper; @= K r - B r method [46]; [] = spallation method [12]; [] (n, 7)36C1 method [47]. Note that the latter three methods yield minimum values for Ro.

of the meteoroids. Aside from probability arguments, direct measurements of induced radionuclides [40] and VH cosmic-ray tracks [43] as well as the position of the points in Fig. 5 show this not to have been the case. In fact, of the 14 samples listed in Table 3, ten come from a depth of about 10 cm or less although the corresponding pre-atmospheric radii vary between 35 and 82 cm. Among the samples for which we obtain a depth of less than 10 cm is that from St. Severin where Marti et al. [40] have presented convincing evidence that their sample, which was adjacent to ours, comes from a distance of 3 - 4 cm from the pre-atmospheric surface. The preponderance of specimens from shallow depths might be expected from the trivial fact that the volume of spherical shells with constant thickness is proportional to their radius, i.e. that there is more mass in the outer shells than in the inner ones. In order to find material from the outside, however, ablation losses during the passage of the meteoroids through the atmosphere must be small while the gross discrepancies between pre-atmospheric and recovered mass indicate large losses. One possible explanation for this apparent contradiction would be a break-up of the meteoroids into many pieces and a very low yield for finding these. Although in case of meteorite showers the recovery certainly will never be 100% we do not think this to be the sole reason. We rather suggest that asymmetrical ablation losses are the main reason as it would reconcile large ablation losses with the predominance of shallow samples.

Finally, a few words must be said with regard to Bondoc which has been included in Table 3 and Fig. 5, although its date of fall is unknown. Two samples of metal from different specimens (AML (2) 684.9 and AML (2) 684,87) have been analysed for 360, 39Ar, and the light rare gases, yielding for 36C1 (3.2 -+ 0.7) and (1.5 + 0.3) dpm/kg, for 39At (0.5 + 0.6) and (0.1 + 0.2) dpm/kg, and 36C1-36Ar exposure ages of (142 + 35) and (156 + 35) m.y., respectively [44]. For both samples the 39Ar content was below the limit of detectability, but due to the large relative errors it is not possible to give a reliable figure for the minimum terrestrial age. (On the lo level it would be about 103 years, on the 2o level it might already be zero!) While the 14C content does not help to solve this problem the position of Bondoc in the 14C-39Ar diagram confirms that the low 14C activity is due to a large pre-atmospheric size of the meteoroid and not to a terrestrial age large compared to the half life of 14C. In our calculations and the discussion so far the assumption has been made of a constant flux of the primary galactic cosmic radiation. As meteoroids do not all have the same orbits (the aphelia of the photographicaily well documented P~ibram and Lost City were 4.04 and 2.56 A.U., respectively), this implies a negligible time-averaged gradient of the cosmic-ray intensity. Of course, if this assumption were false there would be different production rates even in meteoroids of the same size if only their orbital parameters were sufficiently different. The extreme view that different activities in different meteorities are mainly due to such orbital differences has recently been taken by Lawukhina and Ustinova [30] and Cameron and Top [45], both using 26A1. There is no direct information in our data which would allow to decide unambiguously whether different pre-atmospheric masses or different orbits are the essential reason for the different activities observed. We wish to re-emphasize, however, what has been said above, namely that the spread in 14C activities observed for different specimens from the same meteorites which, by the way, is true for 26A1 as well. Hence, there is at least no need to invoke different orbits and a strong gradient of the galactic cosmic-ray intensity to explain the differences observed and any conclusions based on such an interpretation should at present be considered with due caution.

168

Acknowledgements We wish t o t h a n k all d o n o r s o f t h e m e t e o r i t e specim e n s investigated, especially Drs. R.S. Clarke, Jr., J. Labeyrie, J. Meighan, C.B. Moore, A. King, F. K r a u t and E.L. Krinov. W i t h o u t t h e i r g e n e r o u s s u p p o r t this investigation w o u l d have b e e n impossible. We are i n d e b t e d to Mr. H. K r u s e for his able assist a n c e in p e r f o r m i n g t h e c o m p u t e r c a l c u l a t i o n s a n d t o Dr. H. W~nke for h e l p f u l discussions.

References 1 P.S. Goel and T.P. Kohman, Cosmogenic carbon-14 in meteorites and terrestrial ages of "finds" and craters, Science 136 (1962) 875. 2 H.E. Suess and H. W~inke, Radiocarbon content and terrestrial age of twelve stony meteorites and one iron meteorite, Geochim. Cosmochim. Acta 26 (1962) 475. 3 M.A. Tamers, Natural radiocarbon measurements, VI, Radiocarbon 13 (1971) 32. 4 R. Boeckl, Terrestrial age of nineteer~ stony meteorites derived from their radiocarbon content, Nature (London) 236 (1972) 25. 5 J.J. O'Gallagher, Observations of the radial gradient of galactic cosmic radiation over a solar cycle, Rev. Geophys. Space Phys. 10 (1972) 821. 6 W.R. Webber and J.A. Lezniak, Interplanetary radial gradients of galactic cosmic-ray protons and helium nuclei: Pioneer 8"and 9 measurements from 0.75 to 1.10 A.U., J. Geophys. Res. 78 (1973) 1979. 7 M.A. Forman, R.W. Stoenner and R. Davis, Cosmic-ray gradient measured by the argon-37/argon-39 ratio in the Lost City meteorite, J. Geophys. Res. 76 (1971) 4109. 8 E.L. Fireman and G. Spannagel, Radial gradient of cosmic rays from the Lost City meteorite, J. Geophys. Res. 76 (1971) 4127. 9 F. Begemann, Argon-37/argon-39 activity ratios in meteorites and the spatial constancy of the cosmic radiation, J. Geophys. Res. 7 (1972) 3650. 10 G. Heusser and O.A. Schaeffer, The meteorite Canon City and cosmic-rays variations in time and space, Trans. Am. Geophys. Union 55 (1974) 334. 11 P. Signer and A.O. Nier, The distribution of cosmic-rayproduced rare gases in iron meteorites, J. Geophys. Res. 65 (1960) 2947. 12 L. Nyquist, H. Funk, L. Schultz and P. Signer, He, Ne and Ar in chondritic N i - F e as irradiation hardness sensors, Geochim. Cosmochim. Acta 37 (1973) 1655. 13 F. Begemann and E. Vilcsek, Chlorine-36 and argon-39 production rates in the metal of stone and stony-iron meteorites, in: Meteorite Research, ed. P. Millman (1969) 355.

14 W. Born; 14C in Meteoriten und Mondproben: Messungen und flare Deutung durch Vergleich mit berechneten Tiefenprofilen, Dissertation, Mainz (1973). 15 G.P. Vdovykin and C.B. Moore, Carbon, in: Hanbook Elem. Abund. Met., ed. B. Mason (1971) 81, 16 R.S. Clarke, E. Jarosewich, B. Mason, J. Nelen, M. G6mez and J.R. Hyde, The Allende, Mexico, meteorite shower, Smithsonian Contrib. Earth Sci. 5 (1970). 17 C.L. Hubbs and G.S. Bien, La Jolla radiocarbon measurements, V, Radiocarbon 9 (1967) 261. 18 F.G. Houtermans and H. Oeschger, Proportionalz~flalrohr zur Messung schwacher Aktivit~iten weicher #-Strahlung, Helv. Phys. Acta 28 (1955) 464. 19 H. Oeschger, Low-level counting methods; in: Radioactive Dating (Intern. Atomic Energy Agency, Vienna, 1963) 13. 20 H. W~inke, personal communication. 21 H.H. Nininger, Meteorite distribution on the earth; in: The Moon, Meterorites and Comets, eds. B.M. Middlehurst and G.P. Kuiper (1963) 162. 22 M. Honda, Spallation products distributed in a thick iron target bombarded by 3-BeV protons, J. Geophys. Res. 67 (1962) 4847. 23 T.P. Kohman and M.L. Bender, Nuclide production by cosmic rays in meteorites and on the moon; in: HighEnergy Nuclear Reactions in Astrophysics, ed. B.S.P. Shen (1967) 169. 24 B.M.P. Trivedi and P.S. Goel, Production of 22Na and 3He in a thick silicate target and its application to meteorites, J. Geophys. Res. 74 (1969) 3909. 25 B.M.P. Trivedi and P.S. Goel, Nuclide production rates in stone meteorites and lunar samples by galactic cosmic radiation, J. Geophys. Res. 78 (1973) 4885. 26 K.H. Ebert and H. W~inke, Woer die Einwirkung der H~Shenstrahlung auf Eisenmeteorite, Z. Naturforsch. 12a (1957) 766. 27 J.R. Arnold, M. Honda and D. Lal, Record of cosmic-ray intensity in the meteorites, J. Geophys. Res. 66 (1961) 3519. 28 R.C. Reedy and J.R. Arnold, Interaction of solar and galactic cosmic-ray particles with the moon, J. Geophys. Res. 77 (1972) 537. 29 Y. Yokoyama, R. Auger, R. Bibron, R. Chesselet, F. Guichard, C. Leger, H. Mabuchi, J.L. Reyss and J. Sato; Cosmonuclides in lunar rocks, Proc. Third Lunar Sci. Conf., Suppl. 3, Geochim. Cosmochim. Acta 2 (1972) 1733, M.I.T. Press. 30 A.K. Lavrukhina and G.K. Ustinova, Cosmogenic radionuclides in stones and meteorite orbits, Earth Planet. Sci. Lett. 15 (1972) 347. 31 M.A. Tamers and G. Delibrias, Sections efficaces de l'oxygbne-16 pour la production de carbone-14 par des protons de hautes ~nergies, Compt. Rend. 253 (1961) 1202. 32 N. Metropolis, R. Bivins and M. Storm, Monte Carlo calculations on intranuclear cascades, Phys. Rev. 110 (1958) 185,204. 33 F. Begemann, W. Born, H. Palme, E. Vilcsek and H. W~inke, Cosmic-ray produced radioisotopes in Apollo 12 and Apollo 14 samples, Proc. Third Lunar Sci. ConL, Suppl. 3, Geochim. Cosmochim. Acta 2 (1972) 1693, M.I.T. Press.

169 34 R.S. Boeckl, A depth prof'de of 14C in the lunar rock 12002, Earth Planet. Sci. Lett. 16 (1972) 269. 35 R. Davis, R.W. Stoenner and O.A. Schaeffer, Cosmic-ray produced 3TAr and 39At activities in recently fallen meteorites, in: Radioactive Dating (Intern. Atomic Energy Agency, Vienna, 1963) 355. 36 T.W. Armstrong and R.G. Alsmiller, Calculation of cosmogenic radionuclides in the Moon and comparison with Apollo measurements, Proc. 2nd Lunar Sci. Conf., Suppl. 2, Geochim. Cosmochim. Acta 2 (1971) 1729, M.I.T. Press. 37 W.D. Ehmann, Oxygen, Handbook Element. Abundance Met., ed. B. Mason (1971) 99. 38 F. Teschke, private communication (1972). 39 R. Bibron, C. Leger, J. Tobailem, Y.Yokoyama, H. Mabuchi and N. Baillard, Radionuclides produits par le rayonnement cosmique dans h m6t~orite Saint-Sgverin, Geochim. Cosmochim. Acta 38 (1974) 197. 40 K. Marti, J.P. Shedlovski, R.M. Lindstrom, J.R. Arnold and N.G. Bhandari, Cosmic-ray produced radionuclides and rare gases near the surface of St. Severin meteorite, in: Meteorite Research, ed. P. MiUman (1969) 246.

41 P.J. Cressy, Cosmogenic radionuclides in the Lost City and Ucera meteorites, J. Geophys. Res. 76 (1971) 4072. 42 R.E. McCrosky, A. Posen, G. Schwartz and C.-Y. Shao, Lost City meteorite - its recovery and comparison with other fireballs, J. Geophys. Res. 76 (1971) 4090. 43 P.B. Price, R.S. Rajan and A.S. Tamhane, On the pre-atmospheric size and maximum space erosion rate of the Patwar stony-iron meteorite, J. Geophys. Res. 72 (1967) 1377. 44 F. Begemann, H. Hintenberger, E. Vilcsek and H. Weber, in preparation. 45 I.R. Cameron and Z. Top, Measurement of 26A1 in stone meteorites and its use in the derivation of orbital elements, Geochim. Cosmochim. Acta 38 (1974) 899. 46 O. Eugster, P. Eberhardt and J. Geiss, Isotopic analysis of krypton and xenon in fourteen stone meteorites, J. Geophys. Res. 74 (1969) 3874. 47 F. Begemann and E. Volcsek, Durch Spallationsreaktionen und Neutroneneinfang erzeugtes 36C1 in Meteoriten und die praeatmosphiirische GriSsse yon Steinmeteoriten, Z. Naturforsch. 20a (1965) 533.