Rare gases and 36Cl in stony-iron meteorites: cosmogenic elemental production rates, exposure ages, diffusion losses and thermal histories

Geochlmica

et Cosmochimlca

Acta.

1976, Vol.

40, pp. 353 to 368.

Pergamon Press. Printed in Great Brltam

Rare gases and WI in stony-iron meteorites: cosmogenic elemental production rates, exposure ages, diffusion losses and thermal histories HINTENBERCER

F. BEGEMANN,H. W. WEBER, E. VILCSEK and H. Max-Planck-Institut

fiir Chemie (Otto-Hahn-Institut),

(Received

1975; accepted

24 March

Mainz, Germany

in revised form 26 September

1975)

Abstract-Metal

and silicate portions from 13 mesosiderites, one pallasite, Bencubbin (‘unique’) and Udei Station (‘iron with silicate inclusions’) have been analysed for their content of He, Ne and Ar; in most cases 36CI could be determined as well. 36C1-36Ar cosmic ray exposure ages fall between 10 and 160 Myr. Half of the metal samples show a deficit of spallogenic 3He (up to 30%) which we ascribe to a loss of tritium. The observed depletion of 3He in the silicates is correlated with their mineralogical composition: feldspar has lost its 3He in all cases, pyroxene definitely in one and possibly in five others, while olivine has been affected in only two meteorites. The thermal histories during their exposure to the cosmic radiation have been different for different meteoroids. Nevertheless, with the exception of Veramin, the data are compatible with the assumption of a continuous diffusion loss during a considerable fraction of the exposure era. For Veramin, however, an episodic event late in the exposure history is required. The exceptionally high 3gAr/36C1 ratio in the metal, which is due to a high 39Ar activity, indicates that the event occurred during the last 500,000 years or so and resulted in an extremely excentric orbit (large aphelion). Production rates of 3*,39Ar from Ca and 2’,22Ne from Mg are given. The ratio Pz,“/P$ is close to unity. The ratios Pz,8/Ps,” vary between 20 and 50, and are not correlated with the absolute produc, production ratio from Mg is found to be close to tion rate of “Ar from metal. The “Ne/“Ne but below unity. Of the mesosiderites only Veramin shows unambiguous evidence for primordial rare gases with larger amounts and a higher ‘“Ne/36Ar ratio in the olivine, suggesting in situ fractionation to have at least been partly responsible for the abundance pattern found. Bencubbin contains large amounts of strongly fractionated primordial gases, but again part of the fractionation may have occurred in situ. Udei Station shows an excess of (3.5 k 0.6) x lo- ” cm3 STP “‘Xc/g in the non-magnetic portion, OF THE

presently known 1700 or so meteorites about 500 have been studied by noble gas mass spectrometry for their content of spallogenic, radiogenic and primordial rare gases; a much smaller number has been analysed for at least some radioactive cosmogenie nuclides as well. These studies comprise an appreciable fraction of almost all classes or groups of meteorites, with the notable exception of the mesosiderites. Although 20 members of this group are known no systematic investigation of this kind has ever been undertaken. Furthermore, in most of the few cases where data do exist the sample desFription-whether metal or silicate or a mixture of both has been analysed-is so incomplete as to make a reasonable interpretation of the results almost impossible. Hence, a few years ago we decided to start such an investigation. Our aim was, first, to fill the gap in our knowledge about an important class of meteorites by analysing as many different mesosiderites as possible. (We were made to realize very soon, however, that the attainment of this goal would depend very much on the generosity of the wardens of these samples.) Secondly, as mesosiderites consist of about equal amounts of metal and rather coarse-grained different silicates, they seemed to be well suited to determine the absolute and relative production rates of various cosmogenic nuclides from different target elements and to study the behaviour of these nuclides during the subsequent history of the mesosiderites.

In addition to 13 mesosiderites we included in this study one pallasite (Marjalahti) as well as Bencubbin and Udei Station. The latter two at various times in the past have been classified as mesosiderites but they are now considered to be ‘unique’ and an ‘iron with silicate inclusions’, respectively (LOVERING. 1962; POWELL, 197 1). EXPERIMENTAL

PROCEDURE

AND RESULTS

I. Sample preparation

1. Metal. In spite of the heterogeneous structure of mesosiderites the preparation of large amounts of clean metal as required for the determination of 36C1 and “Ar was not always simple. The procedure finally adopted was dry milling at low speed and filing down of fragments larger than 1 mm. The metal was then separated by means of a hand magnet, washed repeatedly with ethanol, and dried at 80°C. Small aliquanta of approximately 0.5 g to be used for the mass spectrometric analyses were in addition treated with HF (38-40’4) and inspected under a binocular microscope, where any visible silicates were removed mechanically. 2. ‘Silicates’. For the analysis of the ‘non-magnetic’ portions for their content of the two radioisotopes, a choice had to be made between either using the samples left over from magnetic separation of the metal directly (i.e. with significant amounts of finely dispersed metal still contained therein), or removing the last traces of ferromagnetic particles at the cost of having very small samples. We decided upon the former but nevertheless shall henceforth refer to these samples as ‘silicates’.

F. BEGEMANN,H. W. WEBER,E. VKCSEK and H. HINTENBERGER

354

Table 1. Description and composition of samples used for radioisotope measurements (Me = metal, St = non-magnetic, * = leached with 10m3N H,SO,). The entries under ‘sample’ are in g, for Cl in ppm, all others in wt “/,. Errors for Fe, Ni and Ca are f30i, for Cl & 107; Neteorire

aarea

NRCN

BO”dOC

II

Buduian

26.3

6.3

83.1

St

IS.8

-

16.5

.45

3.2

9u.3

7.35

Me

33.5

AML (2)684.87

Me’

87.”

Me

86.8

MAS 2432*2439

Me(&)27.5

ANT.

Bondoc

Me

(2)684.9

Me(B)l St

25.7

St

12.5

Me

25.3

St

12.4

Me

20.4

Me

st* St

25.0 9.9 9.9

100

-

6.3

7.52

-

.34 4.4,

.09

-

a.3

-

-

-

450

7.5

86.4

7.08

.I7

C5”

7.6

II.1

81.2

7.13

.24

18”

8.1

8.6

79.1

7.21

.42

560

8.4

100

-

.*I

90

19.0

1.38

2.76

67.3

5.96

.81

-

-

14.5

.61

4.94

-

a.5

8,.4

6.85

.3”

-

7.6

”

-6.88

.49

90

8.

.02

140

.61

6.06

165

7.38

.2L

21.4

10.1 100

78.5 _

-

II.3

5.7

84.9

-

12.6

-

6iD

-

8.1

I

8.0

Me

33.t

St

15.9

Me

22.9

7.3

85.2

7.38

.22

-

8.0

Me

17.6

10.7

B0.i

7.32

.25

-

8.4

St

23.7

-

Me

IS.1

6.6

82.8

9.96

.33

-

10.7

Me

13.1

7.0

84.8

7.24

.22

-

7.9

St

8.0

-

11.4

.06

3.11)

-

____

_

14.8

. 28 4.50 . 33 5.6”

-

_---

iiencubbin

WAN

Me

,2.2

15.1

79.2

5.22

c.25

-

6.2

tiarjalahri

HYGL

Me

85.6

4.5

85.9

8.88

c.10

-

9.4

Udei

RhSA

t4e

28.4

2.5

87.0

9.10

c.,o

-

9.5

Scation

_-

12155c

--------..----___

AML: American Meteorite Laboratory; GSI: Geological Survey of India, Calcutta; HYGL: Helsingin Yliopiston Geologian Laitos, Helsinki; MAS: Museum Academy of Sciences, Moscow; MNH: Museum of Natural History, Chicago; MNCN: Museo National de Ciencias Naturales, Madrid; NM: Naturhistorisches Museum, Vienna; MZPAN: Muzeum Ziemi, Polska Akad. Nauk. Warsaw; WAM: Western Australian Museum. Perth. The samples analysed wufss spectro~netricu~ly were in most instances not true aliquants of these portions. From the bulk silicates only the fraction > 150 pm was used. Furthermore, the milled material was inspected for what to the uninitiated looked like olivine and whenever possible (Bondoc II, Emery, Lowicl Mincy, Veramin) a sample of such crystals was prepared by hand-picking. (The samples Emery II, III came from the interior of our specimen, and the Emery I sample from close to the fusion crust; the maximum distance between the three samples was 5 cm.) In the case of Clover Springs a 1 g sample was pulverized in a corundum mortar to 3.40 g/cm3. II. Measurement tion rates

ofradioisotopesand

calculation

of produc-

The experimental procedures for the dissolution of the samples were those employed routinely in our laboratory (cf. e.g. BEGEMANNand VILCSEK.1969). Metal was dissolved in a solution of ammonium-peroxydisulphate (Marjal~ti 30% in 2 N H,SO,, in all other cases IO or 20% in HzO). For the silicates two alternate methods were used depending on whether or not we expected to find 39Ar: mixing of the silicates with ammonium hydrogen fluoride and addition of cont. HzSOL in a vacuum system, or use of soda as flux. In both cases SO-100 mg of NaCl spiked with

37Cl were added as carrier. When using soda as flux the native Cl-content of the samples could be measured by the isotope dilution method. When treating the samples with NH4HF2 + HzS04, however, appreciable amounts of Cl were introduced from the glass vacuum system although the silicate-fluoride mixtures themselves were contained in a Pt beaker. Here, then, the total amount of Cl calculated from the isotopic analyses could only be used to determine the chemical yield. From the rapid formation of ‘rust’ on the fresh surface of a metal nodule from Bondoc II this sample was suspected to contain large amounts ofC1. presumably as lawrencite. Hence, prior to their dissolution the shavings were treated repeatedly with low3 N H,SO+ In this way a total of 450 ppm Cl was leached; it was processed and counted separately in order to search for (n,y)-produced %Jl. In the same way the silicate fractions of Budulan and Emery were leached for 70 and 20 hr, respectively, before dissolution. Concentrations of the relevant target elements for the production of the radioisotopes were determined by wet chemistry. Table 1 gives a brief description of the specimens availabte to us as well as the results of the chemical analyses for the samples used. With the exception of Clover Springs and Bencubbin all ‘metal’ samples consisted of 290% FeNi. For the mesosiderites the Ni content of the dissolved metal phases falls between 7.5 and 8.476, with

355

Rare gases and 36C1 in stony-iron meteorites n

1

‘1

,

n4

6

4

8

,

,

lb

12

‘1 ~b

6

,

i3

lb

1’2

Powezl9691

Wassr

&

et aL(1974)

[weight %I

Fig. 1. Frequency distribution of Ni-content in metal from mesosiderites. the sole exception of Vaca Muerta. As aqueous solutions used as solvents for this metal, do not of (NH&S&> dissolve troilite to any significant degree no addition 01 iron from this source is to be expected. Similarly, any poss-

ible contribution of iron or nickel contained in the dissolved Ca-bearing minerals can be neglected. Hence, to a good approximation the Ni/(Fe + Ni) ratios determined are those of the meteoritic metal. Figure 1 shows a histogram of the frequency distribution found. For comparison, the data reported by POWELL (1969) and WATSONet al. (1974) are shown as well. AS is apparent, the spread in our ratios determined on much larger samples than used in the other two studies is considerably smaller, which, according to WASSON’S(1974) criterion, makes the conclusion inevitable that our data ‘are clearly superior’. The Ni content of metal from the pallasite Marjalahti was found to be 9.4% while WIIK (1963) gives only 8.16%. The only explanation we can offer is that our sample may have contained relatively more plessite and taenite. Considering that our analysis was performed on a large amount of metal the 9.474 may be more representative of the meteorite which, incidentally, would make it more typical of pallasites. According to the compilation of MASON (1963) WIIK’S (1963) value of 8.167; Ni is the lowest of all pallasites. In the case of the stone phases a comparison with the results obtained by other authors is rather meaningless as our silicate phases contained FeS and some metal. In Table 2 the counting data are compiled. Columns 4 and 5 list the measured specific activities (in dpmfig dissolved matter) which for 39Ar from ‘observed falls’ are corrected for radioactive decay. From these activities and the concentrations of Fe, Ni, and Ca as given in Table 1 the production rates of 36C1 on pure FeNi and Ca have been calculated (columns 6 and 7). For the silicate portions

Table 2. j6C1 and “Ar activities in different phases of the meteorites (Me = metal, St = non-magnetic). The entries in columns 4 and 5 are in disintegrations per min per kg dissolved matter, except for the leach fractions where the units are dpm/lOOmg Cl. For observed falls the 39Ar has been corrected for radioactive decay. In the last three columns the decay rates per kg of pure FeNi and Ca are given; for 39Ar those for pure FeNi are identical with the measured ones in column 5 Neteorice

“ate fall

,842

Of

36cl

Phase

19A,

3bc1 FrNi 24.9’I

Me

36Clca .o

143t,5

3g.4r ca 4820

St Find

Me

3.21.7

Me(leach) Me Find

1.5f.3 13.7’-3.0

Me(A)

1400ti,o*

Me(B) St(leach)

St Find

Me St

Find

Me

Find

Me

Stcleach) St Lowicz

1935

Me

St Find

Me

Find

Me

Find

Me

St Yaca

bfuerra

“erami”

,880

Me St

Bcncubbin

Find

Me

24.7t1.3

8.4f.5

24.3t1.5

Marjalahti

1902

Me

22.Zf1.8

23.Ok1.U

22.2?1.8

Udei

1927

Me

!9.6’.8

23.9t\.,

19.6i.8

Station

-

-

-

-

-

356

F. BEGEMANN, H. W. WEBER, E. VILCSEK and H. HINTENBERGER

III. Mass spectrometric analyses the contribution to the activities from Ti and Mn has been taken into account by increasing the Fe-content by 2% The samples (5@300 mg) were wrapped in 5 pm iron foil (absolute). [Since neither element was determined in this and pre-heated at 100°C for 2 hr. They were dropped by study, we used their mean concentration as given by POWmeans of a hand magnet into a Ta crucible and completely ELL (1971) and a weight factor of two.] K has been negvaporized by induction heating of the crucible to about lected; because of the high ratio Ca/K > 200 this is not 1800°C for 30 min. Of the silicate samples 50 mg aliquants expected to introduce any significant error. Where, due were set aside for chemical analysis. to the lack of sufficient amounts of silicates, only the metal Each sample was preceded and followed by a blank run with either empty foils or metal pieces (5 200 mg) from could be analysed, we assumed a value of 10 for the ratio of the El Taco meteorite which has an exceedingly low conthe production rates P(Ca)/P(Fe) and corrected the activities tent of rare gases (NYQUIST et al., 1967; SCHULTZ and HINmeasured on the metal phases accordingly. As this correction is always ~4% the exact value of the ratio is not critical. TENBERGER,1967). The blank following the sample served as a re-heat run. Only for Emery St11 did it contain a Potentially more serious is the neglect of any contribusignificant fraction (2 lO”d,) of the amounts of the cosmotion to the activity which might be due to neutron capture genie rare gases found in the main run; in all other cases by 35C1. Although mesosiderites, because of their high Feit was < 1%. content, will not allow as many neutrons to reach the The gases were purified by exposure to Ti sponge which epithermal and thermal regions as do chondrites (EBERcooled from 700°C to room temperature within IOmin. HARDT et al.. 1963), this is possibly compensated for by Argon and the heavier rare gases were adsorbed on charthe high Cl-contents if the few data in Table 1 are represencoal at the temperature of boiling nitrogen and He and tative at all-and if the Cl found was indeed native to the Ne admitted to the mass spectrometer. After their analysis meteorites. For Bondoc II, Budulan, and Emery we have they were pumped off, and the heavier gases were released attempted to measure this contribution, with essentially from the charcoal at 12o’C and measured. negative results in case of Bondoc and Emery (Table 2, The mass spectrometer is an all-metal double-focusing column 4j. The Cl leached from Budulan, however, is instrument equipped with a Nier-type ion source, a clearly radioactive. (The sample has been counted 8 times, 1Cstage secondary electron multiplier, and a Cary vibratthe activity remaining constant after a second radiochemiing reed amplifier. Gas amounts were determined by comCal purification.) paring the peak heights with those of calibrated standard Taken together the results for Budulan are very puzzling. They cannot be explained in a self-consistent way, if all samples of comparable size. Each day at least one calibthe Cl in the four samples (Table 1, column 9) is native ration run was performed with standards having atmospheric composition except for He. where two different stanCl with a constant specific activity equal to that measured dards with 4He/3He = 5 and 20, respectively, were used. in the Cl leached from the silicates. An explanation may The He, Ne and Ar concentrations are compiled in be that our specimen of Budulan consisted of many small Table 3. They have been corrected for extraction blanks pieces which were badly corroded. Although they were which for “Ne were in all cases less than l”);, and for cleaned superficially before filing the non-magnetic portion He and “Ar less than So/, (the highest corrections for Dalcontained considerable ‘rust’. If the spallogenic 36CI in garanga). Even after this correction, however, the these corrosion products as well as a terrestrial Cl-contaargon-and occasionally the neon as well&from the metal mination are partly leachable, the measured specific acphases still contains an unusually large non-spallogenic tivity of the leached chlorine may be quite misleading. It component. At least in case of argon it is improbable that can be too high as well as too low. The high activity of this admixture is primordial argon, as in a three-comthe silicate portion, however, cannot be explained in this ponent-plot the points fall at or above the line connecting way. It rather indicates an extremely high production rate atmospheric and spallogenic argon (Fig. 2). [Only in case on Ca, which is in agreement with the result obtained from of Bencubbin (metal and silicates) and Veramin (silicates) a combination of the two metal samples assuming the (n,y)is there clear evidence for primordial Ar.] It appears that produced activity to be zero. Because of these ambiguities the reason may lie in the much larger surface area of the we have assigned rather large errors to the production rates from FeNi and Ca. meteorite filings as compared to the metal samples used as blanks. As it would be surprising for an adsorbed In case of 39Ar the production rates on Ca can be given atmospheric component to persist after pre-heating the only for the three observed falls Barea, Lowicz, and Veramin. (Marjalahti and Udei Station contain too little Ca samples at lOO”C, the gases are perhaps introduced to make a measurement feasible without using too much mechanically during the milling and filing. material.) For the first two, where both P(36C1) and P(39Ar) Included in Table 3 are data obtained by other authors on Ca have been measured, the ratio P(3”Cl)/P(3’Ar) is on the meteorites analysed in this study. Only for Mincyabout three. This implies that the small amounts of Ca metal and Marjalahti-olivine is a comparison justified, however. In all other cases the sample description is inadein the metal phase contribute only of the order of 1% to the 39Ar-activities of the metal phase. Thus, the producquate. Perhaps surprising is nevertheless the good agreetion rates on pure metal are equal to the ones measured ment in the He concentrations with those of PANETH and and listed in column 5. URRY (1931) which are among the earliest such determinaMost of the data for the metal phases have been pubtions on meteorites. In the case of the stone phase of Bonlished previously (BEGEMANNand VILCSEK, 1969). Although dot II the discrepancies between our data and those of some of the samples have been recounted, leading to HINTENBERGER et al. (1964b), which were obtained on slightly different activities, and Budulan and Emery have samples from the same specimen, cannot be due solely to been added, the discussion is not changed. We shall not a different chemical composition. Rather, a re-examination repeat it here. of the old note books has shown that for this particular The ratios of the production rates P(Ca)/P(Fe) will be sample unusually large amounts of inactive gases were discussed in more detail in connection with the rare gas released during the extraction procedure. Part of the gas data. Here it suffices to mention that for 39Ar the mean sample may have been lost through the Hg cut-off valves, value of about three is in good agreement with that of a loss which would not have affected He and Ne to the 2.9 obtained for the enstat& chondrite Abee (BEGEMANN same extent (Winke, private communication). et al., 1967) and of 1.7-3.0 for lunar samples (FIREMAN, In Table 4 are listed the concentrations and ratios of 1972). For .‘%I the range is again the same as found for those cosmogenic nuclides which will be used in the subchondrites and lunar matter (14-22; BEGEMANNet al., 1966; sequent discussion. For ‘He and 2’Ne the entries are iden1970; WAHI.EN er al., 1972). tical with those in Table 3 except that averages are given

Rare gases and 36C1 in stony-iron meteorites

Table 3. He, Ne, and Ar in metal and silicates (~on~ntrations in units of 1oS8 Cm3 STP/g). Un~rt~ties in the absolute amounts are ;t57& in isotope ratios z!z;“i, 3

He

2%

4He

Phase

alltea

He

IO.8

Sf

16.5

Me

Il.1

180

,832

.415

31.3

174

.165

.393

32.4

a46

“0ildQ‘Z

SC Bondoc

11

14.7

120

SKI

30.6

5?7

Strl

30.3

363

8.3

206

Clover

9.69

.I66

,140

2.74

658

-

22

-

,720

192

-

-

-

-

-

-

-

-

(1) -

1.38

1.87

102

2.07

2.8,

461

-

SLII

53.4

1*29

13,1

14.8

13.2

1.62

2.28

270

-

2.72

2.72

322

-

4.68

5.90

886

”

10.5

1.36

f.40

217

-

10.9

I.15

1.18

L61

-

3.09

4.25

125

-

5.25

7.31

474

-

3.93

440

1.25

.925

1050

6.76

I .50 9.30


53.5

544

9.55

0’

38.7

636

9.48

tie

62.9

427

1.26

SC

65.1

1169 23

.997 2.21

i.24 11.4

1.32 18.8

34.7

StI*

333

2030

79.1

95.9

95.3

StIII

132

1270

24.4

27.1

29.0

Me

3.72

15.8

fC.0

.429

.387

.936

13.6

3i.7

,387

.582

13.3

32.6

74.7

Me

78.0

StI

90.3

SCII

145

?

86

,176

1.58

453

929 2100

,060

4.86

I .54

417

2290

.047

5.21

..lO

5,23

9.23 20.0

I .56 16.4

.33

96

i2.3

Jot

29.4

443

2.21

-

44

-

109”

”

1.19 1.33

4.13

4.b6

404

-

1.23

4.51

4.97

420

-

6.89

9.45

853

1.34

21.7

23.6

25.2

43.5

52.4

51.9

18.3

19.2

20.3

25.5

-

.303

1.02

-

40

-

5.4

7.9

655

Me

33.4

498

1.84

1.29

1.45

3.62

3.38

611

-

Me

34.5

600

1.4

1.4

1.4

2.64

3.76

115

(31

_

_

_

53.5

lb30

14.8

14.0

1J.i

3,49

4.04

80”

-

SLI

61.2

t 700

Lb.0

15.3

16.0

2.90

3.87

766

-

SZlI

88.9

lO7_8

26.8

28.1

28.7

I .56

1.65

284

-

SC

69.3

231”

14.7

16.2

17.7

2.t.b

3.18

750

(3)

_

(4)

481

SC

2170

_

-

_

He

30.8

186

se

43.5

656

9.23

721

8.97

8.88

727

1.98

2.07

St

47.7

ne

123

_

.989

_

(4)

_

-

2.58

2.69

287

-

3.81

4.35

587

-

9.73

3.98

4.93

783

2.23

5.40

1.77

. id4 8.96

_

(2)

SCI

Me

.658 10.1

”

88

”

St

179

1380

He

103

651

SL

163

Me

128

542

2.96

2.94

3.17

6.68

9.13

272

-

Me

127

542

2.36

2.54

2.73

6.45

8.91

289

-

SCI

44.4

30.3

32.6

1.40

35.7

1.36

I.50

2700

42.0

47.2

49.1

-

14.6

15.8

17.2

92.4

Station

-

2.01

12.0

29.5

Udei

-

344

1340

1040

Hatjalahti

374

2.14

28.3

He

-372

Ref.

44.6

156

Bencubbin

2.26

2.24

1.68

St1

Yeranin

2.60

11.9

I .67

996

btuerra

. 482 .44u

il.9

188

saca

-

.I54

ne

Patwer

663

10.6

Me

Horriarown

1.37

IO.3

ua1garanga

MiWY

-

1.77

4.68

alne,y

Lowicz

355

10.5

1.73

I SO

4OAir

1.07

St1

SL,OC

Orchatd

4.42

9.52

10.3

38_4r

1.63

. 296

Me

Springs

Crab

4.10

9.30

36A,

.237’

.J40

,210

He

St Budulan

315

%e

*‘Ne

ttetaorice

328

3.34

4.65

273

4.83

1.72

207

-

5.0,

6.82

JO”

-

560

(5)

7.5

10.4

-

33.7

37.1

39.2

6.77

7.15

179

-

St.11

184

1890

66.1

62.4

65.4

4.81

2.38

ti,

-

St11

199

1720

71.3

68.8

70.5

3.70

2.16

113

-

6.17

622

-

Me

36.2

652

St

50.8

1480

Me

204

716

7‘63 39.7

I .63

,968 25.8

3.04

27.8

3.09

3.20

17.3 199

40.5

IO.0

15.3

716

-

90

-

Me

204

III

St

340

1760

84.4

St?

368

1929

89.9

105

106

St

410

1965

99

114

113

Me

149

560

2.80

He

146

633

(4*24)

SC

172

1980

38.9

44.9

45.3

16.9

8.20

4580

-

St

158

1840

36.4

41.0

40.9

14.8

7.39

416”

-

3.15

3.22 97.6

3.40 99.1

9.77

97

-

1.35

t.61

136

-

1.08

1.36

49

.89

14.6

I .33

9

_ (6)

2.61

2.67

7.42

9.96

326

-

2.44

2.73

7.50

9.32

335

-

(1) HIN~NB~GER et al. (1964). (2) KIRSTEN et al. (1963). (3) MILLER and Z.&HRINGER(1969). (4) PANETHand URRY (1931). (5) STAUFFER (1962). (6) MEGRUE(1968).

F. BEGEMANN, H. W. WEBER,E. VILCSEKand H. HINTENBERGER of 36C1,and assume as usual an irradiation constant with time. This yields (in units of 10’ yr) T = 425 x [36Ar,]/[P(3”Cl)],

intensity

(1)

where the concentration of cosmogenic 36Ar, has to be inserted in units of lo-’ cm3 STP/g and the production rate of 36C1 in disintegrations per min per

L

Fig. 2. Three-component diagram of the isotopic composition of Ar from metal (0) and silicates ( x ).

kg. The production rates of 36C1to be used in equation (1) are equal to the measured decay rates only if the terrestrial ages of the meteorites are short compared to the half-life of 36C1 (3.1 x lo5 yr). Obviously, this is the case for the observed falls (Barea, Lowicz, Marjalahti, Udei Station, Veramin) and those ‘finds’ which still contain detectable amounts of 39Ar (Bencubbin, Clover Springs, Morristown). This, then, leaves as uncertain Bondoc, Budulan, Crab Orchard, Emery, Mincy, and Vaca Muerta. Emery has one of the highest 36C1activities of all metal samples. indicating that decay since the time of fall has been insignificant.

Table 4. Concentrations of cosmogenic 3He, “Ne, and for duplicate runs. The (“Ne/“Ne), ratios have been cal38Ar (10-s cm3 STP/g) in metal and silicate samples. culated by assuming (“Ne/‘lNe), = 0.90 and the admixed Different roman numerals denote different samples Ne to be of atmospheric composition. In case of Ar we used (3hAr/38Ar), = 0.63 f 0.03 (see Fig. 2) and for the admixed component a ratio of 5.35. While this procedure introduces an error due to the uncertainty in the spallogenie ratio it avoids the over-correction which would result from ascribing all 40Ar to an atmospheric origin when there is radiogenic 40Ar present as well. Although special care was taken to prepare clean metal, the (2’Ne/38Ar), ratios indicate that the samples were nevertheless contaminated to a variable degree by an lludulan admixture of silicates. An upper limit of the degree of contamination is obtained if one uses a value of 0.15 for the 2’Ne/38Ar-production ratio on metal, corresponding to an exceptionally soft irradiation spectrum (SIGNERand NIER, 1962; LIPSCHUTZet al., 1965; SCHULTZand HINTENBERGER, 1967; NYQUISTet al., 1973). With this assumption the silicate admixtures appear to have always been less than 6”/,, and in the majority of cases less than 2%. (The 3He/4He-ratio in the metal is not a good indicator for the presence of silicates as it may be low due to a loss of tritium from the metal, the in situ decay of uranium and thorium, and the stopping in the metal of cl-particles originating in the silicates.) Even such small contaminations preclude the possibility of deriving information on the hardness of the effective irradiation from the isotope and nuclide ratio plots commonly used for this purpose (SIGNER and NIER, 1962; SCHULTZand HINTENBERGER, 1967). It introduces only a negligible error. however, in the assignment of the cosmogenie 3He and 38Ar to the different phases as the amounts of these two nuclides in the metal and non-magnetic portions were found to be comparable in all but a few cases.

DISCUSSION I. 3hCl-36Ar cosmic ray exposure ages For the calculation of the exposure ages (which strictly speaking pertain only to the metal phases) we use a production ratio P(3”C1)/[P(36Cl) + P(3”Ar)] = 0.82 (SCHAEFFERand HEYMANN, 1965), a branching ratio 1 (p-)/I(total) = 0.983 in the decay

Rare gases and Y.Il in stony-iron meteorites

359

Bondoc II, on the other hand, has by far the lowest duction rate of 25 dpm/kg. For DaIg~ranga, however, 36Cl activity, but nevertheless the r4C content of the we prefer not to speculate on a production rate and stone phase is (45 i: 0.9) dpmfig (BORN, 1973). thus give no exposure age at all. As it formed a crater with a diameter of about 25 m (NIN~GEK and Huss, Hence, its terrestrial age must also be short. Actually, from the 14C-36ClM, correlation a terrestrial age of 1960), the pre-atmospheric mass must have been quite less than 20,OOOyr and a pre-atmospheric radius of considerable. more than 2m are derived (BORN and BEGEMANN, Table 5 shows the results; included is Estherville 1975). A qualitatively similar conclusion was pre- which has been analysed previously by SCHAEFFER (1965). For the mesosiderites the expoviously reached by CREEPYand SHEDLOVSKY (1965). and HEYMANN The large me-atmospheric size together with the con- sure ages cover the range from 10 to 160 Myr, with no significant clustering around a particular value. siderable recovered mass of about 900 kg also explain the difference in 36C1 and spallogenic rare gases in They thus fall between typical exposure ages for stone meteorites and for octahedrites, althou~ there is an the two analysed samples. For the remaining four cases the measured decay overlap with both. The exposure age of the metal from the pallasite rates of less than 25 dpm/kg are in all probability Marjalahti is in good agreement with that estimated a size effect as well. First, it would be very unusual indeed not to find a spread in the production rates for the olivine from rare gas data alone (18s205 Myr, similar to that observed in iron meteorites or the MEGRUE,1968). metal from stone meteorites (cf. e.g. BEGEMANNand II. Nuclide ratio correlations and production rates VILCSEK, 1969). Secondly, the resistance towards a. ~~~~~c~~~ co~~usitio~. The chemical composition weathering of mesosiderites is pres~ably comparof the stone phases used for the mass spectrometric able to that of stone meteorites, and with one possible analyses is given in Table 6; the last three columns exception there are no ‘finds’ of stone meteorites show the approximate mineralogical norm composiknown where even the 14C has decayed completely (GOEL and KOHMAN,1962; SUESS and WANKE, 1962; tion. Note that the samples are not necessarily representative for the bulk silicates. This is especially true BOECKL,1972). Finally, there are Dalgaranga and Patwar for which for those samples which were prepared by hand there was not enough material avaitable to us to picking of crystals which resembled olivine (StII of measure 36C1 at all. From heavy cosmic ray track Bondoc II, Emery, Lowicz, Mincy, Veramin). It is densities PRICE et al. (1967) have concluded that the gratifying to see that these were indeed mostly olivine. The non-magnetic portion of Udei Station conpre-atmospheric radius of Patwar was between 14 and 165 cm. Hence, we shall use the maximum 36C1 pro- tained, in addition to the elements listed in Table 6, 136% S which corresponds to about 37% troilite. This is surprising since according to BUNCH et al. Table 5. Cosmic ray exposure ages of the metal phases. The errors assigned to 36Ar, include those arising from (1970) the inclusions in Udei Station are of the Copiapo-type with <.5% sulfides. As our sample was a true the uncertainty in the (36Ar/3SAr),ratio aliquot (3 different analyses yielded the same results) Meteorite r =*re s =ci of a log non-magnetic portion from different pos(lo-dccsrP/~) (dpmltg) (lOba) itions within the specimen available to us. it is conceivable that this sample is more representative of narea .s5t.04 24.9’1.” 9.4t, the inclusions-if ‘representative’ is a term applicable 8ondoc I .251.“9 3.22 .7 lbbf40 to these inclusions at all. sowioc II .s6*.03 1.52 .3 15’1*35 Figure 3 shows a log-log plot of Ca vs Al contents. dudulan I. 15r.07 13.7k3.0 3618 Plotted are all mesosiderite samples with Ca >05%. Clover Springs i.58f.l” 23.921.3 28?3 While at low concentrations there is a considerable Crab Orchard 2.62t.15 17.3t1.5 61f7 scatter in the Ca/Al ratio, for higher concentrations Emery 7.5st.40 24.Ofl.O 134ti* our points fall between the lines given by MCCARTHY Estherville 3.3 f.Z 22.7t2.2 62?7 and AHRENS(1971) for chondrites and mesosiderites, Lowiez z.tl,t. 10 23.Otl ,3 5324 respectively, where the latter is based essentially on Mincy 1.93f.13 18.9fi.5 431’5 the analyses of POWELL(1971). Mortiscovn 1.58.10 b. elusion loss of 3He. Although the variations in 20.2t1.7 33?& chemical composition of the different non-magnetic Pstvar 4.83*-.25 :25 8226 samples analysed from one and the same meteorite vaca Nuerta 4.,j*.2* 13.3+x .If 133*x2 are quite considerable, the large differences observed Veramin 5..sO~.ZS 23.6t2.0 99ti3 for their 3He-content (Table 4) cannot be due to the different target composition. Rather, different bencubbin 2.08f.20 24.3t1.5 3621 minerals must have been affected to different degrees Narjalahci 9.40t.40 22.2?1.8 18OtlB by a loss of 3He. In order to discuss these losses in a Udei Station 6.0 f.30 19.6? .8 130tli) quantitative way we shall resort to a (3He~3*Ar~~ - 3Hq,/38Ar,, diagram.

F. BEGEMANN, H. W. WEBER,E. VILCSEK and H. HINTENBERGER

360

Table 6. Chemical and approximate mineralogical norm composition of the silicate samples used for the mass spectrometric analyses. Concentrations were determined by atomic absorption spectrometry. Errors are +5x, except for Ca contents of ~1% where they are &O*I% absolute. In the case of Clover Springs no data are available for the concentrations of the fight elements in the two density fractions p CT2.95 and p > 3.4 g/cm3 Al

Si

Cs

Fe

SC

IO.”

4.5

24.0

4.4

14.5

76

aondoc

SL

11.4

4.”

23.6

3.9

13.5

73

St1

19.8

1.5

22.8

I.2

SCII

22.7

SfI

10.9

3.0

22.2

SfII

12.7

3.0

22.3

13.0

t.5

23.8

II

aondoc

Budulan

Clover

Springs

.92


Orchard

Emery

Lovicz

niney

24 7

5”

43

7

f,

85

4

2.7

16.1

72

f2

16

2.4

14.3

67

17

16

89

3

8

40

9.3

2.4

1.9

14.2

.9

16.0

SL

8.0

4.0

23.0

4.8

14.4

73

SCI

5.9

7.8

21.2

7.4

15.2

44

16

27

St11

24.2

.I

18.8

.2

13.6

10

90

SEIIl

5.0

7.2

21.2

7.2

17.5

49

14

SfI

9.0

5.2

24.0

4.6

13.3

72

StII

22.6

1.3

20.3

.5

13.3

2s

SfI

Il.0

3.4

23.8

3.5

13.2

ai

SLII

37 28

71

4 19

25.2

1.9

19.8

1.4

9.2

7

ST

7.1

5.0

24.1

4.5

14.3

73

SL

12.5

1.2

23.5

1.9

17.3

04

IO

6

Sf

12.7

1.9

23.0

2.4

15.5

77

I3

IO

1.6

24.0

2.7

9.2

Sl

5

14

SfI

14.7

St11

26.7

.1

19.2

.2

12.7

83

$0 27

3

97

Eenoubbin

Sf

21.1

2.i

22.9

2.2

7.2

48

4,

Harjalahti

Sf

28.0

c.1

18.8

‘.I

10.2

-

100

Udei

St+

II.3

1.3

15.2

I.0

26.2

52

3

Station

2”

12.4

P’ Crab

*north.

15.i)

.65

19.2

PC

Oliv.

Pytox.

Mg 5area

Ii

7

Numerous measurements on iron meteorites and samples which have lost neither 3He from the silicates the metal from stone meteorites have shown that in nor 3H from metal should fall together in a single this metal the 3He/38Ar ratio is fairly constant at point; if onIy 3He was lost from the silicates they 16 rt 2 (SIGNER and N~ER, 1962; LIPSCH~~ et uE., should plot on a horizontaf line originating from this 1965; SCHULTZand H~TENSERGER,1967; NYQUISTec point; if only tritium was lost from metal, on a vertial., 1973). It appears to be independent of the size cal line from this point. Figure 4 shows the data of of the meteoroids and the hardness of the effective Table 4 plotted on this diagram; when more than radiation; a dependence on Ni content is to be one silicate sample has been analysed the points are expected but measurements on samples enriched in connected by horizontal lines. The slanted line is taenite are not yet very conclusive (SCHULTZet al., 1969). Whenever lower abundance ratios are actuafly observed they are due to a deficiency of 3He which is presumably caused entirely by the loss of tritium, the precursor of 3He (HINTENEIERGER and WXNKE, 1964a; HINTENBERGER et al., 1967; SCHULTZ, 1967). As the data in Table 4 show, at least five of the thirteen mesosiderites as well as Bencubbin and Majalahti have suffered such losses. Similarly constant is the production rate ratio of 3He in the stone phase to that in the metal and by inference that of 3He in the stone phase to 38Ar in I, ,, e the metal. Values very close to the production ratios 6 a1 2 L PJI%l 6 6 are expected even if stone and metal phase are intimately intergrown on a scale small compared to the Fig. 3. Ca vs Al content of silicates from mesosiderites (in wt %). The two straight lines correspond to Ca/ range of the cosmogenic 3He or 3H (HINTENBERGER AI = 1.09 (chondrites) and 088 (mesosiderites) given by et al., 1966; NYQUISTet al., 1973). MCCARTHY and AHRENS(1971) for chondrites, howardites, Hence, in a (3He/38Ar)hl, - 3H&t/38ArM, diagram and eucrites and mesosiderites, respectively. l

Rare gases and 3hCl in stony-iron meteorites

10

20

30

Fig. 4. 3He diffusion loss diagram. Samples with their full complement of 3He in silicates and metal should cluster around the asterisk, determined by the production ratios. (The shading of the bars indicates the mineralogical norm composition of the silicates analysed). Different silicate samples from the same meteorites are connected by horizontal lines.

drawn as a visual aid; it pertains to equal percentage losses of 3He from silicates and tritium from metal if the ratio of the direct production rates 3He/ (3He + 3H) in metal is l/3 (HINTENBERGER et al.. 1967). For any other ratiwr if 3He is lost from metal as well-the intercept with the ordinate must be changed accordingly. Points falling above this line indicate A3Hq, > A3HM, and vice versa. As production ratios (i.e. for the Fig. 4 starting point with A3Hes, = A3HM, = 0) we have chosen (3He/38Ar)hl, = 16 and 3Hest/38ArM, = 29. For Lowicz and Bondoc II both measured ratios are slightly’ higher but this may be due to a small contamination of the metal by troilite, schreibersite or silicates which would all result in an increase of both ratios. The 3Hes,/38Arb,c production ratio of 29 is higher than the value for 3H%,,,/38Ar,, of (20 + 3) found in chondrites by NYQUIST et al. (1973). There are two obvious reasons which might explain this discrepancy. First, bulk chondrites contain 2-3 times more total Fe than the stone phases analysed here which results in an about loo/;, lower ratio in the former. Secondly, the ratio given by NYQUIST et al. (1973) has not been corrected for possible diffusion losses of 3He from the silicates. One other tentative explanation is suggested by the fact that with but one exception all our samples with high 3Hq,/38Ar,, ratios have high Mg contents. Perhaps this indicates a higher yield of 3He from Mg than from Si. As is apparent from Fig. 4 most non-magnetic samples have suffered a more or less severe loss of 3He. Furthermore, a comparison of these losses with the norm composition of the samples (Table 6) reveals a clear correlation between anorthite content and 3He loss. It appears that with the possible exception of Budulan the anorthite in all samples has lost its 3He completely, confirming the result of MEGRUE (1966) and HEVMANN et (II. (1968) that feldspar is the least

361

retentive of the major minerals found in meteorites. In the case of Veramin, Bencubbin, Marjalahti, and Udei Station, however, there is unambiguous evidence (and for Barea, Crab Orchard, Clover Springs, Mincy and Patwar tentative evidence) that some other minerals besides feldspar must have been affected as well. The position of the two Veramin samples in Fig. 4 together with their mineralogical composition indicate that the loss occurred from pyroxene; in Marjalahti it must be from olivine; in Udei Station we presume that the troilite has lost its 3He although this is not yet supported by the analysis of a sample enriched in troilite. From these results it is quite clear that during their exposure to the cosmic radiation the thermal history has been different for different meteorites. For a more quantitative interpretation it is important to realize a decisive distinction between the two nuclides: because of the short (12.3 yr) half-life of tritium a loss of this nuclide will show up as a 3He deficit only if it has occurred during a significant part of the exposure age, while 3He may be lost equally well during an episodic event late in the exposure history. Taking this into account, we offer the following explanation: for seven of the meteorites analysed (Barea, Clover Springs, Mincy, Morristown, Veramin, Bencubbin, Marjalahti) the temperature of the metal was high enough during a considerable fraction of the exposure age to allow quantitative loss of tritium-or it was high enough during the whole exposure era to allow partial loss of tritium. As in these cases the feldspar has lost its 3He as well, it is possible that this loss occurred simultaneously. Thus, there is no need to invoke an additional episodic event late in the exposure history to account for the loss of 3He. Where a loss of 3He from feldspar is not accompanied by a loss of tritium from metal (Bondoc, Budulan, Emery, Lowicz, Patwar, Vaca Muerta) there are two alternative explanations. If the retentivity of feldspar for 3He is lower than that of metal for 3H, the average temperature of these samples during their time of exposure may have been so low that only a loss from feldspar occurred. On the other hand, if the sequence of retentivities is reversed an event late in the exposure history must have occurred. This either led to an instantaneous loss of the “He accumulated until then or it caused a change in orbit of the meteoroid which subsequently raised the temperature high enough for the 3He to be lost gradually. In neither case would the simultaneous loss of tritium from the metal manifest itself in the (3He/38Ark,, ratio if the event took place late enough during the exposure era. For Veramin such an episodic event late in the exposure history is actually required to explain an almost complete loss of 3He from pyroxene and a partial loss from even the most retentive olivine, but only a small loss of 3H from metal. In this connection it is perhaps worth mentioning that the decay rate of 39Ar and the 39Ar!3hC1 ratio of 1.33 in the metal

362

F.

BEGEMANN,H.

W. WEBER, E V~.CSEKand H. HINTEN~ERGER

is only 7-7.5 pm compared to 12-l 3 pm for artificially accelerated 56Fe. The studies of FLEISCHERet al. (1967) and PRICE et al. (1967) are, unfortunately, not comprehensive enough for the present purpose. It should perhaps be mentioned that the present discussion of diffusion losses is based on cosmogenic 3He. For 4He the situation is complicated by the presence of a radiogenic component. Due to the inhomogeneous distribution of the a-decaying nuclides the situation is very confusing, as for example is evidenced by the fact that for Clover Springs, Lowicz, and Mincy the silicate portions which show a large loss of 3He are, nevertheless, the highest in 4He. while The pallasite Ma~alahti shows a clear deficit of for Emery and Veramin the opposite is true. Finaliy we wish to point out that the different ther3He in olivine which is of the same order as the loss mal histories of the mesosiderites during their expoof tritium from metal (Z 20%). The simplest explanasure era are not correlated with their thermal history tion appears to be that these losses occurred during as deduced and used by POWELL(1971) to classify the first part of the exposure history, in a regime them into textural sub-groups. If anything, this lack where the temperature was high enough for both nucof correlation speaks against the instantaneous loss lides to be lost completely. If the losses occurred at of 3He during a catastrophic collision because then some later time during the exposure history, the temone might expect to see evidence of extensive shock perature must have been just right over an extended or remelting. period of time for the continuous loss of tritium from c. Production rates of 38Ar from Ca. As can be seen metal to equal the fractional loss of 3He accumulated from Table 4, in all but a few cases the amounts of until then from the olivine. Although not impossible spallogenic 38Ar in the non-magnetic samples are the latter model seems rather artificial. comparable to or exceed those in the metal. This is From our data alone it is neither feasible to disclearly due to the presence of Ca in the silicates. The tinguish between these two possibilities nor to deduce 38Ar contents for pure Ca, calculated from these data absolute values for the mean or peak temperatures. and the chemical compositions given in Table 6, are It appears, however, that an investigation of different listed in Table 7. Whenever more than one silicate minerals with different annealing temperatures for sample has been analysed, average values are given their content of heavy cosmic ray tracks might help provided the Ca content is >0,5”/,; for lower Ca conto answer this question. At present, the only result with some bearing on this problem is that of FLEROV tents the combined error in the correction for the contribution from Fe and in the Ca determination et a[. (1974), who found that in Marjalahti olivine itself becomes unduly large. the mean track length of Fe-group cosmic ray nuclei

of Veramin are outstandingly high. This, too, might be due to a change in orbit late in the exposure history less than 500,OOOyrago (the approximate mean life of ‘“Cl). Furthermore, as ail evidence-from meteorites as weil as direct observation (FORMAN et al., 1971; FIREMANand SPA~AGEL, 1971; BEGEMANN, 1972; CYGALLAGHER,1972; WE~BER and LEZNIAK, 1973)-points to a positive radial heliocentric gradient of the galactic cosmic ray intensity, the aphelion of Veramin must have been unusually large. Thus, if our interpretation is correct the orbit must have been very eccentric during the last 106 yr or less.

Table 7. Content of cosmogenic 38Ar and 21,22Ne calculated for pure Ca and Mg, respectively (10escm3 STP/g). Production rates (10-acm3 STP/g target element and 1O’yr) are calculated by assuming the exposure age of the silicate phases to be the same as that of the metal. Valies for Bruderheim (BOGARD and CRESSY,1973) and Elenovka and Otis (BOCHSLERet al., 1969) are given for comparison

*area

29.013

oondoc

61.9f6

.37

314

(61.7t10)

42.7L5

.26

4726

(4%.0*3)

86.6!7

2.41

48?5

(89.lt9)

61 .4f7

2.19

24f4

(54.4i7)

2.14

33t5

(1421’28)

Bondor

II

Budulan Springs

Clover Crab

Orchard

137f12

3.08

33%

(30.8’6)

Emery

346t25

2.58

29t3

365?30

Lariez

t88f20

3.55

41f6

218t23

Nincy

94.018

2.19

3lf4

100ti2

Horriscoun

90.3i9

2.74

36?5

(77.4*19)

Patwar

17oi17

2.07

22z3

(187124f

YZXCB Huerfa

235r25

/ .77

36tS

<283137)

Varamin

242f25

2.44

27f4

241’-30

ElellOVka

4.Itl.2

372,

Otis

3.3t.7

*of2” -7

8tuderheim

17.3*2

I

1.23t.17 .14t.17

”

.bit.l8

2.2t.2 -.4 2.2+.j -.4

-__-

Rare gases and 36C1 in stony-iron meteorites Assuming that no loss of 38Ar from feldspar has occurred, the production rates-in units of lo-’ cm3 STP/g and lo6 yr-fall between 0.26 and 3.55, with Bondoc II the lowest and Lowicz the highest. Excluding Bondoc, the variation is only a factor of two. These production rates agree fairly well with those derived by BOCHSLERet al. (1969) for the two chondrites Otis and Elenovka (3.3 + 0.7 and 4.1 i 1.2, respectively) and-for 2 7-cgeometry-that of HUNEKE et a/. (1972) of 18 for lunar samples. Values for the production ratio Pzz/P:f are given in column 4. They are derived under the assumption that for each meteorite the exposure history of metal and silicates has been the same, neglecting any possible differences in the contribution from a potential ‘early irradiation’. These ratios vary again by a factor of two; they are higher than that calculated by STAUFFER(1962) from a combination of measurements on various meteorites, and that given by BOGARD and CRESSY (1973) for Bruderheim, but they agree with those of BOCHSLERet al. (1969) for Elenovka and Otis. In the latter case this agreement may be fortuitous because of the large error (Table 7). There is no simple correlation between the ratios Pz,“/PF”,”and the absolute production rates of 38Ar from metal as inferred from the measured 36C1 (Table 2). As j8ArM, is a high energy product (E 2 200 MeV) and “Arca is produced mainly by knock-on neutrons, this lack of a correlation shows that the shape of the energy spectrum of the nuclear-active particles is not uniquely correlated with the high energy flux, in agreement with calculations by KOHMAN and BENDER (1967) and BORN and BEGEMANN(1975). It was mentioned above (‘Mass spectrometric analyses’) that the amounts of spallogenic “Ar listed in Table 4 have been calculated from the measured concentrations by adopting a cosmogenic production ratio (36Ar/38Ar), = 0.63 f 0.03. As Fig. 2 shows, this value, which has been found in numerous measurements on iron meteorites and the metal from chondrites, is appropriate for the metal from mesosiderites as well. For the production from Ca, however, there is neither much experimental evidence nor any theoretical reason why the ratio should be the same. Provided the exposure age is long compared to the half life of 36C1, and neglecting the small yield of 3hS, both Ar isotopes represent the total isobaric yields at mass 36 and 38, respectively. Hence, for Ca as target the 36Ar/38Ar production ratio is equal to the ratio of the probabilities for the emission of 5 and 3 nucleon& respectively, from the compound nucleus formed upon bombardment of 40Ca. For Fe as target it is equal to the ratio of the probabilities for the emission of 21 and 19 nucleons, respectively. It is not trivial that these ratios should be the same. Figure 5 shows our results obtained on the mesosiderites. The measured Ar concentrations of the nonmagnetic samples (Table 3) have been corrected for the contribution from Fe contained in the silicates, and the corrected 36Ar/3sAr ratios are plotted vs total

1I

363

. l

Fig. 5. 36Ar/38Ar ratios in silicates, corrected for spallogenie contribution from Fe, vs 38Ar-content. The cosmogenie production ratio on Ca appears to be (0.7 k O.l), deviations at low concentrations are presumably due to a small component of atmospheric and/or primordial origin. (The error given is a visual estimate not based on any calculations.) 38Ar content. Veramin has been omitted because it contains primordial Ar. The data suggest that the 36Ar/38Ar production ratio on Ca is indeed (0.7 * O.l), the higher ratios at low concentrations being due to an admixture of atmospheric or primordial Ar which becomes relatively more important at low concentrations. In all cases the ‘excess’ 36Ar is
‘364

F. BEGEMANN,H. W.

WEBER, E. VILCSEK and H. HINTENBERGER

Even with these simplifying assumptions the results are not unambiguous, however, either because only two samples have been measured for the individual meteorites or because the compositions of two samples are too similar (Emery). Thus, we shall calculate the production rates from Mg and Si for various assumed PAL/Psi ratios. Fortunately, for the case of Mg it turns out that within reasonable limits the choice of this ratio is not critical. For 0 ,< PAI/ Psi 5 2, the absolute amounts of both *lNe and *‘Ne to be expected in pure Mg change by less than 6:/, with higher values for higher PAl/Psi ratios. Even for PA, = 0, and reduction of the measured Si content by 1.04 x [Al] (i.e. assuming complete loss of the Ne produced from anorthite), the reduction is less than 4”/,,except for Emery where it would be 10%. The values of *lNehlg and 2ZNGp-in units of 10e8 cm3 STP/g-calculated for PAI/Psi = 03 are listed in Table 7. The errors given are those which would arise from the worst possible combination of the +Su/, uncertainties in the Ne concentrations (St1 + 5% and St11 - 5% or vice versa). Note that the ratios of ‘lNe/‘*Ne produced from Mg are much more accurate than indicated by these uncertainties, as any errors in the absolute amounts of Ne do not influence the isotopic ratios. AS is evident for this choice of P,,/Psi, the **Ne/‘lNe production ratio from Mg is below unity (094-0.98). in accordance with what was anticipated from the uncorrected measurements of the olivine samples where ratios close to or even below unity are found (Table 3). Actually the production ratio changes by less than 2?<, for any other PAI/PSi $ 2 provided the same ratio is used for both isotopes. As this need not be the case, we have tried all combinations of PA,/Psi ratios (different for *‘Ne and 22Ne) with the result that for Lowicz, Mincy, and Veramin there is no combination yielding 22Ne/21Ne production ratios which are larger than unity. Only for Emery could it be as high as 1.02. This is in contradiction with the result of BOCHSLER et al. (1969), who found (22Ne/Z’Ne)M, = 1.07 + 0.03 in Elenovka. The production rates from Si and thus the PMp/Psi ratios, on the other hand, cannot be determined with the same accuracy. They depend more critically on the choice for PAI/Psi and on errors in the measured neon concentrations. This is demonstrated in Fig. 6, where 6a shows the correlation between PMg/Psi and PAliPs; when using the measured neon contents. Such high ratios as are obtained for Veramin and Lowicz are extremely unlikely. Hence, in 6b are shown the correlation lines which result when. as previously, the Ne concentrations in St1 are raised by 5% and those in St11 are lowered by 5%. Almost identical curves are obtained if the measured Ne contents are not used, but instead the assumption is made that the silicate samples rich in feldspar and pyroxene (St1 in Tables 4 and 6) have lost 10% of their spallogenic neon. For Veramin such a loss would not be surprising in view of the fact that the same silicate sample

has been shown above to have almost completely lost its 3He. Because of this strong dependence of PMg/Psi on small errors in the amounts of neon and on small diffusion losses, it is impossible to give a reliable figure for this ratio. For the same reason the (22Ne/ 21Ne),i ratio cannot be determined, especially if one allows different PAI/Psi values for the two isotopes. Obviously, high *‘Ne/*‘Ne ratios in samples rich in Al and Si, compared to samples rich in Mg, can be due either to a high (22Ne/2*Ne)si production ratio or to a higher P~~JP,Z,2than P,$/Pii’. Had it been possible to arrive at an accurate value of P$/P$‘, and had this production ratio been the same for the, different meteorites, then one could use this ratio to calculate 21N~, in those cases where only one silicate sample was measured. In spite of the uncertainties, such a calculation is worthwhile. We adopt P,,/P,, = 6 rt 2 and PA,/PSi = 0.5 _+ 0.5. This appears to be a reasonable choice not only from Fig. 6 but also because BOCHSLERet al. (1969) derived values of (5.3 _+ 3) and (47 ‘2::) for Elenovka and Otis, respectively, B~GARD and CRESSY (1973) a value of (5.3 + 1.8) for Bruderheim, and STAUFFER (1962) a value of 6.8 from a pooling of measurements on various meteorites. The results are given in Table 7, in brackets. The errors assigned are those resulting from the most unfavorable combination of ratios within the spread allowed for them; with but one exception (Morristown) they are less than 20”/,. Column 7 of Table 7 shows the ratios P$Pzf. As both nuclides are produced by low energy nuclearactive particles (mostly neutrons), this ratio should be constant and independent of the pre-atmospheric size of the meteorites as well as the position of the samples within them. This is indeed seen to be the case: the

35 ,/ 30

i

0 /

/

,lavlCZ +$

Fig. 6. Dependence of calculated P$/P$’ on assumed ratio Pi:/P&l and on small errors in the Ne determinations or diffusion losses. In 6a the concentrations of “Ne as measured in the different silicate samples have been used in the calculations, 6b shows the same curves for the most unfavorable combination of errors which is equivalent to a 10% loss of “Ne from the least retentive silicate each. PAI/Psi = -1 corresponds to a complete loss of spallogenie neon from feldspar.

365

Rare gases and “Cl in stony-iron meteorites

value for the four ‘superior’ meteorites is 1.07 + 0.07, and that for all samples is 1.04 + 0.10. For comparison we have included in Table 7 the ratios obtained by other authors. The agreement with Bogard and Cressy (Bruderheim) is fairly good, while values obtained by the Bern group are considerably lower. At least in the case of Elenovka, this is clearly due to the fact that Bochsler et al. explain the discrepancy in the 38Arca values they obtained from feldspar and pyroxene as due to diffusion loss of 38Ar from feldspar, and use the higher production rate derived from pyroxene. The last column of Table 7 lists the absolute production rates of “Ne from Mg in units of lo-’ cm3 STP/g Mg and lo6 yr. As anticipated, they show about the same range as Pz,“, again with Bondoc the lowest and Lowicz the highest. mean

III. Gas retention ages Aliquots of the non-magnetic samples from Crab Orchard and Mincy St1 have been anaiysed by neutron activation for their content of potassium and uranium (W&NICE,1973). Hence for these two meteorites U/He and K/Ar gas retention ages can be calculated, although we realize the shortcomings of such whole rock ages. Corrections for spallogenic 4He are made by adopting a production ratio (4He/3He), = 5 and subtracting 5 times the measured 3He contents which is equivalent to assuming an equal percentage loss of spallogenie 4He and 3He from the silicates. For 4*Ar we define upper and lower limits for the radiogenic comwhere the upper limits are simply ponent, “Arrad = 40Ar,,,. This implies either the absence of primordial Ar and variable (36Ar/3”Ar)C,. ratios of

0.73, 0.89, and 0.76, respectively, or for (3”Ar/ 38ArX. 2 060 the presence of < 0.84, 1.06, and @57 xBIO-*cm3 STP/g of primordial 36Ar. The lower limits for 40Ar,,, on the other hand, are obtained if one again uses (3bAr/“8ArX, = O-60 and ascribes the excess 36Ar to an atmospheric component, reducing the 40Ar accordingly. The ages thus derived are listed in Table 8. For comparison we have included the few other values available for mesosiderites from the literature, listed

in the appropriate columns. The agreement of the K/Ar ages among the duplicate measurements on Mincy St1 is seen to be much better for T,,, than for rmin, indicating that for Mincy the higher value is to be preferred. This, in turn, means that there are indeed small amounts (0*5-i x 10-8cm3 STP/g) of primordial 36Ar present, in agreement with the result of MCJLLER and Z&RINGER (1969) who give 36Ar, = 0.7 x 10-8. For Mincy the U/He age is considerably shorter than the K/Ar age. Although the Th content has not been measured, a wrong choice for the Th/U ratio f = 3.0) cannot possibly be the reason, as the discrepancy would remain even for Th = 0. Hence, there must have been a diffusion loss of radiogenic He. If the ‘true’ U/He age is equal to the measured K/Ar age, more than .50°! of the 4He is missing-a fractional loss which is definitely higher than that of cosmogenic 3He from the same sample. Stepwise heating experiments could possibly rule on whether this is due to loss of radiogenic He before the onset of the exposure era (or during its earIy part), or due to residence in less retentive sites than the cosmogenic 3He. For Crab Orchard the higher K/Ar age again appears to be the more reasonable one, since otherwise the U/He age would be longer than the K/Ar age. Here, the diffusion loss of radiogenic He has been much less severe than for Mincy, although the loss of cosmogenic 3He is higher. IV. P$i~rdial rare gases aad excess lz9Xe In addition to a few marginal cases already mentioned, Bencubbin (‘unique’) and the mesosiderite Veramin show evidence for larger amounts of primordial rare gases, and Udei Station (‘iron with silicate inclusions’) contains a pronounced excess of lz9Xe. Ben~bbjn. Both the metal and silicate samples contain primordial 3bAr (15 and 195 x lo- * cm3 STP/g, respectively) and ‘*Ne (6.5 and 16.5 x lo-‘, respectively). From the ratio of cosmogenic “Ne in metal to that in the silicate (0038), it is obvious that the metal was contaminated by less than 3%0/ of average silicates. As the ratios for primordial 2oNe and 3”Ar are considerably higher, either both metal and silicate must contain the primordial gases or they reside in

Table 8. Gas retention ages of mesosiderites. (Adopted Th/U = 3.0) --~ K(PP4

KIAr-age (109yr)

u (ppb) “ax

Crab Orchard

75’20

Mincy, St1

18it30

Mincy

i 50t20

Nin

7.12.7

4.2Zt.5

3.20t.t.5

3.8”?.35

3.6Zf.4 3.582.4

2.88t.36

2.0Lf.2” *.“bt.*”

3.18A.1

3.932.3

($1 4.u

192

Rei.

-~-

26.7’1.3

Esthervilla Lovicz

_____-U/He-age 6Jgyr,

(2)

3.231.2

(3) _~____~

(1) M~~LLHR and Z;~HRINGER(1969). (2) MEGRUE(1966). (3) KIRSTENet al. (1963).

366

F. BEGEMANN, H. W. WEBER,E. VILCSEKand H. HINTENBEKGER

trace constituents which are more abundant in the non-magnetic portion than in the metal. Further experiments are planned to clarify this point. The primordial gases are of the ‘strongly fractionated’ type: combining the two samples-which may not be justified-the (20Ne/22Ne) ratio turns out to be IO. As silicates and metal show pronounced deficits of cosmogenic 3He, it is not clear at present to what extent the fractionation may have occurred in situ. Veramirz.The most gas-rich sample is the ‘olivine’ St11 (10 and 2.6 x lO-8 of “Ne and 36Ar, respectively). While the 36Ar content of St1 is about half that of St11 there is Lo evidence for 2oNe, in the former; the measured “Ne/“Ne ratio is 0.91. Thus, the ratio (20Ne/36Ar),, in St1 is at least 10 times smaller than that in StII. As it has been shown above that both samples have lost part of their spallogenic rare gases-with St1 the more seriously affected-it is probable that primordial gases have been lost as well. If at least part of the rare gases originally had solar composition, a preferential loss of He and Ne might also account for the (zoNe/36Ar), ratio of 4 in StII, which is between those typical of solar and planetary type rare gases. Udei Station. In the two aliquants of the non-magnetic sample the measured amounts of ‘32Xe--before any blank correction-were 5.1 and 4.5 x lo-” cm3 STP/g, respectively, and the 12gXe/‘32Xe ratios were 1.71 5 0.25 and 1.69 + 0.25. This yields a ‘29Xe excess of (3.5 _t 0.6) x lo-” cm3 STP/g. In both runs the amounts of Kr and Xe were about 10 times larger than those found on the average in the other silicate samples. This, together with the presence of an unusually large non-cosmogenic 36Ar component, strongly suggests that Udei Station contains planetary type primordial gases. We prefer, however, to await the results of further measurements before making more definitive statements. SUMMARY In conclusion the results obtained on mesosiderites can be summarized as follows: (1) the Ni content of the metal is between 7.5 and 8.47; (exception: Vaca Muerta with 10.7%). (2) In the silicates, for absolute concentrations ~3%. the Ca/Al ratio converges to 1.0 + 0.1. (3) 36Cl--36Ar cosmic ray exposure ages range from 10 to 160 Myr, with no significant clustering around any particular value. (4) Five of the 13 meteorites analysed show a deficit of cosmogenic 3He in the metal (up to 30”/,), which is presumably due to the diffusion loss of tritium. Silicate samples have suffered diffusion losses of 3He. For individual meteorites the deficits are correlated with the chemical composition of the samples, indicating the sequence of retentivity to be feldspar < pyroxene < olivine. Anorthite appears to have lost its 3He in all cases, pyroxene definitely shows a loss in the

case of Veramin and possibly in five others as well (Barea, Crab Orchard, Clover Springs, Mincy, Patwar), while olivine has been affected only in Veramin. (5) The data do not allow determination of the position in the above sequence for retentivity of metal for tritium. Thus, it is not possible to decide whether the losses of 3He from the silicates occurred simultaneously with those of tritium from the metal during a considerable fraction of the exposure era, or in an episodic event late in the exposure history. In all cases but one, however, we favor gradual losses over an extended period of time. Only for Veramin is a late episodic event indicated. The high activity ratio 3gAr/36C1 = 1.33 in the metal, due to an outstandingly high 39Ar content, indicates that the event happened less than half a million years ago, and resulted in an extremely large aphelion distance for Veramin. (6) Production rates of 38Ar from Ca are 2&50 times higher than from FeNi; they are equal to those of “Ne from Mg. The cosmogenic (36Ar/38Ar)c, ratio of 0.7 k 0.1 is indistinguishable from that for metal; the distribution of the relative yields along the isobar 36 is markedly different, however. While for metal about 80% of the total 36Ar is produced via 36C1, in the case of Ca this fraction is only 20:&. For the production of neon from Mg the 22Ne/21Ne ratio is below unity (097 rt 0.02). (7) Veramin, in addition to being unusual in several other respects [see (4) and (5)], is the only mesosiderite to show unambiguous evidence for primordial rare gases in the silicates. The larger amounts of 20N% and 36Arp in the olivine compared to those in the ‘pyroxene’ together with a much higher ratio of (20Ne/36Ar& in the former, indicate that diffusion losses of primordial gases have occurred along with losses of cosmogenic rare gases. At least part of the primordial gas appears to have been of solar composition. Acknowledgements-We gratefully acknowledge the generosity of the donors of the meteorite specimens investigated: Prof. A. HALICKA, Warsaw; Dr. HERNANDEZ-PACHECO, Madrid; Prof. E. L. KRINOV,Moscow; Prof. G. KURAT, Vienna; Mr. D. MERRILEES,Perth; Dr. 0. N#YKKI, Helsinki; Dr. E. OLSEN, Chicago and Dr. J. A. PHILPOTTS, Greenbelt. We thank Mrs. G. LOHMANNand C. REITZ for their able help with the chemical and mass spectrometric analyses and Prof. L. SCHULTZ and Prof. H. WANKE for numerous discussions.

REFERENCES BEGEMANN F., VILCSEKE. and WXNKEH. (1966) Die Produktionsraten von 36CI und 39Ar in Metall- und Steinphase des Chondriten Leedey. 2. Naturforsch. Zla, ll(r115. BEGEMANN F., V~LCSEK E. and WANKEH. (I 967) The origin of the “excess” argon-39 in stone meteorites. Eurfh Planet. Sci. Lett. 3, 207-212.

BEGEMANNF. and VILCSEK E. (1969) Chlorine-36 and argon-39 production rates in the metal of stone and

Rare gases and 36Cl in stony-iron meteorites stony-iron meteorites. In Mefeorite Research, (editor P. Millman), pp. 355-362. D. Reidel. B~EMANN F., VILCSEKE., &EVER R., BORNW. and WXNKE H. (1970) Cosmic-ray produced radioisotopes in iunar samples from the Sea of Tranquility. Proc. Apooflo 11 Lunur Sci. Co@ Geochim. Cosmochim. Acta Suppi. 1, Vol. 2, pp. 995-1005. Pergamon Press.

BEGEMANN, F. (1972) Argon 37/argon 39 activity ratios in meteorites and the spatial constancy of the cosmic radiation. J. Geophys. Rex 17, 3650-3659. BOCHSLERP., EBERH~RDTP., GE~SSJ. and GR~~GLERN. (1969) Rare-gas measurements in separate mineral phases of the Otis and Elenovka chondrites. In Meteorire Res~~~r~h.(editor P. ~illman), pp. 857-874. D. Reidel. BOECKL,R. (1972) Terrestrial age of nineteen stony meteorites derived from their radiocarbon content. Nature 236, 25-26.

BOGAKDD. D. and CKESSYP. J. (1973) Spallation production of 3He. “Ne, and “‘Ar from target elements in the Bruderheim chondrite. Geochim. ’ Cosmochim. Acta 37, 5X-546.

BORNW. (1973) “‘C in Meteoriten und Mondproben. Messungen und ihre ~utungen durch Vergleich mit berechneten Tiefenpro~len, Dissertation, Mainz. BORNW. and BEGEMANN F. (1975) “C-3gAr,,-correlations in chondrites and their pre-atmospheric size. Earth Planet. Sci. Lett. 25, 159-169.

BUNCH T. E., KEIL K. and OLSEN E. (1970) Mineralogy and petrology of silicate inclusions in iron meteorites. Contrih. Mineral. Petrol. 25, 297-340.

CRESSYP. J. and SHEDLOVS~Y J. P. (1965) Cosmogenic radionuclides in the Bondoc meteorite. Science 148, 1716-1717. EBERHARDT P., GEI~S J. and LUTZ H. (1963) Neutrons in meteorites. In Earth Science and Meteoritics, (editor J. Geiss and E. D. Goldberg), pp. 143-168. North-Holland. FIREMANE. L. and SPANNAGELG. (1971) The radial gradient of cosmic rays from the Lost City meteorite. J. Geophys. Rex 16, 4127-4134. FIREMANE. L. (1972) Depth variation of argon-37 and argon-39 in lunar material. In The Apollo 15 Lunar Samples, (editor J. W. Chamberlain and C. Watkins), pp, 364-367. Lunar Science Institute, Houston. FLEISCHERR. L., PRICE P. B., WALKERR. M., MAC’RE~ M. and MORGANG. (1967) Tracks of heavy primary cosmic rays in meteorites. d. Geophys. Res. 72, 355-366. FLEROVG. N., ZHOLUDT. P., OTCONSUREN O., PERELYGIN V. P. and Wrr~ H. B. (1974) On search for tracks of superheavy cosmic nuclei in crystals from pallasites. Preprint. FORMANM. A., STOENNER R. W. and DAVISR. (1971) Cosmic ray gradient measured by the argon 37/argon 39 ratio in the Lost City meteorite. J. Geophys. Res. 76, 41094120. GOEL P. S. and KOHMANT. P. (19623 Cosmogenic carbon-14 in meteorites and terrestrial ages of “finds” and craters. Science 136, 875-876. HEYMANND., MAZOR E. and ANDERSE. (1968) Ages of calcium-rich achondrites--I. Eucrites. Geochim. Cosmoc/Tim.Acta 32, 1241-1268. HINTENBERGER H. and WANKE H. (1964a) Helium- und Neonisotope in Eisenmeteoriten. 2. Naturjksch. 19a, 210-218. H~NTENBERGER H., KBNIG H., SCHULTZL. and WKNKEH. ( 1964b) Radiogene, spallogene und primordiale Edelgase in Steinmeteoriten. 2. f~a~~{rforsch. 19a. 327-341. HI~TENBEKGER H., SCHULTZ L.“and WANKEH. (1966) Messung der Diffusionsverluste von radiogenen und spallogenen Edelgasen in Steinmeteoriten II. Z. Naturforsch. 21a, 1147-1159. HINTENBERGER H., SCHULTZL., W~NKE H. and WEBERH. (1967)Helium- und Neonisotope in Eisenmeteoriten und der Tritiumverlust in Hexaedriten. Z. Naturforsch. 22a, 780-787.

367

HUNEKEJ. C., PODOSEKF. A. and WA~SERBURC G. J. (1972) Gas retention and cosmic ray exposure ages of a basalt fragment from Mare Fecunditatis. Earth Planet. Sci. Lett. 13, 375-383. KIRSTENT., KRANKOWSKYD. and Z.&HRINGER J. (1963) Edelgas und Kaliumbestimmungen an einer griisseren Zahl von Steinmeteoriten. Geochim. Cosmochim. Acta 27, 1342.

KOHMANT. P. and BENDERM. L. (1967) Nuclide production by cosmic rays in meteorites and on the moon. In High Energy Nuclear Reactions in Astrophysics, (editor B. S. P. Shen), pp. 169-245. W. A. Benjamin. LIPS~HUTZM. E., SIGNERP. and ANDERSE. (1965) Cosmic ray exposure ages of iron meteorites by the 2’Ne/Z6Almethoh. J. Geoihys. Res. 70, 1473-1489. LOVERING J. F. (1962) The evolution of the meteorites--evidence for the co-existence of chondritic, achondritic and iron meteorites in a typical parent meteorite body. In Researches on Meteo%es, (editor C. B. Moore); pp. 179-197. Wilev. MCCARTHYT. i. and AHRENSL. H. (1971) The composition of stony meteorites (X). The Ca/Al ratio in mesosiderites. Earth Planer. Sci. Lett. 11. 35-36. MACONB. (1963) The aallasites. Amer. Mus. No&ares No. 2163. MEGRUEG. H. (1966) Rare gas chronology of calcium-rich achondrites. J. Geoohvs. Res. 71. 4021-4027. MEGRUEG. H. (1968)‘Rare gas chronology of hypersthene achondrites and pallasites. J. Geophys. Res. 73, 2027-2033. MYJLLERH. W. and ZAHR~NGER J. (1969) Rare gases in stony meteorites. In Meteorite Research, (editor P. Millman), pp. 845-856. D. Reidel. NININGER H. H. and HUSSG. I. (1960) The unique meteorite crater at Dalgaranga, Western Australia. Mineral. Mug. 32, 619-639.

NYQUISTL. E., H~JNEKEJ. C. and SIGNERP. (1967) Spallogenie rare gases in the El Taco meteorite. Earth Planet. Sci. Lett. 2, 241-248.

NYQUISTL., FUNK H., SCHULTZL. and SIGNERP. (1973) He, Ne, and Ar in chondritic Ni-Fe as irradiation hardness sensors. Geochim. Cosmochim. Acta 37, 1655-1685. O’GALLAGHERJ. J. (1972) Observations of the radial gradient of galactic cosmic radiation over a solar cycle. Reu. Geophys. Space Phys. 10, 821-840. PANES F. and URRY W. D. (1931) Heliumuntersuchungen IX. Uber den Heliumgehalt im Eisen und in den akzessorischen Bestandteilen von Eisenmeteoriten sowie in irdischen Metallen. Z. Phys. Chem. 152, 127-149. POWELLB. N. (1969) Petrology and chemistry of mesosiderites---I. Textures and composition of nickel-iron. Geochim. Cosmorhim. Acfa 33, 789-810. POWELLB. N. (1971) Petrology and composition of mesosiderites-II. Silicate textures and compositions and metal-silicate relationships. Geochj~~, Cosmochim. Acta 35, 5-34.

PRICEP. B., RAJANR. S. and TAMHANEA. S. (1967) On the pre-atmospheric size and maximum space erosion rate of the Patwar stony-iron meteorite. J. Geophys. Res. 72, 1377-1388. SCHAEFFER 0. A. and HEYMANND. (1965) Comparison of 36Cl-36Ar and 36Ar--38Ar cosmic ray exposure ages of dated fall iron meteorites. J. Geophys. Res. 70, 215-224. SCHULTZ L. and HIN~~~GER H. (1967) Edelgasmessungen an Eisenmeteoriten. Z. Naturforsch. 22a, 773-779. SCHULTZL. (1967) Tritium loss in iron meteorites. Enrth Planet. Sci. Lett. 2, 87-89.

SCHULTZL., HUNEKE J. C., NYQUISTL. E. and SIGNER P. (1969) Spallogenic rare gases in taenite separated from iron meteorites. In Meteorite Research, (editor P. Millman), pp. 887-894. D. Reidel. SIGNERP. and NIER A. 0. (1962) The measurement and

368

F. BEGEMANN.H. W. WEBER,E. VILCSEKand H. HINTENBERCER

interpretation of rare gas concentrations in iron meteorites. In Rescurches on Meteorites. (editor C. B. Moore), pp. 7-X J. Wiley. STAUFFERH. (1962) On the production ratios of rare gas isotopes in stone meteorites. J. Geophys. Res. 67,

WAS~ONJ. T., SCHAUDYR., BILD R. W. and CHOU C. L. (1974) Mesosiderites. I. Compositions of their metallic portions and possible relationships to other metal-rich meteorite groups. Geochirn. Cosmochim. Actn 38, 135149. WAS+N J. T. (1974) ~ereuriti~s. Cl~s.~i~~~zriolz and Proper2023-2028. SUES H. E. and WANKE H. (1962) Radiocarbon content ties, p. 43. Springer. and terrestrial age of twelve stony meteorites and one WEBBERW. R. and LEZNIAKJ. A. (1973) Interplanetary iron meteorite. Geochim. Cosmochim. ilcta 26, 475-480. radial gradients of galactic cosmic ray protons and WANKE H. (1973) Private communication. helium nuclei: Pioneer 8 and 9 measurements from WAHLENM., HONDA M., IMAMLJRA M., FRUCHTERJ. S., ,75 AU to 1.10 AU. J. Geophys. Rex 78, 197992000. FINKEL R. C.. KOHL C. P., ARNOLDJ. R. and REEDY WIIK H. B. (1963) Quoted by MASON(1963) The pallasites. R. C. (1972) Cosmogenic nuclides in football-sized rocks. Amer. Mus. Nocitutes No. 2163. Proc. Third Lunur Sci. Con& Geochim. Cosmochim. Supp/. 3, Vol. 2, pp. 1719-1732. M.I.T. Press.

Acttr.