Cosmic-ray-produced40K and50V in the metal phase of chondrites

Earth and Planetary Letters, 26 (1975) 54-60

LZA

© Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

COSMIC-RAY-PRODUCED 4°K AND S°V IN THE METAL PHASE OF CHONDRITES KEIKO IMAMURA, MASAKO SHIMA* and MASATAKE HONDA

Institute for Solid State Physics, University of Tokyo, Roppongi, Minato-ku, Tokyo (Japan) Revised version received March 2, 1975

Cosmic-ray-produced 4°K in the metal phase of six chondrites and S°V in that of one chondrite were determined using a surface ionization mass spectrometer. The 22Netotal/4°Kmetal ratios of the chondrites are explained in part by shielding effects during cosmic-ray irradiation. The wide variation of this ratio in some groups of meteorites is explained in terms of partial loss of rare-gas nuclides. Radiation ages for the chondrites were determined using 40K measurements and production-rate estimates from thick target calculations.

1. Introduction

2. Experimental

Many cosmic-ray-produced radioactive and stable nuclides have been measured in meteorites. Difficulties may arise in the interpretation of rare-gas data when used alone because o f the possibility o f raregas loss. This seems to be more serious for stone meteorites than for iron meteorites. The measurement o f spallogenic non-volatile stable nuclides in stone meteorites could help resolve the difficulties involved with rare-gas data. Many spallation-produced rare-gas nuclides in stone meteorites are low-energy products - nuclides with masses close to that of the target element. Such nuclides are produced mainly by secondary particles o f relatively low energy (< 100 MeV). The measurement of b o t h a high-energy product, such as 4°K, and a low-energy product, such as S°v, in the metal phases o f chondrites would give information on shielding effects during their radiation histories [1,2]. We report here the results o f measurements o f spallogenic 4°K in the metal phases o f the chondrites Breitscheid, Richardton, Bruderheim, Peace River, Holbrook and St. S6verin and also o f spallogenic s°V in the metal phase o f Bruderheim.

2.1. Separation o f metal particles from the chondrite

* Present address: Department of Chemistry, Nuclear Research Building, McMaster University, Hamilton, Ont., Canada, L8S 4K1.

In our experimental technique at least 0.1 g of the metallic fraction is necessary to determine spallogenie 4°K and s ° v . Therefore more than 10 g of each sample was crushed in an alumina mortar at liquid nitrogen temperature and the metallic particles were separated with a hand magnet. The magnetic fraction was immersed in pure water and was washed in an ultrasonic bath to remove silicate minerals. It was then treated with 0.1N HC1 to dissolve troilite contamination. From the contents o f potassium which were measured by isotope dilution analysis, the silicate contamination level in our metallic samples were found to be in the range o f 0.1-0.3%.

2.2. Chemical procedure All reagents used were purified by ourselves. Isopropanol was distilled twice. Purification methods for other reagents and the preparation o f the 39K spike solution have been described in our previous papers [ 2 - 4 ] . The s ° v spike and the 49V tracer were obtained from the Oak Ridge National Laboratory, U.S.A. Both solutions were prepared by dissolving them in HC1. The s ° v / s 1V ratio o f the en-

55 riched spike was 0.557 -+ 0.001. The purified magnetic fraction was dissolved in concentrated HC1 and HNO3 after adding the 39K spike and the 49V tracer. An anion-exchange resin column was used for the separation of potassium [2,5] which was recovered in the effluent of the first two column volumes of a mixture of 2.5N HC1 and 74% isopropanot. The resin column was then treated with 7N HC1 to recover Ni and V. Finally Fe was eluted with 0.5N HC1. The target materials, Ni and Fe, were measured by flame-photometry and/or EDTA titration. The K fraction was evaporated to dryness for mass spectrometry. Vanadium was further separated by cupferron extraction, then purified by a small anionexchange resin column. The vanadium fraction was divided into three aliquots: the first for isotopic

ratio measurements, the second for isotope dilution analysis after adding S°v spike, and the third for counting 49V to determine the chemical yield. The 49V activity was counted with a Xe-fiUed proportional counter.

2.3. Mass spectrometry The isotopic ratios of potassium and vanadium were measured using a surface ionization source mass spectrometer with an electron multiplier. K isotope was measured by a similar technique as reported in the previous paper [3]. For V, stable beams were obtained when the sample was m o u n t e d on a single Re fdament together with dispersed carbon suspension.

TABLE 1 Mass spectrometric data of cosmic-ray-produced 4°K in metal phase of chondrites

Chondrite sample

Metal Fe (g)

Ni (g)

Spike added fag K)

Peak height ratio

Cosmic-ray-produced 4°K (× 1011 atoms/g (Fe + Ni*))

Breitscheid

0.353 0.606**

0.038 0.052

0.238 0.543

17.76 ± 0.06 46.48 ± 0.09

2.058±0.025 3.674±0.045

Bruderheim

0.210 0.072** 0.390 1.157"*

0.023 0.058 0.036 0.127

0.202 0.195 0.235 0.840

17.54 20.66 17.96 21.55

1.961±0.014 1.995±0.014 2.016±0.018 2.187±0.010

40/41 (X 103)

39/41

7.2±1.4 6.0±0.2 mean

± ± ± ±

0.09 0.04 0.08 0.04

6.3±0.2 7.0±1.9 6.0±1.4 5.1±1.0 4.6±0.5

mean

5.3±0.4

Richardton

0.290

0.021

0.205

20.79 ± 0.04

2.018 ± 0.02~

3.1±0.6

Peace River

0.439** 0.626**

0.080 0.067

0.220 0.953

16.54 ± 0.03 20.70 ± 0.03

2.021 ± 0.012 2.016 ± 0.010

6.1±1.0 5.6±1.1 mean

5.9±0.7

Holbrook

0.246

0.073

0.178

19.38 ± 0.05

1.923 ± 0.018

2.1±0.4

St. S6verin (DIV2"'b)

0.226

0.071

0.177

21.03 ± 0.06

1.955 ± 0.027

1.9±0.4

Reagent K

May, 1968 - Oct., 1970 Oct., 1970 - Jan., 1971

13.89 +- 0.03 13.94 ± 0.02

1.719±0.006 1.751±0.025

Spike * Fe + 0.74 X Ni (g). ** Oct., 1970 - Jan., 1971.

2668.2 ± 28.8

36.41 ±0.18

56 3. Results The enrichment of 4°K in the potassium samples mainly depends on the cosmic-ray exposure age of the chondrite and on the level of contamination. The errors accounted for are statistical errors and the reproducibilities of isotopic ratio measurements. To calculate the cosmic-ray-produced 4°K, two sets of slightly different data for terrestrial potassium were used. The results for potassium are shown in Table 1. The Ni content in the treated metal phase was relatively high and the contribution of spallogenic products from Ni is not negligible. The production rate, Q, from the Ni target relative to the Fe was estimated as follows [1] :

Q(A) = k(AA)-k2;

~A = 57-A

Here the constant k2 for stone meteorites (radius ~30 cm) is 2.3 ~ 1.9. The normalizing factors (Q(A) from Ni/Q(A) from Fe) were calculated to be 0.72 - 0.76 for 4°K and 0.44 - 0.51 for s°V; mean values of 0.74 and 0.47 were taken for a°K and S°v, respectively. The isotopic ratio of s°Ws 1V in the extract of the Bruderheim metal phase was (2.93 + 0.05) × 10 -3 whereas the mean value of six terrestrial vanadium measurements was (2.49 + 0.04) × 10 -3. From these figures, the isotope dilution analysis and the 49V counting data, the spallogenic s°V was calculated to be (2.7 -+ 0.5) × 1012 atoms/g (Fe + Ni) for 20% Ni. Stauffer and Honda [6] had previously reported (4.5 -+ 2.0) × 1012 atoms/g (Fe + Ni). Using our present data for s°v, the 5°V/4°K ratio of the Bruderheim metal was calculated to be 5.1 -+ 0.9.

4. Discussion The present 4OK and 5°V results in the metal phase of a chondrite may be useful in a discussion of the relative shielding in stone and iron meteorites. The production rates of 4°K, 22Ne and S°V in the metal phase and of 22Na and 22Ne in the total chondrite were estimated using the data obtained from thick target bombardment experiments [7-10]. The calculation was made according to the method employed by Honda [7] and Kohman and Bender

[ 11 ]. From the experimental thick target cross sections, we graphically obtained o(R), the effective cross section at the center of a sphere with radius R. At depth D in the sphere with radius R, the cross section of each nuclide was calculated using the following equation:

o(E,R,D)= ? a(E,x) sin OdO/ j o o

sin 0d0

where E is the bombardment energy and x is the distance from the surface to the point at which the production rates are calculated. Bombardment data exist [7-10] for proton energies of 0.45, 1.0, 3.0 and 6.0 GeV. The production rates of stable nuclides were calculated from radioactive nuclide data using Rudstam's equation [ 12] : In

o(A,Z) =PA- Q-R (Z-SA )2

whereP = 10.2E -°'63 (E in MeV),R = 1.9, andS = 0.474. Thin target cross sections for 42K at 0.73 GeV (3.2 mb) and 29 GeV (3.5 mb) were used for 4°K [13,14], thick target data for 4SV and 51Cr at 3 GeV were used for S°V and data for 22Na at 3 GeV was used for 22Ne [7]. Relative production rates were as follows: 4°K/ 42K = 2.5 (0.73 GeV) and 4.6 (29 GeV), 48V/s°V = 0.95, 51Crf;0V = 1.6 (3 GeV) and (22Ne + 22Na)/22Na = 2.5. Using 22Na and 22Ne contents in Aroos iron meteorite (age: 8 ~ 9 × 108 yr) [1,15,16],(22Ne + 22Na)/22Na was calculated to b~ 2.6. The 22Ne production rate in stone meteorites was calculated from glass-block bombardment data at 3 GeV [10]. The build-up factors (Omax/OR=0)for 47Sc in ironmatrix and glass-matrix were obtained from 1 GeV and 3 GeV experimental data [ 7 - 9 ] , respectively. These factors are 1.3 in iron and 1.2 in glass at 1 GeV and 2.1 in iron and 2.0 in glass at 3 GeV. Thus the build-up factors for iron and glass were assumed to be equal at each energy. The mean absorption lengths were assumed to be 156 g/cm 2 (iron, 1 GeV), 114 g/cm 2 (glass, 1 GeV), 200 g/cm 2 (iron, 3 GeV) and 145 g/cm 2 (glass, 3 GeV). The mean absorption length in a silicate matrix was assumed to be ~0.73 times that in an iron matrix. According to Kohman and Bender [11], the primary cosmic rays may be divided into four energy

57 TABLE 2 The comparison o f the (50V/4°K)metal and (40K/22Ne)metal for chondrites and iron meteorites

Meteorite

(S°v/4°K)metal

Ref.

(4°K/22Ne)metal

Ref. *

Breitscheid Bruderheim

5.1 -+ 0.9

this work

1.9 -+ 0.4 2.1 -+ 0.7

[ 17,18 ] [17,18]

3.8 +- 1:0 2.O -+0.4

[191 [201

-

2.3 -+ 0.8 2.8 -+ 0.7

[ 18] [19]

[2] [22] [22]

3.1 + 0.2 4.0 + 0.2 3.2 -+ 0.2

[2,21] [15,22] [15,22]

Holbrook St. S~verin Treysa Grant-Interior Grant-Surface

5.0 + 0.4 6.5 + 0.4 5.2 + 0.4

* For chondrites, references are only for 22He.

regions: 0.3-0.7 (0.45), 0.7-2.0 (1.0), 2.0-4.5 (3.0) and 4.5 -o0 (6.1) GeV. The energy at which production rates were calculated is indicated in parentheses. We modified the production rates in the 4 . 5 o0 GeV region by using the more suitable value of 10 GeV rather than 6.1 GeV. The cosmic-ray fluxes (proton + a - particle × 4) in each energy region were estimated to be 1.0, 1.2, 1.1 and 0.9 nucleon/ cm 2 sec 4zr, respectively. The (s°v/4°K)meta1 and (4°K/22Ne)metal ratios for chondrites and iron meteorites are compared in Table 2. It can be seen that all of the samples including Breitscheid and Bruderheim had shielding similar to that of the surface samples in the iron meteorites Treysa and Grant. The 22Ne contents in the metal phases of chondrites are uncertain because of stone contamination and recoil gain of 22Ne produced in the stone phases. Table 3 gives the 22Netotal/4°Kmetal ratios for stone meteorites. The values of 22Ne used are the mean value of published data [18-20, 23-36]. For four chondrites the ratio lies in the narrow range 8.5 -+ 0.5. On the other hand Breitscheid and Bruderheim have low values, 4.1 + 0.5 and 5.2 + 0.6, respectively. Fig. 1 is a comparison of these measured ratios with calculated ratios as a function of size and depth. The spallogenic nuclides S3Mn and 22Na, both of which are low-energy products, are indicated in Table 4. The 22Netotal/4°Kmeta 1 ratios together with the S3Mn and 22Na activities of Peace River and St. Sdverin indicate that the four

chondrites with higher 22Netotal/4°Kme ~ ratios are samples from > 10cm depth in bodies of ~ 4 0 60-cm radius. The lower 22Netotal/4°Kmeta I ratios of the sampies from the chondrites Breitscheid and Bruderheim might be explained by less shielding during their irradiation except that their S3Mn and 22Na contents are about twice those in Harleton. This suggests

2OO

. ~ -

,

6O

%O

/

•

/" Richardton,

i

0

Pelce River, St. S~verin and Holbrook

Bruderhelm

A

Oreitscheid

DEPTH ( e m ) •

40

.

Fig. 1 Correlation between Kmetal, c o s m l c - r a y ~ r o d u c e d 4°K c o n t e n t in a metal phase of chondrites, and Ne, the 22Ne in a total chondrite. The solid curves show the result of calculations using thick target data, and indicate a variation of a ratio from the surface to the center of a spherical body. The radius, in cm, is indicated beside the solid line. The ratio at very near the surface, 0 ~ 2 cm, m u s t be somewhat higher than the figure indicated in this graph because of solar proton contribution to 22Ne production.

58 TABLE 3 Relation between spallogenic 22Ne in total chondrite and 4°K in the metal phase, and radiation ages

Chondrite

22Ne total* (10 -8 cm 3 STP/g)

22Netota 1

4°K age** (m.y.)

22Ne age*** (m.y.)

42 35 21 39 14 13

22, 23, 22, 38, 16, 12,

4°Kmeta 1 Breitscheid Bruderheim Richardton Peace River Holbrook St. S~verin

9.7 10.3 10.0 17 7.0 5.4

4.1 5.2 8.7 7.7 9 8

± 0.5 -+ 0.6 +- 1.8 ± 1.2 -+ 2 ±2

±5 ±5 ±4 ±5 -+ 3 ±3

40 42 41 64 26 21

* The values o f 22Netotal are averaged literature data. There are, for Breitscheid, three data, 10.6 [23], 9.9 [24] and 8.5 [18] × 10 -8 cm 3 STP/g; for Bruderheim, twelve data [18 - 20, 24 ~ 30] including lowest value 7.9 [ 18] and highest 11.9 [19] X 10 -8 cm a STP/g; for Richardton, five data [30 - 341, 8.59 [32] to 11.82 [31] × 10 -8 cm 3 STP/g; for Holbrook, four data, 5.7 [18], 6.3 [24], 7.0 [33] and 9.0 [27] × 10 -8 cm 3 STP/g. For Peace River, because o f no data for 22Ne, the data of 21Ne (15.5 × 10 -8 cm 3 STP/g [35]) was multiplied by 1.1. For St. S~verin, Funkhauser et al's data, 5.4 × 10 -8 cm 3 STP/g, sample position D-III2 [36], was taken. This is the nearest location to our sample; DIV2'"'b. ** The estimated production rate (1.5 +- 0.1) × 101° atoms 4°K/g m.y. was used (see text). *** Assumed: 90 dpm 22Na/kg. In first row, the production ratio Q(22Na+22Ne)/Q(22Na)= 2.5 --+ the production rate o f 22Ne 1.2 × 1011 atoms 22Ne/g m.y. was used. In second row, according to Fisher [37] the production ratio Q(22Na + 22Ne)/ Q(22Na) = 1.5 ~ the production rate o f 22Ne ~ 7 × 101° atoms 22Ne/g m.y. was used.

TABLE 4 Typical radioactive nuclides produced by lower-energy particles

Meteorite

Breitscheid

Bruderheim Peace River St. S~verin*

Harleton

53Mn ( t ½ : 3 . 7 × 106 yr) (dpm/kg) -

85 ± 17

Ref.

22Na (t½:2.6 yr) (dpm/kg)

-

89 90 90 96 94

[39]

± 15 +- 10 -+ 6 ±6 ± 10

[38] [39] [40] [41] [42]

(91 ± 9 62±8

[43] [441

~-60 +- 20 (DII2') ~65

[45] 146]

~ 102 ± 3 (DIII1) / 101 ± 8 ( D I I I 2 ( 1 ) ) 53 ± 5 (DIII2(4))

[47] [411 [45]

44 ± 8 (55 + 5

Ii1 [431

( ~

64 ± 7 53 _+6

[1] [40]

]

58 ± 5

(

98 ± 5 69±7

Ref.

k(138 ± 15)

[41] [44]

[41] [431

* 22Na in various locations of St. S~verin was measured by many authors. Here only the data from the places near our sample location (DIV2'"'b) are cited. Unfortunately, 53Mn was not determined in the location near our sample. The data from [46] is a mean value o f the data from four different locations.

59

that Bruderheim and Breitschied are larger than Harleton and similar in size to Peace River and St. S6verin so that their relatively low 22Netotal/4°Kmeta1 ratios would be attributable to partial rare-gas loss. Irradiation ages based on the 4°K content in the metal phase are given in Table 3. The production rate of 4°K in metal was estimated for shielding locations of greater than 15 cm depth in 30-60-cm radius spheres and corresponds to a value of (1.5 -+ 0.1) × 101° atoms 4°K/g-Fe m.y. This value is in accord with the value of 1.4 X 101° atoms 4°K/m.y. obtained from the 4°K content [1] and the radiation age [16] of the small iron meteorite Treysa. The calculated 4°K radiation ages are nearly identical with 22Ne-22Na pair ages for the chondrites Richardton, Peace River, Holbrook and St. S6verin, for which the relative production rate Q(22Ne+22Na)/ Q(22Na) = 2.5 was used. With the 4°K production rate of(1.5 -+ 0.1) × 101° atoms/m.y, the radiation ages of Bruderheim and Breitscheid were calculated to be 35 m.y. and 42 m.y. respectively. These ages are approximately 50-70% higher than estimates based on 22Ne contents alone or on 22Ne-22Na pairs. Recently Fisher [37] discussed the cosmogenic rare-gas production rate in chondrites. According to his estimation, the conventional production rates are too high by a factor of ~1.75, and the production ratio Q(22Na + 22Ne)/Q(22Na) falls to 1.5. With this ratio the 22Ne ages would be 40 and 42 m.y. for Breitscheid and Bruderheim respectively, The age derived from the a6Cl(tl/2:3.1 × 10 s yr) -36Ar pair for Bruderheim is 33 -+ 8 m.y. [39]. For Breitscheid and Bruderheim~ Fisher's estimation agrees with 4°K ages, while for the other four chondrites it seems not to agree so well.

Acknowledgments We are indebted to the following scientists who contributed samples and made this work possible: Professor H. Hintenberger, Max-Planck-Institut for Chemic, Mainz, West Germany, for Breitscheid; Professor C.B. Moore, Center for Meteorite Studies, Arizona State University, Tempe, Arizona, U.S.A. for Richardton; Professor R.E. Folinsbee, University of Alberta, Edmonton, Alta., Canada, for Bruderheim

and Peace River; Dr. B. Mason, Smithsonian Institution, U.S. National Museum, Washington, D.C., U.S.A., for Holbrook; and Dr. J. Labeyrie, Centre d'l~tudes Nucl6aires de Saclay, France, for St. S6verin. We are also grateful to Miss K. Horie for her help in counting technique of 49V.

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35

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38 39

40

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43

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