Tritium and argon39 in the Bruderheim meteorite

Geochimica etCoamochimics Beta 19G2 Vol.26,pp.069to003.Pergamon PrewLtd. Printed inRorthcrn Irelend

Tritium and azgon39 in the Bruderheim meteorite ST. CHARALAMBUS and K. GOEBEL C.E.R.K., Nuclear Physics Division Geneva 23, Switzerland (Received 23 September 1961) &&&-The T and A39 contents of the Bruderheim stone meteorite were measured. At the time of fall the average T activity of three different samples of 0.44 count/min per g was obtained. The A39w&80.01 counts/mm in agreement with measurements in other laboratories. From both pairs, T-He3 and A3s-A39, an exposure age of 28 million years &3 was calculated. This agrees with exposure ages obtained by other methods using NaZZ, Alz6 and CPs, which yielded a value of 30 x lo6 years.

I. INTRODUCTION Or; 4 MARCH, 1960, a stone

meteorite fell near Bruderheim in the Province of Alberta (Canada) (53*54”N, 112.54”W). This chondrite was first described by FOLMSBEE and BAYROCH (1961) and samples of this meteorite were supplied to different laboratories for all kinds of measurements. We will therefore limit ourselves to describing the measurement of T and A39 produced under the influence of cosmic radiation. Samples have also been sent to other places for the same purpose, and so it is possible to test on this particular meteorite the reliability of the measurements in question. One should, however, keep in mind that the Bruderheim meteorite is relatively large and, therefore, different samples can yield different results, as the influence of the shielding of the meteoric material might be of some importance. In this case we will deal with measurements on two different samples. In the following it is of some interest to note that according to BAADSQAARD et al. (1961) the Bruderheim meteorite is a grey bronzite-olivine chondrite of Urey’s low iron group. It contains (see Appendix) about 20% Fe and 1.3% Ni. Other elements heavier than the argon which have been found are Ca (l-2%), Cr (0.5 7$), K (0.12%) and Mn (0.25o;b). It can be seen from the table that the meteorite does not show very important peculiarities in its chemical composition. We shall try to compare the results of the Bruderheim with those found for other chondrites (GOEBEL and SCHMIDLM, 1960). Using the values of He3 and A3* of the same samples which have been measured by Z-RINGER (1961) we could determine a radiation age by means of two different pairs of isotopes. II. EXPERIMENTAL

PROCEDURE

To extract gases quantitatively from the samples we used the widely adopted method of degassing the samples in a vacuum by means of high-frequency heating. The method has already been described elsewhere (GOEBEL and SCHMIDLM, 1959). Samples of lo-15 g were put in an Al,O, crucible and heated for 2 hr at about 1600”. The gases were extracted from the furnace with a diffusion pump and the hydrogen was separated from the other gases by diffusion through palladium and then pumped into a G-M counter. Facilities were provided for cracking hydrocarbons a,nd decomposing water. The residual gas was brought into contact with 659

660

and K. GOEBEL

ST.CHARALAMBUS

hot calcium and copper oxide and the remaining rare gases were put in a small G-M counter for A counting. In every extraction the whole procedure was recycled and the active gases from the second run were put in other counters. After the first heating period the remaining gases were found in this way to be less than 10 per cent of the total amount. Before and after each separation tests were made to check the procedure and the background of the whole vacuum line. The determination of the decay rate of the radioactive gases was done in an ordinary low-level counting system. The G-M counter for the T measurement has a volume of 150 cm3 and a background of about 2.4 counts/mm, whereas the counter for the radioactive A had only about 10 cm3 and a background of about O-23counts/ min. After the T was counted in a G-M counter, the hydrogen-T mixture was transferred to a proportional counter of about 600 cm3 volume. Fig. 1 shows the section through the proportional counter. The TR counter (central counter) and the guard counter (annular counter) are mounted in the same tube and separated by a thin mylar foil (3.4 mg/cm2) coated on both sides with Al. The foil separates the two volumes completely but the two parts have to be kept approximately on the same total pressure. The central counter had a low background of O-36 counts/ min; more details about this counter will be given elsewhere CEARALAMBUS and GOEBEL (1961) . By measuring the same gas with the G-M counter and a proportional counter we could check whether the activity given by the G-M counter can be really attributed to the energy region of the T-8 spectrum. The pulse-height spectrum of the proportional counter which was accepted for the T counting was limited by two discriminators. The lower one was placed at about 1.5 keV, the upper one at maximum T energy. III. RESULTS The results of T measurements are given in Table 1. From this Table we can compare the values for the different samples and for the different counting methods (G-M and proportional counter). Table 1 Tritium activity at time of all. dpm/g Sample

1

2 3

Weight (g)

G-M counting

Proport. counting

17.24 15.50

0.47 0.47

0.40 0.49

13.50

0.39

0.40

For the first sample the proportional counter gave a value which was too low compared with the GM counter. This might be due to the fact that the T-counting gas mixture was kept for a considerably long time (5 months) in a storage vessel, When the Hi and T was transferred to the proportional counter the counting gas (45% A, 50% ethylene) was frozen in a trap. During the storage time there might have been some exchange of the T with the H1 of the ethylene, so that some T was lost for the proportional counting. For the other two samples, however, the T was immediately transferred to the proportional counter. The errors for theT measurement can be estimated as follows.

tifil

Tritium and argona in the Bruderheim meteorike END CORRECTION

TUBE

MYLAR

n

I

n OUT

OUT

Fig. 1. Low-level

proportional counter for tritium

The counting error, including the error of the background and the efficiency of the counter, is below 5 per cent. As could be checked by recycling, the error of the degassing of the meteoric sample is lower than 10 per cent. Systematic errors arising from small contaminations of the sample are not included. We estimate our total error for the T measurements as being 15 per cent. A3Bactivity has been measured for the extracted gases of sample 1. We obtained a value of O-01 dpm/g. The error of this measurement is somewhat higher than in the case of T in the activity measurements (10 per cent). However, in the case of rare gases, the degassing is always more complete than in the case of hydrogen. This was checked in the case of meteorites as well as for irradiated targets CRARALAMBUS and GOEBEL (1961). The total error for the Ass measurement should, therefore, be about the same as in the case of T. We estimate it to be below 20 per cent. As the sample was supplied too late the As7 had already decayed. IV.

DISCUSSION

If we compare the T measurement of the Bruderheim meteorite with those of other stone meteorites it can be seen that it fits fairly well into the curve of Fig. 2 1.0 -j

x

.6 L cn -z

x x

4

X-x-

x

-&

I

I

102

IO Fig. 2.

I

10~ kgr

ST. CIULRALAMBUS and K. GOEBEL

662

which shows the relation between weight of the meteorites and the T decay rate. It should be mentioned, however, that the mass of the meteorite found is only a rough measure of the pre-atmospheric size. Therefore, the fluctuation in the diagram should not be taken too seriously for the lighter meteorites. Table 2 Irradiation

Irradiation 9 (d&g) 0.47

age A%_A39

se3 (omYg)

age 1H3-$Ie3 ( 1OByears)

wA3e (omYg)

1**30 (dpm/g)

(106 years)

46.5 x 10-e

25 h 10%

1.42 x 10-e

0.94 x 10-Z

31 & 10%

In Table 2 the T and A39values are compared with those of He3 and A3*. For the following evaluation of exposure ages we assume that the rate of direct production of T and He3 by the cosmic radiation is the same (GEISS and OESCHOER, 1960). The production ratio for As* and A39is slightly more difficult to estimate. The ratio for Fe haa been measured directly (SCHAEFFER and ZXHRMQER 1958): but for other elements which have a higher atomic number than argon, the ratios are not well known. This is particularly important in the case of Ca because the production cross-section for argon should be fairly high. If we take the production ratio for Fe-Ni (A3*/AS9 = 1.8) and reduce this value to take account of the production from other elements, we arrive at about l-6. As Table 2 shows, the t,wo ages agree within the limit of error. On the other hand, HONDA et ul. (1961) measured the radiation ages using solid isotopes such as Na *2,Alas and C13*,and found a value of 30 x lo6 years, which is in perfect agreement with our results. A3e measurements on the same meteorite were published by FII~EMAN and DE FELICE (1961): they found the same value of O-01 dpm/g for the A 39, but only about half the activity for the T (0.29 dpm/g) . The agreement between the exposure ages obtained from T and He3 on the one side, and from the pair A3* and A39 on the other, can be taken as an indication that diffusion losses are negligible, at least during some 10’ years. If the diffusion were of importance it would be expected that the He would diffuse much quicker (about a factor 10) than the argon (FECHTIU et al., 1960). As9losses are very unlikely as the A-K method gives an age of the order of four billion years. REFERENCES BAAIXX+AAXD H., CA-BELL F. A., CUMMIN~G. L. and FOLINSBEE R. E. (1961) J. Geophya. Res. To be published. CH~US ST. and, GOEBELK. (1961) Low-level proportional counterfor tritium. CERN report. In preparation. CH~~ALAMBUS ST. end GQEBELK. (1961) T,A3 in stone meteorites, Int. S~llation Phenm. Conf. CERN, Septe4nber 1961. Programme abstract. FECX~~I~ H. private oommunication. FECETI~H., GENTNERW. und ZLERINCXSR J. (1960) Dif%.koneverlustevon Argon in Mineralien und ibre Auswirkung auf die Kalium-Argon-Alterebestimmung. Cfeochim. et Cosmochim. Acta 19, 70-79. FIREMANE. and DE FELICEJ. (1961) As9,Ae7 and tritium in recently fallen Bruderheimmeteorite. 42nd ann. meeting Amer. Beophys. Un. p. 56. Programme abstract.

Tritium and argon3” in the Bruderhoim meteorite

663

FOLINSBEE K. and BAYROCK L. (1961) 42nd Ann. .lIeeting Armor. Geophys. Un. p. 56. Programme abstract. GEISS J. and OESCHGER H. (1960) Proc. 1st Int. ,SjxcceSci. Symp., Nice 1960. p. 1076. GOEBEL I(. und SCHMIDLIN P. (1960) Tritium Messugen an Stein-meteoriten. 2. Naturl. 15a, 79. GOEBEL K. und SCHMIDLIN P. (1959) Tritium. Geochim. et Coemochim. Acta 17,342. HONDA M., UMEMOTO 8. and ARNOLD J. (1961) Radioactive species produced by cosmic rays in Bruderheim and stone meteorite. SCHAEFFER 0. und ZHHRINGER J. (1958) Helium und Argon-Erzegung in Eisentargerts durch energiereiche Protonen. 2. Naturl. Ma. ZLHRINGER J. (1961) privat,e communication. APPENDIX Chemical analysis and normative mineral position of the Bruderheim meteorite (BAADSGAARD et al., 1961) SiO TiO, Al,03 Fe” Fe0 FeS MnO MgO CaO Ka,O IGO P,OG H,OH,O+ Ni” Co” Cr,O, C Total

39.94 0.12 1.86 8.59 12.94 6.38 0.33 24.95 1.74 1.01 0.13 0.29 0.01 0.10 1.30 0.05 0.60 0.04 100.38

Norm&i Nickel-iron Troilite Olivine Hypersthene Diopside Albite Anorthite Orthoclase Chromite Apatite Ihnenite

com-

-composition 9.94 6.38 41.65 25.90 5.34 8.52 0.17 0.78 0.92 0.74 0.21

Analysis by BAADSGAARD and STELMACII.