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Tritium loss in iron meteorites
EARTH AND PLANETARY SCIENCE LETTERS 2 (1967) 87-89. NORTH~HOLLAND PUBL. COMP., AMSTERDAM
T R I T I U M LOSS IN I R O N METEORITES L. S C H U L T Z Max-Planck-lnstitut flit Chemie ( Otto-Hahn-Institut), Mainz, Germany Received 10 January 1967
The concentrations of cosmic ray produced 3He in the medium octahedrites Ruff, s Mountain and Roebourne are too low as compared to the concentrations of other spallogenic nuclear species. A number of arguments is compiled which indicate that the 3He anomaly in these octahedrites and in some hexahedrites, as well as the low spallogenic tritium content in freshly fallen iron meteorites, is caused by diffusion loss of tritium due to solar heating of the meteorites in space.
In the c o u r s e of i n v e s t i g a t i o n s of s p a l l o g e n i c r a r e g a s e s in i r o n m e t e o r i t e s - which will be published e l s e w h e r e [1] - an a n o m a l o u s low 3 H e / 4He r a t i o of 0.158 w a s found in the m e d i u m o c t a h e d r i t e R u f f ' s Mountain. Up to now, such low r a t i o s have only been d e t e c t e d in s o m e h e x a h e d r i t e s [2-4] and in the n i c k e l - r i c h a t a x it e Washington County [4, 5]. In Washington County the a n o m a l o u s 3 H e / 4 H e r a t i o i s undoubtedly due to p r i m o r d i a l 4He [5,6]. B a u e r [2] p r e s u m e d that the low 3 He/ 4 He r a t i o of s o m e h e x a h e d r i t e s i s a l s o due to an e x c e s s of 4He; H i n t e n b e r g e r and W~Luke [3], h o w e v e r , c l e a r l y showed that the low 3H e/ 4 He r a t i o of the h e x a h e d r i t e Brannau i s not c a u s e d by an e x c e s s of 4He but r a t h e r by a l o s s of 3He. T h e i r c o n c l u s i o n was that Braunau and m o s t l i k e l y a l s o the h e x a h e d r i t e s m e a s u r e d by B a u e r m o v ed in o r b i t s r e l a t i v e l y c l o s e to the sun. In t h e s e o r b i t s , the t e m p e r a t u r e of the m e t e o r i t e s w a s so high that t r i t i u m diffused into space. Hence, t h e s e m e t e o r i t e s contain the 3Hecomponent p r o d u c e d d i r e c t l y but only a p o r t i o n of the 3He p r o d u c e d by the d e c a y of s p a l l o g e n i c tritium. This, very recently, was also conf i r m e d by H i n t e n b e r g e r et al. [4]. The low 3 He/ 4 He r a t i o in R u f f ' s Mountain a p p e a r e d to be e s p e c i a l l y i n s t r u c t i v e since B u c h wald [7] concluded f r o m m e t a l l o g r a p h i c i n v e s t i gations that a late c o s m i c gentle r e h e a t i n g o c c u r r e d in t h i s m e t e o r i t e . The t e m p e r a t u r e m u s t have been sufficient to r e c r y s t a l l i z e a ll k a m a c i t e to e q u i a x e d f e r r i t e g r a i n s and no Neumann bands a r e p r e s e r v e d . T h e s e e f f e c t s w e r e a l s o found in R o e b o u r n e but not quite to such an extent. T h e r e f o r e , a l s o m e a s u r e m e n t s of the r a r e g a s c o n c e n t r a t i o n s w e r e p e r f o r m e d on R o e b o u r n e , w h e r e a low 3 He/ 4 He r a t i o w a s d e t e c t e d a s well.
A c c o r d i n g to t h e s e o b s e r v a t i o n s t h e r e s e e m s to be a c o r r e l a t i o n between r e c r y s t a l l i z a t i o n e f f e c t s of the Widmanst~ttten p a t t e r n and the l o s s of t r i t i u m . T h i s i n d i c a t e d that the c o n c l u s i o n s by H i n t e n b e r g e r and W~tnke on t r i t i u m l o s s in space v e r y probably a r e c o r r e c t . A d e s c r i p t i o n of the e q u i p m e n t and of the p r o c e d u r e s f o r the m e a s u r e m e n t s is in p r e p a r a tion [1]. The r e s u l t s a r e given in table 1. Of s p e c i a l i n t e r e s t a r e the low v a l u e s of 3 H e / 4 H e , 3He/21Ne and 3 H e / 3 8 A r for R u f f ' s Mountain. In m o s t o t h e r m e t e o r i t e s t h e s e r a t i o s a r e between 0.20 and 0.32; 68 and 100; 15.4 and 18.2, r e s p e c t i v e l y . On the o t h er hand, the v a l u e s of 4 H e / 21Ne and 3 8 A r / 2 1 N e for RufUs Mountain a g r e e r e a s o n a b l y with the c o r r e s p o n d i n g v a l u e s in o t h er i r o n m e t e o r i t e s [1]. Q u a l i t a t i v e l y , the s a m e g e n e r a l f e a t u r e s a r e found for R o e b o u r n e , although the a n o m a l y a p p e a r s q u a n t i t a t i v e l y to be l e s s pronounced. T h e r e i s no doubt that only d e f i c i e n c i e s in 3He can be the r e a s o n f o r t h ese a n o m a l i e s , and that the n o r m a l v a l u e s for 4 H e / 21Ne and 3 8 A r / 2 1 N e t o g e t h e r with B u c h w a l d ' s findings on r e c r y s t a l l i z a t i o n s c l e a r l y indicate that the 3He d e f i c i e n c i e s a r e due to l o s s of t r i t i u m in space. It w as shown in a n u m b e r of publ i c a t i o n s (see e.g. W~nke [8]) that the s p a l l o g e n i c 3 H e / 3 8 A r r a t i o in i r o n m e t e o r i t e s d o e s not d e pend on the e f f e c t i v e h a r d n e s s of the i r r a d i a t i o n . Under the a s s u m p t i o n that the 3 H e / 3 8 A r r a t i o in Ro eb o u r n e and R u f f ' s Mountain without t r i t i u m l o s s w as 16.4 [1], c o r r e c t e d 3He c o n c e n t r a t i o n s and c o n c e n t r a t i o n r a t i o s have been c a l c u l a t e d a s l i s t e d in table 1. The v a l u e s of t h e s e r a t i o s a r e in a r e a s o n a b l e r e l a t i o n to those r a t i o s which do not contain 3He. The m i s s i n g quantity of 3He a m o u n t s to 37.5%
88
L.SCHULTZ
Table 1 Description of meteorite samples investigated and their noble gas concentrations. The missing quantities of 3He are calculated under the assumption 3He/38Ar = 16.4. 3Hecorr are the values without 3He loss. Meteorite
Recovered Ni 3He 4He 21Ne 38Ar 3He/4He 3He 3He 4He 38Ar mass 21Ne 38Ar 21Ne 21Ne (kg) (%) in 10-8cm 3 STP/g 53
8.6
38 31
232 211
0.72 0.62
3.6 3.2
Average
35
222
0.67
3.4
3Hecorr
56
Ruff'sMountain
Roebourne
87
8.3 133 134
544 556
1.74 1.72
8.7 9.1
Average
134
550
1.73
8.9
3Hecorr
146
in Ruff's Mountain and to 8% in Roebourne. U n de r the a s s u m p t i o n that the production r a t e s of t r i t i u m and of 3He a r e equal, the l o s s of t r i t i u m is 75% and 16%, r e s p e c t i v e l y . T h e s e m e t e o r i t e s w e r e e i t h e r not h e a t e d to such a high t e m p e r a t u r e that al l t r i t i u m could diffuse, or - a n d this s e e m s to be m o r e l i k e l y - they w e r e e x p o s e d to a sufficiently high t e m p e r a t u r e for only a f r a c tion of t h e i r l i f e t i m e . The c o s m i c r a y e x p o s u r e a g e s of Roebourne and R u f f ' s Mountain a r e 270 and 120 m . y . , r e s p e c t i v e l y , a s c a l c u l a t e d by the method of Signer and N i e r [5]. T h e s e a g e s a r e v e r y low for m e d i u m o c t a h e d r i t e s ; Voshage [9] pointed out that the c o s m i c r a y e x p o s u r e a g e s of m e d i u m o c t a h e d r i t e s g e n e r a l l y a r e between 570 and 730 m.y. It s e e m s e s p e c i a l l y i n s t r u c t i v e that all m e t e o r i t e s with low e x p o s u r e a g e s ( s o m e h e x a h e d r i t e s , n i c k e l - i r o n phase of stone m e t e o r i t e s [10, 11]) obviously have lost a d e t e c t a b l e amount of t h e i r s p a l l o g e n i c 3He. Arnold [12-14] and W~nke [15] explained the r e l a t i v e l y high e x p o s u r e a g e s (~ 6 × 108 y) of the m e d i u m o c t a h e d r i t e s without 3He l o s s by t h e i r flight in o r b i t s f ar f r o m the e a r t h ' s orbit. The e a r t h - c r o s s ing o r b i t s a r e r e a c h e d (e.g. by c lo s e e n c o u n t e r s with Mars) only a s h o r t t i m e b e f o r e the m e t e o r i t e s a r e cap t u red , since the l i f e t i m e on t h e s e o r b i t s i s s m a l l a s c o m p a r e d to 6 x 108 y. The " n o r m a l " 3 He/ 4 He r a t i o s in o c t a h e d r i t e s can be explained by the fact that the t e m p e r a t u r e of t h e s e m e t e o r i t e s in t h e i r o r b i t s i s m o s t of the t i m e too low for t r i t i u m diffusion. Heating and diffusion of t r i t i u m could take place a f t e r change of the o r b i t , that i s in a r e l a t i v e l y late and s h o r t p e r i o d of t h e i r e x p o s u r e h i s t o r y . The m i s s i n g quantity of 3He, which c o r r e s p o n d s to the amount of t r i t i u m l o s t during this p e r i o d of t i m e , i s
0.158
52
10.3
0.252
83.5
16.4
0.244 0.265
77.5 84.5
15.0 16.4
331
318
5.07
5.15
Source
U.S. National Museum, Washington D.C., 1670 Mineralogical Museum of the Univers~y, Copenhagen, Nr. 1905.1740
v e r y difficult to detect; h o w e v e r , the a b s e n c e of t r i t i u m can be o b s e r v e d in f r e s h l y f a l l e n i r o n m e t e o r i t e s . Since al l e x p e r i m e n t s f a i l e d to find s p a l l o g e n i c t r i t i u m in quantities to be e x p e c t e d (see e.g. Bainbridge et al. [16]), it m u s t be c o n cluded that the m e t e o r i t e s in e a r t h - o r b i t c r o s s ing o r b i t s r e a c h such t e m p e r a t u r e s that t r i t i u m diffuses at l e a s t p a r t i a l l y into space. B u t l e r and J e n k i n s [17] c a l c u l a t e d the t e m p e r a t u r e of an i r o n m e t e o r i t e at a d i s t a n c e of 1 a s t r o n o m i c a l unit f r o m the sun. They obtained a t e m p e r a t u r e of 90oc; at this t e m p e r a t u r e t r i t i u m can diffuse out of i r o n m e t e o r i t e s . T h i s i s shown in a r e c e n t work by F e c h t i g et al. [18] *. T h e s e a u t h o r s c a m e to the conclusion that i r o n m e t e o r i t e s l o s t m o s t of the t r i t i u m p r o d u c e d in space within a few w e e k s a f t e r t h e i r fall. A c c o r d i n g to other a u t h o r s [19-21] the a b s e n c e of t r i t i u m in f r e s h l y fallen m e t e o r i t e s is c a u s e d by other p r o c e s s e s . F o r m e t e o r i t e s with r e l a t i v e l y sh o r t c o s m i c r a y e x p o s u r e a g e s like Roebourne and R u f f ' s Mountain and f o r some of the h e x a h e d r i t e s a s well a s f o r stone m e t e o r i t e s , the l i f e t i m e s in e a r t h c r o s s i n g o r b i t s make up such a c o n s i d e r a b l e p a r t of t h e i r whole e x p o s u r e that the l o s s of t r i t i u m is d e t e c t a b l e by t h e i r 3 H e - c o n c e n t r a t i o n . Under the a s s u m p t i o n that the o r b i t change took place suddenly and that p r i o r to the orbit change no t r i t i u m could diffuse but al l t r i t i u m was l o s t a f t e r the o r b i t had changed, the above m en t i o n ed e x p o s u r e a g e s and the t r i t i u m l o s s allow to c a l c u l a t e the t i m e when the o r b i t change took place: 90 x 106 y ago f o r R u f f ' s Mountain and 45 x 106 y ago for Roebourne. T h e s e v a l u e s * I am indebted to Professor Gentner and the authors for sending me this paper before publication.
TRITIUM LOSS r e p r e s e n t , of c o u r s e , o n l y l o w e r l i m i t s , s i n c e i t m a y b e p o s s i b l e t h a t n o t a l l of t h e t r i t i u m w a s lost after the orbit changed. However, the ages a r e i n t h e s a m e o r d e r of m a g n i t u d e a s t h e e x p o s u r e a g e s of m o s t h e x a h e d r i t e s w h i c h , a c c o r d i n g to W ~ n k e [15], c a m e f r o m t h e m o o n a n d h a d , therefore, o r b i t s w h i c h c r o s s e d t h e o r b i t of earth from the very beginning. T h e s e c o n s i d e r a t i o n s s h o w t h a t t h e a b s e n c e of tritium in freshly fallen meteorites which cont a i n t h e a m o u n t of 3He e x p e c t e d f r o m t h e d e c a y of s p a l l o g e n i c t r i t i u m m o s t l i k e l y i s c a u s e d b y s o l a r h e a t i n g of t h e m e t e o r i t e s d u r i n g t h e l a t e a n d s h o r t p e r i o d of t i m e i n w h i c h t h e y c r o s s e d t h e o r b i t of e a r t h . T h e a u t h o r w i s h e s to e x p r e s s h i s t h a n k s to M r s . S. M u n c k a n d M r . V. F. B u c h w a l d , C o p e n h a gen, and Dr.R.S. Clarke Jr., Washington, for supplying the samples. I a m i n d e b t e d t o P r o f e s s o r D r . H. H i n t e n b e r g e r a n d D r . H. V o s h a g e f o r e n c o u r a g i n g d i s c u s s i o n s , a n d t o M r . U. I>auli f o r h i s a s s i s t a n c e i n the laboratory.
[6] [ 7] [8] [9] [10]
[11]
[12]
[13] [14] [15]
REFERENCES [1] L.Schultz and H. Hintenberger, E d e l g a s m e s s u n g e n an E i s e n m e t e o r i t e n , Z. Naturforsch. (in p r e p a r a tion). [2] C . A . B a u e r , The h e l i u m contents of m e t a l l i c m e t e o r i t e s , J. Geophys. Res. 68 (1963) 6043. [3] H. H i n t e n b e r g e r and H. W~nke, Helium- und Neonisotope in E i s e n m e t e o r i t e n , Z. Naturforsch. 19a (1964} 210. [4] H . H i n t e n b e r g e r , L.Schultz, H.W~nke and H . W e ber, Helium- und Neonisotope in E i s e n m e t e o r i t e n (in p r e p a r a t i o n ) . [5] P. Signer and A. O. Nier, The m e a s u r e m e n t and int e r p r e t a t i o n of r a r e gas c o n c e n t r a t i o n s in iron
[16]
[17] [18] [19] [20] [21]
89
m e t e o r i t e s , in: R e s e a r c h e s on M e t e o r i t e s , ed. C. B . M o o r e (John Wiley, New York, 1962) p. 7. D . T i l l e s , P r i m o r d i a l gas in the Washington County m e t e o r i t e , J. Geophys. Res. 67 (1962) 1687. V. F. Buchwald, The i r o n - n i c k e l - p h 0 s p h o r o u s s y s tem and the s t r u c t u r e of iron m e t e o r i t e s , Acta Polytechn. Scand., Set. Ch. 51 (1966) 46. H.W~inke, Exposure ages for iron m e t e o r i t e s , Nat u r e 188 (1960) 1101. H.Voshage, B e s t r a h l u n g s a l t e r und Herkunft d e r E i s e n m e t e o r i t e , Z. Naturforsch. (in preparation}. H . H i n t e n b e r g e r , E. Vilcsek and H. W~lnke, Z u r F r a g e d e r Diffusionsverluste yon radiogenen und spallogenen Edelgasen aus Steinmeteoriten, Z. Naturforsch. 19a (1964) 219. H. Hintenberger, L. Sehultz and H. W~nke, Messung d e r Diffusionsverluste von radiogenen und spallogenen Edelgasen in Steinmeteoriten II, Z. Naturforsch. 21a (1966) 1147. J . R . Arnold, The origin of m e t e o r i t e s as small bodies, in: Isotope and Cosmic C h e m i s t r y , eds. H. Craig, S . L . Miller and G . J . W a s s e r b u r g (NorthHolland Publ. Co., A m s t e r d a m , 1964). J . R . Arnold, The origin of m e t e o r i t e s as small bodies. II. The model, Astrophys. J. 141 (1965) 1536. J . R . Arnold, The origin of m e t e o r i t e s as s m a l l bodies. HI. General considerations, A s t r o p h y s . J, 141 (1965) 1548. H.W~nke, M e t e o r i t e n a l t e r und verwandte P r o b l e m e d e r Kosmochemie, F o r t s c h . Chem. F o r s c h . 7 (1966) 322. A . E . B a i n b r i d g e , H . E . S u e s s and H.W~lnke, The t r i t i u m content of t h r e e stony m e t e o r i t e s and one iron m e t e o r i t e , Geoehim. Cosmochim. Acta 26 (1962) 471. C . P . B u t l e r and R . J . J e n k i n s , T e m p e r a t u r e of an iron m e t e o r i t e in space, Science 142 (1963) 1567. H. Fechtig, J . G . F e s t a g and H. Schultes, T r i t i u m Diffusionsmessungen an p r o t o n e n b e s t r a h l t e n P r o ben des E i s e n m e t e o r i t e n Sikhote-Alin (preprint). M.A. T a m e r s , Low concentration of t r i t i u m in iron m e t e o r i t e s , Nature 197 (1963) 276. D . T i l l e s , T r i t i u m r e t e n t i o n in iron m e t e o r i t e s , Nature 200 (1963) 563. D. F i s h e r , The cosmogenic t r i t i u m problem in iron m e t e o r i t e s , P r e p r i n t (1966).
T R I T I U M LOSS IN I R O N METEORITES L. S C H U L T Z Max-Planck-lnstitut flit Chemie ( Otto-Hahn-Institut), Mainz, Germany Received 10 January 1967
The concentrations of cosmic ray produced 3He in the medium octahedrites Ruff, s Mountain and Roebourne are too low as compared to the concentrations of other spallogenic nuclear species. A number of arguments is compiled which indicate that the 3He anomaly in these octahedrites and in some hexahedrites, as well as the low spallogenic tritium content in freshly fallen iron meteorites, is caused by diffusion loss of tritium due to solar heating of the meteorites in space.
In the c o u r s e of i n v e s t i g a t i o n s of s p a l l o g e n i c r a r e g a s e s in i r o n m e t e o r i t e s - which will be published e l s e w h e r e [1] - an a n o m a l o u s low 3 H e / 4He r a t i o of 0.158 w a s found in the m e d i u m o c t a h e d r i t e R u f f ' s Mountain. Up to now, such low r a t i o s have only been d e t e c t e d in s o m e h e x a h e d r i t e s [2-4] and in the n i c k e l - r i c h a t a x it e Washington County [4, 5]. In Washington County the a n o m a l o u s 3 H e / 4 H e r a t i o i s undoubtedly due to p r i m o r d i a l 4He [5,6]. B a u e r [2] p r e s u m e d that the low 3 He/ 4 He r a t i o of s o m e h e x a h e d r i t e s i s a l s o due to an e x c e s s of 4He; H i n t e n b e r g e r and W~Luke [3], h o w e v e r , c l e a r l y showed that the low 3H e/ 4 He r a t i o of the h e x a h e d r i t e Brannau i s not c a u s e d by an e x c e s s of 4He but r a t h e r by a l o s s of 3He. T h e i r c o n c l u s i o n was that Braunau and m o s t l i k e l y a l s o the h e x a h e d r i t e s m e a s u r e d by B a u e r m o v ed in o r b i t s r e l a t i v e l y c l o s e to the sun. In t h e s e o r b i t s , the t e m p e r a t u r e of the m e t e o r i t e s w a s so high that t r i t i u m diffused into space. Hence, t h e s e m e t e o r i t e s contain the 3Hecomponent p r o d u c e d d i r e c t l y but only a p o r t i o n of the 3He p r o d u c e d by the d e c a y of s p a l l o g e n i c tritium. This, very recently, was also conf i r m e d by H i n t e n b e r g e r et al. [4]. The low 3 He/ 4 He r a t i o in R u f f ' s Mountain a p p e a r e d to be e s p e c i a l l y i n s t r u c t i v e since B u c h wald [7] concluded f r o m m e t a l l o g r a p h i c i n v e s t i gations that a late c o s m i c gentle r e h e a t i n g o c c u r r e d in t h i s m e t e o r i t e . The t e m p e r a t u r e m u s t have been sufficient to r e c r y s t a l l i z e a ll k a m a c i t e to e q u i a x e d f e r r i t e g r a i n s and no Neumann bands a r e p r e s e r v e d . T h e s e e f f e c t s w e r e a l s o found in R o e b o u r n e but not quite to such an extent. T h e r e f o r e , a l s o m e a s u r e m e n t s of the r a r e g a s c o n c e n t r a t i o n s w e r e p e r f o r m e d on R o e b o u r n e , w h e r e a low 3 He/ 4 He r a t i o w a s d e t e c t e d a s well.
A c c o r d i n g to t h e s e o b s e r v a t i o n s t h e r e s e e m s to be a c o r r e l a t i o n between r e c r y s t a l l i z a t i o n e f f e c t s of the Widmanst~ttten p a t t e r n and the l o s s of t r i t i u m . T h i s i n d i c a t e d that the c o n c l u s i o n s by H i n t e n b e r g e r and W~tnke on t r i t i u m l o s s in space v e r y probably a r e c o r r e c t . A d e s c r i p t i o n of the e q u i p m e n t and of the p r o c e d u r e s f o r the m e a s u r e m e n t s is in p r e p a r a tion [1]. The r e s u l t s a r e given in table 1. Of s p e c i a l i n t e r e s t a r e the low v a l u e s of 3 H e / 4 H e , 3He/21Ne and 3 H e / 3 8 A r for R u f f ' s Mountain. In m o s t o t h e r m e t e o r i t e s t h e s e r a t i o s a r e between 0.20 and 0.32; 68 and 100; 15.4 and 18.2, r e s p e c t i v e l y . On the o t h er hand, the v a l u e s of 4 H e / 21Ne and 3 8 A r / 2 1 N e for RufUs Mountain a g r e e r e a s o n a b l y with the c o r r e s p o n d i n g v a l u e s in o t h er i r o n m e t e o r i t e s [1]. Q u a l i t a t i v e l y , the s a m e g e n e r a l f e a t u r e s a r e found for R o e b o u r n e , although the a n o m a l y a p p e a r s q u a n t i t a t i v e l y to be l e s s pronounced. T h e r e i s no doubt that only d e f i c i e n c i e s in 3He can be the r e a s o n f o r t h ese a n o m a l i e s , and that the n o r m a l v a l u e s for 4 H e / 21Ne and 3 8 A r / 2 1 N e t o g e t h e r with B u c h w a l d ' s findings on r e c r y s t a l l i z a t i o n s c l e a r l y indicate that the 3He d e f i c i e n c i e s a r e due to l o s s of t r i t i u m in space. It w as shown in a n u m b e r of publ i c a t i o n s (see e.g. W~nke [8]) that the s p a l l o g e n i c 3 H e / 3 8 A r r a t i o in i r o n m e t e o r i t e s d o e s not d e pend on the e f f e c t i v e h a r d n e s s of the i r r a d i a t i o n . Under the a s s u m p t i o n that the 3 H e / 3 8 A r r a t i o in Ro eb o u r n e and R u f f ' s Mountain without t r i t i u m l o s s w as 16.4 [1], c o r r e c t e d 3He c o n c e n t r a t i o n s and c o n c e n t r a t i o n r a t i o s have been c a l c u l a t e d a s l i s t e d in table 1. The v a l u e s of t h e s e r a t i o s a r e in a r e a s o n a b l e r e l a t i o n to those r a t i o s which do not contain 3He. The m i s s i n g quantity of 3He a m o u n t s to 37.5%
88
L.SCHULTZ
Table 1 Description of meteorite samples investigated and their noble gas concentrations. The missing quantities of 3He are calculated under the assumption 3He/38Ar = 16.4. 3Hecorr are the values without 3He loss. Meteorite
Recovered Ni 3He 4He 21Ne 38Ar 3He/4He 3He 3He 4He 38Ar mass 21Ne 38Ar 21Ne 21Ne (kg) (%) in 10-8cm 3 STP/g 53
8.6
38 31
232 211
0.72 0.62
3.6 3.2
Average
35
222
0.67
3.4
3Hecorr
56
Ruff'sMountain
Roebourne
87
8.3 133 134
544 556
1.74 1.72
8.7 9.1
Average
134
550
1.73
8.9
3Hecorr
146
in Ruff's Mountain and to 8% in Roebourne. U n de r the a s s u m p t i o n that the production r a t e s of t r i t i u m and of 3He a r e equal, the l o s s of t r i t i u m is 75% and 16%, r e s p e c t i v e l y . T h e s e m e t e o r i t e s w e r e e i t h e r not h e a t e d to such a high t e m p e r a t u r e that al l t r i t i u m could diffuse, or - a n d this s e e m s to be m o r e l i k e l y - they w e r e e x p o s e d to a sufficiently high t e m p e r a t u r e for only a f r a c tion of t h e i r l i f e t i m e . The c o s m i c r a y e x p o s u r e a g e s of Roebourne and R u f f ' s Mountain a r e 270 and 120 m . y . , r e s p e c t i v e l y , a s c a l c u l a t e d by the method of Signer and N i e r [5]. T h e s e a g e s a r e v e r y low for m e d i u m o c t a h e d r i t e s ; Voshage [9] pointed out that the c o s m i c r a y e x p o s u r e a g e s of m e d i u m o c t a h e d r i t e s g e n e r a l l y a r e between 570 and 730 m.y. It s e e m s e s p e c i a l l y i n s t r u c t i v e that all m e t e o r i t e s with low e x p o s u r e a g e s ( s o m e h e x a h e d r i t e s , n i c k e l - i r o n phase of stone m e t e o r i t e s [10, 11]) obviously have lost a d e t e c t a b l e amount of t h e i r s p a l l o g e n i c 3He. Arnold [12-14] and W~nke [15] explained the r e l a t i v e l y high e x p o s u r e a g e s (~ 6 × 108 y) of the m e d i u m o c t a h e d r i t e s without 3He l o s s by t h e i r flight in o r b i t s f ar f r o m the e a r t h ' s orbit. The e a r t h - c r o s s ing o r b i t s a r e r e a c h e d (e.g. by c lo s e e n c o u n t e r s with Mars) only a s h o r t t i m e b e f o r e the m e t e o r i t e s a r e cap t u red , since the l i f e t i m e on t h e s e o r b i t s i s s m a l l a s c o m p a r e d to 6 x 108 y. The " n o r m a l " 3 He/ 4 He r a t i o s in o c t a h e d r i t e s can be explained by the fact that the t e m p e r a t u r e of t h e s e m e t e o r i t e s in t h e i r o r b i t s i s m o s t of the t i m e too low for t r i t i u m diffusion. Heating and diffusion of t r i t i u m could take place a f t e r change of the o r b i t , that i s in a r e l a t i v e l y late and s h o r t p e r i o d of t h e i r e x p o s u r e h i s t o r y . The m i s s i n g quantity of 3He, which c o r r e s p o n d s to the amount of t r i t i u m l o s t during this p e r i o d of t i m e , i s
0.158
52
10.3
0.252
83.5
16.4
0.244 0.265
77.5 84.5
15.0 16.4
331
318
5.07
5.15
Source
U.S. National Museum, Washington D.C., 1670 Mineralogical Museum of the Univers~y, Copenhagen, Nr. 1905.1740
v e r y difficult to detect; h o w e v e r , the a b s e n c e of t r i t i u m can be o b s e r v e d in f r e s h l y f a l l e n i r o n m e t e o r i t e s . Since al l e x p e r i m e n t s f a i l e d to find s p a l l o g e n i c t r i t i u m in quantities to be e x p e c t e d (see e.g. Bainbridge et al. [16]), it m u s t be c o n cluded that the m e t e o r i t e s in e a r t h - o r b i t c r o s s ing o r b i t s r e a c h such t e m p e r a t u r e s that t r i t i u m diffuses at l e a s t p a r t i a l l y into space. B u t l e r and J e n k i n s [17] c a l c u l a t e d the t e m p e r a t u r e of an i r o n m e t e o r i t e at a d i s t a n c e of 1 a s t r o n o m i c a l unit f r o m the sun. They obtained a t e m p e r a t u r e of 90oc; at this t e m p e r a t u r e t r i t i u m can diffuse out of i r o n m e t e o r i t e s . T h i s i s shown in a r e c e n t work by F e c h t i g et al. [18] *. T h e s e a u t h o r s c a m e to the conclusion that i r o n m e t e o r i t e s l o s t m o s t of the t r i t i u m p r o d u c e d in space within a few w e e k s a f t e r t h e i r fall. A c c o r d i n g to other a u t h o r s [19-21] the a b s e n c e of t r i t i u m in f r e s h l y fallen m e t e o r i t e s is c a u s e d by other p r o c e s s e s . F o r m e t e o r i t e s with r e l a t i v e l y sh o r t c o s m i c r a y e x p o s u r e a g e s like Roebourne and R u f f ' s Mountain and f o r some of the h e x a h e d r i t e s a s well a s f o r stone m e t e o r i t e s , the l i f e t i m e s in e a r t h c r o s s i n g o r b i t s make up such a c o n s i d e r a b l e p a r t of t h e i r whole e x p o s u r e that the l o s s of t r i t i u m is d e t e c t a b l e by t h e i r 3 H e - c o n c e n t r a t i o n . Under the a s s u m p t i o n that the o r b i t change took place suddenly and that p r i o r to the orbit change no t r i t i u m could diffuse but al l t r i t i u m was l o s t a f t e r the o r b i t had changed, the above m en t i o n ed e x p o s u r e a g e s and the t r i t i u m l o s s allow to c a l c u l a t e the t i m e when the o r b i t change took place: 90 x 106 y ago f o r R u f f ' s Mountain and 45 x 106 y ago for Roebourne. T h e s e v a l u e s * I am indebted to Professor Gentner and the authors for sending me this paper before publication.
TRITIUM LOSS r e p r e s e n t , of c o u r s e , o n l y l o w e r l i m i t s , s i n c e i t m a y b e p o s s i b l e t h a t n o t a l l of t h e t r i t i u m w a s lost after the orbit changed. However, the ages a r e i n t h e s a m e o r d e r of m a g n i t u d e a s t h e e x p o s u r e a g e s of m o s t h e x a h e d r i t e s w h i c h , a c c o r d i n g to W ~ n k e [15], c a m e f r o m t h e m o o n a n d h a d , therefore, o r b i t s w h i c h c r o s s e d t h e o r b i t of earth from the very beginning. T h e s e c o n s i d e r a t i o n s s h o w t h a t t h e a b s e n c e of tritium in freshly fallen meteorites which cont a i n t h e a m o u n t of 3He e x p e c t e d f r o m t h e d e c a y of s p a l l o g e n i c t r i t i u m m o s t l i k e l y i s c a u s e d b y s o l a r h e a t i n g of t h e m e t e o r i t e s d u r i n g t h e l a t e a n d s h o r t p e r i o d of t i m e i n w h i c h t h e y c r o s s e d t h e o r b i t of e a r t h . T h e a u t h o r w i s h e s to e x p r e s s h i s t h a n k s to M r s . S. M u n c k a n d M r . V. F. B u c h w a l d , C o p e n h a gen, and Dr.R.S. Clarke Jr., Washington, for supplying the samples. I a m i n d e b t e d t o P r o f e s s o r D r . H. H i n t e n b e r g e r a n d D r . H. V o s h a g e f o r e n c o u r a g i n g d i s c u s s i o n s , a n d t o M r . U. I>auli f o r h i s a s s i s t a n c e i n the laboratory.
[6] [ 7] [8] [9] [10]
[11]
[12]
[13] [14] [15]
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m e t e o r i t e s , in: R e s e a r c h e s on M e t e o r i t e s , ed. C. B . M o o r e (John Wiley, New York, 1962) p. 7. D . T i l l e s , P r i m o r d i a l gas in the Washington County m e t e o r i t e , J. Geophys. Res. 67 (1962) 1687. V. F. Buchwald, The i r o n - n i c k e l - p h 0 s p h o r o u s s y s tem and the s t r u c t u r e of iron m e t e o r i t e s , Acta Polytechn. Scand., Set. Ch. 51 (1966) 46. H.W~inke, Exposure ages for iron m e t e o r i t e s , Nat u r e 188 (1960) 1101. H.Voshage, B e s t r a h l u n g s a l t e r und Herkunft d e r E i s e n m e t e o r i t e , Z. Naturforsch. (in preparation}. H . H i n t e n b e r g e r , E. Vilcsek and H. W~lnke, Z u r F r a g e d e r Diffusionsverluste yon radiogenen und spallogenen Edelgasen aus Steinmeteoriten, Z. Naturforsch. 19a (1964) 219. H. Hintenberger, L. Sehultz and H. W~nke, Messung d e r Diffusionsverluste von radiogenen und spallogenen Edelgasen in Steinmeteoriten II, Z. Naturforsch. 21a (1966) 1147. J . R . Arnold, The origin of m e t e o r i t e s as small bodies, in: Isotope and Cosmic C h e m i s t r y , eds. H. Craig, S . L . Miller and G . J . W a s s e r b u r g (NorthHolland Publ. Co., A m s t e r d a m , 1964). J . R . Arnold, The origin of m e t e o r i t e s as small bodies. II. The model, Astrophys. J. 141 (1965) 1536. J . R . Arnold, The origin of m e t e o r i t e s as s m a l l bodies. HI. General considerations, A s t r o p h y s . J, 141 (1965) 1548. H.W~nke, M e t e o r i t e n a l t e r und verwandte P r o b l e m e d e r Kosmochemie, F o r t s c h . Chem. F o r s c h . 7 (1966) 322. A . E . B a i n b r i d g e , H . E . S u e s s and H.W~lnke, The t r i t i u m content of t h r e e stony m e t e o r i t e s and one iron m e t e o r i t e , Geoehim. Cosmochim. Acta 26 (1962) 471. C . P . B u t l e r and R . J . J e n k i n s , T e m p e r a t u r e of an iron m e t e o r i t e in space, Science 142 (1963) 1567. H. Fechtig, J . G . F e s t a g and H. Schultes, T r i t i u m Diffusionsmessungen an p r o t o n e n b e s t r a h l t e n P r o ben des E i s e n m e t e o r i t e n Sikhote-Alin (preprint). M.A. T a m e r s , Low concentration of t r i t i u m in iron m e t e o r i t e s , Nature 197 (1963) 276. D . T i l l e s , T r i t i u m r e t e n t i o n in iron m e t e o r i t e s , Nature 200 (1963) 563. D. F i s h e r , The cosmogenic t r i t i u m problem in iron m e t e o r i t e s , P r e p r i n t (1966).