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Potassium abundances in some ultra-basic and basic rocks
EARTH
AND PLANETARY
S C I E N C E L E T T E R S 3 (1966) 110-112. N O R T H - H O L L A N D
PUBL. COMP., A M S T E R D A M
P O T A S S I U M A B U N D A N C E S IN S O M E U L T R A - B A S I C A N D BASIC R O C K S J. W. MORGAN and A. D. T. GOODE * Depa~'trnent of Geophysics and Geochemistry, Institute of Advanced Studies, Australian National University, Canberra, A. C. T., Australia Received 8 April 1966
i. ~TRODUCTION In a r e c e n t n e u t r o n activation study, u r a n i u m and t h o r i u m w e r e d e t e r m i n e d in a n u m b e r of b a sic and u l t r a - b a s i c rocks [1]. F o r any s u b s e q u e n t heat production c a l c u l a t i o n s it is also n e c e s s a r y to know the p o t a s s i u m a b u n d a n c e s for these rocks. Some d e t e r m i n a t i o n s of this e l e m e n t have b e e n made by a flame p h o t o m e t r i c technique [2]; however, as the r e s u l t s showed little c o r r e l a t i o n to the u r a n i u m and t h o r i u m abundances, and the l e v e l s found were in many c a s e s n e a r the lower l i m i t of the method, it was thought advisable to r e p e a t the d e t e r m i n a t i o n s u s i n g an e n t i r e l y i n dependent p r o c e d u r e . Neutron activation a n a l y s i s i s quite s e n s i t i v e for p o t a s s i u m , and has the great advantage of being f r e e f r o m i n t e r f e r e n c e by r e a g e n t c o n t a m i n a t i o n d u r i n g c h e m i c a l p r o c e s s i n g after i r r a d i a t i o n .
2. ANALYTICAL METHOD About 100 m g aliquots of the powdered s a m ples were weighed into c l e a n s i l i c a a m p o u l e s (4 m m i n t e r n a l d i a m e t e r ) and sealed. Standards w e r e p r e p a r e d by weighing about 20 mg of S p e c pure SiO 2 into s i m i l a r a m p o u l e s and spiking with 50 to 100 ~g of p o t a s s i u m as a s t a n d a r d solution p r e p a r e d f r o m A n a l a r K2CO 3 d r i e d to c o n s t a n t weight at 200oc. The s t a n d a r d s were d r i e d at 80oc and the a m p o u l e s sealed. Batches of six s a m p l e s and two s t a n d a r d s w e r e i r r a d i a t e d in a flux of 3 × 1012 n e u t r o n s , c m - 2 . sec -1 for 24 hr. After i r r a d i a t i o n the a m p o u l e s were opened and the s a m p l e s weighed into p l a t i n u m dishes; 25 mg of p o t a s s i u m c a r r i e r w e r e added and the s a m p l e s dissolved by f u m i n g with HF and H2SO 4. Any insoluble m a t e r i a l (mostly spinel) was u s u ally discarded. (In the case of the Gila, A r i z o n a * Present address: Department of Geology, University of Adelaide, Adelaide, South Australia.
s a m p l e s the insoluble r e s i d u e s weighed 1.8 mg and 2.5 mg and contained < 0.6 x 10-3 ~g and < 0.8 x 10-3 ~g of K r e s p e c t i v e l y . ) After taking finally to d r y n e s s , the contents of the dish w e r e d i s s o l v e d in water, t r a n s f e r r e d to a 250 m l b e a k e r and 10 m l HNO 3 plus 5 m l HC104 added. The b e a k e r was heated on a hot plate u n t i l f u m e s of HC10 4 appeared. The solution was t r a n s f e r r e d to a c e n t r i f u g e tube and cooled. The p r e c i p i t a t e was centrifuged off, washed t h r e e t i m e s with ethanol and d i s s o l v e d in 10 m l of water; 10 mg of f e r r i c i r o n c a r r i e r w e r e added and Fe(OH) 3 p r e c i p i t a t ed by the addition of NaOH solution. The p r e c i p itate was centrifuged and the s u p e r n a t e f i l t e r e d into a c l e a n c e n t r i f u g e tube. The p r e c i p i t a t e was washed with 5 m l of w a t e r and, after f i l t e r i n g , the washings were added to the s u p e r n a t e . The solution was acidified with glacial acetic acid (phenolphthalein indicator) and 10 m l of a f r e s h l y p r e p a r e d 10% solution of s o d i u m - c o b a l t i n i t r i t e w e r e added. After s t a n d i n g for 10 m i n the p r e c i p itate was centrifuged and washed s e v e r a l t i m e s with water, c e n t r i f u g i n g and d i s c a r d i n g the s u p e r n a t e each time. The p r e c i p i t a t e was d i s s o l v e d by w a r m i n g with 2 m l of I : I H N O 3. Up to this stage the p r o c e d u r e was a l m o s t i d e n t i c a l to that r e p o r t e d by Salmon [3]. After cooling, the n i t r i c acid solution was f i l t e r e d and 5 m l of a 5% EDTA solution added. The pH was adjusted to 6.5 by the addition of NaOH solution ( b r o m o c r e s o l purple indicator) and the solution cooled in ice; 10 m l of sodium t e t r a p h e n y l b o r o n solution [4] w e r e added while s t i r r i n g . The p o t a s s i u m t e t r a p h e n y l b o r o n p r e cipitate was centrifuged and s u b s e q u e n t l y washed s e v e r a l t i m e s with water. It was d i s s o l v e d in a m i n i m u m of acetone, cooled in ice, and r e p r e cipitated by the addition of 0.1 N HC1, 15 m l of w a t e r and 2 m l of sodium t e t r a p h e n y l b o r o n s o l u tion. The p r e c i p i t a t e was centrifuged, washed t h r e e t i m e s in w a t e r and again d i s s o l v e d in a m i n i m u m of acetone; 10 m l of a solution of 0.5% sodium t e t r a p h e n y l b o r o n in ethanol was added
111
POTASSIUM ABUNDANCES IN SOME ULTRA-BASIC AND BASIC ROCKS Table 1 P o t a s s i u m a b u n d a n c e s in s o m e u l t r a - b a s i c and b a s i c r o c k s . Specimen
P o t a s s i u m (ppm) This work
P o t a s s i u m (ppm) Other work
Reference
Ultra-basic inclusions P e r i d o t i t e 2604, Mt. L e u r a , Vic. P e r i d o t i t e 2642, Mt. L e u r a , Vic. P e r i d o t i t e 2669, Mt. Shadwell, Vic. P e r i d o t i t e 2700, Mt. Noorat, Vic. P e r i d o t i t e 2728, Mt. G a m b i e r , S.A. P e r i d o t i t e R42, Delegate, N.S.W. P e r i d o t i t e R71, Snowy Mts., N.S.W. Dunite R80, Snowy Mts., N.S.W. P e r i d o t i t e R420, Oahu, Hawaii Dunite, Gila, A r i z o n a G a r n e t P e r i d o t i t e R357, South A f r i c a
70 , 70 , 15 , 14 , 90 , 91 , 109 , 110 , 29 30 , 81 , 80 , 23 , 24 , 37 , 33 , 23 , 23 , 1.6, 1.6, 159 , 160 ,
av. 70 av. 15 av. 90 av. 110 av. 29 av. 80 av. 23 av. 35 av. 23 av. 1.6 av. 159
60 50 90 50 140 4O 170 30
[6] [6] [6] [6] [6] [6] [6] [6]
* * * * * * * *
180
[6] *
C r u s t a l and i n t r u s i v e u l t r a - b a s i c r o c k s High t e m p e r a t u r e p e r i d o t i t e , Tinaquilla, V e n e z u e l a Dunite, $39, B a l s a m Q u a r r y , N.C. Dunite, R461, J a c k s o n Co., N.C. Dunite, Twin S i s t e r s , Washington Dunite 2515, Almklovdalen, N o r w a y G a r n e t P e r i d o t i t e R372, R i v e r T e s s i n , S w i t z e r l a n d
6.3,
5.9, 5.8 27 , 25 , 3.8, 4.2, 14 , 16 , 63 58 , 49 50 ,
av.
6.0
av. av. av. av. av.
26 4.0 15 61 50
, av. 8.7, av.
11 8.8
2.6, av. 5.6, av.
2.2 5.6
32 29 12±4
[3] ** [6] *
5O
[6] *
[7] t
U. S. G. S. s t a n d a r d u l t r a - b a s i c r o c k s Dunite DTS-1, Twin S i s t e r s , Washington P e r i d o t i t e P C C - 1 , S o n o m a Co., C a l i f o r n i a
11 , 8.8,
12
Pallasite olivines Brenham Huckitta
1.8, 5.6,
2.5
[8]
**
Basic inclusions P y r o x e n i t e , Oahu, Hawaii Granulite R52, Delegate, N.S.W. Granulite R140, Delegate, N.S.W. G a b b r o R362, Hualalai, Hawaii Eclogite R616, Dodoma, Tanganyika
174 259
, 260
, av. 260
932
, 945
, av. 938
248
, 249
, av. 249
231
, 229
, av. 230
,
, av.
190 250 900
[71 t
220
[6] *
30
[6] *
[6] * [61 *
Crustal eclogite " B r o n z i t e P y r o p i t e " R375, Sittampundi, India
S p e c p u r e SiO 2
30
1.7,
32
1.2, av.
31 1.4
Methods: * F l a m e p h o t o m e t r y . ** N e u t r o n activation. J" I s o t o p e dilution.
a n d t h e m i x t u r e c o o l e d in i c e f o r 10 r a i n . T h e p r e c i p i t a t e w a s s e p a r a t e d b y s p i n n i n g in a c e n trifuge, and washed three times with ethanol. It was then slurried with half a ml of ethanol , transferred to a w e i g h e d c o u n t i n g p l a n c h e t , a n d dried under a heat lamp. The sources were a l l o w e d to c o o l a n d w e r e r e w e i g h e d t o d e t e r m i n e the radiochemical yield. Except for the first precipitation of potassium
tetraphenylboron from an EDTA solution at pH 6.5, w h i c h w a s t a k e n f r o m a m e t h o d d e s c r i b e d b y C l u l e y [5], t h e s e c o n d p a r t o f t h e c h e m i c a l p r o c e d u r e o u t l i n e d a b o v e w a s b a s e d on a m e t h o d f o r the d e t e r m i n a t i o n of f i s s i o n p r o d u c t c e s i u m r e p o r t e d b y H a n d l e y a n d B u r r o s [4]. T h e c h e m i cal s e p a r a t i o n of the c o m p a r a t o r s t a n d a r d s w a s i d e n t i c a l to t h a t u s e d f o r t h e s a m p l e s . T h e a c t i v i t y of the final s o u r c e s w a s d e t e r -
112
J.W. MORGAN and A. D. T. GOODE
m i n e d by b e t a c o u n t i n g . No s e l f - a b s o r p t i o n c o r r e c t i o n s w e r e n e c e s s a r y b e c a u s e of the h i g h b e t a e n e r g y of 42K, on w h i c h the a n a l y s e s w e r e b a s e d . D e c a y c u r v e s w e r e f o l l o w e d and it w a s o b s e r v e d that a s m a l l c o r r e c t i o n ( g e n e r a l l y 0.5 to 2%) w a s r e q u i r e d f o r the p r e s e n c e of s o m e l o n g e r l i v e d a c t i v i t y , p o s s i b l y of 86Rb ( h a l f - l i f e 18.7 days). W h e n t h i s c o r r e c t i o n w a s m a d e the o b s e r v e d d e c a y c o r r e s p o n d e d v e r y c l o s e l y to the 12.5 h r h a l f - l i f e of 42K.
n e u t r o n a c t i v a t i o n m e t h o d g i v e s b o t h p r e c i s e and a c c u r a t e r e s u l t s in the r a n g e f r o m 2 to 900 p p m .
ACKNOWLEDGEMENTS We w i s h to thank D r . J. F. L o v e r i n g , D r . D . H . G r e e n and D r . S.R. T a y l o r f o r p r o v i d i n g the s a m p l e s u s e d in t h i s w o r k , and a l s o f o r v a l u a b l e c o m m e n t and d i s c u s s i o n . T h i s w o r k w a s s u p p o r t e d in p a r t by a g r a n t f r o m the A u s t r a l i a n I n s t i t u t e of N u c l e a r S c i e n c e and E n g i n e e r i n g .
3. R E S U L T S T h e p o t a s s i u m a b u n d a n c e s found in 25 r o c k s and 2 p a l l a s i t e o l i v i n e s a r e shown in t a b l e 1. T h e e r r o r on e a c h i n d i v i d u a l d e t e r m i n a t i o n i s of the o r d e r of 3%, e s t i m a t e d f r o m the s t a n d a r d d e v i a tion of the f o u r c o m p a r a t o r s t a n d a r d s o u r c e s ( d u p l i c a t e a l i q u o t s f r o m e a c h of t h e two s t a n d ards irradiated with each six samples). The unc e r t a i n t y i n t r o d u c e d by the c o u n t i n g s t a t i s t i c s i s c o n s i d e r a b l y l e s s t h a n t h i s , so that the e s t i m a t e d error will include all the uncertainties involved in the d e t e r m i n a t i o n s , e x c e p t f o r any s y s t e m a t i c e r r o r in the s t a n d a r d p o t a s s i u m s o l u t i o n f r o m w h i c h the s t a n d a r d s w e r e p r e p a r e d . T h i s c a n be s e e n to b e s m a l l by a c o m p a r i s o n w i t h d e t e r m i n a t i o n s m a d e by o t h e r m e t h o d s on s o m e of the s a m e s a m p l e s (table 1). It can be s e e n that in g e n e r a l the a g r e e m e n t i s good, e x c e p t f o r s o m e of the f i a m e p h o t o m e t r y r e s u l t s on s a m p l e s w i t h low p o t a s s i u m a b u n d a n c e s . T h e d i s c r e p a n c y i s a l m o s t c e r t a i n l y due to l a r g e e r r o r s and i n t e r f e r e n c e s w h i c h b e c o m e i n c r e a s i n g l y s e r i o u s in the f l a m e p h o t o m e t r y m e t h o d b e l o w the 100 p p m l e v e l . In g e n e r a l it i s s a f e to s a y that the p r e s e n t
REFERENCES [1] J.W. Morgan, Ph.D. thesis, Australian National University (1965), unpublished. [2] A . J . Easton and J . F . Lovering, Determimation of small quantities of potassium and sodium in stony meteoritic material, rocks and minerals. Anal. Chim. Acta 30 (1964) 543. [3] L. Salmon, The determination of sodium and potassium in a sample of dunite by radioactivation analysis. United Kingdom Atomic Energy Authority Report A.E.R.E. C/M 323 (1957). [4] T.H. Handley and C.L. Burros, Determination of radioactive cesium, Anal. Chem. 31 (1959) 332. [5] I-I. J. Cluley, The determination of potassium by precipitation as potassium tetraphenylboron and its applicatio n to silicate analysis, Analyst 80 (1955) 354. [6] A. J. Easton, unpublished work. [7] G. R. Tilton and G.W. Reed, Radioactive heat production in eclogite and some ultramafic rocks. In: Earth Science and Meteoritics, eds. J. Geiss and E. D. Goldberg (North-Holland Publishing Company, Amsterdam, 1963) p. 31. [8] R. W. Stoenner and O. A. Schaeffer, private communication.
AND PLANETARY
S C I E N C E L E T T E R S 3 (1966) 110-112. N O R T H - H O L L A N D
PUBL. COMP., A M S T E R D A M
P O T A S S I U M A B U N D A N C E S IN S O M E U L T R A - B A S I C A N D BASIC R O C K S J. W. MORGAN and A. D. T. GOODE * Depa~'trnent of Geophysics and Geochemistry, Institute of Advanced Studies, Australian National University, Canberra, A. C. T., Australia Received 8 April 1966
i. ~TRODUCTION In a r e c e n t n e u t r o n activation study, u r a n i u m and t h o r i u m w e r e d e t e r m i n e d in a n u m b e r of b a sic and u l t r a - b a s i c rocks [1]. F o r any s u b s e q u e n t heat production c a l c u l a t i o n s it is also n e c e s s a r y to know the p o t a s s i u m a b u n d a n c e s for these rocks. Some d e t e r m i n a t i o n s of this e l e m e n t have b e e n made by a flame p h o t o m e t r i c technique [2]; however, as the r e s u l t s showed little c o r r e l a t i o n to the u r a n i u m and t h o r i u m abundances, and the l e v e l s found were in many c a s e s n e a r the lower l i m i t of the method, it was thought advisable to r e p e a t the d e t e r m i n a t i o n s u s i n g an e n t i r e l y i n dependent p r o c e d u r e . Neutron activation a n a l y s i s i s quite s e n s i t i v e for p o t a s s i u m , and has the great advantage of being f r e e f r o m i n t e r f e r e n c e by r e a g e n t c o n t a m i n a t i o n d u r i n g c h e m i c a l p r o c e s s i n g after i r r a d i a t i o n .
2. ANALYTICAL METHOD About 100 m g aliquots of the powdered s a m ples were weighed into c l e a n s i l i c a a m p o u l e s (4 m m i n t e r n a l d i a m e t e r ) and sealed. Standards w e r e p r e p a r e d by weighing about 20 mg of S p e c pure SiO 2 into s i m i l a r a m p o u l e s and spiking with 50 to 100 ~g of p o t a s s i u m as a s t a n d a r d solution p r e p a r e d f r o m A n a l a r K2CO 3 d r i e d to c o n s t a n t weight at 200oc. The s t a n d a r d s were d r i e d at 80oc and the a m p o u l e s sealed. Batches of six s a m p l e s and two s t a n d a r d s w e r e i r r a d i a t e d in a flux of 3 × 1012 n e u t r o n s , c m - 2 . sec -1 for 24 hr. After i r r a d i a t i o n the a m p o u l e s were opened and the s a m p l e s weighed into p l a t i n u m dishes; 25 mg of p o t a s s i u m c a r r i e r w e r e added and the s a m p l e s dissolved by f u m i n g with HF and H2SO 4. Any insoluble m a t e r i a l (mostly spinel) was u s u ally discarded. (In the case of the Gila, A r i z o n a * Present address: Department of Geology, University of Adelaide, Adelaide, South Australia.
s a m p l e s the insoluble r e s i d u e s weighed 1.8 mg and 2.5 mg and contained < 0.6 x 10-3 ~g and < 0.8 x 10-3 ~g of K r e s p e c t i v e l y . ) After taking finally to d r y n e s s , the contents of the dish w e r e d i s s o l v e d in water, t r a n s f e r r e d to a 250 m l b e a k e r and 10 m l HNO 3 plus 5 m l HC104 added. The b e a k e r was heated on a hot plate u n t i l f u m e s of HC10 4 appeared. The solution was t r a n s f e r r e d to a c e n t r i f u g e tube and cooled. The p r e c i p i t a t e was centrifuged off, washed t h r e e t i m e s with ethanol and d i s s o l v e d in 10 m l of water; 10 mg of f e r r i c i r o n c a r r i e r w e r e added and Fe(OH) 3 p r e c i p i t a t ed by the addition of NaOH solution. The p r e c i p itate was centrifuged and the s u p e r n a t e f i l t e r e d into a c l e a n c e n t r i f u g e tube. The p r e c i p i t a t e was washed with 5 m l of w a t e r and, after f i l t e r i n g , the washings were added to the s u p e r n a t e . The solution was acidified with glacial acetic acid (phenolphthalein indicator) and 10 m l of a f r e s h l y p r e p a r e d 10% solution of s o d i u m - c o b a l t i n i t r i t e w e r e added. After s t a n d i n g for 10 m i n the p r e c i p itate was centrifuged and washed s e v e r a l t i m e s with water, c e n t r i f u g i n g and d i s c a r d i n g the s u p e r n a t e each time. The p r e c i p i t a t e was d i s s o l v e d by w a r m i n g with 2 m l of I : I H N O 3. Up to this stage the p r o c e d u r e was a l m o s t i d e n t i c a l to that r e p o r t e d by Salmon [3]. After cooling, the n i t r i c acid solution was f i l t e r e d and 5 m l of a 5% EDTA solution added. The pH was adjusted to 6.5 by the addition of NaOH solution ( b r o m o c r e s o l purple indicator) and the solution cooled in ice; 10 m l of sodium t e t r a p h e n y l b o r o n solution [4] w e r e added while s t i r r i n g . The p o t a s s i u m t e t r a p h e n y l b o r o n p r e cipitate was centrifuged and s u b s e q u e n t l y washed s e v e r a l t i m e s with water. It was d i s s o l v e d in a m i n i m u m of acetone, cooled in ice, and r e p r e cipitated by the addition of 0.1 N HC1, 15 m l of w a t e r and 2 m l of sodium t e t r a p h e n y l b o r o n s o l u tion. The p r e c i p i t a t e was centrifuged, washed t h r e e t i m e s in w a t e r and again d i s s o l v e d in a m i n i m u m of acetone; 10 m l of a solution of 0.5% sodium t e t r a p h e n y l b o r o n in ethanol was added
111
POTASSIUM ABUNDANCES IN SOME ULTRA-BASIC AND BASIC ROCKS Table 1 P o t a s s i u m a b u n d a n c e s in s o m e u l t r a - b a s i c and b a s i c r o c k s . Specimen
P o t a s s i u m (ppm) This work
P o t a s s i u m (ppm) Other work
Reference
Ultra-basic inclusions P e r i d o t i t e 2604, Mt. L e u r a , Vic. P e r i d o t i t e 2642, Mt. L e u r a , Vic. P e r i d o t i t e 2669, Mt. Shadwell, Vic. P e r i d o t i t e 2700, Mt. Noorat, Vic. P e r i d o t i t e 2728, Mt. G a m b i e r , S.A. P e r i d o t i t e R42, Delegate, N.S.W. P e r i d o t i t e R71, Snowy Mts., N.S.W. Dunite R80, Snowy Mts., N.S.W. P e r i d o t i t e R420, Oahu, Hawaii Dunite, Gila, A r i z o n a G a r n e t P e r i d o t i t e R357, South A f r i c a
70 , 70 , 15 , 14 , 90 , 91 , 109 , 110 , 29 30 , 81 , 80 , 23 , 24 , 37 , 33 , 23 , 23 , 1.6, 1.6, 159 , 160 ,
av. 70 av. 15 av. 90 av. 110 av. 29 av. 80 av. 23 av. 35 av. 23 av. 1.6 av. 159
60 50 90 50 140 4O 170 30
[6] [6] [6] [6] [6] [6] [6] [6]
* * * * * * * *
180
[6] *
C r u s t a l and i n t r u s i v e u l t r a - b a s i c r o c k s High t e m p e r a t u r e p e r i d o t i t e , Tinaquilla, V e n e z u e l a Dunite, $39, B a l s a m Q u a r r y , N.C. Dunite, R461, J a c k s o n Co., N.C. Dunite, Twin S i s t e r s , Washington Dunite 2515, Almklovdalen, N o r w a y G a r n e t P e r i d o t i t e R372, R i v e r T e s s i n , S w i t z e r l a n d
6.3,
5.9, 5.8 27 , 25 , 3.8, 4.2, 14 , 16 , 63 58 , 49 50 ,
av.
6.0
av. av. av. av. av.
26 4.0 15 61 50
, av. 8.7, av.
11 8.8
2.6, av. 5.6, av.
2.2 5.6
32 29 12±4
[3] ** [6] *
5O
[6] *
[7] t
U. S. G. S. s t a n d a r d u l t r a - b a s i c r o c k s Dunite DTS-1, Twin S i s t e r s , Washington P e r i d o t i t e P C C - 1 , S o n o m a Co., C a l i f o r n i a
11 , 8.8,
12
Pallasite olivines Brenham Huckitta
1.8, 5.6,
2.5
[8]
**
Basic inclusions P y r o x e n i t e , Oahu, Hawaii Granulite R52, Delegate, N.S.W. Granulite R140, Delegate, N.S.W. G a b b r o R362, Hualalai, Hawaii Eclogite R616, Dodoma, Tanganyika
174 259
, 260
, av. 260
932
, 945
, av. 938
248
, 249
, av. 249
231
, 229
, av. 230
,
, av.
190 250 900
[71 t
220
[6] *
30
[6] *
[6] * [61 *
Crustal eclogite " B r o n z i t e P y r o p i t e " R375, Sittampundi, India
S p e c p u r e SiO 2
30
1.7,
32
1.2, av.
31 1.4
Methods: * F l a m e p h o t o m e t r y . ** N e u t r o n activation. J" I s o t o p e dilution.
a n d t h e m i x t u r e c o o l e d in i c e f o r 10 r a i n . T h e p r e c i p i t a t e w a s s e p a r a t e d b y s p i n n i n g in a c e n trifuge, and washed three times with ethanol. It was then slurried with half a ml of ethanol , transferred to a w e i g h e d c o u n t i n g p l a n c h e t , a n d dried under a heat lamp. The sources were a l l o w e d to c o o l a n d w e r e r e w e i g h e d t o d e t e r m i n e the radiochemical yield. Except for the first precipitation of potassium
tetraphenylboron from an EDTA solution at pH 6.5, w h i c h w a s t a k e n f r o m a m e t h o d d e s c r i b e d b y C l u l e y [5], t h e s e c o n d p a r t o f t h e c h e m i c a l p r o c e d u r e o u t l i n e d a b o v e w a s b a s e d on a m e t h o d f o r the d e t e r m i n a t i o n of f i s s i o n p r o d u c t c e s i u m r e p o r t e d b y H a n d l e y a n d B u r r o s [4]. T h e c h e m i cal s e p a r a t i o n of the c o m p a r a t o r s t a n d a r d s w a s i d e n t i c a l to t h a t u s e d f o r t h e s a m p l e s . T h e a c t i v i t y of the final s o u r c e s w a s d e t e r -
112
J.W. MORGAN and A. D. T. GOODE
m i n e d by b e t a c o u n t i n g . No s e l f - a b s o r p t i o n c o r r e c t i o n s w e r e n e c e s s a r y b e c a u s e of the h i g h b e t a e n e r g y of 42K, on w h i c h the a n a l y s e s w e r e b a s e d . D e c a y c u r v e s w e r e f o l l o w e d and it w a s o b s e r v e d that a s m a l l c o r r e c t i o n ( g e n e r a l l y 0.5 to 2%) w a s r e q u i r e d f o r the p r e s e n c e of s o m e l o n g e r l i v e d a c t i v i t y , p o s s i b l y of 86Rb ( h a l f - l i f e 18.7 days). W h e n t h i s c o r r e c t i o n w a s m a d e the o b s e r v e d d e c a y c o r r e s p o n d e d v e r y c l o s e l y to the 12.5 h r h a l f - l i f e of 42K.
n e u t r o n a c t i v a t i o n m e t h o d g i v e s b o t h p r e c i s e and a c c u r a t e r e s u l t s in the r a n g e f r o m 2 to 900 p p m .
ACKNOWLEDGEMENTS We w i s h to thank D r . J. F. L o v e r i n g , D r . D . H . G r e e n and D r . S.R. T a y l o r f o r p r o v i d i n g the s a m p l e s u s e d in t h i s w o r k , and a l s o f o r v a l u a b l e c o m m e n t and d i s c u s s i o n . T h i s w o r k w a s s u p p o r t e d in p a r t by a g r a n t f r o m the A u s t r a l i a n I n s t i t u t e of N u c l e a r S c i e n c e and E n g i n e e r i n g .
3. R E S U L T S T h e p o t a s s i u m a b u n d a n c e s found in 25 r o c k s and 2 p a l l a s i t e o l i v i n e s a r e shown in t a b l e 1. T h e e r r o r on e a c h i n d i v i d u a l d e t e r m i n a t i o n i s of the o r d e r of 3%, e s t i m a t e d f r o m the s t a n d a r d d e v i a tion of the f o u r c o m p a r a t o r s t a n d a r d s o u r c e s ( d u p l i c a t e a l i q u o t s f r o m e a c h of t h e two s t a n d ards irradiated with each six samples). The unc e r t a i n t y i n t r o d u c e d by the c o u n t i n g s t a t i s t i c s i s c o n s i d e r a b l y l e s s t h a n t h i s , so that the e s t i m a t e d error will include all the uncertainties involved in the d e t e r m i n a t i o n s , e x c e p t f o r any s y s t e m a t i c e r r o r in the s t a n d a r d p o t a s s i u m s o l u t i o n f r o m w h i c h the s t a n d a r d s w e r e p r e p a r e d . T h i s c a n be s e e n to b e s m a l l by a c o m p a r i s o n w i t h d e t e r m i n a t i o n s m a d e by o t h e r m e t h o d s on s o m e of the s a m e s a m p l e s (table 1). It can be s e e n that in g e n e r a l the a g r e e m e n t i s good, e x c e p t f o r s o m e of the f i a m e p h o t o m e t r y r e s u l t s on s a m p l e s w i t h low p o t a s s i u m a b u n d a n c e s . T h e d i s c r e p a n c y i s a l m o s t c e r t a i n l y due to l a r g e e r r o r s and i n t e r f e r e n c e s w h i c h b e c o m e i n c r e a s i n g l y s e r i o u s in the f l a m e p h o t o m e t r y m e t h o d b e l o w the 100 p p m l e v e l . In g e n e r a l it i s s a f e to s a y that the p r e s e n t
REFERENCES [1] J.W. Morgan, Ph.D. thesis, Australian National University (1965), unpublished. [2] A . J . Easton and J . F . Lovering, Determimation of small quantities of potassium and sodium in stony meteoritic material, rocks and minerals. Anal. Chim. Acta 30 (1964) 543. [3] L. Salmon, The determination of sodium and potassium in a sample of dunite by radioactivation analysis. United Kingdom Atomic Energy Authority Report A.E.R.E. C/M 323 (1957). [4] T.H. Handley and C.L. Burros, Determination of radioactive cesium, Anal. Chem. 31 (1959) 332. [5] I-I. J. Cluley, The determination of potassium by precipitation as potassium tetraphenylboron and its applicatio n to silicate analysis, Analyst 80 (1955) 354. [6] A. J. Easton, unpublished work. [7] G. R. Tilton and G.W. Reed, Radioactive heat production in eclogite and some ultramafic rocks. In: Earth Science and Meteoritics, eds. J. Geiss and E. D. Goldberg (North-Holland Publishing Company, Amsterdam, 1963) p. 31. [8] R. W. Stoenner and O. A. Schaeffer, private communication.