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Metallic spherules in Bosumtwi crater glasses
EARTH AND PLANETARY SCIENCE LETTERS 1 (1966) 23-24. NORTH-HOLLAND PUBL. COMP., AMSTERDAM
METALLIC SPHERULES IN BOSUMTWI
CRATER GLASSES
Ahmed E L GORESY
Max-Planck-lnstitut flit Kernphysik, Heidelberg, Germany Received 26 November 1965
Several metallic spherules were discovered in some new glass specimens from the Dosumtwi crater in Ghana. The microprobe analysis indicated 95.4% Fe, 5.2~ Ni and 0.6% Co. Pyrrhotite spherules present also contain Ni up to 0.3%. The presence of Ni and Co in the metallic spherules is in agreement with the impact origin of the glasses.
In new g l a s s s p e c i m e n s c o l l e c t e d f r o m the s u e v i t e o u t cr o p n e a r Atasu (Ata) s t r e a m f r o m the B o s u m t w i c r a t e r in Ghana many m e t a l l i c s p h e r u l e s have been found. T h e g l a s s s p e c i m e n s a r e in g e n e r a l v e r y p u m a c e o u s and full of bubbles. Studies of p o l i s h e d s e c t i o n s in r e f l e c t e d light r e ve a led , that the g l a s s e s a r e r a t h e r homogeneous. Only few s c a t t e r e d s t r o n g l y a l t e r e d s i l i c a t e inc l u s i o n s a r e p r e s e n t . Some m a g n e t i t e , r u ti le , and p s e u d o - b r o o k i t e a r e also i n c l o s e d in the g l a s s showing t e x t u r e s s i m i l a r to those d e s c r i b e d b e f o r e f r o m the s a m e l o c a l i t y by E1 G o r e s y [1]. T h e m e t a l l i c s p h e r u l e s a r e of d i f f e r e n t s i z e s , r a n g i n g g e n e r a l l y f r o m 1 to 12 p and a r e p e r f e c t l y s p h e r i c a l without any sign of w e a t h e r ing. The m a j o r i t y of the s p h e r u l e s , h o w e v e r , have an a v e r a g e s i z e of 5 to 7 ~. They a r e c o m p l e t e l y e m b e d d e d in the g l a s s and o c c u r s o m e -
t i m e s in s w a r m s . No signs of s t r a i n have been d e t e c t e d in the g l a s s s u r r o u n d i n g the s p h e r u l e s . D e t a i l e d m i n e r a g r a p h i c st u d i es on those s p h e r u l e s r e v e a l e d , that they c o n s i s t e x c l u s i v e l y of one s t r o n g r e f l e c t i n g and m e t a l l i c phase, p r e su m ab l y k a m a c i t e . No o t h e r i n c l u s i o n s s i m i l a r to t h o se found by Chao et al. [2] in m e t a l l i c s p h e r u l e s of the P h i l i p p i n e t e k t i t e s w e r e o b served. V e r y often the s p h e r u l e s a r e e m b e d d e d in the g l a s s with about s i m i l a r d i s t a n c e s (figs. 1 and 2), a p h e n o m e n o n o b s e r v e d in many m e t e o r i t e s . E l e c t r o n m i c r o p r o b e a n a l y s e s have been c a r r i e d out on many s p h e r u l e s with the C A M E C A m i c r o a n a l y s e r u si n g 25 keV e x c i t a t i o n en er g y . F o r the sake of good conducting, the s u r f a c e s of the s e c t i o n s w e r e c o v e r e d with a thin c a r b o n f i l m (~ 300 ~). S t an d ar d s used a r e p u r e iron, nickel, and cobalt m e t a l s r e s p e c t i v e l y . Th e r e s u l t s o b -
O •
•
"
0 , *
•
O"
O0 50 n T i c r o n s
50 n l i C r O l l S
t
i
Fig'. 1. Metallic spherules of different sizes, Atasu, oil immersion,
Fig. 2. Small metallic spherules embedded in the ~[~ss with about similar distances, Atasu, oil immersion.
24
A. EL GORESY
tained f r o m the a v e r a g e 5 to 7 ~ s p h e r u l e s in s p e c i m e n A-27 w e r e u n s a t i s f a c t o r y s in c e the total m e a s u r e d Fe, Ni, and Co did not r e a c h 100%, though they w e r e d e t e c t e d as m a j o r e l e m e n t s . No sulphur was found. Such r e s u l t s a r e due to the fact, that the s p h e r u l e s a r e only few m i c r o n s thin, i.e., the e l e c t r o n b e a m will p e n e t r a t e the s p h e r u l e s and m e e t the g l a s s below them. M e a s u r e m e n t s at l ow er excitation e n e r g i e s c a u s e d s o m e i n c r e a s e in the p e r c e n t a g e s of the e l e m e n t s but the total s t i l l r e m a i n e d far below 100%. Only a single s p h e r u l e of 15 p d i a m e t e r gave good r e s u l t s . The c h e m i c a l c o m p o s i t i o n of this s p h e r u l e i s given in table 1. Table 1 Microprobe analysis of metallic and pyrrhotite spherules in Bosumtwi glass, Atasu, specimen A-27.
Metallic spherule ( ~ . %/
Pyrrhotite spherule (wt. %)
95.4~0.5 5.2~0.1 0.6±0.1
59.4~0.3 0.3~0.1
also few p y r r h o t i t e s p h e r u l e s . C o m p a r e d to the m e t a l l i c ones they a r e g e n e r a l l y l a r g e r in size, d i s t i n c t l y b r o w n i s h in c o l o r , and s t r o n g l y a n i s o tropic. The c h e m i c a l c o m p o s i t i o n of a 14 ~ p y r r h o t i t e s p h e r u l e was d e t e r m i n e d also by the m i c r o p r o b e u s i n g p u r e synthetic FeS and m e t a l l i c Ni as s t a n d a r d s and 17 keV e x c i t a t i o n energy. The r e s u l t s a r e given in table 1. No o t h e r e l e m e n t s than Fe, Ni and S w e r e r e c o r d e d . The m i c r o p r o b e a n a l y s i s shows, that the m i n e r a l is r e a l l y p y r r h o t i t e and not t r o i l i t e . Both t e r r e s t r i a l p y r r h o t i t e and m e t e o r i t i c t r o i l i t e m ay contain s i m i l a r Ni values. F o r this r e a s o n the p r e s ent a n a l y s i s of the p y r r h o t i t e can not indicate if it is of t e r r e s t r i a l o r of m e t e o r i t i c origin. Howe v e r , the q u a l i t a t i v e Ni and Co i n v e s t i g a t i o n s in many m e t a l l i c s p h e r u l e s in addition to the quant i t a t i v e a n a l y s i s of the 15 ~ one in s a m p l e A-27 f r o m the B o s u m t w i g l a s s is i n d i c a t i v e of the i m pact o r i g i n of the glass.
A cknou'ledgernent Fe
Ni Co S
35.6±0.5 101.2
I am indebted to P r o f . W. G en t n er and Dr. J. Z$thringer f o r p r o v i d i n g the s a m p l e s . The help of Dr. H. F e c h t i g during the m i c r o p r o b e a n a l y s i s is also acknowledged.
95.3 REFERENCES
The r e s u l t s a r e c o r r e c t e d f o r dead t im e , absorption, and f l u o r e s c e n c e . F r o m the m i c r o p r o b e a n a l y s i s of the m e t a l l i c s p h e r u l e it is e v i dent, that the Ni and Co contents a r e quite s i m i l a r to those of k a m a c i t e of s o m e i r o n m e t e o r ites. In the s a m e s p e c i m e n s the author r e c o r d e d
[1] A. E1 Goresy, Die Erzmineralien in den Ries- und Bosumtwi-Krater-Gllisern und ihre genetische Deutung, Geochim. Cosmochim. Acta 28 {1964) 1881.
[2] E. C. T. Chao, I. Adler, E . J . Dwornik and J. Littler, Metallic spherules in tekties from Isabella, Philippine Island. Science 135 (1962) 97.
METALLIC SPHERULES IN BOSUMTWI
CRATER GLASSES
Ahmed E L GORESY
Max-Planck-lnstitut flit Kernphysik, Heidelberg, Germany Received 26 November 1965
Several metallic spherules were discovered in some new glass specimens from the Dosumtwi crater in Ghana. The microprobe analysis indicated 95.4% Fe, 5.2~ Ni and 0.6% Co. Pyrrhotite spherules present also contain Ni up to 0.3%. The presence of Ni and Co in the metallic spherules is in agreement with the impact origin of the glasses.
In new g l a s s s p e c i m e n s c o l l e c t e d f r o m the s u e v i t e o u t cr o p n e a r Atasu (Ata) s t r e a m f r o m the B o s u m t w i c r a t e r in Ghana many m e t a l l i c s p h e r u l e s have been found. T h e g l a s s s p e c i m e n s a r e in g e n e r a l v e r y p u m a c e o u s and full of bubbles. Studies of p o l i s h e d s e c t i o n s in r e f l e c t e d light r e ve a led , that the g l a s s e s a r e r a t h e r homogeneous. Only few s c a t t e r e d s t r o n g l y a l t e r e d s i l i c a t e inc l u s i o n s a r e p r e s e n t . Some m a g n e t i t e , r u ti le , and p s e u d o - b r o o k i t e a r e also i n c l o s e d in the g l a s s showing t e x t u r e s s i m i l a r to those d e s c r i b e d b e f o r e f r o m the s a m e l o c a l i t y by E1 G o r e s y [1]. T h e m e t a l l i c s p h e r u l e s a r e of d i f f e r e n t s i z e s , r a n g i n g g e n e r a l l y f r o m 1 to 12 p and a r e p e r f e c t l y s p h e r i c a l without any sign of w e a t h e r ing. The m a j o r i t y of the s p h e r u l e s , h o w e v e r , have an a v e r a g e s i z e of 5 to 7 ~. They a r e c o m p l e t e l y e m b e d d e d in the g l a s s and o c c u r s o m e -
t i m e s in s w a r m s . No signs of s t r a i n have been d e t e c t e d in the g l a s s s u r r o u n d i n g the s p h e r u l e s . D e t a i l e d m i n e r a g r a p h i c st u d i es on those s p h e r u l e s r e v e a l e d , that they c o n s i s t e x c l u s i v e l y of one s t r o n g r e f l e c t i n g and m e t a l l i c phase, p r e su m ab l y k a m a c i t e . No o t h e r i n c l u s i o n s s i m i l a r to t h o se found by Chao et al. [2] in m e t a l l i c s p h e r u l e s of the P h i l i p p i n e t e k t i t e s w e r e o b served. V e r y often the s p h e r u l e s a r e e m b e d d e d in the g l a s s with about s i m i l a r d i s t a n c e s (figs. 1 and 2), a p h e n o m e n o n o b s e r v e d in many m e t e o r i t e s . E l e c t r o n m i c r o p r o b e a n a l y s e s have been c a r r i e d out on many s p h e r u l e s with the C A M E C A m i c r o a n a l y s e r u si n g 25 keV e x c i t a t i o n en er g y . F o r the sake of good conducting, the s u r f a c e s of the s e c t i o n s w e r e c o v e r e d with a thin c a r b o n f i l m (~ 300 ~). S t an d ar d s used a r e p u r e iron, nickel, and cobalt m e t a l s r e s p e c t i v e l y . Th e r e s u l t s o b -
O •
•
"
0 , *
•
O"
O0 50 n T i c r o n s
50 n l i C r O l l S
t
i
Fig'. 1. Metallic spherules of different sizes, Atasu, oil immersion,
Fig. 2. Small metallic spherules embedded in the ~[~ss with about similar distances, Atasu, oil immersion.
24
A. EL GORESY
tained f r o m the a v e r a g e 5 to 7 ~ s p h e r u l e s in s p e c i m e n A-27 w e r e u n s a t i s f a c t o r y s in c e the total m e a s u r e d Fe, Ni, and Co did not r e a c h 100%, though they w e r e d e t e c t e d as m a j o r e l e m e n t s . No sulphur was found. Such r e s u l t s a r e due to the fact, that the s p h e r u l e s a r e only few m i c r o n s thin, i.e., the e l e c t r o n b e a m will p e n e t r a t e the s p h e r u l e s and m e e t the g l a s s below them. M e a s u r e m e n t s at l ow er excitation e n e r g i e s c a u s e d s o m e i n c r e a s e in the p e r c e n t a g e s of the e l e m e n t s but the total s t i l l r e m a i n e d far below 100%. Only a single s p h e r u l e of 15 p d i a m e t e r gave good r e s u l t s . The c h e m i c a l c o m p o s i t i o n of this s p h e r u l e i s given in table 1. Table 1 Microprobe analysis of metallic and pyrrhotite spherules in Bosumtwi glass, Atasu, specimen A-27.
Metallic spherule ( ~ . %/
Pyrrhotite spherule (wt. %)
95.4~0.5 5.2~0.1 0.6±0.1
59.4~0.3 0.3~0.1
also few p y r r h o t i t e s p h e r u l e s . C o m p a r e d to the m e t a l l i c ones they a r e g e n e r a l l y l a r g e r in size, d i s t i n c t l y b r o w n i s h in c o l o r , and s t r o n g l y a n i s o tropic. The c h e m i c a l c o m p o s i t i o n of a 14 ~ p y r r h o t i t e s p h e r u l e was d e t e r m i n e d also by the m i c r o p r o b e u s i n g p u r e synthetic FeS and m e t a l l i c Ni as s t a n d a r d s and 17 keV e x c i t a t i o n energy. The r e s u l t s a r e given in table 1. No o t h e r e l e m e n t s than Fe, Ni and S w e r e r e c o r d e d . The m i c r o p r o b e a n a l y s i s shows, that the m i n e r a l is r e a l l y p y r r h o t i t e and not t r o i l i t e . Both t e r r e s t r i a l p y r r h o t i t e and m e t e o r i t i c t r o i l i t e m ay contain s i m i l a r Ni values. F o r this r e a s o n the p r e s ent a n a l y s i s of the p y r r h o t i t e can not indicate if it is of t e r r e s t r i a l o r of m e t e o r i t i c origin. Howe v e r , the q u a l i t a t i v e Ni and Co i n v e s t i g a t i o n s in many m e t a l l i c s p h e r u l e s in addition to the quant i t a t i v e a n a l y s i s of the 15 ~ one in s a m p l e A-27 f r o m the B o s u m t w i g l a s s is i n d i c a t i v e of the i m pact o r i g i n of the glass.
A cknou'ledgernent Fe
Ni Co S
35.6±0.5 101.2
I am indebted to P r o f . W. G en t n er and Dr. J. Z$thringer f o r p r o v i d i n g the s a m p l e s . The help of Dr. H. F e c h t i g during the m i c r o p r o b e a n a l y s i s is also acknowledged.
95.3 REFERENCES
The r e s u l t s a r e c o r r e c t e d f o r dead t im e , absorption, and f l u o r e s c e n c e . F r o m the m i c r o p r o b e a n a l y s i s of the m e t a l l i c s p h e r u l e it is e v i dent, that the Ni and Co contents a r e quite s i m i l a r to those of k a m a c i t e of s o m e i r o n m e t e o r ites. In the s a m e s p e c i m e n s the author r e c o r d e d
[1] A. E1 Goresy, Die Erzmineralien in den Ries- und Bosumtwi-Krater-Gllisern und ihre genetische Deutung, Geochim. Cosmochim. Acta 28 {1964) 1881.
[2] E. C. T. Chao, I. Adler, E . J . Dwornik and J. Littler, Metallic spherules in tekties from Isabella, Philippine Island. Science 135 (1962) 97.