C h o n d r u l e s a r e c h a r a c t e r i s t i c c o n s t i t u e n t s i n p r i m i t i v e m e t e o r i t e s , o r c h o n d r i t e s . T h e s e t i n y spherical objects (0.1 to a few mm in diameter) are mainly composed of silicates with minor amounts of metallic iron and/or iron sulfide. Chondrules were f o r m e d i n t h e p r i m o r d i a l s o l a r n e b u l a a b o u t 4 . 5 billion years ago prior to the formation of the Earth. Th ey we re on ce mo lt en du e to th e in st an ta ne ou s heating of solid precursors and cooled rapidly. In order to elucidate their origin and obtain information o n t h e s o l a r n e b u l a , m a n y r e s e a r c h e r s h a v e e x a m i n e d c h o n d r u l e s e x t e n s i v e l y u s i n g d i f f e r e n t m e t h o d s . H o w e v e r , t h e i r t h r e e - d i m e n s i o n a l s t r u c t u r e s h a v e n o t b e e n t h o r o u g h l y i n v e s t i g a t e d so far. X-ray computed tomography (CT) can give i n f o r m a t i o n a b o u t t h e i n t e r n a l s t r u c t u r e w i t h o u t damaging the samples and provide 3-D structures by stacking successive sliced images. In SPring-8, a micro X-ray CT system has been developed ( Fig. 1 ) [ 1 - 2 ] . T h e m a j o r a d v a n t a g e s o f a n X - r a y C T Three-dimensional Structures of Chondrules: their Motions in the Primordial Solar Nebula system using SR are (i) high resolution 3-D images due to the well-collimated beam and ( ii ) elimination o f C T i m a g e a r t i f a c t s ( b e a m h a r d e n i n g ) a n d qualitative correlation of CT image brightness (CT- value) with linear attenuation coefficient by using a monochromatic beam. We applied this system to the chondrules. Chondrules of about 1-2 mm in diameter taken f r o m t h e A l l e n d e m e t e o r i t e ( C V 3 c h o n d r i t e ) w e r e im ag ed at be am li ne BL 20 B2 wi th mo no ch ro ma ti c be am s at 17 .5 - 25 ke V [3 ]. Th e cr os s se ct io na l images (CT images) were reconstructed from 360 p r o j e c t i o n s t a k e n b y s a m p l e r o t a t i o n . T h e 3 - D s t r u c t u r e s w e r e o b t a i n e d f r o m 1 5 0 - 3 0 0 i m a g e s . T h e v o x e l s i z e o f 5 . 8 3 μ m × 5 . 8 3 μ m × 5 . 8 3 μ m (“ voxel” is an element of 3-D images whereas the term “ pixel” is used in 2-D images) gave a spatial r e s o l u t i o n o f a b o u t 1 3 μ m [ 1 ] . F i g u r e 2 s h o w s examples of 3-D CT images of the chondrules. We e x a m i n e d b o t h t h e e x t e r n a l s h a p e s a n d i n t e r n a l s t r u c t u r e s o f t h e c h o n d r u l e s t h r e e - d i m e n s i o n a l l y using image analysis techniques. It is known that the chondrules are not perfect s p h e r e s . T h i s w a s d i s c o v e r e d b y t h e t w o - dimensional observation of thin sections. However, their external 3-D shapes have not been described beam monitor visible light mirror data acquisition fluorescent screen trigger X-rays sample X-ray CT system synchronize CCD CCD camera controller stage controller Fig. 1. A schematic illustration of micro X-ray CT system at BL20B2. qualitatively. We approximated the shapes of t h e c h o n d r u l e s a s e l l i p s o i d s w i t h t h r e e different axial radii (a, b and c) [4]. We found that the shapes of the chondrules are diverse f r o m o b l a t e s h a p e s ( a < b ~ c ) , g e n e r a l ellipsoids (a < b < c) to prolate shapes (a ~ b < c ) with the aspect ratio of 0.73 - 0.97 ( Fig. 3 ). Chondrules contain voids as well as metal a n d s u l f i d e g r a i n s . I t i s h a r d t o r e c o g n i z e voids in thin sections by conventional means because some parts of the samples might be removed when making thin sections. The CT s t u d y s h o w e d t h a t m o s t o f t h e c h o n d r u l e s have voids (<0.001 - 0.9 vol.%). This shows t h a t v o i d s a r e a n i m p o r t a n t c o n s t i t u e n t . V o i d s i n t h e c h o n d r u l e s m a y b e f o r m e d b y 61 b / c 0 . 7 0 0 . 7 5 0 . 8 0 0 . 8 5 0 . 9 0 0 . 9 5 1 . 0 0 a / b 0 . 7 0 0 . 7 5 0 . 8 0 0 . 8 5 0 . 9 0 0 . 9 5 1 . 0 0 N = 29 P BO CC c prolate oblate a sphere ( i ) b u b b l i n g o f v o l a t i l e c o m p o n e n t s , ( i i ) t r a p p i n g o f n e b u l a g a s d u r i n g c h o n d r u l e formation by instantaneous heating of dust b a l l s o r ( i i i ) s h r i n k i n g o f c h o n d r u l e s b y solidification. We found that the voids are c o n c e n t r a t e d a l o n g t h e m i n o r a x i s o f a n o b l a t e c h o n d r u l e [ 5 ] . T h i s s t r o n g l y su gg es ts th at th e ch on dr ul e wa s sp in ni ng during their formation in a molten state and t h e v o i d s w e r e m o v e d t o w a r d s t h e m i n o r a x i s b y c e n t r i f u g a l f o r c e . T h e 3 - D distribution of metal and sulfide grains also shows spinning along the minor axes of the oblate chondrules or along the major axes of the prolate ones [4]. C h o n d r u l e s h a v e d i f f e r e n t t e x t u r e s , such as porphyritic and barred olivine ( BO ). I n B O c h o n d r u l e s , p a r a l l e l s e t s o f b a r - s h a p e d c r y s t a l s o f o l i v i n e ( ( M g , F e ) 2 S i O 4 ) w e r e o b s e r v e d i n t h i n s e c t i o n s . T h e C T st ud y sh ow ed th at BO ch on dr ul es co ns is t (a) (c) Fig. 2. Examples of CT images of chondrules and their 3-D structures. (a) A CT image of porphyritic chondrule. (b) Its 3-D structure. (c) A CT image of barred olivine chondrule. (d) Its 3-D structure. 3-D images are rendered in pseudo color. o f p a r a l l e l s e t s o f t h i n o l i v i n e p l a t e s [ 3 ] . Interestingly, these BO chondrules are oblate and t h e n o r m a l d i r e c t i o n s o f t h e o l i v i n e p l a t e s a r e almost the same as the minor axes of the oblate c h o n d r u l e s ( F i g . 4 ) . I f w e a s s u m e t h a t a l l B O chondrules are oblate or prolate, we can estimate t h e i r 3 - D s t r u c t u r e s f r o m t h e t h i n s e c t i o n s t w o - d i m e n s i o n a l l y . S t a t i s t i c a l o b s e r v a t i o n o f B O c h o n d r u l e s i n t h i n s e c t i o n s s u g g e s t e d t h a t t h e normal directions are nearly parallel to the minor axes of many oblate chondrules or to the major axes of some prolate chondrules. These features m a y b e a l s o e x p l a i n e d b y t h e s p i n n i n g o f t h e c h o n d r u l e s p r o b a b l y b e c a u s e c e n t r i f u g a l f o r c e affects the nucleation and crystallization of olivine a l t h o u g h n o d e f i n i t i v e m e c h a n i s m i s k n o w n a t present. F i g . 3 . A s p e c t r a t i o s , a / b a n d b / c , o f c h o n d r u l e s . T h e a x i a l r a d i i , a , b a n d c , were obtained by ellipsoid fitting for each c h o n d r u l e s . P : p o r p h y r i t i c , B O : b a r r e d olivine, CC: crypto crystalline. a / b = d / c ellipsoid 62 (b) (d) A k i r a T s u c h i y a m a a , T s u k a s a N a k a n o b a n d Kentaro Uesugi c (a) Osaka University (b) Geological Survey of Japan, AIST (c) SPring-8 / JASRI E-mail: akira @ ess.sci.osaka-u.ac.jp The idea of spinning chondrules leads to important information on chondrule formation a n d n e b u l a c o n d i t i o n s . P r o l a t e chon drul es m i g h t b e f o r m e d b y a e r o d y n a m i c drag during m o l t e n s t a t e s . T h e m a j o r a x i s s h o u l d c o r r e s p o n d t o t h e r o t a t i o n a x i s a n d t h e p a r a l l e l a x i s t o t h e d i r e c t i o n o f m o v e m e n t , like a volcanic bomb or a flying football. If this is the case, the presence of prolate chondrules shows that some dense gas was present in t h e n e b u l a w h e r e t h e c h o n d r u l e s w e r e formed, or that the relation between the gas pressure and the relative speed of chondrule m o t i o n c a n b e e s t i m a t e d f r o m t h e a s p e c t r a t i o s o f t h e p r o l a t e c h o n d r u l e s . O n t h e o t h e r h a n d , w e c a n e s t i m a t e t h e s p i n n i n g rates of the oblate chondrules by considering the balance between the surface tension of d r o p l e t c h o n d r u l e s a n d c e n t r i f u g a l f o r c e s b y t h e s p i n n i n g . T h e a n g u l a r v e l o c i t y o f rotation, the equatorial radius of the drop, its d e n s i t y a n d t h e i n t e r f a c i a l s u r f a c e t e n s i o n d e t e r m i n e s t h e s h a p e o f s u c h r o t a t i n g droplet. The spinning rate of the chondrules is estimated to be about 50 - 200 rps. Such a h i g h s p i n n i n g r a t e f a v o r s a s p e c i f i c ch on dr ul e fo rm at io n pr oc es s, su ch as ro ta ti on b y d r a g h e a t i n g o f i r r e g u l a r - s h a p e d d u s t b a l l s b y a s h o c k w a v e o r c o l l i s i o n o f s m a l l heavenly bodies. (c) slice-A (d) slice-B 0.5 mm Fi g. 4. 3- D st ru ct ur e of a ba rr ed ol iv in e ch on dr ul e. Rendered in pseudo color. (a) A side view of the oblate chodrule. (b) A top view of the oblate chodrule. (c) A s l i c e d i m a g e a l o n g A - d i r e c t i o n . ( d ) A s l i c e d i m a g e a l o n g B - d i r e c t i o n . I n d i v i d u a l v o i d s a r e s h o w n a s different colors in (a) and (b) . A parallel set of olivine bars (cyan) is seen in (c) and (d) . References [1] K. Uesugi et al. , Proc. SPIE 3772 (1999) 214. [ 2 ] K . U e s u g i e t a l . , N u c l . I n s t r u m . M e t h . P h y s . Res. A 467-468 (2001) 853. [3] A. Tsuchiyama et al. , Lunar Planetary Sci. XXXI (2000) 1566. [4] Shigeyoshi et al. , Abstr. 2001 Fall Meeting Jpn. Soc. Planet. Sci. (2001) 113 (in Japanese). [ 5 ] A . T s u c h i y a m a e t a l . , L u n a r P l a n e t a r y S c i . XXVIII (1997) 1453. 63 (a) side view (b) top view voids minor axis A B
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