COMPOSITION OF SHERGOTTY PARENT BODY; FURTHER EVIDENCE OF A TWO COMPONENT MODEL OF PLANET FORMATION.

Most d i f f e r e n t i a t e d m e t e o r i t e s have c r y s t a l l i z a t i o n ages o f 4.5 b.y . Except ions a re shergo t t i t es ,nakh l i tes, and chass ign i tes ( a l t o g e t h e r 9 i n d i v i d u a l m e t e o r i t e s ) , which a re more than 2 b.y . younger. These SNC m e t e o r i t e s have i n a d d i t i o n a number o f p r o p e r t i e s i n comnon: they a r e unbrecc iated, have cumulate tex tu res , they c r y s t a l l i z e d under oxygen f u g a c i t y near t h e quartz-fayali te-magneti te b u f f e r , t hey have h i g h con ten ts o f v o l a t i l e elements, t h e i r REE p a t t e r n s a r e complex (Wood and Ashwal ( 1 ) ) . The abundances o f many s i d e r o p h i l e elements i n s h e r g o t t i t e s a re s i m i l a r t o those i n t e r r e s t r i a l b a s a l t s ( 2 ) . I n t h e oxygen th ree i so tope p l o t Shergotty,Zagami, Nakhla, and L a f a y e t t e a r e separated from a l l o t h e r meteor i tes . They l i e s i m i l a r t o e u c r i t e s c l o s e t o t h e oxygen i so tope f r a c t i o n a t i o n l i n e o f Moon and Ear th ( 3 ) . The g r e a t importance o f tungsten as an i n d i c a t o r o f p l a n e t a r y f r a c t i o n a t i o n processes was po in ted o u t by Wanke e t a l . ( 4 , 5 ) , Ramensee and Wanke ( 6 ) and Palme and Ramensee (7 ) . Dreibus e t a l . ( 8 ) n o t i c e d t h a t P behaves s i m i l a r t o W d u r i n g p l a n e t a r y f r a c t i o n a t i o n processes. Ringwood (9 ) noted 8 t o 9 t imes h igher P/Sm and P/Ti r a t i o s i n Shergo t t y compared t o t e r r e s t r i a l oceanic t h o l e i i t e s . He s t a t e d : " I t w i l l be i n t e r e s t i n g t o see whether W i s a l s o compara t i ve ly more abundant i n s h e r g o t t i t e s . The o x i d a t i o n s t a t e o f these meteor i tes i s a l s o s i m i l a r b u t somewhat h igher than t h a t o f t h e E a r t h ' s mant le" . We have s tud ied t h e t r a c e element con ten t o f Brachina, Chassigny, Nakhla, and Shergot ty . P r e l i m i n a r y r e s u l t s a re l i s t e d i n Table 1. As expected, t h e W con ten t o f Shergo t t y tu rned o u t t o be f a r above t h a t o f t e r r e s t r i a l , l u n a r o r e u c r i t i c b a s a l t s w i t h s i m i l a r La abundances (10) . Whi le La/W r a t i o s o f e u c r i t e s , t e r r e s t r i a l and l u n a r rocks a re i n t h e range o f those found i n t h e non-meta l l i c p a r t o f e q u i l i b r a t e d o r d i n a r y chondr i tes , SNC meteor i tes have s i g n i f i c a n t l y l ower La/W r a t i o s (F ig . 1). Lanthanum and W a r e b o t h r e f r a c t o r y elements and hence should n o t f r a c t i o n a t e from each o t h e r i n t h e stages preceding acc re t ion . The d e p l e t i o n o f W r e l a t i v e t o La i n the s i l i c a t e p a r t o f e q u i l i b r a t e d chondr i tes i s due t o p a r t i t i o n i n g o f W i n t o FeNi phase i n which W i s concentrated. Unequ i l i b ra ted c h o n d r i t e s have cons ide rab ly h igher W concen t ra t ions i n t h e s i l i c a t e phase and co r respond ing ly lower concen t ra t ions i n FeNi (11 ) . As d iscussed e a r l i e r (6,7), e q u i l i b r a t i o n w i t h about 30 % met a l cou ld e x p l a i n the observed W d e p l e t i o n i n s i l i c a t e p o r t i o n s o f Ear th, Moon and e u c r i t e pa ren t body (EPB). r I 1 1 1 1 1 1 " ' I ' 1 The d e o l e t i o n o f W i n the mant le o f the Ear th can a l s o be