Kovyazin S. V.

Podgornik N. M.

Šarygin V. V.

English

Sharygin V. V.

Podgornykh N. M.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 INCLUSIONS IN OLIVINE OF THE OMOLON PALLASITE: A PRELIMINARY STUDY S H A R Y G I N . V. V. ' . KOVYAZIN, S. V.1 , P O D G O R N Y K H , N. M. 2 1 Institute of Mineralogy and Petrography, Koptyuga prospect 3, 630090 Novosibirsk, Russia. 2 Siberian Geological Museum of U I G G M S D RAS, Koptyuga prospect 3, 630090 Novosibirsk, Russia. E-mail: sharygin@uiggm.nsc.ru The Omolon meteorite (250 kg) was found in 1982 (it fell in 1981) near the Omolon river (Magadan district, Russia) . This meteorite belongs to the main group of the pallasite family. It contains rounded olivine grains (about 60 vol. %, sizes - up to 3 cm, Fa content - 12.3 mole %) in nickeloan iron matrix consisting of kamacite, taenite and plessite (Plyashkevich et al., 1991; Desrousseaux et al., 1997). In addition, troilite, chromite, schreibersite, nikelphosphide (former rhabdite) and stanfieldi te were found in the meteorite (Plyashkevich et al., 1991; Bondar et al., 1997). The irradiation t ime of this meteori te was determined to at 78±7 Ma (by noble gases); the K-Ar age for olivine crystallization was assessed as 4 .6 Ga, i.e., c lose to the starting t ime for formation of protoplanetary system (Shukolyukov et al., 1992). The calculation of a tmospheric trajectory and orbit showed that the Omolon meteorite was probably a f ragment of an Apollo M-type asteroid and its preatmospheric mass was approximate ly 390-490 kg (Bronshten et al., 1999). According to fossil track studies the depth of ablation for the Omolon pallasite does not exceed 8.2±2.1 cm out of the preatmospheric surface (Bondar et al., 1997). Kolyasnikov and Savva (1997) suggested the presence of forsteritic glass in melting rim of the meteorite. The aim of this work is to study the inclusions occurring in ol ivine grains of the Omolon pallassite. Inclusions in olivine W e studied individual olivine grains f rom the outer zones of the Omolon pallasite. In general , inclusions fo rm planar arrays associated with fractures in the host olivine. Coexisting metal-sulfide blebs, fluid and crystal inclusions and their combina t ions were found in some arrays. The sizes of individual inclusions range f rom 5 to 200 /im (Fig. 1). In addition, olivine hosts large metal-sulfide blebs and oriented needle-like isolations of unidentified Cr-rich phase, unrelated to the arrays described above (Fig. 1A-C). Metal-sulfide and fluid inclusions are most common. Typical phase composi t ion of large metal-sulf ide blebs is troilite + kamacite. Nickelphosphide, taenite, stanfieldite, whitlockite, chromite, and unidentif ied Si-rich and Cr-S-rich occur as minor phases. Taenite and nickelphosphide are predominantly located on the boundary between troilite and kamaci te (Fig. 1A-B). Fluid inclusions contain low-density fluid and sometimes metal-sulf ide isolations. Colorless transparent phase (phosphate?) rarely occurs in large fluid inclusions (>100 /xm). Some mineral associat ions f rom the arrays have an appearance of the silicate melt inclusions; microprobe analyses have demonstrated, however , that they are assemblages of stanfieldite, metal-sulfide blebs, and gas bubbles (Fig. IF). The individual euhedral chromite (up 20 /xm) with adhered metal-sulf ide bleb and Si-rich phase occurs rarely (Fig. ID). Introduction 100 PRA Km - kamacite; \ [ & fc"^-A, B, E - reflected light; C, D, F - ordinary light. g -gas. Nph - nickelphosphide; Stf - stanfieldite; Figure 1. Different types of inclusions in olivine of the Omolon pallasite. Sulf - metal-sulfide bleb; Cr-sp - chromite; Tn - taenite; Tro - troilite; 184 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 Chemistry of minerals from inclusions Troilite is the m a j o r phase of me ta l - su l f ide b lebs . Its c o m p o s i t i o n is u n i f o r m : Fe - 63 .3 -63 .5 ; S - 3 6 . 4 - 3 6 . 7 (all c o m p o s i t i o n s are in wt. % ) and similar to su l f ide f r o m meta l l i c par t of the pa l las i te (P lyashkev ich et al., 1991) . C o m p o s i t i o n s of kamacite a re s o m e w h a t var iable: F e - 90 .3 -94 .9 ; N i - 4 .2 -8 .9 ; C o - 0 .3-0 .8 , Si, P and S - <0 .02 . C o m p o s i t i o n s of taenite are h ighly var iab le : F e - 8 5 . 9 - 5 9 . 6 ; Ni - 13 .1-40.1 ; C o - 0 .05-0 .6 . T h e s imi lar var ia t ions are a lso typical of taeni te f r o m meta l l i c par t of the me teo r i t e (P lyashkev ich et al . , 1991). Nickelphosphide (N i ,Fe ) 3 P (recent ly accep ted as a n e w minera l (Br i tv in et al . , 1999); init ial ly w a s n a m e d rhabdi te , Ni - r ich var ie ty of schre ibers i te ) o c c u r s rare ly in me ta l - su l f ide b l ebs in o l iv ine . Its c o m p o s i t i o n is: F e - 3 1 . 9 - 3 2 . 8 ; Ni - 52 .1 -53 .4 ; C o - 0 -0 .03 ; P - 14 .3-14 .8 ; C u , S and Si - <0 .15 . T h i s minera l is d i f f e r e n t f r o m schre ibers i te ( F e , N i ) 3 P and s imi lar t o n i cke lphosph ide f r o m meta l l i c par t o f the me teor i t e (P lyashkev ich et al. , 1991) . Chromite is vir tual ly f r e e in T i 0 2 ( < 0 . 0 5 wt .%) that is typical o f Cr - sp ine l s f r o m o ther pa l lass i tes (Buseck , 1977) . B a s e d o n m i c r o p r o b e ana lyses , c o m p o s i t i o n s of indiv idual c h r o m i t e c rys ta l s in o l iv ine a re h o m o g e n e o u s . T h e chemica l var ia t ions of this mine ra l a re : S i 0 2 - 0 . 0 4 - 0 . 2 ; C r 2 0 3 - 53 .5 -61 .8 ; A1 2 0 3 - 5 .6 -8 .8 ; F e O - 2 5 . 3 - 3 2 . 6 ; M g O - 3 .9 -5 .5 ; M n O - 0 .6 -0 .8 . T w o d i f f e r e n t p h o s p h a t e s w e r e o b s e r v e d in ol iv ine: stanfieldite C a 4 ( M g , F e 2 + , M n ) 5 ( P 0 4 ) 6 and whitlockite ( C a , M g , F e 2 + ) 3 ( P 0 4 ) 2 . S t an f i e ld i t e is a c o m m o n p h o s p h a t e mine ra l , whi le whi t locki te is rare . C h e m i c a l c o m p o s i t i o n of s tanf ie ld i te : P 2 0 5 - 4 6 . 2 - 4 9 . 2 ; S i 0 2 - 0 - 1 . 4 5 ; F e O - 2 .8 -7 .8 ; M n O - 0 .35 -0 .55 ; M g O - 1 9 . 9 - 2 1 . 2 ; C a O - 23 .5 -28 ; N a 2 0 - 0 .02 -0 .1 . Whi t lock i t e is S i O r r i c h ( n = 2 ) : P 2 0 5 -38 .4 ; S i 0 2 - 16.7; F e O - 2 .95 ; M n O - 0 ; M g O - 2 . 7 ; C a O - 37 .9 ; N a 2 0 - 0 .95 . U n f o r t u n a t e l y , w e cou ld no t iden t i fy s o m e mine ra l s f r o m inc lus ions b e c a u s e of their very smal l s izes and the poss ib l e p r e s e n c e of l ight e l e m e n t s tha t canno t b e d e t e r m i n e d b y m i c r o p r o b e t e c h n i q u e (O, N and C) . Si-rich phase f o r m s 1 -5 /xm gra ins in a s soc ia t ion wi th c h r o m i t e and me ta l - su l f ide b leb in the ar rays . M i c r o p r o b e analys is and E D S s h o w e d that th is p h a s e con ta ins u p to 4 0 w t . % of Si (in these ana lyses the d i a m e t e r of the m i c r o p r o b e b e a m was grea te r than the s ize of the a n a l y z e d minera l ) . T h i s p h a s e cou ld be a S i 0 2 p o l y m o r p h or nier i te S i 3 N 4 (Lee e t al., 1995) or s inoi te S i 2 N 2 0 ( A n d e r s e n et al . , 1964) . In o n e me ta l - su l f ide b l eb w e h a v e f o u n d a Cr-S- r ich p h a s e (<1 | t m ) in troil i te . A c c o r d i n g to E D S , this minera l con t a in Cr , F e and S (in this ana lys i s aga in the d i ame te r of the b e a m w a s greater than the s ize of the ana lyzed minera l ) . T h i s p h a s e cou ld be daubree l i t e F e C r 2 S 4 ( e x t r e m e l y rare in o ther pal lass i tes ; B u s e c k , 1977) , o r b rez ina i te Cr 3 S 4 , o r ca r l sberg i te C r N . Discussion T h e p e t r o g r a p h y of the O m o l o n meteor i te , i.e., genera l recrys ta l l iza t ion kamac i t e , the a p p e a r a n c e of fine-grained k a m a c i t e wi th b o u n d a r y of m i g r a t i o n g rowth , the red is t r ibu t ion of rabdi te (n i cke lphosph ide ) , k ink b a n d i n g in o l iv ine , s h o w s tha t this me teo r i t e has u n d e r g o n e at least t w o shock d e f o r m a t i o n s (P lyashkev ich et al., 1991) . T h e p r e s e n c e of p l ana r a r rays of inc lus ions in o l iv ine s e e m s to indica te pos t -de fo rma t iona l annea l i ng fo r the me teor i t e (Buseck , 1977). Poss ib ly , the inc lus ions o r ig ina ted in r e s p o n s e to pene t ra t ion of the me teor i t e in to the E a r t h ' s a t m o s p h e r e and subsequen t impact . References ANDERSEN, C . A. , KEIL, K. , MASON, B. (1964) : S i l icon oxyni t r ide : A meteor i t ic minera l . Sc ience , 146, 2 5 6 - 2 5 7 . BONDAR, J. V. , PERELYGIN, V. P. , PETROVA, R. I. (1997) : Foss i l t rack s tud ies of the O m o l o n pal las i te c rys ta l s . R a d i a t i o n M e a s u r e m e n t s , 28 , 3 3 7 - 3 4 0 . BRITVIN, S. N . , KOLOMENSKY, V. D „ BOLDYREVA, M. M „ BOGDANOVA, A. N „ KRETSER, YU. L „ BOLDYREVA, O . N „ RYDASHEVSKY, N. S. (1999) : N i c k e l p h o s p h i d e (Ni ,Fe) 3 P, n icke l a n a l o g of schre ibers i te . 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Inclusions in Olivine of the Omolon Pallasite : a preliminary study

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Inclusions in Olivine of the Omolon Pallasite : A preliminary study

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