Sokolov, S. V.

English

Sokolov S. V.

University of Szeged

Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 FLUID COMPONENT OF CARBONATITE MELTS S O K O L O V . S. V.1 1 All-Russian Scientific-Research Institute of Mineral Resources; Staromonetnyi per. 31, M o s c o w 119017, Russia. E-mail: vims@df.ru Fluids play diverse and often major role in many magmatic processes. Their s ignif icance is especially great in carbonat i te pedogenesis , as far as formation of carbonate melts under real values of T-parameter proved to be possible only in the presence of carbon dioxide - water fluid phase (Paterson, 1958; Wyllie, Tuttle, 1960). Later it was established that H 2 0 , C 0 2 and F could be dissolved in considerable quantities in carbonatite melts, in which phase t ransformations occur in the presence of f luid which has substantially carbon dioxide-water composition. Apart f rom indirect evidences being results of experimental petrology concerning model melts, participation of volatile components ( H 2 0 , C 0 2 , F, CI, S) in natural carbonatite forming systems is displayed by: 1) mineral parageneses of carbonat i tes composed by carbonates, micas, amphiboles, apatite, humite, serpentine, chlorites, sulf ides; 2) fenitization halos around carbonatite stocks; 3) exocontact reaction zones, bordering carbonatites, which are enriched in ones or others of the above mentioned minerals; 4) joint occurrence in carbonatite minerals of primary melt inclusions (MI), containing fluid (gas + l iquid) isolations, and syngenetic fluid inclusions (FI). The latter confirms saturation of carbonati te melts by fluids, which in the course of crystallization or during sharp decrease of outer pressure (influence of tectonic movements) separated volatile componen t s as independent phase. Results of inclusion studies f rom carbonatite minerals of Russia, Scandinavia, India, Canada and Eastern Afr ica using the methods of thermometry and cryometry, water leaches, Raman spectroscopy and gas chromatography allowed us to es t imate quantitatively thermodynamic (PTVX) properties of fluid component of carbonati te-forming systems (Andersen, 1986; Jaireth et al„ 1991; Morogan, Lmdblom, 1995; Rankin, 1975, 1977; Samson et al„ 1995,a,b; Sokolov, 1993; Ting et al., 1994 and our new data - see table). Homogenizat ion temperatures of MI in apatites and carbonates f rom carbonatites of the consecutive stages decrease f r o m 950-900 to 500°C (Sokolov. 1996). Synchronous with them FI homogenize, as a rule, into liquid phase. There fo re in this temperature range of carbonatite formation, fluids were in liquid-like states that were characterized by increased density caused by outer pressure and/or rather high salinity. The last assertion is confirmed by presence in gas-liquid inclusions of daughter crystals - in the first place halite, nahcolite and sylvine and also calcite, dolomite, shortite and kalicinite. These data completely coincide with results of bulk and individual analyses of FI, which characterize not only composi t ion and salinity of liquid phase but also density of solutions trapped by inclusions. In many cases as regarding their composi t ion , the cations are exclusively represented by alkalis; Na content being constantly higher than that of K, sometimes for 1-2 orders , and in the Oka massif (Canada) potassium is completely absent. At the same time alkaline-earth elements (Ca>Mg) are found in inclusions of minerals of the Kola Peninsula carbonatites, but these elements being in subordinate quantities relative to alkalis. Anionic composition of solution is rather constant, carbonate-chloride, with various relations between chlorine ion and carbonate or bicarbonate ions; however almost exclusively haloid solutions are known (Gulinskii , Russia; Fen, Norway; Oka complexes). Sulphate ion and fluorine are secondary components of FI carbonati tes of the Kola Peninsula and Oka. Concentrations of soluble salts display significant ranges (from first to almost 50 wt. %). Density of water solutions in inclusions ranges f rom 0.58 to 1.26 g/cm3 . Composit ion of many inclusions corresponds or is very close to the system N a C l + H 2 0 ± C 0 2 , whose phase d iagram is well studied in geological important area of PTVX-parameters . If we know solution concentrat ion, homogenizat ion tempera ture of FI and formation temperature of the host mineral (which may be accepted as T h o m of syngenetic MI), a possibility emerges to estimate a value of fluid pressure. Our and literature data testify to the fact that carbonati t ic magmat ic systems are character ized by high values of fluid pressure, attaining 4.6-5.1 kbar (Kovdor and Gulinskii complexes , Russia; Oka, Canada - our data) and even 6.2 kbar (Sukulu complex, Uganda - Ting et al., 1994). Baric regime was characterized by instability both during all process of carbonatite formation and during its separate stages. By the end of carbonati te process, a decrease of pressure to 3.4-2.25 kbar is observed. Pressure fluctuations resulted in stratification of fluid phase, which is testified by coexis tence in one mineral of FI of different composition - aqueous, of ten of high salinity, and carbon dioxide ones or hydrocarbon ones which occur very seldom. Carbon dioxide inclusions as a rule are distributed in carbonatites locally. Gas-chromatographic analysis displays in general low to moderate C 0 2 contents in carbonati te volatile phase (1.6-5.3 mol/kg H 2 0 ) , which very seldom exceed 20 mol /kg H 2 0 (Vuori-Yarvi complex, Russia). Increased concentrations of C 0 2 by the end of process are established in the carbonat i tes of the Kola Peninsula (Sokolov, 2001). Among hydrocarbons methane occupies a dominant position, its fraction in volatile phase is in average 1-2 orders inferior relative to carbon dioxide. Independent hydrocarbon and aqueous-hydrocarbon (methane) FI found in carbonates and apati tes f rom the Kovdor and Sukulu carbonatites (Sokolov, 1993; Ting et al., 1994) are characterized by low salinity. 190 Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003 Chlo r ine and fluorine b e l o n g to charac ter i s t ic volat i le c o m p o n e n t s of the ca rbona t i t e - fo rming sys tems . C h l o r i n e is eas i ly redis t r ibuted into fluid phase , whe rea s fluorine is re ta ined by c a r b o n a t e mel t s and is b o u n d in minera l lat t ices. T h e r e f o r e , low fluorine con ten t s are fixed in l iquid p h a s e of FI (0 .7-1 .3 mg/1). T h e a b o v e da ta a l low us to m a k e a conc lus ion abou t suf f ic ien t s imilar i ty of compos i t i on and o ther p rope r t i e s of c a rbona t i t e f lu ids re la ted to a lka l ine-u l t rabas ic c o m p l e x e s of var ious reg ions . In the c o u r s e of d e v e l o p m e n t of the ca rbona t i t e p roces s genera l f luid sa tura t ion increases , but fluid p ressure dec reases , ro le of C 0 2 and K increases re la t ive to CI and N a co r r e spond ing ly , h y d r o c a r b o n FI ob ta in wider deve lopmen t . T h e r e f o r e , cond i t ions of genera t ion and evo lu t ion of the fluids themse lves , as well as pa ren t fo r them carbona t i t e mel t s are ra ther s imi lar in genera l fea tures . P rope r t i e s of c a rbona t i t e fluids M a s s i f s C o m p o s i t i o n N a / K C, w t . % d, g / c m 3 P n , kba r Russ ia : K o v d o r , Vuor i -Yarvi , Tur i i M y s , Gul insk i i Na , K , M g , Ca , CI, C 0 2 ± ( S 0 3 , F , C H 4 ) 1 .8-14 6 - 5 0 0 . 7 5 - 0 . 8 3 2 . 7 5 - 5 . 1 5 Scand inav ia : F e n , A l n o Na , K , CI, C 0 2 , M g ( ? ) , Fe (? ) > 1 2 -35 0 . 7 6 - 1 . 1 0 4 .5 India: S u n g Va l l ey Na , K , CI, C 0 2 - 9 - 3 8 - 3 .5-4 .7 C a n a d a : O k a Na , K , CI, C 0 2 , S 0 3 , C a ( ? ) 3 . 5 - > 2 7 5 4 .2 -4 .9 0 . 7 7 - 1 . 2 6 3 .8 -4 .9 Eas t e rn Af r i ca : W a s a k i , Suku lu , T o r o r o N a , K , CI, C02 ± C H 4 2 . 4 - 1 6 12 .1-42.4 0 . 5 8 - 1 . 1 7 2 . 2 5 - 6 . 2 References ANDERSEN, T . 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(1958) : T h e mel t ing of ca lc i te in the p r e s e n c e of water and c a r b o n d iox ide . A m e r i c a n minera log i s t , 43 , 603 -6 0 6 . RANKIN, A. H . (1975) : F lu id inc lus ion s tudies in apat i te f r o m ca rbona t i t e s of the W a s a k i area of wes te rn K e n y a . L i thos , 8, 1 2 3 - 1 3 6 . RANKIN, A. H. (1977) : F lu id - inc lus ion e v i d e n c e for the fo rma t ion cond i t ions of apa t i te for the T o r o r o ca rbona t i t e c o m p l e x of eas te rn U g a n d a . Mina ra log i ca l M a g a z i n e , 41 , 155-164. SAMSON, I. M . , L iu , W „ WILLIAMS-JONES, A. E . (1995a ) : T h e na tu re of o r t h o m a g m a t i c hydro the rma l fluids in the O k a ca rbona t i t e , Q u e b e c , C a n a d a : E v i d e n c e f r o m fluid inclus ions . G e o c h i m i c a et C o s m o c h i m i c a Acta , 59 , 1 9 6 3 - 1 9 7 7 . SAMSON, I. M . , WILLIAMS-JONES, A. E. , L i u , W . (1995b) : T h e chemis t ry of hyd ro the rma l fluids in ca rbona t i t e s : E v i d e n c e f r o m leacha te and S E M - d e c r e p i t a t e analys is of fluid inc lus ions f r o m O k a , Q u e b e k , Canada . G e o c h i m i c a et C o s m o c h i m i c a Acta , 59 , 1979-1989 . SOKOLOV, S. V. (1993) : Geo log ica l fo rmat iona l types , or ig in and f o r m a t i o n cond i t ions of ca rbona t i t e s . G e o i n f o r m m a r k , M o s c o w , 7 3 pp . (in Russ . ) . SOKOLOV, S. V . (1996) : F o r m a t i o n t e m p e r a t u r e s and t empe ra tu r e f ac ies of ca rbona t i t e s f r o m the a lka l ine u l t r amaf i c c o m p l e x e s . G e o c h e m i s t r y In te rna t iona l , 34(1) , 13-18. SOKOLOV, S. V. (2001) : R o l e of c a r b o n d iox ide in r o c k - f o r m i n g of the ca rbona t i t e complexes . P r o c e e d i n g s of the Russ i an Mine ra log i ca l Soc ie ty , 3 , 114-123 (in Russ . ) . TING, W „ BURKE, E . A. J. , RANKIN, A. H „ WOOLEY, A. R. (1994) : T h e charac te r i sa t ion and pe t rogene t i c s ign i f i cance of C 0 2 , H 2 0 and CH4 fluid inc lus ions in apat i te f r o m the Sukulu ca rbona t i t e s , U g a n d a . E u r o p e a n Journa l of M i n a r a l o g y , 6, 7 8 7 - 8 0 3 . WYLLIE, P. J. , TUTTLE, O. F . (1960) : T h e sys t em C a 0 - C 0 2 - H 2 0 and the or ig in of carbona t i tes . Journa l of Pe t ro logy , 1, 1 -96 . 191 

Fluid component of carbonatite melts

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Fluid component of carbonatite melts

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