Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
SILICATE M E L T INCLUSIONS IN PHENOCRYSTS FROM GLASSY BASANITES OF THE TERGESH PIPE (NORTH MINUSA DEPRESSION, KHAKASIA, RUSSIA)
T I M I N A, T. Ju.1, S H A R Y G I N , V. V.2, G O L O V I N , A. V.2, P E T Ő , M.3
1 Department of Geology and Geophysics, Novosibirsk State University, Pirogova st. 2, 630090 Novosibirsk, Russia.
2 Institute of Mineralogy and Petrography, Koptyuga pr. 3, 630090 Novosibirsk, Russia.
3 Lithosphere Research Group, Department of Petrology and Geochemistry, Eötvös University, Pázmány Péter sétány 1/C, H-1117 Budapest, Hungary.
E-mail: sharygin@uiggm.nsc.ru Introduction
The Tergesh pipe contains the deepest mantle garnet-spinel xenoliths among all pipes of the North Minusa depression. T h e eruptive breccias are predominant over basanites in the Tergesh pipe (Kryukov, 1964). The recent Ar/Ar dating of the Tergesh pipe shows age 77+1.9 M a (Bragin et al., 1999). The glassy basanites of this pipe were selected for the study of silicate melt inclusions. Olivine (Fo8i-69) and rare clinopyroxene are phenocrysts (about 10 vol. %). T h e groundmass consists of clinopyroxene, plagioclase (An6 8.5|), Ti-magnetite, F-apatite and glass.
Figure 1. Primary melt inclusions from the core of olivine phenocrysts.
GI - glass; Rho - rhonite; Cpx - Ti- augite; Sulf -sulfide bleb; g - gas bubble.
Petrography of inclusions
Primary silicate melt inclusions were observed only in phenocrystal olivine and clinopyroxenes. Their sizes vary f r o m 5 to 5 0 /xm. In olivine they occasionally decorate the growth zones of the host and are associated with single Cr-spinel crystals and C 02 fluid inclusions. Phase composition of the melt inclusions in olivine is glass + fluid ± daughter/trapped crystals ± Fe-Ni- Cu-sulfide globule. In some inclusions glass is finely devitrified. Rhonite and clinopyroxene are the main daughter phases.
Apatite, ilmenite and Ti-magnetite occur rarely. It should b e noted that rhônite-bearing inclusions are generally confined to the core of the host olivine while rhônite-free ones are situated in the outer parts. The relationships of minerals within olivine- hosted inclusions show that rhonite is the earliest daughter phase (Fig.l). The clinopyroxenes-hosted inclusions contain glass, low-density fluid and sometimes Ti-magnetite.
Gl « i
Thermometry
The heating experiments with olivine-hosted inclusions were performed using two methods: (1) the Vernadsky heating stage with visual observations to get homogenization temperatures; (2) heating experiments (T>1250°C) with single grains in the stage design by Petrushin et al. (2003) to get homogenous inclusions "in blind" for further microprobe study. T h e quenched inclusions heated up to temperatures more than 1270°C have the following phase composition: glass + gas ± sulfide globule.
This indicates that sulfide melt was possibly separated immicibly from basanitic liquid at high temperatures (>>1250°C).
According to thermometric studies olivine-hosted melt inclusions are homogenized at 1270-1310°C, while clinopyroxene ones - at 1100°C. The moderate density of the C 02 inclusions (homogenization of vapor into liquid at +25°C), which associated with melt inclusions in olivine, suggest that their trapping pressure was more than 3 kb.
Chemistry of inclusion glasses
Glasses of heated and unheated of olivine- and clinopyroxene-hosted inclusions as well as daughter phases of unheated inclusions in olivine were analyzed by microprobe at U I G G M , Novosibirsk.
Glasses of inclusions heated in the 1280-1320°C temperature interval are characterized by high contents of F e O (9-15), M g O (4-10), C a O (6-11.5), T i 02 (1.9-3.3), P205 (0.2-1.2) and relatively low abundances of S i 02 (41.2-48.4), A1203 (14.7-
17.8), N a20 + K20 (2.1-8.8 wt. %). These compositions are very similar to host basanites (Fig. 2). Residual glasses of unheated inclusions in olivine and clinopyroxene strongly vary in major elements depending on the amount of daughter crystals within inclusion. In general, they are more acidic than heated inclusions (in wt. %): S i 02 - 47-57.3, T i 02 - 0.9-3.5, A1203 - 21-28.5, 213
Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
F e O - 1.3-6, M g O - 0 . 3 - 3 . 2 , C a O - 1.3-11.5, N a20 - 3 - 9 . 6 , K20 - 1.8-8.6, P205 - 0 . 5 - 1 . 9 , CI - 0 . 1 - 0 . 3 . S i l i c a - r i c h g l a s s e s ( S i 02
> 5 2 wt. % ) in olivine c o r r e s p o n d in c o m p o s i t i o n to g l a s s e s f r o m c l i n o p y r o x e n e - h o s t e d i n c l u s i o n s a n d f r o m t h e b a s a n i t e g r o u n d m a s s , e x c e p t i n g alkalis c o n c e n t r a t i o n s .
Chemistry of daughter phases
C l i n o p y r o x e n e of o l i v i n e - h o s t e d i n c l u s i o n s drastically d i f f e r f r o m p h e n o c r y s t a l a n d g r o u n d m a s s c l i n o p y r o x e n e s in S i 02, A 1203 H T i 02. It b e l o n g s to Ti- ri ch a u g i t e ( T i 02 - 2.5-6.1 wt. % ) with high A 1203 H P205 ( 9 - 1 1 . 9 H 0 . 1 - 1 . 3 wt. % , r e s p e c t i v e l y ) . It should b e noted that the p r e s e n c e of P205 in c l i n o p y r o x e n e is characteristic of a p a t i t e - f r e e i n c l u s i o n s c o n t a i n i n g s m a l l a m o u n t of d a u g h t e r p h a s e s . T h e c o m p o s i t i o n of d a u g h t e r r h ô n i t e a l s o strongly v a r i e s (in wt. % ) : S i 02 - 2 4 . 7 - 3 1 , T i 02 - 8 . 7 - 10.7, C r203- 0.1-1.4, A 1203 - 15.4-18.9, F e O - 16.8-19.9, M g O - 11.2-13.8, C a O - 10.2-13, N a20 - 1.1-1.8. M a g n e t i t e f r o m olivine- and c l i n o p y r o x e n e i n c l u s i o n s is rich in T i 02 ( u p to 2 3 wt. % ) and is similar to g r o u n d m a s s m a g n e t i t e .
O to a +
o a s
Z 5
F i g u r e 2. Compositional variations for silicate melt inclusions in the Tergesh basanites.
Solid s q u a r e - glass of heated inclusions in olivine; open circle - glass of unheated inclusions in olivine; cross - glass of unheated inclusions in clinopyroxenes; o p e n s q u a r e - glass of basanite groundmass; open t r i a n g l e - basanites (Sobolev et al.,
1988; author's data).
40 45 50
SiO,
55 60
Discussion
T h e d i a g r a m s (Fig. 2) s h o w c o m p o s i t i o n a l variations f o r i n c l u s i o n glasses ( h e a t e d a n d u n h e a t e d ) in c o m p a r i s o n with b u l k b a s a n i t e of the T e r g e s h p i p e (as p r o b a b l e initial m a g m a ) a n d their g r o u n d m a s s g l a s s (as d e r i v a t e of initial m a g m a ) . T h e d a t a o b t a i n e d a l l o w us a s s u m e that basanitic melt of the T e r g e s h p i p e d u r i n g c r y s t a l l i z a t i o n e v o l v e d t o w a r d s i n c r e a s i n g of S i 02, A 1203, alkalis, CI a n d d e p l e t i n g of m a f i c c o m p o n e n t s . T h e a b s e n c e of w a t e r - b e a r i n g p h a s e s in silicate m e l t i n c l u s i o n s is e v i d e n c e that initial basanitic m a g m a a n d its d e r i v a t e s c o n t a i n e d l o w H20 a b u n d a n c e s . P r e v i o u s l y the s i m i l a r e v o l u t i o n a r y c h a r a c t e r w a s established f o r the B e l e p i p e of the N o r t h M i n u s a d e p r e s s i o n ( G o l o v i n et al., 2 0 0 0 ) a s well as f o r s o m e a l k a l i n e basalts of the world.
T h i s w o r k is s u p p o r t e d b y R u s s i a n F o u n d a t i o n for B a s i c R e s e a r c h (grant no. 0 2 - 0 5 - 6 4 6 2 0 ) .
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