Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
M E L T AND FLUID INCLUSIONS I N ANORTHOSITE XENOLITH FROM T H E UDACHNAYA KIMBERLITE PIPE, YAKUTIA
T O M I L E N K O . A. A.'. S H A T S K Y , V. S . \ K O V Y A Z I N , S. V . ' , O V C H I N N I K O V Y. I.1 1 Institute of Mineralogy and Petrography, Koptyug Ave., 3, 630090 Novosibirsk, Russia.
E-mail: tomilen@uiggm.nsc.ru
The origin of anorthosites, which always attracted the attention of scientists and was repeatedly discussed in literature, remains an unresolved issue (Simmons, Hanson, 1978; Bogatikov, 1979; Owens, D y m e k , 2001). Of particular interest is the origin of Precambrian autonomous anorthosites, since they are considered characteristic rocks of the early stage of the Earth's evolution. Despite the long-term study of these rocks, their genesis remains controversial.
This paper presents first results of the microscopic, microthermal, cryometric, Raman-spectroscopic, and m i c r o p r o b e investigations of melt and fluid inclusions in the minerals of anorthosite xenolith f r o m the Udachnaya kimberlite pipe, Yakutia.
According to the tectonic scheme of Rozen et al., 2001, this pipe is located within the Markhin granulite-greenstone terrane. Doubly polished plates (100-150 pirn) were used for optical investigations of the melt and fluid inclusions under the transmitted light. Microthermometric investigations were performed in heating stage and freezing stage designed by Osorgin and Tomilenko (Osorgin, Tomilenko, 1990a, 1990b). Raman spectroscopy was performed on a R A M A N O R U - 1 0 0 0 (Jobin Yvon) one-channel Raman-spectrometer using argon laser. T h e chemical composition of minerals and melt inclusions was measured on a Camebax-micro microprobe and on the ion microanalyzer IMS-4f in the standard regime.
The xenolith groundmass mainly consists of hypidiomorphic plagioclase grains (>85 vol %) with o r t h o p y r o x e n e , clinopyroxene, and amphibole in the interstices. Mafic minerals account for less than 15 vol %. A m p h i b o l e is observed as subordinate xenomorphic grains. Occasionally, orthopyroxene is partially replaced by secondary a m p h i b o l e along the margins.
In addition, plagioclase hosts individual crystals or groups of crystals of octahedral spinel. Microstructural relationships between minerals in the studied xenolith suggest the following sequence of crystallization: plagioclase-spinel-orthopyroxene- clinopyroxene-amphibole.
Based on X R F data the chemical composition of xenolilth (Sample Ud 90/4) is as follows (wt %): S i 02 48.0; T i 02 0.22;
A1203 22.4; F e203 5.0; M n O 0.13; M g O 6.8; C a O 15.0; N azO 1.33; K20 0.2; L.O.I. 0.78; total 99.86. In terms of mineralogy and petrochemistry, the xenolith corresponds to anorthosite association. Specific features of the chemical c o m p o s i t i o n of minerals in the xenolith are illustrated by microprobe data presented in the table.
Table. Chemical composition of minerals and melt inclusion in clinopyroxene from the anorthosite xenolith , wt %.
Component Plagioclase Orthopyroxene Clinopyroxene Amphibole Spinel Spinel * Melt
core rim core rim core rim core rim inclusion**
S i 02 47,4 47,2 52,5 52,8 49,6 50,1 40,6 40,9 0,06 0,07 58,1
T i 02 0,03 0,02 0,02 0,02 0,38 0,4 2,0 2,0 0,12 0,16 0,3
A1203 33,3 33,4 4,6 4,9 6,7 6,4 16,5 15,9 58,4 58,2 19,0
F e O 0,08 0,08 16,3 15,3 7,2 7,0 10,8 10,5 23,5 23,3 3,05
M n O 0,0 0,0 0,33 0,28 0,12 0,19 0,1 0,09 0,14 0,15 0,08
C r203 0,0 0,0 0,08 0,09 0,18 0,19 0,19 0,15 2,25 1,77 0,0
M g O 0,06 0,05 25,1 25,6 13,3 13,3 13,8 13,6 14,5 15,4 2,54
C a O 16,1 16,4 0,52 0,81 20,9 21,4 10,7 11,0 0,19 0,36 6,7
N a20 2,19 2,24 0,04 0,07 0,69 0,66 2,96 2,77 0,1 0,03 3,02
K20 0,05 0,04 0,0 0,0 0,02 0,0 0,6 0,8 0,0 0,0 4 , 3 6
Total 99,21 99,43 99,49 99,87 99,09 99,64 98,25 97,71 99,26 99,44 97,15 Ca/Ca+Na+K 0,80 0,80
Notes: * - lamellar spinel in clinopyroxene; ** - quenched glass of the homogenized (Thorn = 1120°C) melt inclusion in clinopyroxene.
It should be noted that all studied grains of plagioclase, clinopyroxene, and amphibole are compositionally h o m o g e n e o u s . However, they significantly differ from minerals of Precambrian autonomous anorthosite massifs located in the northern part of the Kotuikan-Monkhoolin zone and the Magan zone of the Anabar Shield (Sukhanov, 1984). Plagioclase (Ango) is compositionally similar to the calcic end member of this mineral group from the Anabar anorthosite. H o w e v e r , orthopyroxene and clinopyroxene significantly differ, primarily in terms of the A1203 content, f r o m those of the Anabar anorthosite (4.9 and 2.12 wt % in orthopyroxenes and 6.7 and 3.63 in clinopyroxenes, respectively). Amphibole is also enriched in A1203 relative to the Anabar anorthosite. In addition, some clinopyroxene grains contain spinel as by lamellar spinel an exsolution product. T h e octahedral spinel in plagioclase and lamellar spinel in clinopyroxene are compositionally similar (table). T h e lamellar spinel
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Acta Mineralogica-Petrographica, Abstract Series 2, Szeged, 2003
typically restricted to the c l i n o p y r o x e n e c o r e is o c c a s i o n a l l y d e v e l o p e d o v e r the entire grain. T h e higher A 1203 c o n t e n t in the s t u d i e d o r t h o p y r o x e n e relative to the A n a b a r a n o r t h o s i t e indicates its crystallization at a h i g h e r p r e s s u r e ( L o n g h i et al., 1993).
Partially and c o m p l e t e l y crystallized p r i m a r y m e l t inclusions and a s s o c i a t e d fluid i n c l u s i o n s w e r e f o u n d in c l i n o p y r o x e n e and s t u d i e d d u r i n g m i c r o s c o p i c i n v e s t i g a t i o n s o f plates. In s o m e c a s e s , in a d d i t i o n to n o r m a l melt i n c l u s i o n s (i.e. f o r m e d t h r o u g h h o m o g e n e o u s e n t r a p m e n t ) , c o m p o s i t e melt i n c l u s i o n s c o n t a i n i n g initially e n t r a p p e d a n o m a l o u s l y large fluid p h a s e ( h e t e r o g e n e o u s e n t r a p m e n t ) w e r e a l s o f o u n d . T h e associated fluid i n c l u s i o n s typically o c c u r together with the c o m p o s i t e m e l t inclusions. T h e m e l t i n c l u s i o n s in c l i n o p y r o x e n e c o n s i s t of several crystalline p h a s e s , residual glass (in partially c r y s t a l l i z e d inclusions), and fluid p h a s e , w h i c h o f t e n o c c u r s in the interstices b e t w e e n crystalline p h a s e s . T h e 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 ( u p to 2 0 urn) m a k e u p a z o n a l g r o u p s of 2 - 5 inclusions. T h e y a re h o m o g e n i z e d at 1 1 0 0 - 1 1 2 0 °C. T h e fluid i n c l u s i o n s in c l i n o p y r o x e n e are no m o r e than 15 Jim in size. At r o o m t e m p e r a t u r e , all fluid i n c l u s i o n s in c l i n o p y r o x e n e a r e m o n o p h a s e f o r m a t i o n s . C r y o m e t r i c i n v e s t i g a t i o n s s h o w e d that t h e y consist of liquid C 02. T h e h e t e r o g e n i z a t i o n d u r i n g their c o o l i n g in the f r e e z i n g s t a g e is a c c o m p a n i e d by the f o r m a t i o n of gas b u b b l e s . T h e f r e e z i n g t e m p e r a t u r e of liquid C 02 is less than that of C 02 triple p o i n t , w h e r e a s its m e l t i n g t e m p e r a t u r e is a l w a y s about - 5 6 . 6 °C, i n d i c a t i n g nearly p u r e C 02 c o m p o s i t i o n of the inclusions. I n c l u s i o n s in c l i n o p y r o x e n e are h o m o g e n i z e d into a liquid C 02 at t e m p e r a t u r e s r a n g i n g f r o m - 1 6 to - 9 °C a n d indicating C 02 d e n s i t y ( p ) r a n g i n g f r o m 1.01 to 0 . 9 8 g / c m3 ( V a r g a f t i k , 1972). T h e R a m a n s p e c t r o s c o p y of fluid i n c l u s i o n s a n d of fluid p h a s e in m e l t i n c l u s i o n s in c l i n o p y r o x e n e a l s o c o n f i r m e d their nearly p u r e C 02 c o m p o s i t i o n . Fluid p h a s e of m e l t i n c l u s i o n s lack r i m s of liquid water or w a t e r solution, s u g g e s t i n g the a b s e n c e of s i g n i f i c a n t water a c c u m u l a t i o n in the p a r e n t a l m e l t of anorthosites. S u c h h i g h d e n s i t y v a l u e s of liquid C 02 indicate that c l i n o p y r o x e n e crystallized in the m a g m a t i c s y s t e m u n d e r high fluid p r e s s u r e . A t t e m p e r a t u r e s of - 1 1 0 0 - 1 1 2 0 °C ( h o m o g e n i z a t i o n t e m p e r a t u r e of melt i n c l u s i o n s in c l i n o p y r o x e n e ) , the fluid p r e s s u r e is no less than 8.0 k b a r (intersection of the c o r r e s p o n d i n g i s o t h e r m s and i s o c h o r e s in t h e C 02
s y s t e m ) . A n a l o g o u s p r e s s u r e e s t i m a t e s w e r e o b t a i n e d f r o m e x p e r i m e n t a l data o n solubility of the C a - T s c h e r m a k m o l e c u l e c o m p o n e n t in c l i n o p y r o x e n e associated with anorthite, plagioclase, o r t h o p y r o x e n e , and spinel in the C a 0 - M g 0 - A l203- S i 02 s y s t e m .
T h e c o m p a r i s o n the c h e m i s t r y of a n o r t h o s i t e xenolith and q u e n c h e d glasses f r o m the h o m o g e n i z e d m e l t i n c l u s i o n s in c l i n o p y r o x e n e f r o m the s a m e s a m p l e r e v e a l e d a distinct e n r i c h m e n t of residual melt in S i 02 (up to 58.1 wt % ) , N a20 + K20 ( u p to 7 . 3 8 wt %), a n d e s p e c i a l l y K20 (up to 4 . 3 6 wt % ) at the m o m e n t of c l i n o p y r o x e n e crystallization. S i m u l t a n e o u s l y , the m e l t is d e p l e t e d in F e O , M g O , and C a O .
P l a g i o c l a s e and o r t h o p y r o x e n e c o n t a i n o n l y m o n o p h a s e liquid C 02 i n c l u s i o n s that a re similar to t h o s e in c l i n o p y r o x e n e . T h e y are up to 3 0 ^ m in size. T h e i r h o m o g e n i z a t i o n t e m p e r a t u r e s r a n g e s f r o m - 1 9 to - 1 2 °C ( p = 1 . 0 3 - 0 . 9 9 g / c m3) in p l a g i o c l a s e and f r o m - 1 6 to -9 °C ( p = 1.01-0.98 g / c m3) in o r t h o p y r o x e n e . T h e largest fluid inclusions u n i v e r s a l l y e x h i b i t e v i d e n c e of partial o p e n i n g a n d partial l e a k a g e of the s u b s t a n c e (This is s u g g e s t e d b y the p r e s e n c e of h a l o s of s e c o n d a r y tiny fluid inclusions). T h e r e f o r e , the a b o v e d e n s i t y values of t h e s e inclusions are the l o w e s t e s t i m a t e s . T h e m e l t i n g t e m p e r a t u r e of solid C 02 is also - 5 6 . 6 °C. B a s e d on R a m a n s p e c t r o s c o p y data, fluid i n c l u s i o n s in p l a g i o c l a s e and o r t h o p y r o x e n e , like t h o s e in c l i n o p y r o x e n e , c o n t a i n p u r e C 02.
T h u s , the d e t e c t i o n and s t u d y of melt i n c l u s i o n s s h o w e d that the a nor th o s it e s crystallized f r o m a h i g h - C a and h i g h - A l m e l t at t e m p e r a t u r e higher than 1 1 0 0 °C and fluid p r e s s u r e n o less than 8 kbar. T h e o b t a i n e d d a t a support the m a g m a t i c o r i g i n of a n o r t h o s i t e s . T h e pattern of distribution of t r a c e e l e m e n t s in m i n e r a l s f r o m a n o r t h o s i t e , as well as in melt i n c l u s i o n s in c l i n o p y r o x e n e a n d high h o m o g e n i z a t i o n t e m p e r a t u r e s of melt i n c l u s i o n s indicate that the a no rt hos i t e s c o u l d not b e f o r m e d d u r i n g the m e l t i n g of crustal r o c k s u n d e r the g r a n u l i t e - f a c i e s m e t a m o r p h s i m .
T h i s w o r k w a s s u p p o r t e d by the 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 , p r o j e c t s no. 0 0 - 0 5 - 6 5 4 1 8 , 0 3 - 0 5 - 6 4 8 6 4
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