2 mm 0 sec 60 sec 120 sec 150 sec Time Fig. 1. The observed images detected by a high-speed CCD camera at 3.3 GPa and 1600 ° C. The measurable window was about 2 mm, because of the narrow anvil g a p d u e t o t h e c o m p r e s s i o n . E a c h f r a m e o f t h e s e images was captured at intervals of 1/30 s. An un de rs ta nd in g of th e vi sc os it y of s i l i c a t e m e l t s u n d e r h i g h - p r e s s u r e s i s essential in contemplating the behavior of magma and volcanic eruptions. A variety o f s i l i c a t e m e l t s h a v e b e e n i n v e s t i g a t e d , leading to the conclusion that the viscosity o f h i g h l y p o l y m e r i z e d s i l i c a t e m e l t s d e c r e a s e s w i t h i n c r e a s i n g p r e s s u r e , i n sh a rp co n tr a st to th e b e h a vi o r o f n o rm a l l i q u i d s [ 1 ] . T h u s f a r , t h e v i s c o s i t y h a v e b e e n m e a s u r e d u s i n g a q u e n c h - f a l l i n g s p h e r e m e t h o d , i n w h i c h t h e t e r m i n a l si nk in g ve lo ci ty is de te rm in ed by al te ri ng t h e q u e n c h r a t e [ 2 - 4 ] . I n t h i s m e t h o d , however, the determination of the terminal velocity may involves uncertainties, due to th e li mi ta ti on of th e si nk in g di st an ce an d the quench rate. The use of synchrotron radiation has enabled in situ observations of th e fa l l i ng sp he re by i mp l em en ti ng an X - r a y r a d i o g r a p h y t e c h n i q u e . T h i s n e w m e t h o d h a s m a n y a d v a n t a g e s o v e r t h e t r a d i t i o n a l q u e n c h - f a l l i n g s p h e r e m e t h o d [ 4 , 5 ] : ( i ) t h e p r e c i s e t e r m i n a l v e l o c i t y o f the falling sphere can be obtained, ( ii ) P-T condition is experimentally determined by combining in situ X-ray diffraction, and ( iii ) l o w - v i s c o s i t y m e l t s c a n b e m e a s u r e d . H e r e , w e r e p o r t a n i n s i t u v i s c o s i t y m e a s u r e m e n t u n d e r h i g h p r e s s u r e u s i n g a n X - r a y r a d i o g r a p h y f a l l i n g s p h e r e method. The first trial was performed on a l b i t e m e l t , w h i c h i s o n e o f t h e m o s t important silicate melts. VISCOSITY MEASUREMENTS OF ALBITE MELT UNDER HIGH-PRESSURE USING AN IN SITU X-RAY RADIOGRAPHY TECHNIQUE Fig. 2. Sinking distance of a Pt sphere in albite melt as a function of time (4 GPa, 1700 ° C). We ha ve se t up an in si tu vi sc os it y me as ur em en t s y s t e m c o m b i n e d w i t h a m u l t i - a n v i l a p p a r a t u s a t SPring-8 [6] . The system has been installed on a lar ge vol ume mul ti- anv il app ara tus (SP EED -15 00) at beamline BL04B1 [7] . Pressure is generated by a double-stage system with tungsten carbide cubes w i t h a t r u n c a t i o n e d g e l e n g t h o f 1 2 m m . T h e i n c i d e n t w h i t e X - r a y f r o m t h e b e n d i n g m a g n e t i r r a d i a t e s t h e s a m p l e c e l l t h r o u g h t h e a n v i l g a p , a n d a n i m a g e o f t h e s a m p l e i s p r o j e c t e d o n t h e fluorescence screen. This image is then magnified an d de te ct ed by a hi gh -s pe ed CC D ca me ra . Fo r this experiments, a Pt sphere with a radius of 50 - 80 μ m was embedded in the upper part of the albite sample. A fine powdered mixture of MgO and BN was filled surround the sample capsule as the inner pressure marker, and the pressure was calculated f r o m t h e o b s e r v e d l a t t i c e c o n s t a n t o f M g O . A 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 0 5 10 15 20 25 Time (sec) Sinking distance ( mm ) v = 0.029815 mm /sec Pt sphere radius: 70 μ m Terminal velocity 30 thermocouple was placed on the top of the sample capsule. The sample was first compressed at the r o o m t e m p e r a t u r e , f o l l o w e d b y h e a t i n g a t a constant applied load. To avoid the differentiation e f f e c t o r p a r t i a l m e l t i n g , t h e c o m p r e s s e d s a m p l e w a s f i r s t a n n e a l e d a t 1 0 0 0 ° C , a n d t h e n r a m p i n g w a s p e r f o r m e d t o r e a c h t h e t a r g e t t e m p e r a t u r e ( 1 6 0 0 ° C a n d 1 7 0 0 ° C ) . T h e h e a t i n g r a t e w a s r e g u l a t e d t o b e a b o u t 2 0 0 ° C / s e c o n d . O n c e t h e t a r g e t t e m p e r a t u r e w a s a t t a i n e d , t h e P t s p h e r e bega n to fall into the melt . The obse rved imag es from one of the series (3.3 GPa and 1600 ° C) are s h o w n i n F i g . 1 . T h e m e a s u r a b l e w i n d o w w a s about 2 mm, because of the narrow anvil gap due to compression. Each frame of these images was c a p t u r e d a t i n t e r v a l s o f 1 / 3 0 s e c o n d . T h e h i g h - speed and high-resolution CCD camera allowed for v e r y g o o d v i s u a l c o n t r a s t b e t w e e n t h e P t s p h e r e Fig. 3. Comparisons of the pressure dependence of the albite melt viscosity determined by in situ and quench experiments. Kenichi Funakoshi SPring-8 / JASRI E-mail: funakoshi @ spring8.or.jp 1.0 2.0 3.0 4.0 5.0 6.0 0.0 0 1 2 3 4 5 6 Pressure ( GPa ) 1400 ° C ( Q ) Kushiro (1978) Q: Quench experiment 1600 ° C ( Q ) B r e a l e y e t a l . ( 1 9 8 6 ) & K a n z a k i e t a l . ( 1 9 8 7 ) 1700 ° C ( Q ) K a n z a k i e t a l . ( 1 9 8 7 ) 1600 ° ° C 1700 ° ° C Viscosity, Log (poise) and albite melt possible in a short exposure time. Th es e me as ur em en ts we re ca rr ie d ou t un de r the seve ral P-T cond itio ns up to 5.3 GPa at 1600 ° C and 1700 ° C. To determine the terminal velocity of the sinking sphere, we analyzed the images and ob ta in ed th e ge om et ri ca l ce nt er po si ti on of th e Pt s p h e r e f r o m e a c h c a p t u r e d f r a m e . T h e s e t t l e d distance (4 GPa and 1700 ° C) is plotted in Fig. 2 as a function of time. At the moment the temperature reached 1700 ° C, the Pt sphere (with a radius of 70 μ m ) b e g a n to si n k sl o w l y a n d sa n k a t a co n st a n t velocity after 10 seconds. We used the linear part of the plot and determined the terminal velocity of t h e P t s p h e r e u s i n g a l i n e a r l e a s t s q u a r e calculation. The viscosity was calculated from this velocity using Stokes’ equation, including the Faxen correction for the wall effect [2,8] . T h e v i s c o s i t i e s a r e s u m m a r i z e d i n F i g . 3 , together with the data obtained by previous quench ex pe ri me nt s. Th e er ro r of ou r vi sc os it y va lu es is e s t i m a t e d w i t h i n 1 . 5 p o i s e . A s s h o w n i n t h i s fi gu re , ou r va lu es cl ea rl y in di ca te th e de cr ea se of t h e v i s c o s i t y w i t h i n c r e a s i n g p r e s s u r e , w h i c h i s s i g n i f i c a n t l y l o w ( w i t h n o m o r e t h a n o n e o r d e r o f p o i s e ) c o m p a r e d w i t h t h o s e d e t e r m i n e d b y t h e q u e n c h s t u d i e s . F u r t h e r m o r e , a t 1 7 0 0 ° C , t h e minimum viscosity is clearly seen to be around 4-5 GPa, which is consistent with the diffusivity results [ 9 ] , t h e r e f o r e s u g g e s t i n g t h a t s o m e s t r u c t u r a l changes may occur at this pressure range. References [ 1 ] C . M . S c a r f e e t a l . , M a g m a t i c p r o c e s s : Ph ys ic oc he mi ca l Pr in ci pl es 1 (1 98 7) 59 , ( Ge oc he m. Soc. Univ. of Park, Pennsylvania.) [2] I . Ku sh ir o, Ea rt h Pl an et . Sc i. L et t. 41 ( 19 78 ) 87 . [3] M. Brearley et al., Geochim. Cosmochim. Acta 50 (1986) 2563. [4 ] M. Ka nz ak i, Ph . D. Th es is , Th e Un iv er si ty of Tokyo (1987). [5] D.P. Dobson et al., Earth Planet. Sci. Lett. 143 (1996) 207. [ 6 ] K . F u n a k o s h i , M . K a n z a k i , A . Y a s u d a , A . S u z u k i , H . T e r a s a k i a n d S . Y a m a s h i t a , S c i e n c e a n d T e c h n o l o g y o f H i g h P r e s s u r e , P r o c . o f AIRAPT-17, 2 (2000) 1023. [ 7 ] W . U t s u m i e t a l . , R e v . H i g h P r e s s u r e S c i . Technol. 7 (1998) 1484. [8] H.R. Shaw, J. Geophys. Res. 68 (1963) 6337. [9] B.T. Poe et al. , Science 276 (1997) 1245.
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