10 D e n s i t y D i f f e r e n c e b e t w e e n S u b d u c t e d O c e a n i c C r u s t a n d A m b i e n t M a n t l e i n t h e M a n t l e T r a n s i t i o n Z o n e D e n s i t y D i f f e r e n c e b e t w e e n S u b d u c t e d O c e a n i c C r u s t a n d A m b i e n t M a n t l e i n t h e M a n t l e T r a n s i t i o n Z o n e S i n c e t h e b e g i n n i n g o f p l a t e t e c t o n i c s , t h e oc ea ni c li th os ph er e ha s be en co nt in ua ll y su bd uc te d i n t o t h e E a r t h ’ s d e e p m a n t l e f o r 4 . 5 G y . T h e o c e a n i c l i t h o s p h e r e c o n s i s t s o f a n u p p e r b a s a l t i c l a y e r ( o c e a n i c c r u s t ) a n d a l o w e r o l i v i n e - r i c h p e r i d o t i t i c l a y e r . T h e t o t a l a m o u n t o f s u b d u c t e d oceanic crust in this 4.5 Gy period is estimated to be at least ~3 × 23 kg, which is about 8% of the w e i g h t o f t h e p r e s e n t E a r t h ’ s m a n t l e . T h u s , t h e oc ea ni c cr us t, wh ic h is ri ch in pyroxene and garnet, m a y b e t h e s o u r c e o f v e r y i m p o r t a n t c h e m i c a l heterogeneity in the olivine-rich Earth’s mantle. To c l a r i f y b e h a v i o r o f t h e o c e a n i c c r u s t i n t h e d e e p m a n t l e , a c c u r a t e i n f o r m a t i o n a b o u t d e n s i t y d i f f e r e n c e s b e t w e e n t h e o c e a n i c c r u s t a n d t h e ambient mantle is very important. W e d e t e r m i n e d p r e s s u r e - v o l u m e - t e m p e r a t u r e r e l a t i o n s ( P - V - T e q u a t i o n o f s t a t e ) o f r e l a t e d m i n e r a l s a t h i g h - p r e s s u r e and high-temperature conditions, to precisely evaluate the density difference. Garnet and ringwoodite are the most abundant minerals in the oceanic crust and the ambient mantle, re sp ec ti ve ly , in th e co nd it io ns of th e ma nt le t r a n s i t i o n z o n e ( 4 1 0 – 6 6 0 k m d e e p ) . H o w e v e r , P - V - T e q u a t i o n o f s t a t e f o r t h e n a t u r a l co mp os it io ns of th es e mi ne ra ls ar e no t we ll constrained. W e c o n d u c t e d i n s i t u X - r a y d i f f r a c t i o n e x p e r i m e n t s a t u p t o 2 1 G P a a n d 1 2 7 3 K , u s i n g t h e S P E E D - 1 5 0 0 m u l t i - a n v i l h i g h - p r e s s u r e a p p a r a t u s i n s t a l l e d a t b e a m l i n e BL04 B1 [1], and simu ltan eous ly dete rmin ed P - V - T equ ati ons of sta te for maj ori te gar net w i t h o c e a n i c c r u s t c o m p o s i t i o n a n d r i n g w o o d i t e w i t h n a t u r a l F e - b e a r i n g c o m p o s i t i o n . T h e garnet and the ringwoodite were synthesized f r o m n a t u r a l a b y s s a l b a s a l t a n d n a t u r a l olivine, respectively. These starting materials Fig. 1. Pressure-volume-temperature data for garnet w i t h o c e a n i c c r u s t c o m p o s i t i o n w i t h c a l c u l a t e d i s o t h e r m a l c o m p r e s s i o n c u r v e s . C l o s e d a n d o p e n c i r c l e s d e n o t e d a t a c o l l e c t e d u s i n g N a C l a n d M g O capsules, respectively. The isotherms (from lower cell volumes, 300, 473, 673, 873, 1073 and 1273 K) are fit to high-temperature equation of state. were packed separately into NaCl or MgO sample c h a m b e r w i t h a m i x t u r e o f g o l d a n d M g O , a n d co mp re ss ed in th e sa me hi gh -p re ss ur e ce ll . Th e p r e s s u r e w a s d e t e r m i n e d b y t h e c e l l v o l u m e f o r gold using an equation of state of gold [2], and the t e m p e r a t u r e w a s m e a s u r e d b y a t h e r m o c o u p l e . T h e c e l l v o l u m e s o f g a r n e t a n d r i n g w o o d i t e w e r e determined by least squares calculations using the pos iti ons of X-r ay dif fra cti on pea ks. The sam ple s were compressed to the desired pressure at room t e m p e r a t u r e a n d h e a t e d t o t h e m a x i m u m t e m p e r a t u r e to release non-hydrostatic stress. P r e s s u r e - v o l u m e - t e m p e r a t u r e d a t a w e r e c o l l e c t e d under 47 different conditions. Figure 1 shows P - - V T data for garnet. The data collected by using an NaCl capsule after heating to above 873 K and the 67 N a C l c a p s u l e M g O c a p s u l e G a r n e t 0 5 1 0 1 5 2 0 2 5 P r e s s u r e ( G P a ) C e l l V o l u m e ( Å 3 ) 1 6 0 0 1 5 5 0 1 5 0 0 1 4 5 0 1 4 0 0 F i g . 2 . D e n s i t y c h a n g e s i n t h e o c e a n i c c r u s t a n d t h e a m b i e n t m a n t l e ( a ) a g a i n s t d e p t h a l o n g t h e g e o t h e r m a n d ( b ) a g a i n s t t e m p e r a t u r e a t a d e p t h o f 6 0 0 km (pressure is 21.04 GPa ). The bol d sol id and das hed lin es are t h e o c e a n i c c r u s t a n d t h e p y r o l i t i c m a n t l e , r e s p e c t i v e l y . S o l i d c i r c l e s a n d o p e n s q u a r e s a r e g a r n e t i n t h e o c e a n i c c r u s t a n d r i n g w o o d i t e i n t h e m a n t l e , r e s p e c t i v e l y , w h i c h a r e c a l c u l a t e d u s i n g p r e s e n t P - V - T e q u a t i o n s o f s t a t e . S e i s m i c o b s e r v a t i o n s a r e s h o w n a s t h i n s o l i d ( P R E M ) a n d d a s h e d ( ak135 ) lines. Fig. 2 References [1] W. Utsumi et al ., Rev. High Pres. Sci. Tech. 7 (1998) 1484. [ 2 ] O . L . A n d e r s o n e t a l . , J . A p p l . P h y s . 6 5 ( 1 9 8 9 ) 1 5 3 4 . [ 3 ] Y . W a n g e t a l . , P h y s . E a r t h P l a n e t . I n t . 1 0 5 (1998) 59. [ 4 ] L . Z h a n g e t a l . , P h y s . C h e m . M i n e r a l s 2 5 (1998) 301. [5] Y. Meng et al ., Phys. Chem. Minerals 21 (1994) 407. data collected using an MgO capsule after heating t o 1 2 7 3 K s h o w e d c o n s i s t e n t r e s u l t s , a n d t h e s a m p l e conditions are considered to be nearly hydrostatic. Thu s, we cou ld det erm ine the P - V - T P - V - T equ ati ons of state for garnet and ringwoodite precisely. In spite of its high Si, Ca, Fe and Na contents, th e pr es en t P - V - T equ at io n of sta te for gar ne t sh ow s e x c e l l e n t a g r e e m e n t w i t h t h a t o f M g 3 A l 2 S i 3 O 1 2 g a r n e t [ 3 , 4 ] . T h e p r e s e n t e q u a t i o n o f s t a t e f o r r i n g w o o d i t e y i e l d s s i g n i f i c a n t l y h i g h e r t h e r m a l e x p a n s i v i t y t h a n t h a t d e r i v e d b y d i a m o n d a n v i l e x p e r i m e n t s o n M g 2 S i O 4 r i n g w o o d i t e t o 3 0 G P a a n d 7 0 0 K [ 5 ] . T h e d i s c r e p a n c y m a y b e d u e t o lim ite d tem per atu re ran ge (70 0 K at max imu m) in the previous study. T h e d e n s i t y o f o c e a n i c c r u s t a n d a m b i e n t mantl e the mantl e trans ition zone were calcu lated b y u s i n g p r e s e n t r e s u l t s , p r e v i o u s l y r e p o r t e d th er mo el as ti c pa ra me te rs of re la te d mi ne ra ls an d p h a s e r e l a t i o n s i n o c e a n i c c r u s t a n d p y r o l i t i c mantle ( ). Because equations of state for garnet and ringwoodite were determined in the s a m e h i g h - p r e s s u r e e x p e r i m e n t s , t h e c a l c u l a t e d de ns it y di ff er en ce be tw ee n th e oc ea ni c cr us t an d th e am bi en t ma nt le in th e lo we r pa rt of th e tr an si ti on zone (520 – 660 km deep) is considered to be valid e v e n a f t e r c o n s i d e r i n g f l u c t u a t i o n s i n t h e m a n t l e t e m p e r a t u r e s , a n d i n a c c u r a c y o f p r e s s u r e d e t e r m i n a t i o n i n t h e P - V - T e x p e r i m e n t s , p o s s i b l e e r r o r i n m i n e r a l p h y s i c s p a r a m e t e r s u s e d , e t c . A c c o r d i n g l y , t h e o c e a n i c c r u s t i s d e n s e r t h a n t h e a m b i e n t m a n t l e a c r o s s t h e e n t i r e r a n g e o f t h e ma nt le tr an si ti on zo ne al on g ge ot he rm s. Pr es en t re su lt s pr ov id e im po rt an t in fo rm at io n fo r cl ar if yi ng the be hav ior of th e o cea nic cr ust in th e d eep ma ntl e. Yu Nishihara * and Eiichi Takahashi Tokyo Institute of Technology *present address : Yale University, U.S.A. E-mail: yu.nishihara @ yale.edu Temperature (K) Depth ( km ) 68 1 2 0 0 1 6 0 0 2 0 0 0 D e p t h = 6 0 0 k m P = 2 1 . 0 4 G P a ( b ) O c e a n i c c r u s t M a n t l e G a r n e t R i n g w o o d i t e P R E M a k 1 3 5 3 . 4 3 . 6 3 . 8 4 . 0 4 . 2 4 . 4 D e n s i t y ( g c m - 3 ) ( a ) 4 0 0 5 0 0 6 0 0 7 0 0
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