Mg/Si Ratios of Aqueous Fluids in Earth’s Upper Mantle W a t e r i n f l u e n c e s m o s t p h y s i c o c h e m i c a l characteristics of earth-forming materials: melting temperature, partial melt chemistry, density and viscosity, electric conductivity, melt extraction, phase transition boundaries and element distribution. Aqueous fluids can dissolve significant amounts of silicate components under high pressure and high temperature conditions. Such fluids can act, therefore, as transfer agents during metamorphism, magmatism, and volcanism. Therefore, knowledge of the chemical compositions of silicates dissolved in aqueous fluids is essential to the understanding of the effects of H 2 O on the physicochemical properties of rocks. A direct observation of aqueous fluids coexisting with MgSiO 3 enstatite and/or Mg 2 SiO 4 O O forsterite is performed at 0.5 - 5.8 GPa and 800 - 1000 C with a Bassett-modified externally heated diamond anvil cell [1] and synchrotron X-rays at beamline BL04B2 [ 2]. Rigorous temperature control maintains the temperature of each diamond to be within a few C. Pressure was determined from the measured unit-cell volume of gold foils using the P-V-T equation of state for gold. Use of this synchrotron X-ray system enables the mineral coexisting with aqueous fluids under high pressure and high temperature conditions to be identified and the Mg/Si ratios of aqueous fluids coexisting with MgSiO 3 enstatite and Mg 2 SiO 4 Fig. 1. ( a) E xperimental results showing Mg/Si ratios of silicate components in aqueous fluids coexisting with enstatite (En) and forsterite (Fo). The values of 1200 C at 5 GPa and 1000 C and 1100 C at 3 GPa are obtained from previous studies [4,5]. (b, c) Schematic isothermal sections of Mg 2 SiO 4 -SiO 2 -H 2 O system at 1000 C. Stars represent possible H 2 O fluids coexisting with Fo and En. At pressures lower than 3 GPa, the aqueous fluids (F) coexisting with En and Fo have Mg/Si < 1, and at pressures higher than 3 GPa, they have 1 < Mg/Si < 2. fo rsterite, which are the main mineral constituents of Earth’s upper mantle, to be estimated. Possible Mg/Si ratios of aqueous fluids coexisting with MgSiO 3 en statite and Mg 2 SiO 4 O O forsterite are shown in Fig. 1(a) [3]. In the pressure range of 0.5 - 2.8 GPa, Mg 2 SiO 4 O O forsterite crystallizes at 1000 C in the MgSiO 3 -H 2 O system. This indicates that the aqueous fluids can dissolve more SiO 2 -rich components than MgO-rich components, resulting in Mg/Si atomic ratio of less than unity (Fig. 1 (b)). In contrast, at pressures greater than 3.9 GPa, MgSiO 3 en statite dissolves congruently at 1000 C, whereas Mg 2 SiO 4 O O forsterite dissolves congruently up to 5.0 GPa. These data suggest that the aqueous fluids coexisting with Mg 2 SiO 4 O O enstatite and Mg 2 SiO 4 O O forsterite can have Mg/Si ratios in the range 1 Mg/Si < 2 under these conditions (Fig. 1(c)). In the present experimental (c) En+Fo +F ( b) E n+Fo +F Pressure (GPa) (a) Temperature ( C) 800 9 00 1000 1100 1200 6 5 4 3 2 1 0 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 86 8 8 8 8 8 8 86 86 86 86 86 86 86 86 86 86 86 86 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 = Fig. 2. (a) Pressure and Mg/Si ratios of aqueous fluids coexisting with forsterite and enstatite at a temperature range from 1000 to 1100 C [4]. (b) Comparison of aqueous fluid chemistry with partial melt chemistry at 1050 - 1100 C in a mantle peridotite model of 13.7 weight % H 2 O [7]. References [1] W.A. Bassett et al. : R e v . Sci . In st rum . 64 ( 1 993) 2340 . [ 2 ] M . I ss hiki et al. : Nucl . In st rum . M et h . Phy s. R es. A 467 - 468 (200 1 ) 663 . [ 3 ] T . K a w a mo t o, K . N . M ats uk a g e , K . Mib e , K . Ni s himur a , M . I ss hiki, N . I s him ats u, a nd S . Ono: t o b e publi s h e d in A m . Min e r a l (Oc t ob e r, 2004) . [ 4 ] K . Mib e et al. : G e ochim . Co s mochim . A c ta 66 (2002) 2273, a nd r e f e r e nc es ci te d h e r e in . [ 5 ] I . Ku s hiro et al. : G e ol . Soc . A m . B ull . 79 ( 1 968) 1 685 . [ 6 ] T . K a w a mo t o et al. : J . Ch e m . Phy s. 120 (2004) 5867 . [ 7 ] T . K a w a mo t o a nd J . R . Hollow a y: Sci e nc e 276 ( 1 997) 240 . T ats uhiko K a w a mo t o Gr a du ate School of Sci e nc e , Kyo t o Univ e r s i t y Em a il: k a w a mo t o@b e p . vg s. kyo t o-u .a c . jp configur at ion s , t h e Mg/Si r at io s of a qu e ou s fluid s a r e fix e d by t h e sta r t ing m ate ri a l . As w e see in Fig . 1 (b,c), t h e fluid compo s i t ion s c a n ch a ng e a long t h e univ a ri a n t curv e. Th e r e for e , in t h e pr ese n t e xp e rim e n ts , w e c a nno t d ete rmin e t h e Mg/Si r at io s bu t c a n only s ugg est t h e ch e mic a l r a ng es of t h e a qu e ou s fluid s co e xi st ing wi t h e n stat i te a nd for ste ri te : Mg/Si < 1 or 1 < Mg/Si < 2 . Th e pr ese n t e xp e rim e n ta l d ata a nd t h e d ata from pr e viou s qu e nch e xp e rim e n ts [ 4 ] s ugg est t h at t h e Mg/Si r at io s of a qu e ou s fluid s c a n ch a ng e dr ast ic a lly from SiO 2 -rich t o MgO-rich r e gim e at a round 3 GP a (Fig . 2( a )) . Th e origin of s uch a dr ast ic ch a ng e r e m a in s t o b e inv est ig ate d in t h e fu t ur e. Liquid H 2 O m a y ch a ng e i ts st ruc t ur e und e r t h ese condi t ion s a nd c a u se t h e ch a ng e in s olu t ion ch e mi st ry [ 6 ]. Thi s s p e cul at ion i s b ase d on t h e di s con t inui t y in t h e pr ess ur e d e p e nd e nc e of t h e R a m a n OH st r et ching fr e qu e ncy of w ate r at 0 . 4 0 .1 GP a at 25 C, 1. 0 0 .1 GP a at 1 00 C a nd 1. 3 0 .1 GP a at 300 C . Th e ch e mic a l compo s i t ion s of p a r t i a l m e l ts of H 2 O- sat ur ate d m a n t l e p e rido t i te w e r e a l s o ch a r a c te riz e d by high Mg/Si r at io s [ 7 ]. Th e a qu e ou s fluid s t h at co e xi st wi t h m a n t l e min e r a l s h a v e Mg/Si r at io s s imil a r t o t ho se found in t h e p a r t i a l m e l ts of hydrou s p e rido t i te (Fig . 2(b)) . Som e wh e r e wi t hin t h e upp e r m a n t l e t h ese t wo fluid s uni te t o form a s ingl e r e gim e a nd a con t inuou s ch a ng e b et w ee n t h ese t wo fluid s occur s. Th e r e for e , t h e t r a di t ion a l H 2 O- sat ur ate d s olidu s te mp e r at ur e m a y r e pr ese n t a te mp e r at ur e wh e r e t h e conc e n t r at ion s of s ilic ate compon e n ts di ss olv e d in t o a qu e ou s fluid s incr ease dr ast ic a lly a nd m a y t h e r e for e ju st ify i ts d es crip t ion as a “pr a c t ic a l s olidu s. ” Pressure (GPa) 6 4 2 0 8 10 12 0.5 3.0 1.0 0.0 2.0 1.5 2.5 3.5 (Mg+Fe)/Si Atomic Ratios Partial melt compositions Pressure (GPa) 6 4 2 0 8 10 12 (Mg+Fe)/Si Atomic Ratios 0.5 3.0 1.0 0.0 2.0 1.5 2.5 (a) Mibe et al. , 1100 C Stalder et al. , 1000 C Stalder et al. , 1100 C This study, 1000 C Zhang & Frantz, 1000-1100 C Ryabchikov et al. , 1000-1100 C ゜ ゜ ゜ 87
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