09:30 〜 09:45
[SIT20-09] Fluid segregation and chemical compaction through efficient solute transport along wet grain boundaries
キーワード：流体移動、化学圧密、石英多結晶体
Segregation of geological fluids such as supercritical aqueous fluids and silicate melts is an essential elementary process for volatile cycles in subduction zones. To explain the mechanism of fluid segregation, various processes have been examined including matrix deformation1, 2, interfacial tension driven fluid redistribution3, 4, 5 and melt rock reaction6, 7. A common feature in these processes is that fluid migrates through interconnected networks along the grain boundaries and channels. In this presentation, we will report a model of efficient fluid segregation which occurs without fluid interconnections.
The CHO fluid-bearing quartz aggregates were synthesized with nominal fluid fractions between 0 and 0.18 from powdered mixture of Arkansus quartz and amorphous silica prepared by sol-gel method. The mixture was hot-pressing in a piston–cylinder apparatus at 900℃ and 1.0 GPa for 24–382 hours for XCO2 = 0–0.44. The recovered run products were imaged with synchrotron X-ray microtomography and the porosity was measured. The quartzites with high initially added fluid fraction (φadd > 0.056) tend to consist of fluid rich section and fluid poor section, which irrelevantly locate to top and bottom of the run products. The fluid poor sections retained very small fluid fractions (< 0.01) regardless of the high added fluid fraction. Partially deformed platinum sleeves and nickel capsules demonstrate that fluids were squeezed out from this region at the initial stage of the segregation. This deformation-assisted fluid segregation would continue until the fluid interconnection was pinched off. The pinch-off fluid fractions of our samples were 0.031–0.037 for XCO2 = 0 and 0.044–0.048 for XCO2 = 0.28–0.44, implying the occurrence of the second stage fluid segregation to form the very dense section of quartzite after the deformation. We calculated diffusive flux of the dissolved SiO2 driven by the difference in the fluid pressure between the fluid rich section and the fluid poor section that lacks fluid interconnection considering the matrix viscosity. We found that the silica flux from the fluid poor to rich sections was 105 times larger than that expected from the reported grain boundary diffusivity. Our results imply that efficiency of the fluid segregation through silica redistribution along the wet or transient grain boundaries is comparable to the fluid segregation along interconnected fluid networks driven by interfacial tension8 and deformation9 when the spatial scale is less than ~0.1 meter.
Reference:
1. Mckenzie, D. (1984). The generation and compaction of partially molten rock. J. Petrol., 25, 713–765.
2. Holtzman, B. K., & Kohlstedt, D. I. (2007). Stress-driven melt segregation and strain partitioning in partially molten rocks: Effects of stress and strain. J. Petrol., 48, 2379–2406.
3. Stevenson, D. J. (1986). On the role of surface tension in the migration of melts and fluids. Geophys. Res. Lett., 13, 1149–1152.
4. Watson, E. B. (1982). Melt infiltration and magma evolution. Geology, 10, 236–240.
5. Nakamura, M., & Watson, E. B. (2001). Experimental study of aqueous fluid infiltration into quartzite: Implications for the kinetics of fluid redistribution and grain growth driven by interfacial energy reduction. Geofluids, 1, 73–89.
6. Daines, M. J., & Kohlstedt, D. L. (1994). The transition from porous to channelized flow due to melt / rock reaction during melt migration. Geophys. Res. Lett., 21, 145–148.
7. Pec, M., Holtzman, B. K., Zimmerman, M., & Kohlstedt, D. L. (2015). Reaction infiltration instabilities in experiments on partially molten mantle
rocks. Geology, 43, 575–578.
8. Takei, Y., & Hier-Majumder, S. (2009). A generalized formulation of interfacial tension driven fluid migration with dissolution/precipitation. Earth. Planet. Sc. Lett., 288, 138–148.
9. Stevenson, D. J. (1989). Spontaneous small-scale melt segregation in partial melts in undergoing deformation. Geophys. Res. Lett., 16, 1067–1070.
The CHO fluid-bearing quartz aggregates were synthesized with nominal fluid fractions between 0 and 0.18 from powdered mixture of Arkansus quartz and amorphous silica prepared by sol-gel method. The mixture was hot-pressing in a piston–cylinder apparatus at 900℃ and 1.0 GPa for 24–382 hours for XCO2 = 0–0.44. The recovered run products were imaged with synchrotron X-ray microtomography and the porosity was measured. The quartzites with high initially added fluid fraction (φadd > 0.056) tend to consist of fluid rich section and fluid poor section, which irrelevantly locate to top and bottom of the run products. The fluid poor sections retained very small fluid fractions (< 0.01) regardless of the high added fluid fraction. Partially deformed platinum sleeves and nickel capsules demonstrate that fluids were squeezed out from this region at the initial stage of the segregation. This deformation-assisted fluid segregation would continue until the fluid interconnection was pinched off. The pinch-off fluid fractions of our samples were 0.031–0.037 for XCO2 = 0 and 0.044–0.048 for XCO2 = 0.28–0.44, implying the occurrence of the second stage fluid segregation to form the very dense section of quartzite after the deformation. We calculated diffusive flux of the dissolved SiO2 driven by the difference in the fluid pressure between the fluid rich section and the fluid poor section that lacks fluid interconnection considering the matrix viscosity. We found that the silica flux from the fluid poor to rich sections was 105 times larger than that expected from the reported grain boundary diffusivity. Our results imply that efficiency of the fluid segregation through silica redistribution along the wet or transient grain boundaries is comparable to the fluid segregation along interconnected fluid networks driven by interfacial tension8 and deformation9 when the spatial scale is less than ~0.1 meter.
Reference:
1. Mckenzie, D. (1984). The generation and compaction of partially molten rock. J. Petrol., 25, 713–765.
2. Holtzman, B. K., & Kohlstedt, D. I. (2007). Stress-driven melt segregation and strain partitioning in partially molten rocks: Effects of stress and strain. J. Petrol., 48, 2379–2406.
3. Stevenson, D. J. (1986). On the role of surface tension in the migration of melts and fluids. Geophys. Res. Lett., 13, 1149–1152.
4. Watson, E. B. (1982). Melt infiltration and magma evolution. Geology, 10, 236–240.
5. Nakamura, M., & Watson, E. B. (2001). Experimental study of aqueous fluid infiltration into quartzite: Implications for the kinetics of fluid redistribution and grain growth driven by interfacial energy reduction. Geofluids, 1, 73–89.
6. Daines, M. J., & Kohlstedt, D. L. (1994). The transition from porous to channelized flow due to melt / rock reaction during melt migration. Geophys. Res. Lett., 21, 145–148.
7. Pec, M., Holtzman, B. K., Zimmerman, M., & Kohlstedt, D. L. (2015). Reaction infiltration instabilities in experiments on partially molten mantle
rocks. Geology, 43, 575–578.
8. Takei, Y., & Hier-Majumder, S. (2009). A generalized formulation of interfacial tension driven fluid migration with dissolution/precipitation. Earth. Planet. Sc. Lett., 288, 138–148.
9. Stevenson, D. J. (1989). Spontaneous small-scale melt segregation in partial melts in undergoing deformation. Geophys. Res. Lett., 16, 1067–1070.