09:15 〜 09:30
[SMP43-02] 角閃岩相高温部における塩素に富む鉱物を含む脈周辺の組織形成過程の検討
キーワード：塩水、交代作用、界面溶解―再沈殿プロセス、大陸地殻
Fluids in the crust play important roles in heat and mass transfer. Evidence for the presence of fluids in the deep crust is recorded as fluid inclusions or as hydrous minerals. Existence of brines in the deep crust is recently recognized in addition to CO2-rich fluids (e.g., Newton et al., 1998; Shmulovich & Graham, 2004). Brines have higher solubility of minerals and lower viscosity and wetting angle than CO2-rich fluids. This makes it possible to induce mass transfer along grain boundaries over vast distances on the km scale (e.g., Harlov, 2012), while it is difficult to be preserved in rocks as fluid inclusions. Therefore, it is important to establish microstructural indicators of the presence of brine in order to understand the distribution and role of brine in the crust.
This study deals with about 1 cm-thick garnet-hornblende (Grt-Hbl) vein that discordantly cuts the gneissose structure of garnet-orthopyroxene-hornblende (Grt-Opx-Hbl) gneiss from Brattnipene, Sor Rondane Mountains (SRM), East Antarctica. The Grt-Hbl vein is likely to have been formed from the wall rock, because the continuous gneissose structure is preserved as arrangements of biotite inclusions in the vein-forming Grt. With distance from the vein center, Cl concentration of Hbl and biotite (Bt), K content of Hbl, and thickness of Na-richer rim of plagioclase (Pl) decrease and become constant at a few cm away from the vein center. These compositional changes imply that the Grt-Hbl vein was possibly formed by NaCl-KCl-bearing fluid or melt infiltration. The P-T conditions for the vein formation is estimated to be ca. 700°C and 0.7 GPa, using geothermobarometers.
In this study, Zr is confirmed as immobile during the Grt-Hbl vein formation by almost constant bulk rock Zr content with distance from the vein (Higashino et al., 2015). Using Zr as an immobile element, the mass balance analysis was performed based on the fractionation mass change value (Ague, 2003). The bulk rock chemical variation with distance from the vein was evaluated. As a result, elements which are compatible to alkali-chloride-rich fluid (Keppler, 1996) were added to the wall rock rather than melt compatible and chloride-free-fluid compatible elements (Keppler, 1996). This supports that the Grt-Hbl vein was formed by brine infiltration.
In addition to Na, K and Cl concentrations, some trace element concentrations of constituent minerals gradually decrease or increase with distance from the vein center and become constant. It is important to note that distances where the trace element concentrations become constant are dependent on elements, and not on mineral species. These decreasing/increasing trends show diffusion-like profiles with distance from the vein. Trace element zoning within each grain is small, and almost negligible compared to chemical variation with distance from the vein. However, Pl preserves discontinuous zoning in terms of anorthite content. Discontinuous boundary between Pl rim and mantle implies that the brine infiltration caused dissolution-reprecipitation process. The preserved sharp mantle-rim boundary compared to flat zoning profile of trace elements would be explained by the sluggish NaSi-CaAl interdiffusion compared to the lattice diffusion of trace elements (e.g., Grove et al., 1984; Cherniak, 1995). It is likely that the brine infiltrated through the grain boundaries and altered the rim composition of minerals. Therefore, microstructure indicating dissolution-reprecipitation process, such as stepwise zoning of anorthite content of plagioclase, coexisting with Cl-bearing minerals may become an indicator of passage of brines. Field mapping of these microstructures would have a potential to unravel the large-scale distributions and movement of brines in the lower crust.
This study deals with about 1 cm-thick garnet-hornblende (Grt-Hbl) vein that discordantly cuts the gneissose structure of garnet-orthopyroxene-hornblende (Grt-Opx-Hbl) gneiss from Brattnipene, Sor Rondane Mountains (SRM), East Antarctica. The Grt-Hbl vein is likely to have been formed from the wall rock, because the continuous gneissose structure is preserved as arrangements of biotite inclusions in the vein-forming Grt. With distance from the vein center, Cl concentration of Hbl and biotite (Bt), K content of Hbl, and thickness of Na-richer rim of plagioclase (Pl) decrease and become constant at a few cm away from the vein center. These compositional changes imply that the Grt-Hbl vein was possibly formed by NaCl-KCl-bearing fluid or melt infiltration. The P-T conditions for the vein formation is estimated to be ca. 700°C and 0.7 GPa, using geothermobarometers.
In this study, Zr is confirmed as immobile during the Grt-Hbl vein formation by almost constant bulk rock Zr content with distance from the vein (Higashino et al., 2015). Using Zr as an immobile element, the mass balance analysis was performed based on the fractionation mass change value (Ague, 2003). The bulk rock chemical variation with distance from the vein was evaluated. As a result, elements which are compatible to alkali-chloride-rich fluid (Keppler, 1996) were added to the wall rock rather than melt compatible and chloride-free-fluid compatible elements (Keppler, 1996). This supports that the Grt-Hbl vein was formed by brine infiltration.
In addition to Na, K and Cl concentrations, some trace element concentrations of constituent minerals gradually decrease or increase with distance from the vein center and become constant. It is important to note that distances where the trace element concentrations become constant are dependent on elements, and not on mineral species. These decreasing/increasing trends show diffusion-like profiles with distance from the vein. Trace element zoning within each grain is small, and almost negligible compared to chemical variation with distance from the vein. However, Pl preserves discontinuous zoning in terms of anorthite content. Discontinuous boundary between Pl rim and mantle implies that the brine infiltration caused dissolution-reprecipitation process. The preserved sharp mantle-rim boundary compared to flat zoning profile of trace elements would be explained by the sluggish NaSi-CaAl interdiffusion compared to the lattice diffusion of trace elements (e.g., Grove et al., 1984; Cherniak, 1995). It is likely that the brine infiltrated through the grain boundaries and altered the rim composition of minerals. Therefore, microstructure indicating dissolution-reprecipitation process, such as stepwise zoning of anorthite content of plagioclase, coexisting with Cl-bearing minerals may become an indicator of passage of brines. Field mapping of these microstructures would have a potential to unravel the large-scale distributions and movement of brines in the lower crust.