5:15 PM - 7:15 PM
[SIT17-P03] Influence of fluorine on the stability field of hydrous phases at subduction zones: Preliminary results
Keywords:fluorine, hydrous phases, subduction zones, halogen
Earth is the only planet in the solar system with water filled on its surface. Water has played important roles in the physicochemical properties and the chemical evolution of the Earth. In the plate tectonics theory framework, the oceanic crust generated at the mid-ocean ridge experiences water-rock interaction, such as the hydrothermal alteration around the ridge and the seafloor weathering, and then, the altered part of the oceanic crust is the so-called altered mid-ocean ridge basalt (altered MORB), which contains ~2 wt% (avg) of H2O (e.g., Kawahata et al., 1987). The oceanic crust containing the oceanic sediments, containing 7.09 wt% of H2O (Plank, 2014), expands along with plate motion at the seafloor. Then, it is subducted into the mantle from the trench. The subducted slab, containing altered MORB and oceanic sediments, suffers dehydration at the subduction zones, releasing fluids which could induce island arc volcanism in the arc system.
On the other hand, the dehydrated slab is further transported into deep regions of the mantle. It can, then, involve a source material for ocean island basalts. In these recycling processes, the water is trapped in pelagic clays such as smectite at the hydrothermal alteration around the ridge and the seafloor weathering. These clays formed as hydrothermal and weathering products with neritic clays such as illite, which are changed to high crystallinity phases such as chlorite and micas during diagenesis and subsequent metamorphism around the surface regions. High-temperature and high-pressure experiments showed that these hydrous phases change to more stable phases at higher temperatures and pressures such as lawsonite, phengite, and phase egg (AlSiO3(OH)) at the subduction zones in the sub-arc region, and Al-rich phases (Al-rich phase D and Al-rich phase H) around the mantle transition zone and the lower mantle (e.g., Schmidt and Poli, 1994; Ono, 1998; Liu et al., 2019). In those high-pressure and high-temperature experiments, however, the influences of F and Cl on the stability fields of those hydrous phases have not been evaluated well. Fluorine and chlorine are contained 30-1200 and 20-2000 ppm in the lithified sediments and the altered oceanic crust, respectively (Kendrick and Banes, 2022). Florine and chlorine are indispensable to decomposing rock and mineral samples in acid digestion processes at the lab works. During the metamorphism, the fluids containing F and Cl could migrate at the grain boundary during the dehydration processes, which may enhance the influence of F and Cl on the stability fields of mineral phases. Therefore, in this study, we evaluated the influence of F and Cl on the stability fields of hydrous phases at subduction zones, using high-pressure and high-temperature experiments to understand the water cycle on Earth.
We prepared the starting materials using the gel method in the experiments, except for the water and F source materials. Al(OH)3 and CaF2 were used as the sources of water and F and mixed with the primary starting materials. In the presentation, we present the preliminary results of high-pressure and high-temperature experiments that evaluate the influence of F on hydrous phases, such as phase egg, which can be formed as hydrous phases in the oceanic sediment composition at subduction zones at sub-arc region (Ono, 1998).
References
Kawahata, H. et al. (1987) Earth Planet. Sci. Lett. 85, 343-355.
Kendrick, M.A. and Banes, J.D. (2022) Elements. 18, 21-26.
Liu, X., et al. (2019) Am. Mineral. 104, 64-72.
Plank, T. (2014) Treatise on Geochemistry (Second Edition). Vol. 4, 607-629.
Schmidt, M.W. and Poli, S. (1994) Earth Planet. Sci. Lett. 124, 105-118.
Ono, S. (1998) J. Geophys. Res. 103, 18253-18267.
On the other hand, the dehydrated slab is further transported into deep regions of the mantle. It can, then, involve a source material for ocean island basalts. In these recycling processes, the water is trapped in pelagic clays such as smectite at the hydrothermal alteration around the ridge and the seafloor weathering. These clays formed as hydrothermal and weathering products with neritic clays such as illite, which are changed to high crystallinity phases such as chlorite and micas during diagenesis and subsequent metamorphism around the surface regions. High-temperature and high-pressure experiments showed that these hydrous phases change to more stable phases at higher temperatures and pressures such as lawsonite, phengite, and phase egg (AlSiO3(OH)) at the subduction zones in the sub-arc region, and Al-rich phases (Al-rich phase D and Al-rich phase H) around the mantle transition zone and the lower mantle (e.g., Schmidt and Poli, 1994; Ono, 1998; Liu et al., 2019). In those high-pressure and high-temperature experiments, however, the influences of F and Cl on the stability fields of those hydrous phases have not been evaluated well. Fluorine and chlorine are contained 30-1200 and 20-2000 ppm in the lithified sediments and the altered oceanic crust, respectively (Kendrick and Banes, 2022). Florine and chlorine are indispensable to decomposing rock and mineral samples in acid digestion processes at the lab works. During the metamorphism, the fluids containing F and Cl could migrate at the grain boundary during the dehydration processes, which may enhance the influence of F and Cl on the stability fields of mineral phases. Therefore, in this study, we evaluated the influence of F and Cl on the stability fields of hydrous phases at subduction zones, using high-pressure and high-temperature experiments to understand the water cycle on Earth.
We prepared the starting materials using the gel method in the experiments, except for the water and F source materials. Al(OH)3 and CaF2 were used as the sources of water and F and mixed with the primary starting materials. In the presentation, we present the preliminary results of high-pressure and high-temperature experiments that evaluate the influence of F on hydrous phases, such as phase egg, which can be formed as hydrous phases in the oceanic sediment composition at subduction zones at sub-arc region (Ono, 1998).
References
Kawahata, H. et al. (1987) Earth Planet. Sci. Lett. 85, 343-355.
Kendrick, M.A. and Banes, J.D. (2022) Elements. 18, 21-26.
Liu, X., et al. (2019) Am. Mineral. 104, 64-72.
Plank, T. (2014) Treatise on Geochemistry (Second Edition). Vol. 4, 607-629.
Schmidt, M.W. and Poli, S. (1994) Earth Planet. Sci. Lett. 124, 105-118.
Ono, S. (1998) J. Geophys. Res. 103, 18253-18267.