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[SCG46-P01] Formation of brucite reaction zone with antigorite veins from the Oman ophiolite and its comparison with hydrothermal experiments.
★Invited Papers
Keywords:Serpentinite, Reaction zone, Hydrothermal experiment
The reaction zones around veins provide valuable information on the flux and timescale of fluid flow in the Earth’s interior. However, as experimental studies on the formation of reaction zones are limited, the formation mechanism of the reaction zone is still poorly understood. In this study, we analyzed the brucite-rich reaction zones developed around antigorite veins found in fully serpentinized dunite from the crust-mantle transition zone of the Oman ophiolite. We also conducted hydrothermal experiments to form brucite from serpentinite and discussed the formation mechanism of the reaction zone.
The sample observed is a dunite at the crust-mantle boundary obtained from the Oman Ophiolite drilling (Oman Drilling Project CM1A). The dunite is completely serpentinized, showing mesh texture consisting of lizardite, brucite, magnetite, and Cr-rich spinel. This matrix (low-T serpentinite) was cut by later antigorite veins (width of 400 – 500 um). It is noted that distinct reaction zones (width of 0.5 – 4 mm) were developed at both sides of the antigorite veins. The mass balance and thermodynamic calculation indicate that 6.3 molSiO2 kgmatrix-1 of silica should be leached from the matrix during the antigorite precipitation in the vein.
We conducted hydrothermal experiments to reproduce the brucite-rich reaction zone. The experimental samples were pieces of fully serpentinized dunite from the Oman ophiolite (2 mm x 2 mm x 1 mm), which placed in a stainless-steel reaction vessel with different pH solutions (NaOH solution with pH 12.1 - 13.8, MilliQ, and H2SO4 with pH 2.2), and heated in a furnace at 300 °C for 5 days. After each run, the surface was observed by SEM, and the solution chemistry was measured by ICP-OES. As a result, brucite precipitated on the surface in the cases of NaOH solution and MilliQ. In contrast, brucite was not observed in the case of H2SO4. As a result of the plot of the solution composition after the experiment in the activity diagram, it was found that the NaOH solution was saturated with brucite, while the H2SO4 solution was undersaturated, which is consistent with the observations. The solution chemistry after the experiments shows increasing in Si concentration (0.77 ppm – 113 ppm), while the Mg/Si ratio showed a decreasing trend with increasing pH (Mg/Si = 4×10-3 at pH = 13.8 - 7×101 at pH = 2.22). These results indicate that (1) the mineralogy and rates of the reaction zone are a strong function of pH of fluids, and (2) the brucite reaction zone from the Oman ophiolite was formed by leaching of silica-related to neutral to alkaline conditions.
The sample observed is a dunite at the crust-mantle boundary obtained from the Oman Ophiolite drilling (Oman Drilling Project CM1A). The dunite is completely serpentinized, showing mesh texture consisting of lizardite, brucite, magnetite, and Cr-rich spinel. This matrix (low-T serpentinite) was cut by later antigorite veins (width of 400 – 500 um). It is noted that distinct reaction zones (width of 0.5 – 4 mm) were developed at both sides of the antigorite veins. The mass balance and thermodynamic calculation indicate that 6.3 molSiO2 kgmatrix-1 of silica should be leached from the matrix during the antigorite precipitation in the vein.
We conducted hydrothermal experiments to reproduce the brucite-rich reaction zone. The experimental samples were pieces of fully serpentinized dunite from the Oman ophiolite (2 mm x 2 mm x 1 mm), which placed in a stainless-steel reaction vessel with different pH solutions (NaOH solution with pH 12.1 - 13.8, MilliQ, and H2SO4 with pH 2.2), and heated in a furnace at 300 °C for 5 days. After each run, the surface was observed by SEM, and the solution chemistry was measured by ICP-OES. As a result, brucite precipitated on the surface in the cases of NaOH solution and MilliQ. In contrast, brucite was not observed in the case of H2SO4. As a result of the plot of the solution composition after the experiment in the activity diagram, it was found that the NaOH solution was saturated with brucite, while the H2SO4 solution was undersaturated, which is consistent with the observations. The solution chemistry after the experiments shows increasing in Si concentration (0.77 ppm – 113 ppm), while the Mg/Si ratio showed a decreasing trend with increasing pH (Mg/Si = 4×10-3 at pH = 13.8 - 7×101 at pH = 2.22). These results indicate that (1) the mineralogy and rates of the reaction zone are a strong function of pH of fluids, and (2) the brucite reaction zone from the Oman ophiolite was formed by leaching of silica-related to neutral to alkaline conditions.