5:15 PM - 7:15 PM
[SCG54-P08] Mineral replacement and Element transfer during alteration of oceanic lower crust: Insights from flow-through hydrothermal experiments using gabbro and brines
Keywords:gabbro, mineral replacement reaction
At mid-ocean ridges, seawater infiltrates the oceanic crust and reacts with surrounding rocks during heating, leading to the removal of Mg2+ and SO42- while dissolving metals such as Fe2+ and Cu2+. Hydrothermal fluids containing valuable metals are discharged from seafloor hydrothermal vents, forming seafloor hydrothermal deposits and influencing seawater composition. In contrast to porous basaltic rocks at the shallower upper crust, gabbro shows low permeability, making it unclear to what extent hydrothermal alteration proceeds through the lower crust. A volumetric chlorite rocks after gabbro were commonly observed in an exposed ophiolite, suggesting a large scale alteration with mass transfer can occur (Yoshitake et al., 2009; Bucher and Stober, 2021). Previous hydrothermal experiments on alteration of oceanic lithosphere were conducted using basalt powders, but very few have investigated the gabbro-seawater reaction. In this study, we conducted flow-through hydrothermal experiments in a gabbro-brine system to examine how various seawater components influence element leaching and fixation, as well as how reactions progress within the dense structure of gabbro.
The fine-grained gabbro samples of 2.5 x 2.5 x 10 mm was used in the experiments. The gabbro was composed mainly of plagioclase and clinopyroxene, with minor iron/titanium oxides, quartz, and K-feldspar. Three types of reaction solutions were used: NaCl aqueous solution (0.5 M), MgCl2-NaCl aqueous solution (0.03 M MgCl2 + 0.5 M NaCl), and FeCl2-NaCl aqueous solution (0.03 M FeCl2 + 0.5 M NaCl). Three rock samples were fixed inside an alumina tube and reacted with the solution under conditions of 300 C and 25 MPa for 72 hours, with the flow rate was of 0.2 mL/min.
The reactions between the rock samples and the solutions showed various characteristics. In the NaCl solution, plagioclase dissolved selectively without forming new products. In the MgCl2-NaCl solution, plagioclase selectively reacted to form a mesh-like replacement texture composed of Mg-rich chlorite. In the FeCl2-NaCl solution, both plagioclase and clinopyroxene reacted, forming a network-like replacement texture where plagioclase was replaced by Fe-rich chlorite and clinopyroxene was replaced by Fe-rich chlorite and hematite. The chemistry of the output solutions showed that pH was the highest in the NaCl solution (8-9), followed by the MgCl2-NaCl solution (5.5), and the lowest in the FeCl2-NaCl solution (3.7). Regarding element leaching, Si and Ca were leached in the MgCl2-NaCl solution, while Fe was not. In contrast, Si, Ca, and Mg were leached in the FeCl2-NaCl solution. Notably, Al was scarcely leached in all experiments.
Regarding the replacement reactions observed in the MgCl2-NaCl and FeCl2-NaCl solutions, mass balance calculations showed that the solution composition and the calculation results matched when the volume decreased by 30-50%, which is consistent with the formation of the mesh-like replacement textures. Geochemical modeling of the reactions between the solutions and gabbro under the experimental conditions was conducted using a software CHIM-xpt. The modeling of mineral titration into brine revealed that chlorite forms preferentially in the high water-rock ratio greater than 1000. Clinochore forms in MgCl2-NaCl aqueous solutions, while daphnite is formed in the FeCl2-NaCl solution. The geochemical modeling provides us the explanation of the systematics of pH. The experiments with the NaCl aqueous solution is characterized by a pH increase, which is caused by the consumption of H+ associated with the dissolution of plagioclase. In contrast, when chlorite precipitates in addition to the dissolution of plagioclase, H+ is released as a whole reaction, leading to a decrease in pH in the MgCl2-NaCl and FeCl2-NaCl solutions. Such pH changes affect the dissolution rate of plagioclase, as its dissolution tends to proceed more readily under acidic conditions. Therefore, the pH decrease caused by chloritization may accelerate the dissolution of plagioclase, further promoting chloritization.
In the Oman ophiolite, parts of the gabbro have been altered into chlorite rock on a scale of several tens of meters. At the boundary, only clinopyroxene was chloritized, whereas inside the chlorite rock, plagioclase was also replaced by chlorite. The chlorite retained the original mineral texture and had a composition rich in iron (Mg# = 0.32-0.40). Compared to the experimental results, the chloritization in the Oman ophiolite is similar to the experiment using FeCl2-NaCl solution, suggesting the involvement of an iron-rich brines.
The fine-grained gabbro samples of 2.5 x 2.5 x 10 mm was used in the experiments. The gabbro was composed mainly of plagioclase and clinopyroxene, with minor iron/titanium oxides, quartz, and K-feldspar. Three types of reaction solutions were used: NaCl aqueous solution (0.5 M), MgCl2-NaCl aqueous solution (0.03 M MgCl2 + 0.5 M NaCl), and FeCl2-NaCl aqueous solution (0.03 M FeCl2 + 0.5 M NaCl). Three rock samples were fixed inside an alumina tube and reacted with the solution under conditions of 300 C and 25 MPa for 72 hours, with the flow rate was of 0.2 mL/min.
The reactions between the rock samples and the solutions showed various characteristics. In the NaCl solution, plagioclase dissolved selectively without forming new products. In the MgCl2-NaCl solution, plagioclase selectively reacted to form a mesh-like replacement texture composed of Mg-rich chlorite. In the FeCl2-NaCl solution, both plagioclase and clinopyroxene reacted, forming a network-like replacement texture where plagioclase was replaced by Fe-rich chlorite and clinopyroxene was replaced by Fe-rich chlorite and hematite. The chemistry of the output solutions showed that pH was the highest in the NaCl solution (8-9), followed by the MgCl2-NaCl solution (5.5), and the lowest in the FeCl2-NaCl solution (3.7). Regarding element leaching, Si and Ca were leached in the MgCl2-NaCl solution, while Fe was not. In contrast, Si, Ca, and Mg were leached in the FeCl2-NaCl solution. Notably, Al was scarcely leached in all experiments.
Regarding the replacement reactions observed in the MgCl2-NaCl and FeCl2-NaCl solutions, mass balance calculations showed that the solution composition and the calculation results matched when the volume decreased by 30-50%, which is consistent with the formation of the mesh-like replacement textures. Geochemical modeling of the reactions between the solutions and gabbro under the experimental conditions was conducted using a software CHIM-xpt. The modeling of mineral titration into brine revealed that chlorite forms preferentially in the high water-rock ratio greater than 1000. Clinochore forms in MgCl2-NaCl aqueous solutions, while daphnite is formed in the FeCl2-NaCl solution. The geochemical modeling provides us the explanation of the systematics of pH. The experiments with the NaCl aqueous solution is characterized by a pH increase, which is caused by the consumption of H+ associated with the dissolution of plagioclase. In contrast, when chlorite precipitates in addition to the dissolution of plagioclase, H+ is released as a whole reaction, leading to a decrease in pH in the MgCl2-NaCl and FeCl2-NaCl solutions. Such pH changes affect the dissolution rate of plagioclase, as its dissolution tends to proceed more readily under acidic conditions. Therefore, the pH decrease caused by chloritization may accelerate the dissolution of plagioclase, further promoting chloritization.
In the Oman ophiolite, parts of the gabbro have been altered into chlorite rock on a scale of several tens of meters. At the boundary, only clinopyroxene was chloritized, whereas inside the chlorite rock, plagioclase was also replaced by chlorite. The chlorite retained the original mineral texture and had a composition rich in iron (Mg# = 0.32-0.40). Compared to the experimental results, the chloritization in the Oman ophiolite is similar to the experiment using FeCl2-NaCl solution, suggesting the involvement of an iron-rich brines.