Rapid fluid infiltration recorded in the brucite-rich reaction zone along the antigorite veins from the Oman ophiolite

Fluid flow in subduction zones is related to geological processes such as seismic and volcanic activities. However, the timescale of fluid flow and its flux in the supra-subduction setting is unclear. In this study, we report the novel texture of the antigorite veins with a brucite-rich reaction zone in dunite in the crust-mantle transition zone of the Oman ophiolite, and investigated the timescale and time-integrated fluid flux during the vein formation.

The antigorite veins occur in the drilling cores of serpentinized dunite in the crust-mantle transition zone taken from the Oman Drilling Project CM1 site (Wadi Zeeb, Northern Sharqiyah). The dunite samples are completely serpentinized and consist mainly of lizardite, brucite, magnetite, and Cr-rich spinel and are cut by the antigorite vein networks and later chrysotile. These features indicate that the antigorite veins formed at the stage of obduction of the Oman ophiolite. The antigorite veins, which are distinct by ‘bright’ networks by X-ray CT due to magnetite, occur preferentially in dunite (at 160 – 313 m along the CM1A). The veins are filled with a mixture of randomly orientated antigorite crystals and fine chrysotile. Some antigorite vein contains fragments of the host rock. The brucite-rich reaction zone was developed at both sides of the antigorite veins with a thickness of 0.5 – 4 mm. The reaction zone is composed of brucite (39.4 area%), chrysotile (59.3 area%), and magnetite (1.3 area%). The microtexture of the reaction zone indicates the brucite silicification after the brucite reaction zone formation. Mass balance and thermodynamic calculation suggest that silica was leached from the host rock lizardite during antigorite vein crystallization, resulting in the formation of the brucite reaction zone. Given solution chemistry and the amount of leached SiO₂ during vein formation, the time-integrated fluid flux was estimated to be 10⁵ - 10⁶ m³_(fluid) m^-2_(rock). A diffusion-controlled model suggests that the reaction zone was formed in a short time, ~10^-1 - 10⁰ years. These results suggest that a large amount of high-temperature fluid passed through the fracture network over several hundred meters in a short time at the earlier stage of the obduction of the Oman ophiolite.