Paleo-permeability in oceanic lithosphere inferred from vein mapping along the Wadi Haymiliyah of the Oman Ophiolite

Abstract To assess the permeability structure of the oceanic lithosphere, we analyzed the field-based vein distribution and geometry in the Wadi Haymiliyah area of the Oman ophiolite. The mineral-filled veins are a fossil record of fluid circulation, and we divided the veins into four categories: amphibole vein, epidote vein, prehnite vein, and serpentine vein. Amphibole and epidote veins are formed during high-temperature hydrothermal circulation near the oceanic ridge, while prehnite and serpentine veins are likely formed at relatively low temperatures, either away from the ridge axis or during the obduction stages. We applied an equivalent-channel model and tensor theory to calculate permeability based on vein density, aperture, and orientation. The results indicate permeabilities on the order of 10 –8 –10 –10 m 2 for amphibole veins, 10 –7 –10 –10 m 2 for epidote veins, 10 –8 –10 –12 m 2 for prehnite veins, and 10 –6 –10 –9 m 2 for serpentine veins. Epidote veins in the sheeted dyke section exhibit relatively high permeability anisotropy with preferential fluid flow subparallel to the vertical plane of the oceanic crust, while the other vein systems show weak orientation dependence. The calculated permeabilities from the vein mapping are significantly higher than the laboratory permeability measured on rock samples collected from the same outcrops, mainly due to the larger aperture of the veins than the microcracks in the matrix rocks. The fracture systems may have repeatedly opened and closed during fluid circulation, such that the calculated apparent paleo-permeability is the integrated value through the evolution of fluid flux in the oceanic lithosphere rather than the fluid circulation at a specific time. This results in systematically higher permeabilities than those derived from in situ borehole measurements, and the vein-based permeabilities are more representative of the cumulative fluid circulation during multiple hydrothermal events.