Paleo‐Permeability Structure of the Crustal Section of the Samail Ophiolite Based on Automated Detection of Veins in X‐Ray CT Core Images From the Oman Drilling Project.

Autor: Akamatsu, Y., Katayama, I., Okazaki, K., Michibayashi, K.
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Zdroj: Geochemistry, Geophysics, Geosystems: G3; Jun2023, Vol. 24 Issue 6, p1-23, 23p
Abstrakt: To assess the paleo‐permeability structure of oceanic crust, we used 3‐D X‐ray Computed Tomography (XCT) images to quantify the distribution and geometry of mineral veins in core samples from Oman Drilling Project Holes GT1A, GT2A, and GT3A, which correspond to the upper to lower crustal sections of the Samail ophiolite. We developed a new method that automatically detects veins in the XCT core images based on iterative adaption of the two‐step Hough transform combined with multiscale Hessian filtering for identifying an elongate structure. Application of the developed method allowed us to identify the geometry and Computed Tomography number of more than 1500 veins with millimeter‐scale apertures in core sections with a total length of ∼1,200 m. High‐CT (HCT) veins in the drilled cores can be related to relatively high‐temperature fluid circulation near the mid‐ocean ridge, whereas Low‐CT (LCT) veins can be related to subsequent low‐temperature fluid circulation. Applying fracture fluid‐flow models to the geometric information for the detected veins, we found that the HCT and LCT vein systems both yielded bulk permeability of 10−13–10−9 m2 for each hole. This indicates that millimeter‐wide fractures can control crustal‐scale permeability, even in the lower oceanic crust. However, these vein systems show different depth dependencies and anisotropies of permeability, possibly reflecting the different spatial variations of high‐ and low‐temperature fluid circulation in oceanic crust. Plain Language Summary: Water transported through fractures in the oceanic crust plays a key role in various geologic processes, such as global transfers of heat and elements moving with fluid. Such fractures are preserved in crustal rocks as mineral veins; therefore, the geometries of the veins can be used to estimate the paleo‐permeability of the fractured rocks. During this study, we investigated the distribution and geometry of veins in core samples of oceanic crust from the Samail ophiolite in Oman to determine its paleo‐permeability structure. The ophiolite represents fragments of ancient oceanic lithosphere obducted onto the continental margin. We quantified three‐dimensional geometric and material information for the veins using 3‐D X‐ray Computed Tomography images of the core samples. Applying fracture fluid‐flow models to the vein data, we found that the oceanic crust had a markedly high permeability, even at depths of several kilometers immediately after the formation of the fractures. We also found that the hydrothermal systems have different spatial variations and anisotropies depending on the CT numbers of the vein‐filling minerals. Our results indicate that there may be differences in the mechanisms of high‐ and low‐temperature hydrothermal circulation in oceanic crust. Key Points: A new method has been developed to automatically detect vein characteristics from X‐ray Computed Tomography (CT) images of drilled coresThe permeability structure of the crustal section of the Samail ophiolite was estimated from the downhole profiles of detected veinsPermeability structures of High‐CT and Low‐CT vein systems have different spatial variations and anisotropies [ABSTRACT FROM AUTHOR]
Databáze: Complementary Index
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