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[SCG48-P21] Spatial distribution of mineralization developed around the plate boundary fault in the Nankai Trough from X-ray CT data
Keywords:X-ray Computed Tomography, barite, Nankai Trough, IODP, ODP, calcite
The alternation zones within the crust influences crustal deformation due to their different physical properties such as strength, density, and permeability compared to other area. Hydrothermal mineralization at depths of 775–1121 m under the seafloor has been reported at Site C0023, located at the tip of the Nankai accretionary prism off Muroto, drilled during the International Ocean Discovery Program (IODP) (Tsang et al., 2020). While these mineralization zones modify the strength structure of the oceanic crust, there have been no studies on their distribution. Therefore, we investigate the distribution of mineralization using core samples collected during multiple international ocean drilling expeditions along the Nankai Trough off Shikoku and the Kii Peninsula. The core samples are from four sites: IODP Site C0023 off Muroto, and Ocean Drilling Program (ODP) Sites 1173, 1174, and 808. Among these, Site 1173 is located seaward of the Nankai Trough, while the other sites are situated at the tip of the Nankai accretionary prism. These sites exhibit a sequence of mid-Miocene and younger hemipelagic sediments and trench fill sediments overlying basaltic basement, classified into lower Shikoku Basin facies, upper Shikoku Basin facies, and trench-wedge facies based on lithology (e.g., Moore et al., 2001).
First, X-ray Computed Tomography (XCT) data was used to examine the spatial distribution of mineralization zones. Subsequently, mineral identification was carried out using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Fluorescence Analysis (XRF).
The results at Site 1173, for example, depth profile of the mean CT numbers of the sediments gradually increase from about 1100 to 1800 with depth. Against this background, positive spikes with mean CT numbers ranging from 2000 to 10000 are observed in three intervals: from the trench-wedge facies to the upper part of the upper Shikoku Basin facies, from the uppermost part of the lower Shikoku Basin facies to the décollement zone equivalent layer, and within the lowermost part of the lower Shikoku Basin facies. These three intervals exhibit thick sections of high mean CT numbers, distributed at intervals of several to tens of meters with thicknesses of several centimeters. Additionally, it was clarified that the minerals constituting the high CT number regions are calcite (CaCO3) for numbers 3000–4000 and barite (BaSO4) and rhodochrosite (MnCO3) for numbers over 10000. These observations show that the intervals are alteration zones. It should be noted that barite (BaSO4) and rhodochrosite (MnCO3) are found only within the lower two intervals of the alteration zones.
By conducting similar analyses on core samples from other sites, the spatial distribution of alteration zones was investigated. At the other three sites off Muroto, high spikes in mean CT numbers similar to those at Site 1173 were concentrated within the same horizons, suggesting that alteration zones extend widely across the Muroto transect. At the sites off Kumano and Ashizuri, calcite (CaCO3) is more abundant compared to beneath the seabed off Muroto, while barite (BaSO4) and rhodochrosite (MnCO3) were less prevalent. This indicates a variation in the distribution of alteration zones parallel to the trough.
First, X-ray Computed Tomography (XCT) data was used to examine the spatial distribution of mineralization zones. Subsequently, mineral identification was carried out using X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), and X-ray Fluorescence Analysis (XRF).
The results at Site 1173, for example, depth profile of the mean CT numbers of the sediments gradually increase from about 1100 to 1800 with depth. Against this background, positive spikes with mean CT numbers ranging from 2000 to 10000 are observed in three intervals: from the trench-wedge facies to the upper part of the upper Shikoku Basin facies, from the uppermost part of the lower Shikoku Basin facies to the décollement zone equivalent layer, and within the lowermost part of the lower Shikoku Basin facies. These three intervals exhibit thick sections of high mean CT numbers, distributed at intervals of several to tens of meters with thicknesses of several centimeters. Additionally, it was clarified that the minerals constituting the high CT number regions are calcite (CaCO3) for numbers 3000–4000 and barite (BaSO4) and rhodochrosite (MnCO3) for numbers over 10000. These observations show that the intervals are alteration zones. It should be noted that barite (BaSO4) and rhodochrosite (MnCO3) are found only within the lower two intervals of the alteration zones.
By conducting similar analyses on core samples from other sites, the spatial distribution of alteration zones was investigated. At the other three sites off Muroto, high spikes in mean CT numbers similar to those at Site 1173 were concentrated within the same horizons, suggesting that alteration zones extend widely across the Muroto transect. At the sites off Kumano and Ashizuri, calcite (CaCO3) is more abundant compared to beneath the seabed off Muroto, while barite (BaSO4) and rhodochrosite (MnCO3) were less prevalent. This indicates a variation in the distribution of alteration zones parallel to the trough.