5:15 PM - 6:45 PM
[SCG48-P39] Occurrence of hydrothermal alteration zones beneath the seafloor off Muroto in Kochi Prefecture, Japan
Keywords:Nankai Trough, barite, fluid inclusion, X-ray computed tomography, X-ray fluorescence scanning, ODP
The distribution of hydrothermal alteration zones in the crust reflects past fluid circulation and is crucial information for understanding crustal dynamics. In the offshore of Muroto, Kochi Prefecture, hydrothermal alteration zones were reported from Site C0023 drilled during the International Ocean Discovery Program (IODP). While such alteration zones can influence the strength and/or permeability of oceanic crust, research on their distribution and formation processes is limited. Therefore, in this study, we investigated the distribution and characteristics of hydrothermal alteration zones under the sea floor at the Nankai Trough off Muroto, using core samples collected by Ocean Drilling Program (ODP) Sites 808, 1173, 1174, and IODP Site C0023. Initially, we examined the spatial distribution of hydrothermal alteration zones using XCT data. Subsequently, to investigate the characteristics of these zones, we conducted elemental mapping of core samples using X-ray fluorescence (XRF) scanning, identified and observed constituent minerals using X-ray diffraction (XRD) and optical microscopy, and analyzed mineral composition using scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDS).
From the XCT data, we identified depth intervals where regions with high CT numbers, several to tens of centimeters thick, were distributed at intervals of several meters to tens of meters. These intervals of concentrated high CT numbers were found to be consistent across the four sites, indicating lateral continuity. To further examine the characteristics of regions with high CT numbers, elemental mapping was conducted on core samples near 941 meters below the sea floor at Site 1174. These samples exhibited fault zones, with large strip-like crystals several millimeters in size observed adjacent to fault planes, and alteration zones showing high CT numbers located several millimeters to centimeters away from the fault plane. The strip-like crystals showed concentration of sulfur (S) and barium (Ba), while the alteration zones showed concentration of calcium (Ca) and manganese (Mn), with concentrations decreasing further away from the fault plane. Combining XRD data, we identified barite (BaSO4), calcite (CaCO3), and rhodochrosite (MnCO3) in the concentrated regions of Ba, Ca, and Mn, respectively. Fluid inclusions observed in the barite crystals exhibited a homogenization temperature of up to 215.0 ℃, significantly higher than the temperature estimated from the geothermal gradient at the same depth, suggesting that barite crystallization is related to hydrothermal activity.
Based on these observations, we propose the following formation process for hydrothermal alteration zones: after the deposition of hemipelagic deposits, faults form, and alteration occurs due to hydrothermal fluid flow along these faults. During this process, barite fills only near the fault plane, while calcite and rhodochrosite precipitate within a range of about 10 cm. It is important to further investigate the timing of mineral zonation formation and the origin of hydrothermal fluids in the future.
From the XCT data, we identified depth intervals where regions with high CT numbers, several to tens of centimeters thick, were distributed at intervals of several meters to tens of meters. These intervals of concentrated high CT numbers were found to be consistent across the four sites, indicating lateral continuity. To further examine the characteristics of regions with high CT numbers, elemental mapping was conducted on core samples near 941 meters below the sea floor at Site 1174. These samples exhibited fault zones, with large strip-like crystals several millimeters in size observed adjacent to fault planes, and alteration zones showing high CT numbers located several millimeters to centimeters away from the fault plane. The strip-like crystals showed concentration of sulfur (S) and barium (Ba), while the alteration zones showed concentration of calcium (Ca) and manganese (Mn), with concentrations decreasing further away from the fault plane. Combining XRD data, we identified barite (BaSO4), calcite (CaCO3), and rhodochrosite (MnCO3) in the concentrated regions of Ba, Ca, and Mn, respectively. Fluid inclusions observed in the barite crystals exhibited a homogenization temperature of up to 215.0 ℃, significantly higher than the temperature estimated from the geothermal gradient at the same depth, suggesting that barite crystallization is related to hydrothermal activity.
Based on these observations, we propose the following formation process for hydrothermal alteration zones: after the deposition of hemipelagic deposits, faults form, and alteration occurs due to hydrothermal fluid flow along these faults. During this process, barite fills only near the fault plane, while calcite and rhodochrosite precipitate within a range of about 10 cm. It is important to further investigate the timing of mineral zonation formation and the origin of hydrothermal fluids in the future.