14:15 〜 14:30
[SCG55-03] Depth profile of hydration along the crust - mantle section of the Oman ophiolite: insights from holes CM1A and CM2B
キーワード：蛇紋岩化反応、反応誘起応力、海洋底変質
The extent of serpentinization of the oceanic lithosphere is a key factor for evaluating the water content brought into the Earth’s interior. However, the distribution of the serpentinized peridotites is still poorly constrained. The Oman ophiolite is one of the best places showing the complete sections from the upper crust to mantle of the oceanic lithosphere. In this study, we investigated the water content, vein structures and hydration structures of the continuous drilling cores from lower crust to transition zone to upper mantle taken from the Oman Drilling Project at CM1A (404 m) and CM2B (300 m) sites, based on thermogravimetry, Raman spectroscopy and electron microbe analyses. The main lithologies are gabbroic rocks at the lower crust, dunites in the crust-mantle transition zone (CMTZ), and harzburgites in upper mantle section.
All parts of CM1A and CM2B are extensively altered, and mainly characterized by serpentinization of olivine grains. The olivine grains in the gabbroic rocks are serpentinized ranging from 11 to 69 % to produce radial cracks at the surrounding plagioclase and pyroxene. Dunite is completely serpentinized with lizardite + brucite + magnetite, and cut by later antigorite vein networks. Harzburgite from upper mantle section are highly serpentinized (70–90%). The matrix part (lizardite + brucite + less magnetite) of the harzburgites are cut by tremolite veins. The TG analysis shows homogeneous serpentinization along the depth. The higher temperature veins (antigorite vein in the dunties, tremolite vein in the harzburgites) after pervasive lower temperature alteration (lizardite + brucite) suggest that the pervasive serpentinization before or during intra-oceanic overthrust of the Oman ophiolite. 300 m length of the crust-mantle transition zone to upper mantle section of the CM2B are almost completely serpentinized with average water content of 12 wt%.
Given that the gabbro has a low permeability as estimated by the intact sample (k=10-22 m2), 25 Myr of fluid-supply are required to complete the serpentinization observed in the CM1A & CM2B cores. This estimate contradicts with the interval of the igneous activity at ridge center of the Oman ophiolite and obduction of 1-2 Myr (Hacker, 1991). In contrast, the discrete element numerical simulations revealed that microcrack network formation by reaction induced stress during serpentinization could be important for enhancement of the permeability by more than several order of magnitude (Yoshida et al., 2020). In case of intact gabbro with enhanced permeability (k=10-20 m2), the duration for sepentinization is estimated to be ~0.25 Myr. These results suggest that abundant of water for the lower temperature serpentinization is possibly supplied during intra-oceanic overthrust of the ophiolite. The fluid flow triggered by intraoceanic thrusting is enhanced by later reaction-induced fracturing, which results in fluid penetration through the lower crust to the upper mantle. After the low-temperature alteration, hot fluids came from the subducting slab to form antigorite vein networks.
References
1. Hacker (1991). Tectonics, 10(2), 455–473. https://doi.org/10.1029/90TC02779
2. Yoshida et al. (2020). J. Geophys. Res. : Solid Earth, 125, 11. https://doi.org/10.1029/2020JB020268
All parts of CM1A and CM2B are extensively altered, and mainly characterized by serpentinization of olivine grains. The olivine grains in the gabbroic rocks are serpentinized ranging from 11 to 69 % to produce radial cracks at the surrounding plagioclase and pyroxene. Dunite is completely serpentinized with lizardite + brucite + magnetite, and cut by later antigorite vein networks. Harzburgite from upper mantle section are highly serpentinized (70–90%). The matrix part (lizardite + brucite + less magnetite) of the harzburgites are cut by tremolite veins. The TG analysis shows homogeneous serpentinization along the depth. The higher temperature veins (antigorite vein in the dunties, tremolite vein in the harzburgites) after pervasive lower temperature alteration (lizardite + brucite) suggest that the pervasive serpentinization before or during intra-oceanic overthrust of the Oman ophiolite. 300 m length of the crust-mantle transition zone to upper mantle section of the CM2B are almost completely serpentinized with average water content of 12 wt%.
Given that the gabbro has a low permeability as estimated by the intact sample (k=10-22 m2), 25 Myr of fluid-supply are required to complete the serpentinization observed in the CM1A & CM2B cores. This estimate contradicts with the interval of the igneous activity at ridge center of the Oman ophiolite and obduction of 1-2 Myr (Hacker, 1991). In contrast, the discrete element numerical simulations revealed that microcrack network formation by reaction induced stress during serpentinization could be important for enhancement of the permeability by more than several order of magnitude (Yoshida et al., 2020). In case of intact gabbro with enhanced permeability (k=10-20 m2), the duration for sepentinization is estimated to be ~0.25 Myr. These results suggest that abundant of water for the lower temperature serpentinization is possibly supplied during intra-oceanic overthrust of the ophiolite. The fluid flow triggered by intraoceanic thrusting is enhanced by later reaction-induced fracturing, which results in fluid penetration through the lower crust to the upper mantle. After the low-temperature alteration, hot fluids came from the subducting slab to form antigorite vein networks.
References
1. Hacker (1991). Tectonics, 10(2), 455–473. https://doi.org/10.1029/90TC02779
2. Yoshida et al. (2020). J. Geophys. Res. : Solid Earth, 125, 11. https://doi.org/10.1029/2020JB020268