10:00 AM - 10:30 AM
[SCG41-05] Serpentinization and Fe(III) distribution along the crust-mantle section of the oceanic lithosphere: insights from the Oman Drilling CM1A site
★Invited Papers
Keywords:Oman ophiolite, Fe(III)/Fe(II) distribution, serpentinite, antigorite, magnetite, crust-mantle transition zone
Serpentinization of mantle peridotite causes the production of hydrogen and hydrocarbon, which provides energy for sustaining the microbial activities within the oceanic lithosphere. The total production of hydrogen energy depends on the extent of serpentinization and its reaction pathway. During serpentinization, Fe(II) in olivine or orthopyroxene is commonly oxidized to form magnetite Fe3O4, which is coupled with H2O reduction to produce H2. In contrast, the Fe(II)-rich brucite is often reported as a precursor of magnetite+serpentine, and Fe(III) can exist within serpentine minerals. Due to variable occurrences of iron-bearing minerals, the magnetic sustainability of the serpentinized peridotite within the oceanic lithosphere is not always linear relationship with the extent of serpentinization. The CM1site of the Oman Drilling Project provides us a good opportunity to understand where and how much amount of hydrogen produces within the oceanic lithosphere from the lower crust to upper mantle section. In this study, we reported the depth variation of serpentinization and the Fe(III)/Fe(II) ratios along the crust-mantle section, and the spatial and temporal variations on Fe(III), meaning that the hydrogen production. We conducted bulk measurement of Fe(III)/total Fe along the borehole, and micro XANES mapping at Photon Factory, KEK.
The CM1A site shows the complete continuous core with length of 404.15 m, from the lower crust composed of olivine gabbros and troctolite, crust-mantle transition zone composed of dunite, and mantle section mainly composed of harzburgite. All the core samples are highly serpentinized, but it has a variation along the depth. In the olivine gabbros, the extent of the serpentinization is relatively low (11-69%; Yoshida et al., 2021), but serpentinization of olivine grains with mesh texture are observed as black network veins. In the transition zone, dunite is almost 100% serpentinized, and the onboard X-ray CT images reveals that the network of distinct “bright (magnetite-rich)” vein networks. In the upper mantle section, olivine shows mesh texture and bastite formed around the Opx and 70-90% serpentinized. The serpentine minerals in the gabbroic rocks, dunite and hurzburgite are mostly lizardite and associated with brucite, but we found that blight vein network is composed of antigorite.
The bulk Fe(III)/total Fe ratio ranges from 0.21 to 0.66. we found two clear trends, and the ratio is higher in dunite and harzburgite than gabbroic crust, reflecting the extent of serpentinization. In the harzburgite, the local Fe(III)/total Fe ratio is higher in the mesh rim around the olivine, whereas it is low in bastite. Magnetite also preferentially occurs in olivine mesh texture, and it is absent around olivine, due to the difference in silica activity. This indicates that the olivine has higher potential of oxidation of iron to produce hydrogen. The other feature is that antigorite veins in dunite parts shows (1) magnetite was dissolved and reprecipitated at the margin of the veins, (2) the antigorite is high XMg value, and (3) high Fe(III)/total Fe ratio than the other lizardite. These features indicate that the low-temperature alteration characterized by lizardite + brucite occurred pervasively at the earlier stage to produce hydrogen, and the high-temperature oxidizing fluids would have infiltrated along the fracture networks from the deeper parts, probably related to the later obduction/subduction stage.
References: Yoshida, K., Okamoto, A., Oyanagi, R., Shimizu, H., Tsuchiya, N., Oman Drilling Project Science Party II, 2020. Journal of Geophysical Research, 125, e2020JB020268
The CM1A site shows the complete continuous core with length of 404.15 m, from the lower crust composed of olivine gabbros and troctolite, crust-mantle transition zone composed of dunite, and mantle section mainly composed of harzburgite. All the core samples are highly serpentinized, but it has a variation along the depth. In the olivine gabbros, the extent of the serpentinization is relatively low (11-69%; Yoshida et al., 2021), but serpentinization of olivine grains with mesh texture are observed as black network veins. In the transition zone, dunite is almost 100% serpentinized, and the onboard X-ray CT images reveals that the network of distinct “bright (magnetite-rich)” vein networks. In the upper mantle section, olivine shows mesh texture and bastite formed around the Opx and 70-90% serpentinized. The serpentine minerals in the gabbroic rocks, dunite and hurzburgite are mostly lizardite and associated with brucite, but we found that blight vein network is composed of antigorite.
The bulk Fe(III)/total Fe ratio ranges from 0.21 to 0.66. we found two clear trends, and the ratio is higher in dunite and harzburgite than gabbroic crust, reflecting the extent of serpentinization. In the harzburgite, the local Fe(III)/total Fe ratio is higher in the mesh rim around the olivine, whereas it is low in bastite. Magnetite also preferentially occurs in olivine mesh texture, and it is absent around olivine, due to the difference in silica activity. This indicates that the olivine has higher potential of oxidation of iron to produce hydrogen. The other feature is that antigorite veins in dunite parts shows (1) magnetite was dissolved and reprecipitated at the margin of the veins, (2) the antigorite is high XMg value, and (3) high Fe(III)/total Fe ratio than the other lizardite. These features indicate that the low-temperature alteration characterized by lizardite + brucite occurred pervasively at the earlier stage to produce hydrogen, and the high-temperature oxidizing fluids would have infiltrated along the fracture networks from the deeper parts, probably related to the later obduction/subduction stage.
References: Yoshida, K., Okamoto, A., Oyanagi, R., Shimizu, H., Tsuchiya, N., Oman Drilling Project Science Party II, 2020. Journal of Geophysical Research, 125, e2020JB020268