S2: Water Rock Interaction (Special Session)

[S2-05] Early stages of serpentinization in the mantle section of the Salahi mantle section in the northern Oman ophiolite

Yohki Yoshiba¹, *Eiichi TAKAZAWA^1,2, Toshio Nozaka³ (1. Niigata Univ. Sci., 2. JAMSTEC, 3. Okayama Univ.Sci.)

Keywords:Oman ophiolite, oceanic mantle, serpentinization, antigorite

To understand the early stage of serpentinization processe in the oceanic mantle, we conducted microscopic observations, compositional analyses, and Raman spectroscopic analyses of serpentine and associated minerals in the peridotites of the Salahi mantle section of the Oman Ophiolite. The peridotite contains various proportions of low-temperature serpentines such as lizardite and chrysotile. Antigorite, a high-temperature serpentine, is also widely distributed, but its frequency tends to decrease toward the northwestern part of the mantle section. Most of the antigorite forms veins 0.1 mm to 3.0 mm wide. In the antigorite veins, low-temperature serpentines cut parallel to or across the veins. Fe-rich olivine forms 0.02-0.3 mm wide veins, often accompanied by antigorite veins, in the olivine. The Fo content of common olivine is about 90, whereas that of Fe-rich olivine is 71-88. Antigorite veins in contact with Fe-rich olivine are rich in Mg and poor in Fe. Since Mg-Fe interdiffusivity increases with temperature, it is possible that the antigorite veins formed at high temperatures or that they were heated after formation of veins. The cross-cutting relationships observed in the antigorite veins suggest that olivine, antigorite, chrysotile, magnetite, and carbonate minerals formed in that order. Serpentinization is considered to have progressed with a gradual decrease in temperature. The coexistence of olivine, antigorite, and tremolite suggests the possibility of hydrothermal reactions at 500-600°C. Furthermore, the presence of lizardite and chrysotile in all samples suggests that extensive water infiltration occurred at temperatures lower than 300°C. Antigorite and talc are less frequently observed in the northwestern part of the Salahi mantle section suggesting a relationship with paleo-ocean ridge segment structure. Near the end of the ridge segment (south of Salahi brock), seawater may have penetrated deep into the mantle and formed antigorite in the early stages of cooling. On the other hand, near the center of the ridge segment and deeper in the mantle (northwestern part of Salahi block), the formation of antigorite may have been delayed because the temperature remained higher.