[SCG56-14] オマーン掘削計画・BT1B掘削から得られたオマーンのリストベナイト(炭酸塩岩化した超マフィック岩)と関連する蛇紋岩の岩石学
キーワード：リストベナイト、炭酸塩岩、オフィオライト
In recent years, carbon sequestration by minerals has attracted attention as one of the solutions to reduce carbon dioxide emmision in the atmosphere. Serpentinized and carbonated peridotites are formed when peridotite reacts with H2O and CO2 near the surface of the Earth. Temperature conditions of carbonated peridotites (listvenite) were investigated from fluid inclusions, oxygen isotope analysis, and mineral paragenesis (e.g., Belogub et al., 2017, Falk and Kelemen, 2015). Carbonation of peridotite for the formation of listvenite is estimated at 80 to 360 ℃. However, some fundamental questions are still unresolved: how peridotite is converted to listvenite, and elemental mobilization on carbonation of peridotite.
Listvenite occurs locally at the boundary of the mantle section of the Samail ophiolite and the metamorphic sole below the mantle section. Hole BT1B of the Oman Drilling Project sampled listvenites, 2 carbonate-veined serpentinite bands at 80-100 and 180-185 m depth, and 70 cm of fault gouge at 197 m depth, followed by 103 m metamorphic sole. The serpentinite bands were originally dunite and harzburgite. The listvenite mainly consists of magnesite and quartz/chalcedony ± fuchsite (Cr-mica). Dolomite veins often cut early structures and rarely contain fluid inclusions. Dolomite also occurs as matrix of the listvenite. Dolomite-rich listvenite bands are found from near the upper serpentinite band and just above the metamorphic sole. The frequent occurrence of dolomite in the core suggests that calcium was supplied to the core at a later stage. The occurrence of hematite in some parts of the dolomite-listvenite are similar to the occurrence of magnetite in the serpentinite bands.
Trace elements compositions of magnesite vein and dolomite vein were measured. Both of magnesite and dolomite vein have high Ba and Sr contents, wheras only dolomite vein contents high Pb contents. Dolomite could be play a important role as Pb host in carbonated peridotite.
Listvenite occurs locally at the boundary of the mantle section of the Samail ophiolite and the metamorphic sole below the mantle section. Hole BT1B of the Oman Drilling Project sampled listvenites, 2 carbonate-veined serpentinite bands at 80-100 and 180-185 m depth, and 70 cm of fault gouge at 197 m depth, followed by 103 m metamorphic sole. The serpentinite bands were originally dunite and harzburgite. The listvenite mainly consists of magnesite and quartz/chalcedony ± fuchsite (Cr-mica). Dolomite veins often cut early structures and rarely contain fluid inclusions. Dolomite also occurs as matrix of the listvenite. Dolomite-rich listvenite bands are found from near the upper serpentinite band and just above the metamorphic sole. The frequent occurrence of dolomite in the core suggests that calcium was supplied to the core at a later stage. The occurrence of hematite in some parts of the dolomite-listvenite are similar to the occurrence of magnetite in the serpentinite bands.
Trace elements compositions of magnesite vein and dolomite vein were measured. Both of magnesite and dolomite vein have high Ba and Sr contents, wheras only dolomite vein contents high Pb contents. Dolomite could be play a important role as Pb host in carbonated peridotite.