10:45 AM - 12:15 PM
[SCG58-P11] Fluid inclusions of ophicarbonate at the Piemont zone, Western Alps, as a case of paleo oceanic core complex at slow-spreading ridges
Keywords:ophicarbonate, serpentinite, fluid inclusion, salinity, subduction zone, oceanic core complex
<Background>
The mantle is known as the most significant reservoir of carbon, being supplied 10~27 Mt C per year. (Callow et al., 2018, Int. J. Greenhouse Gas Control). Seawater with dissolved CO2 at oceanic ridges and sediments with organic carbon at subduction zones supply carbon to the mantle by carbonation reactions. This process is studied actively by many researchers, because of its relation to slow earthquakes and carbon capture and storage (Okamoto et al., 2021, Comm. Earth Environ.; Kelemen et al., 2011, Earth Planet. Sci. Lett.). Carbonate formation also occurs in oceanic core complexes exposed at the bottom of the ocean, where the mantle is exhumed to the ocean floor along detachment faults, typically at slow-spreading ridges. Few studies have been conducted on the chemical composition of the fluids responsible of carbonation in oceanic core complexes (Scambelluri et al., 2016, Earth Planet. Sci. Lett.; Piccoli et al., 2018, Lithos). This study reports the species and salinity of fluids and their formation temperature of fluid inclusions in ophiocarbonates.
<Sample>
Rock samples were collected from two ophiolites, Chenaillet (France) and Lago Nero (Italy) in the Piedmont belt of the Western Alps, which are located near to the French and Italian boarder. They are obducted oceanic lithosphere containing oceanic core complexes (Manatschal et al., 2010, Lithos) formed in the Paleo-Tethys Sea, which was formed by a slow-spreading ridge. Chenaillet ophicarbonates shows features almost free of subduction-zone metamorphism, and that of Lago Nero show metamorphism of green-schist facies.
<Methods>
Field emission-scanning electron microscopy (FE-SEM) was used for microstructure observation and chemical analysis of thin sections, and Raman microscopy was used to identify mineral and chemical species in fluid inclusions. Salinity and homogenization temperature of fluid inclusions were measured using a heating and cooling stage by the method of microthermometry.
<Results and Discussion>
The Chenaillet ophicarbonate is composed of serpentinite breccias filled with carbonate in the matrix, in which serpentine is locally replaced by spherical carbonate. The Lago Nero ophicarbonate, on the other hand, has well-developed carbonate veins. All carbonate minerals are calcite. Although water was detected as the only fluid species in the fluid inclusions by Raman microscopy, the presence of CO2 clathrates, which melt above 0°C, were observed some fluid inclusions by the heating and cooling experiments. Most of the fluid inclusions are primary inclusions formed during mineral formation or pseudo-secondary inclusions distributed in rows. Microthermometry results are shown in Table 1. All samples have an average salinity of approximately 5 wt.% NaCl eq., which is 1.5 wt.% higher than that of seawater. Formation of serpentine can absorb water and leave salt, resulting salty and CO2-rich fluids (Debure et al., 2019, Sci. Rep.). Alternatively such fluids may be formed through separation between vapor and liquids of hydrothermal fluids (Kelley and Delaney, 1987, Earth Planet. Sci. Lett.). The homogenization temperature of fluid inclusions from Chenaillet show 140-150°C for carbonates in the matrix and spheres, whereas the carbonate veins of the Lago Nero record about 220°C. The reason of this difference remains to be investigated.
The mantle is known as the most significant reservoir of carbon, being supplied 10~27 Mt C per year. (Callow et al., 2018, Int. J. Greenhouse Gas Control). Seawater with dissolved CO2 at oceanic ridges and sediments with organic carbon at subduction zones supply carbon to the mantle by carbonation reactions. This process is studied actively by many researchers, because of its relation to slow earthquakes and carbon capture and storage (Okamoto et al., 2021, Comm. Earth Environ.; Kelemen et al., 2011, Earth Planet. Sci. Lett.). Carbonate formation also occurs in oceanic core complexes exposed at the bottom of the ocean, where the mantle is exhumed to the ocean floor along detachment faults, typically at slow-spreading ridges. Few studies have been conducted on the chemical composition of the fluids responsible of carbonation in oceanic core complexes (Scambelluri et al., 2016, Earth Planet. Sci. Lett.; Piccoli et al., 2018, Lithos). This study reports the species and salinity of fluids and their formation temperature of fluid inclusions in ophiocarbonates.
<Sample>
Rock samples were collected from two ophiolites, Chenaillet (France) and Lago Nero (Italy) in the Piedmont belt of the Western Alps, which are located near to the French and Italian boarder. They are obducted oceanic lithosphere containing oceanic core complexes (Manatschal et al., 2010, Lithos) formed in the Paleo-Tethys Sea, which was formed by a slow-spreading ridge. Chenaillet ophicarbonates shows features almost free of subduction-zone metamorphism, and that of Lago Nero show metamorphism of green-schist facies.
<Methods>
Field emission-scanning electron microscopy (FE-SEM) was used for microstructure observation and chemical analysis of thin sections, and Raman microscopy was used to identify mineral and chemical species in fluid inclusions. Salinity and homogenization temperature of fluid inclusions were measured using a heating and cooling stage by the method of microthermometry.
<Results and Discussion>
The Chenaillet ophicarbonate is composed of serpentinite breccias filled with carbonate in the matrix, in which serpentine is locally replaced by spherical carbonate. The Lago Nero ophicarbonate, on the other hand, has well-developed carbonate veins. All carbonate minerals are calcite. Although water was detected as the only fluid species in the fluid inclusions by Raman microscopy, the presence of CO2 clathrates, which melt above 0°C, were observed some fluid inclusions by the heating and cooling experiments. Most of the fluid inclusions are primary inclusions formed during mineral formation or pseudo-secondary inclusions distributed in rows. Microthermometry results are shown in Table 1. All samples have an average salinity of approximately 5 wt.% NaCl eq., which is 1.5 wt.% higher than that of seawater. Formation of serpentine can absorb water and leave salt, resulting salty and CO2-rich fluids (Debure et al., 2019, Sci. Rep.). Alternatively such fluids may be formed through separation between vapor and liquids of hydrothermal fluids (Kelley and Delaney, 1987, Earth Planet. Sci. Lett.). The homogenization temperature of fluid inclusions from Chenaillet show 140-150°C for carbonates in the matrix and spheres, whereas the carbonate veins of the Lago Nero record about 220°C. The reason of this difference remains to be investigated.