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[SGC37-09] Ophicarbonate Formation During Hydrothermal Seawater Circulation Beneath Mid Oceanic Ridges Based on Isotopic Compositions and Salinity of Fluid Inclusions
Keywords:Ophicarbonate, oxygen isotope, hydrogen isotope
Ophicarbonate is a rock type containing carbonate minerals within serpentinite. The Western Alps, located along the French-Italian border, were formed through the subduction of the Tethys plate beneath the Eurasian plate, and subsequent continental collision. This orogenic belt extends southward to the Ligurian Mountains and the island of Corsica in the Mediterranean Sea. These rocks have experienced a wide range of PT conditions. The Apennines of the Italian Peninsula, formed by the subduction of the Adriatic Plate beneath the Eurasian Plate at a later stage, expose ophiolites that underwent low-grade metamorphism in the northern region. As a result, ophiolites exhibiting oceanic plate stratigraphy and varying metamorphic P-T conditions are observed in the region of the French-Italian border to northern Italy.
The mineral assemblage of ophicarbonate consists of serpentinite, talc, calcite, and dolomite. Thermodynamic modeling using Perple_X 6.9.1 reveals that the coexistence of these four phases is restricted to low-pressure and low-temperature conditions with low CO2 partial pressure in the fluid. Fluid inclusions analyzed from five samples (Chenaillet, Lago Nero, Queyras, Monviso, and the Apennines) contain saline fluids with NaCl equivalents of 3-7 wt.% and homogenization temperatures between 170 and 240 °C, respectively. Oxygen and hydrogen isotope ratios were determined for fluid inclusions in carbonate minerals from three specimens.
Oxygen and hydrogen isotope analyses were conducted at Nagoya University, following Uemura et al. (2014, Geochimica et Cosmochimica Acta). Analyzed samples were selected by an optical inspection for the abundance of fluid inclusions; fluid inclusion-rich calcite selected along with some calcite + dolomite samples. Bulk water content was measured, and samples exceeding a threshold water content were analyzed. The samples were crushed in an iron mortar under vacuum at 130 °C to release fluids from inclusions, which were then analyzed using a Picarro spectrometer with cavity ring-down spectroscopy and a water vapor carrier gas. Isotopic compositions were calibrated using standard solutions, and sample runs were spaced 25-30 minutes apart to minimize instrumental memory effects.
The hydrogen and oxygen isotope ratios suggest that these fluids are comparable to hydrothermal fluids beneath mid-ocean ridges (Kawabata et al., 1987, Geochimica et Cosmochimica Acta), indicating that the ophicarbonate rocks formed below the seafloor, instead of during subduction. In particular, the northern Apennine ophicarbonates, associated with low-grade metamorphic rocks, is said to be formed near the ocean floor (Cannaò et al., 2020, Chemical Geology). Monviso and Lago Nero display lighter isotope signatures than those of Apennines. Lago Nero is associated with low-pressure metamorphic rocks, whereas Monviso is embedded in high-pressure eclogite facies rocks, subducted to depths of 75 km.
A model for the isotopic evolution of fluids derived from subducting oceanic plates (Kusuda et al., 2014, Earth, Planets, and Space) suggests that fluid compositions change with depth (30-100 km). Fluids expelled at 30 km depth share an isotopic character similar to that of mid-ocean ridge hydrothermal waters, and the values matching the Apennine isotopic signature. The proximity of Lago Nero and Monviso to the meteoric water line suggests potential mixing between oceanic plate-derived fluids and meteoric water. However, the high salinity (6 wt.% NaCl) of Monviso fluid inclusions makes meteoric water mixing unlikely. In conclusion, the Italian ophiolite-associated ophicarbonates likely formed through hydrothermal processes beneath mid-ocean ridges. The low-pressure carbonate assemblages preserved their fluid inclusion chemistry with minimal alteration during subsequent subduction, maintaining their original hydrothermal signatures.
The mineral assemblage of ophicarbonate consists of serpentinite, talc, calcite, and dolomite. Thermodynamic modeling using Perple_X 6.9.1 reveals that the coexistence of these four phases is restricted to low-pressure and low-temperature conditions with low CO2 partial pressure in the fluid. Fluid inclusions analyzed from five samples (Chenaillet, Lago Nero, Queyras, Monviso, and the Apennines) contain saline fluids with NaCl equivalents of 3-7 wt.% and homogenization temperatures between 170 and 240 °C, respectively. Oxygen and hydrogen isotope ratios were determined for fluid inclusions in carbonate minerals from three specimens.
Oxygen and hydrogen isotope analyses were conducted at Nagoya University, following Uemura et al. (2014, Geochimica et Cosmochimica Acta). Analyzed samples were selected by an optical inspection for the abundance of fluid inclusions; fluid inclusion-rich calcite selected along with some calcite + dolomite samples. Bulk water content was measured, and samples exceeding a threshold water content were analyzed. The samples were crushed in an iron mortar under vacuum at 130 °C to release fluids from inclusions, which were then analyzed using a Picarro spectrometer with cavity ring-down spectroscopy and a water vapor carrier gas. Isotopic compositions were calibrated using standard solutions, and sample runs were spaced 25-30 minutes apart to minimize instrumental memory effects.
The hydrogen and oxygen isotope ratios suggest that these fluids are comparable to hydrothermal fluids beneath mid-ocean ridges (Kawabata et al., 1987, Geochimica et Cosmochimica Acta), indicating that the ophicarbonate rocks formed below the seafloor, instead of during subduction. In particular, the northern Apennine ophicarbonates, associated with low-grade metamorphic rocks, is said to be formed near the ocean floor (Cannaò et al., 2020, Chemical Geology). Monviso and Lago Nero display lighter isotope signatures than those of Apennines. Lago Nero is associated with low-pressure metamorphic rocks, whereas Monviso is embedded in high-pressure eclogite facies rocks, subducted to depths of 75 km.
A model for the isotopic evolution of fluids derived from subducting oceanic plates (Kusuda et al., 2014, Earth, Planets, and Space) suggests that fluid compositions change with depth (30-100 km). Fluids expelled at 30 km depth share an isotopic character similar to that of mid-ocean ridge hydrothermal waters, and the values matching the Apennine isotopic signature. The proximity of Lago Nero and Monviso to the meteoric water line suggests potential mixing between oceanic plate-derived fluids and meteoric water. However, the high salinity (6 wt.% NaCl) of Monviso fluid inclusions makes meteoric water mixing unlikely. In conclusion, the Italian ophiolite-associated ophicarbonates likely formed through hydrothermal processes beneath mid-ocean ridges. The low-pressure carbonate assemblages preserved their fluid inclusion chemistry with minimal alteration during subsequent subduction, maintaining their original hydrothermal signatures.