3:30 PM - 3:45 PM
[SCG59-07] Hydrothermal alteration of subducting oceanic mantle: Implications from the Kanasaki serpentinites, Kanto Mountains
★Invited Papers
The Earth's habitable conditions are maintained through carbon cycling processes that involve interactions between the atmosphere, oceans, and the deep Earth (e.g., Plank and Manning, 2019, Nature). However, the mechanism of carbon cycle is not yet fully understood. Ultramafic rocks are known to have the potential to store large amounts of carbon as carbonate minerals. Therefore, it is important to understand the diversity of tectonic settings and trigger of carbonate formation in the ultramafic rocks.
In this study, in order to better understand the process of carbonation of ultramafic rocks, we conducted petrological and geochemical analysis to discuss the petrogenesis of ophicarbonate (a rock composed of carbonate minerals and serpentine) from the Kanasaki area of the Kanto Mountains in Japan. In the Kanasaki area, the mafic rocks of the Mikabu unit, pelitic schists, and ultramafic rocks are exposed. Serpentinite and ophicarbonate occur as blocks surrounded by pelitic schists. Serpentinite shows a mesh structure, with a mesh core (either lizardite or chrysotile) and mesh rim (antigorite). Ophicarbonate occurs in four types: (Type I) calcite filling the interstices of brecciated serpentine, (Type II) calcite and dolomite filling the interstices of brecciated serpentine, (Type III) discrete calcite veins, (Type IV) discrete calcite and dolomite veins. The discrete carbonate veins cut a serpentine matrix containing carbonate cement. The carbonate minerals of types II-IV are depleted in 13C (δ13C(VPDB) = -6 - -8‰) and show a wide range of δ18O(VSMOW) values (13 – 22‰). These carbonate minerals also show a positive Eu anomaly in the REE pattern. On the other hand, type I carbonates are enriched in 13C (δ13C(VPDB) = -2 - +2‰) and show relatively narrow δ18O(VSMOW) values (13 – 16‰). Type I carbonates show a variety of REE patterns. Patterns with negative Ce anomalies and positive Y anomalies enriched in HREE, patterns with positive Eu anomalies slightly enriched in LREE, and intermediate patterns have been observed.
These differences in the occurrence and geochemical characteristics of carbonate minerals in serpentine suggest that the carbonate formation of ultramafic rocks was caused by various fluids and occurred in various tectonic settings. In the presentation, we will discuss the role of ultramafic rocks in the global carbon cycle based on the tectonic setting and the fluids that caused the carbonate formation.
In this study, in order to better understand the process of carbonation of ultramafic rocks, we conducted petrological and geochemical analysis to discuss the petrogenesis of ophicarbonate (a rock composed of carbonate minerals and serpentine) from the Kanasaki area of the Kanto Mountains in Japan. In the Kanasaki area, the mafic rocks of the Mikabu unit, pelitic schists, and ultramafic rocks are exposed. Serpentinite and ophicarbonate occur as blocks surrounded by pelitic schists. Serpentinite shows a mesh structure, with a mesh core (either lizardite or chrysotile) and mesh rim (antigorite). Ophicarbonate occurs in four types: (Type I) calcite filling the interstices of brecciated serpentine, (Type II) calcite and dolomite filling the interstices of brecciated serpentine, (Type III) discrete calcite veins, (Type IV) discrete calcite and dolomite veins. The discrete carbonate veins cut a serpentine matrix containing carbonate cement. The carbonate minerals of types II-IV are depleted in 13C (δ13C(VPDB) = -6 - -8‰) and show a wide range of δ18O(VSMOW) values (13 – 22‰). These carbonate minerals also show a positive Eu anomaly in the REE pattern. On the other hand, type I carbonates are enriched in 13C (δ13C(VPDB) = -2 - +2‰) and show relatively narrow δ18O(VSMOW) values (13 – 16‰). Type I carbonates show a variety of REE patterns. Patterns with negative Ce anomalies and positive Y anomalies enriched in HREE, patterns with positive Eu anomalies slightly enriched in LREE, and intermediate patterns have been observed.
These differences in the occurrence and geochemical characteristics of carbonate minerals in serpentine suggest that the carbonate formation of ultramafic rocks was caused by various fluids and occurred in various tectonic settings. In the presentation, we will discuss the role of ultramafic rocks in the global carbon cycle based on the tectonic setting and the fluids that caused the carbonate formation.