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[SCG53-P09] Relationship between hydrogarnet-bearing ultramafic rock and serpentinization: A case study from the Yorii area, Kanto Mountains
Keywords:Serpentinization, Hydrogarnet, Fluid-rock interaction, Kanto Mountains
The serpentinization process driven by ultramafic rock-fluid interactions induces distinct chemical modifications in fluids, providing critical insights into elemental cycling within subduction zones. In the Yorii area of the Kanto Mountains, jadeite-quartz rocks and ultramafic lithologies (e.g., serpentinite and clinopyroxenite) are distributed within the greenstone mélange (Hirajima, 1983). Recently, Segawa and Taguchi (2024) identified reaction selvages composed of an albitization zone and glaucophanite along the margins of jadeite-quartz rocks in the Yorii area, where jadeite coexists with omphacite, highlighting metasomatism during their formation. During our ongoing petrological investigation of this area, we newly discovered an unusual ultramafic rock containing hydrous garnet (hydroandradite). This study presents the petrological and mineralogical characteristics of this rock and discusses its formation processes.
The ultramafic rock containing hydroandradite occurs as a small body approximately 10 m southeast of the jadeite-quartz rocks. The mineral assemblage consists of serpentine (antigorite, lizardite, and chrysotile), diopside, hydroandradite, clinochlore, magnetite, and pyrrhotite. The matrix is primarily composed of coarse-grained diopside (>500 µm; Wo46–50En46–50Fs4–5), fine-grained diopside (<100 µm) exhibiting a decomposition texture, and pool-like antigorite. Both coarse- and fine-grained diopside coexist with magnetite, and hydroandradites are observed crosscutting some diopside grains. Lizardite fills the interstitial spaces between diopside and antigorite as well as those between diopside and hydroandradite. Additionally, in some cases, it occurs along with a minor clinochlore covering fine-grained diopside. Chrysotiles occur as veins crosscutting the matrix minerals, including diopside and hydroandradite. Under a polarized light microscope, hydroandradite exhibits complex domain structures with incomplete extinction, likely due to hydration-induced optical anomalies. Raman spectroscopy confirms the presence of a Raman band at approximately 3570 cm−1, attributed to OH-stretching vibrations in hydroandradite. The chemical composition of hydroandradite indicates an almost andradite endmember (And87–94Grs0–6Uv0–7) with a slight increase in the uvarovite component at the outermost rim. The serpentine polymorphs exhibited slight variations in their XMg (= Mg/(Mg+Fe2+)) values: antigorite (XMg = 0.89–0.95), lizardite (XMg = 0.92–0.97), and chrysotile (XMg = 0.97–0.99).
The matrix antigorite is interpreted to have formed through the hydration of olivine originally present in the protolith (e.g., clinopyroxenite), indicative of serpentinization under relatively high-temperature conditions. The textural relationship between hydroandradite and diopside suggests that diopside was present before hydroandradite formation. Following the formation of matrix antigorite, reactions between diopside and magnetite in the presence of fluids facilitate the growth of lizardite and hydroandradite under relatively low-temperature serpentinization conditions (Frost and Beard, 2007). The chrysotile veins, characterized by their high XMg values, likely precipitated from Mg2+–HCO3−-rich fluids that infiltrated along fractures developed during later-stage weathering. This sample preserves well-defined mineralogical textures indicative of multi-stage fluid infiltration, providing valuable insights into rock-fluid interactions within subduction zones.
References:
Hirajima (1983) J. Japan. Assoc. Min. Petr. Econ. Geol. 78, 77–83.
Segawa and Taguchi (2024) The 131st Annual meeting of the Geological society of Japan, T1-P-4.
Frost and Beard (2007) J. Petrol. 48, 7, 1351–1368.
The ultramafic rock containing hydroandradite occurs as a small body approximately 10 m southeast of the jadeite-quartz rocks. The mineral assemblage consists of serpentine (antigorite, lizardite, and chrysotile), diopside, hydroandradite, clinochlore, magnetite, and pyrrhotite. The matrix is primarily composed of coarse-grained diopside (>500 µm; Wo46–50En46–50Fs4–5), fine-grained diopside (<100 µm) exhibiting a decomposition texture, and pool-like antigorite. Both coarse- and fine-grained diopside coexist with magnetite, and hydroandradites are observed crosscutting some diopside grains. Lizardite fills the interstitial spaces between diopside and antigorite as well as those between diopside and hydroandradite. Additionally, in some cases, it occurs along with a minor clinochlore covering fine-grained diopside. Chrysotiles occur as veins crosscutting the matrix minerals, including diopside and hydroandradite. Under a polarized light microscope, hydroandradite exhibits complex domain structures with incomplete extinction, likely due to hydration-induced optical anomalies. Raman spectroscopy confirms the presence of a Raman band at approximately 3570 cm−1, attributed to OH-stretching vibrations in hydroandradite. The chemical composition of hydroandradite indicates an almost andradite endmember (And87–94Grs0–6Uv0–7) with a slight increase in the uvarovite component at the outermost rim. The serpentine polymorphs exhibited slight variations in their XMg (= Mg/(Mg+Fe2+)) values: antigorite (XMg = 0.89–0.95), lizardite (XMg = 0.92–0.97), and chrysotile (XMg = 0.97–0.99).
The matrix antigorite is interpreted to have formed through the hydration of olivine originally present in the protolith (e.g., clinopyroxenite), indicative of serpentinization under relatively high-temperature conditions. The textural relationship between hydroandradite and diopside suggests that diopside was present before hydroandradite formation. Following the formation of matrix antigorite, reactions between diopside and magnetite in the presence of fluids facilitate the growth of lizardite and hydroandradite under relatively low-temperature serpentinization conditions (Frost and Beard, 2007). The chrysotile veins, characterized by their high XMg values, likely precipitated from Mg2+–HCO3−-rich fluids that infiltrated along fractures developed during later-stage weathering. This sample preserves well-defined mineralogical textures indicative of multi-stage fluid infiltration, providing valuable insights into rock-fluid interactions within subduction zones.
References:
Hirajima (1983) J. Japan. Assoc. Min. Petr. Econ. Geol. 78, 77–83.
Segawa and Taguchi (2024) The 131st Annual meeting of the Geological society of Japan, T1-P-4.
Frost and Beard (2007) J. Petrol. 48, 7, 1351–1368.