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[SCG46-P07] Alteration of clinopyroxene veins during multi-stage serpentinization at the crust-mantle section from the Bayankhongor ophiolite, Mongolia
Keywords:Multistage serpentinization, Crust-mantle transition , Bayankhongor ophiolite , Mantle veins
Alteration of clinopyroxene veins during multi-stage serpentinization at the crust-mantle section from the Bayankhongor ophiolite, Mongolia
Nomin TUMURKHUU, Otgonbayar DANDAR, Masaoki UNO, Manzshir BAYARBOLD, Atsushi OKAMOTO
Graduate School of Environmental Studies, Tohoku University
Serpentinization of mantle rocks is a trigger geological process that initiates the global water cycle, capable of incorporating approximately 13 wt% of lattice-bound water (Guillot and Hattori, 2013). This process serves as an important for elucidating the dynamics of worldwide water circulation, elemental transference, and geological processes. The crust-mantle interface constitutes a significant geological boundary where chemical element transport and metasomatic reactions transpire. Nevertheless, there exists a lack of research concerning the serpentinization processes of mantle rocks within this zone, particularly in oceanic lithosphere settings originating from Mid-Ocean Ridge environments. Therefore, to understand the serpentinization processes occurring at the crust-mantle transition zone, we investigate potential outcrops indicative of the mantle-crust transition zone within the Bayankhongor ophiolite. The Bayankhongor ophiolite, distinguished as the largest of its kind with a mid-ocean ridge origin (εNd (t) = (+7.6) to (+4.7) and 87Sr/86Sr = 0.70279-0.70327) in Mongolia (Jian et al., 2010), presents an exceptional opportunity for investigation.
In the field, the outcrop of the crust-mantle section in the Bayankhongor ophiolite is exposed as 25-30 meters in diameter. The area is characterized by reddish-yellow gabbroic rocks alongside massive and sheared mantle rocks. Green veins, measuring 80-95 cm in width in the area, along with discontinuous white veins, 25-35 cm wide and 80-90 cm long, cut through the mantle rocks. Additionally, black veins, 2-2.5 cm in length, intersect the mantle rocks.
Mantle rocks do not contain remnants of olivine and orthopyroxene and are fully serpentinized. These samples predominantly consist of serpentine in two forms: mesh core (Mg# = 0.95-0.98) with fine magnetite (<100 μm) and vein (Mg# = 0.96-0.98) with vein magnetite (<30 μm width), along with spinel (Mg# = 0.42-0.52 & Cr# = 0.46-0.48), and chlorite (Mg# = 0.87-0.96). The Raman result indicates that both the veins and the mesh serpentine are composed of lizardite. The absence of brucite in the serpentinites suggests that Si-rich fluids infiltrated the ultramafic rocks of the Bayankhongor Ophiolite.
The samples taken from the green vein, which is the largest vein in the field, consist mainly of clinopyroxene (Mg# = 0.92) with minor of serpentine (Mg# = 0.98), epidote, and chlorite (Mg# = 0.92-0.93). Clinopyroxene is replaced by a mixture of chlorite-serpentine and cut by serpentine veins and epidote veins. Black veins are composed of chlorite patches (Mg# = 0.83-0.93) and patches consisting of a mixture of chlorite-serpentine with clear cleavages and fine ore-minerals (Ti-rich). The reaction zone (2.5-3 mm) between host serpentinite and black veins is observed and shows that magnetite veins have disappeared and been replaced by Al-rich (1.1-6.9 wt%) serpentine. Also, from vein to host rock, Al content in serpentine is decreased (from 1.4 to 0.6 wt%). These observations suggest that the protolith mineral of patches (mixture of chlorite-serpentine) could be clinopyroxene whereas the protolith of chlorite patches could be plagioclase.
These events imply that Si, Al, and Fe mobilities could occur locally at the crust-mantle section from the oceanic lithosphere during multi-stage serpentinization. We will discuss the relationship between multi-stage serpentinization and vein formations.
References:
Jian, P., Kroner, A., Windley, B.F., Shi, Y., Zhang, F., Miao, L., Tomurhuu, D., Zhang, W., Liu, D., 2010. Zircon ages of the Bayankhongor ophiolite mélange and associated rocks: Time constraints on Neoproterozoic to Cambrian accretionary and collisional orogenesis in Central Mongolia. Precambr. Res. 177, 162-180.
Guillot and Hattori, Serpentinites: Essential Roles in Geodynamics, Arc Volcanism, Sustainable Development, and the Origin of Life 2013 1811-5209/13/0009-095$2.50 DOI: 10.2113/gselements.9.2.95
Nomin TUMURKHUU, Otgonbayar DANDAR, Masaoki UNO, Manzshir BAYARBOLD, Atsushi OKAMOTO
Graduate School of Environmental Studies, Tohoku University
Serpentinization of mantle rocks is a trigger geological process that initiates the global water cycle, capable of incorporating approximately 13 wt% of lattice-bound water (Guillot and Hattori, 2013). This process serves as an important for elucidating the dynamics of worldwide water circulation, elemental transference, and geological processes. The crust-mantle interface constitutes a significant geological boundary where chemical element transport and metasomatic reactions transpire. Nevertheless, there exists a lack of research concerning the serpentinization processes of mantle rocks within this zone, particularly in oceanic lithosphere settings originating from Mid-Ocean Ridge environments. Therefore, to understand the serpentinization processes occurring at the crust-mantle transition zone, we investigate potential outcrops indicative of the mantle-crust transition zone within the Bayankhongor ophiolite. The Bayankhongor ophiolite, distinguished as the largest of its kind with a mid-ocean ridge origin (εNd (t) = (+7.6) to (+4.7) and 87Sr/86Sr = 0.70279-0.70327) in Mongolia (Jian et al., 2010), presents an exceptional opportunity for investigation.
In the field, the outcrop of the crust-mantle section in the Bayankhongor ophiolite is exposed as 25-30 meters in diameter. The area is characterized by reddish-yellow gabbroic rocks alongside massive and sheared mantle rocks. Green veins, measuring 80-95 cm in width in the area, along with discontinuous white veins, 25-35 cm wide and 80-90 cm long, cut through the mantle rocks. Additionally, black veins, 2-2.5 cm in length, intersect the mantle rocks.
Mantle rocks do not contain remnants of olivine and orthopyroxene and are fully serpentinized. These samples predominantly consist of serpentine in two forms: mesh core (Mg# = 0.95-0.98) with fine magnetite (<100 μm) and vein (Mg# = 0.96-0.98) with vein magnetite (<30 μm width), along with spinel (Mg# = 0.42-0.52 & Cr# = 0.46-0.48), and chlorite (Mg# = 0.87-0.96). The Raman result indicates that both the veins and the mesh serpentine are composed of lizardite. The absence of brucite in the serpentinites suggests that Si-rich fluids infiltrated the ultramafic rocks of the Bayankhongor Ophiolite.
The samples taken from the green vein, which is the largest vein in the field, consist mainly of clinopyroxene (Mg# = 0.92) with minor of serpentine (Mg# = 0.98), epidote, and chlorite (Mg# = 0.92-0.93). Clinopyroxene is replaced by a mixture of chlorite-serpentine and cut by serpentine veins and epidote veins. Black veins are composed of chlorite patches (Mg# = 0.83-0.93) and patches consisting of a mixture of chlorite-serpentine with clear cleavages and fine ore-minerals (Ti-rich). The reaction zone (2.5-3 mm) between host serpentinite and black veins is observed and shows that magnetite veins have disappeared and been replaced by Al-rich (1.1-6.9 wt%) serpentine. Also, from vein to host rock, Al content in serpentine is decreased (from 1.4 to 0.6 wt%). These observations suggest that the protolith mineral of patches (mixture of chlorite-serpentine) could be clinopyroxene whereas the protolith of chlorite patches could be plagioclase.
These events imply that Si, Al, and Fe mobilities could occur locally at the crust-mantle section from the oceanic lithosphere during multi-stage serpentinization. We will discuss the relationship between multi-stage serpentinization and vein formations.
References:
Jian, P., Kroner, A., Windley, B.F., Shi, Y., Zhang, F., Miao, L., Tomurhuu, D., Zhang, W., Liu, D., 2010. Zircon ages of the Bayankhongor ophiolite mélange and associated rocks: Time constraints on Neoproterozoic to Cambrian accretionary and collisional orogenesis in Central Mongolia. Precambr. Res. 177, 162-180.
Guillot and Hattori, Serpentinites: Essential Roles in Geodynamics, Arc Volcanism, Sustainable Development, and the Origin of Life 2013 1811-5209/13/0009-095$2.50 DOI: 10.2113/gselements.9.2.95