14:30 〜 14:45
[SMP25-16] Serpentinization and Carbonation processes of Ultramafic rocks in Manlay Ophiolite, Southern Mongolia
キーワード：Serpentinization, Carbonation, Manlay Ophiolite, South Mongolia
Carbonation is a natural carbon-capturing process that silicate and hydroxide minerals of mafic and ultramafic rock react with CO2 to form carbonate minerals such as magnesite, dolomite, and calcite as long-term carbon sequestration. One of the highly reactive materials for sequestering CO2 is serpentinite formed by the hydration of olivine-rich ultramafic rocks. An ophiolite is a primary focus for studying carbonation and serpentinization, which is interpreted as remnants of oceanic crust and upper mantle suites of ultramafic, mafic, and felsic rocks. Numerous ophiolites are situated in the Central Asian orogenic belt (CAOB), which is one of the largest orogenic belts in the world. However, metamorphic processes of ophiolites in the CAOB have been insufficiently studied. This study aims to understand the mechanism of multi-stage alteration of ultramafic rock in the Manlay Ophiolite, southern Mongolia, which is located at the central part of the CAOB, in order to understand serpentinization and carbonation processes.
The mineral assemblage of collected samples suggests that ultramafic rocks from the Manlay ophiolite are divided into three types; (1) serpentinized ultramafic rock composed of the low-temperature serpentine phase of lizardite (mesh) and chrysotile (mesh, bastite, and vein) with calcite veins and a small amount of magnetite and spinel. Based on X-ray elemental mapping, mesh lizardite shows low Al content which could indicate that firstly it replaced the olivine, and Al and Fe rich chrysotiles were precipitated later. (2) The partially carbonated serpentinite (lizardite and chrysotile >50%) that replaced by calcite and cut by calcite and dolomite veins with a small amount of diopside, magnetite, and spinel, (3) completely carbonated serpentinite that entirely replaced by calcite and quartz, and contains a small amount of dolomite, magnetite and spinel. The completely carbonated sample includes quartz, calcite and dolomite that show zonal texture as dolomite inside the calcite and surrounded by quartz, as well as the remained mesh texture of serpentine is found which was replaced by quartz in the core and magnetite in the rim.
The relation of microtextures demonstrates three stages. The first stage, protolithic ultramafic rock consisted of olivine and orthopyroxene with a small amount of clinopyroxene and spinel. Secondly, the hydration of ultramafic rock produced serpentinite. The last stage is determined as the carbonation of serpentinites. The reactions of olivine to serpentine and serpentine to calcite and dolomite ± quartz suggest that ultramafic rock was hydrated with Si-rich fluid to form serpentine without brucite, and serpentinite was reacted with Ca and CO2-rich fluid, and partially or completely carbonated by calcite and dolomite ± quartz. We are assuming that silica has dissolved during the continuous carbonation process, and pores have appeared due to volume change. After the serpentinite was completely carbonated, silica could precipitate in the pores which developed an uncommon texture. Therefore, the presence of carbonation in the Manlay ophiolite indicates that serpentinized ultramafic rocks could effectively capture and store CO2, and is an attractive occurrence to study calcium carbonate, especially calcite, dolomite and quartz assemblage.
The mineral assemblage of collected samples suggests that ultramafic rocks from the Manlay ophiolite are divided into three types; (1) serpentinized ultramafic rock composed of the low-temperature serpentine phase of lizardite (mesh) and chrysotile (mesh, bastite, and vein) with calcite veins and a small amount of magnetite and spinel. Based on X-ray elemental mapping, mesh lizardite shows low Al content which could indicate that firstly it replaced the olivine, and Al and Fe rich chrysotiles were precipitated later. (2) The partially carbonated serpentinite (lizardite and chrysotile >50%) that replaced by calcite and cut by calcite and dolomite veins with a small amount of diopside, magnetite, and spinel, (3) completely carbonated serpentinite that entirely replaced by calcite and quartz, and contains a small amount of dolomite, magnetite and spinel. The completely carbonated sample includes quartz, calcite and dolomite that show zonal texture as dolomite inside the calcite and surrounded by quartz, as well as the remained mesh texture of serpentine is found which was replaced by quartz in the core and magnetite in the rim.
The relation of microtextures demonstrates three stages. The first stage, protolithic ultramafic rock consisted of olivine and orthopyroxene with a small amount of clinopyroxene and spinel. Secondly, the hydration of ultramafic rock produced serpentinite. The last stage is determined as the carbonation of serpentinites. The reactions of olivine to serpentine and serpentine to calcite and dolomite ± quartz suggest that ultramafic rock was hydrated with Si-rich fluid to form serpentine without brucite, and serpentinite was reacted with Ca and CO2-rich fluid, and partially or completely carbonated by calcite and dolomite ± quartz. We are assuming that silica has dissolved during the continuous carbonation process, and pores have appeared due to volume change. After the serpentinite was completely carbonated, silica could precipitate in the pores which developed an uncommon texture. Therefore, the presence of carbonation in the Manlay ophiolite indicates that serpentinized ultramafic rocks could effectively capture and store CO2, and is an attractive occurrence to study calcium carbonate, especially calcite, dolomite and quartz assemblage.