Reaction of carbon dioxide (CO2) with minerals to generate stable carbonates, also known as CO2 mineralization, has been regarded as one of the most promising methods for safe and permanent carbon storage. As a promising feedstock, basaltic rock has gained special interest, and elevating basalt carbonation efficiency with the reduction of negative environmental impact is the main challenge for CO2 mineralization system development. Considering multiple potential positive effects of the CO2 carrier, NaHCO3, we conducted this study to experimentally evaluate the CO2 storage efficiency during water-basalt-NaHCO3 interactions under hydrothermal conditions at 200–300°C. The inclusion of NaHCO3 was confirmed to drastically promote the alteration of basalt, especially at higher temperatures. As revealed by experiments conducted at the saturated vapor pressure of water, the carbon storage efficiency at 300°C reached 75 g/kg of basalt in 5 days, which was 12 times higher than that at 200°C. In such hydrothermal systems, basalt was carbonated to generate calcite (CaCO3), where the Ca was mainly from plagioclase; Mg and Fe were incorporated into smectite, and Na in the saline system participated in the formation of Na silicates (i.e., analcime in the case of basalt). Due to the presence of additional Na in solution, all the released elements were consumed quickly with generation of secondary minerals in turn promoted basalt dissolution to release more Ca for CO2 storage. This study illuminated the role of NaHCO3 in basalt carbonation and provided technical backup to the design of advanced CO2 mineralization systems.
Magnetite veins are commonly observed in serpentinized peridotite, but the mobility of iron during serpentinization is poorly understood. The completely serpentinized ultramafic rocks (originally dunite) in the Taishir Massif in the Khantaishir ophiolite, western Mongolia, contain abundant antigorite + magnetite (Atg + Mag) veins, which show an unusual distribution of Mag. The serpentinite records multi-stage serpentinization in the order: (1) Atg + lizardite (Lz) with a hourglass texture (Atg-Lz); (2) thin vein networks and thick veins of Atg; (3) chrysotile (Ctl) that cuts all earlier textures. Mg# values of the Atg-Lz (0.94-0.96) are lower than those of the Atg (~0.99) and chrysotile (~0.98). In the Atg-Lz regions, magnetite occurs as arrays of fine grains (<50 µm) around the hourglass texture, and magnetite is absent in the thin Atg vein networks replacing Atg-Lz. Magnetite occurs as coarse grains (100-250 µm) in the center of some thick Atg veins. As the volume ratio of thin Atg veins to Atg-Lz increases, both the modal abundance of Mag and the bulk iron content decrease. These features indicate that hydrogen generation occurred mainly during Atg-Lz formation, and that the Mag distribution was largely modified by dissolution and precipitation in response to the infiltration of the higher temperature fluids associated with the Atg veins. The transport of iron during redistribution of Mag in the late-stage of serpentinization is potentially important for ore deposit formation and modifying the magnetic properties of ultramafic bodies.
Unmanned aerial vehicles (UAVs) or drones have revolutionized scientific research in multiple fields. Drones provide us multiple advantages over conventional geological mapping or high-altitude remote sensing methods, in which they allow us to acquire data more rapidly of inaccessible or risky outcrops, and can connect the spatial scale gap in mapping between manual field techniques and airborne, high-altitude remote sensing methods. Despite the decreased cost and technological developments of platforms, sensors and software, the use of drones for geological mapping in Mongolia has not yet been utilized. In this study, we present using of drone in two areas: the Chandman area in which eclogite is exposed and the Naran massif of the Khantaishir ophiolite in the Altai area. Drone yields images with high resolution that is reliable to use and reveals that it is possible to make better formulation of geological mapping. Our suggestion is that (1) Mongolian geoscientists are encouraged to add drones to their geologic toolboxes and (2) drone could open new advance of geological mapping in Mongolia in which geological map will be created in more effective and more detailed way combined with conventional geological survey on ground.
Hydration, carbonation, and related metasomatism of mantle peridotite play a significant role in the global geochemical cycle. In this study, we combined an analysis of carbonated serpentinite with hydrothermal experiments on carbonation and Ca-metasomatism for samples from the Manlay ophiolite, southern Mongolia to investigate that carbonation mechanism of the serpentinite body after serpentinization. Samples show that the serpentinite was either transected by calcite and dolomite veins or was completely replaced by carbonates (calcite with minor dolomite) and quartz, in which the original mesh texture of serpentinite was preserved. Carbonation occurred after low-temperature serpentinization (lizardite/chrysotile), suggesting that carbonation occurred at temperatures lower than 300 ˚C. Calcite in the serpentinite showed δ13 CVPDB values ranging from -8.83 to -5.11 ‰ and δ18 OVSMOW from + 20.1 to + 24.4 ‰, suggesting that CO2 in the fluids could be derived from the degradation of organic material or methanotrophic processes rather than the origin of seafloor limestone. Three batch-type experiments, i.e., single step experiments (1) Olivine + NaHCO3,aq + CaCl2,aq and (2) Chrysotile + NaHCO3,aq + wollastonite (Ca source), and two steps experiment (3) Olivine carbonation and Ca-metasomatism, were conducted at 275 °C and 5.7 MPa to constrain the mechanism of calcite replacement of serpentinite. We found that calcite precipitated from the solution directly in the first two experiments, but replacement of serpentinite by calcite was not observed. In contrast, the third experiment caused the initial carbonation to form magnesite and then changed to calcite by later alteration. The natural occurrences and experiments revealed the possibility that the carbonation of olivine followed by Ca-rich fluid infiltration produced calcite in the carbonated serpentinite. Such Ca-metasomatism of Mg carbonates could easily occur in the ultramafic bodies and significantly affect the global carbon cycle.
<p>Magnetite commonly forms during serpentinization of mantle peridotite, involving the hydrogen generation within the oceanic lithosphere. Although magnetite is concentrated in veins, the mobility of iron during serpentinization is still poorly understood. The completely serpentinized ultramafic rocks (originally dunite) within the Taishir massif in the Khantaishir ophiolite, western Mongolia, include abundant magnetite + antigorite veins, which manifest novel distribution of magnetite. The serpentinite records the multi-stage serpentinization, in order of (1) Al-rich antigorite + lizardite mixture with hourglass texture (Al<sub>2</sub>O<sub>3</sub> = 0.46-0.69 wt%; Atg+Lz), (2) Al-poor antigorite composed of thick veins and their branches (Atg), and (3) chrysotile that cut all previous textures. The Mg# (= Mg/ (Mg + Fe<sub>total</sub>)) of Atg+Lz (0.94-0.96) is lower than Atg (0.99) and chrysotile (0.98). In the region of Atg+Lz, magnetite occurs as the arrays of fine grains (<50 &#956;m) around the hourglass texture. In the Atg veins replacing Atg+Lz, magnetite disappears and re-precipitated as coarse grains (100-250 &#956;m) in the center of some veins. As the extent of replacement of Atg+Lz by Atg veins increases, both modal abundance of magnetite and the bulk Fe content decrease. These characteristics indicate that hydrogen generation mainly occurred at the stage of Atg+Lz formation, and magnetite distribution was largely modified via dissolution and precipitation in response to later fluid infiltration associated with the Atg veins. This also indicates the high iron mobility within the serpentinized peridotites even after the primary stage of magnetite formation.</p>
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