11:00 〜 11:15
[SCG54-08] Element transport and the breakdown of magnetite during alteration of a gabbroic vein at the crust-mantle transition zone, the Bayankhongor Ophiolite, Mongolia.
キーワード：Element transport, Serpentinization, Gabbroic vein
The hydration of the oceanic lithosphere is an important geological process that plays a crucial role in the global water cycle and element transport. Serpentine minerals contain approximately 13 wt% of structurally bound water. This process also drives changes in the oxidation state of iron, leading to the production of hydrogen and hydrocarbons (Klein et al., 2009; Yoshida et al., 2024), which can serve as an energy source for microbial ecosystems on the seafloor, within submarine hydrothermal systems, and in mid-ocean ridges. Additionally, mantle hydration influences Earth's water circulation, element transport, and metasomatic transformations at the crust-mantle transition zone in the oceanic lithosphere. However, studies on element transport during the multistage hydration of mantle rocks in this zone, particularly in mid-ocean ridge-derived oceanic lithosphere, remain insufficient.
To address this, we examine the serpentinization and element transport processes at the crust-mantle transition within the Bayankhongor ophiolite, the most extensive mid-ocean ridge-derived ophiolite in Mongolia (Jian et al., 2010). This well-preserved ophiolite presents a crucial opportunity to investigate hydration-related metamorphism, including the influence of crustal veins on element transfer within the hydrated mantle. Through this study, we analyzed mantle rocks and associated crustal veins at the crust-mantle transition zone to understand element migration and iron oxidation during serpentinization.
Based on field observation outcrop of the crust-mantle section (~30 m in diameter) in the Bayankhongor is characterized by a brownish gabbroic body with a massive and sheared mantle body fully serpentinized. Mantle rock samples mainly consist of lizardite in two forms: mesh core (Mg# = 0.95-0.98) with fine magnetite (Mgt) and vein (Mg# = 0.94-0.98) with vein Mgt (<30 µm width), along with spinel (Mg# = 0.42-0.52 & Cr# = 0.46-0.48), and chlorite (Chl; Mg# = 0.87-0.96). The absence of brucite in the serpentinites suggests infiltration of Si-rich fluids. Green veins (80-95 cm in width; it mainly consists of clinopyroxene (Cpx; Mg# = 0.92) replaced by a mixture of Chl-serpentine (Srp) and cut by serpentine and epidote (Ep) veins), along with white veins (~15 cm in width; ~40 cm long; it is mostly consisted of Ep and Cpx with a minor amount of Chl) cut through the mantle rocks. Additionally, black veins (~2 cm in width; it is composed of Chl patches (Mg# = 0.83-0.93) and Chl-Srp patches with clear cleavages and fine Ti-rich minerals) intersect the serpentinite.
The reaction zone (~3 mm) between host serpentinite and black vein shows that Mgt disappeared and Mgt is replaced by Al-rich (1.1-6.9 wt%) Srp. Mass balance calculation on black vein (assuming protoliths: Cpx for Chl-Srp and plagioclase for Chl patch) shows gain of Fe (6.85wt%) and Mg (27.63wt%), and loss of Si (-19.44wt%), Al (-1.6wt%), and Ca (-4.6wt%) whereas that on the reaction zone shows loss of Fe (-3.65wt%) and gain of Si (0.25wt%), and Al (0.44wt%). This implies that Mg-rich fluid and chl formation cause Mgt disappearance and mobility of Fe, Si, and Al. The mass balance calculation on the crust-mantle transition zone implies that local mobility of Si, Al, Fe, Mg, and Ca could occur at the crust-mantle section in the oceanic lithosphere during multi-stage hydration. Additionally, Hydrogen is generated through the reduction of water as ferrous iron (Fe2+) undergoes oxidation to ferric iron (Fe3+). Since magnetite is a primary Fe3+bearing mineral in serpentinite, H2 production is associated with the quantity of magnetite formed (Malvoisin et al., 2012; Yoshida et al., 2024). X-ray absorption fine structure (XAFS) analysis provides valuable insights into the oxidation state and coordination environment of iron (Henderson et al., 2014). An investigation of Fe3+ distribution using XAFS analysis in this study reveals that despite the disappearance of magnetite, the Fe3+/ΣFe ratio of the host serpentine (0.65-0.77) remains consistent with that of the magnetite-depleted reaction zone containing chlorite veins (0.65-0.74), suggesting that the overall iron ratio remains stable during alteration.
To address this, we examine the serpentinization and element transport processes at the crust-mantle transition within the Bayankhongor ophiolite, the most extensive mid-ocean ridge-derived ophiolite in Mongolia (Jian et al., 2010). This well-preserved ophiolite presents a crucial opportunity to investigate hydration-related metamorphism, including the influence of crustal veins on element transfer within the hydrated mantle. Through this study, we analyzed mantle rocks and associated crustal veins at the crust-mantle transition zone to understand element migration and iron oxidation during serpentinization.
Based on field observation outcrop of the crust-mantle section (~30 m in diameter) in the Bayankhongor is characterized by a brownish gabbroic body with a massive and sheared mantle body fully serpentinized. Mantle rock samples mainly consist of lizardite in two forms: mesh core (Mg# = 0.95-0.98) with fine magnetite (Mgt) and vein (Mg# = 0.94-0.98) with vein Mgt (<30 µm width), along with spinel (Mg# = 0.42-0.52 & Cr# = 0.46-0.48), and chlorite (Chl; Mg# = 0.87-0.96). The absence of brucite in the serpentinites suggests infiltration of Si-rich fluids. Green veins (80-95 cm in width; it mainly consists of clinopyroxene (Cpx; Mg# = 0.92) replaced by a mixture of Chl-serpentine (Srp) and cut by serpentine and epidote (Ep) veins), along with white veins (~15 cm in width; ~40 cm long; it is mostly consisted of Ep and Cpx with a minor amount of Chl) cut through the mantle rocks. Additionally, black veins (~2 cm in width; it is composed of Chl patches (Mg# = 0.83-0.93) and Chl-Srp patches with clear cleavages and fine Ti-rich minerals) intersect the serpentinite.
The reaction zone (~3 mm) between host serpentinite and black vein shows that Mgt disappeared and Mgt is replaced by Al-rich (1.1-6.9 wt%) Srp. Mass balance calculation on black vein (assuming protoliths: Cpx for Chl-Srp and plagioclase for Chl patch) shows gain of Fe (6.85wt%) and Mg (27.63wt%), and loss of Si (-19.44wt%), Al (-1.6wt%), and Ca (-4.6wt%) whereas that on the reaction zone shows loss of Fe (-3.65wt%) and gain of Si (0.25wt%), and Al (0.44wt%). This implies that Mg-rich fluid and chl formation cause Mgt disappearance and mobility of Fe, Si, and Al. The mass balance calculation on the crust-mantle transition zone implies that local mobility of Si, Al, Fe, Mg, and Ca could occur at the crust-mantle section in the oceanic lithosphere during multi-stage hydration. Additionally, Hydrogen is generated through the reduction of water as ferrous iron (Fe2+) undergoes oxidation to ferric iron (Fe3+). Since magnetite is a primary Fe3+bearing mineral in serpentinite, H2 production is associated with the quantity of magnetite formed (Malvoisin et al., 2012; Yoshida et al., 2024). X-ray absorption fine structure (XAFS) analysis provides valuable insights into the oxidation state and coordination environment of iron (Henderson et al., 2014). An investigation of Fe3+ distribution using XAFS analysis in this study reveals that despite the disappearance of magnetite, the Fe3+/ΣFe ratio of the host serpentine (0.65-0.77) remains consistent with that of the magnetite-depleted reaction zone containing chlorite veins (0.65-0.74), suggesting that the overall iron ratio remains stable during alteration.