2021 年 116 巻 3 号 p. 176-181
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.