5:15 PM - 6:45 PM
[SMP23-P07] Microstructure of serpentine veins containing iron in dunite in Oshika Village, Nagano Prefecture, Japan
Keywords:iron, serpentine, brucite
1. Introduction
Serpentinization is a reaction that produces serpentinite from olivine and hot water. Fe2+ becomes Fe3+ to produce magnetite from Fe2+ in forsterite. Hydrogen is produced and a more reducing environment is formed, and iron, awaruite (Ni2Fe-Ni3Fe) (Frost, 1985). The peridotites in Oshika Village, Nagano Prefecture, are located at Irizawai and Okawara. Iron produced by serpentinization was reported for the first time in Japan in serpentinite veins in these rocks (Okamoto et al., 1981; Sakai & Kuroda, 1983). The serpentine vein has three stages based on the relationship before and after the serpentine vein, and iron was found in the second stage of the serpentine vein, which is composed of clinochrysotile and brucite. In this study, microstructural observations were made on serpentine veins containing elliptical iron.
2. Analytical methods
Mineralogy of the bulk rock and serpentine veins was analyzed by X-ray diffractometer (Rigaku Ultima IV, Rigaku RINT RAPID II). The chemical composition and microstructure were analyzed by SEM (JEOL JSM-7001F) and FE-EPMA (JEOL JXA-8530F). In addition, thin film samples were prepared using FIB-SEM (FEI Quanta 3D 200i, HITACHI MI4000L, Thermo Fisher Scientific Helios 5 UX) at the Ultra Microscopy Research Center, Kyushu University, and transmission electron microscope (JEOL JEM-ARM300F2, JEOL JEM-ARM300F2, JEOL JEM-2100F) for microstructural observation and chemical composition measurement.
3. Result
The peridotite is composed of forsterite and chromite, with linear serpentine veins ranging from a few cm to several tens of μm in width. The serpentine veins are mainly composed of lizardite, polygonal serpentine, chrysotile, brucite, magnetite, iron, and awaruite. The serpentine veins occur in four directions, serpentine veins I, II, III, and IV, from their anteroposterior relationship. The chemical compositions of serpentine and brucite in the veins were measured by SEM-EDS. #Mg=Mg/(Mg+Fe) show that #Mg=0.94 for serpentine in serpentine veins I and II and #Mg=0.66~0.79 for brucite, #Mg=0.96 for serpentine and #Mg=0.77~0.86 for brucite in serpentine veins III and IV. Iron is present in serpentine veins III and IV. Serpentine vein III is 10 μm to 100 μm wide, gray-green in thin sections, and appears colorless and transparent under a polarized light microscope. There is a brucite-rich area in the center of the vein, and at the boundary between the vein and forsterite, a few to 20 μm long needle-like alteration areas are seen from the serpentine vein to the interior of the forsterite. Iron was elliptical in shape and was aligned with the major axis of the ellipse perpendicular to the vein. Awaruite with nm-sized crystals was observed at the rim and center of the vein. Serpentine vein IV was about 300 μm to 2 or 3 mm wide, with brown and colorless areas observed under polarized light microscopy. Brucite-rich areas in serpentine vein IV were circular and vein-like. Irregularly shaped iron and magnetite were observed. Antigorite containing an average of 2.8 wt% Mn was found in and around the veins. The needle-like alteration area was composed of lizardite and brucite, and the boundary with forsterite was serpentine with brucite in the center. The serpentine veins were composed of polygonal serpentine and chrysotile near forsterite. Near the center of the vein was composed of lizardite and brucite. The chemical compositions of serpentine and brucite were analyzed by STEM-EDS mapping. #Mg of serpentine in the needle alteration zone was 0.94 and that of brucite was 0.83. #Mg of lizardite in the vein was 0.96, #Mg of polygonal serpentine + chrysotile was 0.97, and #Mg of brucite was 0.87. Serpentine veins containing iron were lizardite and polygonal serpentine, chrysotile, and brucite. The composition of these serpentines does not contain Fe3+. In addition, magnetite surrounds the natural iron. These results suggest that iron is not formed by hydrogen produced by a change in the valence of Fe.
Serpentinization is a reaction that produces serpentinite from olivine and hot water. Fe2+ becomes Fe3+ to produce magnetite from Fe2+ in forsterite. Hydrogen is produced and a more reducing environment is formed, and iron, awaruite (Ni2Fe-Ni3Fe) (Frost, 1985). The peridotites in Oshika Village, Nagano Prefecture, are located at Irizawai and Okawara. Iron produced by serpentinization was reported for the first time in Japan in serpentinite veins in these rocks (Okamoto et al., 1981; Sakai & Kuroda, 1983). The serpentine vein has three stages based on the relationship before and after the serpentine vein, and iron was found in the second stage of the serpentine vein, which is composed of clinochrysotile and brucite. In this study, microstructural observations were made on serpentine veins containing elliptical iron.
2. Analytical methods
Mineralogy of the bulk rock and serpentine veins was analyzed by X-ray diffractometer (Rigaku Ultima IV, Rigaku RINT RAPID II). The chemical composition and microstructure were analyzed by SEM (JEOL JSM-7001F) and FE-EPMA (JEOL JXA-8530F). In addition, thin film samples were prepared using FIB-SEM (FEI Quanta 3D 200i, HITACHI MI4000L, Thermo Fisher Scientific Helios 5 UX) at the Ultra Microscopy Research Center, Kyushu University, and transmission electron microscope (JEOL JEM-ARM300F2, JEOL JEM-ARM300F2, JEOL JEM-2100F) for microstructural observation and chemical composition measurement.
3. Result
The peridotite is composed of forsterite and chromite, with linear serpentine veins ranging from a few cm to several tens of μm in width. The serpentine veins are mainly composed of lizardite, polygonal serpentine, chrysotile, brucite, magnetite, iron, and awaruite. The serpentine veins occur in four directions, serpentine veins I, II, III, and IV, from their anteroposterior relationship. The chemical compositions of serpentine and brucite in the veins were measured by SEM-EDS. #Mg=Mg/(Mg+Fe) show that #Mg=0.94 for serpentine in serpentine veins I and II and #Mg=0.66~0.79 for brucite, #Mg=0.96 for serpentine and #Mg=0.77~0.86 for brucite in serpentine veins III and IV. Iron is present in serpentine veins III and IV. Serpentine vein III is 10 μm to 100 μm wide, gray-green in thin sections, and appears colorless and transparent under a polarized light microscope. There is a brucite-rich area in the center of the vein, and at the boundary between the vein and forsterite, a few to 20 μm long needle-like alteration areas are seen from the serpentine vein to the interior of the forsterite. Iron was elliptical in shape and was aligned with the major axis of the ellipse perpendicular to the vein. Awaruite with nm-sized crystals was observed at the rim and center of the vein. Serpentine vein IV was about 300 μm to 2 or 3 mm wide, with brown and colorless areas observed under polarized light microscopy. Brucite-rich areas in serpentine vein IV were circular and vein-like. Irregularly shaped iron and magnetite were observed. Antigorite containing an average of 2.8 wt% Mn was found in and around the veins. The needle-like alteration area was composed of lizardite and brucite, and the boundary with forsterite was serpentine with brucite in the center. The serpentine veins were composed of polygonal serpentine and chrysotile near forsterite. Near the center of the vein was composed of lizardite and brucite. The chemical compositions of serpentine and brucite were analyzed by STEM-EDS mapping. #Mg of serpentine in the needle alteration zone was 0.94 and that of brucite was 0.83. #Mg of lizardite in the vein was 0.96, #Mg of polygonal serpentine + chrysotile was 0.97, and #Mg of brucite was 0.87. Serpentine veins containing iron were lizardite and polygonal serpentine, chrysotile, and brucite. The composition of these serpentines does not contain Fe3+. In addition, magnetite surrounds the natural iron. These results suggest that iron is not formed by hydrogen produced by a change in the valence of Fe.