JpGU-AGU Joint Meeting 2020

Presentation information

[E] Poster

S (Solid Earth Sciences ) » S-MP Mineralogy & Petrology

[S-MP37] Supercontinents and crustal evolution

convener:Madhusoodhan Satish-Kumar(Department of Geology, Faculty of Science, Niigata University), Krishnan Sajeev(Centre for Earth Sciences, Indian Institute of Science), Tomokazu Hokada(National Institute of Polar Research), Yasuhito Osanai(Division of Evolution of Earth Environments, Faculty of Social and Cultural Studies, Kyushu University)

[SMP37-P08] Native iron and serpentinization of peridotite, Oshika Village, Nagano Prefecture, Japan

*Yuya Takeda1, Seiichiro Uehara2 (1.Department of Earth and Planetary Science, Graduate School of Science, Kyushu University, 2.Department of Earth and Planetary Science, Faculty of Science, Kyushu University)

Keywords:serpentinization, iron

Serpentinization is a reaction to form serpentine from peridotite and hot water, and Fe2+ becomes Fe3+ since magnetite is formed from Fe2+ in forsterite. At that time, hydrogen is generated, and a more reducing environment is formed iron, awaruite (Ni2Fe-Ni3Fe) (Frost, 1985).
Peridotite in Oshika Village, Shimoina-gun Nagano Prefecture is located mainly in Irisawai and Ogawara, and iron formed by serpentinization is reported for the first time in Japan (Okamoto et al., 1981; Sakai & Kuroda, 1983). Serpentine veins have three stages. Iron is found in veins composed chrysotile-2Mc1 and brucite in the 2nd stage of serpentine veins. In this study, the peculiar serpentine vein was reexamined for the purpose of the elucidation of the formation of iron and awaruite.
Obtained samples were analysed by powder X-ray diffraction (XRD) to determine the constituent serpentine species.The selected samples were analyzed by optical microscope, scanning electron microscope (SEM) with energy dispersive spectrometer (EDS), electron microprobe analyzer (EPMA) and transmission electron microscope (TEM). Experimental devices was Ultima Ⅳ and Rint Rapid Ⅱ (XRD), JSM-7001 (SEM), JMS-8530FF (EPMA). TEM observation was carried out at 300kV using JEM 3200FSK at the Ultramicroscopy Research Center at Kyushu university.
The peridotite is composed of forsterite and chromite, linear serpentine veins ranging from a few cm to 10 of µm in width. There were up to 4 directions of the vein. Serpentinization was made to be 4 stages from the anteroposterior relation of the vein by the observation by the slice. The vein of these 4 stages was made to be serpentine veins I, II, III, IV from the early one. The serpentine veins consisted lizardite, chrysotile-2Mc1, brucite, magnetite, iron, and awaruite. The serpentine veins are characterized in Table 1. Serpentine vein IV have elliptical and striated areas rich in brucite. Iron has irregular shapes, and some change to magnetite. Awaruite is a minute crystal, and it is unevenly distributed in the vein. The chemical compositions of serpentine and brucite in the veins were measured by SEM-EDS, and #Mg=Mg/ (Mg + Fe) was obtained. The serpentine of veins I and II was #Mg=0.94, that of veins III and IV was #Mg=0.96, and that of brucite was #Mg=0.66~0.79 0.77~0.86.
Iron is in serpentine veins III and IV, ranging in size from 1µm to 100µm, and contains up to 3.2% Ni and 3.1% Co. Iron of serpentine vein III was arranged in the position in which the long axis of the ellipse was perpendicular to the serpentine vein, and the iron of serpentine vein IV was irregularly shaped. Native iron was also found in the serpentine veins, which were not linear. Awaruite is found in all serpentine veins and most of them are in the range of 50 nm to several 100 nm, and many of them are automorphic crystal. Awaruite consists of Ni0.79Fe to Ni2.55Fe and is rich in iron.