Japan Geoscience Union Meeting 2019

Presentation information

[J] Poster

S (Solid Earth Sciences ) » S-EM Earth's Electromagnetism

[S-EM18] Geomagnetism, paleomagnetism and rock magnetism

Sun. May 26, 2019 3:30 PM - 5:00 PM Poster Hall (International Exhibition Hall8, Makuhari Messe)

convener:Hisayoshi Shimizu(Earthquake Research Institute, University of Tokyo), Masahiko Sato(Department of Earth and Planetary Science, The University of Tokyo)

[SEM18-P13] Rock-magnetic studies on seafloor hydrothermal deposits in the Okinawa Trough

*Chie Kato1, Masao Ohno1, Tadahiro Hatakeyama2, Jun-ichiro Ishibashi3, Shuhei Totsuka4, Kazuhiko Shimada3 (1.Department of Environmental Changes, Faculty of Social and Cultural Studies, Kyushu University, 2. Information Processing Center, Okayama University of Science, 3.Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University, 4. Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University)

Keywords:Hydrothermal deposits, Cu-Fe-S sulfide, Low-temperature magnetometry

Investigation of the mineral assemblage of seafloor hydrothermal deposits are crucial for understanding its formation environment and process. Rock-magnetic measurements of the deposits are quick and effective to detect magnetic metal sulfides which are sensitive to the environment. We report our results of rock-magnetic measurements conducted on sulfide ores from seafloor hydrothermal deposits in the Okinawa Trough. One sample is a sulfide ore of chimney structure collected from hydrothermal mound in the Noho Site and others are sediment core samples drilled at the Izena Hole.
X-ray diffraction (XRD) measurements and chemical analyses indicate that the chimney structured ore mainly consist of pyrrhotite and a Cu-Fe-S sulfide. Crystal structure of the Cu-Fe-S sulfide is similar to that of sphalerite and its chemical composition is close to CuFe2S3. From these characters, CuFe2S3 isocubanite is the candidate for the Cu-Fe-S sulfide.
Since the magnetic properties of CuFe2S3 isocubanite were rarely found in the literature, we first conducted low-temperature magnetometry measurements and magnetic hysteresis measurements of isocubanite. Isocubanite was synthesized by heating a cubanite sample (from Henderson No.2 mine, Chibougaman, Qubec, Canada) at 300 ℃ in vacuum (e.g. Sawada et al., 1962; Nenasheva and Kravchenko, 2015). Results of the magnetic measurements showed that isocubanite is weakly ferromagnetic at room temperature, and undergo a magnetic transition to relatively strong ferromagnetic at ~100 K. Saturation magnetization was 0.09 Am2/kg at room temperature and 0.4 Am2/kg at 50 K.
Similar magnetic measurements were performed on the Cu-Fe-S sulfide in the chimney ore which was magnetically separated from pyrrhotite. The Cu-Fe-S sulfide was paramagnetic at room temperature and became ferromagnetic at below ~100 K. Saturation magnetization at 50 K was ~20 Am2/kg. Such strong magnetization and sharp magnetic transition at low temperature is inconsistent with the behavior of isocubanite, but rather comparable to that of Cu1-εFe3+εS4 ‘new phase’ reported by Wintenberger et al. (1994) as an ageing product of isocubanite. Results of our magnetic measurements indicate that ‘new phase’ is a solid solution which includes CuFe2S3 composition.
Some of the sediment core samples contain a Cu-Fe-S sulfide with similar XRD pattern and chemical composition to that found in the chimney ore. Those sediment core samples also show a magnetic transition at ~100 K. Intensity of the magnetization at low temperature could be explained by assuming that the Cu-Fe-S sulfide in the sediment core samples is isocubanite, based on rough estimate of mineral proportion from XRD.