16:00 〜 16:15
[SCG52-18] Geochemistry of hydrothermal ferromanganese oxides from Daigo-Kume Knoll in the Okinawa Trough
キーワード：鉄マンガン酸化物、海底熱水活動、熱水変質、沖縄トラフ
Hydrothermal fluids venting from the seafloor form mineral deposits which are sulfides, as potential mineral resources, at high temperature and valueless ferromanganese oxides at low temperature. Hydrothermal activities have been recognized at many sites in the Okinawa Trough, however there is no report about active hydrothermal site and evidence of it at Daigo-Kume Knoll where is the study site of this study.
Ferromanganese oxides, pumices and mudstones were dredged from Daigo-Kume Knoll in the middle segment of the Okinawa Trough during the cruise KH-22-3. In this study, geochemical and mineralogical analyses were performed on the rock samples to reveal the origin and formation process of the ferromanganese oxides. All ferromanganese oxide samples were classified into hydrothermal origin based on the geochemical discrimination diagram1. Birnessite (7 Å manganate) is contained in all ferromanganese oxide samples. In contrast, todorokite (10 Å manganate) is contained in 9 of the 13 samples. Todorokite, precipitated from low-temperature hydrothermal fluids (generally <150℃), shows low structural stability and changes to birnessite due to drying2. Therefore, it is considered that the ferromanganese oxides were precipitated from low-temperature hydrothermal fluids. Parts of the rim of some ferromanganese oxide samples are enriched in Fe, Ni, and Cu. In addition, vernadite, that is manganate of hydrogenetic origin, is detected in some ferromanganese oxide samples. These results indicate that hydrogenetic ferromanganese oxides grew on the rim exposed to seawater after the cessation of the low-temperature hydrothermal activity. Altered pumice and ferromanganese oxide samples contain celadonite, and a mudstone sample contains phillipsite and calcite.
It is suggested that the samples were formed by the two-stage hydrothermal activity: (1) primary hydrothermal fluids precipitating Fe oxides and altering silicate debris and (2) secondary hydrothermal fluids precipitating Mn oxides, based on independent component analysis for the elemental mapping data of the selected samples. The temperature of the primary hydrothermal fluid is considered to be higher than that of the secondary hydrothermal fluid because unaltered silicate debris and the silicate debris hydrothermally altered by the primary hydrothermal fluid are cemented by the Mn oxides precipitated from the secondary hydrothermal fluid. The temperature of the primary hydrothermal fluid would be lower than 150 ℃ because the celadonite, contained in the samples, is caused by hydrothermal alteration lower than 150 ℃3. In contrast, the temperature of the secondary hydrothermal fluid might have been around 33 ℃, because the structural stability of todorokite (depending on the formation temperature)2 in the samples is similar to that of Kaikata Seamount where the temperature of the hydrothermal fluids has been reported to be maximum 33 ℃4.
The median of Mo/Mn ratio of the ferromanganese oxide samples is 3.41×103, which is higher than that of previous studies from hotspots, volcanic arcs, and back arcs (1.14×103, 1.14×103, and 2.44×103, respectively). Because Mo is preferentially leached by water–rock interaction above about 310 ℃5, the high Mo/Mn ratio of the samples indicates that the hydrothermal fluids were originally caused by the high-temperature water–rock interaction. As the samples were obtained from the outer rim of the caldera in Daigo-Kume Knoll, heat source of such high-temperature water–rock interaction might have been magma in the center of the caldera. Therefore, high-temperature hydrothermal activity might be occurred in the caldera. Because high-temperature hydrothermal activity usually forms sulfide deposits, the existence of submarine sulfide deposits in the caldera is implied by this study.
1. Josso, P. et al. Ore Geol. Rev. 87, 3–15 (2017).
2. Usui, A. et al. Mar. Geol. 86, 41–56 (1989).
3. Lowell, R. P. et al. J. Geophys. Res. Solid Earth 100, 327–352 (1995).
4. Shinji, T. et al. JAMSTEC J. Deep Sea Res. 18, 209–215 (2001).
5. Trefry, J. H. et al. J. Geophys. Res. Solid Earth 99, 4925–4935 (1994).
Ferromanganese oxides, pumices and mudstones were dredged from Daigo-Kume Knoll in the middle segment of the Okinawa Trough during the cruise KH-22-3. In this study, geochemical and mineralogical analyses were performed on the rock samples to reveal the origin and formation process of the ferromanganese oxides. All ferromanganese oxide samples were classified into hydrothermal origin based on the geochemical discrimination diagram1. Birnessite (7 Å manganate) is contained in all ferromanganese oxide samples. In contrast, todorokite (10 Å manganate) is contained in 9 of the 13 samples. Todorokite, precipitated from low-temperature hydrothermal fluids (generally <150℃), shows low structural stability and changes to birnessite due to drying2. Therefore, it is considered that the ferromanganese oxides were precipitated from low-temperature hydrothermal fluids. Parts of the rim of some ferromanganese oxide samples are enriched in Fe, Ni, and Cu. In addition, vernadite, that is manganate of hydrogenetic origin, is detected in some ferromanganese oxide samples. These results indicate that hydrogenetic ferromanganese oxides grew on the rim exposed to seawater after the cessation of the low-temperature hydrothermal activity. Altered pumice and ferromanganese oxide samples contain celadonite, and a mudstone sample contains phillipsite and calcite.
It is suggested that the samples were formed by the two-stage hydrothermal activity: (1) primary hydrothermal fluids precipitating Fe oxides and altering silicate debris and (2) secondary hydrothermal fluids precipitating Mn oxides, based on independent component analysis for the elemental mapping data of the selected samples. The temperature of the primary hydrothermal fluid is considered to be higher than that of the secondary hydrothermal fluid because unaltered silicate debris and the silicate debris hydrothermally altered by the primary hydrothermal fluid are cemented by the Mn oxides precipitated from the secondary hydrothermal fluid. The temperature of the primary hydrothermal fluid would be lower than 150 ℃ because the celadonite, contained in the samples, is caused by hydrothermal alteration lower than 150 ℃3. In contrast, the temperature of the secondary hydrothermal fluid might have been around 33 ℃, because the structural stability of todorokite (depending on the formation temperature)2 in the samples is similar to that of Kaikata Seamount where the temperature of the hydrothermal fluids has been reported to be maximum 33 ℃4.
The median of Mo/Mn ratio of the ferromanganese oxide samples is 3.41×103, which is higher than that of previous studies from hotspots, volcanic arcs, and back arcs (1.14×103, 1.14×103, and 2.44×103, respectively). Because Mo is preferentially leached by water–rock interaction above about 310 ℃5, the high Mo/Mn ratio of the samples indicates that the hydrothermal fluids were originally caused by the high-temperature water–rock interaction. As the samples were obtained from the outer rim of the caldera in Daigo-Kume Knoll, heat source of such high-temperature water–rock interaction might have been magma in the center of the caldera. Therefore, high-temperature hydrothermal activity might be occurred in the caldera. Because high-temperature hydrothermal activity usually forms sulfide deposits, the existence of submarine sulfide deposits in the caldera is implied by this study.
1. Josso, P. et al. Ore Geol. Rev. 87, 3–15 (2017).
2. Usui, A. et al. Mar. Geol. 86, 41–56 (1989).
3. Lowell, R. P. et al. J. Geophys. Res. Solid Earth 100, 327–352 (1995).
4. Shinji, T. et al. JAMSTEC J. Deep Sea Res. 18, 209–215 (2001).
5. Trefry, J. H. et al. J. Geophys. Res. Solid Earth 99, 4925–4935 (1994).