Japan Geoscience Union Meeting 2015

Presentation information


Symbol S (Solid Earth Sciences) » S-CG Complex & General

[S-CG64] Ocean Floor Geoscience

Wed. May 27, 2015 6:15 PM - 7:30 PM Convention Hall (2F)

Convener:*Kyoko Okino(Ocean Research Institute, University of Tokyo), Keiichi Tadokoro(Research Center for Seismology, Volcanology and Earthquake and Volcano Research Center, Nagoya University), Osamu Ishizuka(Geological Survey of Japan, AIST), Tomohiro Toki(Faculty of Science, University of the Ryukyus), Narumi Takahashi(Research and Development Center for Earthquake and Tsunami, Japan Agency for Marine-Earth Science and Technology)

6:15 PM - 7:30 PM

[SCG64-P03] Geochemistry of trace alkali elements in the seafloor hydrothermal fluids

*Naoya EBINA1, Jun-ichiro ISHIBASHI2 (1.Department of Earth and Planetary Sciences, Graduate School of Sciences, Kyushu University, 2.Department of Earth and Planetary Sciences, Faculty of Science, Kyushu University)

Keywords:trace alkali elements, hydrothermal fluids, arc, back-arc basin, sediment

Hydrothermal fluid contains many elements at high concentrations as a result of fluid interaction with rock/sediment and seawater dusring fluid circulation beneath the subseafloor. In particular, Rb and Cs are known as "soluble elements" which is easily leached from the rock/sediment into the fluid because of their large ion radii. Thus, trace alkali element compositions of hydrothermal fluids would provide information about water/rock interactions.
We determined Rb and Cs concentrations of hydrothermal fluids collected from four fields in the Izu-Ogasawara arc (Myojin Knoll Caldera, Myojinsho Caldera, Bayonnaise Knoll Caldera and Suiyo Seamount), from six fields in the Mariana Trough (Alice Springs Field, Forecast Vent Field, Pika Site, Archean Site, Snail Site and Urashima Site), and from the Iheya North Knoll hydrothermal field in the Okinawa Trough, to discuss their diversity.
Analysis of Rb and Cs concentrations of each sample was conducted using ICP-QMS. To determine the endmember Rb and Cs compositions for each hydrothermal field, the analytical results of the samples were extrapolated to zero Mg concentration. The endmember concentrations of Rb and Cs are plotted in Figure 1. In addition to the results of this study, data from hydrothermal field in the EPR 21oN[1] and MAR (TAG and MARK)[6] located in sediment-starved mid ocean ridge setting, in the Escanaba Trough and Guaymas Basin[2] located in a sediment-hosted setting, and in the Lau Basin[3] and Manus Basin[4] located in a back-arc basin setting are plotted in the same figure. Moreover, compiled data for volcanic rocks and sediment material around these hydrothermal field are overimposed as shaded region in Figure 1.
A range of Rb/Cs ratio of hydrothermal fluids from an arc setting (square symbols; Rb/Cs=12.8 to 26.7) can be distinctive that from a back-arc setting (circle symbols; Rb/Cs=18.6 to 100.1). Rb and Cs concentrations in hydrothermal fluids from a sediment-hosted hydrothermal field is characterized by their substantially high concentrations. Moreover, it is likely that the range of Rb/Cs ration of hydrothermal fluids are comparable for those of volcanic rocks/sediment surrounding these hydrothermal field. It would be suggest that the distribution of Cs from rocks to hydrothermal fluids in arc setting is higher than one in other tectonic setting.

[1] Palmer and Edmond (1989) Earth and Plan. Sci. Let., 95, 8-14.
[2] Campbell et al. (1994) U.S. Geol. Surv. Bull, No. 2022, 201-221.
[3] Mottl et al. (2011) Geochimica et Cosmochimica Acta, 75, 1013-1038.
[4] Reeves et al. (2011) Geochimica et Cosmochimica Acta, 75, 1088-1123.
[5] de Ronde et al. (2011) Miner Deposita, 46, 541-584.
[6] Campbell et al. (1988) Nature, 335, 514-519.