6:15 PM - 7:30 PM
[SCG64-P03] Geochemistry of trace alkali elements in the seafloor hydrothermal fluids
Keywords:trace alkali elements, hydrothermal fluids, arc, back-arc basin, sediment
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 and MAR (TAG and MARK) located in sediment-starved mid ocean ridge setting, in the Escanaba Trough and Guaymas Basin located in a sediment-hosted setting, and in the Lau Basin and Manus Basin 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.
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