HR: 1340h
AN: V13A-1450    [Abstracts]
TI: Cycling of Li, K, Rb, and Cs at Subduction Zones and Ridge Crests With Implications for Ocean Chemistry: Hydrothermal Experiments at 35-350$\deg$C and 600 bars
AU: * Wei, W
EM: wewei@ucsd.edu
AF: Scripps Inst. of Oceanography, SIO 0212, La Jolla, CA 92093
AU: Kastner, M
AF: Scripps Inst. of Oceanography, SIO 0212, La Jolla, CA 92093
AU: Rosenbauer, R
AF: USGS, 345 Middlefield Rd, Menlo Park, CA 94025
AU: Weinstein, Y
AF: Bar Ilan Univ., Dept of Geography, Ramat Gan, 52900 Israel
AU: Chan, L
AF: LSU, Dept of Geology and Geophysics, Baton Rouge, LA 70803
AB: The alkali metals (Li, K, Rb, Cs) are excellent tracers of fluid-rock reactions and cycling because of strong partitioning into the fluid phase, particularly at moderate to high temperatures. As such, they can provide critical information on the process of fluid recycling at the principal plate boundary, the reaction site temperature, the nature of the reacting solid phases (i.e. the involvement of sediment in arc volcanoes), with links to seawater (SW) chemistry. Our hydrothermal experiments indicate that the reactivity of each alkali metal is distinct; each has a characteristic behavior with respect to partitioning into the fluid phase with temperature. The data thus suggest that alkali concentration ratios together with Li isotopes may be used for geothermometry. MORB-SW and smectite-SW hydrothermal experiments were conducted between $35-350\deg$C at $25\deg$C, 600 bars, and a water/rock mass ratio of $\sim$5, using the Dickson-type rocking autoclave. Run-times continued until steady-state partitioning was observed. In the MORB-SW experiments, fluid K/Cl molar ratios first increase slightly, from 35-$65\deg$C, then decrease to a minimum of 1.1 x10$^{-2}$ at $275\deg$C. At $>$$275\deg$C the ratio sharply increases to 6.0 x10$^{-2}$ at $350\deg$C. In contrast, Li/Cl, Rb/Cl, and Cs/Cl ratios all increase from 35-$350\deg$C, but each at a distinct slope, indicating significant fractionation between the alkali metals. The Li/Cl ratio gently increases between 35-$250\deg$C, and sharply increases to a ratio of $\sim$1.20 x10$^{-3}$ at $350\deg$C. Rb/Cl behaves similarly, except for a higher inflection temperature of $\sim$$300\deg$C, and a steeper slope between 300-$350\deg$C. Cs/Cl behaves distinctly, the ratio increasing steadily with temperature (i.e. no inflection point) to 1.8 x10$^{-8}$ at $350\deg$C. Consequently, Li/K and Cs/K ratios exhibit sharp maxima at $\sim$$275\deg$C, at 35 x10$^{-3}$ $&$ 10 x10$^{-6}$, respectively. In the smectite-SW experiments there is no reversal in slope, thus the sediment influence on the fluid chemistry can be clearly identified. Rb/K, in contrast to Li/K and Cs/K, do not have a maximum, but steadily increase from 35-$350\deg$C in the MORB experiments. In the smectite experiments, the fluid Rb/K ratios are considerably higher, hence, are as well indicators of sediment contribution. At $350\deg$C the enrichment factors (ER) of Li/Cl and Cs/Cl are higher in the smectite than in the MORB experiments ($\sim$70 & 150 vs. $\sim$20 & 30, respectively). The ER of Rb/Cl is $\sim$10 in both experiments, but that of K/Cl is higher in the MORB vs. smectite experiments (3.0 & 1.7, respectively). Like Sr, the fluid Li totally exchanges its isotopes with MORB or with the smectite. In the MORB experiments, the fluid $\delta$$^{7}$Li fall on a SW-MORB mixing line, shifting from 30.95$\permil$, the SW value, to17.4$\permil$ at $150\deg$C, and 7.33$\permil$ at $350\deg$C. Hence, $\delta$$^{7}$Li values together with Li/K, Li/Rb and Li/Cs ratios constrain the reaction temperature ($\pm$20-$30\deg$C) and the nature of the rock involved. Applications of the experimental data (1) to pore fluids from two subduction zones: the Middle America Trench offshore Costa Rica, and the Nankai Trough, and to an 'average' ridge crest hydrothermal fluid composition, and (2) to revised Rb and Cs ocean budgets, will be presented.
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 8149 Planetary tectonics (5475)
DE: 3640 Igneous petrology
DE: 1025 Composition of the mantle
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting