HR: 14:55h
AN: V53B-06    [Abstracts]
TI: Thallium Isotope Constraints on Hydrothermal Water Fluxes at Mid-Ocean Ridge Axes and Flanks
AU: * Rehkamper, M
EM: markr@erdw.ethz.ch
AF: Dept. of Earth Sciences, ETH Zurich, Sonneggstr. 5, CH-8092, Zurich Switzerland
AU: Nielsen, S G
EM: sune@erdw.ethz.ch
AF: Dept. of Earth Sciences, ETH Zurich, Sonneggstr. 5, CH-8092, Zurich Switzerland
AU: Alt, J C
EM: jalt@umich.edu
AF: Dept. of Geological Sciences, University of Michigan, Ann Arbor, MI 48109 United States
AU: Butterfield, D A
EM: David.A.Butterfield@noaa.gov
AF: Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, Seattle, WA 98195 United States
AB: The hydrothermal circulation that occurs at mid-ocean ridge axes and flanks has profound effects on the chemical budgets of the oceans but our understanding of the relevant fluxes is incomplete. Here, we use new thallium (Tl) isotope and concentration data for hydrothermal fluids and rocks from ODP Hole 504B to obtain independent estimates of the high- and low-temperature (T) hydrothermal water fluxes at spreading axes and ridge flanks. Seawater is characterized by relatively uniform Tl isotope compositions and concentrations of $\epsilon^{205}Tl$ = -6 and 65 pmol/kg, respectively ($\epsilon^{205}Tl$ represents the deviation of the $^{205}Tl/^{203}Tl$ ratio of a sample from the standard in parts per 10,000). In contrast, high-T hydrothermal fluids from ridges axes display $\epsilon^{205}Tl = -2\pm1$, indistinguishable from unaltered mantle rocks. The correlation of Tl and Cl abundances indicates an average Tl content of 10-25 nmol/kg for high-T endmember fluids. The low-T alteration of the upper volcanic zone of ODP Hole 504B is associated with Tl-uptake from seawater. The isotope fractionation that occurs during the uptake generates Tl-rich rocks that have $\epsilon^{205}Tl$ as low as -16. The sheeted dike complex displays low Tl contents due to leaching of the rocks by high-T hydrothermal fluids. Taken together, these observations indicate that high-T vent fluids do not acquire significant Tl from the altered Tl-rich rocks of the volcanic section. With this constraint, the high-T axial water flux can be calculated, assuming that Tl is leached with an efficiency of $60-95%$ from 1.0-1.4 km of sheeted dikes and upper gabbros, which have a Tl concentration of $3\pm1$ ppb. These parameters yield a high-T water flux of 0.2-2.5 x $10^{13}$ kg/yr, equivalent to a heat flux of 0.1 to 1.2 TW. This result is in excellent agreement with other geochemical estimates of high-T water fluxes, e.g., those based on Li isotopes (Chan et al., 2002) or the Sr isotope profile of ODP Hole 504B (Teagle et al., 2003). If the total axial hydrothermal power output is about 2 TW, the geochemical data indicate that at least some heat loss at mid-ocean ridges is due to diffuse low-T fluid flow. The Tl data acquired for off-axis fluids and the volcanic zone rocks are in accord with significant ($30-80%$) losses of Tl from circulating seawater. If it is assumed that 15-35 ppb of Tl are added to the top 600 m of ocean crust, the low-T water flux of ridge flanks can be calculated as 1-5 x $10^{16}$ kg/yr, which is equivalent to an average fluid exit temperature of about $3-10\deg$C. This result is in accord with a recent Mg-based estimate (Mottl, 2003), which indicates that about $90-98%$ of the ridge flank power output occurs at cool sites with fluid temperatures of less than $20\deg$C.
DE: 4825 Geochemistry
DE: 4875 Trace elements
DE: 1030 Geochemical cycles (0330)
DE: 1045 Low-temperature geochemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting