HR: 0800h
AN: OS21A-1206    [Abstracts]
TI: Lithium Contents and Isotopic Compositions of Ferromaganese Deposits from the Global Oceans
AU: * Chan, L
EM: lchan@geol.lsu.edu
AF: Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA 70803-4101 United States
AU: Hein, J R
EM: jhein@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, MS 999, Menlo Park, CA 94025-3591 United States
AB: Trace elements and isotopes in marine ferromanganese crusts have been used to reconstruct the history of water-mass movement and composition in global oceans. Here we evaluate the feasibility of using Li concentration and isotopic composition as proxies of past seawater composition in response to varying mantle and continental contributions to the ocean. This is possible because hydrothermal and fluvial inputs have distinct Li isotope signatures. We analyzed a suite of ferromanganese deposits from the Indian, Atlantic and Pacific Oceans. There are three main types of deposits: hydrogenetic crust precipitated from seawater, stratabound deposits from hydrothermal fluids, and mixed hydrogenetic and hydrothermal Fe/Mn oxides. The hydrothermal deposits were collected from midplate hot spots, back-arc basins, volcanic arcs and mid-ocean ridges. Two Li fractions were examined: the more mobile fraction that is leachable by acetic acid and the relatively immobile fraction that remains in the oxide phase. Hydrogenetic and diagenetic deposits contains $<$1 to 10 ppm Li of which $25$ to $73%$ are mobile. Hydrothermal deposits are strongly enriched in Li (150 to 1400 ppm) that is predominantly in the leachable fraction, suggesting that almost all hydrothermal Li is loosely sorbed on the oxides. Mixed hydrogenetic/hydrothermal crusts show intermediate concentration levels. $\delta^{7}$Li ranges of acetic acid extractable Li on hydrogenetic $(23.7$ to $31.6%_o)$, mixed crust $(16.7$ to $32.8%_o)$, and hydrothermal $(30.3$ to $33.5%_o)$ deposits are mostly seawater-like $(32%_o)$. The residual Li in hydrothermal crusts $(28%_o)$ also resembles seawater. Mid-ocean ridge hydrothermal fluids worldwide have an average $\delta^{7}$Li of $7\pm 1 %_o$. Our results suggest that hydrothermal deposits have exchanged Li with seawater and lost the mantle signature with time. We conclude that Li concentration on ferromanganese deposits may serve as an index of hydrothermal flux to the ocean but its mantle isotopic signature may be erased by exchange with seawater.
DE: 4808 Chemical tracers
DE: 4870 Stable isotopes
DE: 4267 Paleoceanography
SC: Ocean Sciences [OS]
MN: 2004 AGU Fall Meeting