Laurie D Benton (202-686-4370-4393; Dept of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC 20015: benton@dtm.ciw.edu); Ivan Savov (Dept of Geology, University of South Florida, Tampa, FL 33620: isavov1@luna.cas.usf.edu)

Jeffrey G Ryan (813-974-1598; Dept. Geology, Univ. South Florida: ryan@chuma.cas.usf.edu)

Lithium isotope ratios were measured for serpentinites from Conical Seamount, a non-volcanic serpentine seamount in the Mariana forearc, to investigate the return flux of subducted Li in forearcs. Examples of low-grade slab metamorphism are usually inaccessible. However, at Conical Seamount, situated ~30 km above the subducting slab, serpentinites generated via interactions between upper mantle rocks and slab-derived fluids provide a unique opportunity to examine forearc chemical fluxes.

Lithium was isolated by cation exchange chromatography using a mixture of methanol and nitric acid, after digestion via HF:HClO4, and dilution in 1M HNO3. Isotopic measurements were made using a VG Plasma 54-30 multi-collector sector ICP-MS. Long term reproducibility is estimated at ±1.1‰. Lithium concentrations were determined by DCP by standard additions techniques, with a reproducibility of ±5%.

Samples were selected from ODP Leg 125 cores from Site 780 on the Conical Seamount summit and Site 779 on the southeastern flank. Clast interiors were chosen to avoid the effects of late stage reactions with seawater. Serpentinite clasts have d 7Li (sample 7Li/6Li relative to the L-SVEC standard) ranging from -0.4 to +7.5‰ (n=12) with Li concentrations ranging from 2 to 19 ppm. The predominantly silt-size serpentine matrix has d7Li between +5.2 to +9.8‰ (n=6) and 3 to 7 ppm Li. The Li isotope compositions do not covary with depth or with boron contents, suggesting that seawater interaction plays a minor role. Variation in clast values is likely due to mixing between lighter mantle-derived Li and a heavier slab component. Higher values for matrix are consistent with greater degrees of Li exchange with the upwelling fluids at the seamount. Given that altered seafloor basalts have significantly lower d 7Li than seawater, the slab-derived fluids upwelling at Conical Seamount are likely to be considerably heavier than the serpentinites analyzed.