HR: 1340h
AN: V23B-0626    [Abstracts]
TI: Lithium and strontium isotope compositions of serpentinite-hosted carbonate veins from the MAR (ODP Leg 209): Records of different stages of seafloor metamorphism
AU: * Rosner, M
EM: rosner@gfz-potsdam.de
AF: GoeForschungsZentrum Potsdam, Telegrafenberg B323, Potsdam, 14473 Germany
AU: * Rosner, M
EM: rosner@gfz-potsdam.de
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543 United States
AU: Bach, W
EM: bach@whoi.edu
AF: Woods Hole Oceanographic Institution, MS#8, Woods Hole, MA 02543 United States
AU: Paulick, H
EM: Holger.Paulick@uni-bonn.de
AF: University Bonn, Poppelsdorfer Schloss, Bonn, 53115 Germany
AU: Erzinger, J
EM: erz@gfz-potsdam.de
AF: GoeForschungsZentrum Potsdam, Telegrafenberg B323, Potsdam, 14473 Germany
AB: Geochemical investigations of carbonate veins hosted in serpentinized peridotites drilled from the Mid-Atlantic Ridge 14$\deg$-16$\deg$N (ODP Leg 209) were conducted to gain insights into temperature and composition of the alteration fluids from which carbonates were precipitated. We have examined carbonate veins that can be grouped into low-T ($<$20$\deg$C) aragonite veins hosted in serpentinite, low-T calcite veins hosted in troctolitic rocks, and high-T (90-220$\deg$C) calcite veins hosted in talc-serpentine schists. The geochemical and radiometric characteristics of these veins is presented in a companion paper (Bach, Paulick, and Rosner) in this session. Here we report on Li and Sr isotope compositions and their relations to age, temperature, and trace element composition. Strontium isotope compositions are similar to, or slightly lower than that of modern seawater. This is consistent with young radiocarbon ages ($<$55 kyrs) and limited Sr exchange between circulating seawater and basement at low temperatures.$\delta$$^{7}$Li values (relative to L-SVEC) of the majority of aragonite samples range between 14 and 20 $\permil$. At sites 1271 and 1274 on rift valley walls of inside corner highs north and south of the 15$\deg$20N fracture zone, the $\delta$$^{7}$Li values of aragonites increase systematically with depth. At site 1274 .$\delta$$^{7}$Li shows a weak positive correlation with 14-C formation ages. Our preliminary interpretation of the Li isotope data is a decrease in the Li isotope fractionation factor as temperatures increase with increasing basement depth. This interpretation would require a fractionation factor at 2$\deg$C of about 1.016 - significantly larger than $\alpha$ =1.011 proposed for biogenic aragonite formation at 25$\deg$C. The large fractionation observed in the upper part of the drilled cores could be due to lower temperature, different precipitation kinetics, variations in fluid chemistry (e.g., strongly variable Mg/Ca), or combination of these effects. Calcite veins from Sites 1271 and 1275 display at least two compositional groups. Low T calcites range $\delta$$^{7}$Li from 22 to 27 $\permil$ and precipitated at T$<$20$\deg$C from a fluid with seawater-like Sr isotope composition. Three high-T calcite veins record an earlier and deeper stage of metamorphism associated with detachment faulting. $\delta$$^{7}$Li values between 3 and 8 $\permil$ correlate with Sr isotope ratios (as low as 87Sr/86Sr=0.70385) and indicate a prominent contribution of Li and Sr leached from the peridotite basement.
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8135 Hydrothermal systems (8424)
DE: 1020 Composition of the crust
DE: 1045 Low-temperature geochemistry
DE: 1050 Marine geochemistry (4835, 4850)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting