HR: 17:00h
AN: V54A-05    [Abstracts]
TI: Microbeam Analyses of Rare-Earth Element (REE) and Sr Isotopes of Anhydrite Veins From the PACMANUS Hydrothermal System
AU: * Craddock, P R
EM: pcraddock@whoi.edu
AF: Woods Hole Oceanographic Institution,, Dept of Marine Chemistry and Geochemistry, 360 Woods Hole Rd,, Woods Hole, MA 02543 United States
AU: Bach, W
EM: wbach@whoi.edu
AF: Woods Hole Oceanographic Institution,, Dept of Marine Chemistry and Geochemistry, 360 Woods Hole Rd,, Woods Hole, MA 02543 United States
AB: Ocean Drilling Program Leg 193 drilled into areas of focused and diffuse venting from the PACMANUS hydrothermal field along Pual Ridge, eastern Manus Basin. Pual Ridge is primarily composed of fresh dacite that is pervasively altered to argillaceous (illite-chlorite) and overprinting acid-sulfate (pyrophyllite-anhydrite) and siliceous secondary mineral assemblages. Anhydrite is an abundant precipitate in all cores recovered beneath the active vent field. Isotopic (Sr, S), elemental (Mg, Sr, REE) and fluid inclusion analyses on anhydrite mineral separates demonstrate that pathways of fluid evolution (magmatic input, variable fluid composition, seawater entrainment) differ on the meter scale within this system. In addition, large variations in REE content, S ($\delta^{34}$S $>$ 5$\permil$) and Sr ($^{87}$Sr/$^{86}$Sr $>$ 1000 ppm) isotope composition are observed within individual anhydrite veins. Using microbeam techniques, we have analyzed this small scale chemical and isotopic heterogeneity in order to unravel the record of hydrothermal processes occurring at PACMANUS. Cathodoluminescence microscopy is used to distinguish between different crystal growth textures (e.g., oscillatory versus sector zoning). A Finnigan Neptune laser-ablation MC-ICP-MS is used to measure $^{87}$Sr/$^{86}$Sr, and a Cameca 3f ion microprobe and a Finnigan Element2 laser-ablation MC-ICP-MS to measure REE and minor element concentrations in these zones. Initial microbeam measurements of sector-zoned crystals reveal REE patterns trending from mid-REE enriched to MREE depleted with distance from crystal center and with a uniform $^{87}$Sr/$^{86}$Sr (0.7087). Analyses for oscillatory zoned crystals reveal variable and increasing $^{87}$Sr/$^{86}$Sr (0.7043 to 0.7089) and REE patterns trending from light-REE enriched to LREE depleted with both negative and positive Eu anomalies during crystal growth. $^{87}$Sr/$^{86}$Sr reproducibility for an isotopically homogenous anhydrite is better than 40 ppm. The Sr isotopic composition range for heterogenous crystals ($>$ 4000 ppm) is hence greater than 100 times the analytical uncertainty of our LA-MC-ICP-MS method. The general trend of these data can be modeled by Rayleigh-type fractional crystallization of anhydrite, with sector zoned anhydrite precipitating from a seawater dominated fluid and oscillatory zoned anhydrite forming from a hydrothermally dominated fluid following introduction of seawater into the basement. Future microbeam REE, Sr and S-isotope analyses are planned to provide a more comprehensive view of fluid evolution and mineral formation within this backarc hydrothermal system.
DE: 8424 Hydrothermal systems (8135)
DE: 4815 Ecosystems, structure and dynamics
DE: 4832 Hydrothermal systems
DE: 4803 Bacteria
DE: 3045 Seafloor morphology and bottom photography
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting