HR: 0830h
AN: V21D-0551    [PDF]
TI: High Temperature Hydrothermal Circulation in the Deep Oceanic Crust - Sr Isotopes and Trace Elements Modelisation Constraints on the Origin of the Fluids
AU: * BOSCH, D
EM: bosch@dstu.univ-montp2.fr
AF: Universite de Montpellier II, Place E. Bataillon, Montpellier, 34095 France
AU: Lamour, M
EM: lamour@dstu.univ-montp2.fr
AF: Universite de Montpellier II, Place E. Bataillon, Montpellier, 34095 France
AU: Jamais, M
AF: Max Planck Institut fur Chemie, Abteilung Geochemie, Postfach 3060, MAINZ, 55020 Germany
AU: BODINIER, J
EM: bodinier@dstu.univ-montp2.fr
AF: Universite de Montpellier II, Place E. Bataillon, Montpellier, 34095 France
AB: Previous field, petrological and geochemical works have identified high temperature hydrous alteration traces throughout the gabbros of the Samail ophiolite. Temperatures have been calibrated for the successive stages of alteration, starting with orthopyroxene-pargasite coronas (above 975 \deg C) and ending with the low temperature (LT) lizardite serpentinisation (below 500 \deg C). Sr isotopic analyses performed on massive gabbros, dikes and veins and associated minerals depart from typical mantle signatures and are characterized by radiogenic Sr isotopic ratios suggesting seawater as the most likely hydrothermal contaminant. The main water channels may be submillimetric microcracks with a dominantly vertical attitude and constituting the recharge hydrothermal system, whereas dikes and veins represent the discharge part. This model requires that these dikes have been generated by hydration of the crystallizing gabbros via seawater penetration, near the internal wall of the LVZ-magma chamber, i.e. at temperatures well above the near 1000 \deg C temperature recorded so far. We used the numerical plate model of VerniŠres et al. (1997) to simulate the chemical evolution of Sr isotopes and some trace elements in fluids through the gabbro column. This approach takes into account mineralogical and porosity variations due to dissolution-precipitation processes, as well as variations of partition coefficients as a function of distance from the fluid source. The aim of modelling was twofold: (1) to provide estimates of the chemical evolution of fluids as a result of high-temperature interaction with gabbros, and (2) to constrain the fluid-rock ratios throughout the gabbros sequence. Such an approach sheds new lights on the importance of high temperature hydrothermal processes and on the geochemical modifications they induced during oceanic crust formation at fast spreading ridge. VerniŠres J., Godard M., Bodinier J.-L., 1997. A plate model for the simulation of trace element fractionation during partial melting and reactive magma transport in the Earth's upper mantle. J. Geophys. Res. 102, 24771-24784.
DE: 1020 Composition of the crust
DE: 1065 Trace elements (3670)
DE: 3035 Midocean ridge processes
DE: 5120 Plasticity, diffusion, and creep
DE: 8005 Folds and folding
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting