HR: 16:45h
AN: V12J-04    [PDF]
TI: Thermal Control on High Temperature Hydrothermal Circulation at Fast Spreading Ridges
AU: * BOUDIER, F
EM: Boudier@dstu.univ-montp2.fr
AF: University Montpellier II, Place Eugene Bataillon, Montpellier, 34090 France
AU: Nicolas, A
EM: Nicolas@dstu.univ-montp2.fr
AF: University Montpellier II, Place Eugene Bataillon, Montpellier, 34090 France
AU: Mainprice, D
EM: Mainprice@dstu.univ-montp2.fr
AF: University Montpellier II, Place Eugene Bataillon, Montpellier, 34090 France
AB: A deep and high-T (up to 1000$\deg$C) hydrothermal contamination of the oceanic crust at the ridge axis has been documented in the Oman ophiolite, and is confirmed by recent Sr and O isotopic investigations (Bosch et al. submitted). In the deep and hot gabbros, the main water channels are submillimetric microcracks with a dominantly vertical attitude. We propose that the mechanism allowing seawater ingression in the lower gabros, at temperatures above 700$\deg$C, is anisotropy of thermal contraction, opening microcracks that are controlled by fabric. The exceptionally large anisotropy of thermal contraction (109%) of single crystal of calcic plagioclase, when coupled with the strong lattice fabrics in the lower gabbros results in the maximum thermal contraction direction to be parallel to maximum crystals elongation L1. This direction in the oceanic ridge referential is horizontal and is perpendicular to microcracks dominant orientation. This high-T hydrothermal alteration in gabbros reaches the Moho. In the underlying peridotites, preliminary Sr isotopic data on clinopyroxene from wehrlites suggest that seawater was able to ingress at crystallization temperature for clinopyroxene. Interestingly, in these peridotites with horizontal foliation, thermal contraction, calculated as above from fabrics and thermal expansion coefficients in olivine, is vertical, being responsible for subhorizontal cracking, as deduced from serpentine dominant veining. Thus, during off-axis drifting of newly accreted lithosphere, thermal contraction opens vertical cracks, favoring seawater ingression down to the Moho. Below, in peridotites, the horizontal microcrack system would favor closing of hydrothermal circuit at Moho level.
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